1

Adsorption Geometry Determines Catalytic

Selectivity in Highly Chemoselective Hydrogenation

of Crotonaldehyde on Ag(111)

Katrin Brandta, May E. Chiu

b, David J. Watson

c*, Mintcho S. Tikhov and Richard M. Lambert

d§

Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, CB2 1EW

CORRESPONDING AUTHORS:

* Email: d.j.watson@surrey.ac.uk; Tel.: +44 1483 686836; Fax: +44 686851

§ Email: rml1@cam.ac.uk; Tel.: +44 1223 336467; Fax: +44 1223 336362

AUTHOR FOOTNOTE: Present corresponding addresses of several of the authors differ from that

given in the affiliation line where the work was undertaken. They are now: a

Freiburg Institute for

Advanced Studies (FRIAS), Albert-Ludwigs-Universität Freiburg, 79104 Freiburg, Germany, b

BP

Global Lubricants Technology, Whitchurch Hill, Pangbourne, Reading, RG8 7QR, UK, and c

Department of Chemistry, University of Surrey, Guildford, GU2 7XH, UK.

d Also at: Consejo Superior de Investigaciones Científicas, Instituto de Ciencia de Materiales,

Universidad de Sevilla, 41092 Sevilla, Spain.

2

ABSTRACT

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature

programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule

adsorbed without decomposition, all three possible hydrogenation products were formed and

desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were

found under TPR conditions.

The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2

catalysts and the present findings permit a rationalization of the catalytic performance in terms of

pronounced coverage-dependent changes in adsorption geometries of the reactant and the products.

Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface

whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring

hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost

parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing

selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall two-

step process: crotonaldehyde � crotyl alcohol � butanol, further hydrogenation of the desired

product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the

unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is

offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective

hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to

C=O π bond, as suggested by C 1s XPS binding energies.

KEYWORDS: adsorption geometry, selectivity, catalysis, crotyl alcohol, butyraldehyde.

3

INTRODUCTION

The chemoselective hydrogenation of α,β-unsaturated aldehydes, especially that of crotonaldehyde

to crotyl alcohol, has been extensively studied by traditional methods of catalytic science due to the

value of such unsaturated alcohols as versatile intermediates in the production of fine chemicals and

pharmaceuticals. An extensive literature exists and comprehensive reviews of the subject include

references [1], [2] and [3]. C=O hydrogenation in the presence of a C=C functionality is

challenging and fundamentally interesting because thermodynamics favors hydrogenation of the

latter to form the (undesirable) saturated aldehyde. Thus a suitable catalyst is required in order to

manipulate kinetic effects so as to favor C=O hydrogenation: however, only a few direct

experimental investigations of the associated phenomena carried out under well-defined conditions

have been reported.

Over the last fifteen years, classical studies involving platinum and copper nanoparticle

catalysts have dominated the field - only recently has attention turned to dispersed silver. Usually

encountered as an oxidation catalyst, silver has emerged as a promising and effective practical

catalyst for chemoselective hydrogenation of unsaturated aldehydes4-7 as first demonstrated by

Claus et al. who used Ag/SiO2 catalysts and found steady-state selectivities as high as 63% for the

production of crotyl alcohol from crotonaldehyde.5 Thus far, no fundamental studies of this system

have been reported.

Here, we give an account of the surface chemistry, adsorption geometry and hydrogenation

behavior of crotonaldehyde on the Ag(111) surface, studied by temperature programmed

desorption/reaction (TPR/TPD), high-resolution X-ray photoelectron (XPS) and near-edge X-ray

absorption fine structure spectroscopy (NEXAFS). Pertinent properties of the three possible

hydrogenation products, crotyl alcohol, butyraldehyde and n-butanol, are also described. The

chemoselective hydrogenation activity observed under ultra-high vacuum conditions corresponds

well with selectivity trends previously reported for Ag/SiO2 catalysts and our findings permit a

rationalization of the catalytic behavior in terms of coverage-dependent changes in adsorption

geometries of the reactant and the products. The clean unmodified Ag(111) surface exhibits very

high intrinsic selectivity towards crotyl alcohol formation (up to 95% under TPR conditions), in

striking contrast to Cu(111).8,9

In that case, the clean surface was entirely inert, whereas addition of

sulfur activated the system towards chemoselective hydrogenation, again in accord with the steady-

state behavior of practical Cu nanoparticle catalysts.10

The observed high selectivity of the Ag(111) surface towards crotyl alcohol formation is in

4

accord with studies using supported silver nanoparticles11,12

where larger particles consisting of a

greater proportion of (111) facets showed enhanced selectivity to crotyl alcohol compared to

smaller particles.

