Defect Chemistry and Catalytic Activity of Nanosized Co₃O₄

Nanometric cobalt oxide Co 3 O 4 powders made of 4 nm isotropic particles, directly precipitated from Co 2+ aqueous solutions under alkaline and oxidizing conditions, are found to exhibit abnormal X-ray diffraction intensities, mainly in (111) and (220) reflections, because of a significant amount of displaced Co atoms from ideal 8a and 16d positions to interstitial 48f and 16c sites. Upon heating, the Co/O stoichiometry is maintained and the delocalized atoms progressively migrate to stable positions through empty neighboring sites. Despite the presence of such defects, the order of reaction ( n ≈ 1) and the activation energy ( E a ≈ 60 ± 5 kJ/mol) versus the decomposition of diluted solutions of hydrogen peroxide solutions is found to be similar to bulk nonfaulted Co 3 O 4 materials, but the intrinsic rate constants k 20 °C are found to be proportionally enhanced by both the defect and microstrains levels. Last, a careful selection of monolithic samples, so as to keep away the catalytic influence of the intercrystallites reactive grain-boundaries, has enabled us to precisely study and isolate the role of the structural characteristics on the catalytic activity of Co 3 O 4 toward H 2 O 2 decomposition. The catalytic activity of such divided oxides toward this reaction is also put forward as a fast selection tool for catalysts in Li-oxygen secondary cells.