Elucidation of Active Sites of Gold Nanoparticles on Acidic Ta₂O₅ Supports for CO Oxidation

Gold nanoparticles of small sizes with a highly uniform dispersion were successfully deposited on several crystalline forms of Ta₂O₅ (pseudohexagonal, orthorhombic, and pyrochlore) by the sol immobilization method. The pseudohexagonal Ta₂O₅ (TT-Ta₂O₅) synthesized by a hydrothermal method showed the highest catalytic activity for CO oxidation as a support for gold catalysts. The temperature at 50% CO conversion was −11 °C for 1 wt % Au/TT-Ta₂O₅, and the temperature was 23 °C at 100% CO conversion (space velocity of 20000 mL h^–1 g_cat^–1). To investigate the reaction mechanism, in situ diffuse reflectance infrared Fourier transform spectroscopy for Au/TT-Ta₂O₅ was performed, and the sequential delivery of adsorbed CO on Ta₂O₅ sites and Au sites to the active sites was observed at −120 °C. The results of temperature-programmed CO reduction suggested that the oxygen adsorbed at −100 °C contributed mainly to the high catalytic activity of CO oxidation, while the lattice oxygen played a minor role for CO oxidation from −60 to 100 °C, which was the same temperature range with the actual reaction condition.