Evolution and stabilization of subnanometric metal species in confined space by in situ TEM

Substance migration in the synthesis of single-atom catalysts

Substance migration is universal and crucial in the synthesis of catalysts, which directly affects their existing form and the micro-structure of their active sites. Realizing migration during the synthesis of single-atom catalysts (SACs) is beneficial for not only increasing their metal loading capacity but also manipulating the electronic structures (coordination structure, long-range interactions, etc.) of their metal sites. This review summarizes the thermodynamics and kinetic processes involved in the synthesis of SACs to unveil the fundamental principles involved in their synthesis. For a better understanding of the effect of migration, the migration of both metal (including ions, atoms, and molecules) and nonmetal species is outlined. Moreover, we propose the research directions to guide the rational design of SACs in the future and deepen the fundamental understanding in the formation of catalysts.

On the Tracks to "Smart" Single-Atom Catalysts

Despite their enormous impact in modern heterogeneous catalysis, single-atom catalysts (SACs) continue to puzzle the catalysis community, which often struggles to draw correct conclusions in SAC-catalyzed experiments. In many cases, the reasons for such an uncertainty originate from the lack of knowledge of the exact single-atom evolution under operative conditions and the fundamental factors controlling the fate of the single atom in relation to the catalytic mechanism. This has led to confusion also about correct definition and terminology, where the coined term single-site catalysts reflects the difficulty in defining the true active species as well as in obtaining long-range ordered homogeneous supports [Chi, S.; et al. J. Catal. 2023, 419, 49-57. DOI: 10.1016/j.jcat.2023.02.003]. Most recent studies have attempted to clarify several of the key aspects that are in play during SAC catalysis. However, one largely overlooked opportunity is to take advantage of all the dynamic phenomena occurring at the single metal site to turn the conventional catalytic sequences into a smart, stimulus-responsive, and controllable evolution of the single atom under operative conditions. Such "smartness" could potentially unleash pathways that mitigate some of the typical drawbacks of SACs, such as selectivity and stability. Here we present our vision on these yet-unexplored opportunities for exploiting the dynamicity of SACs, and we discuss various examples that could be the cornerstones for the advent of a next generation of SACs, that we term here "smart" single-atom catalysts (SSACs). Despite smart-behaving SACs still being far from realization, the clues provided here suggest pathways to achieve this goal.

Understanding the Dynamic Evolution of Active Sites among Single Atoms, Clusters, and Nanoparticles

Catalysis remains a cornerstone of chemical research, with the active sites of catalysts being crucial for their functionality. Identifying active sites, particularly during the reaction process, is crucial for elucidating the relationship between a catalyst's structure and its catalytic property. However, the dynamic evolution of active sites within heterogeneous metal catalysts presents a substantial challenge for accurately pinpointing the real active sites. The advent of in situ and operando characterization techniques has illuminated the path toward understanding the dynamic changes of active sites, offering robust scientific evidence to support the rational design of catalysts. There is a pressing need for a comprehensive review that systematically explores the dynamic evolution among single atoms, clusters, and nanoparticles as active sites during the reaction process, utilizing in situ and operando characterization techniques. This review aims to delineate the effects of various reaction factors on dynamic evolution of active sites among single atoms, clusters, and nanoparticles. Moreover, several in situ and operando techniques are elaborated with emphases on tracking the dynamic evolution of active sites, linking them to catalytic properties. Finally, it discusses challenges and future perspectives in identifying active sites during the reaction process and advancing in situ and operando characterization techniques.

Merger of Single-Atom Catalysis and Photothermal Catalysis for Future Chemical Production

Photothermal catalysis is an emerging field with significant potential for sustainable chemical production processes. The merger of single-atom catalysts (SACs) and photothermal catalysis has garnered widespread attention for its ability to achieve precise bond activation and superior catalytic performance. This review provides a comprehensive overview of the recent progress of SACs in photothermal catalysis, focusing on their underlying mechanisms and applications. The dynamic structural evolution of SACs during photothermal processes is highlighted, and the current advancements and future perspectives in the design, screening, and scaling up of SACs for photothermal processes are discussed. This review aims to provide insights into their continued development in this rapidly evolving field.

