Electron microscopy of solid catalysts--transforming from a challenge to a toolbox

A Simple and Sequential Strategy for the Introduction of Complexity and Hierarchy in Hydrogen-Bonded Organic Framework (HOF) Crystals for Environmental Applications

Hydrogen-bonded organic frameworks (HOFs) are a new class of crystalline porous organic molecular materials (POMMs) with great potential for a diverse range of applications. HOFs face common challenges to POMMs, and in general to purely organic crystals, that is, the difficulty of integrating complexity in crystals. Herein, we propose a simple and sequential strategy for the formation of HOFs with hierarchical superstructures. The strategy is based on controlling the assembly conditions, avoiding the use of any surface functionalization or template, which allows to obtain hierarchical crystalline porous superstructures in an easy manner. As proof of concept, we obtained the first example of core-shell (HOF-on-HOF) crystals and HOFs with hierarchical superstructures having superhydrophobicity and trapping abilities for the capture of persistent water contaminants such as oils and microplastics. We expect that this strategy could serve as inspiration for the construction of more intricate multiscale structures that could greatly expand the library of HOF materials.

Growth Mechanism of Carbon Nanotubes Revealed by in situ Transmission Electron Microscopy

Elucidating the growth mechanism of carbon nanotubes (CNTs) is critical to obtaining CNTs with desired structures and tailored properties for their practical applications. With atomic resolution imaging, in situ transmission electron microscopy (TEM) has been a key technique to reveal the microstructure and dynamics of CNTs in real time. In this review, recent advances in the development of in situ TEM with different types of environmental reactors will be introduced. The catalytic growth mechanisms of CNTs revealed by in situ TEM under realistic conditions are discussed from fundamental thermodynamics and kinetics to the detailed nucleation, growth, and termination mechanisms, including the state and phase of active catalysts, interfacial connections between catalyst and growing CNTs, and catalyst-related growth kinetics of CNTs. Great progresses have been made on how a CNT nucleates, grows and terminates, focusing on the interface dynamics and kinetic fluctuations. Finally, challenges and future directions for understanding the atomic dynamics under the real growth conditions are proposed. It is expected that breakthroughs in the fundamental growth mechanisms will pave the way to the ultimate goal of designing and controlling the atomic structures of CNTs for their applications in various devices.

Directional growth and reconstruction of ultrafine uranium oxide nanorods within single-walled carbon nanotubes

Understanding the atomic structures and dynamic evolution of uranium oxides is crucial for the reliable operation of fission reactors. Among them, U4O9-as an important intermediate in the oxidation of UO2 to UO2+x -plays an important role in the nucleation and conversion of uranium oxides. Herein, we realize the confined assembly of uranyl within SWCNTs in liquid phase and reveal the directional growth and reconstruction of U4O9 nanorods in nanochannels, enabled by in situ scanning transmission electron microscopy (STEM) e-beam stimulation. The nucleation and crystallization of U4O9 nanorods in nanochannels obey the "non-classical nucleation" mechanism and exhibit remarkably higher growth rate compared to those grown outside. The rapid growth process is found to be accompanied by the formation and elimination of U atom vacancies and strain, aiming to achieve the minimum interfacial energy. Eventually, the segments of U4O9 nanorods in SWCNTs merge into single-crystal U4O9 nanorods via structural reconstruction at the interfaces, and 79% of them exhibit anisotropic growth along the specific 〈11̄0〉 direction. These findings pave the way for tailoring the atomic structures and interfaces of uranium oxides during the synthesis process to help improve the mechanical properties and stability of fission reactors.

Non-Invasive Detection of Early-Stage Fatty Liver Disease via an On-Skin Impedance Sensor and Attention-Based Deep Learning

Early-stage nonalcoholic fatty liver disease (NAFLD) is a silent condition, with most cases going undiagnosed, potentially progressing to liver cirrhosis and cancer. A non-invasive and cost-effective detection method for early-stage NAFLD detection is a public health priority but challenging. In this study, an adhesive, soft on-skin sensor with low electrode-skin contact impedance for early-stage NAFLD detection is fabricated. A method is developed to synthesize platinum nanoparticles and reduced graphene quantum dots onto the on-skin sensor to reduce electrode-skin contact impedance by increasing double-layer capacitance, thereby enhancing detection accuracy. Furthermore, an attention-based deep learning algorithm is introduced to differentiate impedance signals associated with early-stage NAFLD in high-fat-diet-fed low-density lipoprotein receptor knockout (Ldlr-/-) mice compared to healthy controls. The integration of an adhesive, soft on-skin sensor with low electrode-skin contact impedance and the attention-based deep learning algorithm significantly enhances the detection accuracy for early-stage NAFLD, achieving a rate above 97.5% with an area under the receiver operating characteristic curve (AUC) of 1.0. The findings present a non-invasive approach for early-stage NAFLD detection and display a strategy for improved early detection through on-skin electronics and deep learning.

Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

In Situ Visualization and Mechanistic Understandings on Facet-Dependent Atomic Redispersion of Platinum on CeO2

Redispersion is an effective method for regeneration of sintered metal-supported catalysts. However, the ambiguous mechanistic understanding hinders the delicate controlling of active metals at the atomic level. Herein, the redispersion mechanism of atomically dispersed Pt on CeO2 is revealed and manipulated by in situ techniques combining well-designed model catalysts. Pt nanoparticles (NPs) sintered on CeO2 nano-octahedra under reduction and oxidation conditions, while redispersed on CeO2 nanocubes above ∼500 °C in an oxidizing atmosphere. The dynamic shrinkage and disappearance of Pt NPs on CeO2 (100) facets was directly visualized by in situ TEM. The generated atomically dispersed Pt with the square-planar [PtO4]2+ structure on CeO2 (100) facets was also confirmed by combining Cs-corrected STEM and spectroscopy techniques. The redispersion and atomic control were ascribed to the high mobility of PtO2 at high temperatures and its strong binding with square-planar O4 sites over CeO2 (100). These understandings are important for the regulation of atomically dispersed platinum catalysts.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

The phenomenon of "dead" metal in heterogeneous catalysis: opportunities for increasing the efficiency of carbon-supported metal catalysts

This review addresses the largely overlooked yet critical issue of "dead" metal in heterogeneous metal catalysts. "Dead" metal refers to the fraction of metal in a catalyst that remains inaccessible to reactants, significantly reducing the overall catalyst performance. As a representative example considered in detail here, this challenge is particularly relevant for carbon-supported metal catalysts, extensively employed in research and industrial settings. We explore key factors contributing to the formation of "dead" metal, including the morphology of the support, metal atom intercalation within the support layers, encapsulation of metal nanoparticles, interference by organic molecules during catalyst preparation, and dynamic behavior under microwave irradiation. Notably, the review outlines a series of strategic approaches to mitigate the occurrence of "dead" metal during catalyst preparation, thus boosting the catalyst efficiency. The knowledge gathered is important for enhancing the preparation of catalysts, especially those containing precious metals. Beyond the practical implications for catalyst design, this study introduces a novel perspective for understanding and optimizing the catalyst performance. The insights are expected to broadly impact different scientific disciplines, empowered with heterogeneous catalysis and driving innovation in energy, environmental science, and materials chemistry, among others. Exploring the "dead" metal phenomenon and potential mitigation strategies brings the field closer to the ultimate goal of high-efficiency, low-cost catalysis.

Controlled Growth of Pd Nanocrystals by Interface Interaction on Monolayer MoS2: An Atom-Resolved in Situ Study

The crystal growth kinetics is crucial for the controllable preparation and performance modulation of metal nanocrystals (NCs). However, the study of growth mechanisms is significantly limited by characterization techniques, and it is still challenging to in situ capture the growth process. Real-time and real-space imaging techniques at the atomic scale can promote the understanding of microdynamics for NC growth. Herein, the growth of Pd NCs on monolayer MoS2 under different atmospheres was in situ studied by environmental transmission electron microscopy. Introducing carbon monoxide can modulate the diffusion of Pd monomers, resulting in the epitaxial growth of Pd NCs with a uniform orientation. The electron energy loss spectroscopy and theoretical calculations showed that the CO adsorption assured the specific exposed facets and good uniformity of Pd NCs. The insight into the gas-solid interface interaction and the microscopic growth mechanism of NCs may shed light on the precise synthesis of NCs on two-dimensional (2D) materials.

