Electrochemical carbon dioxide reduction in ionic liquids at high pressure.

Imidazolium ionic liquids are potentially useful solvents for both carbon dioxide reduction conversion and capture. In particular electrocatalytic CO2 reduction has been shown to occur at low overpotentials using a 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) and water mixed solvent. A limitation of such solvent systems is their viscosity, making it hard to maintain reasonable catalytic current densities without energy intensive stirring/agitation of the electrolyte. Here we explore the electrochemical reduction of CO2 at high pressures (0.1 to 5.1 MPa) and demonstrate a correlation between the volume of expansion of the ionic liquid and the achieved catalytic current density. The improved electrocatalytic behaviour is proposed to be due to both the increased bulk CO2 concentration and the improved mass transport properties of the gas-expanded ionic liquid. These initial studies at pressure represent a step towards realising an integrated CO2 capture and utilisation system based around a common ionic liquid.

Size-selective separation of polydisperse gold nanoparticles in supercritical ethane.

The aim of this study was to use supercritical ethane to selectively disperse alkanethiol-stabilized gold nanoparticles of one size from a polydisperse sample in order to recover a monodisperse fraction of the nanoparticles. A disperse sample of metal nanoparticles with diameters in the range of 1-5 nm was prepared using established techniques then further purified by Soxhlet extraction. The purified sample was subjected to supercritical ethane at a temperature of 318 K in the pressure range 50-276 bar. Particles were characterized by UV-vis absorption spectroscopy, TEM, and MALDI-TOF mass spectroscopy. The results show that with increasing pressure the dispersibility of the nanoparticles increases, this effect is most pronounced for smaller nanoparticles. At the highest pressure investigated a sample of the particles was effectively stripped of all the smaller particles leaving a monodisperse sample. The relationship between dispersibility and supercritical fluid density for two different size samples of alkanethiol-stabilized gold nanoparticles was considered using the Chrastil chemical equilibrium model.

Dependence of the critical temperature on molecular parameters.

At the critical temperature the surface tension between coexisting liquid and vapor phases must be zero, and the repulsive contributions associated with cavity formation must exactly counterbalance those from interactions of a molecule in the cavity and the bulk. An expression for the critical temperature of pure fluids in terms of the parameters of scaled particle theory (SPT) has been obtained, and the calculated critical temperatures are compared with experimental data for a range of pure fluids. These include noble and diatomic gases, short and medium length hydrocarbons, aromatic compounds, halogenated compounds, oxygen-containing compounds, and water. Considering the simplicity of this approach, a remarkably good correlation between calculated and experimental values is found for most of these fluids.

Generalized van der Waals Theory of Interfaces in Simple Fluid Mixtures.

An efficient implementation of the generalized van der Waals theory of fluids is presented for the calculation of surface tension in simple fluid mixtures. While detailed correlation analysis is avoided the dominant binding energy contribution and the negative contribution due to the nonlocal entropy are accounted for in the free energy density functional by simple physical approximations of the type originally introduced by van der Waals. Efficient computation is achieved by the use of a single-parameter optimization of a tanh-shaped profile representing the total density as well as the composition variation across the interface. This simple profile nevertheless incorporates the expected adsorption to the interface of the volatile component. Application is made to argon/krypton mixtures represented by Lennard-Jones potentials and Lorentz-Berthelot combining rules. Surface tension predictions compare well with both experimental observations and computer simulation results which also indicated close agreement in particle density profiles, especially if the Berthelot rule is amended with a binary interaction parameter slightly (3%) less than unity. Copyright 2001 Academic Press.