Atomic-Scale Observations of Catalyst Structures under Reaction Conditions and during Catalysis

Machine Learning Molecular Dynamics Shows Anomalous Entropic Effect on Catalysis through Surface Pre-melting of Nanoclusters

Due to the superior catalytic activity and efficient utilization of noble metals, nanocatalysts are extensively used in the modern industrial production of chemicals. The surface structures of these materials are significantly influenced by reactive adsorbates, leading to dynamic behavior under experimental conditions. The dynamic nature poses significant challenges in studying the structure-activity relations of catalysts. Herein, we unveil an anomalous entropic effect on catalysis via surface pre-melting of nanoclusters through machine learning accelerated molecular dynamics and free energy calculation. We find that due to the pre-melting of shell atoms, there exists a non-linear variation in the catalytic activity of the nanoclusters with temperature. Consequently, two notable changes in catalyst activity occur at the respective temperatures of melting for the shell and core atoms. We further study the nanoclusters with surface point defects, i.e. vacancy and ad-atom, and observe significant decrease in the surface melting temperatures of the nanoclusters, enabling the reaction to take place under more favorable and milder conditions. These findings not only provide novel insights into dynamic catalysis of nanoclusters but also offer new understanding of the role of point defects in catalytic processes.

In-situ and wavelength-dependent photocatalytic strain evolution of a single Au nanoparticle on a TiO2 film

Photocatalysis is a promising technique due to its capacity to efficiently harvest solar energy and its potential to address the global energy crisis. However, the structure-activity relationships of photocatalyst during wavelength-dependent photocatalytic reactions remains largely unexplored because it is difficult to measure under operating conditions. Here we show the photocatalytic strain evolution of a single Au nanoparticle (AuNP) supported on a TiO2 film by combining three-dimensional (3D) Bragg coherent X-ray diffraction imaging with an external light source. The wavelength-dependent generation of reactive oxygen species (ROS) has significant effects on the structural deformation of the AuNP, leading to its strain evolution. Density functional theory (DFT) calculations are employed to rationalize the induced strain caused by the adsorption of ROS on the AuNP surface. These observations provide insights of how the photocatalytic activity impacts on the structural deformation of AuNP, contributing to the general understanding of the atomic-level catalytic adsorption process.

In situ visualizing reveals potential drive of lattice expansion on defective support toward efficient removal of nitrogen oxides

As a sustainable and promising approach of removing of nitrogen oxides (NOx), catalytic reduction of NOx with H2 is highly desirable with a precise understanding to the structure-activity relationship of supported catalysts. In particular, the dynamic evolution of support at microscopic scale may play a critical role in heterogeneous catalysis, however, identifying the in situ structural change of support under working condition with atomic precision and revealing its role in catalysis is still a grand challenge. Herein, we visually capture the surface lattice expansion of WO3-x support in Pt-WO3-x catalyst induced by NO in the exemplified reduction of NO with H2 using in situ transmission electron microscopy and first reveal its important role in enhancing catalysis. We find that NO can adsorb on the oxygen vacancy sites of WO3-x and favorably induce the reversible stretching of W-O-W bonds during the reaction, which can reduce the adsorption energy of NO on Pt4 centers and the energy barrier of the rate-determining step. The comprehensive studies reveal that lattice expansion of WO3-x support can tune the catalytic performance of Pt-WO3-x catalyst, leading to 20% catalytic activity enhancement for the exemplified reduction of NO with H2. This work reveals that the lattice expansion of defective support can tune and optimize the catalytic performance at the atomic scale.

Perspective and prospects of in situ transmission/scanning transmission electron microscopy

In situ transmission/scanning transmission electron microscopy (TEM/STEM) measurements have taken a central stage for establishing structure-chemistry-property relationship over the past couple of decades. The challenges for realizing 'a lab-in-gap', i.e. gap between the objective lens pole pieces, or 'a lab-on-chip', to be used to carry out experiments are being met through continuous instrumental developments. Commercially available TEM columns and sample holder, that have been modified for in situ experimentation, have contributed to uncover structural and chemical changes occurring in the sample when subjected to external stimulus such as temperature, pressure, radiation (photon, ions and electrons), environment (gas, liquid and magnetic or electrical field) or a combination thereof. Whereas atomic resolution images and spectroscopy data are being collected routinely using TEM/STEM, temporal resolution is limited to millisecond. On the other hand, better than femtosecond temporal resolution can be achieved using an ultrafast electron microscopy or dynamic TEM, but the spatial resolution is limited to sub-nanometers. In either case, in situ experiments generate large datasets that need to be transferred, stored and analyzed. The advent of artificial intelligence, especially machine learning platforms, is proving crucial to deal with this big data problem. Further developments are still needed in order to fully exploit our capability to understand, measure and control chemical and/or physical processes. We present the current state of instrumental and computational capabilities and discuss future possibilities.

