(Invited) Spectroelectrochemical Evidence of an Electron Transfer through Viologen Moiety from an ITO Electrode to a Molecular Catalyst for Hydrogen Evolution Reaction Confined within a Viologen Monolayer
Authors: Uosaki Kohei , Noguchi Hidenori , Kurniawan Cepi , Masuda Takuya 
Published: February 2020

To realize highly efficient and selective chemical and electrochemical processes, catalyst is essential. Although precious metals are often key components of many catalysts, they are expensive and of limited resources. Many efforts have been made to develop non-precious metal catalyst and maximize the utilization of these elements. 100% atom efficiency is expected for molecular and single atom catalysts but both have difficulties in practical use. It is difficult to separate molecular catalysts from reactants and products after the reaction as they are usually used as homogeneous catalyst in solution. Preparation of single atom catalysts is difficult and they easily aggregate if they are not prepared on proper supports.
We have recently proposed that these problems can be overcome by confining a metal complex on and within molecular layers so that “confined molecular catalyst” is formed and reported that metal complexes confined within viologen molecular layer, which is constructed on Si(111) electrode, indeed act as electrocatalysts for both hydrogen evolution [1-4] and CO2 reduction [4, 5] reactions, demonstrating the effectiveness of the “confined molecular catalyst”. Although we suggested that viologen moiety of the molecular layer and a Pt complex act as an electron relay and a catalyst, respectively, no direct evidence for electron to be transferred via viologen moiety has been provided. If electron is transferred via viologen moiety, viologen radical cation (V+•), which is formed by one electron reduction of viologen dication (V2+), should be detected as an intermediate state upon potential step to negative potential where hydrogen evolution reaction (HER) takes place.
Here we carried out spectroelectrochemical measurements at a viologen monolayer constructed on indium tin oxide (ITO) with and without a Pt complex. Two redox peaks corresponding to viologen dication/radical cation (V2+/V+•) and viologen radical cation/neutral form (V+•/V0) are observed in cyclic voltammogram (CV) of viologen monolayer modified ITO electrode without Pt complex. Stable spectra corresponding to radical cation (V+•) and neutral form (V0) of viologen are obtained at potentials between V2+/V+• and V+•/V0 redox peaks and more negative than V+•/V0 redox peak, respectively. On the other hand, at viologen monolayer modified ITO electrode with Pt complex, no redox peaks but large current due to HER is observed in CV and UV/visible spectra obtained during the potential scan show no absorption peaks corresponding to V+• or V0. Time-resolved spectroelectrochemical measurements show, however, that V+• is formed upon the potentials step to the potentials more negative than V2+/ V+• redox potential and disappeared within ca. 1 ms, confirming that electron is transferred from ITO electrode to proton via viologen moiety and Pt complex.
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