Dark-field Scanning Transmission Electron Microscopy Research Articles

Single−atom Co anchored on covalent organic framework for remarkable photocatalytic hydrogen evolution efficiency

Single-atom catalysts (SACs) have attracted extensive attention because of their high atomic utilization and superior catalytic activity. Herein, single-atom Co has anchored on the β-ketoenamine-linked covalent organic framework (Tp-Tta COF) pores and used for remarkable photocatalytic hydrogen(H2) production under Ultraviolet–Visible light (UV–Vis light) irradiation. The single-atom Co is visualized by the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM), and the C3–Co–N1 coordination structure is demonstrated by X-ray absorption fine structure (XAFS) measurements. The modified Co0.4-SAC/Tp-Tta COF shows a high photocatalytic H2 production of 1798.5 μmol g−1 h−1 and the Apparent quantum yield(AQY) is 3.02% at 420 nm. The improvement of the activity and stability of the photocatalyst is mainly attributed to the incorporation of Co single-atom that boosts the separation efficiency of photogenerated electron-hole pair and provides well-dispersed active sites. This study provides valuable insights into the development of UV–Vis light-responsive photocatalysts at the atomic level.

RhRu Bimetallic Oxide Cluster Catalysts for Cross-Dehydrogenative Coupling of Arenes and Carboxylic Acids.

Noble-metal-based bimetallic oxide clusters are promising novel catalysts. In this study, we developed carbon-supported RhRu bimetallic oxide clusters (RhRuOx/C) with a mean diameter of 1.2 nm, which showed remarkable catalytic activity for the cross-dehydrogenative coupling (CDC) of arenes and carboxylic acids with O2 as the sole oxidant. RhRu bimetallic oxide cluster formation was confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and synchrotron X-ray absorption spectroscopy. Kinetic isotope and substituent effects indicated that arene C-H bond cleavage was the rate-determining step and proceeded via electrophilic concerted metalation-deprotonation mechanism, with a carboxylate as an internal base. Density functional theory calculations supported the proposed mechanism and indicated that the active center for C-H bond activation was Rh(V) rather than Rh(III), while Ru enhanced the electrophilicity of the Rh(V) site by decreasing the negative charge of the surrounding oxygen atoms. Electron-rich arenes showed relatively high reactivity for the RhRuOx/C-catalyzed CDC reaction, and both aliphatic and aromatic carboxylic acids were applicable to the reaction. The RhRuOx/C catalyst is promising for the CDC reaction of arenes and carboxylic acids to produce aryl esters. This work promotes the development of noble-metal-based bimetallic oxide clusters for C-H bond activation reactions.

On the shear strain associated with T1 precipitation in Al-Cu-Li(-Mg-Ag) alloys

The formation mechanism and the distribution arrangements of T1 precipitate plates in peak-aged samples of an Al-4Cu-1.2Li-0.3Mg-0.4Ag (wt.%) alloy are studied using atomic-resolution high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM). The formation of a T1 plate of a single unit-cell thickness from the α-Al matrix is found to be associated with a large shear strain that is equivalent to the formation and glide of two different Shockley partial dislocations on two alternate {111}α planes. The T1 precipitate plates tend to cluster into various configurations, including T-shaped and V-shaped configurations comprising two plates of two different T1 variants, and X-shaped configurations consisting of three or four plates of two different variants. The signature of the shear strain is clearly visible in these configurations. A single T1 plate of one unit-cell thickness is elastically distorted in a T-shaped configuration, and the two T1 plates of the same variant in an X-shaped configuration are displaced with respect to each other. Different types of stair-rods are also formed in the intersection of the paired T1 plates in the V-shaped configurations.

A new high-pressure die casting Mg−Gd−Sm−Al alloy with high strength-ductility synergy

A high-pressure die casting (HPDC) Mg−5Gd−1.5Sm−0.7Al alloy was newly developed and exhibits outstanding strength-ductility synergy, with the yield strength and the tensile elongation to fracture being approximately 200 MPa and 8.5%, respectively. This alloy has two types of α-Mg grains: coarse α1-Mg ((46 ± 18) μm) and fine α2-Mg ((9.2 ± 2.3) μm) grains, and various Al-GS (GS = Gd and Sm) particles located at grain boundaries while clear solute-atom segregation near grain boundaries with limited or free intermetallic particles. Characterizations using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) indicate the crystal structures of Al-GS phases. After aging, dense β’ precipitates and chain-shaped β’’-like structures precipitated near grain boundaries while a high density of ultrafine β’’-(Mg,Al)3Sm precipitates and Al-GS clusters formed in grain center. Relatively fine grains, Al-GS primary particles, solute-atom segregation near grain boundaries, and/or multiple precipitates contribute to the high strength of the studied alloy, while the multi-scale α-Mg grains, variety of intermetallic particles but discontinuous skeleton, and the multi-typed precipitated lead to its satisfactory ductility.

