Direct observation of proton donor in CO₂ photoreduction electrocatalyst by in-situ surface-enhanced Raman spectroscopy

抄録

Recently, electrochemical carbon dioxide (CO₂) reduction reaction using photocatalysts to convert CO₂ into useful chemicals has attracted much attention. Au/p-GaN photocatalysts, in which gold nanoparticles (AuNPs) are supported on p-type gallium nitride (p-GaN), show high activity for CO₂ reduction due to the plasmon excitation of AuNPs followed by selective hole transfer to the p-GaN substrate remaining high-density electrons at the AuNP surfaces^[1]. The plasmon-assisted electrochemical CO₂ reduction using Au/p-GaN catalysts has been reported to selectively produce carbon monoxide (CO)^[1]. However, the chemical structure of proton donor required for CO₂ reduction with the Au/p-GaN catalysts is not clear. It was proposed that multiple protons are required for CO₂ reduction to proceed^[2]. Therefore, the understanding of proton donor is important for controlling CO₂ reduction. In this study, we attempted to observe the proton donor in a near surface region of the AuNPs by in-situ surface-enhanced Raman spectroscopy (in-situ SERS) under electrochemical conditions.

The Au/p-GaN catalyst was prepared by vacuum deposition of Au on p-GaN substrates according to the previously reported method by Joseph et al^[1]. The Au/p-GaN catalyst exhibited absorption peak at about 570 nm due to a plasmon resonance of AuNPs. In-situ SERS measurements were performed in a CO₂-saturated 0.5 M NaHCO₃ solution (pH=7.4), with CO₂ bubbling during the measurement to prevent pH change in the bulk. The Au/p-GaN catalyst was used as a working electrode, Pt wire as a counter electrode, and Ag/AgCl electrode as a reference electrode. All SERS spectra were collected at a constant potential. All the potentials are referred to the Ag/AgCl standard electrode potential unless noted otherwise.

Fig. 1 shows potential dependence of Raman bands at 1358 cm^-1 and 1380 cm^-1 assigned to bicarbonate (HCO₃^-) and carbonate (CO₃^2-), respectively, in the near surface region of AuNPs. The peak area of HCO₃^- decreases as the potential becomes more negative, while the peak area of CO₃^2- exhibits almost no change. This result indicates that the HCO₃^- is transformed to the CO₃^2- if the peak area of Raman band is assumed to be proportional to the molecular density and the newly transformed CO₃^2- diffuses out from the near surface region due to a repulsion from the negatively charged AuNPs’ surface. The transformation of HCO₃^- to CO₃^2- can be related to the CO₂ reduction, because the bulk pH is constant during the in-situ SERS measurement. This is interpreted as deprotonation of HCO₃^- in the near surface region of AuNPs with acting as a proton donor during the CO₂ reduction. Furthermore, the local pH near the electrode surface was calculated from the peak area ratio of HCO₃^- and CO₃^2- at each potential by the Henderson-Hasselbarch equation. The surface pH was found to increase as the potential becomes more negative. This result suggests that not only the HCO₃^- was converted to a CO₃^2- (eq 1), but also that the HCO₃^- acted as a proton donor (eq 2).

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