Ammonia is commonly used in fertilizers because it has the highest nitrogen content of commercial fertilizers, making it essential for agricultural production. However, two molecules of carbon dioxide are made for every molecule of ammonia produced, contributing to excess carbon dioxide in the atmosphere.
A team from the Artie McFerrin Department of Chemical Engineering at Texas A&M University, including assistant professor Dr. Abdoulaye Djire and graduate student Denis Johnson, has developed a method for producing ammonia through electrochemical processes, helping to reduce carbon emissions. This research aims to replace the Haber-Bosch thermochemical process with a more sustainable and environmentally safe electrochemical process.
The researchers recently published their findings in Nature Science Reports.
Since the early 1900s, the Haber-Bosch process has been used to produce ammonia. This process works by reacting atmospheric nitrogen with hydrogen gas. A disadvantage of the Haber-Bosch process is that it requires high pressure and temperature, which leaves a large energy footprint. The process also requires a hydrogen feedstock, which is derived from non-renewable resources. It is unsustainable and has negative environmental implications, accelerating the need for new environmentally friendly processes.
The researchers proposed using the electrochemical nitrogen reduction reaction (NRR) to produce ammonia from atmospheric nitrogen and water. The advantages of using an electrochemical method include the use of water to supply protons and the ability to produce ammonia at room temperature and pressure. This process would potentially require less energy and be less expensive and more environmentally friendly than the Haber-Bosch process.
The NRR works by using an electrocatalyst. For this process to be successful, nitrogen must bind to the surface and break down to produce ammonia. In this study, the researchers used MXene, a titanium nitride, as an electrocatalyst. What differentiates this catalyst from others is that nitrogen is already in its structure, allowing for a more efficient ammonia formulation.
“It’s easier for ammonia to form because protons can attach to nitrogen in the structure, form the ammonia, and then the ammonia will come out of the structure,” Johnson said. “A hole is made in the structure that can suck in the nitrogen gas and separate the triple bond.”
The researchers found that the use of titanium nitride induces a Mars-van Krevelen mechanism, a popular mechanism for the oxidation of hydrocarbons. This mechanism follows a lower energy pathway that would allow for higher ammonia production rates and selectivity due to the titanium nitride catalyst nitrogen.
Without modifying the materials, the researchers achieved a selectivity of 20%, which is the ratio of the desired product formed compared to the undesirable product formed. Their method could potentially achieve a higher percentage selectivity with modifications, opening a new avenue for the production of ammonia by electrochemical processes.
“The Department of Energy has set a target of 60% selectivity, which is a tough number to achieve,” Johnson said. “We were able to hit 20% using our hardware, showcasing a method we could leverage moving forward. If we update our hardware, can we hit 60% soon? That’s the question which we will continue to work to meet.
This research could potentially reduce the carbon footprint and global energy consumption on a larger scale.
“In the future, this could be a major science reform,” Djire said. “About 2% of the world’s total energy is used for the production of ammonia. Reducing this huge number would significantly reduce our carbon footprint and energy consumption.
Reference: Johnson D, Hunter B, Christie J, King C, Kelley E, Djire A. The Ti2N MXene nitride evokes the Mars-van Krevelen mechanism to achieve high selectivity for the nitrogen reduction reaction. Scientific representative. 2022;12(1):657. do I: 10.1038/s41598-021-04640-7
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