Spectroscopic Detection of Hydrogen Atom Spillover from Au

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J. Phys. Chem. C 2007, 111, 2959-2964

2959

Spectroscopic Detection of Hydrogen Atom Spillover from Au Nanoparticles Supported on TiO2: Use of Conduction Band Electrons Dimitar A. Panayotov† and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: October 11, 2006; In Final Form: December 11, 2006

The spillover of atomic hydrogen from supported Au particles on TiO2 has been studied. IR spectroscopy detects trapped electrons in the TiO2 lattice produced by the atomic H delivered from the Au. The spillover process is one-half order in PH2 and the activation energy for H2 dissociation on Au is 0.52 ( 0.02 eV. It is shown that surface Ti-OH groups are not involved in donating electrons to the trap sites in TiO2.

I. Introduction Hydrogen spillover1 from metal sites onto supporting oxide surfaces is a key surface process often involved in catalytic hydrogenation chemistry. Heterogeneously catalyzed hydrogenation reactions over gold surfaces and supported gold catalysts have been recently reviewed.2 The metal surface provides sites for breaking the H-H bond and for migration of adsorbed atomic H onto the oxide support.3-5 One of the first spectroscopic studies of hydrogen spillover was reported for Rh/Al2O3 catalysts where surface Al-OH and Al-OD groups were observed to form from H2(g) or D2(g) at 300 K as a result of hydrogen dissociation on metallic Rh sites.6 The dissociation of H2 on Au/TiO2 supported catalysts is found to occur at 300 K, and various chemical studies of the activation of H2 on the Au sites have been carried out.7-9 It has been observed that H2 is dissociated at room temperature on Au/TiO2,9 Au/Fe2O3,9 Au/CeO2,8 and Au/Al2O310 catalysts. In this work, we report the observation of H atom spillover from Au/TiO2 catalysts. The entry of atomic H into the TiO2 bulk is observed to cause the production of trapped electrons which produce delocalized conduction band electrons (CBEs) within the bulk material by excitation by photons in the IR region (as illustrated in Figure 8). This electron excitation by IR irradiation produces a distinctive broad-band IR absorbance in the TiO2. The study of the intensity of the broad-band spectrum resulting from these CBEs permits the sensitive measurement of the kinetics of the H atom spillover to TiO2 from the contiguous Au sites on the TiO2 support surface. II. Experimental Section Transmission IR spectroscopy is employed in this work. As shown in Figure 1, the Au/TiO2 catalyst and pure TiO2 powder are separately pressed into 0.7-cm diameter spots into the interstices of a tungsten grid (pressure ) 2.15 × 108 N m2). The density of the deposits in the grid is nominally 15 mg cm-2. The open regions of the grid transmit about 80% of the incident IR radiation. The grid may be heated electrically to above 700 K and cooled to 90 K with liquid nitrogen cooling of a reentrant Dewar through which the heating/cooling leads pass. Temper* To whom correspondence should be addressed. E-mail: johnt@ virginia.edu. † On leave from the Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria.

Figure 1. Schematic of the sample support apparatus for the study by transmission IR of TiO2 and Au/TiO2 catalysts. Transmission IR spectroscopy is performed on the TiO2 spot pressed into the bottom portion of the W mesh or on the middle Au/TiO2 spot. Reference spectra are measured on the top portion of the mesh. When needed, a spiral W filament at 1800 K, optically shielded from the TiO2 samples can supply atomic H.

ature is maintained constant to (1 K by feedback control using a thermocouple welded to the top center of the grid. Infrared measurements are made by mechanically translating the cell to various vertical positions using a translation stage with an accuracy of (1 µm (Newport Corporation). By using two samples on the same grid, we may observe the Au/TiO2 or the pure TiO2. The use of this arrangement for the simultaneous comparative study of the Au/TiO2 and the pure TiO2 materials is especially advantageous, since both samples are subjected to identical conditions of heat treatment and gas exposure. The grid containing the compressed samples and held by Ni support clamps is placed into a stainless steel ultrahigh vacuum transmission IR cell previously described.11 The KBr spectroscopic windows are sealed to the cell using differentially pumped Viton O-ring seals. The cell is connected to a small ultrahigh vacuum system for evacuation with both an ion pump and a turbomolecular pump. Gas pressures in the system are measured with a capacitance manometer (Baratron, 0.001-1000 Torr) or

