Transition Metal Single-atom Catalysts Research Articles

Defective SiC nanotube based single-atom catalysts for electrocatalytic nitrogen fixation with curvature effect

The production of fertilizers, explosives, and various other chemicals relies heavily on ammonia (NH3). Electrochemical reduction of nitrogen to ammonia, also known as the nitrogen reduction reaction (NRR), is a potential alternative to the Haber-Bosch process that can be carried out at ambient conditions. However, many NRR catalysts suffer from low selectivity and require a large overpotential to drive the reaction, limiting their practical application. In this study, we used density functional theory (DFT) calculations to investigate the mechanistic aspects of the NRR and the effect of curvature variation on the catalytic activity of silicon carbide nanotubes (SiCNTs) decorated with 3d-5d transition metal single-atom catalysts. Our results suggest that the Os@SiCNT catalyst exhibits promising activity as an eNRR catalyst with an overpotential of 0.47 V and high selectivity over hydrogen evolution reaction (HER) competition. The electronic “acceptance-donation” interaction between N2 and Os promotes the adsorption and activation of N2 molecules on the catalyst surface. Additionally, our findings demonstrate that as the nanotube diameter decreases, the energy barrier for the NRR reaction decreases as well. However, this decrease in diameter also leads to a weaker capacity for *N2 adsorption, which may ultimately reduce the selectivity of the NRR reaction. These findings offer valuable insights that can aid in the rational design of nanotube-based electrocatalysts for eNRR.

Integration of atomic Co-N5 sites with defective N-doped carbon for efficient zinc-air batteries

Transition metal single-atom catalysts have attracted considerable attention for their applicability in electrocatalytic oxygen reduction reactions (ORR), but it is still very challenging to regulate the interaction between the active sites and oxygen-containing intermediates. In this study, atomically dispersed Co-N5 sites integrated with defective N-doped carbon (Co-N5/DHC) were developed using a facile templating approach, followed by NH3 treatment. NH3 can simultaneously etch inactive amorphous substrates on the surfaces of metal sites and construct intrinsic carbon defects, thereby increasing the number of both metal and metal-free active sites. By bridging the intrinsic carbon defects and metal sites, the optimized Co-N5/DHC catalyst exhibited significantly improved electrocatalytic ORR performance compared to the pristine Co-Nx/HC sample (cobalt-nitrogen sites anchored on heteroatom-doped carbon). Density functional calculations revealed that the strong interaction between the Co-N5 sites and carbon defects modified the electronic localization, thus optimizing the binding energy with oxygen-containing intermediates and resulting in significantly improved ORR catalytic activity with a half-wave potential of 0.877 V. In addition, a zinc-air battery assembled with the Co-N5/DHC as the cathode achieves a maximum power density of 160.7 mW cm−2 and affords a specific capacity of 766.2 mA h gZn−1.

MXene-supported transition metal single-atom catalysts for nitrogen dissociation

Industrial ammonia production follows the Haber-Bosch process, whose rate-limiting step is the dissociation of the nitrogen molecule (N2), hence requiring suitable catalysts to break its triple bond. MXenes, a class of two-dimensional transition metal carbides and nitrides, have been proposed as very efficient catalysts for N2 dissociation. Here, by employing density functional theory-based calculations, we assess whether the deposition of one atom of a transition metal element (TM) on the Ti2C MXene surface further improves the catalytic potential of the MXene, serving as a single-atom catalyst. The results show that, for 21 of the 30 TMs considered, N2 can exothermically bind to the TM adatom, this bonding being favourable with respect to adsorption on the pristine Ti2C MXene surface for TMs of groups 3 to 6 of the Periodic Table. All the 21 TMs that successfully bind to N2 effectively reduce the N2 dissociation energy barrier when compared to the bulk Ti2C MXene by 18 to 84 %. Our results strongly indicate that doping the Ti2C MXene with atoms of transition metal elements significantly reduces the energy required to break the triple bond in N2, which may impact the nitrogen-to-ammonia process.

Co Single Atoms Anchored in N and P Co-doped Porous Carbon Fibers for Efficient Water Splitting.

