Low-temperature ammonia decomposition catalysts for hydrogen generation

Catalytic decomposition of ammonia with complete conversion for generating hydrogen at low temperature (<450 °C) is getting much attention, as onsite hydrogen generation from ammonia bypasses the challenges of hydrogen storage and transportation. Ammonia is a carbon-free hydrogen carrier with a reasonably good volumetric and gravimetric energy density that has available technology for efficient storage and transportation. The state-of-the-art catalysts for decomposition of ammonia are ruthenium doped with potassium, barium and cesium and supported on various oxides and carbon supports; these catalysts are still not sufficiently active for low temperature NH3 decomposition, and, more importantly, will not be economically viable for large-scale applications. This review provides a comprehensive consideration of recent development of various types of nonprecious transition metal catalysts. Proposed mechanisms of ammonia decomposition on various transition metals are examined, and theoretical bases for the design of future catalysts are presented, including the role of electronic promoters and supports. Among studied catalyst formulations, this review emphasizes transition metal nitride-lithium imide composites where a synergy offers great promise for the efficient decomposition of ammonia at low temperatures. Development of such catalysts will make hydrogen storages, transportation, and generation from ammonia a feasible technology for fuel cells and other energy applications.