Charge-Separated Pdδ--Cuδ+ Atom Pairs Promote CO2 Reduction to C2

Experimental and Theoretical Insights into Single Atoms, Dual Atoms, and Sub-Nanocluster Catalysts for Electrochemical CO2 Reduction (CO2RR) to High-Value Products

Electrocatalytic carbon dioxide (CO2) conversion into valuable chemicals paves the way for the realization of carbon recycling. Downsizing catalysts to single-atom catalysts (SACs), dual-atom catalysts (DACs), and sub-nanocluster catalysts (SNCCs) has generated highly active and selective CO2 transformation into highly reduced products. This is due to the introduction of numerous active sites, highly unsaturated coordination environments, efficient atom utilization, and confinement effect compared to their nanoparticle counterparts. Herein, recent Cu-based SACs are first reviewed and the newly emerged DACs and SNCCs expanding the catalysis of SACs to electrocatalytic CO2 reduction (CO2RR) to high-value products are discussed. Tandem Cu-based SAC-nanocatalysts (NCs) (SAC-NCs) are also discussed for the CO2RR to high-value products. Then, the non-Cu-based SACs, DACs, SAC-NCs, and SNCCs and theoretical calculations of various transition-metal catalysts for CO2RR to high-value products are summarized. Compared to previous achievements of less-reduced products, this review focuses on the double objective of achieving full CO2 reduction and increasing the selectivity and formation rate toward C-C coupled products with additional emphasis on the stability of the catalysts. Finally, through combined theoretical and experimental research, future outlooks are offered to further develop the CO2RR into high-value products over isolated atoms and sub-nanometal clusters.

Near-Electrode Concentration Gradients of Bicarbonate and pH within Porous Gas Diffusion Electrode for Optimized Selective CO2 Electroreduction to C2+ Products

With high current density, the intense near-electrode CO2 reduction reaction (CO2RR) will cause the concentration gradients of bicarbonate (HCO3-) and hydroxyl (OH-) ions, which affect the selectivity of high-value C2+ products of the CO2RR. In this work, we simulated the near-electrode concentration gradients of electrolyte species with different porous Cu-based CLs (catalyst layers) of GDE (gas diffusion electrode) by COMSOL Multiphysics. The higher porosity CL exhibits a better buffer ability of local alkalinity while ensuring a sufficient supply of H+ and local CO2 concentration. Subsequently, the different porosity CLs were prepared by vacuum-thermal evaporation with different evaporation rate. Structural characterizations and liquid permeability tests confirm the role of the porous CL structure in optimizing concentration gradients. As a result, the high-porosity CL (Cu-HP) exhibits a higher C2+ Faraday efficiency (FE) of ∼79.61% at 500 mA cm-2 under 1 M KHCO3, far more than the FEC2+ ≈ 38.20% with the low-porosity sample (Cu-LP).

Stabilizing High-Valence Copper(I) Sites with Cu-Ni Interfaces Enhances Electroreduction of CO2 to C2+ Products

In this study, the copper-nickel (Cu-Ni) bimetallic electrocatalysts for electrochemical CO2 reduction reaction(CO2RR) are fabricated by taking the finely designed poly(ionic liquids) (PIL) containing abundant Salen and imidazolium chelating sites as the surficial layer, wherein Cu-Ni, PIL-Cu and PIL-Ni interaction can be readily regulated by different synthetic scheme. As a proof of concept, Cu@Salen-PIL@Ni(NO3)2 and Cu@Salen-PIL(Ni) hybrids differ significantly in the types and distribution of Ni species and Cu species at the surface, thereby delivering distinct Cu-Ni cooperation fashion for the CO2RR. Remarkably, Cu@Salen-PIL@Ni(NO3)2 provides a C2+ faradaic efficiency (FEC2+) of 80.9% with partial current density (jC 2+) of 262.9 mA cm-2 at -0.80 V (versus reversible hydrogen electrode, RHE) in 1 m KOH in a flow cell, while Cu@Salen-PIL(Ni) delivers the optimal FEC2+ of 63.8% at jC2+ of 146.7 mA cm-2 at -0.78 V. Mechanistic studies indicates that the presence of Cu-Ni interfaces in Cu@Salen-PIL@Ni(NO3)2 accounts for the preserve of high-valence Cu(I) species under CO2RR conditions. It results in a high activity of both CO2-to-CO conversion and C-C coupling while inhibition of the competitive HER.

