CO Oxidation over Au/ZnO: Unprecedented Change of the Reaction Mechanism at Low Temperature Caused by a Different O2 Activation Process

Au/ZnO prepared by coprecipitation exhibited extremely high catalytic activity for low-temperature CO oxidation below room temperature. The catalytic activity was influenced by the reduction atmosphere in the preparation and the heat treatment in air before the reaction. Reduction of Au(III )by H-2 not only gave smaller Au particles (Au/ZnO (H-2-xK), H-2 treatment at x = 373-673 K) but also a larger amount of Au-0 species than did calcination in air (Au/ZnO (O-2 -xK)), resulting in better activity. In addition, the catalytic activity of Au/ZnO (H-2-xK) was markedly enhanced by heat treatment in air prior to the CO oxidation. Kinetic measurements revealed that the activation energy (E-a) of Au/ZnO (H-2-xK) suddenly changed from 26 to 1.6 kJ mol(-1 )at a temperature below 253 K while the E a of Au/ZnO (O-2-xK) was constant, suggesting that the reaction mechanism for Au/ZnO (H-2-xK) changed at 253 K. UV-vis spectroscopy suggested a larger amount of defects of ZnO. Electron paramagnetic resonance results indicated that the amount of oxygen vacancies of ZnO or O-2- radicals formed on the oxygen vacancies was increased by H-2 reduction and heat treatment in air. In temperature-programmed O-2 desorption, a desorption peak was observed at a lower temperature for Au/ZnO (H-2-xK) after heat treatment than that for Au/ZnO (H-2-xK) before heat treatment and Au/ZnO (O-2-xK). These results suggested that the heat treatment of Au/ZnO (H-2-xK) created oxygen vacancies of which O-2 is activated around the perimeter interface and the activated oxygen is easily desorbed. These oxygen vacancies may become more efficient at a low temperature, resulting in the change of the reaction mechanism at 253 K. This study showed that the effect of the perimeter interface on activation of O-2 changes depending on the temperature and can be controlled by catalyst preparation and heat treatment.