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81.
Fonseca GS Umpierre AP Fichtner PF Teixeira SR Dupont J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(14):3263-3269
Stable transition-metal nanoparticles of the type [M(0)](n) are easily accessible through the reduction of Ir(I) or Rh(III) compounds dissolved in "dry" 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid by molecular hydrogen. The formation of these [M(0)](n) nanoparticles is straightforward; they are prepared in dry ionic liquid whereas the presence of the water causes the partial decomposition of ionic liquid with the formation of phosphates, HF and transition-metal fluorides. Transmission electron microscopy (TEM) observations and X-ray diffraction analysis (XRD) show the formation of [Ir(0)](n) and [Rh(0)](n) nanoparticles with 2.0-2.5 nm in diameter. The isolated [M(0)](n) nanoparticles can be redispersed in the ionic liquid, in acetone or used in solventless conditions for the liquid-liquid biphasic, homogeneous or heterogeneous hydrogenation of arenes under mild reaction conditions (75 degrees C and 4 atm). The recovered iridium nanoparticles can be reused several times without any significant loss in catalytic activity. Unprecedented total turnover numbers (TTO) of 3509 in 32 h, for arene hydrogenation by nanoparticles catalysts, have been achieved in the reduction of benzene by the [Ir(0)](n) in solventless conditions. Contrarily, the recovered Rh(0) nanoparticles show significant agglomeration into large particles with a loss of catalytic activity. The hydrogenation of arenes containing functional groups, such as anisole, by the [Ir(0)](n) nanoparticles occurs with concomitant hydrogenolysis of the C-O bond, suggesting that these nanoparticles behave as "heterogeneous catalysts" rather than "homogeneous catalysts". 相似文献
82.
Chronoamperograms for gold in solutions containing 0.1 M thiourea, 0.5 M H2SO4, and catalytically active sulfide ions at the concentration c 1 from 1 × 10?5 to 4 × 10?5 M are obtained at different potentials with the aid of an automated setup intended for renewing the electrode surface directly in the solution by cutting off a thin surface layer of the metal. It is shown that the results of measurements of the current practically coincide at a constant value of the product c 1 t, where t is the time period elapsed after the renewal of the electrode surface. Such a coincidence testifies to a diffusion nature of processes that hamper accumulation of sulfide ions at the gold surface. This fact permitted the use of a procedure developed previously for the calculation of polarization curves at constant values of surface coverage θ by catalytically active ions. At θ = const, the voltammetric curves for gold in sulfide-containing thiourea solutions are shown to correspond to the Tafel equation. With the surface coverage increasing, the effective values of the exchange current i 0, transfer coefficient α, and anodic reaction order with respect to thiourea P a increase from the values i }~ 10?5 A cm?2, α }~ 0.12, and P a = 0.2, which are characteristic of pure solutions, to 2 × 10?4 A cm?2, α }~ 0.5, and P a = 1.1 (at θ }~ 0.5). An interpretation to the established regularities is given. 相似文献
83.
X. N. Yang 《Journal of solution chemistry》1998,27(3):261-272
A diaphragm cell has been used to measure mutual diffusion coefficients at 25°C for four binary nonelectrolyte mixtures: ethylbenzene + n-hexane, carbon tetrachloride + ethylbenzene, cyclohexane + p-xylene, and 1,2-dichloroethane + cyclohexane. A free-volume predictive approach for binary mutual diffusion coefficients was developed and tested. Only infinite dilution diffusion coefficients, some readily available pure substance data, and UNIFAC group contribution parameters are used in the model. No binary equilibrium thermodynamic information is required. For 73 binary systems with an overall average absolute deviation of 5.2%, it has been shown that the developed method is better than two commonly available reference methods for the prediction of liquid diffusion coefficients. 相似文献
84.
85.
86.
V. G. Nefedov O. A. Artyushenko E. V. Kashevarova 《Russian Journal of Electrochemistry》2006,42(6):638-642
Mass transfer to a horizontal electrode during electrolytic evolution of oxygen and hydrogen at current densities of 100 to 10 000 A/m2 is studied. The mass transfer intensity is evaluated from the diffusion layer thickness, which varies from 60 to 5 μm at such current densities. Calculations show that the decrease in the diffusion layer thickness is due to bubbles with a stationary interphase surface crossing the diffusion layer. During the hydrogen evolution, the diffusion layer thickness is nearly the same for vertical and horizontal electrodes. During the oxygen evolution, the diffusion layer is much thinner for a horizontal electrode. Additional decrease of the diffusion layer thickness during the evolution of oxygen is associated with the lesser solution density in the near-electrode layer and with its transport away by means of natural convection. 相似文献
87.
Summary The reversed-flow method for measurement of gas diffusion coefficients in binary mixtures is now extended to simultaneous determination of effective diffusion coefficients for each substance in a multicomponent gas mixture. The method is applied to six ternary mixtures, each consisting of two gaseous hydrocarbons and H2, He or N2. The results are in agreement with a limiting case of the Stefan-Maxwell equations. 相似文献
88.
Taking soil colloid and hydrated silica (quartz sand) as the experimental material, the comparative study has been made on the kinetics of ion diffusion and ion exchange in charged colloid and charged coarse disperse systems. The results showed that ion exchange kinetics in the two systems conform to the kinetic law of ion diffusion. Besides, through this comparative study on the kinetics of ion exchange and ion diffusion, a method has been advanced theoretically to estimate the quantity of adsorbed ion that is located in the inner of the Helmholtz layer. As far as hydrated silica is concerned , there were about 33 per cent of the total adsorbed quantity of Mg2+that were located in the inner of the Helmholtz layer under the given experimental conditions, bu tfor soil colloid the percentage was only 7.5. 相似文献
89.
A Taylor dispersion tube has been used to measure mutual diffusion in aqueous solutions of glycine hydrochloride at 25°C and concentrations from 0.0005 to 0.5 M. Analysis of the dispersion profiles shows that the diffusion of glycine hydrochloride (GlyHCl) produces a subtantial additional flow of hydrochloric acid that is liberated by the dissociation: GlyH+ + Cl- Gly + H+ + Cl-. Diffusion in this system is, therefore, a ternary process described by the equations J
1(GlyHCl) = – D
11C
1 – D
12C
2 and J
2(HCl) = –D
21C
1 – D
22C
2 for the coupled fluxes of total glycine hydrochloride (1) and hydrochloric acid (2) components. The ratio D
21/D
11 of measured diffusion coefficients indicates that up to two moles of HCl are cotransported per mole of GlyHCl. Although protonated glycine diffuses with relatively mobile Cl– counterions, the main diffusion coefficient of glycine hydrochloride, D
11, is lower than or nearly identical to the diffusion coefficient of aqueous glycine. A model for the diffusion of protonated solutes is developed to interpret this result and the large coupled flows of HCl. Diffusion coefficients are also reported for the aqueous hydrochlorides of 3- and 4-aminobenzoic acids. 相似文献
90.
2-Azido-3,5-dichloropyridine and 2-azido-5-chloro-3-fluoropyridine were given by reaction of sodium azide with 2,3,5-trichloropyridine, 3,5-dichloro-2-fluoropyridine or 5-chloro-2,3-difluoropyridine in ionic liquids. 1,3-Dipolar cycloaddition of 2-azido-3,5-dichloropyridine or 2-azido-5-chloro-3-fluoropyridine to alkynes in ionic liquids afforded the corresponding 1,4,5-trisubstituted [1,2,3]-triazoles in good yields and regioselectivities. 相似文献