<——<

Theoretical Studies on the Catalytic
Activity of Ag Surface for the
Oxidation of Olefins
HIROSHI NAKATSUJI,1, 2, 3 ZHEN-MING HU,1 HIROMI NAKAI 1, U
1Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto
University, Sakyo-ku, Kyoto 606-01, Japan2
Department of Applied Chemistry, Graduate School of Engineering, University of Tokyo, Hongo,
Tokyo 113, Japan
3
Institute for Fundamental Chemistry, 34-4, Takano-Nishihirakicho, Sakyo-ku, Kyoto 606, Japan
Received 2 March 1997; accepted 14 March 1997

ABSTRACT: Systematic theoretical studies for the mechanisms of the epoxidation andcomplete oxidation of ethylene and propylene over silver surface as well as the reactivity
and the stability of oxygen species on Cu, Ag, and Au surfaces have been presented. The
dipped adcluster model ŽDAM. combined with the ab initio Hartree]Fock ŽHF., secondorder Møller-Plesset ŽMP2., and SACrSAC-CI Žsymmetry-adapted clusterrconfigurationinteraction. methods are used. These studies clarify the origin of silver as a uniqueeffective catalyst for the epoxidation of ethylene and the different mechanisms for theoxidation of olefins over silver surface. For the epoxidation of ethylene, the superoxideOy
2 , which is molecularly adsorbed in bent end-on geometry on the silver surface, is theactive species. The origin of the unique catalytic activity of silver for the epoxidation of
ethylene is due to its ability to adsorb oxygen as the superoxide species. Such anadsorbed species cannot be stable or exist on Cu and Au surfaces. For the oxidation of
propylene, both reaction mechanisms initiated by the activation of olefinic carbon and by
the activation of the allyl hydrogen exist. The activation of the allyl hydrogen is theorigin of the complete oxidation of some olefins over silver surface. The present results
not only let us have a better understanding of the reaction mechanisms of olefins oversilver surface but also supply a basic idea for the new catalyst design of the epoxidation
reaction. Q 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 839]855, 1997
U

Present address: Department of Chemistry, School of Scienceand Engineering, Waseda University, Ohkubo 3-4-1, Shinjukuku, Tokyo 169, Japan.
Correspondence to: H. Nakatsuji.
Contract grant sponsor: Japanese Ministry of Education,
Science, and Culture and the New Energy and Industrial Technology Development Organization ŽNEDO..
International Journal of Quantum Chemistry, Vol. 65, 839]855 (1997)
Q 1997 John Wiley & Sons, Inc.

CCC 0020-7608 / 97 / 050839-17

NAKATSUJI, HU, AND NAKAI

Key words: Reaction mechanism; epoxidation; combustion; catalytic activity of silver,
copper, and gold; superoxide; atomic oxygen; propylene; ethylene; dipped adclustermodel ŽDAM.; SACrSAC-CI method; ab initio Hartree]Fock ŽHF. and second-order
Møller-Plesset ŽMP2. methods

IntroductionThe heterogeneous selective oxidation of
olefins to epoxides on silverC

olefin

1
q O2
2

6

C

Ag

C

C

O
epoxideŽ1.

is an exceedingly important industrial catalytic
reaction, and as such has received long and extensive study w 1]7x . One particularly interesting aspect of the epoxidation reaction is its uniqueness:
Not only is silver a uniquely effective catalyst forheterogeneous epoxidation but also ethylene is the
only hydrocarbon which may be epoxidized withhigh selectivity. Actually, silver catalysts are used
in industry to produce several million tons of
ethylene oxide yearly w 1]4x . Recent experimental
studies show that, except for ethylene, only a few
olefins such as styrene w 8, 9x , 3,3-dimethylbutene
w 10x , norbornene w 11x , and butadiene may be epoxidized over silver with high selectivity. In contrast,
other olefins such as propylene, butenes, andpentenes are epoxidized over silver but with extremely low selectivity w 12]16x .
During the past 20 years, many studies w 12]24x
have been devoted to getting a better understanding of the epoxidation reaction mechanism. However, the origin of this unique catalytic activity ofsilver has not yet been clarified, and also themechanistic details of the epoxidation and that of
the competing combustion to carbon dioxygen andwater are far from understood. One of the mainquestions about the mechanism of ethylene oxidation is the roles of the adsorbed molecular andatomic oxygen. Both mechanisms involving molecular oxygen w 12]18x and atomic oxygen w 19]23x as
the active species have been suggested. It has beensuggested that the uniqueness of silver is associated with its ability to adsorb oxygen as the super-840

oxide species w 24x . More direct evidence for molecular oxygen as the active species has been obtained by several studies: e.g., only complete oxidation occurs when N2 O pulse which gives atomically adsorbed oxygen is used, but the epoxidation
reaction occurs when O 2 pulse which gives molecularly adsorbed oxygen, at least initially, is used
w 25x . The mechanism assuming atomic oxygen as
an active species was suggested from the experimental data that only atomically adsorbed oxygen
exist at high temperature in an industrial condition, since molecular oxygen desorbs or dissociates. The assumption that molecular and atomic
oxygens cause the partial and total oxidation reactions, respectively, gives an upper limit for theselectivity of 6r7. Recent experiments giving
higher selectivity than 6r7 w 26x imply that theatomic oxygen may at least pay an important role
for the epoxidation of ethylene.
Theoretical studies have done much less for the
epoxidation reaction of olefins. Though a few
works w 27]30x studying the epoxidation mechanism of ethylene have ever been published, lessdiscussions were presented w 27, 30x for the oxidation of propylene, and the calculations did not
explicitly include the surface contribution. The
main difficult problem still remaining in the theoretical studies is how to describe chemisorption
species and calculate adsorption energy correctlybased on a selected small cluster. This may be why
no ab initio theoretical study for the full reaction
pathway and the associated energy diagrams of
the epoxidation reaction of olefins has been reported.
In a series of our previous studies w 31]37x , atheoretical model, named the dipped adclustermodel ŽDAM. w 31]33x , has been proposed to study
chemisorptions and surface reactions by involving
the interactions between bulk metal and admolecules with consideration of the electron transfer between them and the image force correction.
Such an interaction is found to be very importantfor studying the chemisorptions and the reactionsof the oxygen on a metal surface, and even a small
DAM could be used to give correct electronic

