Surface Science 401 (1998) 371–391

Oxidation mechanism of propylene on an Ag surface:dipped adcluster model study
Zhen-Ming Hu a, Hiromi Nakai a,1, Hiroshi Nakatsuji a,b,c,*a Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku,
Kyoto 606-01, Japanb Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japanc Institute for Fundamental Chemistry, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606, Japan
Received 14 April 1997; accepted for publication 18 December 1997

Abstract
The mechanisms of the epoxidation and complete oxidation of propylene over a silver surface are studied by the dipped adclustermodel (DAM ) combined with the ab initio HF and MP2 methods. Several postulated reaction pathways involving both molecularly
and atomically adsorbed oxygens are investigated. The energy diagrams of the epoxidation and complete oxidation of propylene on
the silver surface are presented, and the detailed electronic mechanisms for the oxidation of propylene are clarified. The calculations
show that the mechanism involving the single precursor intermediate common to both epoxidation and complete oxidation proposedin some literatures can be ruled out. Instead, the two competitive mechanisms are suggested: the epoxide formation is initiated by
the reaction of the olefinic carbon with the adsorbed oxygen, while the combustion is initiated by the abstraction of the allylichydrogen by the adsorbed oxygen. The former mechanism is essentially the same as that in the epoxidation of ethylene reported
previously. The latter mechanism is energetically more favorable for propylene, which is a reason for the low selectivity of theepoxidation of propylene. The adsorbed allyl species, which is an intermediate in the latter mechanism, is demonstrated to exist in
the form of chemisorbed anion. The mechanisms presented in this paper seem to be useful for explaining the epoxidation andcomplete oxidation of other olefins. A comparison with the previous studies is also presented. © 1998 Elsevier Science B.V. All
rights reserved.
Keywords: Ab initio Hartree–Fock (HF ) and second-order Møller–Plesset (MP2) methods; Atomic oxygen; Catalytic activity ofsilver; Dipped adcluster model (DAM ); Epoxidation vs. complete oxidation; Olefins; Propylene; Superoxide

1. Introduction
The epoxidation of olefins on silver catalyst is
an exceedingly important catalytic reaction inindustry, and as such has received long and extensive study (for reviews, see Refs. [1–7]). During the* Corresponding author. Fax: (+81) 75 7535910.
1 Present address: Department of Chemistry, School of
Science and Engineering, Waseda University, Ohkubo 3-4-1,
Shinjuku-ku, Tokyo 169, Japan.
0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 98 ) 0 00 2 5 -9

past 20 years, much research [1–32] has been
devoted to gain a better understanding of theepoxidation mechanism with the ultimate goal ofincreasing the selectivity for the epoxide formation.
One particularly interesting aspect of the epoxidation reaction is its uniqueness: not only is silvera uniquely effective catalyst for heterogeneousepoxidation with molecular oxygen as oxidant, butalso, ethylene is the only hydrocarbon that may
be epoxidized with a high selectivity [1–7]. Recent
experimental studies show that, except for ethy-

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Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

lene, a few olefins such as styrene [33,34],
3,3-dimethylbutene [35], norbornene [36 ] andbutadiene can be epoxidized over silver with highselectivity. In contrast, other olefins such as propylene, butenes, and pentenes are epoxidized on silver
with extremely low selectivity [8–14]. The selectivity reported for the formation of propylene oxidefrom propylene on silver is from 2 to 4% [8–11].
Due to this low selectivity, the industrial value of
this reaction for the production of propylene oxide
is quite limited, and considerably less attention
has been paid to the oxidation of propylene on
silver than to that of ethylene.
Previous studies have suggested that the lower
epoxidation selectivity for propylene vs. ethylene
is due both to a decreased rate of epoxidation of
propylene [8] and to an increased rate of propylene
combustion [8,12]. The increased favorability
for propylene combustion has been attributed[8,14,27,28] to a greater reactivity of the allylichydrogens in propylene compared to that of thevinyl hydrogens in ethylene. This was consistent
with the observation of the isotope effect in theepoxidation of propylene: the selectivity increases
by as much as 14% when the allylic hydrogen is
replaced with deuterium [8,9].
For the oxidation of ethylene and propylene,
two mechanisms involving molecular oxygen [8–18] and atomic oxygen [22–31] as the active species
have been suggested. The mechanistic sketch summarized by Ayame [2–4] leads to a good insight
into the mechanism of the epoxidation of ethylene.
Based on the mechanism involving molecularoxygen, Cant and Hall [8] proposed that thedominant epoxidation mechanisms were similar
for both ethylene and propylene, and the adsorbed
species containing peroxide linkage was thecommon intermediate. The very low yield of epoxide from propylene was supposed to be due to theintramolecular isomerization of the peroxide intermediate to allylhydroperoxide by breaking theC–H bond of the methyl group. Akimoto et al.
[14] proposed that the complete oxidation of propylene was initiated by the abstraction of allylichydrogen of the adsorbed propylene by theadsorbed molecular oxygen.
Carter and Goddard [29] have used the generalized valence bond (GVB) method to calculate theenergetics of the elementary steps for the olefinoxidation on the silver surface, and suggested thatthe surface atomic oxyradical anion (SAO) is theactive species for the formation of ethylene oxide.
The possible reaction mechanism for the combustion of higher olefins such as propylene, competing
effectively with the epoxidation, was supposed tobe due to the formation of alkoxide intermediate.
Recently, in contrast to the result of Carter andGoddard, Madix et al. [27,28] showed that the
combustion of propylene via an allylic intermediate
was facile on Ag(110) with the oxygen coverages
from 0.05 to 0.5 ML, and proposed that the dominant mechanism for the combustion of propylene
and higher alkenes over silver is the activation of
the allylic C–H bond via an acid–base reaction.
Theoretical studies on the epoxidation reactionsof olefins are quite limited. Though a few papers[29–32] have been published for the epoxidationof ethylene, almost no discussions were presented[29,32] for the oxidation of propylene, and thecalculations did not explicitly include the surface
contribution. The main difficulty in the theoreticalstudies was how to describe chemisorption species
and calculate adsorption energy correctly using
only small clusters. This may be the reason why
no ab initio theoretical study for the full reaction
pathway and the associated energy diagrams of
the epoxidation reaction of olefins has beenreported.
In a series of our previous studies [37–46 ], atheoretical model named dipped adcluster model(DAM ) [37,38] has been proposed to study chemisorptions and surface reactions involving electrontransfer between admolecules and surfaces. The
DAM has been successfully applied to the harpooning, surface chemiluminescence and electron
emission in the halogen/alkali-metal surface system[42], and to the study of the oxygen chemisorption
species on a silver surface [39–41]. Most recently,
we used this model to study the mechanisms of
the reaction of ethylene both with molecularly andatomically adsorbed oxygen on a silver surface[43], and for the first time, we could obtain the
total reaction pathway and energy diagrams forthe epoxidation reaction of ethylene on a silver
surface. The uniqueness of the silver surface for
ethylene epoxidation reaction has been clarified by

