(Digital Presentation) Investigations on the Ethanol/Ethylene Bifurcation and Restructure of Cu/Ag Catalysts for Electrochemical CO₂ Reduction

Abstract

Electrochemical CO2 reduction (ECO2R) converts greenhouse gas CO2 into valuable fuels and chemicals, and thus helps with closing the anthropogenic carbon cycle. Currently, Cu is the only known material being capable of producing a variety of hydrocarbons and alcohols, while the poor selectivity limits its further use. Ethanol and ethylene have been proved to go through similar pathways but their bifurcation is yet to be fully understood. It has been found that introducing Ag atoms into Cu lattice could shift the product distribution toward ethanol compared to ethylene. However, previous studies have proposed contradictory speculations: DFT calculations predict the introduced Ag atoms prefer to dope on the undercoordinated sites on Cu surface [1], while experiments have proved that C-C coupling occurs at these sites and is promoted when they are occupied by Ag [1]–[3]. Literature also interpreted various mechanisms of the interaction between Cu and, such as the constrained effect [1], [4], “spillover” [5], and different C-C coupling pathways between *CO and *CHx (x=1,2) at the boundaries [6]. The oxidation state [7] and faceting [1] as well as the composition of CuAg catalysts over time have been observed during the reaction course, but explicitly real-time information remains scarce. To provide more mechanistic information on the above controversies, we prepared both bimetallic (with miscible Cu and Ag phases s) and surface alloy (with separated Cu and Ag phases) CuAg thin films by physical vapor deposition (PVD) and galvanic exchange, respectively. Ex situ X-ray Photoelectron Spectroscopy (XPS) and O perando X-ray Absorption Spectroscopy (XAS), including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) on surface alloy CuAg are performed under ECOR/ECO2R conditions to investigate the oxidation state and coordination numbers of Cu and Ag. By this means, electron transfer and the interface miscibility between Cu and Ag are identified. Combined with DFT calculations, we speculated possible doping sites of Ag atoms in the Cu lattice and potential adsorption sites of the produced intermediates for ethanol/ethylene formation. Besides, variations of the oxidation state, electric and geometric local structure, as well as the transformation in crystallinity of the CuAg catalysts over time are monitored by correlating XAS with operando Grazing Incidence X-ray Diffraction (GIXRD). The produced CO intermediates are substantiated to be the reason for Cu-enrichment occurring on the CuAg electrodes as speculated. References: [1] D. Higgins et al., “Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility,” ACS Energy Lett., vol. 3, no. 12, pp. 2947–2955, 2018, doi: 10.1021/acsenergylett.8b01736. [2] L. Wang et al., “Selective reduction of CO to acetaldehyde with CuAg electrocatalysts,” Proc. Natl. Acad. Sci. U. S. A., vol. 117, no. 23, pp. 12572–12575, 2020, doi: 10.1073/pnas.1821683117. [3] C. Hahn et al., “Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons,” Proc. Natl. Acad. Sci. U. S. A., vol. 114, no. 23, pp. 5918–5923, 2017, doi: 10.1073/pnas.1618935114. [4] E. L. Clark, C. Hahn, T. F. Jaramillo, and A. T. Bell, “Electrochemical CO2 Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity,” J. Am. Chem. Soc., vol. 139, no. 44, pp. 15848–15857, 2017, doi: 10.1021/jacs.7b08607. [5] S. Lee, G. Park, and J. Lee, “Importance of Ag-Cu Biphasic Boundaries for Selective Electrochemical Reduction of CO2 to Ethanol,” ACS Catal., vol. 7, no. 12, pp. 8594–8604, 2017, doi: 10.1021/acscatal.7b02822. [6] Y. C. Li et al., “Binding Site Diversity Promotes CO2 Electroreduction to Ethanol,” J. Am. Chem. Soc., vol. 141, no. 21, pp. 8584–8591, 2019, doi: 10.1021/jacs.9b02945. [7] S. B. Scott et al., “Absence of Oxidized Phases in Cu under CO Reduction Conditions,” ACS Energy Lett., vol. 4, no. 3, pp. 803–804, 2019, doi: 10.1021/acsenergylett.9b00172.