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

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 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 relatively small Pd nanoparticles (<2 nm) were considered to be actual catalysts for HER and CO2 reduction. Interestingly, at the Pt-modified Si(111) electrode, not only highly efficient HER but also highly selective CO2 reduction in preference to HER were achieved, despite the fact that HER is dominant at Pt pure metal electrodes even in CO2-saturated aqueous solutions. On the basis of XPS and XAFS analysis, Pt complexes were incorporated into the molecular layers and acted as confined molecular catalysts for both HER and CO2 reduction without being converted into Pt metal nanoparticles. This should be the major reason for the anomalously high selectivity of the Pt-modified Si(111) electrode for CO2 reduction, unlike pure Pt metal electrocatalysts.