Reactions of Au(CH3)2(acac) on γ-Al2O3: Characterization of the Surface Organic, Organometallic, Metal Oxide, and Metallic Species

Abstract

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.