Simulating Structural Dynamics of Metal Catalysts under Operative Conditions

Structural reconstructions of metal catalysts have been recognized as common phenomena during catalytic reactions, which play a key role in their activities in heterogeneous catalysis. Precisely identifying the structures under the operative conditions becomes a prerequisite to establish a reliable structure-activity relationship and further rationalize the design of metal catalysts. However, real-time capture of the structural variations of catalysts at the atomic level with high-temporal resolution is a grand challenge for present in situ characterizations. During the past decade, significant progress has been made in theory to couple the structures with the reaction conditions to reproduce the experimental observations and predict the adsorbate-induced changes of catalysts in composition, morphology, size, etc. Modeling the dynamic correlation between the structure and activity of the metal catalysts brings us advanced knowledge of heterogeneous catalysis and becomes indispensable for accurate evaluation of the performance of metal catalysts.

Dynamic Active Sites In Situ Formed in Metal Nanoparticle Reshaping under Reaction Conditions

Understanding the nonequilibrium transformation of nanocatalysts under reaction conditions is important because metastable atomic structures may be created during the process, which offers unique activities in reactions. Although reshaping of metal nanoparticles (NPs) under reaction conditions has been widely recognized, the dynamic reshaping process has been less studied at the atomic scale. Here, we develop an atomistic kinetic Monte Carlo model to simulate the complete reshaping process of Pt nanoparticles in a CO environment and reveal the in situ formation of atomic clusters on the NP surface, a new type of active site beyond conventional understanding, boosting the reactivities in the CO oxidation reaction. Interestingly, highly active peninsula and inactive island clusters both form on the (111) facets and interchange in varying states of dynamic equilibrium, which influences the catalytic activities significantly. This study provides new fundamental knowledge of nanocatalysis and new guidance for the rational design of nanocatalysts.

Disproportionation chemistry in K2PtCl4 visualized at atomic resolution using scanning transmission electron microscopy

The direct observation of a solid-state chemical reaction can reveal otherwise hidden mechanisms that control the reaction kinetics. However, probing the chemical bond breaking and formation at the molecular level remains challenging because of the insufficient spatial-temporal resolution and composition analysis of available characterization methods. Using atomic-resolution differential phase-contrast imaging in scanning transmission electron microscopy, we have visualized the decomposition chemistry of K2PtCl4 to identify its transient intermediate phases and their interfaces that characterize the chemical reduction process. The crystalline structure of K2PtCl4 is found to undergo a disproportionation reaction to form K2PtCl6, followed by gradual reduction to crystalline Pt metal and KCl. By directly imaging different Pt─Cl bond configurations and comparing them to models predicted via density functional theory calculations, a causal connection between the initial and final states of a chemical reaction is established, showcasing new opportunities to resolve reaction pathways through atomistic experimental visualization.

In Situ TEM Characterization and Modulation for Phase Engineering of Nanomaterials

Solid-state phase transformation is an intriguing phenomenon in crystalline or noncrystalline solids due to the distinct physical and chemical properties that can be obtained and modified by phase engineering. Compared to bulk solids, nanomaterials exhibit enhanced capability for phase engineering due to their small sizes and high surface-to-volume ratios, facilitating various emerging applications. To establish a comprehensive atomistic understanding of phase engineering, in situ transmission electron microscopy (TEM) techniques have emerged as powerful tools, providing unprecedented atomic-resolution imaging, multiple characterization and stimulation mechanisms, and real-time integrations with various external fields. In this Review, we present a comprehensive overview of recent advances in in situ TEM studies to characterize and modulate nanomaterials for phase transformations under different stimuli, including mechanical, thermal, electrical, environmental, optical, and magnetic factors. We briefly introduce crystalline structures and polymorphism and then summarize phase stability and phase transformation models. The advanced experimental setups of in situ techniques are outlined and the advantages of in situ TEM phase engineering are highlighted, as demonstrated via several representative examples. Besides, the distinctive properties that can be obtained from in situ phase engineering are presented. Finally, current challenges and future research opportunities, along with their potential applications, are suggested.

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.

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.

Identifying the morphology of Pt nanoparticles for the optimal catalytic activity towards CO oxidation

The morphology of nanoparticles (NPs) is crucial for determining their catalytic performance. The dramatic changes in the morphology of metal NPs during reactions observed in many in situ experiments pose great challenges for the identification of the geometry for optimal catalytic activities, which arouses the controversial understanding of the reaction mechanism. In this work, taking CO oxidation as a model reaction, we coupled a multiscale structure reconstruction model with kinetic Monte Carlo simulations to study the catalytic performance of the Pt NPs with changing morphology and reaction conditions. Through the quantitative analysis of contour plots for turnover frequencies, we show that the NPs with more well-coordinated sites exhibit optimal activity under CO-rich conditions at higher temperatures, while the reactivity of NPs with more low-coordination sites is optimal under O₂-rich conditions at lower temperatures. Further analysis indicates that the competitive adsorption of CO and O₂ plays the key role, in which the structure with optimal activity has a closer CO and O coverage. This work not only reconciles the controversy of the active geometry in the experiments, but offers an efficient method to guide the rational design of high-performance catalysts.

