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PUBLICATIONS

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

The Electron Factor in

Catalysis on Metals

I
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tjtffonal Bureau of Standard

"'P'^ 1 9 1977 The Electron Factor in Catalysis on li/letals

Proceedings of a workshop held at the

National Bureau of Standards

Gaithersburg, Maryland, December 8-9, 1975

Edited by

L. H. Bennett

Institute for Materials Research

National Bureau of Standards

Washington D.C. 20234

Sponsored by

National Bureau of Standards

National Science Foundation

Energy Research and Development Administration

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

£ NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1977
Library of Congress Catalog Card Number: 77-600009

National Bureau of Standards Special Publication 475

Nat. Bur. Stand. (U.S.), Spec. Publ. 475, 217 pages (Apr. 1977)

CODEN. XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent ot Documents, U.S. Government Printing OflSce Washington, D.C- 20402 - Price $2.80

Stock No. 003-003-01764-7
GENERAL ABSTRACT

This book presents the proceedings of a Workshop on the Electron Factor in

Catalysis on Metals held at the National Bureau of Standards, Ga 1 thersburg, Maryland,

on December 8-9, 1975. The Workshop was sponsored by the Institute for Materials

Research, NBS, the Division of Materials Research of the National Science

Foundation, and the Division of Conservation Research and Technology of the

Energy Research and Development Administration. The purpose of the Workshopwas to review the most recent experimental and theoretical investigations

on catalysis on metals and related topics, and to bring together chemists,

chemical engineers, surface scientists, and solid state physicists and chemists

Involved In research related to this topic. These proceedings summarize the

four panel sessions into which the Workshop was organized: Experimental

Techniques, Effect of Alloying, Geometrical Effects, and Electronic Structure.

KEY WORDS: Catalysis; Characterization; ChemI sorpt ion ; Electronic factor;

Geometric factor; Metals; Surfaces.

iii

Table of Contents

WELCOME E. Ambler . 1

OVERVIEW OF THE WORKSHOP - L. H. Bennett 3

KEYNOTE ADDRESS

Catalysis by Metals: Concepts, Factors and Reactions - M. Boudart _ _ _ 5

session' 1: EXPERIMENTAL TECHNIQUES 13

PANEL MEMBERS: J. Katzer, R. Park, T. Rhodin, J. T. Yates

and G. Haller, Chairman- ____________________ 14

INVITED TALK: Experimental Methods in Heterogenous Catalysis

J.T. Yates, Jr 15

RECORDER'S SUMMARY - T.E. Madey . 30

PANEL DISCUSSION 3O

CONTRIBUTED PAPER: Investigation of Support Catalysis by

X-ray Absorption Spectroscopy - F.W. Lytle- __________ 34

PANEL DISCUSSION (continued) 44

SESSION 2: EFFECT OF ALLOYING 53

PANEL MEMBERS: G.D. Gelatt, Jr., V. Ponec,

J.H. Sinfelt and H. Ehrenrich Chairman _ ____________ 54

RECORDER'S SUMMARY- A.J. McAlister , 55

INVITED TALK: Catalysis by Alloys and Bimetallic

Clusters - J.H. Sinfelt ____________________ 55

PANEL DISCUSSION 68

CONTRIBUTED PAPER: Surface Composition of NickelCopper Alloys - V. Ponec _ ___________________ 71

PANEL DISCUSSION (Continued) 81

CONTRIBUTED PAPER Heat of Formation of 3d and 4d Transition

Metal Hydrides G. D. Gelatt, J. A. Weiss and H. Ehrenrich 83

PANEL DISCUSSSION (Continued) 87

SESSION 3: GEOMETRICAL EFFECTS 89

RECORDER'S SUMMARY- A. J. Melmed ^90

PANEL MEMBERS: G. Ertl, C.B. Duke,

J. Turkevich and R. Hansen Chairman ______________ 91

PANEL DISCUSSION 93

INVITED TALK: Geometric Factors in Chemisorption and

Catalysis on Metals -G. Ertl _ _ ______________ 94

PANEL DISCUSSION (Continued) 121

CONTRIBUTED PAPER: The Geometry of Solid Surfaces - C.B. Duke 127

PANEL DISCUSSION (Continued) 131

CONTRIBUTED PAPER: Bonding Properties of Stepped

Transition Metal Surfaces - G. S . Painter, R.O. Jones,

and P.J. Jennings _ ______________________ 136

PANEL DISCUSSION (Continued) I5O

SESSION 4: ELECTRONIC STRUCTURE 151

PANEL MEMBERS: A.B. Anderson, J.W. Gadzuk,

R.P. Messmer, J.R. Smith, and J.W. Gadzuk Chairman 152

PANEL DISCUSSION 153

INVITED PAPER: The Role of Model Hamiltonians in

Chemisorption and Catalysis- J.W. Gadzuk _ ___________ 154

PANEL DISCUSSION 164

V
CONTRIBUTED PAPER: Theory for Chemisorption and

Catalysis - A. B. Anderson- __________________ _ 166

PANEL DISCUSSION (Continued) 171

CONTRIBUTED PAPER: A Compairson of SCF-Xa and

Extended Huckel Methods for Metal Clusters - R.P. Messmer, C.W. Tucker Jr., and K.H. Johnson _ _______ 174

PANEL DISCUSSION (Continued) 178

CONTRIBUTED PAPER: A New Role for Theory in

Surface Science -D.R. Hamann _ _________________ 183

PANEL DISCUSSION (Continued) 193

REGISTRATION LIST : ^^195

AUTHOR INDEX 201

SUBJECT INDEX /'206

MATERIALS INDEX 208

vi
Welcome to the Workshop on the Electron Factor in Catalysis on Metals

E. Ambler

We are pleased to be joining the National Science Foundation and the

Energy Research and Development Administration in the sponsorship of this

workshop. As the Acting Director of the National Bureau of Standards, it

is my pleasant duty to officially welcome you this morning.

As some of you may know, this workshop is being held on the eve of the

Bureau's 75th anniversary. The National Bureau of Standards was founded by

Congressional legislation in 1901. Although that original legislation has

been amended several times, three themes have persisted that reflect the

scope of NBS activities. First, it is our responsibility to provide the

nation's standards of measurement, second, we determine physical constants

and properties of materials, and finally, we provide other agency support.

Approximately 3,500 employees work at our site here in Gaithersburg, our site

in Boulder, Colorado and our radio stations in Fort Collins, Colorado and the

Hawaiian Islands. Our activities range from maintaining the nation's primary

frequency standard and determining precise atomic constants and data, to

carrying out fire research and investigating some aspects of the process of

catalysis. That is to say from making precise measurements on very well

defined systems to attempting to unravel processes that are inherently very

complex.

When I joined the Bureau nearly 25 years ago as a bench level physicist,

the process was of great economic value to this nation's commerce, particularly

to the oil industry. Much has happened in the intervening 25 years enabling

us to better understand the basic nature of the catalytic process.

1
Moreover, catalysis remains not only a significant factor in our industrial

economy, it has become of even greater importance as we strive to clean up

our environment and move toward energy independence. There is an increasing

need to invent less expensive catalysts and to find ways of extending life

of those now in use.

Science has made great progress in this field over the same time period.

Twenty years ago we could only speculate on the role the electron factor might

play in the catalytic process. Today, we are able to investigate in som.e

detail the role played by the valence electrons of the various constituents

Involved in a reaction. Over the past few years, meaningful considerations

of the electron factor in catalysis have been made more detailed and specific

largely because of the new techniques developed, especially within the surface

science community.

I have always felt that a sign of vigor in a given area of science and

technology is when theorists and practitioners stand side by side and work

on the same problems. This is the case with catalysis. This workshop offers

an opportunity for workers from many fields—from theoretical physicists to

surface chemists to catalytic engineers--to broaden and strengthen the common

thread of communication. Hopefully, this type of interaction will send us back

to our respective home bases with increased enthusiasm and renewed dedication.

My best wishes for a most successful workshop.

2
Overview of the Workshop

L. H. Bennett

Traditionally, the term electron factor in catalysis has denoted a

relationship between catalytic activity and the bulk electronic structure of the catalyst. While there is abundant empirical evidence of the

Importance of the electron factor in heterogeneous catalysis, its precise

definition and relation to other factors, particularly the geometric

factor, have yet to be established. The interdependence of electronic

and geometric factors in bulk bonding is well known: examples include

the Hume-Rothery rules of alloy phase stability, relating valence, size,

and electronegativity; and Pauling's famous equation relating bond

length to bond strength. Chemical processes at a solid-fluid interface

have not, thus far, been similarly systematized. However, many new and

powerful techniques for the analysis and characterization of clean

surfaces and small particles, and of chemical complexes on such surfaces

have been developed in recent years. It was the aim of this workshop to

move toward systematic understanding of this problem through interdisciplinary action. Specific panel topics were: Experimental Techniques, the Effects of Alloying, Geometrical Effects, and the Theory of

Electronic Structure.

The workshop brought together workers in a range of fields including

theoretical physicists engaged in band structure and surface calculations.

3
theoretical chemists doing chemical bonding calculations, surface chemists working on clean surfaces and single crystals, practical catalytic

chemists engaged in small particle and support effect studies, electrochemists concerned with activity in aqueous environments, etc.

While progress toward quantitative expression of the Electron

Factor in catalysis was limited, this aim served as a focus for the

clarification of the capabilities and accomplishments of many experimental and computational techniques. The interdisciplinary emphasis of

the conference was one of its most fortunate aspects; many of the

distinguished experts present found clarification of the relation

between their specialties and catalysis.

4
Keynote Address

Catalysis by Metals:

Concepf-s, Factors and Reactions

M. Boudart

Department of Chemical Engineering

Stanford University

Stanford, Galifornia 94305

5
INTRODUCTION

In fhe past few yecrs, there has been a growing number of studies overlapping

the physics and chemlsiry of metal surfaces. TViese have generated inquiries into the

physical basis of heterogeneous catalysis (I) or the relationship between so-called surface

science and catalysis (2) or the impact of the new physical tools on surface chemistry (3),

In spite of the limited thrust of these recent studies, confined as they are to metals which

constitute only a very small fraction of catalytic materials, the new results have already

changed many of the traditional concepts of heterogeneous catalysis. Although these

results obtained often at very low pressures on large chunks of metals may not seem at first

to be relevant to catalytic reactions run at high pressures on metallic clusters containing

about one hundred atoms, closer examination shows how concepts and even reaction

mechanisms can be transposed from low to high pressures (4) or from single crystals to small

clusters (5). The purpose of this introduction is to survey some of the new emerging concepts

or some modifications in the old concepts In heterogeneous catalysis by metals and alloys.

CONCEPTS

The catalytic reaction engineer (6) uses a number of concepts evolved between

about 1920 and I960 in what might be colled the Langmuirian period of surface science. In

the post-LangmuIrlan period, new concepts have emerged which will be taken up by the next

generation of practi tioners of heterogeneous catalysis. As happens frequently in science,

the new concept does not supplant the older one but extends it to new situations which

were not envisaged in the past.

Thus, the Langmuirian Idea of adsorption site remains valid of course but recent work shows

that off-site adsorption Is important not only in the case of physisorption but also in chemlsorbed

compressed layers, for instance of carbon monoxide on palladium (7). This is an Important

new idea, as carbon monoxide is frequently used to titrate metallic sites and also because

6
carbon monoxide and hydrogen are fhe basis of many catalytic processes toward the synthesis

of fuels and chemicals. Besides, off-site chemisorption may be a widespread phenomenon.

In fact, since the compressed layer forms a coincidence lattice over the metal lattice,

off-site chemisorption is related to corrosive chemisorption or surface reconstruction where the

adsorbate forms with the outer layer of metal atoms a coincidence lattice over the subjacent

metal lattice, as for Instance in the case of sulfur over low Index faces of copper (8). Tbis

is very different from the Langmuirian pictures of immutable checkerboards. Corrosive

chemisorption Is the first step toward destructive aging of a metallic catalyst.

Another fundamental Idea of Langmuir was the monolayer due to saturation of the

adsorption sites. Many examples are now known of ordered surface structures consisting of

definite fractions of a monoloyer . As many chemlsorbed molecules are used to titrate metallic

surface sites, a knowledge of these non-classical stolchlometries and of the conditions under

which they are obtained, is very desirable in applied catalysis. Even more important perhaps

is the reason behind these ordered chemlsorbed sub-monolayers, namely attractive forces

between chemlsorbed species on metals. While repulsive forces between chemlsorbed species

hove been discussed many times during the Langmuirian periods, attractive forces between

adsorbotes were recognized only In the case of physlsorption. Quite recently, attraction

between chemlsorbed species on metals has been discovered not only experimentally but

also theoretically (9). The role of these interactions in catalytic reactions remains to be

explored.

Another Langmuirian postulate was that a molecule striking a site would rebound if

that site were already occupied. It is now clear that this site exclusion principle is violated

In many Instances, at least on clean metal surfaces, where the striking molecule may be held

at the surface in a weakly bound precursor state long enough so that it can diffuse to the

site where adsorption takes place (10). These precursor states must be of great Importance

7
In cafalytic reactions. They seem fo be bound to the metal surfaces with energies intermediate between those fognd in physisorption and chemisorption . A striking example Is

undlssoclated methane on tungsten (100) with a binding energy of more than 28 kJmol ' (II).

Another advance of the new era is the identification of adsorption sites with defined

surface atoms. This is still a particularly controversial area but rapid progress is being made (12)

An early example Is Estrup's assignment of hydrogen atom chemisorption to bridging positions

between any pair of tungsten atoms at a W(IOO) surface (13). Thus for the first time, it is now

possible to treat adsorbote-metal complexes with almost normal ideas of molecular structure

and bonding.

As to kinetics of desorption, Langmuir first proposed, besides a rate proportional to

fraction of surface covered 9, a rate decreasing exponentially with 9, the first of a long series

of phenomenologlcal treatments of adsorption, desorption and catalytic reactions to which the

names of Temkin, Zeldovlch and Wagner are attached in particular (14). Quite novel on the

contrary Is the finding by Madix of a self-accelerating (autocatalytic or explosive) rate of

desorption from a NI(IIO) surface on which formic acid has been preadsorbed (15). This Is the

first new kinetic pattern of surface reactivity in forty years and is a remarkable example of

non-classical behavior.

Finally, consider the two modes of catalytic reactions between molecules A and B

either involving reaction between both A and B in chemisorbed states (Langmuir-Hinshelwood)

or involving reaction between non-chemlsorbed B with pre-chemlsorbed A (Rldeal-Eley) . The

distinction between these modes of reaction has generated byzantlne discussions over the past

thirty years. Yet, today, a decision between them can be reached from molecular beam

reactive scattering studies now performed in a number of post-LangmuIrian laboratories.

In summary, off-site chemisorption, corrosive chemisorption, fractional ordered

monolayers, attraction between chemisorbed species, precursor states, adsorption sites with

8
assigned sfrucfures, explosive surface desorpHon, molecular beam scaffering at surfaces,

are {usf a Few examples thaf have already changed profoundly our Langmuirian ideas of

looking af catalytic reactions at metallic surfaces. The new findings should contribute to

the clarification of the traditional factors that were formulated during the Langmuirian

period of catalysis: the electronic and geometric factors.

FACTORS

The old distinction in catalysis by metals between geometric and electronic effects

is confusing and retains by now only historical interest.

Instead of geometric effect, it is better to talk about effect of structure , the latter

being defined by the distribution of surface atoms of given coordination numbers, as varied

by exposing various crystal lographic planes or changing particle size in the critical range

between I and 10 nm (16). Structure is different from geometry. Indeed, another postLangmuirian example consists of hexagonal overlayers on the (100) planes of iridium,

platinum and gold (17,18). But although the geometry of the atoms in these overlayers is

the same as that found on the (III) faces of these metals, the structure, as defined here, is

different in both cases (19) . Besides, the ultimate difference between atoms of different

geometry or structure, must be ascribed in last analysis to electronic effects, as pointed

out a long time ago (20)

.

Thus, it is best to avoid not only the geometric factor but also the electronic factor

as the latter also denotes reasonings associated with the now discarded rigid band theory of

alloys. Perhaps the best alternative to electronic effect would be ligand effect introduced

by Sochtler and his school to denote the change in reactivity of a metallic atom A when some

or all of its A neighbors are replaced by atoms of metal B. Similarly then, the difference in

reactivity between two surfaces of pure metals A and B of Identical structure, would be

ascribed to the different ligand factors of A and B."

9
Anofher factor related to the ill-defined geometric effect is that which Is related

to the need for more than one surface atom In the rate determining process of the catalytic

reaction. What is needed perhaps Is a multiple site, or multlplet, or ensemble, the properties

of which may be affected by either or both of the structural and llgand factors as defined

here. To determine a priori the relative Importance of these factors in catalysis by metals

and alloys, ultimately in a quantitative manner, remains a formidable challenge in our

post-LangmuIrlan era of surface science.

REACTIONS

What can be done today is to survey the empirical evidence in order to rank the

relative importance of structural and llgand factors for various catalytic reactions In the

hope of achieving classifications which suggest future work of a fundamental nature.

Such a survey has been conducted (21) and though the list of reactions Is thus

for limited, it is noteworthy that ten years ago, this simple task would have been Impossible

for lack of data and even five years ago, it would have led to an ever much more restricted

list of reactions.

In essence, what has been found for reactions performed in excess hydrogen on metals

of Group VIII and alloys between metals of Groups VIII and lb is that two classes of

reactions emerge. In the first class are reactions involving breaking or making of H-H bonds

and C-H bonds, e.g. hydrogenation of alkenes and aromatics. in the second class are

reactions where C-C bonds are broken as in hydrogenolysis or cracking as well as ammonia

synthesis or decomposition where N-N bonds ore broken or made.

Reactions of the first class are found to be stnjcture insensitive while reactions of the

second class are structure sensitive. When metals of Group VIII are surveyed for their

catalytic activity, it Is found that rates for reactions of Class I vary much less than rates for

10
reactions of Class II. Finally, in shjdies of the effect of adding Group lb atoms to Group

VIII metals, this latter effect is found to be much more important for Class II than for Class I

reactions.

To explain these findings may not be as formidable a task as predictions of catalytic

specificity of metals. Even so all that can be done today with the available classification

is to speculate. One possibility is that in reactions of Class I, a single surface atom or

maybe at the most a pair of them is required in the rate determining process of the catalytic

reaction. By contrast in reactions of Class II, a multiple site might be required. To check

this simple hypothesis may not be an impossible or remote task in the age of post-Langmulrian

surface science.

REFERENCES

1. The Physicol Basis for Heterogeneous Catalysis , E. Drauglis and

R. I. Jaffee, Eds. Plenum Press, New York, 1975.

2. M. Boudart, Chem. Tech . 4, 748 (1974).

3. J. T. Yates, C & E News, August 26, 1974.

4. M. Boudart, D. M. Collins, F. V. Hanson and W, E. Spicer,

J. Vac. Sci. Technology, In press.

5. M. Boudart, in Interoctions on Metal Surfaces, R. Gomer Ed.,

Springer-Verlog, Berlin Heidelberg 1975, p. 286.

6. J.J. Carberry, Chemical and Catalytic Reaction Engineering,

McGraw Hill, New York 1976.

7. J. C. Tracy and P. W. Palmberg, J. Chem. Phys ., 5], 4852 (1969),

8. J. B^nord, Catal. Rev . 3, 93 (1970).

9. T. L. Einstein and J. R. Schrieffer, J. Vac. Sci. Technol. 9, 956

(1972).

10. G. Ehrlich, J. Chem. Phys. 59, 473 (1955).

11. J. T. Yates and T. E. Modey, Surface Sci. 28, 437 (1971).

11
12. T. N. Rhodin and D. L. Adams, TreoHse on Solid State Chemistry, B. Honnay, Ed., Vol 6A, Chap. 5, in press.

13. P. J. Estrup and J. Anderson, J. Chem. Phys. 45, 2254 (1966).

14. M. Boudart, In Treatise of Physical Chemistry , H. Eyring, W. Jost and

Henderson, Eds., Vol. 7, Chap. 7, Academic Press, Inc., New York 1975.

15. J. L. Falconer, J. G. McCarty and R. J. Madix, Surface Sci . A2, 329 (1974).

16. R. Van Hardeveld and F. Hartog, Surface Sci. 15, 189 (1969).

17. T. N. Rhodin, P. W. Palmberg and E. W. Plummer, in Tlie Structure and

Chemistry of Solid Surfaces , G. A. Somorjai, Ed., Wiley, New York

1969, paper No. 22.

18. T. A. Clarke, I. D. Gray and R. Mason, Surface Sci . 50, 137 (1975).

19. C. R. Helms, personal communication.

20. M. Boudart, J. Am. Chem. Soc , 72, 1040 (1950).

21. M. Boudart, Proc. Vlth Intern. Congr. Catal., Pitman Press, Both,

in press.

12
SESSION 1

EXPERIMENTAL

TECHNIQUES

Moderator:

G.HALLER

Yale University

Lecture by:

J.T.YATES

NBS

13
Session 1.

Experimental Techniques

Panel Members:

J. Katzer,

Univ. of Del.

R. Park,

Univ. of f\/ld.

T. Rhodin,

Cornell Univ.

J. T. Yates,

NBS

Recorder:

T. E. Madey,

NBS

14
EXPERIMENTAL METHODS IN HETEROGENEOUS CATALYSIS

J. T. Yates, Jr.

Siirface Processes and Catalysis Section

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 2023*+

I. Introduction

Research in the field of heterogei eous catalysis currently involves the

use of many types of measurement techniques. Ultimately one vishes to employ

these measurement techniques to design catalysts which enhance the rates and

selectivity of catalytic processes as well as the useful lifetime of the

catalyst. Along the way to achieving these very practical objectives, we

have the exciting intellectual possibility of understanding the atomic and

electronic features of complex chemical processes which are specifically

promoted on the siarface of solid catalysts.

Ideally, fundamental research on a catalytic process should give us

definitive information of three kinds:

1. Character and surface concentration of active sites.

2. Identity of catalytic intermediates and mechanism of the reaction.

3. Rate of the catalytic reaction.

In practice, we must often be satisfied with less than this complete

picture of the catalytic reaction - in many cases this is because of our

limited ability to make the necessary physical or chemical measurements.

This is particularly true in the case of catalytic intermediate identification

since spectroscopic techniques often detect the major adsorbed species which

may not be catalytic intermediates.

In this short paper I want to give a brief summary of some of the best

examples of current catalytic research which illustrate the state of the art

in measuring definitive features of the three types listed above. A longer

15
review is available covering other topics related to this subject.

II. Measurement of the Character and Siirface Concentration of Active

Sites using Electron Spin Resonance

Electron Spin resonance (ESR) techniques are especially well suited for

quantitative studies of the natiire of active sites on catalysts. The method

is exceedingly sensitive, and when applied to high surface area solids, is

capable of detection of ^10 spins/cm . In recent work by Boudart and

(2) coworkers , the active site on MgO which catalyzes '^^I'Q^ exchange has been

characterized, using ESR. It has been found that approximately one site

in 10^ is active for exchange. The site is postulated to be an array of

0 radical ions associated with a nearby hydroxyl group which participates

in exchange. There is a remarkable correlation of the catalytic rate of

H^/Dg exchange with the concentration of active sites measured, over a range

of several orders of magnitude. By appropriate surface treatment, the active

site may be caused to appear and disappear and a concomitant variation in

catalytic activity is observed.

(3)

' In another thoroiigh ESR investigation, Voorhoeve and coworkers have

studied the ESR spectrum of WS^ catalysts. The objective was to determine the

nature of active sites for benzene hydrogenation as well as for other reactions.

Again, it was found that catalytic activity was associated with surface defect

+3 +U sites where W ions are |)resent in contrast to W ions in the normal crystal

+3 -2

lattice. The W sites exhibit lower coordination with neighboring S ions

+U than is found in the case of bulk W ions. These sites are preferentially

located at the edges of WS2 crystallites; enhancement of their concentration

by various methods including preparation of sulphur deficient non-stoichiometric

+3

WSg leads to increased catalytic activity which is proportional to the W

16
ESR signal over a range of more than 3 orders of magnitude as shown in

+3

Fig. 1. It is posttilated that W preferentially binds benzene via a

It - complex interaction and that catalytic hydrogenation of these bound

species occurs.

III. Measurement of the Structure of Adsorbed Species and Catalytic

Intermediates using IR Spectroscopy .

Infrared (IR) spectroscopy has become a widely used technique for the

(U 5) study of heterogeneous catalysis. ' In general, it has been foimd that

the principles employed in interpreting IR spectra of chemical compounds

are also useful in studies of adsorbed species. Thus, the concept of group

vibrational frequencies associated with different functional groups seems

to be valid on surfaces just as in molecules. Also, the belief that electronic

effects cause small shifts in group vibrational frequencies is widely accepted.

IR spectroscopy has been applied to the study of the mode of adsorption

(6 7) of hydrogen by ZnO catalysts. ' Through the use of HD as an adsorbate,

it was found that hydrogen chemisoiTption occurs on a ZnO pair site, with

D...H

preferential adsorption in the | ) conformation at low temperatures.

Zn 0

Heating to 300 K causes an irreversible change of the DH conformation to

H...D

\ I as shown by the IR intensity behavior in Fig. 2. Control experiments

Zn 0

involving or Hg adsorption indicate that preferential Zn-D or 0-H adsorption

(8) does not occur for these homomoleciilar adsorbates.

Kinetic and thermodynamic isotope effects are postulated to be responsible for this unusual behavior. Irrespective of the explanation of

the effect, it is necessary to conclude that adsorbs on a ZnO pair site.

Thus, in this example, it is seen that IR is a powerful tool for ascertaining

the general bonding nature of an adsorbed species.

17
A second exeimple of the utility of IR spectroscopy for structural

studies of adsorted species has to do with CO chemi sorption by transition

metals. It is well known that two general kinds of CO are often observed

to adsorb on transition metals - a form with CO stretching frequency in

the range 2000-2100 cm [linear-CO, sp hybridized] and a form exhibiting

—1 2 peak adsorbance below 2000 cm [bridged-CO, sp hybridized]. There has

long been a controversy about the assignment of the "bridged-CO species".

(9) In a recent study, Sachtler and coworkers investigated the spectrum of

CO on a range of Pd/Ag alloys. On pure Pd, the major infrared band is the

"bridged band". As Ag is alloyed with Pd, there is a diminution of the

bridged band and an increase in intensity of the linear band as shown in

Fig. 3. This is interpreted as being due to the statistical reduction of

Pdg sites due to Ag alloying. A secondary feature of the experiments is

also of importance. Over a wide range of Pd/Ag alloy compositions, the two

CO bands are observed to exhibit almost constant frequencies. This behavior

may be ascribed to the electronically independent nature of the constituent

atoms in the alloy. This conclusion has been confirmed by recent x-ray

photoelectron spectroscopy (ESCA) valence band studies of Pd/Ag alloys where

it is seen that the d-electrons from each atom are behaving atomically, in

contrast to the predictions of the rigid band model for alloys .

^ "''^ ^

IV. Measurement of the Electronic Character of Adsorbed Species Using

Ultraviolet Photoelectron Spectroscopy (UPS)

The use of monochromatic ultraviolet light to eject valence level

photoelectrons from solids and from surfaces containing adsorbed species

has recently become widespread in surface physics and chemistry. For a metal,

the highest energy photoelectrons will be ejected from the top of the conduction band; electrons of lower kinetic energy will be generated in an energy

distribution curve due to photoemission throughout the band to a depth of

(hv-(f), where (J» is the work function of the surface. Because of the short

18
no-loss escape depth for photoelectrons in the range 10-Uo eV, the UPS

method is surface sensitive, sampling only several atomic layers.

For a metal plus an adsorbate, photoemission will include a joint

contribution from both the metal and the adsorbate. Subtraction of the

that obtained for the clean metalj yields a difference spectrum characteristic

of photoemission from the adsorbate; in addition, characteristic intensity

losses from the metal sire seen as the density of states in the metal is

modified by adsorption.

The power of UPS for studies of adsorptive bonding are well illustrated

by the work of Demuth and Eastman^ (Fig. k) , where difference spectra

for CgHg, CgHj^ and C^H^ adsorbed on Ni(lll) are shown, in comparison with

gas phase UPS spectra for the same moleciiles. In the lower panel of Fig. U

a comparison of physically adsorbed C^H^ with C^^ig) is made, and the

correspondence between the broad featiires due to a„„ and a orbitals is good- en Q/C/

In this comparison, the energy scales have been shifted to eliminate the

effect of screening of final electron-hole states by the metal electrons.

Thus, CgH^ in the physically adsorbed mode of bonding is very similar to

C2Hg(g) in its orbital spectrum.

When the same type of comparison is made for chemisorbed CgHj^ and

CgHgj it is seen that the a levels correspond well with the gas phase spectra,

but the ir-levels are shifted to higher binding energies by about 1 eV.

On this basis, it is concluded that chemisorptive bonding of and CgHg

occurs mainly via a ir-interaction. In these cases, ir-bonding to a single

Ni atom is the preferred model, rather than the often postulated di-a bonding

to 2 - Ni atoms with destruction of the 7r-bond. The constancy in energy of

photoelectron energy distribution curves minus

19
the various <y-levels upon adsorption is also consistent with 20% re2 hybridization of the sp orbitals in C^Hj^ upon chemisorption by Ni(lll),

based on an SCF-LCAO calculation, it -bonding of this type is widely recognized in organometallic compounds; recently Zeise's salt (involving CgH^

ir-bonding to one corner of a square planar Pt Cl^ complex) has been studied

(12) by neutron diffraction in order to locate the H's of the CgH^^. The

o CgHj^-hydrogens are pushed back by 0.l8 A from the plane of the C^Hj^ molecule

2 in this compound, suggesting partial rehybridization of the carbon sp orbitals.

In addition, the C=C stretching frequency is decreased by "^6% implying that

for distortion of this magnitude, the ir-bond remains essentially intact in

the ligand.

V. Kinetic Measurements of the Rate of Catalytic Reactions

In the last 10 yeaxs, much progress has been made in the measurement

of the rate of catalytic reactiocs on surfaces. For dispersed catsilysts,

this advance has been due in part to the introduction of the practice of

measuring specific catalytic activity, i.e., the rate of reaction per unit

area of active catalyst. The active surface area may best be determined

by a niimber of techniques involving chemisorptive uptake. ^^^'^^^ For

catalysts being studied using ultrahigh vacuum methods such as Auger

spectroscopy, ESCA, LEED, molecular beams, etc., the use of single crystals

of high bulk and siirface purity has been of major importance.

Boudart and coworkers have classified a nimiber of catalytic reactions

into two general categories - those which are sensitive to the catalyst

surface structiare (demanding reactions) and those which seem to be insensitive to catalyst surface structure (facile reactions ) ^^^^

. In the

case of the hydrogenation of cyclopropane over Pt catalysts (a facile reaction)

it has been found that the rate of this reaction per surface Pt atom is

20
essentially constant over a very wide range of catalyst particle size from

highly dispersed Pt crystallites on Al2^3 ^^^2 ^^PP°^'''2 foil and Ft

( 16

)

single crystals. Resiilts of this type help to establish a strong link

between catalytic studies on dispersed and single crystal catalysts. It

should be possible to employ many of the modern methods of surface physics and

chemistry to the study of this and other similar reactions with assurance that

the results will be applicable to the processes which occur on a practical

catalyst

.

A second example of the study of catalytic rates on single crystals

(17) comes from the work of McAllister and Hansen. The decomposition of

^^(g) at high pressures has been studied on three single crystal planes

of tungsten, starting with atomically clean tungsten in an ultrahigh vacum

environment. It was shown (Fig. 5) that the rate of the reaction follows

the expression,

rate = A -H B P^^2/3

The "A" process is thought to be the rate of desorption from the

essentially fully covered W-N surface. The "B" process is tho\ight to occur

on a complete W-N adlayer with complex intermediates being produced which

eventually lead to and products. It should be noted that the W(lll)

plane is more active than W(lOO) or W(llO) for this decomposition process.

No model is presently available to explain this structural sensitivity.

