April 16, 2020 feature
Simultaneously tuning the surface structure and oxidation state of copper catalysts
Electrical energy derived from renewable sources could be used to rearrange bonds in carbon dioxide (CO2) and water molecules into complex hydrocarbons, which can then be burnt to produce new energy and CO2, ultimately enabling a carbon cycle. Copper is a catalytic material that has been found to be promising for enabling this process and facilitating the CO2 electroreduction reaction (CO2RR).
Two key elements when trying to understand the parameters controlling the CO2RR reaction are a well-defined surface structure and known material composition. Past theoretical and experimental studies have shown that the C―C coupling pathway for generating ethylene is favored on the Cu(100) surface.
More recently, researchers noticed the key catalytic role played by Cuδ+ and sub-surface oxygen for the production of C2-C3 hydrocarbons and alcohol. However, stabilizing copper under the conditions necessary for the CO2 electroreduction reaction (CO2RR) to take place has so far proved to be very challenging.
Researchers at the Fritz-Haber Institute, part of the Max-Plank Society in Berlin, have carried out a study aimed at stabilizing Cu(I), copper in the 1+ oxidation state, in order to better understand its role in the CO2RR reaction. In a recent paper, published in Nature Energy, they reported an enhanced efficiency in producing ethanol using copper, attained by tuning the structure and oxidation state of Cu (I) catalysts.
"So far, the stabilization of Cu(I) species under CO2 reduction conditions has proved to be very difficult," Beatriz Roldan Cuenya, one of the researchers who carried out the study, told TechXplore. "The main goal of our study was to be able to generate Cu(I) species and transiently stabilize them on a well-defined surface, to then study their impact on the CO2RR product selectivity."
In their study, Roldan Cuenya and her colleagues tuned the structure and oxidation state of copper catalysts using a technique known as pulsed electrolysis. This technique allowed them to design a pulsed potential sequence, which enabled the simultaneous tuning of both the surface structure and composition of Cu catalysts during the CO2RR reaction.
The researchers monitored changes in the catalyst's structure as well as the chemical state of its surface. This ultimately led to interesting new findings about the mechanisms through which copper catalysts enable the generation of hydrocarbons via the CO2RR reaction.
"Our findings suggest that the combination of (100) domains, defect sites, and surface Cu2O is the best configuration to enhance the CO2RR reaction pathway leading to C2+ products," Roldan Cuenya explained. "In particular, an increased ethanol selectivity could be linked to the coexistence of Cu(I) and Cu0 species, while the ethylene yield was dominated by the length of the Cu(100) terraces."
The recent study carried out by this team of researchers gathered new interesting findings that shed some light on the role of copper catalysts in facilitating the electrochemical conversion of CO2. In the future, the technique used by Roldan Cuenya and her colleagues could be used to tune electrochemical interfaces with constrained surface structures and compositions, so that they can be used to selectively produce C2 products.
"In our next studies, would like to explore the effect of the continuous regeneration of Cu(I) species on other surface orientations and finally apply this pulsed protocol to other nanoparticulated systems aiming more practical applications in real electrolyzers," Roldan Cuenya said.
Federico Calle-Vallejo et al. Theoretical Considerations on the Electroreduction of CO to C2Species on Cu(100) Electrodes, Angewandte Chemie International Edition (2013). DOI: 10.1002/anie.201301470
Javed Hussain et al. Calculations of Product Selectivity in Electrochemical CO2 Reduction, ACS Catalysis (2018). DOI: 10.1021/acscatal.7b03308
Yoshio Hori et al. Selective Formation of C2 Compounds from Electrochemical Reduction of CO2at a Series of Copper Single Crystal Electrodes, The Journal of Physical Chemistry B (2002). DOI: 10.1021/jp013478d
Yun Huang et al. Electrochemical Reduction of CO2 Using Copper Single-Crystal Surfaces: Effects of CO* Coverage on the Selective Formation of Ethylene, ACS Catalysis (2017). DOI: 10.1021/acscatal.6b03147
Hemma Mistry et al. Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene, Nature Communications (2016). DOI: 10.1038/ncomms12123
Hai Xiao et al. Cu metal embedded in oxidized matrix catalyst to promote CO 2 activation and CO dimerization for electrochemical reduction of CO 2, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1702405114
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