Methane combustion on Mg-doped LaMnO_3 perovskite catalysts

摘要

LaMn_(1-x)Mg_xO_3 perovskite catalysts (x=0-0.5) were synthesised by the so-called "citrates method", charaterised (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 (LaMnO_3 and LaMn_(o.8)Mg_(0.2)O_3). The major results obtained are:(i) Mg substitution in the basic LaMnO_3 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-x)Mg_xO_3 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 deg 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 LaMnO_3, the catalytic combustion over LaMn_(0.8)Mg_(0.2)O_3 seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.