FMR of La(1.8)M(0.2)CuO(4) (M=Pr, Sm, Tb) catalysts for methane flameless combustion

FMR of La1.8 M0.2CuO4 (M = Pr, Sm, Tb) catalysts for methane flameless combustion C.Oliva(a), S.Cappelli(a), A.Kryukov(b), G.L.Chiarello(a), A.V.Vishniakov(b), L.Forni(a) (a)Dip. Chimica Fisica ed Elettrochimica, Università di Milano via C.Golgi, 19 20133 Milano, Italy. (b)D.I.Mendeleev University of Chemical Technology of Russia, Moscow, Russia. A few La1.8M0.2CuO4 samples (M = Pr, Sm or Tb) were prepared by a flame-spray pyrolysis method [1]. Ferromagnetic (FM) systems were observed with the M = Sm biphasic sample when fresh, but not after recalcination, when it became monophasic, as confirmed by XRD. These FM systems could be formed of Magnetic Bound Polarons (MBP) due to the thermally excited J=1 Sm2+ ions forming as impurity through the reactions: 10 La1.8Sm0.2CuO4 ® 9 La2O3 + 10 CuO + Sm2O3 ; Sm23+ O32- ® 2 Sm2+O2- + ½ O2. Indeed, the intensity of the FM resonance strongly increased with increasing temperature (Fig.1). The La1.8Sm0.2CuO4 sample showed worst catalysts than La0.9Sm0.1CoO3 in which no FM system was ever observed. Indeed, biphasic La1.8M0.2CuO4 catalysts may possess high specific surface area, but their catalytic activity can be lowered by the formation of MBP. Indeed, MBP cause a grouping of electrons, which become less available for the catalytic reaction. Sample recalcination is accompanied by sample sintering which would cause a decreased catalytic activity. On the contrary, the same activity per unit mass was measured before and after such a sample treatment. This suggests that the sintering negative consequence on the catalytic activity were overwhelmed by the positive effects of the disappearance of MBP, as indicated by an increased activity per unit surface area (upper curve of Fig.2). The discussion is extended to the cases of M = Pr and Tb [2]. [1] GL.Chiarello, I.Rossetti, L.Forni, J.Catal. 236 (2005)251. [2] C.Oliva, S.Cappelli, A.Kryukov, G.L.Chiarello, A.V.Vishniakov, L.Forni, J.Molec.Catal A, 247(2006)248.