Forms of oxygen in La1-xCaxMnO3+δ (x=0-1) perovskites and their reactivities in oxidation reactions

The effects of substitution in the cationic sublattice and of the synthesis procedure on the reactivity of different forms of oxygen in La1-xCax MnO3+delta perovskites synthesized by mechanochemical and ceramic processing was studied by temperature-programmed reduction (TPR) with hydrogen. As the calcium content of the perovskite is raised, the maxima of the TPR peaks shift to lower temperatures and the extent of reduction of the perovskite increase, implying an increase in the reactivity of the system. Conversely, raising the calcination temperature or extending the calcination time shifts the maxima of the peaks to higher temperatures and diminishes the extent of reduction of the sample. TPR data for the intermediate-composition samples can be explained in terms of the dependence of microstructure on the synthesis procedure (near-surface calcium segregation in the mechanochemically synthesized samples and the microheterogeneous structure of the ceramic samples). The reduction process Mn4+ -> Mn2+ takes place in the low- and medium-temperature regions. According to the literature, the bulk reduction process Mn3+ -> Mn2+ occurs at high temperatures. The activity of the system in CO oxidation is correlated with the amount of the most reactive surface oxygen, which is eliminated in hydrogen TPR runs below 250-300 degrees C.