Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O4

Gold can be highly dispersed on a variety of metal oxides by coprecipitation and deposition-precipitation followed by calcination in air. The small gold particles are hemispherical in shape and stabilized by epitaxial contact, dislocations, or contact with an amorphous oxide layer. Such supported gold differs in catalytic nature from unsupported gold particles and exhibits high catalytic activities for low-temperature oxidation of CO. Especially, gold supported on TiO2, α-Fe2O3, Co3O4, NiO, Be(OH)2, and Mg(OH)2 is very active even at temperatures below 0°C. Among the gold catalysts supported on TiO2, α-Fe2O3, and Co3O4 the turnover frequencies for CO oxidation per surface gold atom are almost independent of the kind of support oxides used and increase sharply with a decrease in diameter of gold particles below 4 nm. Small gold particles not only provide the sites for the reversible adsorption of CO but also appreciably increase the amount of oxygen adsorbed on the support oxides. In the temperature range −10 to 65°C, the activation energies for CO oxidation were 8.2 kcal/mol (Au/TiO2), 8.4 kcal/mol (Au/α-Fe2O3), and 3.9 kcal/mol (Au/Co3O4). The rate of CO oxidation is zero order with respect to CO for the three catalysts, and 0.2-0.3 for Au/TiO2 and Au/Co3O4 and zero order for Au/α-Fe2O3 with respect to O2. By taking into consideration TPD and FT-IR data, a mechanism is proposed in which CO adsorbed on gold particles migrates toward the perimeter on support oxides and there it reacts with adsorbed oxygen to form bidentate carbonate species. The decomposition of the carbonate intermediate is considered to be rate-determining.