Electrochemical Reduction of CO2 to Ethane through Stabilization of an Ethoxy Intermediate

Electrochemical conversion of CO₂ into ethane is seldom observed because of the generally higher selectivity towards methane, ethylene, and ethanol. Consequently, little experimental evidence for its reaction mechanism exists and thus remains largely unknown. Now, by combining electrochemistry with in situ X-ray absorption fine-structure and in situ Raman techniques, iodide-derived copper (ID-Cu) and oxide-derived copper (OD-Cu) systems were studied to obtain a deeper understanding of the CO₂ to ethane mechanism. With trace iodine species on the surface and positively charged Cu species, production of ethane is significantly more favored on ID-Cu compared to OD-Cu, with higher selectivity and faster kinetics. For the first time, it is experimentally found that the formation of ethane follows the same pathway to ethylene and ethanol, and better stabilization of the late stage ethoxy intermediate can steer the reaction to ethane over ethanol.     

In situ Raman spectroscopy studies for electrochemical CO2 reduction over Cu catalysts

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CO/H_2在Cu/ZrO_2催化剂表面吸附行为原位红外表征

用原位FT IR法比较了Cu ZrO2 和ZrO2 催化剂表面对CO及CO H2 的吸附行为。结果表明 ,CO在 5 0℃便可以在Cu ZrO2 表面形成b HCOO Zr、Zr COO- 和b HOCOOZr物种 ,吸附温度升高 ,b HOCOOZr逐渐分解生成Zr OH和CO2 ,而b HCOO Zr吸附物种逐渐增强。b HCOO Zr物种在Cu ZrO2 催化剂表面生成速率远远大于ZrO2 催化剂。在Cu ZrO2 催化剂表面 ,所形成的合成甲醇中间物种 (HCOO Zr和CH3O Zr)均和ZrO2 有关 ,意味着CO加氢反应主要在ZrO2 表面进行 ,铜组分主要向ZrO2 提供吸附CO及H2 物种。     

CeO2担载Cu纳米粒子电催化CO2还原产乙烯：CeO2不同暴露晶面对催化性能的影响

Fossil fuels are expected to be the major source of energy for the next few decades. However, combustion of nonrenewable resources leads to the release of large quantities of CO₂, the primary greenhouse gas. Notably, the concentration of CO₂ in the atmosphere is increasing annually at an astounding rate. Electrochemical CO₂ reduction (ECR) to value-added fuels and chemicals using electricity from intermittent renewable energy sources is a carbon-neutral method to alleviate anthropogenic CO₂ emissions. Despite the steady progress in the selective generation of C₁ products (CO and formic acid), the production of multi-carbon species still suffers from low selectivity and efficiency. As an ECR product, ethylene (C₂H₄) has a higher energy density than do C₁ species and is an important industrial feedstock in high demand. However, the conversion of CO₂ to C₂H₄ is plagued by low productivity and large overpotential, in addition to the severe competing hydrogen evolution reaction (HER) during the ECR. To address these issues, the design and development of advanced electrocatalysts are critical. Here, we demonstrate fine-tuning of ECR to C₂H₄ by taking advantage of the prominent interaction of Cu with shape-controlled CeO₂ nanocrystals, that is, cubes, rods, and octahedra predominantly covered with (100), (110), and (111) surfaces, respectively. We found that the selectivity and activity of the ECR depended strongly on the exposed crystal facets of CeO₂. The overall ECR Faradaic efficiency (FE) over Cu/CeO₂(110) (FE ≈ 56.7%) surpassed that of both Cu/CeO₂(100) (FE ≈ 51.5%) and Cu/CeO₂(111) (FE ≈ 48.4%) in 0.1 mol·L^-1 KHCO₃ solutions with an H-type cell. This was in stark contrast to the exclusive occurrence of the HER over pure carbon paper, CeO₂(100), CeO₂(110), and CeO₂(111). In particular, the FE toward C₂H₄ formation and the partial current density increased in the sequence Cu/CeO₂(111) < Cu/CeO₂(100) < Cu/CeO₂(110) within applied bias potentials from -1.00 to -1.15 V (vs. the reversible hydrogen electrode), reaching 39.1% over Cu/CeO₂(110) at a mild overpotential (1.13 V). The corresponding values for Cu/CeO₂(100) and Cu/CeO₂(111) were FE_C2H4 ≈ 31.8% and FE_C2H4 ≈ 29.6%, respectively. The C₂H₄ selectivity was comparable to that of many reported Cu-based electrocatalysts at similar overpotentials. Furthermore, the FE for C₂H₄ remained stable even after 6 h of continuous electrolysis. The superior ECR activity of Cu/CeO₂(110) to yield C₂H₄ was attributed to the metastable (110) surface, which not only promoted the effective adsorption of CO₂ but also remarkably stabilized Cu⁺, thereby boosting the ECR to produce C₂H₄. This work offers an alternative strategy to enhance the ECR efficiency by crystal facet engineering.     

