Intrinsic catalytic structure of gold nanoparticles supported on TiO2

Effect of Pt Nanoparticle Morphology on the Aerobic Oxidation of Ethylene Glycol to Glycolic Acid in Water

Pt nanoparticles (diameter <3 nm), generated by metal vapor synthesis and supported on a high surface area carbon, were used to catalyze the aerobic oxidation of ethylene glycol to glycolic acid (GA) in water under neutral and basic reaction conditions. Controlled heat treatment of the catalyst under a nitrogen atmosphere brought about the formation of a morphologically well-defined catalyst. A combination of atomic resolution electron microscopy, CO stripping voltammetry, and XPS analyses conducted on as-synthesized and heat-treated catalysts demonstrated the crucial role of the nanoparticles' morphology on the stabilization of catalytically highly active Pt-OH surface species, which were key species for the Pt-catalyzed oxidation of the alcohol to the carbonyl functionality. The boosting effect of base on the catalyst' s activity and GA selectivity has been proved experimentally (autoclave experiments). The effect of base on the nonmetal-catalyzed reaction steps (i.e., aerobic oxidation of carbonyl to acid functionality) has been proved by DFT calculations.

Simultaneous 2D Projection and 3D Topographic Imaging of Gas-Dependent Dynamics of Catalytic Nanoparticles

Catalyst deactivation through pathways such as sintering of nanoparticles and degradation of the support is a critical factor when designing high-performance catalysts. Here, structural changes of supported nanoparticle catalysts are investigated in controlled gas environments (O2, H2O, and H2) at different temperatures by imaging simultaneously the nanoparticle structures in 2D projection and the 3D surface-sensitive topography. Platinum nanoparticles on carbon support as a model system are imaged in an environmental transmission electron microscope (ETEM), with concurrent acquisition of high-angle annular dark field scanning TEM (HAADF-STEM) and secondary electron (SE) images. Particle migration and coalescence occurs and shows gas-dependent kinetics, with nanoparticles moving across and through the support during and after coalescence. The temperature required for motion is lower in O2 than in H2O and H2, explained through the nature of the gas/nanoparticle interactions. In O2 and H2, the carbon support degrades by trench formation along migration pathways, and the particles move continuously, indicating a chemical reaction between gas and support. In H2O gas, motion is more discontinuous and oriented particle attachment occurs, as expected from theoretical predictions. These results suggest that multimodal imaging in ETEM that combines HAADF-STEM and SE data provides comprehensive information regarding catalyst dynamics and degradation mechanisms.

High-precision charge analysis in a catalytic nanoparticle by electron holography

The charge state of supported metal catalysts is the key to understand the elementary processes involved in catalytic reactions. However, high-precision charge analysis of the metal catalysts at the atomic level is experimentally challenging. To address this critical challenge, high-sensitivity electron holography has recently been successfully applied for precisely measuring the elementary charges on individual platinum nanoparticles supported on a titanium dioxide surface. In this review, we introduce the latest advancements in high-precision charge analysis and discuss the mechanisms of charge transfer at the metal-support interface. The development of charge measurements is entering a new era, and charge analyses under conditions closer to practical working environments, such as real-time, real-space, and reactive gas environments, are expected to be realized in the near future.

Deciphering the Metal-Support Interaction of Au/ZnO Catalyst Induced by H2 and O2 Pretreatment

Metal-support interaction (MSI) provides great possibilities to tune the activity, selectivity, and stability of heterogeneous catalysts. Herein, the Au/ZnO catalyst is prepared by commercial ZnO and chloroauric acid, and the structure evolution of the catalyst pretreated by H2 and O2 gas at varied temperature is investigated to provide mechanistic insights of MSI. It is found that the H2 treatment at 300 °C and above can induce the formation of both the ZnOx overlayer and bulk Au-Zn alloy. In contrast, the O2 treatment can form the ZnOx overlayer at 500 °C and above without the formation of Au-Zn alloy. It is also revealed that the ZnOx overlayer is dynamically stable (permeable), which can provide access for reactant molecules during the reaction process. And, the Au-Zn alloy can recover to Au and ZnO under the CO oxidation reaction condition, which can be deemed as a re-activation process that endows H2 -treated samples with the superior activity and stability.

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.

