Surface Science 603 (2009) 1751–1755

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Surface Science

journal homepage: www.elsevier .com/locate /susc

Strong structure sensitivity in the partial oxidation of styrene on silversingle crystals

Ling Zhou, Robert J. Madix *

School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA 02138, United States

a r t i c l e i n f o

Article history:Available online 13 January 2009

Keywords:EpoxidationSilver catalystsStructure sensitivityStyreneStyrene oxidePhenylacetaldehydeOxametallacycleCombustion intermediate

0039-6028/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.susc.2008.08.032

* Corresponding author.E-mail address: rmadix@seas.harvard.edu (R.J. Ma

a b s t r a c t

In contrast to the formation of styrene oxide on Ag(111), phenylacetaldehyde and phenylketene domi-nate the partial oxidation of styrene on Ag(110), even though the reactions follow the same mechanismon both surfaces. The origin of this difference is that on Ag(110) the activation energy for transformationof the oxametallacycle to the combustion intermediate is much lower than on the (111) surface, so thatring-closure of the oxametallacycle to form styrene oxide is short circuited. Also the combustion interme-diate appears more stable on Ag(110) than on Ag(111).

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The heterogeneous epoxidation of olefins using silver-basedcatalysts is an important class of catalytic reactions [1–3]. Overthe past three decades, model studies have been extensively con-ducted on silver single crystals to elucidate the reaction mecha-nism, especially with respect to the origin of the reactionselectivity [4–12]. It is currently believed that epoxidation of ole-fins on silver proceeds through surface oxametallacycle intermedi-ates, and the reactions of the oxametallacycle control the reactionselectivity [13–15]. Further, there is increasing evidence from bothexperiment and theory that the reactivity of oxametallacycle inter-mediates may be strongly dependent on surface structure [16–19].Indeed, we have found that the partial oxidation of styrene exhibitsstrong structure sensitivity on silver single crystals [20]. In contrastto the formation of styrene oxide on Ag(111) [12,17,21] andAg(100) [9,10], little styrene oxide is produced on Ag(110) [20].Therefore, it is of importance to establish the relation betweenthe reactivity of oxametallacycles and the surface structure so thatbetter control over the selectivity of olefin epoxidation might beachieved.

We have recently proposed a detailed reaction mechanism forstyrene oxidation on Ag(111) [21]. The transformation of a surfaceoxametallacycle to a combustion intermediate leads to the bifurca-tion of the reaction, thereby controlling the reaction selectivity(Fig. 1) [21]. In this paper, we report X-ray photoelectron spectros-

ll rights reserved.

dix).

copy (XPS) results that show that the surface oxametallacycle,combustion intermediate, and benzoate detected in the reactionon Ag(111) [21] are also involved in styrene oxidation onAg(110), implying a common reaction mechanism on both sur-faces. However, on Ag(110), due to different reaction barriers,most of surface oxametallacycle is transformed to combustionintermediate, leading to the dominant partial oxidation productsof phenylacetaldehyde and phenylketene instead of styrene oxide.

2. Experimental

All experiments were performed in an ultra-high vacuum (UHV)system that consists of adjoined preparation and spectroscopychambers. The preparation chamber is equipped with a sputter-ion gun for surface cleaning. The spectroscopy chamber isequipped with low energy electron diffraction (LEED) optics, aquadrupole mass spectrometer (Balzers Prisma QMS 200) for tem-perature programmed reaction (TPRS), a dual anode X-ray source(Perkin–Elmer 04-548), and a hemispherical energy analyzer(SPECS EA 10 Plus). During the TPRS and XPS measurements thepreparation chamber was isolated from the spectroscopy chamberby a gate valve. The mass spectrometer was operated with the elec-tron energy of 70 eV and the emission current of 1 mA to ensurecomparable fragmentation patterns with the NIST database. TheAg(110) surface was cleaned by cycles of Ar+ sputtering at roomtemperature followed by oxygen exposure at 500 K and annealingin vacuum at 900 K for 5 min until no impurity was detected byXPS, and a sharp (1 � 1) LEED pattern was observed. The saturation

O

C C

H H

H CO2 + H 2OC C

H H

H

O O

C

OC

HH

H

O

OH

OC

C

OOCO2

CH

OH

OC

H

OO OO O

β-H elimination

C-C bond scission

Ag(111)

oxametallacycle combustion intermediate

benzoate

O

H

CC

Hβα

styrene styrene oxide

benzoic acid

biphenyl

phenyl benzene

+

benzeneacetic acidO

C C

H H

H

O

C C

H H

H

H

H CO2 + H 2OC C

H H

HC C

H H

H

H

H

OO OO

C

OC

HH

HC

OC

HH

H

H

H

OO

OH

OC

OH

OC

OH

OC

C

OO

C

OOCO2

CH

OH

OC

HC

H

OH

OC

OH

OC

H

OOOO OOOO OO

β-H elimination

C-C bond scission

Ag(111)

oxametallacycle combustion intermediate

benzoate

O

H

CC

Hβα

O

H

CC

Hβα

styrene styrene oxide

benzoic acid

biphenyl

phenyl benzene

+

benzeneacetic acid

Fig. 1. Reaction scheme for styrene oxidation on Ag(111) [21].

