Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO2 Photoanode for Water Oxidation in Acid.

We constructed a photoanode comprising the homogeneous water oxidation catalyst (WOC) Na8K8[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (Co9POM) and nanoporous n-type TiO2 photoelectrodes (henceforth "TiO2-Co9POM") by first anchoring the cationic 3-aminopropyltrimethoxysilane (APS) ligand on a metal oxide light absorber, followed by treatment of the metal oxide-APS with a solution of the polyoxometalate WOC. The resulting TiO2-Co9POM photoelectrode exhibits a 3-fold oxygen evolution photocurrent enhancement compared to bare TiO2 in aqueous acidic conditions. Three-element (Co 2p, W 4f, and O 1s) X-ray photoelectron spectroscopy and Raman spectroscopy studies before and after use indicate that surface-bound Co9POM retains its structural integrity throughout all photoelectrochemical water oxidation studies reported here. Extensive charge-transfer mechanistic studies by photoelectrochemical techniques and transient absorption spectroscopy elucidate that Co9POM serves as an efficient WOC, extracting photogenerated holes from TiO2 on the picosecond time scale. This is the first comprehensive mechanistic investigation elucidating the roles of polyoxometalates in POM-photoelectrode hybrid oxygen evolution reaction systems.

Role of Multiple Vanadium Centers on Redox Buffering and Rates of Polyvanadomolybdate-Cu(II)-Catalyzed Aerobic Oxidations.

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0-4 and 6) in TBA salts of PVxMo12-xO40(3+x)- (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s-1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (-236 and -405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10-4 s-1 and 0.036 s-1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer.

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) and (b) Cu(ClO4)2 in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 102 in less than one hour with a turnover frequency (TOF) of 6 × 10-2 s-1 with respect to PV6Mo6. Multiple electrochemical, spectroscopic, and other methods establish that (1) PV6Mo6, a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu(II)/Cu(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV6Mo6 and reoxidation of reduced PV6Mo6 by O2; and (3) the analogous polytungstate-based system, TBA4H5[PW6V6O40] (PV6W6), has nearly identical cyclic voltammograms (CV) as PV6Mo6 but has almost no catalytic activity: it does not exhibit self-redox buffering.

Structurally Precise Two-Transition-Metal Water Oxidation Catalysts: Quantifying Adjacent 3d Metals by Synchrotron X-Radiation Anomalous Dispersion Scattering.

Mixed 3d metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the locations and percent occupancies of different 3d metals in these heterogeneous catalysts. Without such information, it is hard to quantify catalysis, stability, and other properties of the WOC as a function of the catalyst active site structure. This study combines the site selective synthesis of a homogeneous WOC with two adjacent 3d metals, [Co2Ni2(PW9O34)2]10- (Co2Ni2P2) as a tractable molecular model for CoNi oxide, with the use of multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS) that quantifies both the location and percent occupancy of Co (∼97% outer-central-belt positions only) and Ni (∼97% inner-central-belt positions only) in Co2Ni2P2. This mixed-3d-metal complex catalyzes water oxidation an order of magnitude faster than its isostructural analogue, [Co4(PW9O34)2]10- (Co4P2). Four independent and complementary lines of evidence confirm that Co2Ni2P2 and Co4P2 are the principal WOCs and that Co2+(aq) is not. Density functional theory (DFT) studies revealed that Co4P2 and Co2Ni2P2 have similar frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multistep mechanisms, including likely Co-OOH formation, with the energetics of most steps being lower for Co2Ni2P2 than for Co4P2. Synchrotron XRAS should be generally applicable to active-site-structure-reactivity studies of multi-metal heterogeneous and homogeneous catalysts.

A solvent-free solid catalyst for the selective and color-indicating ambient-air removal of sulfur mustard.

Bis(2-chloroethyl) sulfide or sulfur mustard (HD) is one of the highest-tonnage chemical warfare agents and one that is highly persistent in the environment. For decontamination, selective oxidation of HD to the substantially less toxic sulfoxide is crucial. We report here a solvent-free, solid, robust catalyst comprising hydrophobic salts of tribromide and nitrate, copper(II) nitrate hydrate, and a solid acid (NafionTM) for selective sulfoxidation using only ambient air at room temperature. This system rapidly removes HD as a neat liquid or a vapor. The mechanisms of these aerobic decontamination reactions are complex, and studies confirm reversible formation of a key intermediate, the bromosulfonium ion, and the role of Cu(II). The latter increases the rate four-fold by increasing the equilibrium concentration of bromosulfonium during turnover. Cu(II) also provides a colorimetric detection capability. Without HD, the solid is green, and with HD, it is brown. Bromine K-edge XANES and EXAFS studies confirm regeneration of tribromide under catalytic conditions. Diffuse reflectance infrared Fourier transform spectroscopy shows absorption of HD vapor and selective conversion to the desired sulfoxide, HDO, at the gas-solid interface.

