OXYGEN-EXCHANGE BETWEEN MAGNESIUM-OXIDE SURFACE AND CARBON-DIOXIDE

The isotopic distribution of carbon dioxide desorbed from an MgO surface containing adsorbed O-18-labelled carbon dioxide ((CO2)-O-18) 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 (COO)-O-16-O-18 and (CO2)-C-18 in a ratio of nearly 1 : 1. The desorption peak in the range 400-600 K (region II) is composed of (CO2)-O-16, (COO)-O-16-O-18 and (CO2)-O-18, among which (CO2)-O-16 was most dominant. The third peak appeared in the temperature range 600-1000 K (region III). This peak consists mostly of (CO2)-O-16. 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.