Superoxide Ions Formed on Mgo through the Agency of Presorbed Molecules .2. Details on the Mechanism

The interaction of dioxygen with slightly acidic X-H molecules at the MgO surface causes oxidation reactions typically involving O2- 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 + O2-(s) --> X- + OH-), the second is intermolecular electron transfer (X- + O2 --> X. + O2-). On MgO samples outgassed at low temperatures, all O2- 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 O2- species which become EPR visible. The accompanying X. radical species reacts with dioxygen to yield a second O2- 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 O2- and the molecular properties of X-H controlling superoxide formation are also discussed.