Infrared studies of probe molecules adsorbed on calcium oxide

Abstract

Infrared spectra are reported of pyridine, carbon dioxide, methanoic acid, ethanoic acid and carbon monoxide on calcium oxide which had been preheated in vacuum at 1073 K. The oxide surface contains incompletely coordinated weakly Lewis-acidic calcium cation sites and exposed oxide ions with basic character. Pyridine interacts with Lewis-acidic sites whereas carboxylic acids donate protons to oxide ions with the formation of hydroxy groups and adsorbed carboxylate anions. Carbon monoxide undergoes a complex series of reactions involving oxide ions and the availability of vacant ligand sites. The adsorption of carboxylic acids and carbon monoxide is believed to primarily occur on exposed {111} crystal faces or at step and edge sites at which the array of calcium and oxide ions replicates in miniature a (111) face. The chemisorption of hydrogen on calcium oxide is implied by the poisoning effect of hydrogen on carbon dioxide and carbon monoxide adsorption. Oxygen promotes the formation of surface carbonate complexes from adsorbed carbon monoxide.