In situ time-resolved characterization of Au–CeO2 and AuOx–CeO2 catalysts during the water-gas shift reaction: Presence of Au and O vacancies in the active phase

Synchrotron-based in situ time-resolved x-ray diffraction and x-ray absorption spectroscopies were used to study the behavior of nanostructured $\{\mathrm{Au}+\mathrm{Au}{\mathrm{O}}_x\}–\mathrm{Ce}{\mathrm{O}}_2$ catalysts under the water-gas shift (WGS) reaction. At temperatures above $250°\mathrm{C}$ , a complete $\mathrm{Au}{\mathrm{O}}_x\to \mathrm{Au}$ transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a $\mathrm{Ce}{\mathrm{O}}_2\left(111\right)$ single crystal corroborate that cationic ${\mathrm{Au}}^{\delta +}$ species cannot be the key sites responsible for the WGS activity at high temperatures. The rate determining steps for the WGS seem to occur at the gold-ceria interface, with the active sites involving small gold clusters $(<2\mathrm{nm})$ and O vacancies.