Insights on forming N,O-coordinated Cu single-atom catalysts for electrochemical reduction CO2 to methane

Atomically dispersed copper-zinc dual sites anchored on nitrogen-doped porous carbon toward peroxymonosulfate activation for degradation of various organic contaminants

Single-atom catalysts (SACs) have been widely studied in Fenton-like reactions, wherein their catalytic performance could be further enhanced by adjusting electronic structure and regulating coordination environment, although relevant research is rarely reported. This text elucidates fabrication of dual atom catalyst systems aimed at augmenting their catalytic efficiency. Herein, atomically dispersed copper-zinc (Cu-Zn) dual sites anchored on nitrogen (N)-doped porous carbon (NC), referred to as CuZn-NC, were synthesized using cage-encapsulated pyrolysis and host-guest strategies. The CuZn-NC catalyst exhibited high activity in activation of peroxymonosulfate (PMS) for degradation of organic pollutants. Based on synergistic effects of adjacent Cu and Zn atom pairs, CuZn-NC (PMS) system achieved 94.44 % bisphenol A (BPA) degradation in 24 min. The radical pathway predominated, and coexistence of non-radical species was demonstrated for BPA degradation in CuZn-NC/PMS system. More importantly, CuZn-NC/PMS system showed generality for degradation of various refractory contaminants. Our experiments indicate that CuZn-N sites on CuZn-NC act as active sites for bonding PMS molecules with optimal binding energy, while pyrrolic N sites are considered as adsorption sites for organic molecules. Overall, this research designs diatomic site catalysts (DACs), with promising implications for wastewater treatment.

Fine tuning dual active sites in modulating cascade electrocatalytic nitrate reduction over covalent organic framework

Conversion of NO3- to NH3 proceeds stepwise in natural system under two different enzymes involving intermediate NO2-. Artificial electro-driven NO3- reduction also faces the obstacle of low faradaic efficiency due to insufficient utilization of this intermediate. Herein, we demonstrate a bimetallic COF-based electrocatalyst for the cascade catalysis of NO3--to-NO2--to-NH3 for the first time. TpBpy-Cu2Co4 exhibits a significantly improved performance, with an enhancement factor of 1.4-2 compared to monometallic TpBpy-M. The NH3 yield rate achieves 25.6 mg h-1 mgcat.-1 at -0.55 V vs RHE over TpBpy-Cu2Co4, together with excellent faradaic efficiency (93.4 %). This achievement demonstrates cascade catalysis between Co and Cu units, and their distinct roles are investigated through electrochemical experiments and theory calculations. In electrocatalytic process, Cu site facilities *NO3-to-*NO3H step, while the Co site significantly decreases the energy barrier of *NHOH-to-*NH. The present work provides a valuable inspiration in designing efficient catalysts for cascade reaction.

Ligand-tuning copper in coordination polymers for efficient electrochemical C-C coupling

Cu catalyses electrochemical CO2 reduction to valuable multicarbon products but understanding the structure-function relationship has remained elusive due to the active Cu sites being heterogenized and under dynamic re-construction during electrolysis. We herein coordinate Cu with six phenyl-1H-1,2,3-triazole derivatives to form stable coordination polymer catalysts with homogenized, single-site Cu active sites. Electronic structure modelling, X-ray absorption spectroscopy, and ultraviolet-visible spectroscopy show a widely tuneable Cu electronics by modulating the highest occupied molecular orbital energy of ligands. Using CO diffuse reflectance Fourier transform infrared spectroscopy, in-situ Raman spectroscopy, and density functional theory calculations, we find that the binding strength of *CO intermediate is positively correlated to highest occupied molecular orbital energies of the ligands. As a result, we enable a tuning of C-C coupling efficiency-a parameter we define to evaluate the efficiency of C2 production-in a broad range of 0.26 to 0.86. This work establishes a molecular platform that allows for studying structure-function relationships in CO2 electrolysis and devises new catalyst design strategies appliable to other electrocatalysis.

Approach of a small protein to the biomimetic bis-(μ-oxo) dicopper active-site installed in MOF-808 pores with restricted access perturbs substrate selectivity of oxidase nanozyme

Advances in nanozymes have taken shape over the past few years in several domains. However, persisting challenging limitations of selectivity, specificity, and efficiency necessitate careful attention to aid in the development of next-generation artificial enzymes. Despite nanozymes having significant therapeutic and biotechnological prospects, the multienzyme mimetic activities can compromise their intended applications. Furthermore, the lack of substrate selectivity can hamper crucial biological pathways. While working on addressing the challenges of nanozymes, in this work, we aim to highlight the interplay between the substrates and bis-(μ-oxo) dicopper active site-installed MOF-808 for selectively mimicking oxidase. This oxidase mimetic with a small pore-aperture (1.4 nm), similar to the opening of enzyme binding pockets, projects a tight control over the dynamics and the reactivity of substrates, making it distinct from the general oxidase nanozymes. Interestingly, the design and the well-regulated activity of this nanozyme effectively thwart DNA from approaching the active site, thereby preventing its oxidative damage. Crucially, we also show that despite these merits, the oxidase selectivity is compromised by small proteins such as cytochrome c (Cyt c), having dimensions larger than the pore aperture of MOF-808. This reaction lucidly produces water molecules as a result of four electron transfer to an oxygen molecule. Such unintended side reactivities warrant special attention as they can perturb redox processes and several cellular energy pathways. Through this study, we provide a close look at designing next-generation artificial enzymes that can address the complex challenges for their utility in advanced applications.

Constructing Metal(II)-Sulfate Site Catalysts toward Low Overpotential Carbon Dioxide Electroreduction to Fuel Chemicals

Precise regulation of the active site structure is an important means to enhance the activity and selectivity of catalysts in CO2 electroreduction. Here, we creatively introduce anionic groups, which can not only stabilize metal sites with strong coordination ability but also have rich interactions with protons at active sites to modify the electronic structure and proton transfer process of catalysts. This strategy helps to convert CO2 into fuel chemicals at low overpotentials. As a typical example, a composite catalyst, CuO/Cu-NSO4/CN, with highly dispersed Cu(II)-SO4 sites has been reported, in which CO2 electroreduction to formate occurs at a low overpotential with a high Faradaic efficiency (-0.5 V vs. RHE, FEformate=87.4 %). Pure HCOOH is produced with an energy conversion efficiency of 44.3 % at a cell voltage of 2.8 V. Theoretical modeling demonstrates that sulfate promotes CO2 transformation into a carboxyl intermediate followed by HCOOH generation, whose mechanism is significantly different from that of the traditional process via a formate intermediate for HCOOH production.

