Direct electroconversion of air to nitric acid realized under mild conditions

A research group led by Prof. Deng Dehui from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has realized direct conversion of air to (nitric acid) HNO₃ under ambient conditions through a hydroxyl radical (·OH)-mediated hetero-homogeneous electro-chemical route.

In the route, the N₂ was oxidized to HNO₃ by employing heterogeneous electroreduction of O₂ to H₂O₂ on a graphite rod catalyst combined with homogeneous Fe²⁺-induced dissociation of the in situ generated H₂O₂ to OH· as reactive oxidation species.

The study was published in Nature Synthesis on Sept. 25.

HNO₃, as an important chemical building block, is produced industrially from the oxidation of ammonia via the Ostwald process, which requires high temperatures above 800°C. In this process, the feed ammonia gas is industrially synthesized via the energy-intensive Haber-Bosch method typically operating at high temperatures (400 ~ 500°C) and pressures (20 ~ 50 MPa).

Direct conversion of dinitrogen (N₂) and oxygen (O₂) in the air to HNO₃ under mild conditions is a promising and sustainable route for the production of HNO₃. However, the thermodynamic and kinetic restrictions of this reaction, as well as the high stability of N₂ and the low activity of O₂, make it challenging to achieve co-activation and efficient conversion of the two molecules in the air toward HNO₃ synthesis.

In this study, the researchers developed a hetero-homogeneous electro-chemical route for the direct conversion of air to HNO₃ at the cathode compartment. At the cathodic potential of 0 V vs. RHE, the faradaic efficiency of nitric acid was as high as 25.37%, and the selectivity was over 99%, which was superior than the previously reported electrocatalytic oxidation of N₂ at the anode with high potentials above 1.23 V vs. RHE.

Multiple in-situ spectroscopic characterizations and theoretical calculations revealed that ·OH produced from Fe²⁺-induced dissociation of in-situ generated H₂O₂ could efficiently activate N₂. This delivers a high HNO₃ productivity of 141.83 μmol·h^-1·g_Fe^-1, which is 225 times that of directly using H₂O₂ as the oxidant.

"This process for HNO₃ synthesis from N₂ and O₂ effectively avoids the traditional high-temperature and high-pressure reaction conditions, and provides a new route for the co-activation and efficient conversion of N₂ and O₂ under mild conditions," said Prof. Deng.