Recent development of catalytic strategies for sustainable ammonia production

Presently, ammonia is an ideal candidate for future clean energy. The Haber-Bosch process has been an essential ammonia production process, and it is one of the most important technological advancements since its invention, sustaining the explosive growth of military munitions industry and fertilizers in the first half of the 20th century. However, the process is facing great challenges: the growing need for ammonia and the demands of environmental protection. High energy consumption and high CO2 emissions greatly limit the application of the Haber-Bosch method, and increasing research efforts are devoted to “green” ammonia synthesis. Thermocatalytic, electrocatalytic, and photocatalytic ammonia production under mild conditions and the derived chemical looping and plasma ammonia production methods, have been widely developed. Electrocatalytic and photocatalytic methods, which use low fossil fuels, are naturally being considered as future directions for the development of ammonia production. Although their catalytic efficiency of ammonia generation is not yet sufficient to satisfy the actual demands, considerable progress has been made in terms of regulating structure and morphology of catalyst and improving preparation efficiency. The chemical looping approach of ammonia production differs from the thermocatalytic, electrocatalytic, and photocatalytic methods, and is the method of reusing raw materials. The plasma treatment approach alters the overall ammonia production approach and builds up a new avenue of development in combination with thermal, photocatalytic, and electrocatalytic methods as well. This review discusses several recent effective catalysts for different ammonia production methods and explores mechanisms as well as efficiency of these catalysts for catalytic N2 fixation of ammonia.