Activity and Selectivity of Pd-Based Catalysts for the Electrochemical Conversion of CO2 to CO
Authors: Reichert Andreas M. , Paul Michael T. Y. , Löffler Mario , Freakley Simon , Katsounaros Ioannis 
Published: May 2020

Electrochemical conversion allows the production of added-value chemicals by direct use of electricity. If the electrical energy is provided by renewables, electrosynthesis will offer a unique opportunity to harvest surplus renewable energy and to store it in the form of chemical energy, i.e. in the form of chemical bonds.These processes will become highly appealing, if the cost for this production of chemicals is comparable to that of conventional synthesis methods. To achieve that, it is essential to develop catalysts which can carry out the reactions of interest with minimal energy losses and with high selectivity, i.e. with limited formation of less valuable by-products. [1, 2]

One important industrial feedstock chemical is carbon monoxide (CO) which can be produced by steam reforming. Although this process is economically feasible, it is not environmentally friendly since it uses in general natural gas, is energy demanding and requires high temperatures. A “green” alternative would be the electroreduction of carbon dioxide (CO2) using renewable energy and CO2 from irreplaceable sources, e.g. concrete plants. The most prominent representatives of catalysts for the selective electroconversion of CO2 to CO are gold, silver and zinc. Although all three metals produce solely CO as gaseous product and hydrogen from the competing water reduction, silver is regarded as reference catalyst to beat.[3, 4]

Hence, we envisage the exploitation of new catalysts for the selective electrochemical reduction of CO2 to CO, aiming to find materials that catalyze the reaction at a lower overpotential compared to metallic silver. Here, we present results on alternative catalysts, esp. bimetallic palladium-gold nanoparticles on carbon black (PdXAuY/C). We investigated five different compositions of Pd and Au. All of them were able to reduce CO2 to CO. The CO yield was strongly dependent on the Pd:Au ratio and the applied potential. Simplified, the higher the amount of gold in the nanoparticles the lower was the necessary overpotential and the higher were the reached Faradaic efficiencies, up to 87% at -0.59 VRHE for Pd1Au3/C.
We obtained the data by using our three-electrode, two-compartment cell. CO2 was continuously introduced into the electrolyte and the gas-outlet entered an online gas chromatograph to detect gaseous products. The electrolyte was collected after the measurement and analysed via ion chromatography and GC/MS to look for possible liquid products. Several characterization techniques, including XPS and IL-TEM, were used to identify changes in the composition or morphology of the samples before and after electrolysis.

Acknowledgements

This work was supported by the Federal Ministry for Education and Research (BMBF) under the project grants 033RC004C (eEthylen) and 03SFK2Z0 (Power-to-X).

References

1. Haas, T., et al., Technical photosynthesis involving CO2 electrolysis and fermentation. Nature Catalysis, 2018. 1(1): p. 32-39.
2. Martín, A.J., et al., Towards sustainable fuels and chemicals through the electrochemical reduction of CO2: lessons from water electrolysis. Green Chemistry, 2015. 17(12): p. 5114-5130.
3. Hori, Y., et al., Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochimica Acta, 1994. 39(11/12): p. 1833-1839.
4. Artz, J., et al., Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment. Chem Rev, 2018. 118(2): p. 434-504.