DIPOLAR AND CONTACT-PROCESSES IN H-2 O-P-CONVERSION ON IONIC SURFACES

Different channels in the ortho-para conversion of hydrogen molecules physisorbed on a transition metal oxide surface are investigated. The Wigner process, which assumes catalysts to be point dipoles, is analysed and compared to the dipolar and contact processes, which include the orbital degrees of freedom of the surface electrons. Three possible ground states, corresponding to different surface structures, are considered and their relative efficiencies discussed. In particular, the o-p conversion rate is found to be very sensitive to the presence of a metal dangling bond perpendicular to the surface. The molecule-metal electron overlap is shown to strengthen the contact process considerably but the dipolar one only negligibly. Our general expressions are illustrated by a simple model, corresponding qualitatively to chromium impurities dispersed on an alumina surface and discussed in terms of two parameters: the surface-molecule distance d, and the effective metal nuclear charge Z. A fluctuation of about 100% in the o-p conversion rates, when compared to the original Wigner theory, is found.