Steering CO 2 Electroreduction to Methane with an Atomically Precise Copper Nanocluster

Atomically precise copper nanoclusters have recently attracted tremendous attention, as they often exhibit structures and properties distinct to their gold and silver counterparts. Although considerable progress has been made in the preparation and structure determination of copper nanoclusters, the introduction of those newly created materials in more underlying areas such as nonlinear optics is in its infancy. Herein, we report a new class of Cu50 nanoclusters exhibiting a tunable optical limiting effect. The facile preparation prototype reported in this work allows the discovery of up to four clusters with the nearly identical metal framework, namely, Cu50(CF3COO)12(3,5-diMe-PhS)18(PPh3)4H2 (Cu50-1), Cu50(CF3COO)10(4-F-PhS)20(PPh3)6H2 (Cu50-2), Cu50(CF3COO)10(PhS)20(PPh3)6H2 (Cu50-3), and Cu50(PhCOO)10(4-F-PhS)20(PPh3)6H2 (Cu50-4). The molecular structure of the Cu50 series, as revealed by X-ray single crystal analysis, describes the stabilization of Cu44 core by an organometallic shell of thiolate-carboxylate-phosphine-copper. The combinatorial studies by electrospray ionization mass spectroscopy (ESI-MS), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations suggest that the clusters contain 18 free valence electrons, making them probably the first library of copper superatoms featuring 18e. As revealed by the nanosecond Z-scan measurements results, the clusters exhibit typical optical limiting effects, indicating their promising applications in the field of nonlinear optics (NLO). More importantly, the labile surface motifs on Cu50 allow us to tune their NLO properties. This work not only reports a new class of atomically precise copper nanoclusters with facile preparation process, clear structures, and labile surface motifs but also encourages research on the exploration of copper nanoclusters in more exciting areas.

Metal oxide/metal chalcogenide heterostructure materials have garnered intensive attention in both fundamental research and practical applications, particularly in the field of catalysis. For example, the Cu2O/CuS heterostructures have found wide applications in electrocatalysis and photoelectrocatalysis. Despite their wide-ranging applications, the atomic-level insight into the interfacial structures of Cu2O/CuS heterostructure materials and the effect of such interface on catalysis has remained a longstanding challenge. Herein, we present a comprehensive illustration of Cu2O/CuS heterostructures through the complete structural determination of an atomically precise copper nanocluster, Cu40O8(tBuS)24 (tBuSH is 2-methyl-2-propanethiol) whose structure describes the stabilization of Cu4O tetrahedra by −S–Cu–S– units. Notably, the cluster has been prepared in one pot under ambient conditions and exhibits exceptional stability when subjected to heat and highly concentrated acid (1 M HCl) or base (1 M KOH), respectively. Importantly, the cluster exhibits outstanding activity in the electrocatalytic hydrogen evolution reaction (HER), surpassing the performance of most reported copper nanoclusters with an overpotential of 90 mV at 10 mA/cm2. Both the experimental and theoretical investigations suggest that the heterostructures of the cluster play a pivotal role in enhancing its electrocatalytic HER activity. This work not only provides crucial insights into the long-pursued interfacial structures of Cu2O/CuS heterostructure materials but also paves the way for the fabrication of atomically precise metal nanoclusters featuring metal oxide/metal chalcogenide heterostructures.

Owing to distinct physicochemical properties in comparison to gold and silver counterparts, atomically precise copper nanoclusters are attracting embryonic interest in material science. The introduction of copper cluster nanomaterials in more interesting fields is currently urgent and desired. Reported in this work are novel copper nanoclusters of [XCu54Cl12(tBuS)20(NO3)12] (X=S or none, tBuSH=2-methyl-2-propanethiol), which exhibit high performance in photothermal conversion. The clusters have been prepared in one pot and characterized by combinatorial techniques including ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS). The molecular structure of the clusters, as revealed by single crystal X-ray diffraction analysis (SCXRD), shows the concentric three-shell Russian doll arrangement of X@Cu14@Cl12@Cu40. Interestingly, the [SCu54Cl12(tBuS)20(NO3)12] cluster contains 8 free valence electrons in its structure, making it the first eight-electron copper nanocluster stabilized by thiolates. More impressively, the clusters possess an effective photothermal conversion (temperature increases by 71 °C within ~50 s, λex=445 nm, 0.5 W cm-2) in a wide wavelength range (either blue or near-infrared). The photothermal conversion can be even driven under irradiation of simulated sunlight (3 sun), endowing the clusters with great potency in solar energy utilization.

