Langmuir 2003, 19, 3897-3903
3897
Reactions of Au(CH3)2(acac) on γ-Al2O3: Characterization of the Surface Organic, Organometallic, Metal Oxide, and Metallic Species Javier Guzman and Bruce C. Gates* Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received November 6, 2002. In Final Form: January 17, 2003 The reaction of Au(CH3)2(acac) with the partially dehydroxylated surface of γ-Al2O3 (calcined at 673 K) and the subsequent formation of supported gold clusters upon treatment of the supported species in He at various temperatures and 1 atm were investigated with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectorscopies, and X-ray absorption near edge structure (XANES). The IR spectra show that Au(CH3)2(acac) reacts readily at room temperature with both surface OH groups and coordinatively unsaturated aluminum sites of the γ-Al2O3 surface, forming supported mononuclear gold complexes and {Al}-acac species (where the braces refer to surface Al sites). EXAFS data demonstrate the formation of mononuclear gold complexes as the predominant surface gold species, confirmed by the lack of Au-Au contributions in the EXAFS spectra. The mononuclear complex is represented as {AlO}2-AuIII(CH3)2, with the Au-O bonding distance being 2.15 Å. Treatment of {AlO}2-AuIII(CH3)2 in He at 323 K and 1 atm led to formation of supported Au(III) oxide in the form of highly dispersed clusters. Further treatment of the samples led to the formation of metallic gold clusters of increasing size, ultimately those with an average diameter of about 30 Å. The {Al}-acac groups were converted to {Al}-acetate groups upon treatment in He at 423 K; further treatment at increasing temperatures removed all the organic ligands from the surface. The results provide the first evidence of the reaction of a metal acetylacetonate complex with an oxide surface that is characterized by essentially all the resultant surface species.
Introduction Supported metals are important technological catalysts, typically consisting of nanoclusters or particles of metals stably dispersed on the internal surfaces of porous supports such as metal oxides.1 The importance of these materials has motivated research on synthetic methods for the preparation of highly dispersed supported metals and those with tailored structures.2,3 A novel approach involves the reaction of metal acetylacetonate complexes [Mx+(acac)y] (also known as metal β-diketonate complexes; acac is C5H7O2) with the surfaces of metal oxides.4-6 The surface organometallic chemistry involved in these syntheses has been investigated for the supports silica, zirconia, and alumina with metal acetylacetonate complexes of Ir,7 Pd,8 Fe,9 V,10 and Cr.11 It has been suggested9-11 that the * Corresponding author. E-mail:
[email protected]. (1) Gates, B. C. J. Mol. Catal. A 2000, 163, 55. (2) Serp, P.; Kalck, P.; Feurer, R. Chem. Rev. 2002, 102, 3085. (3) Preparation of Solid Catalyst; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1999. (4) Van Der Voort, P.; Mitchell, M. B.; Vansant, E. F.; White, M. G. Interface Sci. 1997, 5, 169. (5) White, M. G. Catal. Today 1993, 18, 73. (6) Baltes, M.; Collart, O.; Van Der Voort, P.; Vansant, E. F. Langmuir 1999, 15, 5841. (7) Locatelli, F.; Didillon, B.; Uzio, D.; Niccolai, G.; Candy, J. P.; Basset, J.-M. J. Catal. 2000, 193, 154. (8) Daniell, W.; Landes, H.; Foaud, N. E.; Kno¨zinger, H. J. Mol. Catal. A 2002, 178, 211. (9) Van Der Voort, P.; van Welzenis, R.; de Ridder, M.; Brongersma, H. H.; Baltes, M.; Mathieu, M.; van de Ven, P. C.; Vansant, E. F. Langmuir 2002, 18, 4420. (10) (a) Van Der Voort, P.; White, M. G.; Vansant, E. F. Langmuir 1998, 14, 106. (b) Van Der Voort, P.; Morey, M.; Stucky, G. D.; Mathieu, M.; Vansant, E. F. J. Phys. Chem. B 1998, 102, 585. (c) Van Der Voort, P.; Baltes, M.; Vansant, E. F. J. Phys. Chem. B 1999, 103, 10102. (d) Van Der Voort, P.; Possemiers, K.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1996, 92, 843. (11) Babitch, I. V.; Plyuto, Y. V.; van Der Voort, P.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1997, 93, 3191.
