CLUSTER MODEL STUDIES ON CATALYTIC PROPERTIES OF VANADIUM PENTOXIDE

ZINDO cluster model calculations are performed to study the electronic structure and chemical reactivity of the V2O5(010) surface. Inter-ato mic binding in vanadium pentoxide is determined to be of a mixed ionic and covalent character. The calculations reveal the difference in the catalytic properties between structurally inequivalent surface oxygen centers and show the increased local reactivity of bridging oxygens w ith respect to the electrophilic adparticles. Convergence of the elect ronic properties with respect to the cluster size is achieved for clus ter as large as V10O31H12. The effect of the second substrate layer on the surface electronic properties is found to be negligible. Further, rather similar electronic parameters of the V10O31H12 cluster in its idealized, bulk and optimized geometry are obtained. The H/H+ species adsorb ct the V2O5(010) surface, always at oxygen sites, forming very stable surface hydroxyl groups. The strongest binding occurs with the oxygen O(c) bridging two bare vanadium atoms. These O(c) oxygens becom e quite mobile in presence of the H/H+ adparticle. Allowing the surfac e oxygens to relax during adsorption of H/H+ leads to different adsorp tion scenarios depending on the surface oxygen site. At the vanadyl ox ygen site a very stable and rigid hydroxyl group O(a)H is formed above the vanadium center, At the doubly coordinated oxygen site O(b) the a dsorbate penetrates between two vanadyl groups to form a local O(b)H g roup, while at the O(c) site an O(c)H group is created (slightly above the surface O(c) position), where O-H binding is strongest. Weak init ial interaction, between triply coordinated oxygens O(d,e) and the inc oming H/H+ species, leads to stabilization of the adsorbate near the c losest vanadyl site resulting in a tilted O(a)H group instead of O(d)H or O(e)H.