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

Multi-electron transfer reactions such as the hydrogen evolution reaction, oxygen evolution reaction and carbon dioxide reduction reaction at semiconductor electrodes have been extensively studied because of the interest in solar-to-chemical energy conversion. Multi-electron transfer reactions at semiconductors are generally slow and a catalyst is required to efficiently promote these reactions. Hence, many groups have examined the modification of the semiconductor surface by catalytic metals. It was, however, found that direct contact of a metal leads to the formation of a Schottky junction and recombination center, resulting in a reduced efficiency.

In this study, we constructed organic molecular layers with viologen moieties as electron transfer mediators and various metal complexes such as PtCl₄^2-, PdCl₄^2- 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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