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Surface Chemistry Underpinning Enantioselective Heterogeneous Catalysis: Supramolecular Self-Assembly of Chiral Modifiers and Pro-Chiral Reagents on Ni{111}† Aoife G. Trant and Christopher J. Baddeley* EaStCHEM School of Chemistry, UniVersity of St Andrews, St Andrews, Fife, KY16 9ST, U.K. ReceiVed: June 11, 2010; ReVised Manuscript ReceiVed: August 3, 2010
One of the most heavily studied examples of enantioselective heterogeneous catalysis is the hydrogenation of β-ketoesters over chirally modified Ni catalysts. We use scanning tunneling microscopy to investigate the interaction of the simplest β-ketoester, methylacetoacetate, with Ni{111} surfaces premodified with (S)glutamic acid. The behavior of methylacetoacetate is strongly dependent on the initial modifier coverage. At an intermediate (S)-glutamic acid coverage, two distinct domains are identified which correspond to ordered arrangements of glutamate and methylacetoacetate in different stoichiometric ratios. The implications of our findings for enantioselective catalysis are discussed. Introduction The ability to selectively produce one enantiomeric product in a catalytic reaction is increasingly important in the pharmaceutical and agrochemical industries. This is largely due to the fact that two enantiomers of a chiral molecule can have vastly different physiological properties. This significance of enantioselective catalysis was underlined by the award, in 2001, of the Nobel Prize for Chemistry to Knowles,1 Noyori,2 and Sharpless3 for their work in this area. To date, homogeneous catalysis and enzyme catalysis have provided the route to the production of most industrial chiral molecules. Despite the huge potential economical advantages of heterogeneous catalysis in terms of ease of product separation and high yields, heterogeneous enantioselective systems have failed to break through into the industrial catalytic arena. The number of heterogeneously catalyzed processes is rapidly growing, but a detailed understanding of their operation is generally lacking. There have been two main approaches to resolving this problem. First, many groups have sought to “heterogenize” homogeneous systems via attaching a chiral homogeneous catalyst to a support, for example, via covalent interactions.4 The second approach involves the utilization of metal catalysts which are “modified” by the adsorption from solution of chiral molecules. The two most heavily researched examples of this type of catalyst are Pt/cinchona systems used for R-ketoester hydrogenation5-8 and Ni/tartrate or Ni/amino acid systems for β-ketoester hydrogenation.7,9 Examples of these reactions are shown schematically in Figure 1. In each case, the reactant molecule (e.g., methyl pyruvate or methylacetoacetate) is close to planar. If the reactant adsorbs with the molecular plane parallel to the metal surface, the act of adsorption results in an adsorbed molecular species which is nonsuperimposable on its mirror image and can therefore be considered chiral. In the absence of a chiral influence, one would anticipate a 50:50 mixture of the two “enantiomers” of the adsorbed molecule. The reaction is thought to occur via attack of atomic hydrogen from beneath the molecular plane of the reactant. The stereochemistry of the reaction product is deter†
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. E-mail: cjb14@ st-and.ac.uk.
Figure 1. Reaction schemes for the enantioselective hydrogenation of (top) R-ketoesters over chirally modified Pt catalysts and (bottom) β-ketoesters over chirally modified Ni catalysts.
mined by which the enantiotopic face (pro-(R) or pro-(S)) of the molecule is attached to the surface. One may therefore conclude that, in order to maximize enantioselectivity, one needs to control the adsorption geometry of the modifier such that exclusively one enantiotopic face of the reactant is adopted on the metallic surface. There are a number of ways in which the energetics of the reaction pathway to the (R) and (S) enantiomeric products could be envisaged to be skewed such as to favor the formation of one product. One-to-One Docking Interactions. The presence of randomly distributed chiral molecules on the surface can provide docking sites (e.g., via intermolecular H-bonding interactions)
10.1021/jp105377p 2011 American Chemical Society Published on Web 09/01/2010
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Figure 2. STM image (4 nm × 4 nm) showing the 2-D supramolecular structure consisting of an ordered array of 1:1 H-bonded complexes of (R,R)-tartrate and methylacetoacetate species on Ni{111} giving a chiral 〈3 1|-3 4〉 structure (adapted from ref 17).
