A General Strategy for Fabricating Isolated Single Metal Atomic Site Catalysts in Y Zeolite

A general strategy for preparing pyrrolic-N4 type single-atom catalysts via pre-located isolated atoms

Single-atom catalysts (SACs) have been applied in many fields due to their superior catalytic performance. Because of the unique properties of the single-atom-site, using the single atoms as catalysts to synthesize SACs is promising. In this work, we have successfully achieved Co₁ SAC using Pt₁ atoms as catalysts. More importantly, this synthesis strategy can be extended to achieve Fe and Ni SACs as well. X-ray absorption spectroscopy (XAS) results demonstrate that the achieved Fe, Co, and Ni SACs are in a M₁-pyrrolic N₄ (M= Fe, Co, and Ni) structure. Density functional theory (DFT) studies show that the Co(Cp)₂ dissociation is enhanced by Pt₁ atoms, thus leading to the formation of Co₁ atoms instead of nanoparticles. These SACs are also evaluated under hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the nature of active sites under HER are unveiled by the operando XAS studies. These new findings extend the application fields of SACs to catalytic fabrication methodology, which is promising for the rational design of advanced SACs.

Hyperporous magnetic catalyst foam for highly efficient and stable adsorption and reduction of aqueous organic contaminants

The facile and low-cost fabrication of free-standing magnetic catalysts with high catalytic efficiency, rapid reaction rate and excellent recoverability has been pursued for various catalysis applications, e.g., treating aqueous organic 4-nitrophenol pollutants. Here, we design and fabricate a free-standing nickel-coated hyperporous polymer foam (Ni-HPF) with adjustable shapes and sizes, hierarchical multiscale porous structures, abundant catalytical interfaces and excellent super-paramagnetic properties. Due to the synergistical effect of abundant binding sites and highly catalytic reduction, the as-prepared Ni-HPF has demonstrated high conversion efficiency (> 90% at extremely low concentration of 7.5 μM) and rapid reaction rate (2.58 × 10^-3 s^-1) for the reduction of organic 4-nitrophenol. Moreover, the magnetic catalyst also holds excellent recoverability (>80% conversion rate even after 1000 cycles) and good reproducibility (>80% conversion rate after 3 months of storage). As such, this work with novel material design and working principle could provide a wide range of potential applications in water purification, chemical catalysis and energy storage devices.

Polyoxometalate-Based Metal-Organic Framework as Molecular Sieve for Highly Selective Semi-Hydrogenation of Acetylene on Isolated Single Pd Atom Sites

Achieving highly selective acetylene semi-hydrogenation in an ethylene-rich gas stream is of great industrial importance. Herein, we construct isolated single Pd atom in a polyoxometalate-based metal-organic framework (POMOF). The unique internal environment allows this POMOF to separate acetylene from acetylene/ethylene gas mixtures and confine it close to the single Pd atom. After semi-hydrogenation, the resulting ethylene is preferentially discharged from the pores, achieving a selectivity of 92.6 %. First-principles simulations reveal that the adsorbed acetylene/ethylene molecules form hydrogen bond networks with oxygen atoms of SiW₁₂ O₄₀ ^4- and create dynamic confinement regions, which preferentially release the produced ethylene. Besides, at the Pd site, the over-hydrogenation of ethylene exhibits a higher reaction energy barrier than the semi-hydrogenation of acetylene. The combined advantages of POMOF and single Pd atom provides an effective approach for the regulation of semi-hydrogenation selectivity.

Hierarchical Zeolites-confined Metal Catalysts and Their Enhanced Catalytic Performances

The confinement of metal species within hierarchical zeolites combines the acidic/basic sites of zeolites, the enhanced mass transfer of mesoporous system, and the inside active metal sites, leading to high activity, unique selectivity, and superior stability in chemicals synthesis, energy and environment catalysis. To date, review on this emerging topic is rarely reported. Herein, we classify five metals-hierarchical zeolites composite (metal@hierarchical zeolites) according to the location of metals on hierarchical structure, including metals located on micropores, intercrystalline mesopores, intracrystalline mesopores, hollow nanobox and mesoporous shells. The synthesis and catalysis applications of metal@hierarchical zeolites composite are provided, highlighting the rational design of catalyst preparation, the improved catalytic efficiency and stability of metal species. Finally, we discuss the current limitations and future opportunities for this emerging field. This Review is expected to inspire more developments and applications of metal@hierarchical zeolites.

