La/Mn molar ratio tuning the activity of La–Mn perovskites for CO and propane oxidation

Abstract

A series of non-stoichiometric LaxMnO3 with La/Mn molar ratio x in the range of 0.80–1.2 have been prepared by a glycine combustion method and calcinated at 700 °C. The Mn-rich (x < 1) and La-rich (x > 1) catalysts presented higher and lower activity for CO and propane oxidation than the stoichiometric LaMnO3, respectively. The LaxMnO3 presented pure perovskite phase without La- or Mn-related phases detected even calcinated at 1000 °C. Notably, the 50% conversion temperature for CO and propane oxidation on the best La0·84MnO3 were around 70 and 35 °C lower than those on the stoichiometric LaMnO3, whereas which on the worst La1·15MnO3 were around 60 and 50 °C higher. The La0·84MnO3 possessing higher activity for CO and propane oxidation was predominately attributed to more Mn4+ cations, thus achieving abundant surface Mn4+–O bridge site as redox sites and surface Mn4+ sites as Lewis acidic sites, while the La1·15MnO3 owning lower activity for the reactions was primarily ascribed to less Mn4+ cations, and the lower reducibility of Mn4+ cations and surface adsorbed oxygens.