Despite recent advances in cluster‐based catalysis for organic synthesis, the substrate scope of reactions catalyzed by metal nanoclusters is typically not superior to previously established catalytic systems. Herein, we develop new atomically precise copper nanoclusters for protosilylation, with scope expanding to alkenes and simple enynes that were not suitable for prior synthetic methodologies with traditional copper complexes. The involvement of a second copper center in the metal kernel during the migratory insertion step is thought to be responsible for the expanded scope. In addition, the reaction is highly compatible with water and can be carried out in open air rather than under inert gas protection. Mechanistic studies suggest that the cluster‐catalyzed protosilylation proceeds in the absence of silyl radicals. The current findings demonstrate the potential of using metal nanoclusters for practical and sustainable chemical synthesis.

Noble metal nanoclusters (NCs) are a new class of nanomaterials which are considered being a missing link between isolated metal atoms and metal nanoparticles (NPs). The sizes of the NCs are comparable to the Fermi wavelength of the conduction electrons, and this renders them to be luminescent in nature. They exhibit size-dependent fluorescence properties spanning almost the entire breath of the visible spectrum. Among all the noble metal NCs being explored, copper NCs (CuNCs) are the most rarely investigated primarily because of their propensity of getting oxidised. In this chapter, we have given a comprehensive understanding as to why these NCs are luminescent in nature. We have also given a detailed overview regarding the various templates used for the synthesis of these CuNCs along with the respective protocols being followed. The various instrumental techniques used to characterize these CuNCs are discussed which provides an in-depth understanding as to how these CuNCs can be properly examined. Finally, we have highlighted some of the most recent applications of these CuNCs which make them unique to serve as the next-generation fluorophores.Graphical Abstract:The Graphical Abstract highlights some of the key spectroscopic signatures of the CuNCs and their applications.

We report the synthesis, structure analysis, and electrocatalytic CO2 reduction application of Ag19Cu2(CCArF)12(PPh3)6Cl6 (abbreviated as Ag19Cu2, CCArF: 3,5-bis(trifluoromethyl)phenylacetylene) nanoclusters. Ag19Cu2 has characteristic absorbance features and is a superatomic cluster with 2 free valence electrons. Single-crystal X-ray diffraction (SC-XRD) revealed that the metal core of Ag19Cu2 is composed of an Ag11Cu2 icosahedron connected by two Ag4 tetrahedra at the two terminals of the Cu-Ag-Cu axis. Notably, Ag19Cu2 exhibited excellent catalytic performance in the electrochemical CO2 reduction reaction (eCO2RR), manifested by a high CO faradaic efficiency of 95.26% and a large CO current density of 257.2 mA cm-2 at -1.3 V. In addition. Ag19Cu2 showed robust long-term stability, with no significant drop in current density and FECO after 14 h of continuous operation. Density functional theory (DFT) calculations disclosed that the high selectivity of Ag19Cu2 for CO in the eCO2RR process is due to the shedding of the -CCArF ligand from the Ag atom at the very center of the Ag4 unit, exposing the active site. This study enriches the potpourri of alkynyl-protected bimetallic nanoclusters and also highlights the great advantages of using atomically precise metal nanoclusters to probe the atomic-level structure-performance relationship in the catalytic field.

Understanding the role of the oxidation state of the Cu surface and surface-adsorbed intermediate species in electrochemical CO2 reduction is crucial for the development of selective CO2-to-fuel electrocatalysts. In this study, the electrochemical CO2 reduction mechanism over the Cu catalysts with various oxidation states was studied by using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), in situ soft X-ray absorption spectroscopy (Cu L-edge), and online gas chromatography measurements. The atop-adsorbed CO (COatop) intermediate is obtained on the electrodeposited Cu surface which primarily has the oxidation state of Cu(I). COatop is further reduced, followed by the formation of C1 product such as CH4. The residual bridge-adsorbed CO (CObridge) is formed on the as-prepared Cu surface with Cu(0) which inhibits hydrocarbon formation. In contrast, the CV-treated Cu electrode prepared by oxidizing the as-prepared Cu surface contains different amounts of Cu(I) and Cu(0) states. The major theme of this work is that in situ SEIRAS results show the coexistence of COatop and CObridge as the reaction intermediates during CO2 reduction and that the selectivity of CO2-to-ethylene conversion is further enhanced in the CV-treated Cu electrode. The Cu catalysts modulated by the electrochemical method exhibit different oxidation states and reaction intermediates as well as electrocatalytic properties.

