Catalyst Makes Ammonia Process Greener

May 29, 2020
A new catalyst does away with the high temperatures, pressures and energy consumption associated with the current Haber-Bosch process for making ammonia.

A new catalyst consisting of CaFH doped with ruthenium nanoparticles does away with the high temperatures, pressures and energy consumption associated with the current Haber-Bosch process for making ammonia, say its developers at the Tokyo Institute of Technology, Tokyo. The scientists note its lower energy demands reduce the carbon dioxide emissions from the use of large amounts of fossil fuels. They add the development raises the possibility of an environmentally sustainable Haber-Bosch process, opening the door to the next revolution in agricultural food production.

The catalyst — a solid solution (i.e., a solid mixture containing a minor component uniformly distributed within the crystal lattice of the major component) of CaF2 and CaH2 that is formed at low temperatures — produced ammonia from nitrogen and hydrogen gases at 50°C. The reaction involved an activation energy of 20 kJ/mol, which the scientists, in a recent article in Nature Communications, describe as extremely small and less than half that reported for conventional catalysts.

After conducting spectroscopic and computational analyses, the scientists proposed a possible mechanism by which the catalyst facilitates ammonia production. They suggest its performance stems from the weak ionic bonds between Ca2+ and H ions in the solid solution and the easy release of hydrogen atoms from H sites. These then desorb from the ruthenium nanoparticles as hydrogen gas. The resultant charge repulsion between trapped electrons and F- ions releases electrons that attack the bonds between the nitrogen atoms in nitrogen gas and, so, promote the production of ammonia.

The next step in the development is to improve the new catalyst’s performance, says Michikazu Hara, a professor at the Institute of Innovative Research at the university. “For example, its surface area is only 30 m2/g, so we want to increase this,” he explains.

This work initially will take place in the current mL-scale reactor but catalyst tests in a pilot plant will occur at some point, he reckons.

“We will also develop other new catalysts on the basis of the knowledge obtained in this work — probably aimed at methane conversion into methanol, biomass conversion to value-added chemicals and polymers, and the efficient electrolysis of water,” Hara adds.

While the effort has yet to attract interest from industrial fertilizer manufacturers, Hara notes the team has been working closely with the local government, which manages Ogata-Mura, a 9,000-ha ( >22,000-acre) plot of reclaimed land where agricultural innovations are tested.

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