Coordination-Controlled Catalytic Activity of Cobalt Oxides for Ozone Decomposition

Abstract

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 Co²⁺ in tetrahedral coordination (Co_Td²⁺) and CoAl spinel with dominant Co_Td²⁺, cubic rock salt CoO-R with Co²⁺ in octahedral coordination (Co_Oh²⁺), MgCo spinel with dominant Co³⁺ in octahedral coordination (Co_Oh³⁺), and Co₃O₄ with mixed Co_Td²⁺ and Co_Oh³⁺. 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_Oh³⁺ ∼ Co_Oh²⁺ ≫ Co_Td²⁺, and Co_Oh³⁺ and Co_Oh²⁺ show a lower apparent activation energy of ∼42-44 kJ/mol than Co_Td²⁺ (∼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.