1752 L. Zhou, R.J. Madix / Surface Science 603 (2009) 1751–1755

of O-(2 � 1) was achieved, corresponding to the 0.5 monolayer(ML) oxygen coverage. TPD was used to set the initial surface oxy-gen coverage to 0.4 ML since this coverage is comparable with thesaturation oxygen coverage on Ag(111). Styrene (Alfa Aesar,99.5%) was stored in a glass dosing tube attached to the spectros-copy chamber through a leak valve. It was purified by repeatedfreeze-pump-thaw cycles. The introduction of styrene to the sur-face was accomplished by direct dosing through a 5 mm diametertube ending about 10 mm from the sample surface. The purity ofstyrene was checked by mass spectrometry.

3. Results and discussion

The reaction of styrene on oxygen-covered Ag(110) followingdosing of styrene at 280 K has been previously reported to producephenylacetaldehyde, phenylketene, benzeneacetic acid, benzoicacid, biphenyl, benzene, CO2, H2O, with no styrene oxide [20]. Inagreement with those results, following styrene deposition on0.4 ML oxygen-covered Ag(110) surface at 170 K with a coverageof 1.1 ML (Fig. 2), phenylacetaldehyde (C6H5CH2CHO, m/z = 120,parent ion, centered at 510 K) and phenylketene (C6H5CHCO, m/z = 118, parent ion, centered at 530 K) dominate the partial oxida-tion products, but a very small amount of styrene oxide(C6H5CHOCH2, m/z = 120, parent ion, centered at 275 K) was alsoproduced (previously unreported) at the same temperature as onAg(111) [21]. These products have been identified by comparingthe measured fragmentations of the product with the NIST data-base, and also with the fragments we measured in our own systemby condensing the molecules on clean silver surface. As shown inTable 1, the product centered at 275 K with parent ion of m/z = 120 is identified as styrene oxide, whereas the product centeredat 510 K with parent ion of m/z = 120 is assigned to phenylacetal-dehyde since it has no fragment of m/z = 119. The product centeredat 530 K with parent ion of m/z = 118 is identified as phenylketene.All other isomers of this product have been ruled out by the com-parison of the fragmentation patterns (Table 2). The detailed anal-ysis of the fragments has been discussed previously [20].

Unreacted styrene (C8H8, m/z = 104, parent ion) desorbed fromthe multilayer at 190 K and from the monolayer at 240 K. CO2

was produced with multiple peaks at a (320 K), b (420 K), c(520 K), and / (590 K). H2O desorbed at 275 K and 320 K. Benzoicacid (C6H5COOH, m/z = 122, parent ion) was evolved between500 K and 620 K. Benzene, phenyl, and biphenyl were producedat 590 K together with the /-CO2 peak. Benzeneacetic acid wasproduced in a very small amount at 550 K. The temperatures atwhich benzoic acid, benzeneacetic acid, benzene, phenyl, andbiphenyl are evolved are nearly identical on Ag(110) andAg(111) [21], as are the relative amounts produced in both sur-

faces, indicative of the same reaction pathways leading to theseproducts.

XPS was employed to quantitatively track the reaction interme-diates and their reaction pathways in styrene oxidation on Ag(110).The same procedure was followed as that utilized in the previousstudy on Ag(111) [21]. Fig. 3 shows both C 1s and O 1s spectra ofa styrene multilayer reacted with a 0.4 ML oxygen-coveredAg(110) surface starting at 170 K. All spectra were taken at 170 Kafter annealing the surface to prescribed reaction temperatureswith the same heating rate as TPRS. The O 1s peak at 528.1 eVagrees well with the former assignments for chemisorbed oxygenon Ag(110) [22]. The initial oxygen coverage of 0.4 ML was usedas reference to evaluate the coverages of other surface species dur-ing the reaction. The same reaction intermediates that we identifiedpreviously for styrene oxidation on Ag(111) [21] were also identi-fied in the reaction on Ag(110); namely, surface oxametallacycle,combustion intermediate, and benzoate. No difference in bindingenergy (BE) of the reaction intermediates was observed betweenthe two surfaces. Surface oxametallacycle exhibits an O 1s BE at529.5 eV, and C 1s features at BE = 284.2 eV (intense peak) and285.6 eV (shoulder), respectively. The combustion intermediateand oxametallacycle have the same O 1s BE; however, they canbe distinguished by their C 1s binding energies at 285.6 eV and286.2 eV, respectively. Surface benzoate was identified by the O1s peak at 530.5 eV, the intense C 1s peak at 284.2 eV, correspond-ing to the phenyl ring in the molecule, and a high BE peak at287.4 eV, corresponding to the carboxyl carbon.

