HYDROGEN ISOTOPIC EXCHANGE BETWEEN D2 AND H2O ON ALUMINA PREPARED BY SPUTTERING

Hydrogen isotopic exchange between D2 and H2O has been investigated on alumina prepared by sputtering at 803 K and at low pressures. The alumina is amorphous, consisting of aluminum with coordination numbers four, five, and six in the ratio 1:2:1 as determined by NMR. A kinetic model is proposed on the basis of the pressure dependence of the exchange rate and the heats of adsorption at adsorbed sites, evaluating by molecular orbital calculations. The adsorbed sites are assumed to be Lewis acid-base pairs composed of a coordinatively unsaturated aluminum and the adjacent unsaturated oxygen site. The site relating to the rate-determining step contains threefold aluminum, which is formed by unsaturation of fourfold aluminum at the surface. As a consequence, the exchange rate depends on the amount of fourfold aluminum when the alumina is heat treated and changes its phase.