Regulating the Critical Intermediates of Dual-Atom Catalysts for CO2 Electroreduction

Electrocatalysis is a very attractive way to achieve a sustainable carbon cycle by converting CO2 into organic fuels and feedstocks. Therefore, it is crucial to design advanced electrocatalysts by understanding the reaction mechanism of electrochemical CO2 reduction reaction (eCO2RR) with multiple electron transfers. Among electrocatalysts, dual-atom catalysts (DACs) are promising candidates due to their distinct electronic structures and extremely high atomic utilization efficiency. Herein, the eCO2RR mechanism and the identification of intermediates using advanced characterization techniques, with a particular focus on regulating the critical intermediates are systematically summarized. Further, the insightful understanding of the functionality of DACs originates from the variable metrics of electronic structures including orbital structure, charge distribution, and electron spin state, which influences the active sites and critical intermediates in eCO2RR processes. Based on the intrinsic relationship between variable metrics and critical intermediates, the optimized strategies of DACs are summarized containing the participation of synergistic atoms, engineering of the atomic coordination environment, regulation of the diversity of central metal atoms, and modulation of metal-support interaction. Finally, the challenges and future opportunities of atomically dispersed catalysts for eCO2RR processes are discussed.

Stabilizing the oxidation state of catalysts for effective electrochemical carbon dioxide conversion

In the electrocatalytic CO2 reduction reaction (CO2RR), metal catalysts with an oxidation state generally demonstrate more favorable catalytic activity and selectivity than their corresponding metallic counterparts. However, the persistence of oxidative metal sites under reductive potentials is challenging since the transition to metallic states inevitably leads to catalytic degradation. Herein, a thorough review of research on oxidation-state stabilization in the CO2RR is presented, starting from fundamental concepts and highlighting the importance of oxidation state stabilization while revealing the relevance of dynamic oxidation states in product distribution. Subsequently, the functional mechanisms of various oxidation-state protection strategies are explained in detail, and in situ detection techniques are discussed. Finally, the prevailing and prospective challenges associated with oxidation-state protection research are discussed, identifying innovative opportunities for mechanistic insights, technology upgrades, and industrial platforms to enable the commercialization of the CO2RR.

Enhanced Molecular Interaction of 3D Plasmonic Nanoporous Gold Alloys by Electronic Modulation for Sensitive Molecular Detection

Three-dimensional gold and its alloyed nanoporous structures possess high surface areas and strong local electric fields, rendering them ideal substrates for plasmonic molecular detection. Despite enhancing plasmonic properties and altering molecular interactions, the effect of alloy composition on molecular detection capability has not yet been explored. Here, we report molecular interactions between nanoporous gold alloys and charged molecules by controlling the alloy composition. We demonstrate enhanced adsorption of negatively charged molecules onto the alloy surface due to positively charged gold atoms and a shifted d-band center through charge transfer between gold and other metals. Despite similar EM field intensities, nanoporous gold with silver (Au/Ag) achieves SERS enhancement factors (EF) up to 6 orders of magnitude higher than those of other alloys for negatively charged molecules. Finally, nanoporous Au/Ag detects amyloid-beta at concentrations as low as approximately 1 fM, with SERS EF up to 10 orders of magnitude higher than that of a monolayer of Au nanoparticles.

