17142
J. Phys. Chem. C 2010, 114, 17142–17147
Dehydrogenation versus Decarbonylation of Oxygenates on Pd(110): Pure, Clean Pd Is a Poor Catalyst† M. Bowker,* L. Gilbert, J. Counsell, and C. Morgan‡ Wolfson Nanoscience Laboratory and Cardiff Catalysis Institute, School of Chemistry, Cardiff UniVersity, Cardiff CF10 3AT, U.K. ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: July 15, 2010
Clean, pure Pd is a dehydrogenation catalyst and will crack many organic molecules interacting with it down to basic components, generally carbon, CO, and hydrogen. However, when the surface is passivated by adsorption in sufficient amount, the balance of reactivity changes, producing a more gentle catalytic material, which has much reduced dehydrogenation capabilities and more favors hydrogenation. In this paper, we describe molecular beam reactor measurements for the adsorption and reaction of C3 oxygenates with the Pd(110) surface, which clearly illustrate these properties. Introduction Surface science has evolved as a methodology for understanding the structure and reactivity of pure, well-prescribed materials and has established a database of parameters for the interaction of molecules with metal surfaces. However, when it comes to the field of catalysis, in a reactive situation of high pressure and high temperature, then the surface is unlikely to be (i) pure and (ii) monocrystalline. Nonetheless, a number of important concepts have been derived from surface science experiments which are absolutely vital for our understanding of catalysis. These include, for instance, the effect of lateral interactions upon the binding of molecules as coverage changes, the role of precursor states in enhancing adsorption rates, and the “flexible surface” concept of Somorjai.1,2 All of these show us that the structure and reactivity of surfaces can depend on the environmental conditions surrounding the surface by affecting surface structure and/or adsorption energies. In this context, we here want to focus on the behavior of a particular catalytically active metal, namely, Pd. Pd is widely used commercially for a variety of heterogeneous reactions and very widely for heterogeneous, liquid-phase hydrogenation catalysis for the production of fine chemicals and pharmaceuticals. Hence, it is important to understand how pure Pd surfaces interact with simple functional organic molecules. In relation to this, Goodman et al. have carried out a wideranging body of work relating to VAM synthesis3-9 using both Pd and Pd-Au powdered catalysts3-5,8 and crystals/model catalysts.6-9 One of the conclusions from this work was that the presence of carbon on the surface as a cracking product has a substantial effect on reactivity, generally reducing the activity of Pd.3,7,8 The effect of carbon on the surface has also been reported for single crystals from the group of Schlogl et al.10-13 and on model catalysts by Freund at al.14,15 These effects will be discussed in more detail below. In earlier work, we used molecular beam reactor methods to evaluate the reactive behavior of simple oxygenates with the Pd(110) surface.16-24 Here, we report on the reactivity of C3 oxygenates with the Pd(110) surface and compare it with the †
Part of the “D. Wayne Goodman Festschrift”. * To whom correspondence should be addressed. ‡ Present address: Diamond Light Source, Harwell, Oxfordshire, U.K.