10.1021/jp066686k CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007

2960 J. Phys. Chem. C, Vol. 111, No. 7, 2007 with an ionization gauge. The base pressure in the vacuum system is 10-8 Torr. Control studies of the influence of externally generated atomic H were carried out on the pure TiO2 sample by exposing it to atomic H produced on a heated coiled tungsten filament as shown in Figure 1. The filament, operating at 1800 K in 0.20 Torr of H2, generates atomic hydrogen which reaches the TiO2 surface.12 The TiO2 sample is optically shielded from the hot W source. Electronic temperature control of the grid maintains the TiO2 temperature constant at 295 K during exposure to scattered radiation from the hot filament.13 The results obtained for atomic H irradiation of pure TiO2 are in accordance with our previous studies.14 Degussa P25 TiO2 powder is employed in this work. Its specific surface area is 50 m2 g-1 and the average particle size calculated on the basis of the specific surface area is about 30-nm diameter. Au/TiO2 is prepared according to the deposition-precipitation method (DP). This method was originally developed by Tsubota et al.15 and uses NaOH for Au precipitation (DP NaOH). Here, we applied a modification of this method which uses urea for precipitation (DP urea) and produces higher Au-loadings than the DP NaOH method, as reported by Zanella et al.16 HAuCl4‚3H2O was used as the gold precursor. Before preparation, the TiO2 powder was dried in an air flow (100 cm3 min-1) at 373 K for 24 h. All of the preparations were performed in the absence of light to avoid any decomposition of the gold precursor. TiO2 (1 g) was added to 100 cm3 of an aqueous solution of the gold precursor (4.2 × 10-3 M) and of urea (0.42 M). The suspension was thermostated at 353 K and was vigorously stirred for 8 h. The initial pH was ∼2 and increases during the DP procedure to ∼7.2. After the deposition of gold onto TiO2, the solid was separated from the precursor solution by centrifugation (8000 rpm for 10 min), was suspended in water (100 cm3), was stirred for 10 min at 323 K, and was centrifuged again. This washing procedure was repeated five times to remove residual Cl- ions as well as Au species not interacting with the support. The solid was dried in an air flow at 373 K for 24 h. The diameter of the supported Au particles produced by this method is in the range 2-3 nm (measured by transmission electron spectroscopy) according to ref 16. The maximum gold loading achieved by the urea method is ∼8 wt %.16 Before H2 adsorption experiments, both Au/TiO2 and TiO2 samples were pretreated at 473 K as follows: (1) in vacuum at 9 × 10-6 to 5 × 10-7 Torr for 2.5 h, (2) in 20 Torr O2 for 1 h, and (3) in vacuum at 1 × 10-6 Torr for 0.5 h; after that, the temperature was decreased to 295 K in vacuum. The IR spectra of the samples prepared by this manner do not show any evidence for optical absorption because of the IR excitation of trapped electrons producing conduction band electrons (CBEs). Between each separate experiment involving exposure to H2, the Au/TiO2 and TiO2 samples were thermally treated in vacuum at 1 × 10-6 Torr at 473 K for 15 min returning the samples to their starting condition. The temperature ramp from 295 to 473 K was at a rate of 12 K min-1. Transmission IR spectra are acquired at 4 cm-1 spectral resolution by summing 200 or 1000 scans in the mid-IR region (4000-500 cm-1). A liquid-nitrogen cooled Hg/Cd/Te IR detector is used in a N2-purged Mattson Research Series I Fourier transform infrared (FTIR) spectrometer. III. Results A. Spectral Observations of Hydrogen Spillover Au/TiO2. Figure 2 shows a comparison of the transmission IR spectrum for both the Au/TiO2 and the pure TiO2 samples before

Panayotov and Yates

Figure 2. Transmission IR spectra of Au/TiO2 or TiO2 prior to treatment with hydrogen. Both Au/TiO2 and TiO2 samples were treated at 473 K as described in the text.

