Stability and migration barriers of small vanadium oxide clusters on the CeO2(111) surface studied by density functional theory

Abstract

By virtue of periodic density functional theory, we investigate structure and thermodynamic stability of (VO)_k and (VO₂)_k (k = 1, 2, 3) clusters deposited on the CeO₂(111) surface, which serve as models for the very active sub-monolayer vanadia catalyst on a ceria support. We find V always completely oxidized (oxidation state +5) and coordinated to four O atoms. As a consequence, Ce⁴⁺ is (partially) reduced to Ce³⁺. Thus, localized Ce-4f states are populated, which requires an onsite U-term (PBE+U) to avoid over-delocalization of f-electrons. Importantly, trimers of VO₂ were found to be extraordinarily stable (agglomeration energy: −1.68 eV), whereas aggregation of VO species on CeO₂(111) is thermodynamically clearly unfavourable (agglomeration energy: 3.45 eV). As a consequence a large area of the V_nO_m phase diagram (for relevant temperatures) is dominated by the VO₂ trimer. The latter is less active towards reduction/oxidation than the active monomer and dimer of VO₂, which are not present in the phase diagram at all, although directly observed by recent STM measurements. This suggests that kinetic effects hinder VO₂ to grow into larger oligomers. The lowest migration energy barrier we found is as high as 1.95 eV, which indicates that adsorbed monomeric VO₂ is “kinetically locked” at low temperatures and explains why monomers are stabilized on the ceria surface.