Reconstruction of Supported Metal Nanoparticles in Reaction Conditions

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.

Constructing Gradient Orbital Coupling to Induce Reactive Metal-Support Interaction in Pt-Carbide Electrocatalysts for Efficient Methanol Oxidation

Reactive metal-support interaction (RMSI) is an emerging way to regulate the catalytic performance for supported metal catalysts. However, the induction of RMSI by the thermal reduction is often accompanied by the encapsulation effect on metals, which limits the mechanism research and applications of RMSI. In this work, a gradient orbital coupling construction strategy was successfully developed to induce RMSI in Pt-carbide system without a reductant, leading to the formation of L12-PtxM-MCy (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) intermetallic electrocatalysts. Density functional theory (DFT) calculations suggest that the gradient coupling of the d(M)-2p(C)-5d(Pt) orbital would induce the electron transfer from M to C covalent bonds to Pt NPs, which facilitates the formation of C vacancy (Cv) and the subsequent M migration (occurrence of RMSI). Moreover, the good correlation between the formation energy of Cv and the onset temperature of RMSI in Pt-MCx systems proves the key role of nonmetallic atomic vacancy formation for inducing RMSI. The developed L12-Pt3Ti-TiC catalyst exhibits excellent acidic methanol oxidation reaction activity, with mass activity of 2.36 A mgPt-1 in half-cell and a peak power density of 187.9 mW mgPt-1 in a direct methanol fuel cell, which is one of the best catalysts ever reported. DFT calculations reveal that L12-Pt3Ti-TiC favorably weakens *CO absorption compared to Pt-TiC due to the change of the absorption site from Pt to Ti, which accounts for the enhanced MOR performance.

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.

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

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

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

Heterogeneous catalysts undergo thermal- and/or adsorbate-induced dynamic changes under reaction conditions, which consequently modify their catalytic behavior. Hence, it is increasingly crucial to characterize the properties of a catalyst under reaction conditions through the so-called "operando" approach. Operando IR spectroscopy is probably one of the most ubiquitous and versatile characterization methods in the field of heterogeneous catalysis, but its potential in identifying adsorbate- and thermal-induced phenomena is often overlooked in favor of other less accessible methods, such as XAS spectroscopy and high-resolution microscopy. Without detracting from these techniques, and while aware of the enormous value of a multitechnique approach, the purpose of this Review is to show that IR spectroscopy alone can provide relevant information in this field. This is done by discussing a few selected case studies from our own research experience, which belong to the categories of both "single-site"- and nanoparticle-based catalysts.

Defect-Mediated Growth of Crystallographic Shear Plane

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

Strong Metal-Support Interaction Facilitated Multicomponent Alloy Formation on Metal Oxide Support

Multicomponent alloy (MA) contains a nearly infinite number of unprecedented active sites through entropy stabilization, which is a desired platform for exploring high-performance catalysts. However, MA catalysts are usually synthesized under severe conditions, which induce support structure collapse and further deteriorate the synergy between MA and support. We propose that a strong metal-support interaction (SMSI) could facilitate the formation of MA by establishing a tunnel of oxygen vacancy for metal atom transport under low reduction temperature (400-600 °C), which exemplifies the holistic design of MA catalysts without deactivating supports. PtPdCoFe MA is readily synthesized on anatase TiO2 with the help of SMSI, which exhibits good catalytic activity and stability for methane combustion. This strategy demonstrates excellent universality on various supports and multicomponent alloy compositions. Our work not only reports a holistic synthesis strategy for MA synthesis by synergizing unique properties of reducible oxides and the mixing entropy of alloy but also offers a new insight that SMSI plays a vigorous role in the formation of alloy NPs on reducible oxides.

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.

Single Metal Atoms Embedded in the Surface of Pt Nanocatalysts: The Effect of Temperature and Hydrogen Pressure