In-situ TEM study from the perspective of holders

During the in situ transmission electron microscopy (TEM) observations, the diverse functionalities of different specimen holders play a crucial role. We hereby provide a comprehensive overview of the main types of holders, associated technologies and case studies pertaining to the widely employed heating and gas heating methods, from their initial developments to the latest advancement. In addition to the conventional approaches, we also discuss the emergence of holders that incorporate a micro-electro-mechanical system (MEMS) chip for in situ observations. The MEMS technology offers a multitude of functions within a single chip, thereby enhancing the capabilities and versatility of the holders. MEMS chips have been utilized in environmental-cell designs, enabling customized fabrication of diverse shapes. This innovation has facilitated their application in conducting in situ observations within gas and liquid environments, particularly in the investigation of catalytic and battery reactions. We summarize recent noteworthy studies conducted using in situ liquid TEM. These studies highlight significant advancements and provide valuable insights into the utilization of MEMS chips in environmental-cells, as well as the expanding capabilities of in situ liquid TEM in various research domains.

A possibility to infer frustrations of supported catalytic clusters from macro-scale observations

Recent experimental and theoretical studies suggest that dynamic active centres of supported heterogeneous catalysts may, under certain conditions, be frustrated. Such out-of-equilibrium materials are expected to possess unique catalytic properties and also higher level of functionality. The latter is associated with the navigation through the free energy landscapes with energetically close local minima. The lack of common approaches to the study of out-of-equilibrium materials motivates the search for specific ones. This paper suggests a way to infer some valuable information from the interplay between the intensity of reagent supply and regularities of product formation.

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.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

Ultra-high vacuum compatible reactor for model catalyst study of ammonia synthesis at ambient pressure

A high sensitivity reactor was developed to study slow reactions, such as ammonia synthesis over low surface area model catalysts at 1 bar and up to 550 °C. The reactor is connected to an ultra-high vacuum system with a transferable sample design, which allows for cleaning, preparation, and spectroscopic characterization of samples before and after the reaction without exposure to any contaminated environment, such as air. A quasi-closed small volume (250 µl) quartz glass reaction cell is integrated through a capillary with a quartz glass sniffer tube connected to a mass spectrometer. The capillary reduces the 1 bar pressure in the cell to 10-7 mbar in the sniffer tube and mass spectrometer chamber. A quartz fiber-guided laser is used to heat up the sample, and the temperature can be regulated by the proportional-integral-derivative controlled laser power output for fast reaction kinetics research. Proof of principle ammonia synthesis experiments in this reactor at 1 bar, 350-500 °C on Fe(111) single crystal and mass-selected Ru clusters supported on CeO2 thin film yield kinetic parameters that agree very well to those reported in the literature.

Size-dependent diffusion of supported metal nanoclusters: mean-field-type treatments and beyond for faceted clusters

Nanostructured systems are intrinsically metastable and subject to coarsening. For supported 3D metal nanoclusters (NCs), coarsening can involve NC diffusion across the support and subsequent coalescence (as an alternative to Ostwald ripening). When used as catalysts, this leads to deactivation. The dependence of diffusivity, DN, on NC size, N (in atoms), controls coarsening kinetics. Traditional mean-field (MF) theory for DNversus N assumes that NC diffusion is mediated by independent random hopping of surface adatoms with low coordination, and predicts that DN ∼ hN-4/3neq. Here, h = ν exp[-Ed/(kBT)] denotes the hop rate, and neq = exp[-Eform/(kBT)] the density of those adatoms. The adatom formation energy, Eform, approaches a finite large-N limit, as does the effective barrier, Eeff = Ed + Eform, for NC diffusion. This MF theory is critically assessed for a realistic stochastic atomistic model for diffusion of faceted fcc metal NCs with a {100} facet epitaxially attached to a (100) support surface. First, the MF formulation is refined to account for distinct densities and hop rates for surface adatoms on different facets and along the base contact line, and to incorporate the exact values of Eform and neqversus N for our model. MF theory then captures the occurrence of local minima in DNversus N at closed-shell sizes, as shown by KMC simulation. However, the MF treatment also displays fundamental shortcomings due to the feature that diffusion of faceted NCs is actually dominated by a cooperative multi-step process involving disassembling and reforming of outer layers on side facets. This mechanism leads to an Eeff which is well above MF values, and which increases with N, features captured by a beyond-MF treatment.

