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.
Highlights
The electrochemical reduction of CO2 (CO2RR) into useful chemicals and fuels has received much attention as a means to build carbon recycling systems.[1,2] efficient and inexpensive electrocatalysts are still required to reduce the thermodynamically stable CO2 molecule while suppressing the H2 evolution reaction (HER)
The representative extended X-ray absorption fine structure (EXAFS) spectrum for a sample after 1 h of CO2RR was obtained after merging 4 spectra collected during 40−80 min of CO2RR
It was confirmed that the composition of the as-prepared samples was consistent with the starting molar ratio of the precursor salts, and that it did not change after the CO2RR (Figure S4)
Summary
The electrochemical reduction of CO2 (CO2RR) into useful chemicals and fuels has received much attention as a means to build carbon recycling systems.[1,2] efficient and inexpensive electrocatalysts are still required to reduce the thermodynamically stable CO2 molecule while suppressing the H2 evolution reaction (HER). Recent studies have demonstrated enhanced reactivity of CuZn catalysts for CO2RR.[30−36] For example, nanoporous CuZn catalysts prepared by annealing and subsequent reduction of commercial CuZn alloy foils showed four and six times higher Faradaic efficiency (F.E.) for CO and HCOOH than those of the untreated CuZn foils.[32] Zn-coated Cu electrodes exhibited higher selectivity for CH4 (52% F.E.) than bare Cu (23% FE for CH4).[33] Oxide-derived CuZn catalysts were favorable toward the formation of C2 products (i.e., C2H4 and C2H5OH), and it was possible to tune the ratio of these products by varying the amount of Zn.[34] This trend in the C2 selectivity was postulated to be due to the spillover of CO from Zn to Cu sites, which was thought to facilitate the production of C2 products at the Cu site.[34] Such CO spillover effects were found to be facilitated when a homogeneous distribution of Cu and Zn atoms was formed in the CuZn catalysts.[35] Despite the former encouraging empirical results, significant discrepancies in the product selectivity of seemingly similar CuZn systems have been reported, and fundamental. The gradual formation of a Cu−Zn alloy could be observed in the course of the CO2RR and correlated to the switch in selectivity from CH4 to CO
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