Efficient Synthesis of Nitric Acid Under Air Atmosphere with Lattice-Confined Ru Clusters
Authors: LI Xin , Leung Michael K.H. 
Published: February 2024

Nitrate (NO3
-) is an important industrial chemical used as fertilizers in agriculture, as well as oxidizing agents in explosives. Today, NO3
- is manufactured predominantly via an Ostwald process by oxidizing ammonia (NH3), and the ammonia used here comes primarily from the Haber–Bosch (HB) process.However, this process operates at high temperatures (300–500 °C) and pressures (200–300 bar), and requires a coupled steam reforming plant for hydrogen (H2) production. As a result, approximately 1.9 metric tons of CO2 is formed per metric ton of NH3 produced, contributing significantly to climate change. In addition, due to the complexity of the Ostwald and Haber-Bosch processes, it is only economical at large scales, leading to centralized production, which situation is poorly matched with the distributed nature of HNO3 utilization. Therefore, it is highly desirable to bypass the ammonia route and develop a direct and sustainable approach for HNO3 synthesis.

Electrochemical synthesis of chemicals has been regarded as an attractive alternative to traditional thermochemical methods. In electrochemical reactions, electric potential can replace high temperature and pressure as the thermodynamic driving force.Therefore, direct electrochemical oxidation of molecular nitrogen appears to be a very potential approach for HNO3 synthesis,which could be sustainable, modular and easily integrated with intermittent renewable electricity. However, due to the lack of natural or artificial electrocatalysts as a reference, the nitrogen oxidation reaction (NOR) remains largely unexplored despite its enormous practical value as a replacement for the fossil fuel-driven two-step HNO3 preparation approach.Until 2019, Zhang et al. reported the first experimental NOR electrocatalyst composed of well-dispersed Pd nanoparticles on MXene nanosheets that can effectively convert N2 into nitrate, which definitely proved the feasibility of using electrochemistry to motivate the endothermic NOR at ambient conditions. Unfortunately, to date, there are still only a few works that reported the electrosynthesis of nitrate from nitrogen, and the proposed electrocatalysts can only achieve limited NOR activity.

An in-depth exploration of the NOR process, which consists of two main steps: the first step is a rate-limiting step that converts inert N2 into the active NO* intermediate; the second step is a nonelectrochemical step where NO* reacts with H2O and the generated O* from electrocatalytic water splitting to form nitrate. Therefore, the main factors restricting the development of the electrochemical nitrate synthesis technology can be inferred as follows: (1) the strong N≡N triple bond in the dinitrogen molecule (bond energy: 940.95 kJ mol−1) is highly challenging to oxidate nitrogen to nitrate under mild conditions; (2) the potential window between the NOR and oxidation evolution reaction (OER) is quite narrow, and thus the latter as a competitive reaction can severely restrict the selectively of the nitrogen oxidation synthesis of nitrate; (3) the extremely low solubility and diffusion coefficient of N2 in aqueous electrolytes can significantly inhibit the nitrogen available for the reaction, resulting in unfavorable NOR performance. Therefore, to overcome the above obstacles, extensive studies should focus on designing efficient nitrogen oxidation catalysts and improving electrochemical reaction systems to promote NOR and suppress OER.

In the present work, we firstly discovered that synthesizing Ru particles grow along the TiO2 lattice (Ru-TiO2) as the working electrode and can achieve excellent NOR performance by using air as a gas source. Textural analysis sufficiently demonstrates that the matching lattice constants of metal Ru and TiO2 allow the Ru to be confined in the TiO2 lattice, leading to the formation of Ru-Ti bonds and the transfer of electrons from TiO2 to Ru metal. Our results reveal that the electron redistribution of Ru-TiO2 can significantly alter the electronic structure of Ru, leading to the formation of highly occupied Ru 4d bands, which can effectively change the adsorption of surface species and improve the NOR activity. In addition, the Ru confined in TiO2 exhibits particularly high stability towards NOR up to potentials above 1.7 V (versus RHE). As a result, Ru-TiO2 could achieve a record-high NOR performance with a nitric acid yield rate of 18.71 ug h-1 cm-2 (150.2 μmol h-1 g-1) and Faraday efficiency of 25.17% in the air atmosphere. This discovery suggests that the surface oxophilicity and electronic structure of metal particles can be effectively modified by lattice confinement in semiconductors and offers an alternative concept for designing catalysts with unique properties for use in catalysis and beyond.

Figure 1