VI. Conclusion

In this talk, I have attempted to select examples of recent work

which illustrate some of the methods employed for studying the character of

catalytic sites, catalytic species, and chemical kinetics at catalytic

surfaces. At present, we are just beginning to achieve new insights through

( 18

)

the use of the newer methods of siarface science for the study of catalysis.

The future is bright for the eventual understanding of the structural chemistry

21
involved in heterogeneous catalysis, and possibly for the application

this knowledge to the design of better catalysts.

22
REFERENCES

(1) T. E. Madey, J. T. Yates, Jr., D. R. Sandstrom, and R.J.H. Voorhoeve,

Treatise on Solid State Chemistry, Plenum Press, Vol. 6, Chapter 6 (1976),

(Ed. N. B. Hannay)

(2) M. Boudart, A. Delhouille, E. G. Deroyane, V. Indorina, and A. B.

Wallers, J. Am. Chem. Soc, 9]+, 6622 ^1972).

(3) R.J.H. Voorhoeve, J. Catal., 23, 236 (1971).

ik) L. H. Little, Infrared Spectra of Adsorbed Species, Academic Press,

London (1966).

(5) M. L. Hair, Infrared Spectroscopy in Surface Chemistry, Marcel Dekker,

New York (I967).

(6) R. P. EischenS, W. A. Pliskin and M.J.D. Low, J. Catal. 1, I80 (1962).

(7) R. J. Kokes, A. L. Dent, C. C. Chang, and L. T. Dixon, J. Am. Chem.

Soc, ^^19 (1972).

(8) A. L. Dent, private communication.

(9) Y. Soma and W.M.H. Sachtler, Japan J. Appl. Phys. Suppl. 2, Pt. 2,

2U1 (197I+).

(10) S. Hufner, G. K. Wertheim, J. H. Wernick, Phys. Rev. B8, 1+511 (1973).

(11) D. E. Eastman and J. E. Demuth, Proc. 2nd Intern. Conference on Solid

Surfaces, Japan J. Appl. Phys., Suppl. 2, Pt. 2, 827 (197^+).

(12) W. C. Hamilton, Acta Cryst., 25A, 5172 (XIV-U6) (1969)-

(13) J. H. Sinfelt, Catal. Rev., 3, 175 (1970).

ilk) J. E. Benson, H. S. Hwang, and M. Boudart, J. Catal., 30, 1^+6 (1973).

(15) M. Boudart, A. Aldag, J. E. Benson, N. A. Dougharty, and C. G. Harkins,

J. Catal., 6., 92 (1966).

23
(l6) D. R. Kahn, E. E. Petersen, and G. A. Somorjai, J. Catal., Sh, 29k (19TU).

(it) J. McAllister and R. S. Hansen, J. Chem. Phys., 59, ^1^+ (19T3).

(18) J. T, Yates, Jr., Chem. and Engineering News, 52, 19 (I97U).

24
10° lo' 10^ 10^ lo'*

INTENSITY OF w'"*" ESR

1. Identification as W as the active site in benzene hydrogenation

over WSg-based catalysts "by the linear relation between the ESR

intensity and the hydrogenation rate constant.

25
Fig. 2. Infrared measurement of the thermal equilibration of species

produced by adsorption of HD on ZnO at -195°C.

26
2100 2000 1900 1800 1700

I in—

Fig. 3. Infrared spectrum in the CO stretching region for CO adsorbed on

Pd or Pd/Ag alloys. The IR band near 1950 cm is assigned to

bridge-bonded CO; the band near 2060 cm is assigned to linear

adsorbed CO.

27
I

'

I

' I

I I

'

I

'

I

' I

'

18 16 14 12 10

J 1 I I I L__i

16 14 12

I I I

'

I I I .

I I 14 12 10 8 6 4 2 Ep=0

ELECTRON BINDING ENERGY (eV)

Fig. k. Ultraviolet photoelectron difference spectra of organic

moleciiles adsorbed on Ni(lll).

28
Fig. 5. Comparison of NH decomposition rates on three tungsten single

crystals at similar temperatures. The NH^ pressure is in units

-3

of 10 Torr.

29
PANEL DISCUSSION: EXPERIMENTAL METHODS IN CATALYSIS

Chairman: Professor Gary Haller, Yale University

Recorder: Dr. Theodore E. Madey, National

Bureau of Standards

Panel Members:

Professor James Katzer, University of Delaware

Professor Robert L. Park, University of Maryland

Professor Thor Rhodin, Cornell University

Dr. John T. Yates, Jr., National Bureau of Standards

The session chairman. Professor Haller, begaji the discussion by

introducing the panel members and asking each of them to define some of

the problem areas in experimental methods related to catalysis. Following

their opening remarks. Dr. Farrel Lytle presented a paper in which he discussed the utility of extended x-ray absorption fine structure (EXAFS) in

studies of supported catalysts. Finally, the audience and panel participated

in a free-wheeling dialogue concerned with modern methods and concepts

in catalysis.

In the following pages, we have attempted to paraphrase the essence

of these three distinct phases of the panel discussion. Except for the

paper by Dr. Lytle, which is printed in the form he supplied, the remainder

of the account is based on our (Haller, Madey, Yates) collective recollection.

I. Opening remarks by Panel Members

Professor Katzer noted that there is too frequently not much

interaction between experiment and theory, and he called for stronger such

interactions. He made a few remarks concerning the proper interaction

30
between experiment and theory and suggested that the minimvun interaction

requires the use of fundamental chemical principles such as the laws of

thermodynamics. Katzer pointed out that one of the major problems is that

experimentalists measure what is most easily measured and theorists calculate

what is most easily calculable, and frequently the two don't really get

together. He proposed reducing catalysis to its most irreducible factor

and suggested, following some earlier remarks of Professor John Turkevich,

that d-electrons and transition metals are the most irreducible factor

on the one side and protons on the other. He realized, of cotirse, that

this is an over-simplification. Another issue which he addressed concerned

Professor Boudart's earlier discussion of the structure sensitivity of reactions,

and he raised the issue as to what is the most irreducible state of a supported

catalyst. Does it consist of 2 to 5 atoms or 5 to 10 atoms: Just what is

the smallest size of active catalyst? Characterization of the structvire

sensitivity of reactions necessarily involves larger samples involving clusters

of many ('-lOOO) atoms. These clearly present special problems for theorists

because of the large number of atoms involved. He then turned from the

discussion of solid state problems to surface problems, in the sense that he

tried to define the most important step in catalysis. What is the rate

limiting step in catalysis? Is it dissociation of the reactant (as in the

case of ammonia synthesis) or is the rate controlling step a bimolecular

reaction on the surface, or is the rate controlling step dissociation of

stable surface intermediates? He noted that real progress and \mderstanding

of gas phase reactions came with the use of spectroscopic tools to follow

the decomposition of intermediates. Similar studies are just now beginning

on surfaces . He made a plea to bridge the gap between work on single crystals

and real practical catalysts, between low and high pressures, as well as a

31
plea to bring experiment and theory closer together. He thought that

the theorists can best accomplish this by calculating trends as one goes

from molecule to moleciile or sample to sample.

Professor Rhodin opened his comments by asking to what extent

does the quantum description of chemical bonding obtained from calculations

and electron spectroscopies of clean surfaces contribute to an understanding

of simple chemical reactions. The corollary question to this is: how

does the understanding of simple chemical reactions contribute to a better

understanding of industrial catalytic processes? He attempted to answer both

questions by stating that the study of simple reactions or adsorption on clean

siirfaces allows one to develop and test new concepts, and that conceptual

principles can be applicable to practical catalysts. X-ray and vacuum ultraviolet photoemission, ion-neutralization, field emission, appearance potential.

Auger and electron loss spectroscopies all provide information on electron

structure of the solid surface and UV photoelectron spectroscopy provides

information on the chemical nature of adsorbed molec\iles as well. While all

spectroscopies are rather limited in theoretical imderstanding, the clarification of the chemisorption process on well defined metal surfaces is a first

step in understanding simple chemical reactions on metals. Rhodin cited

the work of Ertl on CO oxidation in high vacuum on clean metals as an example

of a simple chemical reaction which appears to attain the same characteristics

as when carried out under practical conditions 10 orders of magnitude pressure

change). He indicated that this may be a situation of some generality for

reactions where the pressure ratio of reactants and products (and not absolute

pressxare) is a critical factor. Rhodin believes it may be possible to make a

compilation of rate constants for elementary reaction steps on well defined

surfaces and these could be used in the analysis of a postulated sequence of

32
chemical reactions in a more complex set. He emphasized that measurement

of the rate constants under dynamic conditions may be required, Rhodin

concluded by restating that the study of the physics and chemistry of well

defined systems can provide information on reaction rate mechanisms as well

as the natiire of chemical bonding of molecules at surfaces and these serve

as building blocks in the development of new concepts. The role of critical

design parameters in the engineering of new catalytic processes is essential

but probably not directly amenable from a study of the physics and chemistry

of well defined surfaces.

Professor Park pointed out that as a new technique appears

on the horizon, there is a contrived enthusiasm for the technique which

soon passes when people recognize that it can provide qualitative information

but not quantitative information. There are few techniques which have proven

to be "cure-alls" for the field. One example he used as an illustration is

the difficulty in making Auger spectroscopy quantitative. Auger spectroscopy

is fine for qualitative determination of surface cleanliness, but it is inadequate for quantitative determination of surface composition. This only as

an example - there are many examples where the experimental techniques we

use simply are incapable of giving us quantitative information.

Professor Haller noted that the talk by Dr. Yates precluded

the necessity of further opening remarks by him, and introduced Dr. Farrel Lytle.

33
Investigation of Supported Catalysts by

* X-ray Absorption Spectroscopy

by

Farrel W. Lytle

The Boeing Company

Seattle, Washington 98124

Sunanary of Remarks

The unique ability of x-ray absorption spectroscopy to Isolate

one particular atom in a complex material, and from the energy position

and shape of the absorption edge determine the chemical state and from the

extended x-ray absorption fine structure (EXAFS) determine the radial arrangement of the atoms surrounding the absorbing atom, can be used to good

advantage in the study of heterogeneous catalysts. The absorption edge

spectroscopy has been summarized by Azaroff

.

^ Theories of EXAFS which

2 3

more or less agree have been given by Stem, Lee and Pendry, and by

4 5 Ashley and Doniach. The demonstration by Sayers, et al that EXAFS can be

Fourier transformed into a radial distribution function surrounding the

absorbing atom has created a new interest in the technique for studying

glassy materials,^ complex biological molecules,^ solutions containing

coordination complexes,''"^ gases,

'''^ and supported catalysts,^

The measurement and normalization of EXAFS using conventional

x-ray sources have been described^^ as have been the details of data processing.

18 The recent advent of the EXAFS spectrometer at the Stanford Synchrotron

8 10 Radiation Project (SSRP) now offers an unparalled x-ray flux (10 -10 photons

sec ^) with a band width of 1 eV. The experiments described here were

performed on this instrument.

22 The Ljjj-edge absorption of Au, Pt, Ir, and Ta is shown in

Fig. 1. The data have been normalized to unit absorption and lined up on

the inflection point of the first rise on the low energy side of the edge.

34
These threshold resonance ("white lines") transitions are often observed

in transition metal absorption spectra and are qualitatively understood

as allowed dipole transitions, 2p-to -empty 5d levels. Figure 1 shows this

in that the resonance is present in Au where all the 5d levels are filled

and increases with Pt, Ir and Ta as the 5d shell empties. For metals such

simple electron counts are unjustified as conduction bands are created.

19 The band structure calculations for Au, Pt , and Ir of Smith et. al and

20

for Ta by Matthiess were integrated in the region from the Fermi energy

to 10 eV above in order to more accurately estimate the xinfilled d-density

of states and compare to the absorption resonance. This is summarized in

Table 1. In all cases the resonance (area or amplitude above the Au edge) was found to increase with increasing d-states although the relationship was not linear. The absorption spectra of the Au Ljjj edge (and the band

structure integration) was subtracted from that of each of the other elements

to isolate just the absorption to d-states. In Figure 2 data from Pt

compounds and 1 wt. pet. catalysts supported on Cabosil (Si02) is compared

to Pt metal. Data for PtCl^ (not shown) was nearly identical in amplitude

and position with Pt metal. Again, the Pt compounds show resonance increases

as expected from an estimate of d-vacancies . The catalyst was sensitive to

its surface preparation. The "reduced" sample was prepared by a 500°C

reduction in flowing transferred to an air tight sample cell in a dry

box under N2 and then measured. This same sample after exposure to air

comprised the other sample. The much smaller resonance in the reduced

sample may be due to filling of the Pt d-band by hydrogen as in the familiar

2 quenching of Ni magnetization by hydrogen.

In a similar 5% Pt on Cabosil sample the Ljjj EXAFS was measured

and a Fourier transform obtained. A typical example is shown in Figure 3.

The magnitude of the transform is plotted vs radial distance from the

absorbing Pt atom. It is seen that the environment of Pt in a well-dispersed

(90% by gas adsorption) catalyst is not simple. The identification of

various peaks with possible atomic species has been made considering the

state of the sample and the expected interatomic distances. Bonding to

oxygen and other Pt atoms is expected and found to vary as the sample was oxidized or reduced. CI was also expected in the sample. The "short bond"

may be evidence of epitaxy to the support. The distance 1.7% is the same

35
as Si-0 in the support and may be envisaged as Pt filling a missing Si

site on the three-oxygen-atom "nest" of the (111) plane. This same kind

of "short bond" has been found in supported Au catalysts.

'''^

In summary the technique is a general and powerful one for the

investigation of the electronic and structural environment of the catalytic

atom. Planned experiments will use the high flux of the SSRP facility for

in situ analysis of catalysts during reduction and various chemisorption

experiments.

*For the experimental opportiinity I thank the SSRP staff and NSF and ERDA

who fund the facility.

36
References

1. L. V. Azaroff, "X-ray Spectroscopy", McGraw Hill, N.Y. (1974).

2. E. A. Stem, Phys. Rev. BIO, 3027 (1974).

3. P. A. Lee and J. B. Pendry, Phys. Rev. Bll , 2795 (1975).

4. C. A. Ashley and S. Doniach, Phys. Rev. Bll , 1279 (1975).

5. D. E. Sayers, E. A. Starn, and F. W. Lytle, Phys. Rev. Lett. 27^, p. 24, (1971)

6. D. E. Sayers, F. W. Lytle, and E. A. Stem in "Amorphous and Liquid

Semiconductors", North Holland (1974), p. 403.

7. D. E. Sayers, E. A. Stern, and F. W. Lytle, Phys. Rev. Lett. 35^, 584 (1975).

8. D. E. Sayers, F. W. Lytle, M. Weissbluth, and P. Pianetta, J. Chem.

Phys. 62, 2514 (1975).

9. D. E. Sayers, E. A. Stem, and J. R. Herriott, J. Chem. Phys. 64^, 427 (1975).

10. B. Kincaid, P. Eisenberger, K. 0, Hodgson, and S. Doniach, Proc. Nat.

Acad. Sci. USA 12^, 2340 (1975).

11. R. G. Shulman, P. Eisenberger, W. E. Bliimberg, and N, A. Stombaugh,

Proc. Nat. Acad. Sci. USA (to be published).

12. P. Eisenberger and B. M. Kincaid, Chem. Phys. Lett. 36, 134 (1975).

13. B. M. Kincaid and P. Eisenberger, Phys. Rev. Lett. 34, 1361 (1975).

14. F. W. Lytle, D. E. Sayers, and E. B. Moore, Appl. Phys. Lett. 24^, 45 (1974).

15. I. Bassi, F. W. Lytle, and G. Parravano, J. Catalysis (to be published).

16. F. W. Lytle, D. E, Sayers, and E. A. Stem, Phys. Rev. Bll, 4825 (1975).

17. E. A. Stem, D. E. Sayers, and F. W. Lytle, Phys. Rev. Bll, 4836 (1975).

18. B. M. Kincaid, P. Eisenberger, and D, E. Sayers, Phys. Rev. (to be published)

19. N. Smith, G. Wertheim, S. Hufner, and M. Traum, Phys. Rev. BIO , 3197 (1974).

20. L. Matthiess, Phys. Rev. Bl, 373 (1970).

21. R. Selwood, S. Adler, and T. Phillips, J. Am. Chem. Soc. T±, 2281 (1954):

77^, 1462 (1955).

22. F. W. Lytle, J. Catalysis (to be published).

37
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Figure 1. X-ray adsorption spectra of Au, Pt, Ir, and Ta near the

Ljjj. absorption threshold. The data were normalized by

fitting a straight line 50-200 eV above the peak and extrapolating to below the edge to obtain the jump ratio.

Point-by-point division of the L-j-j-j- threshold absorption

curves by the jump ratio produced these curves normalized

to unit absorption. The zero of energy in each case is the

1st inflection point after the onset of absorption obtained

by numerically differentiating the spectra.

39
I I ' I ' ^ ' I -15 -10 -05 0 05 10 15 20

ENERGY, eV

Figure 2. X-ray absorption spectra of Pt catalyst samples and

a-Pt02 compared to Pt and Au. Same normalization and

energy scale as in Figure 1.

40
Figure 3, Fourier transform of Pt L^^^ EXAFS from a 5% Pt on

Cabosil sample which had been exposed to air.

41
III. Dialogue between Panel and Audience

Since Dr. Lytle's talk was fresh in the minds of the

audience, the first questions dealt with the utility of EXAFS in catalytic

studies. Dr. E. Siegel (Puhlic Service and Gas and Electric) asked whether

or not one can study a real catalytic reaction using EXAFS. The conditions

he suggested are 800° Farenheit, 300 Atmosphere. Lytle answered that temperature is no problem but that pressures in excess of several atmospheres can

be a problem. The basic difficulty is that one needs an x-ray transparent

cell which is able to contain the sample, and he knows of no material strong

enough and transparent enough t6 be used for high pressiire catalytic reactions .

Dr. Paul Citrin (Bell Telephone Laboratories) pointed out

that the EXAFS method seems to have a unique applicability in studies of

surfaces. That is, under certain circumstances, it may be made specifically

sensitive to surface species by observing the fine structure due to elements

in the adsorbed molecule rather than just to changes in substrate species.

He observed that one can detect not only the absorption edge due to a surface

species, hut one can also monitor the photoelectrons ejected or the Auger

electrons ejected as one sweeps the wavelength of synchrotron radiation. He

suggests that specific surface EXAFS (SEXAFS) may provide unique information

about structural sensitivity of catalytic reactions.

Lytle noted that there will often be signal from molecules

not on the active sites of interest just as with other spectroscopies. He

then observed that scientists from Bell Laboratories md U. California (Berkeley)

have also used the x-ray fluorescence being emitted fffOTn the atom absorbing

the x-rays as a very

42
sensitive detector in the EXAFS technique. He further pointed out that

Dr. Mel Kline (Berkeley) has looked at manganese in a leaf at concentrations

of the order of a few parts per million and has obtained an observable EXAFS

signal. Experimental times are of the order of one-half hour to ^^5 minutes per

absorption edge.

Dr. Warren Grobman (IBM) pointed out that one limitation

of EXAFS as applied to catalytic systems, is the fact that many of the atoms

of interest - carbon, oxygen, and hydrogen - are hard or impossible to

detect using this technique. The threshold for x-ray absorption is at a

fairly low energy and self-absorption by the substrate would frequently

cause attenuation of the signal due to surface species. This is par;.icularly true for high Z substrates.

Citrin pointed out that the fluorescence technique and/or

the search for Augei' electrons accompanying the x-ray absorption are the

ideal ways to study the adsorbed surface species.

Park said that for studying low Z materials the decay of

core hole excitations are such that the emission of Auger electrons is much

more probable than x-ray fluorescence.

Dr. Charles Duke (Xerox and the University of Rochester) issued

a warning concerning theoretical calculations. There are two types of calculations one can do. Firstly, one can compute ground state properties y reaction

probabilities, etc., and secondly, one can calculate excitation spectra. In

all of the spectroscopies discussed today, one is concerned with excitation

spectroscopy as a tool for studying ground state properties. Duke points

out that there may be little correlation between the excitation spectra and

the ground state properties and that model calculations appropriate for analyzi

43
one may be entirely inappropriate for the other. As an example, Huckel

and CNDO/2 models are useful for the prediction of ground state properties

(e.g., geometries, dipole moments) of organic molecules whereas spectroscopic

CNDO/S models are required for interpretation of electronic spectra like

optical absorption or photoemission. Thus, in quantum chemistry it is

an accepted (even if undesirable) procedure to utilize different semiempirical models to interpret different properties. Consequently, one

should approach with caution the task of extracting ground state properties

(e.g., geometries) from electronic spectroscopies (e.g., photoemission).

Duke also cautioned against simplistic assessments of the

structure-determination capability of kinematic analyses of EXAFS data. To

extract geometric information from kinematic analyses of such data, \mcertainties in individual atomic electron scattering cross sections, the use

of a finite data base, and multiple scattering phenomena must be assessed

quantitatively. Such assessments have not yet been reported. Therefore,

a substantial effort lies ahead before the present promise of EXAFS structiure

analysis is converted into a reality by the actual determination of previously

unknown structures

Lytle responded by noting that recent calculations by

Lee and Pendry (Phys. Rev. Bll , 2795 (1975) show that multiple scattering

can be observed if one goes beyond the fourth coordination sphere but that

multiple scattering is not a problem if you are only concerned with the first

or second coordination spheres . The data truncation problems inherent in

finite Fourier transforms have been handled using standard techniques.

Siegel concluded the discussion of EXAFS by noting that this

technique may be particularly useful for studying high Z poisons on catalysts.
Seigel then asked questions concerning the role of

d-electrons in catalysis. Basically, he wanted to know: why are all

practical and important catalysts d-hole deficient?

Prof. Michel Boudart (Stanford University) answered by

pointing out that there are many catalysts which do not involve d-electrons

or d-holes. For example, protons, MgO and Alumina are all good catalysts

for certain catalytic, reactions.

Seigel then asked: Could one envision the role of d-holes

as follows. In chemisorption, electron transfer must take place from the

adsorbate to the substrate and vice versa, and it is much easier when

electrons transfer from an adsorbed molecule to the substrate if there

is a d-hole rather than a filled d-orbital.

Prof. John Turkevich (Princeton University) suggested that

this idea may have some merit. In order to activate a molecule at the

45'

I
surface, one simple-minded way of doing so is to take away electrons.

This can he best accomplished if one can make a temporary transfer of electrons

from the molecule to the solid substrate.

Dr. J. W. Gadzuk (NBS) suggested that one new promising technique

for studying catalytic reactions may be the chemiluminescence technique

pioneered by Kasemo in Sweden. He asked for comments from members of the

panel. Park answered with a description of Kasemo 's experiments.

He pointed out that adsorption of oxygen on magnesium and aluminiim gave rise

_7

to a yield of photons with a probability of 10 photons per adsorption event.

He fiorther speculated that chemisorptive luminescence might be a useful tool

for following catalytic reactions even though the probability of a catalytic

reaction might be a lot less than the usual high sticking probability for

adsorption of molecules on svirfaces. One might be able to integrate over a

long time and follow the light emission from a sustained catalytic reaction.

Gadzuk then pointed out that one advantage of chemisorptive luminescence as

a tool for following catalytic reactions is the fact that the quantity detected

is a photon, unlike other s\irface spectroscopies in which the quantities

detected are charged particles. Thus, this technique appears to be applicable

to high pressure catalytic situations. Park agreed.

Turkevich described some experiments in which they

have generated hydrogen atoms by irradiation of molecular hydrogen dissolved

in silica. The hydrogen atoms were detected using ESR spectroscopy. When

the sample was heated to -150°C from LN^, '"he hydrogen atoms recombined at

impurity centers and light was emitted. There is a one-to-one correspondence

between the disappearance of the ESR signal and the ... ->tons emitted. He suggests

that studies involving light emission may be very useful in following energetic

46

!

i
catalytic reactions.

Prof. Ponec (Leiden) had several comments to make with respect

to John Yates' lecture. First of all, he observed that the hydrogenation of

cyclopropane is only structure insensitive on platinum, where the only reaction is the addition of hydrogen. He speculates that on nickel, where a

bond breaking reaction competes with hydrogenation, the reactions involving

hydrogen and cyclopropane are probably /ery structure sensitive. Secondly,

he took issue with the XPS studies of silver-palladium alloys. Although

the center of the bands did not change as the concentration of silver and

palladium in- the alloys changed, there were significant changes in the band

edges. He pointed out that recent infrared studies by Sachtler indicate

that for CO on silver-palladium alloys, there are slight shifts in the

infrared absorption' bands that had not been detected previously. That is,

slight shifts as a function of silver-palladiiun concentrations. In his

next comment, Ponec pointed out the danger of experimentalists trying to

follow theory too closely. He cited, as an example, the rigid band theory.

If one believed this model, then one should not be able to titrate copper

nickel alloy surfaces using hydrogen atoms but in fact there is preferential

absorption of the hydrogen on nickel in complete disagreement with the

concepts of the rigid band pictiire. He also indicated that Turkevich^s

notion of activation of surface species by interaction of the absorbed

molecule with the d-holes or by electron transfer of the absorbed molecule

to the d-holes is an over-simplification of an incorrect idea by Dowden.

Gadzuk inquired as to whether or not it was feasible

or reasonable to study catalytic reactions using ultrahigh vacuum techniques

that is, to pump the system to low pressures following a catalytic reaction

.47
and examine the state of the surface. To what extent does this sort of

procediire help in the understanding of the actual catalytic mechanism?

Yates replied that first of all, one should study the

kinetics of the reaction while the gas is in the system and not simply rely

on the measurement of the surface following the reaction. Secondly, he

pointed out that catalytic intermediates may be removed as the gas is pumped

away from the sample. On the other hand, one can then examine residues,

such as carbon, following the catalytic reaction. If carbon is present

on the surface at that stage, then it is logical to assume that the carbon

was also present d\iring the course of the reaction.

Dr. Galen Fisher (NBS) inquired: what specific experimental

techniques have been useful for determination of catalytic intermediates?

Katzer answered that infrared absorption techniques

are probably the most useful and most widely applied method for looking

at catalytic intermediates. However, it is often difficult to distinguish

between actual reaction intermediates and other stable adsorbed species.

Katzer also issued a warning concerning the use of siirface spectroscopies.

He pointed out that these techniques are useful for studying chemi sorption

processes as well as for looking at the most stable intermediates. However,

they are generally not useful for studying the dynamics of catalytic processes

Fundamental information concerning reaction mechanisms can probably best be

obtained using such techniques as modulated molecular beam methods.

Haller pointed out that infrared and other spectroscopies can be useful for studying catalytic intermediates, provided that

you combine such techniques with the kinetic transient method. The objective

here is to correlate the change in concentration of surface species with

48
changes that are occuring in the gas phase in a transient experiment.

In this way, one can he reasonably certain that you are studying something

which is kinetically important

Park indicated that both optical reflectance and

chemisorptive luminescence would be useful for studying catalytic intermediates under actual high pressure and transient conditions. He further

suggested the use of isotopic labelling in kinetic studies as a means of

getting information about intermediates. (He did not mention the work

11+ of Emmett using radioactive C tracers, but that is an interesting instance

of the applicability of this technique to the detection of intermediates).

Fisher said that in many cases, there may be multiple

pathways to products with very stable reaction intermediates which exist

under certain conditions but which may not be the most kinetically important

intermediates.

Finally, Yates suggested a restaT;?iient of the question:

can one detect the transition state in a catalytic reaction? He indicated

that in homogeneous kinetics, it frequently occurs that the concentration

of the transition state may be very, very low, but one may hope to deduce

the structure of the transition state if the structure of intermediate

precursors can be determined.

Boudart commented that many of the old conventional

chemisorption methods and methods of chemical analysis can frequently be used

to calibrate some new spectroscopies such as Auger spectroscopy. In particxilar, he mentioned an example from his own laboratory in which they used

quantitative chemisorption of CO, CO^ and to calibrate some Auger spectroscopic measurements on ammonia synthesis catalysts. Prof. Boudart further

49
pointed out that such techniques as EXAFS should best be done on wellcharacterized samples, on samples which have been studied in several

different laboratories. He mentioned also that there is a program now,

scientific interchange in matters of catalysis between the US and the USSR,

and that this program is providing a mechanism for exchange of catalyst

samples that have been characterized in different laboratories.

Prof. Theodore Einstein (Univ. of Md. ) asked: how conclusive

is the evidence for the adsorption of atoms in high symmetry sites as is usually

assumed in low energy electron diffraction calculations? He based his skepticism

on the recent electron stimulated desorption ion angular distribution (ESDIAD)

measurements made by Madey, Czyzewski and Yates at the National Bureau of

Standards and on the calculations of Gersten, et al. which indicated that some

ion desorption patterns can be explained on the basis of adsorption at sites

not associated with high symmetry positions on the substrate.

Park indicated that it has not been established theoretically

that adsorbed atoms and molecules usually sit in sites of high symmetry.

The LEED calculations which have been performed to date suggest that- high

symmetry adsorption sites are appropriate in some cases. However, because

of the type of model calculations usually performed by LEED theorists, it is

impossible to test all possible adsorption sites. Perhaps wider use of

inversion techniques will demonstrate this more clearly.

Haller then called for closing remarks by the panelists.

Yates reformulated the question that he had asked at the

end of his talk. If one has knowledge of the atomistic details of catalytic

reactions such as specific adsorption sites, knowledge of surface structures

and intermediates, knowledge of specific reaction rates under careftilly

50
controlled conditions, how can this information be transferred in a

practical sense to assist in the design and construction of a catalytic

reactor for a specific practical catalytic experiment? He acknowledged

that the instrumental methods of the surface scientists are being more

widely used by catalytic chemists (e.g., ESCA is finding increasing use

In catalytic studies) . The real question concerns the utility of basic

concepts as applied to practical catalyst design.

Katzer emphasized the need to understand the chemical

composition of complex catalytic systems, and observed that modern

methods provide this information.

Park suggested that "educated intuition" plays a major

role in catalyst design, and basic research on surfaces provides catalytic

chemists with better models on which to base their intuition.

Finally, Rhodin cautioned that kinetics are essential in

catalysis, and modern methods should be used in conjunction with kinetic

measurements.

Dr. Lawrence Bennett (NBS) closed the panel discussion by

thanking the participants and noting that there were some techniques

that were not considered in any detail. One technique is the Mossbauer

effect, which Professor Boudart has shown has usefulness in real catalytic

situations, and in which one can hope to distinguish between the surface

and bulk particles. Another technique which was not mentioned is one

which is being developed at NBS - perturbed angular correlation. It is

a specialized technique which measures the same type of thing that

Mossbauer techniques do, namely, the hyperfine fields, but it is also

restricted to elements with specific nuclear properties. In the case of

Iron, the NBS group has shown that information can be obtained by

51
combining pertiirbed angular correlation and Mossbauer experiments that are

not observable from either method alone because of resolution problems.

Another isotope which is very useful in perturbed angular correlation is

rhodium, and that may be of some interest in catalysis. There was not any

discussion about nuclear magnetic resonance, but he thinks that is another

technique which has a great deal of usefulness in catalysis.

^2
SESSION 2

EFFECTS OF

ALLOYING

Moderator:

H.EHRENREICH

Harvard University

Lecture by:

J.SINFELT

Exxon

53
Session 2.

Effects of Alloying

Panel Members:

C. D. Geiatt, Jr.,

Harvard Univ.

V. Ponec,

Gorlaeus Lab., Leiden

J. Sinfelt,

Exxon

Recorder:

A. J. McAlister,

NBS

54
SESSION 2. EFFECTS OF ALLOYING

Chairman: H. Ehrenreich, Harvard University

Recorder: A. J. McAlister, N.B.S.

Panel Members: J. H. Sinfelt, Exxon

C. D. Gelatt, Harvard University

V. Ponec, Gorlaeus Lab.

The session on the Effects of Alloying was formally divided into

two parts: an invited talk by Dr. Sinfelt on the topic "Catalysis by

Alloys and Bimetallic Clusters"; and a panel discussion, chaired by Dr.

Ehrenreich, during which Dr. Ponec and Dr. Gelatt made short formal

presentations, and in which the audience participated actively.

