Migration Coalescence Research Articles

Reaction-Driven Migration Dynamics of Nano-Metal Particles Unraveled by Quantitative Electron Microscopies.

The stability of supported nano-metal catalysts holds significant importance in both scientific and economic practice, beyond the long pursuit of enhanced activity. While previous efforts have concentrated on augmenting the interaction between nano-metals and carriers, in the thermodynamic macro-perspective, to achieve optimized repression upon particle migration coalescence and Ostwald ripening, nevertheless, the microscale kinetics of migrating catalyst particles driven by the reaction remains unknown. In this work, the migration of nano-copper particles is investigated during hydrogen oxidation reaction by utilizing high spatiotemporal resolution of environmental transmission electron microscopy. It is shown that there exists a delicate correlation between the migration dynamics of nano-copper particles and the evolution of asymmetrically distributed Cu and Cu2O phases over the particle surface. It is found that the interplay of reduction and oxidation near the surface areas filled with Cu and Cu2O phases can facilitate the pressure gradient, which drives the migration of nano-particles. A driving force model is therefore established which is capable of qualitatively explaining the influences of reaction conditions such as temperature and hydrogen-to-oxygen ratio on the reaction-driven particle migration. This work adds a potential yet critical perspective to understanding particle migration and thus the nano-metal catalyst particle sintering in heterogeneous catalysis.

Electrophoretic coalescence behavior of oil droplets in oil-in-water emulsions containing SDS under DC electric field: A molecular dynamics study

The presence of oil droplets in oil-in-water (O/W) emulsions seriously affects the efficiency of wastewater purification, and the appropriate addition of surfactants can effectively improve the demulsification process. In this paper, the electrophoretic coalescence behavior of oil droplets containing anionic surfactant sodium dodecyl sulfate (SDS) under a direct current (DC) electric field was studied based on molecular dynamics (MD) simulations, and four typical stages of oil droplet migration coalescence were defined. The weak interactions between SDS and water and between SDS and n-hexane are mainly van der Waals interactions, while a small number of hydrogen bonds are formed between SDS and water and between water molecules internally. With the increase of electric field strength (E), the deformation of oil droplets after aggregation increases, and the contact angle with the electrode plate becomes smaller. The interfacial tension between oil and water decreases and then increases with increasing surfactant concentration, reaching a minimum value of 32.52 mN/m when the number of SDS molecules (n) around each oil droplet is 55. When E = 1.2 V/nm and n = 55, the coalescence time of oil droplets reaches the minimum value of 2.425 ns. The migration rate of oil droplet far from the anode is greater than that of oil droplet near the anode, which is due to the larger combined forces on the former. The results reveal the microscopic mechanism of oil droplet migration and coalescence under the coupling effect of DC electric field and anionic surfactant, which could provide theoretical guidance for the improvement of oily wastewater purification process.

Sintering Behaviors of Supported Nanoparticles Related to Spatial Location by a Quasi-Four-Dimensional TEM.

The performance degradation via sintering phenomenon is a critical issue for the application of supported nanoparticles in industrial catalysis. However, the challenges to combine in situ stimulation and three-dimensional (3D) characterization hinder a profound understanding of sintering behaviors, thus the effect of spatial location on nanoparticles sintering has long been neglected. Herein, based on a homemade holder integrated with in situ Joule heating and electron tomography, a quasi-four-dimensional (4D) transmission electron microscope characterization approach is developed to reveal the spatial location of supported nanoparticles and its pronounced impact on size distribution and sintering behaviors. The results of 3D visualization and statistical analysis demonstrate a strong location-dependent sintering behavior of supported nanoparticles, where external nanoparticles sinter via migration coalescence, and internal nanoparticles sinter via Ostwald ripening. The quasi-4D methods developed in this work can also be extended to the study on 3D configuration evolution of other nanomaterials under an external stimulus.

Encapsulation of Metal Clusters within Porous Organic Materials: From Synthesis to Catalysis Applications.

Metal clusters (MCs) with dimensions between a single metal atom and nanoparticles of >2 nm usually possess distinct geometric and electronic structures, and their outstanding performance in catalysis applications have underpinned a broad research interest. However, smaller-sized MCs are easily deactivated by migration coalescence during the catalysis process because of their high surface energy. Therefore, the search of an appropriate stabilizer for MCs is urgently demanded. In recent years, porous organic polymers (POPs) and organic molecular cages (OMCs), as emerging functional materials, have attracted significant attention. Benefiting from the spatial confinement, encapsulating MCs into these porous organic materials is a promising approach to guarantee the uniform size distribution and stability. In this review, we aim to provide a comprehensive summary of the recent progress in the synthetic strategies and catalysis applications of the encapsulated MCs, and seek to uncover promising ideas that can stimulate future developments at both the fundamental and applied levels.