Ferroelectric behavior arising from polar topologies in epitaxially strained SrTiO3-δ ultrathin films

In this work, we show that epitaxially strained SrTiO3-δ thin films, grown by ion-beam sputtering onto (001)Nb:SrTiO3 substrates, exhibit a ferroelectric behavior. At the atomic-scale, through high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, it was possible to identify the presence of polar nanoregions with non-trivial polar topological structures in the SrTiO3-δ film, which are induced through oxygen vacancies. To further confirm the presence of strained regions with a polar structure, Raman spectroscopy and high-resolution X-ray diffraction were employed and it was possible to confirm the presence of a tetragonal structure in the SrTiO3-δ film, with a tetragonality ratio (c/a) of 1.005. Scanning probe microscopy and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior with maximum polarization of ∼1.5 μC/cm2, remnant polarization of ∼0.4 μC/cm2 and coercive field of ∼0.3 MV/cm. This work opens a window for exploring novel polar topological effects in sub-10 nm thin film materials for non-volatile memory application.

Complementary nanodomain structures in topologically close-packed precipitates of Al-Zn-Mg-Cu alloy

High-strength aluminium alloys attract continuous research interest, owing to their wide use in aerospace and terrestrial transportation industries. The performance of the alloys is essentially determined by nanoscale precipitates, thus understanding the structure and formation mechanism of the precipitates during thermal processing is of great importance. Here, we reveal complementary domain structures in precipitates in a high-strength Al-Zn-Mg-Cu alloy using a combination of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. Through the transfer of fivefold symmetry polyhedra at domain boundaries, the intermediate precipitates develop two sets of highly ordered and structurally complementary nanoscale domains in their nucleation and growth. This new type of atom packing maintains the topologically close-packed structure inside the precipitates while reducing the lattice misfit with the Al matrix and the barrier of structural transformations on two interfaces, demonstrating good thermal stability. Our discoveries lead to insight into the precipitate evolution in the Al-Zn-Mg-Cu alloy system and can help understand other precipitation processes of complex structured phases. Furthermore, the evolution process demonstrates an advanced mode of building metastable architectures with nanoscale interlocking pieces in alloy matrixes, which may inspire strategies to obtain high-strength, lightweight and thermostable alloys.

Homogeneous distribution of highly dispersed Pt species over O and P co-doped g-C3N4 and its superior photocatalytic H2 evolution activity

In this study, we synthesized O and P co-doped graphitic C3N4 (OPCN) materials by a one-step calcination method and prepared Pt/OPCN photocatalysts for the photocatalytic H2 evolution reaction. Among the Pt/OPCN catalysts with various O and P contents, Pt/OPCN2 exhibited the highest H2 evolution rate of 2198 μmol·g−1 (AQE = 7.33 %), which was 1.65 times higher than that of Pt/CN (1330 μmol·g−1). Moreover, Pt/OPCN2 demonstrated excellent photocatalytic properties, such as suitable band gap (2.50 eV) for visible light and efficient charge separation of photoexcited electron–hole pairs, which was confirmed by ultraviolet-visible spectroscopy, electrochemical impedance spectroscopy, photoluminescence spectroscopy, and time-resolved photoluminescence measurements. Fundamentally, the Pt species effectively interacted with neighboring PO and CO functional groups that optimally located on defective OPCN2, resulting in a homogeneous formation of highly dispersed PtO evenly distributed over Pt/OPCN2, as confirmed by X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, and density functional theory calculations. However, excessive O and P contents of OPCN3 damaged the morphological structure of g-C3N4, inducing a poor interaction between the Pt species and OPCN3 and forming isolated Pt nanoparticles, resulting in the low photocatalytic activity of Pt/OPCN3.