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

a comparative study of the stability and the activityof oxygen species on Cu, Ag, and Au surfaces[44]. The nature of the transition state ( TS) in thereaction of the atomically adsorbed oxygen with
ethylene to ethylene oxide has been studied indetail [47], since it is the key step to increase theoverall selectivity in the epoxidation of ethylene
on a Ag surface. The electron back-transfer fromthe adcluster to the bulk Ag surface at this TS has
been clarified [47]. Thus, the DAM was essential
for the study of these surface reactions. Even asmall DAM could be used to give correct electronic
structures and reliable adsorption energies in contrast to the cluster model [45].
In this paper, we investigate the mechanisms forthe epoxidation and complete oxidation of propylene on a silver surface using the DAM combinedwith the ab initio Hatree–Fock (HF ) and secondorder Møller–Plesset (MP2) calculations. The aim
is to address why some higher olefins mainly
undergo complete oxidation instead of epoxidation. Several postulated pathways for both theepoxidation and complete oxidation are investigated, and both molecularly and atomicallyadsorbed oxygen species are examined. The paper
is organized as follows. Section 2 gives the outlineof the computational details. In Section 3, we study
the mechanism similar to that of ethylene reported
in the former paper [43]. In Section 4, the mechanistic details for the activation of allylic C–H bondare reported. An overall discussion based on thepresent results compared with the previous studies
is given in Section 5, and finally in Section 6, wegive the conclusion of the present study.

2. Computational details
We use the DAM [37,38] that has been proposed
as a theoretical model for studying chemisorptions
and surface reactions involving electron transfer
between bulk metal and admolecules with consideration of the image force correction. Suchinteractions were found to be very important in
the previous studies [37–46 ] for studying the chemisorptions and the reactions of the oxygen on ametal surface.
As active oxygen species, we study molecularly

373

Fig. 1. Adsorption structure of oxygen on silver surface. (a)Molecularly adsorbed oxygen in end-on bent geometry. (b)On-top form of atomically adsorbed oxygen.

adsorbed superoxide in an end-on bent structure( Fig. 1a) and the atomically adsorbed oxygen( Fig. 1b). The validity of the similar model has
been shown in the study of the mechanism of
ethylene epoxidation on Ag [43], Cu, and Au [44].
The electron exchange between the adcluster andthe bulk metal is taken into account by the DAM.
Since we use the highest spin coupling model[37,38], the number of electrons transferred into
the adcluster is unity (n=1) as shown previously[39–41]. The electrostatic interaction between theadmolecule and the extended surface is estimatedby the image force correction [38]. We calculate
the potential energy of the adcluster by the abinitio HF and MP2 methods. The geometries of
the reactants, products, intermediates, and transition states ( TSs) are optimized by the energy
gradient method at the HF level except for the
Ag–Ag distance and the Ag –Ag –O angle that2
1 awere fixed at 2.8894 Å and 90.0°, respectively. The
electron correlations are considered by the MP2
method. The energy scale is, in all cases, in kilocalories per mole (kcal mol−1), and the bond distancescale is in angstroms (Å). The calculations are374

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

performed using the Gaussian 92 software package [48].
The Gaussian basis for the silver atom is(3s3p4d)/[3s2p2d ] set, and the Kr core was
replaced by the relativistic effective core potential[49]. For oxygen, we used the (9s5p)/[4s2p] set of
Huzinaga–Dunning [50,51] augmented by the
diffuse s and p functions of a=0.059 as anion
bases [52]. The polarization d functions of a=
2.704, 0.535 [53] were added to the oxygen basis
in the MP2 calculations. For carbon and hydrogen,the (9s5p)/[4s2p] and (4s)/[2s] sets, respectively, of
Huzinaga–Dunning were used [50,51]. The validity of the present basis functions as well as theeffects of electron correction have been carefully
examined in the former paper [43]. The heats ofreactions for the epoxidation and complete oxidation of propylene were calculated to be 23.9 and456.6 kcal mol−1, respectively, in the MP2 calculation using the present basis set, which compare
well with the experimental values, 27.5 and460.3 kcal mol−1, respectively.

3. Mechanisms via the CNC double bond breakage
A common view of the epoxidation mechanism
assumed that the reaction takes place between theadsorbed oxygen species and the carbon of the
CNC double bond of olefins. Our previous study
has shown that this was certainly true for the
epoxidation of ethylene [43]. Some studies [8–11,29,32] had assumed that the epoxidation mechanism for ethylene and propylene may be similar.
We begin in this section with the investigation of
the mechanisms for the epoxidation and complete
oxidation of propylene via the CNC double bondbreakage and compare the result with those ofethylene.
3.1. Reaction with superoxide O− on silver
2
A difference between propylene and ethylene isthat propylene has two different olefinic carbons.
Therefore, in double bond opening, two radical
intermediates, i.e. CH CH · –CH –O–O–Ag and32· CH –CH(CH )–O–O–Ag should be considered.
2
3Then, we study here the reactions of two different

Fig. 2. Model adcluster for the olefinic carbon attack (a) on theterminal olefinic carbon, and (b) on the central olefinic carbon.

olefinic carbons with the molecularly adsorbed
superoxide species on a silver surface. The mostfavorable pathway for the attack on the CNC
double bond takes the form of Figs. 2a or b.
Fig. 3 shows the energy diagrams for the oxidation reactions between molecular superoxide andpropylene at the terminal carbon atom (Fig. 2a).
The diagram leading to propylene oxide (PRO) isdenoted by a solid line, and that to aldehyde andthen to complete combustion is denoted by thedashed line. The optimized structures of the intermediates and the TSs are also illustrated.
An obvious point deduced from Fig. 3 is thatthe energy diagram for the oxidation of propylene
is very similar to that of ethylene reported in theprevious paper [43]. It certainly reflects somecommon features between the reactions of the two

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

375

Fig. 3. Energy diagram for the reaction between terminal carbon atom of propylene and molecularly adsorbed superoxide on Ag
surface. The routes leading to propylene oxide and aldehyde are shown by the solid and broken lines, respectively.