The evolution of the surface species during the reaction is quan-tified in Table 3 and compared to analogous data for Ag(111) [21].When styrene was adsorbed on the 0.4 ML oxygen-coveredAg(110) surface at 170 K, 0.07 ML of surface oxametallacycle and0.06 ML of combustion intermediate formed, and the amount ofsurface oxygen decreased by 0.13 ML, in accord with the expectedstoichiometry. Note that the oxygen coverage is conserved, as noproducts evolve in the gas phase at this temperature. In sharp con-trast, no combustion intermediate formed under identical condi-tions on Ag(111). Further, annealing the surface to 230 Ktransforms nearly all of the oxametallacycle to combustion inter-mediate. The change in the coverages of surface oxygen, oxametal-lacycle, and combustion intermediate from 170 K to 230 K clearlydemonstrates the transformation of the oxametallacycle to thecombustion intermediate while annealing. The formation of thecombustion intermediate between 170 and 230 K cannot be dueto the direct reaction of styrene with surface oxygen since the de-crease of the oxygen coverage (0.02 ML) is significantly smallerthan the increase of the coverage of the combustion intermediate(0.08 ML). The same conclusion was reached for the reactions onAg(111) [21]. Interestingly, after annealing the Ag(110) surfaceto 230 K, there was only 0.01 ML oxametallacycle existing on the

Table 2Mass spectra fragmentation patterns for the product centered at 530 K, and forvarious isomers with stoichiometry C8H6O (parent ion m/z = 118). The values given inbrackets are reference data from the NIST database.

m/z Product at 530 K Benzofuran Benzocyclobutenone

89 100 (29) (91)90 94 (34) (100)118 59 (100) (88)119 10 (25) (7)

4x10-9

3

2

1

0

MS

Inte

nsity

[a.u

.]

800700600500400300200

Temperature [K]

m/z = 18

m/z = 44

m/z = 118

m/z = 120

m/z = 122

m/z = 136

m/z = 78

m/z = 154

m/z = 104

m/z = 77

benzeneacetic acid

benzoic acid

biphenyl

styrene

H2O

CO2

styrene oxide

phenylacetaldehyde

phenylketene

phenyl

benzene

x 1

x 1

x 1

x 5

x 5

x 2

x 500

x 50

x 50

x 50

φα

β

γ

3.0x10-9

2.5

2.0

1.5

1.0

0.5

0.0

MS

Inte

nsity

[a.u

.]

800700600500400300200

Temperature [K]

m/z = 18

m/z = 44

m/z = 120

m/z = 122

m/z = 136

m/z = 78

m/z = 104x 1

x 1

x 5

x 5

x 1

x 1

x 50

x 50

x 500

H2O

CO2

styrene oxide

benzoic acid

benzeneacetic acid

styrene

styrene (fragment)

α

α

β

γ

γ

δ

δ

ε

ε

φ

x 5

m/z = 77

benzene

x 2 m/z = 154biphenylphenyl

a b

Fig. 2. Temperature-programmed reaction spectra of a styrene multilayer reacting with (a) a 0.4 ML oxygen pre-covered Ag(110) surface and (b) a 0.375 ML oxygen pre-covered Ag(111) surface. The adsorption temperature of styrene was 170 K. Oxygen was deposited on the surface by dosing NO2 at 480 K. The heating rate was 4 K/s.

Table 1Mass spectra fragmentation patterns for the oxidation products of styrene onAg(110), and for various substances with stoichiometry C8H8O (parent ion m/z = 120).The values given in brackets are reference data from the NIST database.

m/z 91 120 119

Product at 275 K 100 36 22Product at 510 K 100 30 0Styrene oxide 100 (100) 36 (35) 22 (21)Phenylacetaldehyde 100 (100) 30 (28) 0 (0)Acetophenone (6) (100) (0)

L. Zhou, R.J. Madix / Surface Science 603 (2009) 1751–1755 1753

surface; most of surface oxametallacycle was transformed to com-bustion intermediate (0.145 ML). In contrast, on the Ag(111) sur-face, a rather large amount of oxametallacycle (0.09 ML)remained on the surface after annealing to 230 K while only0.08 ML transformed to combustion intermediate. This differenceclearly indicates that the activation energy for the transformationof oxametallacycle to combustion intermediate is lower on

Ag(110) than on Ag(111). When the Ag(110) surface was an-nealed to 280 K, the small amount of oxametallacycle ring-closedand desorbed from surface as styrene oxide (Fig. 2). Compared toAg(111), the amount of styrene oxide produced on Ag(110) isnearly one order of magnitude lower.