Nonlinear optical encoding enabled by recurrent linear scattering

Optical information processing and computing can potentially offer enhanced performance, scalability and energy efficiency. However, achieving nonlinearity-a critical component of computation-remains challenging in the optical domain. Here we introduce a design that leverages a multiple-scattering cavity to passively induce optical nonlinear random mapping with a continuous-wave laser at a low power. Each scattering event effectively mixes information from different areas of a spatial light modulator, resulting in a highly nonlinear mapping between the input data and output pattern. We demonstrate that our design retains vital information even when the readout dimensionality is reduced, thereby enabling optical data compression. This capability allows our optical platforms to offer efficient optical information processing solutions across applications. We demonstrate our design's efficacy across tasks, including classification, image reconstruction, keypoint detection and object detection, all of which are achieved through optical data compression combined with a digital decoder. In particular, high performance at extreme compression ratios is observed in real-time pedestrian detection. Our findings open pathways for novel algorithms and unconventional architectural designs for optical computing.

Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy

Copper and nitrogen co-doped carbon catalysts exhibit a remarkable behavior during the electrocatalytic CO2 reduction (CO2RR), namely, the formation of metal nanoparticles from Cu single atoms, and their subsequent reversible redispersion. Here we show that the switchable nature of these species holds the key for the on-demand control over the distribution of CO2RR products, a lack of which has thus far hindered the wide-spread practical adoption of CO2RR. By intermitting pulses of a working cathodic potential with pulses of anodic potential, we were able to achieve a controlled fragmentation of the Cu particles and partial regeneration of single atom sites. By tuning the pulse durations, and by tracking the catalyst's evolution using operando quick X-ray absorption spectroscopy, the speciation of the catalyst can be steered toward single atom sites, ultrasmall metal clusters or large metal nanoparticles, each exhibiting unique CO2RR functionalities.

Migration of zeolite-encapsulated subnanometre platinum clusters via reactive neural network potentials

The migration of atoms and small clusters is an important process in sub-nanometre scale heterogeneous catalysis, affecting activity, accessibility and deactivation through sintering. Control of migration can be partially achieved via encapsulation of sub-nanometre metal particles into porous media such as zeolites. However, a general understanding of the migration mechanisms and their sensitivity to particle size and framework environment is lacking. Here, we extend the time-scale and sampling of atomistic simulations of platinum cluster diffusion in siliceous zeolite frameworks, by introducing a reactive neural network potential of density functional quality. We observe that Pt atoms migrate in a qualitatively different manner from clusters, occupying the dense region of the framework and avoiding the free pore space. We also find that for cage-like zeolite CHA there exists a maximum in self diffusivity for the Pt dimer beyond which, confinement effects hinder intercage migration. By extending the quality of sampling, NNP-based methods allow for the discovery of novel dynamical processes at the atomistic scale, bringing modelling closer to operando experimental characterization of catalytic materials.

Understanding the Dynamic Aggregation in Single-Atom Catalysis

The dynamic response of single-atom catalysts to a reactive environment is an increasingly significant topic for understanding the reaction mechanism at the molecular level. In particular, single atoms may experience dynamic aggregation into clusters or nanoparticles driven by thermodynamic or kinetic factors. Herein, the inherent mechanistic nuances that determine the dynamic profile during the reaction will be uncovered, including the intrinsic stability and site-migration barrier of single atoms, external stimuli (temperature, voltage, and adsorbates), and the influence of catalyst support. Such dynamic aggregation can be beneficial or deleterious on the catalytic performance depending on the optimal initial state. Those examples will be highlighted where in situ formed clusters, rather than single atoms, serve as catalytically active sites for improved catalytic performance. This is followed by the introduction of operando techniques to understand the structural evolution. Finally, the emerging strategies via confinement and defect-engineering to regulate dynamic aggregation will be briefly discussed.

Operando Mobile Catalysis for Reverse Water Gas Shift Reaction

Metal atoms on the support serve as active sites for many heterogeneous catalysts. However, the active metal sites on the support are conventionally described as static, and the intermediates adsorbed on the support far away from the active metal sites cannot be transformed. Herein, we report the first example of operando mobile catalysis to promote catalytic efficiency by enhancing the collision probability between active sites and reactants or reaction intermediates. Specifically, ligand-coordinated Pt single atoms (isolated MeCpPt- species) are bonded on CeO2 and transformed into mobile MeCpPt(H)CO complexes during the reverse water gas shift reaction for operando mobile catalysis. This strategy enables the conversion of inert carbonate intermediates on the CeO2 support. A turnover frequency (TOF) of 6358 mol CO2  molPt -1  ⋅ h-1 and 99 % CO selectivity at 300 °C is obtained for reverse water gas shift reaction, dramatically higher than those of Pt catalysts reported in the literature. Operando mobile catalysis presents a promising strategy for designing high-efficiency heterogeneous catalysts for various chemical reactions and applications.