EXPERIMENTAL METHODS

The surface chemistry and hydrogenation of crotonaldehyde on Ag(111) were studied by TPD and

TPR in Cambridge, UK, and by high-resolution fast-XPS and NEXAFS on the SuperESCA

beamline at the ELETTRA synchrotron radiation facility in Trieste, Italy. TPD and TPR

experiments were carried out in an ultra-high vacuum chamber of conventional design, using a

linear heating rate of 4 K s-1 with simultaneous detection of all desorbing species by a quadrupole

mass spectrometer. Data are corrected for mass spectrometer sensitivity, molecular ionization cross-

sections and molecular fragmentation patterns so as to be comparable with NIST reference spectra.

Desorption yields and hence conversions and selectivities were calculated by means of calibration

data consisting of sequences of TPD uptake experiments that established the saturation coverage of

each molecule; in each case, onset of a characteristic multilayer desorption feature enabled

identification of the monolayer point. Coverages are thus quoted in terms of nominal monolayers

(ML).

High-resolution XPS and NEXAFS were carried out to study electronic structure and

orientation of C=O and C=C bonds with respect to the surface. Experimental details relevant to the

synchrotron experiments are provided elsewhere13-15

and the NEXAFS data were processed by

standard methodology.16,17

XPS binding energies were corrected for monochromator error and

adsorbate coverages were calibrated by continuously monitoring the C 1s XPS signal during

exposure of the sample to each adsorbate, the monolayer point being identified by shifts in the

binding energies of the peak maxima.

The Ag(111) sample was cleaned by cycles of Ar+ bombardment at room temperature, 600

K, 800 K and again at room temperature, followed by annealing in vacuum at 800 K. Surface

condition was monitored by LEED and Auger spectroscopy in Cambridge and by XPS at

ELETTRA. Crotonaldehyde (≥ 99.5 %) and its three hydrogenation products, crotyl alcohol (≥ 97

%), butyraldehyde (≥ 99 %) and n-butanol (≥ 99.5 %), were obtained from Sigma Aldrich, further

purified by freeze-pump-thaw cycles and dosed by back-filling the vacuum chamber.

5

Dissociative adsorption of dihydrogen on Ag(111) is significantly activated18,19

so that dosing with

H2 is not a practical means of delivering chemisorbed hydrogen to the surface under vacuum

conditions. Accordingly, atomic hydrogen was generated by means of a hot filament atomizer20,21

which delivered a flux of H atoms directly onto the Ag(111) surface. Hydrogen exposures are

quoted as fractions of a monolayer, based on calibration experiments which showed that an

exposure of 60 Langmuirs of H2(g) with the atomizer switched on gave a coverage of ~1 ML of

H(a). In all cases, the dosing sequence of adsorbates onto the clean, liquid nitrogen-cooled single

crystal surface was (1) organic species then (2) hydrogen. This sequence mirrors the procedure

employed during high-pressure catalytic testing.22,23

Pre-covering the surface with hydrogen

passivated it towards reaction with subsequently adsorbed crotonaldhyde which desorbed without

reaction, as also found by Zaera et al. for Pt(111) + H(a) + crotonaldehyde.24

Base temperatures

were 140 K in Cambridge and 123 K at ELETTRA.

RESULTS AND DISCUSSION

Adsorption-desorption chemistry

The adsorption and desorption characteristics of crotonaldehyde and its three hydrogenation

products were determined by TPD and XPS in order to provide reference data necessary for

analysis of the hydrogenation results. TPD profiles for each of these four species are shown in

Figure 1A: submonolayer coverages were used so as to simulate behavior representative of practical

reaction conditions. Reported desorption profiles correspond to the most intense ion observed with

our quadrupole instrument in the case of each molecule - the parent ion of crotonaldehyde and

fragment ions in the case of the three products. In every case, several other m/z signals

corresponding to diagnostic ions in each molecule’s fragmentation pattern were also monitored: the

relative intensities and profiles of these auxiliary data confirmed that the four molecules adsorbed

and desorbed intact, without measurable decomposition, as confirmed by XPS (see later). The

asymmetric peak shapes indicate approximately first order desorption kinetics, the shoulders

observed at high temperatures being ascribed to desorption from defect sites. In the case of the

reactant, this component corresponds to < 10% of the total crotonaldehyde desorption yield, from

which we conclude that the effect of defect sites on the hydrogenation reaction is likely to be minor,

as indeed the TPR results presented in the next section indicate: the observed very substantial

conversions of crotonaldehyde to reaction products cannot be accounted for in terms of defect sites.