behavior of these other molecules in order to generalize the behavior. We especially want to emphasize the role of adsorbate deposition on the clean surface in significantly modifying the reactivity pattern and speculate on the significance of this for catalysis on supported Pd catalysts. Experimental Section A stainless steel UHV system was employed in these studies, with a base pressure of ∼2 × 10-10 mbar. The chamber was equipped with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and a quadrupole mass spectrometer (QMS) for residual gas analysis. Gases were allowed to enter the chamber via leak valves connected to a rotary-backed gas line. Kinetic studies were carried out using a molecular beam reactor, the design and principles of which have been described previously.25,26 An in-beam pressure of ∼5 × 10-8 mbar was obtained from a source pressure of 40 mbar, which was usually used in this work. The system relies on the principles described by King and Wells, which enables the accurate measurement of sticking probabilities.27 The Pd(110) crystal was mounted on two tungsten wires, which pass through grooves in the edges of the crystal, and it was heated by passing current through these wires. The temperature was measured using a chromel-alumel thermocouple, which was attached directly to the sample via a small hole drilled in the side of the crystal. The sample was cleaned by flashing to 900 K, sputtering with 500 eV of Ar+ at 700 K, flashing again, and finally annealing in O2. This procedure required approximately 3 weeks of work to get the crystal completely clean. Results We examined the reactivity of four different but related oxygenated molecules, which have either alcohol, aldehyde, alkene functional groups, or a combination of them. 1. Propan-1-al. Figure 1a shows molecular beam reactor results for the sticking of propanal on the Pd(110) surface at 333 K and shows the evolution of the main products, together with the sticking probability dependence on the time of reaction. Time zero in these data, and those to follow, is the point at which the molecular beam of the reactant is allowed to impact
10.1021/jp104837t 2010 American Chemical Society Published on Web 07/27/2010
Dehydrogenation versus Decarbonylation of Oxygenates
Figure 1. (a) Molecular beam reactor data for the propan-1-al adsorption on Pd(110) at 333 K. Here, the sticking probability (S) and product evolution are shown as a function of time. The bars on the curves represent the mean deviation of the noise on the product curves, while it is the mean error for the sticking values. These errors are similar for the following figures also. The product curves have been normalized to a maximum value of 0.5. (b) As for (a) but at a crystal temperature of 383 (squares) and 473 K (circles). Only one profile is shown for CO and H2 at 473 K since their profiles were identical. The product curves have been normalized in this case to a maximum value of 1.0 for clarity.
the surface by opening the blocker in the main UHV chamber. The initial sticking probability, S0, is 0.41, which then drops to a lower value of 0.28 after about 80 s of reaction time and which remains constant thereafter. This also corresponds with a switch in reactivity and is due to a steady-state reaction taking place after an initial non-steady-state period. Immediately after the beam is introduced, dehydrogenation takes place, as shown by hydrogen evolution for the first 40 s or so of reaction. No other products are seen at the beginning; therefore, presumably, fragments of the rest of the molecule remain on the surface, but after ∼40 s, when the hydrogen evolution has reduced significantly, ethene begins to evolve, together with CO, though it takes some time for them to reach constant evolution rate (it takes ∼2 min). Ethene production seems to start very quickly,
J. Phys. Chem. C, Vol. 114, No. 40, 2010 17143
Figure 2. (a) Molecular beam reactor data for the acrolein adsorption on Pd(110) at 333 K. The product curves have been normalized to a maximum value of 0.5. (b) Molecular beam reactor data for the acrolein adsorption on Pd(110) at 373 K. The product curves have been normalized to a maximum value of 0.5.
reaching a peak at around 30 s of reaction, whereas CO seems lagged. Ethene production then drops to the steady-state level from a higher rate of production, and CO reaches steady state at about the same time (80 s). At a reaction temperature of 383 K, the reaction profile is rather different, with only hydrogen and CO being seen as products (Figure 1b). However, there is no steady-state reaction, and product evolution ceases after ∼1.5 min. At elevated temperatures of 473 K and above, the only products evolved were CO and H2, but the reaction proceeded continuously, with a sticking probability for the propanal of 0.4 (Figure 1b). 2. Acrolein (Prop-2-en-1-al). The sticking probability of acrolein was generally much higher than for propanal, at a value of ∼0.7 at 333 K surface temperature (Figure 2a). It is interesting to note that this is close to the value found for ethene and significantly bigger than it is for propanal above, implying that the presence of the CdC π bond aids adsorption. Like propanal, however, there is also an initial dehydrogenation, which drops after ∼30 s, followed by a sharp increase in CO evolution, which
17144
J. Phys. Chem. C, Vol. 114, No. 40, 2010
Figure 3. Molecular beam reactor data for the allyl alcohol adsorption on Pd(110) at 323 K. The product curves have been normalized to a maximum value of 1.0 for clarity. In the legend, “al” refers to the propanal product.
