Individual Metal Atoms Research Articles

Phosphorus‐Enhanced Bimetallic Single‐Atom Catalysts for Hydrogen Evolution

AbstractSingle‐atom catalysts (SACs), where individual metal atoms are anchored on support, hold great promise for electrocatalytic hydrogen evolution reactions (HERs). The inherent simplicity of the single‐atom center restricts opportunities for further enhancement. Binuclear SACs, incorporating two different metal sites, can further improve HER kinetics. However, the underlying mechanisms of the HER at the binuclear sites are complex and not fully understood, hampering the design of new efficient catalyst structures. Here, a comprehensive investigation is presented into phosphorus‐doped iron and cobalt bimetallic SACs supported on nitrogen‐doped graphitic carbon (P/FeCo‐NC), focusing on the potential mechanisms underpinning their enhanced HER activity. P/FeCo‐NC exhibits overpotentials of 38 and 95 mV at 10 mA cm2 in 0.5 m H2SO4 and 1 m KOH, respectively, nearing the acidic performance of commercial Pt/C (33 mV). Theoretical studies reveal that the phosphorus bridge significantly alters the electronic properties of the Fe active sites, while the adjacent Co atoms modulate the electronic environment, further optimizing the hydrogen adsorption‐free energy toward more favorable kinetics. This work highlights the structure‐activity relationships of bimetallic SACs and opens perspectives for future applications.

Advancing Heterogeneous Organic Synthesis With Coordination Chemistry-Empowered Single-Atom Catalysts.

For traditional metal complexes, intricate chemistry is required to acquire appropriate ligands for controlling the electron and steric hindrance of metal active centers. Comparatively, the preparation of single-atom catalysts is much easier with more straightforward and effective accesses for the arrangement and control of metal active centers. The presence of coordination atoms or neighboring functional atoms on the supports' surface ensures the stability of metal single-atoms and their interactions with individual metal atoms substantially regulate the performance of metal active centers. Therefore, the collaborative interaction between metal and the surrounding coordination environment enhances the initiation of reaction substrates and the formation and transformation of crucial intermediate compounds, which imparts single-atom catalysts with significant catalytic efficacy, rendering them a valuable framework for investigating the correlation between structure and activity, as well as the reaction mechanism of catalysts in organic reactions. Herein, comprehensive overviews of the coordination interaction for both homogeneous metal complexes and single-atom catalysts in organic reactions are provided. Additionally, reflective conjectures about the advancement of single-atom catalysts in organic synthesis are also proposed to present as a reference for later development.

Soluble individual metal atoms and ultrasmall clusters catalyze key synthetic steps of a natural product synthesis

Metal individual atoms and few-atom clusters show extraordinary catalytic properties for a variety of organic reactions, however, their implementation in total synthesis of complex organic molecules is still to be determined. Here we show a 11-step linear synthesis of the natural product (±)-Licarin B, where individual Pd atoms (Pd1) catalyze the direct aerobic oxidation of an alcohol to the carboxylic acid (steps 1 and 6), Cu2-7 clusters catalyze carbon-oxygen cross couplings (steps 3 and 8), Pd3-4 clusters catalyze a Sonogashira coupling (step 4) and Pt3-5 clusters catalyze a Markovnikov hydrosylilation of alkynes (step 5), as key reactions during the synthetic route. In addition, the new synthesis of Licarin B showcases an unexpected selective alkene hydrogenation with metal-free NaBH4 and an acid-catalyzed intermolecular carbonyl-olefin metathesis as the last step, to forge a trans-alkene group. These results, together, open new avenues in the use of metal individual atoms and clusters in organic synthesis, and confirm their exceptional catalytic activity in late stages during complex synthetic programmes.