-55
CATALYSIS BY ALLOYS AND BIMETALLIC CLUSTERS

(A brief summary of Dr. Sinfelt's remarks, with

selected figures and general references)

I. Surface Enrichment

It has long been realized that the surface composition of an alloy

may differ from that of the bulk, but only in recent years have attempts

been made to obtain information about the composition of the actual

surface exposed to reacting molecules. Chemisorptive titration and

Auger spectroscopy have been the principle experimental tools employed

in such studies, and at least qualitative agreement has been obtained

with theory, which predicts, in rough terms, that the component with

lower heat of vaporization in the pure state will be enriched on the

alloy surface."^

II. Specificity of Metals and Alloys

Catalytic activity depends strongly on the reaction considered.

Two reactions were used as examples: a) ethane hydrogenolysis , in which

carbon-carbon bond rupture is believed rate determining, and for which

adjoining active metal atoms are believed necessary as surface sites: b)

cyclohexane dehydrogenation, for which desorption of the product benzene

is believed to be rate determining.

A strong correlation has been observed between rates of ethane

hydrogenolysis over Group VIII and IB metals and the Pauling percent dcharacter in the bonds of the metals. This correlation, shown in Figure

1, is perhaps the strongest available indication of the importance of an

electron factor in catalysis by metals.

The variation in the rates of the example reactions over Cu-Ni

alloys of about 0.1% dispersion is shown in Figure 2. The decrease in

i

56
hydrogenolysis activity as Cu is added to Ni probably involves a strong

structure factor; if for instance two adjacent metal atoms are needed,

these become scarcer as Cu is added. This structure factor is enhanced

by the surface enrichment of Cu indicated by hydrogen chemisorption

measurements on the samples. Cyclohexane dehydrogenation activity first

increases with Cu addition, suggesting a decrease in the strength of

product binding. It then remains roughly constant, decreasing only at

high Cu content, with the rate determining step perhaps moving back

through the reaction sequence to the initial chemisorption of cyclohexane.

The data above were obtained on alloys of metals, which are miscible, at least at high temperatures. It is of interest to consider

what happens if the metals are Immiscible in bulk, for example Cu and

Ru. One might expect the alloy in this case to consist of mixed particles of pure Cu and Ru, and in view of the results of Figure 1, to

find no change in specific activity as Cu is added to Ru. In fact, Cu

has a marked affect on surface processes occurring on Ru. As Figure 3

shows, for unsupported Cu-Ru catalysts, of about 1% dispersion, both the

volume of strongly chemisorbed hydrogen (i.e., that fraction which

cannot be pumped off at room temperature) and the rate of ethane hydrogenolysis are markedly reduced by the addition of Cu. Hence the Cu

appears to go onto the Ru surface much as if it were chemisorbed there.

III. Dispersion Effects in Bimetallic Systems

Suppose we prepare highly dispersed, supported bimetallic samples,

for example by co- impregnation and subsequent heat treatment. Will we

obtain mixtures of pure metal particles, or, as for the unsupported low

dispersion samples cited above, find interactions between the different

metals whose effects are manifested in chemisorption and catalytic

activity? Figures 4 and 5 show the effects of adding Cu to Ru and Os on

H and CO uptake and on the rate of ethane hydrogenolysis. A strong

interaction between the Group VIII and Group IB metals is indicated.

57
Now, how do the results on bimetallic systems for lov," and high dispersion tie together? In Figure 6 are plotted the data for hydrogen

uptake, and in Figure 7, for hydrogenolysis activity, as a function of

composition, for large Ru-Cu aggregates (dispersion '^1%) and for highly

dispersed Ru-Cu clusters (dispersion '^'50%) . These results are consistent with the notion that Cu covers the Ru particles; when the dispersion is greater, a given fraction of Cu will cover a much smaller percentage of the particle surface.

IV. Hydrogenolysis Activity and Strong Chemisorption

Figure 8 shows a striking correlation between ethane hydrogenolysis

activity of Ru-Cu catalysts and their capacity for strong hydrogen

chemisorption. This suggests that we lie on the left side of a volcano

curve, where there is a positive effect between strength of binding and

catalytic activity.

V. Selectivity of Bimetallic Catalysts

We finally consider the selectivity of bimetallic catalysts. In

particular, we look at the rates of cyclohexane dehydrogenation into

benzene, and hydrogenolysis into low molecular weight fragments, principally methane, over Ru-Cu catalysts. We see in Figure 9 that the

addition of Cu to Ru enchances the selectivity to the production of

benzene by an order of magnitude.

REFERENCES

[1] See for example, R. A. Von Santen and M. A. M. Boersma, J. Catal.

34 , 13 (1974) and references therein.

[2] J. H. Sinfelt, Cat. Rev. - Sci. Eng. 9^, 147 (1974) and references

therein

.

[3] J. H. Sinfelt, Y. A. Lam, J. A. Cusumano, and A. E. Barnett, J.

Catal. 42, 227 (1976).

[4] J. H. Sinfelt, J. Catal. 29, 308 (1973).

58
_l I 1 I L- VIIA Vlllj Vlllj VIII3 IB

PERIODIC GROUP NUMBER

Figure 1. Catalytic activities of metals for ethane hydrogenolysis in

relation to the percentage d-character of the metallic bond. The closed

points represent activities compared at a temperature of 205° C and

ethane and hydrogen pressures of 0.030 and 0.20 atm, respectively, and

the open points represent percentage d-character. Three separate fields are shown in the figure to distinguish the metals in the different long

periods of the Periodic Table. (From Ref. [2])

59
Figure 2. Activities of copper-nickel alloy catalysts for the hydrogenolysls

of ethane to methane and the dehydrogenation of cyclohexane to benzene. The

activities refer to reaction rates at 316° C. Ethane hydrogenolysis activities

were obtained at ethane and hydrogen pressures of 0.030 and 0.20 atm,

respectively. Cyclohexane dehydrogenation activities were obtained at

cyclohexane and hydrogen pressures of 0. 17 and 0. 83 atm, respectively.

(From Ref. [2])

60
Figure 3. Hydrogen chemisorption capacity and ethane hydrogenolysis activity

of ruthenium- copper catalysts as a function of copper content. The hydrogen

chemisorption data were obtained at room temperature and represent the

strongly chemisorbed fraction. The ethane hydrogenolysis activities are reaction rates at 245° C and ethane and hydrogen pressures of 0.030 and

0.20 atm, respectively. The catalysts were prepared by heating in hydrogen at 500° C. (From Ref. [3]).

61
Figure 4. The chemisorption of hydrogen and carbon monoxide at room temperature on silica-supported ruthenium-copper and osmium-copper catalysts.

The catalysts all contain 1 wt% ruthenium or osmium, with varying amounts of

copper. The adsorption data are expressed by the quantities H/Ru, CO/Ru, H/Os

and CO/Os, which represent the number of hydrogen atoms or carbon monoxide

molecules chemisorbed per atom of ruthenium or osmium in the catalyst. (From

Ref. [2])

62
0.2 0.4 0.6 0.8 1.0

Atomic Ratio, Cu/Ru

0.2 0.4 0.6 0.8 1.0

Atomic Ratio, Cu/Os

Figure 5. The specific activities of supported ruthenium-copper and osmiumcopper catalysts for ethane hydrogenolysis . Activities are shown for

the same cat=^lysts used in obtaining the chemisorption data in Figure 4.

The activities are compared at 245° C and ethane and hydrogen pressures

of 0.030 and 0.20 atm, respectively. Specific activity is defined

here as the activity per surface site, and is determined by dividing

activity per atom of ruthenium or osmium in the catalyst by the quantity

H/Ru or H/Os, respectively, from Figure 4. (From Ref. [2])

63
Cu/Ru ATOMIC RATIO

Figure 6. Influence of the state of dispersion of ruthenium-copper catalysts

on the relationship between hydrogen chemisorption capacity and catalyst

composition. The square and triangular points represent total hydrogen

chemisorption and strongly chemisorbed hydrogen, respectively, on the

large ruthenium-copper aggregates. (From Ref . [3])

64
Figure 7. Influence of the state of dispersion of ruthenium- copper

catalysts on the relationship between ethane hydrogenolysis activity

and catalyst composition. The large ruthenium-copper aggregates

have a metal dispersion of the order of 1%, while the highly dispersed

ruthenium-copper clusters have a metal dispersion of the order of

50% (From Ref. [3])

65
Figure 8. Correlation of ethane hydrogenolysis activity and amount of

strongly chemisorbed hydrogen for ruthenium- copper catalysts. The

ethane hydrogenolysis activities are rates at 245° C and ethane and

hydrogen pressures of 0.030 and 0.20 atm, respectively. (From Ref.

[3])

66
Figure 9. The rates of dehydrogenation and hydrogeoolysis of

cyclohexane on ruthenium- copper catalysts as a function of composition.

The rates are shown for a temperature of 316° C and for cyclohexane

and hydrogen pressures of 0.17 and 0.83 atm, respectively. The catalysts

were reduced at 400° C. (From Ref. [3])

67
ALLOYING P;\NEL DISCUSSION

The chairman opened the discussion with a request for comments and

questions on Dr. Sinfelt's talk. Dr. Ponec remarked with regard to

hydrogenolysis on Cu-Ni alloys: when Cu is added to Ni, a multiple CuNi bond can still be formed, the main effect of alloying being on the

size of ensembles of Ni sites; for cracking reactions (e.g. hexane) , Ni

and Cu give terminal cracking, while Pt gives mid-cracking; interestingly,

Ni-Cu alloys give Pt-like mid-cracking. Dr. Ponec then asked Dr. Sinfelt

if he could give any insight as to why d- character might be important

in catalysis. Dr. Sinfelt replied that he simply regarded this correlation as an observed fact for hydrogenolysis, and that it is of predictive value for such reactions. He suggested the possibility that absorbate-metal bonds might resemble those of the bulk metal in some

respect. Dr. Gelatt noted that correlation of % d-character with hydrogenolysis activity is no stronger than that of such a geometric

factor as the volume per atom of the transition metals, the latter

correlating as well with hydrogenation data as does % d-character. Dr.

L. H. Bennett noted that surface enrichment or otherwise depends on

preparation, and asked Dr. Sinfelt if reactions could be studied as a

function of surface composition. Also to this point. Prof. Rhodin asked

whether the experimental environment might affect surfaces; could the

catalytic surface be different during reaction from that prepared? Dr.

Sinfelt felt that surfaces can indeed change during the catalytic process, but probably did not change much in the systems he discussed in

his talk. Prof. Ponec cited some B.^ and CO titrations that showed

chemisorption to affect surface composition of Cu-Ni catalysts; he

68
suggested that here Ni was drawn to the surface. Prof. Turkevich showed

o H2 adsorption data on 32A particles of Au-Pt alloys in which the quantity absorbed varied nearly linearly from a maximum at Pt to nothing at

Au. Prof. Turkevich noted that this was at variance with the cherry

model, (see below, particulary Prof. Ponec's contribution) since bulk

phase diagrams show distinct phase change, and concluded that phase

relations must differ in small particles.

Prof. Park raised the question of whether the local character of

the electronic structure of Cu and Ni in Cu-Ni alloys were not in fact

rather like that in the host metals. Dr. Gelatt replied that in fact,

this was largely the case according to detailed calculations within the

coherent potential model. (For example, see Fig. 4 of Dr. Ponec's contribution which shows the local density of states at a Ni site in a

CuNi alloy.)

o Dr. B. J. Evans noted that about lOA resolution was available in

electron microscopy, and wondered why one could not obtain direct evidence of surface enrichment in highly dispersed systems. Dr. Sinfelt

and Dr. Ponec replied that this was not adequate resolution for the

problem.

Dr. Lagally asked what experimental evidence there might be of

persistence of elemental local electronic structure character in alloys.

Prof. Ertl briefly described some soft x-ray appearance potential spectra

from his laboratory which confirm this picture. Dr. Siedle asked what

was known of the morphology of the bimetallic clusters. Dr. Sinfelt

replied that at present, one only knows that they are very small and

69
composed of both metals, and really nothing more. He hoped that the

extended x-ray absorption fine structure technique (see the previous

session) would contribute much to the solution of this problem.

Dr. Ponec then presented a short review of the experimental situation regarding the phase composition and surface enrichment of Cu-Ni

alloys. His notes on this presentation are reproduced below.

70
SURFACE COMPOSITION OF NICKEL-COPPER ALLOYS

(CONTRIBUTED DISCUSSION BY DR. V. PONEC)

The knowledge of the phase composition is essential for the discussion to follow. Let me, therefore, mention several facts in this

respect first.

As can be seen from Fig. 1, several phase diagrams have been

suggested in the literature. Because of the evident uncertainty of the

information on the phase composition of Ni-Cu alloys at catalytically

[61 interesting temperatures, 150-400° C, Franken from our laboratory

reinvestigated this problem with evaporated metal films and x-ray

diffraction. He found that at 420° C the films consisted of only one

phase after a rather short time; at 215° C one-phase films were formed

when the equilibration was performed during a sufficiently long period

(about 40 hours); and at 165° C two phases persisted even after still

longer sintering. Now, we know also from some other additional experiments that the critical temperature lies between 165-200° C, as

predicted by Meij ering ^ .

Once converted into a one- phase system, the films do not show any

detectable segregation of phases at low temperatures (20-100° C) even

r 81 after several days. However, it is known that segregation can be

induced at 20° C by an electrochemical formation and vacuum decomposition of Ni-Cu hydrides. After such segregation, the Cu-rich 3-phase

[81 forms the surface of the whole system . So much for the phase composition,

[91 [31 Sachtler and Dorgelo and Van der Plank and Sachtler suggested

the use of work function (cj)) measurements and selective chemisorption of

71
H2 to determine the surface composition of Ni-Cu alloys. The main

result of these measurements is: films equilibrated at 200°C revealed a

broad range of bulk concentration for which the surface concentration

was approximately constant (indicated by both mentioned methods) and

this "constant" composition was approximately equal to the bulk composition of 3-alloys (as indicated by the chemisorption) . The

[1-31 explanation of the authors was: two phases coexist at this temperature and the 3-phase forms a shell around the a-phase crystallites

("cherry" model).

However, doubts and criticism on this model appeared soon. First,

[6 71 the coexistence of two phases at 200° C was not certain ' . Further,

according to the rigid band theory (RBT) and Dowden's ideas , Ni in

alloys with 60% and more Cu cannot adsorb hydrogen; the chemisorptive

titration was actually "theoretically impossible". On the other hand,

the work function measurements which are undoubtedly sensitive for the

surface composition are not supported by any theory which would provide

us with a theoretical relation for (j) as a function of composition. Many

expectations were, therefore, related to the Auger spectrometry.

Three groups of authors investigated Ni-Cu alloys by Auger spectrometry and they all reported the same final result: alloys equilibrated at 400-500° C (one-phase alloys) reveal the same bulk and surface

composition; no indication of a constant surface-composition with varying

bulk-composition has been obtained '''"^ 131^ However, in contrast to

these data, selective hydrogen chemisorption (see Fig. 2) suggested a

surface composition constant in a wide range of bulk composition and,

moreover, the same composition for films equilibrated at 200° C and

powders equilibrated at 400° c'"'''^

l-(>]

^ This has led to reconsideration

72
of all data and analysis of the assumptions made. We came to the following conclusions '^ . (1) In spite of the use of (j)-measurements for

the surface determination being empirical, one conclusion is apparently

always right: where <^ is constant, the surface composition is constant.

[61

In this light the following result of Franken was important - see

r fil Fig. 3. The materials equilibrated at 420° C , analogous to powders

used by other authors'"''"^ , reveal the same behaviour as that observed

by hydrogen chemisorption (compare Figs. 2 and 3). Further, the onephase (equilibrated at 215° C, 420° C) and two-phase (165° C) films

[61

showed the same function cj) . The new data by Franken reproduced

[9] the old data by Sachtler and Dorgelo very closely. (2) The theoretical objections against the chemisorptive titration by hydrogen have

been essentially removed by the new coherent potential approximation

theory . We can see that this theory predicts that the local Ni

density of states in the alloy is similar to that in the pure metal.

Further, a comparison of the data in Fig. 4 with the data for pure Ni

shows that when for chemisorption the presence of certain states is

necessary, these states are always found on Ni atoms, never on Cu atoms,

also at highest dilution. This gives us confidence in chemisorptive

titration of surface Ni atoms. (3) When suspicions against the two

mentioned methods were eliminated, attention had to be concentrated on

[22 231 Auger spectrometry. Two groups ''^ reinvestigated the problem and

they showed that alloys equilibrated at 400-500° C do not reveal a

substantial surface enrichment when Auger electrons of E 800 eV are

used for analysis. However, when Auger electrons of E < 100 eV are used

a clear enrichment in Cu is detected, approximately up to the values

derived from hydrogen chemisorption! In this way, the previously existing

73
controversy has been removed and the surface composition of Ni-Cu alloys

is now reasonably well established. Recently, Brongersma has confirmed

[27] these results by an independent method - low energy ion scattering

Experience has thus taught us how valuable measurements of (f) can

be for information on surface composition. It is to be regretted that

so far no theory exists relating the variation of the surface dipole

layer (inferred from variation in ()>) to the bulk and surface composition. There is hope that the new methods ''"^ applied in the

theory of surface states will provide such information if the theoreticians accept the challenge.

74
REFERENCES

[I] Sachtler, W. M. H. and Jongepier, R. J., J. Catal. _4 (1965) 665.

[2] Vecher, A. A. and Gerasimov, J. I., Russ. J. Phys. Chem. 37^ (1963) 254.

[3] Van der Plank, P. and Sachtler, W. M. H. , J. Catal. 12^ (1968) 35.

[4] Rapp, R. A. and Maak, F. , Acta Met. 10 (1962) 62.

[5] Elford, L. , Mliller, F. and Kubaschewski, 0., Ber. Bunsenges. 73^ (1969)

61.

[6] Franken, P. E. C, Thesis, Leiden University, 1975. Franken, P. E. C. and

Ponec, v., J. Catal., in press.

[7] Meijering, J. L., Acta Met. 8^ (1957) 257.

[8] Palczewska, W. and Majchrzak, S., Bull. Acad. Polon. Sci. Serie Chim. 17_

(1969) 681.

[9] Sachtler, W. M., H. and Dorgelo, G. J. H. , J. Catal. 4- (1965) 654.

[10] Dowden, D. A., J. Chem. Soc. 1950, 242; Ind. Eng. Chem. 44 (1952) 997.

[II] Harris, L. A., J. Appl. Phys. 39 (1968) 1419.

[12] Quinto, D. T., Sundaram, V. S. and Robertson, W. D. , Surface Sci. 28^

(1971) 504.

[13] Ertl, G. and KUppers, J., J. Vac. Sci. Technol. 9 (1972) 829.

[14] Cadenhead, D. A. and Wagner, N. J., J. Phys. Chem. 72 (1968)

2775.

[15] Ponec, V. and Sachtler, W. M. H., J. Catal. 24 (1972) 250.

[16] Sinfelt, J. H. , Carter, J. L. and Yates, D. J. C, J. Catal. 25. (1972)

283.

[17] Ponec, V., Catal. Rev. - Sci. Eng. 11 (1975) 41.

[18] Soven, P., Phys. Rev. 156 (1967) 809; 178 (1969) 1136.

[19] Velick^, B., Kirkpatrick, S. and Ehrenreich, H. , Phys. Rev. 175^ (1968)
[20] Hufner, S., Wertheim, G. K. and Wernick, J. H. , Phys . Rev. B8 (1973) 4511.

[21] Stocks, G. M., Williams, R. W. and Faulkner, J. S., Phys. Rev. B4 (1971) 4390.

[22] Helms, C. R. , J. Catal. 36 (1975) 114.

[23] Yamashina, T., Watanabe, K. , Fukuda, Y. and Hasluba, M. , Surface Sci.

50 (1975) 591.

[24] Kalkstein, D. and Soven, P., Surface Sci. 26 (1971) 85.

[25] Haydock, R. , Heine, V., Kelly, M. J. and Pendry, J. B. , Phys. Rev.

Letters 29 (1972) 13.

[26] Berk, N. F., Surface Sci. 48 (1975) 289.

[27] Brongersma, H. H., Private Communication (Philips).

76
1200-

at % Cu

Fig. 1. Phase diagrams for Cu-Ni suggested by various authors.

1 - Refs. [1] and [3], calculations according to data in Ref. [2].

2 - Ref. [3], calculations according to data in Ref. [4].

3 - Ref. [5].

4 - Refs. [6J and [7].

77
2^ 40 60 80 % Cu

Fig. 2. Selective chemisorption of hydrogen by Ni-Cu alloys.

ot=hydrogen chemisorption/xenon physisorption ratios for films equilibrated at 200° C, according to Ref . [15]. (squares)

2 B=hydrogen adsorption/cm of alloy, Ref, [16]; 6=1 for Ni. (circles)

78
5.5

5.3

5.1

A.9-

^.5-

20

-A

ts ^

NiAu

—o

NiCu

^0

1^

60

at % Ni

Fig. 3. Work function measurements on Ni-Cu films equilibrated at 420' C

(the films equilibrated at 165° C and 215° C show exactly the same

values)

.

For comparison: data for a similar Ni-Au system (two-phase systems

at all temperatures used here)

.

79
Fig. 4. Comparison of the experimental XPS valence band structure (dots)

with the CPA theory (full line) for an alloy of 40% Ni in Cu.

Schematically, the density of states localized at Ni atoms, as

[21] calculated by CPA is indicated by the dashed curve.

80
CONTINUATION OF THE PANEL DISCUSSION

Dr. Cohen asked whether it was not possible to construct (a) nonequilibrium surfaces and (b) do catalytic measurements on them. Prof.

Boudart replied that such surfaces can be formed. He cited work by

Helms, also by Yowashita, in which surfaces were prepared by high energy

ion sputtering and annealing, and Auger measurements used to show their

non-equilibrium surface compositions. Dr. Russo inquired if, although

there is no theory for interpretation of work function changes , there are

not at least some semi- empirical rules established for this task.

Dr. Ponec said there was, citing published studies from his laboratory.

A query was made about possible substrate effects on metal clusters. How do support effects occur? Are there any electronic effects

of the support on clusters. Dr. Sinfelt replied that in general, such

effects should be possible, although for the systems he discussed,

supported and unsupported samples gave about the same results (when it

was possible to study them in this way) . But he felt it would not be

hard to visualize cases where such effects could occur.

Dr. Breiter pointed out that in his experiments on bulk, solid AuPt alloys, H2 adsorption occurs only on Pt rich phases.

In reply to Dr. Sinfelt's last remarks, it was noted from the floor

that Moss found that bimetallic clusters of Pd and Ag could be formed on

silica, but not on alumina supports, implying some strong interaction

with the alumina.

Dr. Yates asked Dr. Sinfelt if some carbon residue builds up

in the course of his rate measurements; do characterization tests give

the same results before and after reactions. Dr. Sinfelt replied that

81
surely some residue occurs, but it should not be large. His practice is

to monitor activity continuously, and the rates do not drift with time.

Prof. Ehrenreich commented on the reasons for a lack of theoretical

analysis of work function change measurements: (a) electronic structure

calculations for alloys are hard; (b) surface calculations are harder;

(c) dipole layers involve many body effects which are harder still.

Next, Dr. Gelatt presented the results of a systematic calculation

of the band structure and heats of formation of bulk, stoichiometric

transition metal hydrides across the 3d and 4d rows. The motivation for

presenting these bulk results was two fold: in the preparation of supported transition metal catalysts, there is always a high temperature,

H2 reduction step, under conditions suitable to the formation of hydrides;

secondly, it is of interest to try to understand transition metal hydrogen

bonding in a simpler system than surface adsorption. This work is

presented in the following reprint from Solid State Communications, 17 ,

663 (1975). (A more detailed description of these calculations is in

preparation.

)

82
Reprinted with permission of the authors and Pergaraon Press from

Solid State Communications, Vol. 17, pp. 663-666, 1975. Pefgamon Press. Printed in Great Britain

HEATS OF FORMATION OF 3d AND 4d TRANSITION METAL HYDRIDES*

CD. Gelatt, Jr., Jacquelyn A. Weiss and H. Ehrenreich

Division of Engineering and Applied Physics, Harvard University, Cambridge, MA 02 1 38, U.S.A.

(Received 3 March 1975; in revised form 23 April 1975 by E. Burstein)

One electron energy spectra are used to explain the heats of formation of

stoichiometric transition metal hydrides across the 3d and Ad rows. The

trends agree reasonably with existing experimental information. The magnitudes are predominantly (but not exclusively) determined by the formation of a metal—H bonding band. In contrast to the screened proton model,

the result is not directly related to the Fermi level density of states.

THE IMPORTANT factors contributing to the heat of

formation of transition metal hydrides have been identified on the basis of band theory and used to obtain a systematic understanding of the chemical trends

across both the 3d and 4d rows. Three of the principal

ingredients are: (1) the formation of a metal—hydrogen

bonding band; (2) the lowering of the metal d bands;

(3) the binding of the additional electron associated with each hydrogen atom at the top of the Fermi dis- tribution. In contrast to the screened proton model,^

systematic trends across the transition metal rows are not determined by the variation of the Fermi level density of states.

Results for the band structure of PdH, for various concentrations are shown in Fig. 1 . Switendick' has

previously studied the energy bands of stoichiometric

hydrides without, however, calculating the heats of

formation or the energy spectrum of disordered nonstoichiometric hydrides. The present results are based on the Korringa-Kohn-Rostoker (KKR)^ approach

and rcnormalized atom potentials.* The complex energy bands for non-stoichiometric hydrides were obtained

using the average /-matrix approximation (ATA),^ as extended to a rocksalt structure (for example, CuH^)

with randomly distributed hydrogen atoms and vac-

* Research supported by the National Science Foundation under grants GH-32774 and DMR 7203020.

663

ancies. The results for Pd' and stoichiometric PdH^

are in good agreement with those previously published.

In the dilute hydrides [Fig. 1(b)] a Pd-H molecular

bonding level appears below the Pd band structure

[Fig. 1(a)]. Figure 1(b) shows that some of the bands

are damped due to the disorder as indicated by the width of the shading. Within the ATA levels below the

muffin-tin zero, such as the PdH bonding level, are not damped. With increasing hydrogen concentration the molecular level broadens into a band, while simul- taneously the spectral density of the lowest Pd band

becomes increasingly broad and weak. In the stoichio- metric hydride [Fig. 1(c)] this band has been replaced

by one associated with the Pd-H molecular level in the dilute case. Structure corresponding to this band

has been observed in /3-phase PdH photoemission ex- periments.'

The heat of formation per unit cell A£" for the

reaction of metal Mwith hydrogen gas to form hydride

Mff, is

MsoUd) + ix/^2(gas) -> Mr,(solid) -E. ( 1

)

The change in energy is therefore = E(MH,) - E{Af) - \xEiHi), (2)

where EiMH^) and E{M) are respectively the total energies per unit cell of the hydride and the pure metal. EiHi) (= — 2266 Ry^ in the Hartree-Fock approxi83
664 HEATS OF FORMATION OF 3d AND 4d TRANSITION METAL HYDRIDES Vol. 1 7, No. 6

T 1

1 —I

1 1 1 1 r

J I I I I I I I L

Fig. 2. The variation across the 4d row of (a) the upper and lower d band edges, the average energy of

the occupied d bands, (e^), and the metal Fermi energy, ep ; (b) the shift in average energy of the occupied d

bands; (c) the occupation weighted shift in average energy of the lowest band; and (d) the Wigner-Seitz

radius, Ry/aFig. 3. The calculated heats of formation, AE, for stoichiometric hydrides without (sohd line) and with

(dashed line) Coulomb energy corrections. The additional points represent experimental enthaplies of for- mation, AH, of a-phase (reference 12) and non-dilute

(reference 13) 3d and 4d hydrides.

mation) is the energy required to separate a hydrogen

molecule into its constituent protons and electrons.

We shall approximate AE by the difference in the sum of one-electron valence band energies AEi =

84
Vol. 17, No. 6 HEATS OF FORMATION OF 3d AND 4d TRANSITION METAL HYDRIDES 66S

E(MHx) — E(M). The three principal contributions to

for the stoichiometric case are given by the empirical formula

(3)

They consist of (1) the shift in average energy of the lowest band (LB) containing two electrons, (2) the average shift of the d bands multiphed by w^, the number of d electrons contained in the occupied d bands

above the lowest band, and (3) the energy at which

the added electron in the unit cell enters the solid, approximated by the Fermi energy of the metal. ^°

Equation (3) can be used completely empirically, or

the terms can be evaluated by the use of Brillouin zone

integrations utilizing, for example, the special points

averaging technique.^ The results presented here are based on a two special-point average.

The variation of each of the terms of Equation (3)

for f.c.c. metals and rocksalt structure hydrides across the 4d row is illustrated in Fig. 2 . The behaviour of

the d band width, shown in Fig. 2(a), is associated with the variation of the Wigner—Seitz radius, /?ws

[Fig. 2(d)] . As /?ws decreases on the left side of the period, (ej') rises and the bands broaden.* This be- haviour, together with the filling of the d-bands, ac- counts for the initial rise in ep. On the right side of

the period, the increase of 7?vvs ^nd nuclear charge Z

causes the d bands to shift downward and to narrow. These effects compete with d band filling to produce

a net lowering of with increasing Z. The sharp rise of ep near Ag is a result of filling conduction band

states above the top of the d bands.

The shift in d band energies, shown in Fig. 2(b), correlates with the amount of metal charge lying in the

interstitial region. The decrease on the left side of the row is associated with filling of bonding d orbitals and

increasing d band width: greater delocalization ac- companies larger bonding. On the right side the effects of d band narrowing and concomitant d wave function

localization dominate. The decrease near Ag is associated with the conversion of conduction band states in the metal to band states in the hydride. It is apparent

from Fig. 2(c) that the nearly linear shift in the average energy of the lowest band is the dominant and most

rapidly varying contribution to the heat of formation.

The heats of formation of both 3d and 4d stoichio- metric monohydrides calculated from equation (3)^°

are shown in Fig. 3. In the absence of any systematic

studies of monohydrides, we show experimental points

for dilute hydrogen solutions^ and selected non-dQute

hydrides.^' As a result, comparison between theory and

experiment should be limited to an examination of

general trends (or Z dependence) across the period. The

gross features of the AE curve reflect the general be- havior of hydride formation, namely that with the exception of Pd and Ni stable concentrated hydrides

form only on the left side of the period. It is clear from Fig. 2 that the shape of the curve is determined

largely by the variation of the Fermi energy and the lower band shift with Z. In particular, the dip near Pd

and Co is attributable to the first of these effects.

The discrepancy on the left side of the periods is predominantly due to the neglect of the Coulomb re- pulsion associated with the increased charge density near a proton. This can be estimated by suitably defin- ing an effective hydrogen volume. The prescription used

here defines the hydrogen sphere radius to be located where the sum of metal and hydrogen atomic wavefunctions has zero slope along the [100] direction. The number of electrons contained within this sphere

isn„ = 1.23, 1.15. 1.08. and 1.00 for Ti.Cr, Fe, and

Ni respectively. The Coulomb energy is i«H(«H — l)Fo

when nff > 1 and zero otherwise. Fo is the standard

Slater Coulomb integral. The results of including this estimate in the 3d row are shown by the dashed curve

in Fig. 3. The effect is to bring the calculations into somewhat better accord with the heats of formation

suggested by experiment for elements at the left end

of the period. The correction vanishes to the right of

Co. Experience with YH indicates that similar results can be expected for the 4d row. We interpret the large

Coulomb energies of TiH (and YH) as indicating that the formation of a polyhydride (for which the estimated

Coulomb energies are substantially smaller) may be more favorable, as is observed experimentally Switendick's^ calculations for CaFj structure dihydrides indi- cate that a second metal—hydrogen bonding level is formed, implying that the change in one-electron energy

is compatible with EiMHi) - E(M) - E{Hi) < 0.

The present calculations indicate no significant

change in the charge density within the metal muffintin sphere upon hydride formation. To the extent that the metal charge density is properly monitored by this

estimate, the effects of charge transfer appear to be

small. However, even small charge transfer can influence

85
666 HEATS OF FORMATION OF 3d AND 4d TRANSITION METAL HYDRIDES Vol. 1 7, No. 6

the heat of formation significantly and affect the

trends. We also note that the neglect of metal Coulomb

energy changes on hydride formation is supported by

the preceding argument if the redistribution of charge

in the metal sphere is sufficiently small.