Embedding energetically stable single metal atoms in the surface of Pt nanocatalysts exposed to varied temperature (T) and hydrogen pressure (P) could open up new possibilities in selective and dynamical engineering of alloyed Pt catalysts, particularly interesting for hydrogenation reactions. In this work, an environmental segregation energy model is developed to predict the stability and the surface composition evolution of 24 Metal M-promoted Pt surfaces (with M: Cu, Ag, Au, Ni, Pd, Co, Rh and Ir) under varied T and P. Counterintuitive to expectations, the results show that the more reactive alloy component (i.e., the one forming the strongest chemical bond with the hydrogen) is not the one that segregates to the surface. Moreover, using DFT-based Multi-Scaled Reconstruction (MSR) method and by extrapolation of M-promoted Pt nanoparticles (NPs), the shape dynamics of M-Pt are investigated under the same ranges of T and P. The results show that under low hydrogen pressure and high temperature ranges, Ag and Au—single atoms (and Cu to a less extent) are energetically stable on the surface of truncated octahedral and/or cuboctahedral shaped NPs. This indicated that coinage single-atoms might be used to tune the catalytic properties of Pt surface under hydrogen media. In contrast, bulk stability within wide range of temperature and pressure is predicted for all other M-single atoms, which might act as bulk promoters. This work provides insightful guides and understandings of M-promoted Pt NPs by predicting both the evolution of the shape and the surface compositions under reaction gas condition.

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.

Unveiling the Au Surface Reconstruction in a CO Environment by Surface Dynamics and Ab Initio Thermodynamics

Surface reconstruction changes the atomic configuration of the metal surface and thus alters its intrinsic physical and chemical properties. Recent in situ experiments have shown a variety of surface reconstructions under reaction conditions, but how to effectively predict and characterize these structures remains challenging. Herein, we combine a DFT-based kinetic Monte Carlo simulation method and ab initio thermodynamics to explore the low-energy configurations of metal surface reconstructions, which takes the surface dynamics under the reactive environment into account. We systematically simulate 13 Au surfaces ((100), (110), (111), (210), (211), (221), (310), (311), (320), (321), (322), (331), and (332)) in the CO environment and identify 19 candidate reconstruction patterns driven by CO adsorption. The breakup of the original surfaces is attributed to the lateral interactions among the nearest-neighboring adsorbates. This work provides an efficient approach to unveil the reconstructed metal surface structures in reactive environments for guiding the experiments.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Anodic Shock-Triggered Exsolution of Metal Nanoparticles from Perovskite Oxide

Nanoparticles decorated electrodes (NDEs) are useful in fuel cells, electrolyzers, water treatment, and chemical synthesis. Here, we show that by rapidly bringing a mixed ionic-electronic conductor outside its electrochemical stability window, one can achieve uniform dispersion of metallic nanoparticles inside its bulk and at the surface and improve its electrocatalytic performance when back under normal functional conditions. Surprisingly, this can happen under anodic as well as cathodic current/voltage shocks in an ABO₃ perovskite oxide, La_0.4Ca_0.4Ti_0.88Fe_0.06Ni_0.06O_3-δ (LCTFN), across a wide range of H₂/O₂ gas environments at 800 °C. One possible mechanism for bulk Fe⁰/Ni⁰ precipitation under anodic shock condition is the incomplete oxygen oxidation (O^2- → O^α-, 0 < α < 2), migration and escape of oxygen to interfaces, and "whiplash" transition-metal reduction due to low electronic conductivity. We show that both cathodic and anodic shocks can produce NDEs to enhance electrocatalytic performance, potentially improving the flexibility of this approach in practical devices.

Advances in the Catalytic Reductive Amination of Furfural to Furfural Amine: The Momentous Role of Active Metal Sites

One-pot synthesis of sustainable primary amines by catalytic reductive amination of bio-based carbonyl compounds with NH₃ and H₂ is emerging as a promising and robust approach. The primary amines, especially furfuryl amine (FUA) derived from furfural (FUR), with a wide range of applications from pharmaceuticals to agrochemicals, have attracted much attention due to their versatility. This Review is majorly comprised of two segments on the reductive amination of FUR to FUA, one with precious (Ru, Pd, Rh) and the other with non-precious (Co, Ni) metals on different supports and in various solvent systems in the presence of NH₃ and H₂ . The active metal sites generated on multiple supports are accentuated with experimental evidence based on CO-diffuse reflectance infrared Fourier-transform spectroscopy, H₂ temperature-programmed reduction, X-ray photoelectron spectroscopy, and calorimetry. Moreover, this Review comprehensively describes the role of acidic and basic support for the metal on the yield of FUA. Overall, this Review provides an insight into how to design and develop an efficiently robust catalyst for the selective reductive amination of a broad spectrum of carbonyl compounds to corresponding amines.

