The electrochemical oxidation of hydrogen at activated platinum electrodes in room temperature ionic liquids as solvents

The oxidation of hydrogen was studied at an activated platinum micro-electrode by cyclic voltammetry in the following ionic liquids: [C(2)mim][NTf2], [C(4)mim][NTf2], [N-6.2.2.2][NTf2], [P-14.6.6.6][NTf2], [C(4)mim][OTf], [C(4)mim][BF4] [C(4)mim][PF6], [C(4)mim][NO3], [C(6)mim]Cl and [C(6)mim][FAP] (where [C(n)mim](+) = 1-alkyl-3-methylimidazolium, [N-6,N-2,N-2,N-2](+) = n-hexyltriethylammonium, [P-14,P-6,P-6,P-6](+) = tris(n-hexyltetradecyl) phosphonium, [NTf2](-) = bis(trifluoromethylsulfonyl)amide, [OTf] = trifluoromethlysulfonate and [FAP](-) = tris(perfluoroethyl)trifluorophosphate). Activation of the Pt electrode was necessary to obtain reliable and reproducible voltammetry. After activation of the electrode, the H-2 oxidation waves were nearly electrochemically and chemically reversible in [C(n)mim][NTf2] ionic liquids, chemically irreversible in [C(6)mim]Cl and [C(4)mim][NO3], and showed intermediate characteristics in OTf-, [BF4](-), [PF6](-), [FAP](-) and other [NTf2](-)-based ionic liquids. These differences reflect the contrasting interactions of protons with the respective RTIL anions. The oxidation peaks are reported relative to the half-wave potential of the cobaltocenium/cobaltocene redox couple in all ionic liquids studied, giving an indication of the relative proton interactions of each ionic liquid. A preliminary temperature study (ca. 298-333 K) has also been carried out in some of the ionic liquids. Diffusion coefficients and solubilities of hydrogen at 298 K were obtained from potential-step chronoamperometry, and there was no relationship found between the diffusion coefficients and solvent viscosity. RTILs possessing [NTf2](-) and [FAP](-) anions showed the highest micro-electrode peak currents for the oxidation in H-2 saturated solutions, with[C(4)mim][NTf2] toeing the most sensitive. The large number of available RTIL anion/cation pairs allows scope for the possible electrochemical detection of hydrogen gas for use in gas sensor technology. (c) 2008 Elsevier B.V. All rights reserved.

Preparation of AgX (X = Cl, I) nanoparticles using ionic liquids

Nanoparticles of silver halides have been prepared by mixing silver halide powder with a single liquid phase consisting of an ionic liquid, isooctane, n-decanol and water. Much higher nanoparticle concentrations may be formed with ionic liquids using this new simple method than are found with conventionally applied surfactants. This method also emphasizes the applicability of ionic liquids as versatile components in microemulsions and as solvents for the synthesis of nanomaterials. The effect on the nanoparticles of changing the composition of the liquid mixtures and the nature of the ionic liquid is analysed. High nanoparticle concentrations were only found with chloride based ionic liquids, indicating the importance of the ionic liquid anion in the mechanism of the reaction.

Prediction of ionic liquid properties. I. Volumetric properties as a function of temperature at 0.1 MPa

The prediction of molar volumes and densities of several ionic liquids has been achieved using a group contribution model as a function of temperature between (273 and 423) K at atmospheric pressure. It was observed that the calculation of molar volumes or densities could be performed using the "ideal" behavior of the molar volumes of mixtures of ionic liquids. This model is based on the observations of Canongia Lopes et al. (J. Phys. Chem. B 2005, 109, 3519-3525) which showed that this ideal behavior is independent of the temperature and allows the molar volume of a given ionic liquid to be calculated by the sum of the effective molar volume of the component ions. Using this assumption, the effective molar volumes of ions constituting more than 220 different ionic liquids were calculated as a function of the temperature at 0.1 MPa using more than 2150 data points. These calculated results were used to build up a group contribution model for the calculation of ionic liquid molar volumes and densities with an estimated repeatability and uncertainty of 0.36% and 0.48%, respectively. The impact of impurities (water and halide content) in ionic liquids as well as the method of determination were also analyzed and quantified to estimate the overall uncertainty. © 2008 American Chemical Society.

