AMnO(3) (A=La, Nd, Sm) and Sm1-xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO(3) (A = La, Nd, Sm) and Sm1-xSrxMnO3 (x = 0.1 0.3, 0.5) perovskites prepared by citrat e method. The catalysts were characterized by chemical analysis, XRD and TP R techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623-1023 K, space velocity = 40000 N cm(3) g(-1) h(-1), CH4 concentration = 0.4% v/v, O-2 concentration = 10% v/v. Specific surface areas of perovskites were in the range 13-20 m(2) g(-1). X RD analysis showed that LaMnO3, NdMnO3, SmMnO3 and Sm1-xSrxMnO3 (x = 0.1) a re single phase perovskite type oxides. Traces of Sm2O3 besides the perovsk ite phase were detected in the Sm1-xSrxMnO3 catalysts for x = 0.3, 0.5. Che mical analysis gave evidence of the presence of a significant fraction of M n(TV) in AMnO(3). The fraction of Mn(TV) in the Sm1-xSrxMnO3 samples increa sed with x. TPR measurements on AMnO(3) showed that the perovskites were re duced in two steps at low and high temperature, related to Mn(TV) --> Mn(II I) and Mn(III) --> Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO3 > NdMnO3 > SmMnO3. In Sm1-xSrxMnO3 the Sr substitu tion for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave m ethane complete conversion with 100% selectivity to CO2 below 1023 K, The a ctivation energies of the AMnO(3) perovskites varied in the same order as t he onset temperatures in TPR experiments suggesting that the catalytic acti vity is affected by the reducibility of manganese. Sr substitution for Sm i n SmMnO3 perovskites resulted in a reduction of activity with respect to th e unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism. (C) 2000 Elsevier Science B.V. All rights reserved.