What Makes On-Chip Microdevices Stand Out in Electrocatalysis?

Clean and sustainable energy conversion and storage through electrochemistry shows great promise as an alternative to traditional fuel or fossil-consumption energy systems. With regards to practical and high-efficient electrochemistry application, the rational design of active sites and the accurate description of mechanism remain a challenge. Toward this end, in this Perspective, a unique on-chip micro/nano device coupling nanofabrication and low-dimensional electrochemical materials is presented, in which material structure analysis, field-effect regulation, in situ monitoring, and simulation modeling are highlighted. The critical mechanisms that influence electrochemical response are discussed, and how on-chip micro/nano device distinguishes itself is emphasized. The key challenges and opportunities of on-chip electrochemical platforms are also provided through the Perspective.

Atomically Dispersed Zn/Co-N-C as ORR Electrocatalysts for Alkaline Fuel Cells

Hydrogen fuel cells have drawn increasing attention as one of the most promising next-generation power sources for future automotive transportation. Developing efficient, durable, and low-cost electrocatalysts, to accelerate the sluggish oxygen reduction reaction (ORR) kinetics, is urgently needed to advance fuel cell technologies. Herein, we report on metal-organic frameworks-derived nonprecious dual metal single-atom catalysts (SACs) (Zn/Co-N-C), consisting of Co-N4 and Zn-N4 local structures. These catalysts exhibited superior ORR activity with a half-wave potential (E1/2) of 0.938 V versus RHE (reversible hydrogen electrode) and robust stability (ΔE1/2 = -8.5 mV) after 50k electrochemical cycles. Moreover, this remarkable performance was validated under realistic fuel cell working conditions, achieving a record-high peak power density of ∼1 W cm-2 among the reported SACs for alkaline fuel cells. Operando X-ray absorption spectroscopy was conducted to identify the active sites and reveal catalytic mechanistic insights. The results indicated that the Co atom in the Co-N4 structure was the main catalytically active center, where one axial oxygenated species binds to form an Oads-Co-N4 moiety during the ORR. In addition, theoretical studies, based on a potential-dependent microkinetic model and core-level shift calculations, showed good agreement with the experimental results and provided insights into the bonding of oxygen species on Co-N4 centers during the ORR. This work provides a comprehensive mechanistic understanding of the active sites in the Zn/Co-N-C catalysts and will pave the way for the future design and advancement of high-performance single-site electrocatalysts for fuel cells and other energy applications.

In Situ Incorporation of Atomically Precise Au Nanoclusters within Zeolites for Ambient Temperature CO Oxidation

Preserving ultrasmall sizes of metal particles is a key challenge in the study of heterogeneous metal-based catalysis. Confining the ultrasmall metal clusters in a well-defined crystalline porous zeolite has emerged as a promising approach to stabilize these metal species. Successful encapsulation can be achieved by the addition of ligated metal complexes to zeolite synthesis gel before hydrothermal synthesis. However, controlling the metal particle size during post-reduction treatment remains a major challenge in this approach. Herein, an in situ incorporation strategy of pre-made atomically precise gold clusters within Na-LTA zeolite was established for the first time. With the assistance of mercaptosilane ligands, the gold clusters were successfully incorporated within the Na-LTA without premature precipitation and metal aggregation during the synthesis. We have demonstrated that the confinement of gold clusters within the zeolite framework offers high stability against sintering, leading to superior CO oxidation catalytic performance (up to 12 h at 30 °C, with a space velocity of 3000 mL g-1 h-1).

Generation of Subnanometer Metal Clusters in Silicoaluminate Zeolites as Bifunctional Catalysts

Zeolite-encapsulated subnanometer metal catalysts are an emerging class of solid catalysts with superior performances in comparison to metal catalysts supported on open-structure solid carriers. Currently, there is no general synthesis methodology for the encapsulation of subnanometer metal catalysts in different zeolite structures. In this work, we will show a general synthesis method for the encapsulation of subnanometer metal clusters (Pt, Pd, and Rh) within various silicoaluminate zeolites with different topologies (MFI, CHA, TON, MOR). The successful generation of subnanometer metal species in silicoaluminate zeolites relies on the introduction of Sn, which can suppress the migration of subnanometer metal species during high-temperature oxidation-reduction treatments according to advanced electron microscopy and spectroscopy characterizations. The advantage of encapsulated subnanometer Pt catalysts in silicoaluminate zeolites is reflected in the direct coupling of ethane and benzene for production of ethylbenzene, in which the Pt and the acid sites work in a synergistic way.

