Adsorption of hydrogen on highly dispersed MgO

Abstract

Special production techniques enable the preparation of highly dispersed MgO with a specific surface area up to 400 m² g^–1. This results in a significant accumulation of highly reactive, low-coordinated surface centres which are related to two fundamentally different processes of H₂ chemisorption on neat MgO: the homolytic and the heterolytic dissociation of H₂. The homolytic process is initiated by the presence of O^– and Mg⁺ radicals which are created by thermal activation of the oxide sample. This gives rise to an irreversible formation of surface OH groups and to a reversible formation of bridged hydrides (Mg₃H). On the other hand the heterolytic dissociation of H₂ occurs in the course of a reversible process on the Mg_3C²⁺O_3C^2– ion pairs (3C indicates three-coordinated). The stabilization of the resulting OH groups necessarily implies the formation of hydrogen bonds with O^2– surface anions as proton acceptors. The corresponding hydride groups are highly reactive with respect to physisorbed O₂. The related surface redox reaction initiates the formation of O₂^– ions and of OH groups incorporating oxygen which comes from the MgO surface. An additional type of surface reaction is observed on partially hydroxylated MgO. The admission of D₂ initiates H/D isotope exchange reactions involving single and multiple-coordinated surface OH groups. They are observed on time-scales of hours and seconds, respectively.