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

Abstract

The nitrogen oxidation reaction (NOR) to form nitric acid by applying natural air and H 2 O under ambient conditions is a sustainable approach to achieving efficient and selective N 2 fixation for industrial applications. In this study, four kinds of Co 3 O 4 catalysts with a controllable microstructure were prepared to catalyze the direct NOR of N 2 in the air. At the same time, the reaction mechanism of the conversion of N 2 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 N 2 fixation processes for direct NOR to produce NO 3 − . Furthermore, the 15 N isotopic-labeling experiment confirmed that the produced NO 3 − originated from N 2 in the air by the direct NOR process. In the direct NOR mechanism, inert N 2 molecules were captured at the Co 3+ active sites by the acceptance−donation electron conduction mode, and the oxygen vacancies boosted the chemical adsorption of N 2 molecules and greatly reduced the activation energy barrier of N 2 molecules. The active free radicals • OH and • O 2 − generated by the decomposition of O 3 molecules oxidized N 2 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.