[Show abstract][Hide abstract] ABSTRACT: The hydrogenation of acetone to isopropanol has been studied in the vapour phase over Pt/Al2O3 catalysts. The rate law obtained at a total pressure of 1 atm and temperatures between 303 and 363 K is of the form V=kP0aP1/2H exp (-44 kJ mol−1 RT−1). The kinetic results are consistent with a Langmuir-Hinshelwood hydrogenation mechanism involving a half hydrogenated species and a non-competitive chemisorption of acetone and hydrogen on the platinum surface. The specific activity (calculated per platinum surface atom) has been found to be scarcely dependent on the platinum particle size. It is suggested that the chemisorption sites are made of a very small ensemble of platinum atoms.
[Show abstract][Hide abstract] ABSTRACT: Transmission and scanning electron microscopy techniques were used in conjunction with X-ray microchemical analysis to investigate the structure, morphology and composition of direct methanol fuel cells components. Two different preparation methods were adopted for fabrication of membrane—electrode assembly. The first was based on a conventional method involving ionomer spreading on electrodes, the second method concerned the direct mixing of catalyst and ionomer. The latter method allowed improved electrochemical activity and conductivity characteristics to be obtained. According to electron microscopy and X-ray microchemical analyses, the observed results were attributed to an improved bonding between catalyst and ionomer and to a larger extension of the three-phase reaction zone at the electrode—electrolyte interface.
[Show abstract][Hide abstract] ABSTRACT: An investigation was carried out in the electro-oxidation of methanol on a carbon-supported quaternary PtRuSnW catalyst prepared by a liquid-phase reduction method. As derived by X-ray diffraction and X-ray photoelectron spectroscopy, the catalyst was composed of metallic Pt, microcrystalline RuO2 and SnO2 phases and amorphous WO3/WO2 species. The electrochemical analysis was carried out in half-cell containing sulfuric acid electrolyte as well as in a liquid methanol-fed solid polymer electrolyte single-cell. The activity of catalyst in the half-cell varied as a function of the methanol concentration, it increased with CH3OH molarity in the activation-controlled region and showed a maximum in 2 M CH3OH at high currents. IR-free polarization curves showed that the activity of the quaternary catalyst was superior to Pt metal/C samples having the same Pt amount. The presence of semi-insulating metal oxides such as RuO2, SnO2 and WO3 on the electrode surface exhibited a significant uncompensated resistance. The single-cell performance was lower than that predicted by the half-cell experiments mainly due to the methanol cross-over through the Nafion membrane.
Journal of Power Sources 06/1995; 55(2-55):159-166. DOI:10.1016/0378-7753(94)02178-6 · 6.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the sector of solid polymer electrolyte fuel cells, the CNR-TAE activities have been focused on the development of a fuel cell system based on a proprietary dual-layer electrode preparation procedure. The influence of some characteristic parameters of the electrodes, i.e. hydrophobic agent and Nafion® loadings, on electrochemical performance has been explored. It has been concluded that in the diffusion layer, the amount of hydrophobic agent controls the water transfer into the cell and consequently the mass transport phenomena. In the active layer, the Nafion® content mostly influences the electrocatalytic activity and the ionic resistance of the cell.
International Journal of Hydrogen Energy 06/1994; 19(6):523-527. DOI:10.1016/0360-3199(94)90007-8 · 2.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The water transport in a solid polymer electrolyte fuel cell (SPEFC) can be regulated by varying the amount of hydrophobic agent (polyfluoroethylenepropylene) in the gas diffusion layer of the anode, which controls the ionic resistance as well as the gas diffusion in the cell. It has been found that at high water transport the ionic resistance of the electrolyte is lower, while gas diffusion is hindered by the presence of a high water content. A maximum in the cell performance is obtained at intermediate water transport, when the ionic resistance is reasonably low and the mass transport phenomena become negligible. Prototech electrodes, which are morphologically different from ours, also showed a low ionic resistance at low water transport.