Environmental TEM Study of the Dispersion of Au/α-MoC: From Nanoparticles to Two-Dimensional Clusters

The synthesis of highly dispersed Au nanoclusters that are stable under elevated temperatures in heterogeneous catalysis is challenging. Here, we directly observe a strong metal-support interaction (SMSI)-induced dispersion of Au nanoparticles (NPs) on α-MoC using an environmentally atomically resolved secondary imaging technique. Under a realistic environment, Au NPs flatten and spread out on the α-MoC to form two-dimensional atomic layered clusters. The formed highly dispersed Au/α-MoC catalyst shows excellent stability at 600 °C for 160 h in the reverse water-gas shift reaction. The X-ray photoelectron spectrum and extended X-ray absorption fine structure results show that Au NPs gradually become low-coordination-number cluster species and lose electrons to become Auδ+; these form chemical bonds with the α-MoC support and are responsible for the dispersion behavior. This work provides an insightful understanding of dispersion behavior and promotes the rational design and synthesis of reverse sintering catalysts.

Locally Ordered Single-Atom Catalysts for Electrocatalysis

Single-atom catalysts manifest nearly 100 % atom utilization efficiency, well-defined active sites, and high selectivity. However, their practical applications are hindered by a low atom loading density, uncontrollable location, and ambiguous interaction with the support, thereby posing challenges to maximizing their electrocatalytic performance. To address these limitations, the ability to arrange randomly dispersed single atoms into locally ordered single-atom catalysts (LO-SACs) substantially influences the electronic effect between reactive sites and the support, the synergistic interaction among neighboring single atoms, the bonding energy of intermediates with reactive sites and the complexity of the mechanism. As such, it dramatically promotes reaction kinetics, reduces the energy barrier of the reaction, improves the performance of the catalyst and simplifies the reaction mechanism. In this review, firstly, we introduce a variety of compelling characteristics of LO-SACs as electrocatalysts. Subsequently, the synthetic strategies, characterization methods and applications of LO-SACs in electrocatalysis are discussed. Finally, the future opportunities and challenges are elaborated to encourage further exploration in this rapidly evolving field.

Defect-Mediated Growth of Crystallographic Shear Plane

As representative extended planar defects, crystallographic shear (CS) planes, namely Wadsley defects, play an important role in modifying the physical and chemical properties of metal oxides. Although these special structures have been intensively investigated for high-rate anode materials and catalysts, it is still experimentally unclear how the CS planes form and propagate at the atomic scale. Here, the CS plane evolution in monoclinic WO3 is directly imaged via in situ scanning transmission electron microscope. It is found that the CS planes nucleate preferentially at the edge step defects and proceed by the cooperative migration of WO6 octahedrons along particular crystallographic orientations, passing through a series of intermediate states. The local reconstruction of atomic columns tends to form (102) CS planes featured with four edge-sharing octahedrons in preference to the (103) planes, which matches well with the theoretical calculations. Associated with the structure evolution, the sample undergoes a semiconductor-to-metal transition. In addition, the controlled growth of CS planes and V-shaped CS structures can be achieved by artificial defects for the first time. These findings enable an atomic-scale understanding of CS structure evolution dynamics.

Nanoparticles with cubic symmetry: classification of polyhedral shapes

Structural studies of polyhedral bodies can help to analyze geometric details of observed crystalline nanoparticles (NP) where we consider compact polyhedra of cubic point symmetry as simple models. Their surfaces are described by facets with normal vectors along selected Cartesian directions (a, b, c) together with their symmetry equivalents forming a direction family {abc}. Here we focus on polyhedra with facets of families {100}, {110}, and {111}, suggested for metal and oxide NPs with cubic lattices. Resulting generic polyhedra, cubic, rhombohedral, octahedral, and tetrahexahedral, have been observed as NP shapes by electron microscopy. They can serve for a complete description of non-generic polyhedra as intersections of corresponding generic species, not studied by experiment so far. Their structural properties are shown to be fully determined by only three parameters, facet distancesR100,R110, andR111of the three facet types. This provides a novel phase diagram to systematically classify all corresponding polyhedra. Their structural properties, such as shape, size, and facet geometry, are discussed in analytical and numerical detail with visualization of typical examples. The results may be used for respective NP simulations but also as a repository stimulating the structural interpretation of new NP shapes to be observed by experiment.

