Catalytic Ozonation of Air toward Direct Nitric Acid Production Using Hierarchical Co₃O₄ with a Tunable...

The nitrogen oxidation reaction (NOR) to form nitric acid by applying natural air and H₂O under ambient conditions is a sustainable approach to achieving efficient and selective N₂ fixation for industrial applications. In this study, four kinds of Co₃O₄ catalysts with a controllable microstructure were prepared to catalyze the direct NOR of N₂ in the air. At the same time, the reaction mechanism of the conversion of N₂ to nitric acid under catalytic ozonation was explored through experimental research and density functional theory (DFT) calculation. The results showed that the prepared porous nanosheets self-assembled into microflower-structured samples. The HCOF showed extraordinary catalytic performance for direct NOR to produce a high concentration of nitric acid. The maximum rate of nitric acid formation could be as high as 6.67 mmol/(h·g_cat), which was higher than those of most reported photocatalytic or electrocatalytic N₂ fixation processes for direct NOR to produce NO₃^-. Furthermore, the ¹⁵N isotopic-labeling experiment confirmed that the produced NO₃^- originated from N₂ in the air by the direct NOR process. In the direct NOR mechanism, inert N₂ molecules were captured at the Co³⁺ active sites by the acceptance-donation electron conduction mode, and the oxygen vacancies boosted the chemical adsorption of N₂ molecules and greatly reduced the activation energy barrier of N₂ molecules. The active free radicals ^•OH and ^•O₂^- generated by the decomposition of O₃ molecules oxidized N₂ to the intermediate *NO, which was the rate-determining step, and it was then absorbed by water to form nitric acid. The air catalytic ozonation method in this study was proposed as a facile pathway for efficient nitrogen fixation. This research provides a new method for environmental protection and efficient production of nitric acid at distributed sources.