ARTICLE pubs.acs.org/JPCC
Structure-Specific Reactivity of Alumina-Supported Monomeric Vanadium Oxide Species Hacksung Kim,†,‡,§ Glen A. Ferguson,||,§ Lei Cheng,|| Stan A. Zygmunt,||,^ Peter C. Stair,*,†,‡ and Larry A. Curtiss*,||,r Chemical Sciences and Engineering Division, Materials Science Division, and rCenter for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Department of Chemistry, Center for Catalysis and Surface Science, and Institute for Catalysis and Energy Processes, Northwestern University, Evanston, Illinois 60208, United States ^ Department of Physics and Astronomy, Valparaiso University, Valparaiso, Indiana 46383, United States )
†
bS Supporting Information ABSTRACT: Oxidative dehydrogenation (ODH) catalysts based on vanadium oxide are active for the production of alkenes, chemicals of great commercial importance. The current industrial practice for alkene production is based on energy-intensive, dehydrogenation reactions. UV resonance and visible Raman measurements, combined with density functional studies, are used to study for the first time the structurereactivity relationships for alumina-supported monomeric vanadium oxide species. The relationship between the structure of three vanadium oxide monomeric surface species on a θ-alumina surface, and their reducibility by H2 was determined by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H2 at temperatures from 450 to 650 C. The H2 reducibility sequence for the three monomeric species is bidentate > “molecular”> tridentate. The reaction pathways for H2 reduction on the three vanadium oxide monomeric structures on a θ-alumina surface were investigated using density functional theory. Reduction by H2 begins with reaction at the VdO bond in all three species. However, the activation energy, Gibbs free energy change under reaction conditions, and the final V oxidation state are speciesdependent. The calculated ordering of reactivity is consistent with the observed experimental ordering and provides an explanation for the ordering. The results suggest that synthesis strategies can be devised to obtain vanadium oxide structures with greatly enhanced activity for ODH resulting in more efficient catalysts.
I. INTRODUCTION The elucidation and understanding of structurefunction relationships is a long-standing goal of catalysis science.17 In heterogeneous catalysis, achieving this goal is especially challenging because the typical catalytic surface is inhomogeneous, and the knowledge of active site structures at the atomic scale is often unknown or incomplete. This is certainly the case for supported vanadium oxide catalysts where a variety of species coexist on the support (e.g., monomers, dimers, polymers, and crystalline V2O5), and their relative populations depend on the V surface density.8 Vanadium oxide surface species supported on alumina, silica, titania, and other high surface area materials are important catalysts for the selective oxidation and oxidative dehydrogenation (ODH) of alkanes to alkenes, chemicals of great commercial importance.24,815 In these reactions the ability of vanadium species to undergo facile oxidation state changes (redox) is intimately associated with their activity as catalysts. However, in spite of extensive investigations over many years, the relationship r 2011 American Chemical Society
between the molecular structure of vanadium surface species and their activity for redox changes remains unclear. Plausible structures1636 have been proposed that are based on a variety of physical measurements and chemical intuition, but in most cases the proposed structures are merely the simplest among a variety of likely candidates. For vanadium oxide supported on alumina the favored structure for the monomeric species has V bonded to four O-atoms in a pseudotetrahedral geometry with one oxygen atom double-bonded to vanadium and three oxygen atoms bridging to the support to form the socalled tridentate structure. Recently, we have combined speciesselective Raman spectroscopy and density functional theory (DFT) calculations to distinguish molecular, tridentate, and bidentate structures shown in Figure 1 for monomeric vanadium oxide species supported on θ-Al2O3.37,38 Of great interest is how Received: September 27, 2011 Revised: December 1, 2011 Published: December 19, 2011 2927
dx.doi.org/10.1021/jp209326h | J. Phys. Chem. C 2012, 116, 2927–2932
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Schematics of the three unique vanadyl species on θ-Al2O3 characterized by DFT and species-selective Raman spectroscopy. Reproduced with permission from ref 38. The hydrogen attached to the oxygen not bound to aluminum in the “molecular” species is omitted for clarity.
the activity of each species depends on its specific structure, because such knowledge could be used to help in the design and synthesis of vanadium oxide catalyst structures with optimal activity. Since traditional measures of activity will fail to distinguish the activities of the multiple vanadium oxide monomer structures present on the θ-Al2O3 surface, we have used UV Raman and resonance Raman that can detect the VOx vibrations as well as AlO and surface OH vibrations on θ-Al2O3 without background interference at very low vanadium loadings. In this paper we report on the relationship between the structure of the three vanadium oxide monomeric surface species on a θ-alumina surface and their reducibility by H2 by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H2. Density functional calculations are used to help interpret the trends in reactivity. The experimental and theoretical methods are described in section II. The results and discussion are given in section III.