olefins when it takes place via the CNC doublebond breakage.
The reaction proceeds by the two-step mechanism. The first step is the formation of the superoxide species on the Ag surface in which one electron
is transferred from the bulk metal to the adcluster.
The calculated stabilization for this step is
26.9 kcal mol−1, which is the same as in the previous paper [43]. The next step is the attack of theolefin on to the terminal oxygen atom O , thebreactive site of the superoxide, in the geometry
shown in Fig. 2a. The process reaching the TS
includes the formation of a s bond between Oband C and the breaking of the C–C p bond, and1
the energy barrier for this step is 13.7 kcal mol−1
at R of 1.78 Å. The system then arrives at thecointermediate shown in Fig. 3 with the calculated
stabilization energy of 28.8 kcal mol−1. In thisstate, one carbon atom C of propylene is bound1
to the terminal oxygen O of the molecularlybadsorbed oxygen, and the p bond between C and1
C is broken with C being the radical center. The
2
2
C–C bond of this intermediate rotates almost
without any barrier, giving a mixture of the geo-

metrical isomer as shown previously for thereaction of ethylene [43]. In fact, bothcis-/trans-PRO-D and cis-/trans-PRO-D mole2
5
cules were produced from the oxidation ofcis-CH CDCHD and cis-CD CDCHD, respec3
3
tively [9].
When this intermediate is produced, theformation of propylene oxide is a fast process,leaving the atomic oxygen on the Ag surface:the energy barrier of this process is only
3.8 kcal mol−1. The heat of reaction is calculated
to be 39.5 kcal mol−1, which represents the heatof reaction of gaseous propylene oxide plus halfof the dissociative adsorption energy of O on
2
Ag , i.e. 23.9 and 15.6 kcal mol−1, respectively, by2
the MP2 method.
In contrast, the process leading to propylene
aldehyde from this intermediate must overcome alarge energy barrier as high as 69.4 kcal mol−1. In
Fig. 3, this process is shown by a broken line. Theenergy diagram up to this intermediate is common
to the above case. The high barrier is caused by adifficulty of a-H migration, though afterwards, theprocess is energetically favorable giving aldehyde,

376Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Fig. 4. Energy diagram for the reaction between central carbon atom of propylene and molecularly adsorbed superoxide on Ag
surface. The route leading to propylene oxide and aldehyde are shown by the solid and broken lines, respectively.

which is considered to be further oxidized to
CO and H O. Thus, the path leading to complete
22oxidation is energetically forbidden.
The result given in Fig. 3 thus implies that theepoxidation of propylene should occur in highselectivity as for ethylene, but this is in contradiction with the experimental fact. Why? We want toanswer this question in Section 4, but before that,
let us examine another reaction path defined by
Fig. 2b.
The energy diagram for the reaction involving
the attack of the superoxide on the central carbon
of propylene as shown in Fig. 2b is shown in
Fig. 4. Comparing Fig. 4 with Fig. 3, we see thatthe two energy diagrams are essentially thesame except for some rather small differences.
First, the energy barrier leading to epoxide is
15.1 kcal mol−1, which is 11.3 kcal mol−1 higher
than that for the pathway shown in Fig. 3. Theenergy barrier for the a-H migration is, however,almost the same as in Fig. 3, but the stability ofthe products increases by about 9 kcal mol−1.
Though the two paths give the same epoxidation
product, the aldehyde products are different. The

first route gives propylene aldehyde, while thesecond gives acetone. Besides these differences, thesecond reaction given in Fig. 4 also shows a highly
selective production of propylene oxide in contrast
to the experimental fact.
The optimized geometries in these pathways areshown in Figs. 3 and 4. Table 1 shows the normal
mode having the negative force constant at theTS. We refer to such a normal mode as an imaginary mode. The changes in the O –O , O –C anda b b 1
C –C distances, and the mode of the negative
1 2
force constant at TS, clearly reflect the progressof the reaction along the path. Tables 2 and 3
show the net charge and the frontier density (spinpopulation) of the Ag O C H adcluster, calcu2 2 3 6
lated by the MP2 method as the reaction proceeds
as shown in Figs. 3 and 4, respectively. In comparison with the change in the net charge, the frontierdensity changes significantly as the reaction proceeds: the reactive site shifts with the spin movingfrom one atom to another. From Table 2, we seethat in the adsorbed superoxide, the reactive site
is O with a frontier density of +0.784, while inb
the intermediate state, the C atom becomes most2

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Z.-M. Hu et al. / Surface Science 401 (1998) 371–391
Table 1
Normal modes having the negative force constants at the TSs shown in Figs. 3 and 4a
TS structure

Eigen value

Term

Eigen vector

TS1 for approach−0.092TS1 for dissociation−0.170TS2 for approach−0.169TS2 for dissociation−0.145

C –O stretching
1 b
C –C stretching
1 2
O –O stretchinga b
O –O –C bendinga b 1
C –O stretching
1 b
C –C stretching
1 2
O –O stretchinga b
O –O –C bending
a b 1

0.973
−0.168−0.9640.140
0.950−0.166−0.9800.118

aThe TSs shown in Fig. 3 are denoted as TS1, and those shown in Fig. 4 are denoted as TS2, respectively.

Table 2
Net charge and frontier density (spin population) for the Ag O –C H adcluster shown in Fig. 3
2 2 3 6
Adsorbed oxygen

TS for appro.

Intermediate

TS for dissoc.

TS for H mig.

IM for CH CH CHO
3
2

Net chargeAg−0.0281Ag−0.1972
O−0.558aO−0.217bC−0.496
1
C−0.021
2
C−0.580
3
H
0.182
11
H
0.191
12
H
0.180
21
H
0.175
31
H
0.183
32
H
0.183
33

−0.058−0.275−0.616−0.319−0.085−0.119−0.641
0.186
0.191
0.161
0.227
0.184
0.164

−0.033−0.307−0.641−0.2900.003−0.147−0.635
0.152
0.158
0.160
0.236
0.179
0.165

−0.035−0.316−0.658−0.273−0.005−0.147−0.634
0.162
0.166
0.159
0.223
0.186
0.165

−0.014−0.273−0.714−0.2130.088−0.343−0.602
0.184
0.170
0.180
0.209
0.165
0.209

−0.021−0.276−0.635−0.261
0.113−0.397−0.621
0.162
0.163
0.170
0.265
0.169
0.169

Frontier density
Ag+0.1821Ag−0.1692
O+0.203aO+0.784
b
C
0.0
1
C
0.0
2
C
0.0
3
H
0.0
11
H
0.0
12
H
0.0
21
H
0.0
31
H
0.0
32
H
0.0
33