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CATALYTIC ACTIVITY OF AG SURFACE
structures and reliable adsorption energies in contrast to the cluster model. Previous studies w 34]36x
for the chemisorption of oxygen on a silver surface
showed that the molecularly adsorbed oxygen exists in two stable geometries; i.e., an end-on Žbent.
structure in an on-top site and a side-on structure
in a bridge site. The ground state in the former
geometry is superoxide, which is expected to be
more electronically favorable for the reaction with
ethylene w 36x .
In this work, we present a brief account of ourrecent systematic studies on the catalytic activityof silver surface for the oxidation of olefins. We
will try to address the following three questions:Ž1. What species of surface oxygen is responsible
for the ethylene epoxidation? Ž2. Why is silver a
unique catalyst for the epoxidation? Ž3. Why dohigher olefins mainly process complete oxidationon the same silver catalyst? We performed the abinitio Hartree]Fock ŽHF., second-order MøllerPlesset ŽMP2., and the SAC Žsymmetry-adaptedcluster.rSAC-CI Žconfiguration interaction. calculations with the use of the DAM. More details areseen in the literature w 38]40x .

Epoxidation Mechanism of Ethylene on
Silver Surface
In this section we give an account of our research on the mechanism of the epoxidation of
ethylene on a silver surface w 38x . The energy diagrams of the reactions of ethylene with both
molecularly adsorbed and atomically adsorbed
oxygens on a Ag surface are studied. The EleyRideal mode, namely the reaction between theadsorbed oxygen species and a gaseous ethylene isassumed. The DAM was used to include the effects of the bulk metal such as the electron transfer
between adcluster and surface and the image force.
Geometry optimizations for calculating the reaction path were performed using the ab initio unrestricted Hartree]Fock ŽUHF. method and the energies at the optimized geometries were calculatedby the MP2 methods including the image force
correction. The Gaussian basis set used is of double-zeta accuracy, and some additional functions
were added for improvement. They are shown toreproduce well the known geometries and the energetics of the reactions w 38x . The model adclusters
used in the present study are the molecularly
adsorbed end-on Žbent. form in an on-top site andthe atomically adsorbed on-top structure.

REACTIONS WITH MOLECULARLYADSORBED OXYGEN
Epoxide Formation
We first study the epoxidation reaction of ethylene with the molecularly adsorbed superoxide
on a silver surface. For the formation of ethyleneoxide, the reaction takes place by an initial interaction of the oxygen with one of the carbon in
ethylene w 5, 6x . Figure 1 shows the obtained favorable initial attack of ethylene to superoxide species
adsorbed on the silver surface. Starting from thisgeometry, we optimized the geometries of the intermediates and the transition states ŽTSs. along
the reaction pathway. The optimized geometries
and energy diagrams are shown in Figure 2. The
reaction proceeds from the left to the right, and theenergy is in kcalrmol relative to the free system,
i.e., Ag 2 q O 2 q C 2 H 4 .
Figure 2 shows a two-step mechanism of thereactions. The first step of this reaction is theadsorption of molecular O 2 on Ag surface in which
one electron is transferred from the bulk metal tothe adcluster, i.e., n s 1. The transferred electron
mainly occupies the out-of-plane pU orbital of O 2 ,and the in-plane pU molecular orbital ŽMO. of O 2
remains singly occupied. The ground state for theend-on adsorption of O 2 is the superoxide. The
adsorption energy of superoxide is calculated to be

FIGURE 1. Model adcluster used. Geometries were
optimized in the Cs symmetry except for the Ag } Ag
˚distance and the O a } Ag } Ag angle fixed at 2.8894 A
and the 90.08, respectively.

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NAKATSUJI, HU, AND NAKAI

FIGURE 2. Energy diagram for the reaction between ethylene and molecularly adsorbed superoxide on Ag surface.
The route leading to ethylene oxide (EO) and acetaldehyde (AA) are shown by the solid and broken lines, respectively.

7.4 kcalrmol at the UHF level and 26.9 kcalrmol
at the MP2 level. The experimental molecular adsorption energies are 9.2, 9.3, and 24.1 kcalrmol
for the AgŽ111. w 18x , AgŽ110. w 18x , and electrolytic
Ag w 41x surfaces, respectively. The next step is theattack of ethylene onto the terminal oxygen atom
O b which is more reactive than the inside one Oa ,leading to the TS and the intermediate with theenergy barrier of 11.3 kcalrmol. The energy levelof the intermediate is similar to that of initial step.
In the TS the s bond between O b and C a isformed and the in-plane p bond of O 2 and the pbond of C 2 are broken. The geometry parametersat every step clearly reflect the reaction processing
along the reaction path w 38x .
When the intermediate is produced, the formation of ethylene oxide is an easy path as shown by
the real line, leaving an atomic oxygen on the Ag
surface. The calculated heat of reaction is 37.1
kcalrmol. The energy diagram given in Figure 2
shows that the reaction leading to ethylene oxide
proceeds very smoothly: The reaction is exothermic, there are no very high barriers, and there are
no too stable intermediates. We therefore conclude
that the epoxide formation reaction from the su-

