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A Nanoscale Model Catalyst Preparation: Solution Deposition of Phosphine-Stabilized Gold Clusters onto a Planar TiO2(110) Support C. C. Chusuei, X. Lai, K. A. Davis, E. K. Bowers, J. P. Fackler, and D. W. Goodman* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012 Received December 1, 2000. In Final Form: April 23, 2001 Agglomeration of noble metal particles has been a long-standing obstacle in the preparation of planar, finely dispersed, metal cluster-oxide support model catalyst systems. A solution deposition method was devised. Six-atom gold clusters, in the form of [Au6(PPh3)6][BF4]2 (Au6L6), were deposited onto a TiO2(110) single crystal after an acetone pretreatment of the substrate and examined with scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). STM revealed single-unit entities of clusters ca. 1 nm in height, corresponding to the six-atom Au center with the triphenyl phosphine ligands attached. As a check on the success of the homogeneous dispersion, electron-stimulated desorption (ESD) was performed, irradiating the TiO2(110) surface with a 0.12 C cm-2 electron beam flux followed by STM, HREELS, and XPS. Evidence for ligand removal was shown by a pronounced reduction in height in the STM, a disappearance of the aromatic ν(C-H) intensity at ca. 3000 cm-1 in the HREEL spectra, and a +0.4 eV shift in the XPS Au 4f7/2 core level.
Introduction Noble metal clusters supported on metal oxides have recently drawn considerable attention due to their significance for applications in heterogeneous catalysis. In particular, Au when supported as ultrafine, dispersed particles on TiO2 has been found to catalyze important industrial reactions such as the oxidation of CO1-4 and partial oxidation of propylene to propylene oxide.5-7 Catalytic activity and selectivity have been strongly influenced by the size of the Au clusters on the support. In its bulk form, Au is chemically inert; however, when deposited as finely dispersed particles on transition metal oxide supports, its activity is greatly enhanced.6,8 The reasons underlying this phenomenon are poorly understood. The long-term objective of utilizing a surface science approach (i.e., charged particle spectroscopy and scanning probe microscopy) is to obtain a fundamental, microscopiclevel insight into the effects of atomic, electronic, and geometric structure on catalytic activity and selectivity.9 Previously in our laboratory, scanning tunneling microscopy (STM) and spectroscopy (STS) proved to be invaluable for probing the electronic structure of Au clusters deposited onto TiO2(110) as their size (1-6 nm diameter) was * To whom correspondence should be addressed. (1) Bollinger, M. A.; Vannice, M. A. Appl. Catal. B 1996, 8, 417-443. (2) Valden, M.; Pak, S.; Lai, X.; Goodman, D. W. Catal. Lett. 1998, 56, 7-10. (3) Grunwaldt, J.-D.; Baiker, A. J. Phys. Chem. B 1999, 103, 10021012. (4) Grunwaldt, J.-D.; Kiener, C.; Wo¨gerbauer, C.; Baiker, A. J. Catal. 1999, 181, 223-232. (5) Haruta, M. In 3rd World Congress on Oxidation Catalysis; Grasselli, R. K., Oyama, S. T., Gaffney, A. M., Lyons, J. E., Eds.; Elsevier Science: Amsterdam, 1997; Vol. 109, pp 123-134. (6) Haruta, M. Catal. Today 1997, 36, 153-166. (7) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566575. (8) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41-51. (9) Rainer, D. R.; Goodman, D. W. J. Mol. Catal. A 1998, 131, 259283.
