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Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VO/TiO x
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Vasilii I. Avdeev, and Alexander Fedorovich Bedilo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp311322b • Publication Date (Web): 17 Jan 2013 Downloaded from http://pubs.acs.org on January 18, 2013
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The Journal of Physical Chemistry
Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VOx/TiO2 Vasilii I. Avdeev* and Alexander F. Bedilo Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russian Federation
ABSTRACT: Detailed molecular mechanisms of oxygen isotopic exchange over VOx/TiO2 catalyst following the R0, R1, and R2 mechanisms were studied using periodic DFT analysis of possible pathways by the CI-NEB method. The electronic structures of surface VOx species formed on the VOx/TiO2 model surface after interaction of molecular oxygen with fully oxidized O=V5+-O-V5+=O sites and reduced V3+-O-V3+ sites were analyzed. We found a number of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen molecules on a reduced V3+-O-V3+ site leads to the formation of a superoxide complex followed by its transformation into a peroxide complex with low activation energy about E* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir-Hinshelwood mechanism without participation of lattice oxygen along the R0 reaction pathway. We demonstrate that adsorption of molecular oxygen on fully oxidized O=V5+-O-V5+=O sites results in the formation of either monodentate V> VOx/Al2O3 > VOx/SiO2. This sequence is similar to the one commonly observed for the catalytic activity of supported vanadia catalysts in oxidative reactions.28 In the present study we attempted to identify active surface oxygen species involved in the isotopic exchange over supported vanadia oxide catalyst VOx/TiO2. We suggested two models of the VOx/TiO2 catalyst for high-temperature and low-temperature oxygen exchange. Periodic DFT analysis of possible pathways of the oxygen isotopic exchange reactions on the VOx/TiO2 model surfaces was carried out in order to gain deeper understanding of the R0, R1 and R2 molecular mechanisms. We found that the R0 reaction pathway for the low-temperature isotopic exchange proceeds via decay of superoxide and peroxide species, which are formed after contact of two oxygen molecules with two reduced vanadium sites, V3+-O-V3+. For high temperature hetero-exchange R1 the likely pathways were found to include monodentate V