In Situ Atomic-Scale Investigation of Structural Evolution During Sodiation/Desodiation Processes in Na3 V2 (PO4 )3 -Based All-Solid-State Sodium Batteries

Recently, all-solid-state sodium batteries (Na-ASSBs) have received increased interest owing to their high safety and potential of high energy density. The potential of Na-ASSBs based on sodium superionic conductor (NASICON)-structured Na3 V2 (PO4 )3 (Na3 VP) cathodes have been proven by their high capacity and a long cycling stability closely related to the microstructural evolution. However, the detailed kinetics of the electrochemical processes in the cathodes is still unclear. In this work, the sodiation/desodiation process of Na3 VP is first investigated using in situ high-resolution transmission electron microscopy (HRTEM). The intermediate Na2 V2 (PO4 )3 (Na2 VP) phase with the P21 /c space group, which would be inhibited by constant electron beam irradiation, is observed at the atomic scale. With the calculated volume change and the electrode-electrolyte interface after cycling, it can be concluded that the  Na2 VP phase reduces the lattice mismatch between Na3 VP and NaV2 (PO4 )3 (NaVP), preventing structural collapse. Based on the density functional theory calculation (DFT), the Na+ ion migrates more rapidly in the Na2 VP structure, which facilitates the desodiation and sodiation processes. The formation of  Na2 VP phase lowers the formation energy of NaVP. This study demonstrates the dynamic evolution of the Na3 VP structure, paving the way for an in-depth understanding of electrode materials for energy-storage applications.

PdRh-Sensitized Iron Oxide Ultrathin Film Sensors and Mechanistic Investigation by Operando TEM and DFT Calculation

Metal oxide semiconductor (MOS) thin films are of critical importance to both fundamental research and practical applications of gas sensors. Herein, a high-performance H2 sensor based on palladium (Pd) and rhodium (Rh) co-functionalized Fe2 O3 films with an ultrathin thickness of 8.9 nm deposited by using atomic layer deposition is reported. The sensor delivers an exceptional response of 105.9 toward 10 ppm H2 at 230 °C, as well as high selectivity, immunity to humidity, and low detection limit (43 ppb), which are superior to the reported MOS sensors. Importantly, the Fe2 O3 film sensor under dynamic H2 detection is for the first time observed by operando transmission electron microscopy, which provides deterministic evidence for structure evolution of MOS during sensing reactions. To further reveal the sensing mechanism, density functional theory calculations are performed to elucidate the sensitization effect of PdRh catalysts. Mechanistic studies suggest that Pd promotes the adsorption and dissociation of H2 to generate PdHx , while Rh promotes the dissociation of oxygen adsorbed on the surface, thereby jointly promoting the redox reactions on the films. A wireless H2 detection system is also successfully demonstrated using the thin film sensors, certifying a great potential of the strategy to practical sensors.

In Situ and Emerging Transmission Electron Microscopy for Catalysis Research

Catalysts are the primary facilitator in many dynamic processes. Therefore, a thorough understanding of these processes has vast implications for a myriad of energy systems. The scanning/transmission electron microscope (S/TEM) is a powerful tool not only for atomic-scale characterization but also in situ catalytic experimentation. Techniques such as liquid and gas phase electron microscopy allow the observation of catalysts in an environment conducive to catalytic reactions. Correlated algorithms can greatly improve microscopy data processing and expand multidimensional data handling. Furthermore, new techniques including 4D-STEM, atomic electron tomography, cryogenic electron microscopy, and monochromated electron energy loss spectroscopy (EELS) push the boundaries of our comprehension of catalyst behavior. In this review, we discuss the existing and emergent techniques for observing catalysts using S/TEM. Challenges and opportunities highlighted aim to inspire and accelerate the use of electron microscopy to further investigate the complex interplay of catalytic systems.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Pushing the Performance Limit of Cu/CeO2 Catalyst in CO2 Electroreduction: A Cluster Model Study for Loading Single Atoms

