One‐Pot Cooperation of Single‐Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization‐Hydrosilylation Process

Realizing the full potential of oxide‐supported single‐atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one‐pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization‐hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double‐bond migration and anti‐Markovnikov α‐olefin hydrosilylation, respectively. First‐principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single‐pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio‐selectivity (>95 %) even from industrially‐relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide‐supported single‐atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.     

Single-Atom Rh on High-Index CeO2 Facet for Highly Enhanced Catalytic CO Oxidation

Reducible oxide-supported noble metal nanoparticles exhibit high activity in catalyzing many important oxidation reactions. However, atom migration under harsh reaction conditions leads to deactivation of the catalyst. Meanwhile, single-atom catalysts demonstrate enhanced stability, but often suffer from poor catalytic activity owing to the ionized surface states. In this work, we simultaneously address the poor activity and stability issues by synthesizing highly active and durable rhodium (Rh) single-atom catalysts through a “wrap-bake-peel” process. The pre-coated SiO₂ layer during synthesis of catalyst plays a crucial role in not only protecting CeO₂ support against sintering, but also donating electron to weaken the Ce−O bond, producing highly loaded Rh single atoms on the CeO₂ support exposed with high-index {210} facets. Benefiting from the unique electronic structure of CeO₂ {210} facets, more oxygen vacancies are generated along with the deposition of more electropositive Rh single atoms, leading to remarkably improved catalytic performance in CO oxidation.     

Electronic Metal-Support Interaction-Modified Structures and Catalytic Activity of CeOx Overlayers in CeOx/Ag Inverse Catalysts

Electronic metal-support interactions (EMSIs) of oxide-supported metal catalysts strongly modifies the electronic structures of the supported metal nanoparticles. The strong influence of EMSIs on the electronic structures of oxide overlayers on metal nanoparticles employing cerium oxides/Ag inverse catalysts is reported herein. Ce₂O₃ overlayers were observed to exclusively form on Ag nanocrystals at low cerium loadings and be resistant to oxidation treatments up to 250 °C, whereas CeO₂ overlayers gradually developed as the cerium loading increased. Ag cubes enclosed by {001} facets with a smaller work function exert a stronger EMSI effect on the CeO_x overlayers than Ag cubes enclosed by {111} facets. Only the CeO₂ overlayers with a fully developed bulk CeO₂ electronic structure significantly promote the catalytic activity of Ag nanocrystals in CO oxidation, whereas cerium oxide overlayers with other electronic structures do not. These results successfully extend the concept of EMSIs from oxide-supported metal catalysts to metal-supported oxide catalysts.     

Photocatalytic Free Radical-Controlled Synthesis of High-Performance Single-Atom Catalysts

Single-atom catalysts (SACs) have emerged as crucial players in catalysis research, prompting extensive investigation and application. The precise control of metal atom nucleation and growth has garnered significant attention. In this study, we present a straightforward approach for preparing SACs utilizing a photocatalytic radical control strategy. Notably, we demonstrate for the first time that radicals generated during the photochemical process effectively hinder the aggregation of individual atoms. By leveraging the cooperative anchoring of nitrogen atoms and crystal lattice oxygen on the support, we successfully stabilize the single atom. Our Pd₁/TiO₂ catalysts exhibit remarkable catalytic activity and stability in the Suzuki–Miyaura cross-coupling reaction, which was 43 times higher than Pd/C. Furthermore, we successfully depose Pd atoms onto various substrates, including TiO₂, CeO₂, and WO₃. The photocatalytic radical control strategy can be extended to other single-atom catalysts, such as Ir, Pt, Rh, and Ru, underscoring its broad applicability.     

Single-Atom Electrocatalysts from Multivariate Metal–Organic Frameworks for Highly Selective Reduction of CO2 at Low Pressures

Single-atom catalysts (SACs) are of great interest because of their ultrahigh activity and selectivity. However, it is difficult to construct model SACs according to a general synthetic method, and therefore, discerning differences in activity of diverse single-atom catalysts is not straightforward. Herein, a general strategy for synthesis of single-atom metals implanted in N-doped carbon (M₁-N-C; M=Fe, Co, Ni and Cu) has been developed starting from multivariate metal–organic frameworks (MOFs). The M₁-N-C catalysts, featuring identical chemical environments and supports, provided an ideal platform for differentiating the activity of single-atom metal species. When employed in electrocatalytic CO₂ reduction, Ni₁-N-C exhibited a very high CO Faradaic efficiency (FE) up to 96.8 % that far surpassed Fe₁-, Co₁- and Cu₁-N-C. Remarkably, the best-performer, Ni₁-N-C, even demonstrated excellent CO FE at low CO₂ pressures, thereby representing a promising opportunity for the direct use of dilute CO₂ feedstock.     