Further heating of the Ag(110) surface to 450 K resulted in thesteep decrease of the coverages of surface oxygen and combustionintermediate, and the appearance of surface benzoate. Clearly, thereaction of the combustion intermediate and the surface oxygenmust produce the surface benzoate and lead to the CO2 TPRS peaksat 320 K (a) and 420 K (b) (Fig. 2). By difference, 0.06 ML of CO2 isformed in these two peaks. In contrast, this conversion of the com-bustion intermediate to benzoate occurs near 275 K on Ag(111)[21]. The combustion intermediate thus appears more stable onAg(110) than on Ag(111), since no combustion intermediate ex-isted at 450 K on Ag(111), whereas rather large amount of com-bustion intermediate still remained at this temperature onAg(110). With continued heating of the Ag(110) surface to550 K, most of the combustion intermediate disappeared, corre-sponding to the production of phenylacetaldehyde at 510 K, phen-ylketene at 530 K, and c-CO2 at 520 K in TPRS (Fig. 2). Theproduction of phenylacetaldehyde and phenylketene may occurvia disproportionation of the combustion intermediate, one moietyabstracting H from the other to form phenylacetaldehyde andphenylketene. This scenario is consistent with the difference inproduction temperatures of phenylacetaldehyde and phenylketen-e. Surface benzoate vanished when the sample was heated to650 K, commensurate with the production of benzoic acid, ben-zene, phenyl, biphenyl, and CO2 (/) (Fig. 2). At 650 K, about

7000

6000

5000

4000

3000

2000

1000

0

N(E

)/E

536 534 532 530 528 526 524BINDING ENERGY [eV]

O 1s

styrene +O/Ag(110) at 170 K

Oad

oxametallacycle or combustion intermediate

a)

b) 230 K

combustionintermediate

benzoate

c) 280 K

d) 450 K

e) 550 K

f) 650 K

8000

6000

4000

2000

0

N(E

)/E

292 290 288 286 284 282 280BINDING ENERGY [eV]

C 1s

oxametallacycle

combustionintermediate

styrene

phenyl ring

a)

b) 230 K

benzoate

styrene +O/Ag(110) at 170 K

c) 280 K

d) 450 K

residual C

e) 550 K

f) 650 K

Fig. 3. X-ray photoelectron spectra (C 1s and O 1s) of a styrene multilayer reacting with a 0.4 ML oxygen-covered Ag(110) surface formed by dosing NO2 at 480 K. All spectrawere taken at 170 K. (a) After dosing styrene on the oxygen-covered Ag(110) surface at 170 K; and then annealing the surface to (b) 230 K; (c) 280 K; (d) 450 K, (e) 550 K; and(f) 650 K. Dots: original data; curve: fitted curve; and dashed curve: component peak.

Table 3Quantitative evolution of the surface species in styrene oxidation on Ag(110) and Ag(111).

Surfaces Reaction stages Oad Oxametallacycle Combustion intermediate Benzoate Residual C

Ag(110) p(2 � 1)-O 0.4 ML (ref.)Styrene + p(2 � 1)-O at 170 K 0.27 ML 0.07 ML 0.06 ML 0 0230 K 0.25 ML 0.01 ML 0.14 ML 0 0280 K 0.22 ML 0 0.145 ML 0 0450 K 0.04 ML 0 0.08 ML 0.065 ML 0.04 ML550 K 0 0 0.01 ML 0.06 ML 0.1 ML650 K 0 0 0 0 0.1 ML

Ag(111) [21] p(4 � 4)-O 0.375 ML (ref.)Styrene + p(4 � 4)-O at 170 K 0.24 ML 0.135 ML 0 0 0230 K 0.205 ML 0.09 ML 0.08 ML 0 0280 K 0.065 ML 0.025 ML 0.02 ML 0.06 ML 0450 K 0 0 0 0.06 ML 0.08 ML