842

peroxide species on a silver surface proceeds verysmoothly. We note that the exothermicity of theoverall reaction is partially due to the formation of
the atomically adsorbed oxygen on the silver surface.
Acetaldehyde Formation
We next study the formation process of acetaldehyde from ethylene and molecularly adsorbed superoxide on the Ag surface. In Figure 2,
the path is given by the broken line. When acetaldehyde is produced, it is further oxidized to
CO2 and H 2 O by combustion in the presence ofadsorbed oxygen, and therefore it is an intermediate in the complete oxidation process w 42x .
The formation of acetaldehyde starts from theintermediate shown in the center of Figure 2: up to
it the reaction path is common to the epoxidation
reaction. H a bound to C a in the intermediate migrates to C b , giving a stable conformation denotedas acetaldehyde intermediate in Figure 2. However, the energy barrier for this H a migration is ashigh as 71.5 kcalrmol. This step is therefore energetically forbidden. This is a reason for the highVOL. 65, NO. 5

CATALYTIC ACTIVITY OF AG SURFACE
selectivity of the epoxide formation process starting from the superoxide species summarized in
the preceding section.
Effect of Silver Surface
The effect of the silver surface may be twofold:
One is to provide a reactive species adsorbed on
the surface, and the other is to provide electrons to
the reaction site as considered in the DAM. Figure
3 shows the energy diagram for the reaction between ethylene and gaseous oxygen without thesilver surface. Two energy diagrams correspond tothe calculations with n s 1 and n s 0. The former
involves an excess electron but the latter is neutral.
For n s 1, we used the optimized geometries
shown in Figure 2, and, for n s 0, we performed
the geometry optimization for the present purpose.
The geometries illustrated in the upper and lower
sides correspond to those for n s 0 and n s 1,
respectively. For n s 0, the O—O distance is cal˚ for the intermediate, which isculated to be 1.37 A˚ for
smaller than the corresponding value 1.49 A
n s 1. The same is true for all the intermediates
and TSs.

Comparing Figure 3 with Figure 2, we clearly
see the catalytic activity of the silver surface forthe epoxidation reaction. Without the surface, thereaction is endothermic and the pathway has alarge barrier. Electron transfer Ž n s 1. certainly
reduces the barrier, but the direct interaction withthe actual silver atoms on the surface is quite
important. Clearly, the exothermicity of the reaction shown in Figure 2 is due to the formation of
the atomically adsorbed oxygen on the surface.
REACTIONS WITH ATOMICALLY ADSORBEDOXYGEN
When ethylene oxide is ejected out of the Ag
surface, the atomically adsorbed oxygen Oy is left
on the surface. It is also produced by the dissociative adsorption of O 2 on the surface, and thedissociative state is more stable than the molecularadsorption state. In our calculation, the dissociatedoxygen has its singly occupied orbital in the 2 p
orbital parallel to the surface.
Figure 4 shows the energy diagram for the reaction of ethylene with the atomically adsorbed oxygen on the Ag surface. When ethylene attacks

FIGURE 3. Energy diagram for the reactions between ethylene and gaseous oxygens [neutral O 2 ( n = 0) and O 2anion ( n = 1)]. The upper geometries are for n = 0, and the lower geometries for n = 1.

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NAKATSUJI, HU, AND NAKAI

FIGURE 4. Energy diagram for the reaction between ethylene and atomically adsorbed oxygen on Ag surface.
atomic oxygen on the surface, the most favorable
approach is to form the C a —Oa bond as illustrated
in Figure 4. It is similar to the approach of ethylene
to the superoxide species shown in Figure 2. The
calculated energy barrier from the adsorbed oxygen to the intermediate is, however, as small as 1.5
kcalrmol, reflecting the reactivity of the atomic
oxygen.
The next step from this intermediate is very
important, since it determines the selectivity of theatomically adsorbed oxygen. If C b attacks the oxygen to form the C b —Oa bond, ethylene oxide isformed. If C b attacks another C—H bond, causing
the hydrogen migration, acetaldehyde is a product, which is an intermediate for the complete
oxidation product.
For the hydrogen migration reaction, the barrier
was calculated to be 39.1 kcalrmol, which is highbut lower than the barrier, 71.5 kcalrmol, of thereaction in which the superoxide species was involved Žsee Fig. 2.. This process were calculatedusing the adcluster with n s 1. Unfortunately, thestructure of the TS to form the C b —Oa bond could
not be determined by the geometry optimization
procedure using both of the n s 1 and n s 0 adclusters. If the pathway from the intermediate to844

ethylene oxide is one step, it is a quite interesting
step since it involves both geometrical changes
and one electron back-transfer from the admoleculeto the bulk metal. In the DAM picture, the intermediate in Figure 4 is neutrally described withn s 1, i.e., with one additional electron supplied
from the bulk metal to the adcluster, but the product, ethylene oxide plus Ag 2 should be neutral,i.e., n s 0. Therefore, we calculated two potential
curves, one starting from the intermediate and theother from ethylene oxide plus Ag 2 using n s 1
and n s 0, respectively. All the geometrical parameters were assumed to change linearly, except
for the Ag—O distance, which was fixed to˚
2.180 A.
The potential curve for n s 1 increases monotonically from the intermediate to the product,
while the curve for n s 0 also increases monotonically from ethylene oxide and Ag 2 to the intermediate, and the two energy curves cross. The energy
difference between the crossing point and the intermediate is about 36 kcalrmol, which may beconsidered as the energy barrier of this process.
Namely at this TS, one electron is transferred backto the metal.

