LaMO<inf>3</inf> (M = Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation

Abstract

Two new series of perovskite-type oxides LaMO3 (M = Mg, Ti, Fe) with different ratio Mg/Fe (MF) and Ti/Fe (TF) in the B cation site were prepared by annealing the precursor, obtained by the mechanochemical activation (MCA) of constituent metal oxides, at 1000 °C in air. In addition, two closely related perovskites LaFeO3 (LF) and LaTi0.5Mg0.5O3 (TM (50:50)) were synthesized in the similar way. Using MCA method, perovskites were obtained in rather short time and at room temperature. The samples were characterized by X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), temperature programmed desorption of oxygen (TPD), Mössbauer spectroscopy, BET surface area measurements and tested in methane deep oxidation. According to XRPD analysis all synthesized samples are almost single perovskite phase, with trace amounts of La2O3 phase. Data of Mössbauer spectroscopy identify Fe3+ in octahedral coordination. The activity of perovskite in methane deep oxidation increases in the order TM (50:50) < MF series < TF series. Higher activity of TF samples in respect to MF with similar Fe content can be related to the structural characteristic, mainly to the presence of predominantly most labile oxygen species evidenced by TPD at lowest temperature of oxygen evaluation. In used experimental conditions, the Fe substituted perovskite are thermal stable up to the temperature of 850 °C. The stability of Fe active sites is probably the most important parameter responsible for thermal stability of perovskite, but the atomic surface composition also should be taken into account. © 2007 Elsevier B.V. All rights reserved.