Perovskite-type oxides - II. Redox properties of LaMn1-xCuxO3 and LaCo1-xCuxO3 and methane catalytic combustion

Redox properties of high-surface-area LaM1-xCuxO3 (M = Mn or Co) perovskite s prepared by the citrate method were studied by H-2 TPR and O-2 TPD techni ques. We have found that the reduction of Mn4+ occurs in all La-Mn-Cu perov skites at temperatures lower than that of Co3+ in La-Co-Cu samples. The pre sence of copper affects the reduction of both Mn4+ and Co3+, increasing the temperature of reduction. Furthermore, Mn-based perovskites with a low Cu substitution (x<0.4) release a great amount of O-2 at high temperature. Cat alytic activity toward methane combustion was investigated under diluted co nditions (0.4% CH4 and 10% O-2, N-2 as balance) in the temperature range 67 3-1073 It using a fixed bed reactor with a space velocity of 40,000 N cm(3) h(-1) g(-1). All samples catalyze methane oxidation in the temperature ran ge 673-773 K, giving complete methane conversion below 1073 K and producing 100% CO2. No deactivation phenomena were observed. Substitution with coppe r decreases catalytic activity. The higher activity of Mn-based perovskites compared with that of the corresponding Co samples with the same compositi on was attributed to the stronger oxidative properties of LaM1-xCuxO3. An a pparent activation energy of 23 kcal mol(-1) was evaluated on the basis of a first-order kinetic equation for all samples. (C) 1999 Academic Press.