6

Figure 1 (A) TPD spectra of crotonaldehyde, crotyl alcohol, butyraldehyde and butanol on

Ag(111) after adsorption at 140 K, (B) C 1s XPS of the same four molecules on

Ag(111) at 123 K.

C 1s XP spectra for all four species are shown in Figure 1B: each spectrum exhibits two

peaks with a 3:1 intensity ratio. The smaller component at higher binding energy is assigned to

carbon attached to oxygen (hereafter referred to as the C-O peak);25

the larger component at lower

binding energy is due to unresolved C 1s emission from the other three carbons. As expected, the C-

O component of the alcohols appears at lower binding energy than those of the aldehydes, due to

the change from sp2 to sp

3 hybridization. All spectra were stable over the experimental timescale,

indicating that X-ray beam effects were negligible and these results are summarized in Table 1,

along with estimated desorption enthalpies calculated from the TPD data following the method

proposed by Redhead.26

Table 1 Summary of TPD and XPS results.

Monolayer position

(Binding energy)

Desorption

temperature / K

Desorption enthalpy /

kJ mol-1

CC / eV CO / eV

7

Butyraldehyde 177 44.2 285.1 287.9

Crotonaldehyde 183 45.6 285.6 287.9

Crotyl alcohol 203 50.4 285.2 286.7

Butanol 213 52.8 285.0 286.4

Hydrogenation of crotonaldehyde

Crotonaldehyde hydrogenation on the clean Ag(111) surface was studied as a function of both

hydrogen and crotonaldehyde coverage, following the dosing procedures described above.

Figure 2 shows TPR data acquired after adsorption of ~ 0.6 ML crotonaldehyde and

progressively increasing amounts of hydrogen (0.66 ML, 1.0 ML, 1.33 ML in A-C respectively). It

is clear that there was an overall decrease in the production of crotyl alcohol, with increasing yields

of butyraldehyde and, especially, butanol as the coverage of hydrogen increased. This decreasing

chemoselectivity with increasing hydrogen coverage corresponds to progressive activation of the

C=C function and subsequent hydrogenation; as will be seen, it correlates with corresponding

changes in adsorption geometry of the reactant.

The relatively small amounts of butyraldehyde and butanol desorbing at ~ 230 K and ~ 300

K, respectively, are possibly due to hydrogenation occurring at defect sites. Although butanol (m/z

= 56), exhibits some fragment ion intensity at m/z = 31, this makes only a minor contribution to the

m/z = 31 spectra as is apparent from the lack of correlation between the 31 and 56 intensities in

Figure 2.

Corresponding TPR spectra showing differing coverages of crotonaldehyde at constant

hydrogen coverage are provided in the supplementary information.

8

Figure 2 Dependence of reaction selectivity on hydrogen coverage at fixed crotonaldehyde

coverage. TPR spectra for 0.66 ML, 1.0 ML and 1 ML crotonaldehyde (A, B and C

respectively) co-adsorbed with 1 ML H ; A-C: m/z 70 is scaled by 1/5, A-C: m/z 31 is

scaled by 1/2.

The results of 27 such TPR experiments spanning a wide range of crotonaldehyde and hydrogen

coverages are summarized in Figure 3. Crotonaldehyde conversion is plotted as grey bars, with

selectivities to each product plotted as points. In all cases, crotonaldehyde consumption increased

with hydrogen coverage, which is to be expected. It is remarkable that the selectivity towards crotyl

alcohol formation was always very high, reaching 95% when the coverage of both reactants was

low. Butyraldehyde and butanol selectivities showed the opposite trend with reactant coverage

when compared to crotonaldehyde selectivity, and this behavior may be rationalised in the light of

the NEXAFS results presented below.

