The atomic-level regulation of single-atom site catalysts for the electrochemical CO2 reduction reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is viewed as a promising way to remove the greenhouse gas CO₂ from the atmosphere and convert it into useful industrial products such as methane, methanol, formate, ethanol, and so forth. Single-atom site catalysts (SACs) featuring maximum theoretical atom utilization and a unique electronic structure and coordination environment have emerged as promising candidates for use in the CO₂RR. The electronic properties and atomic structures of the central metal sites in SACs will be changed significantly once the types or coordination environments of the central metal sites are altered, which appears to provide new routes for engineering SACs for CO₂ electrocatalysis. Therefore, it is of great importance to discuss the structural regulation of SACs at the atomic level and their influence on CO₂RR activity and selectivity. Despite substantial efforts being made to fabricate various SACs, the principles of regulating the intrinsic electrocatalytic performances of the single-atom sites still needs to be sufficiently emphasized. In this perspective article, we present the latest progress relating to the synthesis and catalytic performance of SACs for the electrochemical CO₂RR. We summarize the atomic-level regulation of SACs for the electrochemical CO₂RR from five aspects: the regulation of the central metal atoms, the coordination environments, the interface of single metal complex sites, multi-atom active sites, and other ingenious strategies to improve the performance of SACs. We highlight synthesis strategies and structural design approaches for SACs with unique geometric structures and discuss how the structure affects the catalytic properties.

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising way to remove CO₂ and convert it into useful industrial products. Single-atom site catalysts provide opportunities to regulate the active sites of CO₂RR catalysts at the atomic level.     

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO₂RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO₂RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C₁ products and formation of C₂₊ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO₂RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO₂RR are raised and possible solutions are provided.

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels.     

Liquid Fluxional Ga Single Atom Catalysts for Efficient Electrochemical CO2 Reduction

Precise design and tuning of the micro-atomic structure of single atom catalysts (SACs) can help efficiently adapt complex catalytic systems. Herein, we inventively found that when the active center of the main group element gallium (Ga) is downsized to the atomic level, whose characteristic has significant differences from conventional bulk and rigid Ga catalysts. The Ga SACs with a P, S atomic coordination environment display specific flow properties, showing CO products with FE of ≈92 % at −0.3 V vs. RHE in electrochemical CO₂ reduction (CO₂RR). Theoretical simulations demonstrate that the adaptive dynamic transition of Ga optimizes the adsorption energy of the *COOH intermediate and renews the active sites in time, leading to excellent CO₂RR selectivity and stability. This liquid single atom catalysts system with dynamic interfaces lays the foundation for future exploration of synthesis and catalysis.     

Recent strategy(ies) for the electrocatalytic reduction of CO2: Ni single-atom catalysts for the selective electrochemical formation of CO in aqueous electrolytes

To decrease the global carbon footprint concerns and to diminish the energy crisis, electrocatalytic reduction of CO₂ which results in the formulation of value-added chemicals is a potential solution. In this review, single-atom catalysts (SACs) which are rapidly growing and being developed as the stimulating catalytic materials for electrocatalytic reduction of CO₂ with improved selectivity, efficiency, and stability are considered. Various factors which are responsible for the efficient CO₂ reduction are discussed. The pyrolytic approach for the preparation of Ni-based SACs and the maximum atom utilization efficiency for the desirable production of CO from CO₂ are highlighted.     

Recent progress in advanced core-shell metal-based catalysts for electrochemical carbon dioxide reduction

Electrochemical carbon dioxide reduction（CO₂RR） plays an important role in solving the problem of high concentration of CO₂in the atmosphere and realizing carbon cycle. Core-shell structure has many unique features including tandem catalysis, lattice strain effect, defect engineering, which exhibit great potential in electrocatalysis. In this review, we focus on the advanced core-shell metal-based catalysts（CMCs） for electrochemical CO₂RR. The recent progress of ...     

Regulating Efficient and Selective Single-atom Catalysts for Electrocatalytic CO2 Reduction

Anchoring transition metal (TM) atoms on suitable substrates to form single-atom catalysts (SACs) is a novel approach to constructing electrocatalysts. Graphdiyne with sp−sp² hybridized carbon atoms and uniformly distributed pores have been considered as a potential carbon material for supporting metal atoms in a variety of catalytic processes. Herein, density functional theory (DFT) calculations were performed to study the single TM atom anchoring on graphdiyne (TM₁−GDY, TM=Sc, Ti, V, Cr, Mn, Co and Cu) as the catalysts for CO₂ reduction. After anchoring metal atoms on GDY, the catalytic activity of TM₁−GDY (TM=Mn, Co and Cu) for CO₂ reduction reaction (CO₂RR) are significantly improved comparing with the pristine GDY. Among the studied TM₁−GDY, Cu₁−GDY shows excellent electrocatalytic activity for CO₂ reduction for which the product is HCOOH and the limiting potential (U_L) is −0.16 V. Mn₁−GDY and Co₁−GDY exhibit superior catalytic selectivity for CO₂ reduction to CH₄ with U_L of −0.62 and −0.34 V, respectively. The hydrogen evolution reaction (HER) by TM₁−GDY (TM=Mn, Co and Cu) occurs on carbon atoms, while the active sites of CO₂RR are the transition metal atoms . The present work is expected to provide a solid theoretical basis for CO₂ conversion into valuable hydrocarbons.     

