UV reflectance and FTIR spectroscopic studies of CO adsorption and reaction on lanthanum oxide

Abstract

Microcrystalline La₂O₃ outgassed at 873 K has a UV absorption edge centred at ca. 250 nm on which absorption ascribable to surface states is superimposed. This latter absorption is diminished by CO₂ adsorption, which produces surface carbonate. UV reflectance and IR results with CO are correlated to show that CO adsorption on the outgassed oxide occurs with rapid formation of carbonite CO₂^2– ions, accompanied at room temperature by a slow reaction forming reduced oligomeric (CO)_n^2– anions and carbonate CO₃^2–. The reaction stages have been resolved by extending the UV and IR measurements to low temperatures. CO₂^2–, identified by UV absorption at 340 nm and IR bands at 1390 and 987 cm^–1, is the only process occurring at 90 K. As the temperature is increased, the ketenic dimer anion C₂O₃^2– is formed, characterized by a UV band at 300 nm and IR bands at 2075 and 1367 cm^–1. The onward disproportionation producing reduced oligomers and carbonates gives a complex IR spectrum in the 1700–1000 cm^–1 range. The bands due to the oligomers are readily discriminated by their sensitivity to oxygen. The discussion illustrates that UV and IR spectroscopy afford a powerful combination in defining and understanding the stages involved in the adsorption and reaction of CO on lanthanum oxide. The nature and extent of the reactivity towards CO shows that the basicity of La₂O₃ is not as great as that of the alkaline-earth-metal oxides.