Graphene Quantum Dots Eradicate Resistant and Metastatic Cancer Cells by Enhanced Interfacial Inhibition

Drug-resistant and metastatic cancer cells such as a small population of cancer stem cells (CSCs) play a crucial role in metastasis and relapse. Conventional small-molecule chemotherapeutics, however, are unable to eradicate drug-resistant CSCs owing to limited interface inhibitory effects. Herein, it is reported that enhanced interfacial inhibition leading to eradication of drug-resistant CSCs can be dramatically induced by self-insertion of bioactive graphene quantum dots (GQDs) into DNA major groove (MAG) sites in cancer cells. Since transcription factors regulate gene expression at the MAG site, MAG-targeted GQDs exert greatly enhanced interfacial inhibition, downregulating the expression of a collection of cancer stem genes such as ALDH1, Notch1, and Bmi1. Moreover, the nanoscale interface inhibition mechanism reverses cancer multidrug resistance (MDR) by inhibiting MDR1 gene expression when GQDs are used at a nontoxic concentration (1/4 × half-maximal inhibitory concentration (IC50)) as the MDR reverser. Given their high efficacy in interfacial inhibition, CSC-mediated migration, invasion, and metastasis of cancer cells can be substantially blocked by MAG-targeted GQDs, which can also be harnessed to sensitize clinical cytotoxic agents for improved efficacy in combination chemotherapy. These findings elucidate the inhibitory effects of the enhanced nano-bio interface at the MAG site on eradicating CSCs, thus preventing cancer metastasis and recurrence.

Local Heating Induced Single-Crystalline Phase Control in Electrochemical Synthesis of Nanomaterials

Development of synthetic strategies selectively yielding single crystals is desired owing to the facet-dependent chemical reactivities. Recent advances in electrochemical materials synthesis yielded nanomaterials that are surfactant-free, however, typically in polycrystalline forms. In this work, an electrochemical synthetic strategy selectively yielding single-crystalline nanoparticles by implementation of surface-selective heating of the working electrode is developed. Single crystals of copper, silver, gold, and platinum are afforded, and the crystallinity verified by electron diffraction and chemical reactivity studies. Notably, Cu (100) surface prepared by electrochemical synthesis yielded high single product selectivity when applied to electrochemical CO2 reduction catalysis.

Tumour microenvironment-responded Fe-doped carbon dots-sensitized cubic Cu2O for Z-scheme heterojunction-enhanced sono-chemodynamic synergistic tumor therapy

The efficacy of electron-hole separation in a single sonosensitizer and the complexities of the tumor microenvironment (TME) present significant challenges to the effectiveness of sonodynamic therapy (SDT). Designing efficient sonosensitizers to enhance electron-hole separation and alleviate TME resistance is crucial yet challenging. Herein, we introduce a novel Z-scheme heterojunctions (HJs) sonosensitizer using Fe-doped carbon dots (CDs) as auxiliary semiconductors to sensitize cubic Cu2O (Fe-CDs@Cu2O) for the first time. Fe-CDs@Cu2O demonstrated enhanced SDT effects due to improved electron-hole separation. Additionally, the introduction of Fe ions in CDs synergistically enhances Fenton-like reactions with Cu ions in Cu2O, resulting in enhanced chemodynamic therapy (CDT) effects. Moreover, Fe-CDs@Cu2O exhibited rapid glutathione (GSH) depletion, effectively mitigating TME resistance. With high rates of 1O2 and OH generated by Fe-CDs@Cu2O, coupled with strong GSH depletion, single drug injection and ultrasound (US) irradiation effectively eliminate tumors. This innovative heterojunction sonosensitizer offers a promising pathway for clinical anti-tumor treatment.

Direct formation of copper nanoparticles from atoms at graphitic step edges lowers overpotential and improves selectivity of electrocatalytic CO2 reduction

A key strategy for minimizing our reliance on precious metals is to increase the fraction of surface atoms and improve the metal-support interface. In this work, we employ a solvent/ligand/counterion-free method to deposit copper in the atomic form directly onto a nanotextured surface of graphitized carbon nanofibers (GNFs). Our results demonstrate that under these conditions, copper atoms coalesce into nanoparticles securely anchored to the graphitic step edges, limiting their growth to 2-5 nm. The resultant hybrid Cu/GNF material displays high selectivity in the CO2 reduction reaction (CO2RR) for formate production with a faradaic efficiency of ~94% at -0.38 V vs RHE and a high turnover frequency of 2.78 × 106 h-1. The Cu nanoparticles adhered to the graphitic step edges significantly enhance electron transfer to CO2. Long-term CO2RR tests coupled with atomic-scale elucidation of changes in Cu/GNF reveal nanoparticles coarsening, and a simultaneous increase in the fraction of single Cu atoms. These changes in the catalyst structure make the onset of the CO2 reduction potential more negative, leading to less formate production at -0.38 V vs RHE, correlating with a less efficient competition of CO2 with H2O for adsorption on single Cu atoms on the graphitic surfaces, revealed by density functional theory calculations.

Altering the CO2 Electroreduction Pathways Towards C1 or C2+ Products via Engineering the Strength of Interfacial Cu-O Bond

Copper (Cu)-based catalysts have established their unique capability for yielding wide value-added products from CO2. Herein, we demonstrate that the pathways of the electrocatalytic CO2 reduction reaction (CO2RR) can be rationally altered toward C1 or C2+ products by simply optimizing the coordination of Cu with O-containing organic species (squaric acid (H2C4O4) and cyclohexanehexaone (C6O6)). It is revealed that the strength of Cu-O bonds can significantly affect the morphologies and electronic structures of derived Cu catalysts, resulting in the distinct behaviors during CO2RR. Specifically, the C6O6-Cu catalysts made up from organized nanodomains shows a dominant C1 pathway with a total Faradaic efficiency (FE) of 63.7 % at -0.6 V (versus reversible hydrogen electrode, RHE). In comparison, the C4O4-Cu with an about perfect crystalline structure results in uniformly dispersed Cu-atoms, showing a notable FE of 65.8 % for C2+ products with enhanced capability of C-C coupling. The latter system also shows stable operation over at least 10 h with a high current density of 205.1 mA cm-2 at -1.0 VRHE, i.e., is already at the boarder of practical relevance. This study sheds light on the rational design of Cu-based catalysts for directing the CO2RR reaction pathway.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

Spin-polarized p-block antimony/bismuth single-atom catalysts on defect-free rutile TiO2(110) substrate for highly efficient CO oxidation

Developing high-loading spin-polarized p-block-element-based single-atom catalysts (p-SACs) upon defect-free substrates for various chemical reactions wherein spin selection matters is generally considered a formidable challenge because of the difficulty of creating high densities of underpinning stable defects and the delocalized electronic features of p-block elements. Here our first-principles calculations establish that the defect-free rutile TiO2(110) wide-bandgap semiconducting anchoring support can stabilize and localize the wavefunctions of p-block metal elements (Sb and Bi) via strong ionic bonding, forming spin-polarized p-SACs. Cooperated by the underlying d-block Ti atoms via a delicate spin donation-back-donation mechanism, the p-block single-atom reactive center Sb(Bi) exhibits excellent catalysis for spin-triplet O2 activation and CO oxidation in alignment with Wigner's spin selection rule, with a low rate-limiting reaction barrier of ∼0.6 eV. This work is crucial in establishing high-loading reactive centers of high-performance p-SACs for various important physical processes and chemical reactions, especially wherein the spin degree of freedom matters, i.e., spin catalysis.

The meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO2 reduction reaction

We report Fe porphyrins bearing different meso-substituents for the electrocatalytic CO2 reduction reaction (CO2RR). By replacing two and four meso-phenyl groups of Fe tetraphenylporphyrin (FeTPP) with strong electron-withdrawing pentafluorophenyl groups, we synthesized FeF10TPP and FeF20TPP, respectively. We showed that FeTPP and FeF10TPP are active and selective for CO2-to-CO conversion in dimethylformamide with the former being more active, but FeF20TPP catalyzes hydrogen evolution rather than the CO2RR under the same conditions. Experimental and theoretical studies revealed that with more electron-withdrawing meso-substituents, the Fe center becomes electron-deficient and it becomes difficult for it to bind a CO2 molecule in its formal Fe0 state. This work is significant to illustrate the electronic effects of catalysts on binding and activating CO2 molecules and provide fundamental knowledge for the design of new CO2RR catalysts.

Advances and challenges in the electrochemical reduction of carbon dioxide

The electrocatalytic carbon dioxide reduction reaction (ECO2RR) is a promising way to realize the transformation of waste into valuable material, which can not only meet the environmental goal of reducing carbon emissions, but also obtain clean energy and valuable industrial products simultaneously. Herein, we first introduce the complex CO2RR mechanisms based on the number of carbons in the product. Since the coupling of C-C bonds is unanimously recognized as the key mechanism step in the ECO2RR for the generation of high-value products, the structural-activity relationship of electrocatalysts is systematically reviewed. Next, we comprehensively classify the latest developments, both experimental and theoretical, in different categories of cutting-edge electrocatalysts and provide theoretical insights on various aspects. Finally, challenges are discussed from the perspectives of both materials and devices to inspire researchers to promote the industrial application of the ECO2RR at the earliest.

Single-Atom Iron Doped Carbon Dots with Highly Efficient Electrochemiluminescence for Ultrasensitive Detection of MicroRNAs

Herein, single-atom iron doped carbon dots (SA Fe-CDs) were successfully prepared as novel electrochemiluminescence (ECL) emitters with high ECL efficiency, and a biosensor was constructed to ultrasensitively detect microRNA-222 (miRNA-222). Importantly, compared with the conventional without single-atom doped CDs with low ECL efficiency, SA Fe-CDs exhibited strong ECL efficiency, in which single-atom iron as an advanced coreactant accelerator could significantly enhance the generation of reactive oxygen species (ROS) from the coreactant S2O82- for improving the ECL efficiency. Moreover, a neoteric amplification strategy combining the improved strand displacement amplification with Nt.BbvCI enzyme-induced target amplification (ISDA-EITA) could produce 4 output DNAs in every cycle, which greatly improved the amplification efficiency. Thus, a useful ECL biosensor was built with a detection limit of 16.60 aM in the range of 100 aM to 1 nM for detecting traces of miRNA-222. In addition, miRNA-222 in cancer cell lysate (MHCC-97L) was successfully detected by using the ECL biosensor. Therefore, this strategy provides highly efficient single-atom doped ECL emitters for the construction of sensitive ECL biosensing platforms in the biological field and clinical diagnosis.

Cu-phen Coordination Enabled Selective Electrocatalytic Reduction of CO2 to Methane

Manipulation of selectivity in the catalytic electrochemical carbon dioxide reduction reaction (eCO2RR) poses significant challenges due to inevitable structure reconstruction. One approach is to develop effective strategies for controlling reaction pathways to gain a deeper understanding of mechanisms in robust CO2RR systems. In this work, by precise introduction of 1,10-phenanthroline as a bidentate ligand modulator, the electronic property of the copper site was effectively regulated, thereby directing selectivity switch. By modification of [Cu3(btec)(OH)2]n, the use of [Cu2(btec)(phen)2]n·(H2O)n achieved the selectivity switch from ethylene (faradaic efficiency (FE) = 41%, FEC2+ = 67%) to methane (FECH4 = 69%). Various in situ spectroscopic characterizations revealed that [Cu2(btec)(phen)2]n·(H2O)n promoted the hydrogenation of *CO intermediates, leading to methane generation instead of dimerization to form C2+ products. Acting as a delocalized π-conjugation scaffold, 1,10-phenanthroline in [Cu2(btec)(phen)2]n·(H2O)n helps stabilize Cuδ+. This work presents a novel approach to regulate the coordination environment of active sites with the aim of selectively modulating the CO2RR.

Engineering NH2-Cu-NH2 Triple-atom Sites in Defective MOFs for Selective Overall Photoreduction of CO2 into CH3COCH3

Selective photoreduction of CO2 to multicarbon products, is an important but challenging task, due to high CO2 activation barriers and insufficient catalytic sites for C-C coupling. Herein, a defect engineering strategy for incorporating copper sites into the connected nodes of defective metal-organic framework UiO-66-NH2 for selective overall photo-reduction of CO2 into acetone. The Cu2+ site in well-modified CuN2O2 units served as a trapping site to capture electrons via efficient electron-hole separation, forming the active Cu+ site for CO2 reduction. Two NH2 groups in CuN2O2 unit adsorb CO2 and cooperated with copper ion to functionalize as a triple atom catalytic site, each interacting with one CO2 molecule to strengthen the binding of *CO intermediate to the catalytic site. The deoxygenated *CO attached to the Cu site interacted with *CH3 fixed at one amino group to form the key intermediate CO*-CH3, which interacted with the third reduction intermediate on another amino group to produce acetone. Our photocatalyst realizes efficient overall CO2 reduction to C3 product acetone CH3COCH3 with an evolution rate of 70.9 μmol gcat -1 h-1 and a selectivity up to 97 % without any adducts, offering a promising avenue for designing triple-atomic sites to producing C3 product from photosynthesis with water.

Anionic Coordination Control in Building Cu-Based Electrocatalytic Materials for CO2 Reduction Reaction

Renewable energy-driven conversion of CO2 to value-added fuels and chemicals via electrochemical CO2 reduction reaction (CO2RR) technology is regarded as a promising strategy with substantial environmental and economic benefits to achieve carbon neutrality. Because of its sluggish kinetics and complex reaction paths, developing robust catalytic materials with exceptional selectivity to the targeted products is one of the core issues, especially for extensively concerned Cu-based materials. Manipulating Cu species by anionic coordination is identified as an effective way to improve electrocatalytic performance, in terms of modulating active sites and regulating structural reconstruction. This review elaborates on recent discoveries and progress of Cu-based CO2RR catalytic materials enhanced by anionic coordination control, regarding reaction paths, functional mechanisms, and roles of different non-metallic anions in catalysis. Finally, the review concludes with some personal insights and provides challenges and perspectives on the utilization of this strategy to build desirable electrocatalysts.

