VOx (x = 1−4) Submonolayers Supported on Rutile TiO2(110) and CeO2(111) Surfaces: The Structure, the Charge of the Atoms, the XPS Spectrum, and the Equilibrium Composition in the Presence of Oxygen

We use density functional theory to examine the properties of VO x ( x = 1−4) isolated clusters supported on rutile TiO 2 (110) and CeO 2 (111) surfaces. We calculate the structure of the clusters, the charge on the atoms in the clusters, the vanadium 2p core-level energies, the energy of the reactions VO x (s) + 1 / 2 O 2 = VO x +1 (s), and the dependence of the equilibrium composition of VO, VO 2, VO 3, and VO 4 on oxygen pressure and surface temperature. We find that, contrary to statements made in the literature, the structure of the clusters supported on ceria is different from that supported on titania and so is their oxidation chemistry. The most stable structures found here are also different from those suggested by previous work. The structure of the VO x is very strongly influenced by the position of the oxygen atoms and of the cations on the surface of the support. We propose that the redox couple in oxidation reactions by supported vanadia cluster is VO 2 /VO 3; the substance being oxidized takes oxygen from VO 3 to form VO 2, which is then oxidized by gas-phase oxygen back to VO 3 . The charge on the vanadium atom in VO x is practically independent of x or the support, and it has no relationship with the formal charges attributed to vanadium. The vanadium 2p core-level energy differs for different clusters, even though the charge on the V atom does not change.