1754 L. Zhou, R.J. Madix / Surface Science 603 (2009) 1751–1755

0.1 ML of residual carbon existed on the surface, as shown by thesmall peak at 284.6 eV in the C 1s spectrum (Fig. 3f).

The quantitative XPS analysis of the surface intermediates com-bined with the TPRS provides clear evidence that the reactivity ofoxametallacycle is strongly dependent on the surface structure.Though the pathway to products appears to be the same on bothsurfaces, with annealing, the transformation of oxametallacycleto combustion intermediates occurs at different rates on the two

surfaces. This pathway competes with the ring-closure of oxame-tallacycle to produce styrene oxide, which appears to have a simi-lar activation energy on Ag(110) and Ag(111), as judged by thefact that it appears at the same temperature on both surfaces.Due to the smaller energy barrier of the transformation of oxame-tallacycle to combustion intermediate on Ag(110) than onAg(111), less styrene oxide is produced on Ag(110). Instead, phe-nylacetaldehyde and phenylketene are the dominant partial oxida-

CH

OH

OC

H

O

C C

H H

H

CO2 + H 2O

C C

H H

H

O O

Ag

α β

Ag

C

OC

HH

H

O

β

α

C-Cbond breakage

CO2

OH

OC

C

OO

Ag

β-H elimination

styrene oxide

O

Ag

Ag(111)Ag(110)

+

CH

H

OC

HC

H

O

C

O

H

CC

Hβα

styrene oxametallacyclecombustion intermediate

phenylacetaldehydephenylketene

benzoate

benzoic acid biphenyl

phenylbenzene

benzeneacetic acid

CH

OH

OC

HC

H

OH

OC

OH

OC

H

O

C C

H H

H

O

C C

H H

H

H

H

CO2 + H 2O

C C

H H

H

O O

Ag

α βC C

H H

HC C

H H

H

H

H

OO OO

AgAg

α β

Ag

C

OC

HH

H

O

β

α

AgAg

C

OC

HH

HC

OC

HH

H

H

H

OO

β

α

CC bond breakage

CO2

OH

OC

OH

OC

OH

OC

C

OO

Ag

C

OO

C

OO

C

OO

Ag

β-H elimination

styrene oxide

OO

AgAg

Ag(111)Ag(110)

+

CH

H

OC

HC

H

H

OC

H

OC

HC

H

O

CCH

O

C

O

H

CC

Hβα

O

H

CC

Hβα

styrene oxametallacyclecombustion intermediate

phenylacetaldehydephenylketene

benzoate

benzoic acid biphenyl

phenylbenzene

benzeneacetic acid

Fig. 4. Reaction mechanistic scheme of styrene oxidation on silver single crystals.

L. Zhou, R.J. Madix / Surface Science 603 (2009) 1751–1755 1755

tion products via the combustion intermediate. Further, the com-bustion intermediate is more stable on Ag(110) than onAg(111). In previous study, we postulated that the combustionintermediate has an extended conjugated electron system thatcouples the p-electrons of the phenyl ring and the carbonyl groupwith the surface [21]. One possible origin of the difference in sta-bility of the combustion intermediate on the two surfaces is thatthis conjugated p-electron system interacts more strongly withthe Ag(110) surface than with the Ag(111) surface. The presenceof the phenyl ring in styrene may affect the stability of the inter-mediates on different silver surfaces and, therefore, affect the ener-getics of the competing reaction pathways. More fundamentalunderstanding of the origin of the structure sensitivity in styreneoxidation on silver single crystals requires theoretical studies ofthe intermediate–surface interaction and the reaction energetics.

4. Conclusions

We have shown that a common reaction mechanism accountsfor the strong structure sensitivity in the partial oxidation of sty-rene on the Ag(111) and Ag(110) surfaces. As shown in Fig. 4, sty-rene reacts with oxygen on silver to form an oxametallacycleintermediate. On the Ag(111) surface the surface oxametallacycleprefers ring-closes to form styrene oxide. However, on the Ag(110)surface, the oxametallacycle largely transform to a more stablecombustion intermediate. The combustion intermediate dispro-portionates to produce phenylacetaldehyde and phenylketene onAg(110). Besides combustion to CO2 and H2O, the combustionintermediate can be also further oxidized to a benzoate intermedi-ate and a very small amount of benzeneacetic acid. At high temper-ature, the benzoate intermediate produces benzoic acid ordecomposes to phenyl and CO2 leading to the production of biphe-nyl and benzene. The transformation of surface oxametallacycle to

the combustion intermediate is a primary kinetic step to controlthe epoxidation selectivity.

Acknowledgements

We gratefully acknowledge the support of the National ScienceFoundation (NSF CHE 9820703). We also appreciate the valuablediscussions with Prof. C.M. Friend and X.Y. Liu.

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