favoring one enantiotopic face. This coupled with a rate enhancement at the modified sites can lead to a very high enantioselectivity. Extensive experimental and computational work by the groups of Baiker,10,11 Blaser,8 and Wells12 have conclusively established that such a mechanism operates in the Pt-catalyzed reaction where enantiomeric excess (ee) values of >97% have been reported.13,14 In the Ni-catalyzed system, no such rate enhancement is observed.9 In this case, similar docking interactions are thought to be important, but the enantioselective reaction at the modified sites is believed to compete with the racemic reaction at bare metal sites. As such, the enantioselectivity of the Ni catalyzed reaction is strongly dependent on the modifier coverage.15 Too high a coverage, and the adsorption of methylacetoacetate is blocked;16,17 too low a coverage, and the rate of the competing racemic reaction limits enantioselectivity. Role of Chiral Surface Defects. A second possible contributor to enantioselective reactions at surfaces involves the induction of chirality on the metal itself. It is now well established that metal surfaces can be inherently chiral.18,19 In the case of fcc surfaces, chirality is manifested by the presence of stepkink defects. The two enantiomers of chiral adsorbates bind with different adsorption energies at such defects.20-22 Similarly, since the adsorbate-substrate complex is itself chiral, such step-kink sites can interact differently with the pro-(R) and pro-(S) adsorption geometries of a pro-chiral reactant. If a surface with a high density of chiral defects could be produced, it could operate as an enantioselective catalyst in the absence of chiral modifiers. The corrosive adsorption of amino acids on Cu surfaces has been shown to yield chiral arrangements of metal atoms stabilized via interactions with the chiral adsorbate.23 Our own work has shown that, under conditions where catalytic enantioselectivity is optimized,24 the coverage of aspartic acid on nickel is below the detection limit of reflection absorption infrared spectroscopy.25 One interesting inference from this work is that either a very low coverage of modifier has an unusually large effect on enantioselectivity or chiral metal arrangements themselves act enantioselectively. In each case, the performance of the catalyst would be dramatically improved if the localized chiral recognition at the asymmetric site (either modifier or chiral defect) could be amplified into terrace regions of the surface. Pascual et al. have shown that the ordering of the pro-chiral carboxylic acid 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) molecules in the vicinity of step-kinks on Ag{110} extends for a few molecular repeats into the Ag terraces.26 Similarly, Chen
and Richardson demonstrated chiral recognition between chains of adenine dimers and phenylglycine on Cu{110}.27 Phenylglycine nucleation at the adenine dimer row resulted in an ordered phenylglycine arrangement. As was the case for the PVBA on silver,26 the extent of ordering observed was relatively shortranged, i.e., just a few molecular repeats.27 Formation of 2-D Chiral Supramolecular Assemblies. In contrast to the Pt/cinchona system, where typical modifiers (e.g., cinchonidine) are relatively large in comparison with the reactant molecules (e.g., methyl pyruvate), in the Ni catalyzed system, the modifiers and reactants are similar in dimensions. In the Pt/cinchona system, the large modifier is able to envelop the reactant, dramatically favoring the adsorption of one enantiotopic face.28 In the Ni system, though models have been presented for 1:1 interactions29-31 which would lead to enantioselective behavior, it is less clear that a significant difference would exist between pro-(R) and pro-(S) adsorption. Earlier work by our group proposed that the mobility of tartrate modifiers enables the formation of highly ordered supramolecular assemblies of tartrate and methylacetoacetate on Ni{111}.17 The pro-(R) face of methylacetoacetate is stabilized in these assemblies by multiple H-bonding interactions with neighboring (R,R)-tartrate species,17 as shown in Figure 2. In recent years, a large body of work has focused on the production of H-bonded networks at surfaces, for example, the coadsorption of perylene tetracarboxylic diimide (PTCDI) and melamine on Ag-terminated Si32 and Au33 surfaces. In these systems, the stoichiometry of the adsorbates is often found to strongly influence the structures produced. In the Ni{111}/(R,R)-tartaric acid/methylacetoacetate system, such structures are observed over a narrow range of tartrate coverage, i.e., approximately 20% of its saturation value on Ni{111}.17 Interestingly, this is precisely the tartrate coverage which is thought to produce optimum enantioselectivity in the catalytic reaction.15 In this manuscript, we demonstrate that similar supramolecular structures are formed when methylacetoacetate is coadsorbed with glutamic acid on Ni{111} at intermediate coverages of glutamate. We therefore provide support for our earlier proposal that hydrogen bond mediated supramolecular self-assembly may play an important role in the mechanism of the enantioselective hydrogenation of β-ketoesters over chirally modified Ni catalysts.17 Experimental Methods Reflection absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM) experiments were con-
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Figure 3. STM images presenting (a) low coverage (S)-glutamic acid adsorbed onto Ni{111} at 300 K (12.3 nm × 12.3 nm, It ) 0.4 nA, Vt ) -1 V), (b) following the adsorption of 10 L of H2 followed by methylacetoacetate onto the surface presented in part (a) (57 nm × 57 nm, It ) 1.4 nA, Vt ) 1.4 V), and (c) showing the effect of time after methylacetoacetate exposure on the surface morphology (100 nm × 100 nm, It ) 0.1 nA, Vt ) 1.5 V).