−0.0090.021
0.475−0.183−0.1550.953−0.1070.003
0.001−0.072
0.007
0.048
0.018

0.017
0.022
−0.0220.055−0.1071.183−0.1330.006
0.006−0.085
0.019
0.063
0.010

−0.0360.051−0.291
0.323−0.1271.191−0.1350.011
0.006−0.087
0.015
0.063
0.016

−0.004
0.002
0.052
0.063
0.974
−0.0890.042−0.015−0.0820.056−0.0040.007−0.0020.006−0.009
0.065
0.044
1.004
−0.1180.005−0.0680.032
0.040−0.001−0.001
0.000

reactive with a spin population of 1.183. A similar
behavior is also seen from Table 3. The energy
barrier is actually the barrier for the spin transfer.
We note that the net charges of the Ag atoms

are relatively small ( less than ~0.3) throughout
the reaction, and the frontier density on silver isalmost zero in the course of the reaction. Thismeans that the additional electron transferred from

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Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Table 3
Net charge and frontier density for the Ag O –C H adcluster shown in Fig. 4
2 2 3 6
Adsorbed oxygen

TS for appro.

Intermediate

TS for dissoc.

TS forH mig.

IM. forCH C(O)CH3
3

Net chargeAg−0.0281Ag−0.1972
O−0.558aO−0.217bC−0.496
1
C−0.021
2
C−0.580
3
H
0.182
11
H
0.191
21
H
0.180
22
H
0.175
31
H
0.183
32
H
0.183
33

−0.082−0.315−0.594−0.2630.273−0.493−0.687
0.178
0.182
0.179
0.287
0.156
0.181

−0.058−0.307−0.629−0.2690.302−0.448−0.672
0.144
0.169
0.175
0.257
0.156
0.181

−0.120−0.305−0.350−0.481
0.373−0.485−0.699
0.141
0.172
0.171
0.256
0.154
0.178

−0.050−0.260−0.709−0.301
0.472−0.816−0.716
0.436
0.185
0.152
0.246
0.171
0.171

−0.053−0.290−0.664−0.2350.496−0.724−0.699
0.220
0.187
0.177
0.238
0.148
0.200

Frontier density
Ag+0.1821Ag−0.1692
O+0.203aO+0.784
b
C
0.0
1
C
0.0
2
C
0.0
3
H
0.0
11
H
0.0
21
H
0.0
22
H
0.0
31
H
0.0
32
H
0.0
33

−0.0110.045
0.490−0.198−0.1600.973−0.0040.000−0.066−0.066−0.001−0.0040.001−0.0200.034−0.0720.101−0.117
1.232
0.003
0.006
−0.084−0.084−0.0010.002
0.000−0.0150.029
0.304−0.301−0.0621.209−0.002−0.001−0.083−0.081−0.0000.005−0.003−0.0080.017
0.067−0.021
0.978−0.090−0.093
0.034
0.032
0.023
0.023
0.009
0.030

−0.0130.014−0.0070.133
0.937−0.109−0.094
0.010
0.060
0.007
0.008
0.030
0.024

the bulk metal to the adcluster in the DAM lies
mainly on the admolecule throughout the courseof the reaction.
3.2. Reactions with atomically adsorbed oxygen
We investigate in this section the energetics andthe pathway of the reaction of the olefinic carbon
with the atomically adsorbed oxygen on an Ag
surface. The net charge and the frontier density of
the atomically adsorbed oxygen on the silver surface are calculated to be −0.642 and +1.031,
respectively. The charge is largely negative, morenegative than the O (inside) atom of the superoxa
ide shown in Table 2. The spin exists on thein-plane 2p orbital, which is parallel to the Ag–Ag
bond. Therefore, the most favorable approach of

propylene to form the C –O bond takes place in1
the form illustrated in Fig. 5.
Fig. 5 shows the geometries and the energy
diagram in the course of the reaction. Table 4shows the net charge and the spin population ofeach atom. In the TS approach, the C –O bond is1
formed by the interaction between the in-plane porbital of O and the p and p* orbitals of propylene.
The C –O distances in the TS and the intermediate1
are calculated to be 1.86 and 1.42 Å, respectively.
The C–C bond distance changes from 1.35 Å in
the reactant to 1.42 Å in the TS, and further to
1.52 Å in the intermediate: it changes from the
double-bond length to the single-bond one. The
calculated stabilization energy of the intermediate
is 35.1 kcal mol−1. The energy barrier to this intermediate is calculated to be −1.0 kcal mol−1, which

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

379

Fig. 5. Energy diagram for the reaction between terminal carbon atom of propylene and atomically adsorbed oxygen on Ag surface.
The routes leading to propylene oxide and aldehyde are shown by the solid and broken lines, respectively.

means that no or little barriers exist in this pathand reflects the high reactivity of the atomicallyadsorbed oxygen. The negative energy barrier hereis due to the present somewhat inconsistent calculations: the geometry was optimized by the HF
method, and the energy was calculated by the MP2
method. In the HF level, this energy barrier was
27.4 kcal mol−1, a positive value.
The next step from this intermediate was very
important for the ethylene oxidation in the former
paper [43], since it determines the selectivity of
the reaction by the atomically adsorbed oxygen.
If C attacks the oxygen to form the C –O bond,2
2 bpropylene epoxide is formed. If C attacks the a2
C–H bond, causing the hydrogen migration asshown in Fig. 5, the aldehyde is a product that isfurther oxidized to CO and H O, the complete
22oxidization product.
For the hydrogen migration reaction, the barrier
was calculated to be 37.6 kcal mol−1, which islower than the barrier 69.4 kcal mol−1 of the reaction in which the superoxide species is involved(see Fig. 3). This means that the complete oxidation has a greater chance of taking place by thereaction with atomically adsorbed oxygen. The
structure of the TS to the epoxide formation was,however, difficult to determine, and the corresponding energy barrier could not be presented.
However, if we take the crossing point of then=1 and n=0 curves as the barrier of this process,the energy barrier is almost the same as that forthe complete oxidation path, showing a competition of the two reaction path. This shows that theatomically adsorbed oxygen is not selective forthe epoxidation of propylene, as it was so for the
epoxidation of ethylene [43].