Single atom catalysts (SACs) show potential application for highly efficient water splitting. Herein, we designed Co single atoms (SAs) dispersed on N and P co-doped porous carbon nanofibers as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The configuration of Co SAs is proved to coordinate with 4 N/O atoms. The long-range interactions between doped P atoms and Co-N4 (O) sites could modulate the electronic structures of M-N4 (O) sites, which greatly reduce the adsorption energies of HER and OER intermediates at metal sites. Density Functional Theory calculations reveal that CoSA /CNFs exhibits optimized HER and OER kinetics when P coordinates with two N atoms. The atomically dispersed Co electrocatalyst exhibits the low overpotentials of 61, 89 and 390 mV for acidic HER, alkaline HER and OER at 10 mA cm-2 current density, along with the Tafel slope of 54, 143 and 74 mV dec-1 , respectively. This work demonstrates the prospect of utilizing di-heteroatom-doping transition metal SACs, and provides a novel and general strategy for the preparation of SACs.

Mimicking Metalloenzyme Microenvironments in the Transition Metal-Single Atom Catalysts for Electrochemical Hydrogen Peroxide Synthesis in an Acidic Medium.

Electrochemical reduction of oxygen into hydrogen peroxide in an acidic medium offers an energy-efficient and green H2 O2 synthesis as an alternative to the energy-intensive anthraquinone process. Unfortunately, high overpotential, low production rates, and fierce competition from traditional four-electron reduction limit it. In this study, a metalloenzyme-like active structure is mimicked in carbon-based single-atom electrocatalysts for oxygen reduction to H2 O2 . Using a carbonization strategy, the primary electronic structure of the metal center with nitrogen and oxygen coordination is modulated, followed by epoxy oxygen functionalities close to the metal active sites. In an acidic medium, CoNOC active structures proceed with greater than 98% H2 O2 selectivity (2e- /2H+ ) rather than CoNC active sites that are selective to H2 O (4e- /4H+ ). Among all MNOC (M=Fe, Co, Mn, and Ni) single-atom electrocatalysts, the CoNOC is the most selective (> 98%) for H2 O2 production, with a mass activity of 10Ag-1 at 0.60V vs. RHE. X-ray absorption spectroscopy is used to identify the formation of unsymmetrical MNOC active structures. Experimental results are also compared to density functional theory calculations, which revealed that the structure-activity relationship of the epoxy-surrounded CoNOC active structure reaches optimum (ΔG*OOH ) binding energies for high selectivity.

Transition Metal (Fe, Zn, Co, and Ni) Single-Atom Catalysts Anchored on N,S-Codoped Hybrid Nanocarbons for Oxygen Reduction Reaction

Atomically dispersed transition metal catalysts have served as promising candidates to substitute noble metals in energy conversion. A majority of carbon-supported transition metal single-atom catalysts (SACs) have been synthesized via the high-temperature sintering method, in which the complex heating and cooling processes are time- and energy-consuming. Herein, we have developed a simple and highly effective microwave-assisted heating method to prepare a series of isolated transition metal SACs anchored on a N,S-codoped nanocarbon matrix, which are denoted as M-NSC-MW, M = Fe, Zn, Co, and Ni. Among the various M-NSC-MW, Fe-NSC-MW possesses the highest ORR electrochemical activity in alkaline electrolytes. Its onset potential (Eonset) is 0.955 V, and its half-wave potential (E1/2) can reach 0.852 V, both of which are the same as those of commercial Pt/C. Moreover, Fe-NSC-MW also exhibits much superior ORR catalytic efficiency than conventional pyrolysis samples. It is worth noting that Zn-NSC–MW is prepared for the first time, and it also shows a high ORR performance with an Eonset of 0.945 V and an E1/2 of 0.832 V. This work has revealed the general application of the microwave-assisted heating method to prepare well-dispersed transition metal SACs anchored on the N,S-codoped graphene/carbon hybrid matrix.

Theoretical Insights into Single-Atom Catalysts Supported on N-Doped Defective Graphene for Fast Reaction Redox Kinetics in Lithium-Sulfur Batteries.

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects.