Hidden Impurities Generate False Positives in Single Atom Catalyst Imaging.

Single-atom catalysts (SACs) are an emerging class of materials, leveraging maximum atom utilization and distinctive structural and electronic properties to bridge heterogeneous and homogeneous catalysis. Direct imaging methods, such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, are commonly applied to confirm the atomic dispersion of active sites. However, interpretations of data from these techniques can be challenging due to simultaneous contributions to intensity from impurities introduced during synthesis processes, as well as any variation in position relative to the focal plane of the electron beam. To address this matter, this paper presents a comprehensive study on two representative SACs containing isolated nickel or copper atoms. Spectroscopic techniques, including X-ray absorption spectroscopy, were employed to prove the high metal dispersion of the catalytic atoms. Employing scanning transmission electron microscopy imaging combined with single-atom-sensitive electron energy loss spectroscopy, we scrutinized thin specimens of the catalysts to provide an unambiguous chemical identification of the observed single-atom species and thereby distinguish impurities from active sites at the single-atom level. Overall, the study underscores the complexity of SACs characterization and establishes the importance of the use of spectroscopy in tandem with imaging at atomic resolution to fully and reliably characterize single-atom catalysts.

Atomic-level investigation of structure and formation process of Y3Al5O12 with super-high content of Ce emitting anomalous orange-red emissions

Ce-doped yttrium aluminum garnet (YAG:Ce, Y3Al5O12:Ce) is the most commonly used yellow phosphor in white light-emitting diodes. Recently, we found that (Y1–xCex)3Al5O12:Ce with a super-high content of Ce (x = 0.11–0.21) can be prepared by a polymerized complex method and exhibits anomalous orange–red emission. To understand the nature of this anomalous behavior from the atomic level, we applied high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDS), a state-of-the-art technique that enables analysis of the atomic-level elemental distributions of Al, Y, and Ce. STEM-EDS images on the atomic level were successfully obtained for YAG:Ce. It was found that Ce in Y site has spatially uneven for few nanometers. Ce–K edge extended X-ray absorption fine structure (EXAFS) analysis was also conducted, which revealed a high level of asymmetry around Ce. By combining the STEM-EDS and EXAFS results, a dynamic model for the red emission was proposed. STEM-EDS also revealed that CeO2 nanocrystals were firstly formed with low-temperature annealing and that the reaction of Y with the CeO2 nanocrystals subsequently yielded YAG:Ce, unlike in the conventional one-step high-temperature synthesis.

Electrodeposition of Cu with Amino Acids toward Electrocatalytic Enhancement of CO2 Reduction Reaction

This study investigates the effect of five amino acids on the electrodeposition of Cu to enhance its electrocatalytic performance in CO2 reduction reactions (CO2RR). The amino acids significantly influenced the deposition potential, crystallite size, and surface morphology of the electrodeposited Cu. Electrodeposited Cu with amino acids exhibit significantly smaller crystallites and higher particle density on carbon paper relative to amino acid-free samples. The integration of amino acids into the electrodeposited Cu was confirmed via high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy and hard X-ray photoelectron spectroscopy. All electrodeposited Cu exhibits a higher faradaic efficiency (FE) in the electrochemical reduction of CO2 to CH4 relative to Cu foil (24.2%), regardless of the presence or absence (55.0%) of amino acids when the electrolysis was conducted at −1.27 V vs RHE. Electrodeposited Cu with L-histidine, containing an imidazole group, demonstrates a higher FE of CH4 (67.6%) and effectively suppressed the hydrogen evolution reaction, highlighting the crucial role of amino acid functional groups, particularly imidazole, in augmenting the electrochemical conversion of CO2 to CH4. The study demonstrates the critical influence of specific functional groups in amino acids on the catalytic efficiency of electrodeposited Cu in CO2RR electrocatalysis applications.

Доменная структура в нестехиометрических сплавах Гейслера на основе Ni-Mn-In

The relationship between antiphase boundaries (APBs), antiphase domains (APDs) and the functional properties of alloys of the Ni-Mn-In system has been studied. The density of APBs was controlled using various thermal treatments. The fine domain structure was observed by transmission electron microscopy (TEM), high-angle dark-field scanning transmission electron microscopy (HAADF-STEM), and electron backscatter diffraction (EBSD). The results obtained are discussed in the context of the possibility of modeling and designing materials with specified characteristics and properties.

