Titania- and zirconia-supported gold particles of 1−5 nm size, prepared by various routes of synthesis, were employed in the partial hydrogenation of acrolein. In-depth characterization of their structural and electronic properties by electron microscopy, electron paramagnetic resonance, and optical absorption spectroscopy aimed at disclosing the nature of the active sites controlling the hydrogenation of CO vs CC bonds. The structural characteristics of the catalysts, as mean particle size, size distribution, and dispersion, distinctly depend on the synthesis applied and the oxide support used whereby the highest gold dispersion (D Au = 0.78, Au/TiO2) results from a modified sol−gel technique. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. Besides the influence of the surface geometry on the adsorption mode of the α,β-unsaturated aldehyde, the marked structure sensitivity of the catalytic properties with decreasing particle size is attributed to the electron-donating character of paramagnetic F-centers forming electron-rich gold particles as active sites. The effect of structural and electronic properties due to the quantum size effect of sufficiently small gold particles on the partial hydrogenation is demonstrated.

The active sites of supported gold catalysts, favoring the adsorption of C=O groups of acrolein and subsequent reaction to allyl alcohol, have been identified as edges of gold nanoparticles. After our recent finding that this reaction preferentially occurs on single crystalline particles rather than multiply twinned ones, this paper reports on a new approach to distinguish different features of the gold particle morphology. Elucidation of the active site issue cannot be simply done by varying the size of gold particles, since the effects of faceting and multiply twinned particles may interfere. Therefore, modification of the gold particle surface by indium has been used to vary the active site characteristics of a suitable catalyst, and a selective decoration of gold particle faces has been observed, leaving edges free. This is in contradiction to theoretical predictions, suggesting a preferred occupation of the low-coordinated edges of the gold particles. On the bimetallic catalyst, the desired allyl alcohol is the main product (selectivity 63%; temperature 593 K, total pressure p(total) = 2 MPa). From the experimentally proven correlation between surface structure and catalytic behavior, the edges of single crystalline gold particles have been identified as active sites for the preferred C=O hydrogenation.

Highly ordered silver nanowire arrays have been obtained by pulsed electrodeposition in self-ordered porous alumina templates. Homogeneous filling of all the pores of the alumina template is achieved. The interwire distance is about 110 nm corresponding to a density of silver nanowires of 61ϫ10 9 in. Ϫ2 and the diameter can be varied between 30 and 70 nm. The silver wires are monocrystalline with some twin lamella defects and grow perpendicular to the ͗110͘ direction. The previously encountered difficulty to obtain 100% filling of the alumina pores is discussed in the framework of electrostatic instabilities taking into account the different potential contributions during electrodeposition. To obtain homogeneously filled pore membranes, a highly conductive metal containing electrolyte, a homogeneous aluminum oxide barrier layer, and pulsed electrodeposition are a prerequisite.

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Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO 2 and Ag/ TiO 2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silver particle size on activity and selectivity toward the unsaturated alcohol. Nanostructural features of the catalysts, as their silver particle size distribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity to the unsaturated alcohol was found to be (59 ( 3)%, independent of the particle size in the range of 3.7 nm e d h Ag e 6.3 nm. Moreover, the specific activities were similar in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO 2 catalysts appears to be structureinsensitive. Titania-supported silver catalysts reduced in hydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), however, showed a quite different behavior. These silver particles exhibit rather narrow size distributions and very low mean particle sizes (d h Ag ) 2.8 ( 1.9 nm for Ag/TiO 2 -LTR, d h Ag ) 1.4 ( 0.5 nm for Ag/TiO 2 -HTR), i.e., an exceedingly high dispersion (D Ag ) 0.46 and 0.69, respectively). The LTR catalyst gave a higher selectivity to crotyl alcohol (53%) than the ultradispersed HTR catalyst (28%). This pronounced change in selectivity suggests the hydrogenation of crotonaldehyde over these Ag catalysts to be qualified as structure-sensitive with the rate-determining step depending critically on the silver particle size and thus on the silver surface structure. If hydrogenation of the CdO group of the R,β-unsaturated aldehyde is favored by face atoms, most likely the increased fraction of Ag(111) planes of the larger silver particles will give higher formation rates of the desired unsaturated alcohol.

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Silicon-rich silicon oxide thin films have been prepared by thermal evaporation of silicon monoxide in vacuum. The SiOx film composition (1.1⩽ x ⩽1.7) has been controlled by varying the deposition rate and residual pressure in the chamber. Long time stability of all films has been ensured by a postdeposition annealing at 523 K for 30 min in Ar atmosphere. Some films were further annealed at 973 K and some others at 1303 K. Raman scattering measurements have implied the formation of amorphous silicon nanoparticles in films annealed at 973 K and Si nanocrystals in films annealed at 1303 K. The latter conclusion is strongly supported by high resolution electron microscopy studies which show a high density of Si nanocrystals in these films. Photoluminescence has been observed from both amorphous and crystalline nanoparticles and interpreted in terms of band-to-band recombination in the nanoparticles having average size greater than 2.5 nm and carrier recombination through defect states in smaller nanoparticles.

Au/Pd bimetallic nanoparticles, even with core-shell structure, were synthesized by successive and simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first obtained by reduction of Pd(NO 3) 2 , followed by adding HAuCl 4 solution. As a result, Au-core/Pd-shell structured particles are formed, instead of Pd-core/Au-shell as usually expected. The successive method is more effective than the simultaneous one in the formation of the core-shell structure. Detailed investigation with optical absorption spectroscopy suggested that the pre-formed Pd atoms or clusters have a reduction effect on Au 3+ ions and the post-formation of Pd-shell can damp the surface plasmon resonance of Au nanoparticles. Theoretical absorption spectra based on Mie-like model for core-shell structured particles yield excellent agreement with the experimental results.