[Show abstract][Hide abstract] ABSTRACT: The location of Naflon in solid-polymer-electrolyte-fuel-cell electrodes was investigated by porosimetry and scanning microprobe analysis. At low Nafion loadings, the polymeric electrolyte uniformly fills the micro- and macropores of the electrode structure and increases its ionic conductivity. The pores are completely filled in the range from 0.8 to 1.0 mg of Nafion/cm2 of electrode, and further addition of polymer results in the formation of a film on the external surface of the electrode. This film causes an additional resistance in series with the ionic resistance in the active layer. Impedance spectroscopic measurements of electrodes with different Nafion loadings provided evidence for a variation of ionic resistance with the amount of polymer. The comparison of impedance data with polarization tests indicates that, at least for our formulations, the ionic resistance is the main factor which controls the electrochemical performance.
Journal of The Electrochemical Society 01/1992; 139(3):761-765. · 2.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hydrogenation of nitrobenzene has been carried out over Pt and Pt-Sn catalysts at 273 K using ethanol as solvent. In the first stage of the reaction the intermediate phenylhydroxylamine is formed, together with aniline. After almost all nitrobenzene is converted, phenylhydroxylamine is hydrogenated further to aniline. Selectivity to phenylhydroxylamine is scarcely influenced by the level of conversion and by the Pt/Sn ratio.Upon addition of tin, catalytic activity increases at low tin content, and then decreases at higher Sn/Pt ratios. It is suggested that at low concentration tin ions act as promoters by activating the oxygen-containing group. At higher tin content, platinum is poisoned and availability of activated hydrogen becomes rate-determining.
Journal of Molecular Catalysis 11/1987; 42(3-42):379-387. DOI:10.1016/0304-5102(87)85014-9
[Show abstract][Hide abstract] ABSTRACT: Hydrogenation of α,β-unsaturated aldehydes has been carried out over supported Pt and PtSn/ catalysts. Over platinum there is preferential formation of saturated adehydes due to the lower energy of the CC double bond with respect to the CO bond. Addition of tin to Pt/Nylon causes a drastic change in the product distribution. On the PtSn samples the reaction proceeds mainly through the hydrogenation of the CO bond with formation of α,β-unsaturated alcohols. The rate of reaction has been found to increase with the addition of small amounts of tin. However, catalysts having a ratio higher than 1.2–1.3 are inactive. It is suggested that the effect of tin ions is related to their acid properties which enhance the reactivity of the CO bond. At higher tin contents the platinum active sites are unable to activate hydrogen molecules.
[Show abstract][Hide abstract] ABSTRACT: Hydrocinnamaldehyde and phenylacetylene have been hydrogenated over Pt and Pt-Sn catalysts. The Pt-Sn system has been studied as a function of the Pt/Sn ratio. In the hydrogénation of the CC triple bond, addition of tin to platinum causes a sharp decrease in the rate of reaction. In the hydrogénation of the CO group, an increase is instead observed at lower tin contents with a subsequent decrease at the higher Sn/Pt ratios.These results are discussed in terms of: (a) an interaction between tin and platinum which deactivates the active metal sites and (b) an interaction between hydrocinnamaldehyde and tin ions which enhances the reactivity of the CO group.
Journal of Molecular Catalysis 06/1986; 35(3-35):365-375. DOI:10.1016/0304-5102(86)87084-5
[Show abstract][Hide abstract] ABSTRACT: The physical properties of Pt and Pt–Sn/Nylon catalysts have been determined using X-ray scattering techniques (SAXS and WAXS). Two different structural conditions of Pt/Nylon were inferred from the characterization. A large amount of the supported metal phase, ca. 100 Å in size, retains the Pt crystalline structure and appears to be embedded in the Nylon. A second fraction with a strong metal–ligand interaction is also present. Addition of tin to the Pt/Nylon system does not change the observed structural framework, although a Pt–Sn solid solution appears with a decrease in the size of the metallic phase.
Journal of the Chemical Society Faraday Transactions 1 01/1985; 81(2). DOI:10.1039/f19858100321 · 4.20 Impact Factor