Oxygen-Bridged Cu Binuclear Sites for Efficient Electrocatalytic CO2 Reduction to Ethanol at Ultralow Overpotential

Electrocatalytic CO2 reduction (CO2RR) to alcohols offers a promising strategy for converting waste CO2 into valuable fuels/chemicals but usually requires large overpotentials. Herein, we report a catalyst comprising unique oxygen-bridged Cu binuclear sites (CuOCu-N4) with a Cu···Cu distance of 3.0-3.1 Å and concomitant conventional Cu-N4 mononuclear sites on hierarchical nitrogen-doped carbon nanocages (hNCNCs). The catalyst exhibits a state-of-the-art low overpotential of 0.19 V (versus reversible hydrogen electrode) for ethanol and an outstanding ethanol Faradaic efficiency of 56.3% at an ultralow potential of -0.30 V, with high-stable Cu active-site structures during the CO2RR as confirmed by operando X-ray adsorption fine structure characterization. Theoretical simulations reveal that CuOCu-N4 binuclear sites greatly enhance the C-C coupling at low potentials, while Cu-N4 mononuclear sites and the hNCNC support increase the local CO concentration and ethanol production on CuOCu-N4. This study provides a convenient approach to advanced Cu binuclear site catalysts for CO2RR to ethanol with a deep understanding of the mechanism.

Fe Single-Atom Carbon Dots Nanozyme Collaborated with Nucleic Acid Exonuclease III-Driven DNA Walker Cascade Amplification Strategy for Circulating Tumor DNA Detection

Circulating tumor DNA (ctDNA), as a next-generation tumor marker, enables early screening and monitoring of cancer through noninvasive testing. Exploring the development of new methods for ctDNA detection is an intriguing study. In this work, a unique electrochemical biosensor for the ctDNA detector was constructed in the first utilizing Fe single-atom nanozymes-carbon dots (SA Fe-CDs) as a signaling carrier in collaboration with a DNA walker cascade amplification strategy triggered by nucleic acid exonuclease III (Exo III). The electrochemical active surface area of AuNPs/rGO modified onto a glassy carbon electrode (AuNPs/rGO/GCE) was about 1.43 times that of a bare electrode (bare GCE), with good electrical conductivity alongside a high heterogeneous electron transfer rate (5.81 × 10-3 cm s-1), that is, as well as the ability to load more molecules. Sequentially, the DNA walker cascade amplification strategy driven by Exo III effectively converted the target ctDNA into an amplified biosignal, ensuring the sensitivity and specificity of ctDNA. Ultimately, the electrochemical signal was further amplified by introducing SA Fe-CDs nanozymes, which could serve as catalysts for 3,3',5,5'-tetramethylbenzidine (TMB) oxidation with facile responding (Vmax = 0.854 × 10-6 M s-1) and robust annexation (Km = 0.0069 mM). The integration of the triple signal amplification approach achieved detection limits as low as 1.26 aM (S/N = 3) for a linearity spanning from 5 aM to 50 nM. In this regard, our proposal for a biosensor with exceptional assay properties in complicated serum environments had great potential for early and timely diagnosis of cancer.

Pulse Electrolysis Turns on CO2 Methanation through N-Confused Cupric Porphyrin

Breaking the D4h symmetry in the square-planar M-N4 configuration of macrocycle molecular catalysts has witnessed enhanced electrocatalytic activity, but at the expense of electrochemical stability. Herein, we hypothesize that the lability of the active Cu-N3 motifs in the N-confused copper (II) tetraphenylporphyrin (CuNCP) could be overcome by applying pulsed potential electrolysis (PPE) during electrocatalytic carbon dioxide reduction. We find that applying PPE can indeed enhance the CH4 selectivity on CuNCP by 3 folds to reach the partial current density of 170 mA cm-2 at >60 % Faradaic efficiency (FE) in flow cell. However, combined ex situ X-ray diffraction (XRD), transmission electron microscope (TEM), and in situ X-ray absorption spectroscopy (XAS), infrared (IR), Raman, scanning electrochemical microscopy (SECM) characterizations reveal that, in a prolonged time scale, the decomplexation of CuNCP is unavoidable, and the promoted water dissociation under high anodic bias with lowered pH and enriched protons facilitates successive hydrogenation of *CO on the irreversibly reduced Cu nanoparticles, leading to the improved CH4 selectivity. As a key note, this study signifies the adaption of electrolytic protocol to the catalyst structure for tailoring local chemical environment towards efficient CO2 reduction.

Coordination Environment Engineering of Metal Centers in Coordination Polymers for Selective Carbon Dioxide Electroreduction toward Multicarbon Products

Electrocatalytic carbon dioxide reduction reaction (CO2RR) toward value-added chemicals/fuels has offered a sustainable strategy to achieve a carbon-neutral energy cycle. However, it remains a great challenge to controllably and precisely regulate the coordination environment of active sites in catalysts for efficient generation of targeted products, especially the multicarbon (C2+) products. Herein we report the coordination environment engineering of metal centers in coordination polymers for efficient electroreduction of CO2 to C2+ products under neutral conditions. Significantly, the Cu coordination polymer with Cu-N2S2 coordination configuration (Cu-N-S) demonstrates superior Faradaic efficiencies of 61.2% and 82.2% for ethylene and C2+ products, respectively, compared to the selective formic acid generation on an analogous polymer with the Cu-I2S2 coordination mode (Cu-I-S). In situ studies reveal the balanced formation of atop and bridge *CO intermediates on Cu-N-S, promoting C-C coupling for C2+ production. Theoretical calculations suggest that coordination environment engineering can induce electronic modulations in Cu active sites, where the d-band center of Cu is upshifted in Cu-N-S with stronger selectivity to the C2+ products. Consequently, Cu-N-S displays a stronger reaction trend toward the generation of C2+ products, while Cu-I-S favors the formation of formic acid due to the suppression of C-C couplings for C2+ pathways with large energy barriers.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Amorphous Modulation of Atomic Nb-O/N Clusters with Asymmetric Coordination in Carbon Shells for Advanced Sodium-Ion Hybrid Capacitors

Anode materials with excellent properties have become the key to develop sodium-ion hybrid capacitors (SIHCs) that combine the advantages of both batteries and capacitors. Amorphous modulation is an effective strategy to realize high energy/power density in SIHCs. Herein, atomically amorphous Nb-O/N clusters with asymmetric coordination are in situ created in N-doped hollow carbon shells (Nb-O/N@C). The amorphous clusters with asymmetric Nb-O3/N1 configurations have abundant charge density and low diffusion energy barriers, which effectively modulate the charge transport paths and improve the reaction kinetics. The clusters are also enriched with unsaturated vacancy defects and isotropic ion-transport channels, and their atomic disordering exhibits high structural stress buffering, which are strong impetuses for realizing bulk-phase-indifferent ion storage and enhancing the storage properties of the composite. Based on these features, Nb-O/N@C achieves notably improved sodium-ion storage properties (reversible capacity of 240.1 mAh g-1 at 10.0 A g-1 after 8000 cycles), and has great potential for SIHCs (230 Wh Kg-1 at 4001.5 W Kg-1). This study sheds new light on developing high-performance electrodes for sodium-ion batteries and SIHCs by designing amorphous clusters and asymmetric coordination.

