Oxide-derived Copper Catalysts Research Articles

Unraveling the rate-determining step of C2+ products during electrochemical CO reduction

The electrochemical reduction of CO has drawn a large amount of attention due to its potential to produce sustainable fuels and chemicals by using renewable energy. However, the reaction’s mechanism is not yet well understood. A major debate is whether the rate-determining step for the generation of multi-carbon products is C-C coupling or CO hydrogenation. This paper conducts an experimental analysis of the rate-determining step, exploring pH dependency, kinetic isotope effects, and the impact of CO partial pressure on multi-carbon product activity. Results reveal constant multi-carbon product activity with pH or electrolyte deuteration changes, and CO partial pressure data aligns with the theoretical formula derived from *CO-*CO coupling as the rate-determining step. These findings establish the dimerization of two *CO as the rate-determining step for multi-carbon product formation. Extending the study to commercial copper nanoparticles and oxide-derived copper catalysts shows their rate-determining step also involves *CO-*CO coupling. This investigation provides vital kinetic data and a theoretical foundation for enhancing multi-carbon product production.

Maximizing Roughness Factors in Oxide-Derived Copper Coatings through Electrodeposition Parameters for Enhanced Electrocatalytic Performance

The pursuit of novel techniques for obtaining dispersed copper-based catalysts is crucial in addressing environmental issues like decarbonization. One method for producing nanostructured metals involves the reduction of their oxides, a technique that has found widespread use in CO2 electroreduction. Currently, the intrinsic activities of oxide-derived copper electrocatalysts produced via different routes cannot be compared effectively due to the lack of information on electrochemically active surface area values, despite the availability of electrochemical methods that enable estimation of surface roughness for highly dispersed copper coatings. In this study, we aim to explore the potential of oxide-derived copper to achieve a high electrochemically active surface area by examining samples obtained from acetic and lactic acid deposition solutions. Our results revealed that Cu2O oxides had distinct morphologies depending on the electrodeposition solution used; acetate series samples were dense films with a columnar structure, while electrodeposition from lactic acid yielded a fine-grained, porous coating. The roughness factors of the electroreduced films followed linear relationships with the deposition charge, with significantly different slopes between the two solutions. Notably, a high roughness factor of 650 was achieved for samples deposited from lactic acid solution, which represents one of the highest estimates of electrochemically active surface area for oxide-derived copper catalysts. Our results highlight the importance of controlling the microstructure of the electrodeposited oxide electrocatalysts to maximize surface roughness.

Creating interfaces of Cu0/Cu+ in oxide-derived copper catalysts for electrochemical CO2 reduction to multi-carbon products

Electrochemical carbon dioxide reduction reaction (CO2RR) is an effective approach to capture CO2 and convert it into value-added chemicals and fuels, thereby reducing excess CO2 emissions. Recent reports have shown that copper-based catalysts exhibit excellent performance in converting CO2 into multi-carbon compounds and hydrocarbons. However, theselectivityto the couplingproductsispoor. Therefore, tuningCO2-reductionselectivitytoward C2+productsover Cu-based catalyst is one of the most important issues in CO2RR. Herein, we prepare a nanosheet catalyst with interfaces of Cu0/Cu+. The catalyst achieves Faraday efficiency (FE) of C2+ over 50% in a wide potential window between − 1.2 V to − 1.5 V versus reversible hydrogen electrode (vs. RHE). Moreover, the catalyst exhibits maximum FE of 44.5% and 58.9% towards C2H4 and C2+, with a partial current density of 10.5 mA cm−2 at − 1.4 V. Density functional theory (DFT) calculations show that the interface of Cu0/Cu+ facilitates CC coupling to form C2+ products, while inhibits CO2conversion toC1products.

The nature of active sites for carbon dioxide electroreduction over oxide-derived copper catalysts

The active sites for CO2 electroreduction (CO2R) to multi-carbon (C2+) products over oxide-derived copper (OD-Cu) catalysts are under long-term intense debate. This paper describes the atomic structure motifs for product-specific active sites on OD-Cu catalysts in CO2R. Herein, we describe realistic OD-Cu surface models by simulating the oxide-derived process via the molecular dynamic simulation with neural network (NN) potential. After the analysis of over 150 surface sites through NN potential based high-throughput testing, coupled with density functional theory calculations, three square-like sites for C–C coupling are identified. Among them, Σ3 grain boundary like planar-square sites and convex-square sites are responsible for ethylene production while step-square sites, i.e. n(111) × (100), favor alcohols generation, due to the geometric effect for stabilizing acetaldehyde intermediates and destabilizing Cu–O interactions, which are quantitatively demonstrated by combined theoretical and experimental results. This finding provides fundamental insights into the origin of activity and selectivity over Cu-based catalysts and illustrates the value of our research framework in identifying active sites for complex heterogeneous catalysts.

The in situ study of surface species and structures of oxide-derived copper catalysts for electrochemical CO2 reduction.

