Hydrogen activation on zinc oxide

Abstract

Three types of hydrogen adsorption have previously been characterized on zinc oxide at low temperatures, designated Types I, II and III. One of these, Type III, is a molecular form of adsorption which can be specifically titrated by nitrogen adsorption on 10–20% of the surface with formation of a molecular nitrogen species. This selective titration by a molecular nitrogen adsorption was employed to study the participation of molecularly adsorbed hydrogen in the allotropie (ortho-para) hydrogen conversion and isotopic (H 2D 2) hydrogen exchange. The promotional effect of oxygen on the conversion reaction was also investigated. These results at low temperatures clearly indicate that the conversion reaction proceeds predominantly by a molecular paramagnetic mechanism involving this type III hydrogen below 140 °K, while the exchange reaction proceeds via a Bonhoeffer-Farkas mechanism involving type I hydrogen at both low temperature and also at room temperature. At low temperatures, the exchange and conversion reactions are independent, but this is not necessarily true at room temperature. A mathematical model is presented for the paramagnetic conversion based on Wigner's theory involving longer times of interaction. The result is an enhancement of the conversion rate by several orders of magnitude with the deuterium conversion being enhanced more than the molecular hydrogen conversion.