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

DFT investigation into efficient transition metal single-atom catalysts supported on N-doped graphene for nitrate reduction reactions

The electrochemical conversion of nitrate (NO3−), an environmental pollutant, into ammonia (NH3), a hydrogen energy carrier, is attracting attention as a promising alternative to the conventional Haber–Bosch process. Single-atom catalysts (SACs) based on N-doped graphene (NDG) have shown excellent efficiency for the nitrate reduction reaction (NO3RR). However, a fundamental understanding of the effect of the arrangement and number of N atoms on the NO3RR has not yet been achieved. Herein, we designed various 3d, 4d, and 5d transition metal (TM)-SAC/NDG systems and proposed specific patterns and quantities of N doping to develop highly stable and efficient candidate catalysts for the NO3RR. The activities of the screened TM-SACs (e.g., Ru- and Ir-SACs/NDG) were investigated by analyzing the density of states and excess Bader charges, and the stabilities of the catalysts were evaluated based on nanocluster formation energies and ab initio molecular dynamic (AIMD) simulations.

Promotion effect of CO oxidation via activation of surface lattice oxygen by single atom Cu/MnO2 catalyst

The catalytic oxidation of CO by single-atom transition metal catalysts has always been the focus of researchers. Herein, we report a strategy for the preparation of Cu single-atom anchored on MnO2 by oxalate chelation-assisted hydrothermal method, and the obtained Cu/MnO2–0.25 catalyst with the optimal molar ratio achieved about 90% CO conversion at 80 °C. The strong electronic interaction between Cu and MnO2 not only effectively inhibits the aggregation of Cu single atoms, but also the formed Cu-O-Mn hybrid structure activates the adjacent surface lattice oxygen, further improving the catalytic oxidation performance of CO at low and medium temperatures. The elevated activity is achieved at the single-atom active site by the Mars-van Krevelen (MvK) mechanism. This work provides a simple and reliable method for the rational design of stable transition metal single-atom catalysts and deepens the understanding of metal-support strong interactions (SMSI).

A universal strategy for green synthesis of biomass-based transition metal single-atom catalysts by simple hydrothermal and compression treatment

Carbon-based single-atom catalysts (SACs) have attracted increasing extensive attention due to their excellent catalytic performance and high atomic utilization efficiency. Traditional porous carbon materials are not suitable supports for SACs synthesis owing to the inert surface and heteroatoms such as N and S are necessarily introduced as increased anchoring sites for single metal atoms. Herein, we reported a novel universal strategy to synthesize carbon-based transition metal (Fe, Co, Ni, Cu, and Zn) SACs by one-pot hydrothermal treatment combined with a mechanochemical method by virtue of abundant active oxygen naturally in biomass feedstock. The hydrothermal treatment resulted in relatively monodispersed metal ions within the carbonaceous matrix and compression process played a vital role in the formation and anchoring of the single atom, preventing self-aggregation during pyrolysis by confinement effect. The characterization results showed the metal atoms were atomically dispersed on carbon support in the form of M-O3C coordination and had relatively high metal contents of up to 1.15 wt%. Due to the in-situ formation, unique coordination surroundings, and high content of metal atoms, the prepared catalyst exhibited excellent reactivity performance. This work demonstrated the sustainability and economic feasibility of carbon-based SACs synthesis simultaneously by co-using natural oxygen as anchoring sites and abundant carbon as the skeleton in biomass feedstock and offered a novel approach for high-value utilization of biomass.

A High-Throughput Screening toward Efficient Nitrogen Fixation: Transition Metal Single-Atom Catalysts Anchored on an Emerging π-π Conjugated Graphitic Carbon Nitride (g-C10N3) Substrate with Dirac Dispersion.

