Elucidating the Electrocatalytic CO2 Reduction Reaction over a Model Single-Atom Nickel Catalyst

Designing effective electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) is an appealing approach to tackling the challenges posed by rising CO₂ levels and realizing a closed carbon cycle. However, fundamental understanding of the complicated CO₂RR mechanism in CO₂ electrocatalysis is still lacking because model systems are limited. We have designed a model nickel single-atom catalyst (Ni SAC) with a uniform structure and well-defined Ni-N₄ moiety on a conductive carbon support with which to explore the electrochemical CO₂RR. Operando X-ray absorption near-edge structure spectroscopy, Raman spectroscopy, and near-ambient X-ray photoelectron spectroscopy, revealed that Ni⁺ in the Ni SAC was highly active for CO₂ activation, and functioned as an authentic catalytically active site for the CO₂RR. Furthermore, through combination with a kinetics study, the rate-determining step of the CO₂RR was determined to be *CO₂⁻+H⁺→*COOH. This study tackles the four challenges faced by the CO₂RR; namely, activity, selectivity, stability, and dynamics.     

Adaptive Catalysts for the Selective Hydrogenation of Bicyclic Heteroaromatics using Ruthenium Nanoparticles on a CO2-Responsive Support

Ruthenium nanoparticles (NPs) immobilized on an amine-functionalized polymer-grafted silica support act as adaptive catalysts for the hydrogenation of bicyclic heteroaromatics. Whereas full hydrogenation of benzofuran and quinoline derivatives is achieved under pure H₂, introducing CO₂ into the H₂ gas phase leads to an effective shutdown of the arene hydrogenation while preserving the activity for the hydrogenation of the heteroaromatic part. The selectivity switch originates from the generation of ammonium formate species on the surface of the materials by catalytic hydrogenation of CO₂. The CO₂ hydrogenation is fully reversible, resulting in a robust and rapid switch between the two states of the catalyst adapting its performance in response to the feed gas composition. A variety of benzofuran and quinoline derivatives were hydrogenated to fully or partially saturated products in high selectivity and yields simply by altering the composition of the feed gas from H₂ to H₂/CO₂. The adaptive catalytic system thus provides controlled access to valuable products using a single catalyst rather than two specific and distinct catalysts with static reactivity.     

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH⁻ and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO₂RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH⁻ and H₂O activity that in turn can possibly affect activity, stability, and selectivity of the CO₂RR. We determine the local OH⁻ and H₂O activity in close proximity to a CO₂-converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO₂RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.     

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO₂ER) to produce multicarbon (C₂₊) feedstocks (e.g., C₂H₄). However, the high energy barriers for CO₂ activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C₂H₄ selectivity of 62.5 % and an industrial-level current density of 160 mA cm⁻² at a low potential of −0.72 V were achieved. Our studies revealed that the isolated NiN₃ active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO₂ER to C₂H₄.     

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.     

Rare‐Earth Single Erbium Atoms for Enhanced Photocatalytic CO2 Reduction

The solar‐driven photocatalytic reduction of CO₂ (CO₂RR) into chemical fuels is a promising route to enrich energy supplies and mitigate CO₂ emissions. However, low catalytic efficiency and poor selectivity, especially in a pure‐water system, hinder the development of photocatalytic CO₂RR owing to the lack of effective catalysts. Herein, we report a novel atom‐confinement and coordination (ACC) strategy to achieve the synthesis of rare‐earth single erbium (Er) atoms supported on carbon nitride nanotubes (Er₁/CN‐NT) with a tunable dispersion density of single atoms. Er₁/CN‐NT is a highly efficient and robust photocatalyst that exhibits outstanding CO₂RR performance in a pure‐water system. Experimental results and density functional theory calculations reveal the crucial role of single Er atoms in promoting photocatalytic CO₂RR.     

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.     