Microstructural Evolution and Tensile Properties of Al-Si Piston Alloys during Long-Term Thermal Exposure

The present study investigated microstructural evolution and changes in tensile properties of an Al-Si piston alloy subjected to thermal exposures at 250 and 350 °C for 150, 300, and 500 h. Microstructural and nanoscale precipitates were characterized using a combination of high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images and scanning electron microscopy (SEM). The tensile testing was performed. The results demonstrated that the thermal exposure induced granulation of the δ-Al3CuNi particles, alongside precipitation of the θ-Al2Cu phase particles and AlCu clusters within the matrix. Specifically, an increase in the size and number density of the θ-Al2Cu phase particles was observed with exposure time at 250 °C. Conversely, at 350 °C, the θ-Al2Cu particles exhibited a gradual increase in size with prolonged thermal exposure, coupled with a decrease in their number density. AlCu clusters precipitated solely at a thermal exposure temperature of 350 °C, with precipitation intensifying over time. Moreover, a decrease in the alloy’s tensile strength and an increase in elongation were noted after thermal exposure. Finally, the present study discussed the precipitation mechanisms of θ-Al2Cu particles and AlCu clusters within the grains, suggesting that the AlCu clusters exerted a more effective strengthening effect compared to the θ-Al2Cu particles.

Universal Synthesis of Single-Atom Catalysts by Direct Thermal Decomposition of Molten Salts for Boosting Acidic Water Splitting.

Single-atom catalysts (SACs) are considered prominent materials in the field of catalysis due to their high metal atom utilization and selectivity. However, the wide-ranging applications of SACs remain a significant challenge due to their complex preparation processes. Here, a universal strategy is reported to prepare a series of noble metal single atoms on different non-noble metal oxides through a facile one-step thermal decomposition of molten salts. By using a mixture of non-noble metal nitrate and a small-amount noble metal chloride as the precursor, noble metal single atoms can be easily introduced into the non-noble metal oxide lattice owing to the cation exchange in the in situ formed molten salt, followed by the thermal decomposition of nitrate anions during the heating process. Analyses using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure spectroscopy confirm the formation of the finely dispersed single atoms. Specially, the as-synthesized Ir single atoms (10.97 wt%) and Pt single atoms (4.60 wt%) on the Co3O4 support demonstrate outstanding electrocatalytic activities for oxygen evolution reaction and hydrogen evolution reaction, respectively.

Scaling-Up Microwave-Assisted Synthesis of Highly Defective Pd@UiO-66-NH2 Catalysts for Selective Olefin Hydrogenation under Ambient Conditions.

The need to develop green and cost-effective industrial catalytic processes has led to growing interest in preparing more robust, efficient, and selective heterogeneous catalysts at a large scale. In this regard, microwave-assisted synthesis is a fast method for fabricating heterogeneous catalysts (including metal oxides, zeolites, metal-organic frameworks, and supported metal nanoparticles) with enhanced catalytic properties, enabling synthesis scale-up. Herein, the synthesis of nanosized UiO-66-NH2 was optimized via a microwave-assisted hydrothermal method to obtain defective matrices essential for the stabilization of metal nanoparticles, promoting catalytically active sites for hydrogenation reactions (760 kg·m-3·day-1 space time yield, STY). Then, this protocol was scaled up in a multimodal microwave reactor, reaching 86% yield (ca. 1 g, 1450 kg·m-3·day-1 STY) in only 30 min. Afterward, Pd nanoparticles were formed in situ decorating the nanoMOF by an effective and fast microwave-assisted hydrothermal method, resulting in the formation of Pd@UiO-66-NH2 composites. Both the localization and oxidation states of Pd nanoparticles (NPs) in the MOF were achieved using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively. The optimal composite, loaded with 1.7 wt % Pd, exhibited an extraordinary catalytic activity (>95% yield, 100% selectivity) under mild conditions (1 bar H2, 25 °C, 1 h reaction time), not only in the selective hydrogenation of a variety of single alkenes (1-hexene, 1-octene, 1-tridecene, cyclohexene, and tetraphenyl ethylene) but also in the conversion of a complex mixture of alkenes (i.e., 1-hexene, 1-tridecene, and anethole). The results showed a powerful interaction and synergy between the active phase (Pd NPs) and the catalytic porous scaffold (UiO-66-NH2), which are essential for the selectivity and recyclability.

