Methane combustion on Mg-doped LaMnO3 perovskite catalysts

LaMn_1−xMg_xO₃ perovskite catalysts (x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO₃ and LaMn_0.8Mg_0.2O₃). The major results obtained are: (i) Mg substitution in the basic LaMnO₃ perovskite has a positive effect on the catalytic activity only at low x values (x≤0.2); (ii) as opposed to the results of previous studies on the LaCr_1−xMg_xO₃ system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO₃, the catalytic combustion over LaMn_0.8Mg_0.2O₃ seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.