Figure 3. Reversible production and loss of conduction band electron absorbance (A) during hydrogen spillover and (B) during electronhole pair recombination in vacuum at 295 K on Au/TiO2. Spectra C and D are control experiments for H2 exposure to pure TiO2.

experiments were done with hydrogen. It may be seen that both samples contain small coverages of isolated surface Ti-OH species17 with O-H stretching modes near 3700 cm-1 as well as CO32- species. These surface contaminants are characteristic of high-area TiO2 surfaces which are not heated to higher temperatures in vacuum, and these surface species do not contribute to the findings reported here. In addition, the initial spectra show a low absorbance and broad IR band in the range ∼3600 cm-1 to ∼3200 cm-1 because of small coverages of self-associated Ti-OH species resulting from interspecies hydrogen bonding.18-20 The spectra shown in Figure 2 are employed as baseline spectra for the studies of atomic H adsorption to be described later, where difference spectra will be shown. Figure 3A shows the continuous development of the difference spectra (∆A) during treatment of Au/TiO2 with H2 at 1 Torr pressure at 295 K over a time period of several hours. In addition, in Figure 3B, the reversible behavior of the spectra is shown upon evacuation of the H2. It is observed that in the range 4000 cm-1 to ∼1000 cm-1, the exposure of the Au/TiO2

Hydrogen Atom Spillover from Au Nanoparticles

Figure 4. Comparison of the kinetics of production of CBE and Ti-OH groups on Au/TiO2 at 295 K.

sample to molecular H2 produces a significant increase in the absorbance across this spectral range. In comparison, almost no changes are observed for the pure TiO2 sample either upon exposure to molecular H2 or upon evacuation, as shown in Figure 3C and 3D. The profound changes in the background spectra for Au/TiO2 shown in Figure 3A and B are accompanied by the development of a band near 3420 cm-1 which grows and shifts to 3310 cm-1 as the time of exposure to H2 increases. This behavior is evidence for the production of additional self-associated (hydrogen-bonded) surface Ti-OH groups as the Au/TiO2 surface is exposed to molecular H2. Spectral changes in the CO32region are also observed on the Au/TiO2 surface during the experiment. At the same time, no significant change is observed in either the Ti-OH spectral region or the CO32- spectral region as pure TiO2 is exposed to H2 or as evacuation takes place. These observations clearly indicate that chemical processes occur on the Au/TiO2 surface when it is exposed to molecular H2 at 298 K and that these processes do not occur on the pure TiO2 during the same exposure to H2. We now focus on the behavior of the broad featureless absorbance produced by exposure of Au/TiO2 to molecular H2. Separate studies17,21,22 have shown that the behavior of the broad-band IR absorbance may be accurately monitored by observation at a single wavenumber, 1740 cm-1, which is located near the maximum in the observed difference spectra. Figure 4A shows the kinetic behavior of the broad-band absorbance for Au/TiO2 during H2 adsorption and during evacuation at 295 K. At point a, 1.0 Torr of H2 is admitted, and the development of the broad-band absorbance begins immediately and continues to point b, where H2 evacuation is carried out. At point b, a decay process, which is due to