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.     

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.     

Redox Replacement of Silver on MOF-Derived Cu/C Nanoparticles on Gas Diffusion Electrodes for Electrocatalytic CO2 Reduction

Bimetallic tandem catalysts have emerged as a promising strategy to locally increase the CO flux during electrochemical CO₂ reduction, so as to maximize the rate of conversion to C−C-coupled products. Considering this, a novel Cu/C−Ag nanostructured catalyst has been prepared by a redox replacement process, in which the ratio of the two metals can be tuned by the replacement time. An optimum Cu/Ag composition with similarly sized particles showed the highest CO₂ conversion to C₂₊ products compared to non-Ag-modified gas-diffusion electrodes. Gas chromatography and in-situ Raman measurements in a CO₂ gas diffusion cell suggest the formation of top-bound linear adsorbed *CO followed by consumption of CO in the successive cascade steps, as evidenced by the increasingνC−H bands. These findings suggest that two mechanisms operate simultaneously towards the production of HCO₂H and C−C-coupled products on the Cu/Ag bimetallic surface.     

Controllable Hydrocarbon Formation from the Electrochemical Reduction of CO2 over Cu Nanowire Arrays

In this work, the effect of Cu nanowire morphology on the selective electrocatalytic reduction of CO₂ is presented. Cu nanowire arrays were prepared through a two‐step synthesis of Cu(OH)₂ and CuO nanowire arrays on Cu foil substrates and a subsequent electrochemical reduction of the CuO nanowire arrays to Cu nanowire arrays. By this simple synthesis method, Cu nanowire array electrodes with different length and density were able to be controllably synthesized. We show that the selectivity for hydrocarbons (ethylene, n‐propanol, ethane, and ethanol) on Cu nanowire array electrodes at a fixed potential can be tuned by systematically altering the Cu nanowire length and density. The nanowire morphology effect is linked to the increased local pH in the Cu nanowire arrays and a reaction scheme detailing the local pH‐induced formation of C₂ products is also presented by a preferred CO dimerization pathway.     

铜电极上二氧化碳还原机理的研究进展

本文从历史角度综述了二氧化碳在铜基催化剂上的还原机理的最新研究进展,对区分C1和C2产物路径发生的机制,以及调控二氧化碳还原产物选择性的影响因素和方法进行了重点阐述,着重讨论了如何利用电化学红外光谱与微分电化学质谱等技术在揭示反应机理方面的研究思路与方法学.     

New Insight into CO2 Reduction to Formate by In Situ Hydrogen Produced from Hydrothermal Reactions with Iron

To reveal the nature of CO₂ reduction to formate with high efficiency by in situ hydrogen produced from hydrothermal reactions with iron, DFT calculations were used. A reaction pathway was proposed in which the formate was produced through the key intermediate species, namely iron hydride, produced in situ in the process of hydrogen gas production. In the in situ hydrogenation of CO₂, the charge of H in the iron hydride was −0.135, and the Fe–H bond distance was approximately 1.537 Å. A C-H bond was formed as a transition state during the attack of H^δ− on C^δ+. Finally, a HCOO species was formed. The distance of the C-H bond was 1.107 Å. The calculated free energy barrier was 16.43 kcal/mol. This study may provide new insight into CO₂ reduction to formate in hydrothermal reactions with metal.     