4. Mechanism via allylic C–H bond activation
The results presented above show that the reaction between propylene and adsorbed oxygen via
the CNC double bond breakage is favorable forthe epoxide formation likewise in the reaction of
ethylene. However, experimentally, the oxidationof ethylene and propylene over the same catalysts
produces markedly different products: propylene
is mainly oxidized to CO and H O. The observed
2
2

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Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Table 4
Net charge and frontier density for the Ag O–C H adcluster shown in Fig. 52
3 6Adsorbed oxygen

TS for approach

Intermediate

TS for H migration

Intermediate for CH CH CHO
3
2

Net charge
Ag−0.058
a
A
0.300
b
O
−0.642aC−0.496
1
C−0.021
2
C−0.580
3
H
0.182
11
H
0.191
12
H
0.180
21
H
0.175
31
H
0.183
32
H
0.183
33

−0.195−0.248−0.651−0.180−0.117−0.655
0.150
0.158
0.156
0.163
0.255
0.164

−0.219−0.272−0.651−0.029−0.140−0.636
0.107
0.136
0.165
0.165
0.253
0.154

−0.150−0.252−0.638−0.010−0.296−0.642
0.154
0.126
0.205
0.162
0.169
0.173

−0.167−0.269−0.5990.066−0.380−0.630
0.094
0.154
0.145
0.284
0.152
0.151

Frontier densityAg−0.108
a
Ag
0.077
b
O
1.031
a
C
0.0
1
C
0.0
2
C
0.0
3
H
0.0
11
H
0.0
12
H
0.0
21
H
0.0
31
H
0.0
32
H
0.0
33

0.356
−0.3310.460−0.3170.902−0.0960.015
0.011−0.066
0.030
0.034
0.001

0.036
−0.0150.022−0.0851.155−0.1280.008
0.000−0.082
0.027
0.059
0.004

−0.008
0.009
0.105
0.442
0.545
−0.041−0.054−0.002−0.031
0.001
0.004
0.028

−0.017
0.015
0.165
0.938
−0.1080.006−0.061
0.034
0.031−0.001−0.001−0.001much lower selectivity, i.e. 2–5%, of propylene toepoxide than that of ethylene is therefore notunderstandable based on the reaction mechanism
discussed above.
In this section, we investigate a different oxidation pathway of propylene via an allylic hydrogen
activation. The reaction pathway first posturated
and finally concluded in this section is illustrated
in Fig. 6. The reaction with the molecularly
adsorbed superoxide species is given in the upper
part, and that with the atomically adsorbed oxygenis given in the lower part. Though the detailed
explanation is given below, the point is the formation of the (O)–O–H species and the allylic anion
adsorbed on a Ag surface. The symmetry of thereaction is C : the allylic CCH and (O)–O–Ag
1
2
are in the same plane, and the other part of
propylene is out of plane.

4.1. Reaction with molecularly adsorbed oxygen
We use the model adcluster illustrated in Fig. 7a
to study the reaction of the allylic hydrogen of
propylene with the molecularly adsorbed superoxide. A two-step path is involved there as illustrated
in Fig. 6a: the allylic hydrogen abstraction stepand the O–O decomposition step. In the first step,
the allylic hydrogen of propylene is abstracted
by the terminal oxygen of the superoxide, leading
to the adsorbed hydroperoxyl species [ HOO ] and(a)
the adsorbed allyl species [C H ]. The next step
3 5(a)
is the decomposition of the adsorbed hydroperoxyl
group to the adsorbed surface hydroxyl group[ HO ] and the adsorbed atomic oxygen [O ].
(a)(a)
Two surface hydroxyl groups can be disproportionated to produce water and one adsorbed
oxygen [28,29]. The adsorbed oxygen atom pro-

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

381

Fig. 6. Mechanism of the complete oxidation of propylene over silver surface. (a) Reaction with molecularly adsorbed superoxide.
(b) Reaction with atomically adsorbed oxygen.

Fig. 7. Model adcluster for the allylic H attack mechanism. (a)Reaction with molecularly adsorbed superoxide. (b) Reaction
with atomically adsorbed oxygen.

duced in this process has the reactivity to process
a further oxidation reaction, which will be studied
below in this section. The adsorbed hydrocarbon
species should be readily oxidized to CO and2
H O in the presence of adsorbed oxygens, though2
the detailed mechanism of the further oxidation is
not clear.
Fig. 8 shows the fully optimized geometries andthe energy diagram for the first reaction step. The
process reaching to the TS includes the formationof a s bond between an allylic hydrogen and theterminal oxygen atom (O ) and the breakage ofbthe s bond between this hydrogen and C atom.
1
The H –O distance is 1.17 Å, and the C –H
a b
1 a
distance is 1.38 Å which is elongated by about
0.3 Å in comparison with 1.09 Å in the free propylene molecule. The O –O distance is also elongateda bfrom 1.39 Å in the adsorbed superoxide state to
1.47 Å. The C –C and C –C distances are also
1 2
2 3
different to the typical C–C single and CNC
double bonds, respectively, but the charges are
small. The imaginary mode at TS shown in Table 5
clearly shows the nature of the TS in this reaction
path, i.e. O –H and C –H stretching.
b
1
The energy barrier in this first step is calculated
to be 10.8 kcal mol−1, which is smaller than thoseof 13.7 and 12.7 kcal mol−1 for the reactions leading to the epoxide shown in the preceding section.
This shows that the pathway via the abstractionof the allylic hydrogen is easier. The stabilization

382

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Fig. 8. Energy diagram for the reaction between molecularly adsorbed oxygen and allylic hydrogen of propylene on silver surface.
Table 5
Normal modes having the negative force constants at the TSs shown in Figs. 8 and 9
TS structure

Eigen value

Term

Eigen vector

TS for approach−0.232

TS for O–O dissociation−0.140O –H stretchingb
H–C stretching1
H –C –C bending
11 1 2
H C C O torsion
11 1 2 b
O –O stretchinga b
H–O –O bending
a b−0.6820.639
0.151−0.257−0.9280.369

energy after the barrier at the intermediate is
29.7 kcal mol−1 as shown in Fig. 8 in comparison
with 28.8 and 30.0 kcal mol−1 for the stabilizationenergy of the intermediates shown in Figs. 3 and4. The geometry of this intermediate state given in
Fig. 8 shows that the allyl group is adsorbed tothe surface very weakly, due to the small cluster
used here. We note that the allyl group would be
more stably adsorbed on the surface as discussed
later, and the actual stabilization energy in theadsorbed intermediate state would be larger thanthe energy obtained here.
Fig. 9 shows the calculated results for the second
reaction step: the decomposition of the adsorbedhydroperoxyl group. The geometry optimization
was carried out assuming the O–Ag–Ag angle tobe rectangular. The hydroperoxyl group in theadsorbed intermediate shown in Fig. 8 is not stablebecause of the interaction with the allyl anion
species adsorbed on the adjacent silver atom. The

optimized geometry shown in Fig. 9, in which nosuch interaction exists, is very different to that in
the final adsorbed intermediate shown in Fig. 8.
In Fig. 9, the O –O distance and the O O Aga bb aangle are 1.532 Å and 84.9°, respectively, in comparison with 1.50 Å and 102.5° in Fig. 8. Thischange is caused by the interaction between Oband the adjacent silver atom Ag . At the TS for
2
dissociation, the O–O distance increases and the
O–Ag distance decreases, and the energy barrier1
is only 2.8 kcal mol−1. After the TS, the system
gains a very high stabilization energy of
42.6 kcal mol−1 and becomes the intermediate
composed of the adsorbed hydroxyl group and theadsorbed oxygen atom. The net charge and frontierdensity shown in Table 6 reflect a shift of thereactive site from silver atom Ag to O . The net
2acharge and the frontier density of the adsorbed
oxygen atom are −0.662 and +1.089, similar to−0.642 and +1.031 (see Table 4) of the free