Efficient heterogeneous asymmetric catalysis of the Mukaiyama aldol reaction by silica- and ionic liquid-supported Lewis acid copper(II) complexes of bis(oxazolines)

Lewis acid complexes based on copper(II) and an imidazolium-tagged bis(oxazoline) have been used to catalyse the asymmetric Mukaiyama aldol reaction between methyl pyruvate and 1-methoxy-1-tri-methylsilyloxypropene under homogeneous and heterogeneous conditions. Although the ees obtained in ionic liquid were similar to those found in dichloromethane, there was a significant rate enhancement in the ionic liquid with reactions typically reaching completion within 2 min compared with only 55% conversion after 60 min in dichloromethane. However, this rate enhancement was offset by lower chemoselectivity in ionic liquids due to the formation of 3-hydroxy-1,3-diphenylbutan-1-one as a by-product. Supporting the catalyst on silica or an imidazolium-modified silica using the ionic liquid or in an ionic liquid-diethyl ether system completely suppressed the formation of this by-product without reducing the enantioselectivity. Although the heterogeneous systems were characterised by a drop in catalytic activity the system could be recycled up to five times without any loss in conversion or ee.

Recent advances in understanding CO oxidation on gold nanoparticles using density functional theory

Since the discovery of a series of Au-based catalysts by Haruta et al. considerable progress has been made in understanding the active role of Au in CO oxidation catalysis. This review provides a summary of recent theoretical work performed in this field; in particular it addresses DFT studies of CO oxidation catalysis over free and supported gold nanoparticles. Several properties of the Au particles have been found to contribute to their unique catalytic activity. Of these properties, the low-coordination state of the Au atoms is arguably the most pertinent, although other properties of the Au cluster atoms, such as electronic charge, cannot be ignored. The current consensuses regarding the mechanism for CO oxidation over Au-based catalysts is also discussed. Finally, water-enhanced catalysis of CO oxidation on Au clusters is summarized.

Recyclable copper catalysts based on imidazolium-tagged bis(oxazolines): A marked enhancement in rate and enantioselectivity for Diels-Alder reactions in ionic liquid

Imidazolium-tagged bis(oxazolines) have been prepared and used as chiral ligands in the copper(II)-catalysed Diels-Alder reaction of N-acryloyl- and N-crotonoyloxazolidinones with cyclopentadiene and 1,3-cyclohexadiene in the ionic liquid 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [emim][NTf2]. A significant and substantial enhancement in the rate and enantioselectivity was achieved in [emim][NTf2] compared with dichloromethane. For example, complete conversion and enantioselectivities up to 95 % were obtained for the reaction between N-acryloyloxazolidinone and cyclopentadiene within 2 min in [emim][NTf2] whereas the corresponding reaction in dichloromethane required 60 min to reach completion and gave an ee of only 16 %. The enhanced rates obtained in the ionic liquid enabled a catalyst loading as low as 0.5 mol % to give complete conversion within 2 min while retaining the same level of enantioselectivity. The imidazolium-tagged catalysts can be recycled ten times without any loss in activity or enantioselectivity and showed much higher affinity for the ionic liquid phase during the recycle procedure than the analogous uncharged ligand.

Electrochemical reduction of nitrobenzene and 4-nitrophenol in the room temperature ionic liquid [C(4)dmim][N(Tf)(2)]

The reductions of nitrobenzene and 4-nitrophenol were studied by cyclic voltammetry in the room temperature ionic liquid 1-butyl2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C(4)dmim][N(Tf)(2)] on a gold microelectrode. Nitrobenzene was reduced reversibly by one electron and further by two electrons in a chemically irreversible step. The more complicated reduction of 4-nitrophenol revealed three reductive peaks (two irreversible and one reversible) which were successfully simulated using the digital simulation program DigiSim((R)) using a mechanism of rapid self-protonation, given below.

