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 vanadiu m pentoxide were determined by a molecular mechanics simulation. The adsorb ate molecules were modeled both as rigid molecules and as flexible molecule s described by the consistent valence force field. The energetically most f avored sites for adsorption are located between two double rows of vanadyl oxygens. In this area the potential energy surfaces exhibit broad minima fo r ethane and propane adsorption energies with values of -22.8 and -33.3 kJ mol(-1), respectively. The shortest distances between the alkane hydrogen a nd different surface lattice oxygen species increase in the following order : double-coordinated bridging oxygen < vanadyl oxygen < threefold coordinat ed oxygen. All C-H bonds of the adsorbates facing the surface are shortened and those facing outward are elongated. The physisorbed states may be tent atively considered as precursors to chemisorption and the subsequent cataly tic reaction. Enthalpies of physisorption of ethane and propane on crystall ine V2O5 have been determined by microcalorimetry. These experimental value s agree well with the adsorption energies obtained from the simulation.