The lattice strain energy, computed from the volume

expansion and the bulk modulus, is ~ 0.01 Ry. Although

this quantity is too small to affect AE appreciably, it

is important in determining the phase diagram of the

various metal hydrides.*^

Despite these reservations, we should note that the

predictions of the present calculations are considerably

superior to those of the screened proton model^ and

do not involve adjustable parameters. The binding

energies of hydrides are weak on the scale of ^E(H2) =

1.13 Ry [equation (2)] and the discrepancies between

theory and experiment on this scale are actually fairly

smalL

Acknowledgements — The authors would like to thank

Dr. K. Kritayakirana for assistance in the early stages

of this work, and Prof. G.W. Pratt, Jr., and Drs. A. Bansil and H.L. Davis for help with the NaCl struc- ture programs.

REFERENCES

1 . EBISUZAKl Y. & O'KEEFFE M., Prog. Solid State Chem. 4, 1 87 (1967).

2. SWITENDICK \£., Solid State Commun. 8, 1463 (1970); SWITENDICK k.C.Jnt. J. Quantum Chem. 5,459

(1971); SWITENDICK A.C., Ber. Bunsenges. Phys. Chem. 76, 535 (1972).

3. KOHN W. & ROSTOKER N., Phys. Rev. 94, 1 1 1 1 (1954).

4. HODGES L., WATSON R.E. & EHRENREICH H., Phys. Rev. 85, 3953 (1972); GELATT CD., Jr., Unpublished

Ph.D. thesis. Harvard University (1974); GELATT CD., Jr., EHRENREICH H. & WATSON R.E. (to be published).

5. BANSIL A., EHRENREICH H., SCHWARTZ L. & WATSON R.E., Phys. Rev. B9, 445 (1974),

6. HUlSMAN L.& WEISS J.A., Solid State Commun. 16, 983 (1975).

7. MUELLER F.M., FREEMAN AJ., DIMMOCK J.O. & FURDYNA A.M., Phys. Rev. Bl, 4617 (1970).

8. EASTMAN D.E., CASHION J.K. & SWITENDICK A.C., Phys. Rev. Lett. 27, 35 (1971).

9. PARR R.G., Quantum Theory ofMolecular Electronic Structure, Benjamin, NY (1972).

10. For the noble metals it is necessary to modify equation (3) by substituting for ep the explicit energy (~ ep") at which the electron is added.

11. BALDERESCHI A., Phys. Rev. B7, 5212 (1973); CHADI D.J. & COHEN M.,Phys. Rev. B8, 5747 (1973).

12. MCLELLAN R.B.&OATESVi.A., Acta Met. 21, 181 (1973).

1 3. LIBOWITZ G.C, 77ie Solid State Chemistry ofBinary Metal Hydrides, Benjamin, NY (1965); MEULLER W.A.,

BLACKLEDGE J.P. & LIBOWITZ G.C, Metal Hydrides, Academic, NY (1968).

14. GELATT CD., Jr. & EHRENREICH H., Phys. Rev. BIO, 398 (1974).

15. ALEFELD G.,Ber Bunsenges. Phys. Chem. 76, 746 (1972); WAGNER H. & HORNER li.,Adv. Phys. 23, 587

(1974).

86
CONCLUSION OF THE PANEL DISCUSSION

With reference to Fig. 3 of the reprint, Dr. Gelatt observed that

bulk bonding of hydrogen is weak in the region of active catalysts, as

one might expect.

Dr. Ponec questioned the validity of this analogy. He pointed out

that Mo has one of the highest heats of adsorption for hydrogen, but

that it does not form a hydride. He suggested that some factor related

to the rupture of metallic bonding must be added to the present considerations.

It was enquired from the audience whether the potential for the

calculations included core-core interactions, or was it all orbital.

Dr. Gelatt replied that it was all orbital; the systems are metallic,

and there is little evidence of charge transfer.

Dr. Gadzuk wondered about the apparent lack of importance of many

body effects to the bulk calculations, in contrast to the frequency with

which many body effects are invoked in chemisorption calculations. Dr.

Gelatt replied that it was not clear that a good one-electron job has

yet been done on the chemisorption problem.

Dr. Siegel noted that the calculated d band widths appear to remain

the same upon hydride formation. Dr. Gelatt pointed out that in the

NaCl structure assumed for the calculations, the states which define the

top and bottom of the d bands are of a symmetry which does not see the

proton.

Dr. Messmer noted that Yang, Johnson, and he have done cluster

calculations, with hydrogen at tetrahedral and octahedral sites, and

note the same d band width effect. But he noted that binding energies

are different for hydrogen at the separate sites.

87
Dr. Duke asked Dr. Messraer if his cluster geometries were self

consistently calculated or fixed. Dr. Messmer said they were fixed at

the bulk geometry.

At this point, the chairman asked for comments as to whether these

calculations are relevant to the problems of interest, or useful in some

way; they are certainly easier to make than calculations of the real

thing. Dr. Ponec made one such remark already. Were there others?

Dr. Bennett pointed out that bulk hydrogen seems to affect the

catalytic properties of Raney Ni.

Dr. Sinfelt remarked that calculational insight into the ways

molecules are adsorbed would be quite useful.

Prof. Boudart noted the importance of knowing about the bulk electronic structure of a variety of interstitial compounds, hydrides,

carbides, and nitrides-some of which are active catalysts - with a view

to predictions of new, useful catalytic materials.

Dr. Duke wondered if, since a high temperature reduction in hydrogen is always employed in the preparation of small supported particles, the hydrogen might not be incorporated in some essential way in

their structure. This possibility was acknowledged by Dr. Sinfelt, and

again the tenuous nature of our knowledge of the structure of these

small particles was emphasized.

88
SESSION 3

GEOMETRICAL

EFFECTS

Moderator:

R.HANSEN

Ames Laboratory, Energy

Research & Development Admin.

Lecture by:

G.ERTL

University of l\^unctien

89
Session 3.

Geometrical Effects

Panei (Members:

G. ErtI,

Univ. of Munchen

C. B. Duke,

Xerox Corp.

J. Turkevich,

Princeton

Recorder:

A. J. Melmed,

NBS

90

ll
WORKSHOP ON THE ELECTRON FACTOR IN CATALYSIS

SESSION 3: GEOMETRICAL EFFECTS

THE RECORDER'S STORY

Allan J. Helmed

National Bureau of Standards

Washington, D.C. 20234

In a large sense, discussion of "geometrical effects" took place at

many random times throughout this Workshop as well as during the approximately two hours which were formally devoted to the subject. If one

attended the Workshop with the expectation of learning about geometrical

effects on the electron factor in catalysis, however, one perforce would

be disappointed. For, despite the wealth of discussion of geometrical

effects, there was a dire paucity of comments on geometrical effects in

catalysis and an even less densely populated set of comments attempting

to relate geometry of surfaces to something one might recognize as an

"electron factor."

It is appropriate to ask whether it is possible to separately

investigate electron and geometry effects for real surfaces. In order

to achieve a separation, it would be necessary to use surfaces with

constant geometry and varying electron factors and surfaces with

constant electron factor and varying geometry. In a strict sense, this

does not appear to be possible. The closest approach to these conditions

seems to be the use of surfaces of materials with similar crystal structure

in order to examine electron factor effects and the use of various

surface modifications of one material in order to examine geometrical

effects. But neither of these approaches actually separates the electron

factor and the geometrical factor. The hope is that in the two types of

experimentation, one factor or the other will be the dominant factor.

Probably a more accurate description would be to speak of the material

factor.

As a Recorder, it was not my responsibility to react emotionally to the

Workshop discussions. For those attendees who might have suffered some

disappointment, however, I will first attempt to explicitly recall those

91
isolated comments which somehow directly related geometrical effects to

an electron factor (Part 1) . Then will follow a much larger collection

of material which related to geometrical effects in surface physics,

surface chemistry, and surface metallurgy; the formal contribution of the

invited speaker, G. Ertl (Part 2), and the contributions from the panel

(Parts 3-6) . Finally, general discussion items are recorded (Part 7)

and a succinct summary is given (Part 8)

.

92
Part 1. Discussion Related to Geometrical Effects

on an Electron Factor

V. Ponec mentioned E. W. Miiller's explanation of certain aspects of

image contrast in the field ion microscope in terms of directed, or

dangling bonds protruding from the surface (Z. Knor and E. W. Miiller,

Surf. Sci. 10 (1968) 21). (In the model referred to, the probability of

electron transfer from an image gas molecule or atom to the specimen

surface is strongly influenced by the geometry of the unpaired, virtual

bonds of the surface atoms, and also by the degree of occupancy of these

orbitals .

)

G. Ertl, during his formal lecture (which follows), discussed

Smoluchowski' s ideas relating electron work function differences to

differences in surface atomic geometry. He then discussed experimental

results which might be partially understood in terms of such electron

work function differences.

93
Part 2. The Geometric Factor in Chemisorption and

Catalysis on Metals

G. Ertl

Institut fUr Physikalische Chemie,

Universitat Munchen

I. Introduction

The idea that the geometric arrangement of the atoms in the surface

of a catalyst might be of importance for its activity and selectivity

emerged already in the early days of systematic research on the catalytic

action. The role of this principle becomes strikingly evident in the

field of biology, in which reactions of complex molecules are catalyzed

with extreme specificity by enz3nnes leading to the picture that these

catalysts fit their substrates, just as a key fits into a lock.

H. S. Taylor ''^ suggested that similar effects might be of considerable

importance for reactions catalyzed by solid surfaces. He introduced the

concept of "active centres" which were believed to be regions characterized

by particular configurations of the surface atoms. Their actual geometry

[2]

was discussed particularly by Balandin in the framework of his socalled "multiplet theory."

Systematic experiments on these effects with "real" catalysts are

complicated by the fact that they consist of polycrystalline material

exposing different crystal planes and a whole spectrum of structural

imperfections, i.e., without any well-defined surface topography. It is

only possible to investigate this phenomenon by studying the catalytic

[3] activity with particles of varying mean size . Very small particles

o (with diameters less than about 50A) are predicted to expose a larger

number of atoms exhibiting a high degree of unsaturated valences. In

particular special adsorption sites at which an adsorbed particle is

surrounded by five surface atoms (B5 sites) were predicted to occur

° [4] predominantly with particles of about 25A diameter . As a result of

experimental studies in this direction surprisingly many catalytic

[3] reactions were found to be structure- insensitive ("facile" reactions) , and even in those cases in which the rate and selectivity were dependent

94
on the particle size ("demanding" reactions) it is not always clear

whether these effects are really due to the operation of the geometric

[3] factor . Since most of these investigations were performed with fee

transition metals, at least for this class of materials, the conclusion

may be drdwn that their catalytic activity with most reactions is not

influenced dramatically by the surface morphology.

Systematic studies can be perfoinned by using single crystal surfaces,

probably themselves exhibiting well-defined structural imperfections

(arrays of steps) , and apparently some of the recent results from this

latter approach seem to indicate a considerable influence of the surface

geometry—in contrast to most of the investigations on the particle-size

effect.

The rate of a catalyzed reaction, r, is in the simplest way determined

by the rate constant k(T) and by a function of the concentrations of the

reacting species f(c^), i.e.,

r = k(T) X f(C^).

If the reacting particles are in equilibrium with the fluid phase

through adsorption—desorption steps (which is frequently the case)

,

their surface concentrations depend exponentially on the adsorption

E /RT energies E_^; therefore f(c^) = g(e i ). The rate constant may be

written as

1

, AS|/R -E*/RT

k(T) = k • e ' • e

o

in which AS | denotes the activation entropy and E* the (apparent) activation

energy. The latter will, in general, also be related to the adsorption

energies of the reacting particles. In the simplest case this will be a

linear relationship similar to the Br^nsted law in acid-base catalysis.

As a consequence, even relatively small variations of the adsorption

energies (all the other parameters being constant) are predicted to

appreciably influence the reaction rate due to their exponential relationship.

From a simplified point of view, it may, therefore, be predicted that the

geometric factor will be of importance in the following cases:

a) if relatively strong variations of the energy of the bond

between the surface and the adsorb ate are observed;

95
b) with reactions of larger molecules or with cooperative

processes, i.e., if not only a single surf ace-adsorbate

bond is involved in the elementary step of the reaction.

Enzyme catalysis is an extreme example for this situation

which is expected essentially to affect also the activation

entropy. In the following, this aspect will, however, not be

discussed in more detail.

According to a rather naive picture, one would assume that the

strength of the adsorbate-substrate bond increases as the number of

"unsaturated" valences of the surface atoms increases, i.e., if their

coordination number decreases. This assumption generally does not hold

as will be shown by several examples. (The energy for dissociation of

an H-atom from H-O-H is 119 kcal/mole, but only 102 kcal/mole from HOl''^"'.) Using the picture of a surface molecule, variations of the adsorption energy are to be expected if the surface geometry influences

markedly either the energies and/or occupancies of those orbitals of the

surface atoms which are involved in the bond, or the overlap between the

adatom orbitals and the corresponding group orbitals from the surface

atoms. It can be assumed that these effects are more pronounced with

solids exhibiting strongly directed bonds in the bulk which probably

also persist at the surface ("dangling bonds") . The occurrence of

electronic surface states represents a further complication of this very

rude picture.

Bonds of this latter type are, for example, present with the elemental

semiconductors (Ge, Si) which crystallize with the diamond lattice, and

in fact a reaction with clean germanium surfaces has been found to be

[61 highly structure-sensitive : Rather heterogeneous surfaces were

created by crushing thin Ge slabs in UHV with a magnetically operated

hammer. The decomposition of leading to gaseous ^2 oxygen

remaining attached to the surface served as a test reaction. Since

oxygen does not desorb, the 0 atoms formed continuously block the "active

sites" where the reaction takes place. As a consequence during the

progress of the reaction the activation energy increases continuously

from 7 to 45 kcal/mole (although even in the latter case the oxygen

coverage was still far below saturation) thus demonstrating very strong

variations of the catalytic activity across the surface (Fig. 1).

96
II. Metal Single Crystal Surfaces

With the transition metals variations of the electronic properties

with the surface orientation become evident from theoretical as well as

from experimental investigations.

[7] Using a tight-binding approximation Hydock and Kelly calculated

the local densities of d-states at atoms in different low index surfaces.

Figure 2 shows the results for the (110), (100), and (111) surfaces of a

bcc crystal which have to be compared with those reproduced in Fig. 3

for the three most densely packed surfaces of an fee lattice.

r 81 Ultraviolet photoelectron spectra from a bcc metal (W)^ , as well as

[9] from an fee metal (Ni) , exhibit pronounced differences between different]

oriented surfaces. Moreover, the W(IOO) surface is the first example of

clear evidence for the existence of metallic surface states . However,

at least in the case of nickel, the anisotropy of the chemical behaviour

is much smaller than would probably be expected on the basis of the

different electron energy distributions.

The adsorption energies for hydrogen on different W single crystal

planes, as measured as a function of coverage by Domke, et al. , are

[12] reproduced in Fig. 4. Similar data are reported by Schmidt . The

initial heat of adsorption varies between 32 and 40 kcal/mole. Since

the adsorption is dissociative this means that the strength of the

metal-hydrogen bond varies between 68 and 72 kcal/mole, i.e., only by

about 6%. At higher coverages the adsorption energies change due to

energetic heterogeneities or due to the onset of repulsive interactions.

Much stronger differences were reported for the adsorption kinetics

[131 of nitrogen on tungsten. Adams and Germer concluded that the sticking

coefficient at room temperature is appreciably high only on those planes

which contain sites with fourfold coordination for the adsorbate. In

particular for the W(lll) face (which is suspected to play a dominant

role in ammonia synthesis at iron catalysts) the sticking coefficient

[14] is reported to be quite small

One of the few examples reported in the literature, for which the

rate of a catalytic reaction has been studied with different single

crystal surfaces under UHV conditions is the work of McAllister and

97
Hansen'"''"^"' on the ammonia decomposition on tungsten. Some of their

results are reproduced in Fig. 5 and indicate that the rate of NH^

decomposition on W(lll) is considerably higher than on the (100) and

(110) surfaces. This appears to be in some contrast to the findings on

the adsorption kinetics of N- on tungsten, since on the (111) plane the

[14] sticking coefficient is rather low , although a direct comparison

between the kinetic data of these two different processes is problematic.

Since the kinetic laws describing the rate of ammonia decomposition are

rather complicated, it is not possible to see in a simple manner in

which elementary step of the reaction the plane specificity comes into

play. It is interesting to notice that the (apparent) activation energies

on the (111) and (100) planes are found to be nearly identical but on

the (110) surface are appreciably higher.

The most important catalytic reaction occurring at a bcc metal is

certainly the synthesis of ammonia over iron catalysts. Despite enormous

r 16

1

efforts in the past , the mechanism of this reaction is still unclear,

although mostly the adsorption of nitrogen is considered as being the

rate-determining step. Some years ago Brill, et al.,'"''"^'' suggested from

some rather qualitative observations by means of the field emission

microscope, that the NH^ formation takes place preferentially on the

(111) faces. This assumption was recently supported by some work from

[181 Boudart's laboratory using small iron particles. These authors

concluded—mainly based on the magnetic properties and CO adsorption

data of their small catalyst particles—that the so-called C^-sites as

present on the Fe(lll) surface are the most active ones in ammonia synthesis.

This picture is confirmed by very recent studies on the kinetics of

nitrogen adsorption on clean Fe(lOO) '"'^^^ and (111) single crystal

surfaces using Auger electron spectroscopy to monitor the surface concentration. The variation of this quantity with the exposure is reproduced

in Fig. 6 for both surface orientations. Whereas on the (100) surface

the initial sticking coefficient is rather small (in the order of 10 ^)

,

on the (111) surface chemisorption proceeds more rapidly by at least one

order of magnitude. By means of low energy electron diffraction the

formation of ordered adsorbate structures could be observed, but no

nitrogen induced surface reconstruction as has been suggested to occur

98
ri8i [171 with small catalyst particles or field emission tips was observed.

We thus believe that ammonia synthesis on Fe catalysts represents, in

fact, an example of a structure-sensitive reaction. However, whether

the C^-sites play indeed the dominant role still remains somewhat speculative.

Variations with the surface orientation of the initial adsorption

[21] [22] energies of hydrogen on nickel , as well as of CO on nickel and

[23] palladium are listed in Tables 1-3 and may serve as examples for the

role of the geometric factor in chemisorption on fee metals. With the

H/Ni system the differences are particularly small and the strength of

[24] the metal-H bond is quite similar to that in the diatomic NiH molecule

A similar behaviour is found for the adsorption of CO on nickel for

which again the adsorption energies are comparable to the dissociative

[25]

energy (35 kcal/mole ) of Ni(CO)^. It is felt that this close correspondence indicates that cluster calculations (using the SCF-Xa technique)

are a successful approach to a theoretical treatment of chemisorption on

metals.

The last column of Table 2 contains the maximum number of CO molecules

2 -4

adsorbed per cm at room temperature and with CO pressures below 10

Torr. Although the density of surface atoms varies between the three

planes by about 60% the adsorbed amounts are quite similar, irrespective

of any features characterizing the coordination of numbers of "dangling"

bonds of the surface atoms. Moreover the maximum densities of adsorbed

CO molecules with all fee metal surfaces which were studied so far, are

determined by the tendency for the formation of close-packed layers,

whereby, an effective diameter of about 3A has to be attributed to the

adsorbed CO. This aspect is of some importance for those techniques in

which the metallic surface area of small catalyst particles is derived

from selective CO adsorption.

A series of LEED observations on the adsorption of CO on fee metals

revealed that saturation of the adsorbate layer is achieved by a continuous

compression of the unit cell of the adsorbate. This means that fixed

adsorption sites do not exist but rather that the binding energy changes

only slightly along certain directions on the surface.

Such a behaviour is also predicted on the basis of energy profiles

calculated by means of a modified Anderson-Grimley formalism under the

99

[22,23,26]
assumption of maximum overlap between the 2fT*-orbital of CO and the

[27] metallic d-orbitals . As an example the theoretical energy profile

for CO on Pd(llO) is reproduced in Fig. 7 together with the structural

models (corresponding to increasing coverage) as derived from LEED

observations. Aside from being a satisfactory explanation of the experimentally

derived surface configurations of the CO molecules, this semi-empirical

theory also predicts only relatively small variations of the binding

energies with surface orientation which is also in agreement with the

experimental findings.

The catalytic oxidation of CO over different Pd single crystal

surfaces, as well as with a polycrystalline wire, was studied in some

["28 291 detail in our laboratory ' . The conclusions on the kinetics of

this reaction were recently confirmed by a series of papers by White and

coworkers . Figure 8 shows the variation of the steady-state rate of

CO2 formation with temperature at constant partial pressures of the

reactants with different Pd surfaces. Obviously, there is no noticeable

influence of the surface crystallography on the reaction rate. At

temperatures below about 200 °C the rate is determined by the desorption

of CO. (CO^^ inhibits the dissociative adsorption of O2 which is a

necessary prerequisite for the reaction to occur) . From Table 3 it

becomes evident that the heat of CO adsorption (and therefore, obviously,

also its rate of desorption) is nearly independent of the surface orientation.

Even with Pd(llO) for which the highest value for the initial adsorption

energy was observed E drops rapidly to about 35 kcal/mole with increasing

[23]

coverage . The observed decrease of the reaction rate at higher

temperatures is due to the onset of oxygen desorption. Although data on

the adsorption energies of oxygen on different Pd surfaces are not

available in such detail as for CO, there is certainly again no strong

variation with the surface orientation. Thus, it becomes plausible why

the oxidation of CO on Pd is a structure- insensitive ("facile") reaction.

III. The Role of Steps

Structural imperfections have been frequently discussed as playing

the role of "active centres" in heterogeneous catalysis. Particularly,

100
this was believed to be the case with dislocations which are of decisive

importance for the kinetics of crystal growth. Unfortunately, so far

there exists no experimental possibility to introduce dislocations with

defined densities and structures at clean single crystal surfaces under

UHV conditions. However, an alternative possibility for deviations from

the perfect lattice structure is the preparation of surfaces with periodic

arrays of monoatomic steps. Surfaces of this type are frequently quite

[31] stable and can easily be studied by means of LEED . Considerable

differences of the reactivities between low index planes and stepped

surfaces with reactions involving hydrocarbons were reported by Somorjai,

[32]

et al. . However, in these cases the surfaces became covered during

the reaction by carbonaceous overlayers whose structure and degree of

periodicity depended strongly on the presence and type of steps. Thus,

one might argue that the variations of the catalytic activity are mainly

caused by the structure of the decomposition products which in turn

might be influenced (as in normal crystal growth) by the presence of the

steps.

The effect of steps on the nucleation and growth of domains of

[33] ordered adsorbed layers was recently studied with the system 0/W(110) : Oxygen adsorption on a W(llO) surface causes the formation of two domain

orientations of a p(2xl) structure. With the presence of periodic steps

along the (111) directions it was observed that one type of domains

appeared quite preferentially.

The effect of periodic step arrays on the adsorption energy has

been studied for CO and H2 adsorption on a Pd(lll) surface. The

results are reproduced in Figs. 9 and 10. For CO the isosteric heat of

adsorption, as a function of coverage, is practically identical for both

types of surfaces thus supporting the structure-insensitivity of the CO

oxidation reaction. However, for hydrogen the initial adsorption energy

is higher by about 3 kcal/mole for the stepped surface (thus being

similar to the value for the (110) plane) and approaches the data for the

low index surface with increasing coverage. This result has to be

interpreted in terms of a somewhat higher binding energy of the H-atoms

at the adsorption sites near the steps (64.5 kcal/mole instead of 63

kcal/mole)

.

101
Quite dramatic effects of the presence of steps were reported for

r 3 2 361

the interaction of hydrogen with Pt(lll) surfaces. Somorjai, et al. ,

'

reported that there is nearly no hydrogen adsorption on low index platinum

planes, whereas, this is readily the case with stepped surfaces. These

findings made those techniques in "practical" catalysis research questionable

for which the surface area of Pt particles is determined from the hydrogen

uptake and which have so far been quite successful. Recently Bernasek

[35] and Somorjai studied the '^^I'D^ exchange reaction on Pt(lll) surfaces

with a molecular-beam technique and found that with the low index plane,

practically no HD formation could be observed, whereas the stepped

surfaces were apparently quite reactive. The conclusion was that the

atomic steps play a decisive role in dissociating the H2 molecules.

Recent results from our laboratory are in contrast to these con-

[381 elusions : It was found that even at 100 K hydrogen adsorbs dissociatively on a Pt(lll) surface with an appreciably high sticking coefficient

(s^ ~ 0.1) without any indication for the existence of an activation

barrier. The isotopic exchange reaction was also observed to take place

[39] quite readily in agreement with earlier results of Lu and Rye

However, the adsorption energy was determined to be rather low ('^^10

kcal/mole) even at small coverages so that far below room temperature

complete desorption takes place after evacuation of the vacuum system.

The variation with coverage of the isosteric heat of hydrogen adsorption

on a Pt(lll) plane, as well as on a stepped Pt(lll) surface''^^^ is

reproduced in Fig. 11. Similar to the behaviour on Pd(lll) E^^ increases

for the stepped surface with decreasing coverage to values which are

about 3-4 kcal/mole higher. With respect to the strength of the Pt-H

bond this corresponds to a variation of only about 3%! A difference of

the adsorption energy by 4 kcal/mole, however, is at room temperature

equivalent to a variation of the mean residence time by nearly three

orders of magnitude. This means that under steady-state conditions a

large variation of the surface concentrations occurs and this might

explain the pronounced differences observed in the experiments of Bernasek

102
[351 and Somorjai with the activity for the ^2^^2 ^^^^^'^S^ reaction.

This example nicely demonstrates how under certain conditions even

rather small variations of the adsorbate bond strength may drastically

effect the catalytic activity so that structural imperfections may

indeed play the role of active centres.

It is evident that a quantitative theoretical understanding of such

small energetic variations will be rather difficult. An interesting

alternative to the naive picture in which the valences of the edge atoms

are less saturated and therefore may form a stronger bond with the

[41] adsorbate was recently proposed by Ibach in connection with the

discussion of the influence of steps on semiconductor surfaces on the

kinetics of oxygen adsorption: In an extended study with W(llO) surfaces,

[421 Wagner and Besocke observed that the presence of steps leads to a

lowering of the work function which they interpreted in terms of an

[43] early hypothesis of Smoluchowski . According to this model any

deviation from a flat surface should cause such an effect since the

electron gas does not follow sharp edges. As a consequence, the binding

energy of electronegative species should increase mainly for electrostatic

reasons.

While this concept probably holds for the interaction between

oxygen and semiconductor surfaces , no direct applicability to the examples

discussed in the present context may be found: On Pd(lll) the adsorption

energy of CO is totally unaffected by steps, although the adsorbed CO

molecule carries a negative charge. With H2/Pt(lll) steps increase the

adsorption energy, although hydrogen adsorption lowers the work function.

And finally for the three mostly densed packed clean nickel planes, the

1.3 e^ [21]

[44] work function is reported to vary by 0.3 eV whereas the adsorption

energies for hydrogen are nearly equal

103
References

[I] Taylor, H. S. , J. Phys. Chem. 30 (1926), 145.

[2] Balandin, A. A., Z. Phys. Chem. B2 (1929), 289; B3 (1929), 167.

[3] Boudart, M. , Adv. Catalysis 20 (1969), 153.

[4] van Hardeveld, R. , and van Montfoort, A., Surface Sci. h_ (1966),

396.

[5] CRC Handbook of Chemistry and Physics.

[6] Ertl, G., Z. Phys. Chem. N.F. 50 (1966), 46.

[7] Haydock, R. , and Kelly, M. J., Surface Sci. 38 (1973), 139.

[8] Feuerbacher, B. , Surface Sci. 47 (1975), 115.

[9] Page, P. J., and Williams, P. M. , Faraday Disc. 58 (1974), 80.

[10] a) Feuerbacher, B., and Fitton, B., Phys. Rev. Lett. 29 (1972),

786.

b) Waclawski, B. J., and Plummer, E. W. , Phys. Rev. Lett. 29^ (1972),

783.

[II] Domke, M. , JShnig, G. , and Drechsler, M. , Surface Sci. 42 (1974), 389.

[12] Schmidt, L. D., Catalysis Rev. 9^ (1974), 115.

[13] Adams, D. L. , and Germer, L. H. , Surface Sci. 27^ (1971), 21.

[14] Delchar, T. A., and Ehrlich, G. , J. Chem. Phys. 42 (1965), 2686.

[15] McAllister, J., and Hansen, R. S., J. Chem. Phys. 59 (1973), 414.

[16] Emmett, P. H. , in "The physical basis of heterogeneous catalysis"

(R. I. Jaffee, ed.). Plenum Press 1976.

[17] Brill, R. , Richter, E. L. , and Ruch, E. , Angew. Chem. Int. Ed. 6^

(1967), 882.

[18] a) Dumesic, J. A., Topsoe, H. , Khammouma, S., and Boudart, M. , J.

Catalysis ^

(1975), 503.

b) Dumesic, J. A., Topsoe, H. , and Boudart, M. , J. Catalysis _37

(1975), 513.

[19] Ertl, G., Grunze, M. , and Weib, M. , J. Vac. Sci. Techn. , 1976 (in

press)

.

[20] Ertl, G., Grunze, M. , and Weib, M. , in preparation.

[21] Christmann, K. , Schober, 0., Ertl, G. , and Neumann, M. , J. Chem.

Phys. 60 (1974), 4528.

[22] Christmann, K. , Schober, 0., and Ertl, G. , J. Chem. Phys. 60

(1974), 4719.

104
[23] Conrad, H. , Ertl, G. , Koch, J., and Latta, E. E. , Surface Sci. 43

(1974), 462.

[24] Gaydon, "Dissociation Energies and Spectra of Diatomic Molecules,"

London 1953.

[25] Skinner, H. A., Adv. in Organometallic Chemistry (Eds. F. G. A. Stone

and R. West) , 1 (1964)

.

[26] a) Tracy, J. C. , and Palmberg, P. W. , J. Chem. Phys. 51 (1969), 4852.

b) Track, J. C. , J. Chem. Phys. 56 (1972), 2736.

[27] Doyen, G. , and Ertl, G. , Surface Sci. 43 (1974), 197. This treatment

was later improved by including also the coupling of the CO 56-Orbital

to free electron-like metallic s-states, however, without significantly

affecting the shape of the calculated energy profiles or the predicted

changes of the adsorption energy with surface orientation (G. Doyen and

G. Ertl, to be published).

[28] a) Ertl, G. , and Rau, P., Surface Sci. 15 (1969), 443.

b) Ertl, G. , and Koch, J., in "Adsorption-Desorption Phenomena"

(F. Ricca, ed.). Academic Press 1972, p. 345.

c) Ertl, G. , and Neumann, M. , Z. Phys. Chem. N. F. 60 (1974), 127.

[29] Ertl, G. , and Koch, J. Proc. 5th Internat. Congr. on Catalysis,

North Holland 1973, p. 969.

[30] a) Matsushima, T. , and White, J. M. , J. Catalysis 39 (1975), 265.

b) Matsushima, T., Almy, D. B. , Foyt, D. C, Close, J. S., and

White, J. M. , idib. , 277.

[31] Lang, B., Joyner, R. W. , and Somorjai, G. A., Surface Sci. ^

(1972) 440, 454.

[32] Lang, B. , Joyner, R. W. , and Somorjai, G. A., J. Catalysis 7J_

(1972), 405.

[33] Engel, T. , von dem Hagen, T., and Bauer, E. , to be pulished.

[34] Conrad, H. , Ertl, G. , and Latta, E. E. , Surface Sci. 41 (1974), 435.

[35] Bernasek, S. L., and Somorjai, G. A., J. Chem. Phys. §2^ (1975),

3149.

[36] Morgan, A. E. , and Somorjai, G. A., Surface Sci. 12 (1968), 405.

[37] Benson, J. E. , and Boudart M. , J. Catalysis h_ (1965), 704.