Electrochemistry of phenol in bis{(trifluoromethyl)sulfonyl}amide ([NTf2](-)) based ionic liquids

The electrochemistry of phenol and 4-tert-butyl-phenol is described in [C(2)mim][NTf2] and [C(4)mpyrr][NTf2] ionic liquids. Oxidation of phenol and phenolate is observed at E-p(a) = +1.64 and +0.24 V vs. Ag in both ionic liquids. On the cathodic sweep at a potential of -2.05 P 02 V vs. Ag under an oxygen atmosphere, the production of O-2(2-) dianions triggers the formation of phenolate anions which undergo chemical oxidation to the phenoxyl radical. The phenoxyl radical then reacts with the [NTf2](-) anion of the ionic liquid to form the corresponding phenyl triflate molecule. (c) 2005 Elsevier B.V. All rights reserved.

A mechanistic study of the electro-oxidation of bromide in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at platinum electrodes

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C(4)mim][NTf2]), and the conventional aprotic solvent. acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim(R) was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2Br(-) --> Br-2 + 2e(-)

Double potential step chronoamperometry at microdisk electrodes: simulating the case of unequal diffusion coefficients

An efficient approach to the simulation of the double potential step chronoamperometry at a microdisk electrode based on an exponentially expanding time grid and conformal mapping of the space is presented. The dimensionless second potential step flux data are included as a function of the first potential step duration and the ratio of the diffusion coefficients of the reacting species allowing instant analysis of the experimental double potential step chronoamperograms without a need for simulation. The values of the diffusion coefficients are determined for several test systems and found to be in good agreement with existing literature data. (C) 2004 Elsevier B.V. All rights reserved.

Oxidation of N,N,N ',N '-tetraalkyl-para-phenylenediamines in a series of room temperature ionic liquids incorporating the bis(trifluoromethylsulfonyl)imide anion

The five room temperature ionic liquids: 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CnMIM][N(Tf)(2)], n = 2, 4, 8, 10) and n-hexyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N-6222][N(Tf)(2)]) were investigated as solvents in which to study the electrochemical oxidation of N,N,N',N'-tetramethyl-para-phenylenediamine (TMPD) and N,N,N',N'-tetrabutyl-paraphenylenediamine (TBPD), using 20 mul micro-samples under vacuum conditions. The effect of dissolved atmospheric gases on the accessible electrochemical window was probed and determined to be less significant than seen previously for ionic liquids containing alternative anions. Chronoamperometric transients recorded at a microdisk electrode were analysed via a process of non-linear curve fitting to yield values for the diffusion coefficients of the electroactive species without requiring a knowledge of their initial concentration. Comparison of experimental and simulated cyclic voltammetry was then employed to corroborate these results and allow diffusion coefficients for the electrogenerated species to be estimated. The diffusion coefficients obtained for the neutral compounds in the five ionic liquids via this analysis were, in units of 10(-11) m(2) s(-1), 2.62, 1.87, 1.12, 1.13 and 0.70 for TMPD. and 1.23, 0.80, 0.40, 0.52 and 0.24 for TBPD (listed using the same order for the ionic liquids as stated above). The most significant consequence of changing the cationic component of the ionic liquid was found to be its effect on the solvent viscosity; the diffusion coefficient of each species was found to be approximately inversely proportional to viscosity across the series of ionic liquids, in accordance with Walden's rule. (C) 2003 Elsevier B.V. All rights reserved.

