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

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

Controlled Synthesis of a Vacancy-Defect Single-Atom Catalyst for Boosting CO2 Electroreduction

The reaction of precursors containing both nitrogen and oxygen atoms with Ni^II under 500 °C can generate a N/O mixing coordinated Ni-N₃O single-atom catalyst (SAC) in which the oxygen atom can be gradually removed under high temperature due to the weaker Ni−O interaction, resulting in a vacancy-defect Ni-N₃-V SAC at Ni site under 800 °C. For the reaction of Ni^II with the precursor simply containing nitrogen atoms, only a no-vacancy-defect Ni-N₄ SAC was obtained. Experimental and DFT calculations reveal that the presence of a vacancy-defect in Ni-N₃-V SAC can dramatically boost the electrocatalytic activity for CO₂ reduction, with extremely high CO₂ reduction current density of 65 mA cm⁻² and high Faradaic efficiency over 90 % at −0.9 V vs. RHE, as well as a record high turnover frequency of 1.35×10⁵ h⁻¹, much higher than those of Ni-N₄ SAC, and being one of the best reported electrocatalysts for CO₂-to-CO conversion to date.     

First‐principle calculations on CO oxidation catalyzed by a gold nanoparticle

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first‐principle density‐functional theory (DFT) calculations. Calculations on selected model show that the low‐coordinated Au atoms of the Au₂₉ nanoparticle carry slightly negative charges, which enhance the O₂ binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH). The overall LH reaction O_2(ads) + CO_(gas) → O_2(ads) + CO_(ads) → OOCO_(ads) → O_(ads) + CO_2(gas) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states ( TS_LH1 and TS_LH2 ) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au₂₉ + O_2(gas) + CO_(gas) → Au₂₉–O_2(ads) + CO_(gas) → Au₂₉–CO_3(ads) → Au₂₉–O_(ads) + CO_2(gas) requires an overall activation barrier of 0.19 eV, and the formation of Au₂₉–CO_3(ads) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au₂₉ nanoparticle is likely to occur at or even below room temperature. To gain insights into high‐catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Formation Mechanism,Geometric Stability and Catalytic Activity of a Single Iron Atom Supported on N-Doped Graphene

Based on density functional theory (DFT) calculations, the formation geometries, stability and catalytic properties of single-atom iron anchored on xN-doped graphene (xN-graphene-Fe, x=1, 2, 3) sheet are systemically investigated. It is found that the different kinds and numbers of gas reactants can effectively regulate the electronic structure and magnetic properties of the 3 N-graphene-Fe system. For NO and CO oxidation reactions, the coadsorption configurations of NO/O₂ and CO/O₂ molecules on a reactive substrate as the initial state are comparably analyzed. The NO oxidation reactions through the Langmuir–Hinshelwood (LH) and Eley-Rideal (ER) mechanisms have relatively smaller energy barriers than those of the CO oxidation processes. In comparison, the preadsorbed 2NO reacting with 2CO molecules (2NO+2CO→2CO₂+N₂) through ER reactions (<0.4 eV) are energetically more favorable processes. These results can provide beneficial references for theoretical studies on NO and CO oxidation and designing graphene-based catalyst for toxic gas removal.     

Comprehensive Mechanism of CO2 Electroreduction on Non-Noble Metal Single-Atom Catalysts of Mo2CS2-MXene

In this work, a series of non-noble metal single-atom catalysts of Mo₂CS₂-MXene for CO₂ reduction were systematically investigated by well-defined density-functional-theory (DFT) calculations. It is found that nine types of transitional metal (TM) supported Mo₂CS₂ (TM-Mo₂CS₂) are very stable, while eight can effectively inhibit the competitive hydrogen evolution reaction (HER). After comprehensively comparing the changes of free energy for each pathway in CO₂ reduction reaction (CO₂RR), it is found that the products of TM-Mo₂CS₂ are not completely CH₄. Furthermore, Cr-, Fe-, Co- and Ni-Mo₂CS₂ are found to render excellent CO₂RR catalytic activity, and their limiting potentials are in the range of 0.245–0.304 V. In particular, Fe-Mo₂CS₂ with a nitrogenase-like structure has the lowest limiting potential and the highest electrocatalytic activity. Ab initio molecular dynamics (AIMD) simulations have also proven that these kinds of single-atom catalysts with robust performance could exist stably at room temperature. Therefore, these single TM atoms anchored on the surface of MXenes can be profiled as a promising catalyst for the electrochemical reduction of CO₂.     