The Guidelines for the Design and Synthesis of Transition Metal Atom Doped Carbon Dots

Atoms doping is a practical approach to modulate the physicochemical properties of carbon dots (CDs) and thus has garnered increasing attention in recent years. Compared to non-metal atoms, transition metal atoms (TMAs) possess more unoccupied orbitals and larger atomic radii. TMAs doping can significantly alter the electronic structure of CDs and bestow them with new intrinsic characteristics. TMAs-doped CDs have exhibited widespread application potential as a new class of single-atom-based nanomaterials. However, challenges remain for the successful preparation and precise design of TMAs-doped CDs. The key to successfully preparing TMA-doped CDs lies in anchoring TMAs to the carbon precursors before the reaction. Herein, taking the formation mechanism of TMAs-doped CDs as a starting point, we systematically summarized the ligands employed for synthesizing TMAs-doped CDs and proposed the synthetic strategy involving multiple ligands. Additionally, we summarize the functional properties imparted to CDs by different TMA dopants to guide the design of TMA-doped CDs with different functional characteristics. Finally, we describe the bottlenecks TMAs-doped CDs face and provide an outlook on their future development.

Unraveling Valence Electron Number Dependent Excitonic Effects over M1-N3C1 Sites in Single-Atom Catalysts

Excitonic effects significantly influence the selective generation of reactive oxygen species and photothermal conversion efficiency in photocatalytic reactions; however, the intrinsic factors governing excitonic effects remain elusive. Herein, a series of single-atom catalysts with well-defined M1-N3C1 (M = Mn, Fe, Co, and Ni) active sites are designed and synthesized to investigate the structure-activity relationship between photocatalytic materials and excitonic effects. Comprehensive characterization and theoretical calculations unveil that excitonic effects are positively correlated with the number of valence electrons in single metal atoms. The single Mn atom with 5.93 valence electrons exhibits the weakest excitonic effects, which dominate superoxide radical (O2•-) generation through charge transfer and enhance photothermal conversion efficiency. Conversely, the single Ni atom with 9.27 valence electrons exhibits the strongest excitonic effects, dominating singlet oxygen (1O2) generation via energy transfer while suppressing photothermal conversion efficiency. Based on the valence electron number dependent excitonic effects, a reaction environment with hyperthermia and abundant cytotoxic O2•- is designed, achieving efficient and stable water disinfection. This work reveals single metal atom dependent excitonic effects and presents an atomic-level methodology for catalytic application targeted reaction environment tailoring.

N and OH-Immobilized Cu3 Clusters In Situ Reconstructed from Single-Metal Sites for Efficient CO2 Electromethanation in Bicontinuous Mesochannels

Cu-based catalysts hold promise for electrifying CO2 to produce methane, an extensively used fuel. However, the activity and selectivity remain insufficient due to the lack of catalyst design principles to steer complex CO2 reduction pathways. Herein, we develop a concept to design carbon-supported Cu catalysts by regulating Cu active sites' atomic-scale structures and engineering the carbon support's mesoscale architecture. This aims to provide a favorable local reaction microenvironment for a selective CO2 reduction pathway to methane. In situ X-ray absorption and Raman spectroscopy analyses reveal the dynamic reconstruction of nitrogen and hydroxyl-immobilized Cu3 (N,OH-Cu3) clusters derived from atomically dispersed Cu-N3 sites under realistic CO2 reduction conditions. The N,OH-Cu3 sites possess moderate *CO adsorption affinity and a low barrier for *CO hydrogenation, enabling intrinsically selective CO2-to-CH4 reduction compared to the C-C coupling with a high energy barrier. Importantly, a block copolymer-derived carbon fiber support with interconnected mesopores is constructed. The unique long-range mesochannels offer an H2O-deficient microenvironment and prolong the transport path for the CO intermediate, which could suppress the hydrogen evolution reaction and favor deep CO2 reduction toward methane formation. Thus, the newly developed catalyst consisting of in situ constructed N,OH-Cu3 active sites embedded into bicontinuous carbon mesochannels achieved an unprecedented Faradaic efficiency of 74.2% for the CO2 reduction to methane at an industry-level current density of 300 mA cm-2. This work explores effective concepts for steering desirable reaction pathways in complex interfacial catalytic systems via modulating active site structures at the atomic level and engineering pore architectures of supports on the mesoscale to create favorable microenvironments.

A replacement strategy for regulating local environment of single-atom Co-SxN4-x catalysts to facilitate CO2 electroreduction

The performances of single-atom catalysts are governed by their local coordination environments. Here, a thermal replacement strategy is developed for the synthesis of single-atom catalysts with precisely controlled and adjustable local coordination environments. A series of Co-SxN4-x (x = 0, 1, 2, 3) single-atom catalysts are successfully synthesized by thermally replacing coordinated N with S at elevated temperature, and a volcano relationship between coordinations and catalytic performances toward electrochemical CO2 reduction is observed. The Co-S1N3 catalyst has the balanced COOH*and CO* bindings, and thus locates at the apex of the volcano with the highest performance toward electrochemical CO2 reduction to CO, with the maximum CO Faradaic efficiency of 98 ± 1.8% and high turnover frequency of 4564 h-1 at an overpotential of 410 mV tested in H-cell with CO2-saturated 0.5 M KHCO3, surpassing most of the reported single-atom catalysts. This work provides a rational approach to control the local coordination environment of the single-atom catalysts, which is important for further fine-tuning the catalytic performance.

Cu-Based Materials for Enhanced C2+ Product Selectivity in Photo-/Electro-Catalytic CO2 Reduction: Challenges and Prospects

Carbon dioxide conversion into valuable products using photocatalysis and electrocatalysis is an effective approach to mitigate global environmental issues and the energy shortages. Among the materials utilized for catalytic reduction of CO2, Cu-based materials are highly advantageous owing to their widespread availability, cost-effectiveness, and environmental sustainability. Furthermore, Cu-based materials demonstrate interesting abilities in the adsorption and activation of carbon dioxide, allowing the formation of C2+ compounds through C-C coupling process. Herein, the basic principles of photocatalytic CO2 reduction reactions (PCO2RR) and electrocatalytic CO2 reduction reaction (ECO2RR) and the pathways for the generation C2+ products are introduced. This review categorizes Cu-based materials into different groups including Cu metal, Cu oxides, Cu alloys, and Cu SACs, Cu heterojunctions based on their catalytic applications. The relationship between the Cu surfaces and their efficiency in both PCO2RR and ECO2RR is emphasized. Through a review of recent studies on PCO2RR and ECO2RR using Cu-based catalysts, the focus is on understanding the underlying reasons for the enhanced selectivity toward C2+ products. Finally, the opportunities and challenges associated with Cu-based materials in the CO2 catalytic reduction applications are presented, along with research directions that can guide for the design of highly active and selective Cu-based materials for CO2 reduction processes in the future.