High-Rate CO2 Electrolysis to Formic Acid over a Wide Potential Window: An Electrocatalyst Comprised of Indium Nanoparticles on Chitosan-Derived Graphene

Realizing industrial-scale production of HCOOH from the CO₂ reduction reaction (CO₂RR) is very important, but the current density as well as the electrochemical potential window are still limited to date. Herein, we achieved this by integration of chemical adsorption and electrocatalytic capabilities for the CO₂RR via anchoring In nanoparticles (NPs) on biomass-derived substrates to create In/X−C (X=N, P, B) bifunctional active centers. The In NPs/chitosan-derived N-doped defective graphene (In/N-dG) catalyst had outstanding performance for the CO₂RR with a nearly 100 % Faradaic efficiency (FE) of HCOOH across a wide potential window. Particularly, at 1.2 A ⋅ cm⁻² high current density, the FE of HCOOH was as high as 96.0 %, and the reduction potential was as low as −1.17 V vs RHE. When using a membrane electrode assembly (MEA), a pure HCOOH solution could be obtained at the cathode without further separation and purification. The FE of HCOOH was still up to 93.3 % at 0.52 A ⋅ cm⁻², and the HCOOH production rate could reach 9.051 mmol ⋅ h⁻¹ ⋅ cm⁻². Our results suggested that the defects and multilayer structure in In/N-dG could not only enhance CO₂ chemical adsorption capability, but also trigger the formation of an electron-rich catalytic environment around In sites to promote the generation of HCOOH.     

Surface Modification of Polyimide Membranes by Diamines for H2 and CO2 Separation

Summary: The separation of H₂/CO₂ is technologically important to produce the next generation fuel source, hydrogen, from synthesis gas. However, the separation efficiency achieved by polymeric membranes is usually very low because of both unfavourable diffusivity selectivity and solubility selectivity between H₂ and CO₂. A series of novel diamino‐modified polyimides has been discovered to enhance the separation capability of polyimide membranes especially for H₂ and CO₂ separation. Both pure gas and mixed gas tests have been conducted. The ideal H₂/CO₂ selectivity in pure gas tests is 101, which is far superior to other polymeric membranes and is well above the Robeson's upper‐bound curve. Mixed gas tests show an ideal selectivity of 42 for the propane‐1,3‐diamine‐modified polyimide. The lower selectivity is a result of the sorption competition between H₂ and the highly condensable CO₂ molecules. However, both pure gas and mixed gas data are better than other polymeric membranes and above the Robeson's upper‐bound curve. It is evident that the proposed modification methods can alter the physicochemical structure of polyimide membranes with superior separation performance for H₂ and CO₂ separation.

Both pure gas and mixed gas separation properties of H₂/CO₂ for membranes derived from 6FDA‐durene with respect to the upper‐bound curve.     

Enhancing Activity and Reducing Cost for Electrochemical Reduction of CO2 by Supporting Palladium on Metal Carbides

Electrochemical CO₂ reduction reaction (CO₂RR) with renewable electricity is a potentially sustainable method to reduce CO₂ emissions. Palladium supported on cost‐effective transition‐metal carbides (TMCs) are studied to reduce the Pd usage and tune the activity and selectivity of the CO₂RR to produce synthesis gas, using a combined approach of studying thin films and practical powder catalysts, in situ characterization, and density functional theory (DFT) calculations. Notably, Pd/TaC exhibits higher CO₂RR activity, stability and CO Faradaic efficiency than those of commercial Pd/C while significantly reducing the Pd loading. In situ measurements confirm the transformation of Pd into hydride (PdH) under the CO₂RR environment. DFT calculations reveal that the TMC substrates modify the binding energies of key intermediates on supported PdH. This work suggests the prospect of using TMCs as low‐cost and stable substrates to support and modify Pd for enhanced CO₂RR activity.     

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.     

Highly Active Hydrogen-rich Photothermal Reverse Water Gas Shift Reaction on Ni/LaInO3 Perovskite Catalysts with Near-unity Selectivity

Photo-assisted reverse water gas shift (RWGS) reaction is regarded green and promising in controlling the reaction gas ratio in Fischer Tropsch synthesis. But it is inclined to produce more byproducts in high H₂ concentration condition. Herein, LaInO₃ loaded with Ni-nanoparticles (Ni NPs) was designed to obtain an efficient photothermal RWGS reaction rate, where LaInO₃ was enriched with oxygen vacancies to roundly adsorbing CO₂ and the strong interaction with Ni NPs endowed the catalysts with powerful H₂ activity. The optimized catalyst performed a large CO yield rate (1314 mmol g_Ni⁻¹ h⁻¹) and ≈100 % selectivity. In situ characterizations demonstrated a COOH* pathway of the reaction and photoinduced charge transfer process for reducing the RWGS reaction active energy. Our work provides valuable insights on the construction of catalysts concerning products selectivity and photoelectronic activating mechanism on CO₂ hydrogenation.     

