Preparation and characterization of the non-stoichiometric La-Mn perovskites

Abstract Six La-Mn oxide samples with La/Mn atomic ratio x = 1.03-0.56 (denoted as sample LaxMn) were prepared by the citrate method with calcination at 700 °C for 5 h, and characterized by X-ray diffraction (XRD), N₂ adsorption-desorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). It is confirmed that the four samples with La/Mn atomic ratio at 1.03-0.72 are all single phase perovskites by XRD patterns. Lattice parameters of the perovskites are varying with the La/Mn atomic ratio. As the La/Mn atomic ratio further lowers to 0.63 and 0.56, Mn₃O₄ phase is formed besides the main phase of perovskite. Lattice vacancy at the A-sites of the perovskites is present for all the six samples, and there are an appreciable number of Mn⁴⁺ ions in the perovskite crystal according to the refinement results of the Rietveld method. XPS analyses indicate that Mn ions are enriched on the surfaces of all the samples. In addition, catalytic activity for methane oxidation is in an order of sample La_0.89Mn > La_1.03Mn > La_0.81Mn > La_0.72Mn > La_0.63Mn > La_0.56Mn.