Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported An catalysts (Au/CeO2, Au/Al2O3, An/TiO2) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (similar to 1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O-2/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania>ceria>alumina. The as-prepared samples were inactive catalysts, but Au/TiO2 and Au/CeO2 developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al2O3 remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania>ceria>alumina similar to 0. The EXAFS of samples pretreated in air at 773 K and in H-2 at 573 K reveals the presence of only metallic particles for Au/TiO2 and Au/Al2O3. Au(III) supported on CeO2 remains unreduced after calcination, but reduces during the treatment with H-2. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO2 is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.