Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TEM

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TE

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TEM

Lithiation of Rutile TiO₂‑Coated Si NWs Observed by in Situ TE

Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage via DFT Calculations Coupled with <i>in Situ</i> TEM and IR

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by <i>in situ</i> TEM and quantitatively with sample-averaged measurements of surface atom configurations by <i>in situ</i> infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact

Tunable, Endotaxial Inclusion of Crystalline Pt-Based Nanoparticles Inside a High-Quality Bronze TiO₂ Matrix

A series of high-quality bronze titanium oxide films containing endotaxially embedded Pt-based nanoparticles was fabricated using pulsed laser deposition under various oxygen partial pressures (0 to 50 mTorr). We found that morphological control over the embedded Pt nanoparticles is possible by varying the oxygen partial pressure during growth. We also found that the titanium oxide matrix plays an important role in controlling composition, shape, and distribution of the endotaxially embedded Pt-based nanoparticles over this range of oxygen partial pressure by affecting (1) the formation of a segregated layer of Pt–Ti alloy nanoparticles, in addition to the pure Pt nanoparticles, under vacuum, (2) the generation of crystallographic twinning, steps, and kinks within the Pt nanoparticles, and (3) the localized precipitation of Pt nanoparticles spatially confined and morphologically adapted to the extended defects within the matrix

Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO₂

Oxide-supported precious metal nanoparticles are widely used industrial catalysts. Due to expense and rarity, developing synthetic protocols that reduce precious metal nanoparticle size and stabilize dispersed species is essential. Supported atomically dispersed, single precious metal atoms represent the most efficient metal utilization geometry, although debate regarding the catalytic activity of supported single precious atom species has arisen from difficulty in synthesizing homogeneous and stable single atom dispersions, and a lack of site-specific characterization approaches. We propose a catalyst architecture and characterization approach to overcome these limitations, by depositing ∼1 precious metal atom per support particle and characterizing structures by correlating scanning transmission electron microscopy imaging and CO probe molecule infrared spectroscopy. This is demonstrated for Pt supported on anatase TiO₂. In these structures, isolated Pt atoms, Pt_iso, remain stable through various conditions, and spectroscopic evidence suggests Pt_iso species exist in homogeneous local environments. Comparing Pt_iso to ∼1 nm preoxidized (Pt_ox) and prereduced (Pt_metal) Pt clusters on TiO₂, we identify unique spectroscopic signatures of CO bound to each site and find CO adsorption energy is ordered: Pt_iso ≪ Pt_metal < Pt_ox. Pt_iso species exhibited a 2-fold greater turnover frequency for CO oxidation than 1 nm Pt_metal clusters but share an identical reaction mechanism. We propose the active catalytic sites are cationic interfacial Pt atoms bonded to TiO₂ and that Pt_iso exhibits optimal reactivity because every atom is exposed for catalysis and forms an interfacial site with TiO₂. This approach should be generally useful for studying the behavior of supported precious metal atoms

New Atomic-Scale Insight into Self-Regeneration of Pt-CaTiO₃ Catalysts: Incipient Redox-Induced Structures Revealed by a Small-Angle Tilting STEM Technique

A small-angle tilting technique, applied to scanning transmission electron microscopy (STEM), was used together with multislice image simulation to reveal new atomic-scale information about the structural evolution of single-crystalline Pt-doped CaTiO₃ thin films, grown by pulsed laser deposition, that occurs in response to reduction and reoxidation treatments. Specifically, we were able to confirm that Pt atoms are randomly dispersed throughout the as-grown film, most often occupying Ti sites, show that the smallest (∼1 nm) Pt-rich clusters embedded within CaTiO₃ after reduction have a structure consistent with metallic Pt, and demonstrate that the Pt atoms from these clusters occupy mainly Ca sites, in the form of ordered Pt-atom arrays, after reoxidation

In Situ Atomic-Scale Observation of the Two-Dimensional Co(OH)₂ Transition at Atmospheric Pressure