Multi-Tasking POM Systems.

Polyoxometalate (POM)-based materials of current interest are summarized, and specific types of POM-containing systems are described in which material facilitates multiple complex interactions or catalytic processes. We specifically highlight POM-containing multi-hydrogen-bonding polymers that form gels upon exposure to select organic liquids and simultaneously catalyze hydrolytic or oxidative decontamination, as well as water oxidation catalysts (WOCs) that can be interfaced with light-absorbing photoelectrode materials for photoelectrocatalytic water splitting.

Synergetic Catalysis of Copper and Iron in Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen.

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O2-based oxidations is reoxidation of the reduced POM by O2. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POMred (α-PW12O404- and α-SiVW11O406-) reacting with O2, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POMred by Fe(III) and Cu(I) by O2 to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PW12O403- and α-SiVW11O405-, are far better oxidants of organic substrates than the previously studied POMs, α-SiW12O404- and α-AlW12O405-, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O2/air-based processes.

Stabilization of Polyoxometalate Water Oxidation Catalysts on Hematite by Atomic Layer Deposition.

Fast and earth-abundant-element polyoxometalates (POMs) have been heavily studied recently as water oxidation catalysts (WOCs) in homogeneous solution. However, POM WOCs can be quite unstable when supported on electrode or photoelectrode surfaces under applied potential. This article reports for the first time that a nanoscale oxide coating (Al2O3) applied by the atomic layer deposition (ALD) aids immobilization and greatly stabilizes this now large family of molecular WOCs when on electrode surfaces. In this study, [{RuIV4(OH)2(H2O)4}(γ-SiW10O34)2]10- (Ru4Si2) is supported on hematite photoelectrodes and then protected by ALD Al2O3; this ternary system was characterized before and after photoelectrocatalytic water oxidation by Fourier transform infrared, X-ray photoelectron spectroscopy, energy-dispersive X-ray, and voltammetry. All these studies indicate that Ru4Si2 remains intact with Al2O3 ALD protection, but not without. The thickness of the Al2O3 layer significantly affects the catalytic performance of the system: a 4 nm thick Al2O3 layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. Al2O3 layers thicker than 6.5 nm appear to completely bury the Ru4Si2 catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.

Electrooxidation of Ethanol and Methanol Using the Molecular Catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10.).

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-)). Voltammetric studies with dissolved and surface-confined forms of [1(γ-SiW10O36)2](10-) suggest that the oxidized forms of 1 can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of 1 also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO3. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that 1 is one of the most active molecular electrocatalysts for methanol and ethanol oxidation.

[{Ni4 (OH)3 AsO4 }4 (B-α-PW9 O34 )4 ](28-) : A New Polyoxometalate Structural Family with Catalytic Hydrogen Evolution Activity.

A new structural polyoxometalate motif, [{Ni4 (OH)3 AsO4 }4 (B-α-PW9 O34 )4 ](28-) , which contains the highest nuclearity structurally characterized multi-nickel-containing polyanion to date, has been synthesized and characterized by single-crystal X-ray diffraction, temperature-dependent magnetism and several other techniques. The unique central {Ni16 (OH)12 O4 (AsO4 )4 } core shows dominant ferromagnetic exchange interactions, with maximum χm T of 69.21 cm(3) K mol(-1) at 3.4 K. Significantly, this structurally unprecedented complex is an efficient, water-compatible, noble-metal-free catalyst for H2 production upon visible light irradiation (photosensitizer=[Ir(ppy)2 (dtbbpy)][PF6 ]; sacrificial electron donor=triethylamine or triethanolamine). The highest turnover number of approximately 580, corresponding to a best quantum yield of approximately 4.07 %, is achieved when using triethylamine as electron donor in the presence of water. The mechanism of this photodriven process has been probed by time-solved luminescence and by static emission quenching.

Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design.