drops slowly to a steady-state level after ∼180 s. There is also evolution of ethene, following CO, quickly reaching steady state by ∼100 s. At steady-state reactivity, the sticking has dropped to 0.3 and continues at that adsorption rate. At 373 K, the behavior is rather different (Figure 2b); no ethene is evolved, and the sticking probability continuously diminishes from its initial value of 0.48. Like propanal above, the only products at 473 K (not shown) are CO and hydrogen, which evolve continuously with a sticking probability of ∼0.5. 3. Allyl Alcohol (Prop-2-en-1-ol). As seen in Figure 3, this alcohol sticks to the surface with a similar probability as that for propanal and lower than that for acrolein. As for the other molecules, the initial reaction is dehydrogenation, proceeding during the initial stages of adsorption, followed by CO and ethene evolution, beginning after ∼30 s of beaming. The CO evolution goes through a maximum before reaching steady state after about 150 s, while the ethene rises to reach steady state after 30 s. Also evolved with these products is propanal; its evolution is lagged, slightly more than CO and ethene. Propanal production was also reported by Shekhar and Barteau in TPD experiments.30 The sticking drops from an initial value of ∼0.42 to ∼0.21 at steady state. The behavior at more elevated temperatures then evolves in the same way as it does for the other molecules, giving continuous steady-state dehydrogenation at 473 K, but the sticking drops little with temperature, with a continuous sticking probability of 0.37 at that temperature. 4. Propan-1-ol. The behavior of the C3 alcohol appears to be rather different than that of the other molecules. Figure 4 shows molecular beam data for the sticking of propanol on the Pd(110) surface. Here again, we see the dehydrogenating properties of the Pd surface during first impact of the molecule with the surface. However, a major difference from the molecules above is that reactivity appears to stop after this initial phase; there is no steady state part to the reaction. Further, the initial sticking probability diminishes significantly as temperature increases, so that by 473 K, it is only ∼0.05 ( 0.03 (data not shown). The reasons for this difference in behavior are considered below in more detail. Discussion 1. Pd(110) as Dehydrogenator. It is clear from the work above with the C3 oxygenates that the clean Pd surface is a
Bowker et al.
Figure 4. Molecular beam reactor data for the propanol adsorption on Pd(110) at 333 K. In this case, the only gas-phase product is hydrogen.
vigorous dehydrogenator. The evidence for this is that hydrogen is always evolved, from each type of molecule impinged on the surface, as soon as the beam is allowed to hit it. This was also found to be the case earlier for the C131 and C2 primary alcohols,16,31 and for ethanal,21,24 using the molecular beam reactor. Similar behavior was found in several works by Barteau et al. Thus, for instance, for propanal adsorption at 120 K, they found that the only desorption product after dosing small amounts was hydrogen, CO, and surface carbon, and this was also the case for propanol,29 allyl alcohol, and acrolein.30 This dehydrogenation behavior is undoubtedly true at elevated temperature. For all of the molecules above, except for propanol, CO and hydrogen evolve continuously at higher temperatures, the temperature for reaching steady-state evolution generally being ∼450 K; this was also the case for ethanal.21,24 It is somewhat surprising that steady state is achieved at all since at that temperature carbon is being deposited on the surface continuously, and therefore, one would expect the surface to become poisoned. However, the surface continues to be reactive since it appears to be “clean” due to efficient diffusion of carbon from the surface region into the subsurface and, indeed, into the bulk.20,21 We have very recently reported measurements of both dissolution of carbon itself into the bulk of the crystal and segregation back to the surface.31 In that work, we showed that dissolution happens at surprisingly low temperatures and is certainly happening by 450 K, whereas segregation of bulk carbon back to the surface does not occur until very high temperatures (∼750 K). This behavior is due to the balance of the kinetics of dissolution and the thermodynamics of segregation back to the surface (an endothermic process). Thus, the mechanism of decomposition at high temperature, where steadystate dehydrogenation occurs, can be generally expressed as follows
y CxHyOg f COg + H2g + (x - 1)Cb 2 where subscript b refers to bulk carbon, g refers to gas-phase species, and x and y are stoichiometric numbers. That it is carbon that is deposited on the surface at the elevated temperatures,
