New affordable catalyst relies on nitrogen vacancies to produce ammonia

Ammonia (NH₃) is one of the most commonly produced chemicals worldwide, due to its use as an important ingredient in a wide range of industrial manufacturing processes. For example, it is essential in the production of fertilizers, and more than 150 million tons are applied each year to increase the yields of various crops. Ammonia is naturally produced by many living organisms, but artificially synthesized using nitrogen (N_two) and hydrogen (H_two) gases are challenging because the strong bond between N atoms is difficult to break.

While a method of producing NH₃ On an industrial scale, called the Haber-Bosch process, it has existed since the beginning of the 20th century, the current best-performing approach involves the use of ruthenium, an expensive and scarce metal, as a catalyst to trigger the necessary reactions. Recently, Professor Hideo Hosono and colleagues at the Tokyo Institute of Technology (Tokyo Tech), Japan have developed a new strategy to produce NH₃ using lanthanum (La), a much more abundant element, in combination with nickel (Ni).

In his article, published in Nature, explain how they were inspired by previously informed NH₃ production catalyst with formula Co₃Month₃N, which has nitrogen vacancies, places where the presence of a nitrogen atom would be expected but which are actually empty. These vacancies were found to divide N_two easier molecules, prompting the Hosono team to take a new direction of exploration to achieve more available and effective NH₃ Synthesis catalysts. He explains: “The critical role of nitrogen vacancies in Co₃Month₃N inspired us to consider other nitrogen-containing materials on which vacancies could easily be generated as the basis for new Ni-based catalysts. “

The catalyst they developed consists of LaN crystals loaded with Ni nanoparticles. Ni dissociates easily H_two into H atoms Thus, the pretreatment of the catalyst with H_two easily generates H atoms, which then react with N atoms in the crystal structure to form NH₃ and create N vacancies in the LaN bracket. Each of these empty sites captures an N atom from an N_two molecule of the incoming nitrogen gas, causing the molecule’s NN bond to weaken. Another dissociated H atom breaks the weakened NN bond to produce more NH₃, leaving behind an N atom to fill the original vacancy. These cycles are repeated, continuously generating nitrogen vacancies and maintaining the synthesis process.

This concept of a “dual active site” catalyst proved very promising. The performance of the proposed catalyst far exceeds that of the more conventional nickel-based cobalt catalysts and is comparable even to that of ruthenium-based catalysts: not only does it consistently produce high yields of ammonia at moderate temperature and pressure, its structure is maintains even after 100 hours of continuous reaction, demonstrating its high stability.

Hosono says: “We anticipate that our work will stimulate further exploration of catalyst designs that make use of more abundant elements. In particular, our results illustrate the potential of using vacant sites in reaction cycles and point to a new concept of design for catalysts for ammonia synthesis. ”

The new strategy could make ammonia production simpler and more affordable, thereby facilitating a multitude of significant industrial processes.