Prospects and challenges of green ammonia synthesis

Ammonia is a chemical commodity in high demand, owing to its use in agriculture as well as its potential as a chemical vector for renewable energy storage and transportation. At present, ammonia synthesis consumes 1–2% of the world’s total energy output while producing 1% of the world’s total carbon emissions. Thus, the development of greener synthetic routes to ammonia is urgently required. In this Review, we discuss the progress and challenges in regard to the technological and economic aspects of various routes to green ammonia synthesis. Fundamental mechanisms, including the classical N2 dissociative process, the newly identified associative process for catalytic N2 conversion to NH3 under milder conditions and the chemical looping pathway, are discussed to guide novel catalyst designs. In particular, associative N2 activation can be achieved at low pressure, which is more adaptable for coupling to renewable energy (such as solar, wind or tidal), offering a new industrial production route to green ammonia. Additional possibilities for direct large-scale green ammonia synthesis through electrochemical and photochemical approaches are also discussed. Finally, a scaleup roadmap for ammonia synthesis is described alongside recent industrial developments, highlighting the rapid evolution and prosperous future of green ammonia generation. Green ammonia synthesis is important for future sustainable manufacturing of fuels and chemicals. This Review highlights the recent progress and challenges in both fundamental research in catalysis and potential industrial scaleup using renewables.