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

Electrocatalytic CO₂ reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti₃C₂T_x MXene-regulated Ag−ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag−ZnO/Ti₃C₂T_x catalyst achieves an outstanding CO₂ conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm⁻² at −0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag−ZnO interfaces contributes the high selectivity of CO. The CO₂ conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO₂ reduction.     

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.     

Strain boosts CO oxidation on Ni single-atom-catalyst supported by defective graphene

In order to realize the sulfur and water resistance and facilitate the CO oxidation reactions,the effects of strain on the adsorption of CO,O₂,SO₂ and H₂O molecules on Ni single-atom-catalyst supported by single-carbon-vacancy graphene(Ni-SG) have been studied based on first principles calculations.It shows that the compressive strain increases the adsorption energies of all above mentioned molecules on Ni-SG,where SO₂ is adsorbed more strongly on Ni-S...     

CO oxidation on MXene (Mo2CS2) supported single-atom catalyst: A termolecular Eley-Rideal mechanism

Finding transition metal catalysts for effective catalytic conversion of CO to CO₂ has attracted much attention. MXene as a new 2D layered material of early transition metal carbides, nitrides, and carbo-nitrides is a robust support for achoring metal atoms. In this study, the electronic structure, geometries, thermodynamic stability, and catalytic activity of MXene (Mo₂CS₂) supported single noble metal atoms (NM = Ru, Rh, Pd, Ir, Pt and Au) have been systematically examined using first-principles calculations and ab initio molecular dynamic (AIMD) simulations. First, AIMD simulations and phonon spectra demonstrate the dynamic and thermal stabilities of Mo₂CS₂ monolayer. Three likely reaction pathways, Langmuir-Hinshelwood (LH), Eley-Rideal (ER), and Termolecular Eley–Rideal (TER) for CO oxidation on the Ru₁- and Ir₁@Mo₂CS₂ SACs, have been studied in detail. It is found that CO oxidation mainly proceeds via the TER mechanism under mild reaction conditions. The corresponding rate-determining steps are the dissociation of the intermediate (OCO-Ru₁-OCO) and formation of OCO-Ir₁-OCO intermediate. The downshift d-band center of Ru₁- and Ir₁@Mo₂CS₂ help to enhance activity and improve catalytst stability. Moreover, a microkinetic study predicts a maximum CO oxidation rate of 4.01 × 10² s^-1 and 4.15 × 10³ s^-1 (298.15 K) following the TER pathway for the Ru₁- and Ir₁@Mo₂CS₂ catalysts, respectively. This work provides guideline for fabricating and designing highly efficient SACs with superb catalyts using MXene materials.     

Lithium storage properties of Ti3C2Tx (Tx = F,Cl, Br) MXenes

In this work, Ti₃C₂T_x MXene with -F, -Cl and -Br surface terminations are synthesized and the effect of these halogen terminations on the lithium storage properties is investigated. A maximum Li⁺ storage capacity of 189 mAh/g is achieved with Ti₃C₂Br_x MXene much higher than Ti₃C₂Cl_x and Ti₃C₂F_x with 138 mAh/g and 123 mAh/g, respectively. Density functional theory (DFT) calculation shows that the adsorption formation energy of halogen atoms on Ti atoms follows the trend of Ti-F > Ti-Cl > Ti-Br, leading to the same trend in the content of terminations on corresponding MXenes. In addition, inevitable exposure of MXene to oxygen causes competition between halogen and oxygen. Theoretical results show Ti₃C₂Br_x MXene has the highest Ti to O ratio and the lowest Ti to Br ratio, the high lithium affinity of O explains the maximum Li-ion storage capacity with Ti₃C₂Br_x MXene. This work shed light on the opportunity for achieving improved lithium storage properties of MXene electrodes by regulating the surface chemistry.     

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.     

