Scientists unveil mechanism behind greener ammonia production

Tokyo, Japan – Researchers from Tokyo Metropolitan University have revealed how a catalyst in a promising chemical reaction for industry helps make ammonia, a major ingredient in fertilizer. Copper oxide is a key catalyst in the electrochemical nitrate reduction reaction, a greener alternative to the existing Haber-Bosch process. They discovered that copper particles are created mid-reaction, helping convert nitrite ions to ammonia. This insight into the underlying mechanisms promises leaps forward in developing new industrial chemistry.

As an ingredient in fertilizer, ammonia is an important chemical in industrial agriculture. The most widely adopted way to make ammonia is the Haber-Bosch process, where nitrogen and hydrogen are reacted at high temperature and pressure. This makes the process energy intensive; it is said to account for around 1.4% of global carbon dioxide emissions. As a chemical underpinning so much food production, the hunt is on for greener ways to make ammonia.

A team led by Professor Fumiaki Amano from Tokyo Metropolitan University has been studying the electrochemical nitrate reduction reaction, a promising alternative that can make ammonia from nitrates at room temperature and pressure. Electrochemical processes work by putting electrodes into a chemical mixture and applying a voltage to drive reactions. Despite numerous studies identifying specific reactions occurring at the electrodes as ammonia is produced, the exact mechanism has proven elusive.

Using cutting-edge techniques, the team gained unprecedented insight into how ammonia is produced in the presence of a copper oxide catalyst, one of the most effective electrocatalysts for this kind of reaction. They used operando X-ray absorption, a method combining insights into electronic states with knowledge of local bonding and structure. Mounting small particles of copper oxide onto carbon fibers, they succeeded in extracting how things change as the voltage is made more negative during the reaction. Under a positive voltage, it was shown that nitrate ions “passivate” the catalyst by absorbing onto them and preventing the conversion of copper oxide into metallic copper, making nitrite ions instead. As the voltage is made more negative, ammonia production is seen to ramp up abruptly. This happens at the same time as the appearance of metallic copper particles, evidenced by a dramatic increase in the number of copper-copper bonds. They discovered that the metallic copper is helping to add hydrogen to the nitrite ions to make ammonia.

The team’s measurements have shown how surface passivation affects the efficiency of the copper oxide catalyst, and how the production of metallic copper is crucial to the efficient production of ammonia. Their work highlights a broad class of strategies to optimize green ammonia production and design new electrochemical catalysts.

This work was supported by Tokyo Metropolitan University and the Tokyo Global Partner Scholarship Program and was based on results obtained from project JPNP14004 commissioned by the New Energy and Industrial Technology Development Organization (NEDO).