Structure-Property Relationships for Nickel Aluminate Catalysts in Polyethylene Hydrogenolysis with Low Methane Selectivity

Earth-abundant metals have recently been demonstrated as cheap catalyst alternatives to scarce noble metals for polyethylene hydrogenolysis. However, high methane selectivities hinder industrial feasibility. Herein, we demonstrate that low-temperature ex-situ reduction (350 °C) of coprecipitated nickel aluminate catalysts yields a methane selectivity of <5% at moderate polymer deconstruction (25-45%). A reduction temperature up to 550 °C increases the methane selectivity nearly sevenfold. Catalyst characterization (XRD, XAS, 27Al MAS NMR, H2 TPR, XPS, and CO-IR) elucidates the complex process of Ni nanoparticle formation, and air-free XPS directly after reaction reveals tetrahedrally coordinated Ni2+ cations promote methane production. Metallic and the specific cationic Ni appear responsible for hydrogenolysis of internal and terminal C-C scissions, respectively. A structure-methane selectivity relationship is discovered to guide the design of Ni-based catalysts with low methane generation. It paves the way for discovering other structure-property relations in plastics hydrogenolysis. These catalysts are also effective for polypropylene hydrogenolysis.

Direct Observation of Ni Nanoparticle Growth in Carbon-Supported Nickel under Carbon Dioxide Hydrogenation Atmosphere

Understanding nanoparticle growth is crucial to increase the lifetime of supported metal catalysts. In this study, we employ in situ gas-phase transmission electron microscopy to visualize the movement and growth of ensembles of tens of nickel nanoparticles supported on carbon for CO2 hydrogenation at atmospheric pressure (H2:CO2 = 4:1) and relevant temperature (450 °C) in real time. We observe two modes of particle movement with an order of magnitude difference in velocity: fast, intermittent movement (vmax = 0.7 nm s-1) and slow, gradual movement (vaverage = 0.05 nm s-1). We visualize the two distinct particle growth mechanisms: diffusion and coalescence, and Ostwald ripening. The diffusion and coalescence mechanism dominates at small interparticle distances, whereas Ostwald ripening is driven by differences in particle size. Strikingly, we demonstrate an interplay between the two mechanisms, where first coalescence takes place, followed by fast Ostwald ripening due to the increased difference in particle size. Our direct visualization of the complex nanoparticle growth mechanisms highlights the relevance of studying nanoparticle growth in supported nanoparticle ensembles under reaction conditions and contributes to the fundamental understanding of the stability in supported metal catalysts.

Recent advances in in-situ transmission electron microscopy techniques for heterogeneous catalysis

The process of heterogeneous catalytic reaction under working conditions has long been considered a "black box", which is mainly because of the difficulties in directly characterizing the structural changes of catalysts at the atomic level during catalytic reactions. The development of in situ transmission electron microscopy (TEM) techniques offers opportunities for introducing a realistic chemical reaction environment in TEM, making it possible to uncover the mystery of catalytic reactions. In this article, we present a comprehensive overview of the application of in situ TEM techniques in heterogeneous catalysis, highlighting its utility for observing gas-solid and liquid-solid reactions during thermal catalysis, electrocatalysis, and photocatalysis. in situ TEM has a unique advantage in revealing the complex structural changes of catalysts during chemical reactions. Revealing the real-time dynamic structure during reaction processes is crucial for understanding the intricate relationship between catalyst structure and its catalytic performance. Finally, we present a perspective on the future challenges and opportunities of in situ TEM in heterogeneous catalysis.