Manipulating Electrocatalytic Activity of Perovskite Oxide Through Electrochemical Treatment

Perovskite oxides are widely used in electrochemical cells, profiting from their excellent accommodation of different elements and structure stability. Here, it is reported that when rapidly exceeding the electrochemical stability window of a perovskite oxide through electrochemical treatment, nanoparticles can dynamically exsolve from the perovskite lattice, yielding a nanoparticle decorated material (NDM) with fascinating particle population and distribution. It is reported that as compared to the NDM produced by chemical gas reduction, electrochemical treatment fabricated NDM shows much better electrochemical performance. At 900 °C, a peak power density (PPD) of 896 mW cm^-2 (more than tenfold enhancement) is obtained for a yttrium stabilized zirconia (YSZ) electrolyte-supported symmetrical cell with La_0.43 Ca_0.37 Ti_0.8 Co_0.1 Fe_0.1 O_{3- δ} (LCTCF) electrode after electrochemical treatment for several minutes, while it only reaches to 210 mW cm^-2 after chemical gas treatment for tens of hours using humidified hydrogen as fuel. The study establishes a new fairyland for tuning the performance of-but not limited to-the electrochemical cells.

Reversing sintering effect of Ni particles on γ-Mo2N via strong metal support interaction

Reversing the thermal induced sintering phenomenon and forming high temperature stable fine dispersed metallic centers with unique structural and electronic properties is one of the ever-lasting targets of heterogeneous catalysis. Here we report that the dispersion of metallic Ni particles into under-coordinated two-dimensional Ni clusters over γ-Mo2N is a thermodynamically favorable process based on the AIMD simulation. A Ni-4nm/γ-Mo2N model catalyst is synthesized and used to further study the reverse sintering effect by the combination of multiple in-situ characterization methods, including in-situ quick XANES and EXAFS, ambient pressure XPS and environmental SE/STEM etc. The under-coordinated two-dimensional layered Ni clusters on molybdenum nitride support generated from the Ni-4nm/γ-Mo2N has been demonstrated to be a thermally stable catalyst in 50 h stability test in CO2 hydrogenation, and exhibits a remarkable catalytic selectivity reverse compared with traditional Ni particles-based catalyst, leading to a chemo-specific CO2 hydrogenation to CO.

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).

Single-Crystal Cobalt Phosphide Nanorods as a High-Performance Catalyst for Reductive Amination of Carbonyl Compounds

The development of metal phosphide catalysts for organic synthesis is still in its early stages. Herein, we report the successful synthesis of single-crystal cobalt phosphide nanorods (Co₂P NRs) containing coordinatively unsaturated Co-Co active sites, which serve as a new class of air-stable, highly active, and reusable heterogeneous catalysts for the reductive amination of carbonyl compounds. The Co₂P NR catalyst showed high activity for the transformation of a broad range of carbonyl compounds to their corresponding primary amines using an aqueous ammonia solution or ammonium acetate as a green amination reagent at 1 bar of H₂ pressure; these conditions are far milder than previously reported. The air stability and high activity of the Co₂P NRs is noteworthy, as conventional Co catalysts are air-sensitive (pyrophorous) and show no activity for this transformation under mild conditions. P-alloying is therefore of considerable importance for nanoengineering air-stable and highly active non-noble-metal catalysts for organic synthesis.

Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

We showed recently that the catalytic efficiency of ammonia synthesis on Fe-based nanoparticles (NP) for Haber-Bosch (HB) reduction of N₂ to ammonia depends very dramatically on the crystal surface exposed and on the doping. In turn, the stability of each surface depends on the stable intermediates present during the catalysis. Thus, under reaction conditions, the shape of the NP is expected to evolve to optimize surface energies. In this paper, we propose to manipulate the shape of the nanoparticles through doping combined with chemisorption and catalysis. To do this, we consider the relationships between the catalyst composition (adding dopant elements) and on how the distribution of the dopant atoms on the bulk and facet sites affects the shape of the particles and therefore the number of active sites on the catalyst surfaces. We use our hierarchical, high-throughput catalyst screening (HHTCS) approach but extend the scope of HHTCS to select dopants that can increase the catalytically active surface orientations, such as Fe-bcc(111), at the expense of catalytically inactive facets, such as Fe-bcc(100). Then, for the most promising dopants, we predict the resulting shape and activity of doped Fe-based nanoparticles under reaction conditions. We examined 34 possible dopants across the periodic table and found 16 dopants that can potentially increase the fraction of active Fe-bcc(111) vs inactive Fe-bcc(100) facets. Combining this reshaping criterion with our HHTCS estimate of the resulting catalytic performance, we show that Si and Ni are the most promising elements for improving the rates of catalysis by optimizing the shape to decrease reaction barriers. Then, using Si dopant as a working example, we build a steady-state dynamical Wulff construction of Si-doped Fe bcc nanoparticles. We use nanoparticles with a diameter of ∼10 nm, typical of industrial catalysts. We predict that doping Si into such Fe nanoparticles at the optimal atomic content of ∼0.3% leads to rate enhancements by a factor of 56 per nanoparticle under target HB conditions.

