Superoxide ions formed on MgO through the agency of presorbed molecules. Part 2.—Details on the mechanism

Abstract

The interaction of dioxygen with slightly acidic X—H molecules at the MgO surface causes oxidation reactions typically involving O^–₂ as intermediate. EPR-detectable species are observed only on samples outgassed at 1070 K, together with small amounts of by-products, revealed by IR spectroscopy. This latter reaction and the overall oxidation are part of the same network. The first step is the dissociative adsorption of X—H (X—H + O^2–(s)→ X^–+ OH^–), the second is intermolecular electron transfer (X^–+ O₂→ X˙+ O^–₂). On MgO samples outgassed at low temperatures, all O^–₂ species are unstable and react quantitatively with the X˙ radicals to yield oxidation products. In addition, on MgO outgassed at 1070 K strongly coordinatively unsaturated cations act as traps for O^–₂ species which become EPR visible. The accompanying X˙ radical species reacts with dioxygen to yield a second O^–₂ and an oxygenated species XO^– responsible for the IR signals. The nature of the XO^– species is discussed in the cases of propene, benzene and toluene; further reactions of O^–₂ and the molecular properties of X—H controlling superoxide formation are also discussed.