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CATALYTIC ACTIVITY OF AG SURFACE
The selectivity giving either ethylene oxide or
acetaldehyde would be dependent on the heightsof the barriers of the two processes and on thestabilities of the two products. The calculated energy barriers for the two processes are similar, 39.1
and about 36 kcalrmol, respectively. On the other
hand, the energy difference between the two products and the intermediate are q15.6 and y13.7
kcalrmol, respectively, which means endothermicity and exothermicity from the intermediate, respectively. From the above data alone, it is difficult to decide which is a preferential process,though the acetaldehyde formation may be favorable for the exothermicity. However, it may besaid that both ethylene oxide and acetaldehyde areformed from ethylene and atomically adsorbed
oxygen on the Ag surface. Furthermore, this fact isvery important since it would be a reason for theexperimental maximum selectivity more than 6r7
w 5]7x .
OVERALL MECHANISM OF EPOXIDATION OFETHYLENE
We summarize here our results on the mechanism of the epoxidation of ethylene on a silver
surface. The calculated energy barriers and theheats of the reactions are summarized in Table I.
The primarily important species for the epoxidation of ethylene on a silver surface is the superoxide Oy
which is molecularly adsorbed on the2
surface in the bent end-on geometry. Ethylene attacks the terminal oxygen atom of the superoxide,
as shown in Figure 1, with the barrier of about 11
kcalrmol, and then the reaction proceeds quitesmoothly leading to ethylene oxide as shown in

Figure 2. The overall reaction is exothermic by 37
kcalrmol from C 2 H 4 q O 2 q Ag surface or by 10
kcalrmol from C 2 H 4 q superoxide on Ag surface.
The second exothermicity is due to the larger adsorption energy of the atomic oxygen than that of
the superoxide.
On the contrary, the complete oxidation of ethylene from the superoxide species is forbidden due
to the existence of the large barrier Ž72 kcalrmol.
in the hydrogen migration step, though this reaction is largely exothermic by 66 kcalrmol from theinitial compounds or by 40 kcalrmol from thesuperoxide on Ag and C 2 H 4 . When silver surface
does not exist, the epoxidation reaction is a very
unfavorable reaction having a high-energy barrier
and endothermicity. Thus, the epoxidation of ethylene by the superoxide on the Ag surface is highly
efficient and selective.
The atomically adsorbed oxygen, which is left
on the surface after completion of the epoxidation
reaction by the superoxide or which may exist by
the dissociative adsorption of O 2 on the surface,
has two reaction channels leading to ethylene oxide and to complete oxidation. The selectivity here
seems to be small and both products would beobtained. This would be the reason why someexperiments reach the selectivity larger than 6r7.
In the process leading to ethylene oxide, theelectron back-transfer from the reaction adsorbate
complex to the metal surface should be importantand related to the barrier. We note that both electron acceptor and donor may work to reduce thebarrier of the electron transfer step: They work tostabilize the n s 1 and n s 0 curves, respectively.
The effects of Ce and halogen as promoters ofcatalyst are well known experimentally w 5]7x . We

TABLE I
Comparison of the energy barrier and the heat of reaction for the epoxidation and acetaldehyde formation by
the molecularly and atomically adsorbed oxygens (kcal / mol).
Reaction
Molecularly adsorbed oxygen
O 2( a ) / Ag + C 2 H 4 ª C 2 H 4 O + O( a ) / Ag
O 2( a ) / Ag + C 2 H 4 ª CH3 CHO + O( a ) / Ag
Atomically adsorbed oxygenO( a ) / Ag + C 2 H 4 ª C 2 H 4 O + Ag
O( a ) / Ag + C 2 H 4 ª CH3 CHO + Ag
Without Ag surface
O 2y+ C 2 H 4 ª O y+ C 2 H 4 O
O 2 + C 2H 4 ª O + C 2H 4O
aEnergy barrier

11.3
71.5

Heat of reactiona

10.2
39.5

;36
39.1

21.5 (24.7)
50.8 (52.3)

31.2
52.9

y13.2 (y11.3)
y38.0 (y35.4)

Values in parentheses show the experimental values.

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NAKATSUJI, HU, AND NAKAI
speculate that these promoters act in this electrontransfer step. Design of the promoter or co-catalyst, which is effective to this electron-transfer step,is of crucial importance since it would be a key forraising up the selectivity over 6r7.

Activation of O 2 on Cu, Ag, and Au
Surfaces
The experimental results show that silver is aunique catalyst for the epoxidation of ethylene.
However, it is not yet well understood why only
silver is so exceptional. In the former section, we
showed that the superoxide species adsorbed in
the end-on form on a Ag surface is very active andselective for the epoxidation reaction. Aiming toclarify the reason why only silver is an effective
catalyst for the epoxidation of ethylene, in thissection, we perform a comparable study for thechemisorption and activation of oxygen on Cu, Ag,
and Au surfaces.
We take M 2 O 2 ŽM s Cu, Ag, Au. as adclusters.
Both electronic structures and the reactivity of themolecular end-on and side-on adsorptions arestudied. The geometries of the molecular end-on
and side-on species are optimized at the UHFlevel. The relative stabilities of the adsorbed species
is studied by SACrSAC-CI method w 43]45x , since
the roles of lower ground and excited states arequite important in this kind of surface electronic
processes w 34]36x . The DAM w 31]33x is again
adopted to investigate the electron transferabilityof these surfaces.
REACTIVITY AND STABILITY OF
SUPEROXIDE SPECIES
Figure 5 shows an illustration of the geometriesof the end-on and side-on forms. The calculations
are performed by assuming one-electron transfer

FIGURE 5. Illustration of the geometrical conversion
from end-on (a1) to side-on (a4) forms. The M } M(M = Cu, Ag, and Au) distances were fixed at the lattice
constants.
from the bulk to the adcluster occurs. The opti˚ in the endmized O—O distances are 1.36]1.38 A
on form, which are close to or slightly greater than˚ . w 46x , and that in thethat of a free O 2 anion Ž1.35 A
˚ The calculatedside-on form are all about 1.51 A.
Cu—O distance are shorter than the Au—O and
Ag—O distances, reflecting a large adsorption energy and a short lattice constant of Cu.
The calculated electronic structure of O 2 in the
end-on and side-on adsorption forms is superoxide2yOy
2 and peroxide O 2 , respectively. Table II shows
the net charge and frontier density Žor spin population., which are related to the reactivity of thesuperoxide species. In all three species, the net
charges on the inside ŽOa . and outside ŽO b . oxygen
atoms are calculated to be about y0.6 and y0.15,
respectively, while the outside oxygen atoms havea frontier density of about 0.86, which is muchlarger than that of the inside oxygen atoms Žabout
0.12.. This shows that the outside oxygen is much
more reactive than the inside one and that theformer is not so negative as the latter one, thesame result as that reported before w 36x for a linear
Ag—O—O system. Actually, in the epoxidation
reaction with ethylene, the oxygen that reacts withTABLE IINet charge and frontier density of the end-on superoxide species on Cu, Ag, and Au surfaces.
Net charge