ducted in ultrahigh vacuum (UHV) conditions in an Omicron variable temperature-STM system with a base pressure of 1 × 10-11 mbar which also has facilities for sample cleaning via argon ion bombardment and characterization via low energy electron diffraction (LEED). A Nicolet Nexus 860 FTIR spectrometer was used to acquire RAIRS data (512 scans at a resolution of 4 cm-1) using a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. A background spectrum was taken of the Ni{111} surface prior to exposure to glutamic acid and methylacetoacetate. In this case, RAIRS was primarily used to ensure that similar surfaces were created to those reported in the earlier study by Jones and Baddeley.16 All STM experiments were acquired in constant current mode by use of an electrochemically etched W tip. STM topographic images were processed using WSxM software and were limited to low-pass filtering.34 In all experiments, the Ni{111} sample was cleaned by cycles of ion sputtering (Ar+, 1.5 kV, sample ∼8 µA) and annealing to 900 K until a sharp (1 × 1) LEED pattern was obtained. The Ni{111} surface was then exposed to glutamic acid, via sublimation from a solid doser. For purposes of qualitative comparison of relative exposures, glutamic acid exposures are quoted in Langmuirs (L) where 1 L ) 10-6 Torr s and the pressure measurement corresponds to the ion gauge reading in the background of the chamber. The temperature of glutamic acid was monitored via a thermocouple and was maintained at 403 K for all experiments. Methylacetoacetate (Sigma-Aldrich,
99% purity) was purified by freeze-pump-thaw cycles before use (verified by mass spectroscopy) and introduced to the Ni{111} sample via background dosing using a high precision leak valve at 300 K with a dosing pressure of typically ∼1 × 10-10 Torr. Results Adsorption of Methylacetoacetate onto Ni{111} Premodified by (S)-Glutamic Acid. (i) Low (S)-Glutamic Acid CoWerage. The Ni{111} surface was exposed to 1 L of (S)-glutamic acid at 300 K so as to ensure a low glutamic acid coverage. As previously reported,35 the only identifiable glutamic acid species under these conditions are located at step sites, as shown in Figure 3a. The dimensions of these features indicate that the species are likely to consist of dimers of glutamate species. The surface was then exposed to 10 L of H2 at a hydrogen partial pressure of 1 × 10-7 Torr. This hydrogen exposure was chosen because this was found to give a saturation coverage by Christmann et al. exhibiting a (1 × 1) LEED pattern.36 We did not use LEED in our experiments, as we found that the glutamic acid overlayers were strongly susceptible to beam damage on the Ni{111} surface. The coadsorption of hydrogen was carried out to more accurately simulate the conditions encountered in the heterogeneous catalytic reaction. It has previously been shown by Lambert and co-workers that the surface chemistry of methyl pyruvate on Pt surfaces depends strongly on the presence or absence of coadsorbed hydrogen.37 In our case, the
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Figure 4. STM topographic image presenting the formation of cocrystalline arrays between methylacetoacetate and H2/(S)-glutamic and on Ni{111} (38 nm × 38 nm, It ) 0.45 nA, Vt ) -0.5 V). The domain labeled A is assigned to a pure glutamate domain. The domain labeled B is assigned to a mixed glutamate/methylacetoacetate domain.
