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.

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.

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The viable production of solar fuels requires a visible-light absorbing unit, a H2O (or CO2) reduction catalyst (WRC) and a water oxidation catalyst (WOC) that work in tandem to split water or reduce CO2 with H2O rapidly, selectively and for long periods of time. Most catalysts and photosensitizers developed to date for these triadic systems are oxidatively, thermally and/or hydrolytically unstable. Polyoxometalates (POMs) constitute a huge class of complexes with extensively tunable properties that are oxidatively, thermally and (over wide and adjustable pH ranges) hydrolytically stable. POMs are some of the fastest and most stable WOCs to date. This Microreview updates the very active POM WOC field, reports the first POM WRCs and initial selfassembling metal oxide semiconductor-photosensitizer-POM catalyst triad photoanodes. The complexities of investigating these POM systems, including but not limited to the study of POM-hydrated metal ion-metal oxide speciation processes, are outlined. The achievements and challenges in POM WOC, WRC and triad research are outlined. IntroductionMeasurements and models make it ever more certain that the planet will face a serious energy shortage as the availability of economically accessible fossil fuels fails to keep pace with global energy needs. [1] Data and analysis also indicate that the environmental change caused by fossil fuel combustion will become increasingly problematic. Although green and alternative energy sources are rapidly becoming more available and less expensive, the net consumption of environmentally worrisome fossil fuel is not dropping significantly. Increases in both global population and average global standard of living paint a less-thanrosy picture for our energy future. [1b, 1g, 2] Solar remains the most likely source of sustainable energy for the medium and longer-term future. The other renewable sources of energy, with the arguable exception of biofuels provided the energy production efficiency (photosynthesis and other efficiencies) can be significantly increased, will not likely be sufficient to power the planet. In addition, high density energy will be needed in enormous quantities moving forward; electricity and other sources of energy will not provide sufficient energy density for our major transportation needs (ships, aircraft). Unlike the production of solar electricity, which is a now a rapidly maturing technical area and a major and growing market sector, production of solar fuel is in its infancy.The principal reactions for the generation of solar fuel are H2O splitting to produce H2 and O2 (eq. 1) and H2O splitting coupled to CO2 reduction (eq. 2). Technology is needed so both these processes can be driven by terrestrial sunlight and proceed with high rates and selectivity to the desired products. A factor in the slow rates observed for H2O oxidation by many systems is that it is a four-electron, four-proton process, hence the need for a catalyst that can facilitate the multiple proton-coupled electron transfer (PCET) processes with lo...

A POM can perform four functions simultaneously, a theme of potential value in the construction of energy converting multicomponent assemblies.

Solutions to two general limitations in the immobilization of molecular water oxidation catalysts (WOCs) on nanoparticle or photoelectrode surfaces have been investigated: (a) a straightforward electrostatic method to bind charged WOCs more effectively to these surfaces and (b) a method to increase the concentration of the semiconductor- and/or electrode-immobilized WOCs so they can be spectroscopically characterized in this form. Polyoxometalate (POM) WOCs, known to be fast, selective, and oxidatively stable under homogeneous conditions, and their high negative charges are a good test case to assess the viability of electrostatic surface immobilization. The POM WOCs, [RuIV 4O5(OH)­(H2O)4(γ-PW10O36)2]9– (Ru 4 P 2 ) and [{RuIV 4(OH)2(H2O)4}­(γ-SiW10O34)2]10‑ (Ru 4 Si 2 ), have been immobilized by silanization on TiO2 nanoparticles and nanoporous electrodes and have been found to retain catalytic water oxidation activity. In photoelectrochemical systems, increased current density of WOC-TiO2/FTO electrodes is consistent with water oxidation occurring on the derivatized, modified semiconductor surface. The simple use of semiconductor nanoparticles (versus conventional larger particles or surfaces) provides sufficient concentrations of immobilized POM WOCs to enable their spectroscopic and electron microscopic characterization after photoelectrocatalytic use. Multiple techniques have been used to observe the effectiveness of silanization for POM WOC immobilization on nanoparticle surfaces as well as TiO2/FTO electrodes before and after photoelectrocatalysis.

Improved sensitizer design dramatically enhances visible light-driven water oxidation from dye-sensitized TiO2 photoanodes treated with polyoxometalate water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10–.

Two MOF-like but all-inorganic polyoxometalate-based networks, [Na7X2W18Sn9Cl5O68·(H2O)m]n (1, X = Si, m = 35; 2, X = Ge, m = 41), and the molecular tetramer Na6[{Na(μ-OH2)(OH2)2}6{Sn6(B-SbW9O33)2}2]·50H2O (3) have been prepared and characterized by X-ray diffraction and spectroscopic methods. All three compounds exhibit unique structural features, and networks 1 and 2 incorporate the highest nuclearity of Sn(II)-containing POMs to date. Tetramer 3 comprises bridging Sn(II) ions with [B-SbW9O33](9-) units and exhibits two highly unusual features, a long-range Sb···Sb interaction and an intramolecular charge-transfer transition involving donation of the lone-pair electron density on both Sb(III) and Sn(II) to the POM. The electronic structure and excited-state dynamics have been studied by transient spectroscopy, spectroelectrochemistry, DFT calculations, and resonance Raman spectroscopy. The synergistic effect of two types of stereoactive lone-pairs on Sb(III) and Sn(II) is critical for the charge-transfer absorption feature in the visible.