9

Figure 3 Summary of TPR experiments as a function of hydrogen doses for coverages of 0.3

ML, 0.6 ML and 1 ML (A, B and C respectively) crotonaldehyde. Selectivities

toward crotyl alcohol, butyraldehyde and butanol are plotted as points, overlaid onto

total conversion (plotted as grey bars).

NEXAFS of crotonaldehyde

The adsorption geometry of crotonaldehyde on the otherwise clean surface was determined at low

coverage (0.2 ML; Figure 4A) and also high coverage (0.8 ML). C K-edge data were acquired at

five angles of photon incidence (θ) at both coverages and the raw data were processed following

standard methodology16,17

by first extracting the integrated π* intensities and then analyzing their

angular dependence.

10

Figure 4 C K-edge NEXAFS of 0.2 ML crotonaldehyde on Ag(111) at 123 K (A) and full

analysis of normalized π* resonance intensities plotted against the photon-incidence

angle (θ), showing the tilt angles (αααα) of each double bond with respect to the surface

for 0.2 ML (B) and 0.8 ML (C) crotonaldehyde.

The spectra exhibit two sharp 1s � π* resonances whose dependence on photon incidence angle is

markedly different: they are assigned to the C=C bond (282.5 eV) and the C=O bond (284.3

eV),13,15

the intensity of the latter transition decreased to almost zero at normal photon incidence, at

which point substantial C=C intensity remained. From the well known selection rules for 1s � π*

transitions of adsorbates on (111) surfaces we may draw the qualitative conclusion that C=O bond

is almost parallel to the metal surface whereas the C=C is appreciably tilted. A more accurate

evaluation of the tilt angles (α) of the C=C and C=O bonds with respect to the metal surface was

carried out following the established procedure16,17

which involves fitting the observed θ

dependence of the normalized π* intensities to the theoretically calculated dependence for a series

of trial α values, as shown in Figure 4B. The marked difference in tilt angles of the C=C (31o) and

C=O (5o) bonds is clearly apparent, and a possible adsorption geometry consistent with these values

is illustrated in Figure 5. This geometry plausibly accounts for the observed very high selectivity at

low hydrogen coverage (Figure 3) - the C=O functionality lies close to the surface where it can

interact strongly with the metal and is accessible for reaction with adsorbed hydrogen; conversely,

the C=C moiety is tilted away from the surface where its hydrogenation is disfavored. Thus both

11

effects act to enhance selectivity towards formation of the unsaturated alcohol. The influence of co-

adsorbed H(a) on crotonaldehyde geometry is dealt with below and also found to be in line with the

reactive behavior.

Figure 5 Schematic showing possible adsorption geometries of crotonaldehyde at 0.2 ML and

0.8 ML. The C=C (blue bond) tilts more at 0.2 ML than at 0.8 ML whereas the C=O

(red bond) tilt angle remains the same.

To examine possible effects of increased crotonaldehyde coverage, corresponding analysis of π*

resonance intensities acquired at a substantially higher crotonaldehyde coverage (0.8 ML) was also

carried out and the results are shown in Figure 4C. Here, the tilt angle of the C=O bond is almost

unchanged (7°) whereas that of the C=C bond is distinctly reduced. This change is consistent with

the reaction data (decreased chemoselectivity, Figure 3), and is mainly due to enhanced

butraldehyde formation as C=C hydrogenation competes more effectively with C=O hydrogenation.

Figure 3 also depicts a very pronounced decrease in chemoselectivity, up to ~ 25%, caused

by increasing hydrogen coverage at fixed crotonaldehyde coverage—the corresponding NEXAFS

data again provide an explanation. Figures 6A and B show how the C=C and C=O bond

orientations were affected by systematic increases in coverage of co-adsorbed hydrogen. (NEXAFS

spectra were acquired successively after sequential doses of hydrogen were applied to a surface pre-

covered with 0.2 ML crotonaldehyde.) It is clear that while C=O orientation was almost unaffected

by H(a), C=C tilt was strongly perturbed, decreasing from 30° to almost flat-lying, as the hydrogen

coverage increased. These observations provide rather direct insight into why partial hydrogenation

selectivity shifted from crotyl alcohol formation towards butyraldehyde formation as hydrogen

coverage increased: as the C=C bond is forced towards the surface, it becomes increasingly