Structural Size Effect in Capped Metallic Nanoparticles

The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle. First, our electron diffraction measurements reveals that NiPt nanoparticles around 4 nm in diameter covered by a mixture of oleylamine and oleic acid (50:50) display a lattice parameter expansion around 2% when compared to the same particles without surfactant. Using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) techniques, we show that this expansion can not be explained by crystal defects, twinning, oxidation, or atoms insertion. Then, using covered NPs in the 4-22 nm size range, we show that the lattice parameter evolves linearly with the inverse of the NP size, as it is expected when a surface stress is present. Finally, the study is extended to pure nickel and pure platinum NPs, with different sizes, coated by different surfactants (oleylamine, trioctylphosphine, polyvinylpyrrolidone). The surfactants induce lattice parameter variations, whose magnitude could be related to the charge transfer between the surfactant and the particle surface.

Insight into the transient inactivation effect on Au/TiO2 catalyst by in-situ DRIFT and UV–vis spectroscopy

Au catalysts have drawn broad attention for catalytic CO oxidation. However, a molecular-level understanding of the reaction mechanism on a fast time-resolved scale is still lacking. Herein, we apply in situ DRIFTS and UV-Vis spectroscopy to monitor the rapid dynamic changes during CO oxidation over Au/TiO₂. A pronounced transient inactivation effect likely due to a structural change of Au/TiO₂ induced by the reactants (CO and O₂) is observed at the beginning of the reaction. The transient inactivation effect is affected by the ratio of CO and O₂ concentrations. More importantly, during the unstable state, the electronic properties of the Au particles change, as indicated by the shift of the CO stretching vibration. UV-Vis spectroscopy corroborates the structure change of Au/TiO₂ surface induced by the reactants, which leads to a weakening of the Au catalyst’s ability to be oxidized (less O₂ adsorption), resulting in the transient inactivation effect.

A molecular-level understanding of the Au-catalyzed CO oxidation on a fast time-resolved scale is still lacking. Here the authors monitor the rapid dynamic changes during CO oxidation over Au/TiO2 using in situ DRIFTS and UV-Vis spectroscopy, and reveal that the catalyst undergoes a surprising structural change at the beginning of the reaction.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper–Mixed Oxide Interfaces

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu_xMn_1–xO₃ (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu_yPd_x/perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N₂ selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N₂O at low temperatures, enhancing the selectivity toward N₂. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal–mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

Ultra-high thermal stability of sputtering reconstructed Cu-based catalysts

The rational design of high-temperature endurable Cu-based catalysts is a long-sought goal since they are suffering from significant sintering. Establishing a barrier on the metal surface by the classical strong metal-support interaction (SMSI) is supposed to be an efficient way for immobilizing nanoparticles. However, Cu particles were regarded as impossible to form classical SMSI before irreversible sintering. Herein, we fabricate the SMSI between sputtering reconstructed Cu and flame-made LaTiO₂ support at a mild reduction temperature, exhibiting an ultra-stable performance for more than 500 h at 600 °C. The sintering of Cu nanoparticles is effectively suppressed even at as high as 800 °C. The critical factors to success are reconstructing the electronic structure of Cu atoms in parallel with enhancing the support reducibility, which makes them adjustable by sputtering power or decorated supports. This strategy will extremely broaden the applications of Cu-based catalysts at more severe conditions and shed light on establishing SMSI on other metals.

Strong metal-support interactions induced by an ultrafast laser

Supported metal catalysts play a crucial role in the modern industry. Constructing strong metal-support interactions (SMSI) is an effective means of regulating the interfacial properties of noble metal-based supported catalysts. Here, we propose a new strategy of ultrafast laser-induced SMSI that can be constructed on a CeO2-supported Pt system by confining electric field in localized interface. The nanoconfined field essentially boosts the formation of surface defects and metastable CeOx migration. The SMSI is evidenced by covering Pt nanoparticles with the CeOx thin overlayer and suppression of CO adsorption. The overlayer is permeable to the reactant molecules. Owing to the SMSI, the resulting Pt/CeO2 catalyst exhibits enhanced activity and stability for CO oxidation. This strategy of constructing SMSI can be extended not only to other noble metal systems (such as Au/TiO2, Pd/TiO2, and Pt/TiO2) but also on non-reducible oxide supports (such as Pt/Al2O3, Au/MgO, and Pt/SiO2), providing a universal way to engineer and develop high-performance supported noble metal catalysts.

