Colloidal metal oxide particles loaded with synthetic catalysts for solar H2 production

Abstract

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 H₂ 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 μmol H₂ h⁻¹ (g TiO₂)⁻¹. A total turnover number of 108 ± 9 mol H₂ (mol CoP)⁻¹, the evolution of 4340 ± 240 μmol H₂ (g TiO₂)⁻¹ and approximately 87 μmol H₂ (m² 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 H₂ production rate (182 ± 8 μmol H₂ 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.