Electron Donor Sites on the Surface of Magnesium Oxide Powder studied by Electron Spin Resonance Spectroscopy

Abstract

The electron donor properties of MgO powders, activated in vacuo in the temperature range 300–1200 K have been investigated by adsorption of nitrobenzene and some of its para substituted derivatives. The results confirm that the donor properties of MgO are an intrinsic property of the surface, being practically independent of impurity. Two types of nitrobenzene radical were detected: the first one, formed on low temperature activated samples (<1200 K), is the nitrobenzene radical anion interacting with surface protons; the second one, observed on high temperature (1200 K) activated MgO, is the “free” radical anion. No correlation between electron affinity of the adsorbed molecules and radical surface concentration was found. The proton affinity of the anion radical has to be taken into account to rationalize the results. A model of the electron donor surface site consisting of a dual site (O2–cus, OH–) is presented. The effect of adding Ni2+ ions in solid solution has also been examined.