Anomalous trapping of carbon monoxide by carbon dioxide on magnesium oxide

Abstract

An anomalous increase in the adsorption of carbon monoxide by adsorbed carbon dioxide has been observed on magnesium oxide at temperatures in the range 290–320 °C. Two or three molecules of adsorbed CO₂ are necessary to trap one molecule of adsorbed CO. The activation energy for desorption of the trapped CO is estimated by temperature-programmed desorption to be 184 kJ mol^–1, which is much higher than the value on CO₂-free magnesium oxide (88 ± 5 kJ mol^–1). On the other hand, the adsorption and desorption of CO₂ are not affected by the adsorption of CO, either in terms of the amount adsorbed or with regard to the heat of adsorption of CO₂. Carbon dioxide is strongly and competitively adsorbed on the same active sites as CO, and therefore no CO is absorbed on a surface fully pre-covered with adsorbed CO₂. The adsorption sites are ca. 1% of the number of total active sites, and we speculate that these consist of surface ions of lower coordination.