Frontier density

Systema

Ma

Mb

Oa

Ob

Ma

Mb

Oa

Ob

Cu 2 O 2
Ag 2 O 2
Au 2 O 2+0.060+0.058+0.095y0.271y0.295y0.285y0.643y0.605y0.644y0.146y0.158y0.166

0.007
0.014
0.007

0.002
0.003
0.002

0.133
0.119
0.116

0.858
0.864
0.876

a

The geometry of the end-on form is shown in Fig. 1.

846

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CATALYTIC ACTIVITY OF AG SURFACE
ethylene is the outside one as shown in the above
section. Based on the analyses of the net charge
and the frontier density, similar reactivity is expected for the end-on superoxide species on Cu,
Ag, and Au surfaces.
The reactivities of the superoxide species on Cu,
Ag, and Au surface for ethylene are also very
similar w 39x . The differences among Cu, Ag, andAu surfaces are small, if the superoxide species
exists on the metal surfaces. Large observed differences in the catalytic activity of these metal surfaces are therefore attributed to the different stabilities of the adsorbed oxygen species, particularly
the stability of the superoxide species.
Next, we examine a conversion from the end-on
to the side-on geometries. This process accompanies a conversion from the superoxide to the peroxide, since the ground state of the end-on andside-on species are the superoxide and peroxide,
respectively, as shown in some details in this section. Thus, the method to be used here should be
able to describe both ground and excited states ina good accuracy. For this purpose, the HF method
is inadequate, we use the SACrSAC-CI methodw 43]45x . The geometries optimized at the HF levelare used for the end-on Ža1. and side-on Ža4.
forms, respectively. The intermediate geometries
a2 and a3, which are illustrated in Figure 5, areapproximated as a linear function of the OMM
angle.
Figure 6 shows the potential energy curvesŽPECs. calculated for the conversion process between the end-on and side-on forms on the Cu,
Ag, and Au surfaces. In the end-on geometry, thesuperoxide Ž2 B2 . is always the ground state, but inthe side-on geometry, the ground state is the peroxide species Ž2A1 .. The superoxide species withthe 2A 2 symmetry is less important since it never
becomes the ground state in this conversion process. In the 2 B2 superoxide, the in-plane p U orbitalof oxygen is singly occupied while the out-of-planep U orbital is doubly occupied. These occupations
are reversed in the 2A 2 state. The crossing of the
PECs of the superoxide and peroxide occurs between the end-on and side-on geometries.
Relative stabilities of the superoxide and peroxide in the end-on and side-on geometries and thebarrier of the conversion are quite important forunderstanding the catalytic activities of metals forthe epoxidation of ethylene. Such information is
given in Figure 6. In the end-on form, the superoxide species is more stable than the peroxide species:
the energy difference is about 20 kcalrmol for Ag

FIGURE 6. Potential energy curves calculated by the
SAC / SAC-CI method for the conversion between the
end-on (a1) and side-on (a4) adsorption states in thelowest three states of the Cu 2 O 2 , Ag 2 O 2 , and Au 2 O 2
adclusters with n = 1.

and Au and 5 kcalrmol for Cu. In the side-on
form, the peroxide species is more stable than the
superoxide species: The energy difference is 50.1,
27.6, 21.2 kcalrmol for Cu, Ag, and Au, respectively. The energy difference between the end-on
superoxide and the side-on peroxide is 50.8, 23.1,
and y1.2 kcalrmol for Cu, Ag, and Au, respectively. On the Cu surface, the superoxide species
in the end-on form easily converts into the peroxide species in the side-on form because of a large
stabilization energy and for a lack of the barrier:
The lifetime of the end-on superoxide species
should be very short on the Cu surface. On theother hand, an energy barrier of about 20 kcalrmol
is calculated for the conversion on Au. On the Ag
surface, the end-on superoxide should have a considerable lifetime as seen from the PECs shown in
Figure 6. Based on the above results, we can conclude that the reason for the nonselectivity of Cu
for the epoxidation of ethylene is due to the absence of the stable adsorbed superoxide species.
However, for Au, the results obtained from Figure

