Oligomeric Vanadium Oxide Species Supported on the CeO2(111

Article pubs.acs.org/JPCC

Oligomeric Vanadium Oxide Species Supported on the CeO2(111) Surface: Structure and Reactivity Studied by Density Functional Theory Christopher Penschke, Joachim Paier,* and Joachim Sauer Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany S Supporting Information *

ABSTRACT: We examine (VO)k and (VO2)k (k = 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+U). As reactivity descriptors, we calculate oxygen defect formation and hydrogenation energies. In agreement with experiment, our results suggest that vanadylterminated 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.

I. INTRODUCTION Supported vanadium oxides represent very active and selective catalysts for many reactions of industrial relevance. Important examples are the oxidation of o-xylene to phthalic anhydride, the selective catalytic reduction of NOx with ammonia, and the selective oxidation of methanol to formaldehyde.1−5 The latter, a frequently studied model system for oxidative dehydrogenation (ODH) reactions, is a way to chemically activate and functionalize C−H bonds.6 Selectivity of this process is indispensable due to the thermodynamic preference of combustion to CO2 and H2O. However, activity and selectivity of the vanadium oxide catalyst are driven by several factors. First, they critically depend on the nature of the support material. Reducible oxides such as TiO2 or CeO2 perform significantly better than inert supports like SiO2.7−11 Note that reducibility of the support material plays a major role because hydrogen transfer, the ratedetermining step in the ODH reaction,12−14 leaves an electron on the supported vanadium oxide catalyst. Second, it is known experimentally that the vanadia catalyst exhibits only high activity when present as submonolayer or monolayer species on the support.7,15,16 Depositing vanadia on ceria leads to a particularly active catalytic system.7,10,11 The high redox activity of ceria is explained by its ease to free and recapture oxygen,17 but only recently a combined experimental and computational study of VO·CeO2(111) and VO2·CeO2(111) has shown that this © 2013 American Chemical Society

remarkable activity is due to the ability of ceria to easily accommodate electrons in localized Ce-4f orbitals.12 Several other studies have also addressed the partial reduction of the V,Ce-mixed metal oxide system.18−22 Shapovalov and Metiu studied VOn clusters deposited on the TiO2(110) and CeO2(111) surfaces and applied density functional theory (DFT) within the generalized gradient approximation without correcting for onsite Coulomb correlation. They suggested VO3/VO2 as the active redox couple in oxidation reactions and found almost constant Bader charges for vanadium, which appeared to be insensitive to the support material supplied.18 For VOn particles (n = 0, 1) on a CeO2(111) surface, photoelectron spectroscopy in agreement with DFT+U (i.e., DFT corrected for the Coulomb onsite correlation) demonstrated that vanadium becomes fully oxidized (+5 oxidation state), whereas reduced cerium (+3) is formed.20 Furthermore, a direct relationship between the morphology of vanadia/ceria and the vanadyl frequency was found. Experimentally a blue shift of the vanadyl stretch frequency of ∼25 cm−1 is observed, which could be attributed to the onset of dipole coupling between neighboring VO groups. Calculation of the DFT harmonic frequencies for the VO·CeO2(111) and (VO)3·CeO2(111) system could reproduce the blue shift.20 Received: January 16, 2013 Revised: February 12, 2013 Published: February 13, 2013 5274

dx.doi.org/10.1021/jp400520j | J. Phys. Chem. C 2013, 117, 5274−5285

The Journal of Physical Chemistry C

Article

5s25p64f15d16s2). Two partial waves were used for each orbital, and the cutoff radius for the partial waves amounted to 1.5 and 1.65 for 5s and 6s, 1.8 for 5p, 2.3 for 5d, and 2.57 au for the 6f states, respectively. For oxygen and vanadium, 6 ([He] 2s22p4) and 11 ([Mg] 3p64s23d3) valence electrons were used, respectively. The respective partial wave cutoff radii amount to 1.2 and 1.52 au for 2s and 2p states in oxygen and 2.0, 2.3, and 2.3 au for the p, s, and d states in vanadium. For the 1s orbital of hydrogen, a partial wave cutoff radius of 1.10 au was employed. Some selected results were obtained using the Heyd− Scuseria−Ernzerhof (HSE)38 hybrid functional combined with a screening parameter of 0.207 Å−1 (cf. ref 39). Structural optimizations were performed until all forces acting on the relaxed atoms were below 0.02 eV/Å. Vibrational frequencies use central differences for the force derivative with atomic displacements of ±0.015 Å. Only vanadium atoms and oxygen atoms bound to vanadium were included to build the partial Hessian, whereas all other atoms were kept fixed. Because of the relatively large unit cell used (see below), the sampling of the Brillouin zone was restricted to the Γ point. Convergence with respect to the plane-wave kinetic energy cutoff and k-point sampling was tested by (separate) calculations using a cutoff of 900 eV and a (2 × 2 × 1) k mesh. Both parameters affect relative energies by