Review on Ru-Based and Ni-Based Catalysts for Ammonia Decomposition: Research Status, Reaction Mechanism, and Perspectives

Hydrogen (H2) is a zero-carbon and high-energy-density fuel promising to replace fossil fuels for power generation and clean energy. However, hydrogen still faces enormous challenges in terms of production, transportation, and storage. Ammonia (NH3) is a promising H2 (17.7 wt %) carrier that easily overcomes the difficulties associated with H2 storage and transport. However, for the NH3 decomposition hydrogen production reaction, the biggest challenge at present is to achieve complete conversion of ammonia under a relatively high space velocity (about 30,000 mL·gcat–1·h–1) at low-temperature conditions (about 350 °C) with reasonable price catalysts. At present, the most efficient ammonia decomposition catalyst is a Ru-based catalyst doped with K, Ba, and Cs and supported on various carbon supports and metal oxides. Otherwise, the catalysts that exhibited the most outstanding activity among non-noble metal catalysts are nickel-based, and because of their low cost, nickel is regarded as a reasonable alternative candidate material for NH3 decomposition. Advances in the study of reaction kinetics of ammonia decomposition reactions and whether the rate-determining step of the ammonia decomposition reaction is the cleavage of the first N–H bond or the desorption of nitrogen gas are also discussed. This review provides a comprehensive consideration of the recent development of Ru-based and Ni-based catalysts and proposed mechanisms of ammonia decomposition on them are examined. The effects of preparation methods, support, and promoters on catalyst activity were studied and theoretical bases for the design of future catalysts are presented. At last, a brief introduction to catalytic membrane reactor technology in recent years is given. This review can serve as a comprehensive work for designing novel catalysts.