Thermal Stability and Chemical Reactivity of (02-)s Species Adsorbed on MgO Surfaces

Abstract

The thermal stability and the chemical reactivity of (O2)s species adsorbed on a MgO surface have been studied by e.s.r. spectroscopy over a wide range of experimental conditions. The (O2)s radicals (0.4-3.3 x 1015 spins m-2, depending on the activation temperature) were reproducibly formed according to the following sequence: (i) MgO was heated in vacuo at 873-1173 K, (ii) H2 (1 kN m-2) was added at 298 K, (iii) O2 (1 kN nv2) was added at 298 K. The thermal stability has been studied by heating specimens containing (O2)s in vacuo up to 550 K. At this temperature the radical was destroyed, but it could be restored to some extent by first adding H2 and then O2. The chemical reactivity of (O;)s at different temperatures (298-550 K) with CO, H2, C2H4, NH3 and H20-vapour was also investigated. A volumetric determination of H2 and O2 adsorption on MgO evacuated at 1173 K has also been performed. A comparison of the oxygen coverage values so obtained, with those determined by e.s.r., indicates that practically all the oxygen is adsorbed as (O2)s. Three different, although closely related, electron donor centres at the MgO surface are proposed.