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 alu mina is amorphous, consisting of aluminum with coordination numbers fo ur, five, and six in the ratio 1:2:1 as determined by NMR. A kinetic m odel is proposed on the basis of the pressure dependence of the exchan ge rate and the heats of adsorption at adsorbed sites, evaluating by m olecular orbital calculations. The adsorbed sites are assumed to be Le wis acid-base pairs composed of a coordinatively unsaturated aluminum and the adjacent unsaturated oxygen site. The site relating to the rat e-determining step contains threefold aluminum, which is formed by uns aturation of fourfold aluminum at the surface. As a consequence, the e xchange rate depends on the amount of fourfold aluminum when the alumi na is heat treated and changes its phase.