Porous Materials Confining Single Atoms for Catalysis

In recent years, single-atom catalysts (SACs) have received extensive attention due to their unique structure and excellent performance. Currently, a variety of porous materials are used as confined single-atom catalysts, such as zeolites, metal-organic frameworks (MOFs), or carbon nitride (CN). The support plays a key role in determining the coordination structure of the catalytic metal center and its catalytic performance. For example, the strong interaction between the metal and the carrier induces the charge transfer between the metal and the carrier, and ultimately affects the catalytic behavior of the single-atom catalyst. Porous materials have unique chemical and physical properties including high specific surface area, adjustable acidity and shape selectivity (such as zeolites), and are rational support materials for confined single atoms, which arouse research interest in this field. This review surveys the latest research progress of confined single-atom catalysts for porous materials, which mainly include zeolites, CN and MOFs. The preparation methods, characterizations, application fields, and the interaction between metal atoms and porous support materials of porous material confined single-atom catalysts are discussed. And we prospect for the application prospects and challenges of porous material confined single-atom catalysts.

Spatially isolated cobalt oxide sites derived from MOFs for direct propane dehydrogenation

The "active site isolation" strategy has been proved to be efficient for enhancing the catalytic performance in propane dehydrogenation (PDH). Herein, spatially isolated cobalt oxide sites within nitrogen-doped carbon (NC) layers supported on silicalite-1 zeolite (CoO_x@NC/S-1) were synthesized by a two-step process consisting of the pyrolysis of bimetallic Zn/Co zeolitic imidazole frameworks loaded on silicalite-1 (ZnCo-ZIF/S-1) under N₂ and the subsequent calcination in air atmosphere. This catalyst possesses exceptional catalytic performance for PDH with the propane conversion of 40% and the propene selectivity of >97%, and no apparent deactivation is observed after 10 h PDH reaction at 600 °C. With intensive characterizations and experiments, it is indicated that the real active sites of CoO_x@NC/S-1 are isolated CoO sites during the PDH process. In situ FT-IR spectroscopy shows the same intermediate product (Co-C₃H₇) during both propane dehydrogenation and propene hydrogenation, indicating that they have a reverse reaction process, and a reaction mechanism for PDH is proposed accordingly.

Fabricating polyoxometalates-stabilized single-atom site catalysts in confined space with enhanced activity for alkynes diboration

Effecting the synergistic function of single metal atom sites and their supports is of great importance to achieve high-performance catalysts. Herein, we successfully fabricate polyoxometalates (POMs)-stabilized atomically dispersed platinum sites by employing three-dimensional metal-organic frameworks (MOFs) as the finite spatial skeleton to govern the accessible quantity, spatial dispersion, and mobility of metal precursors around each POM unit. The isolated single platinum atoms (Pt₁) are steadily anchored in the square-planar sites on the surface of monodispersed Keggin-type phosphomolybdic acid (PMo) in the cavities of various MOFs, including MIL-101, HKUST-1, and ZIF-67. In contrast, either the absence of POMs or MOFs yielded only platinum nanoparticles. Pt₁-PMo@MIL-101 are seven times more active than the corresponding nanoparticles in the diboration of phenylacetylene, which can be attributed to the synergistic effect of the preconcentration of organic reaction substrates by porous MOFs skeleton and the decreased desorption energy of products on isolated Pt atom sites.

It is of great significance to exert the synergistic effect between single atom and support. Here, the authors prepare polyoxometalates-stabilized single-atom site catalysts in confined space with enhanced activity for alkynes diboration.

Low-Temperature Synthesis of Single Palladium Atoms Supported on Defective Hexagonal Boron Nitride Nanosheet for Chemoselective Hydrogenation of Cinnamaldehyde

Metal-support interactions are of great importance in determining the support-activity in heterogeneous catalysis. Here we report a low-temperature synthetic strategy to create atomically dispersed palladium atoms anchored on defective hexagonal boron nitride (h-BN) nanosheet. Density functional theory (DFT) calculations suggest that the nitrogen-containing B vacancy can provide stable anchoring sites for palladium atoms. The presence of single palladium atoms was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. This catalyst showed exceptional efficiency in chemoselective hydrogenation of cinnamaldehyde, along with excellent recyclability, sintering-resistant ability, and scalability. We anticipate this synthetic approach for the synthesis of high-quality SACs based on h-BN support is amenable to large-scale production of bench-stable catalysts with maximum atom efficiency for industrial applications.