TM-Nx is becoming a comforting catalytic center for sustainable and green ammonia synthesis under ambient conditions, resulting in increasing interest in single-atom catalysts (SACs) for the electrochemical nitrogen reduction reaction (NRR). However, given the poor activity and unsatisfactory selectivity of existing catalysts, it remains a long-standing challenge to design efficient catalysts for nitrogen fixation. Currently, the two-dimensional (2D) graphitic carbon-nitride substrate provides abundant and evenly distributed holes for stably supporting transition-metal atoms, which presents a fascinating prospect for overcoming this challenge and promoting single-atom NRR. An emerging holey graphitic carbon-nitride skeleton with a C10N3 stoichiometric ratio (g-C10N3) from a supercell of graphene is constructed, which provides outstanding electric conductivity for achieving high-efficiency NRR due to the Dirac band dispersion. Herein, a high-throughput first-principles calculation is carried out to evaluate the feasibility of π-d conjugated SACs resulting from a single TM atom anchored on g-C10N3 (TM = Sc-Au) for NRR. We find that W metal embedded in g-C10N3 (W@g-C10N3) can compromise the ability to adsorb the key target reaction species (N2H and NH2), hence acquiring an optimal NRR behavior among 27 TM-candidates. Our calculations demonstrate that W@g-C10N3 shows a well-suppressed HER ability and, impressively, a low energy cost of -0.46 V. Additionally, all-around descriptors are proposed to uncover the fundamental mechanism of NRR activity, among which a 3D volcano plot (limiting potential, screening strategy, and electron origin) uncovers the NRR activity trend, achieving a quick and high-efficiency prescreening for numerous candidates. Overall, the strategy of the structure- and activity-based TM-Nx-containing unit design will offer useful insight for further theoretical and experimental attempts.

Computational screening of high activity and selectivity of CO2 reduction via transition metal single-atom catalysts on triazine-based graphite carbon nitride.

Single-atom catalysts (SACs) are emerging as promising catalysts in the field of the electrocatalytic CO2 reduction reaction (CO2RR). Herein, a series of 3d to 5d transition metal atoms supported on triazine-based graphite carbon nitride (TM@TGCN) as a CO2 reduction catalyst are studied via density functional theory computations. Eventually, four TM@TGCN catalysts (TM = Ni, Rh, Os, and Ir) are selected using a five-step screening method, in which Rh@TGCN and Ni@TGCN show a low limiting potential of -0.48 and -0.58 V, respectively, for reducing CO2 to CH4. The activity mechanism shows that the catalysts with a negative d-band center and optimal positive charge can improve the CO2RR performance. Our study provides theoretical guidance for the rational design of highly active and selective catalysts.

Bifunctional electrocatalytic activity of Fe-embedded biphenylene for oxygen reduction and evolution reactions.

Transition metal single-atom catalysts have attracted great attention because of their great potential applications in the chemical industry. Except for graphene, there are few single-layer materials that can act as substrates to support the dispersive metal atoms. Recently, a biphenylene layer, a new two-dimensional allotrope of graphene, was synthesized in experiments, providing a new substrate layer to fabricate single-atom catalysts (SACs). In this work, we predict three transition metal SACs MN4-biphenylene (M = Fe, Co, and Ni) based on first-principles calculations. The results indicate that FeN4-biphenylene is a promising bifunctional catalyst with low overpotentials for both the oxygen reduction reaction and oxygen evolution reaction, ηORR = 0.11 V and ηOER = 0.27 V. The high catalytic activities are explained by the position of the d-band center of the Fe atom in the biphenylene network and the strength of interaction between FeN4-biphenylene and the reaction intermediates.

Theoretical investigations of electrochemical nitrogen reduction on single transition metal atom catalysts supported by 1T-MoSe2

The search for stable and efficient single-atom catalysts (SACs) for the electrocatalytic nitrogen reduction reaction (eNRR) has garnered significant theoretical interest. In this study, we evaluate the eNRR performance of eighteen two-dimensional 1T-MoSe2-supported transition metal single-atom catalysts (TM@1T-MoSe2, TM=V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Tc, Ru, Rh, Pd, Ta, W, Re, Os, Ir, and Pt) using density functional theory calculations. We assess the stability of each TM@1T-MoSe2, as well as the limiting potential of eNRR and the ammonia selectivity on each stable TM@1T-MoSe2. Our results show that W@1T-MoSe2 is the most promising single-atom catalyst for eNRR, with a limiting potential of -0.23 V via the distal pathway starting from three co-adsorbed nitrogen molecules. Furthermore, the multi-adsorption of N2 on W@1T-MoSe2 effectively suppress the hydrogen evolution reaction, thus enhancing the selectivity of the eNRR. This research provides a promising avenue for the development of a new class of 1T-MoSe2-based single-atom catalysts for electrocatalytic ammonia synthesis.

