The Functionality of Surface Hydroxy Groups on the Selectivity and Activity of Carbon Dioxide Reduction over Cuprous Oxide in Aqueous Solutions

Carbon dioxide (CO₂) reduction in aqueous solutions is an attractive strategy for carbon capture and utilization. Cuprous oxide (Cu₂O) is a promising catalyst for CO₂ reduction as it can convert CO₂ into valuable hydrocarbons and suppress the side hydrogen evolution reaction (HER). However, the nature of the active sites in Cu₂O remains under debate because of the complex surface structure of Cu₂O under reducing conditions, leading to limited guidance in designing improved Cu₂O catalysts. This paper describes the functionality of surface‐bonded hydroxy groups on partially reduced Cu₂O(111) for the CO₂ reduction reaction (CO₂RR) by combined density functional theory (DFT) calculations and experimental studies. We find that the surface hydroxy groups play a crucial role in the CO₂RR and HER, and a moderate coverage of hydroxy groups is optimal for promotion of the CO₂RR and suppression of the HER simultaneously. Electronic structure analysis indicates that the charge transfer from hydroxy groups to coordination‐unsaturated Cu (Cu_CUS) sites stabilizes surface‐adsorbed COOH*, which is a key intermediate during the CO₂RR. Moreover, the CO₂RR was evaluated over Cu₂O octahedral catalysts with {111} facets and different surface coverages of hydroxy groups, which demonstrates that Cu₂O octahedra with moderate coverage of hydroxy groups can indeed enhance the CO₂RR and suppress the HER.     

Selective CO2 Reduction to Ethylene Mediated by Adaptive Small-molecule Engineering of Copper-based Electrocatalysts

Electrochemical CO₂ reduction reaction (CO₂RR) over Cu catalysts exhibits enormous potential for efficiently converting CO₂ to ethylene (C₂H₄). However, achieving high C₂H₄ selectivity remains a considerable challenge due to the propensity of Cu catalysts to undergo structural reconstruction during CO₂RR. Herein, we report an in situ molecule modification strategy that involves tannic acid (TA) molecules adaptive regulating the reconstruction of a Cu-based material to a pathway that facilitates CO₂ reduction to C₂H₄ products. An excellent Faraday efficiency (FE) of 63.6 % on C₂H₄ with a current density of 497.2 mA cm⁻² in flow cell was achieved, about 6.5 times higher than the pristine Cu catalyst which mainly produce CH₄. The in situ X-ray absorption spectroscopy and Raman studies reveal that the hydroxyl group in TA stabilizes Cu^δ+ during the CO₂RR. Furthermore, theoretical calculations demonstrate that the Cu^δ+/Cu⁰ interfaces lower the activation energy barrier for *CO dimerization, and hydroxyl species stabilize the *COH intermediate via hydrogen bonding, thereby promoting C₂H₄ production. Such molecule engineering modulated electronic structure provides a promising strategy to achieve highly selective CO₂ reduction to value-added chemicals.     

Unexpected high selectivity for acetate formation from CO2 reduction with copper based 2D hybrid catalysts at ultralow potentials

Copper-based catalysts are efficient for CO₂ reduction affording commodity chemicals. However, Cu(i) active species are easily reduced to Cu(0) during the CO₂RR, leading to a rapid decay of catalytic performance. Herein, we report a hybrid-catalyst that firmly anchors 2D-Cu metallic dots on F-doped Cu_xO nanoplates (Cu_xOF), synthesized by electrochemical-transformation under the same conditions as the targeted CO₂RR. The as-prepared Cu/Cu_xOF hybrid showed unusual catalytic activity towards the CO₂RR for CH₃COO⁻ generation, with a high FE of 27% at extremely low potentials. The combined experimental and theoretical results show that nanoscale hybridization engenders an effective s,p-d coupling in Cu/Cu_xOF, raising the d-band center of Cu and thus enhancing electroactivity and selectivity for the acetate formation. This work highlights the use of electronic interactions to bias a hybrid catalyst towards a particular pathway, which is critical for tuning the activity and selectivity of copper-based catalysts for the CO₂RR.

