Photocatalytic overall water splitting using semiconductor powders has gained tremendous interest as a solar energy conversion technology. Overall water splitting reaction produces both H2 and O2 in one reactor, making the system simple. One of the issues of such reaction is the reverse reaction of the products forming H2O. Using semiconductor powder, it is required or often essential to have hydrogen evolution catalyst on the surface. This modification can be metal particles, but many of such electrocatalysts are also active for the back reaction. The nano-sized protection layer to cover the metal surfaces is effective to selectively suppress back reaction without losing H2 evolution activity. Domen and coworkers demonstrated that the overall water splitting can be achieved once the metal particles (Rh, Pt, and many transition metals) are decorated by Cr species. Using electrochemical approach, it was proposed that the Cr layer acts as a kind of selective membrane: O2 molecules cannot penetrate through this Cr layer, but H2 molecules can, consistent with electrochemical and spectroscopic measurements. This study demonstrates our continuous efforts to find the identity and function of the cocatalysts for selective H2 evolution without introducing back reaction. Electrochemical approach gives quantitative information of the electrochemical materials how current would flow at a given potential. Firstly, the electrochemistry of Pt modified with other element, such as Cr and Mo, was conducted to monitor the catalysis in different atmosphere. Secondly, the working state of the material was investigated using in-operando X-ray absorption spectroscopy (XAS). The electrochemical measurement in H2 and O2 confirmed that the oxygen reduction reaction (ORR) is drastically suppressed, whereas hydrogen evolution reaction (HER) is maintained. The XAS results showed that the Pt oxidation state can be freely changed at potential sweeping with and without the coating layer. The Mo species that covers This interesting approach will provide design of cocatalyst for photocatalytic overall water splitting.

Oxygen-insensitive hydrogen evolution sites coated by Cr and Mo species for overall water splitting

254th National Meeting (2017)

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