Metal Single-atom Catalysts Research Articles

Engineering d-p Orbital Hybridization in Single-Atom Metal-Embedded Three-Dimensional Electrodes for Li-S Batteries.

Single-atom metal catalysts (SACs) are used as sulfur cathode additives to promote battery performance, although the material selection and mechanism that govern the catalytic activity remain unclear. It is shown that d-p orbital hybridization between the single-atom metal and the sulfur species can be used as a descriptor for understanding the catalytic activity of SACs in Li-S batteries. Transition metals with a lower atomic number are found, like Ti, to have fewer filled anti-bonding states, which effectively bind lithium polysulfides (LiPSs) and catalyze their electrochemical reaction. A series of single-atom metal catalysts (Me = Mn, Cu, Cr, Ti) embedded in three-dimensional (3D) electrodes are prepared by a controllable nitrogen coordination approach. Among them, the single-atom Ti-embedded electrode has the lowest electrochemical barrier to LiPSs reduction/Li2 S oxidation and the highest catalytic activity, matching well with the theoretical calculations. By virtue of the highly active catalytic center of single-atom Ti on the conductive transport network, high sulfur utilization is achieved with a low catalyst loading (1wt.%) and a high area-sulfur loading (8mg cm-2 ). With good mechanical stability for bending, these 3D electrodes are suitable for fabricating bendable/foldable Li-S batteries for wearable electronics.

High performance SACs for HER process using late first-row transition metals anchored on graphyne support: A DFT insight

For ever-growing demand of clean and renewable energy resources, finding a low-cost, earth abundant, and an efficient electrocatalyst to replace platinum-based catalysts for hydrogen evolution reaction (HER) has drawn the interest of scientific community. In present work, first-row transition metal single atom catalysts supported on graphyne surface have been designed and investigated using density functional theory approach. The results indicate that among all considered systems, the highest thermodynamic stability and best HER catalytic performance is computed for Ni single atom catalyst (SAC) anchored on graphyne support with low ΔGH∗ value of 0.08 eV. We have calculated density of states, energies of HOMO, LUMO and HOMO-LUMO gap for our designed single atom catalysts as well as hydrogen adsorption. Our results indicate that Ni anchored on graphyne support can be a promising candidate for noble metal free, earth abundant, and low cost electrocatalyst to efficiently catalyze hydrogen evolution reaction process.

Central metal and ligand effects on oxygen electrocatalysis over 3d transition metal single-atom catalysts: A theoretical investigation

Metal single-atom catalysts (SACs) have recently emerged as promising alternatives to precious metal-based electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) under alkaline conditions. Currently, the role of the metal centre and local coordination environment on the ORR/OER activity of metal SACs remains unclear. Herein, we employed density functional theory (DFT) calculations to systematically study oxygen electrocatalysis on 60 different 3d transition metal SACs (scandium to zinc, supported on heteroatom-doped graphene supports), encompassing three symmetric nitrogen coordination configurations (i.e., M−Nn−C, n = 4, 3, 2) and three asymmetric coordination configurations (i.e., M−N3X−C, X = P, S, B). The calculations reveal the central metal and the coordinating atoms strongly influence oxygen electrocatalysis over 3d transition metal SACs, predominantly by tuning the adsorption free energy of adsorbed hydroxyl (ΔG*OH, a key descriptor of both ORR and OER activity). Dual limiting potential volcano curves were constructed for ORR and OER, with Ni-N2-C identified as the optimal synthetic target for bifunctional ORR/OER electrocatalysis, closely followed by Fe-N4-C, Co-N4-C, Co-N2-C, and Ni-N3P-C.

Modulation effect in adjacent dual metal single atom catalysts for electrochemical nitrogen reduction reaction

Nitrogen reduction reaction (NRR) is a clean mode of energy conversion and the development of highly efficient NRR electrocatalysts under ambient conditions for industrial application is still a big challenge. Metal-nitrogen-carbon (M-N-C) has emerged as a class of single atom catalyst due to the unique geometric structure, high catalytic activity, and clear selectivity. Herein, we designed a series of dual metal single atom catalysts containing adjacent M-N-C dual active centers (MN4/M'N4-C) as NRR electrocatalysts to uncover the structure-activity relationship. By evaluating structural stability, catalytic activity, and selectivity using density functional theory (DFT) calculations, 5 catalysts, such as CrN4/M'N4-C (M’ = Cr, Mn, Fe, Cu and Zn), were determined to exhibit the best NRR catalytic performance with the limiting potential ranging from −0.64 V to −0.62 V. The CrN4 center acted as the main catalytic site and the adjacent M'N4 center could enhance the NRR catalytic activity by modulation effect based on the analysis of the electronic properties including the charge density difference, partial density of states (PDOS), and Bader charge variation. This study offers useful insights on understanding the structure-activity relationship of dual metal single atom catalysts for electrochemical NRR.

