A Kinetic and Adsorption Study of CO Oxidation over Unsupported Fine Gold Powder and over Gold Supported on Titanium Dioxide

Abstract

The catalytic oxidation of CO with O2 and adsorption of CO2 and O2 on Au deposited on TiO2 (Au/TiO2, mean diameter of gold particles 3.5 nm), TiO2, and unsupported Au powder (mean diameter 76 nm) have been investigated by using a closed recirculation reaction system. The catalytic oxidation of CO with O2 on Au/TiO2 measured at 253–293 K proceeded very rapidly. In the same temperature range, fine gold powder was catalytically active, while TiO2 alone was entirely inactive for the oxidation. The rate constant of CO oxidation per unit of surface area of gold was larger by two orders of magnitude for Au/TiO2 than for Au powder. Adsorption of CO and CO2 occurred almost instantaneously and reversibly on preoxidized Au/TiO2, whereas a slow increase in the O2 pressure was observed at 273 K in the absence of evacuation. A similar increase of O2 pressure was observed from oxidized TiO2, but Au powder did not show any increase. The increase was ascribed to the desorption of weakly adsorbed O2 on the support surface of Au/TiO2. The rate of the reduction of preoxidized Au/TiO2 with CO almost corresponded to the increase in the O2 pressure and was far smaller than the rate of the catalytic oxidation of CO with O2 on the catalyst, indicating that molecular oxygen weakly adsorbed on the support surface of Au/TiO2 contributes only partly to the catalytic oxidation of CO. During the oxidation, O2 in the gas phase may be directly activated on the surface of deposited gold particles and/or on the very narrow perimeter interface between the gold and the support.