Mechanism of Water Oxidation by Single-Site Ruthenium Complex Catalysts

Concepcion, Javier J.; Tsai, Ming Kang; Muckerman, James T.; Meyer, Thomas J.

doi:10.1021/ja904906v

Cited by 453 publications

(610 citation statements)

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“…The mechanism, established earlier for single-site molecular catalysts (12,13), is shown in Scheme 2 and reenters the catalytic cycle. In Scheme 1, "RDS" denotes the rate-determining step.…”

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confidence: 60%

Solar water splitting in a molecular photoelectrochemical cell

Alibabaei

Brennaman

Norris

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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211

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Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core-shell nanostructured photoanode with the core a high surface area conductive metal oxide film--indium tin oxide or antimony tin oxide--coated with a thin outer shell of TiO 2 formed by atomic layer deposition. A "chromophore-catalyst assembly" 1, [(PO 3 H 2 ) 2 bpy) 2 Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH 2 )] 4+ , which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO 2 shell. Visible photolysis of the resulting core-shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.P hotosynthesis uses the energy of the sun with water as the reducing agent to drive the reduction of carbon dioxide to carbohydrates with oxygen as a coproduct through a remarkably complex process. At photosystem II, a subsystem imbedded in the thylakoid membrane where O 2 is produced, light absorption, energy migration, electron transfer, proton transfer, and catalysis are all used in multiple stepwise chemical reactions which are carefully orchestrated at the molecular level (1, 2).Photosynthesis solves the problem of energy storage by biomass production but with low solar efficiencies, typically <1%. In artificial photosynthesis with solar fuels production, the goal is similar but the targets are either hydrogen production from water splitting, Eq. 1, or reduction of carbon dioxide to a carbon-based fuel, Eq. 2 (3, 4). Different strategies for solar fuels have evolved (5, 6). In one, direct bandgap excitation of semiconductors creates electron-hole pairs which are then used to drive separate halfreactions for water oxidation (2H 2 O → O 2 + 4H + + 4e − ) and water/proton reduction (2H + + 2e − → H 2 ) (7-9).Here, we report a hybrid strategy for solar water splitting, the dye sensitized photoelectrosynthesis cell (DSPEC). It combines the electron transport properties of semiconductor nanocrystalline thin films with molecular-level reactions (10). In this approach, a chromophore-catalyst molecular assembly acts as both light absorber and catalyst. It is bound to the surface of a "core-shell," nanostructured, transparent conducting oxide film. The core structure consists of a nanoparticle film of either tin-doped indium oxide (nanoITO), or antimony-doped tin oxide (nanoATO), deposited on a fluoride-doped tin oxide (FTO) glass substrate. The shell consists of a conformal TiO 2 nanolayer applied by atomic layer deposition (ALD). The resulting "photoanode," where water oxidation occurs, is connected to a Pt cathode for proton reduction to complete the water splitting cell. A diagram for the photoanode in the DSPEC device is shown in Fig. 1. It illustrates...

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confidence: 60%

Solar water splitting in a molecular photoelectrochemical cell

Alibabaei

Brennaman

Norris

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

211

232

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“…2 is initiated by accumulating multiple oxidative equivalents at the catalyst through stepwise proton coupled electron transfer (PCET) reactions, Ru II - (22,25,41).…”

Section: Ru-oh 2 2+mentioning

confidence: 99%

“…Retention of electrocatalytic reactivity on the surface is demonstrated and water oxidation catalysis investigated over a wide pH range. Clear evidence is found in these studies that added proton acceptor bases enhance the kinetic pathways in the key, rate-limiting step (O-O bond formation) via an atom-proton transfer (APT) mechanism (22,35). In addition, a facile pathway has been identified with direct attack by OH − on an activated oxo form of the catalyst with rate enhancements of up to 10 6 for water oxidation.…”

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confidence: 96%

“…Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16)(17)(18)(19)(20)(21)(22)(23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.…”

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confidence: 99%

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Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Vannucci

Alibabaei

Losego

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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127

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Enhancing the surface binding stability of chromophores, catalysts, and chromophore-catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1-12) by atomic layer deposition of an overlayer of TiO 2 . Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases −H 2 PO − 4 /HPO 2− 4 , B(OH) 3 /B(OH) 2 O − , HPO 2− 4 /PO 3− 4 − and with a pathway identified in which O-atom transfer to OH − occurs with a rate constant increase of 10 6 compared to water oxidation in acid.electrocatalysis | surface stabilization H eterogeneous catalysis plays an important role in industrial chemical processing, fuel reforming, and energy-producing reactions. Examples include the Haber-Bosch process, steam reforming, Ziegler-Natta polymerization, and hydrocarbon cracking (1-8). Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16-23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.Procedures are available for immobilization of organometallic and coordination complexes on the surfaces of solid supports. Common strategies include surface derivatization of metal oxides by carboxylate, phosphonate, and siloxane bindings (24-27), carbongrafted electrodes (28-30), and electropolymerization (31-33). These approaches provide a useful bridge to the interface and a way to translate mechanistic understanding and ease of synthetic modification of solution catalysts to heterogeneous applications with a promise of higher reactivity under milder conditions. A significant barrier to this approach arises from the limited stability of surface binding. Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic...

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“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

et al. 2017

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In this work we describe the synthesis, structural characterization and redox properties of two Ru complexes containing the dianionic potentially pentadentate [2,2':6',2''-terpyridine]-6,6''-dicarboxylate (tda 2-) ligand that coordinates Ru at the equatorial plane and with additional pyridine or dmso acting as monondentate ligand in the axial positions: [Ru II (tda--N 3 O)(py)(dmso)], 1 II and [Ru III (tda--N 3 O 2 )(py)(H2O) ax ] + , 2 III (H2O) + . Complex 1 II has been characterized by single crystal XRD in the solid state and in solution by NMR spectroscopy. The redox properties of 1 II and 2 III (H2O) + have been thoroughly investigated by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Complex 2 II (H2O) displays poor catalytic activity with regard to the oxidation of water to dioxygen and its properties have been analyzed based on foot of the wave analysis (FOWA) and catalytic Tafel plots. The activity of 2 II (H2O) has been compared with related water oxidation catalysts (WOCs) previously described in the literature. Despite its moderate activity, 2 II (H2O) constitutes the cornerstone that has triggered the rationalization of the different factors that govern overpotentials as well as efficiencies in molecular water oxidation catalysts (WOCs). The present work uncovers the interplay between different parameters namely, coordination number, number of anionic groups bonded to the first coordination sphere of the metal center, water oxidation catalysis overpotential, pKa and hydrogen bonding and the performance of a given WOC. It thus establishes the basic principles for the design of efficient WOCs operating at low overpotentials.

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Mechanism of Water Oxidation by Single-Site Ruthenium Complex Catalysts

Cited by 453 publications

References 50 publications

Solar water splitting in a molecular photoelectrochemical cell

Solar water splitting in a molecular photoelectrochemical cell

Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

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