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO2, polyoxometalate (POM) water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (1), and both known photosensitizer [Ru(bpy)2(H4dpbpy)]2+ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)2(H2dpbpy)](H22). Both triads, containing catalyst 1, and catalyst-free dyads, produce O2 with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H22 absorbs light more strongly than P2, and increases O2 quantum yields by up to 270%. TiO2-2 based photoelectrodes are also more stable to desorption of active species than TiO2-P2: losses of catalyst 1 are halved when pH > TiO2 point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O2 are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO2, and is also consistent with the 4th1-to-dye electron transfer limiting performance rather than catalyst TOFmax. Transient absorption reveals that TiO2-2-1 has similar 1st electron transfer dynamics to TiO2-P2-1, with rapid (ps timescale) formation of long-lived TiO2(e-)-2-1(h+) charge separated states, and demonstrates that metallation of the crown ether groups (Na+/Mg2+) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO2-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO2 can oxidize water without a catalyst.

A noble-metal-free, tetra-nickel polyoxotungstate catalyst for efficient photocatalytic hydrogen evolution.

A tetra-nickel-containing polyoxotungstate, Na6K4[Ni4(H2O)2(PW9O34)2]·32H2O (Na6K4-Ni4P2), has been synthesized in high yield and systematically characterized. The X-ray crystal structure confirms that a tetra-nickel cluster core [Ni4O14] is sandwiched by two trivacant, heptadentate [PW9O34](9-) POM ligands. When coupled with (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate [Ir(ppy)2(dtbbpy)][PF6] as photosensitizer and triethanolamine (TEOA) as sacrificial electron donor, the noble-metal-free complex Ni4P2 works as an efficient and robust molecular catalyst for H2 production upon visible light irradiation. Under minimally optimized conditions, Ni4P2 catalyzes H2 production over 1 week and achieves a turnover number (TON) of as high as 6500 with almost no loss in activity. Mechanistic studies (emission quenching, time-resolved fluorescence decay, and transient absorption spectroscopy) confirm that, under visible light irradiation, the excited state [Ir(ppy)2(dtbbpy)](+)* can be both oxidatively and reductively quenched by Ni4P2 and TEOA, respectively. Extensive stability studies (e.g., UV-vis absorption, FT-IR, mercury-poison test, dynamic light scattering (DLS) and transmission electron microscopy (TEM)) provide very strong evidence that Ni4P2 catalyst remains homogeneous and intact under turnover conditions.

Mediator enhanced water oxidation using Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2] film modified electrodes.

The water insoluble complex Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2], ([Ru(II)bpy]5[Ru(III)4POM]), was synthesized from Rb8K2[{Ru(IV)4O4(OH)2(H2O)4}(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [Ru(II)bpy]5[Ru(III)4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)3](3+)([Ru(III)bpy]) acting as the electron transfer mediator and [Ru(V)4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s(-1) was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [Ru(II)bpy]5[Ru(III)4POM] → [Ru(III)bpy]5[Ru(V)4POM] + 13 e(-) and [Ru(III)bpy]5[Ru(V)4POM] + 2H2O → [Ru(III)bpy]5[Ru(IV)4POM] + O2 + 4H(+). The voltammetry of related water insoluble [Ru(II)bpy]2[S2M18O62] (M = W and Mo) and [Fe(II)Phen]x[Ru(III)4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [Ru(III)bpy] is the electron transfer mediator and [Ru(V)4POM] is the species responsible for oxygen evolution.

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.

Collecting meaningful early-time kinetic data in homogeneous catalytic water oxidation with a sacrificial oxidant.

As the field of water oxidation catalysis grows, so does the sophistication of the associated experimental apparatuses. However, problems persist in studying some of the most basic aspects of catalytic water oxidation including acquisition of satisfactory early-reaction-time kinetics and rapid quantification of O2 concentration. We seek to remedy these problems and through better experimental design, elucidate mechanistic aspects of catalytic water oxidation with theory backed by experimental data. Two new methods for evaluating homogeneous water oxidation catalysts by reaction with a stoichiometric oxidant are presented which eliminate problems of incomplete fast mixing and O2 measurement response time. These methods generate early-reaction-time kinetics that have previously been unavailable.

An inorganic chromophore based on a molecular oxide supported metal carbonyl cluster: [P2W17O61{Re(CO)3}3{ORb(H2O)}(µ3-OH)]9-.