Nanoscale mapping of residual stresses in Al 2024 alloys using correlative and multimodal scanning transmission electron microscopy

A methodology for the mapping of residual stresses in metal alloys has been developed by analyzing an isotropic and homogeneous Al2024 alloy with scanning transmission electron microscopy (STEM), combined with diffraction (4DSTEM) and electron energy loss spectroscopy (STEM-EELS) techniques of TEM. The investigations on the alloy's microstructure and elemental distributions were also carried out with conventional dark-field STEM (DFSTEM) and X-ray energy dispersive (EDS) techniques, respectively. Using the STEM-EELS technique, the Young's modulus (YM) is mapped in the (001) plane of the Al alloy in the same regions where the residual strain maps are generated in [1‾ 00] and [010] directions by using 4DSTEM technique. The YM vs. residual strain plot for the Al 2024 alloy revealed that the value of YM decreased by about ∼ 7 % after the tensile residual strain reached 0.02 %. Whereas such a decrease in YM happens after the compressively residual strain reaches −0.015 %. The residual stress maps were also obtained in accordance with the Hooke's law i.e., by multiplying YM map with the corresponding residual strain maps.

Sampling Real-Time Atomic Dynamics in Metal Nanoparticles by Combining Experiments, Simulations, and Machine Learning.

Even at low temperatures, metal nanoparticles (NPs) possess atomic dynamics that are key for their properties but challenging to elucidate. Recent experimental advances allow obtaining atomic-resolution snapshots of the NPs in realistic regimes, but data acquisition limitations hinder the experimental reconstruction of the atomic dynamics present within them. Molecular simulations have the advantage that these allow directly tracking the motion of atoms over time. However, these typically start from ideal/perfect NP structures and, suffering from sampling limits, provide results that are often dependent on the initial/putative structure and remain purely indicative. Here, by combining state-of-the-art experimental and computational approaches, how it is possible to tackle the limitations of both approaches and resolve the atomistic dynamics present in metal NPs in realistic conditions is demonstrated. Annular dark-field scanning transmission electron microscopy enables the acquisition of ten high-resolution images of an Au NP at intervals of 0.6 s. These are used to reconstruct atomistic 3D models of the real NP used to run ten independent molecular dynamics simulations. Machine learning analyses of the simulation trajectories allow resolving the real-time atomic dynamics present within the NP. This provides a robust combined experimental/computational approach to characterize the structural dynamics of metal NPs in realistic conditions.

Characteristics of Oxide Films on Zr702 and Their Corrosion Performance in Boiling Fluorinated Nitric Acid

Fluoride ions, which interfere with the oxide formation on zirconium have been over-looked until recently. The effect of fluoride ions on oxide formation and dissolution behaviors in zirconium was investigated in this study. A detailed quantitative characterization of the oxide films formed on Zr702 immersed in a fluorinated nitric acid solution was performed using X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and representative high-angle annular dark-field scanning Transmission Electron Microscope, (TEM). The corrosion performance in a fluorinated nitric acid solution was discussed. The results reveal that the thickness of the oxide films immersed in the fluorinated nitric acid solution was between 42–48 nm, which is much thinner than that of the oxide layer (~98.85 nm thickness) in the F− free HNO3 solution. The oxide film was identified to be a nanocrystalline cluster, comprised of outermost HfO2 and HfF4 layers, sub-outer ZrO2 and ZrF4 layers, and an innermost Zr (F, O)3.6 layer. This fluoride species penetration through the oxide films indicated that the fluoride ions are responsible for the dissolution of the oxide film of Zr702.

Insights into the Construction of Robust Pt Clusters with Satisfactory Stability on CeO2 for the Catalytic Oxidation of CO.