Dehydrogenation versus Decarbonylation of Oxygenates and not hydrogenated species, comes both from the fact that the species can diffuse into the subsurface (unlikely for hydrogenated species) and, more importantly, from the fact that subsequent treatment in oxygen results in CO and/or CO2 evolution but not H2O evolution.20,21 It is evident that such dehydrogenation occurs at ambient temperature too, at least at the start of the adsorption process, in all cases here (Figures 1-4). This temperature though is below the normal desorption temperature of CO and is below the temperature at which C can dissolve into the subsurface region. Thus, there is a build-up of these species on the surface during adsorption, and this results in a passivation of the dehydrogenation process and a change in reactivity, as described in the following section. It is likely that the carbonaceous species left on the surface is still partially hydrogenated. For instance, for ethene, it was proposed to be a C2H (ethynyl) species, and upon carrying out TPD from low-temperature adsorption, hydrogen desorption was seen up to ∼450 K.20,32,33 However, it is generally observed that most of the hydrogen from such molecules is lost by 350 K, and a recent example of this is the work of Li and Tysoe,34 who found that most hydrogen was lost from adsorbed butan-2-ol on Pd(100) in TPD by ∼360 K. A similar observation was made by Madix et al. for alkenes also on Pd(100),35 and therefore, it is likely that the dominant surface species above ∼360 K are carbon and some adsorbed CO. The steady-state CO coverage will diminish significantly above ∼400 K due to desorption and is obviously already evolving at steady state at as low as 333 K (Figure 1a, for instance), its desorption rate being enhanced by the carbonaceous/carbon adlayer which is coadsorbed. 2. Adsorbate-Covered Pd(110) as a Decarbonylator, Hydrogenator. Thus, as we have described above, the clean Pd surface is most definitely a powerful dehydrogenation catalyst. However, as can be seen in the data for lower temperatures, after the initial dehydrogenation phase, the surface is passivated in this respect and becomes an effective decarbonylation catalyst, proceeding in the following way for the simple aldehydes; the first of these, ethanal, was reported in detail previously21-24
CH3CHO f CH4 + CO C2H5CHO f C2H4 + CO + H2 C2H3CHO f C2H4 + CO It appears that this reaction proceeds on a surface which contains two species, namely, adsorbed CO, and also a carbonaceous species The exact nature of the carbonaceous species is not clear, but it is significantly dehydrogenated as a large amount of hydrogen is always evolved initially when no other gas-phase products evolve. Furthermore, it appears that CO alone can certainly act as the passifier. As shown in Figure 5, if CO is dosed first onto the surface to saturation before acrolein dosing, then several facets of behavior are apparent: (i) Acrolein can stick well notwithstanding the presence of CO (note that oxygen cannot adsorb under these circumstances36,37). (ii) No hydrogen is evolved from the surface initially; dehydrogenation has been stopped. (iii) CO evolution begins immediately, instead of being delayed as it is in Figure 2a. Note that some CO is desorbed from the initial layer (rise and decay of CO in the first 30 s before the steady state in CO is established).
J. Phys. Chem. C, Vol. 114, No. 40, 2010 17145
Figure 5. The effect of predosing CO to saturation on the adsorption and reaction of acrolein, to compare with Figure 2a. Effectively, it is very similar to that figure after ∼70 s of reaction.
(iv) Steady-state sticking and evolution of ethene is established quickly, and the sticking probability is similar to that seen in Figure 2a at steady state, after ∼70 s of reaction. Thus, this behavior shows such a much lower ability of the CO-passivated surface to dehydrogenate the molecules and that H atoms transfer can efficiently occur on the passivated surface, and therefore, partially hydrogenated intermediates are stabilized. In a detailed investigation of this for the case of the ethanal reaction with the surface,21,24 it was clear that the intermediate was the methyl group, which was stabilized by the presence of carbon on the surface, formed by the initial dehydrogenation. In a similar vein, we have shown that acetate is stabilized by carbon.18 That the methyl group was the intermediate was confirmed by isotopic labeling experiments with predosed hydrogen and fully deuterated ethanal. There, the main product from the surface was CD3H, and it was clear that methane was produced from coadsorbed hydrogen and methyl groups rather than by some intramolecular process. The methyl group was also identified by Shekhar et al. by EELS to be present near ambient temperature after heating an ethanal layer adsorbed at low temperature.28 In the case of propanal here, we might ask why ethane is not evolved as a product. This is due to the greater instability of terminal methyl groups, which are more liable to dehydrogenate due to weaker