EPR investigations on La(1-x)Ce(x)CoO(3) (x=0, 0.05, 0.1, 0.2)perovskite-like catalysts for methane flameless oxidation

We have analyzed at temperature ranging between 95 and 295 K a series of low-surface-area (ca. 4.5 m2/g) La(1-x)Ce(x)CoO(3) perovskitic samples (x = 0, 0.05, 0.1, 0.2), not used (series LN) and used (series LU) as catalysts for the flameless combustion of methane. A strongly asymmetric EPR signal was observed in both cases at room temperature, corresponding to anisotropic A and g tensors. When progressively lowering temperature these features became less asymmetric, finally ending in a narrowing Lorentzian shape. With LN samples the transition from asymmetric to Lorentzian-shaped EPR pattern occurred at higher temperature than with LU ones. Furthermore, in the latter also the magnetic field value CF at the centre of the EPR line was temperature dependent, moving toward lower values at lower temperatures. The asymmetric EPR pattern has been attributed to the formation of Co3+/O2- pairs, due to incomplete oxygen activation, or to incomplete reoxidation of the sample after catalytic use. Incomplete reoxidation, together with the substitution of Ce4+ for La3+, would also be at the origin of the spin bags formation causing the temperature-dependent shift of the EPR line observed with LU samples. At last, a comparison with some high-surface-area (ca. 22 m2/g) samples (series HN and HU, not used and used as catalyst, respectively) has been also made. A relation between sample properties and catalytic performance is here proposed.