Challenges in modelling dynamic processes in realistic nanostructured materials at operating conditions

The question is addressed in how far current modelling strategies are capable of modelling dynamic phenomena in realistic nanostructured materials at operating conditions. Nanostructured materials used in applications are far from perfect; they possess a broad range of heterogeneities in space and time extending over several orders of magnitude. Spatial heterogeneities from the subnanometre to the micrometre scale in crystal particles with a finite size and specific morphology, impact the material's dynamics. Furthermore, the material's functional behaviour is largely determined by the operating conditions. Currently, there exists a huge length-time scale gap between attainable theoretical length-time scales and experimentally relevant scales. Within this perspective, three key challenges are highlighted within the molecular modelling chain to bridge this length-time scale gap. Methods are needed that enable (i) building structural models for realistic crystal particles having mesoscale dimensions with isolated defects, correlated nanoregions, mesoporosity, internal and external surfaces; (ii) the evaluation of interatomic forces with quantum mechanical accuracy albeit at much lower computational cost than the currently used density functional theory methods and (iii) derivation of the kinetics of phenomena taking place in a multi-length-time scale window to obtain an overall view of the dynamics of the process. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

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.

Neutron Scattering Studies of Heterogeneous Catalysis

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Time-Resolved Formation and Operation Maps of Pd Catalysts Suggest a Key Role of Single Atom Centers in Cross-Coupling

An approach to the spatially localized characterization of supported catalysts over a reaction course is proposed. It consists of a combination of scanning, transmission, and high-resolution scanning transmission electron microscopy to determine metal particles from arrays of surface nanoparticles to individual nanoparticles and individual atoms. The study of the evolution of specific metal catalyst particles at different scale levels over time, particularly before and after the cross-coupling catalytic reaction, made it possible to approach the concept of 4D catalysis-tracking the positions of catalytic centers in space (3D) over time (+1D). The dynamic behavior of individual palladium atoms and nanoparticles in cross-coupling reactions was recorded with nanometer accuracy via the precise localization of catalytic centers. Single atoms of palladium leach out into solution from the support under the action of the catalytic system, where they exhibit extremely high catalytic activity compared to surface metal nanoparticles. Monoatomic centers, which make up only approximately 1% of palladium in the Pd/C system, provide more than 99% of the catalytic activity. The remaining palladium nanoparticles changed their shape and could move over the surface of the support, which was recorded by processing images of the array of nanoparticles with a neural network and aligning them using automatically detected keypoints. The study reveals a novel opportunity for single-atom catalysis─easier detachment (capture) from (on) the carbon support surface is the origin of superior catalytic activity, rather than the operation of single atomic catalytic centers on the surface of the support, as is typically assumed.

Mechanistic Insights Gained by High Spatial Resolution Reactivity Mapping of Homogeneous and Heterogeneous (Electro)Catalysts

The recent development of high spatial resolution microscopy and spectroscopy tools enabled reactivity analysis of homogeneous and heterogeneous (electro)catalysts at previously unattainable resolution and sensitivity. These techniques revealed that catalytic entities are more heterogeneous than expected and local variations in reaction mechanism due to divergences in the nature of active sites, such as their atomic properties, distribution, and accessibility, occur both in homogeneous and heterogeneous (electro)catalysts. In this review, we highlight recent insights in catalysis research that were attained by conducting high spatial resolution studies. The discussed case studies range from reactivity detection of single particles or single molecular catalysts, inter- and intraparticle communication analysis, and probing the influence of catalysts distribution and accessibility on the resulting reactivity. It is demonstrated that multiparticle and multisite reactivity analyses provide unique knowledge about reaction mechanism that could not have been attained by conducting ensemble-based, averaging, spectroscopy measurements. It is highlighted that the integration of spectroscopy and microscopy measurements under realistic reaction conditions will be essential to bridge the gap between model-system studies and real-world high spatial resolution reactivity analysis.

Size Effects in Gas-phase C-H Activation

The gas-phase clusters reaction permits addressing fundamental aspects of the challenges related to C-H activation. The size effect plays a key role in the activation processes as it may substantially affect both the reactivity and selectivity. In this paper, we reviewed the size effect related to the hydrocarbon oxidation by early transition metal oxides and main group metal oxides, methane activation mediated by late transition metals. Based on mass-spectrometry experiments in conjunction with quantum chemical calculations, mechanistic discussions were reviewed to present how and why the size greatly regulates the reactivity and product distribution.