Graphdiyne Enabled Nitrogen Vacancy Formation in Copper Nitride for Efficient Ammonia Synthesis

The electrocatalytic reduction of nitrate is promising for sustainable ammonia synthesis but suffers from slow reduction kinetics and multiple competing reactions. Here, we report a catalyst featuring copper nitride (Cu3N) anchored on a novel graphdiyne support (termed Cu3N/GDY), which is used for electrocatalytic reduction of nitrate to produce ammonia. The GDY absorbed hydrogen and enabled nitrogen (N) vacancy formation in Cu3N for the fast nitrate reduction reaction (NO3RR). Further, the distinct absorption sites formed by GDY and N vacancy enabled the excellent selectivity and stability of NO3RR. Notably, the Cu3N/GDY catalyst achieved a high ammonia yield (YNH3) up to 35280 μg h-1 mgcat.-1 and a high Faradaic efficiency (FE) of 98.1% using 0.1 M NO3- at -0.9 V versus a reversible hydrogen electrode (RHE). Using electron paramagnetic resonance (EPR) technology and in situ X-ray absorption fine structure (XAFS) spectroscopy measurement, we visualized the N vacancy formation in Cu3N and electrocatalytic NO3RR enabled by GDY. These findings show the promise of GDY in sustainable ammonia synthesis and highlight the efficacy of Cu3N/GDY as a catalyst.

Tailoring the *CO and *H Coverage for Selective CO2 Electroreduction to CH4 or C2H4

In the electrochemical CO2 reduction reaction (CO2RR), the coverages of *CO and *H intermediates on a catalyst surface are critical for the selective generation of C1 or C2 products. In this work, we have synthesized several CuxZnyMnz ternary alloy electrocatalysts, including Cu8ZnMn, Cu8Zn6Mn, and Cu8ZnMn2, by varying the doping compositions of Zn and Mn, which are efficient in binding *CO and *H adsorbates in the CO2 electroreduction process, respectively. The increase of *H coverage allows to promotion of the CH4 and H2 formation, while the increase of the *CO coverage facilitates the production of C2H4 and CO. As a result, the Cu8ZnMn catalyst presented a high CO2-to-CH4 partial current density (-418 ± 22 mA cm-2) with a Faradaic efficiency of 55 ± 2.8%, while the Cu8Zn6Mn catalyst exhibited a CO2-to-C2H4 partial current density (-440 ± 41 mA cm-2) with a Faradaic efficiency of 58 ± 4.5%. The study suggests a useful strategy for rational design and fabrication of Cu electrocatalysts with different doping for tailoring the reduction products.

Customizing catalyst surface/interface structures for electrochemical CO2 reduction

Electrochemical CO2 reduction reaction (CO2RR) provides a promising route to converting CO2 into value-added chemicals and to neutralizing the greenhouse gas emission. For the industrial application of CO2RR, high-performance electrocatalysts featuring high activities and selectivities are essential. It has been demonstrated that customizing the catalyst surface/interface structures allows for high-precision control over the microenvironment for catalysis as well as the adsorption/desorption behaviors of key reaction intermediates in CO2RR, thereby elevating the activity, selectivity and stability of the electrocatalysts. In this paper, we review the progress in customizing the surface/interface structures for CO2RR electrocatalysts (including atomic-site catalysts, metal catalysts, and metal/oxide catalysts). From the perspectives of coordination engineering, atomic interface design, surface modification, and hetero-interface construction, we delineate the resulting specific alterations in surface/interface structures, and their effect on the CO2RR process. At the end of this review, we present a brief discussion and outlook on the current challenges and future directions for achieving high-efficiency CO2RR via surface/interface engineering.

Integrating Host Design and Tailored Electronic Effects of Yolk-Shell Zn-Mn Diatomic Sites for Efficient CO2 Electroreduction