Engineering d-band center of nickel in nickel@nitrogen-doped carbon nanotubes array for electrochemical reduction of CO2 to CO and Zn-CO2 batteries

Self-supported transition-metal single-atom catalysts (SACs) facilitate the industrialization of electrochemical CO₂ reduction, but suffer from high structural heterogeneity with limited catalytic selectivity. Here we present a facile and scalable approach for the synthesis of self-supported nickel@nitrogen-doped carbon nanotubes grown on carbon nanofiber membrane (Ni@NCNTs/CFM), where the Ni single atoms and nanoparticles (NPs) are anchored on the wall and inside of nitrogen-doped carbon nanotubes, respectively. The side effect of Ni NPs was further effectively inhibited by alloying Ni with Cu atoms to alter their d-band center, which is theoretically predicted and experimentally proved. The optimal catalyst Ni₉Cu₁@NCNTs/CFM exhibits an ultrahigh CO Faradic efficiency over 97% at ?0.7 V versus reversible hydrogen electrode. Additionally, this catalyst shows excellent mechanical strength which can be directly used as a self-supporting catalyst for Zn-CO₂ battery with a peak power density of ～0.65 mW/cm² at 2.25 mA/cm² and a long-term stability for 150 cycles. This work opens up a general avenue to facilely prepare self-supported SACs with unitary single-atom site for CO₂ utilization.     

Phosphorene-Supported Transition-Metal Dimer for Effective N2 Electroreduction

The electrochemical reduction of N₂ to NH₃ at ambient conditions is a promising alternative to the energy-intensive, high-temperature, high-pressure Haber-Bosch process. But it is extremely challenging to find an electrocatalyst that can effectively activate N₂ and reduce it to NH₃. From first principles density functional theory, we found that the Ti dimer supported on single-layer phosphorene can be used as a promising electrocatalyst for N₂ capture and conversion to NH₃. The overpotential (relative to the standard hydrogen electrode) was found to be as low as 0.20, much lower than those predicted on the Ti surface (1 to 1.5 V) or their nitrides (0.5 to 1 V). In addition, we found that hydride is involved in the N₂ reduction on the Ti dimer catalyst via formation of Ti₂-H species, and the hydride would favorably transfer onto the adsorbed N₂* to form *NNH intermediate and further reduced to NH₃. Moreover, we also examined other first-row transition metal dimers, and found that Sc and Fe dimer to be potential catalysts which could catalyze N₂ reduction at a low overpotential of about 0.21 and 0.45 V, respectively. Our predictions hence suggest Ti, Sc and Fe dimer clusters supported on phosphorene as promising electrocatalysts for N₂ reduction to NH₃.     

A theoretical study of electrocatalytic ammonia synthesis on single metal atom/MXene

Electrocatalytic ammonia synthesis under mild conditions is an attractive and challenging process in the earth's nitrogen cycle, which requires efficient and stable catalysts to reduce the overpotential. The N₂ activation and reduction overpotential of different Ti₃C₂O₂-supported transition metal (TM) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, and Au) single-atom catalysts have been analyzed in terms of the Gibbs free energies calculated using the density functional theory (DFT). The end-on N₂ adsorption was more energetically favorable, and the negative free energies represented good N₂ activation performance, especially in the presence Fe/Ti₃C₂O₂ (?0.75 eV). The overpotentials of Fe/Ti₃C₂O₂, Co/Ti₃C₂O₂, Ru/Ti₃C₂O₂, and Rh/Ti₃C₂O₂ were 0.92, 0.89, 1.16, and 0.84 eV, respectively. The potential required for ammonia synthesis was different for different TMs and ranged from 0.68 to 2.33 eV. Two possible potential-limiting steps may be involved in the process: (i) hydrogenation of N₂ to *NNH and (ii) hydrogenation of *NH₂ to ammonia. These catalysts can change the reaction pathway and avoid the traditional N–N bond-breaking barrier. It also simplifies the understanding of the relationship between the Gibbs free energy and overpotential, which is a significant factor in the rational designing and large-scale screening of catalysts for the electrocatalytic ammonia synthesis.     

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

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

Electronic Perturbation of Copper Single-Atom CO2 Reduction Catalysts in a Molecular Way

Fine-tuning electronic structures of single-atom catalysts (SACs) plays a crucial role in harnessing their catalytic activities, yet challenges remain at a molecular scale in a controlled fashion. By tailoring the structure of graphdiyne (GDY) with electron-withdrawing/-donating groups, we show herein the electronic perturbation of Cu single-atom CO₂ reduction catalysts in a molecular way. The elaborately introduced functional groups (−F, −H and −OMe) can regulate the valance state of Cu^δ+, which is found to be directly scaled with the selectivity of the electrochemical CO₂-to-CH₄ conversion. An optimum CH₄ Faradaic efficiency of 72.3 % was achieved over the Cu SAC on the F-substituted GDY. In situ spectroscopic studies and theoretical calculations revealed that the positive Cu^δ+ centers adjusted by the electron-withdrawing group decrease the pK_a of adsorbed H₂O, promoting the hydrogenation of intermediates toward the CH₄ production. Our strategy paves the way for precise electronic perturbation of SACs toward efficient electrocatalysis.     