[38] Christmann, K. , Ertl, G., and Pignet, T., Surface Sci. (in press).

[39] Lu, K. E., and Rye, R. R. , Surface Sci. 45 (1974), 677.

[40] Christmann, K. , and Ertl, G. , to be published.

105
[41] Ibach, H. , Surface Scl. (in press).

[42] Besocke, K. , and Wagner, H. , Phys. Rev. B8 (1973), 4597.

[43] Smoluchowski, R. , Phys. Rev. 60 (1941), 661.

[44] Maire, G. , Anderson, J. R. , and Johnson, B. B., Proc. Roy. Soc

A320 (1970), 227.

106
Table 1: Initial adsorption energies for hydrogen on Ni single crystal

. [21] surfaces

Plane (111) (100) (110)

E j[kcal/mole] 23 23 21.5

acl

Table 2: Adsorption of CO on Nickel. Initial adsorption energy E^^

and maximum densities of adsorbed molecules, n , at T=300 K

_ / r 9 9 1 max

and p_- $ 10 Torr^ .

Plane (111) (100) (110)

E ,[kcal/mole] 26.5 30 30 ad

n X 10"'"^[cm"^] 1.1 1.1 1.14

a

Table 3: Initial adsorption energies for CO on Pd single crystal

surfaces

.

Plane (111) (100) (110) (210) (311)

E^^[kcal/mole] 34 36.5 40 35.5 35

107
12 16 20

1/T [''K-'J

24 -W

Fig. 1. Arrhenius plots for the rate of decomposition on crushed

Ge surfaces at different stages of inhibition by adsorbed

[6]

oxygen

108
200 + 200+ 200f

•0.1 0 0.1 -0.1 0 0.1 -0.1 0 0.1

(111) (110) (100)

Fig. 2. Local densities of d-states at the (111), (110), and (100)

surface of bcc crystal as calculated by Haydock and Kelly [7]

109
110
( 1001

••••« (III

I

..... (IIJJ

Fig. 4. Adsorption energy versus coverage for hydrogen on different W

single crystal planes. (Domke, et al.

)

111
Fig. 5. Rate of NH„ decomposition as a function of p on different

3 [15] W single crystal surfaces (McAllister and Hansen ).

112
Fig. 6. Adsorption kinetics of nitrogen on Fe(lll) and Fe(lOO) surfaces

at 235°C. Variation of the relative coverage with N2 exposure

(Ertl, et aL^^^-").

113
tioou

Fig. 7. a) - c) Structure models (with increasing coverage) for CO

adsorbed on Pd(llO)^^^^.

d) Theoretical energy profile for the variation of the CO

adsorption energy within the unit cell of the Pd(llO) surface

114
steady -state rate of CO2 formation at Pd surfaces

Fig. 8. Steady-state rate of CO^ formation as a function of temperature on

-7 [291 different Pd surfaces. ~ Pn =10 Torr

L>(J U2

115
Fig. 9. Isosteric heat of CO adsorption as a function of the work function

increase A(J) on a Pd(lll) surface (dark circles) and on a stepped

Pd(lll) surface (open circles). The latter consisted of terraces

with (111) orientation, 9 atomic rows in width and separated by

[23] monoatomic steps also with (111) orientation

116
Fig. 10. Isosteric heat of H2 adsorption as a function of the work function

increase A(|) on a Pd(lll) surface (dark circles) and on a stepped

[34] Pd(lll) surface (open circles)

117
Fig. 11. Adsorption energy as a function of coverage for H2 on a Pt(lll)

surface (curve b, open triangles) and on a stepped Pt(lll) surface

(curve a, dark circles) .

118
Part 3. Abbreviation of Comments

Robert S. Hansen

Ames Laboratory, ERDA

Iowa State University

Ames , Iowa

Since my paper with McAllister (J. Chem. Phys. 59, 41A-22 (1973))

has been cited by two of the speakers, I would like to add some comments

of my own. Briefly, we found that the rate of ammonia decomposition on

the (111), (100), and (110) faces of tungsten followed the law Rate = A

2/3 + BP.^„ , and was independent of nitrogen and hydrogen pressures, over

the temperature range 800 < T < 970 K and pressure range 0.5 < P < lOOy.

Catalytic activities were in the order (111) > (100); > (110); the small

activity observed for (110) was likely due to crystal edges with the

(110) face itself being inactive. For a given face (100) and temperature

the constant A was the same for NH^ and ND^, but the constant B was

significantly greater for NH^. These results imply that two processes

are carrying the decomposition process, represented by the terms A and

2/3 BP^^ in the rate equation. We called these the A and B processes,

respectively. Using the (100) face as a model, we proposed that both

processes occurred on a nearly complete surface WN structure ((1X1)

structure). The WN notation is intended to represent surface stoichiometry only, i.e., one nitrogen atom per tungsten atom. The two

processes suggested were then

k

A process 2WN -> W^N + I/2N2

B process 3WN + NH^(g) | + WNH

^1

W^N^H^ W^N + +

^2

2WNH ^ 2WN +

Presuming the surface fraction of WN to be nearly unity, the fractions

of W2N, and WNH correspondingly small, the independence of the A

process on reactant and product pressures and the fact that the constant

119
A is the same for NH and ND are accounted for. A steady state treatment

2/3

of the B process under the same assumptions leads to the P^^^ dependence,

and since NH bonds are broken in both rate determining steps^the constant

B is expected to differ for NH^ and ND^ as observed. The (IXI) WN and

C(2X2) W^N structures have been established by LEED; the WNH and W^N^H^

structures have not been established (and in the above model would not

have sufficiently large concentrations to form recognizable phases)

.

I would like to use these and related findings as a background to

discuss what I think is the outstanding problem in catalysis—the development

of a conceptual framework for discussing, in structural detail, the

rates of surface reactions, i.e., a conceptual framework for discussing

the transition state in such reactions. We generally know the initial

reactants and final products of a catalytic reaction, and sometimes have

an idea of structures of adsorbed species immediately preceding and

immediately following the rate determining step. But we need much

better patterns of thought for discussing the rate determining step

itself. Let me illustrate the problem with some reactions on the (100)

face of tungsten (not because it is catalytically most important, for it

surely isn't, but because it is geometrically simple).

Suppose we superimpose a Cartesian coordinate system on W(IOO) with

o the unit cell length 3.16A as length unit and the center of a surface

tungsten atom at (0,0), so that each position (m,n) , with m,n integers,

is located at the center of a surface tungsten atom. The position

(1/2, 1/2) is then a hole with 4 tungsten atoms surrounding it in its

plane and one below it, so that it is a CN-5 (coordination number 5)

position; of course all positions (1/2 + m, 1/2 + n) are similar positions.

Nitrogen adsorbs very readily (sticking coefficient about 0.2) on

W(IOO) until a stoichiometry W2N is reached, and this produces on annealing

the well-defined LEED C(2X2) patterns mentioned several times at this

meeting. We believe the nitrogen atoms are in the CN-5 positions, in

which case exactly half of these positions are filled. Why does the N2

sticking coefficient fall several orders of magnitude after the W2N

stoichiometry is reached, when half of the adsorption sites are still

vacant?

The bond energy in N2 is 10 e.V. ; it is an extremely strong bond,

and can be broken rapidly only if other very strong bonds are in the

process of forming as the nitrogen-nitrogen bond is in the process of

120
breaking. Suppose for simplicity that these new bonds are forming in

the CN-5 positions. The empty surface furnishes adjacent pairs of such

positions, so that bonds to both nitrogen atoms can be forming as the

nitrogen-nitrogen bond is breaking. The C(2X2) configuration is one

in which all CN-5 positions immediately adjacent to an empty CN-5 position

are filled, so only one nitrogen atom in N2 can be forming a bond in the

transition state which is correspondingly much less favorable. If

nitrogen atoms are provided (by causing N^Cg) ^ 2N(g) by electron bombardment) they are immediately adsorbed until WN stoichiometry (readily

annealed to (IXI) structure) is achieved—in this case new bonds forming

in the transition state do not have to pay a 10 e.V. bond dissociation

price as it has already been paid.

The B process previously outlined for the ammonia decomposition on

tungsten also suggests possible transition states. The C(2X2)

structure has a lower work function than tungsten, so must have a surface

double layer positive out. Electronegativity considerations indicate

that nitrogen is negatively charged with respect to tungsten; if the

nitrogen bond to the underlying tungsten in the CN-5 position is very

strong the nitrogen adatom can be sufficiently "buried" to account for

the positive out double layer. The work function for WN (IXI) is greater

than that of tungsten, so the nitrogen centers must lie above the plane

of centers of the surface tungsten atoms, which could result simply if

the bond of the nitrogen to the underlying tungsten atom were weaker

than in the W2N structure as it surely would be. There are of course

other positions which would achieve the double layer negative out result,

but for model purposes let us assume that the nitrogen atoms are still

in the CN-5 position.

The B process then occurs on top of complete nitrogen adlayer, with

a tungsten atom, positively charged, "visible" in the middle of each

elementary square of nitrogen atoms. This tungsten atom is hence functionally a Lewis acid, and is an attractive place for the Lewis base ammonia

to sit, bonding to the tungsten through its unshared pair of electrons.

Further, the surrounding nitrogen atoms are negatively charged, and so

are receptive sites for proton transfer. These ideas provide models as

to how the proposed species W2N2H2 (which thus really means NH2 coordinated

to w"*" in the middle of the elementary square of nitrogen atoms in a WN

structure) and WNH (which thus means h"*" coordinated to N in the WN

12]
structure) . The charges are doubtless incomplete but sho ild be understood

as representing bond polarity in each case.

122
Part 4. Contribution of John Turkevich,

Department of Chemistry,

Princeton University

Princeton, New Jersey

Palladium catalyst particles (Plate 6067) made from palladium sol

at Princeton Laboratories and examined by ultra high electron microscopy

by Lazlo L. Ban of Petrochemical Research, Cities Service Company. The

magnification on the plate is 2,600,000X. The lattice of the palladium

metal is easily discerned in the polycrystalline particles. Lattice

planes up to the very edge of the particles can be seen on some sides,

though rounded-off amorphous surfaces seem to predominate.

123
Colloidal gold (175A diameter plate 3268) prepared at the Princeton

Laboratories and examined with high resolution electron microscopy by

Lazlo L. Ban of Petrochemical Research, Cities Service Company. The

magnification of the plate is 4,000,000X. One millimeter corresponds to

o o 2.5A. The lattice spacing resolution is about 1.2A. The spacing of the

gold lattice can be seen under the coarser Moire pattern. In the case

o of the particle on the bottom of the plate, a spacing of 2.5A corresponds

to the 111 plane and in the particle, second from the top, a spacing of

o 3.3A corresponds to 100 plane. The particles themselves are not single

crystals but are either multicrystalline (twinned) or have amorphous

areas. Spherical shape predominates though there are areas indicating

flat surface planes. Flat crystal face appears only on one particle

(one face on the particle at the bottom of the plate) . In all other

cases, the surface is either amorphous or with lattices coming out at an

angle. The amorphous nature may be due either to contamination

of the surface or to a

disordered atomic state of

the surface. The appearance of lattice at an

angle to the surface and

no evidence of lattice

parallel to the surface may

indicate partial ordering

of the gold atoms. The

formation of bridges between

particles indicates gold

atom migration at room

temperature after mounting

of the particles on the

carbon support membrane

used in electron microscopy.

124
The separation of the electronic factor from the geometrical factor

has been in the focus of attention in catalysis. One approach to this

problem is to determine the catalytic activity of alloys. We have

synthesized platinum gold alloy particles in aqueous solution by simultaneous

reduction of gold and platinum chlorides with sodium citrate. The

resultant product in which we varied the platinum to gold ratio was

examined optically. The results obtained by the 50-50% alloy are shown

in the figure (1) together with adsorption spectrum of pure platinum

sol, pure gold sol and a mixture of the two sols. It is seen that the

peak characteristic of gold at 540 nra is absent in the alloy. This is

taken as evidence that the platinum has affected the electronic properties

of gold.

Absorption Spectra of Gold, Platinum, M Mixture and 1-( Alloy

( 320 m/x - 620 m/x )

O Pure Gold Sol A M Pt/ Au Mixture of Sols

LiJ a

<

CD

cro

COm

<

<

o

H

a.o

100-

0,50-

Pure Platinum Sol X M Pt/Au Alloy Sol

0.00

320 380 440 500 560

WAVELENGTH (m^)

620

125
Colloidal platinum (Plate 6064) particles mounted on carbon film in

cluster form and examined with high resolution electron microscopy by

Lazlo L. Ban of the Petrochemical Research, Cities Service Company. The

magnification on the plate is 2,500,000X and one millimeter corresponds

o to 4. A. The size of the individual particles of platinum is about 20-

o o 30A. The lattice image of the particle has a spacing of about 3. OA

which may correspond to the 111 plane of platinum. This and the optical

spectra of platinum colloid solutions indicate that particles of the

o platinum as small as 20 to 30A have metallic properties of bulk platinum.

126
Part 5» The Geometry of Solid Surfaces

C. B. Duke

Xerox Webster Research Center

800 Phillips Road

Webster, New York 14580

A discussion of the atomic geometry of solid surfaces may be divided

conveniently into consideration of three topics. How are such structures

determined? What is the present state of our knowledge of these structures?

What does this inforaation teach us about surface chemistry? In this

presentation each of these subjects is examined, in turn.

Two techniques commonly are used for the determination of the

atomic geometry of the surfaces of crystalline solids. Field-ion microscopy

consists of the imaging on a hemispherical screen of ions generated in

the vicinity of a small tip (a few thousand angstroms in radius) following

[1 2] field evaporation to produce a suitable tip surface ' . In this

fashion an image of the surface is produced on the screen, suitable for

photographing. This technique is discussed and such photographs are

displayed in this session by Dr. A. J. Melmed. The second widelyemployed structure-analysis technique is low-energy electron diffraction

("LEED"). In particular, the analysis of the configuration of beams of

electrons elastically reflected from the surface of a crystalline solid

can be analyzed to extract the translational symmetry parallel to a

planar surface, whereas the intensities of these beams must be examined

[3 A] in order to determine the atomic geometry of the surface ' . These

experiments are sensitive to surface structure because the strong interactions

of such low-energy (5 eV $ E $ 500 eV) electrons with the constituents

of solids requires that for their elastic emission from a solid, they

o must emanate from a depth of, at most, about lOA from its surface. The

details of these interactions and their consequences have been described

in the literature'"^ Here, it is appropriate only to observe that

these strong interactions are responsible for the surface sensitivity of

electron scattering and emission spectroscopies (e.g., LEED, photoemission

and Auger-electron-emission) , and that care in interpreting such spectra

must be exercised for exit electron energies E > 200 eV, in which case

they begin to reflect the bulk as well as surface properties of the

sample. 127
A number of reviews of the status of surface-structure determination

via LEED intensity analysis have appeared recently '"'^'^'^ The

general trend which emerges both from these reviews and from more recent

[11-13] results (especially for semiconductors ) is an intimate relationship

between the nature of the chemical bonding of a bulk solid and the

structure of its clean low-index surfaces (e.g., cleavage planes). Most

metals exhibit surface geometries essentially identical to those in the

bulk with the possibility of a small (i.e., 10% or less) contraction of

the uppermost lattice spacing on more open (higher index) surfaces.

Homopolar semiconductors (Si and Ge) are characterized by translationalsjnnmetry-breaking atomic rearrangements even on their (111) cleavage

planes, apparently driven by the tendency of "dangling" covalent bonds

to yield insulating rather than metallic behavior parallel to the surface

("Peierls instabilities") . The cleavage planes of heteropolar semiconductors

[the (110) plane for zincblende and (lOlQ (1120) planes for wurtzite

geometries] exhibit the same translational sjnmnetries as their bulk

counterparts. Subtle bond- length-conserving rotations of the uppermost

atoms may occur, however, giving a rippled appearance to the surface

much like waves on a choppy sea. These result from the competition

between ionic and covalent contributions to the surface energy. Such

competition is not relevant for the polar zincblende (111) and wurtzite

(0001) and (0001) surfaces, which are less stable and tend to exhibit

contractions of the upper layer spacing on the cation but not anion

faces (due to the presence of lone-pair electrons on the anions but not

[13] the cations ) . Little is known about the surface structure of molecular

solids, alkali halides, and transition-metal oxides. The cleavage faces

of transition-metal layer dichalcogenides are thought to exhibit surface

[14] structures identical to the corresponding bulk structure . All of

the low- index surfaces of the transition metal oxides of interest for

catalysis are amenable to structure determination via LEED intensity

analysis. Thus, only the absence of the requisite intensity data precludes

the determination of these important structures.

Analyses of the structures of adsorbed overlayers are in a more

primitive state than those of clean low- index surfaces. Although some

controversy has occurred in the literature, the structure of the most

extensively examined system, Ni(100)-C(2x2)-S, is now agreed upon by all

128
the groups which examined it'"'''^"'. In spite of the small number of

systems studied'"^'' and the uncertainties in the determined structures,

however, one trend seems to be emerging. The adsorption of even reactive

gases (e.g., 0) on the low- index faces of fee metals (e.g., Ni) does not

seem to lead to the formation of surface compounds (e.g. , NiO) except

under high temperatures or pressures. Thus, the adsorbed atoms seem to

occupy the hollows of the metal surfaces without substantial distortion

of the metal substrate to a far greater extent that anticipated by early

workers

.

Whereas it is commonly supposed that such ultra-high-vacuum cleansurface and low- coverage adsorbate structures have little relevance to

the surface chemistry of "practical" catalysts, such is not the case in

[131 the semiconductor electronics industry. Indeed, it is well-known

that surface strains on the cation faces of III-V crystals preclude

crystal growth, inhibit mechanical damage, and reduce solution oxidation

rates. Obviously, such structure-property relationships are highly

useful in the processing of semiconductors. Moreover, recently Rowe et

al. have proposed an intimate relationship between surface structure

and the formation of rectifying metal-semiconductor contacts. Consequently,

while the gloomy prognosis for the impact of ultra-high vacuum surfacestructure work on catalytic chemistry advanced by many speakers at this

[171 symposium may well prove correct , it certainly will not mitigate the

substantial importance of such studies for materials processing in the

electronics and electrophotographic industries. Perhaps a more fruitful

approach even in the area of catalysis (in which kinetics are probably

defect-dominated and hence ill suited for direct study via structure

determination) may be the discernment and exploitation of structureproperty relationships analogous to those which have proven so valuable

in semiconductor materials science.

References

[1] MUller, E. W. , in Proceedings of the International School of Physics

'Enrico Fermi' , Course LVIII. (Enditrice-Compositori , Bologna, 1975),

p. 23.

[2] MUller, E. W. , and Tsong, T. T., Field Ion Microscopy (American Elsevier,

New York, 1969) . 129
[3] Duke, C. B., in Proceedings of the International School of Physics , 'Enrico Fermi' , Course LVIII . (Endltrice Composltorl, Bologna, 1975),

pp. 99, 174.

[4] Duke, C. B., Adv. Chem. Phys. 27, 1 (1974).

[5] Duke, C. B., Crlt. Rev. Solid State Sci. 4^, 371 (1974).

[6] Duke, C. B. , in Proceedings of the International School of Physics

'Enrico Fermi' , Course LVIII . (Endltrice Composltorl, Bologna,

1975), p. 52.

[7] Duke, C. B., Japan. J. Appl. Phys. Suppl. 2^ 641 (1974).

[8] Duke, C. B. , Llpari, N. 0., and Laramore, G. E., Nuovo Clmento

23B , 241 (1974).

[9] Lagally, M. G. , Bucholz, J. C, and Wang, G. C. , J. Vac. Sci. Technol.

12, 213 (1975).

[10] Strozler, J. A., Jr., Jepsen, D. W. , and Jona, F. , in Surface Physics

of Materials , J. M. Blakely, Ed., (Academic, New York, 1975), Vol. I,

p. 2.

[11] Duke, C. B., and Lublnsky, A. R. , Surface Sci. 50, 605 (1975).

[12] Lublnsky, A. R. , Duke, C. B., Chang, S. C, Lee, B. W. , and Mark, P.,

J. Va. Sci. Technol., Jan/Feb. (1976).

[13] Gatos, H. C, J. Electrochem. 122, 287C (1975).

[14] long, S. Y. , and Van Hove, M. , private communication.

[15] Duke, C. B., Llpari, N. 0., and Laramore, G. E., J. Vac. Sci.

Technol. 12, 222 (1975).

[16] Rowe, J. E. , Chrlstman, S. B. , and Margaritondo , G. , Phys. Rev.

Lett. 35, 1471 (1975).

[17] Duke, C. B., Crlt. Rev. Solid State Sci. 4, 541 (1974).

130
Part 6. Contribution of A. J. Melmed

National Bureau of Standards,

Washington, DC 20234

One experimental technique which can provide structural information,

expecially surface structural information about particles in the size

range of some practical catalysts is field- ion microscopy (FIM) . To

my knowledge FIM has not yet been applied to actual catalyst particles,

but in principle it is possible.

I will address just one aspect of FIM because I believe that it is

relevant to some natural confusion that has crept into the minds of

those who think about catalysts in terms of atomic surface geometry.

Most field-ion micrographs published are Images of surfaces which have

been prepared, in the final stage, by low- temperature fieldevaporation''"''"'. This process results in surfaces which look similar to

what one would expect on the basis of constructions using hard spheres

to represent atoms. However, this process is very artificial in terms

of processes occurring outside the field- ion microscope. Thus, the

near-ideal structures produced by field-evaporation do not occur as the

result of ordinary annealing, for example.

The surface atomic structures of thermally annealed platinum,

iridium, and tungsten imaged by FIM clearly showed a large degree of

thermal disorder. An example of disorder introduced mechanically in an

iridium specimen was also shown. The intended message: Be aware of

atomic surface disorder, as well as atomic order, which may well be

present on the surfaces of real catalysts.

[1] E. W. Miiller and T. T. Tsong, Field Ion Microscopy; Principles and

Applications (Elsevier, New York, 1969).

131
Part 7. General Discussion for the Session

A. B. Anderson

Chemistry Department

Yale University,

New Haven, CT

A. B. Anderson: I have a question directed to Professor Ertl. You have

shown calculated electronic densities of states for various crystal

faces of fee and bcc metals. It seemed that in one case some kind of

correlation with surface properties may have existed and in the other

case there was apparently no correlation. Can a case be made for the

pertinence of detailed electronic density of states determinations to

problems in chemisorption and catalysis? I have seen these density of

states plots presented in various places, but sometimes without being a

part of an argument. Now I am not against exploring the possible implications of densities of states for surface events, but until we know what

bond shapes really mean to a particular reaction on a surf ace , their

presentation, merely because they are something calculatable , seems glib.

Frequently, adsorbate levels such as sigma and pi energy levels in

hydrocarbons lie several electron volts beneath the bottom of transition

metal s-d bonds. When these orbitals interact with bond orbitals they

may not be sensitive, as far as some properties of the interaction are

concerned, to details of the s-d bonds. We find adsorbed molecules have

an ability to induce a strong interaction with a metal surface because

of the high density of atomic d-orbitals in the surface which find ways,

by appropriate linear combinations, to form strong bonds for various

[2 3] adsorbate locations above various surfaces '

So my question is: Can a case be made for the pertinence of detailed

density of states calculations to our present understanding of chemisorption

and catalysis or has one been made?

Professor Ertl: I am in agreement with what you say and believe the

results in my talk did not show any such pertinence. However, sometimes

adsorbate levels may be in the s-d bond.

132
A. B. Anderson: Yes, as in the case of it* orbitals in some instances.

I have a comment and a question directed to Professor Turkevich. You

have shown how microcrystals of alumina supported platinum appear to

have bulk structures inside but apparently a random structure at and

near the surface. I have calculated binding energies and structures for

two to six atom clusters of tin, titanium, chromium, iron, and nickel

[3] atoms and the structures have no resemblance to the bulk structures.

This seems to be in agreement with your observations. Now I cannot be

entirely certain of the transition metal calculations, as there are no

experimental binding energies for comparison but in the case of tin,

[4] Gingerich and coworkers at Texas A and M University have found by

experimental methods, the binding energy per atom for these clusters and

my calculations agree within around a kilocalorie per mole or two. The

preferred structures were found by varying the coordinates of all the

atoms to find the lowest energy. The structures were quite independent

of the bulk diamond-like structure; for example, the five atom cluster

took the form of a trigonal bipyramid. Because of the accuracy of the

energies, it seems likely that the calculated structures are correct.

And so my question has to do with the temperature of your platinum

microparticles , the possibility of metastability and our understanding

of the nucleation process.

Professor Turkevich: The particles are in structural equilibrium.

A. B. Anderson: Perhaps some interesting things are going on here.

Perhaps the nucleation process involves condensation of tiny crystallites

with various structures, or perhaps one atom adds at a time. Frequently,

according to the calculations, for a particular number of atoms in a

microcrystal, a unique structure is preferred and will be preferred up

to quite high temperatures. But as condensation occurs there is a

transition to the bulk structure in the inner part of the particle. At

least, this is what your work suggests to me.

133
References

Some efforts are being made for generalized interactions between

model surface densities of states and model adsorbates. See J. W.

Gadzuk, Surface Science 43, 44 (1974), M. J. Kelly, J. Phys. C:

Solid State Phys. _7, L157 (1974), and references in these papers

to work by T. L. Einstein, J. R. Schrieffer, T. B. Grimley, D. M.

Newns and others.

Anderson, A. B., and Hoffmann, R. , J. Chem. Phys. 61, 4545 (1974).

See works in footnotes to the talks by A. B. Anderson at this

conference.

Gingerich, K. A., Desideri, A., and Cocke, D. L. , J. Chem.

Phys. _62, 731 (1975).

134
Comments by Arthur Wm. Aldag , Jr.

School of Chemical Engineering and Materials Science

University of Oklahoma

Norman, Oklahoma

We have been engaged in an experimental study!^^'' of the decomposition

f 2

1

of formic acid on a single crystal nickel (100) McCarthy, et al. have

studied the nickel (110) surface. Both studies employ the flash desorption)

technique where formic acid is absorbed at room temperature and the

evolution of the products followed mass spectrometrically as the crystal

is heated. On both the (100) and (110) surfaces and CO^ are liberated

first with the two peaks superimposed on each other. At a higher temperature, CO is liberated leaving a surface oxide that can be detected by

Auger Spectroscopy. A study of the interaction of the reaction products

H^, CO and CO^ , alone on each surface indicates differences in the

binding energies and sticking coefficients not uncommon to many other

studies of simple absorbates on well defined metal surfaces. However

there is a pronounced difference in the formic acid decomposition kinetics

on Ni (100) and Ni (110) . Madix finds that the H (and CO2) appears in a

remarkably sharp flash peak with a half-width of about 6° K. The only

plausible explanation is that the decomposition on Ni (110) is autocatalytic leading to a "kinetic explosion." By contrast, we find that

on Ni (100), and CO2 peaks are of "normal" half-width and appear

to obey 2nd order decomposition kinetics. This would appear at face

value, to be a rather s':riking example of the structure factor referred

to earlier. Possibly the less dense (110) surface offers a more favorable

environment for propagation of the branching chain. .We also find that

there is a small (2%) amouni: of residual oxygen on the "clean" Ni (100)

surface which cannot be removed. An alternate explanation might be that

this oxygen serves to terminate the branching step on Ni (100) . This

alternative would then fall more under the heading of the "ligand factor"

referred to by Professor Boudart.

References

[1] Lee, C-0, Peavey, J. H. and Aldag, A. W. , Submitted to J. of Catal.

[2] McCarthy, J., Falconer, J., Madix, R. J., J. of Catal., 30, 235 (1973).

135
Bonding Properties of Stepped Transition Metal Surfaces

G. S. Painter

Metals and Ceramics Division

2 Oak Ridge National Laboratory

Oak Ridge, Tennessee 37830

R. 0. Jones

Institut fvir Festkbrperforschung der Kernf orschungsanlage

Jiilich, 517 Jtilich, Germany

P. J. Jennings

School of Mathematical and Physical Sciences

Murdoch University, Murdoch, WA 6153 Australia

A great deal of interest has been shown recently in the use

of theoretical clusters models to determine: (1) how electrons

are distributed spatially and energetically in small crystallites

r 2 — 5

1

and at surfaces , (2) how electrons respond to surface disorder

[7 81 such as steps and kinks ' , and (3) how electronic characteristics

are related to chemically active sites on surfaces'"^ Much of

this interest has been stimulated by experimental observations of

large differences in surface phenomena associated with prepared smooth

(low index) surfaces as compared with corresponding behavior for typical

disordered surfaces. The origin of structure insensitivity in certain

reactions has been phenomenologically developed and discussed recently by

[12] Boudart . Surface cluster models offer new insight into these problems

Guest scientist with the Institut fiir Festkorperforschung der

Kernforschungsanlage Jiilich on exchange from ORNL (1974-75)

.

ORNL is operated for the Energy Research and Development Administration

by the Union Carbide Corporation.

136
of surface interactions since they provide detailed information about

the electronic energy distribution and local bonding characteristics at

the surface. The basic formulation of the various cluster theories

involves interactions among a finite number of atoms, thus these models

form a useful complement to those approaches based on the extended

[13-15] nature of the substrate

As part of a series of studies of surface bonding characteristics,

we have carried out calculations of the electronic structure of clusters

chosen to simulate simple stepped transition metal surfaces in the

absence of an adsorbate. Some interesting features of the orbital

[7 81 density near the step region were recently reported ' , and a possible

relation to observed enhanced activities of small particle and stepped

surface catalysts was suggested. In connection with the conference

topic "geometrical effects" we wish to briefly summarize some of these

results and discuss the origin of the various orbital features which

occur at simple surface irregularities. The origin and general occurrence

of these features near edges and corners suggest that similar behavior

should be expected for more extended and complicated types of disorder

as would be found for actual surfaces.

For simplicity we chose a thirteen atom cluster with atoms positioned

to simulate a simple step on the (100) face of a transition metal surface.

Specifically, we chose nine atoms in a square arrangement 2A on a side

where A = the near-neighbor separation in the plane, with four atoms in

interstitial sites a distance h above the plane where h = A/2 for bcc

and \\ = kl /l for fee symmetries. The cluster symmetry is thus C^^. A

one-electron model hamiltonian was used with Slater's statistical exchange

approximation, and the potential function average to "muff in- tin" form

which is spherically symmetric inside touching spheres centered on each

atom and outside a sphere surrounding the cluster of atoms, and constant

in the volume between the spheres. Past experience with this model

suggests that the muffin-tin approximation is in general too restrictive

for treating the energetics of chemisorption, however for qualitative

and comparative studies the simplicity of the model offers attractive

advantages over alternative methods. In the present scattered-wave

r

1 model the simple form of the secular matrix allows us to include

more atoms in the cluster than would otherwise be possible, and this

137
aspect of the calculation was deemed more important for our consideration

than say the inclusion of the non-muffin- tin corrections to the potential.

The calculation was carried out with programs constructed for solving

the multiple-scattering equations for the bound electron states in a

[81 system of muffin-tin scatterers and is described in more detail elsewhere . For the present discussion, the results to be presented are not very

sensitive to the particular details of the technique or parameters of

the model. The features emphasized here should emerge as characteristics

of the system in either a scattered wave or an LCAO approach.

The eigenvalue spectra and histogram densities-of-states for the

clusters of the first transition metal series show some interesting

similarities with corresponding features of the bulk solid. In Fig. 1

we compare our histogram density-of-states for a cluster of thirteen

iron atoms (top panel) with the band structure results of Wood'"''"^'' for

bcc iron (bottom panel) . The cluster distribution has been shifted to

align the Fermi energies (dashed lines) of the two curves, and note that

the energy scales are not the same. The reduced number of interactions

among neighboring atoms in a small cluster naturally leads to a set of

d-levels whose breadth is not as great as that obtained in the bulk

limit. In addition the different boundary conditions for the cluster as

compared to the extended crystal lead to different band widths. In Fig.

