Molecular Catalysts for Water Oxidation

Blakemore, James D.; Crabtree, Robert H.; Brudvig, Gary W.

doi:10.1021/acs.chemrev.5b00122

Cited by 1,055 publications

(886 citation statements)

References 378 publications

(608 reference statements)

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“…This implies that small changes in the systems such as orientation of the active groups, or pH, will drive a change in reaction mechanism. 60 The formation of Ti-O-O-Ti groups have previously been observed in the literature. Timeresolved diffuse reflectance FT-IR studies have detected the presence of surface peroxo and oxyhydroxo bonds on TiO2.…”

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confidence: 72%

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

et al. 2017

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ABSTRACT. It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this article, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent (TPC) and 2 transient absorption spectroscopy (TAS), we measure both the kinetics of electron extraction (t50% ~200 µs, 1.5 VRHE) and the kinetics of hole oxidation of water (t50% ~100 ms, 1.5 VRHE) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water splitting system. Photo-induced absorption spectroscopy (PIAS) measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photo-electrochemical water oxidation. Under these conditions we find that the surface density of these holes increases non-linearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is 3 rd order with respect to surface hole density, with a rate constant = 22 ± 2 nm 4 .s -1 . Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions the rate law was 2 nd order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20 -25 s -1 at ~1 sun irradiance). This showed that the rate limiting step does not involve OH -nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.TOC graphic

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Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

et al. 2017

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“…[23,168,169] Heterogeneity problems have also been well documented by Crabtreei nareview on the study of homogeneoust ransition-metal catalysis. [170] Various experimental controlm ethods have been suggested to solve this problem.…”

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confidence: 97%

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

Liao

Siegbahn

2017

ChemSusChem

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The design of efficient and robust water oxidation catalysts has proven challenging in the development of artificial photosynthetic systems for solar energy harnessing and storage. Tremendous progress has been made in the development of homogeneous transition‐metal complexes capable of mediating water oxidation. To improve the efficiency of the catalyst and to design new catalysts, a detailed mechanistic understanding is necessary. Quantum chemical modeling calculations have been successfully used to complement the experimental techniques to suggest a catalytic mechanism and identify all stationary points, including transition states for both O−O bond formation and O2 release. In this review, recent progress in the applications of quantum chemical methods for the modeling of homogeneous water oxidation catalysis, covering various transition metals, including manganese, iron, cobalt, nickel, copper, ruthenium, and iridium, is discussed.

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“…The TONs are > 5300 and are the largest ever reported for first row transition metal-based molecular catalysts (See Table S4 in the SI). 17 The low Faradaic efficiency is then likely due to graphene oxidation in parallel to water oxidation reaction in basic solutions. 7 Nevertheless, the molecular catalyst remains intact after catalysis as evidenced by CV and XAS spectroscopy.…”

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confidence: 99%

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets

et al. 2017

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A molecular water oxidation catalyst based on the copper complex of general formula [(Lpy)Cu II ] 2-, 2 2-, (Lpy is 4-pyrenyl-1,2-phenylenebis(oxamidate) ligand) has been rationally designed and prepared to support a more extended π-conjugation through its structure in contrast with its homologue, the [(L)Cu II ] 2-water oxidation catalyst, 1 2-(L is ophenylenebis(oxamidate)). The catalytic performance of both catalysts has been comparatively studied in homogeneous phase and in heterogeneous phase by π-stacking anchorage to graphene-based electrodes. In the homogeneous system, the electronic perturbation provided by the pyrene functionality translates into a 150 mV lower overpotential for 2 2-respect to 1 2-and an impressive increase in the kcat from 6 s -1 to 128 s -1 . Upon anchorage, π-stacking interactions with the graphene sheets provide further π-delocalization that improves the catalytic performance of both catalysts. In this sense, 2 2-turned out to be the most active catalyst due to the double influence of both the pyrene and the graphene, displaying an overpotential of 538 mV, a kcat of 540 s -1 and producing more than 5300 TONs.Heterogenized water-oxidation catalysis based on earth abundant transition metals, such as Mn, Fe, Co, Ni and Cu, are highly desired for sustainable energy technologies that exploit direct solar water-splitting. 1 An advantage of heterogenized homogeneous catalysts, when compared to heterogeneous catalysts, 2 is that they can be improved by ligand design. Yet first-row transition metal complexes pose several challenges. They usually get deactivated when immobilized on electrode surfaces and they suffer from instability due to hydrolytic behavior and decomposition into metal-oxides upon oxidation of the organic ligands. 3 However, from an engineering perspective, solid-state electroanodes are desired due to the simplicity of assembly for potential devices. Therefore, it is imperative to understand the influence of the anchoring functionality on the performance of the immobilized catalysts to learn how to anchor and stabilize functional molecular catalysts on electrode surfaces. 4,5 Here, we focus on water oxidation by Cu(II) molecular catalysts heterogenized on graphene surfaces.A family of copper complexes based on tetraamide ligands, such as [(L)Cu II ] 2-, 1 2-, (L = o-phenylenebis(oxamidate)) shown in Figure 1, have been recently reported to be effective at catalyzing oxygen evolution by water oxidation at basic pH. 6 Remarkably, the rate determining step (rds) was found to involve reversible oxidation of the phenyl ring. Here, we explore whether the catalytic properties of these complexes can be manipulated by electronic perturbation of the tetraamide π-system, either by modification of the ligand or by π-stacking to graphitic electrode surfaces.

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Molecular Catalysts for Water Oxidation

Cited by 1,055 publications

References 378 publications

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets

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Molecular Catalysts for Water Oxidation

Cited by 1,055 publications

References 378 publications

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets

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Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO₂ Photoanode: A Rate Law Analysis