Preparation of highly dispersed Ni single-atom doped ultrathin g-C3N4 nanosheets by metal vapor exfoliation for efficient photocatalytic CO2 reduction

Single-atom photocatalysts can modulate the utilization of photons and facilitate the migration of photogenerated carriers. However, the preparation of single-atom uniformly doped photocatalysts is still a challenging topic. Herein, we propose the preparation of Ni single-atom doped g-C3N4 photocatalysts by metal vapor exfoliation. The Ni vapor produced by calcining nickel foam at high temperature accumulates in between g-C3N4 layers and poses a certain vapor pressure to destroy the interlayer van der Waals forces of g-C3N4. Individual metal atoms are doped into the structure while exfoliating g-C3N4 into nanosheets by metal vapor. Upon optimization of Ni content, the Ni single atom doped g-C3N4 nanosheets with 2.81 wt% Ni exhibits the highest CO2 reduction performance in the absence of sacrificial agents. The generation rates of CO and CH4 are 19.85 and 1.73 μmol g−1h−1, respectively. The improved photocatalytic performance is attributed to the anchoring Ni of single atoms on g-C3N4 nanosheets, which increases both carrier separation efficiency and reaction sites. This work provides insight into the design of photocatalysts with highly dispersed single-atom.

Electron beam-induced demetallation of Fe, Co, Ni, Cu, Zn, Pd, and Pt metalloporphyrins: insights in e-beam chemistry and metal cluster formations.

Electron beams are versatile tools for nanoscale fabrication processes, however, the underlying e-beam chemistry remains in its infancy. Through operando transmission electron microscopy investigations, we elucidate a redox-driven cargo release of individual metal atoms triggered by electron beams. The chosen organic delivery molecule, tetraphenylporphyrin (TPP), proves highly versatile, forming complexes with nearly all metals from the periodic table and being easily processed in solution. A comprehensive cinematographic analysis of the dynamics of single metal atoms confirms the nearly instantaneous ejection of complexed metal atoms under an 80 kV electron beam, underscoring the system's broad versatility. Providing mechanistic insights, we employ density functional theory to support the proposed reductive demetallation pathway facilitated by secondary electrons, contributing novel perspectives to electron beam-mediated chemical reaction mechanisms. Lastly, our findings demonstrate that all seven metals investigated form nanoclusters once ejected from TPP, highlighting the method's potential for studying and developing sustainable single-atom and nanocluster catalysts.

Nanostructured single-atom catalysts derived from natural building blocks

The emerging single-atom catalysts derived from biomass sources to date have been comprehensively summarized and discussed, including synthesis strategies, various biomass precursors, catalytic applications, existing challenges, and perspectives.

Cu site differentiation in tetracopper(i) sulfide clusters enables biomimetic N2O reduction.

Copper clusters feature prominently in both metalloenzymes and synthetic nanoclusters that mediate catalytic redox transformations of gaseous small molecules. Such reactions are critical to biological energy conversion and are expected to be crucial parts of renewable energy economies. However, the precise roles of individual metal atoms within clusters are difficult to elucidate, particularly for cluster systems that are dynamic under operating conditions. Here, we present a metal site-specific analysis of synthetic Cu4(μ4-S) clusters that mimic the Cu Z active site of the nitrous oxide reductase enzyme. Leveraging the ability to obtain structural snapshots of both inactive and active forms of the synthetic model system, we analyzed both states using resonant X-ray diffraction anomalous fine structure (DAFS), a technique that enables X-ray absorption profiles of individual metal sites within a cluster to be extracted independently. Using DAFS, we found that a change in cluster geometry between the inactive and active states is correlated to Cu site differentiation that is presumably required for efficient activation of N2O gas. More precisely, we hypothesize that the Cu δ+⋯Cu δ- pairs produced upon site differentiation are poised for N2O activation, as supported by computational modeling. These results provide an unprecedented level of detail on the roles of individual metal sites within the synthetic cluster system and how those roles interplay with cluster geometry to impact the reactivity function. We expect this fundamental knowledge to inform understanding of metal clusters in settings ranging from (bio)molecular to nanocluster to extended solid systems involved in energy conversion.

Evaluation of electrocatalysis of hourglass-shaped polyoxometallates with different transition metals towards hydrogen peroxide transformed from superoxide radicals in living cell mitochondria