II. METHODS a. Experiment. The sample preparation method is described elsewhere.39 Vanadium oxide was supported on θ-Al2O3 by incipient wetness impregnation using ammonium metavanadate. A very low surface density of 0.2 V atoms/nm2 on θ-Al2O3 is used, which corresponds to 2.5% monolayer if 8 V/nm2 is assumed to be the surface density for 1 monolayer. The sample was first calcined (oxidized) in flowing 5% O2/N2 at 550 C for 4 h and then cooled to room temperature. After that, the sample was reduced by heating to 400650 C (ramp rate of 8 C/min, held for 2 h) in flowing 4% H2/He. Raman spectra were collected in flowing helium (100 mL/min) at room temperature before and after the reduction. The excitation wavelengths for Raman measurements are provided by a high repetition rate (4 kHz), nanosecond pulsed, wavelength-tunable Ti:Sapphire laser (Coherent, Indigo-S).37 The scattered light is refocused with a 90 off-axis ellipsoidal reflector onto a triple-grating spectrometer (Princeton Instruments, Trivista 555) where Rayleigh light is filtered out and stray light is significantly suppressed. A liquid N2-cooled CCD detector was used to detect Raman spectra. b. Theory. All calculation of energies, forces, and force constants were performed using the B3LYP40,41 hybrid-density functional method with the Pople-style 6-31g(d,p) basis set, model chemistry B3LYP/6-31g(d,p), as implemented in the Gaussian 09 suite of programs.42 To study the potential energy surface, the triplet, singlet, and open-shell singlet states were calculated for each stationary point. The singlet and open-shell singlet were neglected at points where they should not be expected, e.g., an open-shell singlet at the reactant. For the hydrogen dissociation
Figure 2. Visible Raman and UV resonance spectra in the fundamental VdO and VO stretching region for vanadium oxide monomers (∼0.2 V/nm2) supported on θ-alumina (A) excited at 420 nm and (B) excited at 210 nm. The VdO stretching vibrations significantly change after the reduction by H2 at 450 C (R450 C), 550 C (R550 C), and 650 C (R650 C) compared with before the reduction and after the oxidation by O2/N2 at 550 C (O550 C). Bottom spectrum is for V-free θ-alumina.
barrier both the singlettriplet intersystem crossing (ISC) point and the open-shell singlet (antiferromagnetic) transition state were located. The ISCs were well-converged for the oxygen hydrogen bond breaking and estimated for the open-shell singlet triplet crossing. To calculate the open-shell singlet transition states, we employed a fragment guess technique in which the system is divided into fragments, an initial guess is performed for each fragment determinant, and the fragment determinants are combined and orthogonalized. The ISC was found using the method described in Harvey et al.43 and Bearpark et al.44 with the exception that the force and energy evaluations used the same model chemistry. Convergence to the minimum energy ISC point was determined when chemical accuracy was reached (ΔE < 0.07 mhartree and ∂E/∂qi < 0.001 hartree/a.u.). The algorithm was implemented locally with the energy and forces calculated using the Gaussian 09 suite of programs. Force constants were used to ensure the correct character of the potential energy surface (PES) critical points.