C-H bonding.38 Thus, it appears to proceed to that stage and then hydrogenate at the vinyl surface intermediate rather than via the ethyl, and Shekhar et al. observed this as an intermediate after heating a layer of propanal to near-ambient temperatures.28 The results for acrolein are similar, except, of course, that the ethyl cannot form in that case, and this supports the model for the similar behavior of propanal just described. What about the role of adsorbed carbon? In general, it seems to be a poison for the reaction. Thus, in Figures 1b and 2b, at intermediate temperatures, reactivity drops. As shown previously for ethene20 and ethanal adsorption,21,24 this is due to the buildup of carbon in the surface and subsurface region because at that temperature, diffusion of carbon into the bulk is too slow. At the higher temperature of 473 K, diffusion is fast, and therefore, the surface remains reactive. However, there are several reports of C presence being useful, at least in terms of selectivity and changing the course of catalytic reactions. Thus, for alumina-
17146
J. Phys. Chem. C, Vol. 114, No. 40, 2010
supported Pd, Kennedy et al.39 report selective, partial hydrogenation of propyne in the presence of carbon in/on the catalyst, whereas the clean surface totally hydrogenates the molecule. Similarly, Schlogl et al.11-13 have shown, mainly using highpressure XPS, that C can be present under circumstances of low hydrogen/alkyne ratio, which favors the partial hydrogenation on a variety of forms of Pd, including single-crystal Pd(111). Freund at al. have shown similar effects for methanol partial dehydrogenation on Pd(111)14 and for alkene hydrogenation,15 though they feel that the main effect of this C is to modify the nature of the hydrogen adsorbed on/absorbed in the nanoparticles. Although not reported here, it should be noted that there is no big structural effect in the broad picture of this reactivity pattern. We have examined (and recently reported24) the reactivity pattern of Pd(111) for ethanal, which was described in detail previously on (110).21 Ethanal dehydrogenated on the clean (111) surface at ambient temperature and at steady state above 450 K, while decarbonylation to CH4 + CO occurred at steady state at ambient temperature after the surface was passivated by an initial dehydrogenation reaction. 3. Alcohols and Molecular Sticking Efficiency. The behavior for the primary alcohols is rather different. It appears that they have much more difficulty in adsorbing on the surface and especially onto the passivating layer. The initial sticking probability is lower, and after the initial dehydrogenation phase, the reactivity diminishes to low levels. There is no evidence of a significant rate of either steady-state dehydrogenation at high temperature or of steady-state decarbonylation at lower temperatures. Thus, alcohol adsorption appears to be self-poisoned by the layer formed by dehydrogenation. Nonetheless, the general behavior seen for the aldehydes can also be seen in some data for the alcohols. For instance, in TPD experiments, methane desorption is seen from ethanol,16,40 and methane is also seen transiently in molecular beam experiments, although it was not seen to evolve at a significant rate at steady state.16 Thus, alcohols generally seem to have a lower reactivity with the Pd surface, as reflected in the initial sticking probability data of Figure 6. Here, we see that, at the extreme, methanol has a very low sticking probability at ambient temperature (∼0.0341), and the sticking appears to follow the mass of the molecule, with butanol having the highest value of 0.48.42 This, then is probably due to an enhanced lifetime in a precursor state, into which adsorption must occur before entering the chemisorbed state, and in turn, the significant decrease of sticking with increasing temperature reflects this involvement. The sticking diminishes with temperature due to the reduced lifetime in the precursor state before desorption of the molecule back into the gas phase. This reduced adsorption ability is then also reflected elsewhere in the reactivity pattern; in particular, the steady-state reactivity seen for the other molecules is not seen for the alcohols. It appears that they are much more easily poisoned in their reactivity than are the other three molecules, which can continue to react even when a preadsorbed layer is there. The exception to this pattern of behavior for the alcohols is the allyl alcohol. It does give products, and the profile of sticking probability dependence on temperature is more similar to the aldehydes than it is to the alcohols (Figure 6). This is probably due to tautomerism, which enables the alcohol to H-transfer to form the aldehyde species, propanal. 4. General Conclusions We have shown that the reactivity of clean Pd is such that it simply dehydrogenates all molecules which impinge on it at
Bowker et al.