Applications of Transmission Electron Microscopy in Phase Engineering of Nanomaterials

Phase engineering of nanomaterials (PEN) is an emerging field that aims to tailor the physicochemical properties of nanomaterials by precisely manipulating their crystal phases. To advance PEN effectively, it is vital to possess the capability of characterizing the structures and compositions of nanomaterials with precision. Transmission electron microscopy (TEM) is a versatile tool that combines reciprocal-space diffraction, real-space imaging, and spectroscopic techniques, allowing for comprehensive characterization with exceptional resolution in the domains of time, space, momentum, and, increasingly, even energy. In this Review, we first introduce the fundamental mechanisms behind various TEM-related techniques, along with their respective application scopes and limitations. Subsequently, we review notable applications of TEM in PEN research, including applications in fields such as metallic nanostructures, carbon allotropes, low-dimensional materials, and nanoporous materials. Specifically, we underscore its efficacy in phase identification, composition and chemical state analysis, in situ observations of phase evolution, as well as the challenges encountered when dealing with beam-sensitive materials. Furthermore, we discuss the potential generation of artifacts during TEM imaging, particularly in scanning modes, and propose methods to minimize their occurrence. Finally, we offer our insights into the present state and future trends of this field, discussing emerging technologies including four-dimensional scanning TEM, three-dimensional atomic-resolution imaging, and electron microscopy automation while highlighting the significance and feasibility of these advancements.

Quantitative Analysis of the Interface between Titanium Dioxide Support and Noble Metal by Electron Energy Loss Spectroscopy

The interface structure of supported catalysts plays a significant role in the strong metal-support interactions (SMSI). However, it remains limited on interpreting interface structures, thus affecting the understanding of SMSI origin and impact on catalytic performance. Herein, electronic energy loss spectroscopy was adopted to characterize the interface microstructures of Pt/TiO2 materials. After high-temperature reduction processing, it was observed that the coating on the surface of the Pt metal particles was TiOx. Then, based on Gaussian function fitting, an effective and valid method was established for quantitative analysis on Ti L edge loss spectrum. This method allowed us to accurately determine the stoichiometric number of TiOx phases. In order to probe the classical phenomenon of strong metal-support interactions in more detail, we also discussed and analyzed the origin of TiOx and its effect on the electronic structure of the material using density functional theory calculations. The structure of surfaces and interfaces as well as the chemical evolution of supported catalysts on a microscale have been revealed, thereby providing a new analysis method and research perspectives for the future study of metal-support interactions.

Phase Segregation in PdCu Alloy Nanoparticles During CO Oxidation Reaction at Atmospheric Pressure

Bimetallic nanoparticle (NP) catalysts are widely used in many heterogeneous gas-based reactions because they often outperform their monometallic counterparts. During these reactions, NPs often undergo structural changes, which impact their catalytic activity. Despite the important role of the structure in the catalytic activity, many aspects of how a reactive gaseous environment affects the structure of bimetallic nanocatalysts are still lacking. Here, using gas-cell transmission electron microscopy (TEM), it is shown that during a CO oxidation reaction over PdCu alloy NPs, the selective oxidation of Cu causes the segregation of Cu and transforms the NPs into Pd-CuO NPs. The segregated NPs are very stable and have high activity for the conversion of CO into CO2 . Based on the observations, the segregation of Cu from Cu-based alloys during a redox reaction is likely to be general and may have a positive impact on the catalytic activity. Hence, it is believed that similar insights based on direct observation of the reactions under relevant reactive conditions are critical both for understanding and designing high-performance catalysts.

Chemical Electron Microscopy (CEM) for Heterogeneous Catalysis at Nano: Recent Progress and Challenges

Chemical electron microscopy (CEM), a toolbox that comprises imaging and spectroscopy techniques, provides dynamic morphological, structural, chemical, and electronic information about an object in chemical environment under conditions of observable performance. CEM has experienced a revolutionary improvement in the past years and is becoming an effective characterization method for revealing the mechanism of chemical reactions, such as catalysis. Here, we mainly address the concept of CEM for heterogeneous catalysis in the gas phase and what CEM could uniquely contribute to catalysis, and illustrate what we can know better with CEM and the challenges and future development of CEM.

4D-STEM Ptychography for Electron-Beam-Sensitive Materials

Recent advances in high-speed pixelated electron detectors have substantially facilitated the implementation of four-dimensional scanning transmission electron microscopy (4D-STEM). A critical application of 4D-STEM is electron ptychography, which reveals the atomic structure of a specimen by reconstructing its transmission function from redundant convergent-beam electron diffraction patterns. Although 4D-STEM ptychography offers many advantages over conventional imaging modes, this emerging technique has not been fully applied to materials highly sensitive to electron beams. In this Outlook, we introduce the fundamentals of 4D-STEM ptychography, focusing on data collection and processing methods, and present the current applications of 4D-STEM ptychography in various materials. Next, we discuss the potential advantages of imaging electron-beam-sensitive materials using 4D-STEM ptychography and explore its feasibility by performing simulations and experiments on a zeolite material. The preliminary results demonstrate that, at the low electron dose required to preserve the zeolite structure, 4D-STEM ptychography can reliably provide higher resolution and greater tolerance to the specimen thickness and probe defocus as compared to existing imaging techniques. In the final section, we discuss the challenges and possible strategies to further reduce the electron dose for 4D-STEM ptychography. If successful, it will be a game-changer for imaging extremely sensitive materials, such as metal-organic frameworks, hybrid halide perovskites, and supramolecular crystals.

