USTC makes hydroxylamine from air and water

A research team led by Prof. ZENG Jie and Prof GENG Zhigang from University of Science and Technology of China (USTC) proposed a new model for sustainable hydroxylamine (NH₂OH) synthesis, plasma-electrochemical cascade pathway (PECP). They achieved the green and sustainable synthesis of NH₂OH from ambient air and water at mild conditions. The study was published in Nature Sustainability.

NH₂OH, as an important chemical intermediate, is widely used in the fine chemical fields of medicine, pesticides, textiles etc. The traditional production methods of NH₂OH mainly include the Raschig process, nitric oxide reduction method, and nitric acid reduction method. However, the Raschig process causes a large amount of nitrogen loss and environmental pollution; while the other two methods give out great carbon emissions. It is, therefore, urgent to develop a new green, low-carbon, and sustainable synthesis process for NH₂OH.

The electrosynthesis process driven by green electricity and using water as a proton source is expected to overcome the drawbacks of traditional NH₂OH production processes. However, due to the thermodynamic stability of nitrogen molecules, it is difficult to achieve efficient activation of nitrogen molecules in the direct electrocatalytic process of nitrogen.

Researchers have achieved the green and sustainable synthesis of NH₂OH using only air and water as raw materials by developing a new process that couples plasma nitrogen fixation to nitric acid production with the electrocatalytic reduction of nitric acid to NH₂OH. In addition, researchers have designed a plasma discharge device with multiple parallel tips so as to enlarge the overlapping zone for the efficient activation of nitrogen gas.

Firstly, researchers introduced air into the plasma parallel arc discharge device and used a water solution containing methyl orange as the exhaust gas absorbent to convert the solution from neutral to acidic. By optimizing the air flow rate, researchers obtained a nitric acid solution with a maximum concentration of 20.3 millimoles per liter. With each reaction cycle lasting 30 minutes, the plasma discharge device maintained excellent stability over 20 cycles. The obtained nitric acid solution could be directly used for the electrocatalytic synthesis of NH₂OH after dilution and addition of electrolytes.

In addition, researchers also prepared a bismuth metal thin film catalyst via magnetron sputtering and applied it to the electrocatalytic reduction of nitric acid to produce NH₂OH.

The accumulation of NH₂OH in the electrolyte during long-term electrolysis of a 100 mmol L nitric acid solution by a bismuth thin film catalyst was investigated. After continuous electrolysis for 5 h, the highest concentration of NH₂OH reached 77.7 mmol L. Finally, 1.887 g high-purity NH₂OH sulfate product was prepared.

This study proposes a viable way to efficiently synthesize hydroxylamine from simpler feedstock at milder conditions, which contributes to the sustainability transformation of the chemical industry.