Masaaki Kitano and his team at Tokyo Tech point out that the main barrier to a facile synthesis of ammonia from hydrogen and nitrogen gas is the surmounting the high energy barrier needed to split diatomic nitrogen. Nitrogen-fixing plants, of course, can handle this process with a range of enzymes evolved over millions of years and metals catalysts coupled with high temperatures and pressures are the mainstays of the industrial process. There have been efforts to make perovskites in which some of their oxygen atoms have been replaced with hydrogen and nitrogen ions to act as ammonia forming materials, but these too only work at a high temperature of more than 800 degrees Celsius and the reaction takes weeks to proceed to completion. These two factors had until now meant perovskites for all the hyperbole were perhaps not going to create a new ammonia process. [Kitano, M. et al., J. Am. Chem. Soc. (2019); DOI: 10.1021/jacs.9b10726]
However, Kitano and his team think they have addressed the various problems. They have devised a novel method for the low-temperature synthesis of one of such oxygen-substituted perovskite, BaCeO3-xNyHz. They have tested their cerium-containing perovskite and found its performance as a catalyst for making ammonia to quite tenable. Their perovskite was prepared via a slightly unconventional approach using barium amide, instead of carbonate, and cerium dioxide as precursors Barium amide reacts easily with cerium dioxide under ammonia gas flow to directly form the new perovskite at a relatively low temperature and on a much shorter timeframe than previous efforts.
"This is the first demonstration of a bottom-up synthesis of such a material, referred to as perovskite-type oxynitride-hydride," explains Kitano. The team tested its catalytic prowess under a range of low-temperature conditions and found it could produce ammonia much more efficiently and effectively than even the state-of-the-art competitors in this area, especially when it is combined with an adjunct metal catalyst in the form of ruthenium. However, it worked even better with cheaper metals, such as cobalt and iron, which could make the whole process much more economically viable than any process that requires costly precious or rare metals.
"Our results will pave the way in new catalyst design strategies for low-temperature ammonia synthesis," adds Kitano. These findings will hopefully make the synthesis of useful materials cleaner and more energy efficient.