Construction and evolution of active palladium species on phase-regulated reducible TiO2 for methane combustion

Non-traditional amorphous Pd²⁺ species on the surface of Pd/TiO₂ catalysts facilitate CH₄ combustion, while formed Pd_xTi_1−xO₂ would be detrimental.

Selective Hydrogenation of Cinnamaldehyde over the Stepped and Plane Surface of Pd Nanoparticles with Controlled Morphologies by CO Chemisorption

Carbon monoxide (CO) molecules are attracting attention as capping agents that control the structure of metal nanoparticles. In this study, we aimed to control the shape and surface structure of Pd particles by reducing the supported Pd precursor with CO. The reduction of Pd nanoparticles with CO promoted the exposure of step sites and generated spherical and concave-tetrahedral Pd particles on carbon and SiO₂ supports. On the other hand, conventional H₂-reduced Pd particles show a flattened shape. The preferential exposure of the step sites by the adsorbed CO molecules was supported by the density functional theory-calculated surface energy and the Wulff construction. Morphology- and surface-controlled Pd nanoparticles were used to study the surface structure and morphology effects of Pd nanoparticles on cinnamaldehyde (CAL) hydrogenation. With an increase in the fraction of step sites on Pd nanoparticles, the hydrogenation activity and selectivity of hydrocinnamaldehyde (HCAL) increased. On step sites, the adsorption of the C═C bond of CAL proceeded preferentially, and HCAL was efficiently and selectively generated.

Visualizing H2O molecules reacting at TiO2 active sites with transmission electron microscopy

Imaging a reaction taking place at the molecular level could provide direct information for understanding the catalytic reaction mechanism. We used in situ environmental transmission electron microscopy and a nanocrystalline anatase titanium dioxide (001) surface with (1 × 4) reconstruction as a catalyst, which provided highly ordered four-coordinated titanium “active rows” to realize real-time monitoring of water molecules dissociating and reacting on the catalyst surface. The twin-protrusion configuration of adsorbed water was observed. During the water–gas shift reaction, dynamic changes in these structures were visualized on these active rows at the molecular level.

Theoretically predicted surface morphology of FCC cobalt nanoparticles induced by Ru promoter

The addition of Ru promoter has an important role in tuning the stability of the exposed facets of FCC Co NPs, accompanied by the change of surface morphology.

Capsule-Structured Copper-Zinc Catalyst for Highly Efficient Hydrogenation of Carbon Dioxide to Methanol

To develop a new and efficient CO₂ -to-methanol catalyst is of extreme significance but still remains a challenge. Herein, an innovative indirect two-step strategy is reported to synthesize a highly efficient capsule-structured copper-based CO₂ -to-methanol catalyst (CZA-r@CZM). It consists of a structurally reconstructed millimeter-sized Cu/ZnO/Al₂ O₃ core (CZA-r) with intensified Cu-ZnO interactions, which is made by a facile hydrothermal treatment in an alkaline aqueous solution, and a Cu/ZnO/MgO (CZM) shell prepared by an ethylene glycol-assisted physical coating method. The CZA-r core displays 2.7 times higher CO₂ hydrogenation activity with 2.0 times higher CO selectivity than the previously reported Cu/ZnO/Al₂ O₃ (CZA-p), whereas the CZM shell can efficiently catalyze hydrogenation of the as-formed CO from the CZA-r core to methanol as it passes through the shell. As a result, the developed capsule-structured CZA-r@CZM catalyst exhibits 2.4 times higher CO₂ conversion with 1.8 times higher turnover frequency and 2.3-fold higher methanol space-time yield than the CZA-p catalyst (729.8 vs. 312.6 g_MeOH  kg_cat ^-1  h^-1 ). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) experiments reveal that the CO₂ hydrogenation reaction proceeds through a reverse water-gas shift reaction followed by a CO hydrogenation pathway via an *H₃ CO intermediate. This work not only produces an efficient CO₂ -to-methanol catalyst, but also opens a new avenue for designing superior catalysts for other consecutive transformations.

