Insight into the mechanism of the water-gas shift reaction over Au/CeO catalysts using combined operando spectroscopies.

The mechanism of the low-temperature water-gas shift (LT-WGS) reaction over Au/CeO catalysts with different ceria terminations, i.e., (111), (110), and (100) facets, was investigated. Using combined operando Raman and UV-Vis spectroscopy as well as isotope exchange experiments, we are able to draw conclusions about the reducibility behaviour and the exchange of surface oxygen. Additional density functional theory (DFT) calculations facilitate the vibrational bands assignments and enhance the interpretation of the results on a molecular level. A facet-dependent role of gold is observed with respect to the oxygen dynamics, since for the CeO(111) facet the presence of gold is required to exchange surface oxygen, whereas the CeO(110) facet requires no gold, as rationalized by the low defect formation energy of this facet. This behaviour suggests that surface properties (termination, stepped surface) may have a strong effect on the reactivity. While the reduction of the support accompanies the reaction, its extent does not directly correlate with activity, highlighting the importance of other properties, such as the dissociative adsorption of water and/or CO/H desorption. The results of our facet-dependent study are consistent with a redox mechanism, as underlined by HO isotopic exchange experiments demonstrating the ready exchange of surface oxygen.