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.

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.

Composition-dependent ordering transformations in Pt-Fe nanoalloys

SignificanceDynamically understanding the microscopic processes governing ordering transformations has rarely been attained. The situation becomes even more challenging for nanoscale alloys, where the significantly increased surface-area-to-volume ratio not only opens up a variety of additional freedoms to initiate an ordering transformation but also allows for kinetic interplay between the surface and bulk due to their close proximity. We provide direct evidence of the microscopic processes controlling the ordering transformation through the surface-bulk interplay in Pt-Fe nanoalloys and new features rendered by variations in alloy composition and chemical stimuli. These results provide a mechanistic detail of ordering transformation phenomena which are widely relevant to nanoalloys as chemical ordering occurs in most multicomponent materials under suitable environmental bias.

Structure reconstruction of metal/alloy in reaction conditions: a volcano curve?

Recent in situ works have shown extensive evidence of the dramatic and reversible structure reconstructions of metal and alloy materials in reaction conditions. The reconstructions are of primary interest because they could lead to alternative catalytic mechanisms during real reactions. However, how the catalyst structure evolves under the pressures relevant to industrial applications (>1 atm) is so far unexplored. In our recent works, we have developed multiscale theoretical models to give reliable and precise predictions of the equilibrium shapes of metal nanoparticles and of the segregation properties of alloy surfaces at a given temperature and gas pressure. The theoretical predictions have been successfully used in interoperations of various in situ experimental observations. In this work, we applied these methods to study the detailed structural information of metal NPs and of bimetallic alloys at the temperature from 300 to 1000 K and the gas pressure from 10 to 10⁷ Pa. The results show, in some cases, both the gas-induced shape change and the gas-induced segregation change are maximized when the gas adsorption is 'just right'. The fraction of the low-coordinated sites of the metal NP shows a volcano-like curve with pressure at a constant temperature. A similar volcano shape could also be found in the plot of the environmental segregation energy as functions of temperature and pressure. The similar gas effects at low pressure and at high pressure indicate the structural information obtained in laboratory environments (<1 atm) could be of use to understanding the catalysts structure reconstruction in industrial conditions (>1 atm).

“Storage-Discharge” Ethanol Cold Plasma for Synthesizing High Performance Pd/Al2O3 Catalysts

Atmospheric pressure cold plasma is an environmentally friendly and novel method to synthesize supported metal catalysts, which usually uses active hydrogen species to reduce metal ions. Ethanol is a hydrogen-rich renewable liquid hydrogen source, and it is more convenient to store and transport than H2. In this study, a “storage-discharge” ethanol cold plasma was used to prepare Pd/Al2O3-EP catalysts, and the obtained catalysts are used for CO oxidation. The complete oxidation of CO temperature (T100) over Pd/Al2O3-EP was 145 °C, which was comparable to the performance of Pd/Al2O3-HP that was synthesized by atmospheric pressure hydrogen cold plasma. Pd/Al2O3-EP-C obtained by calcining Pd/Al2O3-EP at 450 °C for 2 h in air atmosphere in order to remove residual carbon species showed much higher CO oxidation activity, and T100 was 130 °C. The Pd/Al2O3 catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron diffraction (XPS), Brunauer–Emmett–Teller (BET), and transmission electron microscopy (TEM), and the structure-performance relationship was analyzed. The results indicate that the “storage-discharge” ethanol cold plasma can reduce the Pd precursor ions into metallic Pd state, and the dissociation of ethanol forms lots of highly active chemisorbed oxygen species, which can enhance the performance of Pd/Al2O3-EP for CO oxidation. In contrast, Pd/Al2O3-EP-C shows much higher CO oxidation activity, which is mainly attributed to the removal of the residual carbon species, and the exposure of more Pd active sites and chemisorbed oxygen species. The “storage-discharge” ethanol cold plasma is a safe and efficient novel method for synthesizing supported Pd catalysts, and it has important potential for the preparation and application of supported metal catalysts.