In situ creation of multi-metallic species inside porous silicate materials with tunable catalytic properties

Porous metal silicate (PMS) material PMS-11, consisting of uniformly distributed multi-metallic species inside the pores, is synthesized by using a discrete multi-metal coordination complex as the template, demonstrating high catalytic activity and selectivity in hydrogenation of halogenated nitrobenzenes by synergistically activating different reactant molecules via Ni and Co transition metal centers, while GdIII Lewis acid sites play a role in tuning the catalytic properties.

Atomically Dispersed Reactive Centers for Electrocatalytic CO2 Reduction and Water Splitting

Developing electrocatalytic energy conversion technologies for replacing the traditional energy source is highly expected to resolve the fossil fuel exhaustion and related environmental problems. Exploring stable and high-efficiency electrocatalysts is of vital importance for the promotion of these technologies. Single-atom catalysts (SACs), with atomically distributed active sites on supports, perform as emerging materials in catalysis and present promising prospects for a wide range of applications. The rationally designed near-range coordination environment, long-range electronic interaction and microenvironment of the coordination sphere cast huge influence on the reaction mechanism and related catalytic performance of SACs. In the current Review, some recent developments of atomically dispersed reactive centers for electrocatalytic CO2 reduction and water splitting are well summarized. The catalytic mechanism and the underlying structure-activity relationship are elaborated based on the recent progresses of various operando investigations. Finally, by highlighting the challenges and prospects for the development of single-atom catalysis, we hope to shed some light on the future research of SACs for the electrocatalytic energy conversion.

Incorporation of Active Metal Species in Crystalline Porous Materials for Highly Efficient Synergetic Catalysis

The design and development of efficient catalytic materials with synergistic catalytic sites always has long been known to be a thrilling and very dynamic research field. Crystalline porous materials (CPMs) mainly including metal-organic frameworks and zeolites with high scientific and industrial impact have recently been the subject of extensive research due to their essential role in modern chemical industrial processes. The rational incorporation of guest species in CPMs can synergize the respective strengths of these components and allow them to collaborate with each other for synergistic catalysis, leading to enhanced catalytic activity, selectivity, and stability in a broad range of catalytic processes. In this review, the recent advances in the development of CPMs-confined active metal species, including metal nanoparticles, metal/metal oxides heteroparticles, metal oxide, subnanometric metal clusters, and polyoxometalates, for heterogeneous catalysis, with a particular focus on synergistic effects between active components that result in an enhanced performance are highlighted. Insights into catalysts design strategies, host-guest interactions, and structure-property relationships have been illustrated in detail. Finally, the existing challenges and possible development directions in CPMs-based encapsulation-structured synergistic catalysts are discussed.

Zeolite-Encapsulated Ultrasmall Cu/ZnOx Nanoparticles for the Hydrogenation of CO2 to Methanol

Selective hydrogenation of CO₂ to methanol is a "two birds, one stone" technology to mitigate the greenhouse effect and solve the energy demand-supply deficit. Cu-based catalysts can effectively catalyze this reaction but suffer from low catalytic stability caused by the sintering of Cu species. Here, we report a series of zeolite-fixed catalysts Cu/ZnO_x(Y)@Na-ZSM-5 (Y is the mass ratios of Cu/Zn in the catalysts) with core-shell structures to overcome this issue and strengthen the transformation. Fascinatingly, in this work, we first employed bimetallic metal-organic framework, CuZn-HKUST-1, nanoparticles (NPs) as a sacrificial agent to introduce ultrasmall Cu/ZnO_x NPs (∼2 nm) into the crystalline particles of the Na-ZSM-5 zeolite via a hydrothermal synthesis method. The catalytic results showed that the optimized zeolite-encapsulated Cu/ZnO_x(1.38)@Na-ZSM-5 catalyst exhibited the space time yield of methanol (STY_MeOH) of 44.88 g_MeOH·g_Cu^-1·h^-1, much more efficient than the supported Cu/ZnO_x/Na-ZSM-5 catalyst (13.32 g_MeOH·g_Cu^-1·h^-1) and industrial Cu/ZnO/Al₂O₃ catalyst (8.46 g_MeOH·g_Cu^-1·h^-1) under identical conditions. Multiple studies demonstrated that the confinement in the zeolite formwork affords an intimate surrounding for the active phase to create synergies and avoid the separation of Cu-ZnO_x interfaces, which results in an improved performance. More importantly, in the long-term test, the Cu/ZnO_x(1.38)@Na-ZSM-5 catalyst exhibited constant STY_MeOH with superior durability benefitted from its fixed structure. The current findings demonstrate the importance of confinement effects in designing highly efficient and stable methanol synthesis catalysts.