A two-dimensional (2D) copper hybrid catalyst (Cu/Cu_xOF) composed of metallic Cu well dispersed on 2D F-doped Cu_xO nanoplates (Cu_xOF) is reported, which shows high catalytic activity toward the CO₂RR for acetate generation.     

Copper-Sulfur-Nitrogen Cluster Providing a Local Proton for Efficient Carbon Dioxide Photoreduction

Atomically precise Cu clusters are highly desirable as catalysts for CO₂ reduction reaction (CO₂RR), and they provide an appropriate model platform for elaborating their structure–activity relationship. However, an efficient overall photocatalytic CO₂RR with H₂O using assembled Cu-cluster aggregates as single component photocatalyst has not been reported. Herein, we report a stable crystalline Cu−S−N cluster photocatalyst with local protonated N−H groups (denoted as Cu₆−NH ). The catalyst exhibits suitable photocatalytic redox potentials, high structural stability, active catalytic species, and a narrow band gap, which account for its outstanding photocatalytic CO₂RR performance under visible light, with ≈100 % selectivity for CO evolution. Remarkably, systematic isostructural Cu-cluster control experiments, in situ infrared spectroscopy, and density functional theory calculations revealed that the protonated pyrimidine N atoms in the Cu₆−NH cluster act as a proton relay station, providing a local proton during the photocatalytic CO₂RR. This efficiently lowers the energy barrier for the formation of the *COOH intermediate, which is the rate-limiting step, efficiently enhancing the photocatalytic performance. This work lays the foundation for the development of atomically precise metal-cluster-based photocatalysts.     

Surface Adsorbed Hydroxyl: A Double-Edged Sword in Electrochemical CO2 Reduction over Oxide-Derived Copper

Oxide-derived Cu (OD−Cu) featured with surface located sub-20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO₂ reduction (ECO₂RR). With surface adsorbed hydroxyls (OH_ad) identified during ECO₂RR, it is realized that OH_ad, sterically confined and adsorbed at OD−Cu by surface located sub-20 nm NPs, should be determinative to the multi-carbon (C₂) product selectivity. In situ spectral investigations and theoretical calculations reveal that OH_ad favors the adsorption of low-frequency *CO with weak C≡O bonds and strengthens the *CO binding at OD−Cu surface, promoting *CO dimerization and then selective C₂ production. However, excessive OH_ad would inhibit selective C₂ production by occupying active sites and facilitating competitive H₂ evolution. In a flow cell, stable C₂ production with high selectivity of ∼60 % at −200 mA cm⁻² could be achieved over OD−Cu, with adsorption of OH_ad well steered in the fast flowing electrolyte.     

Electrochemically Driven Cation Exchange Enables the Rational Design of Active CO2 Reduction Electrocatalysts

Metal oxides or sulfides are considered to be one of the most promising CO₂ reduction reaction (CO₂RR) precatalysts, owing to their electrochemical conversion in situ into highly active electrocatalytic species. However, further improvement of the performance requires new tools to gain fine control over the composition of the active species and its structural features [e.g., grain boundaries (GBs) and undercoordinated sites (USs)], directly from a predesigned template material. Herein, we describe a novel electrochemically driven cation exchange (ED‐CE) method that enables the conversion of a predesigned CoS₂ template into a CO₂RR catalyst, Cu₂S. By means of ED‐CE, the final Cu₂S catalyst inherits the original 3 D morphology of CoS₂, and preserves its high density of GBs. Additionally, the catalyst's phase structure, composition, and density of USs were precisely tuned, thus enabling rational design of active CO₂RR sites. The obtained Cu₂S catalyst achieved a CO₂‐to‐formate Faradaic efficiency of over 87 % and a record high activity (among reported Cu‐based catalysts). Hence, this study opens the way for utilization of ED‐CE reactions to design advanced electrocatalysts.     