Patterning the consecutive Pd3 to Pd1 on Pd2Ga surface via temperature-promoted reactive metal-support interaction

Atom-by-atom control of a catalyst surface is a central yet challenging topic in heterogeneous catalysis, which enables precisely confined adsorption and oriented approach of reactant molecules. Here, exposed surfaces with either consecutive Pd trimers (Pd₃) or isolated Pd atoms (Pd₁) are architected for Pd₂Ga intermetallic nanoparticles (NPs) using reactive metal-support interaction (RMSI). At elevated temperatures under hydrogen, in situ atomic-scale transmission electron microscopy directly visualizes the refacetting of Pd₂Ga NPs from energetically favorable (013)/(020) facets to (011)/(002). Infrared spectroscopy and acetylene hydrogenation reaction complementarily confirm the evolution from consecutive Pd₃ to Pd₁ sites of Pd₂Ga catalysts with the concurrent fingerprinting CO adsorption and featured reactivities. Through theoretical calculations and modeling, we reveal that the restructured Pd₂Ga surface results from the preferential arrangement of additionally reduced Ga atoms on the surface. Our work provides previously unidentified mechanistic insight into temperature-promoted RMSI and possible solutions to control and rearrange the surface atoms of supported intermetallic catalyst.

Recent Developments of Microscopic Study for Lanthanide and Manganese Doped Luminescent Materials

Luminescent materials are indispensable for applications in lighting, displays and photovoltaics, which can transfer, absorb, store and utilize light energy. Their performance is closely related with their size and morphologies, exact atomic arrangement, and local configuration about photofunctional centers. Advanced electron microscopy-based techniques have enabled the possibility to study nanostructures with atomic resolution. Especially, with the advanced micro-electro-mechanical systems, it is able to characterize the luminescent materials at the atomic scale under various environments, providing a deep understanding of the luminescent mechanism. Accordingly, this review summarizes the recent achievements of microscopic study to directly image the microstructure and local environment of activators in lanthanide and manganese (Ln/Mn²⁺ )-doped luminescent materials, including: 1) bulk materials, the typical systems are nitride/oxynitride phosphors; and 2) nanomaterials, such as nanocrystals (hexagonal-phase NaLnF₄ and perovskite) and 2D nanosheets (Ca₂ Ta₃ O₁₀ and MoS₂ ). Finally, the challenges and limitations are highlighted, and some possible solutions to facilitate the developments of advanced luminescent materials are provided.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parental MXenes, as a Chinese proverb says, "Indigo blue is extracted from the indigo plant, but is bluer than the plant it comes from." Therefore, 0D QDs harvest larger surface-to-volume ratio, outstanding optical properties, and vigorous quantum confinement effect. Currently, MQDs trigger enormous research enthusiasm as an emerging star of functional materials applied to physics, chemistry, biology, energy conversion, and storage. Since the surface properties of small-sized MQDs include the type of surface functional groups, the functionalized surface directly determines their performance. As the Nobel Laureate Wolfgang Pauli says, "God made the bulk, but the surface was invented by the devil," and it is just on the basis of the abundant surface functional groups, there is lots of space to be thereof excavated from MQDs. We are witnessing such excellence and even more promising to be expected. Nowadays, MQDs have been widely applied to catalysis, whereas the related reviews are rarely reported. Herein, we provide a state-of-the-art overview of MQDs in catalysis over the past five years, ranging from the origin and development of MQDs, synthetic routes of MQDs, and functionalized MQDs to advanced characterization techniques. To explore the diversity of catalytic application and perspectives of MQDs, our review will stimulate more efforts toward the synthesis of optimal MQDs and thereof designing high-performance MQDs-based catalysts.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution

Transition metal single-atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn-containing zeolitic imidazolate framework. Aberration-corrected environmental TEM coupled with in-situ EELS is used for direct visualization of the evolution process at 200-1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co-single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation-driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high-temperature processes.