Structural changes in noble metal nanoparticles during CO oxidation and their impact on catalyst activity

The dynamical structure of a catalyst determines the availability of active sites on its surface. However, how nanoparticle (NP) catalysts re-structure under reaction conditions and how these changes associate with catalytic activity remains poorly understood. Using operando transmission electron microscopy, we show that Pd NPs exhibit reversible structural and activity changes during heating and cooling in mixed gas environments containing O₂ and CO. Below 400 °C, the NPs form flat low index facets and are inactive towards CO oxidation. Above 400 °C, the NPs become rounder, and conversion of CO to CO₂ increases significantly. This behavior reverses when the temperature is later reduced. Pt and Rh NPs under similar conditions do not exhibit such reversible transformations. We propose that adsorbed CO molecules suppress the activity of Pd NPs at lower temperatures by stabilizing low index facets and reducing the number of active sites. This hypothesis is supported by thermodynamic calculations.

Revealing Sintering Kinetics of MoS2-Supported Metal Nanocatalysts in Atmospheric Gas Environments via Operando Transmission Electron Microscopy

The decoration of two-dimensional (2D) substrates with nanoparticles (NPs) serve as heterostructures for various catalysis applications. Deep understanding of catalyst degradation mechanisms during service conditions is crucial to improve the catalyst durability. Herein, we studied the sintering behavior of Pt and bimetallic Au-core Pt-shell (Au@Pt core-shell) NPs on MoS₂ supports at high temperatures under vacuum, nitrogen (N₂), hydrogen (H₂), and air environments by in situ gas-cell transmission electron microscopy (TEM). The key observations are summarized as effect of environment: while particle migration and coalescence (PMC) was the main mechanism that led to Pt and Au@Pt NPs degradation under vacuum, N₂, and H₂ environments, the degradation of MoS₂ substrate was prominent under exposure to air at high temperatures. Pt NPs were less stable in H₂ environment when compared with the Pt NPs under vacuum or N₂, due to Pt-H interactions that weakened the adhesion of Pt on MoS₂. Effect of NP composition: under H₂, the stability of Au@Pt NPs was higher in comparison to Pt NPs. This is because H₂ promotes the alloying of Pt-Au, thus reducing the number of Pt at the surface (reducing H₂ interactions) and increasing Pt atoms in contact with MoS₂. Effect of NP size: The alloying effect promoted by H₂ was more pronounced in small size Au@Pt NPs resulting in their higher sintering resistance in comparison to large size Au@Pt NPs and similar size Pt NPs. The present work provides key insights into the parameters affecting the catalyst degradation mechanisms on 2D supports.

Nanoscale temperature measurement during temperature controlled in situ TEM using Al plasmon nanothermometry

Over recent years, the advent of microelectromechanical system (MEMS)-type microheaters has pushed the limits of temperature controlled in situ transmission electron microscopy (TEM). In particular, by enabling the observation of the structure of materials in their application environments, temperature controlled TEM provides unprecedented insights into the link between the properties of materials and their structure in real-world problems, a clear knowledge of which is necessary for rational development of functional materials with new or improved properties. While temperature is the key parameter in such experiments, accessing the precise temperature of the sample at the nanoscale during observations still remains challenging. In the present work, we have applied aluminium plasmon nanothermometry technique that monitors the temperature dependence of the volume plasmon of Al nanospheres using electron energy loss spectroscopy for in situ local temperature determination over MEMS-type microheaters. With access to local temperatures between room temperature to 550 ^∘C, we have assessed the spatial and temporal stabilities of these microheaters when they operate at different setpoint temperatures both under vacuum and in the presence of a static H₂ gas environment. Temperature comparisons performed under the two environments show discrepancies between local and setpoint temperatures.

Oxide Catalysts with Ultrastrong Resistance to SO2 Deactivation for Removing Nitric Oxide at Low Temperature

Nitrogen oxides are one of the major sources of air pollution. To remove these pollutants originating from combustion of fossil fuels remains challenging in steel, cement, and glass industries as the catalysts are severely deactivated by SO₂ during the low-temperature selective catalytic reduction (SCR) process. Here, a MnO_X /CeO₂ nanorod catalyst with outstanding resistance to SO₂ deactivation is reported, which is designed based on critical information obtained from in situ transmission electron microscopy (TEM) experiments under reaction conditions and theoretical calculations. The catalysts show almost no activity loss (apparent NO_X reaction rate kept unchanged at 1800 µmol g^-1 h^-1 ) for 1000 h test at 523 K in the presence of 200 ppm SO₂ . This unprecedented performance is achieved by establishing a dynamic equilibrium between sulfates formation and decomposition over the CeO₂ surface during the reactions and preventing the MnO_X cluster from the steric hindrance induced by SO₂ , which minimized the deactivation of the active sites of MnO_X /CeO₂ . This work presents the ultralong lifetime of catalysts in the presence of SO₂ , along with decent activity, marking a milestone in practical applications in low-temperature selective catalytic reduction (SCR) of NO_X .