Emerging Xene-Based Single-Atom Catalysts: Theory, Synthesis, and Catalytic Applications

In recent years, the emergence of novel 2D monoelemental materials (Xenes), e.g., graphdiyne, borophene, phosphorene, antimonene, bismuthene, and stanene, has exhibited unprecedented potentials for their versatile applications as well as addressing new discoveries in fundamental science. Owing to their unique physicochemical, optical, and electronic properties, emerging Xenes have been regarded as promising candidates in the community of single-atom catalysts (SACs) as single-atom active sites or support matrixes for significant improvement in intrinsic activity and selectivity. In order to comprehensively understand the relationships between the structure and property of Xene-based SACs, this review represents a comprehensive summary from theoretical predictions to experimental investigations. Firstly, theoretical calculations regarding both the anchoring of Xene-based single-atom active sites on versatile support matrixes and doping/substituting heteroatoms at Xene-based support matrixes are briefly summarized. Secondly, controlled synthesis and precise characterization are presented for Xene-based SACs. Finally, current challenges and future opportunities for the development of Xene-based SACs are highlighted.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS)

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

Single atom catalyst-mediated generation of reactive species in water treatment

Water is one of the most essential components in the sustainable development goals (SDGs) of the United Nations. With worsening global water scarcity, especially in some developing countries, water reuse is gaining increasing acceptance. A key challenge in water treatment by conventional treatment processes is the difficulty of treating low concentrations of pollutants (micromolar to nanomolar) in the presence of much higher levels of inorganic ions and natural organic matter (NOM) in water (or real water matrices). Advanced oxidation processes (AOPs) have emerged as an attractive treatment technology that generates reactive species with high redox potentials (E0) (e.g., hydroxyl radical (HO˙), singlet oxygen (1O2), sulfate radical (SO4˙-), and high-valent metals like iron(IV) (Fe(IV)), copper(III) (Cu(III)), and cobalt(IV) (Co(IV))). The use of single atom catalysts (SACs) in AOPs and water treatment technologies has appeared only recently. This review introduces the application of SACs in the activation of hydrogen peroxide and persulfate to produce reactive species in treatment processes. A significant part of the review is devoted to the mechanistic aspects of traditional AOPs and their comparison with those triggered by SACs. The radical species, SO4˙- and HO˙, which are produced in both traditional and SACs-activated AOPs, have higher redox potentials than non-radical species, 1O2 and high-valent metal species. However, SO4˙- and HO˙ radicals are non-selective and easily affected by components of water while non-radicals resist the impact of such constituents in water. Significantly, SACs with varying coordination environments and structures can be tuned to exclusively generate non-radical species to treat water with a complex matrix. Almost no influence of chloride, carbonate, phosphate, and NOM was observed on the performance of SACs in treating pollutants in water when nonradical species dominate. Therefore, the appropriately designed SACs represent game-changers in purifying water vs. AOPs with high efficiency and minimal interference from constituents of polluted water to meet the goals of water sustainability.

Catalytic ozonation mechanism over M1-N3C1 active sites

The structure-activity relationship in catalytic ozonation remains unclear, hindering the understanding of activity origins. Here, we report activity trends in catalytic ozonation using a series of single-atom catalysts with well-defined M1-N3C1 (M: manganese, ferrum, cobalt, and nickel) active sites. The M1-N3C1 units induce locally polarized M - C bonds to capture ozone molecules onto M atoms and serve as electron shuttles for catalytic ozonation, exhibiting excellent catalytic activities (at least 527 times higher than commercial manganese dioxide). The combined in situ characterization and theoretical calculations reveal single metal atom-dependent catalytic activity, with surface atomic oxygen reactivity identified as a descriptor for the structure-activity relationship in catalytic ozonation. Additionally, the dissociation barrier of surface peroxide species is proposed as a descriptor for the structure-activity relationship in ozone decomposition. These findings provide guidelines for designing high-performance catalytic ozonation catalysts and enhance the atomic-level mechanistic understanding of the integral control of ozone and methyl mercaptan.

Gaining Deep Understanding of Electrochemical CO2 RR with In Situ/Operando Techniques

Electrocatalysis for CO2 conversion has been extensively studied to mitigate the energy shortage and environmental issues, which are gaining ever-increasing attention. However, the complicated CO2 reduction process and the dynamic evolution occurring on electrocatalyst surface make it hard to understand the catalytic mechanism. The development of advanced in situ/operando techniques intelligently coupled with electrochemical cells sheds light on the related study via capturing surface atomic rearrangement, tracing chemical state change of catalysts, monitoring the behavior of intermediates and products, and depicting microenvironment near the electrode surface. In this review, fundamentals of the state-of-the-art in situ/operando techniques are clarified first. Case studies on the in situ/operando techniques performed to probe the CO2 reduction reaction processes are then discussed in detail. Finally, conclusions and outlook on this field are presented.

A universal approach to dual-metal-atom catalytic sites confined in carbon dots for various target reactions

Here, a molecular-design and carbon dot-confinement coupling strategy through the pyrolysis of bimetallic complex of diethylenetriamine pentaacetic acid under low-temperature is proposed as a universal approach to dual-metal-atom sites in carbon dots (DMASs-CDs). CDs as the "carbon islands" could block the migration of DMASs across "islands" to achieve dynamic stability. More than twenty DMASs-CDs with specific compositions of DMASs (pairwise combinations among Fe, Co, Ni, Mn, Zn, Cu, and Mo) have been synthesized successfully. Thereafter, high intrinsic activity is observed for the probe reaction of urea oxidation on NiMn-CDs. In situ and ex situ spectroscopic characterization and first-principle calculations unveil that the synergistic effect in NiMn-DMASs could stretch the urea molecule and weaken the N-H bond, endowing NiMn-CDs with a low energy barrier for urea dehydrogenation. Moreover, DMASs-CDs for various target electrochemical reactions, including but not limited to urea oxidation, are realized by optimizing the specific DMAS combination in CDs.

Atomic Cu Sites Engineering Enables Efficient CO2 Electroreduction to Methane with High CH4/C2H4 Ratio

Electrochemical reduction of CO2 into high-value hydrocarbons and alcohols by using Cu-based catalysts is a promising and attractive technology for CO2 capture and utilization, resulting from their high catalytic activity and selectivity. The mobility and accessibility of active sites in Cu-based catalysts significantly hinder the development of efficient Cu-based catalysts for CO2 electrochemical reduction reaction (CO2RR). Herein, a facile and effective strategy is developed to engineer accessible and structural stable Cu sites by incorporating single atomic Cu into the nitrogen cavities of the host graphitic carbon nitride (g-C3N4) as the active sites for CO2-to-CH4 conversion in CO2RR. By regulating the coordination and density of Cu sites in g-C3N4, an optimal catalyst corresponding to a one Cu atom in one nitrogen cavity reaches the highest CH4 Faraday efficiency of 49.04% and produces the products with a high CH4/C2H4 ratio over 9. This work provides the first experimental study on g-C3N4-supported single Cu atom catalyst for efficient CH4 production from CO2RR and suggests a principle in designing highly stable and selective high-efficiency Cu-based catalysts for CO2RR by engineering Cu active sites in 2D materials with porous crystal structures.