Decarboxylation-Induced Defects in MOF-Derived Single Cobalt Atom@Carbon Electrocatalysts for Efficient Oxygen Reduction.

Developing transition metal single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is of great importance. Zeolitic imidazolate frameworks (ZIFs) as a subgroup of metal-organic frameworks (MOFs) are distinguished as SAC precursors, due to their large porosity and N content. However, the activity of the formed metal sites is limited. Herein, we report a decarboxylation-induced defects strategy to improve their intrinsic activity via increasing the defect density. Carboxylate/amide mixed-linker MOF (DMOF) was chosen to produce defective Co SACs (Co@DMOF) by gas-transport of Co species to DMOF upon heating. Comparing with ZIF-8 derived SAC (Co@ZIF-8-900), Co@DMOF-900 with more defects yet one fifth Co content and similar specific double-layer capacitance show better ORR activity and eight times higher turnover frequency (2.015 e s-1 site-1 ). Quantum calculation confirms the defects can weaken the adsorption free energy of OOH on Co sites and further boost the ORR process.

Ionic-liquid-assisted synthesis of metal single-atom catalysts for benzene oxidation to phenol

Ionic liquids (ILs) have the advantages of low cost, eco-friendliness, abundant heteroatoms, excellent solubility, and coordinated ability with metal ions. These features make ILs a suitable precursor for fabricating metal single-atom catalysts (SACs). Herein, we prepared various metal single atoms anchored on ultrathin N-doped nanosheets (denoted as Cu1/NC, Fe1/NC, Co1/NC, Ni1/NC, and Pd1/NC) by direct pyrolysis using ILs and g-C3N4 nanosheets as templates. Taking benzene oxidation to phenol with H2O2 as a model reaction to evaluate their catalytic performance and potential applications, Cu1/NC calcined at 1000°C (denoted as Cu1/NC-1000) exhibits the highest activity with a turnover frequency of about 200 h−1 in the first 1 h at 60°C, which is better than that of most metal SACs reported in the literature. High benzene conversion of 82% with high phenol selectivity of 96% and excellent recyclability were achieved using the Cu1/NC-1000 catalyst. This study provides an efficient general strategy for fabricating SACs using ILs for catalytic applications.

Atomically dispersed low-cost transition metals catalyze efficient hydrogen evolution on two-dimensional SnO nanosheets

Two-dimensional (2D) electrocatalyst plays an important role in hydrogen production via water splitting. In this work, the first-principles calculation was used to investigate the hydrogen evolution reaction (HER) performance of transition metal (TM) single atom catalysts (SACs) on 2D SnO nanosheets. Among the TM considered (TM = V, Cr, Mn, Fe, Co, Ni, Cu and Pt), V, Fe, Co, Ni, Cu and Pt can effectively improve the catalytic activity of SnO. More importantly, the low-cost Co can exhibit promising HER performance with the Gibbs free energy as low as ~0.015 eV, which is competitive with the precious catalyst Pt. The theoretical exchange current densities of Co SACs can reach ~10−16 A/site. The exciting HER activity is mainly facilitated through the d-d hybridization between the TM and Sn atoms on the SnO surface, which introduces new electron states near the Fermi level. Our work highlights the complexity and diversity of the effect of TM SACs on SnO nanosheets and implies their potential applications as efficient HER electrocatalysts.