1 the "band" of cluster states constructed by summing energy-normalized

Gaussians at each eigenvalue, is somewhat greater than half the width of

the bulk d-bands. Corresponding to this reduction is an overall increase

in the density-of-states of the cluster over the energy range including

the d-levels, since the total number of states per atom is the same in

each case. Thus the density-of-states at the Fermi energy (highest

occupied level) is 5.90 electrons per atom per eV for the cluster compared

with 3.49 for the bulk case. The distribution of discrete levels contributing

to the density-of-states of the cluster is given in Fig. 1 with the

various C^^ symmetry representations noted at the left.

In considering the relationship between clean substrate characteristics

and general surface activity, the calculated narrowing and enchancement

of the density-of-states in small clusters may be significant as far as

a density-of-states factor is concerned. Surely one of the most noticeable

138
aspects of the state density for small metal clusters is the similarity

observed in the shape of the density-of-states to that for the extended

solid, particularly at the top of the band'-'^ ' . The main structural

features that appear in the bulk limit, particularly the position of the

Fermi level in the main peak, have emerged even with this limited number

of atoms. Apparently the principal influence of including more neighbors

in this cluster model is a broadening of the band and an attenuation of

the peaks with no great alteration of the main structure. To the extent

that the results for this finite cluster represent the local density of

states at the surface of the extended solid, the similarities with the

bulk may explain instances in which correlations of surface activity

r 181 with bulk density-of-states have been observed

One primary motivation for using the cluster surface molecule

approach to study surface interactions is that it offers the advantages

inherent in molecular orbital methods for extracting information about

the nature of the bonding In the system. A knowledge of surface bonding

is important not only for a quantitative description of the reordering

of d-levels at clean transition metal surfaces but also for a qualitative

understanding of precursor and intermediate states in chemisorption and

molecular dissociation. Some features which are rather characteristic

of the orbitals calculated for the surface clusters of our studies will

be discussed in the following.

It is of course incomplete to discuss surface bonding properties

without specifying the adsorbate since the respective energy spectra of

the surface cluster and adsorbate determine the interaction. However,

the complexity of a given composite system, and the existence of the

vast number of possible reaction pathways have led to searches for

establishing less precise but more general concepts of surface activity

through correlations between properties of the isolated adsorbate and

clean metal substrate. Discussions in terms of bonding have often

involved models utilizing the free atomic d-orbital properties or metal

[19] bond character of the bulk . Most of our discussion will concern

calculated cluster eigenfunctions in the absence of an absorbate, and

emphasis will be placed on the role that bonding among the surface atoms

plays in determing the bonding properties of the surface with an adsorbate.

139
In particular, our interest is concerned with the influence of surface

geometrical disorder on the bonding properties, for example, in the

vicinity of a step.

In Fig. 2 we show contour plots of the orbital density, |ij;(r)| , for an eigenstate which lies near the Fermi level in vanadium, and is

representative of one type of solution of the B2 symmetry species.

The plane of the plot passes through the four atoms comprising the step

in (a) and in (b) the plane is parallel to that in (a) but h/2 above it,

where "h" is the atomic step height. The density magnitudes corresponding

to adjacent contours differ by a factor of two. The sign shown in each

quadrant is that associated with the wavefunction , ^(r) , in the x > 0,

y > 0 quadrant. Within the latter quadrant, the sign changes of

c(r) are unspecified. The density in the plane through the step atoms

clearly shows antibonding between partial waves with large components of

d^^ symmetry on each step site. A function x = + (p^ + + (j)^) , where is a d^^ orbital on the i^^ step atom belongs to representation

and forms an anti-bonding component of the wavefunction. The nearneighbor step atoms form ddTf symmetry bonds through the destructive

superposition of the partial waves in the overlap region between adjacent

sites. Corresponding to this reduction in density in the region between

the sites, the density is shifted to the exterior lobes, i.e., this

state is associated with the formation of charge lobes at the step

corners. In the plane above the step layer (Fig. 2b) similar behavior

occurs originating mainly in this region from a B2 basis function formed

from step side d^_^ and d^^ orbitals. Specifically, the major B2 component

in this region can be written as a combination of d and d orbitals

xz yz

drawn from each step site such that each orbital is anti-bonding with

each near-neighbor orbital through either ddTr or dd6 interactions. This

results in a reduced density along the z-axis (although there is an

important density contribution in the hole centered location) with a

compensating increase in density in the lobes extending away from the

corners. Thus the origin of the extended lobe characteristics of the

electron distribution for this state is simply the anti-bonding interactions

among t„ orbitals (d , d , d ) on neighboring step sites subject to

2g xy xz yz

*' the orthonormality constraint on the state which shifts charge out of

the overlap region. Although the features of this orbital density over

the step are rather simple to analyze, this is not generally the case,

140
as illustrated in Fig. 3 for a plane midway between the step layer and

the layer of nine atoms for a state in iron. Also, the density in a

given plane for a given symmetry type can change significantly with

energy—although an atomic orbital analysis assists in understanding the

interactions which determine a given state, it can be misleading to

describe the nature of the state from postulated interactions among an

assumed set of atomic orbitals.

The symmetry characteristics of the various cluster orbitals provide

information about the types of bonding allowed for adsorbates at different

sites of the cluster, based on rules for the conservation of orbital

symmetry in a reaction. A knowledge of the distribution of substrate

orbital density makes it possible to qualitatively describe the relative

bond strengths for an absorbate at various sites, to the extent that

simple orbital overlap plays a role in the bonding. The orbital distributions

of symmetry type shown in Fig. 2 illustrate these points in a simple

way. Consider an identical atom (vanadium) approaching the cluster

along the z-axis. A d^ orbital on the z-axis belongs to the

representation and can form a dd6 bonding configuration with the step

orbital distribution, which is made up of d , d and d single site

xy xz yz ° contributions and has d^^ symmetry about the z-axis itself. This mechanism

for the formation of a bond at a (001) hole-centered site in fact is

operative in the bonding of the V atom at the origin of the cluster with

the step atoms above. Furthermore, a position on the z-axis over the

step atoms would be the stable bonding site for a V atom for growth of

the cluster to form the crystalline solid.

The cluster wavefunctions for the totally symmetric A^ representation

are significant for a-bonding in the hole-centered (001) site (Fig. 4).

The partial wave contributions from the step sites are of d^2 symmetry

and are summed in the A^ representation to produce a totally symmetric

contribution to the orbital density which is concentrated over the step

atoms. In contrast to the B- representation, the d , d step orbitals

2 xz yz ^ are involved in the formation of bonding ddir, dd6 combinations between

near-neighbors in the step plane, thus enhancing the density about the

z-axis and producing a state conducive for a-bonding at a (001) holecentered site on the step (Fig. 4b).

141
The importance of the directionality of the d , d step orbital ^ xz yz

contributions in a-bonding of an adsorbate can be illustrated in the

case of CO chemisorbed at the (001) hole site of a Ni^^ cluster. In

Fig. 5, we show contours in the x = y plane for a a-bonding eigenstate

in the Ni^^-CO system. The plane of the figure passes through nickel

atoms at the center (left) and corner (right) of the layer of nine;

through one of the step atoms above, and the CO is along the z-axis with

the C atom located nearer the Ni cluster. The d and d partial wave

xz yz

components of the "substrate" (particularly the step sites) admix with

other orbitals to give a hybrid directed toward the CO, forming a good

bond with the CO molecular orbital fomed from s-p combinations. In

^z

Fig. 6, we show density contours for this state in a plane passing

parallel to the step layer and somewhat more than h/2 above it (the

plane is actually £/2 below the C atom where S, = 2.13234 a.u., the CO

bond length). Clearly, a significant amount of charge is associated

with this a-bond, and involves the metal electron distribution which

initially was concentrated over the step layer similarly to that shown

in Fig. 4b.

In conclusion, we note that the great complexities of most systems

of practical concern in catalysis make it unfeasible to apply cluster

models directly to the problems of interest. However, the fundamental

information obtained in simpler systems and processes, e.g., chemisorption,

should play a useful role in the interpretation of processes involved in

complex catalytic reactions. A large amount of spectral data from

various surface probe experiments performed on adsorbate covered surfaces

now exists. Theoretical attention to this problem has grown

. [5,9,10,21-25] , ^, ^ . ^. . recently ^ 1

, and it appears that a productive interplay

between theory and experiment has emerged in relating calculated and

measured spectral features. In order to study the energetics of intermediate

products formation and molecular dissociation at surfaces, various

refinements of the present models are required, and efforts to implement

these are currently in progress. Within the present model, the cluster

solutions may be useful as a starting point for simplified calculations

of the energetics of adsorption and dissociation, as for example, in the

r 25

1

perturbation approach employed by Deuss and van der Avoird . In the

near future however, we anticipate that the greatest use of the cluster

142
models will involve interaction with experiment to study bonding configurations

and stable adsorption sites on surfaces through correlations of surface

spectral features.

G.S.P. and P.J.J, would like to express their appreciation to

Professor G. Eilenberger and the members of Theorie I for their interest

in this work and the hospitality extended to us during the period of our

visit with the Institute when most of this work was carried out.

Referent es

(11 See, for example, review by T. B. Crlaley in Dynamical Aspects of Surfioe

Physics . Varenna Lectures (1974).

12] Baetzold, R. C, and Mack, R. E., J. Chem. Phya. 62, 1513 (1975) and

references therein.

[3] Johnson, K. H., and Messmer, R. P., J. Vac. Scl. Technol. 11, 236

(1974) .

[4] Messmer, R. P., Tucker, C. U., Jr., and Johnson, K. B. , Chem. Phya.

Letters 36, 423 (1975)

.

[5] Fassaert, D. J. M. , Verbeek, R. , and van der Avolrd, A., Surface

Scl. 29, 501 (1972).

16] Gadzuk, J. W. , in Surface Physics of Crystalline Materials , edited by

Blakely,'J. M., (Academic Press, N.Y., 1974).

[7] Painter, G. S., Jennings, P. J., and Jones, R. 0., J. Phys. C: Solid State Phya. 8, L199 (1975).

[8] Jones, R. 0., Jennings, P. J., and Painter, G. S., Surface Sci. (In

press)

.

[9] Anderson, A. B., and Hoffman, R., J. Chem. Phys. 61, 4545 (1974).

[10] Baerends, E. J., Ellis, D. E., and Ros, P., Theoret. Chlm. Acta

(Berl.) 27, 339 (1972).

[11] Somorjal, G. A., and Blakely, D. H. , Nature 258, 580 (1975) and

references therein.

(12J Boudart, M. , J. Vac. Scl. Technol. 12, 329 (1975).

{131 Smith, J. R. , In Interactions on Metal Surfaces , edited by

R. Gomer (Springer-Verlag, Berlin, 1975) and references therein.

[14] Caruthers, E. B. , and Rleinman,, L., Phys. Rev. Lett. 35, 738

(1975) and references therein.

[IS] Kesmodel, L. L., and Falicov, L. M. , Solid State Coant. 16, 1201 (1975).

[16] See, for example, K. B. Johnson, Adv. Quant. Chem. 2> 1^3 (1973).

[17] Wood, J. H. , Phys. Rev. 126, 517 (1962).

[18] Douden, D. A., in 1975 Proc. Trieste Winter College on Surface

Science, IAEA, Wicn (to be published).

[19] Sinfelt, J. H., CRC Critical Review of Solid State Sciences

1, 311 (1974).

[20] Woodward, R. B., and Hoffman, R. , The Conservation- of Orbital

Symmetry (academic Press, New York, 1970).

[21] Niemczyk, S. J., J. Vac. Sci. Technol. 12, 246 (1975).

[22] Batra, I. P., and 0. Robaux, J. Vac. Sci. 49, 653 (1975).

[23] Anders, L. W. , Hansen, R. S. , and Bartell, L. S., J. Chem.

Phys. 62, 1641 (1975).

[24] Lang, N. D., and Williams, A. R., Phys. Rev. Lett. 34, 531 (1975).

[25] Harris, J., and Painter, G. S., Phys. Rev. Lett. 36, 151 (1976).

[26] Deuss, H. , and van der Avolrd, A., Phys. Rev. B8, 2441 (1973).

143
ORNL-DWG 76-2189

-0.2 -0.1 0

£-£^ (Ry)

0.4 0.5 0.6 0.7 0.8

£(Ry)

Figure 1. Comparison of the densities-of-states for iron calculated

£r&m (top panel) the discrete levels of a thirteen atom cluster

and (bottom panel) band structure of bcc crystalline iron by

J. W. Wood. Distributions have been shifted to align the Fermi

levels; note that energy scales differ by a factor of nearly 2.

144
+

+

-

Figure 2. Charge density contours from a symmetry orbital of a vanadium

thirteen atom cluster in planes (a) through the four step atoms

(z = h) (b) above the step (z = 3h/2) ; h = step height. Adjacent

contours differ by a factor of two; signs in each quadrant denote

symmetry of orbital.

145
Figure 3. Charge density contours for an A^^ symmetry orbital of an iron

thirteen atom cluster in a plane (z = h/2) midway between the

nine atoms of the "flat" surface (z = 0) and the four step atoms

(z = h).

146
"

I

I I I I I I I J I L I I I L I I I 1 L I L ] 1 I I i I I I 1 I I L I L ] 1 ] I I t 1 L I I I I I 1 I i I I I L

I

I

I I I I I I 1 I | -

Figure 4. Charge density contours for an A^^ symmetry orbital of the vanadium

cluster in planes (a) z = h and (b) z = 3h/2 as described in Fig. 2.

147
Figure 5. Orbital charge density contours for an S3rTMnetry state

in a Ni^2"C0 cluster. The plane of the figure (x = y) passes

through the central and a comer Ni atom of the "flat" surface,

a Ni step atom above and the CO molecule in the (001) holecentered site with the carbon atom down.

148
Figure 6. Charge density of the A^^ orbital of Fig. 5 in a plane parallel

to that containing the four step atoms and ill below the C atom

where £ is the CO bond length.

149
Part 8. Recorder's Summarizing Comment

The discussion encompassed an assortment of geometrical considerations.

Experimental methods for determining atomic surface geometry were briefly

reviewed, and bond geometry at surface defects was theorized. Geometric

specificity in chemisorption was discussed at length and questions about

geometric specificity in some catalytic reactions were pondered. The

overall implication clearly seemed to be an awareness of a great lack of

knowledge about the geometrical details of real catalysts. To the

extent that this gap was realized by the participants , this session

succeeded

.

150
SESSION 4

ELECTRONIC

STRUCTURE

Moderator:

R.E.WATSON

Btookhaven National Laboratory

Lecture by:

J.W.GADZUK

NBS

151
Session 4.

Electronic Structure

Panel Members:

A. B. Anderson,

Yale Univ.

J. W. Gadzuk,

NBS

D. R. Hamann,

Bell Tel.

R. P. Messmer,

Gen. Elec.

J. R. Smith.

Gen. Motors

Recorder:

J. W. Gadzuk,

NBS

152
PANEL DISCUSSION: ELECTRONIC STRUCTURE

Chairman: Dr. R. E. Watson, Brookhaven National Laboratory

Recorder: Dr. J. W. Gadzuk, National Bureau of Standards

Panel Members:

Dr. A. B. Anderson, Yale University

Dr. J. W. Gadzuk, National Bureau of Standards

Dr. D. R. Hamann, Bell Telephone Laboratories

Dr. R. D. Messmer, General Electric

Dr. J. R. Smith, General Motors

I. Opening Remarks by Chairman

The session was called to order by Chairman Watson with the

announcement that "Now theorists from both the fields of chemistry and

physics will have some time to claim their virtues and hide their

shortcomings." The chairman explained that Prof. Grimley was not able

to attend and so in his place, J. W. Gadzuk would speak on a topic about

which Prof. Grimley might have spoken, had he been present. However, it

was noted that Prof. Grimley might have emphasised more strongly the

tenuous (at best) link between current state of the art electronic

surface calculations and catalysis. This notion continued to raise its

head throughout this session.

Watson suggested the following points to be dealt with, in either

this session or future calculational programs. What questions can or

cannot be addressed by a particular model approach? With the arrival of

modern spectroscopic probes, calculations of single electron energy

levels are proliferating as it is believed by many that these numbers

can be compared with say observed photoemission spectra. The chairman

rhetorically asked whether such energy level calculations have seen a

variational principle. Do the potentials generated from the wavefunctions

make sense? He stressed that good charge densities do not follow automatically from a set of well-liked one electron levels (where "wellliked" is probably defined as those agreeing with experiment)

.

153
With this charge, the podium was turned over to Dr. Gadzuk who spoke on

the pragmatic utility of model Hamiltonian approaches for describing

chemical events at surfaces, as outlined in the following report.

II. The Role of Model Hamiltonians in

Chemisorption and Catalysis

J. W. Gadzuk

National Bureau of Standards

Washington, DC 20234

Great simplification in the description of the electronic structure

of a coupled atom-metal system results if the chemisorbed state can be

characterized in terms of some properties of the individual, uncoupled

constituents and a few parameters which in principle are calculable, but

in practice are usually adjusted to agree with some experimental spectroscopic data. The picture of chemisorption we envision is shown in fig.

1. The uncoupled atom and metal are shown in fig. la together with

characteristic wavefunctions for each entity. As the atom is brought to

the surface, bonding orbitals are formed between the atom and metal,

which shift and broaden the originally discrete atomic level as in figs.

lb and Ic. System wavefunctions now extend throughout the atom and metal.

Those states whose energies are within the resonance have disproportionately large charge densities in the vicinity of the atom.

The basic advantages of a model description of such a state of

affairs are physical transparency and minimal computation. Thus the

model approach is ideally suited as a testing ground for new ideas.

Benchmark theoretical or spectroscopic data can easily be incorporated

into the model. Such chemically desirable things as potential energy

surfaces should be relatively easy to calculate.

The principle disadvantage of the approach is that it is not a

"first principles theory." Thus, one has parameters available which are

often treated in an arbitrary manner. (This is also true in so-called

ab initio theories, although the parameters and "adjustable assumptions"

154
are hidden in more subtle ways.) In order to write down a model Hamiltonian,

one must first independently decide what is important in the chemisorption

bond. The model Hamiltonian only provides a mathematical vehicle for

displaying the physics which was already decided upon. In otherwords,

you get nothing out which was not put in. The reader is referred to

either recent review articles or research papers'^l tor further

details.

The most widely used model is the so-called Anderson Hamiltonian:

H = E e, n, „ k,a 'V ^' a

+ (V 1 c t c, + H.C.) + U n n . (1) ak aa ka aa a-a k,a 'V.

The various quantities in this operator, written in the occupation

number representation, are identified as follows, is the metal "band

structure" energy and n^^ is the number of electrons occupying state k.

The adatom ionization energy (suitably modified to include surface

shifts) is e . The coupling term V , = <a|H|^> transfers electrons from

the adatom to metal and vice versa and is just a quantum chemical resonance

integral. The last term U is a measure of the coulomb repulsion between

electrons of opposite spin which are simultaneously on the adatom.

The field theoretic treatments provide succinct expressions for the

properties of the coupled system which quantum chemists would describe,

in the U=0 limit, by wavefunctions of the form

^SYS = a(q)'^a(^) ^ ^ b(q,^)if;^(;^) (2)

where a and b are coefficients and q is the set of quantum numbers of

the coupled system. The main difference between the field theoretic

versus chemical approach is that )^ is well represented as a continuous

variable in the solid state, whereas it is taken as a discrete set of

quantum numbers in a molecule. From either point of view, a little bit

of a lot of states \b are on the adatom.

sys

The model Hamiltonian worker has well defined procedures for constructing various Green's functions from Eq. 1. The Green's function

associated with the electron charge on the adatom is labeled G (e) and

Sidi 155
— Im G (e) is the adatom local density of states, in its simplest form

ir aa y » r a Lorentzian shown in figs, lb and Ic. Equivalently , the eigenvalues

of Eq. 1 are given by a secular determinant in which the only off diagonal

terms,

^^j^' appear in one row and one column. These eigenvalues satisfy

e- e -2^ =0 (3)

a e-e,

' k k

Stated otherwise, they are the poles of the Green's function

-1

IV |2

= e- e -E — = U- - A(e) - iA (e))

aa a , a a

L k J. J

with the local density of states

A^(e)

p^^ (e) = ^ Im G^^ (e) = ^ . (4)

(e-e^ - A(e))^ + A^^(e)

a a

If the level shift A(e) and width A^(e) functions are independent of )^ and thus e, then p would be a Lorentzian. For most interesting cases

of chemisorption, both the magnitude as well as the ]^ (and thus

e(^)) dependence are all important in determining the electronic

structure.

In the language of quantum chemistry, the local density of states

on the adatom is the square of the energy resolved projection of the

system wavefunction onto the atomic state. That is

p^ (e) = Zl<'i' J'l'_ (q)>12 6ie-e) = I | a(q) 1 ^6 (e-e^) (5)

aa ' a ' sys q q

q q

which should be equivalent to Eq. 4. In both cases the orthogonality

condition<a I ]1^> = 0 has been assumed . We must now make some connection between localized bonding involving

both discrete and continuum states. Suppose, as shown in fig. 2a, that a

hydrogenic atom tries to bridge-bond to the d group orbitals of the

substrate. The net overlap of ^ with

a

I- , = - [d (1) + d (2)1 (6a) nb ^2 L

xy

J

156
vanishes whereas the overlap and thus bonding is nonzero for the rephased

group orbital

^ v/2 L

d (D-d (2)

xy xy

(6b)

The resulting 3 atom molecular orbital spectrum, obtained from the

coupled group orbitals and adatom, are shown in fig. 2b. ^ and ^, 3 D

form bonding and antibonding orbitals whereas ^ , remains unperturbed.

nb

In the case of an infinite solid, the discrete energy levels merge into

a band of energies shown on the right side of fig. 2c. The Bloch eigenfunctions, written in a tightbinding representation are

^ (r) =^i: e (j) (r-R ) (7) ^ /S" n

where N is the number of atoms in the solid and R = na (with n an

n

integer and a the primitive translation vector of the lattice) is the location of atom n. For k=0, ^y^_Q (j^) = — 2 (j) i]^ -

]^^) is just an LCAO

with the phase of all orbitals identicaf j'^as with the no-bond group

orbital. For k = , at the zone boundary, e'^'^^n = e''"^'^ and _ JL ~

1. XTTXl 3 — E e ^^li ~ l^j^) which is an LCAO with phases of alternate orbitals

reversed, as in the bonding group orbital.

Intuitively one should expect then that the adatom, while interacting

with the semi-infinite metal, sees the k^ir/a states near the bottom of

the band as bonding group orbitals and the kwO states near the top of

the band as non-bonding group orbitals. The resulting local density of

states might look like that shown in fig. 2c. Here a state is split

off below the band and closely resembles a localised bonding orbital.

A distribution of virtual states throughout the d-band appears with a

resonance near which could be called an anti-bonding virtual state.

Due to a mild repulsion between e and states k > 0, a localised nonbonding state is likely to be pushed out above the band. The degree to

which discrete level cluster elgnvalues (fig. 2b) resemble continuous

local densities of states (fig. 2c) determines the usefulness of a cluster

approach to this solid state problem.

If the adatom orbital couples mainly to a single group orbital of

the substrate, the hopping or resonance integral might be approximated

157
by

V , = <a|H|k> ^ <a|g> <g|H|k> .

This makes life and calculations much more manageable since V , is now

separable into a product of an atomic overlap integral S(^^) = <a|g>

depending on the position of the adatom, but not on times a substrate

hopping integral F(k,g)^ <8|H|k> = E a e^^'^i «{) (R.) |h| (1)(R )>.

Here the sum on j is over the centers in the group orbital with coefficients

a,. Claiming that F is a function only of )^ and g, but not j^^ is an

approximation since H is a function of j^^ and thus <j|H|i> does vary

with R . However we can hope that this is a small factor since it

a

measures the change in hopping between substrate orbitals due to the

perturbation outside the solid. (This is equivalent to neglecting

a Vj^^, term in the Anderson model.) With this factorization, the eigenvalues

given by Eq. 3 can be written

The beauty of Eq. 8 is that the sum on ]^ quantity, call it Q(e) , can be

calculated once and for all for a given substrate, group orbital, and

energy. QCe) is independent of jR^ (within our approximations). Thus

the adatom Green's function becomes:

[41 Kj oilers trom, Scalapino, and Schrieffer have given an expression for

the electronic interaction energy, within the Anderson model, which with

our eigenvalues is simply

(^a) =27i A^a""^-^ |s(^^)|2Q(e) -2c

^ (9)

X |s(j^^) |2Q(e)) X G^^(e) de - . Equation 9 yields potential energy surfaces of an atom or molecule

interacting with a surface. Such surfaces form the backbone from which

reaction coordinates needed to understand kinetics, and thus catalysis,

can be obtained. As it stands, the model Hamiltonian based Eq. 9 can be

158

AE(
handled quite simply. The only function of adatom position is the

[5] overlap integral S(]^^). Mulllken has given formulae for overlap

integrals of Slater functions on different centers. Alternatively, S can

be parameterized and fitted to experimental data by setting

S(R ) = S (R = E exp (-3|R - R .J)

'^a expt '^a '^^equil "v^a '\^equil'

where S^^^^ is chosen to give the correct desorption energy or to agree

with spectroscopic data. The range parameter 3 could be determined from

Mullikan's formulae. More will be heard about this procedure in Hamann's

talk.

A simple example of the type of result I have in mind is the set

of potential energy surfaces and reaction paths shown in fig. 3. Deuss

and van der Avoird'"^'' considered the problem of dissociative chemisorption of H2 on transition metal surfaces: The model they considered is

the 4 atom cluster, also shown in fig. 3. In this particular calculation

they calculated the interaction energy of the broadside molecule with

the 3d 2 Ni group orbital as a function of molecule surface and H-H

z separation. Contours of constant interaction energy are shown in fig. 3

and the dotted path is the reaction path. From this figure it is seen

that if the H--Ni interaction is solely through a single 3d 2 gi^oup

orbital, then the will dissociate with no activation barrier to

overcome. As cautioned by Deuss and van der Avoird, both this model of

chemisorption and the theory used are overly simplified so the results

are not to be compared with real experiments. Nonetheless, this type of

calculation appears to be almost feasible with more realistic models,

especially using a model Hamiltonian procedure such as that one sketched

out in these notes. When this stage of development soon arrives, we

should be in a much better position to really know why the electron

factor in catalysis is a factor.

159
References

Some current review articles which stress model Hamiltonians are:

T. B. Grimley, Adv. in Surface and Membrane Sci. 71 (1975);

J. W. Gadzuk, in "Surface Physics of Materials," Ed. J. M.

Blakely (Academic, N.Y., 1975); R. Comer, Solid State Phys. 30>

94 (1975).

Examples of model Hamiltonian research papers include: P. W.

Anderson, Phys. Rev. 124 , 41 (1961); D. M. Newns, Phys. Rev.

178 , 1123 (1969); J. W. Gadzuk, J. K. Hartman, and T. N. Rhodin,

Phys. Rev. B4_, 241 (1971); G. Doyen and G. Ertl, Surface Sci. 43, 197

(1974); W. Brenig and K. Schonhammer, Z. Phys. 267, 201 (1974); R. H.

Paulson and J. R. Schrieffer, Surface Sci. 48, 329 (1975).

A. Madhukar, Phys. Rev. B 8^, 4458 (1973); A. Bagchi and M. H.

Cohen, Phys. Rev B £, 4103 (1974).

B. KjollerstrOm, D. J. Scalapino, and J. R. Schrieffer, Phys. Rev.

148, 665 (1966).

R. S. Mulliken, C. A. Rieke, D. Orloff, and H. Orloff , J.Chem. Phys.

17, 1248 (1949).

H. Deuss and A. van der Avoird, Phys. Rev. B 8, 2441 (1973).

160
(b) (c)

Fig. 1. (a) Schematic potential and energy level diagram for noninteracting atom and metal. The occupied portion of the

conduction band lies within the range -E < e < 0. A

r narrow d-band is centered at e =

a

(b) Adsorption for which the broadened atomic virtual

state lies below the Fermi level and is thus totally

unoccupied.

(c) Ionic adsorption for which the broadened valence level

lies above the Fermi level and is thus almost totally

unoccupied.

161
NO BOND BOND

GROUP

ORBITAL MOLECULE ADATOM COMPLEX^ SOLID

NB

B

pQoiE)

°

Fig. 2. (a) Adatom with a valence s orbital together with nonbonding

bridge d^^ group orbital (left) and bonding group orbital

(right)

(b) Molecular energy level diagram for the orbitals shown

in (a)

(c) Local density of states formed on the adatom when it

bonds to a surface through the group orbitals shown in (a)

Here the discrete bonding and non-bonding group orbital

energies are replaced by the energy band continuum.

162
(a)

d

I • )

—( • 1 V Is ) V Is y

H

Ni

h(a.)-

5 -

U -

(b) 3

H(1sl H|1s)

Ni(3dz2) Ni(3dz2)

U diaol

Fig. 3. (a) Model for H2 interaction with Ni surface through the Ni

d 2 orbitals.

(b) Potential energy surfaces (Kcal/mole) from the model

in a. The dotted line shows the most favorable reaction

path followed by the H2 molecule. The size of the dots is

a measure for the value of the interaction energy.

163
III. Discussion Following First Paper

Dr. Messmer reiterated and added to the points brought up in the

previous talk concerning the similarity in physical content, if not

mathematical appearance, between the P. W. Anderson model Hamiltonian

held dear by many solid state surface theorists, and simple (not extended)

Hiickel theory appearing in 30 year old quantum chemical papers by such

people as Coulson. A translation dictionary was presented in which

Hiickel concepts were expressed in terms of "modern" Green's function

language. For instance, the imaginary part of a Green's function, often

called the local density of states, is none other than the Hiickel

orbital coefficients (modulus squared) in the limit of a molecule with

an infinite number of centers. In addition, Hiickel theorists discuss

and calculate such quantities as bond order, atom polarizability , and

bond polarizability which Messmer suggests is still virgin territory to

the Green's function users. It was further pointed out that when the

discrete levels obtained in a quantum chemical calculation on a finite

molecule are Gaussian broadened, then the structure in the resulting

density of states is compellingly similar to that obtained from a full

band calculation, even for clusters containing only 27 (3x3x3) atoms.

[Ed. note: A word of caution though; the relative peak heights and

widths may be reasonable but the absolute values for the energies are

often way of f .

]

Prof. A. B. Kunz (Univ. of Illinois) suggested that Schrieffer and

Soven (Physics Today, April, 1975) had contrasted the band and broadened

cluster density of states in a way which pointed out significant differences.

Dr. Messmer replied that the general shapes did agree, although

edge effects in clusters could cause some discrepancies. [Ed. note:

Since all but one atom in a 27 atom cluster is either at an edge or has

a nearest neighbor edge atom, the resolution of this question with

existing cluster calculations cannot really be achieved.]

164
Dr. D. R. Hamann mentioned a similar controversy related to cluster

versus semi- infinite calculations on Si surface states. Calculations

due to Batra and Ciraci on 14 atom Si clusters (in which the dangling

bonds that would be connected to other Si atoms in a semi-infinite solid

are saturated with H atoms) , give surface states which differ from those

obtained with the procedure of Appelbaum and Hamann, to be discussed

shortly. Spatial relaxation of surface atoms give rise to new bands of

surface states which appear quite differently in cluster or continuum

models. Hamann does agree that clusters, when treated with care, could

be fine for describing localized bonding.

Dr. Messmer concurred with Hamann and added that if one considers

questions which depend on energies comparable with the width of the

broadening function (or level spacing) then the cluster method cannot be

expected to provide numbers which can be meaningfully compared with

those generated from a continuum model.

Dr. C. B. Duke (Xerox) observed that a "Stradivarius in the hands

of a village fiddler is still a fiddle." [Ed. note: It has been

suggested to us by Dr. A. Helmed that a corollary to this exists: The

average village audience is incapable of distinguishing between the

sounds of a Stradivarius and a common fiddle, when played by a persuasive musician. ] Thus with the many different kinds of cluster

techniques available, those "in the hands of a skilled man are fine but

in the hands of an amateur, can lead to lots of mistakes." In otherwords, you better understand your problem or "know" the answer. At this

point Chairman Watson intervened with the admonition that clusters were

to come later, and to keep to the schedule the remaining four presentations

will be given without major interruption.

