Various Active Metal Species Incorporated within Molecular Layers on Si(111) Electrodes for Hydrogen Evolution and CO2 Reduction Reactions

Abstract

Organic molecular layers with viologen moieties as electron transfer mediators were constructed on hydrogen-terminated Si(111) surfaces, and metal catalysts for multielectron transfer reactions were incorporated into the molecular layers by immersing the viologen-modified Si(111) electrodes in aqueous solutions containing various metal complexes (K2PdCl4, NaAuCl4, and K2PtCl4). Significant enhancements were achieved for CO2 reduction at the Au-modified Si(111) electrode and for both hydrogen evolution reaction (HER) and CO2 reduction at the Pd-modified Si(111) electrode. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analysis showed that Au complexes were spontaneously reduced to metal nanoparticles during the metal insertion, and therefore, actual catalysts for CO2 reduction at the Au-modified Si(111) electrode were Au metal nanoparticles. In contrast, Pd complexes were inserted into the molecular layers and partly reduced during HER and CO2 reduction. Pd complexes and ...