Modulating the surface and spatial structure of the host is associated with the reactivity of the active site, and also enhances the mass transfer effect of the CO2 electroreduction process (CO2 RR). Herein, we describe the development of two-step ligand etch-pyrolysis to access an asymmetric dual-atomic-site catalyst (DASC) composed of a yolk-shell carbon framework (Zn1 Mn1 -SNC) derived from S,N-coordinated Zn-Mn dimers anchored on a metal-organic framework (MOF). In Zn1 Mn1 -SNC, the electronic effects of the S/N-Zn-Mn-S/N configuration are tailored by strong interactions between Zn-Mn dual sites and co-coordination with S/N atoms, rendering structural stability and atomic distribution. In an H-cell, the Zn1 Mn1 -SNC DASC shows a low onset overpotential of 50 mV and high CO Faraday efficiency of 97 % with a low applied overpotential of 343 mV, thus outperforming counterparts, and in a flow cell, it also reaches a high current density of 500 mA cm-2 at -0.85 V, benefitting from the high structure accessibility and active dual sites. DFT simulations showed that the S,N-coordinated Zn-Mn diatomic site with optimal adsorption strength of COOH* lowers the reaction energy barrier, thus boosting the intrinsic CO2 RR activity on DASC. The structure-property correlation found in this study suggests new ideas for the development of highly accessible atomic catalysts.

Enhanced CO2 Electroreduction Selectivity toward Ethylene on Pyrazolate-Stabilized Asymmetric Ni-Cu Hybrid Sites

Metal-organic frameworks (MOFs) possess well-defined, designable structures, holding great potential in enhancing product selectivity for electrochemical CO2 reduction (CO2R) through active site engineering. Here, we report a novel MOF catalyst featuring pyrazolate-stabilized asymmetric Ni/Cu sites, which not only maintains structural stability under harsh electrochemical conditions but also exhibits extraordinarily high ethylene (C2H4) selectivity during CO2R. At a cathode potential of -1.3 V versus RHE, our MOF catalyst, denoted as Cu1Ni-BDP, manifests a C2H4 Faradaic efficiency (FE) of 52.7% with an overall current density of 0.53 A cm-2 in 1.0 M KOH electrolyte, surpassing that on prevailing Cu-based catalysts. More remarkably, the Cu1Ni-BDP MOF exhibits a stable performance with only 4.5% reduction in C2H4 FE during 25 h of CO2 electrolysis. A suite of characterization tools─such as high-resolution transmission electron microscopy, X-ray absorption spectroscopy, operando X-ray diffraction, and infrared spectroscopy─and density functional theory calculations collectively reveal that the cubic pyrazolate-metal coordination structure and the asymmetric Ni-Cu sites in the MOF catalyst synergistically facilitate the stable formation of C2H4 from CO2.

Generating Multi-Carbon Products by Electrochemical CO2 Reduction via Catalytically Harmonious Ni/Cu Dual Active Sites

Despite the unique advantages of single-atom catalysts, molecular dual-active sites facilitate the C-C coupling reaction for C2 products toward the CO2 reduction reaction (CO2 RR). The Ni/Cu proximal dual-active site catalyst (Ni/Cu-PASC) is developed, which is a harmonic catalyst with dual-active sites, by simply mixing commercial Ni-phthalocyanine (Ni-Pc) and Cu-phthalocyanine (Cu-Pc) molecules physically. According to scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) energy dispersive spectroscopy (EDS) data, Ni and Cu atoms are separated, creating dual-active sites for the CO2 RR. The Ni/Cu-PASC generates ethanol with an FE of 55%. Conversely, Ni-Pc and Cu-Pc have only detected single-carbon products like CO and HCOO- . In situ X-ray absorption spectroscopy (XAS) indicates that CO generation is caused by the stable Ni active site's balanced electronic state. The CO production from Ni-Pc consistently increased the CO concentration over Cu sites attributed to subsequent reduction reaction through a C-C coupling on nearby Cu. The CO bound (HCOO- ) peak, which can be found on Cu-Pc, vanishes on Ni/Cu-PASC, as shown by in situ fourier transformation infrared (FTIR). The characteristic intermediate of *CHO instead of HCOO- proves to be the prerequisite for multi-carbon products by electrochemical CO2 RR. The work demonstrates that the harmonic dual-active sites in Ni/Cu-PASC can be readily available by the cascading proximal active Ni- and Cu-Pc sites.

