A Theoretical Investigation of the Selective Oxidation of Methanol to Formaldehyde on Isolated Vanadate Species Supported on Titania

Abstract

The selective oxidation of methanol to formaldehyde occurring on titania-supported vanadate species has been analyzed theoretically with the aim of understanding why the activity of VO x /TiO 2 is ∼10 3 faster than that of VO x /SiO 2 . The active site was represented by a [(O) 3 VdO] group located at the corner of a cubic TiO x cluster, a model similar to that used successfully to describe the oxidation of methanol on isolated vanadate species supported on silica. Density functional theory was used to calculate the geometry, vibrational frequencies, and energy of all ground state and transition state structures. The equilibrium constants and rate coefficients for each elementary reaction step were calculated using statistical mechanics and absolute rate theory. Methanol oxidation to formaldehyde was taken to proceed via two key steps: the reversible adsorption of methanol across a V-O-Ti bond followed by the transfer of a hydrogen atom from an adsorbed methoxy group to a vanadyl O atom. The rate parameters and the apparent first-order rate coefficient determined for VO x /TiO 2 were found to be very similar to those reported earlier in a theoretical analysis of VO x /SiO 2 [J. Phys. Chem. C 2007, 111, 14753], indicating that the significantly higher rate of reaction seen experimentally for VO x /TiO 2 is not due to an intrinsic electronic effect of the support on the catalytic properties of the active center. Introduction of an O-vacancy adjacent to the vanadate species results in a reduction in the activation barrier for the rate-limiting step and to close agreement between the rate parameters predicted and those found experimentally. The effect of O-vacancies in the support on the rate of methanol on metal oxidesupported vanadate species is further evidenced by a strong correlation between the turnover frequency for methanol oxidation and the energy required to form an O-atom defect on metal oxide supports.

Density functional theory study of the oxidation of methanol to formaldehyde on a hydrated vanadia cluster

“…Broken-symmetry has also been used in previous works where oxidation processes have been studied in gas phase models. 32,[56][57][58][59] In those studies, the results are in good agreement with the experimental data confirming the appropriate use of such approach applied to catalytic systems.…”

“…8,9,28,30,32,56,57,[68][69][70][71][72][73] Sauer and co-workers have studied the oxidation of methanol to formaldehyde by means of a O¼ ¼V(OCH 3 ) 3 model and the corresponding activation energy for the CÀ ÀH bond breaking process is 35.1 kcal/mol. 32 In the case of the oxidation of methane to methanol which involves similar steps, the activation energy of CÀ ÀH bond breaking is calculated to be of 36.6 kcal/mol with the same O¼ ¼V(OH) 3 model cluster.…”

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

“…This would result in the formation of a hydroxyl on the ceria support and a vanadylmethoxy complex (i.e., O=V -OCH 3 ), as previously suggested by DFT studies of methanol ODH on vanadia monomers supported by SiO 2 or TiO 2 [39,[74][75][76] (structure II). Another possibility is that isolated vanadyl species are not as sterically hindered as vanadyl groups in vanadia thin films, and both the hydroxyl and the methoxy are bound to the vanadia forming a HO-V-OCH3 species (structure III).…”

Propane Ammoxidation Over the Mo–V–Te–Nb–O M1 Phase: Reactivity of Surface Cations in Hydrogen Abstraction Steps

“…For the cluster-adsorbate systems employed in this study, the ground energy state occurred in all cases with one of the lowest spins that are usually energetically spaced within 0.1 eV. Cluster models of metal-oxygen species on oxide surfaces, including oxygen attached to V [32,[40][41][42], Mo, and Bi [29, 43], have been fruitfully employed by various groups previously for studies of alkane activation and oxidation on metal oxide surfaces. Our cluster models consisted of three truncated M1 ab planes containing the proposed active site, which contained V in the S2 lattice site, Mo in the S4 and S7 lattice sites, and Te in the S12 channel sites (Fig.…”