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.