Defect LaCuO3-delta (delta=0.05-0.45) perovskites - Bulk and surface structures and their relevance in CO oxidation

Polycrystalline LaCuO3-delta (delta=0.45, 0.35, 0.17 and 0.05) powders have been prepared by a citrate technique followed by annealing under 200 bar O -2 pressure at temperatures 1073-1273 K. The crystal structure of three sel ected samples was refined by the Rietveld method from room temperature neut ron powder diffraction data. Thermogravimetric analysis of the samples in a H-2/N-2 Row allowed the determination of the oxygen nonstoichiometry. Thes e TG profiles showed two clear reduction processes: the first one correspon ds to a partial reduction to give intermediate perovskites and in the secon d step the sample is completely reduced to Cu-0 and La2O3. However, surface analysis of a representative sample (LaCuO2.55) revealed that at intermedi ate reduction temperatures, a fraction of copper is already reduced to meta llic state while the other fraction still contains Cu-III 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-delta phases as delta increases appears to be responsible for th e enhancement of the catalytic properties of this perovskite based system. (C) 2000 Elsevier Science B.V. All rights reserved.