High-performance water gas shift induced by asymmetric oxygen vacancies: Gold clusters supported by ceria-praseodymia mixed oxides

Modifying and controlling sites at the metal/oxide interface is an effective way of tuning catalytic activity, beneficial for bifunctional catalysis by reducible oxide supported metal nanoparticles. We employed mixed ceria-praseodymia supported Au clusters for the water gas shift reaction (WGSR). Varying the Ce: Pr ratio (4:1, 2:1, 1:4) not only allows to control the number of oxygen vacancies but, even more important, their local coordination, with asymmetrically coordinated O# being most active for water activation. These effects have been examined by X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, temperature programmed desorption/reduction (TPD/TPR), and density functional theory (DFT). Using the WGSR performance of Au/CeO_x as reference, Au/Ce₄Pr₁O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300°, which is about 5-times that of Au/CeO_x. Au/Ce₄Pr₁O_x also showed excellent stability, with the conversion still being 70% after 50 h time-on-stream at 300 °. Although a higher Pr content leads to more O vacancies, the catalytic activity showed a "volcano behavior". Based on DFT, this was rationalized via the formation energy of oxygen vacancies, the binding energy of water, and the asymmetry of the O# site. The presented route of creating active vacancy sites should also be relevant for other heterogeneous catalytic systems.