behavior of glutamic acid did not appear to be influenced by the presence of hydrogen with similar adsorption behavior to that observed previously.35 After pretreatment in hydrogen, the sample was exposed to methylacetoacetate at 300 K. No ordered
Trant and Baddeley domains of methylacetoacetate were observed on the surface with STM. However, as shown in Figure 3b, there is evidence for considerable corrosion of Ni steps. In addition to the reshaping of the Ni steps, a large number of nanosized bright features are observed on the terraces. The height of these features is ∼2 Å relative to the surrounding terrace. For purposes of comparison, the Ni{111} step height is 2.04 Å and the apparent height of glutamate species is ∼0.5 Å. We attribute these structures to Ni clusters etched from the steps by a combination of glutamic acid and methylacetoacetate. After several hours, the cluster-like material on the terraces disappears and the step edges adopt a more faceted appearance, as shown in Figure 3c. This signifies extensive mass transport of Ni mediated by the organic adsorbates. (ii) Intermediate (S)-Glutamic Acid CoWerage. At intermediate coverages of (S)-glutamic acid deposited at 300 K, no ordered arrangements of glutamic acid species were observed. Following exposure to 10 L of H2 at 300 K followed by methylacetoacetate, the surface is covered with a number of identifiable phases, as shown in Figure 4. Phase A (Figure 5a) is identified as a (7 × 7) R19.1° phase previously observed for (S)-glutamic acid on Ni{111} by Jones et al.35 Phase B (Figure 5b) is the most commonly observed phase, giving a rhombus-shaped unit cell of dimensions ∼11 Å × ∼11 Å containing four “footprint”-like features at the four corners of the unit cell and an additional, apparently different, feature at the center of the unit cell. We are unable to use LEED to determine the registry of the overlayer, since the LEED beam damages the molecular overlayer. If the overlayer is commensurate with the underlying Ni surface, the most likely structure involves a (21 × 21) R10.9° overlayer. This overlayer would give a rhombic unit cell with sides 11.4 Å in length. The close packed direction is consistent with the LEED
Figure 5. STM topographic image presenting the formation of cocrystalline arrays between (S)-glutamic and methylacetoacetate on Ni{111}. Three types of arrays were identified: (a) phase A (5.5 nm × 5.5 nm, It ) 0.45 nA, Vt ) -0.5 V); (b) phase B (7.8 nm × 7.8 nm, It ) 0.45 nA, Vt ) -0.5 V); (c) phase C (4.4 nm × 4.4 nm, It ) 0.49 nA, Vt ) 0.5 V).
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Figure 6. STM topographic image presenting saturation coverage of (S)-glutamic acid adsorbed on Ni{111} at 300 K (29 nm × 29 nm, It ) 0.68 nA, Vt ) -1.2 V).
pattern of the clean Ni{111} surface. (This structure has not been previously identified following the adsorption of either (S)glutamic acid or methylacetoacetate on Ni{111}). Under analogous conditions, Jones and Baddeley identified that methylacetoacetate adopts a flat lying configuration likely in the diketo tautomeric form and an interaction was proposed between methylacetoacetate, acting as a H-bond acceptor and zwitterionic (S)-glutamic acid acting as a H-bond donor.16 A third, minority phase was identified elsewhere on the surface. This phase (phase C) has unit cell dimensions of ∼21 Å × 9 Å with an angle of 97° between the unit cell vectors, as shown in Figure 5c. The unit cell contains two well-defined molecular features and an additional poorly defined feature. (iii) High (S)-Glutamic Acid CoWerage. At high (S)-glutamic acid exposure, a dense but poorly ordered coverage of molecular species is observed, as exemplified in Figure 6. Under these conditions, the sticking probability of methylacetoacetate is found to be close to zero.16 Discussion The coverage of modifier is known to be an important parameter in the optimization of enantioselectivity.15 A reaction requiring coadsorbed β-ketoester and hydrogen is highly likely to be retarded by a very dense modifier overlayer. At low glutamic acid coverages, exposure to methylacetoacetate causes substantial reshaping of step edges. This process is considerably slower in the absence of glutamic acid and may indicate that the amino acid facilitates the corrosion process. Eventually, the step edges of the Ni surface become refaceted. It is likely that the influence of the combination of the chiral modifier and pro-chiral reactant may lead to the production of chiral facets analogous to the behavior of lysine on Cu{001}.23 A very low coverage of modifier could, in principle, lead to a highly selective catalytic system by a combination of chiral recognition (i.e., the preferred docking of one enantiotopic face with a modifier, or a chiral step-kink site) and chiral amplication (i.e., the propagation of the favored enantiotopic face into terrace sites away from the modifier). Wilson and Baddeley used
Figure 7. Proposed models for the molecular structures for (a) phase B and (b) phase C. In each case, all methylacetoacetate species are in the pro-(R) configuration with the exception of the circled molecules which are in the pro-(S) configuration.