varied.10 From this study, a correlation of cluster size with activity was observed, and a physical basis for understanding enhanced activities of small, dispersed metal clusters related to quantum size effects was developed, yielding insight into corresponding high-surface area (HSA) catalyst systems. An array of methods have been developed for depositing size-selected, nanosized metal particles (Pd, Ag, Ir, Ni, Au) on metal oxide supports (Al2O3, MgO) as free clusters11-15 and using metal-centered (Mn, Ru), CO ligand-stabilized organometallic precursors16,17 to tailor catalytic behavior. Recently, Iwasawa and coworkers have used phosphine-stabilized Au complexes, Au(PPh3)(NO3) and Au9(PPh3)8(NO3)3, to prepare highly dispersed Au particles on HSA TiO2 and as-precipitated supports.18-21 Upon deposition and calcination on HSA TiO2, the Au aggregated into 30 nm diameter particles, inhibiting its catalytic properties. More success was obtained on the as-precipitated Ti(OH)4, yielding clusters (10) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650. (11) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.-D.; Ha¨kkinen, H.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 1999, 103, 95739578. (12) Harbich, W.; Fedrigo, S.; Buttet, J.; Lindsay, D. M. Mater. Res. Soc. Symp. Proc. 1991, 206, 369-374. (13) Heiz, U.; Vanolli, F.; Sanchez, A.; Schneider, W.-D. J. Am. Chem. Soc. 1998, 120, 9668-9671. (14) Xu, Z.; Xiao, F.-S.; Purnell, S. K.; Alexeev, O.; Kawi, S.; Deutsch, S. E.; Gates, B. C. Nature 1994, 372, 346-348. (15) Abbet, S.; Sanchez, A.; Heiz, U.; Schneider, W.-D.; Ferrari, A. M.; Pacchioni, G.; Ro¨sch, N. J. Am. Chem. Soc. 2000, 122, 3453-3457. (16) Pierantozzi, R.; Valagene, E. G.; Nordquist, A. F.; Dyer, P. N. J. Mol. Catal. 1983, 21, 189-202. (17) Vanolli, F.; Heiz, U.; Schneider, W.-D. Surf. Sci. 1998, 414, 261270. (18) Yuan, Y.; Asakura, K.; Wan, H.; Tsai, K.; Iwasawa, Y. Chem. Lett. 1996, 755-756. (19) Yuan, Y.; Asakura, K.; Wan, H.; Tsai, K.; Iwasawa, Y. Catal. Lett. 1996, 42, 15-20. (20) Yuan, Y.; Kozlova, A. P.; Asakura, K.; Wan, H.; Tsai, K.; Iwasawa, Y. J. Catal. 1997, 170, 191-199. (21) Yuan, Y.; Asakura, K.; Kozlova, A. P.; Wan, H.; Tsai, K.; Iwasawa, Y. Catal. Today 1998, 44, 333-342.
10.1021/la001684r CCC: $20.00 © 2001 American Chemical Society Published on Web 06/02/2001
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3 nm in diameter, which exhibited high activity for CO oxidation.21 Recently in our laboratory, we have succeeded in using phosphine-stabilized hexagold clusters as an organometallic precursor (described fully in the Experimental Section) to prepare a finely dispersed Au/TiO2 highsurface area catalyst, which showed high activity for CO oxidation with very minimal decrease in turnover frequency after subsequent regeneration treatments.22 (Residual carbon from the ligands was removed by calcination in order to render the gold effective for the reaction.) There is much interest in probing atomic-scale structure and composition of the Au/TiO2 system in order to gain insight for tailoring its catalytic activity. To this end, we investigate the feasibility of using solution depositions of a hexagold phosphine-stabilized Au complex for preparing nanosized metal adclusters on planar single-crystal TiO2(110) surfaces, making them amenable to surface sensitive probes. Experimental Section The complex used for deposition, [Au6(PPh3)6][BF4]2 (Au6L6), was synthesized by decomposing [(Ph3PAu)3NNMe2]BF4 in CH2Cl2 at 295 K:23-25
2[(Ph3PAu)3NNMe2]BF4 f [Au6(PPh3)6][BF4]2 + Me2NNdNNMe2 The hexagold cluster product was isolated, purified, and characterized by 31P magic angle spinning NMR, X-ray diffraction (XRD), and infrared (IR) spectroscopy. The dimensions of the Au6L6 center, obtained from its X-ray crystal structure, are well established.26 The CH2Cl2 solvent was evaporated and the complex stored in powder form until use. A TiO2(110) single crystal (Commercial Crystal Laboratories) was chosen as the model planar oxide support due to its suitability for atomically resolved STM and STS analysis.27 This n-type semiconductor is sufficiently conductive for STM and electron spectroscopy after cycles of Ar+ bombardment and annealing to 700-1100 K. The temperature of the TiO2(110) crystal, mounted onto a tantalum sample holder, was monitored using an optical pyrometer (OMEGA OS3700) during the cleaning procedure. The crystal was then removed from the ultrahigh-vacuum (UHV) environment and treated with acetone (Startex Chemical, Inc.; 99+% purity) to assist dispersion of the Au6L6 upon deposition. The Au6L6 complex was applied to the TiO2(110) and then surface analyzed with X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and