Researchers at the University of Tokyo have achieved a significant breakthrough in sustainable chemistry by developing a method to synthesize ammonia using only sunlight, atmospheric nitrogen, and water. This innovative process mimics the natural nitrogen-fixation methods employed by cyanobacteria in symbiotic relationships with plants. According to a university press release, this development opens the door to ammonia production with dramatically lower energy requirements and environmental impact.

Ammonia is a cornerstone of global agriculture, primarily used in the production of urea-based fertilizers essential for large-scale crop cultivation. With approximately 200 million tonnes of ammonia produced annually—over 80 percent of which is used in agriculture—finding a cleaner production method is critical. Currently, ammonia is synthesized through the Haber-Bosch process, which requires high temperatures and pressures, making it energy-intensive and responsible for about 2% of global carbon emissions.

Led by Professor Yoshiaki Nishibayashi of the Department of Applied Chemistry, the University of Tokyo research team created a system in which nitrogen from the atmosphere reacts with water in the presence of sunlight to form ammonia—mimicking how nitrogen-fixing bacteria operate in nature.

Since replicating bacterial nitrogen fixation outside of living cells is extremely challenging, the researchers turned to catalysis to facilitate the reaction. Their approach relied on a novel combination of catalysts to enable the reaction under mild, solar-powered conditions.

The researchers employed two transition metal-based catalysts: one containing iridium and the other molybdenum. The iridium-based photocatalyst was used to activate tertiary phosphines and water, while the molybdenum-based catalyst activated atmospheric dinitrogen (N₂).

“We used an iridium photocatalyst and another chemical called a tertiary phosphine, which enabled photochemical activation of water molecules,” Nishibayashi explained. “The reaction efficiencies were higher than expected, compared to previous reports of visible-light-driven photocatalytic ammonia formation.”

When operating under optimal conditions, the system successfully converts two nitrogen atoms and three water molecules into two ammonia molecules, leaving oxygen as the only byproduct.

The reaction mechanism involves the iridium photocatalyst absorbing sunlight and entering an excited state, which allows it to oxidize tertiary phosphines. These oxidized phosphines then interact with water, forming a chemical bond that releases protons—crucial for forming ammonia. Simultaneously, the molybdenum catalyst facilitates the reduction of nitrogen molecules, completing the synthesis of ammonia.

This study represents a promising step toward cleaner, more sustainable ammonia production. If scaled effectively, it could help reduce the carbon footprint of global agriculture and reduce reliance on fossil-fuel-based chemical processes.

By Impact Lab