RAIRS to show that when aspartic acid modified Ni catalysts are prepared via deposition of aspartic acid from solution, under conditions where catalytic enantioselectivity is optimized, the coverage of modifier is below the detection limit of RAIRS.25 This implies that, for aspartic acid, chiral step-kink sites may play a role in the catalysis. However, aspartic acid is a relatively poor modifier compared to tartaric acid and glutamic acid, so any role of chiral step-kinks may be limited. At intermediate glutamic acid coverages, there is clear evidence for the formation of 2-D supramolecular assemblies. We conclude that phase B, the (21 × 21) R10.9° domain shown in Figure 5b, contains a 1:1 mixture of (S)-glutamate and methylacetoacetate. The local coverage of (S)-glutamate in this structure is 0.05 ML. This compares with the local coverage of 0.07 ML (R,R)-tartrate in the analogous structure observed on Ni{111} by Jones and Baddeley.17 The coverage of tartaric acid reported by Keane for optimum enantioselective behavior is 20% of a saturated monolayer.15 A saturated monolayer is believed to equate to 0.28 tartaric acid molecules per surface Ni atom. Hence, the coverage regime where the supramolecular assemblies are observed for both tartrate and glutamate on Ni is precisely in the optimum regime defined by Keane.15 In order to ascertain the nature of the coadsorbed structure, we have assumed that the “heel” of the “footprint” observed in
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the STM image corresponds to the aliphatic carboxylic acid functionality. The dimensions of the unit cell are ideal for methylacetoacetate to form intermolecular H-bonds to the surrounding modifiers. Computational modeling would be required to ascertain the optimum structure of methylacetoacetate and (S)-glutamate in this assembly, but the proposed model in Figure 7a would allow a number of H-bonds to occur. The interaction between the pro-(S) configuration of methylacetoacetate (circled) is subtly different from the pro-(R). Even a stablization of a few kJ mol-1 could lead to a preference for one geometry and hence enantioselective behavior. If each methylacetoacetate is equivalent in such a domain and the pro(R) face is exposed to the surface, this would yield an enantiomeric excess of (R)-methyl-3-hydroxybutyrate. This is the preferred product when methylacetoacetate is hydrogenated over Ni catalysts modified by (S)-glutamic acid at 300 K.9 Figure 7b shows our schematic model for phase C. In this case, we conclude that two glutamate species and one methylacetoacetate molecule are contained within the unit cell. The dimensions of the molecular features are consistent with the formation of a row of glutamate dimers interacting via their aliphatic carboxylic acid groups. Our earlier RAIRS study concluded that, under these conditions, glutamate exists in the zwitterionic configuration with the carboxylate of the amino acid functionality bound to the surface and the -NH3+ functionality available for H-bonding interactions.16 In phase C, it appears that methylacetoacetate in the pro-(R) configuration could form two N-H · · · O hydrogen bonds, while only one hydrogen bond would be possible in the pro-(S) configuration (circled). The role of 2-D self-assembly appears to be important in the Ni/tartrate or Ni/amino acid system, whereas the larger modifiers in the Pt/cinchona system are able to control enantioselectivity at a single modifier site. It is very important to note that in neither the tartaric acid17 nor the glutamic acid case is there a requirement for ordered modifier structures prior to the adsorption of methylacetoacetate. Indeed, it seems to be actively beneficial to have a disordered and mobile coverage of modifier in order to facilitate the formation of the ordered mixed domains of modifer and pro-chiral reagent. Conclusions The adsorption of methylacetoacetate onto Ni{111} surfaces premodifed by (S)-glutamic acid is strongly dependent on modifier coverage. At low modifier coverage, methylacetoacetate adsorption causes extensive etching of Ni step