Role of bulk and grain boundary oxygen mobility in the catalytic oxidation activity of LaCo1–xFexO3

In this study, oxygen mobility in LaCoO3 was studied by means of oxygen isotopic exchange. Seven LaCoO3 were synthesized by different methods and characterized. The solids present large differences in specific surface area, crystallite size, and redox properties (as measured by oxygen desorption and reduction by H2). The results obtained for the exchange reaction showed that the activity was strongly influenced by the crystal size. However, it was observed for all of the solids, that all of the bulk oxygen atoms are available for exchange. A mechanism of exchange is proposed that supposes the adsorption of an O2 molecule to a reduced cobalt site. Then the adsorbed O2 exchanges with the surface, and the exchanged atom can diffuse into the grain boundaries and into the bulk of the solid. It is supposed, from these results, that grain boundary diffusion proceeds quickly in comparison with bulk diffusion. These results were compared with those obtained for the CH4 oxidation reaction. The mechanism of oxidation was discussed on the basis of a comparison of the results obtained for these two reactions.