Pebax‐Based Membrane Filled with Two‐Dimensional Mxene Nanosheets for Efficient CO2 Capture

Mixed matrix membranes (MMMs) made from inorganic fillers and polymers is a kind of promising candidate for gas separation. In this work, two‐dimensional MXene nanosheets were synthesized and incorporated into a polyether‐polyamide block copolymer (Pebax) matrix to fabricate MMM for CO₂ capture. The physicochemical properties of MXene nanosheets and MXene/Pebax membranes were studied systematically. The introduction of MXene nanosheets provided additional molecular transport channels and meanwhile enhanced the CO₂ adsorption capacity, thereby enhancing both the CO₂ peremance and CO₂/N₂ selectivity of Pebax membrane. The optimized MXene/Pebax membrane with a MXene loading of 0.15 wt % displayed a high separation performance with a CO₂ permeance of 21.6 GPU and a CO₂/N₂ selectivity of 72.5, showing potential application in CO₂ capture.     

Intermediates-induced CO2 Reduction Reaction Activity at Single-Atom M−N2 (M=Fe,Co, Ni) Sites

Single-atom M−N₂ (M=Fe, Co, Ni) catalysts exhibit high activity for CO₂ reduction reaction (CO₂RR). However, the CO₂RR mechanism and the origin of activity at the single-atom sites remain unclear, which hinders the development of single-atom M−N₂ catalysts. Here, using density functional theory calculations, we reveal intermediates-induced CO₂RR activity at the single-atom M−N₂ sites. At the M−N₂ sites, the asymmetric *O*CO configuration tends to split into *CO and *OH intermediates. Intermediates become part of the active moiety to form M−(CO)N₂ or M-(OH)N₂ sites, which optimizes the adsorption of intermediates on the M sites. The maximum free energy differences along the optimal CO₂RR pathway are 0.30, 0.54, and 0.28 eV for Fe−(OH)N₂, Co−(CO)N₂, and Ni−(OH)N₂ sites respectively, which is lower than those of Fe−N₂ (1.03 eV), Co−N₂ (1.24 eV) and Ni−N₂ (0.73 eV) sites. The intermediate modification can shift the d-band center of the spin-up (minority) state downward by regulating the charge distribution at the M sites, leading to less charge being accepted by the intermediates from the M sites. This work provides new insights into the understanding of the activity of single-atom M−N₂ sites.     

Two-dimensional Pt2P3 monolayer: A promising bifunctional electrocatalyst with different active sites for hydrogen evolution and CO2 reduction

Green hydrogen production and CO₂ fixation have been identified as the fundamental techniques for sustainable economy. The open challenge is to develop high performance catalysts for hydrogen evolution reaction (HER) and CO₂ electroreduction (CO₂ER) to valuable chemicals. Under such context, this work reported computational efforts to design promising electrocatalyst for HER and CO₂ER based on the swarm-intelligence algorithm. Among the family of transition-metal phosphides (TMPs), Pt₂P₃ monolayer has been identified as excellent bifunctional catalysts due to high stability, excellent conductivity and superior catalytic performance. Different from typical d-block catalysts, p-band center presented by P atoms within Pt₂P₃ monolayer plays the essential role for its reactivity towards HER and CO₂ER, underlining the key value of p-electrons in advanced catalyst design and thus providing a promising strategy to further develop novel catalysts made of p-block elements for various energy applications.     

Single atom catalytic oxidation mechanism of formaldehyde on Al doped graphene at room temperature

Formaldehyde (HCHO) is one kind of common indoor toxic pollutant, the catalytic oxidation degradation of formaldehyde at room temperature is desired. In this work, a new single atomic catalyst (SAC), Al doped graphene, for the catalytic oxidation of HCHO molecules was proposed through density function theory (DFT) calculations. It is found that Al atoms can be adsorbed on graphene stably without aggression. Then HCHO can be effectively oxidized into CO₂ and H₂O in the presence of O₂ molecules on Al doped graphene with a low energy barrier of 0.82 eV and releasing energy of 2.29 eV with the pathway of HCHO → HCOOH → CO → CO₂. The oxidation reaction can happen promptly with reaction time τ = 56.9 s at the speed control step at room temperature. Therefore, this work proposed a high-performance catalyst Al-doped graphene without any noble metal for HCHO oxidation at ambient temperature, and corresponding oxidation pathway and mechanism are also deeply understood.