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.

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

Copper-based complexes have been largely neglected as potential water reduction catalysts. This article reports the synthesis and characterization of a tetra-copper-containing polyoxotungstate, Na3K7[Cu4(H2O)2(B-α-PW9O34)2]·30H2O (Na3K7-Cu4P2). Cu4P2 is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate ([Ir(ppy)2(dtbbpy)][PF6]) as a light absorber and triethanolamine (TEOA) as sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of ∼1270 per Cu4P2 catalyst is obtained after 5 h of irradiation (light-emitting diode; λ = 455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9% is achieved. Both oxidative and reductive quenching pathways are observed by measuring the luminescence intensity of excited state [Ir(ppy)2(dtbbpy)](+*) in the presence of Cu4P2 or TEOA, respectively. Many stability studies (e.g., UV-vis absorption, FT-IR, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy) show that catalyst Cu4P2 undergoes slow decomposition under turnover conditions; however, both the starting Cu4P2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuOx particles. Considering the high abundance and low cost of copper, the present work provides considerations for the design and synthesis of efficient, molecular, water-compatible Cu-based water reduction catalysts.

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.

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 (AlO) 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, [{Ru(OH)(HO)}(γ-SiWO)] (RuSi) is supported on hematite photoelectrodes and then protected by ALD AlO; 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 RuSi remains intact with AlO ALD protection, but not without. The thickness of the AlO layer significantly affects the catalytic performance of the system: a 4 nm thick AlO layer provides optimal performance with nearly 100% faradaic efficiency for oxygen generation under visible-light illumination. AlO layers thicker than 6.5 nm appear to completely bury the RuSi catalyst, removing all of the catalytic activity, whereas thinner layers are insufficient to maintain a long-term attachment of the catalytic POM.

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A Noble-Metal-Free, Tetra-Nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution. -Na 6K4[Ni4(H2O)2(PW9O34)2]·32H2O is synthesized by refluxing an aqueous solution of Na 2WO4, Na2HPO4, and Ni(OAc)2 for 2.5 h, followed by addition of KOAc (73% yield). The compound crystallizes in the triclinic space group P1 with Z = 1 (single crystal XRD). The structure contains a tetra-nickel cluster core [Ni 4O14], which is sandwiched by two trivacant, heptadentate [PW9O34] 9ligands. The material works as an efficient and robust molecular catalyst for H 2 production upon visible light irradiation. Under minimally optimized conditions, it catalyzes H2 production over one week and achieves a turnover number of as high as 6500 with almost no loss in activity. -(LV, H.; GUO, W.; WU, K.; CHEN, Z.; BACSA, J.; MUSAEV, D. G.; GELETII, Y. V.; LAUINGER, S. M.; LIAN, T.; HILL*, C. L.; J. Am. Chem. Soc. 136 (2014) 40, 14015-14018, http://dx.

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