Comparison of silver nanoparticles confined in nanoporous silica prepared by chemical synthesis and by ultra-short pulsed laser ablation in liquid

Hexagonally ordered mesoporous silica materials, MCM-41 and SBA-15, have been synthesized and loaded with Ag nanoparticles, utilizing both chemical synthesis and ultra-short pulsed laser ablation in liquid. In laser ablation, a silver target, immersed in aqueous suspension of ordered mesoporous silica SBA-15, was irradiated by ultra-short laser pulses to generate silver nanoparticles. For comparison, samples of similar silver contents were prepared either by incorporating silver into the SBA-15 during a hydrothermal synthesis or by introducing silver in MCM-41 by template ion-exchange. Samples were characterized by XRD, N₂ physisorption, TEM and UV–vis spectroscopy. All preparations contained significant amount of 5–50 nm size silver agglomerates on the outer surface of the silica particles. The laser ablation process did not cause significant destruction of the SBA-15 structure and metallic silver (Ag⁰) nanoparticles were mainly generated. It is demonstrated that by laser ablation in aqueous silica suspension smaller and more uniform metallic silver particles can be produced and loaded on the surface of the silica support than by synthesis procedures. Catalytic properties of the samples have been tested in the total oxidation of toluene. Because of its favorable Ag dispersity, the Ag/SBA-15 catalyst, generated by the laser ablation method, had better catalytic stability and, relative to its Ag load, higher activity than the conventional Ag/SBA-15 preparations.     

Mixed constructions for generalized topologies

As generalization of the construction of I-τ where τ is a topology and I is an ideal [11], and similarly of $ \mathcal{H} $ -µ where µ is a generalized topology and $ \mathcal{H} $ is a hereditary class [5], we introduce generalized topologies starting from two generalized topologies µ and µ′.     

Palladium-Catalyzed Cyclization of a Pyryne Precursor to Higher Pyrenylenes

The palladium catalyzed cyclotrimerization of ortho-silylaryl triflates as aryne precursors is meanwhile an established method to synthesize polycyclic aromatic hydrocarbons (PAHs) with triphenylene cores. During the palladium-catalyzed reaction of a pyrene with an o-silylaryl triflate moiety in the K-region higher homologues with central eight- and ten-membered rings (the pyrenylenes) were found, besides the expected trimer and a protocol was developed to isolate all members of this series. This unprecedented new class of PAHs was fully investigated by all means, including X-ray diffraction of single-crystals, UV/Vis and fluorescence spectroscopy and theoretical calculations. Supported by density-functional theory (DFT) calculations, a mechanism of all higher cyclooligomers is proposed.     

Measurement of interfacial adhesion between glassy polymers using the JKR method

The surface and interfacial energies of polymers are measured using the JKR-type experiments. A novel method has been developed to prepare samples of glassy polymers for adhesion measurements. A thin layer of a polymer is coated on the surface of an O₂-plasma modified cross-linked poly(dimethylsiloxane) [PDMS] spherical cap resulting in the formation of a composite. Using the JKR theory, the surface energies of polystyrene [PS] and poly(methyl methacrylate) [PMMA] are determined from the measurements of the contact radius as a function of applied load. The results of the JKR-type experiments are compared to adhesion measurements done using the surface forces apparatus (SFA). Adhesion hysteresis was observed for PS-PS contact as well as PMMA-PMMA contact. However, no hysteresis was observed for PDMS-PDMS, PDMS-PS, and PDMS-PMMA contacts. The exact origin of the hysteresis is not clear at present. The current evidence suggests that hysteresis is due to rearrangement of the interface during contact.