Design and Development of Ruthenium-Based Catalysts for Enabling Hydrogen Storage

Abstract

For past decades, hydrocarbon-based fuels have been the primary source of energy as a necessity for modern machines and automotive vehicles. Despite use in combustion engines and power plants, fossil fuels reserves are unpredictable and the associated CO2 emissions are now causing terrible effects on the environment and human health. Researchers have realized H2 as a potentially safe and efficient energy source, with an energy density of 33.3 kWh per kg. Moreover, H2 is significantly more environmentally friendly as H2O is the only by-product. Currently, the development of on-board storage technology is progress for use in the transportation industry. The chemical storage of hydrogen, in which H2 is bound to a carrier molecule, provides a more practical and safer method to store and release immense amounts of H2. Ammonia Borane (H3N-BH3, AB) is a useful material for storing up to 19.6 wt% of and 0.145kg L-1 of H2 with the ability to release H2 under mild ambient conditions through exposure to a suitable catalyst. This project investigates the use of pyrazole-based compounds as ligands for η6-arene Ru-complexes in order to formulate efficient catalysts for AB dehydrogenation. Pyrazoles are interesting diaza-five member heterocycles and have strong electron-donating properties. Mono- and di-coordinating substituted pyrazole complexes and an unusual Cl-bridging dimeric Ru complex were successfully synthesized and characterized (NMR, ESI-MS and X-ray diffraction crystallography) before undergoing evaluation for AB dehydrogenation studies in a high-pressure stainless steel reactor. The dimeric Ru Cl-bridging catalyst released the largest H2 equivalent per mol of AB with 1.45 with a total of 7.82 bar of H2 pressure, proving to be the most successful catalyst in the series. However, from the H2 release profiles, it appears that catalysts undergo slow transformation where it is possible that the pyrazole ligands are lost. Further mechanistic studies are required to determine the decomposition process.