III. RESULTS AND DISCUSSION a. Reducibility of Vanadium Oxide Monomeric Structure from UV and Visible Raman Spectroscopy. Since there are
multiple vanadium oxide monomer structures present on the θ-Al2O3 surface, traditional measures of activity will fail to distinguish their activities. Rates of reactant consumption or product 2928
dx.doi.org/10.1021/jp209326h |J. Phys. Chem. C 2012, 116, 2927–2932
The Journal of Physical Chemistry C formation will only provide an average activity of the structures. Likewise, many spectroscopic techniques are not sensitive enough to distinguish different vanadium oxide species on a metal oxide support. High-surface-area oxides with large band gaps (e.g., Al2O3, MgO) have intrinsic defects (vacancies) and surface OH groups that generate a broad and intense background with visible or near-IR laser excitation, which renders the Raman signals obscure or completely undetectable.37 The most commonly used support, γ-Al2O3, has been designated45,46 as “Raman-silent or -featureless”. In contrast, UV Raman can detect the AlO and surface OH vibrations on θ-Al2O3 without background interference39,47,48 at very low vanadium loadings. UV resonance Raman (UVRR) spectroscopy, a combination of UV Raman and resonance Raman, has very high (by ∼107) detection sensitivity and selectivity for VdO and VO bonds in solid metal oxide materials37 and under working conditions of high temperature (>∼200 C) and pressure (∼1 atm). UVRR is also exceptionally useful at low V loadings on Al2O3 or MgO, where both the Raman background and detection sensitivity become important issues. Use of UVRR at an excitation wavelength of 210 nm, which corresponds to the energy where VdO stretching intensities are almost maximal,39 allows for identification of the vanadium oxide structures on θ-Al2O3 as shown in Figure 2. Use of 420 nm excitation produces high-quality visible Raman spectra for both the oxidized and reduced samples as shown in Figure 2. The uncommon excitation wavelengths at 210 and 420 nm are provided by a wavelength-tunable Ti:Sapphire laser. The relationship between the structure of vanadium surface species and their reducibility by H2 was determined by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H2 at temperatures from 450 to 650 C. Figure 2 shows the Raman spectral change in position and relative intensity of VdO stretching bands appearing in the range ∼ 9701030 cm1 before and after reduction by H2. At the reduction temperature below ∼400 C, Raman spectral features are identical to those of the calcined sample (before reduction), indicating no structural changes on the surface with the heat treatment below 400 C by H2. Three VdO stretching frequencies at 997, 1015, and 1023 cm1 have been assigned to the molecular, tridentate, and bidentate structures, respectively (see ref 38 for the assignment and Figure 1 for the three structures reproduced from ref 38). When the sample is reduced at 450 C, the bidentate band at 1023 cm1 significantly decreases in intensity by 64%, while the tridentate band intensity, ∼1015 cm1, hardly changes as clearly shown in visible Raman spectra (Figure 2A). The molecular band at ∼997 cm1 and the VO stretching vibrations appearing at ∼900 cm1 are very weak in visible excitation Raman spectra. The UVRR spectrum excited at 210 nm (Figure 2B) provides complementary information about the hydrogen-induced structural changes. Change in the molecular band and the appearance of VO band are quite clear. The molecular band intensity increases by 39%, while the bidentate band at ∼1023 cm1 is significantly weaker after H2 reduction at 450 C. This reveals that the bidentate is the most easily reduced by H2 among the three monomers and is greatly reduced at 450 C. The Raman data also suggest a conversion of the bidentate to the molecular at 450 C by H2. The structure of the molecular monomer (Figure 1) has two more hydrogen atoms on each of the VOAl bridges than the bidentate (to be precise, the molecular structure also has an additional oxygen atom on the alumina surface corresponding overall to the net
ARTICLE
Figure 3. UVRR spectra in the fundamental VdO and VO stretching, combination, and first overtone region for the same sample described in Figure 2. The VdO stretching vibrations nearly disappear, and VO stretching notably red-shifts at 650 C (R650 C) reduction compared with the oxidation at 550 C (O550 C) and reduction at 550 C (R550 C), evidencing the decrease in the V oxidation state of +5 to +4 (elongation of axial VO bonds) or to +3 by H2 reduction at 650 C.
