Oligomeric Vanadium Oxide Species Supported on the CeO2(111) Surface: Structure and Reactivity Studied by Density Functional Theory

摘要

We examine (VO)_k and (VO2)_k (k = 1, 2, 3) species supported on CeO2(III) by periodic density functional theory as models for ceria-supported vanadia catalysts. We use the PBE functional and correct for onsite Coulomb correlation (PBE+U), As reactivity descriptors, we calculate oxygen defect formation and hydrogenation energies. In agreement with experiment, our results suggest that vanadyl-terminated monomers, that is, VO2, represent the most active species. This system has a remarkably low oxygen defect formation energy of 0.84 eV (clean surface: 1.84 eV), and hydrogenation proceeds more exothermic (—1.46 vs —1.07 eV for clean surface). The VO2 dimer, preferring an open, chain-like structure at the CeO2(III) surface, is the only other system with a similarly high reactivity. The active species are thermodynamically less favored compared with the VO2 trimer, which forms a ring structure and binds solely via vanadium atoms to CeO2(III). Thus, for this case, relaxation effects are minor. In contrast, the active species bind via oxygen atoms of the VO2 moiety to surface Ce atoms necessitating substantial surface relaxation. Calculated IR vibrational spectra of the supported VO2 monomer and trimer confirm the experimentally observed blue shift of the V=O stretching mode upon aggregation.