Cu纳米针上电场促进C-C耦合增强CO2电还原生成C2产物

电催化CO₂减排技术利用电能将过量的CO₂转化为有附加值的化学品,是解决能源危机、实现碳中和的有效途径之一.电催化CO₂还原反应(CO₂RR)中的多碳产物(C₂),如乙烯和乙醇,因其比C1产物具有更高的能量密度和更广泛的应用而受到较大关注.目前为止,Cu基催化剂被认为是获得C₂产物的独特材料.研究者在提高Cu基催化剂C₂产物的活性和选择性方面做了大量的工作,如催化剂形貌工程、活性位点设计和中间吸附性能调控等.许多理论和实验研究已经证明,Cu基催化剂上的C-C偶联过程是C₂产物生成的速率决定步骤.优化C-C偶联过程的能垒是提高C₂产物活性和选择性的重要而直接的策略.CO₂RR在Cu上是由CO₂还原吸附CO(*CO)并二聚生成C₂产物引起的.C-C偶联过程与*CO的吸附性能密切相关.众所周知,CO是一种典型的极性分子,因此其在催化剂表面的吸附性能可能会受到活性位点周围的局部电场的影响.构建合适的局部电场是调节CO吸附性能和C-C偶联过程的潜在手段之一.前期工作(Nature,2016,537,382-386)证明了高曲率金纳米针可以在尖端产生高的局部电场.高局域电场诱导K+聚集,使活性位点周围CO₂浓度升高,大大促进了Au纳米针上的CO生成.基于Au纳米针的局域电场促进了CO₂RR的CO生成.本文利用Cu纳米针促进并优化C-C偶联反应来提高C₂产物活性和选择性.结果表明,局部电场可以促进C-C偶联过程,进而增强CO₂电还原生成C₂产物.有限元模拟结果表明,高曲率铜纳米针处存在较强的局部电场;密度泛函理论计算结果表明,强电场能促进C-C耦合过程.在此基础上,制备了一系列不同曲率的Cu催化剂,其中,Cu纳米针(CuNNs)的曲率最高,Cu纳米棒(CuNRs)和Cu纳米颗粒(CuNPs)曲率次之.实验测得CuNNs上吸附的K+浓度最高,证明了纳米针上的局部电场最强.同时,CO吸附传感器测试表明,CuNNs对CO的吸附能力最强,原位傅里叶变换红外光谱显示,CuNNs的*COCO和*CO信号最强.由此可见,高曲率铜纳米针可以诱导高局部电场,从而促进C-C耦合过程.催化性能测试结果表明,在低电位(-0.6 V vs.RHE)下,Cu NNs对CO₂RR的生成C₂产物的法拉第效率值为44%,约为Cu NPs的2.2倍.综上,本文为CO₂RR过程中提高多碳产物提供了新的思路.     

Stabilization of Cu/Ni Alloy Nanoparticles with Graphdiyne Enabling Efficient CO2 Reduction

Electrocatalysis has become an attractive strategy for the artificial reduction of CO2 to high-value chemicals.However,the design and development of highly selective and stable non-noble metal electrocatalysts that convert CO2 to CO are still a challenge.As a new type of two-dimensional carbon material,graphdiyne(GDY),is rarely used to explore the application in carbon dioxide reduction reaction(CO2RR).Therefore,we tried to use GDY as a substrate to stabilize the copper-nickel alloy nanoparticles(NPs)to synthesize Cu/Ni@GDY.Cu/Ni@GDY requires an overpotential(-0.61 V)to 10 mA/cm2 for the formation of CO,and it shows better activity than Au and Ag,achieving a higher Faraday efficiency of about 95.2％and high stability of about 26 h at an overpotential(-0.70 V).The electronic interaction between GDY substrate and Cu/Ni alloy NPs and the large specific surface area of GDY is responsible for the high performance.     

Sn/Cu电极电化学还原CO2的研究

采用电沉积法制备Sn/Cu电极,由SEM观察并研究了电沉积电流密度对电极形貌的影响.在碱性三电极体系中考察了Sn/Cu电极对析氢、CO2还原的影响.发现10 mA.cm-2和15 mA.cm-2电沉积电流密度下制得的电极活性较高,尤以15 mA.cm-2时电极性能更佳,并指出了电还原CO2关键材料的结构特性.     