CO2 Reduction Mechanism on the Cu2O(110) Surface: A First-Principles Study

Cu₂O is an attractive catalyst for the selective reduction of CO₂ to methanol. However, the mechanism of the reaction and the role of the Cu species in different oxidation states are not well understood yet. In this work, by first-principles calculations, we investigate the mechanism of the reaction on the Cu₂O(110) surface, which is the most selective for methanol, in different degrees of reduction: ideal surface, slightly reduced surface (SRS), and partially reduced surface (PRS). The most favorable reaction pathways on the three surfaces were identified. We found that Cu(I) on the ideal surface is not capable of chemisorbing CO₂, but surface oxygen serves as the active site which selectively converts CO₂ to CH₃OH with a limiting potential of −0.77 V. The Cu(0) on the SRS and PRS promotes the adsorption and reduction of CO₂, while the removal of the residue O* becomes potential/rate limiting with a more negative limiting potential than the ideal surface. The SRS is selective to methanol while the PRS becomes selective to methane. The result suggests that the key to high methanol selectivity is to avoid the reduction of Cu(I), which provides a new strategy for the design of more efficient catalysts for selective CO₂ reduction to methanol.     

Unveiling the Activity Origin of a Copper‐based Electrocatalyst for Selective Nitrate Reduction to Ammonia

Unveiling the active phase of catalytic materials under reaction conditions is important for the construction of efficient electrocatalysts for selective nitrate reduction to ammonia. The origin of the prominent activity enhancement for CuO (Faradaic efficiency: 95.8 %, Selectivity: 81.2 %) toward selective nitrate electroreduction to ammonia was probed. ¹⁵N isotope labeling experiments showed that ammonia originated from nitrate reduction. ¹H NMR spectroscopy and colorimetric methods were performed to quantify ammonia. In situ Raman and ex situ experiments revealed that CuO was electrochemically converted into Cu/Cu₂O, which serves as an active phase. The combined results of online differential electrochemical mass spectrometry (DEMS) and DFT calculations demonstrated that the electron transfer from Cu₂O to Cu at the interface could facilitate the formation of *NOH intermediate and suppress the hydrogen evolution reaction, leading to high selectivity and Faradaic efficiency.     

Unveiling the Activity Origin of a Copper-based Electrocatalyst for Selective Nitrate Reduction to Ammonia

Unveiling the active phase of catalytic materials under reaction conditions is important for the construction of efficient electrocatalysts for selective nitrate reduction to ammonia. The origin of the prominent activity enhancement for CuO (Faradaic efficiency: 95.8 %, Selectivity: 81.2 %) toward selective nitrate electroreduction to ammonia was probed. ¹⁵N isotope labeling experiments showed that ammonia originated from nitrate reduction. ¹H NMR spectroscopy and colorimetric methods were performed to quantify ammonia. In situ Raman and ex situ experiments revealed that CuO was electrochemically converted into Cu/Cu₂O, which serves as an active phase. The combined results of online differential electrochemical mass spectrometry (DEMS) and DFT calculations demonstrated that the electron transfer from Cu₂O to Cu at the interface could facilitate the formation of *NOH intermediate and suppress the hydrogen evolution reaction, leading to high selectivity and Faradaic efficiency.     

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate''s orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.     

Self-Supported Nanoporous Au3Cu Electrode with Enriched Gold on Surface for Efficient Electrochemical Reduction of CO2

The key to the electrochemical conversion of CO₂ lies in the development of efficient electrocatalysts with ease of operation, good conductivity, and rich active sites that fulfil the desired reaction direction and selectivity. Herein, an oxidative etching of Au₂₀Cu₈₀ alloy is used for the synthesis of a nanoporous Au₃Cu alloy, representing a facile strategy for tuning the surface electronic properties and altering the adsorption behavior of the intermediates. HRTEM, XPS, and EXAFS results reveal that the curved surface of the synthesized nanoporous Au₃Cu is rich in gold with unsaturated coordination conditions. It can be used directly as a self-supported electrode for CO₂ reduction, and exhibits high Faradaic efficiency (FE) of 98.12 % toward CO at a potential of −0.7 V versus the reversible hydrogen electrode (RHE). The FE is 1.47 times that over the as-made single nanoporous Au. Density functional theory reveals that *CO has a relatively long distance on the surface of nanoporous Au₃Cu, making desorption of CO easier and avoiding CO poisoning. The Hirshfeld charge distribution shows that the Au atoms have a negative charge and the Cu atoms exhibit a positive charge, which separately bond to the C atom and O atom in the *COOH intermediate through a bidentate mode. This affords the lowest *COOH adsorption free energy and low desorption energy for CO molecules.     

