Transition Metal Catalysts Anchored to TiO2 Nanoparticles by Functionalised Bipyridine Ligands for the Purpose of CO2 reduction

The overarching goal of this thesis was to enhance the efficiency of photocatalytic transition metal complexes in their ability to reduce CO2 to CO by anchoring them to TiO2 surfaces. Four rhenium tricarbonyl complexes are synthesised starting with the modification of bipyridine ligands to contain -PO3OH or -COOH anchoring groups.
Re(dcbpy)(CO)3Cl was anchored successfully to TiO2 nanoparticles with a maximum absorbance capacity of 35.5 mg g-1. The hybrid could reduce CO2 to CO under irradiation at 365 or 456 nm in a mixture of DMF and TEOA, in a home-build set-up using LEDs. The production of CO was monitored using gas chromatography.
The turn-over numbers and turn-over frequencies were determined for all studied systems. A turn-over number of 183 ± 10.22 was achieved using the hybrid at 365 nm. When using just Re(dcbpy)(CO)3Cl at the same wavelength in the reaction mixture without Ru(bpy)3(PF6)2 as a photosensitiser the maximum value was 24.46 ± 0.14. For the same experiment using the 456 nm LED, these values were 166.67 ± 13.5 and 3.64 ± 0.04 for the anchored and, unanchored, photosensitiser free mixture, respectively. Electron relay using the conduction band of TiO2 was demonstrated through the inability to produce CO with the hybrid when using the 456 nm LED without a photosensitiser.
Three new photosensitising complexes containing osmium and ruthenium centres were prepared using a 1,2,3’-triazole organic bridging ligand and characterised by 1H NMR spectroscopy and mass-spectrometry. Using absorption spectroscopy, emission spectroscopy, and emission lifetime studies it has been shown they are viable as near-IR photosensitisers due to strong absorption in the red part of the visible spectrum, and have comparatively long, hundreds of nanoseconds, excited state lifetimes. Electrochemical measurements confirmed osmium centres maintain their high spin orbit coupling due to more negative oxidation potentials compared to their ruthenium analogues.
Finally, in a step towards the development of light-absorbing electrodes containing complex catalysts, an electrode was fabricated using two techniques, electron beam evaporation and doctor blade method. This gave a sandwich type structure, consisting of a thin film, titanium, or chromium on glass covered with a TiO2 nanoparticulate paste. The surfaces were characterised using Atomic Force Microscopy. Preliminary cyclic voltammetry experiments using the glass/chromium/TiO2 construct confirmed it can function as an electrode.