Combustion synthesis of LaMn1−xAlxO3+δ (0 ≤ x ≤ 1): tuning catalytic properties for methane deep oxidation

Abstract

The objective of this work was to study the effect of aluminum incorporation into the B sublattice of lanthanum manganite on the thermal stability and catalytic activity in CH₄ complete oxidation in relation to the physico-chemical properties. Single-step solution combustion synthesis under stoichiometric conditions of glycine/nitrate was used to prepare LaMn_1−xAl_xO_3+δ oxides (0 ≤ x ≤ 1). The obtained powders were characterized by XRD, SEM, TPD-O₂ and XPS techniques and tested as catalysts in the deep oxidation of methane. XRD analysis showed a crystallized single perovskite phase over a large domain of aluminum content (x ≤ 95%). The specific surface area decreases with the incorporation of aluminum into the perovskite lattice and shows optimum value for x = 0.1. All the Mn-containing oxides exhibit oxidative non-stoichiometry and a mixture of Mn⁴⁺–Mn³⁺. Compared to the bulk, a decrease in surface element enrichment and an increase in the superficial Mn⁴⁺/Mn³⁺ ratio were observed with a progressive introduction of aluminum into the perovskite lattice. Catalytic activity of Al-doped oxides was governed by the superficial Mn concentration and the amount of desorbed oxygen involved in Mn⁴⁺ ↔ Mn³⁺ redox reactions. These parameters were found to be optimal for the best catalyst obtained with x = 0.1 which also proved to be the most stable catalyst.