Mechanistic Investigation of Iridium-Catalyzed Oxidation of Water and Ammonia

Our planet is facing the grand challenge due to fossil fuel depletion and global warming. In the search of other carbon-free renewable energy sources, hydrogen fuel with water as the only bi- product, has been regarded as one of the promising alternative. However, the usage is limited due to its storage and transportation. Therefore, extensive efforts have been devoted in developing cost-effective catalysts for the efficient conversion of earth-abundant resources to molecular hydrogen. In particular, splitting of water into hydrogen and oxygen, inspired by photosynthesis has been extensive studied in past few decades. On the other hand, hydrogen generation from ammonia has also received incredibly attention in recent years because of its high energy density. Unfortunately, the development of these two metal-catalyzed processes are currently hindered by the oxidation steps, i.e. 2H2O → O2 + 4H+ + 4e– in water splitting and 2NH3 → N2 + 6H+ + 6e– in ammonia conversion. To overcome this oxidation barrier in facilitating the design of robust molecular catalysts for effective hydrogen production, the understanding of fundamental O–H/N–H bond-breaking and O–O/N–N bond-forming in these oxidation processes is necessary.

In this proposed project, we design to study the oxidation of water and ammonia in detail using various iridium complexes. Our preliminary studies demonstrate the newly synthesized polypyridyl iridium(III) complex is a promising homogeneous catalyst in oxidizing water to molecular oxygen efficiently using cerium(IV) nitrate as the terminal oxidant. This iridium complex also functions as a catalyst in ammonia oxidation to produce molecular nitrogen upon addition of oxidants and bases. In this project, a variety of iridium(III) complexes bearing different polypyridyl ligands will be synthesized as potential catalysts for the oxidation of water and ammonia. Then, the mechanism of these iridium-catalyzed oxidation of water and ammonia reactions will be investigated in detail using various spectroscopic techniques, including isolation of the key intermediates, kinetic studies and theoretical calculations. Finally, the catalytic efficacy in water oxidation and ammonia oxidation by these iridium complexes will be evaluated by chemical and electrochemical methods. The success of this project should provide significant insights for the development of effective molecular transition metal catalysts in hydrogen production, an important renewable energy source to maintain our high quality of life