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

With the energy transition looming, the search for carbon-freefuels is imperative. Hydrogen is an excellent alternative to fossilfuels, but there are significant challenges in transport and production.Ammonia is an excellent hydrogen carrier with 17.6 wt % hydrogen contentand zero carbon, and the infrastructure for its production, storage,and transport is already well-established. Currently, ammonia decompositionfor hydrogen only reacts at high temperatures; therefore, the challengeis to optimize highly active catalysts for ammonia decomposition atlow temperatures. This review will start from the background of ammoniadecomposition for hydrogen, thoroughly introduce the ammonia catalyticdecomposition systems, mainly covering ruthenium-based, iron-based,cobalt-based, nickel-based, metal nitride, metal carbide, and newalkali metal amide-imide systems, analyze in detail the influenceof different catalyst preparation methods, supports, promoters, andother methods on the catalytic activity, and discuss the general rulesfor optimizing the efficiency of catalysts. Moreover, the reactionkinetics of different catalysts, including reaction mechanisms, reaction-determiningsteps, and activation energies, are presented, and future prospectson ammonia decomposition catalysts are proposed.