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single-atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single-atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single-atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂-supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.     

Regulating the Coordination Environment of MOF-Templated Single-Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single-atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi-MOF-74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single-atom Ni catalysts (named Ni_SA-N_x-C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA-N₂-C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h⁻¹), far superior to those of Ni_SA-N₃-C and Ni_SA-N₄-C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single-atom Ni sites in Ni_SA-N₂-C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

Key factors for designing single-atom metal-nitrogen-carbon catalysts for electrochemical CO2 reduction

The electrochemical CO₂ reduction (CO₂RR) is a sustainable approach to mitigate the increased CO₂ emissions and simultaneously produce value-added chemicals and fuels. Metal-nitrogen-carbon (M-N-C) based single-atom catalysts (SACs) have emerged as promising electrocatalysts for CO₂RR with high activity, selectivity, and stability. To design efficient SACs for CO₂RR, the key influence factors need to be understood. Here, we summarize recent achievements on M-N-C SACs for CO₂RR and highlight the significance of the key constituting factors, metal sites, the coordination environment, and the substrates, for achieving high CO₂RR performance. The perspective views and guidelines are provided for the future direction of developing M-N-C SACs as CO₂RR catalysts.     

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeN_x) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN₄ moiety and spin-state transition in the Fe^II center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN₄ and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.     

The atomic-level regulation of single-atom site catalysts for the electrochemical CO2 reduction reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is viewed as a promising way to remove the greenhouse gas CO₂ from the atmosphere and convert it into useful industrial products such as methane, methanol, formate, ethanol, and so forth. Single-atom site catalysts (SACs) featuring maximum theoretical atom utilization and a unique electronic structure and coordination environment have emerged as promising candidates for use in the CO₂RR. The electronic properties and atomic structures of the central metal sites in SACs will be changed significantly once the types or coordination environments of the central metal sites are altered, which appears to provide new routes for engineering SACs for CO₂ electrocatalysis. Therefore, it is of great importance to discuss the structural regulation of SACs at the atomic level and their influence on CO₂RR activity and selectivity. Despite substantial efforts being made to fabricate various SACs, the principles of regulating the intrinsic electrocatalytic performances of the single-atom sites still needs to be sufficiently emphasized. In this perspective article, we present the latest progress relating to the synthesis and catalytic performance of SACs for the electrochemical CO₂RR. We summarize the atomic-level regulation of SACs for the electrochemical CO₂RR from five aspects: the regulation of the central metal atoms, the coordination environments, the interface of single metal complex sites, multi-atom active sites, and other ingenious strategies to improve the performance of SACs. We highlight synthesis strategies and structural design approaches for SACs with unique geometric structures and discuss how the structure affects the catalytic properties.

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising way to remove CO₂ and convert it into useful industrial products. Single-atom site catalysts provide opportunities to regulate the active sites of CO₂RR catalysts at the atomic level.     

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.     

A Universal Graphene Quantum Dot Tethering Design Strategy to Synthesize Single-Atom Catalysts

A general graphene quantum dot-tethering design strategy to synthesize single-atom catalysts (SACs) is presented. The strategy is applicable to different metals (Cr, Mn, Fe, Co, Ni, Cu, and Zn) and supports (0D carbon nanosphere, 1D carbon nanotube, 2D graphene nanosheet, and 3D graphite foam) with the metal loading of 3.0–4.5 wt %. The direct transmission electron microscopy imaging and X-ray absorption spectra analyses confirm the atomic dispersed metal in carbon supports. Our study reveals that the abundant oxygenated groups for complexing metal ions and the rich defective sites for incorporating nitrogen are essential to realize the synthesis of SACs. Furthermore, the carbon nanotube supported Ni SACs exhibits high electrocatalytic activity for CO₂ reduction with nearly 100 % CO selectivity. This universal strategy is expected to open up new research avenues to produce SACs for diverse electrocatalytic applications.     

Activity And Stability of Single- And Di-Atom Catalysts for the O2 Reduction Reaction

Efficient and inexpensive catalysts for the O₂ reduction reaction (ORR) are needed for the advancement of renewable energy technologies. In this study, we designed a computational catalyst-screening method to identify single and di-atom metal dopants from first-row transition elements supported on defect-containing nitrogenated graphene surfaces for the ORR. Based on formation-energy calculations and micro-kinetic modelling of reaction pathways using intermediate binding free energies, we have identified four potentially interesting single-atom catalysts (SACs) and fifteen di-atom catalysts (DACs) with relatively high estimated catalytic activity at 0.8 V vs RHE. Among the best SACs, MnNC shows high stability in both acidic and alkaline media according to our model. For the DACs, we found four possible candidates, MnMn, FeFe, CoCo, and MnNi doped on quad-atom vacancy sites having considerable stability over a wide pH range. The remaining SACs and DACs with high activity are either less stable or show a stability region at an alkaline pH.     

