Supported gold catalysts for CO oxidation: Effect of calcination on structure, adsorption and catalytic behaviour

Adsorption behaviour and CO oxidation activity of TiO2- and ZrO2-supported gold catalysts, prepared by adsorption of gold colloids on the supports, have been investigated by pulse thermal analysis and diffuse reflectance FTIR spectroscopy. Structural investigations using transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy showed that Au particles, mainly in the metallic state and with similar particle size distribution (2.0-3.0 nm), could be prepared on both supports, but the as-prepared samples contained carbonaceous impurities from the preparation procedure. Upon calcinations in 20 vol.% oxygen/He only slight sintering occurred up to 600 degreesC. Above 600 degreesC, the gold particles sintered on both supports. CO oxidation activity, studied by injection of CO pulses into an oxygen stream in the thermoanalyzer, depended strongly on the calcination temperature of the catalysts. Maximal activity was obtained for the catalysts calcined at 500 degreesC (Au/TiO2) and 560 degreesC (Au/ZrO2), respectively. Catalysts calcined at higher temperatures exhibited lower activity due to sintering of the gold particles. CO adsorption was found to be weak on both catalyst systems. No significant adsorption of O-2 and CO2 could be detected by pulse thermal analysis. CO adsorption increased for catalysts calcined up to 200-400 degreesC. Calcination temperatures leading to maximal CO adsorption did not coincide with calcination temperatures affording maximal CO oxidation activity. Moreover, the removal of carbonaceous residues during calcination was found to be correlated to the activity. The results of the adsorption studies do not support a Mars-van Krevelen mechanism, where lattice oxygen would be involved. CO oxidation by the reaction of adsorbed CO with molecularly adsorbed oxygen seems more likely.