Theoretical insights into CO oxidation activities on CeO2(111) steps

Abstract

Direct CO oxidation on four types of CeO2(111) surface steps were studied by using density functional theory (DFT) calculations. Our results showed that CO adsorption at step edges can be enhanced through the alignment of C-O and surface Ce-O bonds. Reactivities of various step edges on the CeO2(111) surface were found to be similar to those of the corresponding extended facets (i.e., {110} and {100}). In general, the oxygen sites at step edges with lower coordination numbers can give smaller vacancy formation energies and higher reactivities. In particular, nearly all the O species at the lower position of the step edges exhibit superior catalytic activities than those on the upper position. Moreover, it was found that the unusual decrease of the Ce 4f orbital energy level after CO adsorption on the Type II* step can contribute to its excellent catalytic activity towards CO oxidation, which is caused by the synergistic effect of structural and electronic distortions on the low-coordinated surface Ce5c at the step edge. Our results disclosed the unique role CeO2(111) steps play that influences the reactant adsorption, activation barriers, product desorption and thus the CO catalytic oxidation activities overall. These findings may inspire persistent research interests on the structure-reactivity relationships over surface defect sites on rare-earth oxide catalysts.