edges. This process appears to be enhanced by the presence of glutamate species at step sites. Eventually, the step edges are able to refacet, but no evidence for ordered methylacetoacetate domains is observed under these conditions. At high modifier coverage, the densely packed glutamate species are unable to accommodate methylacetoacetate adsorption. At intermediate coverages of glutamate, the adsorption of methylacetoacetate results in the formation of 2-D supramolecular domains presumably stabilized by intermolecular Hbonding interactions. The chirality of the (S)-glutamate species is very likely to cause each methylacetoacetate species to adsorb
Trant and Baddeley exclusively via one of its two possible enantiotopic adsorption geometries. The formation of supramolecular self-assemblies is very similar to the behavior of methylacetoacetate with tartaric acid covered Ni{111}.17 The relevance of these findings to enantioselective catalysis is underlined by the fact that in each case the modifier coverage is in a similar regime to the optimum coverage measured in catalytic experiments.15 Acknowledgment. A.G.T. acknowledges the Engineering and Physical Sciences Research Council for financial support of her postdoctoral position via grant EP/E047580/1. We are very grateful to Professor Neville Richardson for access to his STM/ RAIRS apparatus and to Dr. Steve Francis for his technical advice and support throughout the work. References and Notes (1) Knowles, W. S. Angew. Chem., Int. Ed. 2002, 41, 1999. (2) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008. (3) Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2024. (4) Fraile, J. M.; Garcia, J. I.; Herrerias, C. I.; Mayoral, J. A.; Pires, E. Chem. Soc. ReV. 2009, 38, 695. (5) Orito, Y.; Imai, S.; Niwa, S. Nippon Kagaku Kaishi 1979, 1118. (6) Baiker, A. Catal. Today 2005, 100, 159. (7) Webb, G.; Wells, P. B. Catal. Today 1992, 12, 319. (8) Studer, M.; Blaser, H. U.; Exner, C. AdV. Synth. Catal. 2003, 345, 45. (9) Izumi, Y. AdV. Catal. 1983, 32, 215. (10) Baiker, A. Curr. Opin. Solid State Mater. Sci. 1998, 3, 86. (11) Baiker, A. J. Mol. Catal. A: Chem. 1997, 115, 473. (12) Wells, P. B.; Wilkinson, A. G. Top. Catal. 1998, 5, 39. (13) Zuo, X. B.; Liu, H. F.; Guo, D. W.; Yang, X. Z. Tetrahedron 1999, 55, 7787. (14) Torok, B.; Balazsik, K.; Torok, M.; Szollosi, G.; Bartok, M. Ultrason. Sonochem. 2000, 7, 151. (15) Keane, M. A.; Webb, G. J. Catal. 1992, 136, 1. (16) Jones, T. E.; Baddeley, C. J. Langmuir 2006, 22, 148. (17) Jones, T. E.; Baddeley, C. J. Surf. Sci. 2002, 519, 237. (18) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483. (19) Jenkins, S. J.; Pratt, S. J. Surf. Sci. Rep. 2007, 62, 373. (20) Ahmadi, A.; Attard, G.; Feliu, J.; Rodes, A. Langmuir 1999, 15, 2420. (21) Horvath, J. D.; Gellman, A. J. J. Am. Chem. Soc. 2001, 123, 7953. (22) Sholl, D. S.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771. (23) Zhao, X. Y. J. Am. Chem. Soc. 2000, 122, 12584. (24) Izumi, Y.; Imaida, M.; Fukawa, H.; Akabori, S. Bull. Chem. Soc. Jpn. 1963, 36, 155. (25) Wilson, K. E.; Baddeley, C. J. J. Phys. Chem. C 2009, 113, 10706. (26) Pascual, J. I.; Barth, J. V.; Ceballos, G.; Trimarchi, G.; De Vita, A.; Kern, K.; Rust, H. P. J. Chem. Phys. 2004, 120, 11367. (27) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324. (28) Schwalm, O.; Minder, B.; Weber, J.; Baiker, A. Catal. Lett. 1994, 23, 271. (29) Petrov, Y. I.; Klabunovskii, E. I. Kinet. Catal. 1967, 8, 814. (30) Hoek, A.; Sachtler, W. M. H. J. Catal. 1979, 58, 276. (31) Ito, K.; Harada, T.; Tai, A.; Izumi, Y. Chem. Lett. 1979, 1049. (32) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (33) Perdigao, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539. (34) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78. (35) Jones, T. E.; Urquhart, M. E.; Baddeley, C. J. Surf. Sci. 2005, 587, 69. (36) Christmann, K.; Behm, R. J.; Ertl, G.; Vanhove, M. A.; Weinberg, W. H. J. Chem. Phys. 1979, 70, 4168. (37) Bonello, J. M.; Lambert, R. M.; Kunzle, N.; Baiker, A. J. Am. Chem. Soc. 2000, 122, 9864.
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