addition of one water molecule). Thus, the structural conversion induced by dissociative addition of hydrogen molecules is compatible from a structural point of view. Moreover, the Raman data (Figure 2) show that the VdO in the tridentate structure reacts with hydrogen at 550650 C, but not at 450 C and below. Under H2 at 550 C, about 90% of the VdO units in the bidentate, about 70% in the molecular, and about 50% in the tridentate structures disappear on the basis of the decrease in the VdO band intensities. Therefore, the H2 reducibility sequence is bidentate > “molecular” > tridentate. Except for a small enhancement in intensity of the AlO deformation modes (∼410 450 cm1) associated with a slight elongation of VO bonds at 550 C (Figure 3), no significant change in basal VO bands is observed at 550 C and below (Figure 2). Hence, the reaction with H2 from 450 to 550 C occurs primarily at the VdO bonds rather than the basal VO bonds in VO(Al) linkages. b. Reactivities of Vanadium Oxide Monomeric Structures from Density Functional Theory. The reactivities of vanadium oxide monomeric structures on θ-alumina were investigated using density functional theory (DFT) calculations with the θ-alumina surface modeled by the same cluster used in our previous study.38 The reaction pathways for H2 reduction on the three vanadium oxide monomer structures (Figure 1) were computed. The H2 reaction pathways for the bidentate and tridentate structures are shown in Figure 4. The tridentate structure has a reaction pathway that begins with the H2 reacting asymmetrically with the vanadyl oxygen to break the hydrogenhydrogen bond followed by breaking of the vanadiumoxygen double bond to form a hydroxyl group bound to the vanadium and a weakly bound hydrogen radical. The pathway is completed by formation of a second oxygenhydrogen bond that results in an aqua ligand bound to vanadium. Following this the aqua ligand can desorb, leading to an overall endothermic reaction at the temperatures considered in this study. The reaction of H2 with the bidentate species proceeds through the same transition-state structure as the tridentate species. Details of the reaction energetics at the three temperatures studied by Raman spectroscopy are given in the Supporting Information. 2929
dx.doi.org/10.1021/jp209326h |J. Phys. Chem. C 2012, 116, 2927–2932
The Journal of Physical Chemistry C
ARTICLE
Figure 4. Reaction pathways of the hydrogen interacting with the bidenate and tridentate surface vanadium oxide monomers. All energies are shown relative to the tridentate reactant. After the initial interaction of hydrogen with the tridentate monomer, the bidenate and tridentate structures are the same. Dotted lines represent bond forming and breaking in the reaction. All values are free energies in kilocalories per mole at 450 C and 0.04 atm pressure.
Figure 5. Minimum energy pathway of the molecular vanadium oxide monomer reacting with hydrogen. All energies are shown relative to the molecular reactant. The reduction occurs by a hydrogen-assisted hydrogen transfer and desorption of H2 to form a water-desorbed structure. Dotted lines represent bond forming and breaking in the reaction. All values are free energies at 450 C and 0.04 atm pressure in kilocalories per mole.
A reaction pathway involving bond breaking of H2 by the vanadyl oxygen similar to that of the bidentate and tridentate structures was investigated for the molecular monomer. However, a more favorable pathway, shown in Figure 5, involves reaction of the H2 with the surface hydroxyl groups bound to the vanadium. This reaction involves a hydrogen-assisted hydrogen transfer from the surface hydroxyl to the H2, breaking the OH and HH bonds and formation of a three-centered HHH transition state. The net result of this reaction is hydrogen exchange with loss of H 2 and transformation of VdO to VOH. A similar H2-mediated exchange reaction has recently been proposed in small organic systems.49 Subsequent hydrogen abstraction from a surface hydroxyl to the newly created VOH group leads to formation of an aqua ligand, similar to the products in the bidentate and tridenate reactions. The aqua ligand desorption in this case is downhill and the entire pathway nearly thermoneutral. Thus, DFT predicts that the final state in the reaction has the aqua ligand for bidenate/tridentate
monomers and that it loses the (extra) aqua ligand for the molecular monomer. The ordering of the activity of the three structures from theory is consistent with what is observed experimentally. In Figure 4 the free energy barrier for the H2 reduction at 450 C with the tridentate structure is 70 kcal/mol compared to 53 kcal/mol for the bidentate. The bidentate structure is more active than the tridentate structure due to the fact that it is less stable by 17 kcal/mol at 450 C (by 16 and 15 kcal/mol at 550 and 650 C, respectively; Figure 4 and Table S1). This accounts for the H2 activation barrier being 17 kcal/mol lower, since the resulting transition state and products are the same for the two structures. The molecular structure contains one more water molecule than the bidentate and tridentate structures, with two hydrogens on two of the VO oxygens and an additional oxygen in a bridging site. The free energy barrier for H2 reduction on the molecular structure is 57 kcal/mol, between the values obtained for the bidenate and tridentate species. These calculated results 2930
dx.doi.org/10.1021/jp209326h |J. Phys. Chem. C 2012, 116, 2927–2932
The Journal of Physical Chemistry C are also consistent with the observed experimental ordering. The reaction barriers show that the molecular structure would be expected to have an activity between those of the bidentate and tridentate structures. The DFT calculations are also consistent with a reduction in the V oxidation state and (VdO and VO) bond distance changes. At 650 C H2 reduction (Figure 3) results in substantial changes in the VdO and VO stretching modes. The VdO bands disappear almost completely in the fundamental (∼1015 cm1) and first overtone (∼2020 cm1) regions. This indicates that the terminal (axial) VO bond of the monomers is elongated (e.g., the bond order is reduced to ∼1) or even broken. An associated reduction in the V oxidation state is supported by a significant decrease of calculated Mulliken charges on V (from ∼+1.27e to ∼+1.08e on average, Figure S1) for the DFT geometryoptimized structures. A large shift in the broad band at ∼899 cm1, and its combination in the 17001800 cm1 region,39 is evident in the UVRR spectra after H2 reduction at 650 C (Figure 3). This band is assigned to the symmetric and asymmetric VO stretching vibrations.39 The red shift to ∼820 cm1 (Δ ∼ 79 cm1) primarily results from the elongation of the basal VO bonds. The magnitude of the bond elongation is estimated to be ∼0.05 Å using a diatomic VO approximation.50 The optimized DFT structures (Figure S2 and Table S2) indicate an average elongation of the three basal VO bonds of ∼0.08 Å, in agreement with the Raman.