Highly Selective CO2 Electroreduction to CH4 by In Situ Generated Cu2O Single-Type Sites on a Conductive MOF: Stabilizing Key Intermediates with Hydrogen Bonding

It is still a great challenge to achieve high selectivity of CH₄ in CO₂ electroreduction reactions (CO₂RR) because of the similar reduction potentials of possible products and the sluggish kinetics for CO₂ activation. Stabilizing key reaction intermediates by single type of active sites supported on porous conductive material is crucial to achieve high selectivity for single product such as CH₄. Here, Cu₂O(111) quantum dots with an average size of 3.5 nm are in situ synthesized on a porous conductive copper-based metal–organic framework (CuHHTP), exhibiting high selectivity of 73 % towards CH₄ with partial current density of 10.8 mA cm⁻² at −1.4 V vs. RHE (reversible hydrogen electrode) in CO₂RR. Operando infrared spectroscopy and DFT calculations reveal that the key intermediates (such as *CH₂O and *OCH₃) involved in the pathway of CH₄ formation are stabilized by the single active Cu₂O(111) and hydrogen bonding, thus generating CH₄ instead of CO.     

Molecular Evidence for Metallic Cobalt Boosting CO2 Electroreduction on Pyridinic Nitrogen

Nitrogen‐doped carbon materials (N‐C_mat) are emerging as low‐cost metal‐free electrocatalysts for the electrochemical CO₂ reduction reaction (CO₂RR), although the activities are still unsatisfactory and the genuine active site is still under debate. We demonstrate that the CO₂RR to CO preferentially takes place on pyridinic N rather than pyrrolic N using phthalocyanine (Pc) and porphyrin with well‐defined N‐C_mat configurations as molecular model catalysts. Systematic experiments and theoretic calculations further reveal that the CO₂RR performance on pyridinic N can be significantly boosted by electronic modulation from in‐situ‐generated metallic Co nanoparticles. By introducing Co nanoparticles, Co@Pc/C can achieve a Faradaic efficiency of 84 % and CO current density of 28 mA cm^?2 at ?0.9 V, which are 18 and 47 times higher than Pc/C without Co, respectively. These findings provide new insights into the CO₂RR on N‐C_mat, which may guide the exploration of cost‐effective electrocatalysts for efficient CO₂ reduction.     

Crystal facet-dependent electrocatalytic performance of metallic Cu in CO2 reduction reactions

Developing high-performance electrocatalysts for CO₂ reduction reaction (CO₂RR) is crucial since it is beneficial for environmental protection and the resulting value-add chemical products can act as an alternative to fossil feedstocks. Nonetheless, the direct reduction of CO₂ into long-chain hydrocarbons and oxygenated hydrocarbons with high selectivity remains challenging. Copper (Cu) shows a distinctive advantage that it is the only pure metal catalyst for reducing CO₂ into multi-carbon (C₂₊) products and the certain facets (e.g., (100), (111), (111)) of Cu nanocrystals exhibit relatively low energy barriers for the formation of specific products (e.g., CO, HCOOH, CH₄, C₂H₄, C₂H₅OH, and other C₂₊ products). Therefore, extensive studies have been carried out to explore the relationship between the facets of Cu nanocrystals and corresponding catalytic products. In this review, we will discuss the crystal facet-dependent electrocatalytic CO₂RR performance in metallic Cu catalysts, meanwhile, the detailed reaction mechanisms will be systematically summarized. In addition, we will provide a personal perspective for the future research directions in this emerging field. We believe this review is helpful to guide the design of high-selectivity Cu-based electrocatalysts for CO₂RR.     

