Reactive removal of surface oxygen by H2, CO and CO/H2 on a Au/CeO2 catalyst and its relevance to the preferential CO oxidation (PROX) and reverse water gas shift (RWGS) reaction

Abstract

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 H₂ by quantitative temporal analysis of products (TAP) measurements over a wide range of temperatures (30–300 °C) and compared it with the removal of active oxygen by reaction with CO and a CO–H₂ mixture. It is demonstrated that a surface-oxidized Au/CeO₂ catalyst can be reduced by H₂ pulses only at temperatures higher than 80 °C, whereas significant reduction of the catalyst surface by CO or CO/H₂ pulses is possible already at 30 °C. Even at 300 °C, removal of surface oxygen by H₂ 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 H₂ 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 H₂ to surface reduce a Au/CeO₂ catalyst is much lower than that of CO or of a CO–H₂ 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 H₂, 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 H₂-rich gases are discussed.