Pushing the performance limit of catalysts is a major goal of CO₂ electroreduction toward practical application. A single-atom catalyst is recognized as a solution for achieving this goal, which is, however, a double-edged sword considering the limited loading amount and stability of single-atom sites. To overcome the limit, the loading of single atoms on supports should be well addressed, requiring a suitable model system. Herein, we report the model system of an ultrasmall CeO₂ cluster (2.4 nm) with an atomic precise structure and a high surface-to-volume ratio for loading Cu single atoms. The combination of multiple characterizations and theoretical calculations reveals the loading location and limit of Cu single atoms on CeO₂ clusters, determining an optimal configuration for CO₂ electroreduction. The optimal catalyst achieves a maximum Faradaic efficiency (FE) of 67% and a maximum partial current density of -364 mA/cm² for CH₄, and can maintain high CH₄ FE values over 50% in a wide range of applied current densities (-50 ∼ -600 mA/cm²), exceeding those of the reported catalysts.

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

The Progress and Outlook of Metal Single-Atom-Site Catalysis

Single-atom-site catalysts (SASCs) featuring maximized atom utilization and isolated active sites have progressed tremendously in recent years as a highly prosperous branch of catalysis research. Varieties of SASCs have been developed that show excellent performance in many catalytic applications. The major goal of SASC research is to establish feasible synthetic strategies for the preparation of high-performance catalysts, to achieve an in-depth understanding of the active-site structures and catalytic mechanisms, and to develop practical catalysts with industrial value. This Perspective describes the up-to-date development of SASCs and related catalysts, such as dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs), analyzes the current challenges encountered by these catalysts for industrial applications, and proposes their possible future development path.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

Catalysts composed of platinum dispersed on zeolite supports are widely applied in industry, and coking and sintering of platinum during operation under reactive conditions require their oxidative regeneration, with the platinum cycling between clusters and cations. The intermediate platinum species have remained only incompletely understood. Here, we report an experimental and theoretical investigation of the structure, bonding, and local environment of cationic platinum species in zeolite ZSM-5, which are key intermediates in this cycling. Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at six-membered rings in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls. This conversion, which weakens the platinum-zeolite bonding, is a first step toward platinum migration and aggregation into clusters. X-ray absorption and infrared spectra provide evidence of the reductive and oxidative transformations in various gas environments. The chemistry is general, as shown by the observation of platinum gem-dicarbonyls in several commercially used zeolites (ZSM-5, Beta, mordenite, and Y).

Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles

We report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (-NH₂ or -OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

Identification of the Stable Pt Single Sites in the Environment of Ions: From Mechanism to Design Principle

For single-atom (SA)-based catalysis, it is urgent to understand the nature and dynamic evolution of SA active sites during the reactions. In this work, an example of Pt SA-Zn_0.5 Cd_0.5 S (Pt SA-ZCS) is found to display interesting phenomena when facing the Brownian collision of ions in aqueous photocatalysis. Via synchrotron radiation, surface techniques, microscopy, and theory calculations, the results show that two kinds of Pt sites exist: Pt_Zn-sub -S₃ (Pt substituting the Zn site) and Pt_ads -S₂ (Pt adsorbing on the surface). In Na₂ S, the S^2- can coordinate with Pt atoms and peel them from the Pt_ads -S₂ sites, but leaves more stable Pt_Zn-sub -S₃ sites, bringing a low but stable catalytic activity (19.40 mmol g^-1 h^-1 ). Meanwhile, in ascorbic acid, the ascorbic acid ions show less complex ability with Pt atoms, but can decrease the migration barrier of Pt_ads -S₂ sites (67.18 down to 35.96 mmol g^-1 h^-1 , 52.03% drop after 6 h). Therefore, the Pt_ads -S₂ sites gradually aggregate into nanoclusters, bringing a high but decayed catalytic activity. Moreover, a Pt SA-ZCS-Sulfur composite is designed mainly covered by Pt_Zn-sub -S₃ sites accordingly (max: 79.09 mmol g^-1 h^-1 , 5% drop after 6 h and QE: 14.0% at 420 nm), showing a beneficial strategy "from mechanism to design principle."