Revealing Surface Restraint-Induced Hexagonal Pd Nanocrystals via In Situ Transmission Electron Microscopy

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

Revealing synergetic structural activation of a CuAu surface during water-gas shift reaction

Significance Atomic-level in situ environmental transmission electron microscopy observation of the dynamic activation process on a low-indexed CuAu surface under water-gas shift reaction (WGSR) condition has been revealed. The atomic-scale structural activation of a CuAu surface features a gas-dependent periodic surface restructuring and elemental ordering, explaining the "synergy effect" from the structural point of view. These real-time changes under relevant reactant gases and temperature are correlated with the reaction route of WGSR corroborated by density functional theory-based calculation and ab initio molecular dynamics simulation and can provide insights for atom-precision catalyst design.

Quantitative Evaluation of Supported Catalysts Key Properties from Electron Tomography Studies: Assessing Accuracy Using Material-Realistic 3D-Models

Electron Tomography (ET) reconstructions can be analysed, via segmentation techniques, to obtain quantitative, 3D-information about individual nanoparticles in supported catalysts. This includes values of parameters out of reach for any other technique, like their volume and surface, which are required to determine the dispersion of the supported particle system or the specific surface area of the support; two figures that play a major role in the performance of this type of catalysts.

However, both the experimental conditions during the acquisition of the tilt series and the limited fidelity of the reconstruction and segmentation algorithms, restrict the quality of the ET results and introduce an undefined amount of error both in the qualitative features of the reconstructions and in all the quantitative parameters measured from them.

Here, a method based on the use of well-defined 3D geometrical models (phantoms), with morphological features closely resembling those observed in experimental images of an Au/CeO2 catalyst, has been devised to provide a precise estimation of the accuracy of the reconstructions. Using this approach, the influence of noise and the number of projections on the errors of reconstructions obtained using a Total Variation Minimization in 3D (TVM-3D) algorithm have been determined. Likewise, the benefits of using smart denoising techniques based on Undecimated Wavelet Transforms (UWT) have been also evaluated.

The results clearly reveal a large impact of usual noise levels on both the quality of the reconstructions and nanometrological measurement errors. Quantitative clues about the key role of UWT to largely compensate them are also provided.

Deep learning detection of nanoparticles and multiple object tracking of their dynamic evolution during in situ ETEM studies

In situ transmission electron microscopy (TEM) studies of dynamic events produce large quantities of data especially under the form of images. In the important case of heterogeneous catalysis, environmental TEM (ETEM) under gas and temperature allows to follow a large population of supported nanoparticles (NPs) evolving under reactive conditions. Interpreting properly large image sequences gives precious information on the catalytic properties of the active phase by identifying causes for its deactivation. To perform a quantitative, objective and robust treatment, we propose an automatic procedure to track nanoparticles observed in Scanning ETEM (STEM in ETEM). Our approach involves deep learning and computer vision developments in multiple object tracking. At first, a registration step corrects the image displacements and misalignment inherent to the in situ acquisition. Then, a deep learning approach detects the nanoparticles on all frames of video sequences. Finally, an iterative tracking algorithm reconstructs their trajectories. This treatment allows to deduce quantitative and statistical features about their evolution or motion, such as a Brownian behavior and merging or crossing events. We treat the case of in situ calcination of palladium (oxide) / delta-alumina, where the present approach allows a discussion of operating processes such as Ostwald ripening or NP aggregative coalescence.

Multi-atom cluster catalysts for efficient electrocatalysis

This review presents recent developments in the synthesis, modulation and characterization of multi-atom cluster catalysts for electrochemical energy applications.

Atomic origins of the strong metal-support interaction in silica supported catalysts

Silica supported metal catalysts are most widely used in the modern chemical industry because of the high stability and tunable reactivity. The strong metal–support interaction (SMSI), which has been widely observed in metal oxide supported catalysts and significantly affects the catalytic behavior, has been speculated to rarely happen in silica supported catalysts since silica is hard to reduce. Here we revealed at the atomic scale the interfacial reaction induced SMSI in silica supported Co and Pt catalysts under reductive conditions at high temperature using aberration-corrected environmental transmission electron microscopy coupled with in situ electron energy loss spectroscopy. In a Co/SiO₂ system, the amorphous SiO₂ migrated onto the Co surface to form a crystallized quartz-SiO₂ overlayer, and simultaneously an interlayer of Si was generated in-between. The metastable crystalline SiO₂ overlayer subsequently underwent an order-to-disorder transition due to the continuous dissociation of SiO₂ and the interfacial alloying of Si with the underlying Co. The SMSI in the Pt–SiO₂ system was found to remarkably boost the catalytic hydrogenation. These findings demonstrate the universality of the SMSI in oxide supported catalysts, which is of general importance for designing catalysts and understanding catalytic mechanisms.

