Temperature-programed surface reaction study of CO oxidation over Au/TiO2 at low temperature: An insight into nature of the reaction process

The reaction behavior of CO over Au/TiO2 has been investigated by a temperature-programed surface reaction (TPSR) testing. We have found that there maybe existed four reaction processes of oxidizing CO over Au/TiO2 in the O2-poor stream containing a trace H2O at or below 80 °C: (i) CO adsorbed at TiO2 sites is oxidized by H2O to form Ti–COOH species below 80 °C, which is desorbed into CO2 and H2O at 80 °C; (ii) CO adsorbed at Au sites is oxidized by H2O to form CO2; (iii) CO adsorbed at Au sites is oxidized by H2O to form H2 and CO2 according to the water-gas shift reaction, which only occurs with the desorption of Ti–COOH species at TiO2 sites into CO2 at 80 °C; (iv) CO adsorbed at Au sites is oxidized by molecular O2 to form CO2. For the three processes of H2O oxidizing CO, the chemisorption result of CO and O2 over Au/TiO2 shows that it may be the Ti-bonded hydroxyl species (Ti–OH) but not the adsorbed molecular H2O at TiO2 sites to react with CO adsorbed at Au or TiO2 sites. For the process of O2 oxidizing CO, the non-promotion of pre-adsorption of O2 on the formation of CO2 below 80 °C indicates that it may be the physisorbed molecular O2 not the chemisorbed O2 to directly react with CO adsorbed at Au sites. This work also indicates that the process of CO oxidation in the presence of H2O over the supported Au catalyst may be simultaneously oxidized by O2 and H2O according to the respective process.