12

vulnerable to attack by H(a). One might suppose that in the almost fully-flat geometry, with both

C=C and C=O close to the surface, conditions would be optimal for direct double-hydrogenation of

crotonaldehyde to form butanol. However, as we shall show, this is a minor channel. Most butanol

formation is the result of crotyl alcohol hydrogenation; i.e, an overall two-step process:

crotonaldehyde � crotyl alcohol � butanol

Figure 6 Change in 0.2 ML crotonaldehyde NEXAFS intensities of (a) C=C and (b)

C=O transitions with increasing hydrogen coverages. The vectors

r n ,

r

M and r

E (inset)

correspond to the surface normal, transition dipole and electric field vectors, respectively.

Secondary chemistry: hydrogenation of crotyl alcohol to butanol

Understanding the (undesirable) subsequent hydrogenation of crotyl alcohol to butanol is of interest

because this process could limit catalytic selectivity under practical conditions. Accordingly, we

studied this reaction by both NEXAFS and XPS. Figure 7A shows C K-edge NEXAFS spectra

taken at five angles of photon incidence for 0.2 ML crotyl alcohol in the absence of co-adsorbed

hydrogen. The single π* resonance at 283.1 eV is assigned to the C=C bond of the unsaturated

alcohol27,28

and curve fitting analysis of these data is shown with black symbols in Figure 7B: the

C=C tilt angle in the absence of hydrogen is estimated as 10°.

Subsequently, the surface was exposed to hydrogen atoms for 8 minutes and then a further

12 minutes corresponding to coverages of approximately 0.8 and 1.2 ML, NEXAFS data being

acquired after each hydrogen dose. At the temperature of these experiments (123 K) no significant

13

hydrogenation occurred, as will be apparent from the XPS data that follow. Analysis of the photon

angle-dependence of these data, also shown in Figure 7B, revealed that co-adsorbed hydrogen

systematically decreased the tilt of the C=C bond such that it eventually lay parallel to the surface.

Thus with increasing hydrogen coverage one might expect increased hydrogenation of reactively-

formed crotyl alcohol to butanol, leading to decreased overall selectivity towards the unsaturated

alcohol, in accord with the results shown in Figure 3. Note that reactively-formed crotyl alcohol can

persist on the surface up to ~ 250 K (Figure 2) at which temperature Ag(111) is indeed very active

for crotyl alcohol + H(a) � butanol conversion, as shown below. Therefore this reaction is likely to

be the most significant cause of the ~ 25% decrease in crotyl alcohol selectivity that is observed as

hydrogen coverage is increased (Figure 2). (On the other hand, analysis of C K-edge NEXAFS data

for butyraldehyde similarly acquired as a function of co-adsorbed H(a) coverage showed that the

adsorption geometry of this molecule was not affected by adsorbed hydrogen: see supplementary

information).

Figure 7 (A) C K-edge NEXAFS acquired of 0.2 ML crotyl alcohol (123 K). (B) Curve fitting

analysis of NEXAFS intensities of 0.2 ML crotyl alcohol (black), and the same

surface after exposure to 0.8 ML (red) and 2 ML hydrogen (blue). The vectors

r n ,

r

M and r

E (inset) correspond to the surface normal, transition dipole and electric

field vectors, respectively.

14

The hydrogenation of crotyl alcohol to butanol could be observed directly by means of temperature-

programmed C 1s XPS, as shown in Figure 8. Initial coverages at 123 K were 0.2 ML of

crotonaldehyde and 2 ML of hydrogen. Raw data are shown in Figure 8A where a discernible shift

of the spectra to lower binding energy can be seen, commencing at around 190 K: by 355 K the C

1s intensity had vanished, indicating a “clean” surface reaction, i.e. with no deposition of

carbonaceous residue. The final C 1s positions match those of butanol determined previously (see

Figure 1B) by XPS, showing that crotyl alcohol is converted directly to butanol during this

temperature ramp. By use of reference spectra for pure crotyl alcohol and pure butanol, a subset of

the data in Figure 8A was treated by curve fitting so as to extract the separate contributions from the

two species, as illustrated in Figure 8B. The temperature dependence of the resulting integrated

intensities is shown in Figure 8C. Under those conditions, crotyl alcohol � butanol conversion

commenced immediately upon raising the temperature, the reaction levelling off at ~ 190 K.

