While ammonia production is critical to the world’s food supply—up to 90% of ammonia goes toward making fertilizer for agriculture—the process of converting hydrogen and nitrogen to ammonia leaves a significant carbon footprint, contributing to 1 to 2% of global energy consumption and CO2 emissions annually.
Associate professor
of chemical an
biomolecular
engineering
Global ammonia production was nearly 180 million tons in 2021, so any mechanism that uses less energy during production is a net positive. In addition, ruthenium, the catalyst typically used for this reaction, is expensive, and those capital costs matter even if catalysts are recovered after production.
Two chemical and biomolecular engineers, Chao Wang, associate professor, and Brandon Bukowski, assistant professor, have developed a new catalyst that could improve how ammonia is made. Based on experimental and computational studies, their carbon-coated manganese nitride promises to operate at temperatures at least 100 °C lower than current catalysts and ensures the integrity of the air-sensitive catalyst until it’s activated in situ. Another advantage: Manganese is readily available and about 10,000 times less expensive than ruthenium.
Associate professor
chemical and
biomolecular
engineering
The researchers, who published their results in ACS Catalysis, explain that nitrogen vacancies present on the manganese nitride surface readily adsorb nitrogen to gradually weaken the strong nitrogen-nitrogen bond and proceed with the hydrogenation required to produce ammonia. Such an “associative” instead of a dissociative mechanism requires much less activation energy and makes it possible to synthesize ammonia production at lower temperatures and pressures.
“Chemical plants operating the Habor-Bosch process spend a lot of energy to pressurize the gaseous reactants and heat up the reactor. So, reduced operation temperatures and pressures enabled by our novel manganese nitride catalyst would make the ammonia production more energy efficient,” Wang explains.
To arrive at the manganese nitride catalyst, Wang and Bukowski ran models to predict the energy it takes to break a chemical bond and evaluated catalysts by their potential to lower that number. While laboratory experiments confirmed the successful use of manganese nitride, molecular models helped confirm the associative mechanism down to the molecular level.
“The molecular model can try to make inferences on the data and shortcut the process of doing very expensive physics calculations,” Bukowski says. It took the team hours, instead of days, to crunch the data.
The researchers say that the multiple advantages of manganese nitride for synthesizing ammonia are good news for an industrial process that has long expended too much energy.
— POORNIMA APTE