Aberration-corrected scanning transmission electron microscopy in single-atom catalysis: Probing the catalytically active centers

Deep Learning-Enabled STEM Imaging for Precise Single-Molecule Identification in Zeolite Structures

Observing chemical reactions in complex structures such as zeolites involves a major challenge in precisely capturing single-molecule behavior at ultra-high spatial resolutions. To address this, a sophisticated deep learning framework tailored has been developed for integrated Differential Phase Contrast Scanning Transmission Electron Microscopy (iDPC-STEM) imaging under low-dose conditions. The framework utilizes a denoising super-resolution model (Denoising Inference Variational Autoencoder Super-Resolution (DIVAESR)) to effectively mitigate shot noise and thereby obtain substantially clearer atomic-resolved iDPC-STEM images. It supports advanced single-molecule detection and analysis, such as conformation matching and elemental clustering, by incorporating object detection and Density Functional Theory (DFT) configurational matching for precise molecular analysis. the model's performance is demonstrated with a significant improvement in standard image quality evaluation metrics including Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity Index Measure (SSIM). The test conducted using synthetic datasets shows its robustness and extended applicability to real iDPC-STEM images, highlighting its potential in elucidating dynamic behaviors of single molecules in real space. This study lays a critical groundwork for the advancement of deep learning applications within electron microscopy, particularly in unraveling chemical dynamics through precise material characterization and analysis.

Rice HUSK silica: A review from conventional uses to new catalysts for advanced oxidation processes

The rice industry is of great importance worldwide and within the cereal industrialization process, rice husk is obtained as waste, a by-product with various alternative uses, among others, the obtaining of amorphous silica, a covalent oxide with chemical, structural and textural properties suitable for use as catalytic support. This review shows the potential of rice husk silica in the synthesis of heterogeneous catalysts with transition metals for the oxidation of different polluting molecules present in water, as well as the limitations of the catalytic system and the way to overcome them through new synthesis routes, to obtain single atom catalysts - SACs. The main preparation strategies applied for aqueous phase systems are summarized, as well as the studies of single atom catalysts in oxidation reactions of recalcitrant compounds using silica as support and, finally, the perspectives and opportunities regarding this novel topic.

Upcycle polyethylene terephthalate waste by photoreforming: Bifunction of Pt cocatalyst

Upcycle polyethylene terephthalate (PET) waste by photoreforming (PR) is a sustainable and green approach to tackle environmental problems but with challenges to obtain valuable oxidation products and high purity hydrogen simultaneously. Noble metal cocatalysts are essential to enhance the overall PR reaction efficacy. In this work, TiO2 nanotubes (TiO2 NTs) decorated with single Pt atoms (Pt1/TiO2) or Pt nanoparticles (PtNPs/TiO2) are used in the photoreforming reaction (in one batch), and the oxidation products from ethylene glycol (EG, hydrolysed product of PET) in liquid phase and hydrogen are detected. With Pt1/TiO2, EG is oxidized to glyoxal, glyoxylate or lactate, and hydrogen evolution rate (r H2) reaches 51.8 μmol⋅h-1⋅gcat-1, that is 30 times higher than that of TiO2. For PtNPs/TiO2 (size of Pt NPs: 1.97 nm), hydrogen evolution reaches 219.1 μmol⋅h-1⋅gcat-1, but with the oxidation product of acetate only. DFT calculation demonstrates that for Pt NPs, the reaction path for hydrogen evolution is preferred thermodynamically, due to the formation of Schottky junction. On the oxidation of EG, theoretical and spectroscopic analysis suggest that bidentate adsorption of EG at the interface is facile on Pt1/TiO2, compared to that on PtNPs/TiO2 (two Pt sites), but oxidation products, adsorb less strongly, compared to PtNPs/TiO2, that eventually regulates the distribution of oxidation products. The results thus demonstrate the bifunctions of Pt in the PR reaction, i.e., electron transfer mediator for hydrogen evolution and reactive sites for molecules adsorption. The oxidation reaction is dominated by the adsorption-desorption behavior of molecules but the reduction reaction is controlled by the electron transfer. In addition, acidification of pretreated PET alkaline solution achieves separation of pure terephthalic acid (PTA), which further improves the reaction efficiency possibly by offering high density of active sites and acidic environment. Our work thus demonstrates that to upcycle PET plastics, an optimized process can be reached by atomic design of photocatalysts and proper treatment on the plastic wastes.