Recent advances in carbon-based materials for electrochemical CO2 reduction reaction

The increase of atmospheric CO₂ concentration has caused many environmental issues. Electrochemical CO₂ reduction reaction（CO₂RR） has been considered as a promising strategy to mitigate these challenges. The electrocatalysts with a low overpotential, high Faradaic efficiency, and excellent selectivity are of great significance for the CO₂RR. Carbon-based materials including metal-free carbon catalysts and metal-based carbon catalysts have shown great p...     

Cu cluster embedded porous nanofibers for high-performance CO2 electroreduction

Metal-doped carbon materials, as one of the most important electrocatalytic catalysts for CO₂ reduction reaction (CO₂RR), have attracted increasing attention. Herein, a series of Cu cluster embedded highly porous nanofibers have been prepared through the carbonization of electro-spun MOF/PAN nanofibers. The obtained Cu cluster doped porous nanofibers possessed fibrous morphology, high porosity, conductivity, and uniformly dispersed Cu clusters, which could be applied as promising CO₂RR catalysts. Specifically, best of them, MCP-500 exhibited high catalytic performance for CO₂RR, in which the Faradaic efficiency of CO (FE_CO) was as high as 98% at ?0.8 V and maintained above 95% after 10 h continuous electrocatalysis. The high performance might be attributed to the synergistic effect of tremendously layered graphene skeleton and uniformly dispersed Cu clusters that could largely promote the electron conductivity, mass transfer and catalytic activity during the electrocatalytic CO₂RR process. This attempt will provide a new idea to design highly active CO₂RR electrocatalyst.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single‐Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen‐doped carbon supported single‐atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single‐atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm^?2) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single‐atom configurations for the H₂ and CO evolution. The results present a useful case on how non‐precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single-atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm⁻²) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H₂ and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Design and Preparation of Electrocatalysts by Electrodeposition for CO2 Reduction

With the increasing emission of carbon dioxide (CO₂), the conversion and utilization of CO₂ have become a topic of increasing concern. Electrochemical CO₂ reduction reaction (CO₂RR) is an attractive and sustainable approach for solving energy and environmental problems. Design of efficient catalysts is crucial for achieving highly efficient CO₂RR. Different methods to prepare catalysts have been reported and used. Among them, electrodeposition is one of the common approaches, which has some obvious advantages, such as requiring simple equipment, environmentally benign. Especially, it can direct deposit catalysts on different substrates to prepare electrodes for CO₂RR. In this review, we discuss recent advances in design and preparation of the catalysts by electrodeposition and their applications in CO₂RR. Furthermore, the perspective of this promising area is also discussed.     

Understanding the Catalytic Sites of Metal–Nitrogen–Carbon Oxygen Reduction Electrocatalysts

The development of low-cost catalysts containing earth-abundant elements as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for the large-scale commercial application of proton exchange membrane fuel cells (PEMFCs). Nonprecious metal–nitrogen–carbon (M-N-C) materials represent the most promising candidates to replace Pt-based catalysts for PEMFCs applications. However, the high-temperature pyrolysis process for the preparation of M-N-C catalysts frequently leads to high structural heterogeneity, that is, the coexistence of various metal-containing sites and N-doped carbon structures. Unfortunately, this impedes the identification of the predominant catalytic active structure, and thus, the further development of highly efficient M-N-C catalysts for the ORR. This Minireview, after a brief introduction to the development of M-N-C ORR catalysts, focuses on the commonly accepted views of predominant catalytic active structures in M-N-C catalysts, including atomically dispersed metal–N_x sites, metal nanoparticles encapsulated with nitrogen-doped carbon structures, synergistic action between metal–N_x sites and encapsulated metal nanoparticles, and metal-free nitrogen-doped carbon structures.     

Positive Valent Metal Sites in Electrochemical CO2 Reduction Reaction

The discovery of high-performance catalysts for the electrochemical CO₂ reduction reaction ( CO₂RR) has faced an enormous challenge for years. The lack of cognition about the surface active structures or centers of catalysts in complex conditions limits the development of advanced catalysts for CO₂RR. Recently, the positive valent metal sites (PVMS) are demonstrated as a kind of potential active sites, which can facilitate carbon dioxide (CO₂) activation and conversation but are always unstable under reduction potentials. Many advanced technologies in theory and experiment have been utilized to understand and develop excellent catalysts with PVMS for CO₂RR. Here, we present an introduction of some typical catalysts with PVMS in CO₂RR and give some understanding of the activity and stability for these related catalysts.     

