Catalytic pathways for efficient ammonia-to-hydrogen conversion towards a sustainable energy future

Abstract

A sustainable and smooth transition from fossil-fuel-based energy to a clean hydrogen economy requires affordable hydrogen storage and transportation solutions. Ammonia is a desirable hydrogen carrier option due to its high hydrogen content (17.6 wt%), being devoid of a carbon footprint, its ease of liquefaction (∼33.4 °C at 1 atm or 20 °C at 8.46 atm), and the century-old well-established infrastructure for the manufacture and transportation of NH₃. However, breaking the NH₃ bonds to regain the stored hydrogen requires catalysts for dehydrogenation of NH_x (x = 1–3) and then quick associative desorption of N from the active metal center under reaction conditions. This review highlights recent advancements in catalyst design strategies, performance, and challenges associated with understanding the intricate relationship between the catalyst structure and activity. Here, mechanisms of decomposition/oxidation of noble and transition metals are discussed, which provide a strong foundation for heterogeneous catalyst design in terms of charge transfer and the synergistic effects between active metal sites and supports. This evolves as a crucial factor for the reduction at decomposition temperatures. This review also emphasizes the recent development of homogeneous catalytic ammonia decomposition (AD)/oxidation (AO) at low temperatures (<100 °C) using a series of metal (M = Cr, Mn, Fe, Ni, Cu, Mo, Os and Ru) complexes. Its molecular reaction mechanisms and pathways to develop efficient catalysts have been discussed extensively.