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

We examine (VO)_<i>k</i> and (VO₂)_<i>k</i> (<i>k</i> = 1, 2, 3) species supported on CeO₂(111) by periodic density functional theory as models for ceria-supported vanadia catalysts. We use the PBE functional and correct for onsite Coulomb correlation (PBE+<i>U</i>). 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(111) 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(111). 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