Solar Hydrogen Generation Using Homogeneous, Heterogeneous and Biphasic Catalytic Systems

Using sunlight to generate hydrogen from water has captivated the attention of researchers as it would prove to be a completely renewable method of fuel production. For several decades, scientists have been trying to achieve ‘total
water splitting’, which is a highly complex and energetically demanding process. To overcome this barrier, the process is treated separately as two half reactions, namely water oxidation and reduction, with the majority of the research directed at the reduction side which pertains to H₂ evolution. Of late, research is also being conducted on using other H₂ sources like acids and alcohols which are
thermodynamically easier to dehydrogenate. The usual components for photocatalytic hydrogen evolution include a light-absorbing chromophore/photosensitizer, a proton reducing catalyst and a sacrificial donor/ redox mediator which mimics the oxidative half of water photolysis.
These reactions have been carried out using both molecular and heterogeneous systems, mainly employing noble metal-based photoharvesters and catalysts, due to their inherent low overpotentials, high activity and stability. To ensure
commercial viability of the hydrogen generation process, recent focus has been on utilizing earth-abundant materials. With this background in mind, this dissertation aims to explore different routes to photocatalytic hydrogen
generation. Both molecular and heterogeneous methods of water reduction were probed, using an earth-abundant Ni-based hexameric cluster and a metal-free nanocarbon, respectively as proton reducing catalysts. Both systems were photodriven using state-of-the-art iridium-based photosensitizers, which are robust and well-known to probe the capability of new catalysts. While the Nibased molecular system achieved up to 30,000 catalyst turnovers, the metal-free nanocarbon was able to outperform platinum catalysts in terms of total hydrogen generation. As an example of replacing synthetic sacrificial agents with naturally
occurring compounds, this thesis also demonstrates the use of oxalic acid as an electron donor, along with metallosurfactant Ir and Rh complexes in a novel biphasic H2 generation system. Oxidative quenching of the Ir excited state by the Rh, followed by electron donation by oxalic acid via phase transfer is proposed to be the dominant pathway, although further studies need to be conducted to prove
this mechanism. Finally, this work contributes to the growing field of alcohol dehydrogenation, using visible light, a rhodium polypyridine catalyst and iodide in an acidic medium. This system was found to generate hydrogen and acetone from isopropyl alcohol, linearly for ~100 h, highlighting the robustness of the catalytic system.