18.1% single palladium atom catalysts on mesoporous covalent organic framework for gas phase hydrogenation of ethylene

Noble metal single-atom catalysts maximize metal utilization and offer opportunities to design heterogeneous catalysts at the molecular scale. Mesoporous covalent organic frameworks provide an ideal support to stabilize metal single atoms with specific ligand configuration similar to a homogeneous catalyst. In this work, a high loading of single Pd atoms, 18.1 wt %, on mesoporous imine-linked covalent organic framework was synthesized, characterized, and evaluated for ethylene hydrogenation. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and diffuse-reflectance infrared Fourier transform spectroscopy of adsorbed CO demonstrate that the Pd is atomically dispersed with a highly homogeneous local coordination. The Pd single atoms are active for hydrogenation of ethylene to ethane at room temperature. The study demonstrates that mesoporous COFs provide a large number of identical metal binding sites that are good candidates for immobilizing metal single atoms and their use in gas-phase catalytic applications.

Theoretical considerations on activity of the electrochemical CO2 reduction on metal single-atom catalysts with asymmetrical active sites

Electrochemical CO2 reduction to higher-value hydrocarbons beyond C1 products has attracted much attention recently. Single-atom catalysts (SACs) are regarded as promising CO2 reduction electrocatalysts. However, most SACs only show activity to C1 products. In this work, we considered the activity and the synergetic effect of dual active sites for metal SACs supported on graphitic carbon nitride (g-C3N4) as CO2 reduction electrocatalysts. Density functional theory (DFT) calculations are employed. First, by using the adsorption energies of CO* on the metal site and that of H* on the nitrogen site as bi-descriptors, we predicted seven out of 14 metal centers have the propensity to generate beyond CO products. To further evaluate the catalyst activity on beyond CO product formation, we established reaction pathways towards ethylene through M/N or M/C. Ru has the best performance (the limiting potential is −0.90 V) by taking M/N as active sites. A dual volcano-shaped plot is built up based on the CO adsorption energies on metal sites, which can be used to indicate whether M/C or M/N shows better performance for a specific metal center. Our work shed light on developing criteria to guide the design of CO2 reduction electrocatalysts with dual active sites.

Polymer-metal complexes as emerging catalysts for electrochemical reduction of carbon dioxide

A class of metal-doped polyanilines (PANIs) was synthesized and investigated as electrocatalysts for the carbon dioxide reduction reaction (CO2RR). These materials show good affinity for the electrode substrate and allow to obtain stable binder-free electrodes, avoiding the utilization of expensive ionomer and additives. The emeraldine-base polyaniline (EB-PANI), in absence of metal dopant, shows negligible electrocatalytic activity and selectivity toward the CO2RR. Such behavior significantly improves once EB-PANI is doped with an appropriate cationic metal (Mn, Cu or Sn). In particular, the Sn-PANI outperforms other metal-doped samples, showing a good turnover frequency of 72.2 h−1 for the CO2RR at − 0.99 V vs the reversible hydrogen electrode and thus satisfactory activity of metal single atoms. Moreover, the Sn-PANI also displays impressive stability with a 100% retention of the CO2RR selectivity and an enhanced current density of 4.0 mA cm−2 in a 10-h test. PANI, a relatively low-cost substrate, demonstrates to be easily complexed with different metal cations and thus shows high tailorability. Complexing metal with conductive polymer represents an emerging strategy to realize active and stable metal single-atom catalysts, allowing efficient utilization of metals, especially the raw and precious ones.Graphic abstract

Electrocatalytic Nitrogen Reduction by Transition Metal Single-Atom Catalysts on Polymeric Carbon Nitride

Electrocatalytic Nitrogen Reduction by Transition Metal Single-Atom Catalysts on Polymeric Carbon Nitride

Direct Observation of Metal Oxide Nanoparticles Being Transformed into Metal Single Atoms with Oxygen-Coordinated Structure and High-Loadings.

Direct conversion of bulk metal or nanoparticles into metal single atoms under thermal pyrolysis conditions is a highly efficient and promising strategy to fabricate single-atom catalysts (SACs). Usually, nitrogen-doped carbon is used as the anchoring substrate to capture the migrating metal ion species at high temperatures, and stable isolated SACs with nitrogen coordination are formed during the process. Herein, we report unexpected oxygen-coordinated metal single-atom catalysts (Fe-, Co-, Ni-, Mn-SACs) with high loadings (above 10 wt %) through direct transformation of metal oxide nanoparticles (Fe-, Co-, Ni-, Mn-NPs) in an inert atmosphere at 750 °C for 2 h. The atomic dispersion of metal single atoms and their coordinated structures were confirmed by aberration-corrected scanning transmission electron microscopy and X-ray absorption fine structures. In addition, the dynamic process of nanoparticles to atoms was directly observed by in situ transmission electron microscopy. The as-prepared Fe SAC exhibited high activity and superior selectivity for catalytic oxidation of benzene to phenol with hydrogen peroxide.

