Surface reactivity of magnesium oxide doped with manganese: an e.s.r. and chemisorption study

Abstract

At 600°C hydrogen and oxygen are adsorbed on magnesium oxide doped with manganese to an extent which is dependent on the manganese concentration. E.s.r. studies indicate that adsorption of hydrogen results in a reduction of higher valence states to Mn²⁺, whereas adsorption of oxygen converts Mn²⁺ into Mn³⁺ and Mn⁴⁺, which concentrate at the oxide surface. The stoichiometry and mechanism of these processes have been investigated in powders of small (100 Å) and large (600 Å) average particle sizes. At 20°C the adsorption of hydrogen is little affected by manganese impurity whereas the adsorptions of oxygen is directly proportional to the manganese concentration. These adsorptions are weak forms of chemisotorption and do not appear to involve charge transfer. The e.s.r. spectra of Mn²⁺ in the small particles of MgO are interpreted in terms of a D term in the Hamiltonian and the presence of this axial component in the normally octahedral environment of Mn²⁺ is thought to arise from the proximity of the surface.