a-AluminaCatalyst design Ethylene oxide Particle size effect a b s t r a c t Currently, for the industrial ethylene epoxidation a-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. a-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20-500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.

The catalytic performance and optical properties of bimetallic nanoparticles critically depend on the atomic distribution of the two metals in the nanoparticles. However, at elevated temperatures, during light-induced heating, or during catalysis, atomic redistribution can occur. Measuring such metal redistribution in situ is challenging, and a single experimental technique does not suffice. Furthermore, the availability of a well-defined nanoparticle system has been an obstacle for a systematic investigation of the key factors governing the atomic redistribution. In this study, we follow metal redistribution in precisely tunable, single-crystalline Au-core, Ag-shell nanorods in situ, both at a single particle and an ensemble-averaged level, by combining in situ transmission electron spectroscopy with in situ extended X-ray absorption fine structure validated by ex situ measurements. We show that the kinetics of atomic redistribution in Au–Ag nanoparticles depend on the metal composition and particle volume, such that a higher Ag content or a larger particle size led to significantly slower metal redistribution. We developed a simple theoretical model based on Fick’s first law that can correctly predict the composition- and size-dependent alloying behavior in Au–Ag nanoparticles, as observed experimentally.

acquire many insights in interesting topics such as interparticle interactions and colloidal self-assembly. [1][2][3] Liquid-cell (scanning) transmission electron microscopy [4,5] (LC(S)TEM) has recently emerged as a powerful tool to observe dynamic processes of nanoparticles (NPs) in liquid with nanometer spatial resolution. However, the electron beam significantly influenced the observed phenomena in many cases. So far, strongly slowed down diffusion of NPs was observed in LC(S)TEM studies. Possible explanations for this phenomenon, apart from trivial difficulties such as the imaging system not being fast enough to image free Brownian motion, include hydrodynamic slowing down near the window's surface, [10,16] a highly viscous ordered liquid layer near the windows, [10,15] and strong (sometimes beam-induced) interactions with the liquid-cell windows. [6,10,15,16,18] Observing 3D Brownian motion in the electron microscope that is not significantly altered by the electron beam and/or the presence of the windows would open the way for many experiments, including studies on colloidal self-assembly of NP dispersions. [30] The objective of this work is to find conditions and identify key experimental parameters for which 3D Brownian motion is observable in LC(S)TEM.In this study, we combine a low dose scanning transmission electron microscopy (STEM) technique with viscous liquid media having a high dielectric constant to observe bulk diffusion of gold NPs and titania particles in LC(S)TEM. The significantly faster diffusion of particles in comparison to many previous liquid-cell electron microscopy studies that we report on in this work underlines the importance of choosing a suitable electron microscopy imaging technique, electron dose rate and solvent in order to study dynamic processes in LC(S)TEM without artefacts. Results and DiscussionFor this work, we studied two different systems. One with bigger particles in a less viscous solvent and one with smaller particles in a more viscous solvent. The bigger particles serve as a first check whether free diffusion is at all possible within the In theory, liquid-cell (scanning) transmission electron microscopy (LC(S)TEM) is the ideal method to measure 3D diffusion of nanoparticles (NPs) on a single particle level, beyond the capabilities of optical methods. However, particle diffusion experiments have been especially hard to explain in LC(S) TEM as the observed motion thus far has been slower than theoretical predictions by 3-8 orders of magnitude due to electron beam effects. Here, direct experimental evidence of undamped diffusion for two systems is shown; charge-neutral 77 nm gold nanoparticles in glycerol and negatively charged 350 nm titania particles in glycerol carbonate. The high viscosities of the used media and a low electron dose rate allow observation of Brownian motion that is not significantly altered by the electron beam. The resulting diffusion coefficient agrees excellently with a theoretical value assuming free diffusion. It is confirmed that the par...

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The recent development of liquid cell (scanning) transmission electron microscopy (LC-(S)TEM) has opened the unique possibility of studying the chemical behavior of nanomaterials down to the nanoscale in a liquid environment. Here, we show that the chemically induced etching of three different types of silica-based silica nanoparticles can be reliably studied at the single particle level using LC-(S)TEM with a negligible effect of the electron beam, and we demonstrate this method by successfully monitoring the formation of silica-based heterogeneous yolk–shell nanostructures. By scrutinizing the influence of electron beam irradiation, we show that the cumulative electron dose on the imaging area plays a crucial role in the observed damage and needs to be considered during experimental design. Monte-Carlo simulations of the electron trajectories during LC-(S)TEM experiments allowed us to relate the cumulative electron dose to the deposited energy on the particles, which was found to significantly alter the silica network under imaging conditions of nanoparticles. We used these optimized LC-(S)TEM imaging conditions to systematically characterize the wet etching of silica and metal(oxide)–silica core–shell nanoparticles with cores of gold and iron oxide, which are representative of many other core–silica–shell systems. The LC-(S)TEM method reliably reproduced the etching patterns of Stöber, water-in-oil reverse microemulsion (WORM), and amino acid-catalyzed silica particles that were reported before in the literature. Furthermore, we directly visualized the formation of yolk–shell structures from the wet etching of Au@Stöber silica and Fe 3 O 4 @WORM silica core–shell nanospheres.

Yolk–shell or rattle-type particles consist of a core particle that is free to move inside a thin shell. A stable core with a fully accessible surface is of interest in fields such as catalysis and sensing. However, the stability of a charged nanoparticle core within the cavity of a charged thin shell remains largely unexplored. Liquid-cell (scanning) transmission electron microscopy is an ideal technique to probe the core–shell interactions at nanometer spatial resolution. Here, we show by means of calculations and experiments that these interactions are highly tunable. We found that in dilute solutions adding a monovalent salt led to stronger confinement of the core to the middle of the geometry. In deionized water, the Debye length κ –1 becomes comparable to the shell radius R shell , leading to a less steep electric potential gradient and a reduced core–shell interaction, which can be detrimental to the stability of nanorattles. For a salt concentration range of 0.5–250 mM, the repulsion was relatively long-ranged due to the concave geometry of the shell. At salt concentrations of 100 and 250 mM, the core was found to move almost exclusively near the shell wall, which can be due to hydrodynamics, a secondary minimum in the interaction potential, or a combination of both. The possibility of imaging nanoparticles inside shells at high spatial resolution with liquid-cell electron microscopy makes rattle particles a powerful experimental model system to learn about nanoparticle interactions. Additionally, our results highlight the possibilities for manipulating the interactions between core and shell that could be used in future applications.

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