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

383

Fig. 9. Energy diagram for the decomposition of hydroperoxyl group on silver surface.

atomically adsorbed oxygen species. This atomically adsorbed oxygen should be active.
The mechanism that the complete oxidation of
olefin proceeds via allylic hydrogen abstraction bymolecular superoxide is supported by some experimental results. Tezuka et al. [54] showed in theoxidation of 9,10-dihydroanthracene that thesuperoxide ion is a hydrogen abstracting agent,and the initial step is the formation of radical
intermediate and HO− group. Moro-oka et al.
2[55] further studied the reactivity of the superoxide
ion with some substrates having labile hydrogens,and indicated that the superoxide ion abstractshydrogen from the substrate only when the formed
radical can be stabilized by conjugating its pelectrons. Hydrogen abstraction by the superoxide
ion depends obviously on the C–H bond dissociation energy of the substrate, the borderline being
roughly 80 kcal mol−1 at room temperature [55],which is similar to the C–H bond energy of
propylene studied here.
4.2. Reaction with atomically adsorbed oxygen
The atomically adsorbed oxygen on the Ag
surface is produced through the peroxide speciesof the molecularly adsorbed oxygen [42]. It is also
produced, as shown in Fig. 9, as a product of theoxidation reaction of propylene with the molecu-

larly adsorbed superoxide species. These twoadsorbed atomic oxygen species have the same
electronic structure and therefore, the same reactivity. We study in this section the reaction of theatomically adsorbed oxygen with propylene. A
pictorial sketch of the reaction path is shownin Fig. 6b.
Fig. 10 shows the optimized reaction path andthe energy diagram for the reaction of propylene
with atomically adsorbed oxygen. Table 7 shows
the net charge and frontier density for this reaction
path. The geometry of the TS is similar to that of
the reaction with the molecularly adsorbed oxygen
shown in Fig. 8: it shows a formation of the sbond between allylic hydrogen and oxygen atom
and a breakage of the s bond between thishydrogen and C atom. The geometry of the1
adsorbed allyl group is also similar to that inFig. 8, but the C –Ag distance is shortened to12
2.74 Å, which shows a stronger interaction between
the allyl group and the surface. The reason of thischange would be the neighboring effect of theadjacently hydroperoxyl adsorbed group, i.e. thesteric and electrostatic effects of the hydroperoxyl
group (HOO), as compared to hydroxyl one (HO),lead to a longer allyl carbon-surface distance. The
calculated energy barrier in this reaction path is
only +0.1 kcal mol−1, which means that essentially no barrier exists in this reaction path. A

384

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Table 6
Net charge and frontier density for the adcluster shown in Figs. 8 and 9
Adsorbed
oxygenTS forH-appro.

Adsorbedintermediate

Adsorbed HOO
species

TS for HO
2
dissoc.

Adsorbed O andHO species−0.028−0.197−0.558−0.2170.182−0.580−0.021−0.496
0.183
0.183
0.175
0.191
0.180

−0.073−0.224−0.597−0.3840.405−0.5490.009−0.523
0.205
0.206
0.168
0.169
0.187

0.012
−0.291−0.611−0.5740.443−0.323−0.068−0.494
0.196
0.200
0.163
0.163
0.184

−0.053−0.210−0.689−0.5160.467−0.107−0.180−0.645−0.541
o.473

0.092
0.187
−0.662−1.1090.493

Frontier density
Ag+0.1821Ag−0.1692
O+0.203aO+0.784
b
H
0.0
a
C
0.0
1
C
0.0
2
C
0.0
3
H
0.0
11
H
0.0
12
H
0.0
21
H
0.0
31
H
0.0
32

0.487
−0.5120.424−0.0280.000
0.560−0.1850.349−0.028−0.030−0.001−0.018−0.017−0.0130.022−0.012
0.008
0.000
0.829
−0.4080.780−0.057−0.0580.017−0.052−0.0530.876
1.155−0.0670.040−0.004

0.903
1.118
0.067
−0.093
0.005

0.457
0.388
1.089
0.070
−0.003

Net charge
Ag
1
Ag
2
O
a
O
b
H
a
C
1
C
2
C
3
H
11
H
12
H
21
H
31
H
32

similar result was also presented in Section 3.2.
The stabilization energy of the adsorbed intermediate is 38.3 kcal mol−1 as shown in Fig. 10, whichis slightly more stable than the adsorbed intermediate shown in Section 3.2 (35.1 kcal mol−1 asshown in Fig. 5).
The formation of the adsorbed stable allyl groupis a key factor for the reaction path discussed
above. In order to understand the nature of theinteraction between the allyl species and the silver
surface more clearly, we further carried out aseparate comparable calculation using Ag C H
2 3 5
adcluster.
Fig. 11 shows the results for the Ag C H
2 3 5
adclusters calculated with and without electron
supply from the bulk metal, respectively. The net
charge and frontier density are listed in Table 7.

The geometries are very similar in the twoadsorbed species, the C –Ag distances are 2.28 Å
1
and 2.36 Å, and the calculated stabilization energyis 7.9 kcal mol−1 and 18.1 kcal mol−1 for the
n=0 and n=1 calculations, respectively. The net
charge of C H is −0.27 and −0.52, respectively.
3 5These indicate that the allyl intermediate on thesilver surface is anionic in nature. The evidence
for the anionic nature had been reported experimentally by Madix et al. [27,28,56 ].

5. Overall discussion
Figs. 12 and 13 show the comparisons of theenergy diagrams for the reaction via the CNC
double bond breakage presented in Section 3 and385

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Fig. 10. Energy diagram for the reaction between allylic hydrogen of propylene and atomically adsorbed oxygen on silver surface.

Table 7
Net charge and frontier density for the adclusters shown in Figs. 10 and 11
Adsorbed
oxygenTS forH-appro.