Molecular nitrogen induced structural evolution of single transition metal atoms supported by B/N co-doped graphene for enhanced nitrogen electroreduction performance

The structural evolution of local coordination environments of single-atom catalysts (SACs) under reaction conditions plays an important role in the catalytic performance of SACs. Using density functional theory calculations, the possible structural evolution of transition metal single atoms supported by B/N codoped-graphene (TM-B2N2/G) under nitrogen reduction reaction (NRR) conditions is explored and the catalytic performance based on reconstructed SACs is theoretically evaluated. A novel nitrogen adsorption mode on TM-B2N2/G is discovered and the protonation of one of the N atoms results in the TM atoms binding with three N atoms, among which one associates with two B atoms (TM-N3B2/G). It is suggested that the N3B2/G supported tungsten single atom (W-N3B2/G) exhibits excellent N2 activity with a limiting potential of -0.27 V and high ammonia selectivity. Electronic structure analysis indicates that the coordination of N3B2/G redistributes the charge density of central W, shifts its d band center upward and strengthens the interaction of W and the adsorbed nitrogen molecule, thereby endowing it with better NRR performance, compared with that supported by pyridine-3N-doped graphene and pyrrolic-3N-doped graphene.

Tailoring the coordination environment of double-atom catalysts to boost electrocatalytic nitrogen reduction: a first-principles study

Tailoring the coordination environment is an effective strategy to modulate the electronic structure and catalytic activity of atomically dispersed transition-metal (TM) catalysts, which has been widely investigated for single-atom catalysts but received less attention for emerging double-atom catalysts (DACs). Herein, based on first-principles calculations, taking the commonly studied N-coordinated graphene-based DACs as references, we explored the effect of coordination engineering on the catalytic behaviors of DACs towards the electrocatalytic nitrogen reduction reaction (NRR), which is realized through replacing one N atom by the B or O atom to form B, N or O, N co-coordinated DACs. We found that B, N or O, N co-coordination could significantly strengthen N2 adsorption and alter the N2 adsorption pattern of the TM dimer active center, which greatly facilitates N2 activation. Moreover, on the studied DACs, the linear scaling relationship between the binding strengths of key intermediates can be attenuated. Consequently, the O, N co-coordinated Mn2 DACs, exhibiting an ultralow limiting potential of -0.27 V, climb to the peak of the activity volcano. In addition, the experimental feasibility of this DAC system was also identified. Overall, benefiting from the coordination engineering effect, the chemical activity and catalytic performance of the DACs for NRR can be significantly boosted. This phenomena can be understood from the adjusted electronic structure of the TM dimer active center due to the changes of its coordination microenvironment, which significantly affects the binding strength (pattern) of key intermediates and changes the reaction pathways, leading to enhanced NRR activity and selectivity. This work highlights the importance of coordination engineering in developing DACs for the electrocatalytic NRR and other important reactions.

Insight on Atomically Dispersed Cu Catalysts for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (ECO2R) with renewable electricity is an advanced carbon conversion technology. At present, copper is the only metal to selectively convert CO2 into multicarbon (C2+) products. Among them, atomically dispersed (AD) Cu catalysts have received great attention due to the relatively single chemical environment, which are able to minimize the negative impact of morphology, valence state, and crystallographic properties, etc. on product selectivity. Furthermore, the completely exposed atomic Cu sites not only provide space and bonding electrons for the adsorption of reactants in favor of better catalytic activity but also provide an ideal platform for studying its reaction mechanism. This review summarizes the recent progress of AD Cu catalysts as a chemically tunable platform for ECO2R, including the atomic Cu sites dynamic evolution, the catalytic performance, and mechanism. Furthermore, the prospects and challenges of AD Cu catalysts for ECO2R are carefully discussed. We sincerely hope that this review can contribute to the rational design of AD Cu catalysts with enhanced performance for ECO2R.

Titania-Supported Cu-Single-Atom Catalyst for Electrochemical Reduction of Acetylene to Ethylene at Low-Concentrations with Suppressed Hydrogen Evolution

Electrochemical acetylene reduction (EAR) is a promising strategy for removing acetylene from ethylene-rich gas streams. However, suppressing the undesirable hydrogen evolution is vital for practical applications in acetylene-insufficient conditions. Herein, Cu single atoms are immobilized on anatase TiO2 nanoplates (Cu-SA/TiO2 ) for electrochemical acetylene reduction, achieving an ethylene selectivity of ≈97% with a 5 vol% acetylene gas feed (Ar balance). At the optimal Cu-single-atom loading, Cu-SA/TiO2 is able to effectively suppress HER and ethylene over-hydrogenation even when using dilute acetylene (0.5 vol%) or ethylene-rich gas feeds, delivering a 99.8% acetylene conversion, providing a turnover frequency of 8.9 × 10-2  s-1 , which is superior to other EAR catalysts reported to date. Theoretical calculations show that the Cu single atoms and the TiO2 support acted cooperatively to promote charge transfer to adsorbed acetylene molecules, whilst also inhibiting hydrogen generation in alkali environments, thus allowing selective ethylene production with negligible hydrogen evolution at low acetylene concentrations.

Selective and Stable CO2 Electroreduction to CH4 via Electronic Metal-Support Interaction upon Decomposition/Redeposition of MOF

The CO2 electroreduction to fuels is a feasible approach to provide renewable energy sources. Therefore, it is necessary to conduct experimental and theoretical investigations on various catalyst design strategies, such as electronic metal-support interaction, to improve the catalytic selectivity. Here a solvent-free synthesis method is reported to prepare a copper (Cu)-based metal-organic framework (MOF) as the precursor. Upon electrochemical CO2 reduction in aqueous electrolyte, it undergoes in situ decomposition/redeposition processes to form abundant interfaces between Cu nanoparticles and amorphous carbon supports. This Cu/C catalyst favors the selective and stable production of CH4 with a Faradaic efficiency of ≈55% at -1.4 V versus reversible hydrogen electrode (RHE) for 12.5 h. The density functional theory calculation reveals the crucial role of interfacial sites between Cu and amorphous carbon support in stabilizing the key intermediates for CO2 reduction to CH4 . The adsorption of COOH* and CHO* at the Cu/C interface is up to 0.86 eV stronger than that on Cu(111), thus promoting the formation of CH4 . Therefore, it is envisioned that the strategy of regulating electronic metal-support interaction can improve the selectivity and stability of catalyst toward a specific product upon electrochemical CO2 reduction.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Achievements and challenges of copper-based single-atom catalysts for the reduction of carbon dioxide to C2+ products

Copper is the only metal that can convert CO2 into C2 and C2+ in electrocatalytic carbon dioxide reduction (CO2RR). However, the Faraday efficiency of CO2 conversion to C2 and C2+ products at high current densities is still low, which cannot meet the actual industrial demand. Here, the design methods of single-atom copper catalysts (including regulating the coordination environment of single-atom copper, modifying the carbon base surface and constructing diatomic Cu catalysts) are reviewed, and the current limitations and future research directions of copper-based single-atom catalysts are proposed, providing directions for the industrial conversion of CO2 into C2 and C2+ products.

