BIOMIMETIC APPROACHES TO THE CONSTRUCTION OF SUPRAMOLECULAR CATALYSTS - TITANIUM-DIOXIDE PLATINUM ANTENNA CATALYSTS TO REDUCE WATER USING VISIBLE-LIGHT

Thermally and photochemically stable catalysts in systems that convert the energy of visible light into the chemical energy of dihydrogen gas have yet to be fully developed and all proposed systems suffer, to varying extents, from loss of activity with use. We have attempted to model nature's photochemical reaction center through the use of TiO2 antenna catalysts and very small amounts of Pt that form active centers on the TiO2 semiconductor surface. This system is relatively inexpensive, reproducible, extremely stable, efficient in conversion of light to dihydrogen in aqueous solutions and displays higher linearity in dihydrogen evolution with greater efficiency in recycling than do other previously reported catalysts. Here we report the self-hydrogenation properties of our TiO2-Pt catalysts in comparison with well known Pt, Ru and Os catalysts as well as their catalytic efficiency in systems for the sacrificial photoreduction of water. The sensitizers employed in this study were a recently synthesized new class of bis-heteroleptic complexes [Ru(bpy)2(PP)]2+, [Ru(bpy)2(PPB)]2+, [Ru(bpy)2(PPB-pCl)]2+ containing one pyridyl-pyrimidine ligand and [Ru(bpy)3]2+ and [Ru(bpz)3]2+ as well known standard sensitizers. In order to obtain maximum information about the TiO2-Pt catalysts we varied the components and conditions in our sacrificial systems for water reduction: MV2+, PVS MPVS were used as electron relays, EDTA and TEOA as sacrificial electron donors. Furthermore, the dependence of the dihydrogen evolution rates on the pH of the photolysis solution and its ionic strength were investigated. The techniques used in this study are cyclic voltammetry, steady state and time resolved luminescence spectroscopy, Stern-Volmer quenching, measurements of the quantum yields for PVS reduction, and volumetric measurements of dihydrogen photoproduction. A linear dependence was found between the dihydrogen production rate of the TiO2-Pt catalysts and the quantum yield for the MV2+ and PVS photoreduction at low ionic strength, showing that the new catalysts are reduced by the electron relays and convert protons into dihydrogen in a highly efficient manner.