Thermal stability and chemical reactivity of (O –2)s species adsorbed on MgO surfaces

Abstract

The thermal stability and the chemical reactivity of (O^–₂)_s species adsorbed on a MgO surface have been studied by e.s.r. spectroscopy over a wide range of experimental conditions. The (O^–₂)_s radicals (0.4–3.3 × 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 m^–2) was added at 298 K.

The thermal stability has been studied by heating specimens containing (O^–₂)_sin 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^–₂)_s at different temperatures (298–550 K) with CO, H₂, C₂H₄, NH₃ and H₂O-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 (O^–₂)_s.

Three different, although closely related, electron donor centres at the MgO surface are proposed.