165
IV. Theory For Chemisorption And Catalysis

by

Alfred B. Anderson

Chemistry Department, Yale University

New Haven, Connecticut 06520

Chemistry, by virtue of its regularities, is, at certain theoretical levels, not a difficult or perverse field of study. The regularities

in structures, binding energies, vibrational force constants and electronic energy levels in molecules have lent themselves to simple interpretations. A conceptually and computationally simple theoretical

procedure has been newly developed to deal with these properties. In it

rigid atoms are superimposed in molecular geometric configurations and

repulsive two-body forces are calculated from the charge densities

according to the Hellmann-Feynman force theorem. Charge redistributions

yield attractive energy components which might be calculated using the

Hellmann-Feynman theorem, but which are conveniently gotten as approximate one electron orbital energies. The method has been useful in

understanding small molecules and promises to be useful for large systems

because of its simplicity and low cost.

Chemical problems of catalysis on metal surfaces are complicated

and demand that theory and experiment join forces to establish rapid

progress. Three theoretical case studies are presented here. The first

is a simple orbital analysis of the catalysis of 1,3 sigmatropic shifts

by transition metal atoms, clusters and surfaces. The transition state

is stabilized through a bonding stabilization of a filled hydrocarbon

orbital with metal d orbitals. The second shows the energy levels

representing the bonding interactions and geometric distortions accompanying chemisorption of 0 and CO on an iron surface, compared with

experimental photoemission spectra by T. Rhodin and C. Brucker. The

third is the dissociative chemisorption of acetylene, HC = CH on iron

surfaces yielding two CH groups bonding perpendicular to the surface, H

166
ends up. The calculations predict such a reaction and recent photoemission experiments by C. Brucker and T. Rhodin show the CH a level

growing in time as acetylene dissociates on iron. Without the calculation, interpretation of the experimental spectrum is difficult. For

such a reaction, with manifestly strong and localized interactions,

small clusters of atoms representing a surface are adequate, but for

considerations of adsordate-adsorbate interactions and coverage-dependent

phenomena, larger clusters will be required.

Current References to Theory and Applications

[1] Derivation of the Extended Hiickel Method with Corrections ; One

Electron Molecular Orbital Method for Energy Level and Structure

Determinations , A. B. Anderson, J. Chem. Phys. 62^, 1187 (1975).

[2] Vibrational Potentials and Structures in Molecular and Solid Carbon , Silicon , Germanium and Tin , A. B. Anderson, J. Chem. Phys. 63^, 4330

(1975)

.

[3] Transition Metal Catalysis of Olefin Isomerizations , A. B. Anderson

Chem. Phys. Letters 498 (1975).

[4] Molecular Orbitals and Bonding in Ar2 , Kr2 , ArKr , (CI2) 2 > ArHCl and

Solid Chlorine , A. B. Anderson, J. Chem. Phys. 00, 0000 (1976).

[5] Structures , Binding Energies and Charge Distributions for Two to

Six Atom Ti , Cr , Fe and Ni Clusters and their Relationship to

Nucleatlon and Cluster Catalysis , A. B. Anderson, to be published.

[6] Interaction of Hydrogen , Carbon, Ethylene , Acetylene and Alkyl

Fragments with Iron Surfaces : Catalytic Hydrogenation , Dehydrogenation . Carbon Bond Breakage and Hydrogen Embrittlement , A. B.

Anderson, to be published.

167
Figure 1. Molecular orbital energy level diagram for an Fe atom.

Acetylene, and acetylene bonded as shown to Fe.

168
-5

-10

>

>%

c

liJ

-15

-20

TT

Fe Band

FeCH

FeCH

pFeCH

Half

Dissociated

^sFeCH

Dissociated

Figure 2. Molecular orbital energy level diagram for acetylene

o dissociating on two Fe atoms spaced 1.866 A. The Adsorbed

o position corresponds to a C-C bond length of 1.7 A and the

o Half Dissociated position corresponds to 2.3 A.

169
Figure 3. Total energy and components for acetylene dissociating on two

o Fe atoms 2.866 A apart.

170
V. Cluster Techniques Discussed by R. P. Messmer

The article reproduced as Appendix A discusses the merits of the

Xa - scattered wave techniques, pioneered by Messmer and Johnson, in

contrast with other standard methods. It is a good example of the type

of work presented by Dr. Messmer in his talk.

Another interesting example which was presented was a small Li

cluster, particularly since Dr. Smith showed results for the same clusters

using an alternative calculational scheme. The following is excerpted

from an article by Messmer and K. H. Johnson, published in J. Vac. Sci.

Technol. 11, 236 (1974).

As a prelude to investigating the most catalytically important

systems, we have tested the SCF-Xa-SW computational procedure on several

simple prototype metal clusters. For example, the simplest metal cluster

is the diatomic molecule. Even the most elaborate HF-SCF-LCAO

method does not yield a proper binding curve for such molecules. In

contrast, an SCF-Xa-SW calculation (requiring only a small fraction of

the computer time expended in the HF-SCF-LCAO calculation) leads to a

total energy, equilibrium internuclear distance, and separated-atom

limit for Li2 in relatively good agreement with experiment.

To investigate the relative stabilities of larger aggregates of Li

atoms, calculations have been carried out on clusters such as tetrahedral Li^, square-planar Li^, simple cubic Lig, body-centered cubic

Lig, cubo-octahedral Li-j^^* icosahedral ^^-^2'

It was found that the Lig cluster is considerably more stable

energetically than eight separate Li atoms or four Li2 molecules. It is

also interesting to observe that the equilibrium Li-Li internuclear

distance is much closer to the internuclear distance in bulk crystalline

lithium than it is to the Li2 bond length. No experimental value for

the bond length in Lig is available, although there is mass-spectroscopic

evidence for the occurrence of similar alkali-metal clusters in vapor.

It has originally been shown that the SCF-Xa-SW method facilitates

the computation of molecular orbital wavefunctions and densities, thus

permitting the visual display (via computer-generated contour maps) of

171
electronic charge distributions and chemical bonds. This facility is

particularly valuable for analyzing the fundamental nature of interfacial (e.g., adsorbate-substrate) chemical bonds. In fig. 1, for

example, we display a contour map of the valence electronic charge

distribution for a Lig cluster, computed at the equilibrium internuclear

distance and plotted in the plane of the cube face. It is especially

interesting to note the significant amount of charge density located

between the nuclei and directed toward the center of the cube face

(indicated in fig. 1 by the contours labeled 10). The pileup of charge

in the cube face is important, not only because it relates to the bonding

and stability of the Lig cluster, but also because it may be relevant to

the type of charge overlap which is essential to the reactive chemisorption of hydrogen on small lithium particles.

172
Fig. 1. Contour map of valence electronic charge density of Li^

o in the plane of a cube face, calculated by the SCF-Xa-SW

method. Density of contour nearest each Li nucleus is

0.092 e/a^S; density of contour 10 is 0.012 e/a^S; density

of contour 8 is 0.003 £/a„3 (a =Bohr atomic radius).

173
Reprinted with permission of the authors and Elsevier North-Holland, Inc. from

Volume 36. number 4 CHEMICAL PHYSICS LETTERS 1 December 1975

A COMPARISON OF SCF-Xa AND EXTENDED HUCKEL METHODS FOR METAL CLUSTERS

R.P. MESSMER*, C.W. TUCKER Jr.*

General Electric Corporate Research and Development, Schenectady, New York 12301, USA

and

K.H. JOHNSON**

Department ofMaterials Science and Engineering, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139, USA

Received 8 July 1975

Revised manuscript received 18 August 1975

A comparison of some results from Xo-scattered wave (Xa-SW) and extended Hiickel (EH) calculations for metal clusters

is given. It is found that small clusters of atoms (= 13 atoms) using the Xa-SW method reproduce many of the features of

the electronic structure of the bulk metals, whereas this is not the case for the same clusters using the EH method. A more

systematic approach to EH parametrizations is suggested in order to make this method a more viable approach to treating

metal clusters.

Recently a number of papers have appeared in

which the extended Hiickel (EH) method has been

used to investigate the electronic structure of transition — or noble — metal clusters [1—5] and the interaction of these clusters with adsorbates [3,5]. It had

also been used previously to investigate adsorbate—

substrate interactions in a non-metalhc system [6]. A

number of problems and shortcomings of the EH

method for treating metal clusters [3] and chemisorption systems [6] have been recognized and discussed.

One major problem iS the proper treatment of elec- tron transfer in a self-consistent manner. Another is the determination of the necessary parameters to

treat transition metals.

In the general context of using clusters of metal

atoms as a theoretical model to represent the substrate

for chemisorption studies, the question arises: how

many atoms are needed to give a reasonable representation of the electronic structure of a true metal? In the present letter we will compare the answers pro-

* Research sponsored in part by the Air Force Office of

Scientific Research (AFSC) Contract F44620-72-C-0008.

** Research sponsored by the National Science Foundation,

the Petroleum Research Fund (administered by the American Chemical Society), and the Shell Foundation.

vided by calculations made to date using the EH and

Xa-SW methods.

It is well known that the SCF-Xa-SW method re- duces to the KKR method of band theory when applied to the perfect bulk metal [7] and that the latter has been very successfully used to describe the elec- tronic structure of many metals [8]. Hence the

Xa-SW method should provide a useful starting point

for the investigation of finite metal clusters in an at- tempt to answer the above question. On the other

hand, the extended Huckel method has not been applied properly to obtain a band structure of any metal

and hence its efficacy for treating metal clusters is

a priori in doubt. However, the utility of the extended

Huckel method for obtaining the band structures of

certain semiconductors as well as the electronic structure of finite, clusters of atoms representing these

semiconductors is well established [9].

The SCF-Xa-scattered wave method has been employed to investigate clusters of up to 13 atoms of

U [10],andof Cu,Ni,Pd and Pt [11]. The parameters of the Xa-SW method in its muffin-tin form consist of (i) the atomic sphere radii, (ii) the exchange

parameter a and (iii) the basis set, i.e., the number of

partial waves on each center. For a cluster of 13 atoms

174
Volume 36, number 4 CHEMICAL PHYSICS LETTERS 1 December 1975

representing the nearest neighbor environment in an

fee lattice, all the atomic sphere radii are constrained

by the geometry of the problem to be one-half the

nearest neighbor distance of the lattice and the ex- change parameter is the atomic value tabulated by

Schwarz [12]. Partial waves up to / = 2 on the atomic

centers and / = 4 on the outer sphere are used. This

completely sets the calculations and hence there are no further parameters to affect the outcome of the

calculation. This is in marked contrast to the EH

method as applied to such systems, where changes in possible parametrization can yield rather different results and interpretations [1,5].

In the EH calculations of Anderson and Hoffmann

(AH) charge differences on neighboring atoms of over one electron occur in some cases. No such large charge

differences have been found in the Xa-SW calculations on metal clusters [11]. AH note, however, that al- though the charge buildups on atoms are overestimated due to the approximate nature of the EH calculations, "the sign and relative magnitude of the

charges are nevertheless useful for qualitative dis- cussion". Thus they clearly attribute physical or chemical significance to these charge differences; this

is in contrast to other workers such as Fassaert et al.

[1] who do not attribute any significance to the initial charges of the atoms in the metal cluster but only

consider changes in charge distribution relative to the

isolated metal cluster in discussing chemisorption.

We have repeated the calculations for Nig and W9

and reproduced the AH results. Moreover we have ex- tended the calculations (using the AH parameters) to

clusters of Wj3 and Ni|3. For the latter cluster we

may make a direct comparison with results obtained

by the Xa-SW method. In the first four rows of table 1 , the EH net atomic charge results are presented

for the W9, Nig, Wj3, and Nij3 clusters using the AH

parameters. The calculated charges for Ni]3 are of the same sign as those found from the Xa-SW calculations

(see table I); the energy levels which will be discussed

below are not, however, in very good agreement. One

thing which stands out immediately when comparing

the first four rows of table I is the rather notable qualitative differences between the tungsten and nickel

results. Such differences in charges according to Anderson and Hoffmann should be physically meaningful, if not in a quantitative sense nevertheless for

quaUtative discussion.

Table 1 EH net atomic charges for W and Ni clusters *)

MeinOQ v-iusicr atom

in-piaiie

atoms b)

UUl-Oi-pi

atoms

AH W9 + 0.74 + 0.41 -0.59

AH H.52 -0.39 + 0.39

AH Ni9 -0.25 -0.11 + 0.17

AH Nil 3 -0.17 + 0.01 + 0.01 SZAH Ni9 + 0.38 + 0.20 -0.30

SZAH Nii3 + 2.73 -0.23 -0.23

FVA Ni,3 + 2.54 -0.21 -0.21

SCF-Xa Ni,3 -0.72 + 0.06 + 0.06

3) The clusters for W and Ni have somewhat different geometries

because W has a bcc structure and Ni an fee structure. The four atoms which aie in the same plane as the center atom

The four (or eight) atoms which are in the plane(s) above

(and below) the plane containing the center atom.

There is, however, an anomalous difference in basis

functions between the AH calculations for tungsten

and nickel. In the former case a single Slater function

(single zeta = SZ) is used to represent the d-orbitai

whereas for nickel a double zeta (DZ) function is used.

When the Nig and Nij3 calculations are repeated using an SZ function for Ni (f = 2.0) and keeping all other

AH parameters the same, which is comparable to the

AH tungsten calculations, a rather different charge dis- tribution is obtained (see table 1). In comparing rows

3 and 4 with 5 and 6 of the table we find that the net charges change not only in magnitude but also in sign

and that the SZAH results gre qualitatively similar to the single zeta results of AH for tungsten. Thus we are

led to the conclusion that the large differences in net charges in the AH results between Ni and W clusters

are not physically significant but reflect differences in

parametrization. This is further supported by the results of ref. [1 ] for an Nij3 cluster using the EH

method, but with a different parametrization. The re- sultant charges are given in the seventh row of table I and labelled FVA.

In fig. 1 a comparison is provided for the one elec- tron energies obtained from the SCF-Xa-SW and EH

calculations. The EH calculations shown are for the

AH parameters. To the left all the occupied valence

levels are shown, to the right the higher occupied

levels are shown on an enlarged scale. The comparison

175
Volume 36, number 4 CHEMICAL PHYSICS LETTERS 1 December 1975

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

-0.9

-1.0

SCF-Xa-SW EH

NI|B

_-0.38

-0.40

-0.42

-0.44

i-0.46'

I

-0.48

--0.50

-0.52

SCF-Xo-SW

h, -4

— >S'*fa — 'ti =- s — "t,

-0.651-

-0.67

-0.69:

-0.71

-0.73-

-0.75 —cll; -0.77

-0.79

EH _ 40 - "I,

"I

-A.

='!"

Fig. 1. A comparison of energy levels for Nii3 as determined

by the spin-restricted SCF-Xa-SW and EH methods. The ar- rows indicate the calculated Fermi levels. In the EH results

the highest t2g level is completely occupied. In the SCF-Xor

SW results the highest tjg, ti^, and ajg levels are nearly de- generate and have an occupancy of 8/14.

between the two calculations shows rather little agreement.

Returning to the question posed earlier of how well

a small cluster represents the electronic structure of a metal, it is convenient to present the results of fig. 1

in a somewhat different form in order to compare

with the density of states (DOS) of bulk nickel.

Namely, we calculate the density of states for the

Nij3 cluster by replacing each discrete eigenvalue

(see fig. 1) by a gaussian with a width of 0.01 rydberg,

weighted by the degeneracy of the orbital. The result

is shown in fig. 2 where it is compared to the DOS

results from the SCF-LCAO-Xa band structure of

Callaway and Wang for bulk Ni [13]. As the extended

Hiickel method is a spin-restricted procedure we compare here only the Xa-spin restricted results and like- wise use only the majority-spin DOS results of Callaway

and Wang rather than their total spin-unrestricted results. In a spin-restricted calculation there would be

the same number of majority and minority spins and

hence no shift in the minority spin DOS with respect

to the majority spin DOS. A comparison such as presented in fig. 2 represents one very useful and graphic

criterion for assessing the similarity in the electronic structures of clusters of atoms to that of the bulk

metal.

It is clear from fig. 2 that the results of EH and

Xa-SW for Nii3 are quite different and that the Xa-SW

as 0.3 02 01

ENERGY (RYDBERGSI

Fig. 2. A comparison of the density of states of Ni as deter- mined from: (i) a bulk band structure calculation [13], (ii) a 13-atom Ni cluster using the extended Hiickel method and

(iii) a 13-atom Ni cluster using the SCF-Xa-SW method. The

energy scales for the clusters have been shifted so as to line up the Fermi levels which are indicated by arrows.

176
Volume 36, number 4 CHEMICAL PHYSICS LETTERS 1 December 1975

results have many features in common with the bulk

DOS, whereas this is not the case for the EH results. Xa-SW calculations have also been performed for

Cuj3, Pdj3, and Pt|3 and will be discussed in detail

elsewhere [11 ]. It is of interest here, however, to

briefly compare some results of these calculations for

Cui3 and Pdj3 with recent EH calculations for Cu

and Pd clusters by Baetzold and Mack [4] . Experimentally it is known that the d-band width increases

through the series Cu, Ni, Pd, Pt — this trend is reproduced by the Xa-SW calculations for 13-atom clusters

[11]. The results for even larger clusters, i.e., 19

atoms by Baetzold and Mack (BM) using the EH

method give results for Cu and Pd which are inconsistent with experiment and with the Ni results using the

AH parameters. BM calculate d-band widths for CU19

and Pdi9 of * 0.2 eV and 0.3 eV, respectively

(cf. fig. 9 of ref. [4]), whereas the experimental d- band widths as well as those calculated by the Xa-SW

method are of the order of several eV. BM state: "In

most properties, the clusters in the size range reported

here are different from the bulk metals". Hence this seems to represent the answer to the question posed

above — as provided by extended Hiickel calculations.

This conflicts, however, with the answer which

emerges from Xa-SW calculations carried out thus far. These latter calculations suggest that much of the bulk

band width and DOS structure can be obtained with a

cluster of approximately a dozen atoms.

We suggest that this discrepancy arises from the

EH calculations due to the current arbitrariness in parametrizations [1,5] used for transition metals in

this method. A much more systematic approach to

the problem would be to obtain paranteters by matching^he occupied bands as obtained from an EH band

structure calculation of the metal with those obtained

from more rigorous band calculations, in much the same spirit as previously used for semiconductors [9].

These parameters would then provide a reasonable

starting point for calculating clusters of metal atoms.

Alternatively, EH parameters might be chosen by

matching to the results of Xa-SW calculations on clusters.The importance of having a reasonable description

of the electronic structure of a metal cluster, before

using this cluster to study the chemisorption of molecules cannot be over-emphasized. The inadequacy of

the AH parametrization (as seen in table 1 and fig. 2)

must be a strong contributing factor, along with the

effects of self-consistency in charge transfer, to the

fact that the Anderson—Hoffmann explanation of

the photoemission results for CO chemisorbed on Ni

is inconsistent with the most recent and definitive

experimental data [14].

References

{11 D.J.M. Fassaert, H. Verbeek and A. van der Avoird,

Surface Sci. 29 (1972) 501. .

[2] R.C. Baetzold, J. Catal. 29 (1973) 129.

[3] L.W. Anders, R.S. Hansen and L.S. Barlell, J. Chem.

Phys. 59 (1973) 5277.

[4] R.C. Baetzold and R.E. Mack, J. Chem. Phys. 62 (1975)

1513.

(5] A.B. Anderson and R. Hoffmann, J. Chem. Phys. 61

(1974)4545.

(6] AJ. Bennett, B. McCarroIl and R.P. Messmer, Surface

Sci. 24 (1971) 191.

(7) K.H. Johnson and F.C. Smith Jr., in: Computational

methods in band theory, eds. P.M. Marcus, J.F. Janak

and A.R. Williams (Plenum Press, New York, 1971)

p. 377.

[8] B. SegaU and F.S. Ham, in: Methods in computational

physics. Vol. 8, eds. B. Alder, S. Fernbach and M.

Rotenberg (Academic Press, New York, 1968) ch. 7.

(9] RJ. Messmer, Chem. Phys. Letters 11 (1971) 589;

G.D. Watkins and R.P. Messmer, in: Computational

methods for large molecules and localized states in

solids, eds. F. Herman, A.D. McLean and R.K. Nesbet

(Plenum Press, New York. 1973) p. 133;

R.P. Messmer and G.D. Watkins, Phys. Rev. B7 (1973)

2568;

C. Weigel, RJ>. Messmer and J.W. Corbett, Phys. SUt.

SoL 57b (1973)455.

[10] J.G. Fripiat, K.H.Chow, M. Boudart, J.B. Diamond and

K.H. Johnson, J.Mol. Catal., submitted for publication.

1 11 ] R.P. Messmer, S.K. Knudson, K.H. Johnson, J.B. Diamond and C.Y. Yang, to be published.

(12) K. Schwarz, Phys. Rev. B5 (1972) 2466.

(13] J. CaUaway and C.S. Wang, Phys. Rev. B7 (1973) 1096.

[14] T. Gustafsson, E.W. Plummer, D.E. Eastman and J.L. Freeouf, BuU. Am. Phys. See. 20 (1975) 304.

177
VI. Essential Points Presented by J. R. Smith

The approaches to the surface electronic structure problem discussed in this presentation are based on models that are simple enough

to be solved self-consistently from first principles, i.e., with no

adjustable parameters. The jellium model is rather accurate for work

function calculations. A comparison between calculated and measured

values is shown in fig. 1. It fails for all but a few alkalies in

surface energy calculations, however, due to the energetic consequencies

of replacing the discrete periodic lattice by a uniform smeared out

charge. In an attempt to correct for this, a pseudopotential perturbation

calculation has been performed which yields reasonably accurate surface

energies and adhesive energy profiles. Bulk ion core pseudopotentials

were used. The question of the general validity of bulk pseudopotentials

in surface calculations must still be answered"^. A linear response

function has been computed for the jellium model and applied to hydrogen

2 chemisorption . The interaction energy versus distance from the surface

is shown in fig. 2. It should be possible to establish a connection

between the linear response function and the calculation of potential

surfaces. To study catalysis on transition metals, one must go beyond a

(crystalline) jellium model. The generalized Wannier function (GWF)

method is a promising approach. The efficiency and accuracy of the GWF.

3 method has been tested on a one-dimensional calculation . The first

three-dimensional calculation is done on a lithium eight atom particle.

4 Such a particle was treated earlier . Our cohesive energy is '^'12%

larger. A plausible explanation is that the larger value is due to the

removal of the muffin tin constraint. Our charge contours (shown in Fig.

3) in the surface (chemisorptive) region of the particle reflect much

more strongly the cubic symmetry of the particle rather than a spherical

symmetry. The method is currently being applied to adsorbate covered

transition metal surfaces.

Recommended entries to the literature: Intevaotions of Metal

SurfaceSj Vol. 4, Topics in Applied Physics, edited by R. Gomer (SpringerVerlag, NY, 1975). Surface Physios of Crystalline Solids, edited by J.

M. Blakeley (Academic, NY, 1975).

178
[Ed. note: As Smith emphasized, the non-spherical character of the

valence electron density around a Li site is predicted to be quite

different in his calculations compared with those of Messmer and

Johnson. In fact, viewing one or the other plot, one would make rather

different statements concerning the character of the valence states

which are available for bonding with adatoms. Band theory has had

difficulty in predicting aspherical charge character in the bulk of

solids as well. For example, Marvin Cohen and coworkers have found it

necessary to introduce a non-local pseudopotential in order to reproduce

the aspherical bonding density inferred experimentally for Si; standard

local pseudopotentials did not suffice.

]

N.D. Lang, Solid State Phys. 28, 225 (1973).

S. C. Ying, J. R. Smith, and W. Kohn, Phys. ^ Rev. B 11, 1483 (1975).

I'j. R. Smith and J. G. Gay, Phys. Rev. B 12, 4238 (1975).

K. H. Johnson and R. P. Messmer, J. Va. Sci. Technol. % 561 (1974).

179
Fig. 1. Workfunction versus r , the nondimensional inter-electron ° s spacing from jellium calculations and from experiment.

180
Fig. 2. Proton-metal interaction energy versus separation distance.

The nuclei of the surface plane of the metal are located at

-d/2, where d is the distance between planes parallel to the

surface.

181
Fig. 3. Contours of constant valence charge density in the cube face of

a Lio cluster. The nonband charge contours are in units of

3 electrons/nm . Note that in the interstitial regions, the

charge density peaks in the near-neighbor directions. In the

region outside the atoms, note that the contours are more

square than circular. Outside contour 20, the value of

successive contours has a ratio of 2.

182
VII. A New Role For Theory In Surface Science

by

D. R. Hamann

Bell Laboratories

Murray Hill, New Jersey 07974

Several recently developed theoretical techniques are presently

yielding detailed information about the structural, chemical, and

spectroscopic properties of semiconductor surfaces. The chemisorption of H on the Si (111) surface and the reconstruction of the

Si (100) surface are discussed, emphasizing the interplay of theory

and experiment.

Great advances have been made in the last few years in the application of theoretical techniques to provide detailed information about

the electronic structure of solid surfaces and their chemical activity.

Progress has been most rapid in the field of semiconductor surfaces,

which has today reached the point at which meaningful comparisons and

cross-stimulation between experimental and theoretical studies are

regularly taking place. I will discuss two examples from this field in

a quasi-historical fashion—the chemisorption of H on the Si (111) surface and the reconstruction of the Si (100) surface—after briefly describing the most productive current theoretical approaches. These have

as a common thread a characteristic that I feel is extremely important:

they employ realistic models of specific systems, and methods which have

been demonstrated to produce accurate electronic structure for these

solids in the bulk.

The first approach, developed by Joel A. Appelbaum and myself,

[1] employs a semi-infinite geometry . Its basic assumption, which has

been verified by explicit calculation, is that the disturbance produced

[2] by a surface is "healed" within several atomic layers . A fully selfconsistent quantum mechanical calculation is performed, representing the

ion cores by model potentials, treating the Hartree potential of the

183
valence electrons exactly and approximating the exchange and correlation

potential by a local function of the electron density. No artificial

discontinuities or "muffin tins" are introduced. An approach employing

an essentially similar procedure to model the physical problem, but

using a slab geometry and different calculational techniques^ has recently

been applied to a variety of problems by Cohen and coworkers . The empirical tight-binding approach has recently been applied to

[4] several surface problems by Pandey and Phillips using a slab geometry . While not a new method, these workers showed for the first time that a

very good fit to the entire valence band spectrum could be obtained

using a small number of parameters and a sophisticated fitting procedure. They also demonstrated that a simple overlap scaling of matrix

elements could represent the effects of changes in bond lengths at the

surface, and that matrix elements fitted to molecular levels could be

transferred to surface situations to describe chemisorption.

The story of H chemisorption on Si (111) begins, logically at least,

with the ultraviolet photoemission measurement of the surface density of

states by Rowe and Ibach''^''. They showed that atomic H readily adsorbs

on this surface, and adds a broad peak to the density of states centered

5 to 6 eV below the valence band maximum. The geometry of the chemisorptive bond seemed simple in this case—a single bond is broken at

each surface atom when the surface is formed, and each broken bond can

be saturated by a single H. Adapting this geometry, Appelbaum and I

calculated the electronic structure of the surface with an ordered

[21 monolayer of H . The self-consistent potential found in this calculation is shown in fig. 1. By varying the normal spacing of the H layer,

we found the bond length at which the forces on the layer went to zero,

and the bond stretching frequency. The length was in excellent agreement with the empirical chemical value, and the force constant in agreer 6

ment with infrared measurements . The density of states spectrum,

shown in fig. 2, presented a problem, however. It showed a distinct two

peak structure, unlike the single peak in Rowe and Ibach's data'"^''.

Publication of these results stimulated Hagstrum and Sakurai to

attempt an independent measurement of the surface density of states

using ion neutralization spectroscopy. They simultaneously performed a

184
UV photoemission measurement, however, and found, to everyone's surprise,

a two-peaked spectrum as predicted by the theory'"^''. The ion neutralization spectrum proved difficult to interpret, however. Returning to

the theoretical results, Appelbaum and I showed that this spectrum could

o be fit if one assumed that the neutralizing electron tunneled 2 A out

into the vacuum, but that the Auger-emitted electron originated within

r 8 1 i the surface layer . An apparent anomalbusly large shift in the high

energy threshold of the emitted electrons in going from clean to H

covered Si was also explained by these calculations, and shown to be

simply a large reduction in the amplitude of the spectrum over a 2 eV

range, and not an actual threshold shift.

The next chapter in the Si-H story came when Hagstrum and Sakurai

found that a Si surface prepared by quenching from high temperatures

could adsorb a great deal of additional hydrogen beyond that required to

saturate the amplitude of the previously mentioned two peak structure.

This structure disappeared, and two new large peaks appeared at consider-

[9] ably lower energies . Pandey showed that these puzzling results could

be explained by assuming that the surface Si layer was eroded away, and

that three H atoms bonded to the three available bonds of what was

originally the second Si layer. A spectrum calculated using this geometry

[91 produced peaks of the correct position and shape . The latest chapter in this story, a joint experimental effort by

Hagstrum and Sakurai and theoretical effort by Appelbaum and me, has, in

a sense, closed the circle. A partial H monolayer displays distinctly

different spectra depending on whether it is ordered or disordered '''^

,

and this then-unappreciated effect explains the difference between the

ipec

[9]

[5] initial photoemission spectrum of Rowe and Ibach and the later results

of Hagstrum and Sakurai

The second topic which I will discuss concerns the Si(lOO) surface,

which has long been known to occur only in a reconstructed form with a

doubled translational periodicity. Two bonds are broken for each atom

on this surface, so the ideal geometry is clearly not an optimum one.

Two models for the atomic geometry of the reconstructed surface were

[11] proposed quite a while ago by Schlier and Farnsworth . In one model,

185
pairs of surface atoms move together to rebond one of the broken bonds

on each, while bending but not stretching their bonds to the second

layer. In the second, half of the surface atoms are removed, so that

those remaining can saturate all the broken bonds by forming double

bonds. The pairing model has recently been supported by Levine in the

course of explaining the extremely low work function of Si(lOO) with

:-int:

[13]

[121 coadsorbed Cs and 0 . The vacancy model has recently been re-introduced

by Phillips who argues that it is favored by thermochemical data

Appelbaum, Baraff and I began to study this surface by calculating

[14] the electronic structure of the ideal geometry . Two partially

occupied bands of surface states were found. While such a "metallic"

surface should generally be unstable, no instability corresponding to

the 2x1 reconstruction mode appeared when the dielectric response of

these bands was calculated.

We proceeded to calculate the electronic structure of the reconstructed surface for both the pairing and vacancy models The

general nature of the states found for the latter destroyed the hypothesis

that double bonds could form. Only one additional bond resonating

between the two bond directions of the surface atom to the second layer

occurred, and the remaining electron pair occupied two partially filled

surface state bands. The pairing model, on the other hand, did better

than anticipated in its bonding. The bonds bent towards each other

joined to look like a normal bulk Si bond, as may be seen from the

valence charge density shown in fig. 3. The other two broken bonds of

the pair formed two nearly split surface state bands, and contributed an

additional nearly saturated pi-like bond. A comparison of the surface

region density of states for each model with UV photoemission data taken

r -j^^ 1 by Rowe is shown in fig. 4. While matrix element effects wipe out

the photoemission from the low- lying s-like bands, the structure from

the higher p-like bands clearly is better fit by the pairing model.

Although seemingly complete, the results described had an annoying

loose end. The surface was still metallic because the bands of bonding

and antibonding pi-like surface states overlapped slightly. With the

increased bonding from this band, it seemed plausible to expect a pair

186
bond length somewhat shorter than the assiamed single bond length.

Another calculation was carried out for a shortened bond, and while the

overlap decreased, the bands did not separate'"''"'''"'.

ri8i At this time, we learned of new LEED data by Webb , which showed

that very clean and carefully annealed surfaces display an additional

very weak set of spots indicating a fourth order reconstruction superimposed on the 2x1 structure. We calculated the dielectric response of

our overlapping bands of surface states for the pairing model, and found

a strong tendency to instability for a deformation with just the needed

ri9i periodicity to explain the additional spots . The instability persisted

for both choices of pair bond length. This result strongly suggests

that a charge density wave state such as found in layered transition

metal compounds '^^' exists on this surface and is responsible for the

higher-order reconstruction.

