Theoretical Study of the Role of a Metal-Cation Ensemble at the Oxide-Metal Boundary on CO Oxidation

Identification of the active sites in heterogeneous catalysis is important for a mechanistic understanding of the structure-reactivity relationship and rationale of the design of new catalysts, but remains challenging. Among others, the boundaries at metal nanoparticles and supported oxides were found to be important and attributed to the active sites in various catalytic reactions. To reveal the nature of the active sites at the boundaries, the catalytic role of the inverse 3d transition-metal oxide nanoislands on Pt(111) for low-temperature CO oxidation was studied by density functional theory calculations. A characteristic Pt-cation ensemble at the oxide/metal boundaries as the active sites is identified. In Pt-cation ensembles, coordinate-unsaturated (CUS) cations exposed at the edges of oxide nanoislands are highly active for O-2 adsorption and dissociation, and less-reactive Pt binds modestly with dissociated O responsible for the facile CO oxidation. Inverse VIIIB-oxide/Pt boundaries exhibit high activities for low-temperature CO oxidation, and the corresponding activity decreases gradually from Fe to Co to Ni. The results rationalize a wide range of the experimental findings. To take advantage of the high oxidizing activity of low-valent VIIIB cations, FeO/Pt and CoO/Pt catalysts are appropriate for the reactions under oxygen-poor conditions, whereas NiO/Pt for the reactions under oxygen-rich conditions. The dependence of the activity and valence state of the Pt-cation ensemble at the oxide/metal boundaries is discussed, and the insight of the metal-cation ensemble as the active sites is highlighted.