Synergistic effects of Ir-Au/TiO2 catalysts in the total oxidation of propene: influence of the activation conditions.

Iridium was added to the Au/TiO2 system to try to enhance its catalytic activity in the reaction of propene oxidation, performed under conditions close to those used in the studies of decomposition of volatile organic compounds (1200 ppm propene and 9 vol% O2 in He). Titania supported Ir-Au (Ir/Au = 1) was prepared by sequential deposition-precipitation with urea (DPU) of Ir then Au. The effect of the activation conditions (hydrogen or air at 400 °C) was investigated. The study of the activation conditions of Ir-Au/TiO2 showed that activation under hydrogen at 400 °C generated a catalyst more active than the monometallic ones, while Ir-Au/TiO2 activated in air remained as poorly active as Au/TiO2. TEM characterization showed the formation of metallic particles of similar size (2-3 nm) in both monometallic Au/TiO2 and bimetallic Ir-Au/TiO2. Characterization especially by DRIFTS using CO as a probe molecule suggests the presence of Ir-Au interaction, IrO2-Au(0) interaction when the sample is calcined and Ir(0)-Au(0) bimetallic particles when it is reduced. XPS and TPR characterization techniques showed that gold hinders to some extent the reoxidation of iridium in the reduced bimetallic Ir-Au/TiO2 catalyst. The enhanced catalytic activity of the reduced bimetallic Ir-Au/TiO2 catalyst is attributed to a surface Ir(0)-Au(0) synergism.

Aging of thin polymer films cast from a near-theta solvent.

We have examined the aging behavior of spin-cast thin polymer films as a function of their processing history. Films prepared from solutions close to the Θ temperature were aged for varying times at room temperature, followed by a dewetting experiment above the glass transition temperature of the polymer. The characteristic aging time varied strongly with the quality of the solvent, which is attributed to distorted chain conformations in the as-cast films. This is an indication for the nonequilibrium nature of thin polymer films, possibly causing some of their unexplained properties.

Silica-Supported Au-Ag Catalysts for the Selective Hydrogenation of Butadiene.

Gold and silver are miscible over the entire composition range, and form an attractive combination for fundamental studies on bimetallic catalysts. Au-Ag catalysts have shown synergistic effects for different oxidation and liquid-phase hydrogenation reactions, but have rarely been studied for gas-phase hydrogenation. In this study 3 nm particles of Au, Ag and Au-Ag supported on silica (SBA-15) were investigated as catalysts for selective hydrogenation of butadiene in an excess of propene. The Au catalyst was over an order of magnitude more active than the Ag catalyst at 120 °C. The initial activity of the Au-Ag catalysts scaled linearly with the Au-content, suggesting a direct correlation between the surface and overall compositions of the nanoparticles and the absence of synergistic effects. All Au-containing catalysts were highly selective to butenes (>99.9 %). The Au catalysts were stable, whereas the Au-Ag catalysts lost about half of their activity during 20 h run time at 200 °C, but the initial activity was restored by a consecutive oxidation-reduction treatment. Near ambient pressure x-ray photoelectron spectroscopy showed that exposure to H2 at elevated temperatures led to a gradual enrichment of the surface of the Au-Ag nanoparticles by Ag. These observations highlight the importance of considering progressive atomic rearrangements in bimetallic nanocatalysts under reaction conditions.

Simple and Specific Grafting of Antibacterial Peptides on Silicone Catheters.

To fight against nosocomial infection initiated by colonization of medical devices, a strategy enabling the direct and fast functionalization of silicone surfaces is proposed. This strategy proceeds in a site-specific way using original hybrid silylated antibacterial peptides. This safe and up-scalable method guarantees a covalent and robust immobilization with the correct orientation of the bioactive moiety. Importantly it also avoids multi-step chemical modifications of the surface or multi-layer polymer coatings. As proof of concept, antibacterial silicone catheter has been prepared whose immediate and long term efficiency is superior by comparison to similar silver-embedded materials.

Biocidal Properties of a Glycosylated Surface: Sophorolipids on Au(111).

Classical antibacterial surfaces usually involve antiadhesive and/or biocidal strategies. Glycosylated surfaces are usually used to prevent biofilm formation via antiadhesive mechanisms. We report here the first example of a glycosylated surface with biocidal properties created by the covalent grafting of sophorolipids (a sophorose unit linked by a glycosidic bond to an oleic acid) through a self-assembled monolayer (SAM) of short aminothiols on gold (111) surfaces. The biocidal effect of such surfaces on Gram+ bacteria was assessed by a wide combination of techniques including microscopy observations, fluorescent staining, and bacterial growth tests. About 50% of the bacteria are killed via alteration of the cell envelope. In addition, the roles of the sophorose unit and aliphatic chain configuration are highlighted by the lack of activity of substrates modified, respectively, with sophorose-free oleic acid and sophorolipid-derivative having a saturated aliphatic chain. This system demonstrates thus the direct implication of a carbohydrate in the destabilization and disruption of the bacterial cell envelope.

Reduction and oxidation of SrCoO2.5 thin films at low temperatures.

Brownmillerite SrCoO(2.5) (010) thin films synthesized by pulsed laser deposition became amorphous when reduced at low temperatures by CaH(2), indicating that the infinite-layer structure with the square planar Co(2+)O(4) configuration is unstable. Ferromagnetic and conducting perovskite SrCoO(3) epitaxial thin films, on the other hand, were obtained topotactically at room temperature by oxidation with NaClO.