Hydrogen adsorption on magnesium oxide powders

Abstract

Hydrogen adsorption on MgO powders has been studied using t.p.d. spectra, adsorption isotherms and e.s.r. spectra. When H₂ is adsorbed at 308 K, MgO outgassed at 773 K shows only one t.p.d. peak (M₂) at 385 K. On the other hand, MgO outgassed at 1123 K shows three new peaks at 480 (H₁), 550 (H₂) and 680 K (H₄) as well as peak M₂. Both samples have another adsorbed species (M₁) which is reversible at 308 K. These five adsorbed species come from different adsorption sites and the surface concentrations of the sites are given.

A model is proposed such that every adsorption site consists of a pair of O^2–_cus and Mg²⁺_cus, in highly coordinative unsaturation, formed by high-temperature activation; the model further proposes that H₂ is dissociatively adsorbed on the sites to form H⁺ and H^–. The coordination number (3 or 4) of O^2–_cus seems to be the main factor in determining the adsorption energy. This type of H₂ adsorption may be widespread in other oxide systems and useful as a probe for some coordinatively unsaturated surface ions.