1,721,312 research outputs found
Guide archeologiche. Preistoria e Protostoria in Italia. Emilia Romagna
No full textQuesto volume, che fa parte della "collana Guide Archeologiche. Preistoria e Protostoria in Italia", è dedicato all'Emilia Romagna presentandone i più importanti musei e siti archeologic
Reductive catalytic dehalogenation of light chlorocarbons
No full textSeveral palladium and platinum supported catalysts have been employed in the gas-phase dehalogenation of chloromethanes (CCl4, CHCl3 and CH2Cl2) and chlorobenzene by using molecular hydrogen under mild conditions. A new porous glass material (Vycor) has been found to be a promising carrier for platinum in the selective hydrogenation of CCl4 to CHCl3 ensuring high selectivity and durability. Palladium on alumina showed the best performance in the deep dehalogenation of CHCl3 and CH2Cl2 to CH4 and HCl. Chlorobenzene dehalogenation is a fast reaction with many catalytic systems. However, palladium catalysts produce mostly C6H6, whereas platinum ones induce overhydrogenation to cyclohexane. A comparison of reductive and oxidative processes for chloromethane disposal is presented. (C) 1999 Elsevier Science B.V. All rights reserved
Catalytic synthesis of C-nitroso compounds by cis-Mo(O)(2)(acac)(2)
No full textThe ortho, meta and para mono substituted anilines R-C6H4NH2 (R = 4-Me, 3-Me, 2-Me, 4-Et, 2-Et, 4-Br, 3-Br 2-Br; 4-Cl, 3-Cl, 2-Cl,4-F, 4-Pr-i, 4-Bu-t) were catalytically oxidised by H2O2, in the presence of cis-Mo(O)(2)(acac)(2), (acacH = CH3C(O)CH2C(O)CH3), producing the corresponding C-nitroso derivatives, R-C6H4NO. High conversions and selectivities were obtained. (C) 2000 Elsevier Science B.V. 2000 rights reserved
Gold Colloids : From Quasi-Homogeneous to Heterogeneous Catalytic Systems
No full textRuby red colloids of gold have been used for thousands of years and in the past have attracted much attention due to their optical properties. Surface plasmon resonance (SPR) bands are responsible for gold colloid colors and typically appear for nanometer-sized gold nanoparticles (GNPs). These lie in the visible range and their position (and intensity) depends on the size, distribution of size, and shape of GNPs but also on their interaction with other materials (i.e., support).
Scientists consider colloids as quasi-homogeneous systems, but because of their intrinsic thermodynamic instability, they need different capping agents providing sufficient stability. The strength and the nature of the interaction between the protective (or functionalizing) molecule and the GNP surface is of utmost importance. It can determine the catalytic properties of the nanoparticles, as they mainly interact with the active sites, thus interfering with reactant. Therefore, the protective layer should contribute to the colloid stability, but at the same time, it should not be irreversibly adsorbed on the active site of the GNP surface providing convenient accessibility to reactant.
From a catalytic point of view, the milder the interaction is between the particle surface and the capping agent, the more the activity increases. Unfortunately, the reaction conditions often do not allow the required stability of GNPs, which constitutes a fundamental prerequisite for stable catalytic activity. Anchoring GNPs on suitable supports can circumvent the problem, and this technique is now considered a valuable alternative to classical methods to produce highly dispersed gold catalysts.
In this Account, we describe the advantages in using this technique to produce gold heterogeneous catalysts of high metal dispersion on a large variety of supports with the possibility of tuning to a large extent the size and (even partially) the shape of GNPs. We also review our recent progress on the sol-immobilization technique. Specifically, we highlight how, depending on its nature, the protective agent not only mediates the activity of GNPs in alcohol oxidation process but also actively participates in the anchoring process and to the stability of GNPs depending on the support surface. We can also use the modification of the metal surface operated by the capping agent to prepare bimetallic species and influence the surface potential, which modifies the intrinsic activity of the GNP.