Controlled Synthesis and Structure Engineering of Transition Metal-based Nanomaterials for Oxygen and Hydrogen Electrocatalysis in Zinc-Air Battery and Water-Splitting Devices

Electrocatalytic energy conversion plays a crucial role in realizing energy storage and utilization. Clean energy technologies such as water electrolysis, fuel cells, and metal-air batteries heavily depend on a series of electrochemical redox reactions occurring on the catalysts surface. Therefore, developing efficient electrocatalysts is conducive to remarkably improved performance of these devices. Among numerous studies, transition metal-based nanomaterials (TMNs) have been considered as promising catalysts by virtue of their abundant reserves, low cost, and well-designed active sites. This Minireview is focused on the typical clean electrochemical reactions: hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. Recent efforts to optimize the external morphology and the internal electronic structure of TMNs are described, and beginning with single-component TMNs, the active sites are clarified, and strategies for exposing more active sites are discussed. The summary about multi-component TMNs demonstrates the complementary advantages of integrating functional compositions. A general introduction of single-atom TMNs is provided to deepen the understanding of the catalytic process at an atomic scale. Finally, current challenges and development trends of TMNs in clean energy devices are summarized.

Which is Better for Nanomedicines: Nanocatalysts or Single-Atom Catalysts?

With the rapid advancements in nanotechnology and materials science, numerous nanomaterials have been used as catalysts for nanomedical applications. Their design and modification according to the microenvironment of diseases have been shown to achieve effective treatment. Chemists are in pursuit of nanocatalysts that are more efficient, controllable, and less toxic by developing innovative synthetic technologies and improving existing ones. Recently, single-atom catalysts (SACs) with excellent catalytic activity and high selectivity have attracted increasing attention because of their accurate design as nanomaterials at the atomic level, thereby highlighting their potential for nanomedical applications. In this review, the recent advances in nanocatalysts and SACs are briefly summarized according to their synthesis, characterizations, catalytic mechanisms, and nanomedical applications. The opportunities and future scope for their development and the issues and challenges for their application as nanomedicine are also discussed. As far as it is known, the review is the systematic comparison of nanocatalysts and SACs, especially in the field of nanomedicine, which has promoted the development of nanocatalytic medicine.

Prototype Atomically Dispersed Supported Metal Catalysts: Iridium and Platinum

When metal nanoparticles on supports are made smaller and smaller-to the limit of atomic dispersion-they become cationic and take on new catalytic properties that are only recently being discovered. The synthesis of these materials is reviewed, including their structure characterization-especially by atomic-resolution electron microscopy and X-ray absorption and infrared spectroscopies-and relationships between structure and catalyst performance, for reactions including hydrogenations, oxidations, and the water gas shift. Structure determination is challenging because of the intrinsic nonuniformity of the support surfaces-and therefore the structures on them-but fundamental understanding has advanced rapidly, benefiting from nearly uniform catalysts consisting of metals on well-defined-crystalline-supports and their characterization by spectroscopy and microscopy. Recent advances in atomic-resolution electron microscopy have spurred the field, providing stunning images and deep insights into structure. The iridium catalysts have typically been made from organoiridium precursors, opening the way to understanding and control of the metal-support bonding and ligands on the metal, including catalytic reaction intermediates. Platinum catalysts are usually made with less precision, from salt precursors, but they catalyze a wider array of reactions than the iridium, typically being stable at higher temperatures and seemingly offering rich prospect for discovery of new catalysts.

