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 analys ed and compared to the dipolar and contact processes, which include th e orbital degrees of freedom of the surface electrons. Three possible ground states, corresponding to different surface structures, are cons idered 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 e lectron overlap is shown to strengthen the contact process considerabl y but the dipolar one only negligibly. Our general expressions are ill ustrated by a simple model, corresponding qualitatively to chromium im purities 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 ra tes, when compared to the original Wigner theory, is found.