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

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• 作者：Takuya Masuda ; Yu Sun ; Hitoshi Fukumitsu ; Hiromitsu Uehara ; Satoru Takakusagi ; Wang-Jae Chun ; Toshihiro Kondo ; Kiyotaka Asakura ; Kohei Uosaki
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：July 28, 2016
• 年：2016
• 卷：120
• 期：29
• 页码：16200-16210
• 全文大小：714K
• 年卷期：0
• ISSN：1932-7455

文摘

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 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.