Rates of adsorption of hydrogen, carbon monoxide and their mixtures on zinc oxide

Abstract

The rates of adsorption of hydrogen on zinc oxide at 24.6°C at various pressure (50-300 Torr) have been measured. After an initial period of time, the Elovich equation was closely obeyed. Out-gassing at 410°C for 30 h (high temperature regeneration) did not reproduce the original surface and the rates continuously decreased with each high temperature regeneration. A detailed study was therefore made of the effect of the presence of residual absorbate left on the surface due to ineffective outgassing, viz., evacuation at 24.6°C. Neither a model of a surface having a site heterogeneity but absence of induced effects, nor one of a uniform surface on which induced heterogeneity was developed, can account for the kinetics of readsorption, but a combined model in which both site and induced effects are postulated gives an adequate representation of the experimental results. Some general features of the distribution of site activation energies can also be deduced.

Carbon monoxide was rapidly adsorbed by zinc oxide at 24.6°C but the subsequent increase of adsorption with time was negligible. For a binary gaseous mixture of carbon monoxide and hydrogen, the rate of adsorption of hydrogen was decreased but an activated rate of adsorption subsequently developed. This mutual enhancement of the rate of adsorption of both gases is brought about by strong interaction between the two adsorbates on the surface. By introduction of an attractive potential into the analysis of the combined model, the kinetics of adsorption of each gas from a binary mixture can be adequately explained; furthermore, a comparison of the rates for the two components enables the empirical formula of the surface complex, viz., H₂CO, to be deduced. This method of detecting the presence of a surface compound formation and determining its composition should have general validity for simple systems.