Typical transition metal single-atom catalysts with a metal-pyridine N structure for efficient CO2 electroreduction

It is of great importance to establish a definite relationship between structure and catalytic properties for developing highly efficient single-atom catalysts (SACs); however, this remains a challenge. Herein, three single atoms anchored on a nitrogen-doped carbon matrix (M-N-C, M = Fe, Co, Ni) with a metal-pyridine N structure were prepared and investigated as catalysts for electrochemical CO2 reduction. M-N-C exhibit promising capability for CO2-to-CO conversion with the selectivity order of Ni > Co > Fe and the activity order of Co > Ni > Fe, in contrast to the previously reported performance order of Ni > Fe > Co. Among M-N-C catalysts, Ni-N-C shows superior selectivity (approximately 100% CO Faradaic efficiency from −0.66 V to −0.96 V), whereas Co-N-C exhibits high activity (CO current density = −24.24 mA cm−2 and CO turnover frequency = 7182 h−1 at −0.86 V) and capability of yielding syngas (CO/H2 = 0.5∼2.11). Density functional theory calculations support the experimental selectivity and activity orders. This study presents a new possibility for improving the catalytic performance of SACs by investigating the coordinated environments of metal atoms.

Vapor-Phase Self-Assembly to Generate Single Atom Catalysts with Weak Metal-Support Interaction

Preparation of thermally-stable metal single atom catalysts (SACs) is a challenge in heterogeneous catalysis, especially on conventional supports that provide a weak metal-support interaction. In this work, we report that conventional support MgAl2O4 can stabilize Pt single atoms by a mechanism of vapor-phase self-assembly when using a potassium-containing salt at a high temperature treatment (800 oC, air). The obtained Pt/K/MgAl2O4 SAC has a high surface area of ca. 100 m2/g, which is ten times higher than that when supported on ceria with strong metal-support interaction involving the same method. We found that the Pt single atoms via this vapor-phase self-assembly preferentially locate at the MgAl2O4 (111) plane for the Pt/K/MgAl2O4 SAC. The experimental results on the formation mechanism and the structure are discussed and validated by density functional theory calculations and ab initio molecular dynamics simulations. We infer that stable triangular K3O3 structures help to stabilize Pt single atoms at high temperatures in oxidizing conditions, showing the higher reactivity than a Pt/CeO2 single atom catalyst in methane oxidation. The obtained Pt/K/MgAl2O4 SAC presents excellent stability in methane oxidation after steam treatment at elevated temperature, whereas a Pt/MgAl2O4 suffers from rapid deactivation due to Pt nanoparticle growth. This work paves the way for preparing thermally-stable and highly active SACs using conventional high-surface-area supports, despite providing only a weak metal-support interaction.

Carbon and Oxygen Coordinating Atoms Adjust Transition Metal Single-Atom Catalysts Based On Boron Nitride Monolayers for Highly Efficient CO2 Electroreduction.

Although single-atom catalysts (SACs) with transition metal-nitrogen complexes have been studied widely, investigations that use light-element atoms to adjust the coordination environment of the central metal atoms in metal-nitrogen complexes are still rare but show enormous potential for various electrocatalytic reactions. Herein, we design novel SACs based on monolayer BN adjusted by B, C, or O coordinating atoms as catalysts for the CO2 reduction reaction (CRR). These SACs are denoted as M@BN_D (BN = monolayer boron nitride; D = B, C, or O atom; M = Co, Cr, Fe, Mn, Mo, Pd, Pt, Ru, V, W, Ni, Zn, Zr, Ag, Au, Cu, or Ti atom) and are investigated as CRR catalysts using density functional theory calculations. Among these structures, we identified some promising candidate catalysts for CRR with impressive low limiting potential (UL): Pt@BN_C with a UL of -0.18 for the product CH4 and Co@BN_C and Au@BN_O with UL of -0.41 and -0.37 V, respectively, for the product CH3OH. In particular, Pt@BN_C shows a remarkable reduction in UL for the product CH4 compared to any existing catalysts, synthesized or predicted. In addition, the ultralow UL for CRR on Pt@BN_C was derived from the unique bonding feature between the single metal atom and adsorbates and the modulation of ionic interactions induced by the coordination effect of the C atom.