formation of oxide-supported metal catalysts from metal acetylacetonate complexes involves the irreversible adsorption of the metal complex by hydrogen bonding to the surface or by ligand exchange followed by subsequent thermal decomposition of the supported complexes. Evidence for these suggestions is based on infrared (IR) and ultraviolet-visible (UV-vis) spectra and thermogravimetric data characterizing the surface species, but there is only fragmentary information characterizing the structures of the metal-containing species and the metalsupport interface in the materials. Thus, we have used X-ray absorption spectroscopy (XAS) and other techniques to characterize the surface species at various intermediate stages as supported metal particles were formed. Gold was chosen as the metal because highly dispersed supported gold has been found to be catalytically active for reactions including CO oxidation,12 the water gas shift (WGS),13 and NO reduction,14 as well as selective for oxidation of propene to propene oxide.15 Methods for synthesizing such catalysts include16,17 depositionprecipitation, coprecipitation, ion exchange, and chemical vapor/liquid deposition; the use of gold acetylacetonate complexes has also been reported, with preparations by chemical vapor deposition18,19 and adsorption of the complex from the liquid phase.20-22 The objective of this research was to investigate the reaction of Au(CH3)2(acac) with partially dehydroxylated (12) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (13) Andreeva, D.; Idakiev, V.; Tabakova, T.; Andreev, A.; Giovanoli, R. Appl. Catal. A 1996, 134, 275. (14) Cant, N. W.; Ossipoff, N. J. Catal. Today 1997, 36, 125. (15) Stangland, E. E.; Stavens, K. B.; Andres, R. P.; Delgass, W. N. J. Catal. 2000, 191, 332. (16) Haruta, M. Catal. Today 1996, 36, 153. (17) Bond, G. C.; Thompson, D. T. Catal. Rev.-Sci. Eng. 1999, 41, 319.
10.1021/la0268101 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/21/2003
3898
Langmuir, Vol. 19, No. 9, 2003
alumina and determine the structures of the products on the support surface. We report IR and extended X-ray absorption fine structure (EXAFS) spectra characterizing the surface species formed by reaction of Au(CH3)2(acac) with γ-Al2O3 and X-ray absorption near edge structure (XANES) data characterizing the electron density and ligand environment of the gold. This is the first report of the reaction of a metal acetylacetonate complex with an oxide surface that includes a full accounting for the products formed on the surface, including organic and organometallic species and metal oxide and metal clusters. The data give evidence of (a) ligand exchange of the acac ligand of Au(CH3)2(acac) with support oxygen, (b) hydrogen-bonding interactions between the acac ligands and support OH groups, and (c) the structural and electronic changes of the supported gold during He treatment. Experimental Section Reagents and Materials. He and N2 (Matheson, 99.999%) were purified by passage through traps containing reduced Cu/ Al2O3 and activated zeolite 4A to remove traces of O2 and moisture, respectively. Reagent grade n-hexane (Aldrich) was dried over sodium benzophenone ketyl and deoxygenated by sparging with N2 prior to use. γ-Al2O3 powder (Aluminum Oxide C, Degussa) was made into a paste by adding deionized water, followed by drying overnight at 393 K. It was then ground and calcined, evacuated at 673 K, and stored in a drybox (Vacuum Atmospheres HE-63-P) until subsequent use. The BET surface area of γ-Al2O3 calcined at 673 K was approximately 100 m2 g-1.23 The precursor Au(CH3)2(acac) [dimethyl(acetylacetonate) gold(III); Strem, 98%] and the reference compounds Al(acac)3 [aluminum acetylacetonate; Strem 99%] and Hacac [acetylacetone or 2,4-pentanedione; Aldrich 99%] were used as supplied. Caution: Hacac is toxic and explosive. Sample Preparation. Syntheses and transfers of samples were performed with exclusion of air and moisture on a doublemanifold Schlenk vacuum line and in a drybox purged with N2 that was recirculated through traps containing particles of supported Cu and zeolite 4A for removal of O2 and moisture, respectively. The sample, containing 1 wt % Au, was prepared by forming a slurry of Au(CH3)2(acac) in n-hexane with γ-Al2O3 powder that had been partially dehydroxylated under vacuum at 673 K. The slurry was stirred for 1 day, and the solvent was removed by evacuation (pressure
