A plasma-electrochemical cascade pathway converts ambient air and water into hydroxylamine



Zeng Jie and Prof. Geng Zhigang from the University of Science and Technology of China (USTC) developed a new method to synthesize hydroxylamine under mild conditions with high selectivityused plasma di. They used plasma disscharge to converted air and water into high-purity nitric acid, and then reduced the nitric acid using an electrocatalytic process to hydroxyl amine.

Hydroxylamine is usually made from ammonia, with hydrogen or sulfur dioxide as the reducing agent, and its production process will not only consume a large amount of fossil resources, but also emit a large amount of carbon dioxide, causing environmental pollution. In addition, ammonia, the raw material for making hydroxylamine from nitrogen, also requires a lot of energy. This is mainly because the current industrial ammonia synthesis mostly uses the Haber process, which needs to be carried out in a high-temperature and high-pressure environment, which will lead to the production of 300 million tons of carbon emissions per year, consuming about 2% of the world’s energy.

High-pressure generated by lightning oxidises air nitrogen  into nitrogen oxides. Using plasma discharge technology, renewable electricity can be used as the driving force at room temperature and pressure to convert nitrogen into nitrogen oxides, mainly nitrogen dioxide. The researchers found that alkaline liquids absorb nitrogen dioxide efficiently, but the target product hydroxylamine is not stable in alkaline solutions and decomposes easily. Moreover, the metal salts of the alkaline solution would also adversely affect the separation and purification of hydroxylamine. Therefore, the researchers switched to using pure water as an absorbent for nitrogen dioxide and designed a multistage gas recirculation absorber tower device as a way to obtain high-purity nitric acid solutions more efficiently. They realized the continuous production of nitric acid solution with a concentration of up to 7.5 grams per liter using only air and water as raw materials through the structural design of the plasma discharge unit and the gas absorption unit. Guided by theoretical calculations, the researchers developed a highly selective hydroxylamine-making catalyst, a bismuth-based catalyst, capable of inhibiting competitive byproducts. The yield of hydroxylamine production by electrocatalytic nitric acid reduction over the bismuth-based catalyst reached 200 grams per square meter per hour at room temperature and pressure, and the selectivity of hydroxylamine was as high as 95% in all nitrides.

To further increase the cumulative concentration of hydroxylamine in solution, the researchers subjected the nitric acid solution to continuous electrolysis for 5 hours, ultimately obtaining a hydroxylamine solution with a content of up to 2.5 g per liter. This verified that prolonged electrolysis time can increase the cumulative concentration of hydroxylamine, and the accumulated hydroxylamine will not be reduced again to produce ammonia. Zeng Jie said that this highly concentrated hydroxylamine solution can be obtained as a solid, high-purity hydroxylamine sulfate after simple decontamination and evaporation and crystallization.

A plasma-electrochemical cascade pathway converts ambient air and water into hydroxylamine
Scroll to top