Abstract
The electrochemical carbon dioxide (CO2) reduction to ethylene (C2H4) and ethanol (C2H5OH) using renewable energy is a viable method for the production of these commercially vital chemicals. Copper (Cu) based materials are by far the most promising electrocatalysts for this purpose. However, the formation of ethanol using Cu and its oxides is generally less favored as compared to that of ethylene. Here, we demonstrate that the selectivity of CO2 reduction towards ethanol could be systematically tuned by introducing a co-catalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amount of Zn dopants (Cu, Cu10Zn, Cu4Zn and Cu2Zn) were prepared via electrodeposition and used as catalysts under ambient pressure in aqueous 0.1 M KHCO3 electrolyte. By varying the concentration of Zn in the bimetallic catalysts, we found that the selectivity of ethanol versus ethylene formation, defined by the ratio of their faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of up to ~12.5. Ethanol selectivity was maximized on Cu4Zn at -1.05 V vs. RHE, with a remarkable faradaic efficiency and current density of 29.1% and -8.2 mA/cm2 respectively. The Cu4Zn catalyst was also catalytically stable towards ethanol formation for at least 5 hrs. Operando Raman spectroscopy revealed that the as-deposited oxides were reduced to the metallic Cu/Zn during CO2 reduction, after which only signals belonging to CO adsorbed on Cu sites were recorded. This indicated that the reduction of CO2 might preferably occur on metallic sites rather than on metal oxides. The importance of Zn as a CO producing site was also demonstrated by performing CO2 reduction on Cu-Ni and Cu-Ag bimetallic catalysts. Thus, the mechanism of ethanol formation was proposed as following: Incoming CO2 molecules could first bind to either Cu or Zn sites, and be reduced to CO. On the Cu sites, CO could be reduced further to CHO or CHx (x=1,2,3) intermediates, while CO adsorbs weakly on Zn sites and is likely to desorb. The desorbed CO could diffuse and spill over onto the Cu sites. The spilled-over CO may then insert itself into the bond between the Cu surface and *CH2, to form *COCH2. Further reduction of *COCH2 will produce acetaldehyde and finally ethanol. 1) Ren, D.; Ang, B. S. H.; Yeo, B. S. ACS Catal. 2016, 8239-8247. 2) Ren, D.; Deng, Y.; Handoko, A. D.; Chen, C. S.; Malkhandi, S.; Yeo, B. S. ACS Catal. 2015, 5, 2814-2821. 3) Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2309-2326. 4) Calle-Vallejo, F.; Koper, M. T. M. Angew. Chem. Int. Ed. 2013, 52, 7282-7285.
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