Reactive removal of surface oxygen by H-2, CO and CO/H-2 on a Au/CeO2 catalyst and its relevance to the preferential CO oxidation (PROX) and reverse water gas shift (RWGS) reaction

Aiming at further insight into the mechanism of the preferential CO oxidation (PROX) and the (reverse) water gas shift (R)WGS reaction over Au/CeO2 catalysts, we investigated the removal of stable, active surface oxygen from a Au/CeO2-supported catalyst by H-2 by quantitative temporal analysis of products (TAP) measurements over a wide range of temperatures (30-300 degrees C) and compared it with the removal of active oxygen by reaction with CO and a CO-H-2 mixture. It is demonstrated that a surface-oxidized Au/CeO2 catalyst can be reduced by H-2 pulses only at temperatures higher than 80 degrees C, whereas significant reduction of the catalyst surface by CO or CO/H-2 pulses is possible already at 30 degrees C. Even at 300 degrees C, removal of surface oxygen by H-2 or CO pulses is possible only in the presence of Au nanoparticles, underlining that these processes are Au assisted. At all temperatures investigated, the amount of H-2 necessary to remove the available active surface oxygen is much higher than that of CO. Hence, over the whole range of temperatures the efficiency of H-2 to surface reduce a Au/CeO2 catalyst is much lower than that of CO or of a CO-H-2 mixture. On the other hand, it is significantly higher than that for active oxygen deposition from CO2, indicating that under steady-state reaction conditions during the RWGS reaction surface lattice oxygen vacancies are present on the surface. Furthermore, the influence of adsorbates resulting from H-2, such as hydroxyl groups or water, on the oxygen storage capacity (OSC) of Au/CeO2 catalysts and on the active oxygen deposition from CO2 was elucidated. Implications of these results for the mechanistic understanding of the PROX and the (R) WGS reaction in H-2-rich gases are discussed.