9.4 and 240 GHz EPR Spectra of Perovskite Catalysts for the Catalytic Flameless Combustion of Methane

La1-xMxCoO3+ (M=Ce, Eu; x=0, 0.05, 0.1, 0.2) samples, prepared by a new flame-hydrolysis (FH) procedure [1], showed nanostructured, having particle size of 20-80 nm (instead of 400-1000 nm shown by their analogues with identical chemical composition but prepared through the usual calcination-milling (CM) and sol-gel citrate (SGC) procedures). These samples are thermally very stable and characterised by highly crystalline perovskite-like structure and high surface area (ca. 20 m2g-1). Their activity as catalysts for the flameless combustion of methane was by ca. one order of magnitude higher than that of their analogues. Adsorption of oxygen was accompanied by formation of paramagnetic species. Immediately after their preparation, all the samples showed an EPR pattern composed of more contributions. That at g  2.35 was attributed to Co3+/O2- pairs, formed by the O2 adsorption on Co2+. A few days later the intensity of this spectral region decreased significantly with undoped LaCoO3+ sample, at difference with the Ce- and Eu-doped ones, indicating that Co3+/O2- pairs are more stable in the doped catalysts. This has been also confirmed by analysing the last samples by 240 GHz EPR spectroscopy after nine months ageing. References; 1) R.Leanza, I.Rossetti, L.Fabbrini, C.Oliva, L.Forni, Appl.Catal.B:Environmental, 2000, 709, in press.