S-Block Potassium Single-atom Electrocatalyst with K−N4 Configuration Derived from K+/Polydopamine for Efficient Oxygen Reduction

Currently, single-atom catalysts (SACs) research mainly focuses on transition metal atoms as active centers. Due to their delocalized s/p-bands, the s-block main group metal elements are typically regarded as catalytically inert. Herein, an s-block potassium SAC (K−N−C) with K-N₄ configuration is reported for the first time, which exhibits excellent oxygen reduction reaction (ORR) activity and stability under alkaline conditions. Specifically, the half-wave potential (E_1/2) is up to 0.908 V, and negligible changes in E_1/2 are observed after 10,000 cycles. In addition, the K−N−C offers an exceptional power density of 158.1 mW cm⁻² and remarkable durability up to 420 h in a Zn-air battery. Density functional theory (DFT) simulations show that K−N−C has bifunctional active K and C sites, can optimize the free energy of ORR reaction intermediates, and adjust the rate-determining steps. The crystal orbital Hamilton population (COHP) results showed that the s orbitals of K played a major role in the adsorption of intermediates, which was different from the d orbitals in transition metals. This work significantly guides the rational design and catalytic mechanism research of s-block SACs with high ORR activity.     

Non-Metal Single-Phosphorus-Atom Catalysis of Hydrogen Evolution

Non-metal-based single-atom catalysts (SACs) offer low cost, simple synthesis methods, and effective regulation for substrates. Herein, we developed a simplified pressurized gas-assisted process, and report the first non-metal single-atom phosphorus with atomic-level dispersion on unique single-crystal Mo₂C hexagonal nanosheet arrays with a (001) plane supported by carbon sheet (SAP-Mo₂C-CS). The SAP-Mo₂C-CS is structurally stable and shows exceptional electrocatalytic activity for the hydrogen evolution reaction (HER). A so-called high-active “window” based on the active sites of P atoms and their adjacent Mo atoms gives a ΔG_H* close to zero for hydrogen evolution, which is the most ideal ΔG_H* reported so far. Meanwhile, the moderate d-band center value of SAP-Mo₂C-CS can be also used as an ideal standard value to evaluate the HER performance in non-metal-based SACs.     

单原子催化:沟通均相催化与多相催化的桥梁（英文）

催化在现代化学工业中占据着极为重要的地位.催化剂是催化过程的核心.均相催化剂由于具有均一、孤立的活性位点,往往具有高活性与高选择性;但是分离困难限制了其实际应用.多相催化剂由于金属原子利用效率低、活性组分不均匀,活性与选择性相对较低;但其稳定易分离的特点使得目前大多数工业催化过程都是多相催化过程.近年来,单原子催化逐渐成为催化领域新的研究热点与前沿,受到相关研究人员的广泛关注.作为一种多相催化剂,单原子催化剂具有稳定易分离的优势.此外,单原子催化剂具有类似均相催化剂的孤立活性位点,可能具有高活性与高选择.因此单原子催化的概念一经提出,便被认为有望成为架起多相催化与均相催化的桥梁;但几年来并未从实验上得到证实.2016年开始,逐渐有单原子催化剂在经典均相催化反应过程中的应用报道,为该观点提供了实验上的证据.本综述概述了2016至2017年单原子催化剂在典型均相催化反应中的成功应用,包括:1)氢甲酰化反应.以烯烃和合成气为原料合成精细化学品醛类化合物的氢甲酰化反应是目前化工生产中典型的均相催化反应之一.2016年,张涛课题组和曾杰课题组先后报道了Rh/ZnO和Rh/CoO单原子催化剂在该反应中的成功应用.催化剂都表现出优异的催化性能,活性与经典均相Wilkinson’s催化剂相当;2)氢硅加成反应.作为合成有机硅产品的重要反应之一,工业上硅氢加成反应主要由Pt基均相催化剂催化.2016年Beller课题组首次报道了将Pt/Al_2O_3单原子催化剂用于烯烃硅氢加成反应中.该催化剂除表现出良好的催化活性和区域选择性外,还具有较高的稳定性和底物普适性;3)C–H键选择性氧化.烷烃部分氧化反应在学术研究和工业应用方面都有重要意义.刘文刚等将M-N-C单原子催化剂(其中M为Fe,Co等金属)成功应用于C–H键的活化反应中,并对催化剂的结构进行了深入剖析.以上实例表明通过调控金属与载体组合、设计开发合适的单原子催化剂,可以达到结合均相催化高活性、高选择性与多相催化稳定易分离的目的,为均相催化多相化提供了一条新途径,也证明单原子催化可望成为沟通均相催化与多相催化的桥梁.     