Fabricating pyridinic N-B sites in porous carbon as efficient metal-free electrocatalyst in conversion CO2 into CH4

Electrochemical reduction of CO₂ (CO₂RR) to value-added chemicals is an attractive strategy for greenhouse gas mitigation and carbon recycle. Carbon material is one of most promising electrocatalysts but its product selectivity is limited by few modulating approaches for active sites. Herein, the predominant pyridinic N-B sites (accounting for 80% to all N species) are fabricated in hierarchically porous structure of graphene nanoribbons/amorphous carbon. The graphene nanoribbons and porous structure can accelerate electron and ion/gas transport during CO₂RR, respectively. This carbon electrocatalyst exhibits excellent selectivity toward CO₂ reduction to CH₄ with the faradaic efficiency of 68% at −0.50 V vs. RHE. As demonstrated by density functional theory, a proper adsorbed energy of *CO and *CH₂O are generated on the pyridinic N-B site resulting into high CH₄ selectivity. Therefore, this study provides a novel method to modulate active sites of carbon-based electrocatalyst to obtain high CH₄ selectivity.     

Dynamic Changes in the Structure,Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂ electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_x species observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄ and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^I species under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁ product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.     

Dynamic Changes in the Structure,Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂ electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_x species observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄ and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^I species under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁ product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.     

Polyoxometalate-Based Compounds for Photo- and Electrocatalytic Applications

Photo/electrocatalysis of water (H₂O) splitting and CO₂ reduction reactions is a promising strategy to alleviate the energy crisis and excessive CO₂ emissions. For the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and CO₂ reduction reaction (CO₂RR) involved, the development of effective photo/electrocatalysts is critical to reduce the activation energy and accelerate the sluggish dynamics. Polyoxometalate (POM)-based compounds with tunable compositions and diverse structures are emerging as unique photo/electrocatalysts for these reactions as they offer unparalleled advantages such as outstanding solution and redox stability, quasi-semiconductor behaviour, etc. This Minireview provides a basic introduction related to photo/electrocatalytic HER, OER and CO₂RR, followed by the classification of pristine POM-based compounds toward different catalytic reactions. Recent breakthroughs in engineering POM-based compounds as efficient photo/electrocatalysts are highlighted. Finally, the advantages, challenges, strategies and outlooks of POM-based compounds on improving photo/electrocatalytic performance are discussed.     

Insight into the Formation and Transfer Process of the First Intermediate of CO2 Reduction over Ag-Decorated Dendritic Cu

It is still poorly understood how the first intermediates of CO₂ reduction are formed and converted to multi-carbon products over Cu-based electrodes. Herein, Ag is used to decorate dendritic Cu and a high Faradaic efficiency (FE) for C₂H₄ (25 %) is obtained on a CuAg electrode, which is about five times higher than dendritic Cu. The intermediates including *CO₂⁻, OH groups, Cu-CO, C-O rotation, and CH_x species are investigated by in situ Raman spectroscopy. This work provides spectroscopic evidence that the first intermediate of CO₂ reduction on Ag-decorated Cu is carboxylate anion *CO₂⁻ bonded with the catalyst surface through the C and O atom. The formation and evolution process of the *CO₂⁻ intermediate over the applied potential are investigated in depth as well. This research contributes to a better understanding of the mechanism of CO₂ reduction and multi-carbon product formation pathways over Ag-decorated Cu.     

Decrypting the Controlled Product Selectivity over Ag−Cu Bimetallic Surface Alloys for Electrochemical CO2 Reduction

Electrochemical CO₂ reduction reaction (ECO₂RR) with controlled product selectivity is realized on Ag−Cu bimetallic surface alloys, with high selectivity towards C2 hydrocarbons/alcohols (≈60 % faradaic efficiency, FE), C1 hydrocarbons/alcohols (≈41 % FE) and CO (≈74 % FE) achieved by tuning surface compositions and applied potentials. In situ spectral investigations and theoretical calculations reveal that surface-composition-dependent d-band center could tune *CO binding strengths, regulating the *CO subsequent reaction pathways and then the product selectivity. Further adjusting the applied potentials will alter the energy of participated electrons, which leads to controlled ECO₂RR selectivity towards desired products. A predominant region map, with an indicator proposed to evaluate the thermodynamic predominance of the *CO subsequent reactions, is then provided as a reliable theoretical guidance for the controllable ECO₂RR product selectivity over bimetallic alloys.