Realistic Modeling of the Electrocatalytic Process at Complex Solid-Liquid Interface

The rational design of electrocatalysis has emerged as one of the most thriving means for mitigating energy and environmental crises. The key to this effort is the understanding of the complex electrochemical interface, wherein the electrode potential as well as various internal factors such as H-bond network, adsorbate coverage, and dynamic behavior of the interface collectively contribute to the electrocatalytic activity and selectivity. In this context, the authors have reviewed recent theoretical advances, and especially, the contributions to modeling the realistic electrocatalytic processes at complex electrochemical interfaces,  and illustrated the challenges and fundamental problems in this field. Specifically, the significance of the inclusion of explicit solvation and electrode potential as well as the strategies toward the design of highly efficient electrocatalysts are discussed. The structure-activity relationships and their dynamic responses to the environment and catalytic functionality under working conditions are illustrated to be crucial factors for understanding the complexed interface and the electrocatalytic activities. It is hoped that this review can help spark new research passion and ultimately bring a step closer to a realistic and systematic modeling method for electrocatalysis.

Single-Atom Catalysis: Insights from Model Systems

The field of single-atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modeling SAC utilize periodic boundary conditions and low-index facets of an idealized support. Thus, the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modeling, i.e., comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh-vacuum (UHV) environment. In this review, we report the status of the surface-science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular, the surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well as CeO2 and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low-index surfaces of typical supports. In the Perspective section, we discuss the possibility for expanding such studies into other relevant supports.

Single-Atom Nanozymes: Fabrication, Characterization, Surface Modification and Applications of ROS Scavenging and Antibacterial

Nanozymes are nanomaterials with intrinsic natural enzyme-like catalytic properties. They have received extensive attention and have the potential to be an alternative to natural enzymes. Increasing the atom utilization rate of active centers in nanozymes has gradually become a concern of scientists. As the limit of designing nanozymes at the atomic level, single-atom nanozymes (SAzymes) have become the research frontier of the biomedical field recently because of their high atom utilization, well-defined active centers, and good natural enzyme mimicry. In this review, we first introduce the preparation of SAzymes through pyrolysis and defect engineering with regulated activity, then the characterization and surface modification methods of SAzymes are introduced. The possible influences of surface modification on the activity of SAzymes are discussed. Furthermore, we summarize the applications of SAzymes in the biomedical fields, especially in those of reactive oxygen species (ROS) scavenging and antibacterial. Finally, the challenges and opportunities of SAzymes are summarized and prospected.

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.

Single atom catalysis for electrocatalytic ammonia synthesis

This review points out major challenges and outlook of NH3 synthesis via SACs. Summarizing the deficiencies of existing research can help researchers to continuously innovate and improve, and explore new research approaches.

Dual-Single-Atom Tailoring with Bifunctional Integration for High-Performance CO2 Photoreduction

Single-atom photocatalysis has been demonstrated as a novel strategy to promote heterogeneous reactions. There is a diversity of monoatomic metal species with specific functions; however, integrating representative merits into dual-single-atoms and regulating cooperative photocatalysis remain a pressing challenge. For dual-single-atom catalysts, enhanced photocatalytic activity would be realized through integrating bifunctional properties and tuning the synergistic effect. Herein, dual-single-atoms supported on conjugated porous carbon nitride polymer are developed for effective photocatalytic CO₂ reduction, featuring the function of cobalt (Co) and ruthenium (Ru). A series of in situ characterizations and theoretical calculations are conducted for quantitative analysis of structure-performance correlation. It is concluded that the active Co sites facilitate dynamic charge transfer, while the Ru sites promote selective CO₂ surface-bound interaction during CO₂ photoreduction. The combination of atom-specific traits and the synergy between Co and Ru lead to the high photocatalytic CO₂ conversion with corresponding apparent quantum efficiency (AQE) of 2.8% at 385 nm, along with a high turnover number (TON) of more than 200 without addition of any sacrificial agent. This work presents an example of identifying the roles of different single-atom metals and regulating the synergy, where the two metals with unique properties collaborate to further boost the photocatalytic performance.

