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

Catalytic methane combustion and CO oxidation were investigated over AFeO(3) (A = La, Nd, Sm) and LaFe1-xMgxO3 (x = 0.1,0.2, 0.3, 0.3, 0.5) perovskites prepared by citrate method and calcined at 1073 K. 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 2.3-9.7 m(2) g(-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 AFeO(3) 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 I = 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 773 K, respectively. For the AFeO(3) 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 ix = 0.3). Concerning the CO oxidation, the order of activity fur the AFeO(3) materials is Nd > La > Sm, while the activity (per unit SA) of the LaFe1-xMgxO3 catalysts decreases at high magnesium content. (C) 2001 Elsevier Science B.V. All rights reserved.