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Water oxidation with all-inorganic earth-abundant catalysts José Ramón Galán-Mascarós,1,2 1- Institute of Chemical Research of Catalonia (ICIQ), Avenida Països Catalans 16, E-43007 Tarragona, Spain. E-mail: [email protected] 2- Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluis Companys 23, E-08010 Barcelona, Spain Water oxidation catalysis is one of the biggest challenges chemistry is facing today. The discovery of a fast, robust and readily available catalyst would be key for the realization of artificial photosynthesis, an achievement that could probably solve the energy problem worldwide. Oxygen evolution from water is a complex redox process. It occurs at high oxidation potentials and it involves four electrons. Furthermore, turnover conditions are very demanding. An efficient catalyst able to work at low overpotential for optimized energy conversion will also need to be stable to air, light, water, heat and oxidative deactivation. 2H2O ā O2 +4H+ +4eā E = +1.229ā0.059(pH)V vs NHE at 25ºC Pure inorganic water oxidation catalysts (WOCs) are a remarkable alternative to typical coordination chemistry candidates, where organic ligands support the metal complex. The presence of organic ligands precludes long-term stability under turnover conditions because of oxidative deactivation. Here we will present our latest results in two different types of promising candidates: i) Polyoxometalates (POMs) are a versatile family of inorganic molecular clusters. In these polyanions a transition-metal core can be stabilized by discreet metal-oxide frameworks. Several POMs have been reported as homogeneous water oxidation catalysts (WOCs) in recent years.[1,2,3] However, the actual performance of POMs has been put into question[4,5] and they are the subject of some controversy. Although stable to oxidative deactivation, they exhibit a complex solution equilibria that may yield adventitious cobalt oxides in certain experimental conditions. On the contrary, POMs can also be immobilized onto solid state supports, remaining remarkably active in heterogeneous conditions. ii) Transition metal hexacyanometallates (Prussian blues) are stable in a wide pH range, and able to promote water oxidation for days without significant appearance of fatigue, at high turnover frequencies.[6] This family of coordination polymers represents a novel alternative to metal oxide WOCs, while adding the typical advantages of molecule-based materials: well-defined crystal structure, high porosity, low density, flexibility, processability and transparency to visible light. [1] H. Lv, Y. V. Geletii, C. Zhao, J. W. Vickers, G. Zhu, Z. Luo, J. Song, T. Lian, D. G. Musaev, C. L. Hill, Chem. Soc. Rev. 2012, 41, 7572. [2] Q. Yin, J. M. Tan, C. Besson, Y. V. Geletii, Djamaladdin G. Musaev, A. E. Kuznetsov, Z. Luo, K. I. Hardcastle, C. L. Hill , Science 2010, 328, 342. [3] S. Goberna-Ferron, L. Vigara, J. Soriano-Lopez, J. R. Galan-Mascaros, Inorg. Chem. 2012, 51, 11707. [4] J. J. Stracke, R. G. Finke, J. Am. Chem. Soc. 2011, 133, 14872. [5] M. Natali, S. Berardi, A. Sartorel, M. Bonchio, S. Campagna, F. Scandola, Chem. Commun. 2012, 48, 8808. [6] S. Pintado, S. Goberna-Ferrón, E. C. Escudero-Adán, J. R: Galan-MAscaros, J. Am. Chem. Soc. 2013, 135, 13270.