Emerging Dual-Atomic-Site Catalysts for Efficient Energy Catalysis

Atomically dispersed metal catalysts with well-defined structures have been the research hotspot in heterogeneous catalysis because of their high atomic utilization efficiency, outstanding activity, and selectivity. Dual-atomic-site catalysts (DASCs), as an extension of single-atom catalysts (SACs), have recently drawn surging attention. The DASCs possess higher metal loading, more sophisticated and flexible active sites, offering more chance for achieving better catalytic performance, compared with SACs. In this review, recent advances on how to design new DASCs for enhancing energy catalysis will be highlighted. It will start with the classification of marriage of two kinds of single-atom active sites, homonuclear DASCs and heteronuclear DASCs according to the configuration of active sites. Then, the state-of-the-art characterization techniques for DASCs will be discussed. Different synthetic methods and catalytic applications of the DASCs in various reactions, including oxygen reduction reaction, carbon dioxide reduction reaction, carbon monoxide oxidation reaction, and others will be followed. Finally, the major challenges and perspectives of DASCs will be provided.

Advances and Applications of Atomic-Resolution Scanning Transmission Electron Microscopy

Although scanning transmission electron microscopy (STEM) images of individual heavy atoms were reported 50 years ago, the applications of atomic-resolution STEM imaging became wide spread only after the practical realization of aberration correctors on field-emission STEM/TEM instruments to form sub-Ångstrom electron probes. The innovative designs and advances of electron optical systems, the fundamental understanding of electron–specimen interaction processes, and the advances in detector technology all played a major role in achieving the goal of atomic-resolution STEM imaging of practical materials. It is clear that tremendous advances in computer technology and electronics, image acquisition and processing algorithms, image simulations, and precision machining synergistically made atomic-resolution STEM imaging routinely accessible. It is anticipated that further hardware/software development is needed to achieve three-dimensional atomic-resolution STEM imaging with single-atom chemical sensitivity, even for electron-beam-sensitive materials. Artificial intelligence, machine learning, and big-data science are expected to significantly enhance the impact of STEM and associated techniques on many research fields such as materials science and engineering, quantum and nanoscale science, physics and chemistry, and biology and medicine. This review focuses on advances of STEM imaging from the invention of the field-emission electron gun to the realization of aberration-corrected and monochromated atomic-resolution STEM and its broad applications.

Structure evolution of single-site Pt in a metal-organic framework

Heterogeneous single-metal-site catalyst or single-atom catalyst research has grown rapidly due to the accessibility of modern characterization techniques that can provide invaluable information at the atomic-scale. Herein, we study the structural evolution of isolated single Pt sites incorporated in a metal-organic framework containing bipyridine functional groups using in situ diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule. The structure and electronic properties of the isolated Pt sites are further corroborated by x-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. We find the prerequisite of high temperature He treatment for Pt activation and CO insertion and inquire into the structural transformation of Pt site process by dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance spectroscopy.

Unraveling CO adsorption on model single-atom catalysts

Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper₁, silver₁, gold₁, nickel₁, palladium₁, platinum₁, rhodium₁, and iridium₁ species on Fe₃O₄(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag₁-CO) or weaken it (as for Ni₁-CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.

Construction of Dual-Active-Site Copper Catalyst Containing both CuN3 and CuN4 Sites

Clear recognition and rational construction of suitable active center for specific reaction is always of great significance in designing highly efficient catalysts. Herein, a dual-active-site copper catalyst (DAS-Cu) containing both CuN₃ and CuN₄ sites is reported. Such catalysts show extremely high catalytic performance (yield: up to 97%) toward oxyphosphorylation of alkenes, while catalysts with single active site (CuN₃ or CuN₄ ) are chemically inert in this reaction. Combined with theoretical and experimental results, the different roles of two different Cu active sites in this reaction are further identified. CuN₃ site captures the oxygen and trigger further oxidizing process, while CuN₄ site provides moderate adsorption sites for the protection of phosphonyl radicals. This work deeply discloses the significant cooperated role with two single-atomic sites in one catalytic active center and brings up a valuable clue for the rational design of better-performing heterogeneous catalyst.