Visualization of catalytic edge reactivity in electrochemical CO2 reduction on porous Zn electrode

Energy recycling and production using abundant atmospheric CO2 and H2O have increasingly attracted attention for solving energy and environmental problems. Herein, Pt-loaded Ti sheets were prepared by sputter-deposition and Pt4+-reduction methods, and their catalytic activities on both photocatalytic CO2 reduction and electrochemical hydrogen evolution were fully demonstrated. The surface chemical states were completely examined by X-ray photoelectron spectroscopy before and after CO2 reduction. Gas chromatography confirmed that CO, CH4, and CH3OH were commonly produced as CO2 reduction products with total yields up to 87.3, 26.9, and 88.0 μmol/mol, respectively for 700 °C-annealed Ti under UVC irradiation for 13 h. Pt-loading commonly negated the CO2 reduction yields, but CH4 selectivity was increased. Electrochemical hydrogen evolution reaction (HER) activity showed the highest activity for sputter-deposited Pt on 400 °C-annealed Ti with a HER current density of 10.5 mA/cm2 at −0.5 V (vs. Ag/AgCl). The activities of CO2 reduction and HER were found to be significantly dependent on both the nature of Ti support and the oxidation states (0,II,IV) of overlayer Pt. The present result could provide valuable information for designing efficient Pt/Ti-based CO2 recycle photocatalysts and electrochemical hydrogen production catalysts.

Boosting C–C coupling to multicarbon products via high-pressure CO electroreduction

Copper‐based 3D fractal nanostructures are integrated on the electrodes using a scalable and ink‐free flame aerosol synthesis technique for electrochemical CO2 reduction. The effects of tortuosity and oxidation state of copper are respectively investigated by isolating each effect from the others. By balancing the intermediate confinement and local availability of CO2, CuO‐derived Cu with optimal tortuosity exhibits a Faradaic efficiency of 65% toward C2+ products at an applied potential of −1.04 V versus reversible hydrogen electrode. A subsequent study of the effects of the oxidation state, which is free from the influence of tortuosity, reveals that Cu2+‐derived Cu demonstrates suppressed hydrogen evolution reaction and a higher C2+/CH4 ratio than metallic Cu. The preference for the formation of both ethanol and n‐propanol versus ethylene, is found to follow the trend from metallic Cu > Cu2+‐derived Cu > Cu+‐derived Cu toward alcohols’ formation. These findings elucidate the underlying causes for the effects of tortuosity of porous Cu electrodes on selectivity and provide insights into the specific effects of the initial oxidation state on various reaction pathways during electrochemical CO2 reduction.

The acidic electrochemical CO2 reduction reaction (CO2RR) offer potential for enhanced carbon utilization efficiency in hydrocarbon production, but its selectivity is constrained by the competing hydrogen evolution reaction (HER) and the poor C2/C1 product control. Herein, we demonstrate that precise tailoring of copper nanoclusters enables exceptional HER suppression while promoting selective CO2-to-CH4 conversion under acidic conditions (pH 2). Cu36 nanoclusters achieve superior methane selectivity across a wide current density range (300–700 mA cm–2), achieving a CH4 partial current density (jCH4) of 288 mA cm–2. By analyzing the correlation between post-activation CO2RR performance and crystal structure, we reveal that the structural evolution of Cu nanoclusters significantly influences their product distribution under electrochemical conditions, and confirm that ligands stabilize the Cu36 cluster to avoid significant aggregation. Ligand-stabilized Cu exhibits enhanced stability in acidic media, maintaining operation for 70 h at 300 mA cm–2. Our approach addresses key acidic CO2RR challenges by simultaneously delivering high selectivity, activity, and stability─critical metrics for practical electrochemical carbon conversion.

