Ammonia synthesis in mild conditions – Recent developments of new catalytic systems

In 1913, the BASF company developed the industrial synthesis of ammonia based on the discovery of German chemist Fritz Haber, who five years earlier had succeeded in demonstrating the possibility of fixing nitrogen from the air using heterogeneous catalysis: this was the invention of the Haber-Bosch process. The production, from atmospheric nitrogen, of ammonia (NH ₃ ), a molecule containing one nitrogen atom for every three hydrogen atoms, has since been considered the industrial process with the greatest impact on society, with applications in the military (explosives), economic (chemical and textile industries) and agricultural (fertilizers) fields. In particular, it is recognized that 40-50% of the world's population could not feed themselves without nitrogen fertilizers derived from ammonia synthesis. Since its development, the Haber-Bosch process has saved billions of human lives. Paradoxically, it has also had significant adverse effects on biodiversity and climate, since this energy-intensive process was designed and optimized using fossil fuels. For, while the nitrogen in the reaction (N ₂ + 3H ₂ → NH ₃ ) comes from the atmosphere, the dihydrogen is generated from coal, oil or, more generally today, natural gas, with the co-product carbon dioxide (CO ₂ ), known for its greenhouse gas properties. The process's heat and power requirements are also very high, to reach temperatures in excess of 500°C and pressures in excess of 200 bar in most units to achieve sufficient ammonia yield and make production economically viable. The amount of CO ₂ emitted per tonne of ammonia produced is thus currently estimated at 2.4 tonnes. Yet it is forecast in some energy transition scenarios that ammonia production could triple by 2050 for additional needs in particular as a fuel to decarbonize the maritime sector. Logically, this growth can only be achieved on the basis of decarbonized ammonia production, which would radically change the process as conceived over a century ago. One of the challenges is to discover new catalytic formulations enabling ammonia to be synthesized under milder temperature and pressure conditions. This article reviews recent research work that has paved the way for the design of promising new catalysts on a laboratory scale.