Deposition of highly dispersed gold on alumina support

Gold catalysts of Au/Al2O3, prepared by the methods of deposition precipitation (DP, AuCl3 was the precursor of gold) and chemical vapor deposition (CVD, an organic gold complex was the precursor), were tested in the reaction of CO oxidation at subambient temperatures. The activity observed for prepared catalysts generally increased with their gold dispersion characterized by transmission electron microscopy. Finely dispersed Au/Al2O3 (with d(Au) < 5 nm) catalysts may be easily prepared by the CVD method. Au/Al2O3 samples prepared by the traditional DP method contained many large gold crystallites (d(Au) > 7 nm). They originated, during the impregnation step, from the occlusion of gold complex solution in pores of Al2O3 However, small gold particles (d(Au) similar to 3.5 nm) may be deposited on Al2O3 by a modified DP method featuring prealkalinization of the Al2O3 surface. Furthermore, a mild hydrogen reduction is effective in eliminating the incubation period required for catalysts freshly prepared by the DP method. Characterization of the temperature-programmed reduction revealed that the function of reduction was to remove poisonous chloride ions from deposited gold crystallites. (C) 2001 Academic Press.