A polyoxometalate-supported trirhenium carbonyl cluster, mimicking metal oxide supported interfacial dyadic structures, has been synthesized and characterized. Multiple techniques, including computational and transient absorption spectroscopy, have been applied to characterize the charge-transfer dynamics occurring at the interfaces of this "double cluster". The stepwise kinetics of charge separation and recombination has been thoroughly investigated.

Voltammetric determination of the reversible potentials for [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- over the pH range of 2-12: electrolyte dependence and implications for water oxidation catalysis.

Voltammetric studies of the Ru-containing polyoxometalate water oxidation molecular catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-) where 1 represents the {Ru4O4(OH)2(H2O)4} core and 1(0) stands for its initial form with all ruthenium centers in the oxidation state IV) have been carried out in aqueous media over a wide range of pH (2-12 using Britton-Robinson buffer) and ionic strength. Well-defined voltammograms in buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO3, NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, MgSO4, or Na2SO4). A combination of data derived from dc cyclic, rotating disk electrode, and Fourier transformed large amplitude ac voltammetry allow the assignment of two processes related to reduction of the framework and the complete series of Ru(III/IV) and Ru(IV/V) redox processes and also provide their reversible potentials. Analysis of these data reveals that K(+) has a significantly stronger interaction with 1(1) (the number inside bracket stands for the number of electrons removed from 1(0)) than found for the other cations investigated, and hence its presence significantly alters the pH dependence of the 1(0)/1(1) reversible potential. Comparison of experimental data with theory developed in terms of equilibrium constants for process 1(0)/1(1) reveals that both H(+) and K(+) interact competitively with both 1(0) and 1(1). Importantly, reversible potential data reveal that (i) proton transfer does not necessarily need to be coupled to all electron transfer steps to achieve catalytic oxidation of water, (ii) the four-electron oxidized form, 1(4), is capable of oxidizing water under all conditions studied, and (iii) under some conditions, the three-electron oxidized form, 1(3), also exhibits considerable catalytic activity.

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.

Di- and tri-cobalt silicotungstates: synthesis, characterization, and stability studies.

Di- and tricobalt silicotungstate complexes, K(5)Na(4)H(4)[{Na(3)(µ-OH(2))(2)Co(2)(µ-OH)(4)} (Si(2)W(18)O(66))]·37H(2)O (1) and K(6)Na(3)[Na(H(2)O){Co(H(2)O)(3)}(2){Co(H(2)O)(2)}(Si(2)W(18)O(66))]·22H(2)O (2), have been synthesized through reaction of cobalt chloride and [A-α-SiW(9)O(34)](10-) in acidic buffer solution. They have been characterized by X-ray crystallography, elemental analysis, cyclic voltammetry, infrared, and UV-vis spectroscopy. In 1, two cobalt atoms as well as three sodium atoms are incorporated in the central pocket of the [Si(2)W(18)O(66)](16-) polyanion. In 2, one cobalt atom and one sodium atom are incorporated in the pocket of [Si(2)W(18)O(66)](16-); two other cobalt atoms in this complex protrude outside the pocket and connect with WO(6) units of other [Si(2)W(18)O(66)](16-) polyanions to form a one-dimensional polymeric structure. The crucial parameters in the synthesis of these two compounds are discussed, and their stability in different buffer solutions is studied. The decomposition of 1 or 2 in heated potassium acetate buffer (pH 4.8, 1 M) yields K(11)[{Co(2)(H(2)O)(8)}K(Si(2)W(18)O(66))]·17H(2)O (3) based on spectroscopic studies and an X-ray crystal structure.

Detailed electrochemical studies of the tetraruthenium polyoxometalate water oxidation catalyst in acidic media: identification of an extended oxidation series using Fourier transformed alternating current voltammetry.

The electrochemistry of the water oxidation catalyst, Rb(8)K(2)[{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)] (Rb(8)K(2)-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A'/A to H/H') that are readily detected under dc voltammetric conditions. The six most positive processes (A'/A to F/F'), each involve a one electron ruthenium based charge transfer step (A'/A, B'/B are Ru(IV/V) oxidation and C/C' to F/F' are Ru(IV/III) reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B'/B, E/E', and F/F' processes are quasi-reversible, while the others are close to reversible. A third Ru(IV/V) oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb(8)K(2)-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G') assigned to Ru(III/II) reduction and the multiple electron transfer reduction process (H/H') that arise from the tungstate polyoxometalate framework. A significant shift of E°' to a more positive potential value for process H/H' was observed on removal of K(+) (~100 mV in H(2)SO(4) and ~50 mV in HCl).