Defect LaCuO3−δ (δ=0.05−0.45) perovskites: Bulk and surface structures and their relevance in CO oxidation

Polycrystalline LaCuO3−δ (δ=0.45, 0.35, 0.17 and 0.05) powders have been prepared by a citrate technique followed by annealing under 200 bar O2 pressure at temperatures 1073–1273 K. The crystal structure of three selected samples was refined by the Rietveld method from room temperature neutron powder diffraction data. Thermogravimetric analysis of the samples in a H2/N2 flow allowed the determination of the oxygen nonstoichiometry. These TG profiles showed two clear reduction processes: the first one corresponds to a partial reduction to give intermediate perovskites and in the second step the sample is completely reduced to Cu0 and La2O3. However, surface analysis of a representative sample (LaCuO2.55) revealed that at intermediate reduction temperatures, a fraction of copper is already reduced to metallic state while the other fraction still contains CuIII stabilized in the lattice. These samples were tested in the catalytic oxidation of CO under flow condition, and the intrinsic activity was found to depend strongly on the oxygen nonstoichiometry. The increasing structural instability observed in LaCuO3−δ phases as δ increases appears to be responsible for the enhancement of the catalytic properties of this perovskite based system.