XAFS Study on the Structures of Catalysts for Photoelectrochemical Hydrogen Evolution and CO2 Reduction Prepared By the Incorporation of Metal Complexes within Viologen Multi-Layers Formed on Si(111) Surfaces
In this study, we constructed organic molecular layers with viologen moieties as electron transfer mediators and various metal complexes such as PtCl42-, PdCl42- and AuCl4- as catalysts for multi-electron transfer reactions on hydrogen terminated Si(111) surfaces and the modified Si(111) electrodes were utilized for photoelectrochemical hydrogen evolution and CO2 reduction reactions. Precise structures of the metal catalysts incorporated with the molecular layers were determined by polarization-dependent total reflection fluorescence (PTRF)-XAFS.
High selectivity for CO2 reduction in preference to the hydrogen evolution reaction was achieved at the Si(111) electrodes modified by molecular layers with viologen moiety and Pd and Au complexes, which are reduced to metallic nanoparticle under operation, and, therefore, actual catalysts for CO2 reduction were Pd and Au metal nanoparticles, which are known to be good electrocatalysts for CO2 reduction. High selectivity for CO2 reduction was also achieved at the Si(111) electrode modified by viologen moiety and Pt complex despite the fact that hydrogen evolution reaction is dominant at Pt metal electrode even in CO2 saturated solution. Pt complex is not reduced to metallic nanoparticle under operation, and, therefore, Pt complex acts as a confined molecular catalyst for CO2 reduction with high selectivity.
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