Single-atom catalysis enabled by high-energy metastable structures

Owing to limited degrees of freedom, the active sites of stable single-atom catalyst (SAC) often have one structure that is energetically much lower than other local-minimum structures. Thus, the SAC adopts one lowest-energy structure (LES) with an overwhelmingly larger proportion than any other high-energy metastable structure (HEMS), and the LES is commonly assumed to be solely responsible for the catalytic performance of an SAC. Herein, we demonstrate with SACs anchored on CeO₂ that the HEMS of an SAC, even though its proportion remains several orders of magnitude lower than the LES throughout the catalytic reaction, can dictate catalysis with extraordinary activity arising from its unique coordination environment and oxidation state. Thus, we unravel the key role of HEMS-enabled catalysis in single-atom catalysis, which shakes the common assumption in the studies of SACs and urges new developments in both experiment and theory to identify and exploit catalysis via HEMSs.

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.

Periodic structural changes in Pd nanoparticles during oscillatory CO oxidation reaction

Nanoparticle (NP) catalysts are ubiquitous in energy systems, chemical production, and reducing the environmental impact of many industrial processes. Under reactive environments, the availability of catalytically active sites on the NP surface is determined by its dynamic structure. However, atomic-scale insights into how a NP surface reconstructs under reaction conditions and the impact of the reconstruction on catalytic activity are still lacking. Using operando transmission electron microscopy, we show that Pd NPs exhibit periodic round-to-flat transitions altering their facets during CO oxidation reaction at atmospheric pressure and elevated temperatures. This restructuring causes spontaneous oscillations in the conversion of CO to CO₂ under constant reaction conditions. Our study reveals that the oscillatory behavior stems from the CO-adsorption-mediated periodic restructuring of the nanocatalysts between high-index-faceted round and low-index-faceted flat shapes. These atomic-scale insights into the dynamic surface properties of NPs under reactive conditions play an important role in the design of high-performance catalysts.

Epitaxial Electrocrystallization of Magnesium via Synergy of Magnesiophilic Interface, Lattice Matching, and Electrostatic Confinement

Rechargeable magnesium batteries are particularly advantageous for renewable energy storage systems. However, the inhomogeneous Mg electrodeposits greatly shorten their cycle life under practical conditions. Herein, the epitaxial electrocrystallization of Mg on a three-dimensional magnesiophilic host is implemented via the synergy of a magnesiophilic interface, lattice matching, and electrostatic confinement effects. The vertically aligned nickel hydroxide nanosheet arrays grown on carbon cloth (abbreviated as "Ni(OH)₂@CC") have been delicately designed, which satisfy the essential prerequisite of a low lattice geometrical misfit with Mg (about 2.8%) to realize epitaxial electrocrystallization. Simultaneously, the ionic crystal nature of Ni(OH)₂ displays a periodic and hillock-like electrostatic potential field over its exposed facets, which can precisely capture and confine the reduced Mg⁰ species onto the local electron-enriched sites at the atomic level. The Ni(OH)₂@CC substrate undergoes sequential Mg-ion intercalation, underpotential deposition, and electrocrystallization processes, during which the uniform, lamellar Mg electrodeposits with a locked crystallographic orientation are formed. Under practical conditions (10 mA cm^-2 and 10 mAh cm^-2), the Ni(OH)₂@CC substrate exhibits stable Mg stripping/plating cycle performances over 600 h, 2 orders of magnitude longer than those of the pristine copper foil and carbon cloth substrates.

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.

Revealing the Origin of Low-Temperature Activity of Ni-Rh Nanostructures during CO Oxidation Reaction with Operando TEM

In bimetallic heterostructured nanoparticles (NPs), the synergistic effect between their different metallic components leads to higher catalytic activity compared to the activity of the individual components. However, how the dynamic changes through which these NPs adopt catalytically active structures during a reaction and how the restructuring affects their activity are largely unknown. Here, using operando transmission electron microscopy, structural changes are studied in bimetallic Ni-Rh NPs, comprising of a Ni core whose surface is decorated with smaller Rh NPs, during a CO oxidation reaction. The direct atomic-scale imaging reveals that, under O₂ -rich conditions, Ni core partially transforms into NiO, forming a (Ni+NiO)-Rh hollow nanocatalyst with high catalytic activity. Under O₂ -poor conditions, Rh NPs alloy with the surface of the core to form a NiRh-alloy surface, and the NPs display significantly lower activity. The theoretical calculations indicate that NiO component that forms only under O₂ -rich conditions enhances the activity by preventing the CO poisoning of the nanocatalysts. The results demonstrate that visualizing the structural changes during reactions is indispensable in identifying the origin of catalytic activity. These insights into the dynamic restructuring of NP catalysts under a reactive environment are critical for the rational design of high-performance nanocatalysts.

