QUANTUM-CHEMICAL DESCRIPTION OF THE ACTIVE-SITES FOR THE SELECTIVE OXIDATION OF HYDROCARBONS

Different aspects of quantum chemical modelling of catalytic systems a re discussed. Further, the electronic structure of various VnOm cluste rs representing the vanadium pentoxide (010) surface is studied and th e reactivity of different surface oxygen sites with respect to adsorpt ion of hydrogen as well as propylene is examined. Results of ab initio HF and DFT calculations are compared with those of the semiempirical INDO-type method. It is found that the different quantum chemical meth ods lead to the same qualitative results. Satisfactory convergence in cluster properties is achieved for clusters of ten vanadium atoms. The saturation of dangling bonds by hydrogen atoms does not influence the properties of the oxygen centers significantly. Among different oxyge n centers the oxygens bridging two bare vanadium atoms are most negati vely charged. Hydrogen binds to all inequivalent oxygen sites with the strongest binding occurring for oxygens bridging two vanadium atoms. INDO cluster studies for propylene adsorption/reaction on V2O5 (010) s how that different approaches of the molecule to the surface yield dif ferent surface complexes which can then react to form different produc ts.