Cooperative Copper Single-Atom Catalyst in 2D Carbon Nitride for Enhanced CO2 Electrolysis to Methane

Renewable-electricity-powered carbon dioxide (CO2 ) reduction (eCO2 R) to high-value fuels like methane (CH4 ) holds the potential to close the carbon cycle at meaningful scales. However, this kinetically staggered 8-electron multistep reduction suffers from inadequate catalytic efficiency and current density. Atomic Cu-structures can boost eCO2 R-to-CH4 selectivity due to enhanced intermediate binding energies (BEs) resulting from favorably shifted d-band centers. In this work, 2D carbon nitride (CN) matrices, viz. Na-polyheptazine (PHI) and Li-polytriazine imides (PTI), are exploited to host Cu-N2 type single-atom sites with high density (≈1.5 at%), via a facile metal-ion exchange process. Optimized Cu loading in nanocrystalline Cu-PTI maximizes eCO2 R-to-CH4 performance with Faradaic efficiency (FECH4 ) of ≈68% and a high partial current density of 348 mA cm-2 at -0.84 V vs reversible hydrogen electrode (RHE), surpassing the state-of-the-art catalysts. Multi-Cu substituted N-appended nanopores in the CN frameworks yield thermodynamically stable quasi-dual/triple sites with large interatomic distances dictated by the pore dimensions. First-principles calculations elucidate the relative Cu-CN cooperative effects between the matrices and how the Cu local environment dictates the adsorbate BEs, density of states, and CO2 -to-CH4 energy profile landscape. The 9N pores in Cu-PTI yield cooperative Cu-Cu sites that synergistically enhance the kinetics of the rate-limiting steps in the eCO2 R-to-CH4 pathway.

Selective CO2 reduction to CH3OH over atomic dual-metal sites embedded in a metal-organic framework with high-energy radiation

The efficient use of renewable X/γ-rays or accelerated electrons for chemical transformation of CO2 and water to fuels holds promise for a carbon-neutral economy; however, such processes are challenging to implement and require the assistance of catalysts capable of sensitizing secondary electron scattering and providing active metal sites to bind intermediates. Here we show atomic Cu-Ni dual-metal sites embedded in a metal-organic framework enable efficient and selective CH3OH production (~98%) over multiple irradiated cycles. The usage of practical electron-beam irradiation (200 keV; 40 kGy min-1) with a cost-effective hydroxyl radical scavenger promotes CH3OH production rate to 0.27 mmol g-1 min-1. Moreover, time-resolved experiments with calculations reveal the direct generation of CO2•‒ radical anions via aqueous electrons attachment occurred on nanosecond timescale, and cascade hydrogenation steps. Our study highlights a radiolytic route to produce CH3OH with CO2 feedstock and introduces a desirable atomic structure to improve performance.

Modulating adsorbed hydrogen drives electrochemical CO2-to-C2 products

Electrocatalytic CO₂ reduction is a typical reaction involving two reactants (CO₂ and H₂O). However, the role of H₂O dissociation, which provides active *H species to multiple protonation steps, is usually overlooked. Herein, we construct a dual-active sites catalyst comprising atomic Cu sites and Cu nanoparticles supported on N-doped carbon matrix. Efficient electrosynthesis of multi-carbon products is achieved with Faradaic efficiency approaching 75.4% with a partial current density of 289.2 mA cm^-2 at -0.6 V. Experimental and theoretical studies reveal that Cu nanoparticles facilitate the C-C coupling step through *CHO dimerization, while the atomic Cu sites boost H₂O dissociation to form *H. The generated *H migrate to Cu nanoparticles and modulate the *H coverage on Cu NPs, and thus promote *CO-to-*CHO. The dual-active sites effect of Cu single-sites and Cu nanoparticles gives rise to the catalytic performance.

Performance Regulation of Single-Atom Catalyst by Modulating the Microenvironment of Metal Sites

Metal-based catalysts, encompassing both homogeneous and heterogeneous types, play a vital role in the modern chemical industry. Heterogeneous metal-based catalysts usually possess more varied catalytically active centers than homogeneous catalysts, making it challenging to regulate their catalytic performance. In contrast, homogeneous catalysts have defined active-site structures, and their performance can be easily adjusted by modifying the ligand. These characteristics lead to remarkable conceptual and technical differences between homogeneous and heterogeneous catalysts. As a recently emerging class of catalytic material, single-atom catalysts (SACs) have become one of the most active new frontiers in the catalysis field and show great potential to bridge homogeneous and heterogeneous catalytic processes. This review documents a brief introduction to SACs and their role in a range of reactions involving single-atom catalysis. To fully understand process-structure-property relationships of single-atom catalysis in chemical reactions, active sites or coordination structure and performance regulation strategies (e.g., tuning chemical and physical environment of single atoms) of SACs are comprehensively summarized. Furthermore, we discuss the application limitations, development trends and future challenges of single-atom catalysis and present a perspective on further constructing a highly efficient (e.g., activity, selectivity and stability), single-atom catalytic system for a broader scope of reactions.

Single-site decorated copper enables energy- and carbon-efficient CO2 methanation in acidic conditions

Renewable CH₄ produced from electrocatalytic CO₂ reduction is viewed as a sustainable and versatile energy carrier, compatible with existing infrastructure. However, conventional alkaline and neutral CO₂-to-CH₄ systems suffer CO₂ loss to carbonates, and recovering the lost CO₂ requires input energy exceeding the heating value of the produced CH₄. Here we pursue CH₄-selective electrocatalysis in acidic conditions via a coordination method, stabilizing free Cu ions by bonding Cu with multidentate donor sites. We find that hexadentate donor sites in ethylenediaminetetraacetic acid enable the chelation of Cu ions, regulating Cu cluster size and forming Cu-N/O single sites that achieve high CH₄ selectivity in acidic conditions. We report a CH₄ Faradaic efficiency of 71% (at 100 mA cm^-2) with <3% loss in total input CO₂ that results in an overall energy intensity (254 GJ/tonne CH₄), half that of existing electroproduction routes.

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated Pt^II species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous Pt^II catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of Pt^II moieties on the single-atom catalysts beyond ideally four-coordinated Pt^II-N₄. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated Pt^II species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d⁸ metal ions.

Carbon-based single-atom catalysts in advanced oxidation reactions for water remediation: From materials to reaction pathways

Single-atom catalysts (SACs) have been widely recognized as state-of-the-art catalysts in environment remediation because of their exceptional performance, 100% metal atomic utilization, almost no secondary pollution, and robust structures. Most recently, the activation of persulfate with carbon-based SACs in advanced oxidation processes (AOPs) raises tremendous interest in the degradation of emerging contaminants in wastewater, owning to its efficient and versatile reactive oxidant species (ROS) generation. However, the comprehensive and critical review unraveling the underlying relationship between structures of carbon-based SACs and the corresponding generated ROS is still rare. Herein, we systematically summarize the fundamental understandings and intrinsic mechanisms between single metal atom active sites and produced ROS during AOPs. The types of emerging contaminants are firstly elaborated, presenting the prior pollutants that need to be degraded. Then, the preparation and characterization methods of carbon-based SACs are overviewed. The underlying material structure-ROS type relationship in persulfate-based AOPs is discussed in depth to expound the catalytic mechanisms. Finally, we briefly conclude the current development of carbon-based SACs in AOPs and propose the prospects for rational design and synthesis of carbon-based SACs with on-demand catalytic performances in AOPs in future research.