Figure 6. The dependence of initial sticking probability upon crystal temperature for alcohols (including data from refs 16, 17, 37, and 38) and for the molecules reported here, together with ethanal reported earlier.21,24
low pressure, producing CO, hydrogen, and carbon/carbonaceous species. On the other hand, passivating layers, including adsorbed CO, allow milder reactions to take place, such as the decarbonylation reported here. It appears that clean, pure Pd is a dehydrogenation catalyst and will crack many organic molecules interacting with it down to basic components, generally carbon, CO, and hydrogen. Surface carbon appears to be a poison for reactivity, as shown by the molecular beam measurements here at ∼370 K, where reactivity stops after the buildup of carbon on the surface. Finally, let us return to the related work of Goodman et al. In studying the vinyl acetate synthesis reaction,3-9 they concluded that C plays a detrimental role in the catalysis (see especially refs 3, 4, and 7). In fact, Pd itself was shown to have high selectivity,4 but it deactivated quickly to a low steadystate rate;3 therefore, he postulated that, in that case, carbon (shown to be present as carbide by XRD3,8) acted as a poison for the reaction, although selectivity remained high. He went on to suggest, as have we,18,19 that a role of Au in that case (which is added in the industrial process43-45) may be (i) to prevent C buildup on the catalyst surface and hence maintain activity at a higher level than would otherwise be the case and (ii) to isolate Pd into monomeric surface ensembles which are essential for the reaction.3,9 It could be that Au would play a similar role for the reactions reported here. Acknowledgment. EPSRC is thanked for studentships for J.C. and C.M., while BP provided partial support for J.C. and C.M. We also thank Richard Holroyd and Neil Perkins for their contribution to experimental developments. References and Notes (1) Somorjai, G. A. J. Mol. Catal. A: Chem. 1996, 107, 39. (2) Tang, D.; Hwang, K.; Salmeron, M.; Somorjai, G. A. J. Phys. Chem. B 2004, 108, 13300. (3) Han, Y. F.; Kumar, D.; Sivadinarayana, C.; Clearfield, A.; Goodman, D. W. Catal. Lett. 2004, 94, 131.
Dehydrogenation versus Decarbonylation of Oxygenates (4) Han, Y. F.; Kumar, D.; Goodman, D. W. J. Catal. 2005, 230, 353. (5) Han, Y. F.; Wang, J. H.; Kumar, D.; Yan, Z.; Goodman, D. W. J. Catal. 2005, 232, 467. (6) Chen, M S.; Kumar, D.; Yi, C W.; Goodman, D W. Science 2005, 310, 291. (7) Chen, M.; Luo, K.; Wei, T.; Yan, Z.; Kumar, D.; Yi, C.-W.; Goodman, D. W. Catal. Today 2006, 117, 37. (8) Chen, M.; Goodman, D. W. Chin. J. Catal 2008, 29, 1178. (9) Kumar, D.; Chen, M. S.; Goodman, D. W. Catal. Today 2007, 123, 77. (10) Gabasch, H.; Hayek, K.; Klotzer, B.; Knop-Gericke, A.; Schlogl, R. J. Phys. Chem. B 2006, 110, 4947. (11) Teschner, D.; Borsodi, J.; Wootsch, A.; Re´vay, Z.; Ha¨vecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlo¨gl, R. Science 2008, 320, 86. (12) Teschner, D; Revay, Z.; Borsodi, J.; Havecker, M.; Knop-Gericke, A.; Schlogl, R.; Milroy, D.; Jackson, S. D.; Torres, D.; Sautet, P. Angew. Chem., Int. Ed. 2008, 47, 9274. (13) Vass, E. M.; Havecker, M.; Zafeiratos, S.; Teschner, D.; KnopGericke, A.; Schlogl, R. J. Phys.: Condens. Matter 2008, 20, 184016. (14) Borasio, M.; Rodrıguez de la Fuente, O.; Rupprechter, G.; Freund, H.-J J. Phys. Chem. B 2005, 109, 17791. (15) Wilde, M.; Fukutani, K.; Ludwig, W.; Brandt, B.; Fischer, J.-H.; Schauermann, S.; Freund, H.-J. Angew. Chem., Int. Ed. 2008, 47, 9289. (16) Bowker, M.; Holroyd, R.; Sharpe, R.; Corneille, J.; Francis, S.; Goodman, D. W. Surf. Sci. 1997, 370, 113. (17) Holroyd, R.; Bennett, R. J.; Jones, I.; Bowker, M. J. Chem. Phys. 1999, 110, 8703. (18) Bowker, M.; Morgan, C.; Couves, J. Surf. Sci. 2004, 555, 145. (19) Bowker, M.; Morgan, C. Catal. Lett. 2004, 98, 67. (20) Bowker, M.; Morgan, C.; Perkins, N.; Holroyd, R.; Fourre, E.; Grillo, F.; MacDowall, S. J. Phys. Chem. B 2005, 109, 2377. (21) Bowker, M.; Holroyd, R.; Perkins, N.; Bhantoo, J.; Counsell, J.; Carley, A. F.; Morgan, C. Surf. Sci. 2007, 601, 3651. (22) Bowker, M.; Morgan, C.; Zhdanov, V. Phys. Chem. Chem. Phys. 2007, 9, 5700. (23) Morgan, C.; Bowker, M. Surf. Sci. 2009, 603, 54.
