Energy-friendly ammonia production for fertilisers and fuel

A Japanese research team, led by Satoshi Kamiguchi from the RIKEN Center for Sustainable Resource Science (CSRS), has discovered a greener way to produce ammonia, an essential compound used in fertilisers. Because ammonia is a good way to safely store hydrogen, as well as an excellent alternative fuel in its own right, this discovery will make it easier to switch from fossil fuels to a carbon-neutral and green-energy economy.

The nitrogen in fertilisers comes from ammonia, which is made by breaking apart hydrogen (H₂) and nitrogen (N₂) molecules and joining the individual elements together into ammonia gas (NH₃) through the Haber–Bosch process. The reaction requires an iron catalyst as well as extremely high pressure and temperatures — about 200 atm and 500°C. Because ammonia is so widely used in fertilisers and other industries, worldwide production consumes a huge amount of energy.

To help reduce ammonia’s energy footprint, the researchers set out to create a more energy-friendly reaction that can proceed stably at much lower temperatures without becoming deactivated. The biggest hurdle was breaking down nitrogen gas, because there is a strong triple bond between the two nitrogen atoms within a molecule of nitrogen gas.

“The trick was to use ultrasmall molybdenum metal particles prepared from a hexanuclear molecular metal halide cluster, which was then activated with hydrogen gas,” Kamiguchi said. Once activated, multiple molybdenum atoms work together to quickly break the strong nitrogen-nitrogen bonds and drive ammonia synthesis.

When tested, this new method was able to create ammonia from nitrogen and hydrogen gases continuously for more than 500 hours at 200°C, greatly reducing the required temperature when using the conventional Haber–Bosch process. The results were published in the journal Chemical Science.

In addition to impacting the fertiliser industry, the new way of producing ammonia would indirectly help reduce carbon emissions if ammonia fuel was used worldwide. Ammonia fuel can be burned directly in internal combustion engines without emitting any CO₂, but it has not become a practical alternative because of the high-energy Haber–Bosch process. With the new method in place, this would no longer be a problem.

Furthermore, at the same time that ammonia is storing nitrogen for fertilisers, it’s also storing hydrogen, which some consider to be the ideal source of energy. When the stored hydrogen is needed, it could be released from ammonia and used as fuel without emitting carbon dioxide.

“Replacing the Haber–Bosch process with our new method should result in worldwide energy saving,” Kamiguchi said. “If ammonia fuel and hydrogen fuel are used in much larger amounts, vastly reducing the energy needed to synthesise ammonia will lead to lower CO₂ emissions and help prevent further global warming.”

One problem still remains: the hydrogen needed to make ammonia is itself still produced using fossil fuels, and in the necessary quantities would also lead to tremendous CO₂ emissions and energy consumption. Currently, the research team is focusing on adding promoters to the molybdenum-based catalyst that will make ammonia synthesis more efficient.

“When our catalyst system is combined with green H₂ production from renewable energy, the emission of global-warming CO₂ could be reduced even more,” Kamiguchi said.

Image credit: iStock.com/FrankRamspott