Scientists at Tokyo Institute of Technology (Tokyo Tech) have found a way to synthesize a special type of perovskite that promotes the production of ammonia, which has key applications in fertilizer production and hydrogen energy. This new synthesis process can be carried out at temperatures immensely lower than that of traditional methods for the synthesis of perovskite-oxide materials and in much less time, and the produced perovskite outperforms all its competitors for producing ammonia.

A paper describing their work is publish in the Journal of the American Chemical Society.

Perovskites are a class of synthetic materials that have a crystalline structure similar to that of the naturally occurring mineral calcium titanate. They have been the subject of many studies because they exhibit exciting and unique properties that can be tuned according to their composition. One of their potential applications is as catalysts for the synthesis of ammonia.

In other words, specific perovskites can be placed inside a reaction chamber with nitrogen and hydrogen to promote the reaction of these gases to form ammonia.

Ammonia is a useful substance that can be employed in the production of fertilizers and artificial chemicals, and even as a clean-energy carrier (in the form of hydrogen), which may be key in eco-friendly technologies. However, there are various challenges associated with the synthesis of ammonia and perovskites themselves.

The synthesis rate for ammonia is generally limited by the high energy required to dissociate nitrogen molecules. Some researchers have had some success using precious metals, such as ruthenium. Recently, perovskites with some of their oxygen atoms replaced by hydrogen and nitrogen ions have been developed as efficient catalysts for ammonia synthesis. However, the traditional synthesis of perovskites with such substitutions usually has to be carried out at high temperatures (more than 800 ˚C) and over long periods of time (weeks).

Synthesis rates for ammonia of various catalysts

To address these issues, in a recent study carried out at Tokyo Tech, a group of researchers led by Prof. Masaaki Kitano devised a novel method for the low-temperature synthesis of one of such oxygen-substituted perovskites with the chemical name BaCeO_3-xN_yH_z and tested its performance as a catalyst to produce ammonia.

To achieve this, they made an innovative alteration to the perovskite synthesis process. The use of barium carbonate and cerium dioxide as precursors involves a very high temperature to have them combine into the base perovskite, or BaCeO₃, because barium carbonate is very stable. In addition, one would then have to substitute the oxygen atoms with nitrogen and hydrogen ions.

The team found that the compound barium amide reacts easily with cerium dioxide under ammonia gas flow to directly form BaCeO_3-xN_yH_z at low temperatures and in less time.

The researchers attempted to elucidate the mechanisms behind the improved synthesis rate for ammonia. Overall, the insight provided in this study serves as a protocol for the synthesis of other types of materials with nitrogen and hydrogen ion substitutions and for the intelligent design of catalysts.

Resources

• Masaaki Kitano, Jun Kujirai, Kiya Ogasawara, Satoru Matsuishi, Tomofumi Tada, Hitoshi Abe, Yasuhiro Niwa, and Hideo Hosono (2019) “Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts” Journal of the American Chemical Society doi: 10.1021/jacs.9b10726