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

Engineering single-atom catalysts toward biomedical applications

Due to inherent structural defects, common nanocatalysts always display limited catalytic activity and selectivity, making it practically difficult for them to replace natural enzymes in a broad scope of biologically important applications. By decreasing the size of the nanocatalysts, their catalytic activity and selectivity will be substantially improved. Guided by this concept, the advances of nanocatalysts now enter an era of atomic-level precise control. Single-atom catalysts (denoted as SACs), characterized by atomically dispersed active sites, strikingly show utmost atomic utilization, precisely located metal centers, unique metal-support interactions and identical coordination environments. Such advantages of SACs drastically boost the specific activity per metal atom, and thus provide great potential for achieving superior catalytic activity and selectivity to functionally mimic or even outperform natural enzymes of interest. Although the size of the catalysts does matter, it is not clear whether the guideline of "the smaller, the better" is still correct for developing catalysts at the single-atom scale. Thus, it is clearly a new, urgent issue to address before further extending SACs into biomedical applications, representing an important branch of nanomedicine. This review begins by providing an overview of recent advances of synthesis strategies of SACs, which serve as a basis for the discussion of emerging achievements in improving the enzyme-like catalytic properties at an atomic level. Then, we carefully compare the structures and functions of catalysts at various scales from nanoparticles, nanoclusters, and few-atom clusters to single atoms. Contrary to conventional wisdom, SACs are not the most catalytically active catalysts in specific reactions, especially those requiring multi-site auxiliary activities. After that, we highlight the unique roles of SACs toward biomedical applications. To appreciate these advances, the challenges and prospects in rapidly growing studies of SACs-related catalytic nanomedicine are also discussed in this review.

Direct assessment of confinement effect in zeolite-encapsulated subnanometric metal species

Subnanometric metal species confined inside the microporous channels/cavities of zeolites have been demonstrated as stable and efficient catalysts. The confinement interaction between the metal species and zeolite framework has been proposed to play the key role for stabilization, though the confinement interaction is elusive to be identified and measured. By combining theoretical calculations, imaging simulation and experimental measurements based on the scanning transmission electron microscopy-integrated differential phase contrast imaging technique, we have studied the location and coordination environment of isolated iridium atoms and clusters confined in zeolite. The image analysis results indicate that the local strain is intimately related to the strength of metal-zeolite interaction and a good correlation is found between the zeolite deformation energy, the charge state of the iridium species and the local absolute strain. The direct observation of confinement with subnanometric metal species encapsulated in zeolites provides insights to understand their structural features and catalytic consequences.

Zeolite-encapsulated metal nanoparticles have important catalytic properties, but their effect on the zeolite local structure has been difficult to characterize. Here the authors, using DFT calculations and scanning transmission electron microscopy, characterize the local strain due to confinement effects in metal-zeolite catalysts.

The 2D or 3D morphology of sub-nanometer Cu5 and Cu8 clusters changes the mechanism of CO oxidation

The mechanism of the CO oxidation reaction catalysed by planar Cu₅, three dimensional (3D) Cu₅, and 3D Cu₈ clusters is theoretically investigated at the B3PW91/Def2TZVP level. All three clusters are able to catalyse the reaction with similar activation energies for the rate determining step, about 16-18 kcal mol^-1, but with remarkable differences in the reaction mechanism depending on cluster morphology. Thus, for 3D Cu₅ and Cu₈ clusters, O₂ dissociation is the first step of the mechanism, followed by two consecutive CO + O reaction steps, the second one being rate determining. In contrast, on planar Cu₅ the reaction starts with the formation of an OOCO intermediate in what constitutes the rate determining step. The O-O bond is broken in a second step, releasing the first CO₂ and leaving one bi-coordinately adsorbed O atom which reacts with CO following an Eley-Rideal mechanism with a low activation energy, in contrast to the higher barriers obtained for this step on 3D clusters.

Migration of zeolite-encapsulated Pt and Au under reducing environments

Simulations reveal accelerated migration of Pt@zeolite by reducing adsorbates and the importance of PtCO in early stages of particle growth.

Exploring the materials space in the smallest particle size range: From heterogeneous catalysis to electrocatalysis and photocatalysis

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such...