Adsorbedintermediate−0.0580.300−0.6420.175−0.580−0.021−0.496
0.183
0.183
0.175
0.191
0.180

−0.051−0.279−0.9490.395−0.5120.023−0.523
0.185
0.185
0.173
0.174
0.180

0.038
−0.042−1.1840.440−0.5660.019−0.576
0.218
0.231
0.146
0.149
0.163

Frontier densityAg−0.108
a
Ag
0.077
b
O
1.031
a
H
0.0
a
C
0.0
1
C
0.0
2
C
0.0
3
H
0.0
11
H
0.0
12
H
0.0
21
H
0.0
31
H
0.0
32

0.033
−0.0060.349−0.0480.641−0.1340.265−0.035−0.036−0.006−0.012−0.0100.019
0.023−0.0270.008
0.736−0.399
0.603
0.020
0.020
−0.0040.006
0.006

Net charge
Ag
a
Ag
b
O
a
H
a
C
1
C
2
C
3
H
11
H
12
H
21
H
31
H
32

Free Ag
2
and C H
3 5

Adsorbed radical
species

Anionic
adsorbate

0.000
0.000

0.176
0.095

−0.265−0.212−0.4720.016−0.472
0.188
0.184
0.184
0.188
0.184

−0.7180.059−0.600
0.205
0.197
0.175
0.205
0.204

−0.7120.087−0.661
0.156
0.149
0.140
0.153
0.166

0.000
0.000

0.219
0.712

0.811
1.034

0.809
−0.4120.809−0.057−0.0530.019−0.057−0.0530.039−0.1180.181−0.009−0.0030.013−0.018−0.0160.140
0.674−0.719−0.0040.006−0.072
0.066
0.065

386

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Fig. 11. Stability of the allyl species adsorbed on Ag cluster in the neutral (n=0) and anion (n=1) adcluster states.
2

Fig. 12. Comparison of the energy diagram for the reactions between propylene and molecularly adsorbed oxygen. The route forallylic H attack is shown by the solid line, and those for carbon attack are shown by the broken and waved lines.

the reaction via allylic C–H breaking presented in
Section 4. In both cases involving molecular andatomic oxygen species, the reaction path via allylic
C–H breaking has lower energy barriers and morestable adsorbed intermediates. These results clearlyshow that the mechanisms via an allylic C–H
breakage should have a greater chance of taking
place with both molecularly adsorbed oxygen andatomically adsorbed oxygen. The low epoxide yieldfrom propylene as compared with ethylene ismainly due to the activity of the allylic hydrogenand the stability of the allyl anion species on the
Ag surface.
Based on the present results, we can reasonably
explain the available experimental facts. The sharp
contrast in the epoxide yield between propylene

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

387

Fig. 13. Comparison of the energy diagram for the reactions between propylene and atomically adsorbed oxygen. The route fortallylic H attack is shown by the solid line, and those for carbon attack are shown by the broken and waved lines.

and ethylene is due to the existence and nonexistence of the allylic hydrogen. The feasibility of
the C–H bond breakage in propylene is due to theformation of the resonance-stabilized allyl radical
or anion. Generally speaking, the olefins containing allylic C–H bonds, such as propylene, butenes
and pentenes [14], suffer the complete oxidation
initiated by the abstraction of the allylic hydrogenas shown in Section 4. Norbornene also possesses
an allylic C–H bond, but no subsequent resonancestabilized allyl radical or anion is formed, andtherefore, it favors the epoxidation reaction initiated by the CNC double bond breakage as shownin Section 3. Ethylene, styrene, 3,3-dimethylbutene
and butadiene contain no allylic C–H bonds, andtherefore, they mainly produce the epoxides
following the epoxidation mechanism as shown in
Section 3.
The present theoretical results partially support
some experimental findings. Early kinetic isotope
effects with propylene for the oxidation of
CD CHCH and CD CDCD studied by Cant3232and Hall [8] implied that further oxidation takes
place by the reaction at the methyl group rather
than at the vinyl or olefinic position. Akimoto

et al. [14] studied the catalytic oxidation of ethylene, propylene, butenes, and pentenes over silver,
and concluded that the complete oxidation of
propylene is initiated by the abstraction of allylichydrogen by the adsorbed molecular oxygen.
Madix et al. [27,28] also proposed that the dominant mechanism for combustion of propylene andhigher alkenes on silver is the activation of an
allylic C–H bond. Further, p-allyl species havebeen demonstrated on a number of metal oxides
active for propylene oxidation [57,58].
Cant and Hall [8] may have been the first to
propose a mechanistic sketch for the oxidation of
ethylene and propylene, and succeeded in explaining the observed kinetic isotope effects of thehydrogens for the formation of epoxide and CO .
2
They assumed that the oxidation of olefins (ethylene and propylene) to epoxide and CO takes
2place via the preliminary formation of an intermediate that is common to both product formations.
The very low yield of epoxide from propylene is
attributed to the intramolecular isomerization of
the common peroxide linkage intermediate involving the breakage of the C–H bond in the methylgroup to the adsorbed hydroperoxide. The latter

388

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

Fig. 14. Energy diagram for the reaction between terminal carbon atom of propylene and molecularly adsorbed superoxide on Ag
surface. The route leading to propylene oxide is given by the solid line, and the intramolecular isomerization of methyl hydrogen isshown by the broken line.

would decompose rapidly at the reaction temperature. We have checked this mechanism as it seems
to be an attractive model based on the molecularly
adsorbed oxygen. Fig. 14 shows the results of ourcalculation. Propylene approaches the adsorbed
superoxide species in a similar way as shown in
Fig. 3 but with the methyl group left in the sameside as O (the inside oxygen atom), which mayalead to a possible intramolecular isomerization of
one H(c)–C to the HOa in the next reaction step.
However, the energy levels of the TS and intermediate are 16.8 and 20.1 kcal mol−1, which is higher
than 13.7 and 28.8 kcal mol−1 shown in Fig. 3,
showing that this reaction path is less favorable.
If the intermediate is formed, the next reaction
step leading to propylene oxide is more favorable.
However, the energy barrier for the intramolecularisomerization of an allylic hydrogen seems to bemuch higher, though a reasonable TS geometry
could not be calculated. Also, the hydroperoxide
species suggested by Cant and Hall [8] may not
be a stable adsorbed species.