J. Phys. Chem. C, Vol. 114, No. 40, 2010 17147 (24) Bowker, M.; Cookson, L.; Bhantoo, J.; Carley, A.; Hayden, E.; Gilbert, L.; Morgan, C.; Counsell, J. Appl. Catal. Submitted. (25) Bowker, M.; Pudney, P.; Barnes, C. J. J. Vac. Sci. Technol., A 1990, 8, 816. (26) Bowker, M. Appl. Catal. 1997, 160, 89. (27) King, D. A.; Wells, M. G. Surf. Sci. 1972, 29, 454. (28) Shekhar, R.; Barteau, M. A.; Plank, R. V.; Vohs, J. M. J. Phys. Chem. B 1997, 101, 7939. (29) Shekhar, R.; Barteau, M. A. Catal. Lett. 1995, 31, 221. (30) Shekhar, R.; Barteau, M. A. Surf. Sci. 1994, 319, 298. (31) Bowker, M.; Counsell, J.; El-Abiary, K.; Gilbert, L.; Morgan, C.; Nagarajan, S.; Gopinath, C. S. J. Phys. Chem. C 2010, 114, 5060. (32) Nishijima, M.; Yoshinobu, J.; Sekitani, T.; Onchi, M. J. Chem. Phys. 1989, 90, 5114. (33) Sekitani, T.; Takaoka, T.; Fujisawa, M.; Nishijima, M. J. Phys. Chem. 1992, 96, 8462. (34) Li, Z.; Tysoe, W. E. Surf. Sci. 2010, 604, 1377. (35) Guo, X.-C.; Madix, R. J. J. Catal. 1995, 155, 336. (36) Jones, I. Z.; Bennett, R. A.; Bowker, M. Surf. Sci. 1999, 439, 235. (37) Bowker, M.; Jones, I. Z.; Bennett, R. A.; Esch, F.; Baraldi, A.; Lizzit, S.; Comelli, G. Catal. Lett. 1998, 51, 187. (38) Gribov, L.; Novakov, I.; Pavlyuchko, A.; Shumovskii, O. J. Struct. Chem. 2007, 48, 607. (39) Kennedy, D. R.; Webb, G.; Jackson, S. D.; Lennon, D. Appl. Catal., A 2004, 259, 109. (40) Shekhar, R.; Barteau, M. A. Catal. Lett. 1995, 31, 221. (41) Holroyd, R; Bowker, M. Surf. Sci. 1997, 377-9, 786. (42) Holroyd, R. Ph.D. Thesis, University of Reading, U.K., 1998. (43) Crathorne, E.; MacGowan, D.; Morris, S.; Rawlinson, A. J. Catal. 1994, 149, 254. (44) Kragten, D.; van Santen, R. A.; Crawford, M.; Povine, W.; Lerou, L. Inorg. Chem. 1999, 38, 331. (45) Provine, W.; Mills, P.; Lerou, J. Stud. Surf. Sci. Catal. 1996, 101, 191.
JP104837T