Dynamics of Heterogeneous Catalytic Processes at Operando Conditions

The rational design of high-performance catalysts is hindered by the lack of knowledge of the structures of active sites and the reaction pathways under reaction conditions, which can be ideally addressed by an in situ/operando characterization. Besides the experimental insights, a theoretical investigation that simulates reaction conditions-so-called operando modeling-is necessary for a plausible understanding of a working catalyst system at the atomic scale. However, there is still a huge gap between the current widely used computational model and the concept of operando modeling, which should be achieved through multiscale computational modeling. This Perspective describes various modeling approaches and machine learning techniques that step toward operando modeling, followed by selected experimental examples that present an operando understanding in the thermo- and electrocatalytic processes. At last, the remaining challenges in this area are outlined.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

Single-Site vs. Cluster Catalysis in High Temperature Oxidations

The behavior of single Pt atoms and small Pt clusters was investigated for high-temperature oxidations. The high stability of these molecular sites in CHA is a key to intrinsic structure-performance descriptions of elemental steps such as O₂ dissociation, and subsequent oxidation catalysis. Subtle changes in the atomic structure of Pt are responsible for drastic changes in performance driven by specific gas/metal/support interactions. Whereas single Pt atoms and Pt clusters (> ca. 1 nm) are unable to activate, scramble, and desorb two O₂ molecules at moderate T (200 °C), clusters <1 nm do so catalytically, but undergo oxidative fragmentation. Oxidation of alkanes at high T is attributed to stable single Pt atoms, and the C-H cleavage is inferred to be rate-determining and less sensitive to changes in metal nuclearity compared to its effect on O₂ scrambling. In contrast, when combustion involves CO, catalysis is dominated by metal clusters, not single Pt atoms.

Recent advances in single-atom catalysts for advanced oxidation processes in water purification

Single-atom catalysts (SACs) have attracted considerable attention from researchers because of their distinct structures and characteristics, especially in maximizing atomic utilization and elevating the intrinsic catalytic activity. More recently, SACs have been becoming a burgeoning area of the environmental field and are extensively applied to remove various refractory organic pollutants. This review summarizes the emerging synthetic and characterization strategies of SACs and analyzes their development tendency. Besides, the application of SACs in advanced oxidation processes (AOPs, e.g., catalysis of H₂O₂, activation of persulfates and photocatalysis) is discussed. The excellent removal of pollutants depends on the fast generation of reactive oxygen species (SO₄^•-, ^•OH, ¹O₂, and O₂^•-). The advantages of SACs in AOPs are summarized, and constructive opinions are put forward for the stability and activity of the catalyst. Finally, the opportunities and challenges faced by SACs and its future development direction in the AOPs catalytic field are proposed.

Size-activity threshold of titanium dioxide-supported Cu cluster in CO oxidation

Development of non-noble metal cluster catalysts, aiming at concurrently high activity and stability, for emission control systems has been challenging because of sintering and overcoating of clusters on the support. In this work, we reported the role of well-dispersed copper nanoclusters supported on TiO₂ in CO oxidation under industrially relevant operating conditions. The catalyst containing 0.15 wt% Cu on TiO₂ (0.15 CT) exhibited a high dispersion (59.1%), a large specific surface area (381 m²/g_Cu), a small particle size (1.77 nm), and abundant active sites (75.8% Cu₂O). The CO oxidation activity measured by the turnover frequency (TOF) was found to be enhanced from 0.60 × 10^-3 to 3.22 × 10^-3 mol_CO·mol_Cu^-1·s^-1 as the copper loading decreased from 5 to 0.15 wt%. A CO conversion of approximately 60% was still observed in the supported cluster catalyst with a Cu loading of 5 wt% at 240 °C. No deactivation was observed for catalysts with low copper loading (0.15 and 0.30 CT) after 8 h of time-on-stream, which compares favorably with less stable Au cluster-based catalysts reported in the literature. In contrast, catalysts with high copper loading (0.75 and 5 CT) showed deactivation over time, which was ascribed to the increase in copper particle size due to metal cluster agglomeration. This study elucidated the size-activity threshold of TiO₂-supported Cu cluster catalysts. It also demonstrated the potential of the supported Cu cluster catalyst at a typical temperature range of diesel engines at light-load. The supported Cu cluster catalyst could be a promising alternative to noble metal cluster catalysts for emission control systems.

