Proceedings of nanoGe Fall Meeting19 (NFM19)
Publication date: 18th July 2019
Following the lead of natural photosynthesis, CO2 reduction combined with water oxidation is a promising route to mitigate CO2 emission. Shining light onto the mechanism and especially the intermediates involved in the electrochemical CO2 reduction reactions is an important step for the development of outstanding catalysts. Here, we adopted operando Raman spectroscopy to investigate the reaction mechanism. The Cu foam employed for this study is an effective SERS active substrate, easily electrodeposited, with no need for further surface modifications.[1] We found that the raw data is dominated by a strong background which is heavily dependent on the electrochemical potential and the chemical state of the Cu surface. We suggest that appropriate subtraction of a higher polynomial function and additional suitable normalization of the data is necessary before any interpretation of peak/intensity trends can be derived from the operando Raman data. Surface oxides are present on the Cu foam and gradually disappear until -0.4 VRHE, and only metallic Cu is present at catalytically relevant potentials. While surface oxides getting reduced, a pair of peaks around 1535 cm-1 appeared which also shift in carbon isotope experiment, we tentatively assign them to COO- intermediate. Carbonate band was prominent but gradually desorbed at catalytically relevant potential. Two distinct vibration modes of *CO adsorption were observed as a broad intramolecular CO band (present at potentials lower than -0.2 VRHE) and a sharp band at higher frequency at catalytically relevant potentials when Cu-CO bands appeared. A wide peak composed of several sub-bands related to either intermediates or products are observed and do not shift in either hydrogen or carbon isotope experiments. Further investigation is required for assignment to specific species.
Figure 1. Operando Raman spectra of the reduction behavior and the adsorbents during CO2 reduction on a Cu foam in CO2 saturated KHCO3 (0.1M, pH=6.8). a) Raw data, b) after background subtraction (paper background was subtracted) and normalization, an offset was applied for clarity.
We acknowledge financial support from the Bundesministerium f¨ur Bildung und Forschung (BMBF, CO2EKAT Project No. 03SF023), the Deutsche Forschungsgemeinschaft (DFG, priority Program No. SPP-1613), and the Chinese Scholarship Council (CSC, Ph.D. fellowship to S.J.). We thank Dr. Luca D’Amario for valuable discussions and help about the data evaluation. We thank Dr. Tintula Kottakkat and Professor Christina Roth (Department of Chemistry, FUB) for joint work on CO2 reduction by copper foams, and Dr. Stefan Mebs (Department of Physics, FUB) for his support regarding the operation of the Raman spectrometer.
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