Impregnating Subnanometer Metallic Nanocatalysts into Self-Pillared Zeolite Nanosheets

Impregnation is the most commonly used approach to prepare supported metal catalysts in industry. However, this method suffers from the formation of large metal particles with uneven dispersion, poor thermal stability, and thus unsatisfied catalytic performance. Here, we demonstrate that the self-pillared MFI zeolite (silicalite-1 and ZSM-5) nanosheets with larger surface area and abundant Si-OH groups are ideal supports to immobilize ultrasmall monometallic (e.g., Rh and Ru) and various bimetallic clusters via simple incipient wetness impregnation method. The loaded subnanometric metal clusters are uniformly dispersed within sinusoidal five-membered rings of MFI and remain stable at high temperatures. The Rh/SP-S-1 is highly efficient in ammonia borane (AB) hydrolysis, showing a TOF value of 430 mol_H2 mol_Rh^-1 min^-1 at 298 K, which is more than 6-fold improvement over that of nanosized zeolite-supported Rh catalyst and even comparable with that of zeolite-supported Rh single-atom catalyst. Because of the synergistic effect between bimetallic Rh-Ru clusters and zeolite acidity, the H₂ generation rate from AB hydrolysis over Rh_0.8Ru_0.2/SP-ZSM-5-100 reaches up to 1006 mol_H2 mol_metal^-1 min^-1 at 298 K, and also shows record activities in cascade hydrogenation of various nitroarenes by coupling with the hydrolysis of AB. This work demonstrates that zeolite nanosheets are excellent supports to anchor diverse ultrasmall metallic species via the simple impregnation method, and the obtained nanocatalysts can be applied in various industrially important catalytic reactions.

Gram-scale synthesis of single-atom metal-N-CNT catalysts for highly efficient CO2 electroreduction

Single-atom catalysts (SACs) have attracted much interest for electrochemical CO2 reduction because of their high metal utilization and excellent catalytic activity. However, the practical applications of SACs were restricted by the low production yield. Herein, we developed a facile synthetic strategy for fabricating metal-nitrogen-carbon nanotube (M-N-CNT, M = Ni, Co, Cu, Fe, Mn, Zn, Pt, or Ru) SACs at scale (>1 g) by direct pyrolysis of metal cations, phenanthroline and CNTs at high temperature. The pyrolysis leads to forming coordinated Ni-N active sites anchored on CNTs. The prepared Ni-N-CNT catalyst with a remarkable Ni loading of 2 wt% determined by ICP exhibits the highest activity for CO2-to-CO conversion with a high faradaic efficiency of 94% and excellent stability. Aberration-corrected high-angle annular dark-field transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the presence of isolated Ni single atoms in Ni-N-CNT, which act as the active centers for CO2 electroreduction while the CNT support offers fast pathways for electron and mass transports. This work laid foundations for future practical applications in CO2 electroreduction, oxygen reduction reactions, water splitting and nitrogen reduction and beyond.

Transformation of atomically dispersed platinum in SAPO-37 into platinum clusters: catalyst for ethylene hydrogenation

Atomically dispersed supported platinum catalysts were synthesized by the reaction of Pt(acac)2 (acac = acetylacetonato) with the silicoaluminophosphate molecular sieve SAPO-37, with infrared spectra showing that the reaction involved SAPO OH groups.

Multilayer stabilization for fabricating high-loading single-atom catalysts

Metal single-atom catalysts (M-SACs) have emerged as an attractive concept for promoting heterogeneous reactions, but the synthesis of high-loading M-SACs remains a challenge. Here, we report a multilayer stabilization strategy for constructing M-SACs in nitrogen-, sulfur- and fluorine-co-doped graphitized carbons (M = Fe, Co, Ru, Ir and Pt). Metal precursors are embedded into perfluorotetradecanoic acid multilayers and are further coated with polypyrrole prior to pyrolysis. Aggregation of the metals is thus efficiently inhibited to achieve M-SACs with a high metal loading (~16 wt%). Fe-SAC serves as an efficient oxygen reduction catalyst with half-wave potentials of 0.91 and 0.82 V (versus reversible hydrogen electrode) in alkaline and acid solutions, respectively. Moreover, as an air electrode in zinc–air batteries, Fe-SAC demonstrates a large peak power density of 247.7 mW cm⁻² and superior long-term stability_. Our versatile method paves an effective way to develop high-loading M-SACs for various applications.

