Structure, Chemical Composition, and Catalytic Behaviour of Stoichiometric and Non-Stoichiometric LaMnO3 Toward Deep Oxidation of Ethanol

A series of structurally modified LarMnO3±d (r = 0.80, 1.00, 1.25) perovskites have been investigated for ethanol deep oxidation in the temperature range 100–300°C. The catalysts were characterized by X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), redox titration and CO2 adsorption. All samples are single perovskite phases without segregated phases. For La- and Mn-under-stoichiometric samples, the defects generated to maintain structure electroneutrality are positive holes and anion vacancies whereas for stoichiometric solids, the defects are positive holes and cation vacancies. The surface oxidation state and composition are critically controlled by the phase composition of the bulk. XPS results showed that the decrease in bulk La/Mn ratio induced a decrease in surface La enrichment. A concomitant decrease of basic site concentration was shown by CO2 adsorption. With lower surface La content, excess Mn sites and increasing concentrations of surface Mn4+ were observed. Catalytic properties in the oxidation reaction of ethanol are attributed to the variability of the manganese oxidation state, basic character of the material surface, which is related to the La/Mn atomic ratio, and to the oxygen storage capacity in the crystalline lattice of the catalysts. It was shown that La0.8MnO3–d presents the best catalytic activity, due to low surface basicity (lowest La/Mn ratio) and high redox properties (highest Mn4+/Mn3+ ratio).