Quantification of Active Site Density and Turnover Frequency: From Single-Atom Metal to Nanoparticle Electrocatalysts.

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe–N–C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe–Nx sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1–13). The SD and TOF quantification can afford an improved understanding of the structure–activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Unique Coordination Structure of Cobalt Single-Atom Catalyst Supported on Dopant-Free Carbon

Metal single-atom catalysts (SACs) supported on doped-carbon materials have attracted extensive attention for the electrocatalytic oxygen reduction reaction (ORR); however, dopant-free carbon suppo...

Elucidating the electro-catalytic oxidation of hydrazine over carbon nanotube-based transition metal single atom catalysts

Elucidating the reaction mechanism of hydrazine oxidation reaction (HzOR) over carbon-based catalysts is highly propitious for the rational design of novel electrocatalysts for HzOR. In present work, isolated first-row transition metal atoms have been coordinated with N atoms on the graphite layers of carbon nanotubes via a M-N4-C configuration (MSA/CNT, M=Fe, Co and Ni). The HzOR over the three single atom catalysts follows a predominant 4-electron reaction pathway to emit N2 and a negligible 1-electron pathway to emit trace of NH3, while their electrocatalytic activity for HzOR is dominated by the absorption energy of N2H4 on them. Furthermore, FeSA/CNT reverses the passivation effect on Fe/C and shows superior performance than CoSA/CNT and NiSA/CNT with a recorded high mass activity for HzOR due to the higher electronic charge of Fe over Co and Ni in the M-N4-C configuration and the lowest absorption energy of N2H4 on FeSA/CNT among the three MSA/CNT catalysts.

Electronic Spin Moment As a Catalytic Descriptor for Fe Single-Atom Catalysts Supported on C2N.

The electrocatalytic activity of transition-metal-based compounds is strongly related to the spin states. However, the underlying relationship connecting spin to catalytic activity remains unclear. Herein, we carried out density functional theory calculations on oxygen reduction reaction (ORR) catalyzed by Fe single-atom supported on C2N (C2N-Fe) to shed light on this relationship. It is found that the change of electronic spin moments of Fe and O2 due to molecular-catalyst adsorption scales with the amount of electron transfer from Fe to O2, which promotes the catalytic activity of C2N-Fe for driving ORR. The nearly linear relationship between the catalytic activity and spin moment variation suggests electronic spin moment as a promising catalytic descriptor for Fe single-atom based catalysts. Following the revealed relationship, the ORR barrier on C2N-Fe was tuned to be as low as 0.10 eV through judicious manipulation of spin states. These findings thus provide important insights into the relationship between catalytic activity and spin, leading to new strategies for designing transition metal single-atom catalysts.

Non-noble metal single atom catalysts with S, N co-doped defective graphene support: A theoretical study of highly efficient acetylene hydration

The Zn-catalyzed acetylene hydration reaction is an important process to produce acetaldehyde in the chemical industry. Heteroatom-doped Zn catalyst has emerged as one of the most promising candidates to replace noble metal-based catalysts for highly efficient acetylene hydration reaction. Single atom catalyst (SAC) has unique performance due to its maximum atomic efficiency, unsaturated metal coordination and electron confinement effect. In this work, geometric and electronic structures of defective graphene supported Zn single atom catalysts with N/S-doping have been systematically studied. The reaction mechanism and activity of acetylene hydration catalyzed by ZnNx (x = 1, 2, 3, 4) and ZnN4S2 are investigated with DFT calculation. Studies have shown that, due to the addition of S atoms, the electronic structure of acetylene is changed when acetylene is adsorbed, which makes the reaction easier. The energy barrier of the ZnN4S2 catalyst is only 11.17 kcal/mol, which is a potential single atom catalyst for acetylene hydration. These results provide insights for the development of highly efficient SACs for acetylene hydration with non-noble single atom metal.