Sinter Resistance and Activity Enhancement via a Facet-Dependent Metal–Support Interaction of Pd/ZnO Catalysts in CO Oxidation

Metal–support interaction (MSI) has been extensively investigated regarding its structural and catalytic modification in heterogeneous catalysts. Herein, we reveal how facet-dependent MSI impacts on the structure and performance of Pd/ZnO in the CO oxidation reaction. ZnO nanosheets with orthohexagonal shape which are exposed mainly with {001} facets exhibit remarkable thermal stability under air atmosphere until 600 °C, while ZnO nanorods bounded with {100} facets transform at 400 °C and totally change after calcination at 600 °C. Thus, the Pd species supported on ZnO nanorods and nanosheets also display different sintering behaviors after thermal treatment, and the particle size of Pd grows from 1.1 to 5.7 nm on {100} facets while it keeps high-dispersion on {001} facets after 600 °C calcination. In situ transmission electron microscopy was used to visualize the sintering process, demonstrating that Pd displays the Ostwald ripening and particle migration coalescence mechanism on ZnO {100} facets, while shows extraordinary stability on ZnO {001} facets. Therefore, Pd supported on ZnO nanorod exhibits deactivation behavior after high temperature treatment in CO oxidation, during which the T50 delays from 106 to 120 and 145 °C after 200, 400, and 600 °C calcination. Interestingly, Pd on ZnO nanosheet shows obvious activity enhancement from 137 to 90 and 75 °C of T50 after the same treatment, which is ascribed to the synergy between Pd and ZnO {001} facets during catalysis. This work gives in-depth understanding of the facet-dependent MSI of Pd/ZnO catalysts, and it sheds light on the rational design of heterogeneous catalysts with high efficiency and sinter resistance.

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.

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

AbstractPreventing 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.

Growth mechanism of bioglass nanoparticles in polyacrylonitrile-based carbon nanofibers

Polyacrylonitrile (PAN) electrospinning in combination with sol–gel method has been a common technique to produce inorganic nanoparticles containing composite carbon nanofibers (CNFs) for diverse applications. To investigate the morphology evolution and crystal transformation of inorganic components along with CNF formation, bioactive glass (BG) containing CNFs (CNF/BG) were prepared by sintering as-spun PAN/precursor composite nanofibers in a nitrogen atmosphere at temperatures of 800, 1000 and 1200 °C. Comprehensive characterizations were performed with TEM, SEM-EDXA and XRD. For samples sintered at 800 °C, numerous BG nanoparticles were observed inside the CNFs and mainly in an amorphous state. With the sintering temperature raised to 1000 °C, a number of spherical BG nanoparticles were detected on the surface of the resulting CNFs, with a crystal structure of wollastonite (β-CaSiO3) polycrystals. When the samples were sintered at 1200 °C, the BG nanoparticles on the surface of CNFs merged into forms with cuboid-like geometry, mainly consisting of pseudowollastonite (Ca3(Si3O9)) single crystals. Based on the geometry evolution and dynamic size distribution function analyses (Ostwald ripening and Smoluchowski equations), it was concluded that the growth of BG nanoparticles conformed to the ripening mechanism at 800 °C and migration–coalescence mechanism at 1200 °C, while the process involved both ripening and migration–coalescence mechanisms at 1000 °C.

Growth and migration of nanocavities in He+ multi-implanted Si measured by in situ small-angle X-ray scattering

Abstract Multi-implantations of He ions were performed on Si samples with (111) or (100) surface orientation in the kiloelectron volt (keV) energy range. The fluencies were chosen to keep the local He concentration constant and above the threshold to get nanocavities upon annealing. Small-angle X-ray scattering (SAXS) experiments were carried out, for the first time ever, to follow in situ the evolution of the size distribution of nanocavities created in the Si bulk. The cavity distribution was measured in the as-implanted state and followed during annealing from 623 K up to 973 K. The final spatial distribution was obtained ex situ by transmission electron microscopy (TEM). Whatever the surface orientation, the increase of the porosity is triggered at 973 K which is a temperature at least 200 K higher than expected. This ongoing increase proves that the growth stage is not complete even after a 2-h annealing. The onset of a new nanocavity population in the (111) Si sample is an experimental evidence that the migration–coalescence (MC) process, although weak at such a low temperature range, contributes to the evolution of the nanocavity size. Basic mechanisms for the growth and coarsening of nanocavities/bubbles are discussed in view of the experimental data arising from in situ SAXS measurements coupled with ex situ TEM observations.

Molecular dynamics simulation of the initial stages of He bubbles formation in silicon

The initial stages of the formation of bubbles due to helium implantation in silicon remain largely unknown, for they occur at time and space scales hardly accessible to experiments. To improve the current knowledge, we have developed an interatomic potential to describe the He : Si system, which can model helium for the pressure conditions expected in bubbles, and reproduce the behaviour of a single helium atom in silicon. Then molecular dynamics calculations were performed. Starting from an initial random distribution of helium interstitials and silicon vacancies, simulations revealed a two-step formation of small HenVm clusters. First, available vacancies quickly formed vacancy clusters Vm, due to their high mobility. In a second step, all clusters included a slowly increasing number of helium atoms, which were captured by diffusion or by cluster migration. Migration coalescence coarsening mechanisms were also identified during the simulation.