Oxide-derived copper (OD-Cu) has been discovered to be an effective catalyst for the electroreduction of CO2 to C2+ products. The structure of OD-Cu and its surface species during the reaction process are interesting topics, which have not yet been clearly discussed. Herein, in situ surface-enhanced Raman spectroscopy (SERS), operando X-ray absorption spectroscopy (XAS), and 18O isotope labeling experiments were employed to investigate the surface species and structures of OD-Cu catalysts during CO2 electroreduction. It was found that the OD-Cu catalysts were reduced to metallic Cu(0) in the reaction. CuOx species existed on the catalyst surfaces during the CO2RR, which resulted from the adsorption of preliminary intermediates (such as *CO2 and *OCO−) on Cu instead of on the active sites of the catalyst. It was also found that abundant interfaces can be produced on OD-Cu, which can provide heterogeneous CO adsorption sites (strong binding sites and weak binding sites), leading to outstanding performance for obtaining C2+ products. The Faradaic efficiency (FE) for C2+ products reached as high as 83.8% with a current density of 341.5 mA cm−2 at −0.9 V vs. RHE.

Active and Selective Ensembles in Oxide-Derived Copper Catalysts for CO2 Reduction

Copper catalysts are unique in CO2 reduction as they allow the formation of C2+ products. Depending on the catalysts’ synthesis, product distribution varies significantly: while Cu nanoparticles pr...

Pulsed Electrochemical Carbon Monoxide Reduction on Oxide-Derived Copper Catalyst.

Efficient electroreduction of carbon dioxide has been a widely pursued goal as a sustainable method to produce value-added chemicals while mitigating greenhouse gas emissions. Processes have been demonstrated for the electroreduction of CO2 to CO at nearly 100 % faradaic efficiency, and as a consequence, there has been growing interest in the further electroreduction of carbon monoxide. Oxide-derived copper catalysts have promising performance for the reduction of CO to hydrocarbons but have still been unable to achieve high selectivity to individual products. A pulsed-bias technique is one strategy for tuning electrochemical selectivity without changing the catalyst. Herein a pulsed-bias electroreduction of CO was investigated on oxide-derived copper catalyst. Increased selectivity for single-carbon products (i.e., formate and methane) was achieved for higher pulse frequencies (<1 s pulse times), as well as an increase in the fraction of charge directed to CO reduction rather than hydrogen evolution.

Time-Resolved Analysis of CO2 Reduction Products on Copper and Oxide-Derived Copper Electrodes

Electrochemical conversion of excess carbon dioxide to useful chemicals and fuels is one way of lowering the anthropogenic carbon footprint, and realizing a route to synthetic reactions. Seminal work from Hori and coworkers1, 2 found that copper stands out from a plethora of materials to catalyze the formation of hydrocarbons such as methane or ethylene. More recent efforts were made to give a more complete description of the product distribution, to selectively direct the reaction towards the formation of C2 and C3 products, and to suppress the competing hydrogen evolution reaction in aqueous electrolytes3-7. Li and Kanan succeeded in promoting the C-C bond formation on an oxide-derived copper electrode, prepared by thermal annealing of copper in air, followed by reduction of the formed oxide8, 9. The performance of such systems is most commonly assessed in steady-state electrolysis by ex situ analytics such as online gas chromatography, or nuclear magnetic resonance7, 10. These techniques have proven useful to obtain quantitative information about the product distribution on a time scale of minutes to hours. To capture transient events on dynamic interfaces, to detect short-living products, and for fast screening of catalysts, we developed the electrochemical real-time mass spectrometry (EC-RTMS)11.Herein, we present a novel and straight-forward approach to quantify the liquid products of the CO2 electrochemical reduction in real time, just seconds after their formation on the electrode. We successfully couple mass spectrometry to an electrolytic flow cell and we monitor seven liquid reaction products at unique mass ion signals. The rate of formation of products is resolved down to a time interval of a few seconds. Polycrystalline and oxide-derived copper catalysts are compared and the role of intermediately formed products on both catalysts is discussed.

Electrochemical CO2 reduction reaction on cost-effective oxide-derived copper and transition metal–nitrogen–carbon catalysts

Abstract The electrochemical CO2 reduction reaction (CO2RR) either to generate multicarbon (C2+) or single carbon (C1) value-added products provides an effective and promising approach to mitigate the high CO2 concentration in the atmosphere and promote energy storage. However, cost-effectiveness of catalytic materials limits practical application of this technology in the short term. Herein, we summarize and discuss recent and advanced works on cost-effective oxide-derived copper catalysts for the generation of C2+ products (hydrocarbons and alcohols) and transition metal–nitrogen–doped carbon electrocatalytic materials for C1 compounds production from CO2RR. We think they represent suitable electrocatalyst candidates for scaling up electrochemical CO2 conversion. This short review may provide inspiration for the future design and development of innovative active, cost-effective, selective and stable electrocatalysts with improved properties for either the production of C2+ (alcohols, hydrocarbons) or carbon monoxide from CO2RR.

