Coordination-Controlled Catalytic Activity of Cobalt Oxides for Ozone Decomposition.

Nowadays, it is still elusive and challenging to discover the active sites of cobalt (Co) cations in different coordination structures, though Co-based oxides show their great potency in catalytic ozone elimination for air cleaning. Herein, different Co-based oxides are controllably synthesized including hexagonal wurtzite CoO-W with Co 2+ in tetrahedral coordination (Co Td 2+ ) and CoAl spinel with dominant Co Td 2+ , cubic rock salt CoO-R with Co 2+ in octahedral coordination (Co Oh 2+ ), MgCo spinel with dominant Co 3+ in octahedral coordination (Co Oh 3+ ), and Co 3 O 4 with mixed Co Td 2+ and Co Oh 3+ . The valences are proved by X-ray photoelectron spectroscopy, and the coordinations are verified by X-ray absorption fine structure analysis. The ozone decomposition performances are Co Oh 3+ ∼ Co Oh 2+ ≫ Co Td 2+ , and Co Oh 3+ and Co Oh 2+ show a lower apparent activation energy of ∼42-44 kJ/mol than Co Td 2+ (∼55 kJ/mol). In specific, MgCo shows the highest decomposition efficiency of 95% toward 100 ppm ozone at a high space velocity of 1,200,000 mL/gh, which still retains at 80% after a long-term running of 36 h at room temperature. The high activity is explained by the d-orbital splitting in the octahedral coordination, favoring the electron transfer in ozone decomposition reactions, which is also verified by the simulation. These results show the promising prospect of the coordination tuning of Co-based oxides for highly active ozone decomposition catalysts.