A molecular mechanics study of the adsorption of ethane and propane on a V2O5(001) surface

Adsorption geometries (positions and orientations) and adsorption energies of ethane and propane molecules physisorbed on the (001) surface of vanadium pentoxide were determined by a molecular mechanics simulation. The adsorbate molecules were modeled both as rigid molecules and as flexible molecules described by the consistent valence force field. The energetically most favored sites for adsorption are located between two double rows of vanadyl oxygens. In this area the potential energy surfaces exhibit broad minima for ethane and propane adsorption energies with values of -22.8 and -33.3 kJ mol(-1), respectively. The shortest distances between the alkane hydrogen and different surface lattice oxygen species increase in the following order: double-coordinated bridging oxygen < vanadyl oxygen < threefold coordinated oxygen. All C-H bonds of the adsorbates facing the surface are shortened and those facing outward are elongated. The physisorbed states may be tentatively considered as precursors to chemisorption and the subsequent catalytic reaction. Enthalpies of physisorption of ethane and propane on crystalline V2O5 have been determined by microcalorimetry. These experimental values agree well with the adsorption energies obtained from the simulation.