Metal single-atom catalysts offer great potential in bridging the gap between heterogeneous and homogeneous catalysis. Here the authors demonstrate a multilayer stabilization strategy for fabricating high-loading single-atom catalysts including non-precious and noble metals.

Single-Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside

Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "inside" thanks to the subatomic resolution of synchrotron X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure and associated methods will be shown to be powerful tools in identifying the single-atom site and can provide details into the coordination environment and bonding disorder of SACs. The X-ray absorption near-edge structure will be demonstrated as a valuable method in probing the electronic properties of SACs by analyzing the white line intensity, absorption edge shift, and pre-/postedge features. Emphasis is also placed on in situ/operando XAS using state-of-the-art equipment, which can unveil the changes in structure and properties of SACs during the dynamic catalytic processes in a highly sensitive and time-resolved manner.

Metal@Zeolite Hybrid Materials for Catalysis

The fixation of metal nanoparticles into zeolite crystals has emerged as a new series of heterogeneous catalysts, giving performances that steadily outperform the generally supported catalysts in many important reactions. In this outlook, we define different noble metal-in-zeolite structures (metal@zeolite) according to the size of the nanoparticles and their relative location to the micropores. The metal species within the micropores and those larger than the micropores are denoted as encapsulated and fixed structures, respectively. The development in the strategies for the construction of metal@zeolite hybrid materials is briefly summarized in this work, where the rational preparation and improved thermal stability of the metal nanostructures are particularly mentioned. More importantly, these metal@zeolite hybrid materials as catalysts exhibit excellent shape selectivity. Finally, we review the current challenges and future perspectives for these metal@zeolite catalysts.

Zeolite-Encaged Pd-Mn Nanocatalysts for CO2 Hydrogenation and Formic Acid Dehydrogenation

A CO₂ -mediated hydrogen storage energy cycle is a promising way to implement a hydrogen economy, but the exploration of efficient catalysts to achieve this process remains challenging. Herein, sub-nanometer Pd-Mn clusters were encaged within silicalite-1 (S-1) zeolites by a ligand-protected method under direct hydrothermal conditions. The obtained zeolite-encaged metallic nanocatalysts exhibited extraordinary catalytic activity and durability in both CO₂ hydrogenation into formate and formic acid (FA) dehydrogenation back to CO₂ and hydrogen. Thanks to the formation of ultrasmall metal clusters and the synergic effect of bimetallic components, the PdMn_0.6 @S-1 catalyst afforded a formate generation rate of 2151 mol_formate  mol_Pd ^-1  h^-1 at 353 K, and an initial turnover frequency of 6860 mol H 2  mol_Pd ^-1  h^-1 for CO-free FA decomposition at 333 K without any additive. Both values represent the top levels among state-of-the-art heterogeneous catalysts under similar conditions. This work demonstrates that zeolite-encaged metallic catalysts hold great promise to realize CO₂ -mediated hydrogen energy cycles in the future that feature fast charge and release kinetics.

Revealing the Correlation between Catalytic Selectivity and the Local Coordination Environment of Pt Single Atom

Single atom catalysts (SACs) have recently attracted great attention in heterogeneous catalysis and have been regarded as ideal models for investigating the strong interaction between metal and support. Despite the huge progress over the past decade, the deep understanding on the structure-performance correlation of SACs at a single atom level still remains to be a great challenge. In this study, we demonstrate that the variation in the coordination number of the Pt single atom can significantly promote the propylene selectivity during propyne semihydrogenation (PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to 94.1% as the coordination number of Pt-O increases from ∼3.4 to ∼5, whereas the variation in the coordination number of Pt-O slightly influences the turnover frequency values of SACs. We anticipate that the present work may deepen the understanding on the structure-performance of SACs and also promote the fundamental research in single atom catalysis.

