Ammonia production is energy intensive. A new approach could help change this. Kittisak Kaewchalun/iStock
Researchers at the University of Tokyo have successfully used ‘artificial photosynthesis’ to produce ammonia using atmospheric nitrogen, water, and sunlight, much like how cyanobacteria synthesize the gas in a symbiotic association with plants. The achievement could unlock ammonia products with low energy costs in the future, a press release said.
Ammonia production is a critical input in global agriculture since it is used to make urea, a fertilizer needed for large-scale production of crops. Estimates suggest that 200 million tonnes of ammonia are produced every year, with over 80 percent of them used in agriculture.
Composed of just two elements, nitrogen, and hydrogen, ammonia has been produced at industrial scales using the Haber-Bosch process, where the gases react under conditions of high temperature and pressure. However, this is an energy-intensive process, which also contributes two percent to global carbon emissions. This is why a more environmentally friendly alternative has been sought for ammonia production.
Artificial photosynthesis to make ammonia
A research team led by Yoshiaki Nishibayashi, a professor at the Department of Applied Chemistry at the University of Tokyo, has now succeeded in developing a system where atmospheric nitrogen and water react to make ammonia in the presence of sunlight. This is similar to the process that symbiotic bacteria use while fixing nitrogen for plants.
However, the reaction cannot be replicated with similar ease outside the bacterial cell. Therefore, The research team turned to catalysts – substances that help carry out a reaction by either lowering their temperature or time requirement without being consumed. The key to their success was using a combination of catalysts to carry out the reaction.
“We used an iridium photocatalyst and another chemical called a tertiary phosphine, which enabled photochemical activation of water molecules,” explained Nishibayashi in the press release. “The reaction efficiencies were higher than expected, compared to previous reports of visible-light-driven photocatalytic ammonia formation.”
When the reaction takes place in optimal conditions, two nitrogen atoms and three water molecules form two ammonia molecules with only oxygen left over. Image credit: Nishibayashi et al
What did the catalysts do?
The researchers used two catalysts, both based in transition metals, iridium and molybdenum in their experiments. The iridium-based catalyst was used to activate tertiary phosphines and water, while the molybdenum-based catalyst activated dinitrogen. Tertiary phosphines also played a role in turning water molecules into protons that helped achieve ammonia as the final product.
“When the iridium photocatalyst absorbs sunlight, its excited state can oxidize the tertiary phosphines. The oxidized tertiary phosphines then activate water molecules via forming a chemical bond between the phosphine’s phosphorous atom and the water, yielding protons,” explained Nishibayashi in the press release.
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“The molybdenum catalyst then enables nitrogen to bond with these protons to become ammonia. The use of water for producing dihydrogen or hydrogen atoms is one of the most important processes for achieving green ammonia production,” he added.
Comparing the reaction to the photosynthesis of ammonia, Nishibayashi said, “Here, the electrons for the reaction are supplied by photosynthesis, and protons are derived from water. Therefore, the findings of our recent study can be regarded as a successful example of the artificial photosynthesis of ammonia.”
The research findings were published in the journal Nature Communications.
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ABOUT THE AUTHOR
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
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