Subsequently, crotyl alcohol desorption became the dominant loss process, with butanol desorption

occurring more slowly and at higher temperatures - note that these observations accord well with

the desorption behavior of crotyl alcohol and butanol, shown in Figure 1.

Figure 8 (A) TP-XPS of 0.2 ML crotyl alcohol and 2 ML hydrogen (123 - 355 K). The grey

coloring highlights a shift in the spectra to lower binding energy at 190 K. (B) Peak

positions for CC and CO carbon 1s in crotyl alcohol and butanol used for peak

fitting. (C) Integrated C 1s intensities from TP-XPS (every fifth scan shown for

clarity).

Silver versus copper

The striking difference between these two metals in regard to their ability to catalyze

chemoselective hydrogenation activity is worthy of comment, not least because in each case the

15

reactivity of extended single crystal surfaces correlates well with the catalytic behavior of practical

nanoparticle catalysts. Thus, as shown here, clean Ag(111) is extremely active and highly selective

towards formation of crotyl alcohol. On the other hand Cu(111) is totally inert towards

crotonaldehyde + H(a). Sulfur adatoms promote both activity and selectivity toward crotyl alcohol

formation.8,9 Based on a simple initial state argument, the XPS data suggest a possible explanation,

as follows. The C 1s binding energies of the carbonyl carbon in crotonaldehyde are 287.9 eV and

288.2 eV for Ag(111) and Cu(111), respectively. This corresponds to a greater degree of electron

transfer from metal to C=O π bond in the case of Ag, resulting in enhanced weakening of the

carbonyl bond relative to the Cu case, and hence activation towards hydrogenation. This view is

supported by the activating effect of S on Cu/crotonaldehyde. Sulfur markedly decreases the C=O C

1s binding energy (to 287.8 eV) relative to that for clean Cu, which, by the above argument signals

appreciable weakening of this bond and hence activation towards reaction with H(a).

CONCLUSIONS

The overall selectivity toward crotyl alcohol formation in the chemoselective hydrogenation of

crotonaldehyde depends strongly on the adsorption geometry of reactant and product species, and

the way in which these geometries vary with the coverage of both reactants, H(a) and

crotonaldehyde.

At low surface coverage, the C=O bond in crotonaldehyde is almost parallel to the metal

surface whereas the C=C is appreciably tilted, a geometry that plausibly accounts for the very high

selectivity towards formation of the unsaturated alcohol (~ 95 %). With increasing coverage of

either reactant, the C=C bond in crotonaldehyde becomes almost parallel to the surface, favoring its

hydrogenation and thus markedly decreasing selectivity towards formation of crotyl alcohol.

Butanol formation is a two-step process, mainly the result of the subsequent hydrogenation

of crotyl alcohol. This selectivity-limiting effect is exacerbated at high H(a) coverage where the

C=C bond is forced parallel to the surface, rendering it vulnerable to attack by H(a).

The strikingly different behavior of Ag(111) and Cu(111) in the chemoselective

hydrogenation of crotonaldehyde may be rationalized in terms of the different degrees of charge

transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

ACKNOWLEDGEMENTS

16

KB gratefully acknowledges financial support from the German Research Foundation (Deutsche

Forschungsgemeinschaft, DFG). MEC acknowledges financial support from the Cambridge

University Oppenheimer Trust. The authors gratefully acknowledge ELETTRA and the European

Community for financial support under the EU contract RII3-CT-2004-506008 (IA-SFS). They also

thank Dr A. Goldoni and Dr M. Barnaba for their assistance during synchrotron experiments and

Dr. S. Lizzit for his assistance during preparation for the beamtime.

SUPPORTING INFORMATION PARAGRAPH

Supporting information includes additional information concerning the orientation of butyraldehyde

with changing concentrations of hydrogen on the surface. This takes the form of a full analysis of

the C K-edge NEXAFS spectra. This information is available free of charge via the Internet at

http://pubs.acs.org."

17

REFERENCES

(1) Ponec, V., Appl. Catal. A 1997, 149, 27-48.

(2) Maeki-Arvela, P.; Salmi, J.; Murzin, T.; Yu, D.; Appl. Catal. A 2005, 292, 1-49.

(3) Bernard Coq, F. F.; Coord. Chem. Rev. 1998, 178–180, 1753-1783.

