Theoretical study on the reactivity of the surface of pure oxides: The Influence of the support and oxygen vacancies

Abstract

The surface reactivity of three oxides widely used as heterogeneous catalysts, CeO2 (ceria), Ga2O3 (gallia), and ZnO, with CO, CO2, and H2 was investigated. The most stable perfect (dehydroxylated) oxide surfaces, surfaces that contain oxygen vacancies, and monolayers of Ga2O3 and ZnO epitaxially grown over CeO2(1 1 1) were investigated using DFT calculations. As expected, CO2 exhibited the highest adsorption energies on almost every surface. The only observed exceptions were the ZnO surfaces, viz., the ZnO(0 0 0 1) perfect surface and a ZnO monolayer grown on ceria, with which the CO molecule interacts more strongly and generates CO2 species. In contrast, H2 interacts weakly with the majority of the surfaces, with the exception of gallia/ceria, where this molecule dissociates. The oxides become considerably more reactive when oxygen vacancies are present on the surface. The reactivity of the CeO2(1 1 1) and Ga2O3(1 0 0) surfaces that contain oxygen vacancies increases up to ten-times with respect to the perfect surfaces. In addition, both Ga2O3 and ZnO also exhibit an important increase of their reactivity when they are supported on ceria. Thin films of these oxides that are epitaxially grown onto ceria surfaces have shown to be highly suitable catalysts for oxidizing CO and CO2 molecules and for dissociating the H2 molecule.