IV. CONCLUSIONS UV resonance and visible Raman measurements, combined with density functional studies, are used to study for the first time the structurereactivity relationships for alumina-supported monomeric vanadium oxide species. The relationship between the structure of three vanadium oxide monomeric surface species on a θ-alumina surface and their reducibility by H2 was determined by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H2 at temperatures from 450 to 650 C. The H2 reducibility sequence for the three monomeric species is bidentate > “molecular”> tridentate. The reaction pathways for H2 reduction on the three vanadium oxide monomeric structures on a θ-alumina surface were investigated using density functional theory. Reduction by H2 begins with reaction at the VdO bond in all three species. However, the activation energy, Gibbs free energy change under reaction conditions, and the final V oxidation state are all species-dependent. The calculated ordering of reactivity is consistent with the observed experimental ordering and provides an explanation for the ordering. The higher activity observed for the bidentate species from experiment and theory suggest that if techniques could be designed to synthesize this structure predominately, significantly higher catalytic activity for CH activation by VOx monomers could be achieved, resulting in more efficient ODH catalysts. ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed energetics, Milliken charges, and geometric parameters. This information is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
ARTICLE
Author Contributions §
These authors contributed equally to this work.
’ ACKNOWLEDGMENT We thank the U.S. Department of Energy, Division of Basic Energy Sciences Contract No. DE-AC-02-06CH11357, for supporting the work performed at Argonne National Laboratory with UChicago Argonne, LLC, the operator of Argonne National Laboratory. We also thank the Argonne Center for Nanoscale Materials for computing resources. ’ REFERENCES (1) Ferreira, M. L.; Volpe, M. J. Mol. Catal. A:Chem. 2002, 184, 349. (2) Gao, X. T.; Wachs, I. E. J. Phys. Chem. B 2000, 104, 1261. (3) Keller, D. E.; de Groot, F. M. F.; Koningsberger, D. C.; Weckhuysen, B. M. J. Phys. Chem. B 2005, 109, 10223. (4) Kim, H. Y.; Lee, H. M.; Pala, R. G. S.; Metiu, H. J. Phys. Chem. C 2009, 113, 16083. (5) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1988, 84, 2987. (6) Wachs, I. E.; Weckhuysen, B. M. Appl. Catal., A 1997, 157, 67. (7) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25. (8) Argyle, M. D.; Chen, K. D.; Bell, A. T.; Iglesia, E. J. Catal. 2002, 208, 139. (9) Cavani, F.; Trifiro, F. Catal. Today 1995, 24, 307. (10) Dinse, A.; Schomacker, R.; Bell, A. T. Phys. Chem. Chem. Phys. 2009, 11, 6119. (11) Feng, H.; Elam, J. W.; Libera, J. A.; Pellin, M. J.; Stair, P. C. J. Catal. 2010, 269, 421. (12) Harlin, M. E.; Niemi, V. M.; Krause, A. O. I.; Weckhuysen, B. M. J. Catal. 2001, 203, 242. (13) Redfern, P. C.; Zapol, P.; Sternberg, M.; Adiga, S. P.; Zygmunt, S. A.; Curtiss, L. A. J. Phys. Chem. B 2006, 110, 8363. (14) Rozanska, X.; Fortrie, R.; Sauer, J. J. Phys. Chem. C 2007, 111, 6041. (15) Stair, P. C.; Marshall, C.; Xiong, G.; Feng, H.; Pellin, M. J.; Elam, J. W.; Curtiss, L.; Iton, L.; Kung, H.; Kung, M.; Wang, H. H. Top. Catal. 