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

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

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

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

Hydrogen spillover-driven synthesis of high-entropy alloy nanoparticles as a robust catalyst for CO2 hydrogenation

High-entropy alloys (HEAs) have been intensively pursued as potentially advanced materials because of their exceptional properties. However, the facile fabrication of nanometer-sized HEAs over conventional catalyst supports remains challenging, and the design of rational synthetic protocols would permit the development of innovative catalysts with a wide range of potential compositions. Herein, we demonstrate that titanium dioxide (TiO₂) is a promising platform for the low-temperature synthesis of supported CoNiCuRuPd HEA nanoparticles (NPs) at 400 °C. This process is driven by the pronounced hydrogen spillover effect on TiO₂ in conjunction with coupled proton/electron transfer. The CoNiCuRuPd HEA NPs on TiO₂ produced in this work were found to be both active and extremely durable during the CO₂ hydrogenation reaction. Characterization by means of various in situ techniques and theoretical calculations elucidated that cocktail effect and sluggish diffusion originating from the synergistic effect obtained by this combination of elements.

Facile fabrication of high-entropy alloys (HEAs) nanoparticles (NPs) on conventional catalyst supports remains challenging. Here the authors show TiO2 is a promising platform for the low-temperature synthesis of supported CoNiCuRuPd HEA NPs with excellent activity and durability in CO2 hydrogenation.

Oxidative Strong Metal-Support Interactions between Metals and Inert Boron Nitride

The strong metal-support interaction (SMSI) is one of the most important concepts in heterogeneous catalysis, which has been widely investigated between metals and active oxides triggered by reductive atmospheres. Here, we report the oxidative strong metal-support interaction (O-SMSI) effect between Pt nanoparticles (NPs) and inert hexagonal boron nitride (h-BN) sheets, in which Pt NPs are encapsulated by oxidized boron (BO_x) overlayers derived from the h-BN support under oxidative conditions. De-encapsulation of Pt NPs has been achieved by washing in water, and the residual ultrathin BO_x overlayers work synergistically with surface Pt sites for enhancing CO oxidation reaction. The O-SMSI effect is also present in other h-BN-supported metal catalysts such as Au, Rh, Ru, and Ir within different oxidative atmospheres including O₂ and CO₂, which is determined by metal-boron interaction and O affinity of metals.

Structure Sensitivity of Au-TiO2 Strong Metal-Support Interactions

Strong metal-support interactions (SMSI) is an important concept in heterogeneous catalysis. Herein, we demonstrate that the Au-TiO₂ SMSI of Au/TiO₂ catalysts sensitively depends on both Au nanoparticle (NP) sizes and TiO₂ facets. Au NPs of ca. 5 nm are more facile undergo Au-TiO₂ SMSI than those of ca. 2 nm, while TiO₂ {001} and {100} facets are more facile than TiO₂ {101} facets. The resulting capsulating TiO_2-x overlayers on Au NPs exhibit an average oxidation state between +3 and +4 and a Au-to-TiO_2-x charge transfer, which, combined with calculations, determines the Ti:O ratio as ca. 6:11. Both TiO_2-x overlayers and TiO_2-x -Au interface exhibit easier lattice oxygen activation and higher intrinsic activity in catalyzing low-temperature CO oxidation than the starting Au-TiO₂ interface. These results advance fundamental understanding of SMSI and demonstrate engineering of metal NP size and oxide facet as an effective strategy to tune the SMSI for efficient catalysis.

Light-Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

Splitting of alcohols into hydrogen and corresponding carbonyl compounds, also called acceptorless alcohol dehydrogenation, is of great significance for both synthetic chemistry and hydrogen production. Light-Driven Alcohol Splitting (LDAS) by heterogeneous photocatalysis is a promising route to achieve such transformations, and it possesses advantages including high selectivity of the carbonyl compounds, extremely mild reaction conditions (room temperature and irradiation of visible light) and easy separation of the photocatalysts from the reaction mixtures. Because a variety of alcohols can be derived from biomass, LDAS can also be regarded as one of the most sustainable approaches for hydrogen production. In this Review, recent advances in the LDAS catalyzed by the heterogeneous photocatalysts are summarized, focusing on the mechanistic insights for the LDAS and aspects that influence the performance of the photocatalysts from viewpoints of metallic co-catalysts, semiconductors, and metal/semiconductor interfaces. In addition, challenges and prospects have been discussed in order to present a complete picture of this field.