A quasi-stable molybdenum sub-oxide with abundant oxygen vacancies that promotes CO2 hydrogenation to methanol

Production of methanol from anthropogenic carbon dioxide (CO₂) is a promising chemical process that can alleviate both the environmental burden and the dependence on fossil fuels. In catalytic CO₂ hydrogenation to methanol, reduction of CO₂ to intermediate species is generally considered to be a crucial step. It is of great significance to design and develop advanced heterogeneous catalysts and to engineer the surface structures to promote CO₂-to-methanol conversion. We herein report an oxygen-defective molybdenum sub-oxide coupled with Pt nanoparticles (Pt/H_xMoO_3−y) which affords high methanol yield with a methanol formation rate of 1.53 mmol g_-cat⁻¹ h⁻¹ in liquid-phase CO₂ hydrogenation under relatively mild reaction conditions (total 4.0 MPa, 200 °C), outperforming other oxide-supported Pt catalysts in terms of both the yield and selectivity for methanol. Experiments and comprehensive analyses including in situ X-ray absorption fine structure (XAFS), in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and density functional theory (DFT) calculations reveal that both abundant surface oxygen vacancies (V_O) and the redox ability of Mo species in quasi-stable H_xMoO_3−y confer the catalyst with enhanced adsorption and activation capability to subsequently transform CO₂ to methanol. Moreover, the Pt NPs act as H₂ dissociation sites to regenerate oxygen vacancies and as hydrogenation sites for the CO intermediate to finally afford methanol. Based on the experimental and computational studies, an oxygen-vacancy-mediated “reverse Mars–van Krevelen (M–vK)” mechanism is proposed. This study affords a new strategy for the design and development of an efficient heterogeneous catalyst for CO₂ conversion.

Oxygen-defective molybdenum sub-oxide coupled with Pt nanoparticles affords high methanol yield in liquid-phase CO₂ hydrogenation via reverse Mars–van Krevelen mechanism.     

Elucidating the Electrocatalytic CO2 Reduction Reaction over a Model Single‐Atom Nickel Catalyst

Designing effective electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) is an appealing approach to tackling the challenges posed by rising CO₂ levels and realizing a closed carbon cycle. However, fundamental understanding of the complicated CO₂RR mechanism in CO₂ electrocatalysis is still lacking because model systems are limited. We have designed a model nickel single‐atom catalyst (Ni SAC) with a uniform structure and well‐defined Ni‐N₄ moiety on a conductive carbon support with which to explore the electrochemical CO₂RR. Operando X‐ray absorption near‐edge structure spectroscopy, Raman spectroscopy, and near‐ambient X‐ray photoelectron spectroscopy, revealed that Ni⁺ in the Ni SAC was highly active for CO₂ activation, and functioned as an authentic catalytically active site for the CO₂RR. Furthermore, through combination with a kinetics study, the rate‐determining step of the CO₂RR was determined to be *CO₂^?+H⁺→*COOH. This study tackles the four challenges faced by the CO₂RR; namely, activity, selectivity, stability, and dynamics.     

Structure and gas transport characteristics of triethylene oxide-grafted polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene

Polymeric membrane-based gas separation technology has significant advantages compared with traditional amine-based CO₂ separation method. In this work, SEBS block copolymer is used as a polymer matrix to incorporate triethylene oxide (TEO) functionality. The short ethylene oxide segment is chosen to avoid crystallization, which is confirmed by differential scanning calorimetry and wide-angle X-ray scattering characterizations. The gas permeability results reveal that CO₂/N₂ selectivity increased with increasing content of TEO functional group. The highest CO₂ permeability (281 Barrer) and CO₂/N₂ selectivity (31) were obtained for the membrane with the highest TEO incorporation (57 mol%). Increasing the TEO content in these copolymers results in an increase in CO₂ solubility and a decrease in C₂H₆ solubility. For example, as the grafted TEO content increased from 0 to 57 mol%, the CO₂ solubility and CO₂/C₂H₆ solubility selectivity increased from 0.72 to 1.3 cm³(STP)/cm³ atm and 0.47 to 1.3 at 35°C, respectively. The polar ether linkage in TEO-grafted SEBS copolymers exhibits favorable interaction with CO₂ and unfavorable interaction with nonpolar C₂H₆, thus enhancing CO₂/C₂H₆ solubility selectivity.     