Nanoscopic Porous Iridium/Iridium Dioxide Superstructures (15 nm): Synthesis and Thermal Conversion by In Situ Transmission Electron Microscopy

Porous particle superstructures of about 15 nm diameter, consisting of ultrasmall nanoparticles of iridium and iridium dioxide, are prepared through the reduction of sodium hexachloridoiridate(+IV) with sodium citrate/sodium borohydride in water. The water-dispersible porous particles contain about 20 wt % poly(N-vinylpyrrolidone) (PVP), which was added for colloidal stabilization. High-resolution transmission electron microscopy confirms the presence of both iridium and iridium dioxide primary particles (1-2 nm) in each porous superstructure. The internal porosity (≈58 vol%) is demonstrated by electron tomography. In situ transmission electron microscopy up to 1000 °C under oxygen, nitrogen, argon/hydrogen (all at 1 bar), and vacuum shows that the porous particles undergo sintering and subsequent compaction upon heating, a process that starts at around 250 °C and is completed at around 800 °C. Finally, well-crystalline iridium dioxide is obtained under all four environments. The catalytic activity of the as-prepared porous superstructures in electrochemical water splitting (oxygen evolution reaction; OER) is reduced considerably upon heating owing to sintering of the pores and loss of internal surface area.

Multiscale atomistic simulation of metal nanoparticles under working conditions

With the fast development of in situ experimental methodologies, dramatic structure reconstructions of nanomaterials that only occur under reaction conditions have been discovered in recent years, which are critical for their application in catalysis, biomedicine, and biosensors. A big challenge for theoreticians is thus to establish reliable models to reproduce the experimental observations quantitatively, and further to make predictions beyond experimental conditions. Herein, we briefly summarize the recent theoretical advances involving the quantitative predictions of equilibrium shapes of metal nanoparticles under reaction conditions and the real-time simulations of nanocrystal transformations. The comparisons between the theoretical and experimental results are presented. This minireview not only helps researchers understand the in situ observations at the atomic level, but also is beneficial for prescreening and optimizing the NPs for practical use.

Atomic-scale selectivity of hydrogen for storage sites in Pd nanoparticles at atmospheric pressure

Hydrogen-storage materials are important carriers for a viable hydrogen economy. Despite palladium being the most studied storage material, the hydrogen-storage mechanism of Pd remains ambiguous owing to the lack of atomic-scale evidence of the diffusion and storage of H atoms in its lattice. In the study reported here, this classical process was investigated on the atomic scale using an in situ transmission electron microscope equipped with an atmospheric-pressure sample holder. The expansion of the Pd interplanar spacings was found to comprise three distinct stages during the diffusion of H atoms. Moreover, the expansion in d-spacing of Pd{111} was markedly different from that of Pd{220}. First-principles calculations indicate that H atoms mainly occupy the centers of the tetrahedral cages in the Pd unit cells during the diffusion stage, and they eventually occupy the octahedral cage centers in the equilibrium state. Moreover, H atoms were detected in substantially high densities in defects such as stacking faults and twin boundaries. These observations on the preferred hydrogen-storage domains can help clarify the hydrogen-storage mechanism and offer guidelines on the future design of higher-capacity hydrogen-storage materials.

Morphology evolution of fcc Ru nanoparticles under hydrogen atmosphere

Tuning the morphology and structural evolution of metal nanoparticles to expose specific crystal facets in a certain reaction atmosphere is conducive to designing catalysts with a high catalytic activity. Herein, coverage dependent hydrogen adsorption on seven fcc Ru surfaces was investigated using density functional theory (DFT) calculations. The morphology evolution of the fcc Ru nanoparticles under the reactive environment was further illustrated using the multiscale structure reconstruction (MSR) model, which combines the DFT results with the Fowler-Guggenheim (F-G) adsorption isotherm and the Wulff construction. At constant pressure, the shape of a fcc Ru nanoparticle changes from a rhombic dodecahedron to a truncated octahedron with an increase of the temperature. More importantly, the desired Ru morphology, with abundant open facets, was predicted to occur at a high temperature and low pressure. Our results provide an insightful understanding of the reshaping of Ru nanoparticles during real reactions, which is crucial for its rational design for use as a nanocatalyst.

Surface faceting and compositional evolution of Pd@Au core–shell nanocrystals during in situ annealing

A step-wise transformation process of a Pd@Au nanoparticle both structurally and compositionally was observed. Monte Carlo simulation was used to explain the results.

Insights into Hydrogen Gas Environment-Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable-Energy Positron Annihilation Spectroscopy

Environmental transmission electron microscopy (ETEM) and variable-energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free palladium (Pd) foils and stressed Pd thin films grown on rutile TiO₂ substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures ( p_H < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found, and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H₂ pressure. The comparison of the two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

Unexpected refacetting of palladium nanoparticles under atmospheric N2 conditions

In situ TEM observations and DFT calculations reveal that the “inert” gas N₂ has the ability to modify the structure of metal nanoparticles.