The adsorption of CO on MgO: The effect of γ-irradiation

Abstract

The adsorption of carbon monoxide on magnesium oxide has been investigated between 294 K and 873 K with and without the influence of continuous γ-irradiation. The adsorption exhibits a fast initial step, which above 573 K is associated with an adsorbed bidentate species requiring an activation energy, E_a of 40.1 ± 6.0 kJ mol^-1. This fast adsorption is followed by a slower process which obeys first-order kinetics with respect to the presence of CO. The Arrhenius plot of this slow adsorption on unirradiated MgO, at 673 K and above, indicated that two processes, of E_a 181.4 ± 5.0 kJ mol^-1 and 78.1 ± 6.0 kJ mol^-1, were operative above and below 773 K respectively. Below 773 K a (CO₃^2-)_ads species is produced, whereas above this temperature the incorporation of (CO₃^2-)_ads species into the bulk of the MgO lattice and the regeneration of the surface by outward diffusion of lattice oxygen ions became rate-determining. The adsorption of CO during γ-irradiation occurred over the complete temperature range 573-773 K, with an E_a of 69.0±6.0 kJ mol^-1; this implies that the rates of these diffusion processes are enhanced by the radiation.    

The G_ads values at 373, 573 and 673 K show a maximum in the initial adsorption region. This is caused by the destruction of the shallow trapped positive holes-which are associated with impurity atoms and used in the adsorption process-by radiation-generated electrons.