Electrochemical reduction of CO2, an artificial way of carbon recycling, represents one promising solution for energy and environmental sustainability. Despite the many advantages, electrochemical reduction of CO2 is challenged by the absence of efficient catalysts for this reaction. Copper (Cu) is the most studied material capable of catalyzing CO2 reduction at significant rates, but it still requires large overpotentials; e.g., to reach a current density of 1 mA/cm2 on polycrystalline Cu electrode, it typically requires an overpotential of >0.5 V for producing CO and HCOOH (two-electron processes) and >0.8 V for further reduced products such as CH4 and C2H4. Moreover, hydrogen evolution competes with CO2 reduction, reducing the Faradaic efficiency (FE) towards carbon-containing compounds. Here we report the synthesis of highly dense Cu nanowires and tailoring of their surface structures to achieve superior performance for electrocatalytic CO2 and CO reduction, with particular attention to low-overpotential conditions. CuO nanowires were first grown by oxidizing Cu mesh in air which were then subjected to electrochemical reduction or annealing in a reducing atmosphere to form Cu nanowires. By tailoring the conditions for the reduction, high activity and selectivity were achieved for the production of CO from CO2, and ethanol/acetate from CO, at E < 0.4 V. It was found that the catalytic performances of the Cu nanowires are strongly correlated to the crystalline and surface structures in the nanoscale, based on which the active sites capable of selective reduction of CO2 and CO were identified. Our work indicates the great potential of electrochemical conversion of CO2 for synthesis of fuel molecules and production of chemical feedstocks.

The electrochemical reduction of CO2 has the potential to store energy from intermittent renewable sources and to produce carbon-neutral fuels and chemicals1. In recent years, theoretical studies of CO2 reduction have usually applied the computational hydrogen electrode model, which allows for the determination of the energies of reaction intermediates without explicitly treating the potential and the ions in solution2. This thermochemical approach has been shown to correlate well with experimental onset potentials 3,4 and applied to computational screening of new catalysts 5,6. However, an understanding of charge transfer barriers, kinetics, selectivity, and pH effects all require explicit consideration of solvent and charge. In this talk, I will discuss new developments in the explicit treatment of the electrochemical interface, which is based on a simple capacitor model that uses the interfacial charge to obtain barriers at constant potential 7,8. I will then discuss the application of such a scheme to CO2 reduction: the determination of reaction pathways on transition metals, field and solvation effects9,10, the resultant kinetics, and catalyst design criteria. 1 Whipple, D. T. & Kenis, P. J. A. Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. The Journal of Physical Chemistry Letters 1, 3451-3458, doi:10.1021/jz1012627 (2010). 2 Nørskov, J. K. et al. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 108, 17886--17892 (2004). 3 Shi, C., Hansen, H. A., Lausche, A. C. & Nørskov, J. K. Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces. Physical Chemistry Chemical Physics 16, 4720-4727, doi:10.1039/C3CP54822H (2014). 4 Kuhl, K. P. et al. Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. Journal of the American Chemical Society, doi:10.1021/ja505791r (2014). 5 Chan, K., Tsai, C., Hansen, H. A. & Nørskov, J. K. Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction. ChemCatChem 6, 1899-1905, doi:10.1002/cctc.201402128 (2014). 6 Asadi, M. et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nature communications 5 (2014). 7 Chan, K. & Nørskov, J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett., 2663--2668 (2015). 8 Chan, K. & Nørskov, J. K. Potential Dependence of Electrochemical Barriers from ab Initio Calculations. The Journal of Physical Chemistry Letters, 1686-1690, doi:10.1021/acs.jpclett.6b00382 (2016). 9 Montoya, J. H., Shi, C., Chan, K. & Norskov, J. K. Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction. The journal of physical chemistry letters 6, 2032--2037 (2015). 10 Chen, L. D., Urushihara, M., Chan, K. & Nørskov, J. Electric Field Effects in Electrochemical CO2 Reduction. ACS Catalysis (2016). 11 Liu, X. et al. Understanding trends in CO2 reductin on transition metals. (2016).

Modulating the oxidation state of copper (Cu) is crucial for enhancing the electrocatalytic CO2 reduction reaction (CO2RR), particularly for facilitating deep reductions to produce methane (CH4) or multi-carbon (C2+) products. However, Cuδ+ sites are thermodynamically unstable, fluctuating their oxidation states under reaction conditions, which complicates their functionality. Incorporating interfacial metal oxides has emerged as an effective strategy for stabilizing these oxidation states. This review provides an in-depth examination of the reaction mechanisms occurring at oxide-modified Cuδ+ sites, offering a comprehensive understanding of their behavior. We explore how Cu/metal oxide interfaces stabilize Cu oxidation states, showing that oxides-modified Cu catalysts often enhance selectivity for C2+ or CH4 products by stabilizing Cu+ or Cu2+ sites. In addition, we discuss innovative strategies for the rational design of efficient Cu catalytic sites tailored for specific deep CO2RR products. The review concludes with an outlook on current challenges and future directions, offering new insights into the rational design of selective and efficient CO2RR catalysts.