Sergey Sokolov

Mechanism of Gold Nanoparticle Formation in the Classical Citrate Synthesis Method Derived from Coupled In Situ XANES and SAXS Evaluation

Although gold nanoparticles (GNP) are among the most intensely studied nanoscale materials, the actual mechanisms of GNP formation often remain unclear due to limited accessibility to in situ-derived time-resolved information about precursor conversion and particle size distribution. Overcoming such limitations, a method is presented that analyzes the formation of nanoparticles via in situ SAXS and XANES using synchrotron radiation. The method is applied to study the classical GNP synthesis route via the reduction of tetrachloroauric acid by trisodium citrate at different temperatures and reactant concentrations. A mechanism of nanoparticle formation is proposed comprising different steps of particle growth via both coalescence of nuclei and further monomer attachment. The coalescence behavior of small nuclei was identified as one essential factor in obtaining a narrow size distribution of formed particles.

Nucleation and Growth of Gold Nanoparticles Studied via in situ Small Angle X-ray Scattering at Millisecond Time Resolution

Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer. The online characterization and screening of this fast process ( approximately 2 s) was enabled by coupling a micromixer operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This online characterization technique enables the time-resolved investigation of the growth process of the nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure microscopy, scanning electron microscopy, and UV-vis spectroscopy the obtained data allow the deduction of a two-step mechanism of gold nanoparticle formation. The first step is a rapid conversion of the ionic gold precursor into metallic gold nuclei, followed by particle growth via coalescence of smaller entities. Consequently it could be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.

Broad-spectrum enhancement of polymer composite dielectric constant at ultralow volume fractions of silica-supported copper nanoparticles.

A new strategy for the synthesis of high permittivity polymer composites is demonstrated based on well-defined spatial distribution of ultralow amounts of conductive nanoparticles. The spatial distribution was realized by immobilizing Cu nanoparticles within the pore system of silica microspheres, preventing direct contact between individual Cu particles. Both Cu-loaded and unloaded silica microspheres were then used as fillers in polymer composites prepared with thermoplastic SEBS rubber as the matrix. With a metallic Cu content of about 0.10 vol % [corrected] in the composite, a relative increase of 94% in real permittivity was obtained. No Cu-induced relaxations were observed in the dielectric spectrum within the studied frequency range of 0.1 Hz to 1 MHz. When related to the amount of conductive nanoparticles, the obtained composites achieve the highest broad-spectrum enhancement of permittivity ever reported for a polymer-based composite.

ZrO2-Based Alternatives to Conventional Propane Dehydrogenation Catalysts: Active Sites, Design, and Performance

Non-oxidative dehydrogenation of propane to propene is an established large-scale process that, however, faces challenges, particularly in catalyst development; these are the toxicity of chromium compounds, high cost of platinum, and catalyst durability. Herein, we describe the design of unconventional catalysts based on bulk materials with a certain defect structure, for example, ZrO2 promoted with other metal oxides. Comprehensive characterization supports the hypothesis that coordinatively unsaturated Zr cations are the active sites for propane dehydrogenation. Their concentration can be adjusted by varying the kind of ZrO2 promoter and/or supporting tiny amounts of hydrogenation-active metal. Accordingly designed Cu(0.05 wt %)/ZrO2 -La2 O3 showed industrially relevant activity and durability over ca. 240 h on stream in a series of 60 dehydrogenation and oxidative regeneration cycles between 550 and 625 °C.

Effect of VOx Species and Support on Coke Formation and Catalyst Stability in Nonoxidative Propane Dehydrogenation

VOx/SiO2–Al2O3 catalysts were prepared by grafting vanadyl acetylacetonate onto the supports with a SiO2 content between 0 and 100 wt. %. The degree of polymerization of VOx species and acidity both of pristine supports and the catalysts were evaluated. To determine their on‐stream stability and carbon deposition activity in nonoxidative propane dehydrogenation, continuous‐flow tests and in situ thermogravimetric measurements were performed. The rate constants of catalyst deactivation and carbon deposition were derived from kinetic evaluation of these experiments. Gathered experimental evidence pointed out that VOx species were significantly more active for coke formation than acid sites of the supports. The rate constant of carbon formation was found to increase with the degree of polymerization of VOx species, whereas no correlation between catalyst acidity and the rate constants of coking or deactivation could be drawn.

Enhanced de‐N2O Performance of Cellulose‐Templated CaO‐Based Catalysts

eral temperature cycles from 673 to 1023 K (in total 45 h onstream) with no visible NO reduction observed. To demonstrate the benefits of our preparation method, we compared de-N2O performance of the templated materials with that of commercial calcium oxide doped with sodium oxide by incipient wetness impregnation. Two methods used for the catalyst preparation were cellulose-templating (CT) and incipient wetness impregnation (IW). In the former method, filter paper (Whatman 50, 0.015 % ash) was soaked in aqueous solution of calcium (Riedel–de Haen) and sodium nitrates (Merck). While still wet, the paper was transferred into a muffle furnace preheated to 1173 K and calcined for 0.5 or 2 h in air. In the IW method, calcium oxide (Riedel-de Haen) was impregnated with an aqueous solution of sodium nitrate. Four catalysts selected for this study were commercial CaO (6.4 m 2 g) � 1 , commercial CaO impregnated with sodium Na0.001CaO_IW (4.3 m 2 g � 1 ), templated CaO_CT (6.0 m 2 g � 1 ), and templated Na0.001CaO_CT (10.4 m 2 g � 1 ). X-ray diffraction (XRD) analysis did not reveal presence of sodium phases in our samples; the main constituents were CaO (PDF 70-4068) and Ca(OH)2 (PDF 84-1273). The latter was probably formed during storage of the catalysts in air. Figure 1 shows scanning electron microscopy (SEM) images of commercial CaO and the catalysts prepared by using the IW and CT methods. The micrographs show the effect of the preparation method on the morphology of the particles. Whereas CaO and Na0.001CaO_IW materials consisted of mm-scale irregularly shaped plate-like particles, in the case of Na0.001CaO_IW with large openings, particles of the CT catalysts were significantly smaller (on the scale of 100 nm), close to spherical geometry and similar in morphology.