A temperature-programmed desorption and IR spectroscopic study of the mechanism of carbon monoxide oxidation on copper-containing catalysts

Abstract

With the use of the temperature-programmed desorption of CO and IR spectroscopy, it was found that, after the adsorption of carbon monoxide on the oxidized 5% CuO/CeO2, 5% CuO/ZrO2, 5% CuO/Al2O3, and 5% CuO/SiO2 samples at 20°C, the greatest amount of adsorbed molecules (2.5 × 1020 g−1) was present on 5% CuO/ZrO2. Of these molecules, 1.0 × 1020 and 1.4 × 1020 g−1 formed carbonyl and carbonate adsorption complexes with the participation of copper-containing sites, respectively. The oxidized sample of 5% CuO/ZrO2 contained only the oxidation sites Cu2+O2−, which participated in the formation of carbonates upon the adsorption of CO. A portion of Cu2+ cations was reduced to Cu+ in the course of reaction, and two types of the carbonyl complexes Cu+CO were formed on them. They were characterized by the presence of absorption bands at 2110 and 2107 cm−1 in the IR spectrum and decomposed with the desorption of CO at 100 and 170°C. Carbonyls were oxidized by adsorbed oxygen at 20°C to carbonates. The temperature of their decomposition accompanied by the desorption of CO2 (Tmax = 170°C) was lower than that of carbonates (240 and 350°C) formed upon the adsorption of CO on the oxidized surface in the absence of oxygen from a gas phase. The properties of adsorption complexes and their participation in the reaction CO + O2 → CO2 at low temperatures, in particular, with the use of a hydrogen-containing mixture, were considered. The oxidation of CO on CuO clusters in 5% CuO/CeO2 and 5% CuO/ZrO2 were discussed.