The most fascinating aspect of the Si (100) story is that its reconstruction apparently involves two different members of a yet to be

enumerated list of reconstruction mechansims. A "first order" effect

involving a major repopulation of states takes it from the grossly

unstable ideal structure to the paired 2x1 geometry. A "second order"

instability effect then takes over to produce additional small atom

displacements with a longer periodicity. This deformation presumably

wipes out all the remaining metallic character of the surface, and

leaves a stable electronic structure.

The two examples discussed indicate the kinds of understanding of

the detailed physics and chemistry of surfaces that can be achieved

today. The interplay of chemical effects, geometry, and spectra can be

untangled through the use of adequate theoretical tools and an active

give-and-take between theory and experiment. There is every reason to

be confident that continuing efforts will explain many additional effects

for semiconductor surfaces, and that a similar approach to the study of

transition metal surfaces will meet with success in the near future.

187
References

[I] J. A. Appelbaum and D. R. Hamann, Phys. Rev. B6, 2166 (1972); Phys.

Rev. Lett. 31, 106 (1973); ibid. 32, 225 (1974); in Proc. of the

Twelfth International Conference on the Physics of Semiconductors ,

ed. by M. H. Pilkuhn (B.G. Teubner, Stuttgart, West Germany, 1974),

p. 675.

[2] J. A. Appelbaum and D. R. Hamann, Phys. Rev. Lett. 34, 806 (1975).

[3] M. Schluter, et al. , Phys. Rev. Lett. 34, 1385 (1975); S. G. Louie

and M. L. Cohen, ibid. 35, 866 (1975).

[4] K. C. Pandey and J. C. Phillips, Phys. Rev. Lett. 32, 1433 (1974);

ibid. 34, 1450 (1975); and to be published.

[5] H. Ibach and J. E. Rowe, Surf. Sci. 43, 481 (1974).

[6] G. E. Becker and G. W. Gobeli, J. Chem. Phys. 35, 458 (1961).

[7] T. Sakurai and H. D. Hagstrum, Phys. Rev. B12, (1975).

[8] J. A. Appelbaum and D. R. Hamann, Phys. Rev., to be published.

[9] K. C. Pandey, T. Sakurai and H. D. Hagstrum, to be published.

[10] J. A. Appelbaum, H. D. Hagstrum, D. R. Hamann, and T. Sakurai, to

be published.

[II] R. E. Schlier and H. Farnsworth, in Semiconductor Surface

Physics , ed. by R. H. Kingston (University of Pennsylvania

Press, Philadelphia, 1957); J. Chem. Phys. 30, 917 (1959).

[12] J. Levine, Surf. Sci. 34, 90 (1973).

[13] J. C. Phillips, Surf. Sci. 40, 459 (1973).

[14] J. A. Appelbaum, G. A. Baraff and D. R. Hamann, Phys. Rev. Bll , 3822 (1975) and to be published.

[15] J. A. Appelbaum, G. A. Baraff and D. R. Hamann, Phys. Rev. Lett.

35, 729 (1975).

[16] J. E. Rowe, Phys. Lett. 46A, 400 (1974).

[17] J. A. Appelbaum, G. A. Baraff and D. R. Hamann, to be published.

[18] M, B. Webb, private communication.

[19] J. A. Appelbaum, G. A. Baraff and D. R. Hamann, to be published.

[20] J. A. Wilson, F. J. DiSalvo and S. Mahajan, Phys. Rev. Lett. 32y

882 (1974); D. E. Moncton, J. D. Axe and F. J. DiSalvo, Phys.

Rev. Lett. 34, 734 (1975).

188
H ON Sl(l I I )-SURFACE POTENTIAL

VACUUM

1. Contour plot of the self-consistent potential of H chemisorbed

on Si (111) . The plane of the plot is normal to the surface, and

the dots indicate the positions of the H, first-layer Si, and

second-layer Si atoms. The contours are at 0.2 Hartree

intervals, and the scale is such that the valence-band

maximum falls at +0.066.

189
Fig. 2. Valence-band density of states for bulk Si and local density

of states on the chemisorbed H atoms. The histograms are

normalized for equal areas.

190
TOTAL CHARGE DENSITY - PAIR ING MODEL

Fig. 3. Charge-density contours on a plane normal to the 2x1 reconstructed

Si (100) surface passing through the paired surface atoms and

fourth-layer atoms (shown by dots) . Second- and third-layer

-3

atoms lie out of this plane. Density is in atomic units xlO

191
Fig. 4. Calculated surface-region density of states for the 2x1 reconstructed Si (100) surface compared to Rto = 21.2 eV photoelectronenergy distribution from Ref . 16, with estimated secondaries

subtracted.

192
VIII. Open Discussion Following Presentations

Prof. M. Boudart (Stanford) , sensing a defensiveness on the part of

the solid state theorists, indicated that jellium calculations may not be

as far afield from catalysis as some people might think, at least if

jellium could be prepared in a dispersed state. For instance, Na dispersed on alumina is an excellent catalyst for reactions involving

carbon ion intermediates. Also, one should not belittle cluster calculations, because people like John Sinfelt deal with clusters daily in real

life. Clusters are much more important to those in catalysis than the

ideal virgin surface of the surface physics laboratory. The comments

inspired Dr . Duke to add a few words of caution related to the ubiquitous adjustable parameters in most models. One must ask the question of

how these parameters are determined? A current method is to adjust the

parameters until the calculations agree with an experimental photoemission

spectrum. However, one must still know if both the spectrum and its

cluster geometry are simultaneously correct. It was suggested that

there are a broad class of models which get the spectrum right but not

the geometry. This is okay for single crystal work since you already

know the geometry. On the other hand, for catalytic systems it is important to have a model which gets both the spectra and the geometry

right.

Prof. Anderson noted that he obtained his atomic parameters from

Clement! wavefunctions and required his clusters to have the correct

diatomic equilibrium distances, force constants, and binding energies.

Prof. John Turkevich (Princeton University) offered some questions

to focus on. One is 100 years old. Does the catalyst distort the

molecule? Is the distortion of the molecule you put on the surface

different from atom to atom? Are electrons transferred more easily?

Chemisorption is secondary to catalysis. What makes a molecule break up,

what makes it active, what makes O2, which are dead, all of a

sudden react? What is the activation process? Maybe by going to really

simple things you may reduce the whole essence of the problem. There

must be an irreducible minimum where the essence of the catalytic act,

[Ed. note: natural or unnatural] as we practice it, is retained but is

193
not that complicated that we can't swallow it. Photoelectron spectra

are very interesting for surface chemistry, for all sorts of solid state

effects, but that's not catalysis.

Dr . Duke interjected that models of catalysis are going to be

checked for a small particle by some sort of spectroscopy. The reply

from Dr. Turkevich , "yes, but spectroscopies are very indirect. It may

give you a living but it will not give us a living" summarized the gulf

(which has slowly narrowed in the past year or two) still existing

between the work of the surface scientist and the catalytic scientist.

194
WORKSHOP ON THE ELECTRON FACTOR IN CATALYSIS ON METALS

December 8-9, 1975

REGISTRATION LIST

Alayne A. Adams

American University

Dept. of Chemistry

Massachusetts & Nebraska Ave. , NW

Washington, DC 20016

Radoslav Adzic

Institute of Electrochemistry

Karnegieva 4

Belgrade, Yugoslavia 11001

A. W. Aldag

University of Oklahoma

202 W. Boyd

Norman, OK 73069

Alfred B. Anderson

Yale University, Chemistry Dept.

225 Prospect St.

New Haven, CT 06520

Joseph A. Baglio

GTE Labs

40 Silvan Avenue

Waltham, MA 02154

Alfonso L. Baldi

Alloy Surfaces Company

100 S. Justison Street

Wilmington, DE 19899

Ian R. Bartky

National Bureau of Standards

Matls Bldg. , B354

Washington, DC 20234

Lawrence H. Bennett

National Bureau of Standards

Matls Bldg. , B152

Washington, DC 20234

Harold Berger

National Bureau of Standards

Matls Bldg. , B254

Washington, DC 20234

George Birnbaum

National Bureau of Standards

Matls Bldg. , B354

Washington, DC 20234

M. Boudart

Stanford University

Stanford, CA 94305

E. 0. Box, Jr.

Phillips Petroleum Co.

334A Res. Bldg //I

Research Center

Bartlesville, OK 74003

•Manfred W. Breiter

General Electric Co.

Corp. Research & Development

P.O. Box 8

Schenectady, NY 12345

Boris D. Cahan

Case Western Reserve Univ.

2074 Adelbert Road

Cleveland, OH 44106

Julio Calderon

Dept. of the Array

AMXFB-CF

Ft. Belvoir, VA 22060

Patrick J. Call

RCA Laboratories

Washington Road

Princeton, NJ 08540

Robert C. Cargill

Physical Elect. Industries, Inc.

278 Fairtree Plaza

Severna Park, MD 21146

Divna Cipris

Allied Chem. Corp.,

Columbia Road

Morristown, NJ 07960

195
Paul H. Citrin

Bell Laboratories

600 Mountain Avenue

Murray Hill, NJ 07974

William L. Clinton

Georgetown University

37 & 0 Streets, NW

Washington, DC 20057

Ernest M. Cohn

103 G Street, SW

Apt. B-620

Washington, DC 20024

Amos Coleman

Dept. of the Army

AMXFB-CF

Ft. Belvior, VA 22060

Bernard R. Cooper

West Virginia University

Dept. of Physics

Morgantown, WV 26506

John R. Cuthill

National Bureau of Standards

Matls Bldg., B148

Washington, DC 20234

Victor V. Damiano

Franklin Institute Res. Labs

20th & Benjamin Franklin Pkwy

Philadelphia, PA 19103

H. L. Davis

Solid State Div.

Oak Ridge National Lab.

Oak Ridge, TN 31830

Jean F. Delord

Reed College

3203 SE Woodstock Blvd.

Portland, OR 97222

Thomas W. DeWitt

National Science Foundation

1800 G Street, NW

Washington, DC 20550

Richard A. Diffenbach

ERDA

4800 Forbes Avenue

Pittsburgh, PA 14213

C. B. Duke

Xerox Corp.

800 Phillips Road, W-114

Webster, NY 14580

Henry Ehrenreich

Harvard University

Cambridge, MA 02138

T. L. Einstein

Dept. of Physics & Astronomy

University of Maryland

College Park, MD 20742

Nils E. Erickson

National Bureau of Standards

Chem Bldg. B248

Washington, DC 20234

Gerhard Ertl

Universitat Munchen

Sophienstr. 11

8 Munchen 2, W. Germany

Billy J. Evans

National Bureau of Standards

Matls Bldg. , B150

Washington, DC 20234

Galen B. Fisher

National Bureau of Standards

Chem Bldg. , B248

Washington, DC 20234

Robert Mark Friedman

Monsanto Company

Mail Zone Q3B

800 N. Lindbergh Blvd.

St. Louis, MO 63166

Yasuo Fukuda

Center of Materials Research

University of MD

College Park, MD 20742

196
J. William Gadzuk

National Bureau of Standards

Metrology Bldg., B21A

Washington, DC 20234

C. D. Gelatt

Harvard University

Pierce Hall

Cambridge, MA 02138

Heinz Gerischer

Fritz-Haber-Institute der

Max-Planck-Gesellschaf

t

Faradayweg 4-6

1 Berlin 33, W. Germany

William L. Gordon

Case Western Reserve Univ.

Dept. of Physics

Cleveland, OH 44106

Warren Grobman

IBM Thomas Watson Res. Ctr.

P.O. Box 218

Yorktown Heights, NY 10598

Laszle Guczi

Worcester Polytechnic Institute

Chem. Eng. Dept.

Worcester, MA 01609

Gary L. Haller

Yale University

Dept. of Eng. & Applied Science

9 Hillhouse Ave.

New Haven, CT 06520

Robert S. Hansen

Ames Laboratory, USERDA

Iowa State University

Ames, lA 50011

D. R. Hamann

Bell Labs

Murray Hill, NJ 07974

Victor E. Henrich

MIT, Lincoln Lab.

Wood Street

Lexington, MA 02155

Jan F. Herbst

National Bureau of Standards

Metrology Bldg., B214

Washington, DC 20234

John D. Hoffman

National Bureau of Standards

Matls Bldg. , B364

Washington, DC 20234

Emanuel Horowitz

National Bureau of Standards

Matls Bldg. , B364

Washington, DC 20234

Howard S. Jarrett

E.I. duPont de Nemours & Co.

Experimental Station

Wilmington, DE 19898

Leslie H. Jenkins

Oak Ridge National Laboratory

P.O. Box X

Oak Ridge, TN 37830

Johann A. Joebstl

Electrochemical Div/Lab 3000

AMXFB-EC, USAMERDC

Ft. Belvoir, VA 22060

Harlan B. Johnson

PPG Industries

P.O. Box 31

Rittman, OH 44203 -

James R. Katzer

Department of Chemical Engineering

University of Delaware

Newark, DE 19711

Richard D. Kelley

National Bureau of Standards

Chem Bldg. , B248

Washington, DC 20234

Chris E. Kuyatt

Natiorral Bureau of Standards

Metrology Bldg., B212

Washington, DC 20234

197
M. G. Lagally

University of Wisconsin

1109 Eng. Res. Bldg.

Madison, WI 53706

John Lambe

Ford Motor Company

P.O. Box 2053

Dearborn, MI 48121

Lennart Larsson

University of Lund, Chem. Ctr.

P.O.B. 740

S-220 07 Lund 7, Sweden

Richard V. Lawrence

Dept. of the Army

AMXFB-CF

Ft. Belvoir, VA 22060

Max Lipslcas

Yeshiva University

Belfer Graduate School of Sci.

2495 Amsterdam Avenue

New York, NY 10033

Farrel Lytle

The Boeing Company

P.O. Box 3999, 2T-04

Seattle, WA 98124

Archie J. McAlister

National Bureau of Standards

Matls Bldg. , B150

Washington, DC 20234

J. D. E. Mclntyre

Bell Lab, Rm ID-354

600 Mountain Avenue

Murray Hill, NJ 07974

Brian D. McNicol

Shell Research Ltd.

Thornton Research Centre

P.O. Box 1, CHI 3SH

Chester, England

Theodore E. Madey

National Bureau of Standards

Chem. Bldg., B248

Washington, DC 20234

198

Robert F. Martin

National Bureau of Standards

Matls Bldg. , B348

Washington, DC 20234

Allan J. Melmed

National Bureau of Standards

Matls Bldg. , B258

Washington, DC 20234

Anton Menth

Brown Boveri Research Center

5401 Baden, Switzerland

Richard P. Messmer

General Electric Corp.

Research St Development

Schenectady, NY 12301

S. R. Montgomery

W. R. Grace & Co.

Davison Chem. Div., Tech. Ctr.

5500 Chemical Road

Baltimore, MD 21226

Bruno Morosin

Sandia Labs

Albuquerque, NM 87115

S. Roy Morrison

Stanford Research Institute

Menlo Park, CA 94025

David L. Nelson

Office of Naval Research, Code 472

800 N. Quincy Street

Arlington, VA 22217

Gayle S. Painter

Oak Ridge National Lab.

P.O. Box X

Oak Ridge, TN 37830

Robert L. Park

University of Maryland

College Park, MD 20742

Daniel T. Pierce

National Bureau of Standards

Metrology Bldg. , B214

Washington, DC 20234
V. Ponec

Gorlaeus Lab

University of Leiden

Wassenaarseweg 76

Leiden, Netherlands, PB 75

G. T. Pott

Koninklijke/Shell-Laboratorium

Badhuisweg 3

Amsterdam-Noord, The Netherlands

C. J. Powell

National Bureau of Standards

Metrology Bldg., B214

Washington, DC 20234

A. K. Price

Allied Chemical

Columbia Road

Morristown, NJ 07960

Eldon B. Priestley

RCA Laboratories

Route 1 & Washington Road

Princeton, NJ 08540

Curt W. Reimann

National Bureau of Standards

Matls Bldg. , B354

Washington, DC 20234

R. J. Reucroft

University of Kentucky

Dept. of Met Eng & Matls Sci

Lexington, KY 40506

Thor Rhodin

Cornell University

Applied Physics

Ithaca, NY 14853

Dana H. Ridgley

Hooker Chem. & Plastics

Long Road

Grand Island, NY 14072

William M. Riggs

Physical Electronics Industries

6509 Flying Cloud Drive

Edina, MN 55343

Ralph Roberts

The Mitre Corp.

1820 Dolley Madison Blvd.

McLean, VA 22101

Carl J. Russo

Mass. Institute of Tech.

Dept. of Matls Sci. & Engr.

Cambridge, MA 02139

Gordon A. Sargent

University of Kentucky

Lexington, KY 40506

Edward Siegel

Public Service Electric & Gas

Energy Laboratory

200 Boyden Avenue

Maplewood, NJ 07040

John H. Sinfelt

Exxon Research and Eng. Co.

P.O. Box 45

Linden, NJ 07036

John R. Smith

General Motors

510 Henley

Birmingham, MI 48008

Paul Soven

University of Pennsylvania

Philadelphia, PA 19101

Fred E. Stafford

National Science Foundation

Solid State Chem.

1800 G Street, NW

Washington, DC 20550

Richard J. Stein

National Bureau of Standards

Metrology Bldg. , B214

Washington, DC 20234

F. Dee Stevenson

ERDA

Div. Phys. Research

Washington, DC 20545

199
Paul Stonehart

United Technologies

P.O. Box 611

Middletown, CT 06457

John A. Strozier, Jr.

Brookhaven National Lab

Dept of Physics

Upton, Long Island, NY 11973

Gary W. Stupian

The Aerospace Corporation

P.O. Box 92957

Los Angeles, CA 90009

Lydon J. Swartzendruber

National Bureau of Standards

Matls Bldg., B150

Washington, DC 202 34

Edward Taborek

Dept. of the Army

AMXFB-CF

Ft. Belvoir, VA 22060

Howard G. Tennent

Hercules Inc. Res. Ctr.

Market Street

Wilmington, DE 19899

John Turkevich

Princeton University

109 Rollingmead

Princeton, NJ 08540

Jens Ulstrup

Technical Univ. of Denmark

Chem. Dept. A, Bldg 207

Lyngby, Denmark 2800

George A. Uriano

National Bureau of Standards

Matls Bldg., B354

Washington, DC 20234

Alan E. Van Til

UOP Inc.

10 UOP Plaza

Des Plaines, IL 60016

Thomas H. Vanderspurt

Celanese Research Company

P.O. Box 1000 - Morris Court

Summit, NJ 07901

Theodore Vorburger

National Bureau of Standards

Metrology Bldg., B214

Washington, DC 20234

Bernard Waclawski

National Bureau of Standards

Metrology Bldg., B214

Washington, DC 20234

Grayson Walker

Dept. of the Amy

AMXFB-CF

Ft. Belvoir, VA 22060

R. E. Watson

Brookhaven National Labs

Upton, NY 11719

Wendell Williams

University of Illinois

Physics Department

Urbana, IL 61801

Thomas Wolfram

University of Missouri

Physics Bldg, 223

Columbia, MO 65201

Chiang Yuan Yang

Dept. of Matls Sci. & Engr.

Mass Institute of Tech

Cambridge, MA 02139

John T. Yates

National Bureau of Standards

Chem Bldg., B268

Washington, DC 20234

200
Author Index

(CITED IN TEXT)

A

Adams, D.L. - 97

Aldag, Jr. , A.W. - 135

Ambler, E. - 1

Anderson, A.B. - 134, 166

Appelbaum, J.R. - 165, 183, 184, 185, 186

Ashley, C.A. - 34

Azaroff, L.V. - 34

B

Ban, L.L. - 123, 124

Baetzold, R.C. - 177

Baraff, G.A. - 186

Batra, I.- 165

Bernasek, S.L. - 102

Besocke, K. - 103

Bennett, L.H. - 51, 68, 88

Blakely, M. - 178

Boudart, M. - 5, 16, 20, 46, 49, 81, 88, 98, 135, 136, 193

Breiter, MW. - 81

Brill, R. - 98

Brucker, C. - 166, 167

Brongersma, H.H. - 74

C

Callaway, J. - 176

dementi , E. - 193

Citrin, P. - 3, 43

Ciraci, S. - 165

Cohen, M.L. - 179, 184

Coulson, C.A. - 164

Czyzewski, J.J. -50

D

Demuth, J.E. - 19

Deuss, J. - 159

Doniach, S. - 34

Domke, M. - 97, 111

Dorgelo, G.J.H. - 71, 73

Dowden, D.A. - 47, 72

Duke, C. - 43, 44, 88, 127, 167, 193, 194

201
E

Ehrenreich, H. - 82, 83

Eilenberger, G. - 143

Einstein, T. - 50

Enimett, P.H. - 49

Ertl, G. - 32, 69, 92, 93, 94, 113, 132

Estrup, P.J. - 8

Evans, B.J. - 69

F

Farnsworth, H. - 185

Fassaert, D.J.M. - 175

Fisher, G. - 48, 49

Franken, P. E.G. - 71, 73

G

Gadzuk, J.W. - 46, 47, 87, 153, 154

Gelatt, Jr., G.D. - 68, 69, 82, 87

Germer, L.H. - 97

Gersten, J. - 50

Gingerich, K.A. - 133

Gomer, R. - 178

Grimley, R.T. - 153

Grobman, W. - 43

H

Hagstrum, H.D. - 184, 185

Haller, G. - 30, 33, 48, 51

Hansen, R.S. - 21, 98, 112, 119

Hamann, D.R. - 159, 165, 183

Haydock, R. - 109

Helms, C.R. - 81

I

Ibach, H. - 103, 184, 185

J

Jacquelyn, A. - 83

Jennings, P.J. - 136

Johnson, K.H. - 87, 171, 174, 179

Jones, R.O. - 136

202
K

Kasemo, B. - 46

Katzer, J. - 30, 31, 48, 51

Kjollerstrom, B. - 158

Kline, M. - 43

Knor, Z. - 95

Kunz, A.B. - 155

L

Lagally, M.G. - 69

Levlne, J. - 183

Lu, K.E. - 102

Lytle, F.W. - 30, 33, 34, 42, 44

M

Madey T. - 50

Mack, R.E. - 177

Matthiess, L. - 35

Meijering, J.L. - 71

Messmer, R.P. - 87, 88, 164, 165, 171, 179

Melmed, A.J. - 91, 127, 131, 165

Moss, R.L. - 81

Miiller, E.W. - 93

Mulliken, R.S. - 159

Mc

McAlister, A.J. - 55

McAllister, J. - 21, 97, 112, 119

P

Painter, G.S. - 136

Pandey, K.C. - 184, 185

Park, R.L. - 33, 43, 46, 49, 51, 69

Pendry, J.B. - 34, 44

Phillips, J.C. - 184

Ponec, V. - 47, 68, 88, 69, 70, 71, 93

R

Rodin, T. - 32, 33, 68, 166, 167

Rowe, J.E. - 129, 184, 185, 186

Russo, C.J. - 81

Rye, R.R. - 102

203
sSachtler, W.M.H. - 9, 47, 71, 73

Sayers, D.E. - 34

Sakurai, T. - 184, 185

Scalapino, D.J. - 158

Schlier, R.E. - 185

Schmidt, L.D. - 97

Schrieffer, J.R. - 158, 164

Siegel, E. - 42, 44, 45, 69

Sinfelt, J.H. - 68, 69, 81, 88, 193

Smoluchowski, R. - 103

Somorjai, G.A. - 101, 102, 103

Soven, P. - 164

Smith, J.R. - 171, 178, 179

Smith, N. - 35

Stern, E.A - 34

Swartz, K. - 175

T

Taylor, H.S. - 94

Temkin, M.T. - 8

Tucker, Jr. , C.W. - 174

Turkevich, J. - 31,46, 47, 69, 123, 133, 193, 194

V

van der Avoird, A. - 159

Van der Plank, P. - 71

Voorhoeve, R.J.H. - 16

W

Wagner, C. - 8

Wagner, H. - 103

Wang, C.S. - 176

Watson, R.E. - 153, 165

Webb, M.B. - 187

White, J.M. - 100

Wood, J.H. - 138, 145

Y

Yang, C.Y. - 87

Yates, Jr., J.T. - 15, 33, 48, 49, 81

Z

Zeldovich, Ya.B. - 8

204
Subject Index

A

absorptive bonding - 19

active centres - 94, 100

appearance potential spectroscopy - 32

Anderson - Grimley formalism - 99

Anderson - Hoffman (AH) - 175

Anderson model Hamitlonian - 155, 159, 164

Auger Spectroscopy - 20, 32, 33, 42, 49, 56, 72, 73, 81, 98, 127, 135, 185

Average- t-matrix (ATA) -84

B

benzene hydrogenation - 16

biological molecules - 34

Bloch functions - 157

Br^nsted Law - 95

bridged bond - 18, 156

bulk electronic properties - 3, 56, 83, 87, 88, 155

C

CaF2 structure - 85

chemisorptive bonding - 19

chemisorptive titration - 56, 68, 73

chemisorptive luminescence - 46

cherry model - 69, 72

clusters - 69, 81, 88, 157, 164, 165

CNDO/2 - 44

coherent potential approximation - 69, 73

coordination complexes - 34

corehole excitations - 43

cyclohexane dehydrogenation - 56, 57

D

dangling bonds - 96, 99, 128, 165

demanding reactions - 20, 95

double zeta function (DZ) - 175

E

electron microscopy - 69

electron loss spectroscopy - 32

electron spin resonance (ESR) - 16, 46

electron stimulated desorption ion angular distribution (ESDIAD) - 50

enzymes - 94, 96

ethane hydrogenolysis - 56, 57

evaporated metal films - 71

extended Hiickel model (EH) - 174, 175, 177

extended x-ray absorption fine structure (ESAFS) - 30, 34, 42, 43, 50

205
F

facile reactions - 20, 94, 100

Fermi level - 83, 139, 140

field-ion microscopy (FIM) - 127, 131

G

geometric effects - 91, 137

Green's functions - 155, 156, 158, 164

H

Hartree^Fock-approximation - 83

Hartree potential - 183

HF-SCF-LCAO calculation - 171

Hellmann-Faynman theorm - 166

high z poisons - 44

high z substrates - 43

homopolar semiconductors - 128

hydrides - 82, 83, 88

hydrogen chemisorption - 57

hyperfine fields - 51

Hiickel model - 44, 164

I

infrared spectroscopy (IR) - 17, 18, 47, 48, 184

ion-neutralization spectroscopy - 32, 184, 185

K

kinetic transient method - 48

Korringa - Kohn - Rostoker (KKR) - 83, 174

L

Langmuirian surface science - 6, 7, 9, 11

Langmuir-Hinshelwood reaction - 8

LCAO - 138, 157

LEED - 20, 50, 99, 100, 101, 120, ,127, 128, 187

ligand factor - 9, 135

low temperature field evaporation - 131

low z substrates - 43

M

metal clusters - 81

mid-cracking - 68

modulated molecular beam method - 20, 48, 102

Mossbauer effect - 51

muffin-tin - 83, 85, 137, 138, 174, 184

multiplet theory - 94

206
N

nuclear magnetic resonance - 52

P

Pauling percent d-charater - 56

Pauling equation - 8

Peierl instabilities - 128

perturbed angular correlation - 51

photo emission spectroscopy - 44, 12 7, 166, 167, 193

R

rigid-band model - 18, 47, 72

S

SCF-Xa-SW method - 99, 171, 173

semiconductors - 96

scattered-wave method - 137

Slater Coulomb intergral - 85

Slater exchange - 137, 176

short bond - 36

soft x-ray appearence potential spectroscopy - 69

supported catalysts - 34, 57

surface defect - 16

surface ESAFS (SEXAFS) - 42

T

terminal cracking - 68

tight-binding approximation - 97, 184

ultraviolet photoelectron spectroscopy (UPS) - 18, 19, 32, 97, 184, 185, 186

W

Wigner-Seitz radius - 85

X

xa-scattered wave (Xa-SW) - 171, 174, 175

x-ray absorption spectroscopy - 34

x-ray photoelectron spectrocopy (XPS) (ESCA) - 18, 20, 51, 194

x-ray diffraction - 71

207
Material Index

A

acetylene - 167, 168, 169, 170

Ag-81

Ag-Pd-47

alumina - 45, 81, 133

Au - 34, 36, 69, 124

Au-Pt - 125

B

benzene - 56

C

C - 43, 48, 42, 193

C2H2 - 19

C2H4 - 19, 20

C2H6 - 19

cabosil (SIO2) - 35

CO - 47, 49, 57, 68, 98, 99, 103, 116, 135, 142, 148, 149, 166, 177

CO2 - 49, 100, 115, 135

Cu - 57, 73, 174

CuH - 83

Cu-Ni - 47, 56, 68, 69, 70, 71, 74

Cu-Ni-hydrides - 71

Cu-Ru - 57

Cr - 133

cyclopropane - 47

F

Fe - 133, 138, 141, 144, 166, 167, 169, 170

Fe catalysis - 97, 98, 99

formic acid - 135

G

gases - 34

glassy materials - 34

H

H2 - 43, 46, 57, 68, 72, 87, 102, 117, 118, 135, 183, 184

H2/D2 - 102

hexane - 68

I

Ir - 34, 35, 131

208
L

Lewis acid - 121

Li - 171, 174, 182

M

methane - 58

Mg - 46

MgO - 16, 45

Mo - 87

N

N2 - 21, 49, 97, 120, 121

N2O - 96, 108

NH^ - 21, 49, 97, 99, 119, 120, 121

Na - 193

Ni - 47, 57, 73, 97, 99, 129, 133, 135, 142, 148, 174, 175, 176, 177

NiO - 129

0

0„ - 43, 46, 100, 101, 186

Os - 57

P

Pd - 18, 81, 83, 85, 99, 100, 101, 115, 116, 117, 123, 174

PdH - 83

Pt - 21, 34, 35, 47, 69, 81, 102, 118, 125, 126, 131, 133, 174, 177

PtCl2 - 35

R

Rancy Ni - 88

Rh - 52

S

Si - 179, 183, 184, 186

SiO - 36

Sn - 133

T

Ta - 34, 35

Ti - 133

TiH - 85

V

V - 140, 141, 146

209
w

W - 21, 97, 19, 120, 121, 131, 175

- 119, 120, 121

WS - 18

wurtzite - 128

Y

YH - 85

Z

zincblende - 128

Zeise's salt - 20

ZnO - 17

•ftU.S. GOVERNMENT PRINTING OFFICE: 19 7 7-240-848/87

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BIBLIOGRAPHIC DATA

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1. PUBLICATION OR REPORT NO.

NBS SP 475

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Electron Factor in Catalysis on Metals

Proceedings of a workshop held at the National Bureau of

Standards, Gaithersburg, Md. , December 8-9, 1975

5. Publication Date

April 1977

6. Performing Organization Code

7. AUTHOR(S)

1 . H. RpnnPff fdltnr

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards, Washington, D. C. 20234

National Science Foundation, Washington, D. C. 20550

Energy Research & Development Administration,

Washington, D. d. 20'5Zt'5

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15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-600009

16. ABSTRACT (A 200-word or less factual summary ol most sigpificant information. If document includes a significant

bibliography or literature survey, mention it here.)

This book presents the proceedings of a Workshop on the Electron Factor in

Catalysis on Metals held at the National Bureau of Standards, Gaithersburg, Maryland,

on December 8-9, 1975. The Workshop was sponsored by the institute for Materials

Research, NBS, the Division of Materials Research of the National Science

Foundation, and the Division of Conservation Research and Technology of the

Energy Research and Development Administration. The purpose of the Workshop

was to review the most recent experimental and theoretical Investigations

on catalysis on metals and related topics, and to bring together chemists,

chemical engineers, surface scientists, and solid state physicists and chemists

Involved In research related to this topic. These proceedings summarize the

four panel sessions into which the Workshop was organized: Experimental

Techniques, Effect of Alloying, Geometrical Effects, and Electronic Structure.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Catalysis; Characterization; ChemI sorpt Ion ; Electronic factor; Geometric factor;

Metals; Surfaces.

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