In this work, we study the electrocatalytic detection of H2O2 at electrodes correspondingly modified by four hourglass-shaped polyoxometallates (POMs) with Cu, Co, Mn and Cs as their individual central metal atoms, and four representative Wells-Dawson POMs with the same metal atoms. In this way, we have related their analytical performance to the open structures of hourglass-shaped POMs and the cage-like structures of Wells-Dawson POMs. Among them, we found the three-dimensional hourglass phosphomolybdate (POM 1) with plentiful exposed Cu active sites exhibits a dynamic range of 0.1–10 μM, a sensitivity of 11.6 μA μM−1 cm−2, and a limit of detection as low as 44 nM for H2O2. It also offers high selectivity, fast response time (2 s), a reproducibility of ∼1.69% and retains ∼97.9% of the initial current after 15 days, verifying its good stability. Accordingly, the POM 1-modified electrode was applied to monitoring H2O2 converted from superoxide anion radical (O2−) generated by mitochondria isolated and purified from human liver cancer cells (HepG2) without pharmaceutical stimulation. Meanwhile, by exploiting four specific inhibitors (rotenone, 2-thenoyltrifluoroacetone, antimycin A and paclitaxel) known to bind with targeted sites in the corresponding complex within mitochondria to produce a high H2O2 concentration, our results indicated that O2− was mainly produced by mitochondrial complex I (ubiquinone oxidoreductase) via electron leakage in the mitochondrial electron transport chain of liver cancer. The electrochemical detection strategy in our work has provided a direct insight into the electron leakage of mitochondrial complexes caused by targeted oxidative stress diseases.

Probing individual single atom electrocatalyst sites by advanced analytical scanning transmission electron microscopy

Single atom electrocatalysts (SAEs) are promising next-generation materials for promoting a variety of important reactions, such as the oxygen reduction, nitrogen reduction, and CO2 reduction reactions. While bulk characterization techniques such as X-ray absorption spectroscopy and Mössbauer spectroscopy have significantly enhanced our understanding of these catalysts, direct probing of individual single metal atom sites at the atomic scale is necessary to understand local variations in the properties of these sites and accelerate design and synthesis of improved SAEs. Aberration-corrected scanning transmission electron microscopy (STEM) has become a powerful tool for providing this type of atomic-scale information about SAE metal sites. These sites are typically unstable under the electron beam, however, which, in combination with conventional acquisition methods and detectors, has limited the type and quantity of information obtainable by spectroscopic STEM techniques. Here, we map multiple individual SAE metal sites in a nitrogen-doped carbon containing atomically dispersed Fe and Re (FeReNC) at the atomic scale by direct electron detection electron energy-loss spectroscopy (EELS). Direct electron detection provides an improved signal-to-noise ratio over conventional scintillator-based detectors and enables detection and real space localization of weak signals. In addition, we demonstrate an automated method for identification of metal atom positions, placement of the probe on these sites, and simultaneous EELS and energy dispersive X-ray spectroscopic (EDS) signal acquisition. This simultaneous acquisition of EELS and EDS provides access to the composition and bonding of a wide range of SAE metal sites. Focusing the probe directly on the metal sites also increases the relevant data acquisition rate by more than an order of magnitude over two-dimensional mapping, enabling improved statistical measurements of site properties. The versatility, sensitivity, and speed that these techniques provide enhances our ability to probe the local elemental and chemical environment of a large number of individual SAE metal site structures at the atomic scale, enabling an improved understanding of the variations in the local properties of these electrocatalysts to be gained. As a result, significantly increased information about individual metal sites will be available to future electrochemical studies through these techniques, accelerating the development of advanced SAEs.

A first principles study on the catalytic performance of methylcyclohexane dehydrogenation on a monoatomic catalyst

SummaryThis research explores how methylcyclohexane (MCH) interacts with individual metal atoms M (M= Pt, Pd, and Cu) based on density functional theory (DFT) total energy calculations. We analyzed the adsorption energies of MCH on these metal atoms to understand their stability and reactivity. The results show that MCH prefers to attach to single Pt atoms, which exhibited the strongest adsorption energy. Compared with Pt(111) surfaces, the adsorption energies on Pt atoms were slightly higher, indicating enhanced stability at the nanoscale. Adsorption on Pd and Cu atoms resulted in lower adsorption energies than on Pt. We also focused on the initial step of dehydrogenation, where we studied the removal of one hydrogen atom (H1) from MCH. The presence of a single metal atom significantly lowered the energy required for breaking the C1–H1 bond, with Pt showing the lowest energy barrier compared with Pd and Cu due to the excessive of charge transfer. We analyzed the electronic properties and charge transfer during dehydrogenation, revealing the influence of the metal atom's electronic structure on catalytic behavior. Overall, this research provides valuable insights into how MCH interacts with monoatomic metal catalysts, emphasizing the role of the metal atom's electronic configuration in determining reactivity.

