Research Status, Optimization Strategies, and Future Prospects of Ammonia Decomposition Catalysts for COx-Free Hydrogen

With the energy transition looming, the search for carbon-free fuels is imperative. Hydrogen is an excellent alternative to fossil fuels, but there are significant challenges in transport and production. Ammonia is an excellent hydrogen carrier with 17.6 wt % hydrogen content and zero carbon, and the infrastructure for its production, storage, and transport is already well-established. Currently, ammonia decomposition for hydrogen only reacts at high temperatures; therefore, the challenge is to optimize highly active catalysts for ammonia decomposition at low temperatures. This review will start from the background of ammonia decomposition for hydrogen, thoroughly introduce the ammonia catalytic decomposition systems, mainly covering ruthenium-based, iron-based, cobalt-based, nickel-based, metal nitride, metal carbide, and new alkali metal amide-imide systems, analyze in detail the influence of different catalyst preparation methods, supports, promoters, and other methods on the catalytic activity, and discuss the general rules for optimizing the efficiency of catalysts. Moreover, the reaction kinetics of different catalysts, including reaction mechanisms, reaction-determining steps, and activation energies, are presented, and future prospects on ammonia decomposition catalysts are proposed.