SURFACE CLUSTER-MODELS FOR V2O5 - STUDIES OF THE IMPORTANCE OF LOCAL GEOMETRY

Catalytic properties of the vanadium pentoxide (010) surface are discu ssed based upon semiempirical quantum chemical calculations using clus ter 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 oxyge n sites are analyzed with the help of Mulliken populations and Meyer b ond order indices. Different optimization procedures (with various bou ndary 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 optimi zed geometry of the cluster remains much closer to the surface geometr y when the optimization is done for the whole cluster, excluding the s aturated hydrogen atoms. Optimization of the small fragment of the clu ster, results in the significant rearrangement of the cluster structur e and leads to the ''warped'' geometry (bridging oxygen as well as van adium atoms are shifted out of the surface). Two types of boundary con ditions assumed during the optimization process lead to similar result s, the optimization of all atoms in the cluster (with saturating hydro gen atoms kept frozen) and the same optimization in the presence of th e second layer. The presence of the second layer stabilizes the surfac e geometry. The role of the second layer is also shown in a formation of an oxygen vacancy at the bridging position.