Imaging the facet surface strain state of supported multi-faceted Pt nanoparticles during reaction

Nanostructures with specific crystallographic planes display distinctive physico-chemical properties because of their unique atomic arrangements, resulting in widespread applications in catalysis, energy conversion or sensing. Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here, we reveal in situ, in three-dimensions and at the nanoscale, the volume, surface and interface strain evolution of single supported platinum nanocrystals during reaction using coherent x-ray diffractive imaging. Interestingly, identical {hkl} facets show equivalent catalytic response during non-stoichiometric cycles. Periodic strain variations are rationalised in terms of O₂ adsorption or desorption during O₂ exposure or CO oxidation under reducing conditions, respectively. During stoichiometric CO oxidation, the strain evolution is, however, no longer facet dependent. Large strain variations are observed in localised areas, in particular in the vicinity of the substrate/particle interface, suggesting a significant influence of the substrate on the reactivity. These findings will improve the understanding of dynamic properties in catalysis and related fields.

Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here the authors demonstrate how the 3D lattice displacement and strain evolution depend on the crystallographic facets of Pt nanoparticles during CO oxidation reaction, providing new insights in the relationship between facet-related surface strain and chemistry.

Modeling surface spin polarization on ceria-supported Pt nanoparticles

In this work, we employ density functional theory simulations to investigate possible spin polarization of CeO₂-(111) surface and its impact on the interactions between a ceria support and Pt nanoparticles. With a Gaussian type orbital basis, our simulations suggest that the CeO₂-(111) surface exhibits a robust surface spin polarization due to the internal charge transfer between atomic Ce and O layers. In turn, it can lower the surface oxygen vacancy formation energy and enhance the oxide reducibility. We show that the inclusion of spin polarization can significantly reduce the major activation barrier in the proposed reaction pathway of CO oxidation on ceria-supported Pt nanoparticles. For metal-support interactions, surface spin polarization enhances the bonding between Pt nanoparticles and ceria surface oxygen, while CO adsorption on Pt nanoparticles weakens the interfacial interaction regardless of spin polarization. However, the stable surface spin polarization can only be found in the simulations based on the Gaussian type orbital basis. Given the potential importance in the design of future high-performance catalysts, our present study suggests a pressing need to examine the surface ferromagnetism of transition metal oxides in both experiment and theory.

Exploring Blob Detection to Determine Atomic Column Positions and Intensities in Time-Resolved TEM Images with Ultra-Low Signal-to-Noise

Spatially resolved in situ transmission electron microscopy (TEM), equipped with direct electron detection systems, is a suitable technique to record information about the atom-scale dynamics with millisecond temporal resolution from materials. However, characterizing dynamics or fluxional behavior requires processing short time exposure images which usually have severely degraded signal-to-noise ratios. The poor signal-to-noise associated with high temporal resolution makes it challenging to determine the position and intensity of atomic columns in materials undergoing structural dynamics. To address this challenge, we propose a noise-robust, processing approach based on blob detection, which has been previously established for identifying objects in images in the community of computer vision. In particular, a blob detection algorithm has been tailored to deal with noisy TEM image series from nanoparticle systems. In the presence of high noise content, our blob detection approach is demonstrated to outperform the results of other algorithms, enabling the determination of atomic column position and its intensity with a higher degree of precision.