STM. After treatment of the TiO2(110) crystal with acetone, aliquots of an 8.3 × 10-5 M Au6L6 colloidal solution [dissolved in CH2Cl2 (Sigma; 99.9% purity)] were deposited onto the crystal with an Eppendorf micropipet. Assuming ∼1 × 1015 surface sites cm-2 and a homogeneous distribution upon solution deposition, a ca. 0.25 monolayer equivalent (MLE) coverage of the complex is estimated to have been deposited. The actual STM image obtained of this surface suggests a lower coverage (see Results and Discussion section). To promote the uniform, unagglomerated dispersion of the Au6L6 complex, the TiO2(110) crystal was immersed in acetone for ca. 3 h. and sonicated for an additional 15 min. Immediately upon evaporation of the acetone, the Au6L6 was deposited. STM before and after this solution treatment revealed flat, atomically resolved TiO2(110) terraces. The acetone (22) Choudhary, T. V.; Sivadinarayana, C.; Chusuei, C. C.; Goodman, D. W., manuscript in preparation. (23) Ramamoorthy, V.; Wu, Z.; Yi, Y.; Sharp, P. R. J. Am. Chem. Soc. 1992, 114, 1526-1527. (24) Dyson, P. J.; Mingos, D. M. P. In Gold: Progress in Chemistry, Biochemistry and Technology; Schmidbaur, H., Ed.; John Wiley & Sons: New York, 1999; pp 511-556. (25) Flint, B. W.; Yang, Y.; Sharp, P. R. Inorg. Chem. 2000, 39, 602608. (26) Briant, C. E.; Hall, K. P.; Mingos, D. M. P.; Wheeler, A. C. J. Chem. Soc., Dalton Trans. 1986, 687-692. (27) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. Rev. B 1997, 56, 13464-13482.
Figure 1. XP survey scan (three sweeps) of Au6L6 deposited onto TiO2(110) prior to ESD.Signal from the Ti 2p and O 1s core levels originate largely from the single crystal. treatment contributed to the deposition of the Au6L6 complex as single-entity units on the surface; without treatment the Au agglomerated into large clusters. Spin-coating the TiO2 substrate to increase dispersion of the particles was attempted but proved to be experimentally impractical due to the rapid evaporation rate of the CH2Cl2 solvent ( 6).36,37 Configuration interaction and density functional theory calculations of the Au6L6 complex reveals that the +2 charge of the molecule is delocalized onto the outer perimeter ligands,32 which, in conjunction with the negative surface charge induced on the TiO2(110) substrate, is the driving force for bonding to the surface. In addition, lateral electrostatic repulsions of like charge would also enhance dispersion of the clusters. ESD was then performed on the Au6L6/TiO2(110) system to desorb the triphenylphosphine ligands by using an electron flood gun. STS of the Au cluster was attempted but proved to be inconclusive. The STS band gap measurements of the underlying substrate and the adsorbed clusters could not be distinguished as a result of agglomeration of the Au clusters and/or surface roughening (35) Rodrigues, F. A.; Monteiro, P. J. M.; Sposito, G. J. Colloid Interface Sci. 1999, 211, 408-409. (36) Brunelle, J. P. Pure Appl. Chem. 1978, 50, 1211-1229. (37) Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. In Preparation of Catalysts VI; Poncelet, G., Martens, J., Delmon, B., Jacobs, P. A., Grange, P., Eds.; Elsevier Science: Amsterdam, 1995; Vol. 91, pp 227-235.
Figure 4. (A) HREEL spectra of Au6L6/TiO2(110) before and after ESD. The broad vibrational peaks (elastic peak fwhm ) 20 meV) were due to surface roughening upon the TiO2(110) crystal exposure to ambient pressures. (B) XP spectra of Au6L6/ TiO2(110) before and after ESD. The data were curve-fitted using Gaussian line shapes. A cumulative function was used to subtract the background intensity in each spectrum.
after the ESD treatment. Mean STM line profiles of the Au6L6/TiO2(110) systems before and after ESD are shown in parts A and B of Figure 3, respectively. A pronounced reduction in average particle size was observed after the electron beam treatment, from an average height of 8 ( 1 to 1.7 ( 0.9 Å. The line profiles (dashed lines) show the mean particle sizes with error bars denoting the upper and lower boundaries of the heights measured. The relatively wide horizontal dimension is attributed to the convolution of the STM tip during lateral scans. XPS results of the P 2p orbital could not conclusively verify whether the P atom was still attached to the Au after the ESD. Compared to the weak Au 4f7/2 signal, the P 2p core level has an atomic sensitivity factor (0.25) approximately one-eighth of that for Au 4f7/2 (1.9), and obtaining sufficient signal-to-noise proved to be difficult. Judging from the high P-Au bond dissociation energy calculated from a density functional theory study (315 kJ mol-1),38 a 0.12 (38) Kickelbick, G.; Schubert, U. Inorg. Chim. Acta 1997, 262, 6164.
