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The molecular structure of the catalyst strongly influences the kinetics of charge separation and recombination., The photoinduced reduction of three Co electrocatalysts immobilised on TiO2 is 104 times faster than the reverse charge recombination. Both processes show an exponential dependence on the distance between the semiconductor and the catalytic core.

A visible-light driven H(2) evolution system comprising of a Ru(II) dye (RuP) and Co(III) proton reduction catalysts (CoP) immobilised on TiO(2) nanoparticles and mesoporous films is presented. The heterogeneous system evolves H(2) efficiently during visible-light irradiation in a pH-neutral aqueous solution at 25 °C in the presence of a hole scavenger. Photodegradation of the self-assembled system occurs at the ligand framework of CoP, which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H(2) (mol CoP)(-1) and above 200,000 mol H(2) (mol TiO(2) nanoparticles)(-1) in water. Our studies support that a molecular Co species, rather than metallic Co or a Co-oxide precipitate, is responsible for H(2) formation on TiO(2). Electron transfer in this system was studied by transient absorption spectroscopy and time-correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO(2) in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long-lived TiO(2) conduction band electron is formed with a half-lifetime of approximately 0.8 s. Electron transfer from the TiO(2) conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge-recombination with the oxidised RuP. This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors.

Sunlight provides us with a practically endless amount of carbon-neutral energy and there is currently great interest in harvesting some of this electromagnetic energy for the production of a solar fuel. An attractive approach is sunlight-driven water splitting, where renewable H2 and the by-product O2 are released simultaneously from water during irradiation. An economical H2-evolution catalyst must not only contain inexpensive materials, but must also be active under O2 during turnover, because a real-world device will be exposed to atmospheric O2 and produce O2 in situ as a result of water splitting. Noble-metal catalysts such as platinum are excellent H2evolution catalysts, but they are expensive and show crossselectivity for the reduction of O2. [3] In biology, the [FeFe]and [NiFe]-hydrogenases are H2-evolution catalysts that work at high rates and small over-potentials. However, isolated hydrogenases are fragile and often extremely sensitive towards O2. [5] Many significant advances were reported recently in the development of small-molecule H2-evolution catalysts. Nevertheless, there has been very little progress in the development of homogeneous H2-generating catalysts that operate in the presence of O2. A noteworthy exception is an expensive rhodium catalyst that operates under O2. [7] To the best of our knowledge, there are no reports of smallmolecule 3d transition-metal catalysts that reduce protons efficiently under high levels of O2. Herein, we report on an inexpensive cobalt catalyst that evolves H2 electroand photocatalytically in pH-neutral water and in the presence of atmospheric O2. We employ (Et3NH)[Co Cl(dimethylglyoximato)2(pyridyl-4-hydrophosphonate)] ((Et3NH)[CoP], Scheme 1), which is a member of cobaloxime-type catalysts that currently receive much attention in electrochemical and photochemical applications. The phosphonic acid group in (Et3NH)[CoP] enables the complex to dissolve in water and allows for its immobilization on metal oxide surfaces for heterogeneous applications. First, we studied the electrocatalytic activity of the complex anion [CoP] in a pH-neutral electrolyte solution in the absence and presence of air (21% O2 in N2). Cyclic voltammetry (CV) scans were recorded in a three-electrode cell with (Et3NH)[CoP] (1 mm) on a glassy carbon working electrode in an aqueous solution of triethanolamine (TEOA) and Na2SO4 (0.1m each) at pH 7 and 25 8C. A cathodic wave at Ep= 0.13 V versus the normal hydrogen electrode (NHE), assigned to the Co!Co reduction process, and reduction of Co with the onset of electrocatalytic proton reduction at approximately 0.55 V versus NHE was observed at a scan rate of 100 mVs 1 under a N2 atmosphere (Figure 1a, dashed Scheme 1. Chemical structures of the cobalt catalyst [CoP] , the organic dye eosin Y (EY), and the ruthenium dye [RuP].

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 micromol H2 h(-1) (g TiO2)(-1). A total turnover number of 108 +/- 9 mol H2 (mol CoP)(-1), the evolution of 4340 +/- 240 micromol H2 (g TiO2)(-1) and approximately 87 micromol 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 micromol 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.

We report on a self-assembled system comprising of a molecular H(2) production cobalt catalyst attached on a ruthenium dye-sensitised TiO(2) nanoparticle. Visible light irradiation of the dispersed nanoparticles in the presence of the sacrificial electron donor triethanolamine produces H(2) photocatalytically in pH neutral water and at room temperature.