Published on Web 05/12/2009
Homogeneous Light-Driven Water Oxidation Catalyzed by a Tetraruthenium Complex with All Inorganic Ligands Yurii V. Geletii, Zhuangqun Huang, Yu Hou, Djamaladdin G. Musaev, Tianquan Lian,* and Craig L. Hill* Department of Chemistry and the Cherry L. Emerson Center for Scientific Computation, Emory UniVersity, Atlanta, Georgia 30322 Received February 22, 2009; E-mail:
[email protected];
[email protected] Providing renewable pollution-free fuels is one of the most important scientific challenges of the 21st century, and in this context, the direct, efficient, sustained sunlight-driven splitting of water into H2 and O2 remains one of the most desirable targets. Development of O2 evolution/H2O oxidation catalysts (WOCs) in artifical photosynthesis (AP) schemes [comprising coupled visiblelight photosensitizers, H2 evolution catalysts (HECs), and WOCs] has frequently been success-limiting, despite significant progress on heterogeneous1-4 and homogeneous5-13 WOCs. Recently, H2O oxidation by [Ru(bpy)3]3+ (eq 1)14 and Ce(IV)15 catalyzed by the tetraruthenium polyoxometalate complex [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (1) has been reported. This complex exhibits the stability advantages of heterogeneous catalysts with the tunability and other advantages of homogeneous catalysts. In this work, we demonstrate that 1 catalyzes efficient water oxidation in a totally homogeneous visible-light-driven AP system at neutral (physiological) pH (Scheme 1).
4[Ru(bpy)3]3+ + 2H2O f 4[Ru(bpy)3]2+ + O2 + 4H+ (1)
Figure 1. Kinetics of O2 formation (O) and persulfate consumption (4) in
the Scheme 1 photocatalytic system. Conditions: Xe lamp, 420-520 nm bandpass filter, 50 mW light beam with a diameter of ∼1.5 cm focused on the reaction solution, 1.0 mM [Ru(bpy)3]2+, 5.0 mM Na2S2O8, 5.0 µM 1, 20 mM sodium phosphate buffer (initial pH 7.2), total reaction volume 8 mL, vigorous agitation using a magnetic stirrer. Scheme 1. Light-Induced Catalytic Water Oxidation by Tetraruthenium Polyoxometalate 1 Using [Ru(bpy)3]2+ as a Photosensitizer and Persulfate as a Sacrificial Electron Acceptor
Figure 1 plots the kinetics of O2 formation and persulfate (S2O82-) consumption catalyzed by 1 (the net reaction is eq 2). The quantum efficiency (defined as the number of molecules of O2 formed per two absorbed photons) is ∼9%, which, to our knowledge, is among the highest reported for photocatalytic water oxidation using molecular catalysts.6
2S2O82- + 2H2O + 2hν f 4SO42- + O2 + 4H+
(2) 3+
In the photocatalytic system reported here, [Ru(bpy)3] is generated from [Ru(bpy)3]2+ (λmax ) 454 nm, ε ) 1.4 × 104 M-1 cm-1) by photooxidation using S2O82- as a sacrificial electron acceptor. This process has been well-studied and is believed to proceed via S2O82- quenching of the visible-light-accessible metalto-ligand charge-transfer excited state, [Ru(bpy)3]2+*.16 The products, [Ru(bpy)3]3+ and SO4•- [E°(SO4•-/SO42-) ≈ 2.4 V]17 are both strong oxidants, and the latter oxidizes [Ru(bpy)3]2+ to form a second [Ru(bpy)3]3+.18 The absorption of two photons and the consumption of 2 equiv of S2O82- generates four [Ru(bpy)3]3+ (eq 3), sequentially oxidizing 1, which in turn oxidizes H2O to O2 (eq 1) and regenerates [Ru(bpy)3]2+ (net reaction in eq 2 via Scheme 1).
4[Ru(bpy)3]2+ + 2S2O82- + 2hν f 4[Ru(bpy)3]3+ + 4SO42(3) The photocatalytic system was evaluated under the experimental conditions described in Figure 1. Dioxygen was formed quickly under visible-light illumination (420-520 nm), while persulfate was 7522
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J. AM. CHEM. SOC. 2009, 131, 7522–7523
consumed. A gradual decrease in pH from 7.2 to 6.3 and a gradual