Probing the Atomistic Reaction Pathways in CuO/C Catalysts

CuOx/C catalysts have been used in the selective catalytic reduction of NOx because of the exceptional low-temperature denitration (de-NOx) activity. A fundamental understanding of the reaction between CuO and C is critical for controlling the component of CuOx/C and thus optimizing the catalytic performance. In this study, a transmission electron microscope equipped with an in situ heating device was utilized to investigate the atomic-scale reaction between CuO and C. We report two reaction mechanisms relying on the volume ratio between C and CuO: (1) The reduction from CuO to Cu2O (when the ratio is < ∼31%); (2) the reduction of CuO into polycrystalline Cu (when the ratio is > ∼34%). The atomistic reduction pathway can be well interpreted by considering the diffusion of O vacancy through the first-principle calculations. The atomic-scale exploration of CuO/C offers ample prospects for the design of industrial de-NOx catalysts in the future.

Regulating charge distribution of Cu3PdN nanocrystals for nitrate electroreduction to ammonia

As-synthesized Cu3PdN nanocrystals displayed high faradaic efficiency and selectivity for nitrate-to-ammonia conversion. The excellent performances can be attributed to the charge redistribution in Cu3PdN as a result of modulations of the electronic structures of Pd and Cu atoms, which altered the adsorption activation energy of the intermediates during the nitrate reduction reaction process.

Atomic Layers of B2 CuPd on Cu Nanocubes as Catalysts for Selective Hydrogenation

The search for highly active and selective catalysts with high precious metal atom utilization efficiency has attracted increasing interest in both the fundamental synthesis of materials and important industrial reactions. Here, we report the synthesis of Pd-Cu nanocubes with a Cu core and an ordered B2 intermetallic CuPd shell with controllable atomic layers on the surface (denoted as Cu/B2 CuPd), which can efficiently and robustly catalyze the selective hydrogenation of acetylene (C2H2) to ethylene (C2H4) under mild conditions. The optimized Cu/B2 CuPd with a Pd loading of 9.5 at. % exhibited outstanding performance in the C2H2 semi-hydrogenation with 100% C2H2 conversion and 95.2% C2H4 selectivity at 90 °C. We attributed this outstanding performance to the core/shell structure with a high surface density of active Pd sites isolated by Cu in the B2 intermetallic matrix, representing a structural motif of single-atom alloys (SAAs) on the surface. The combined experimental and computational studies further revealed that the electronic states of Pd and Cu are modulated by SAAs from the synergistic effect between Pd and Cu, leading to enhanced performance compared with pristine Pd and Cu catalysts. This study provides a new synthetic methodology for making single-atom catalysts with high precious metal atom utilization efficiency, enabling simultaneous tuning of both geometric and electronic structures of Pd active sites for enhanced catalysis.

CO Electroreduction Mechanism on Single-Atom Zn (101) Surfaces: Pathway to C2 Products

Electrocatalytic reduction of carbon dioxide (CO₂RR) employs electricity to store renewable energy in the form of reduction products. The activity and selectivity of the reaction depend on the inherent properties of electrode materials. Single-atom alloys (SAAs) exhibit high atomic utilization efficiency and unique catalytic activity, making them promising alternatives to precious metal catalysts. In this study, density functional theory (DFT) was employed to predict stability and high catalytic activity of Cu/Zn (101) and Pd/Zn (101) catalysts in the electrochemical environment at the single-atom reaction site. The mechanism of C2 products (glyoxal, acetaldehyde, ethylene, and ethane) produced by electrochemical reduction on the surface was elucidated. The C-C coupling process occurs through the CO dimerization mechanism, and the formation of the *CHOCO intermediate proves beneficial, as it inhibits both HER and CO protonation. Furthermore, the synergistic effect between single atoms and Zn results in a distinct adsorption behavior of intermediates compared to traditional metals, giving SAAs unique selectivity towards the C2 mechanism. At lower voltages, the Zn (101) single-atom alloy demonstrates the most advantageous performance in generating ethane on the surface, while acetaldehyde and ethylene exhibit significant certain potential. These findings establish a theoretical foundation for the design of more efficient and selective carbon dioxide catalysts.