Review of the Decomposition of Ammonia to Generate Hydrogen

Because of the problems associated with the generation and storage of hydrogen in portable applications, the use of ammonia has been proposed for on-site production of hydrogen through ammonia decomposition. First, an analysis of the existing systems for ammonia decomposition and the challenges for this technology are presented. Then, the state of the art of the catalysts used to date for ammonia decomposition is described considering the catalysts composed of noble and non-noble metals and their combinations, as well as novel materials such as alkali metal amides and imides. The effect of the supports and promoters used is analyzed in detail, and the catalytic activity obtained is compared. An analysis of the kinetics of the reaction obtained with different catalysts is also presented and discussed, including the reaction mechanism, the determining step of the reaction, and the apparent activation energy. Finally, the structured reactors used to date for the decomposition reaction of ammonia are explored, as well as the possibilities offered by catalytic membrane reactors, which allow the on-site simultaneous production and separation of hydrogen.