A perspective on oxide-supported single-atom catalysts

Single-atom catalysts (SACs) can not only maximize the metal atom utilization efficiency, but also show drastically improved catalytic performance for various important catalytic processes. Insights into the working principles of SACs provide rational guidance to design and prepare advanced catalysts. Many factors have been claimed to affect the performance of SACs, which makes it very challenging to clarify the correlation between the catalytic performance and physicochemical characteristics of SACs. Oxide-supported SACs are one of the most extensively explored systems. In this minireview, some latest developments on the determining factors of the stability, activity and selectivity of SACs on oxide supports are overviewed. Discussed also are the reaction mechanisms for different systems and methods that are employed to correlate the properties with the catalyst structures at the atomic level. In particular, a recently proposed surface free energy approach is introduced to fabricate well-defined modelled SACs that may help address some key issues in the development of SACs in the future.

Theoretical insights into single-atom catalysts

Schematic diagram of theoretical models and applications of single atom catalysts. A review on the theoretical models, intrinsic properties, and the related application of SACs.

Insight into the Microstructure and Deactivation Effects on Commercial NiMo/γ-Al2O3 Catalyst through Aberration-Corrected Scanning Transmission Electron Microscopy

Atom-resolved microstructure variations and deactivation effects on the commercial NiMo/γ-Al2O3 catalysts were revealed by aberration-corrected scanning transmission electron microscope (Cs-STEM) equipped with enhanced energy dispersive X-ray spectroscopy (EDS). Structural information parallel to and vertical to the electron beam provides definitive insight toward an understanding of structure–activity relations. Under the mild to harsher reaction conditions, “fragment” structures (like metal single atoms, metal clusters, and nanoparticles) of commercial NiMo/γ-Al2O3 catalysts, gradually reduces, while MoS2 nanoslabs get longer and thinner. Such a result about active slabs leads to the reduction in the number of active sites, resulting in a significant decrease in activity. Likewise, the average atomic ratio of promoter Ni and Ni/(Mo + S) ratio of slabs decrease from 2.53% to 0.45% and from 0.0788 to 0.0326, respectively, by means of EDS under the same conditions stated above, reflecting the weakening of the promotional effect. XPS result confirms the existence of NixSy species in deactivated catalysts. This could be ascribed to the Ni segregation from active phase. Furthermore, statistical data give realistic coke behaviors associated with the active metals. With catalytic activity decreasing, the coke on the active metals regions tends to increase faster than that on the support regions. This highlights that the commercial NiMo/γ-Al2O3 catalyst during catalysis is prone to produce more coke on the active metal areas.

Remarkable active-site dependent H2O promoting effect in CO oxidation

The interfacial sites of supported metal catalysts are often critical in determining their performance. Single-atom catalysts (SACs), with every atom contacted to the support, can maximize the number of interfacial sites. However, it is still an open question whether the single-atom sites possess similar catalytic properties to those of the interfacial sites of nanocatalysts. Herein, we report an active-site dependent catalytic performance on supported gold single atoms and nanoparticles (NPs), where CO oxidation on the single-atom sites is dramatically promoted by the presence of H₂O whereas on NPs’ interfacial sites the promoting effect is much weaker. The remarkable H₂O promoting effect makes the Au SAC two orders of magnitude more active than the commercial three-way catalyst. Theoretical studies reveal that the dramatic promoting effect of water on SACs originates from their unique local atomic structure and electronic properties that facilitate an efficient reaction channel of CO + OH.

The issue that whether single-atom sites possess similar catalytic properties to the interfacial sites of nanocatalysts remains unresolved. Here, the authors demonstrate a large H₂O promotional effect on CO oxidation over Au single-atom sites due to their unique local atomic structure and electronic properties.

Spectroscopy Identification of the Bimetallic Surface of Metal-Organic Framework-Confined Pt-Sn Nanoclusters with Enhanced Chemoselectivity in Furfural Hydrogenation

Research and development in bimetallic nanoparticles have gained great interest over their monometallic counterparts because of their distinct and unique properties in a wide range of applications such as catalysis, energy storage, and bio/plasmonic imaging. Identification and characterization of these bimetallic surfaces for application in heterogeneous catalysis remain a challenge and heavily rely on advanced characterization techniques such as aberration-corrected electron microscopy and synchrotron X-ray absorption studies. In this article, we have reported a strategy to prepare sub-2 nm bimetallic Pt-Sn nanoclusters confined in the pores of a Zr-based metal-organic framework (MOF). The Pt-Sn nanoclusters encapsulated in the Zr-MOF pores show enhanced chemoselectivity from 51 to 93% in an industrially relevant reaction, furfural hydrogenation to furfuryl alcohol. The presence of bimetallic Pt-Sn surfaces was investigated by a surface-sensitive characterization technique utilizing diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO to probe the bimetallic surface of the encapsulated ultrafine Pt-Sn nanocluster. Complementary techniques such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were also used to characterize the Pt-Sn nanoclusters.

