AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Abstract

Catalytic methane combustion and CO oxidation were investigated over AFeO3 (A=La,Nd,Sm) and LaFe1−xMgxO3 (x=0.1,0.2,0.3,0.4,0.5) perovskites prepared by citrate method and calcined at 1073K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe4+ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7m2g−1 range. XRD analysis showed that LaFeO3, NdFeO3, SmFeO3 and LaFe1−xMgxO3 (x·0.3) are single phase perovskite-type oxides. Traces of La2O3, in addition to the perovskite phase, were detected in the LaFe1−xMgxO3 catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO3 of a very small fraction of Fe4+ which reduces to Fe3+. The fraction of Fe4+ in the LaFe1−xMgxO3 samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO2 below 973 and 773K, respectively. For the AFeO3 materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe1−xMgxO3 catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO3 materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe1−xMgxO3 catalysts decreases at high magnesium content.