A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts

Abstract

The structural and electronic properties of CeO(x) species supported on the rutile TiO(2)(110) surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce(3+) ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce(2)O(3) dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeO(x) nanoparticles (NPs) on the TiO(2) surface is highly exothermic and our calculations show that the redox properties of the Ce(III)-Ce(IV) couple are significantly altered when it is supported on TiO(2). In particular the reactivity against CO/O(2) indicates that on the surface the presence of Ce(III) is favored over Ce(IV) species. Our results also indicate that the CeO(x)/TiO(2) interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M/CeO(x)/TiO(2) (M=Au,Cu,Pt) systems in the water-gas shift reaction, we have examined the dissociation of water on the CeO(x)/TiO(2) surface and estimated a barrier as small as 0.04 eV, i.e. approximately 8 times smaller than that computed for a TiO(2) oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M/CeO(x)/TiO(2) systems over M/TiO(2).