Ni-single atom decorated mesoporous carbon electrocatalysts for hydrogen evolution reaction

A single atom catalyst (SAC) is a type of heterogeneous catalyst in which individual metal atoms are dispersed on a support material, such as carbon, metal oxides, or zeolites, rather than forming nanoparticles or clusters. SACs have unique catalytic properties, such as high selectivity and activity, due to their high surface area and the exposed active metal sites, allowing them to interact more efficiently with the reactants. SACs also have the advantage of being more environmentally friendly and cost-effective than traditional catalysts, as they require smaller amounts of metal and can be easily recycled. In this study, we report the synthetic method of mesoporous single atom catalysts (MSACs) containing nickel (Ni), platinum (Pt), iridium (Ir), cobalt (Co), and iron (Fe) as metal-centered atoms for hydrogen evolution reaction (HER). MSACs are successfully synthesized with metal-chelation and soft-templating methods. The results show that highly uniform MSACs can be fabricated with high reproducibility. The metal contents in MSACs can be reduced to under 1wt%, keeping good HER reactivity. For HER, the Ni mesoporous single atom catalysts (Ni-MSACs) show 270 mV overpotential at 10 mA/cm2 at the initial polarization curve and reduces to 190 mV after 2,000 cycles, which outperforms Ni bulk catalyst (Ni plate). In contrast, porous pure carbon without Ni atoms shows inferior performance, indicating that Ni metal center plays an important role in the catalytic activity. The computational calculation using density functional theory (DFT) tells that Volmer-Heyrovsky pathway is dominant for HER using the Ni-MSACs. The performance of MSACs has a high potential for improvement of performance and extreme reduction of metal usage by doping nitrogen, phosphorous, and two or more metal atoms into MSACs.

Limits of Detection for EXAFS Characterization of Heterogeneous Single-Atom Catalysts

Single-atom catalysts (SACs), consisting of individual metal atoms dispersed on a support, attract attention due to their unique reactivity, efficient use of precious metals, and precise chemical tunability. Characterization of the metal species is crucial to substantiate structure–function relationships. Authors often use─and referees often require─X-ray absorption spectroscopy (XAS) data to prove the absence of clustered metal (or metal oxide) structures after pre-treatment and under in situ or operando conditions. However, there has been no critical assessment of the limitations of XAS in substantiating such conclusive statements, which is particularly important given the potential outsized influence of minority catalyst structures in dictating catalytic activity. In this article, we quantitatively assess the detection limits of XAS to identify metal (or metal oxide) clusters in samples containing predominantly single atoms by modeling the extended X-ray absorption fine structure (EXAFS) of mixtures of structures. We identified that a significant fraction of clusters can coexist with SAC active sites (e.g., ∼10% metallic Pt or ∼40% oxidized Pt clusters in Pt/CeO2 SACs), while eluding detection via EXAFS with any statistical significance. To generalize these conclusions, a descriptor-based screening of bulk metal oxides using a continuous Cauchy wavelet transform was proposed that suggests certain materials for which differentiating atomically dispersed metal species and metal oxide clusters would be infeasible by EXAFS (e.g., ReOx). Based on this analysis, we suggest best practices for the study of SACs using EXAFS and provide recommendations to ensure that conclusions do not outpace the evidence used to support them. In this rapidly expanding research area, rigorous characterization will lead to greater understanding of the behavior of SACs and ultimately improved catalytic materials.

Reasonable Design of MXene-Supported Dual-Atom Catalysts with High Catalytic Activity for Hydrogen Evolution and Oxygen Evolution Reaction: A First-Principles Investigation.

