Synthesis and catalytic properties of perovskite-related phases in the La–Sr–Co–Cu–Ru–O system

Abstract

A series of oxides of the composition La_0.8Sr_0.2Co_1–2yCu_yRu_yO₃ with the perovskite structure were synthesized and characterized by their X-ray diffraction (XRD) patterns and by element analysis in a transmission electron microscope (TEM) equipped with an energy-dispersive spectrometer (EDS). The XRD and TEM–EDS studies confirmed that almost monophasic samples with the perovskite structure having rhombohedral (hexagonal) symmetry were obtained for 0 ⩽y⩽ 0.20, although the simultaneous formation of a by product with the K₂NiF₄-type structure was observed for high y values. The lattice parameters and rhombohedral distortion increased monotonically with increasing y value due to the fact that the average ionic radius of the substituting Cu/Ru pair is larger than that of the host Co ion. Temperature-programmed desorption (TPD) studies showed that the substitution of the Cu/Ru pair for Co caused a decrease in the amount of oxygen desorbed from the bulk of the oxide, and the catalytic activity for CO oxidation and CO–NO reactions decreased. The Cu/Ru substitution did, however, selectivity increase the N₂ formation and decreased that of N₂O in the CO–NO reactions. Synthesis of K₂NiF₄-type oxides with the overall compositions (La_0.8Sr_0.2)₂Co_0.50Cu_0.50–zRuzO₄ with z= 0.05 and 0.10 was also attempted. In both cases, tetragonal K₂NiF₄-type oxides having a composition close to (La_0.8Sr_0.2)₂(Co,Cu)_0.97Ru_0.03O₄ were obtained, implying that the solubility of Ru into the given K₂NiF₄-type framework is very low.