2006, 39, 181. (16) Brazdova, V.; Ganduglia-Pirovano, M. V.; Sauer, J. J. Phys. Chem. B 2005, 109, 23532. (17) Gijzeman, O. L. J.; van Lingen, J. N. J.; van Lenthe, J. H.; Tinnemans, S. J.; Keller, D. E.; Weckhuysen, B. M. Chem. Phys. Lett. 2004, 397, 277. (18) Goodrow, A.; Bell, A. T. J. Phys. Chem. C 2007, 111, 14753. (19) Islam, M. M.; Costa, D.; Calatayud, M.; Tielens, F. J. Phys. Chem. C 2009, 113, 10740. (20) Izumi, Y.; Kiyotaki, F.; Yoshitake, H.; Aika, K.; Sugihara, T.; Tatsumi, T.; Tanizawa, Y.; Shido, T.; Iwasawa, Y. Chem. Commun. (Cambridge, U. K.) 2002, 2402. (21) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J. Phys. Chem. 1983, 87, 5172. (22) Kubicki, J. D.; Apitz, S. E. Am. Mineral. 1998, 83, 1054. (23) Li, C. J. Catal. 2003, 216, 203. (24) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry, 6 ed.; Wiley-Interscience: New York, 2009. (25) Nielsen, U. G.; Topsoe, N. Y.; Brorson, M.; Skibsted, J.; Jakobsen, H. J. J. Am. Chem. Soc. 2004, 126, 4926. (26) Olsthoorn, A. A.; Boelhouwer, C. J. Catal. 1976, 44, 197. (27) Ruitenbeek, M.; van Dillen, A. J.; de Groot, F. M. F.; Wachs, I. E.; Geus, J. W.; Koningsberger, D. C. Top. Catal. 2000, 10, 241. (28) Todorova, T. K.; Ganduglia-Pirovano, M. V.; Sauer, J. J. Phys. Chem. B 2005, 109, 23523. (29) Todorova, T. K.; Ganduglia-Pirovano, M. V.; Sauer, J. J. Phys. Chem. B 2007, 111, 5141. 2931
dx.doi.org/10.1021/jp209326h |J. Phys. Chem. C 2012, 116, 2927–2932
The Journal of Physical Chemistry C
ARTICLE
(30) Topsoe, N. Y. J. Catal. 1991, 128, 499. (31) Tran, K.; Hanninglee, M. A.; Biswas, A.; Stiegman, A. E.; Scott, G. W. J. Am. Chem. Soc. 1995, 117, 2618. (32) Vittadini, A.; Selloni, A. J. Phys. Chem. B 2004, 108, 7337. (33) Vuurman, M. A.; Wachs, I. E. J. Phys. Chem. 1992, 96, 5008. (34) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94, 4240. (35) Wu, Z.; Stair, P. C.; Rugmini, S.; Jackson, S. D. J. Phys. Chem. C 2007, 111, 16460. (36) Zhou, R. S.; Snyder, R. L. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 617. (37) Kim, H.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. Chem. Soc. Rev. 2010, 39, 4820. (38) Kim, H. S.; Zygmunt, S. A.; Stair, P. C.; Zapol, P.; Curtiss, L. A. J. Phys. Chem. C 2009, 113, 8836. (39) Kim, H. S.; Stair, P. C. J. Phys. Chem. A 2009, 113, 4346. (40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (41) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02 ed.; Gaussian: Wallingford, CT, 2009. (43) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95. (44) Bearpark, M. J.; Robb, M. A.; Schlegel, H. B. Chem. Phys. Lett. 1994, 223, 269. (45) Dyer, C.; Hendra, P. J.; Forsling, W.; Ranheimer, M. Spectrochim. Acta A 1993, 49, 691. (46) Wachs, I. E. Catal. Today 1996, 27, 437. (47) Wu, Z. L.; Kim, H. S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. J. Phys. Chem. B 2005, 109, 2793. (48) Stair, P. C.; Li, C. J. Vac. Sci. Technol., A 1997, 15, 1679. (49) Asatryan, R. Chem. Phys. Lett. 2010, 498, 263. (50) Hardcastle, F. D.; Wachs, I. E. J. Phys. Chem. 1991, 95, 5031.
2932
dx.doi.org/10.1021/jp209326h |J. Phys. Chem. C 2012, 116, 2927–2932