MXene-supported single-atom catalysts (SACs) for water splitting has attracted extensive attention. However, the easy aggregation of individual metal atoms used as catalytic active centers usually leads to the relatively low loading of synthetic SACs, which limits the development and application of SACs. Herein, by performing first-principles calculations for Pt and 3d transition metal single atoms immobilized on a two-dimensional (2D) Mo2TiC2O2 MXene surface, we systematically studied the performance of heterogeneous dual-atom catalysts (h-DACs) in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Significantly, h-DACs exhibit higher metal atom loading and more flexible active sites compared to SACs. Benefiting from these features, we found that Pt/Cu@Mo2TiC2O2 heterogeneous DACs exhibits excellent HER activity with ultra-low overpotential |ΔGH∗| (0.04 eV), lower than the corresponding Pt@Mo2TiC2O2 (0.14 eV) and Cu@Mo2TiC2O2 (0.33 eV) SACs, and even lower than that of Pt (0.09 eV). Meanwhile, Pt/Ni@Mo2TiC2O2 exhibits superior OER activity with ultra-low overpotential ηOER (0.38 V), lower than that of Pt@Mo2TiC2O2 (1.11 V) and Ni@Mo2TiC2O2 (0.57 V) SACs, and even lower than that of RuO2 (0.42 V) and IrO2 (0.56 V). Our finding paves the way for the rational design of h-DACs for HER and OER with excellent activity, which provides guidance for other catalytic reactions.

Two-Dimensional Transition Metal-Hexaaminobenzene Monolayer Single-Atom Catalyst for Electrocatalytic Carbon Dioxide Reduction.

Electrocatalytic reduction of CO2 to valuable fuels and chemicals can not only alleviate the energy crisis but also improve the atmospheric environment. The key is to develop electrocatalysts that are extremely stable, efficient, selective, and reasonably priced. In this study, spin-polarized density function theory (DFT) calculations were used to comprehensively examine the catalytic efficacy of transition metal-hexaaminobenzene (TM-HAB) monolayers as single-atom catalysts for the electroreduction of CO2. In the modified two-dimensional TM-HAB monolayer, our findings demonstrate that the binding of individual metal atoms to HAB can be strong enough for the atoms to be evenly disseminated and immobilized. In light of the conflicting hydrogen evolution processes, TM-HAB effectively inhibits hydrogen evolution. CH4 dominates the reduction byproducts of Sc, Ti, V, Cr, and Cu. HCOOH makes up the majority of Zn's reduction products. Co's primary reduction products are CH3OH and CH4, whereas Mn and Fe's primary reduction products are HCHO, CH3OH, and CH4. Among these, the Ti-HAB reduction products have a 1.14 eV limiting potential and a 1.31 V overpotential. The other monolayers have relatively low overpotentials between 0.01 V and 0.7 V; therefore, we predict that TM-HAB monolayers will exhibit strong catalytic activity in the electrocatalytic reduction of CO2, making them promising electrocatalysts for CO2 reduction.

Cerium oxide stabilizes single-atom catalysts

By using a molecular “glue” to hold atoms in place, researchers have come up with a method that disperses individual metal atoms across a solid surface and prevents them from coalescing. The advance may lead to practical procedures for making commercial catalysts that maximize use of expensive metals such as platinum. Many industrial-scale chemical processes are driven by solid catalysts consisting of tiny chunks of a noble metal such as platinum or rhodium dispersed on a solid support—often a metal oxide. Known as heterogeneous catalysts because the catalytic material is solid and the reactants are gases and liquids, these catalysts drive petroleum refining, polymerization chemistry, emissions cleanup, and other processes. Smaller is better when it comes to supported metal catalysts. That’s because the catalytic hot spots that convert reactants to products reside on the surfaces of the particles. Reactant molecules cannot reach metal atoms in the interior of a metal

(Invited) Tracking Metal Electrodeposition Dynamics from Nucleation and Growth of a Single Atom to a Crystalline Nanoparticle

In electrodeposition the key challenge is to obtain better control over nanostructure morphology. Currently, a lack of understanding exists concerning the initial stages of nucleation and growth, which ultimately impact the physicochemical properties of the resulting entities. Using identical location scanning transmission electron microscopy (STEM), with boron-doped diamond (BDD) serving as both an electron-transparent TEM substrate and electrode, we follow this process, from the formation of an individual metal atom through to a crystalline metal nanoparticle, under potential pulsed conditions. In doing so, we reveal the importance of electrochemically driven atom transport, atom cluster formation, cluster progression to a nanoparticle, and the mechanism by which neighboring particles interact during growth. Such information will help formulate improved nucleation and growth models and promote wider uptake of electrodeposited structures in a wide range of societally important applications. This type of measurement is possible in the TEM because the BDD possesses inherent stability, has an extremely high thermal conductivity, is electron beam transparent, is free from contamination, and is robust enough for multiple deposition and imaging cycles. Moreover, the platform can be operated under conditions such that we have confidence that the dynamic atom events we image are truly due to electrochemically driven deposition and no other factors, such as electron-beam-induced movement. Figure 1