Gold Clusters on TiO2(110) Support
C cm-2 flux may have been insufficient to break the P-Au bond. Nevertheless, a pronounced reduction of the adcluster sizes after the electron beam irradiation was observed in the STM, suggesting scission between the Au-P bonds. The Au clusters still retained a hemispherical shape but decreased to a mean height of 1.7 ( 0.9 Å (Figure 3B, n ) 26), approximately an 80% reduction in size. The measured average height (ca. 2 Å) is smaller than expected for the six-atom Au cage structure. Actual distances between the Au atoms in the cage range from 2.6 to 4.9 Å between the nearest neighboring and farthest atoms, respectively. We attribute the discrepancy to the imprecision of the STM measurement at this size regime; heights obtained should be viewed as approximate values. Furthermore, some agglomeration of the Au particles were observed in the STM image after ESD. HREEL spectra (Figure 4A) before and after the ESD showed a reduction in intensity at ca. 3000 cm-1, which we assign to the ν(C-H) asymmetric stretching mode arising from the aromatic rings in the triphenylphosphine ligands. The 2950 cm-1 peak center is lower than that reported for the aromatic ν(C-H) stretch. For example, the closest value for this stretching mode frequency was observed at 2975 cm-1 for C6H5I adsorbed on Si(111).39 This discrepancy may be due to interference from C-containing moieties not originating from the ligands themselves. Given the relatively wide fwhm of the peak, however, the position of the peak envelope is in general agreement (within the fwhm of the HREELS peak) with vibrational spectra reported for the aromatic ν(C-H) asymmetric stretching mode for triphenylphosphine adsorbed on Au40 at 3050 cm-1. The decreased intensity after ESD suggests removal of the aromatic hydrogens from the adsorbed Au6L6 and implies phenyl group decomposition or desorption. Since the C/Ti ratio (obtained from XPS) of the substrate remained the same after ESD, the ligands may have decomposed and remained on the surface. Frequencies at 420, 750, and 1440 cm-1 correspond to the ν1 and ν2 phonon modes and the 2ν2 overtone of the underlying TiO2(110) substrate, consistent with previously reported HREELS (39) Cabibil, H.; Ihm, H.; White, J. M. Surf. Sci. 2000, 447, 91-104. (40) Westermark, G.; Persson, I. Colloids Surf. A 1998, 144, 149166.
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data.31,41 The XPS Au 4f7/2 core level showed a shift from the bulk value at 83.9 to 84.3 eV (Figure 4B). We attribute the +0.4 eV shift to increased screening (i.e., final state effects) of the outgoing Au 4f photoelectrons due to changes in the electronic environment of the hexagold center upon ligand decomposition/desorption. The screening of the admetal cluster photoelectrons were affected due to the fact that the triphenylphosphines are highly polarizable; hence, their removal resulted in a higher BE. In summary, the feasibility of a solid-liquid deposition approach, employing a phosphine-stabilized hexagold complex to produce a Au/TiO2 model catalyst system amenable to STM, STS, HREELS, and XPS probes, has been demonstrated. A reduction in size following ESD of the triphenylphosphine groups was observed by STM, which provided corroborating evidence for intact, singleunit entities on the surface. Complementary HREELS and XPS data were also consistent with ligand removal and/or decomposition from the electron beam irradiation. A wide selection of available phosphine-stabilized Au complexes (e.g., Au1, Au9, Au55) can be similarly used to deposit Au clusters of varying sizes (controlling the number of metal atoms per cluster) for surface-sensitive studies. The solution deposition methodology shows promise for expanding our ability to model practical catalyst systems while maintaining the ability to probe their atomic-scale electronic, chemical, and morphological properties at the admetal-oxide support interface. Acknowledgment. We acknowledge with pleasure the support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, the Robert A. Welch Foundation, and the Dow Chemical Company. We also thank Dr. Mohammad Omary for helpful research discussions. We are indebted to Prof. Paul R. Sharp (University of Missouri at Columbia) for supplying us with the Au6L6 sample used for these studies. C.C.C. gratefully acknowledges support from the Associated Western Universities, Inc., and the Pacific Northwest National Laboratory operated by Battelle Memorial. LA001684R (41) Henderson, M. A. Surf. Sci. 1998, 400, 203-219.
