STUDIES OF THE HYDROGEN HELD BY SOLIDS .23. LOW-TEMPERATURE ADSORPTION OF H-2 AND D-2 AND THEIR SEPARATION FACTORS ON ALUMINA, FLUORIDED ALUMINA, AND MOLYBDENA-ALUMINA SURFACES

Adsorption isotherms of H2 and of D2 at 78 K have been determined for alumina and modified alumina surfaces. In all cases plots of PH2 vs. PD2 for constant amounts adsorbed were linear. This behavior has been predicted by statistical thermodynamics and is shown to follow from equations used to describe chemisorption data. Values of separation factors (S) were derived from the slopes of these curves. This was justified by the earlier statistical thermodynamic formulation and by derivations from Henry's law and Langmuir adsorption isotherms. The adsorption data, however, obeyed accurately the Freundlich law and it is shown that this does not lead to the usual definition of S, i.e., a ratio of adsorption equilibrium constants. The surfaces could also be characterized in terms of fractional coverage (θ) or the amount adsorbed (nH2) at constant pressure. When the parent aluminas were dehydroxylated at increasingly higher temperature, both θ and S increased, but when surfaces were fluorided, S increased while θ decreased. Impregnation with Ga3+ greatly increased S with little change in θ, but when a molybdenum monolayer was grown onto the surface the changes in these properties were relatively small. Reduction of the molybdenum-alumina surface with H2, however, effected increases in both θ and S, whereas the latter was substantially decreased when the reducing agent was CO. Comparison is made of the present values of S with those obtained by the Chromatographic method. © 1979 American Chemical Society.