J. Phys. Chem. C, Vol. 111, No. 7, 2007 2961 electron-hole recombination,14 begins and continues for 100 min where the experiment is interrupted; the decay is not exponential and appears to consist of at least two processes. Since H2 evacuation is complete in a few seconds, the decay process is a natural process occurring within the Au/TiO2 sample which becomes visible after hydrogen spillover has been interrupted by removal of the hydrogen supply. This decay process is due to the recombination of the trapped electrons with holes. At point c, where 1.0 Torr of H2 is admitted for a second time, the development of the broad-band background is observed to occur again, rising to higher absorbance than originally achieved at point b. At point d, evacuation is again initiated, and evidence for slow decay in the broad-band absorbance is again seen. In Figure 4B, the behavior of the integrated Ti-OH absorbance is plotted during the H2 spillover experiments on Au/TiO2. It is seen, in comparison to Figure 4A, that the production of Ti-OH species is essentially irreversible. The Ti-OH species are rapidly produced starting at point a and the Ti-OH absorbance quickly reaches saturation. Evacuation, followed by exposure to molecular hydrogen again, results in only minor changes in the Ti-OH absorbance in progressing from points b to c to d. In their study of room-temperature H2 adsorption on oxidized Au/TiO2, Boccuzzi et al.9 have observed a band in the OH stretching region (≈3300 cm-1, full width at half-maximum (fwhm) ≈ 400 cm-1) which they attributed to newly formed non-hydrogen-bonded OH species produced as a result of atomic hydrogen spillover and reductive interaction with support surface sites. Our results from Figure 4 indicate that the interaction of hydrogen with reactive surface species to produce OH-species is an irreversible process. These experiments indicate that the production kinetics of the broad-band absorbance in the IR range 4000 to ∼1000 cm-1 is unrelated to the production kinetics of the Ti-OH surface species; the two spectroscopic observations are not indications of the same phenomenon in the TiO2. Loss of IR transmittance in the 4000-1000 cm-1 region has been reported previously for room-temperature hydrogen adsorption on other metal-metal oxide systems: Ru/ZnO;23 Cu/Zn/O, Pt/ZnO, and Ru/ZnO;24 Ru/TiO2,25 Pt/TiO2, Pt/ZrO2, and Pt/Al2O3.26 Hydrogen dissociation on the metal and hydrogen atom spillover over the oxide support, where it acts as a fully ionized donor to populate the oxide donor levels, is proposed as the most probable mechanism for the observed increase of conductivity and the production of a broad band in the IR region.24,25 We find that these donor levels in pure TiO2 are located 0.12 eV below the conduction band edge.14 B. Comparison of H-Spillover to Effect on TiO2 using Atomic Hydrogen. Figure 5 compares the effect of hydrogen spillover on Au/TiO2 resulting from molecular hydrogen exposure to the effect of gas-phase atomic H exposure on the pure TiO2 sample. The solid curves compare the two experiments, where in each case the Dewar in the IR cell is maintained at 77 K using liquid-nitrogen cooling for elimination, by condensation, of traces of H2O produced in the system during the experiments. In both cases, the broad-band spectral background is observed to form in several minutes, indicating that atomic H is responsible for this spectral change in the TiO2. The Au/TiO2 sample in addition exhibits evidence of spectral features because of CO32- and possibly other unresolved features in the 1500 cm-1 region superimposed on the broad band. When the reentrant Dewar supporting the sample is not cooled, evidence for small amounts of surface Ti-OH production in the 3400 cm-1 region is observed on both samples because of

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Figure 7. Temperature dependence of CBE production rate on Au/TiO2 during spillover. P(H2) ) 10 Torr. Figure 5. Comparison of the CBE spectra produced by hydrogen spillover on Au/TiO2 and by atomic H incident on pure TiO2. The sample temperature T ) 295 K in all experiments. By cooling a reentrant Dewar in the IR cell to 90 K, surface Ti-OH species do not form as H2O vapor is condensed on the cold Dewar surface.

on the Au surface. Atomic H slowly migrates off of the Au surface and spills over to the TiO2 support with a rate which does not significantly disturb the equilibrium of H(a) with H2(g) on the Au. The activation energy controlling the kinetics of the spillover process was determined by measuring the kinetics of the production of conduction band electrons as a function of the surface temperature of the Au/TiO2 catalyst over a small temperature range. These data are shown in Figure 7A, where the initial rate is measured at 10 Torr H2 pressure. A large temperature dependence of the initial rate is observed. An Arrhenius plot of the data is shown in Figure 7B, yielding an activation energy for the process of 0.52 ( 0.02 eV. IV. Discussion

Figure 6. One-half order H2 pressure dependence of CBE production on Au/TiO2 by spillover. T ) 295 K.

small amounts of water vapor produced in the gas phase which dissociate on the TiO2 surface. Comparison of the spectra in Figure 5A and 5B also indicates that for the Au/TiO2 sample, the shape of the cutoff edge beginning at 1000 cm-1 for the broad-band feature differs in sharpness, compared to the cutoff for the pure TiO2 sample exposed to atomic H, which is sharper. C. Kinetic Study of the Rate of H-Spillover on Au/TiO2. A kinetic study of the rate of hydrogen spillover on the Au/TiO2 sample is informative about the nature of the elementary chemical step controlling the rate of the process. By measuring the initial rate of development of absorbance of the broad band related to conduction band electrons (∆A, CBE), we determined the dependence of the rate on the pressure of H2 in the range 0.2-20 Torr, as shown in Figure 6. The data show a monotonic increase in the rate as a function of H2 pressure as shown in Figure 6A. Figure 6B shows that the initial rate of production of CBEs is proportional to PH21/2, indicating that the H atom flux as a result of hydrogen spillover from the Au to the TiO2 is controlled by the equilibrium process

