Oxygen exchange between magnesium oxide surface and carbon dioxide

Abstract

The isotopic distribution of carbon dioxide desorbed from an MgO surface containing adsorbed ¹⁸O-labelled carbon dioxide (C¹⁸O₂) has been measured by temperature-programmed desorption (TPD), in order to study acid–base pair sites on the surface. Three desorption peaks, differing in both isotopic distribution and temperature of desorption, were observed. The desorption peak in the temperature range 300–400 K (region I) is due to C¹⁶O¹⁸O and C¹⁸O₂ in a ratio of nearly 1 : 1. The desorption peak in the range 400–600 K (region II) is composed of C¹⁶O₂, C¹⁶O¹⁸O and C¹⁸O₂, among which C¹⁶O₂ was most dominant. The third peak appeared in the temperature range 600–1000 K (region III). This peak consists mostly of C¹⁶O₂. For most of the adsorbed species, the surface Mg²⁺ contributes to CO₂ adsorption. It is concluded that the adsorbed CO₂ undergoes multiple oxygen exchange with the surface while migrating over it. Based on IR measurements of the adsorbed CO₂, it is suggested that migration takes place during heating of the sample to 473 K in the TPD run.