Two-dimensional (2D) materials have been recognized as one of the promising materials for various applications due to their unique characteristics. However, the formation and transformation mechanisms behind the 2D structure still largely remain unknown, which is significant for synthesis controlling and property tuning. In this scope, in situ microscopy characterization of two-dimensional (2D) materials under atmospheric reaction conditions offers a powerful tool for understanding their structural evolution at the atomic scale, which, however, has been seldom reported. Here, taking the 2D CoO as the model material which is the promising electro-/photoelectro-catalyst, we report real-time visualization of the structural transition of 2D Co­(OH)₂ nanosheets to CoO using in situ electron microscopy. Three intermediate phases, including one pseudo-Co­(OH)₂ phase, one transition phase, and one pseudo-CoO phase, are identified and characterized during the transition process. The detailed transition pathways and mechanisms are discussed based on the combined in situ STEM and FTIR data. The transition starts with the rapid dehydration process followed by two rearrangement periods and one relaxation process, respectively. The complete transition process is as follows: Co­(OH)₂ → (dehydration) → Co­(OH)_2,p → (rearrangement) → transition phase → (rearrangement) → CoO_p → (relaxation) → CoO

Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-Dependent Oxidation of Pt₃Co Nanoparticles via <i>in Situ</i> Transmission Electron Microscopy

Since catalytic performance of platinum–metal (Pt–M) nanoparticles is primarily determined by the chemical and structural configurations of the outermost atomic layers, detailed knowledge of the distribution of Pt and M surface atoms is crucial for the design of Pt–M electrocatalysts with optimum activity. Further, an understanding of how the surface composition and structure of electrocatalysts may be controlled by external means is useful for their efficient production. Here, we report our study of surface composition and the dynamics involved in facet-dependent oxidation of equilibrium-shaped Pt₃Co nanoparticles in an initially disordered state via <i>in situ</i> transmission electron microscopy and density functional calculations. In brief, using our advanced <i>in situ</i> gas cell technique, evolution of the surface of the Pt₃Co nanoparticles was monitored at the atomic scale during their exposure to an oxygen atmosphere at elevated temperature, and it was found that Co segregation and oxidation take place on {111} surfaces but not on {100} surfaces

Smart Pd Catalyst with Improved Thermal Stability Supported on High-Surface-Area LaFeO₃ Prepared by Atomic Layer Deposition

The concept of self-regenerating or “smart” catalysts, developed to mitigate the problem of supported metal particle coarsening in high-temperature applications, involves redispersing large metal particles by incorporating them into a perovskite-structured support under oxidizing conditions and then exsolving them as small metal particles under reducing conditions. Unfortunately, the redispersion process does not appear to work in practice because the surface areas of the perovskite supports are too low and the diffusion lengths for the metal ions within the bulk perovskite too short. Here, we demonstrate reversible activation upon redox cycling for CH₄ oxidation and CO oxidation on Pd supported on high-surface-area LaFeO₃, prepared as a thin conformal coating on a porous MgAl₂O₄ support using atomic layer deposition. The LaFeO₃ film, less than 1.5 nm thick, was shown to be initially stable to at least 900 °C. The activated catalysts exhibit stable catalytic performance for methane oxidation after high-temperature treatment

Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability

Bimetallic catalytic materials are in widespread use for numerous reactions, as the properties of a monometallic catalyst are often improved upon addition of a second metal. In studies with bimetallic catalysts, it remains challenging to establish clear structure–property relationships using traditional impregnation techniques, due to the presence of multiple coexisting active phases of different sizes, shapes, and compositions. In this work, a convenient approach to prepare small and uniform Pt/Pd bimetallic nanocrystals with tailorable composition is demonstrated, despite the metals being immiscible in the bulk. By depositing this set of controlled nanocrystals onto a high-surface-area alumina support, we systematically investigate the effect of adding platinum to palladium catalysts for methane combustion. At low temperatures and in the absence of steam, all bimetallic catalysts show activity nearly identical with that of Pt/Al₂O₃, with much lower rates in comparison to that of the Pd/Al₂O₃ sample. However, unlike Pd/Al₂O₃, which experiences severe low-temperature steam poisoning, all Pt/Pd bimetallic catalysts maintain combustion activity on exposure to excess steam. These features are due to the influence of Pt on the Pd oxidation state, which prevents the formation of a bulk-type PdO phase. Despite lower initial combustion rates, hydrothermal aging of the Pd-rich bimetallic catalyst induces segregation of a PdO phase in close contact to a Pd/Pt alloy phase, forming more active and highly stable sites for methane combustion. Overall, this work unambiguously clarifies the activity and stability attributes of Pt/Pd phases which often coexist in traditionally synthesized bimetallic catalysts and demonstrates how well-controlled bimetallic catalysts elucidate structure–property relationships