Adsorption and reactions at the (0 1 0) V2O5 surface: cluster model studies

The use of quantum chemical approaches in the description of electronic pro perties of a catalyst and in understanding the mechanism of catalytic react ions is discussed. The electronic structure of vanadium pentoxide, V2O5, is studied based upon the cluster model with ab initio DFT and semiempirical INDO-type methods. Inter-atomic binding in vanadium pentoxide is determined to be of a mixed ionic and covalent character. Convergence of the electron ic properties with respect to the cluster size is achieved for clusters as large as V10O31H12. Similar electronic parameters of the V10O31H12 cluster in its idealized, bulk and optimized geometry are obtained. The effect of t he second substrate layer on the electronic properties is found to be negli gible. The calculations reveal differences in the catalytic properties betw een structurally inequivalent surface oxygen centers and show the increased local reactivity of bridging oxygens with respect to the electrophilic adp articles. The results of the adsorption of hydrogen, treated as a probe rea ction to model the first step in the selective oxidation of hydrocarbons at structurally different oxygen sites, are compared with the adsorption/acti vation of aliphatic (propene) and aromatic (toluene) hydrocarbons at the va nadium pentoxide(010) surface. The H/H+ species adsorbs at the V2O5(010) su rface always at oxygen sites forming stable surface hydroxyl groups. The de tailed mechanism of H/H+ stabilization depends on the structural and electr onic properties of the adsorption site. The strongest binding occurs with t he oxygen O(c) bridging two ban vanadium atoms. These O(c) oxygens become q uite mobile in presence of the H/H+ adparticle. Oxidation of propene and to luene on V2O5(010) into the aldehyde species proceeds through the formation of C-O bond with the bridging oxygen, abstraction of two hydrogen atoms fr om the same carbon atom of CH3-group, and generation of two OH-surface grou ps. (C) 1999 Elsevier Science B.V. All rights reserved.