Improving the efficiency of platinum group metals (Pt, Pd, Rh, etc.) in catalytic oxidation reactions remains an urgent topic. The conflict between the low-temperature activity and high-temperature stability of noble metals can hardly reach a consensus. For instance, Pt cluster catalysts supported on CeO2 with high low-temperature activity will suffer from deactivation due to the redispersion under high-temperature lean-burn reaction conditions. Herein, two Pt1/CeO2 prepared by the incipient wetness impregnation method using different Pt precursors possessed varied Pt-O and Pt-O-Ce coordination numbers (CNs). They showed various priorities in CO oxidation versus NH3 selective catalytic oxidation, materials with higher CNPt-O-Ce selectively catalyzing NH3 oxidation to N2 more superior, conversely materials with lower CNPt-O-Ce performing better in CO oxidation. After activation by H2 reduction, both formed massive Pt clusters on the CeO2 surface but showed drastically distinct stability in lean-burn CO oxidation reactions. By summarizing the experimental results of high-angle annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, Raman spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, etc., it is beyond doubt that the difference in the initial states of Pt1 due to distinct precursors indeed determine the redispersion behavior of the reduced Pt clusters on CeO2. Materials with lower CNPt-O-Ce and higher CNPt-O are more likely to form robust Pt clusters, as they are not conducive to Pt anchoring, thus restricting the reversible structural evolution occurring under lean-burn CO oxidation and reductive conditions. This approach serves as a guide for the convenient and efficient construction and exploration of robust Pt cluster catalysts.

Observation of broad triangular Frank-type stacking faults and characterization of stacking faults with emission wavelengths below 430 nm in 4H–SiC epitaxial layers

Frank-type stacking faults in 4H–SiC epitaxial layers were investigated using room-temperature photoluminescence mapping and high-angle annular dark-field high-resolution scanning transmission electron microscopy. Remarkably, the intrinsic Frank-type stacking fault (5,2) and intrinsic multilayer Frank-type stacking fault (4,2), which have been reported to have elongated triangular shapes to date, were found to have broader triangular shape, which was typical shape of Shockley-type stacking faults. The shaping of stacking faults could be explained based on the directions of the partial dislocations bounding the stacking faults. Formation models for the intrinsic (5,2) and (4,2) stacking faults were proposed. In addition, we observed Frank-type stacking faults (3,2,2,3) and (3,3) and Shockley-type stacking faults (3,3,3,2,3,2) and (3,2,3,3,3,2), which were not reported before. All of these stacking faults were composed with the stacking numbers 2 and 3 in the Zhdanov notation and showed very close characteristic photoluminescence emission wavelengths of 427–428 nm. Formation models for the stacking faults of (3,2,2,3), (3,3), (3,3,3,2,3,2), and (3,2,3,3,3,2) were proposed, also. Based on the proposed formation models of (4,2) and (3,3) stacking faults, formation models for the Frank-type stacking faults (4,1) and (3,2) were discussed. Finally, we pointed out that the (4,1) Frank-type stacking fault should be the intrinsic not the extrinsic fault, and the (3,2) Frank-type stacking fault is the extrinsic fault.

The Development of the Regenerable Catalytic System in Selective Catalytic Oxidation of Ammonia with High N2 Selectivity.

Supported particulate noble-metal catalysts are widely used in industrial catalytic reactions. However, these metal species, whether in the form of nanoparticles or highly dispersed entities, tend to aggregate during reactions, leading to a reduced activity or selectivity. Addressing the frequent necessity for the replacement of industrial catalysts remains a significant challenge. Herein, we demonstrate the feasibility of the 'regenerable catalytic system' exemplified by selective catalytic oxidation of ammonia (NH3-SCO) employing Ag/Al2O3 catalysts. Results demonstrate that our highly dispersed Ag catalyst (Ag HD) maintains >90% N2 selectivity at 80% NH3 conversion and >80% N2 selectivity at 100% NH3 conversion after enduring 5 cycles of reducible aggregation and oxidative dispersion. Moreover, it consistently upholds over 98% N2 selectivity at 100% NH3 conversion after 10 cycles of Ar treatment. During the aggregation-dispersion process, the Ag HD catalyst intentionally aggregated into Ag nanoparticles (Ag NP) after H2 reduction and exhibited remarkable regenerable capabilities, returning to the Ag HD state after calcination in the air. This structural evolution was characterized through in situ transmission electron microscopy, atomically resolved high-angle annular dark-field scanning transmission electron microscopy, and X-ray absorption spectroscopy, revealing the on-site oxidative dispersion of Ag NP. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy provided insights into the exceptional N2 selectivity on Ag HD catalysts, elucidating the critical role of NO+ intermediates. Our findings suggest a sustainable and cost-effective solution for various industry applications.