Quantitatively Unraveling the Redox Shuttle of Spontaneous Oxidation/Electroreduction of CuO x on Silver Nanowires Using in Situ X-ray Absorption Spectroscopy.

Oxide-derived copper catalysts have been shown to enhance CO2 reduction reaction (CO2RR) activity with high selectivity toward hydrocarbon products. However, the chemical state of oxide-derived copper during the CO2RR has remained elusive and is lacking in situ observations. Herein, a two-step process was developed to synthesize Ag nanowires coated with various thicknesses of a CuOx layer for the CO2RR. By employing in situ X-ray absorption spectroscopy, a strong correlation between the chemical state under reaction conditions and the CO2RR product profile can be revealed to validate another competing reaction (i.e., the spontaneous oxidation of Cu(0) in aqueous electrolyte) that significantly governs the chemical state of active centers of Cu. In situ Raman spectroscopy reveals the existence of reoxidation behavior under cathodic potential, and the quantification analysis of reoxidized behavior is revealed to indicate that the reoxidation rate is independent of surface morphology and strongly proportional to the electrochemically surface area. The steady oxidation state of Cu in an in situ condition is the paramount key and dominates the products’ profile of the CO2RR rather than other factors (e.g., crystal facets, atomic arrangements, morphology, elements) that have been investigated in numerous reports.

On the Oxidation State of Cu2 O upon Electrochemical CO2 Reduction: An XPS Study.

The encouraging selectivity of copper oxides for the electroreduction of CO2 into ethylene and alcohols has led to a vivid debate on the possible relation between their operando (sub-)surface oxidation state (i. e. fully reduced or partially oxidized) and this distinct reactivity. The high roughness of the Cu oxides used in previous studies on this matter adds complexity to this controversy and motivated us to prepare quasi-planar Cu2 O thin films that displayed a CO2 reduction selectivity similar to that of oxide-derived copper catalysts reported in previous studies. Most importantly, when the post-mortem thin films were transferred for characterization in an air-free environment, X-ray photoelectron spectroscopy measurements confirmed their complete reduction in the course of the CO2 reduction reaction. Thus, our results indicate that the selectivity of the Cu oxides featured in previous studies stems from their enhanced roughness, highlighting the importance of controlled sample transfer upon post-mortem characterization with ex situ techniques.

Operando Spectroscopic Investigations of Copper and Oxide-Derived Copper Catalysts for Electrochemical CO Reduction

Oxide-derived copper (OD-Cu) has been shown to favor C2 and C3 products in electrochemical CO reduction at lower overpotentials than polycrystalline copper (Cu-poly). Despite numerous studies and proposed mechanisms, the exact nature of the active phase is still a topic of discussion. In this work, we employ operando attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) to investigate different sites available on Cu-poly and OD-Cu surfaces using CO as a probe molecule. We identify a CO adsorption band on OD-Cu at 2058 cm–1 that is different from those on Cu-poly but resembles CO bound to the Cu(100) facet. In accordance with reactivity studies, we propose that this band corresponding to distinct CO binding sites is responsible for OD-Cu’s enhanced CO reduction activity.

–CH3 Mediated Pathway for the Electroreduction of CO2 to Ethane and Ethanol on Thick Oxide-Derived Copper Catalysts at Low Overpotentials

Oxide-derived copper is known for its unique ability to catalyze the selective electroreduction of CO2 to C2 and higher carbon compounds at low overpotentials. To understand this phenomenon, mechanistic studies typically chose ethylene (C2H4) as the model compound. The pathways to form other C2 compounds such as ethane (C2H6) and ethanol are then generally considered to be similar to that of C2H4. However, regular detection of C2H6 or ethanol on thick oxide-derived Cu at low overpotentials, often with selectivities exceeding that of C2H4, raises an important question: does the formation of these two C2 molecules really share a common route with C2H4? In this work, through an investigation of CO2 electroreduction on oxide-derived Cu of different thicknesses and oxidation states, we show that the formation of C2H6 and ethanol on thick oxide-derived Cu films could proceed through routes distinct from that of C2H4 at low overpotentials. Investigations using select molecular precursors such as diacetyl [(CH3CO...

Optimizing C–C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction

Copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO2 and better selectivity toward C2 and C3 (C2+) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO2 reduction, with particular attention given to the selectivity to C2+ products. All catalysts were tested for CO2 reduction in 0.1 M KHCO3 and 0.1 M CsHCO3 at applied voltages in the range from −0.7 to −1.0 V vs RHE. The best performing oxide-derived catalysts show up to ∼70% selectivity to C2+ products and only ∼3% selectivity to C1 products at −1.0 V vs RHE when CsHCO3 is used as the electrolyte. In contrast, the selectivity to C2+ products decreases to ∼56% for the same catalysts tested in KHCO3. By studying all catalysts under identical conditions, the key factors affecting product selectivit...

Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene.

There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper+ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper+ is key for lowering the onset potential and enhancing ethylene selectivity.