Nanocarbon-Edge-Anchored High-Density Pt Atoms for 3-nitrostyrene Hydrogenation: Strong Metal-Carbon Interaction

Strong metal-support interaction (SMSI) has been widely used to improve catalytic performance and to identify reaction mechanisms. We report that single Pt atoms anchored onto hollow nanocarbon (h-NC) edges possess strong metal-carbon interaction, which significantly modifies the catalytic behavior of the anchored Pt atoms for selective hydrogenation reactions. The strong Pt-C bonding not only stabilizes single Pt atoms but also modifies their electronic structure, tunes their adsorption properties, and enhances activation of reactants. The fabricated Pt₁/h-NC single-atom catalysts (SACs) demonstrated excellent activity for hydrogenation of 3-nitrostyrene to 3-vinylaniline with a turnover number >31,000/h, 20 times higher than that of the best catalyst for such selective hydrogenation reactions reported in the literature. The strategy to strongly anchor Pt atoms by edge carbon atoms of h-NCs is general and can be extended to construct strongly anchored metal atoms, via SMSI, onto surfaces of various types of support materials to develop robust SACs.

Catalysis with Two-Dimensional Materials Confining Single Atoms: Concept, Design, and Applications

Two-dimensional materials and single-atom catalysts are two frontier research fields in catalysis. A new category of catalysts with the integration of both aspects has been rapidly developed in recent years, and significant advantages were established to make it an independent research field. In this Review, we will focus on the concept of two-dimensional materials confining single atoms for catalysis. The new electronic states via the integration lead to their mutual benefits in activity, that is, two-dimensional materials with unique geometric and electronic structures can modulate the catalytic performance of the confined single atoms, and in other cases the confined single atoms can in turn affect the intrinsic activity of two-dimensional materials. Three typical two-dimensional materials are mainly involved here, i.e., graphene, g-C₃N₄, and MoS₂, and the confined single atoms include both metal and nonmetal atoms. First, we systematically introduce and discuss the classic synthesis methods, advanced characterization techniques, and various catalytic applications toward two-dimensional materials confining single-atom catalysts. Finally, the opportunities and challenges in this emerging field are featured on the basis of its current development.

Stability investigation of a high number density Pt1/Fe2O3 single-atom catalyst under different gas environments by HAADF-STEM

Catalysis by supported single metal atoms has demonstrated tremendous potential for practical applications due to their unique catalytic properties. Unless they are strongly anchored to the support surfaces, supported single atoms, however, are thermodynamically unstable, which poses a major obstacle for broad applications of single-atom catalysts (SACs). In order to develop strategies to improve the stability of SACs, we need to understand the intrinsic nature of the sintering processes of supported single metal atoms, especially under various gas environments that are relevant to important catalytic reactions. We report on the synthesis of high number density Pt₁/Fe₂O₃ SACs using a facial strong adsorption method and the study of the mobility of these supported Pt single atoms at 250 °C under various gas environments that are relevant to CO oxidation, water-gas shift, and hydrogenation reactions. Under the oxidative gas environment, Fe₂O₃ supported Pt single atoms are stable even at high temperatures. The presence of either CO or H₂ molecules in the gas environment, however, facilitates the movement of the Pt atoms. The strong interaction between CO and Pt weakens the binding between the Pt atoms and the support, facilitating the movement of the Pt single atoms. The dissociation of H₂ molecules on the Pt atoms and their subsequent interaction with the oxygen species of the support surfaces dislodge the surface oxygen anchored Pt atoms, resulting in the formation of Pt clusters. The addition of H₂O molecules to the CO or H₂ significantly accelerates the sintering of the Fe₂O₃ supported Pt single atoms. An anchoring-site determined sintering mechanism is further proposed, which is related to the metal-support interaction.

Hollow carbon anchored highly dispersed Pd species for selective hydrogenation of 3-nitrostyrene: metal-carbon interaction

Hollow nanocarbon supported Pd species are highly active (TOF of 21 845 h⁻¹), selective (97%), and stable (4 cycles) for selective hydrogenation of 3-nitrostyrene to 3-ethylnitrobenze.