Supported gold complexes and clusters: Synthesis, characterization, and catalytic properties

Gold complexes on La2O3 and zeolite NaY were synthesized from initially adsorbed Au(CH3)2(C5H 7O2) and characterized by X-ray absorption and infrared spectroscopies. Under conditions of CO oxidation catalysis at 298 K and 760 Torr in a flow reactor, the gold complexes remained as mononuclear species as demonstrated by extended X-ray absorption fine structure spectra indicating the lack of Au-Au contributions. X-ray absorption near edge structure (XANES) data and infrared (IR) spectra in the nuCO region characterizing the zeolite-supported complexes indicate that the AuIII was reduced to Au I as the catalyst lost activity during operation in the flow reactor, whereas XANES spectra characterizing the La2O3-supported complexes under the same conditions show no evidence of reduction of the Au III species, as the activity of catalyst remained constant. The catalytic activity of the AuIII complexes on both supports was comparable to that of the most active catalysts reported, containing gold nanoparticles. These results are the first evidence of supported mononuclear cationic gold complexes as CO oxidation catalysts and suggest that the identity of the support is important in stabilizing highly active AuIII complexes.; The decomposition and conversion of mononuclear AuIII complexes bonded to the partially dehydroxylated surface of TiO2 into gold nanoclusters was investigated by time-resolved X-ray absorption spectroscopy as the initially present AuIII complexes were treated in flowing helium at increasing temperature. The results show the simultaneous reduction of AuIII to Au0 and the aggregation of the gold to form gold nanoclusters on TiO2. Similar supported gold clusters were prepared on gamma-Al2O 3 and tested for CO oxidation catalysis. Various pretreatments under oxidative or reductive conditions were found to affect the relative amounts of cationic and zerovalent gold, thereby affecting the catalytic activity of the samples. The results show that catalysts containing predominantly cationic gold were more active than catalysts containing predominantly Au0. These results are consistent with those showing that supported cationic gold complexes are active in the absence of Au0.