Hydroxylamine is a crucial chemical intermediate widely employed in various sectors such as pharmaceuticals, agrochemicals, textiles, electronics, and fine chemicals.
Recently, Professor Zeng Jie and his team from the University of Science and Technology of China have devised an innovative and sustainable method to synthesize hydroxylamine. They have successfully transformed air and water into high-purity nitric acid through plasma discharge, followed by electrochemical reduction of nitric acid to selectively produce hydroxylamine under mild conditions. This breakthrough was published in Nature Sustainability on April 19.
Professor Xi Zhenfeng, an academician of the Chinese Academy of Sciences and a professor at Peking University, commented, "This work utilizes a plasma-electrochemical cascade approach to efficiently convert air and water in the environment into high-value-added hydroxylamine, providing a new potential pathway for nitrogen source transformation in the chemical industry."
The image shows hydroxylamine sulfate. Image provided by the research team.
Air + water to produce nitric acid.
Zeng Jie explained that industrial production of hydroxylamine typically relies on ammonia as a raw material and hydrogen or sulfur dioxide as a reducing agent. This process not only consumes a large amount of fossil resources but also emits significant carbon dioxide, leading to environmental pollution.
Additionally, obtaining ammonia from nitrogen gas also requires substantial energy due to the prevalent use of the Haber process in industrial ammonia synthesis. This process, conducted under high temperature and pressure, results in annual carbon emissions of 300 million tons and consumes approximately 2% of global energy.
As the saying goes, "thunderstorms benefit crops." Zeng Jie explained that the scientific principle behind this is that localized high-pressure environments generated by lightning can oxidize nitrogen gas in the air into nitrogen oxides. These nitrogen oxides dissolve in rainwater to form nitrates, which can be absorbed by crops as nitrogen fertilizer, ultimately promoting crop growth.
Inspired by this natural phenomenon, researchers have successfully converted air into nitrogen oxides at ambient conditions using plasma discharge technology powered by renewable electricity.
Plasma discharge generates nitric oxide, nitrogen dioxide, and nitric oxide; among these, nitrogen dioxide is the primary raw material for nitric acid production. To improve the efficiency of nitric acid production, researchers developed a parallel plasma arc discharge device.
With nitrogen dioxide obtained, nitric acid can be further synthesized.
Researchers found that alkaline liquids efficiently absorb nitrogen dioxide, but the target product hydroxylamine is unstable in alkaline solutions and prone to decomposition. Moreover, the metal salts in alkaline solutions also adversely affect the separation and purification of hydroxylamine.
Therefore, researchers switched to using pure water as the absorbent for nitrogen dioxide and designed a multi-stage gas circulation absorption tower device to obtain high-purity nitric acid solution more efficiently.
"Through the structural design of the plasma discharge device and gas absorption device, we have achieved 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," said Zeng Jie.
Catalyst development boosts efficient hydroxylamine production
After obtaining nitric acid, researchers began exploring the selective synthesis of hydroxylamine using electrocatalysis.
The conversion from nitric acid to hydroxylamine is a reduction process. However, among the various forms of nitrogen, hydroxylamine is not in the lowest oxidation state; ammonia is. In other words, hydroxylamine is an intermediate product, and ammonia is the final reduced product. This competitive production of ammonia complicates the reduction of nitric acid to hydroxylamine.
Meanwhile, in electrocatalytic reactions in aqueous solutions, both nitric acid and water can be reduced. Water electrolysis produces hydrogen gas, which competes as a byproduct in hydroxylamine production from nitric acid.
To suppress these competitive byproducts and selectively produce hydroxylamine, researchers, guided by theoretical calculations, developed a highly selective hydroxylamine synthesis catalyst that can simultaneously suppress ammonia and hydrogen production: a bismuth-based catalyst. Under ambient conditions, the bismuth-based catalyst achieved a hydroxylamine production rate of 200 grams per square meter per hour with a high selectivity of 95% among all nitrogen species.
"In actual production, the cost of product separation accounts for a high proportion of the total production cost. If only low-concentration hydroxylamine, such as milligrams or even micrograms per liter, is obtained, the separation cost will be exorbitant," said Zeng Jie. To reduce the cost of product separation, it is necessary to further increase the cumulative concentration of hydroxylamine in the solution.
Therefore, researchers conducted continuous electrolysis of nitric acid solution for 5 hours, ultimately obtaining a hydroxylamine solution with a concentration as high as 2.5 grams per liter. This verifies that extending the electrolysis time can increase the cumulative concentration of hydroxylamine, and the accumulated hydroxylamine will not be further reduced to produce ammonia.
Zeng Jie stated that this high-concentration hydroxylamine solution can be easily obtained as solid high-purity hydroxylamine sulfate through simple purification and evaporation crystallization.
Renewable energy-driven novel nitrogen fixation
Nitrogen fixation refers to the process of converting chemically inert nitrogen gas in the air into ammonia or other nitrogen-containing compounds. Nitrogen gas, which constitutes up to 78% of the atmosphere, is an inexhaustible nitrogen resource. However, nitrogen gas molecules have strong chemical inertness and are highly stable.
In traditional nitrogen fixation processes, converting nitrogen gas typically requires harsh reaction conditions, which is why modern industrial synthesis of ammonia from nitrogen gas requires high temperature and pressure.
Zeng Jie explained that their developed parallel plasma arc discharge device, coupled with electrocatalysis, can break the inert chemical bonds in nitrogen gas molecules under mild conditions, achieving efficient nitrogen fixation and selective catalytic conversion at ambient temperature and pressure.
Wu Lizhu, academician of the Chinese Academy of Sciences and researcher at the Institute of Physical Chemistry, Chinese Academy of Sciences, believes, "This work, through plasma discharge coupled with electrocatalysis, successfully synthesized high-value-added hydroxylamine from air and water under mild conditions, providing a new paradigm for developing green artificial nitrogen fixation processes driven by electricity and is an important direction for sustainable utilization of nitrogen species."
Zeng Jie stated, "Next, to further improve the economic efficiency of hydroxylamine synthesis, we will focus on upgrading the plasma discharge device and optimizing efficient electrocatalysts to further reduce the energy consumption of nitric acid production and increase the energy utilization efficiency of electrochemical hydroxylamine synthesis."
Related paper information: Link
