The surface chemistry of some perovskite oxides

Abstract

The surface hydroxyl concentrations of BaTiO 3, SrTiO 3, and LaCoO 3 were determined by exchange with D 2 as a function of the dehydroxylation temperature. The results suggest that the surface chemistry of these materials resembles that of certain other oxide systems such as alumina and titania. Values approaching the number of hydroxyls required to terminate the lattice (1.4 × 10 15/cm 2) were obtained for samples evacuated at room temperature, but these values fell by about an order of magnitude as the pretreatment temperature was increased to 450 °C and by another factor of about 2 at 600 °C. In this way coordinatively unsaturated centers were produced. The experimental procedure was complicated somewhat by the fact that all three oxides were reduced to a greater or lesser extent when contacted with D 2 at elevated temperatures. This amount was negligibly small at 450 °C (the temperature required for equilibration) with BaTiO 3 and SrTiO 3 but it amounted to about 1 e/mole for LaCoO 3 ( e = electrons). The former two oxides could be reduced by about 0.4 e/mole at 1200 °C. In all cases an amount of H 2O equivalent to that of the H 2 consumed was produced in the reduction step and the oxygen removed could be readily replaced on contact with O 2 at the same temperature. LaCoO 3 could be reduced by up to 3 e/mole at 500 °C and the oxygen again replaced stoichiometrically. X-Ray patterns of the parent perovskites were in good agreement with literature values. The pattern for LaCoO 3 changed slightly when it was reduced by 1 e/mole, but returned to that of the parent sample on reoxidation at 450 °C. The pattern of La 2O 3 appeared when the sample was reduced by 3 e/mole, but it again became nearly identical with that of the parent perovskite upon reoxidation. Lines for metallic Co did not appear at 500 °C but became evident when the reduced sample was sintered in He at 800 °C. In the latter case the perovskite lines returned only very slowly as Co 3O 4 reacted with La 2O 3 at this temperature. The catalytic implications of these observations are briefly discussed.