A Nickel Thiolate Catalyst for the Long‐Lived Photocatalytic Production of Hydrogen in a Noble‐Metal‐Free System

Abstract

The splitting of water through artificial photosynthesis (AP) is a key transformation toward the conversion of solar energy into stored chemical potential in the form of fuel and oxidizer.[1] For water splitting, the reductive side of the reaction involves the light-driven conversion of aqueous protons into H 2 . To perform this half-reaction, photocatalytic systems typically consist of a catalyst, photosensitizer (PS), and sacrificial electron donor.[2] Recent studies on noblemetal-based [3] and noble-metal-free [4] homogeneous systems for light-driven hydrogen production have shown high activity. However, significant problems in the noble-metalfree molecularly based systems include relatively low catalyst turnover numbers (TON < 500 mole H 2 /mole catalyst) for hydrogen formation, and photodecomposition of the systems within a few hours. For most organic dye based systems that have recently been reported, the photochemical quenching step of the excited-state dye (PS*) is reductive, thus leading to unstable PS À radical anions that undergo decomposition.[4b]Thus, the development of more active catalysts, specifically ones that quench PS* oxidatively, would be of great value for obtaining long-lived homogeneous AP systems. Herein, we describe a new homogenous catalyst for H 2 production that has both high activity and the ability to oxidatively quench PS*, thus leading to a much longer system lifetime. Nickel cathodes are used in commercial electrolyzers, suggesting that nickel may be a worthwhile basis for homogeneous catalysts as well.[5] Nickel thiolate complexes have received special attention in recent years because sulfurligated nickel complexes mimic the [Fe-Ni]-hydrogenase active site, [6] and dimeric metal complexes based on nickel thiolate hydrides have been shown to be catalytically active for proton reduction.[7] DuBois and co-workers have also shown that mononuclear nickel(II) bis(diphosphine) complexes are effective catalysts for electrochemical hydrogen generation.[8] While photocatalytic hydrogen generation from the nickel-phosphine complexes is long-lived, the activity of the photocatalytic system with the nickel phosphine catalyst is low, with a turnover frequency (TOF) of approximately 20 equivalents of H 2 per hour.[9] Related nickel(II) complexes containing pyridine-2-thiolate ligands have been known for over two decades, [10] but their catalytic properties for proton reduction have not been reported. In the present study, the complex [Ni(pyS) 3 ] À (1 À ; pyS = pyridine-2-thiolate) is found to have impressive catalytic activity for the photocatalytic production of H 2 in a homogeneous system with fluorescein (Fl) as the PS and triethylamine (TEA) as the sacrificial electron donor.Photolysis of a solution of Fl and 1 À in EtOH/H 2 O (1:1) using a green-light-emitting diode (LED) (l = 520 nm, 0.12 W) at 15 8C results in H 2 generation which was monitored in real time by the pressure change in the reaction vessel, and quantified at the end of the photolysis by GC analysis of the headspace gase...

Nickel Complex with Internal Bases as Efficient Molecular Catalyst for Photochemical H2 Production

“…According to recent reports, as a PS, Fl 2À displays a higher activity and better stability than other halogen-substituted xanthene dyes such as EY 2À and Rose Bengal (RB 2À ) for photochemical H 2 production in basic aqueous solutions with pH > 10, [25][26][27] whereas it is inactive or exhibits a low activity in aqueous solutions with pH < 9.…”

A cobalt-based polyoxometalate catalyst for efficient visible-light-driven H2 evolution from water splitting

“…The photosensitizer excited state can function as either an oxidant or reductant, and thus can be quenched by an electron donor or an acceptor [19,20]. To investigate the quenching mechanism of our system, the luminescence of EY has been measured in the presence of sample …”

General Scheme for Oxidative Quenching of a Copper Bis‐Phenanthroline Photosensitizer for Light‐Driven Hydrogen Production

“…[9] To make a water splitting system commercially viable, these rare and expensive noble metals should be replaced by cheaper and more abundant materials, as was already demonstrated in the case of Eosin Y, [10] fluorescein, [11] Al-, [12] Zn- [13] and Sn-porphyrins [14] as well as quantum dots. [15] Copper based chromophores, in particular copper(I) bis-1,10-phenanthroline complexes represent another very attractive alternative.…”