Microenvironment modulation of single-atom catalysts and their roles in electrochemical energy conversion

Single-atom catalysts (SACs) have become the most attractive frontier research field in heterogeneous catalysis. Since the atomically dispersed metal atoms are commonly stabilized by ionic/covalent interactions with neighboring atoms, the geometric and electronic structures of SACs depend greatly on their microenvironment, which, in turn, determine the performances in catalytic processes. In this review, we will focus on the recently developed strategies of SAC synthesis, with attention on the microenvironment modulation of single-atom active sites of SACs. Furthermore, experimental and computational advances in understanding such microenvironment in association to the catalytic activity and mechanisms are summarized and exemplified in the electrochemical applications, including the water electrolysis and O2/CO2/N2 reduction reactions. Last, by highlighting the prospects and challenges for microenvironment engineering of SACs, we wish to shed some light on the further development of SACs for electrochemical energy conversion.

Recent Advances in Single-Atom Electrocatalysts for Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) plays significant roles in electrochemical energy storage and conversion systems as well as clean synthesis of fine chemicals. However, the ORR process shows sluggish kinetics and requires platinum-group noble metal catalysts to accelerate the reaction. The high cost, rare reservation, and unsatisfied durability significantly impede large-scale commercialization of platinum-based catalysts. Single-atom electrocatalysts (SAECs) featuring with well-defined structure, high intrinsic activity, and maximum atom efficiency have emerged as a novel field in electrocatalytic science since it is promising to substitute expensive platinum-group noble metal catalysts. However, finely fabricating SAECs with uniform and highly dense active sites, fully maximizing the utilization efficiency of active sites, and maintaining the atomically isolated sites as single-atom centers under harsh electrocatalytic conditions remain urgent challenges. In this review, we summarized recent advances of SAECs in synthesis, characterization, oxygen reduction reaction (ORR) performance, and applications in ORR-related H₂O₂ production, metal-air batteries, and low-temperature fuel cells. Relevant progress on tailoring the coordination structure of isolated metal centers by doping other metals or ligands, enriching the concentration of single-atom sites by increasing metal loadings, and engineering the porosity and electronic structure of the support by optimizing the mass and electron transport are also reviewed. Moreover, general strategies to synthesize SAECs with high metal loadings on practical scale are highlighted, the deep learning algorithm for rational design of SAECs is introduced, and theoretical understanding of active-site structures of SAECs is discussed as well. Perspectives on future directions and remaining challenges of SAECs are presented.

Regioselective Generation of Single-Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite

Preparation of supported metal catalysts with uniform particle size and coordination environment is a challenging and important topic in materials chemistry and catalysis. In this work, we report the regioselective generation of single-site Ir atoms and their evolution into stabilized subnanometric Ir clusters in MWW zeolite, which are located at the 10MR window connecting the two neighboring 12MR supercages. The size of the subnanometric Ir clusters can be controlled by the post-synthesis treatments and maintain below 1 nm even after being reduced at 650 °C, which cannot be readily achieved with samples prepared by conventional impregnation methods. The high structure sensitivity, size-dependence, of catalytic performance in the alkane hydrogenolysis reaction of Ir clusters in the subnanometric regime is evidenced.

Cation Exchange Strategy to Single-Atom Noble-Metal Doped CuO Nanowire Arrays with Ultralow Overpotential for H2O Splitting

Single-atom site catalysts (SACs) have aroused enormous attention and brought about new opportunities for many applications. Herein, we report a versatile strategy to rhodium (Rh) SAC by a facile cation exchange reaction. Remarkably, the Rh SAC modified CuO nanowire arrays on copper foam (Rh SAC-CuO NAs/CF) show unprecedented alkaline oxygen evolution reaction (OER) activity with a high current density of 84.5 mA cm^-2@1.5 V vs reversible hydrogen electrode (RHE), 9.7 times that of Ir/C/CF. More strikingly, when used as an anode and a cathode for overall water splitting, the Rh SAC-CuO NAs/CF can achieve 10 mA cm^-2 at only 1.51 V. Density functional theory calculations reveal that the high OER and HER intrinsic catalytic activities result from moderate adsorption energy of intermediates on Rh SAC. Finally, we demonstrate the general synthesis of different single-atom noble-metal catalysts on CuO NAs (M SAC-CuO NAs/CF, where M = Ru, Ir, Os, and Au).