Surface reactivity of nickel and nickel hydride films as catalysts in parahydrogen conversion

Abstract

The kinetics of the parahydrogen conversion were studied on nickel and nickel hydride thin films as catalysts. The rate of conversion was measured by a conventional method from 1 to 30 Torr and from 165 to 195 °K. The transformation of nickel into the hydride phase under the influence of atomic hydrogen was attested by observing the change of electrical resistance of the film. The transformation diminished 300-fold the rate of the p- o hydrogen conversion, due to a decrease in the preexponential factor of the Arrhenius equation. The results are in keeping with the poisoning effect of “hydride” hydrogen in nickel or palladium hydrides already reported in hydrogenation reactions. The effect is discussed in terms of the surface heterogeneity of the hydrided nickel film. Nickel crystallite patches which are not transformed into hydride are suggested to be responsible for the residual activity. The nickel hydride areas are catalytically inactive, which can be explained on the basis of earlier proposals that the metal d-band is filled by 1 s electrons from hydrogen atoms.