Collect. Czech. Chem. Commun. 1998, 63, 1355-1367

Surface Cluster Models for V2O5 - Studies of the Importance of Local Geometry

Małgorzata Witkoa, Renata Tokarza and Klaus Hermannb

a Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30 239 Cracow, Poland
b Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14 195 Berlin, Germany


Catalytic properties of the vanadium pentoxide (010) surface are discussed based upon semiempirical quantum chemical calculations using cluster models. Special attention is paid to the role of the second layer in discussing the geometrical factor by using the semiempirical ZINDO approach. Local electronic properties near the different surface oxygen sites are analyzed with the help of Mulliken populations and Meyer bond order indices. Different optimization procedures (with various boundary conditions) are performed for diverse V-O clusters modeling one and two layers. Electronic parameters of the clusters are found to be similar for the cluster in the bulk and optimized geometry. The optimized geometry of the cluster remains much closer to the surface geometry when the optimization is done for the whole cluster, excluding the saturated hydrogen atoms. Optimization of the small fragment of the cluster, results in the significant rearrangement of the cluster structure and leads to the "warped" geometry (bridging oxygen as well as vanadium atoms are shifted out of the surface). Two types of boundary conditions assumed during the optimization process lead to similar results, the optimization of all atoms in the cluster (with saturating hydrogen atoms kept frozen) and the same optimization in the presence of the second layer. The presence of the second layer stabilizes the surface geometry. The role of the second layer is also shown in a formation of an oxygen vacancy at the bridging position.

Keywords: Vanadium oxide; V2O5; Cluster model calculations; Local geometries; ZINDO; Semiempirical calculations.