H2,CO和H2/CO在ZrO2催化剂上吸附行为研究

用insituFTIR法研究了H₂、CO及CO/H₂在ZrO₂表面的吸附行为.结果表明,H₂在ZrO₂表面吸附存在两种形态的羟基(即ZrOH和ZrOHZr),吸附温度增加,羟基数量增加.CO在200℃易与ZrO₂表面羟基作用形成甲酸盐物种,吸附温度升高时,该物种逐渐分解生成CO和ZrOH.当CO和H₂共存时,表面甲酸盐的量明显增加,并随温度增加,逐渐加氢形成甲氧基,最后生成甲烷.甲氧基的加氢过程较慢,所需反应温度也较高,被认为是CO加氢合成醇的速控步骤.     

In Situ Bismuth Nanosheet Assembly for Highly Selective Electrocatalytic CO2 Reduction to Formate

The reduction of carbon dioxide (CO₂) into value-added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high-performance electrocatalysts for CO₂ reduction reaction (CO₂RR) with high formate selectivity and good stability need to be improved. Earth-abundant Bi has been demonstrated to be active for CO₂RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi-NSs) assembly via an in situ electrochemical transformation of (BiO)₂CO₃ nanostructures. The as-prepared material exhibits high activity and selectivity for CO₂RR to formate, with nearly 94% faradaic efficiency at −1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from −0.83 to −1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi-NSs based CO₂RR/methanol oxidation reaction (CO₂RR/MOR) electrolytic system for overall CO₂ splitting was constructed, evidencing the feasibility of its practical implementation.     

棒状金属有机框架结构PCN-222(Cu)/TiO2复合材料的制备及其高效光催化CO2还原性能

The photocatalytic reduction of CO₂ has attracted considerable attention owing to the dual suppression of environmental pollution and energy shortage. The technology uses solar energy to convert carbon dioxide into hydrocarbon fuel, which is of great significance for achieving the carbon cycle. The development of low-cost photocatalytic materials is critical to achieving efficient solar energy to fuels conversion. One of the most commonly employed photocatalysts is TiO₂. However, it suffers from broad band gap as well as the recombination of photo-excited holes and electron. Hence, in this work, we report the photochemical reduction of CO₂ using rod-like PCN-222(Cu)/TiO₂ composites as photocatalyst through a simple hydrothermal method, in which TiO₂ nanoparticles are anchored at the interface of the SiC rod PCN-222(Cu). Multiple characterization techniques were used to analyze the structure, morphology, and properties of the PCN-222(Cu)/TiO₂ composite. A series of characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared spectroscopy, photo-electrochemical, and photoluminescence (PL) confirm the successful preparation of PCN-222(Cu)/TiO₂ composites. SEM reveals that the TiO₂ nanoparticles are uniformly distributed on the surface of the rod-shaped PCN-222(Cu)/TiO₂. XRD results show that PCN-222(Cu) and PCN-222(Cu)/TiO₂ composite photocatalysts with good crystal structure were successfully synthesized. According to the DRS results, the prepared PCN-222(Cu)/TiO₂ composite samples exhibit characteristic absorption peaks of metalloporphyrins in the visible region. PL spectroscopy, transient photocurrent response, and electrochemical impedance spectroscopy further confirm that the rod-like PCN-222(Cu)/TiO₂ samples have high electron-hole pair separation efficiency. By controlling the mass ratio of PCN-222(Cu) and TiO₂, the photocatalytic CO₂ reduction performance test shows that the 10% PCN-222(Cu)/TiO₂ composite achieves optimal catalytic performance, yielding 13.24 μmol·g⁻¹·h⁻¹ CO and 1.73 μmol·g⁻¹·h⁻¹ CH₄, respectively. All the rod-like PCN-222(Cu)/TiO₂ composites exhibit better photocatalytic CO₂ activity than that of TiO₂ nanoparticles or PCN-222(Cu) under the illumination of xenon lamps, which is attributed to charge transport and electron-hole separation capabilities. After three test cycles, the catalytic activity of PCN-222(Cu)/TiO₂ photocatalyst was virtually unchanged. The reduction yield of the catalyst increased for 8 h under continuous illumination, indicating that PCN-222(Cu)/TiO₂ composites have acceptable stability. The estimation of the band gap curve and the Mote-Schottky curve test show that the lowest unoccupied molecular orbital position of PCN-222(Cu) is more negative than the TiO₂ of the conduction band; hence, a possible photocatalytic reaction mechanism of the PCN-222(Cu)/TiO₂ composite is proposed. This study provides a new strategy for the integration of metal-organic frameworks and oxide semiconductors to construct efficient photocatalytic systems.     