Automated search for optimal surface phases (ASOPs) in grand canonical ensemble powered by machine learning

The surface of a material often undergoes dramatic structure evolution under a chemical environment, which, in turn, helps determine the different properties of the material. Here, we develop a general-purpose method for the automated search of optimal surface phases (ASOPs) in the grand canonical ensemble, which is facilitated by the stochastic surface walking (SSW) global optimization based on global neural network (G-NN) potential. The ASOP simulation starts by enumerating a series of composition grids, then utilizes SSW-NN to explore the configuration and composition spaces of surface phases, and relies on the Monte Carlo scheme to focus on energetically favorable compositions. The method is applied to silver surface oxide formation under the catalytic ethene epoxidation conditions. The known phases of surface oxides on Ag(111) are reproduced, and new phases on Ag(100) are revealed, which exhibit novel structure features that could be critical for understanding ethene epoxidation. Our results demonstrate that the ASOP method provides an automated and efficient way for probing complex surface structures that are beneficial for designing new functional materials under working conditions.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Molecular-Level Insights on Reactive Arrangement in On-Surface Photocatalytic Coupling Reactions Using Tip-Enhanced Raman Spectroscopy

Plasmon-enhanced photocatalytic coupling reactions have been used as model systems in surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) research for decades. However, the role of reactive arrangement on efficiency of these model reactions has remained largely unknown to date often leading to conflicting interpretations of experimental results. Herein, we use an interdisciplinary toolbox of nanoscale TERS imaging in combination with molecular-resolution ambient scanning tunnelling microscopy (STM) and density functional theory (DFT) modeling to investigate the role of reactive arrangement in photocatalytic coupling of 4-nitrobenzenethiol (4-NTP) to p,p'-dimercaptoazobisbenzene on single-crystal and polycrystalline Au surfaces for the first time. TERS imaging with 3 nm resolution clearly revealed a significantly higher catalytic efficiency inside a kinetically driven disordered phase of the 4-NTP adlayer on Au compared to the thermodynamically stable ordered phase. Furthermore, molecular level details of the self-assembled structures in the disordered and ordered phases obtained using ambient high-resolution STM enabled an unambiguous structure-reactivity correlation of photocatalytic coupling. Finally, quantitative mechanistic insights obtained from DFT modeling based on the accurate parameters determined from STM imaging emphatically confirmed that a combination of steric hindrance effect and energetic barrier leads to a lower reaction efficiency in the ordered phase of the 4-NTP adlayer. This fundamental study establishes the first direct structure-reactivity correlation in photocatalytic coupling and highlights the critical role of reactive arrangement in the efficiency of on-surface coupling reactions in heterogeneous catalysis at large.

Investigation of Deoxidation Process of MoO3 Using Environmental TEM

In situ environmental transmission electron microscope (ETEM) could provide intuitive and solid proof for the local structure and chemical evolution of materials under practical working conditions. In particular, coupled with atmosphere and thermal field, the behavior of nano catalysts could be directly observed during the catalytic reaction. Through the change of lattice structure, it can directly correlate the relationship between the structure, size and properties of materials in the nanoscale, and further directly and accurately, which is of great guiding value for the study of catalysis mechanism and the optimization of catalysts. As an outstanding catalytic material in the application of methane reforming, molybdenum oxide (MoO₃)-based materials and its deoxidation process were studied by in situ ETEM method. The corresponding microstructures and components evolution were analyzed by diffraction, high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectrum (EELS) techniques. MoO₃ had a good directional deoxidation process accompanied with the process of nanoparticles crushing and regrowth in hydrogen (H₂) and thermal field. However, in the absence of H₂, the samples would exhibit different structural evolution.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

CO organization at ambient pressure on stepped Pt surfaces: first principles modeling accelerated by neural networks

Step and kink sites at Pt surfaces have crucial importance in catalysis. We employ a high dimensional neural network potential (HDNNP) trained using first-principles calculations to determine the adsorption structure of CO under ambient conditions (T = 300 K and P = 1 atm) on these surfaces. To thoroughly explore the potential energy surface (PES), we use a modified basin hopping method. We utilize the explored PES to identify the adsorbate structures and show that under the considered conditions several low free energy structures exist. Under the considered temperature and pressure conditions, the step edge (or kink) is totally occupied by on-top CO molecules. We show that the step structure and the structure of CO molecules on the step dictate the arrangement of CO molecules on the lower terrace. On surfaces with (111) steps, like Pt(553), CO forms quasi-hexagonal structures on the terrace with the top site preferred, with on average two top site CO for one multiply bonded CO, while in contrast surfaces with (100) steps, like Pt(557), present a majority of multiply bonded CO on their terrace. Short terraced surfaces, like Pt(643), with square (100) steps that are broken by kink sites constrain the CO arrangement parallel to the step edge. Overall, this effort provides detailed analysis on the influence of the step edge structure, kink sites, and terrace width on the organization of CO molecules on non-reconstructed stepped surfaces, yielding initial structures for understanding restructuring events driven by CO at high coverages and ambient pressure.

