This article analyses the role of gold nanoparticles supported on TiO 2 for the gas-phase ethanol condensation. Previously, the original P25 surface was modified for increasing the Au-Ti interaction, in order to minimize the thermal deactivation. Catalysts were tested both in absence and presence of hydrogen (523-673 K, WHSV = 7.9 h-1 ; y EtOH = 0.32; y H2 = 0-0.1; 0.1 MPa). Parent TiO 2 is mainly selective for dehydration reactions yielding diethyl ether (favoured at low temperatures) and ethylene (favoured at higher temperatures). The presence of Au i n the catalyst promotes dehydrogenation pathways, yielding acetaldehyde, as well as condensation products (mainly butanol, with selectivities close to 10%). According to DRIFT spectroscopy results, the strong ethanol adsorption on the TiO 2 surface justifies the low yie lds and the high relevance of side-reactions produced by inter-or intra-molecular dehydration routes (diethyl ether, and ethylene formation). The gold addition minimizes this adsorption and enhances the main route by a double role: an improvement in the dehydrogenation rate (yielding more acetaldehyde) and an enhancement in the hydrogenation steps.

Metallic silver particles in the nanometer size range were obtained in SiO2 matrix by the reduction of AgNO3 with the non-ionic diblock copolymer (Brij 58). Hexagonal mesostructured sol-gel films were synthesized by dip-coating method using the surfactant Brij58 to produce channels into the film, which house the silver nanoparticles. Optical properties of the metallic nanoparticles were studied by UV-Vis spectroscopy, TEM and HRTEM images. The experimental absorption spectrum of the metallic silver nanoparticles exhibits an absorption band located at 438 nm and a shoulder at longer wavelength. The TEM images show randomly distributed silver nanoparticles (Type I) along with some oriented as long line (Type II). Both distributions exhibit a silver oxide shell around of them. The second shell covering the silver core - silver oxide shell system is related to the surfactant. The optical absorption spectrum was modelled using the Gans theory. The fit shows two main contributions related to metallic silver nanoparticles with different axial ratios, and surrounding of a dielectric medium with high refractive index. Presence of the high refractive index silver oxide shell was confirmed by X-ray diffraction technique. The contributions of silver core and silver oxide shell play important roles in the optical properties of the films.

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Silver nanoparticles in sol–gel silica films were obtained by annealing in hydrogen atmosphere and subsequently in oxygen atmosphere. Their properties were measured by UV-vis spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electronic microscopy (HRTEM) and X-ray diffraction analysis. Samples prepared in a reducing atmosphere exhibited a surface plasmon resonance (SPR) located at 399 nm. Silver nanoparticles in an oxidizing atmosphere exhibited a red shift and damping of the SPR. These optical properties were explained due to the oxidation on the surface of silver nanoparticles to silver oxide yield in an oxygen atmosphere. Silver core–silver oxide shell nanostructures were observed by HRTEM. The average size of the metallic nanoparticles obtained by TEM was used for modeling the UV-vis spectra by using the Gans theory. Good fits to the spectra under an oxidizing atmosphere were obtained considering variable refractive indexes coming from the silver oxide shells surrounding to the nanoparticles. Therefore, the interaction between oxygen and the metallic surface of the nanoparticles, sensitively alters their optical properties.