CO2 和C2 H4 光催化合成丙烯酸的Cu/ZnO-TiO2 催化剂

The metallic Cu modified n-p coupled semiconductor Cu/ZnO-TiO2 was prepared by the sol gel method, and its surface structure, absorptivity of ultraviolet light, chemisorption properties and photocatalytic behaviors were investigated by techniques of X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), BET surface area (BET), Infrared absorption spectra (IR), UV-visible diffused reflectance spectra (UV-vis) and photo stimulated surface catalytic reaction. The results show that the main crystal structure of the catalyst is anatase TiO2 with the particle size about 10 nm. The Zn-O-Ti bond is formed and the absorption intensity in the wavelength region of 250～400 nm is enhanced through the coupling of ZnO and TiO2, while the absorption region is expanded to visible light with the UV absorbance limit shifting to the shorter wavelength region after Cu supported on the coupled semiconductor ZnO-TiO2. The adsorption states of reactants are important factors influencing the results of photocatalytic reaction, while the bridge adsorption state of CO2 formed on Cu and Lewis acid sites Ti4+ and undissociated of C2H4 formed on Cu and Lewis base sites bridge-O are the effective intermediates to synthesize the crylic acid with the selectivity over 87% under the temperature of 100℃.     

Sulfur Changes the Electrochemical CO2 Reduction Pathway over Cu Electrocatalysts

Electrochemical CO₂ reduction to value-added chemicals or fuels offers a promising approach to reduce carbon emissions and alleviate energy shortage. Cu-based electrocatalysts have been widely reported as capable of reducing CO₂ to produce a variety of multicarbon products (e.g., ethylene and ethanol). In this work, we develop sulfur-doped Cu₂O electrocatalysts, which instead can electrochemically reduce CO₂ to almost exclusively formate. We show that a dynamic equilibrium of S exists at the Cu₂O-electrolyte interface, and S-doped Cu₂O undergoes in situ surface reconstruction to generate active S-adsorbed metallic Cu sites during the CO₂ reduction reaction (CO₂RR). Density functional theory (DFT) calculations together with in situ infrared absorption spectroscopy measurements show that the S-adsorbed metallic Cu surface can not only promote the formation of the *OCHO intermediate but also greatly suppress *H and *COOH adsorption, thus facilitating CO₂-to-formate conversion during the electrochemical CO₂RR.     

Breaking K+ Concentration Limit on Cu Nanoneedles for Acidic Electrocatalytic CO2 Reduction to Multi-Carbon Products

Electrocatalytic CO₂ reduction reaction (CO₂RR) to multi-carbon products (C₂₊) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C₂₊ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K⁺) concentration at the cathode's surface can inhibit proton-diffusion and accelerate the desirable carbon-carbon (C−C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K⁺ concentration at the reaction sites and thus the overall acidic CO₂RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K⁺ concentrations (4.22 M) – thus breaking the K⁺ solubility limit (3.5 M) – which enables a highly efficient CO₂RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C₂₊ (FE_C2+) can be achieved at 1400 mA.cm⁻², simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO₂ flow rate of 7 sccm.     

金属铈表面氧化还原过程的原位拉曼光谱研究

采用显微拉曼光谱仪原位监测了金属铈表面的氧化和还原过程。2132和2239 cm-1处尖锐Ce3+电子散射峰的出现及462 cm-1处CeO2特征振动峰的消失表明,在高温、高真空条件下金属铈表层的CeO2已转变为Ce2O3。根据1503,1796和2113 cm-1处多个Ce3+电子散射峰及CeO2特征振动峰强度随时间的变化关系,可将金属Ce在空气中的氧化过程分为3个阶段,首先不同价态铈氧物层均快速增长,随后外层的CeO2陷入停顿,含Ce3+的铈氧物层继续增长,最后随着3603 cm-1处OH-的出现,CeO2再次快速增长。