Nanometal Thermocatalysts: Transformations, Deactivation, and Mitigation

Nanometals have been proven to be efficient thermocatalysts in the last decades. Their enhanced catalytic activity and tunable functionalities make them intriguing candidates for a wide range of catalytic applications, such as gaseous reactions and compound synthesis/decomposition. On the other hand, the enhanced specific surface energy and reactivity of nanometals can lead to configuration transformation and thus catalytic deactivation during the synthesis and catalysis, which largely undermines the activity and service time, thereby calling for urgent research effort to understand the deactivating mechanisms and develop efficient mitigating methods. Herein, the recent progress in understanding the configuration transformation-induced catalytic deactivation within nanometals is reviewed. The major pathways of configuration transformations, and their kinetics controlled by the environmental factors are presented. The approaches toward mitigating the transformation-induced deactivation are also presented. Finally, a perspective on the future academic approaches toward in-depth understanding of the kinetics of the deactivation of nanometals is proposed.

Electronic Oxide-Metal Strong Interaction (EOMSI)

Strong metal-support interaction of supported metal catalysts is an important concept to describe the effect of metal-support interactions on the structures and catalytic performances of supported metal particles. By using an example of CeO_x adlayers supported on Ag nanocrystals, herein a concept of electronic oxide-metal strong interaction (EOMSI) is put forward; this interaction significantly affects the electronic structures of oxide adlayers through metal-to-oxide charge transfer. The EOMSI can stabilize oxide adlayers in a low oxidation state under ambient conditions, which individually are not stable; moreover, the oxide adlayers experiencing the EOMSI are resistant to high-temperature oxidation in air to a certain extent. Such an EOMSI concept helps to generalize the strong influence of oxide-metal interactions on the structures and catalytic performance of oxide/metal inverse catalysts, which have been attracting increasing attention.

Heterometallic Ag2Ti10 and Ag4Ti8-oxo clusters with different silver doping models: synthesis, structure, and theoretical studies

Two heterometal-oxo Ag2Ti10 (PTC-221) and Ag4Ti8 (PTC-222) clusters were successfully synthesized and characterized, with the doped Ag atoms surrounded by the Ti-O core and exposed to the cluster surface, respectively. Density functional theory (DFT) calculations were carried out to study the electronic structures of PTC-221 and PTC-222, including the frontier orbitals and partial density of states (PDOS). The solid-state UV-vis absorption spectra of PTC-221 and PTC-222 were also recorded. Interestingly, PTC-221 shows intense visible light absorption with an absorption edge around 590 nm and exhibits good photocurrent response in the visible region.

In Situ TEM Studies of Catalysts Using Windowed Gas Cells

For decades, differentially pumped environmental transmission electron microscopy has been a powerful tool to study dynamic structural evolution of catalysts under a gaseous environment. With the advancement of micro-electromechanical system-based technologies, windowed gas cell became increasingly popular due to its ability to achieve high pressure and its compatibility to a wide range of microscopes with minimal modification. This enables a series of imaging and analytical technologies such as atomic resolution imaging, spectroscopy, and operando, revealing details that were unprecedented before. By reviewing some of the recent work, we demonstrate that the windowed gas cell has the unique ability to solve complicated catalysis problems. We also discuss what technical difficulties need to be addressed and provide an outlook for the future of in situ environmental transmission electron microscopy (TEM) technologies and their application to the field of catalysis development.

Size-Dependent Pt-TiO2 Strong Metal-Support Interaction

The strong metal-support interaction (SMSI) is one of the most important concepts in heterogeneous catalysis. Herein we report a study of Pt-TiO₂ SMSI using Pt/rutile TiO₂(110) model catalysts with different Pt particle sizes by means of X-ray photoelectron spectroscopy, ion scattering spectroscopy, and the adsorption of probe molecules. The acquired results unambiguously demonstrate a size dependence of the Pt-TiO₂ SMSI, in which SMSI occurs barely between supported Pt clusters and the TiO₂(110) substrate but obviously between supported Pt nanoparticles and the TiO₂(110) substrate. Such a size-dependent SMSI could be associated with size-dependent electronic structures of the supported Pt particles. These results highlight the sensitivity of the SMSI to the size of supported metal particles, which greatly advances the fundamental understanding.

