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

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 O2– 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.