FeMn、FeMnNa和FeMnK催化剂上合成气制低碳烯烃的比较

研究了钠、钾助剂对FeMn 合成低碳烯烃催化剂结构及性能的影响. 低温N₂吸附、X射线光电子能谱（XPS）、X射线衍射（XRD）、H₂程序升温还原（H₂-TPR）、CO/CO₂程序升温脱附（CO/CO₂-TPD）、Mössbauer 谱和CO+H₂反应的研究结果表明,增加Mn助剂含量促进了活性相的分散和低碳烯烃的生成,而过多锰助剂在催化剂表面的富集则降低了费托合成反应的CO转化率;钾助剂和钠助剂的加入均抑制了催化剂的还原并且促进了CO₂和CO的吸附. 比较还原后（H₂/CO摩尔比为20）和反应后（H₂/CO摩尔比为3.5）催化剂的体相结构可以发现,在FeMn、FeMnNa和FeMnK催化剂中,由于钾助剂的碱性和CO吸附能力较强,因此体相中FeC_x的含量相对较高;而活性测试结果表明,FeMnNa催化剂拥有最好的CO转化率（96.2%）和低碳烯烃选择性（30.5%,摩尔分数）.     

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.     

Facile Synthesis of Sub-Nanometric Copper Clusters by Double Confinement Enables Selective Reduction of Carbon Dioxide to Methane

Previous density-functional theory (DFT) calculations show that sub-nanometric Cu clusters (i.e., 13 atoms) favorably generate CH₄ from the CO₂ reduction reaction (CO₂RR), but experimental evidence is lacking. Herein, a facile impregnation-calcination route towards Cu clusters, having a diameter of about 1.0 nm with about 10 atoms, was developed by double confinement of carbon defects and micropores. These Cu clusters enable high selectivity for the CO₂RR with a maximum Faraday efficiency of 81.7 % for CH₄. Calculations and experimental results show that the Cu clusters enhance the adsorption of *H and *CO intermediates, thus promoting generation of CH₄ rather than H₂ and CO. The strong interactions between the Cu clusters and defective carbon optimize the electronic structure of the Cu clusters for selectivity and stability towards generation of CH₄. Provided here is the first experimental evidence that sub-nanometric Cu clusters facilitate the production of CH₄ from the CO₂RR.     

Modeling gas separation in metal-organic frameworks

The gas adsorption and CO₂ separation properties of 9 different metal-organic frameworks (MOFs) have been modelled with grand canonical Monte Carlo (GCMC) adsorption simulations. Adsorption of both pure gases and gas mixtures has been studied. MOFs are shown to have high selectivity for polar gases such as CO₂ over non-polar gases such as N₂. Selectivity of one polar gas from another can be altered by changing the polarity of the framework, pore geometry and also temperature. Often features that lead to good selectivity of CO₂ from N₂ also lead to poor selectivity of CO₂ from H₂O.     

Cation-Radius-Controlled Sn−O Bond Length Boosting CO2 Electroreduction over Sn-Based Perovskite Oxides

Despite the intriguing potential shown by Sn-based perovskite oxides in CO₂ electroreduction (CO₂RR), the rational optimization of their CO₂RR properties is still lacking. Here we report an effective strategy to promote CO₂-to-HCOOH conversion of Sn-based perovskite oxides by A-site-radius-controlled Sn−O bond lengths. For the proof-of-concept examples of Ba_1−xSr_xSnO₃, as the A-site cation average radii decrease from 1.61 to 1.44 Å, their Sn−O bonds are precisely shortened from 2.06 to 2.02 Å. Our CO₂RR measurements show that the activity and selectivity of these samples for HCOOH production exhibit volcano-type trends with the Sn−O bond lengths. Among these samples, the Ba_0.5Sr_0.5SnO₃ features the optimal activity (753.6 mA ⋅ cm⁻²) and selectivity (90.9 %) for HCOOH, better than those of the reported Sn-based oxides. Such optimized CO₂RR properties could be attributed to favorable merits conferred by the precisely controlled Sn−O bond lengths, e.g., the regulated band center, modulated adsorption/activation of intermediates, and reduced energy barrier for *OCHO formation. This work brings a new avenue for rational design of advanced Sn-based perovskite oxides toward CO₂RR.