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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]. |
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