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Osamu Tomita, Speaker at Chemical Engineering Conferences
Kyoto University, Japan
Title : Two-step water splitting under visible light by using Polyoxometalate as shuttle redox mediator Osamu Tomita


Water splitting systems based on two-step photoexcitation, so-called Z-scheme systems, have recently been developed and proven as a promising approach to harvesting a wider range of visible light, because the water splitting reaction is separated into two parts, i.e., H2- and O2-evolving systems. Although the introduction of Z- scheme systems enables us to employ various visible-light responsive photocatalysts, the choice of redox has been limited to simple ion couples such as IO3–/I– and Fe3+/Fe2+. This is mainly due to the problems with mismatching redox potentials and/or irreversibility presented by other materials. The development of redox couples with appropriate redox potentials and sufficient reversibility under mild pH conditions is thus required in order to achieve highly efficient Z-scheme systems.

Here, we have paid attention to the use of transition metal-substituted-polyoxometalates, most of which are known to exhibit reversible redox behavior derived from valence differences between the incorporated transition metals, as effective shuttle redox mediators. We have recently reported a new Z-scheme water splitting system using a polyoxometalate (POM) as a redox mediator; the use of a Mn-substituted silicotungstate or Mo-substituted one (K6[SiW11O39MnII(H2O)] or K4[SiW11O40MoVI], denoted as SiW11Mn and SiW11Mo, respectively) with appropriate photocatalysts enabled a stoichiometric evolution of H2 and O2 under visible light. In the present study, V-substituted silicotungstate (K5[SiW11O40VV], denoted as SiW11V)6 was prepared and employed as redox mediator to pursue the availability of POM. The SiW11VV/SiW11VIV was confirmed to function as electron donor in H2-evolution system or as electron acceptor in O2-evolution system. The SiW11VV/SiW11VIV was revealed function as effective shuttle redox mediator between the two photocatalysts under visible light irradiation.


Osamu Tomita received his BS (2006) from Tokyo University of Science, and his MS (2009) and PhD (2012) from Hokkaido University. He then worked as a postdoctoral fellow (2012–2016) at Kyoto University. He is currently an assistant professor (2017–) at Kyoto University. His research interests are photocatalytic water splitting and photocatalytic selective oxidation of organic compounds.