Chemical Insights into Interfacial Effects in Inorganic Nanomaterials

The interfaces between inorganic functional nanomaterials and their surface modifiers play important roles in determining their chemical and physical properties. In numerous situations, interfaces created by organic ligands or secondary inorganic components on inorganic nanomaterials induce significant effects to promote their performances. However, it still remains challenging to understand those interfacial effects at the molecular level. Herein, strategies via the design of model inorganic nanomaterials with well-defined and detectable interfaces to simplify the investigation of interfacial effects in inorganic nanomaterials are summarized. While atomically precise metal nanoclusters enable "seeing" how organic ligands are coordinated on metal surface to create nanoscale metal-organic interfaces, ultrathin low-dimensional nanomaterials modified with organic ligands make it possible to extract the metal-organic interface structure from the average signal to investigate how steric and electronic effects enhance catalysis. The molecular mechanisms of the interfacial effects in supported metal catalysts are disclosed by designing two unique structures of supported catalysts. The interfacial engineering approach will be further extended to optimize the performance and stability of perovskite solar cells. Finally, a perspective on the development of operando characterization techniques is provided to track the dynamic interfacial structures during catalysis.

Single alloy nanoparticle x-ray imaging during a catalytic reaction

The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts’ dynamics. We report the facet-resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment–dependent chemical composition. We find that the initially Pt-terminated nanoparticle surface gets Rh-enriched under CO oxidation reaction conditions. The local composition is facet orientation dependent, and the Rh enrichment is nonreversible under subsequent CO reduction. Tracking facet-resolved strain and composition under operando conditions is crucial for a rational design of more efficient heterogeneous catalysts with tailored activity, selectivity, and lifetime.

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.

Phase Coexistence and Structural Dynamics of Redox Metal Catalysts Revealed by Operando TEM

Metal catalysts play an important role in industrial redox reactions. Although extensively studied, the state of these catalysts under operating conditions is largely unknown, and assignments of active sites remain speculative. Herein, an operando transmission electron microscopy study is presented, which interrelates the structural dynamics of redox metal catalysts to their activity. Using hydrogen oxidation on copper as an elementary redox reaction, it is revealed how the interaction between metal and the surrounding gas phase induces complex structural transformations and drives the system from a thermodynamic equilibrium toward a state controlled by the chemical dynamics. Direct imaging combined with the simultaneous detection of catalytic activity provides unparalleled structure-activity insights that identify distinct mechanisms for water formation and reveal the means by which the system self-adjusts to changes of the gas-phase chemical potential. Density functional theory calculations show that surface phase transitions are driven by chemical dynamics even when the system is far from a thermodynamic phase boundary. In a bottom-up approach, the dynamic behavior observed here for an elementary reaction is finally extended to more relevant redox reactions and other metal catalysts, which underlines the importance of chemical dynamics for the formation and constant re-generation of transient active sites during catalysis.

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.

The Controllable Reconstruction of Bi-MOFs for Electrochemical CO2 Reduction through Electrolyte and Potential Mediation

Monitoring and controlling the reconstruction of materials under working conditions is crucial for the precise identification of active sites, elucidation of reaction mechanisms, and rational design of advanced catalysts. Herein, a Bi-based metal-organic framework (Bi-MOF) for electrochemical CO₂ reduction is selected as a case study. In situ Raman spectra combined with ex situ electron microscopy reveal that the intricate reconstruction of the Bi-MOF can be controlled using two steps: 1) electrolyte-mediated dissociation and conversion of Bi-MOF to Bi₂ O₂ CO₃ , and 2) potential-mediated reduction of Bi₂ O₂ CO₃ to Bi. The intentionally reconstructed Bi catalyst exhibits excellent activity, selectivity, and durability for formate production, and the unsaturated surface Bi atoms formed during reconstruction become the active sites. This work emphasizes the significant impact of pre-catalyst reconstruction under working conditions and provides insight into the design of highly active and stable electrocatalysts through the regulation of these processes.