Dynamic Interconversion of Metal Active Site Ensembles in Zeolite Catalysis

Catalysis science is founded on understanding the structure, number, and reactivity of active sites. Kinetic models that consider active sites to be static and noninteracting entities are routinely successful in describing the behavior of heterogeneous catalysts. Yet, active site ensembles often restructure in response to their external environment and even during steady-state catalytic turnover, sometimes requiring non-mean-field kinetic treatments to describe distance-dependent interactions among sites. Such behavior is being recognized more frequently in modern catalysis research, with the advent of experimental methods to quantify turnover rates with increasing precision, an expanding arsenal of operando characterization tools, and computational descriptions of atomic structure and motion at chemical potentials and timescales increasingly relevant to reaction conditions. This review focuses on dynamic changes to metal active site ensembles on zeolite supports, which are silica-based crystalline materials substituted with Al that generate binding sites for isolated and low-nuclearity metal site ensembles. Metal sites can become solvated and mobilized during reaction, facilitating interactions among sites that change their nuclearity and function. Such intersite communication can be regulated by the zeolite support, resulting in non-single-site and potentially non-mean-field kinetic behavior arising from mechanisms of catalytic action that combine elements of those canonically associated with homogeneous and heterogeneous catalysis.We discuss recent literature examples that document dynamic active site behavior in metal-zeolites and outline methodologies to identify and interpret such behavior. We conclude with our outlook on future research directions to develop this evolving branch of catalysis science and harness it for practical applications.

Directly Probing the Local Coordination, Charge State, and Stability of Single Atom Catalysts by Advanced Electron Microscopy: A Review

The drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X-ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and micro-electro-mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub-Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges and opportunities for further development of TEM to study SACs.

Quantitative Structure-Activity Relationship of Nanowire Adsorption to SO2 Revealed by In Situ TEM Technique

A quantitative structure-activity relationship (QSAR) is revealed based on the real-time sulfurization processes of ZnO nanowires observed via gas-cell in situ transmission electron microscopy (in situ TEM). According to the in situ TEM observations, the ZnO nanowires with a diameter of 100 nm (ZnO-100 nm) gradually transform into a core-shell nanostructure under SO₂ atmosphere, and the shell formation kinetics are quantitatively determined. However, only sparse nanoparticles can be observed on the surface of the ZnO-500 nm sample, which implies a weak solid-gas interaction between SO₂ and ZnO-500 nm. The QSAR model is verified with heat of adsorption (-ΔH°) and aberration-corrected TEM characterization. With the guidance of the QSAR model, the following adsorbing/sensing applications of ZnO nanomaterials are explored: (i) breakthrough experiment demonstrates the application potential of the ZnO-100 nm sample for SO₂ capture/storage; (ii) the ZnO-500 nm sample features good reversibility (RSD = 1.5%, n = 3) for SO₂ sensing, and the detection limit reaches 70 ppb.

Fully Exposed Cluster Catalyst (FECC): Toward Rich Surface Sites and Full Atom Utilization Efficiency

Increasing attention has been paid to single-atom catalysts (SACs) in heterogeneous catalysis because of their unique electronic properties, maximized atomic utilization efficiency, and potential to serve as a bridge between the heterogeneous and homogeneous catalysis. However, SACs can have limited advantages or even constrained applications for the reactions that require designated metallic states with multiple atoms or surface sites with metal-metal bonds. As a cross-dimensional extension to the concept of SACs, fully exposed cluster catalysts (FECCs) offer diverse surface sites formed by an ensemble of metal atoms, for the adsorption and transformation of reactants/intermediates. More importantly, FECCs have the advantage of maximized atom utilization efficiency. Thus, FECCs provide a novel platform to design effective and efficient catalysts for certain chemical processes. This outlook summarizes recent advances and proposes prospective research directions in the design of catalysts and characterizations of FECCs, together with potential challenges.

Structural transformations of solid electrocatalysts and photocatalysts

Heterogeneous catalysts often undergo structural transformations when they operate under thermal reaction conditions. These transformations are reflected in their evolving catalytic activity, and a fundamental understanding of the changing nature of active sites is vital for the rational design of solid materials for applications. Beyond thermal catalysis, both photocatalysis and electrocatalysis are topical because they can harness renewable energy to drive uphill reactions that afford commodity chemicals and fuels. Although structural transformations of photocatalysts and electrocatalysts have been observed in operando, the resulting implications for catalytic behaviour are not fully understood. In this Review, we summarize and compare the structural evolution of solid thermal catalysts, electrocatalysts and photocatalysts. We suggest that well-established knowledge of thermal catalysis offers a good basis to understand emerging photocatalysis and electrocatalysis research.