H2(g) h H2(a) h 2H(a)

(1)

A. Use of TiO2 as a Detector of Atomic H. The entry of H atoms into the rutile TiO2 lattice is expected on theoretical grounds to result in the formation of Ti-O(H)-Ti species containing an internal O-H bond of 0.98 Å length located on an O2- ion bound between two Ti4+ ions in the lattice. The H atom is ionized to H+ (a hole), donating an electron into a shallow trap state near the bottom of the conduction band,14,27-30 thereby making H an n-type dopant. In this case, the O-H σ* antibonding state lies far above the conduction band minimum and the O-H ground electronic state lies above and near the valence band maximum in TiO2.28,29 The internal O-H bond produced by atomic H in the TiO2 lattice is the strongest bond in the system.28,29 The donated electron enters into a shallow defect state located in the band gap about 0.12 eV below the bottom of the conduction band14 and upon thermal excitation or by IR photons is excited into the conduction band where it is highly delocalized. The excited electrons, produced over a wide range of IR photon energies, sample a high density of electronic states; this effect is termed “metallization”. Metallization effects involving various scenarios for n-doping or for electron excitation into the CB by different means are wellknown. Atomic hydrogen induced metallicity in metal/metal oxide systems,23-26,31 TiO2,14 and ZnO(101h0)33 has been reported. The n-doped electronic system can absorb IR radiation over a continuum of IR photon energies, leading to the observed increase in the background absorbance in the IR. The donation of electrons into the shallow trap sites in TiO2 when atomic H diffuses into the TiO2 lattice provides a sensitive IR spectroscopic detection method for atomic H produced on the surface of the TiO2 by spillover or by any other method.

Hydrogen Atom Spillover from Au Nanoparticles This method possesses superior sensitivity over the use of vibrational spectroscopy to detect the produced internal Ti-O(H)-Ti species.14 IR active -OH modes at ∼3280 cm-1 because of internally bound H in TiO2 have been observed experimentally32-35 and have been theoretically predicted36 for H incorporation in TiO2. The corresponding frequency for the -OD modes is 2439 cm-1 for the D atom bonding.32,34 A careful search for vibrational modes due to Ti-O(H)-Ti or Ti-O(D)-Ti in the range 3300-2400 cm-1 was unsuccessful14 at the levels of atomic H irradiation used here. B. Lack of Correlation of CBE Spectra to Surface Ti-OH Groups. The binding of atomic H to TiO2 is often attributed to the production of surface Ti-OH groups which are reported to act as electron donors .29 Our results suggest that this is not the case, since in Figure 4 the kinetics of the formation of surface Ti-OH differs significantly from the kinetics of formation and disappearance of the CBE spectral signature. As shown in Figure 5 (TDewar ) 90 K), the lack of formation of spectroscopically detectable quantities of surface Ti-OH groups on either the Au/TiO2 (exposed to H2) or on pure TiO2 (exposed to atomic H) indicates that the surface Ti-OH groups are not related to the production of electrons in shallow traps which may be excited into the conduction band by IR photons. C. Kinetics of Hydrogen Spillover. Using the TiO2 as a detector of atomic H spillover, we have shown that the initial rate of the spillover process follows a PH21/2 power dependence and that the activation energy for the process is 0.52 eV ( 0.02 eV. This indicates that the rate of H2 dissociation on Au is ratecontrolling in the development of filled electron trap states. The activation energy for this process is about one-eight of the dissociation energy of gas-phase H2. There is a small amount of experimental and theoretical information about the activation barrier of H2 dissociation on Au surfaces. Kislyuk and Tretyakov37 reported an activation energy of 18 kcal/mol (0.78 eV) for thermal atomization of H2 on gold wire at 950-1250 K. Hammer and Nørskov using density functional theory (DFT) calculations38 have shown that the dissociation of H2 on Au is an activated process: for large molecule-surface distance, d, the interaction energy is repulsive with a high barrier of 1.1 eV (around d ) 1.5 Å) for the H2 f H + H reaction. Strømsnes et al.39 calculated that dissociative chemisorption of molecular hydrogen on seven atom gold clusters has an activation barrier of 33 kcal/mol (1.43 eV). Varganov et al.40 have theoretically found that for neutral Au2 and Au3 clusters the activation energies for producing dissociatively adsorbed H2 are 1.10 and 0.59 eV, respectively. Jelinek et al.41 calculated that molecular hydrogen experiences a total barrier of 0.1 eV for the reaction H2 f H + H on a freely suspended Au nanowire. A very recent DFT study by Barrio et al.42 showed that on gold nanoparticles (sizes from 0.7 (Au14) to 1.2 nm (Au29)) dissociation of H2 is a spontaneous process with a stabilization energy of 30-40 kJ/mol (0.3-0.4 eV) when a flexible array of Au atoms is concerned. When the structure of gold atoms was constrained, the H2 f 2H(a) reaction has an endothermicity of 25 kJ/mol (0.26 eV). Separate studies have shown that the activation energy of atomic H diffusion into the bulk of pure TiO2 to produce n-doping is only 0.09 ( 0.01 eV.14 V. Summary A schematic diagram of the processes observed in this study is given in Figure 8. The kinetics of H atom spillover from H2