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NAKATSUJI, HU, AND NAKAI
6 does not give a clear explanation for its inabilityof this reaction. As far as one-electron transferfrom the bulk metal into the adcluster occurs for
Cu, Ag, and Au, the reactivity of the superoxideon these metals should be similar.
ELECTRON TRANSFERABILITY OF Cu, Ag,
AND AuThe results listed in the preceding section showthat the oxygen superoxide species adsorbed on
Au surface has essentially the same reactivity as
those on Cu and Ag surfaces. The stabilities of themolecularly adsorbed oxygen species are mainly
due to the electron-transfer effect from the bulk
metal as already shown previously for silver
w 34]36x and palladium w 31, 32x . The chemical potentials Žwork functions. of the Cu, Ag, and Au
surfaces are 4.48, 4.52, and 5.37 eV for the Ž110.
surfaces, 4.94, 4.74, and 5.31 eV for the Ž111. surfaces, and 4.59, 4.64, and 5.47 eV for the Ž100.
surfaces w 47x . The Cu surface is expected to have
similar properties to the Ag surface. However, it is
not clear whether the electron transfer also occurs
for the Au surface. We therefore investigate here
the EŽ n. curves of the M 2 —O 2 systems in the
DAM in order to clarify why Au cannot be a good
catalyst for the epoxidation of ethylene.
Figure 7 is a display of the EŽ n. curves, namely
the energy of the adcluster calculated as a functionof n. It was calculated with the use of the highest
spin coupling model w 39x . The optimized end-on
geometries of M 2 O 2 adclusters are adopted since
they are the most stable geometries of the superoxide adsorbed species. The EŽ n. curves are upper
convex, and at n s 1.0, the systems are more stable than at n s 0.0, and the order of stability is
Cu 2 O 2 ) Ag 2 O 2 ) Au 2 O 2 . According to DAM
w 31x , the electron transfer would occur between theadcluster and the bulk metal when the chemical
potential of the adcluster becomes equal to thechemical potential of the solid surface, i.e.,­ EŽ n.r­ n s ym. For Cu 2 O 2 and Ag 2 O 2 systems,
the tangents of the EŽ n. curves become equal withthe experimental chemical potentials at about n s
0.9. Therefore, from the concept of DAM w 31x , one
electron should flow from the Cu and Ag bulk
metals into the adcluster after some barrier. This is
the case we used to study the activation of O 2 on
the surfaces shown in the preceding section. However, for the Au 2 O 2 system, though the energy at

848

FIGURE 7. E ( n ) curves for the Cu 2 O 2 , Ag 2 O 2 , andAu 2 O 2 adclusters in the highest spin coupling model atthe optimized end-on geometries.

n s 1.0 is lower than that at n s 0.0, the tangent of
the EŽ n. curve is still smaller than the experimental chemical potential. Therefore, different from Cu
and Ag metals, one-electron flow does not occur
from the Au metal into the adcluster. This point of
the Au surface is essentially different from theother metals and should be the reason why themolecularly adsorbed oxygen species cannot exist
on the clean gold surface. Thus, the inactivity of
the gold surface for the oxygen is explained by the
DAM as being due to its poor donating property.
The chemical potential of Au is much larger
than those of Cu and Ag. We note that this difference is attributed actually to the relativistic effectin gold w 48, 49x . The importance of the relativistic
effects to the valence-electron properties such as
chemical shift has been recently clarified in thislaboratory w 50]52x .
We further note that the electron flow is difficult to occur between the clean gold metal and theadcluster because it does not satisfy the condition
clarified by the DAM w 31x . However, as the energy
at n s 1.0 is lower than that at n s 0.0, the electron transfer can be realized if the chemical potentials m of the gold metal can be lowered by anaddition of promoters, etc. Experimentally, the recent works performed by Haruta et al. w 53, 54x
show that gold is remarkably active for the oxidation of CO and hydrocarbons when it is supported
on suitable metal oxides. This may be explained bysuch effect of the metal-oxide support.

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CATALYTIC ACTIVITY OF AG SURFACEBRIEF SUMMARY
The electron transfer from metal to O 2 is a key
factor for the chemisorption of oxygen on a metal
surface, as shown previously w 32]37x . The differences in the relative stabilities of different oxygen
species on different metal surfaces are attributed
to the differences in the electron-donating ability.
The present DAM calculations show that, for Cu
and Ag surfaces, one-electron transfer from thebulk metal into the adcluster occurs after somebarriers. On the other hand, electron transfer isdifficult to occur for the Au surface since the gold
surface has the higher work function. This is why
molecular adsorption of O 2 does not occur on theAu surface.
On Cu surface, peroxide is much more stable
than superoxide, and there is essentially no barrier
for the conversion from superoxide to peroxide, sothat the lifetime of the superoxide species should
be too short to react with ethylene. On Ag surface,
however, the superoxide species would have considerable lifetime on the surface. On Au surface,
the superoxide species does not exist because themolecular adsorption of O 2 does not occur due tothe difficulty of the electron transfer from the clean
bulk metal. We think that this difference in thestabilities of the end-on superoxide species on Cu,
Ag, and Au surfaces is a reason of the observed
difference in the catalytic activity of these metals
for the epoxidation of ethylene.
The above conclusion for the origin of the different catalytic activity of Cu, Ag, and Au surfaces
for the epoxidation of ethylene suggests a possibility of making a new catalyst for this reaction. The
key is an appropriate lifetime of the end-on superoxide species on the metal. Silver realizes it without any special modification. However, if one canrealize such superoxide species on some metals
using some modification, we think that the essential factor is realized.

Oxidation Mechanism of Propylene on
Silver Surface
Though silver is an effective catalyst for the
epoxidation of ethylene Ž85]87% conversion. w 5]7x ,it is a very poor catalyst for the epoxidation of
propylene Ž2]5% conversion. w 12]16x . A relatively

small substitution of hydrogen with methyl group,
H

H
C

C

H

H
C

H
ethylene

CH 3
C

H
H
propylenecauses such a drastic change of the reaction. It isinteresting to understand why. To know the difference of the oxidation mechanism between ethylene
and propylene will lead to a better understandingof the general oxidation mechanism of other olefins
on silver surface. Further, in the chemical industry,
propylene oxide is an important material, so thatto know why silver is so poor for the conversion of
propylene to propylene oxide is a starting point fora design of a new effective catalyst. This section isa summary of our recent study on the oxidation
mechanism of propylene w 40x .
Two possible reasons may be considered for thedifference in the reaction of ethylene and propylene on a silver surface. One possibility is that thereaction routes are the same but the methyl substituent causes a large change in the barrier height
andror in the stability of the intermediates in thecourse of the reaction. Another possibility is thatan entirely different reaction route comes out for
the existence of the methyl group. We have investigated these two possibilities. As in the case ofethylene, we study the reactions of the two active
oxygen species on a silver surface: the molecularly
adsorbed superoxide species and the dissociatively
adsorbed atomic oxygen species.
The calculational methods are the same as those
discussed in the second section. The DAM w 31]33x
was used to include the effect of the bulk metalsuch as the electron transfer and the image force.
The basis sets are the same as before, and thegeometries of the reactants, intermediates, transition states, and the products were optimized by
the Hartree]Fock method and the energies were
calculated by the MP2 method in order to include
the effects of electron correlations.
ATTACK ON OLEFINIC CARBON
We first study the reaction route, which is essentially the same as that of ethylene w 38x : The
reaction starts from the interaction between theadsorbed oxygen and the doubly bonded carbon
atom of propylene. Figure 8 is an illustration of theadcluster for the initial interaction of propylene

