Infrared spectroscopic study of the adsorption of pyridine, carbon monoxide and carbon dioxide on the perovskite-type oxides LaMO3

Abstract

Infrared spectra of adsorbed pyridine, CO and CO₂ on the perovskite-type oxides LaCrO₃, LaMnO₃, LaFeO₃ and LaCoO₃ have been recorded. These oxides did not exhibit significant Brønsted acidity either before or after the adsorption of water. Some of the samples showed Brønsted acidity after reduction with H₂. This is accounted for by a heterolytic dissociative mechanism of H₂ adsorption. An increase in Brønsted acidity after H₂O adsorption on reduced samples is ascribed to the dissociative adsorption of H₂O on pair sites each consisting of an adjacent O^2– ion and an anion vacancy. The spectra obtained after CO and CO₂ adsorption indicate that several stable species coexist in the systems adsorbent–adsorbate studied. These results point to a highly heterogeneous surface of these LaMO₃ oxides.