The mechanism proposed by Carter andGoddard [29] is somewhat similar to that suggested by Cant and Hall [8], though two atomicallyadsorbed oxygens are considered to be involved as
the active species. Unfortunately, it is difficult tochecked this mechanism exactly by the present
model used. However, our calculations show thatthe abstraction of the c-hydrogen by the nearest
neighbor oxygen atom as suggested by Carter andGoddard [29] seems to be difficult. The C–H(c)bond distance in the optimized intermediate state
by atomically adsorbed oxygen shown in Fig. 5 is1.088 Å, which is shorter than the C–H(a) one of
1.098 Å in the same intermediate, showing that theC–H bond in the methyl group of the intermediateis stronger than that of C–H(a) bond. This meansthat the c-H of the adsorbed intermediate cannot
be easily abstracted, and also that it cannot betransferred via the intramolecular isomerization
appearing in Cant and Hall’s model [8] discussed
above. A similar judgement can be made frombond dissociation energies for hydrocarbons [59]:Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

the C–H bond in the methyl group of the intermediate involved in the oxidation of propylene isstronger than that in the methylene group of theintermediate involved in the oxidation of ethylene
as pointed out by Akimoto et al. [14].
Furthermore, both models [8,29] suggested thealkoxide intermediate, which may be oxidized further to CO and H O. The alkoxide intermediate22
was calculated to be 123.5 kcal mol−1 (shown in
Fig. 14), more stable than the free reactant state,
and if it exists, it should be easily checked out by
the infra-red spectroscopic study and acroleinshould be one of the products. Though one infrared spectroscopic study [13] seems to support anadsorbed acrolein intermediate, however, as
pointed out by Madix et al. [27], additional work
is necessary to characterize further this intermediate state. In fact, most of the experimental results[8–11] show that only propylene oxide, CO and2
H O are the products of the oxidation of propyl2ene. This mechanism is also inconsistent with theexperimental fact that the epoxidation of norbornene occurs readily, regardless of the oxygen
adatom coverage between 0.05 and 0.5 [36 ] aspointed out by Madix et al. [28]. Therefore, themechanisms involving a single common precursor
intermediate to both epoxidation and complete
combustion are less favorable.
The mechanism for the complete oxidation of
propylene had been proposed by Madix et al. [28],and an acid–base reaction between atomicallyadsorbed oxygen and an allylic C–H bond wasassumed. The reaction was explained to be controlled by the acidity of the C–H bond, i.e. theheterolytic C–H bond dissociation, not the homolytic dissociation energy. The present results confirm that the dominant mechanism for olefin
combustion is the activation of an allylic C–Hbond by the adsorbed oxygen, and the anionicadsorbed intermediate is indeed stable on the silver
surface which can be naturally considered by the
DAM used in this study.
The calculated results presented in Sections 3
and 4 clearly enable us to address the mechanismof the epoxidation reaction as well as that of thecomplete combustion of propylene: epoxidation
reaction is initiated by the attack of the adsorbed
oxygen to the CNC double bond. The reaction389

may be written as:

(1)

.
(2)
In Eq. (2), aldehyde is a product of the terminal
C attack, ketone is a product of the central C
attack, and they are further oxidized to CO and2
H O.
2
The complete oxidation route is initiated by theabstraction of the allylic hydrogen of propylene
by the adsorbed oxygen, and the reaction proceeds
as follows:(3)CH –CH–CH /Ag+H–O–O/Ag
2
2
CH –CH–CH /Ag+O/Ag+HO/Ag
2
2

(4)

(5)
The allyl intermediate would lie on the surface,
therefore being amenable to further attack ofadsorbed oxygens and finally oxidized to CO and2
H O.
2
When allylic hydrogen exists in the olefin, theoxidations (3) and/or (4) would proceed, but whensuch hydrogen does not exist, the epoxidation
reaction (1) would proceed. It is difficult forreaction (2) to proceed for the superoxide species
because of the existence of a high energy barrier,but it may proceed partially for the atomicallyadsorbed species.

390

Z.-M. Hu et al. / Surface Science 401 (1998) 371–391

6. Conclusion
We studied, in this paper, the mechanism of theepoxidation and complete oxidation reaction of
propylene on a silver surface. We performed boththe ( U )HF and MP2 calculations with the use of
the dipped adcluster model (DAM ), which
involves the interaction between bulk metal andadmolecules with consideration of electron transfer
and image force correction.
The dominant mechanism for the epoxidationof olefins involves the reaction of the adsorbed
oxygen species with the olefinic carbon atom witha formation of a s bond between the oxygen andthe olefinic carbon and with a breaking of the
C–C p bond. The further reaction is an easy paththat proceeds quite smoothly, leading to an epoxide product. The competitive reaction is complete
oxidation via aldehyde intermediate. The molecular superoxide species is the main active species
that is selective, but the atomic oxygen species isalso active though non-selective.
The dominant mechanism for the complete oxidation of propylene is due to the activation of theallylic C–H bond. The initial step is the abstractionof an allylic hydrogen by the adsorbed oxygen,
resulting in the formation of a surface hydroperoxyl [ HOO ] and/or hydroxyl group [HO ] and(a)(a)
a chemisorbed allyl species [C H ]. The adsorbed
3 5(a)
surface hydroperoxyl [HOO ] is further decom(a)posed to an adsorbed hydroxyl group and oxygen
atom [O ] without any difficulty. The adsorbed(a)allyl species is stable for p conjugation and isanionic in nature. It is further combusted to form
CO and H O in the presence of oxygen. HO
22(a)can react subsequently with the second hydroxyl
via disproportionation to form gaseous water andregenerated O at the reaction temperature. This(a)combustion pathway is more favorable than theepoxidation mechanism for the olefins containing
allylic C–H bonds such as propylene studied inthis paper.
The above proposed two competitive mechanisms explain rather well the available experimental facts for the epoxidation and complete
oxidation of olefins: the olefins contain no allylic
C–H bond and/or no subsequent resonance-stabilized allyl radical or anion formation, such as

ethylene, styrene, 3,3-dimethylbutene, and norbornene mainly producing epoxide according to themechanism via the CNC bond breaking, whereas
the olefins contain allylic C–H bonds, such as
propylene, butenes and pentenes mainly leading to
the complete oxidation through the reaction of theallylic hydrogen with the adsorbed oxygen. The
DAM was essential for the theoretical study ofthese reactions.
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 isjust between 2 and 5% experimentally. Since propylene oxide is an important material in the chemical industry, it is interesting to determine whether
we can block the route leading to the allylic
intermediate.

Acknowledgements
Some calculations were performed using thecomputers at the Institute for Molecular Science.
Part of this study was supported by a Grant-inAid for Scientific Research from the Ministry of
Education, Science, and Culture of Japan and by
the New Energy and Industrial Technology
Development Organization (NEDO).

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