Exploration of single Fe atom supported on anatase TiO2(001) for methane oxidation: A DFT study

Partial oxidation of methane is a promising alternative strategy for methanol production under mild reaction conditions; however, significant challenges hinder the development of appropriate catalysts. In this study, based on first-principles calculations, we demonstrate that a single Fe atom supported on anatase TiO 2 (001) provides double active sites (Fe and Ti 5C ) to activate gas-phase O 2 and form O-assisted intermediates. The triple state Fe-O/TiO 2 (001) system exhibited better activity for methane activation (Δ G max = 1.02 eV). Our findings offer new insights into the design of non-noble-3 d transition metal single-atom catalysts on TiO 2 (001) for partial methane oxidation via an inexpensive O 2 oxidant under mild reaction conditions.

Original exploration of transition metal single-atom catalysts for NOx reduction

Various atomically dispersed transition metal-coordinated graphite carbon materials are, for the first time, used for NOx reduction. Proper K-doping promotes NOx conversion and N2 selectivity.

Boosting photo-assisted efficient electrochemical CO2 reduction reaction on transition metal single-atom catalysts supported on the C6N6 nanosheet.

CO2 reduction to value-added chemicals turns out to be a promising and efficient approach to resolve the increasing energy crisis and global warming. However, the catalytic efficiency of CO2 reduction reaction (CO2RR) to form C1 products (CO, HCOOH, CH3OH, CH4) needs to be quite efficient. Herein with the help of density functional theory, CO2RR towards C1 products was investigated on a transition metal (TM = Fe, Co, Ni) embedded C6N6 framework. The stable geometry of the catalysts, CO2 adsorption configurations, and CO2RR mechanisms were systematically studied for all the systems considered. The possible different adsorption configurations and adsorption energy calculations indicated that CO2 could be chemically adsorbed on the Co@C6N6 system. On the other hand, physical adsorption of CO2 is more preferable on Fe@C6N6 and Ni@C6N6 systems. As a competitive reaction, hydrogen evolution reaction (HER) was investigated and the systems were found to show more selectivity for CO2RR than for HER. OCHO formation turned out to be more favorable than COOH formation as initial protonation intermediates for CO2RR on the TM@C6N6 systems. The present work demonstrates that the Co@C6N6 catalyst can favor the electrocatalytic CO2RR among all systems. In addition, the photocatalytic activity of the systems was also investigated. The systems are found to be active for photoreduction of CO2 to CH3OH and CH4 in the presence of reducing agents such as H2 and H2O as they possess appropriate absorption spectrum in the visible region as well as suitable band edge positions. These findings open a way for designing single atom catalysts for important catalytic reactions.

First principles screening of transition metal single-atom catalysts for nitrogen reduction reaction

The discovery of metals as catalytic centers for nitrogen reduction reactions (NRR) has stimulated great enthusiasm for single-atom catalysts. However, the poor activity and low selectivity of available single-atom catalysts (SACs) are far away from the industrial requirement. Through the first principles screening, the doping engineering can effectively regulate the NRR performance of β-Sb monolayer. Especially, the origin of activated N2 is discovered from the perspective of the electronic structure of the active center. Among the 24 transition metal dopants, Re@Sb showed the best NRR catalytic performance with a low limiting potential. The Re@Sb also could significantly inhibit hydrogen evolution reaction (HER) and achieve a high theoretical Faradaic efficiency of 100%. Our findings not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.

Effect of Nitrogen Ligand Type on 3d Orbital Level Rearrangement of Cobalt Single Atom Catalysts

Transition metal single atom catalysts have recently emerged in acid oxygen evolution reactions (OER) due to their maximum atomic efficiency and high durability. Herein, we report a 3d orbital level rearrangement in square planar symmetry according to the ligand type of a cobalt single atom catalyst. We fabricate cobalt single atom catalyst supported on pyrrole type nitrogen doped crumpled graphene (Pyrrolic CoN4-CG) for acidic OER and it shows orbital rearrangement phenomenon because of their longer Co-N bond distance than pyridine type CoN4 sites. When pyrrole type nitrogen is introduced as a ligand of a cobalt single atom catalyst, the degree of oxidation during OER reaction is much greater than when pyridine type nitrogen ligand is introduced, which is confirmed by Operando X-ray absorption spectroscopy measurements. In addition, the reduction of OH- adsorption energy according to the orbital level rearrangement of Pyrrolic CoN4 and the change in the rate determination step are revealed by density functional theory calculations.