Dynamic Atom Clusters on AuCu Nanoparticle Surface during CO Oxidation

Supported alloy nanoparticles are prevailing alternative low-cost catalysts for both heterogeneous and electrochemical catalytic processes. Gas molecules selectively interacting with one metal element induces a dynamic structural change of alloy nanoparticles under reaction conditions and largely controls their catalytic properties. However, such a multicomponent dynamic-interaction-controlled evolution, both structural and chemical, remains far from clear. Herein, by using state-of-the-art environmental TEM, we directly visualize, in situ at the atomic scale, the evolution of a AuCu alloy nanoparticle supported on CeO₂ during CO oxidation. We find that gas molecules can "free" metal atoms on the (010) surface and form highly mobile atom clusters. Remarkably, we discover that CO exposure induces Au segregation and activation on the nanoparticle surface, while O₂ exposure leads to the segregation and oxidation of Cu on the particle surface. The as-formed Cu₂O/AuCu interface may facilitate CO-O interaction corroborated by DFT calculations. These findings provide insights into the atomistic mechanisms on alloy nanoparticles during catalytic CO oxidation reaction and to a broad scope of rational design of alloy nanoparticle catalysts.

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.

Shapes of epitaxial gold nanocrystals on SrTiO3 substrates

Morphological control of gold nanocrystals is important as their catalytic and optical properties are highly shape dependent.

Synthesis of gold nanoparticle-loaded magnetic carbon microsphere based on reductive and binding properties of polydopamine for recyclable catalytic applications

A hierarchical nanostructure of Fe₃O₄@C–Au, with Fe₃O₄ as a core and carbon as a shell, was synthesized using a simple method.

Divergent influence of {1 1 1} vs. {1 0 0} crystal planes and Yb3+ dopant on CO oxidation paths in mixed nano-sized oxide Au/Ce1−xYbxO2−x/2 (x = 0 or 0.1) systems

In this paper, the fundamental information on interactions in systems concerning nanocrystalline gold disperses on the shaped (octahedron-like or cube-like) Ce_1−xYb_xO_2−x/2 (x = 0 or 0.1) support has been discussed.

Electronic Metal-Support Interaction-Modified Structures and Catalytic Activity of CeOx Overlayers in CeOx /Ag Inverse Catalysts

Electronic metal-support interactions (EMSIs) of oxide-supported metal catalysts strongly modifies the electronic structures of the supported metal nanoparticles. The strong influence of EMSIs on the electronic structures of oxide overlayers on metal nanoparticles employing cerium oxides/Ag inverse catalysts is reported herein. Ce₂ O₃ overlayers were observed to exclusively form on Ag nanocrystals at low cerium loadings and be resistant to oxidation treatments up to 250 °C, whereas CeO₂ overlayers gradually developed as the cerium loading increased. Ag cubes enclosed by {001} facets with a smaller work function exert a stronger EMSI effect on the CeO_x overlayers than Ag cubes enclosed by {111} facets. Only the CeO₂ overlayers with a fully developed bulk CeO₂ electronic structure significantly promote the catalytic activity of Ag nanocrystals in CO oxidation, whereas cerium oxide overlayers with other electronic structures do not. These results successfully extend the concept of EMSIs from oxide-supported metal catalysts to metal-supported oxide catalysts.

First-Principles Investigation of the Structure and Properties of Au Nanoparticles Supported on ZnO

We present a first-principles investigation of the structure, stability, and reactivity of Au nanoparticles (NPs) supported on ZnO. The morphologies of supported Au NPs are predicted using the formation energy of Au surfaces and the adhesion energy between Au and the dominant ZnO surfaces exposed on ZnO tetrapods. We show how Zn interstitials (a stable intrinsic defect in ZnO) are attracted toward the Au/ZnO interface and in the presence of oxygen can lead to the encapsulation of Au by ZnO, an effect that is observed experimentally. We find that O₂ molecules absorb preferentially at the perimeter of the NP in contact with the ZnO support. These results provide atomistic insight into the structure of ZnO-supported Au NPs with relevance to CO oxidation.