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Figure 8. Schematic diagram of the hydrogen spillover process, injecting electrons into shallow trap states near the bottom of the conduction band. The trapped electrons are then excited by IR photons into the conduction band where they are highly delocalized, producing a broad IR absorbance in the range 4000-1000 cm-1.

dissociation on supported Au particles in the 2-3 nm diameter range has been studied at 295 K. It has been found that the TiO2 is a sensitive detector for spillover-H atoms, where trapped electrons in shallow trap states near the bottom of the conduction band edge are produced as electron-donor H atoms enter the bulk TiO2. The development of a continuum IR absorbance in the range 4000-1000 cm-1, because of the delocalized nature of the conduction band electrons excited by the IR, provides a sensitive spectroscopic detector for atomic H made by the spillover process. The activation energy for H2 dissociation on the supported Au particles is measured to be 0.52 ( 0.02 eV. The rate of the spillover process is proportional to PH21/2, indicating that the mobile H atoms originate from the equilibrium dissociative adsorption of adsorbed H2 on the Au surface. Surface Ti-OH species are not related to the production of trapped electrons that feed the conduction band states. Acknowledgment. We acknowledge with thanks the support of this work by the Department of Energy, Office of Basic Energy Sciences. References and Notes (1) Boudart, M.; Vannice, M. A.; Benson, J. E. Z. Phys. Chem. New Folge 1969, 64, 171. (2) Claus, P. Appl. Catal., A 2005, 291, 222. (3) Bond, G. C. Stud. Surf. Sci. Catal. 1983, 1. (4) Rozanov, V. V.; Krylov, O. V. Russ. Chem. ReV. 1997, 66, 107. (5) Pajonk, G. M. Appl. Catal., A 2000, 202, 157. (6) Cavanagh, R. R.; Yates, J. T., Jr. J. Catal. 1981, 68, 22. (7) Bagotzky, V. S.; Skundin, A. M. Electrochim. Acta 1984, 29, 757. (8) Venezia, A. M.; Pantaleo, G.; Longo, A.; Di Carlo, G.; Casaletto, M. P.; Liotta, F. L.; Deganello, G. J. Phys. Chem. B 2005, 109, 2821. (9) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Andreeva, D.; Tabakova, T. J. Catal. 1999, 188, 176. (10) Bus, E.; Miller, J. T.; van Bokhoven, J. A. J. Phys. Chem. B 2005, 109, 14581. (11) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. ReV. Sci. Instrum. 1988, 59, 1321. (12) Yates, J. T., Jr. Experimental InnoVations in Surface Science. A Guide to Practical Laboratory Methods and Instruments; AIP Press and Springer-Verlag: New York, 1998. (13) Muha, R. J.; Gates, S. M.; Yates, J. T., Jr.; Basu, P. ReV. Sci. Instrum. 1985, 56, 613. (14) Panayotov, D. A.; Yates, J. T., Jr. submitted, Chem. Phys. Lett. 2006. (15) Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. Stud. Surf. Sci. Catal. 1995, 91, 227. (16) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B 2002, 106, 7634. (17) Panayotov, D. A.; Yates, J. T., Jr. Chem. Phys. Lett. 2004, 399, 300. (18) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216. (19) Tanaka, K.; White, J. M. J. Phys. Chem. 1982, 86, 4708.

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