This work tracked at the atomic scale the interfacial reaction induced strong metal–support interaction between SiO₂ and metal catalysts and evolution under reactive conditions by aberration-corrected environmental transmission electron microscopy.

Anchoring Sites Engineering in Single-Atom Catalysts for Highly Efficient Electrochemical Energy Conversion Reactions

Single-atom catalysts (SACs) have been at the frontier of research field in catalysis owing to the maximized atomic utilization, unique structures and properties. The atomically dispersed and catalytically active metal atoms are necessarily anchored by surrounding atoms. As such, the structure and composition of anchoring sites significantly influence the catalytic performance of SACs even with the same metal element. Significant progress has been made to understand structure-activity relationships at an atomic level, but in-depth understanding in precisely designing highly efficient SACs for the targeted reactions is still required. In this review, various anchoring sites in SACs are summarized and classified into five different types (doped heteroatoms, defect sites, surface atoms, metal sites, and cavity sites). Then, their impacts on catalytic performance are elucidated for electrochemical reactions based on their distance from the metal center (first coordination shell and beyond). Further, SACs anchored on two typical types of hosts, carbon- and metal-based materials, are highlighted, and the effects of anchoring points on achieving the desirable atomic structure, catalytic performance, and reaction pathways are elaborated. At last, insights and outlook to the SAC field based on current achievements and challenges are presented.

Structural Evolution of Cu/ZnO Catalysts during Water-Gas Shift Reaction: An In Situ Transmission Electron Microscopy Study

Supported metal catalysts experience significant structural evolution during the activation process and reaction conditions, which is critical to achieve a desired active surface and interface enabling efficient catalytic processes. However, such dynamic structural information and related mechanistic understandings remain largely elusive owing to the limitation of real-time capturing dynamic information under reaction conditions. Here, using in situ environment transmission electron microscopy, we demonstrate the atomic-scale structural evolution of the model Cu/ZnO catalyst under relevant water-gas shift reaction (WGSR) conditions. Under a CO gas environment, Cu nanoparticles decompose into smaller Cu species and redistribute on ZnO supports with either the crystalline Cu₂O or amorphous CuO_x phase due to a strong CO-Cu interaction. In addition, we visualize various metal-support interactions between Cu and ZnO under reaction conditions, e.g., ZnO clusters precipitating on Cu nanoparticles, which are critical to understand active sites of Cu/ZnO as catalysts for WGSR. These in situ atomic-scale observations highlight the dynamic interplays between Cu and ZnO that can be extended to other supported metal catalysts.

In Situ Resolving the Atomic Reconstruction of SnO2 (110) Surface

Understanding surface reconstruction of nanocrystals is of great importance to their applications, however it is still challenging due to lack of atomic-level structural information under reconstruction conditions. Herein, through in situ spherical aberration corrected scanning transmission electron microscopy (STEM), the reconstruction of nanocrystalline SnO₂ (110) surface was studied. By identifying the precise arrangements of surface/subsurface Sn and O columns through both in situ bright-field and high-angle annular dark-field STEM images, an unexpected added Sn₂O model was determined for SnO₂ (110)-(1 × 2) surface. The protruded Sn^δ+ of this surface could act as the active sites for activating O₂ molecules according to our density functional theory (DFT) calculations. On the basis of in situ observation of atomic-level reconstruction behaviors and DFT calculations, an energy-driven reconstruction process was also revealed. We anticipate this work would help to clarify the long-standing debate regarding the reconstruction of SnO₂ (110) surface and its intrinsic property.

Deep reinforcement learning for predicting kinetic pathways to surface reconstruction in a ternary alloy

The majority of computational catalyst design focuses on the screening of material components and alloy composition to optimize selectivity and activity for a given reaction. However, predicting the metastability of the alloy catalyst surface at realistic operating conditions requires an extensive sampling of possible surface reconstructions and their associated kinetic pathways. We present CatGym, a deep reinforcement learning (DRL) environment for predicting the thermal surface reconstruction pathways and their associated kinetic barriers in crystalline solids under reaction conditions. The DRL agent iteratively changes the positions of atoms in the near-surface region to generate kinetic pathways to accessible local minima involving changes in the surface compositions. We showcase our agent by predicting the surface reconstruction pathways of a ternary Ni3Pd3Au2(111) alloy catalyst. Our results show that the DRL agent can not only explore more diverse surface compositions than the conventional minima hopping method, but also generate the kinetic surface reconstruction pathways. We further demonstrate that the kinetic pathway to a global minimum energy surface composition and its associated transition state predicted by our agent is in good agreement with the minimum energy path predicted by nudged elastic band calculations.

