Hydrogen Release From Ammonia: Size and Support Effects in Heterogeneous Transition Metal Catalysis

Abstract

Ammonia, industrially produced by the Haber–Bosch process, can serve as a promising renewable energy carrier based on its high hydrogen content and energy density as well as its full-fledged infrastructure worldwide for transportation. Renewable hydrogen will be converted firstly in ammonia synthesis, stored and/or transported bound in ammonia, and released on demand by ammonia decomposition. So far, the most active catalysts for the decomposition reaction are Ru-based due to its optimal nitrogen binding energy compared to other transition metals. However, due to the high cost of Ru, the development of alternative catalysts for ammonia decomposition is attractive, especially non-noble transition metals such as Fe, Ni, or Co. For supported metal catalysts, size and support effects play important roles in the catalytic reaction, resulting in a change of their geometric and/or electronic properties. In this review, we first discuss and comment on representative existing studies of the size and support effects of Ru, Fe, Ni, and Co catalysts in ammonia decomposition from an experimental and theoretical view, suggesting possible reasons for geometrical and electronic effects. Afterward, we will summarize the available catalytic data in the literature in the form of TOF_H2 and reaction rate of each supported transition-metal catalyst with different supports as a function of the particle size, attempting to identify an optimum particle size and a trend for the different supports. Finally, we will discuss the challenges and perspectives of future-oriented research on the size and support effect in ammonia decomposition.