Comparative study of Au/TiO2 and Au/ZrO2 catalysts for low-temperature CO oxidation

Gold catalysts for low-temperature CO oxidation were prepared by immobilizi ng gold colloids of about 2 nm size on TiO2 and ZrO2 in aqueous solution. T he gold particles nearly retained their size after immobilization on both s upports. Slight sintering was observed by X-ray diffraction and high-resolu tion transmission electron microscopy after calcination at 673 K. Some of t he gold particles on ZrO2 showed hexagonal symmetry with close-packed Au(11 1) surfaces. X-ray photoelectron spectroscopy investigations indicated that the gold was in a metallic state. Whereas gold particles on TiO2 showed CO conversion directly after preparation and drying, significant activity of gold on ZrO2 was only observed after calcination in air. CO adsorption was reversible on all catalysts and weaker on the most active catalysts, as sho wn by diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and pulse thermal analysis. Probing of the gold sites by CO adsorption reve aled the existence of different gold sites in these catalysts. Step/kink si tes (2111-2123 cm(-1)) and positively polarized gold sites (2128-2135 cm(-1 )) were identified on uncalcined and in situ calcined gold catalysts. The n umber of low-coordinated gold sites was much higher on TiO2, whereas on ZrO 2 more positively polarized gold atoms were found. This behaviour was trace d to the shape of the gold particles, which affects the number of low-coord inated gold atoms and is dependent on support and treatment, in situ treatm ent in oxygen led to a temporarily inactive catalyst with additional bands in the DRIFTS spectrum in the 2000-2400 cm(-1) region, stemming from CO ads orbed on positively polarized gold sites (2130-2150 cm(-1)) and on the supp ort (2180-2200 cm(-1)), and from molecularly adsorbed CO2 (2353 cm(-1)). No bands arising from CO on low-coordinated, metallic gold sites were identif ied in that case, (C) 1999 Academic Press.