Colloidal metal oxide particles loaded with synthetic catalysts for solar H-2 production

This discussion describes a study of a particulate semiconductor assembly consisting of a photoactive ruthenium dye-sensitised metal oxide nanoparticle loaded with a synthetic cobaloxime proton reduction catalyst (CoP). The colloidal system evolves H-2 during visible light illumination in pH neutral aqueous solution in the presence of a sacrificial electron donor. The ruthenium photosensitiser (RuP) to cobaloxime loading ratio is simple to vary and optimise and a range of metal oxide nanoparticles were tested. A maximum photoactivity was identified with TiO2 nanoparticles modified with CoP and RuP in a 2 : 1 ratio in the colloidal reaction mixture: visible light irradiation yielded 600 +/- 32 mmol H-2 h(-1) (g TiO2)(-1). A total turnover number of 108 +/- 9 mol H-2 (mol CoP)(-1), the evolution of 4340 +/- 240 mu mol H-2 (g TiO2)(-1) and approximately 87 mu mol H-2 (m(2) TiO2)(-1) were observed after 10 h irradiation. Linkage of the catalysts to TiO2 is critical for the system to work efficiently, and CoP and RuP contain one and two phosphonic acid linker moieties, respectively. The novel phosphonate ester analogue of CoP, [CoCl(dimethylglyoximato)(2)(diethyl pyridyl-4-phosphonate)] (1) was also synthesised and studied. Complex 1 adsorbs only to a small extent to TiO2 and a reduced H-2 production rate (182 +/- 8 mmol H-2 h(-1) (g TiO2)(-1)) was observed when 1 was irradiated with RuP-modified TiO2. Thus, the lower TiO2 affinity of 1 results in a reduced photoactivity of the dispersion. The described semiconductor particles are also presented in the light of being advantageous over more established homogenous multi-component systems and supramolecular dyad complexes: the reported semi-heterogeneous system is straightforward to assemble and it works in a purely aqueous environment.