Single-atom catalysis for organic reactions

Metal-based catalysis, including homogeneous and heterogeneous catalysis, plays a significant role in the modern chemical industry. Heterogeneous catalysis is widely used due to the high efficiency, easy catalyst separation and recycling. However, the metal-utilization efficiency for conventional heterogeneous catalysts needs further improvement compared to homogeneous catalyst. To tackle this, the pursing of heterogenizing homogeneous catalysts has always been attractive but challenging. As a recently emerging class of catalytic material, single-atom catalysts (SACs) are expected to bridge homogeneous and heterogeneous catalytic process in organic reactions and have arguably become the most active new frontier in catalysis field. In this review, a brief introduction and development history of single-atom catalysis and SACs involved organic reactions are documented. In addition, recent advances in SACs and their practical applications in organic reactions such as oxidation, reduction, addition, coupling reaction, and other organic reactions are thoroughly reviewed. To understand structure-property relationships of single-atom catalysis in organic reactions, active sites or coordination structure, metal atom-utilization efficiency (e.g., turnover frequency, TOF calculated based on active metal) and catalytic performance (e.g., conversion and selectivity) of SACs are comprehensively summarized. Furthermore, the application limitations, development trends, future challenges and perspective of SAC for organic reaction are discussed.     

Co Single-Atom Catalysts Boost Chemiluminescence

Co single-atom catalysts (SACs) with good aqueous solubility and abundant labelling functional groups were prepared in Co/Fe bimetallic metal-organic frameworks by a facile solvothermal method without high-temperature calcination. In contrast to traditional chemiluminescence (CL) catalysts, Co SACs accelerated decomposition of H₂O₂ to produce a large amount of singlet oxygen (¹O₂) rather than superoxide (O₂^.−) and hydroxyl radical (OH^.). They were found to dramatically enhance the CL emission of the luminol-H₂O₂ reaction by 1349 times, and, therefore, were employed as very sensitive signal probes for conducting CL immunoassay of cardiac troponin I. The detection limit of the target analyte was as low as 3.3 pg mL⁻¹. It is the first time that employment of SACs for boosting CL reactions has been validated. The Co SACs can also be employed to trace other biorecognition events with high sensitivity.     

Water Activation Triggered by Cu−Co Double-Atom Catalyst for Silane Oxidation

High-performance catalysts sufficient to significantly reduce the energy barrier of water activation are crucial in facilitating reactions that are restricted by water dissociation. Herein we present a Cu−Co double-atom catalyst (CuCo-DAC), which possesses a uniform and well-defined CuCoN₆(OH) structure, and works together to promote water activation in silane oxidation. The catalyst achieves superior catalytic performance far exceeding that of single-atom catalysts (SACs). Various functional silanes are converted into silanols with up to 98 % yield and 99 % selectivity. Kinetic studies show that the activation energy of silane oxidation by CuCo-DAC is significantly lower than that of Cu single-atom catalyst (Cu-SAC) and Co single-atom catalyst (Co-SAC). Theoretical calculations demonstrate two different reaction pathways where water splitting is the rate-determining step and it is accelerated by CuCo-DAC, whereas H₂ formation is key for its single-atom counterpart.     

Turning the Inert Element Zinc into an Active Single-Atom Catalyst for Efficient Fenton-Like Chemistry

Amongst various Fenton-like single-atom catalysts (SACs), the zinc (Zn)-related SACs have been barely reported due to the fully occupied 3d¹⁰ configuration of Zn²⁺ being inactive for the Fenton-like reaction. Herein, the inert element Zn is turned into an active single-atom catalyst (SA−Zn−NC) for Fenton-like chemistry by forming an atomic Zn−N₄ coordination structure. The SA−Zn−NC shows admirable Fenton-like activity in organic pollutant remediation, including self-oxidation and catalytic degradation by superoxide radical (O₂⋅⁻) and singlet oxygen (¹O₂). Experimental and theoretical results unveiled that the single-atomic Zn−N₄ site with electron acquisition can transfer electrons donated by electron-rich pollutants and low-concentration PMS toward dissolved oxygen (DO) to actuate DO reduction into O₂⋅⁻ and successive conversion into ¹O₂. This work inspires an exploration of efficient and stable Fenton-like SACs for sustainable and resource-saving environmental applications.