Yurii V. Geletii

A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals

Bulking Up Water Oxidation Storing solar energy by water oxidation, in a process akin to photosynthesis, is a promising approach for building a renewable energy infrastructure. Unfortunately, many of the most active synthetic catalysts for this process fall prey to degradation by the generated oxygen. Yin et al. (p. 342, published online 11 March; see the Perspective by Hurst) used bulky polyoxometalate ligands to protect a catalytic cobalt center from this fate. The full complex was easily prepared by mixing proper ratios of inexpensive tungsten, cobalt, and phosphate salts in boiling water. After isolating and redissolving the catalyst in slightly basic aqueous solution, rapid oxygen generation was observed with a ruthenium-based oxidant. Bulky polytungstate ligands stabilize a cobalt-based catalyst highly active for splitting water. Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10–, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2′-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 production ≥5 s−1 at pH = 8. The rate’s pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10– is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.

Homogeneous light-driven water oxidation catalyzed by a tetraruthenium complex with all inorganic ligands.

A totally homogeneous, molecular, visible-light-driven water oxidation system is reported. The three system components are (i) a water oxidation catalyst, 1 (a Ru(IV)(4)O(4) cluster stabilized by oxidatively resistant [SiW(10)O(32)](8-) ligands); (ii) a photosensitizer, [Ru(bpy)(3)](2+); and (iii) a sacrificial electron acceptor, S(2)O(8)(2-). Dioxygen is formed rapidly with an initial turnover frequency of approximately 8 x 10(-2) s(-1) and an estimated quantum yield (defined as the number of O(2) molecules formed per two photons absorbed) of approximately 9%.

Efficient Light-Driven Carbon-Free Cobalt-Based Molecular Catalyst for Water Oxidation

The abundant-metal-based polyoxometalate complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is a hydrolytically and oxidatively stable, homogeneous, and efficient molecular catalyst for the visible-light-driven catalytic oxidation of water. Using a sacrificial electron acceptor and photosensitizer, it exhibits a high (30%) photon-to-O(2) yield and a large turnover number (>220, limited solely by depletion of the sacrificial electron acceptor) at pH 8. The photocatalytic performance of this catalyst is superior to that of the previously reported precious-metal-based polyoxometalate water oxidation catalyst [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)](10-).

An Exceptionally Fast Homogeneous Carbon-Free Cobalt-Based Water Oxidation Catalyst

An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Structural, Physicochemical, and Reactivity Properties of an All-Inorganic, Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

Differentiating homogeneous and heterogeneous water oxidation catalysis: confirmation that [Co4(H2O)2(α-PW9O34)2]10- is a molecular water oxidation catalyst.

Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) [Co4(H2O)2(PW9O34)2](10-) (Co4POM), initial reports of an efficient, molecular catalyst have been challenged by studies suggesting that formation of cobalt oxide (CoOx) or other byproducts are responsible for the catalytic activity. Thus, we describe a series of experiments for thorough examination of active species under catalytic conditions and apply them to Co4POM. These provide strong evidence that under the conditions initially reported for water oxidation using Co4POM (Yin et al. Science, 2010, 328, 342), this POM anion functions as a molecular catalyst, not a precursor for CoOx. Specifically, we quantify the amount of Co(2+)(aq) released from Co4POM by two methods (cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry) and show that this amount of cobalt, whatever speciation state it may exist in, cannot account for the observed water oxidation. We document that catalytic O2 evolution by Co4POM, Co(2+)(aq), and CoOx have different dependences on buffers, pH, and WOC concentration. Extraction of Co4POM, but not Co(2+)(aq) or CoOx into toluene from water, and other experiments further confirm that Co4POM is the dominant WOC. Recent studies showing that Co4POM decomposes to a CoOx WOC under electrochemical bias (Stracke and Finke, J. Am. Chem. Soc., 2011, 133, 14872), or displays an increased ability to reduce [Ru(bpy)3](3+) upon aging (Scandola, et al., Chem. Commun., 2012, 48, 8808) help complete the picture of Co4POM behavior under various conditions but do not affect our central conclusions.

Cs(9)[(gamma-PW(10)O(36))(2)Ru(4)O(5)(OH)(H(2)O)(4)], a new all-inorganic, soluble catalyst for the efficient visible-light-driven oxidation of water.

The tetraruthenium-substituted polyoxometalate Cs(9)[(gamma-PW(10)O(36))(2)Ru(4)O(5)(OH)(H(2)O)(4)] was synthesized and structurally, spectroscopically and electrochemically characterized; it was shown to be a catalyst for visible-light-induced water oxidation.

A nickel containing polyoxometalate water oxidation catalyst

A new pentanickel silicotungstate complex, K(10)H(2)[Ni(5)(OH)(6)(OH(2))(3)(Si(2)W(18)O(66))]·34H(2)O (KH-), has been synthesized and characterized by X-ray crystallography and several other methods. Dynamic light scattering, kinetics and other experiments confirm that in the presence of [Ru(bpy)(3)](2+) (the photosensitizer for light-driven water oxidations) and [Ru(bpy)(3)](3+) (the oxidant in the dark water oxidations) exists in an equilibrium between solution (soluble) and a [Ru(bpy)(3)](n+)- complex (minimally soluble) form. This new pentanickel polyoxometalate catalyzes efficient water oxidation in both the dark and on irradiation with 455 nm LED light with 1.0 mM [Ru(bpy)(3)](2+) photosensitizer and 5.0 mM Na(2)S(2)O(8), sacrificial electron acceptor. Four lines of evidence indicate that in this solution [symbol:see text] Ru(bpy)(3)](n+)- complex equilibrium remains molecular and does not decompose to nickel hydroxide particles.

Structurally Characterized Iridium(III)-Containing Polytungstate and Catalytic Water Oxidation Activity

The first structurally characterized iridium-substituted polyoxometalate, K(14)[(IrCl(4))KP(2)W(20)O(72)].23H(2)O [1; orthorhombic Pnnm, with a = 18.6546(7) A, b = 19.5192(6) A, c = 14.8670(5) A, V = 5413.4(3) A(3), and Z = 2, final R = 0.0730], is reported. Elemental analysis, X-ray crystallography, and NMR all indicate one Ir atom in each molecule, while IR and electronic absorption spectroscopy, thermal gravimetric analysis, and electrochemistry all indicate its purity in both solid and solution states. Complex 1 is a molecular model of iridium supported on redox-active metal oxides, and aqueous solutions of 1 catalyze oxidation of water to O(2).

Graphene-supported [{Ru4O4(OH)2(H2O)4}-(gamma-SiW10O36)2]10- for highly efficient electrocatalytic

The electrochemistry of the Ru-containing polyoxometalate (POM) water oxidation molecular catalyst, Rb8K2[{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10− (Rb8K2-1), has been studied by cyclic and rotating disk electrode voltammetric methods in aqueous media under acidic and neutral pH conditions using bare glassy carbon (GC) and graphene modified GC electrodes. High concentrations of supporting electrolyte are needed in neutral pH conditions to overcome the electrical double layer effect associated with the highly negatively charged 1. Complex 1 can be confined within the highly porous wet graphene film to form a stable modified electrode, which shows excellent catalytic activity and high stability toward the water oxidation reaction under neutral pH conditions, particularly in the presence of 1.0 M Ca(NO3)2. The catalytic activity of the graphene supported 1electrode is nearly two orders of magnitude higher than that reported with a polymer coated multiwalled carbon nanotube supported 1electrode when both are employed at a moderate overpotential of 0.35 V.