Effect of thermal treatment on the EPR spectrum and on catalytic properties of pure Co3O4

Thermal treatment of Co3O4 decreases the number of cationic vacancies by reducing the oxygen excess in the ''as prepared'' sample. Correspondingly, a narrower Lorentzian-shaped EPR line is detectable after higher temperature treatment, because of a more efficient Co-O-Co super-exchange effect, favored by the Co2+ enrichment of the sample. Furthermore, the linewidth increases linearly with electron paramagnetic resonance (EPR) recording temperature. We attribute this effect to the onset of antisymmetric exchange in layer-like structures. Catalytic activity tests have been carried out at 323, 373, and 423 K on oxidation of CO by air oxygen as a probe reaction. Samples calcined at 536 and 773 K were tested. Both the catalysts proved to be practically inactive at 323 K. However, at 423 K the specific activity of the catalyst calcined at higher temperature was approximately twice as high as that shown by the sample calcined at lower temperature. Also, this effect has been attributed to the enrichment in Co2+ ions, which would act as catalytic centers when located at the solid surface.