Laser‐Irradiated Holey Graphene‐Supported Single‐Atom Catalyst towards Hydrogen Evolution and Oxygen Reduction

AbstractSingle‐atom catalysts (SAC) can boost the intrinsic catalytic activity of hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). However, the challenge remains due to the complex synthesis process and insufficient stability. A sustainable approach is applied to synthesizing SACs through laser irradiation and gaining mesoporous graphene oxide (MGO). The surface dangling bonds of nitrogen‐doped MGO (NMGO) extract metal atoms species from Co or Fe metal foams and convert them to SAC via an appropriate synthesis approach. Notably, the Co‐NMGO electrocatalyst requires low potentials of 146 mV to convey a current density of 10 mA cm−2 towards HER. Similarly, the Fe‐NMGO electrocatalyst offers an onset of 0.79 V towards ORR in acidic solution. The individual metal atoms are confirmed via aberration‐corrected scanning transmission electron microscopy, and X‐ray absorption near‐edge structure and extended X‐ray absorption fine structure. Density functional theory calculations by applying the grand canonical potential kinetics model revealed that Co‐NMGO shows the optimum free reaction energy of −0.17 eV at −0.1 V for HER, and Fe‐NMGO has less limiting potential than that of Co‐NMGO for ORR case. This work opens a new approach towards the synthesis of SAC and its mechanistic understandings.

Inspecting the nonbonding and antibonding orbitals in a surface-supported metal–organic framework

By using low-temperature scanning tunnelling microscopy, we have revealed the spatial distribution and energy separation of the nonbonding and antibonding orbitals associated with individual metal atoms in a surface-supported metal–organic framework.

Structural Regulation and Support Coupling Effect of Single‐Atom Catalysts for Heterogeneous Catalysis

AbstractSingle atom catalysts (SACs) that integrate the merits of homogeneous and heterogeneous catalysts have been attracting considerable attention in recent years. The individual metal atoms of SACs can be stabilized on supports through various unsaturated chemical sites or space confinement for achieving the maximized atom utilization efficiency. Aside from the development of strategies for preparing high loading and high purity SACs, another key challenge in this field is precisely manipulating the geometric and electronic structure of catalytically active single metal sites, thus rendering the catalysts exceptionally reactive, selective, and stabile compared to their bulk counterparts. This review summarizes recent advancements in SACs for heterogeneous catalysis from the perspective of local structural regulation and the synergistic coupling effect between metal species and supports. Special emphasis is placed on the elucidation of the catalytic structure‐performance relationship in terms of coordination environment, valence state and metal‐support interactions by advanced characterization and theoretical studies. Select in situ or operando characterization techniques for tracking the SACs’ structure evolution under realistic conditions are highlighted. Finally, the challenges and opportunities are discussed to offer insight into the rational design of more intriguing SACs with high activity and distinct chemoselectivity.

MoS2-Supported Fe2 Clusters Catalyzing Nitrogen Reduction Reaction to Produce Ammonia

Synergistic interactions within metal clusters help to improve the catalytic activity of individual metal atoms and contribute to advance the understanding of mechanisms in multisite catalysis. Herein, the MoS2-supported Fe2 cluster was chosen as the catalyst for nitrogen reduction reaction (NRR) to produce ammonia. First-principles calculations reveal that the Fe2/MoS2 exhibits high catalytic activity for electrochemical NRR and sheds light on the enzymatic mechanism which bears a low overpotential of 0.21 V. Compared with a single Fe atom on MoS2, the presence of a vicinal Fe atom (i.e., Fe2/MoS2) enables side-on adsorption of N2, which is propitious to effective N–N bond activation. Whereas the Fe3/MoS2 catalyst, simply with one more Fe atom involved, reduces electron depletion among Fe atoms and hence represses N2 adsorption. The origin of reasonable NRR catalysis of Fe2/MoS2 is further unveiled by natural charge population and projected crystal orbital Hamilton population (pCOHP) analyses. This investigation provides an effective strategy for designing active catalysts with low cost and multisite synergy catalysis.