Facet-Dependent Oxidative Strong Metal-Support Interactions of Palladium-TiO2 Determined by In Situ Transmission Electron Microscopy

The strong metal-support interaction (SMSI) is widely used in supported metal catalysts and extensive studies have been performed to understand it. Although considerable progress has been achieved, the surface structure of the support, as an important influencing factor, is usually ignored. We report a facet-dependent SMSI of Pd-TiO₂ in oxygen by using in situ atmospheric pressure TEM. Pd NPs supported on TiO₂ (101) and (100) surfaces showed encapsulation. In contrast, no such cover layer was observed in Pd-TiO₂ (001) catalyst under the same conditions. This facet-dependent SMSI, which originates from the variable surface structure of the support, was demonstrated in a probe reaction of methane combustion catalyzed by Pd-TiO₂ . Our discovery of the oxidative facet-dependent SMSI gives direct evidence of the important role of the support surface structure in SMSI and provides a new way to tune the interaction between metal NPs and the support as well as catalytic activity.

Tuning the shape and crystal phase of TiO2 nanoparticles for catalysis

Synthesis of TiO₂ nanoparticles with tunable shape and crystal phase has attracted considerable attention for the design of highly efficient heterogeneous catalysts. Tailoring the shape of TiO₂, in the crystal phases of anatase, rutile, brookite and TiO₂(B), allows tuning of the atomic configurations on the dominantly exposed facets for maximizing the active sites and regulating the reaction route towards a specific channel for achieving high selectivity. Moreover, the shape and crystal phase of TiO₂ nanoparticles alter their interactions with metal species, which are commonly termed as strong metal-support interactions involving interfacial strain and charge transfer. On the other hand, metal particles, clusters and single atoms interact differently with TiO₂, because of the variation of the electronic structure, while the surface of TiO₂ determines the interfacial bonding via a geometric effect. The dynamic behavior of the metal-titania interfaces, driven by the chemisorption of the reactive molecules at elevated temperatures, also plays a decisive role in elaborating the structure-reactivity relationship.

Deep neural network analysis of nanoparticle ordering to identify defects in layered carbon materials

Smoothness/defectiveness of the carbon material surface is a key issue for many applications, spanning from electronics to reinforced materials, adsorbents and catalysis. Several surface defects cannot be observed with conventional analytic techniques, thus requiring the development of a new imaging approach. Here, we evaluate a convenient method for mapping such "hidden" defects on the surface of carbon materials using 1-5 nm metal nanoparticles as markers. A direct relationship between the presence of defects and the ordering of nanoparticles was studied experimentally and modeled using quantum chemistry calculations and Monte Carlo simulations. An automated pipeline for analyzing microscopic images is described: the degree of smoothness of experimental images was determined by a classification neural network, and then the images were searched for specific types of defects using a segmentation neural network. An informative set of features was generated from both networks: high-dimensional embeddings of image patches and statics of defect distribution.

In situandoperandoelectron microscopy in heterogeneous catalysis-insights into multi-scale chemical dynamics

This review features state-of-the-artin situandoperandoelectron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolvedoperandomeasurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed.

Single-Entity Electrocatalysis of Individual "Picked-and-Dropped" Co3 O4 Nanoparticles on the Tip of a Carbon Nanoelectrode

Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co3 O4 nanoparticle supported on a carbon-based nanoelectrode is determined by employing an efficient SEM-controlled robotic technique for picking and placing a single catalyst particle onto a modified carbon nanoelectrode surface. The stable nanoassembly is microscopically investigated and subsequently electrochemically characterized. The hexagonal-shaped Co3 O4 nanoparticles demonstrate size-dependent electrochemical activity and exhibit very high catalytic activity with a current density of up to 11.5 A cm-2 at 1.92 V (vs. RHE), and a turnover frequency of 532±100 s-1 at 1.92 V (vs. RHE) towards catalyzing the oxygen evolution reaction.