Constructing low-valent Ni nanoparticles for highly selective CO2 reduction

The electroreduction of CO₂（CO₂RR） into value-added chemicals is a sustainable strategy for mitigating global warming and managing the global carbon balance. However, developing an efficient and selective catalyst is still the central challenge. Here, we developed a simple two-step pyrolysis method to confine low-valent Ni-based nanoparticles within nitrogen-doped carbon（Ni-NC）. As a result, such Ni-based nanoparticles can effectively reduce CO₂ to CO, with a max...     

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.     

Single-Atom Electrocatalysts from Multivariate Metal–Organic Frameworks for Highly Selective Reduction of CO2 at Low Pressures

Single-atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single-atom catalysts is not straightforward. Herein, a general strategy for synthesis of single-atom metals implanted in N-doped carbon (M₁-N-C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal–organic frameworks (MOFs). The M₁-N-C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single-atom metal species. When employed in electrocatalytic CO₂ reduction, Ni₁-N-C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe₁-, Co₁- and Cu₁-N-C. Remarkably, the best-performer, Ni₁-N-C, even demonstrated excellent CO FE at low CO₂ pressures, thereby representing a promising opportunity for the direct use of dilute CO₂ feedstock.     

Designs of Tandem Catalysts and Cascade Catalytic Systems for CO2 Upgrading

Upgrading CO₂ into multi-carbon (C2+) compounds through the CO₂ reduction reaction (CO₂RR) offers a practical approach to mitigate atmospheric CO₂ while simultaneously producing high value chemicals. The reaction pathways for C2+ production involve multi-step proton-coupled electron transfer (PCET) and C−C coupling processes. By increasing the surface coverage of adsorbed protons (*H_ad) and *CO intermediates, the reaction kinetics of PCET and C−C coupling can be accelerated, thereby promoting C2+ production. However, *H_ad and *CO are competitively adsorbed intermediates on monocomponent catalysts, making it difficult to break the linear scaling relationship between the adsorption energies of the *H_ad/*CO intermediate. Recently, tandem catalysts consisting of multicomponents have been developed to improve the surface coverage of *H_ad or *CO by enhancing water dissociation or CO₂-to-CO production on auxiliary sites. In this context, we provide a comprehensive overview of the design principles of tandem catalysts based on reaction pathways for C2+ products. Moreover, the development of cascade CO₂RR catalytic systems that integrate CO₂RR with downstream catalysis has expanded the range of potential CO₂ upgrading products. Therefore, we also discuss recent advancements in cascade CO₂RR catalytic systems, highlighting the challenges and perspectives in these systems.     

One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization-Hydrosilylation Process

Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov α-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.     

Transition metal-nitrogen sites for electrochemical carbon dioxide reduction reaction

Electrochemical CO₂ reduction reaction (CO₂RR) powered by renewable electricity has emerged as the most promising technique for CO₂ conversion, making it possible to realize a carbon-neutral cycle. Highly efficient, robust, and cost-effective catalysts are highly demanded for the near-future practical applications of CO₂RR. Previous studies on atomically dispersed metal-nitrogen (M-N_x) sites constituted of earth abundant elements with maximum atom-utilization efficiency have demonstrated their performance towards CO₂RR. This review summarizes recent advances on a variety of M-N_x sites-containing transition metal-centered macrocyclic complexes, metal organic frameworks, and M-N_x-doped carbon materials for efficient CO₂RR, including both experimental and theoretical studies. The roles of metal centers, coordinated ligands, and conductive supports on the intrinsic activity and selectivity, together with the importance of reaction conditions for improved performance are discussed. The mechanisms of CO₂RR over these M-N_x-containing materials are presented to provide useful guidance for the rational design of efficient catalysts towards CO₂RR.     

Halogen-Incorporated Sn Catalysts for Selective Electrochemical CO2 Reduction to Formate

Electrochemically reducing CO₂ to valuable fuels or feedstocks is recognized as a promising strategy to simultaneously tackle the crises of fossil fuel shortage and carbon emission. Sn-based catalysts have been widely studied for electrochemical CO₂ reduction reaction (CO₂RR) to make formic acid/formate, which unfortunately still suffer from low activity, selectivity and stability. In this work, halogen (F, Cl, Br or I) was introduced into the Sn catalyst by a facile hydrolysis method. The presence of halogen was confirmed by a collection of ex situ and in situ characterizations, which rendered a more positive valence state of Sn in halogen-incorporated Sn catalyst as compared to unmodified Sn under cathodic potentials in CO₂RR and therefore tuned the adsorption strength of the key intermediate (*OCHO) toward formate formation. As a result, the halogen-incorporated Sn catalyst exhibited greatly enhanced catalytic performance in electrochemical CO₂RR to produce formate.