Operando Insight into the Correlation between the Structure and Composition of CuZn Nanoparticles and Their Selectivity for the Electrochemical CO₂ Reduction

Abstract

Bimetallic CuZn catalysts have been recently proposed as alternatives in order to achieve selectivity control during the electrochemical reduction of CO2 (CO2RR). However, fundamental understanding of the underlying reaction mechanism and parameters determining the CO2RR performance is still missing. In this study, we have employed size-controlled (∼5 nm) Cu100–xZnx nanoparticles (NPs) supported on carbon to investigate the correlation between their structure and composition and catalytic performance. By tuning the concentration of Zn, a drastic increase in CH4 selectivity [∼70% Faradaic efficiency (F.E.)] could be achieved for Zn contents from 10 to 50, which was accompanied by a suppression of the H2 production. Samples containing a higher Zn concentration displayed significantly lower CH4 production and an abrupt switch in the selectivity to CO. Lack of metal leaching was observed based on quasi in situ X-ray photoelectron spectroscopy (XPS). Operando X-ray absorption fine structure (XAFS) spectroscopy measurements revealed that the alloying of Cu atoms with Zn atoms takes place under reaction conditions and plays a determining role in the product selectivity. Time-dependent XAFS analysis showed that the local structure and chemical environment around the Cu atoms continuously evolve during CO2RR for several hours. In particular, cationic Zn species initially present were found to get reduced as the reaction proceeded, leading to the formation of a CuZn alloy (brass). The evolution of the Cu–Zn interaction with time during CO2RR was found to be responsible for the change in the selectivity from CH4 over Cu-ZnO NPs to CO over CuZn alloy NPs. This study highlights the importance of having access to in depth information on the interplay between the different atomic species in bimetallic NP electrocatalysts under operando reaction conditions in order to understand and ultimately tune their reactivity.

Linking the evolution of catalytic properties and structural changes in copper–zinc nanocatalysts using operando EXAFS and neural-networks

“…As discussed in our previous study, 13 different catalytic selectivities for the CuZn NPs were obtained as a function of the Cu to Zn ratio. Moreover, the new experimental data in Fig.…”

“…Previously this feature was attributed to an additional long Zn-O bond not present in wurtzite-type ZnO, and/or to the presence of disordered hydroxide or amorphous ZnO phases. 13,47,69 Due to the low intensity of this feature, however, it is challenging to assign it to any particular structural motif in conventional EXAFS analysis. Results of the NN analysis of the Zn K-edge EXAFS spectra of the as-prepared samples (Fig.…”

“…Synthesis and characterization details are given in ref. 13. The elemental composition was modied by controlling the molar ratio of the Cu and Zn precursor salts, CuCl 2 and Zn(CH 3 COO) 2 .…”

“…[1][2][3][4][5][6] In addition to the structural characteristics that are known to affect the catalytic activity and selectivity in monometallic systems (e.g., oxidation state, structure, nite-size effects, particle-support interactions), the additional degrees of freedom in bimetallic materials provide new paths to control the interactions with adsorbates, resulting in unique catalytic properties and applications. For example, the addition of a secondary metal, such as zinc, [7][8][9][10][11][12][13] is a common route to tailor the selectivity of copper and other metallic catalysts for the CO 2 electroreduction reaction (CO 2 RR) and to steer the reaction towards the production of valuable chemicals and fuels. [14][15][16][17][18][19][20][21][22][23] To understand the working mechanism of a bimetallic catalyst and the roles of various structural characteristics, the multielement structural motifs need to be detected and characterized experimentally.…”

“…We have recently shown that the catalytic selectivity of this system can be tuned by varying the copper to zinc ratio, and that signicant structural changes take place under reaction conditions. 13,47 Here we take advantage of the sensitivity of our NN-EXAFS method 44 to monitor subtle chemical and structural changes taking place during the CO 2 RR as a function of the Cu-Zn NP composition and time. In particular, we will show that the reduction of copper and zinc species takes place on different time-scales, and that gradual alloying and a transition from a close-packed to a more disordered structure take place.…”