In situ three-dimensional imaging of strain in gold nanocrystals during catalytic oxidation

The chemical properties of materials are dependent on dynamic changes in their three-dimensional (3D) structure as well as on the reactive environment. We report an in situ 3D imaging study of defect dynamics of a single gold nanocrystal. Our findings offer an insight into its dynamic nanostructure and unravel the formation of a nanotwin network under CO oxidation conditions. In situ/operando defect dynamics imaging paves the way to elucidate chemical processes at the single nano-object level towards defect-engineered nanomaterials.

Transformations of supported gold nanoparticles observed by in situ electron microscopy

Oxide supported metal nanoparticles play an important role in heterogeneous catalysis. However, understanding the metal/oxide interface and their evolution under reaction conditions remains challenging. Herein, we investigate the interface between Au nanoparticles and a CeO2 substrate by environmental transmission electron microscopy with atomic resolution. We find that the Au nanoparticles have two preferential epitaxial relationships with the substrate, i.e. Type I (111)[-110]CeO2//(111)[-110]Au and Type II (111)[-110]CeO2//(111)[1-10]Au orientation relationships, where Type I is preferred. In situ observations in the presence of O2 show that the gas can stimulate the supported Au nanoparticles to transform between these two orientations even at room temperature. Moreover, when increasing the temperature to 973 K, the transformation of an Au nanoparticle between the two orientation states and a non-crystalline state in the presence of O2 is also observed. DFT calculations of the binding between Au and CeO2 in the two relationships are strongly influenced by the presence of oxygen vacancies. For a given position of a vacancy, there is a significant energy difference between the energy of the two types. However, for some positions, Type I is preferred, and for others, Type II, but the most favourable position of the vacancy for the two types has a very similar energy. This is consistent with the observation of both types of adhesion.

Oxidation and hydrogenation of Pd: suppression of oxidation by prolonged H2 exposure

We investigate the phase transition of a Pd surface in both oxidizing and reducing environments by environmental transmission electron microscopy (ETEM). ETEM allows us to study sequential exposure of Pd to O2 and H2 in the same TEM conditions. First, under ETEM observation, oxidation occurs at step edges but it can also occur at terraces. Second, as the most important result, we observed a novel process where previous exposure to H2 suppresses new oxidation of the Pd surface. Third, we show by electron energy loss spectroscopy (EELS) that this process, suppression of oxidation by previous exposure to H2, is not due to the formation of bulk β-phase Pd hydride. We also demonstrate that this process is not present in Pt. Finally, we discuss the hypothesis to explain this phenomenon: formation of surface-Pd-hydride suppresses the new oxidation. This observation, suppression of oxidation by H2 exposure, may eventually lead to new breakthroughs.

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.

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.

Interface Engineering of Gold Nanoclusters for CO Oxidation Catalysis

Catalysts based on atomically precise gold nanoclusters serve as an ideal model to relate the catalytic activity to the geometrical and electronic structures as well as the ligand effect. Herein, we investigate three series of ligand (thiolate)-protected gold nanoclusters, including Au₃₈(SR)₂₄, Au₃₆(SR')₂₄, and Au₂₅(SR″)₁₈, with a focus on their interface effects using carbon monoxide (CO) oxidation as a probe reaction. The first comparison is within each series, which reveals the same trend for the three series that, rather than the bulkiness of carbon tails as commonly thought, the steric hindrance of ligands at the interface between the thiolate, Au, and CeO₂ inhibits CO adsorption onto Au sites and hence adversely affects the activity of CO oxidation. The second comparison is between the sets Au₃₈(SR)₂₄ and Au₃₆(SR')₂₄ of nearly the same size, which reveals that the Au₃₆(SR')₂₄ nanoclusters (with face centered cubic structure) are not sensitive to thermal pretreatment conditions, whereas the Au₃₈(SR)₂₄ catalysts (icosahedral structure) are and an optimum activity is observed at a pretreatment temperature of 150 °C. Overall, the atomically precise Au _n(SR) _m nanoclusters have revealed unprecedented details on the catalytic interface and atomic structure effects. It is hoped that such insights will benefit the ultimate goal of catalysis in future design of enzymelike catalysts for environmentally friendly green catalysis.