Operando Spectroscopy and DFT Modeling of Gold/Ceria Catalysts for CO Oxidation and Water-Gas Shift Reaction

The aim of this work is to elucidate the reaction mechanism of the CO oxidation and the water-gas shift reaction over gold/ceria as a prototype heterogeneous oxide supported metal catalyst. In this context a combined setup was developed to apply Raman, UV-Vis and infrared spectroscopy under reaction conditions to the active catalyst (operando approach). Along with density functional theory calculations (DFT) of ceria and gold/ceria model systems the experimental results are interpreted on the molecular level. Employing DFT calculations Raman and infrared active vibrations are calculated and experimental bands are assigned to the vibrational modes properly. With this approach a comprehensive interpretation of the Raman spectra of nanoparticle ceria is possible, while two bands are reassigned to distinct modes of the ceria(111) surface. At well defined ceria nanocrystals the facet dependent oxygen activation behavior is studied and an overall facilitated oxygen adsorption and peroxide formation at the ceria(100) surface facet is observed. In addition superoxide formation is observed at this surface facet for the first time. The results on gold/ceria catalysts reveal that the subsurface oxidation state of the ceria support alters the activity of a gold/ceria catalyst for CO oxidation. An oxidized gold species in direct contact with the ceria support is evidenced as the site for CO adsorption and proposed as the active site for oxidation reactions over gold/ceria catalysts. The results underline the potential of vibrational spectroscopy for operando characterization of catalyst materials. Together with DFT calculations, which allow a proper assignment of the bands in Raman or infrared spectra, this provides an interpretation on the molecular scale and direct conclusions for the reaction mechanism. The results may be directly transferable to other oxide supported metal catalysts highlighting the general relevance of the results presented in this study