Photoproduction of Hydrogen by Decamethylruthenocene

Splitting water to produce hydrogen (H2) fuel appears very promising to address the challenges of solar energy storage and global warming as the only by-product generated is oxygen (O2). An alternative strategy, called batch water-splitting, based on Interfaces between Two Immiscible Electrolyte Solutions (ITIES) and artificial light-induced water-splitting is presented in the following thesis. This approach consists of two biphasic systems for the photo-production of H2 and O2. The following thesis focuses on the realisation of the H2 side. In first instance, the photo-induced hydrogen evolution reaction (HER) by decamethylruthenocene (Cp2Ru(II)) is reported as a strategy to facilitate water splitting in biphasic systems. Hydrogen evolution by Cp2Ru(II) was studied in detail. The study highlights that Cp2Ru(II) is an attractive molecule capable of photo-reducing hydrogen without the need for an additional sensitizer. Electrochemical, gas chromatographic and spectroscopic (UV/vis, 1H and 13C NMR) measurements indicate that the production of hydrogen occurs by a two-step process. First, the decamethylruthenocene hydride ([Cp2Ru(IV)(H)]+) is formed in the presence of acids, followed by the reduction of this complex via a hetero-dissociation reaction leading to a first release of hydrogen. Thereafter, the resultant decamethylruthenocenium ion ([Cp2Ru(III)]+) is further reduced leading to a second release of hydrogen by subtraction of a proton from a methyl group of [Cp2Ru(III)]+. Experimental results showed an excitation of [Cp2Ru(IV)(H)]+ at ï ¬ = 243 nm to evolve H2 for the first oxidation. [Cp2Ru(III)]+ was produced from the reduction of protons by Cp2Ru(II) at ï ¬ = 365 nm and electrochemically regenerated in situ on a Fluorinated Tin Oxide (FTO) electrode surface. A promising internal quantum yield of 25 % was obtained for HER by Cp2Ru(II) combined with electrochemical recycling. Thereafter, HER by Cp2Ru(II) was performed at ITIES. Shake-flask experiments demonstrated the production of H2 only when the biphasic system was positively polarized, to favor proton transfer. Kinetics/thermodynamics for decamethylruthenocene hydride formation were electrochemically evaluated at liquidÇ liquid interface. Simulated curves developed using COMSOL Multiphysics software and compared to experimental data, indicate a modified EC (electrochemicalâ chemical) mechanism for the [Cp2Ru(IV)(H)]+ formation at polarised interfaces. In the proposed pathway, [Cp2Ru(IV)(H)]+ is sufficiently stable in dichloroethane to transfer at negative potentials to the aqueous phase where it quickly dissociates. Additionally, the SHG response of [Cp2Ru(IV)(H)]+ as function of the polarisation applied confirmed this mechanism. Finally, an alternative method using homogeneous catalysts, Co(dmgh)2(py)Cl and Fe2(ï ­-SCH2C6H4CH2S)(CO)6 at liquidÇ liquid interfaces was investigated. Coupled with a sacrificial electron donor and a sensitizer, H2 production was achieved for b