Light-Driven Water Oxidation Catalyzed By Mixed-Valence Cobalt Containing Keggin Type Polyoxometalate

H2 is considered to be a clean and renewable energy because H2O is the only product when H2 combines with O2. Artificial photosythesis water splitting is currently the most ideal energy conversion mode, and it has attracted many researchers’ interest. Water oxidation is identifited as the bottleneck of water splitting because it requires a multielectron stepwise building up of very high redox potentials. So, develepment of efficient and stable water oxidation catalysts (WOCs) is the key in overall water splitting. Plyoxometalates (POMs) are a versatile family of early transition-metal oxide nanoclusters. Owing to the unmatched physical and chemical properties of POMs, they have been widely applied in catalysis, medicine, nanotechnology and materials science. Develepment of POMs as WOCs have given rise to more and more interesting of researchers because they can resist the strong acidity environment, which is a shortcoming of many ogarnometallic complex WOCs. There are many kinds of POMs have been designed as WOCs. Cobalt-containing POM-WOCs as a kind of non-noble metal WOC has a better application prospect. To our best knowledge, POM-WOC with Keggin type has not been reported, which is a class of simple structure and most stable compounds in POMs family. Furthermore, among all the cobalt-containing POM water oxidation catalysts develeped to date, POM with the mixed-valance cobalt has been paid any attention. Here, we report a Keggin POM composing of only earth-abundant elements (cobalt, tungsten, and oxygen) and a unique mixed-valence Co(Ⅲ) and Co(Ⅱ) structural feature, is an efficient molecular WOC catalyst under both thermal conditions and the visible light irradiation.1. synthesis and characterization of K7[CoⅢCoⅡ(H2O)W11O39] (1), which owns both Keggin structure and mixed-valence Co(Ⅲ) and Co(Ⅱ). At the same time, six other kinds of Keggin type POMs:K6[CoⅡW12O40] (2), K5[CoⅢW12O40] (3), K8[CoⅡCoⅡ(H2O)W11O39] (4), K6[SiCoⅡ(H2O)W11O39] (5), K5[PCoⅡ(H2O)W11O39] (6) and K6[GeCoⅡ(H2O)W11O39] (7) were also synthesized. The X-ray single crystal structure of 1 was also firstly reported by us.2. The catalytic reactivity in water oxidation using 1 was compared with other six kinds of Keggin type POMs, and the result indicates that the unique mixed-valence Co(Ⅲ) and Co(Ⅱ) structural feature endow the unusual nature and stability to 1. Variables of the photocatalytic reaction conditions, including catalyst concentrations, buffer types and concentrations, pHs, dye concentrations, oxidant concentrations, etc., were systemically studied. Under the optimal photocatalytic conditions [photoirradiation at≥420 nm, [Ru(bpy)3]Cl2 as the photosensor, Na2S2O8 as the oxidant in borate buffer (pH=9.0)],the turnover number (TON) can reach as high as 360, the initial quantum yield and the initial turnover frequency (TOF) for the first 60 seconds was 27% and 0.5 s-1, respectively.3. The stability of 1 was tested and confirmed with multiple experiments (DLS, CV, laser flash photolysis, and catalyst recycling) within the photocatalytic water oxidation duration. All the evidence stated here supports that 1 is stable at least within the water oxidation duration.