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NAKATSUJI, HU, AND NAKAI
CH 3

H
C

q O 2 or O

H-migration
Ag

H
H
CH 3 CH 2 CH O aldehydeŽCH 3 . 2 C O ketone½

FIGURE 8. Model adclusters for the olefinic carbon
attack mechanism: (a) on the terminal olefinic carbon,(b) on the central olefinic carbon.
with the superoxide species. Since two carbons inpropylene are different, two different attacks wereconsidered. The interaction with atomic oxygen
was similarly considered. The reaction may process as follows:
CH 3

H
C

H

H

CH 3
H

H
H

C

C

O
propylene oxideqO 2 or O

6

C

Ag

Ž2.

6

C

Ž3.

In Ž3., aldehyde is a product of the terminal C
attack and ketone is a product of the central C
attack, and they are further oxidized to CO2 andH 2 O.
Figure 9 shows the optimized geometries andthe energy diagrams for the reaction of the molecularly adsorbed oxygen with the terminal C of
propylene. It is similar to Figure 2 for the reaction
with ethylene: The substitution of one hydrogenwith methyl group does not cause a large change
for the reaction route and the energetics. In thisreaction mechanism, the route going to propyleneoxide should be quite smooth without any highenergy barrier and without any too stable intermediate, while the route leading to the aldehyde
should be essentially blocked up by the existenceof a high-energy barrier in the hydrogen migration
step. The diagram for the reaction of the superoxide with the central carbon atom of propylene w Fig.
8Žb.x was similar to Figure 9 w 40x . Thus, if thereaction proceeds along this reaction route, propylene oxide should be produced in high selectivity due to the catalytic activity of the superoxide

FIGURE 9. Energy diagram for the reaction between the terminal carbon atom of propylene and the molecularly
adsorbed superoxide on a Ag surface. The route leading to propylene oxide and aldehyde are shown by the solid andbroken lines, respectively.

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CATALYTIC ACTIVITY OF AG SURFACE
species adsorbed on silver. Also, for the reaction
with atomically adsorbed oxygen, a similar conclusion as that of ethylene may be obtained as thereaction route and the energy diagrams of propylene are similar to ethylene w 38x .
The results shown above mean that if the reaction proceeds through the oxygen attack on theolefinic carbon of propylene, silver should be as
good a catalyst as for ethylene for the epoxidation.
However, this result contradicts with the experimental observation that the conversion of propylene into propylene oxide is only 2]5% on thesilver catalyst w 12]17x . We therefore examine another possibility that a reaction route entirely different from those of ethylene may exist. Since
propylene has a methyl group, but ethylene does
not, we next examine the reaction route involving
the reaction of this methyl group with the reactive
oxygen on a silver surface.
ATTACK ON ALLYLIC HYDROGEN
Propylene is different from ethylene by an existence of a methyl group. We examine here thereaction route involving the reaction of this methylgroup with the superoxide and the atomic oxygen
on a silver surface. Figure 10 shows the initialattack of the superoxide w Fig. 10Ža.x and atomic
oxygen w Fig. 10Žb.x on the methyl hydrogen, andthe reactions process as follows:
CH 3

H

C
H
6

CH 2

q O 2rAg
H
CH CH 2rAg q H

C

O

OrAg
Ž4.

CH

6

CH 2

CH 3

CH 2rAg q OrAg q HOrAg
Ž5.

H

C
H
6

CH 2

q OrAgH
CH CH 2rAg q HOrAg

C

Ž6.

These reactions produce the allyl intermediate adsorbed on the silver surface, which is oxidized
further to CO2 and H 2 O. Since allyl radical oranion is a rather stable intermediate, this reaction
route is feasible.
Figure 11 shows the optimized reaction routefor the attack of superoxide on the methyl hydrogen of propylene as given by Ž4. and Ž5.. The
present result was added to the previous one for

FIGURE 10. Model adclusters for the allylic H attack
mechanism: (a) reaction with molecularly adsorbed
superoxide species, (b) reaction with atomicallyadsorbed oxygen.

the carbon attack given in Figure 9. The optimized
geometries of the intermediates and the transition
states for the present H attack are given in thelower side, while those for the previous C attack
are given on the upper side. The energy levels of
the intermediates and the transition states for thepresent reaction route are lower than those of theprevious one. The barrier for the attack on the
methyl hydrogen was calculated to be 10.8
kcalrmol, which is lower than the barrier for the
carbon attack, 13.7 kcalrmol, and then the system
becomes allyl intermediate and the hydroperoxygroup adsorbed on silver w Eq. Ž4.x . This intermediate is also more stable than the previous one, but
the hydroperoxy group is unstable and further
converted into OH and O as given by Eq. Ž5.: Thebarrier in this step is only 2.8 kcalrmol and thestabilization energy is as large as 42.6 kcalrmol.
When two-site interaction between allyl and silver
surface is further allowed, the system is further
stabilized by 18.1 kcalrmol. Thus, the reaction
route starting from the allylic H attack is always
lower than that starting from the olefinic C attack,so that the reactions represented by Eqs. Ž4. and Ž5.
should proceed more easily than the reactions
given by Eqs. Ž2. and Ž3..
We next examine the similar reaction given by
Eq. Ž6. involving the atomic oxygen adsorbed on
silver. Figure 12 shows the optimized reaction
route and its energetics. The lower figures give theoptimized geometries for the allylic H attack andthe upper ones for the olefinic C attack. The barrier
for the allylic H attack is essentially zero: only 0.1
kcalrmol. The allyl intermediate is more stable
than the intermediate of the previous C attack:when two-site interaction between allyl and silver
surface is allowed, it is further stabilized by 18.1
kcalrmol. Thus, again, for the atomic oxygen on

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NAKATSUJI, HU, AND NAKAI

FIGURE 11. Comparison of the energy diagrams for the reactions between propylene and the molecularly adsorbed
oxygen. The route for the allylic H attack is shown by the solid line, and those for the carbon attack are shown by thebroken and waved lines.