(4) Bron, M.; Kondratenko, E.; Trunschke, A.; Claus, P.; Z. Phys. Chem. 2004, 218, 405-423.

(5) Bron, M.; Teschner, D.; Knop-Gericke, A.; Steinhauer, B.; Scheybal, A.; Hävecker, M.;

Wang, D.; Födisch, R.; Hönicke, D.; Wootsch, A.; Schlögl, R.; Claus, P.; J. Catal. 2005, 234,

37-47.

(6) Grünert, W.; Brückner, A.; Hofmeister, H.; Claus, P., J. Phys. Chem. B. 2004, 108, 5709-

5717.

(7) Volckmar, C.E. Bron, M.; Bentrup, U.; Martin, A.; Claus, P.; J. Catal. 2009, 261, 1-8.

(8) Chiu, M. E.; Watson, D. J.; Kyriakou, G.; Tikhov, M. S.; Lambert, R. M.; Angew. Chem. Int.

Ed. 2006, 45, 7530-7534.

(9) Chiu, M. E.; Kyriakou, G.; Williams, F. J.; Watson, D. J.; Tikhov, M. S.; Lambert, R. M.

Chem. Commun. 2006, 12, 1283-1285.

(10) Hutchings, G. J.; King, F.; Okoye, I. P.; Padley, M. B.; Rochester, C. H.; J. Catal. 1994, 148,

453-463.

(11) Claus, P; Hofmeister, H; J. Phys. Chem. B 1999, 103, 2766-2775

(12) Yang, X; Wang, A; Wang, X; Zhang, T; Han, K; Li, J; J. Phys. Chem. C 2009, 113, 20918-

20926.

(13) Cropley, R. L.; Williams, F. J.; Urquhart, A. J.; Vaughan, O. P. H.; Tikhov, M. S.; Lambert,

R. M.; J. Am. Chem. Soc. 2005, 127, 6069-6076.

(14) Brandt, K.; Chiu, M. E.; Watson, D. J.; Tikhov, M. S.; Lambert, R. M.; J. Am. Chem. Soc.

2009, 131, 17286-17290.

(15) Beaumont, S. K.; Kyriakou, G.; Watson, D. J.; Vaughan, O. P. H.; Papageorgiou, A. C.;

Lambert, R. M.; J. Phys. Chem. C 2010, 114, 15075-15077.

(16) Outka, D. A.; Stohr, J.; J. Chem. Phys. 1988, 88, 3539-3554.

(17) Stohr, J.; Outka, D. A.; Phys. Rev. B: Condens. Matter, 1987, 36, 7891-7905.

(18) Mijoule, C.; Russier, V.; Surf. Sci. 1991, 254, 329-340.

(19) Healey, F.; Carter, N.; Worthy, G.; Hodgson, A.; Chem. Phys. Lett. 1995, 243, 133-139.

18

(20) Luo, M. F.; MacLaren, D. A.; Shuttleworth, I. G.; Allison, W.; Chem. Phys. Lett. 2003, 381,

654-659.

(21) Kammler, T.; Kuppers, J.; J. Chem. Phys. 1999, 111, 8115-8124.

(22) Campo, B.; Volpe, M.; Ivanova, S.; Touroude R.; Journal of Catalysis 2006, 242, 162–171.

(23) Bron, M.; Teschner, D.; Knop-Gericke, A.; Scheybal, A.; Steinhauer, B.; Havecker, M.;

Fodisch, R.; Honicke, D.; Schlogl, R.; Claus, P.; Catalysis Communications 2005, 6, 371–

374.

(24) de Jesus, J.C.; Zaera, F.; Surf. Sci. 1999, 430, 99-115.

(25) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.; Handbook of X-ray Photoelectron

Spectroscopy, Perkin-Elmer Corporation, Physical Electronics; Eden Prairie, MN, 1992.

(26) Redhead, P. A.; Vacuum, 1962, 12, 203-211.

(27) Outka, D. A.; Stöhr, J.; Madix, R. J.; Rotermund, H. H.; Hermsmeier, B.; Solomon, J.; Surf.

Sci. 1987, 185, 53-74.

(28) Ishii, I.; Hitchcock, A. P; J. Electron Spectrosc. Relat. Phenom. 1988, 46, 55-84.

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TABLE OF CONTENTS GRAPHIC