Surface States and Catalytic Properties of La1-xSrxCoO3

The surface state of perovskite-type La1-xSrxCoO3 was investigated by means of X-ray photoelectron spectroscopy (XPS). As for fresh (as-prepared) oxides, the surface composition of A site cations (La+Sr) was larger than the bulk composition, though Sr tended to segregate more easily at the surface than La. It was found that surface state of the oxides was stable at temperatures below 500°C but subject to change drastically at the higher temperatures. When the oxide was evacuated at 800°C, the surface was reduced to (La, Sr)4Co3O10 or (La, Sr)2CoO4 and, in the subsequent cooling under an oxygen atmosphere, the reduced surface layer was reoxidized to a Sr enriched perovskite-type oxide. The surface state was also affected by the calcination temperature in the catalyst preparation. As calcination temperature increased, La content increased while Sr content decreased. This change may be caused by the sublimation of Sr from the surface during the calcination at high temperatures. These changes in surface state and composition were found to lead to the deactivation of catalytic properties.