In conclusion, this technique implies many contributions (sometimes not yet clarified factors) that are not simply concerning dimension and dispersion of GNPs or type of support. Chemists should make careful selection of the protective agent and reaction parameters depending on which support is used in which reaction
Gold or silver-decorated multiwalled carbon nanotubes modified electrodes for trace electroanalysis
No full textTrace analysis [1] (i.e. the analysis of analytes in concentration low enough to cause difficulty, generally under 1 ppm) albeit very challenging, in the last years has shown a tremendous growth, prompted by the urgent need of many International Organizations (US Environmental Protection Agency EPA, U.S. Food and Drug Administration FDA, European Food Safety Authority EFSA, World Health Organization WHO) looking for new analytical techniques for the detection of different molecules in different and increasingly more complex matrixes.
The determination of trace analytes requires reliable and robust analytical methods characterized by high level of sensitivity, precision, accuracy, selectivity and specificity. Among different analytical techniques electroanalytical ones and particularly those based on pulsed voltammetry, seem to be a promising independent alternative in terms of very high precision, accuracy and sensitivity. Advantages in using these latter systems lie on simplicity of use, portability, easy automation and possibility of on-line and on-site monitoring without sample pre-treatments and low costs.
In this context, the use of nanosized and/or nanostructured materials for the modification of electrodes is growing in importance, with the aim of increasing the affinity for the analyte, increasing sensitivity, lowering the limits of detection and minimizing or completely avoiding interferences, i.e. increasing their selectivity. Carbon nanomaterials coupled with metal nanoparticles [2, 3] present unique peculiar properties, dependent on metal nanoparticle size and shape and therefore are extensively employed in electroanalysis as tunable materials.
In this communication, we will present the electrochemical characterization (by Cyclic Voltammetry and Electrochemical Impedance Spectroscopy) and the electroanalytical application of modified electrodes based on carbon nanotubes decorated with gold or silver nanoparticles. In particular, the synergic effect of both metal and carbon nanomaterials was investigated. Moreover, the use of PVA protective polymer for metal NPs allows better electroanalytical performances due to the protection from oxidation, fouling products and interferences. The optimized electrodes were then tested for the determination of relevant or toxic analytical substances for environmental monitoring such as glycerol and chlorinated compounds, with interesting results [4].
[1] D.T. Pierce, J.X. Zhao, Trace Analysis with Nanomaterials, Wiley-VCH, Weinheim (Germany), (2010).
[2] L. Rassaei, M. Amiri, C.M. Cirtiu, M. Sillanpaa, F. Marken, M. Sillanpaa, Trends in Analytical Chemistry 30(11) (2011) 1705-1715.
[3] V. Pifferi, V. Marona, M. Longhi, L. Falciola, Electrochimica Acta 109 (2013) 447-453.
[4] V. Pifferi, G. Facchinetti, A. Villa, L. Prati, L. Falciola, Catalysis Today, (2014), in press, doi:10.1016/j.cattod.2014.10.00
Gold catalysts for the direct oxidation of aminoalcohols to aminoacids
No full textGold catalysts on different supports, prepared by sol immobilization and deposition-precipitation, were tested in aminoalcohol oxidation for the direct preparation of aminoacids. Nitrogen substitution appeared to be crucial for avoiding parallel reaction pathway, but also the reaction conditions play an important role. Basic conditions and moderate temperature (40 °C) represent a good compromise for enhancing selectivity to aminoacids. TiO 2 represents the most versatile support
Selective oxidation of glycerol to sodium glycerate with gold on carbon catalyst: an insight into reaction selectivity
No full textGlycerol was oxidised with oxygen in the presence of gold on carbon as the catalyst. Two types of catalysts were identified: one, characterised by well-dispersed nanoparticles with a mean diameter centred at 6 nm, did not maintain the initial selectivity of the oxidation at full
conversion; the other, characterised by larger particles (> 20 nm), showed constant selectivity from the beginning to the end of the reaction.
The experimental conditions were studied to optimise glycerate production with selected catalysts, with particular regard to the effect of the NaOH/glycerol ratio, glycerol concentration, temperature, and glycerol/catalyst ratio. The best result was 92% selectivity to glycerate at full
conversion, obtained by oxidising glycerol at 30 ◦C, with a NaOH/glycerol ratio of 4, a glycerol/Au = 500, and 0.3 M concentration
- …