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 Simulation of Morphology Evolution of Supported Pt Nanoparticles via Interfacial Control

The structural and electronic properties of the interface are critical for the morphology of supported metal nanoparticles and thus the performance in catalysis, photonics, biomedical research, and other areas. To reveal the intrinsic mechanism of the formation of various morphologies, a multiscale simulation strategy is adopted to bridge the macroscopic structures by experimental observations and microscopic properties by theoretical calculations. This strategy incorporates the density functional theory (DFT) for the interaction energy calculation, the molecular dynamics (MD) simulation for the structure evolution, and theoretical model for the correlation with contact angles. The interaction energies between Pt atoms (four-atom clusters) and substrates are applied for the force field parametrization in the following MD simulation. Simulation results show the binding energies and structural properties such as radial distribution function and coordination number for supported metal nanoparticles with various sizes in detail. Notably, the contact angles of supported nanoparticles are well correlated by the strength of metal-support interactions. This work yields guidelines on the structure modulation of supported metal nanoparticles via interfacial control.

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.

Reshaping Dynamics of Gold Nanoparticles under H2 and O2 at Atmospheric Pressure

Despite intensive research efforts, the nature of the active sites for O₂ and H₂ adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i. e., at high pressures and high temperatures. We hereby report real-space observations of the equilibrium shapes of titania-supported Au NPs under O₂ and H₂ at atmospheric pressure using gas transmission electron microscopy. In situ TEM observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O₂ from 400 °C to room temperature. In comparison, no instant change in equilibrium shape is observed under a H₂ environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O₂, atomic oxygen, and H₂ is predicted using a multiscale structure reconstruction model. Excellent agreement between TEM observations and theoretical modeling of Au NPs under O₂ provides strong evidence for the molecular adsorption of oxygen on the Au NPs below 120 °C on specific Au facets, which are identified in this work. In the case of H₂, theoretical modeling predicts no interaction with gold atoms that explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O₂ and H₂ adsorption properties of Au NPs under real working conditions and shows a way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

Visualizing Formation of Intermetallic PdZn in a Palladium/Zinc Oxide Catalyst: Interfacial Fertilization by PdHx

Controllable synthesis of well-defined supported intermetallic catalysts is desirable because of their unique properties in physical chemistry. To accurately pinpoint the evolution of such materials at an atomic-scale, especially clarification of the initial state under a particular chemical environment, will facilitate rational design and optimal synthesis of such catalysts. The dynamic formation of a ZnO-supported PdZn catalyst is presented, whereby detailed analyses of in situ transmission electron microscopy, electron energy-loss spectroscopy, and in situ X-ray diffraction are combined to form a nanoscale understanding of PdZn phase transitions under realistic catalytic conditions. Remarkably, introduction of atoms (H and Zn in sequence) into the Pd matrix was initially observed. The resultant PdH_x is an intermediate phase in the intermetallic formation process. The evolution of PdH_x in the PdZn catalyst initializes at the PdH_x /ZnO interfaces, and proceeds along the PdH_x ⟨111⟩ direction.

Silver/Copper Interface for Relay Electroreduction of Carbon Dioxide to Ethylene

CO₂ electroreduction provides an effective solution on CO₂ emission and greenhouse effect. However, it is a big challenge to produce hydrocarbon fuels with high energy density via the electroreduction of CO₂. Here we report the efficient production of ethylene by constructing Ag-Cu bimetallic catalyst with sharp interface; the Faradaic efficiency for ethylene formation is enhanced to 42%, more than 2 times that of pure Cu catalyst. The high yield of ethylene can be rationalized by the relay catalysis of Ag and Cu component around the Ag/Cu interface.

Direct In Situ TEM Visualization and Insight into the Facet-Dependent Sintering Behaviors of Gold on TiO2

Preventing sintering of supported nanocatalysts is an important issue in nanocatalysis. A feasible way is to choose a suitable support. However, whether the metal-support interactions promote or prevent the sintering has not been fully identified. Now, completely different sintering behaviors of Au nanoparticles on distinct anatase TiO₂ surfaces have been determined by in situ TEM. The full in situ sintering processes of Au nanoparticles were visualized on TiO₂ (101) surface, which coupled the Ostwald ripening and particle migration coalescence. In contrast, no sintering of Au on TiO₂ anatase (001) surface was observed under the same conditions. This facet-dependent sintering mechanism is fully explained by the density function theory calculations. This work not only offers direct evidence of the important role of supports in the sintering process, but also provides insightful information for the design of sintering-resistant nanocatalysts.