FIGURE 12. Comparison of the energy diagrams for the two reactions between propylene and the atomicallyadsorbed oxygen on a Ag surface.

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CATALYTIC ACTIVITY OF AG SURFACE
the silver surface, the allylic H attack is an easier
reaction than the olefinic C attack. The allyl intermediate thus produced is stable on the silver surface so that it will be further attacked by theoxygen species on the surface and would finally beconverted into CO2 and H 2 O.
Thus, for both superoxide and atomic oxygen
on a silver surface, the reaction starting from theallylic H attack proceeds more easily than the one
starting from the olefinic C attack. Therefore, theconversion to propylene oxide does not effectively
occur on a silver surface.
The origin of the activation of this allyl hydrogen is due to the stability of the preceding adsorbed allyl intermediate. We have investigated
the nature of the allyl intermediate adsorbed on asilver surface. Figure 13 shows the optimized geometry of the allyl intermediate on Ag 2 calculatedwith n s 0 and n s 1. We see that the adsorptionenergy is larger with n s 1 than with n s 0, indicating that the allyl species on silver is actually an
allyl anion species. Actually, the charge on theallyl group was y0.52 for n s 1, while it was
y0.27 in the case of n s 0. The existence of theanion allyl species on silver surface had been suggested experimentally by Madix et al. w 23x to be
the intermediate leading to the complete oxidation.
BRIEF SUMMARY
Figure 14 is a summary of the reaction mechanism of propylene on a silver surface. The allylichydrogen is more reactive than the olefinic carbon,

due to the stability of the allyl anion species on thesilver surface, so that propylene is converted toallyl intermediate and further converted to CO2
and H 2 O. If an olefin does not have an allylichydrogen, like ethylene, styrene, etc., it is converted to epoxide in high selectivity. Even if it has
an allylic hydrogen but the allyl intermediate is
not stable like in norbornene, the route leading toepoxide is more preferable. This explains the selectivity of the silver surface for the epoxidation reactions of olefins.
We note that the present results clearly showthat the route leading to propylene oxide also
exists on a silver surface. However, it is not themost favorable reaction path. The selectivity is justbetween 2 and 5% experimentally. Since propyleneoxide is an important material in chemical industry, it is interesting whether we can block up theroute leading to the allylic intermediate.

Concluding Remarks
We reported, in this study, the mechanisms of
the epoxidation and complete oxidation reaction of
propylene on a silver surface and the stabilities
and activities of the oxygen species adsorbed on
Cu, Ag, and Au surfaces. We performed the ŽU.HF,
MP2, and SACrSAC-CI calculations with the useof the DAM which involved the interaction between bulk metal and admolecules with consideration of electron transfer and image force correction.

FIGURE 13. Stability of the allyl species adsorbed on Ag 2 cluster in the neutral ( n = 0) and anion ( n = 1) adcluster
states.

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FIGURE 14. Mechanism of the complete oxidation of propylene over a silver surface; (a) with molecularly adsorbed
oxygen, (b) with atomically adsorbed oxygen.

For the epoxidation of ethylene, the active
species is the superoxide, which is molecularly
adsorbed on the surface in the bent end-on geometry. Ethylene attacks the terminal oxygen atom of
the superoxide leading to ethylene oxide quitesmoothly. The importance of the silver surface isquite evident. On the other hand, the atomicallyadsorbed oxygen species is not selective: Bothepoxidation and complete oxidation products
would be obtained. However, to promote the selectivity of the atomically adsorbed species willcertainly promote the total selectivity to epoxidation product. In the process leading to ethyleneoxide, the electron back-transfer from the reaction
adsorbate complex to the metal surface should be
important.
The origin of the catalytic activity of the silver
surface is due to its ability to stabilize the superoxide species. On Cu surfaces, peroxide is much
more stable than superoxide, so that the lifetime ofthe superoxide species should be too short to reactwith ethylene. On Au surfaces, the superoxide
species does not exist because the molecular adsorption of O 2 does not occur due to the difficultyof the electron transfer from the clean bulk metal.
For the oxidation of propylene on silver surface,
two competitive mechanisms exist. The dominantmechanism for the epoxidation of olefins is initiated by the activation of the olefinic carbon. The
dominant mechanism for the complete oxidationof propylene is due to the activation of the allylic
C—H bond. The latter is more favorable due tothe formation of the stable adsorbed allyl interme-854

diate, which is the motivation force for the complete oxidation of propylene.
We note that the proposed two competitive
mechanisms not only reflect the difference between ethylene and propylene but also explain
rather well the available experimental facts for the
epoxidation and complete oxidation of olefins.
Therefore, it reflects a general mechanism to understand the oxidation of olefins over silver surface.
ACKNOWLEDGMENTS
Some calculations were performed using thecomputers at the Institute for Molecular Science.
Part of this study was supported by a Grant-in-Aid
for Scientific Research from the Japanese Ministryof Education, Science, and Culture and by the
New Energy and Industrial Technology Development Organization ŽNEDO..

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