CO oxidation and NO reduction on metal surfaces: density functional theory investigations

This article reviews the accumulated theoretical results, in particular density functional theory calculations, on two catalytic processes, CO oxidation and NO reduction on metal surfaces. Owing to their importance in automotive emission control, these two reactions have generated a lot of interest in the last 20 years. Here the pathways and energetics of the involved elementary reactions under different catalytic conditions are described in detail and the understanding of the reactions is generalized. It is concluded that density functional theory calculations can be applied to catalysis to elucidate mechanisms of complex surface reactions and to understand the electronic structure of chemical processes in general. The achieved molecular knowledge of chemical reactions is certainly beneficial to new catalyst design.

On the need to use steady-state or operando techniques to investigate reaction mechanisms: An in situ DRIFTS and SSITKA-based study example

The nature of the surface species formed at the surface of 2 wt.% Pt/CeO2 catalyst during the forward water-gas-shift (WGS, CO + H2O -> CO2 + H-2) and the reverse reaction (RWGS) were essentially identical. More, the surface concentration of formate, carbonate and carbonyl species was similar in each case. The presence of well-resolved IR bands allowed an unequivocal relative quantitative analysis of each species, avoiding the use of the carboxylate stretching region (1600-1200 cm(-1)). However, the quantitative analysis in the case of an isotopic study was complicated due to the overlapping of the various isotope bands, yet this problem could be overcome by integrating the high-wavenumber part of the bands. The reactivity of the surface species formed under RWGS conditions was followed under two different gaseous streams. Firstly, the reactivity of these intermediates were followed under an inert gas (i.e., At), in which case carbonates were essentially stable and less reactive than formates. Secondly, the reactivity of the same surface species was followed when switching to the corresponding C-13-labelled feed (i.e., (CO2)-C-13 + H-2), in which case carbonates were exchanged significantly faster than formates. While carbonates species have been reported as reaction intermediate under reaction conditions, the increased stability or surface poisoning by these carbonates in the absence of reaction mixture was highlighted. Ultimately, this work re-emphasises the need to use steady-state conditions if the true operando reactivity of the adsorbates and structure of the solid are to be determined. (c) 2005 Elsevier B.V. All rights reserved.

The investigation of mechanisms in environmental catalysis using time-resolved methods

The formation of nitrogen oxides (NOx) during a combustion process is difficult to avoid because of the large exotherm and the consequent problem of avoiding local high-temperature spikes. Consequently, for many applications, such as for automotive power generation, there will be a continuing need to use catalytic after-treatment to reduce harmful emissions. The investigation of the mechanisms of the key catalytic reactions in environmental catalysis can provide an insight into the action of the catalyst, and time-resolved methods offer a powerful means to study these processes under realistic conditions. The use of Temporal Analysis of Products (TAP) and Steady State Isotopic Transient Kinetic Analysis (SSITKA) methods to investigate the reduction of NOx under various experimental conditions is described. From a detailed analysis of the SSITKA profiles, it is shown that at low temperatures the mechanism for the formation of N-2 and N2O from NO may differ from the conventional high-temperature mechanism. This is supported by density functional theory calculations, which show that the barrier to the formation of N2O from the reaction of N(ads) and NO(ads) may be too high to allow this process to occur at low temperatures. The alternative reaction of NO(ads) + NO(ads) = N2O(g) + O(ads) is shown to be much more favorable and is consistent with the SSITKA analysis. The remarkable effect of hydrogen as a reductant at low temperatures is described, and alternative interpretations of the role of hydrogen are discussed.

Sorption behavior of cationic and anionic dyes from aqueous solution on different types of activated carbons

The effect of dye molecular charges on their adsorption from solution was investigated by using different types of activated carbon adsorbents. Two types of model systems were used representing cationic and anionic dyes. Screening investigations using single point tests were used throughout the study. Cationic dyes, of which Methylene Blue is an example, showed a higher adsorption tendency towards activated carbon over anionic dyes represented by an ate-type reactive compound. Of the number of activated carbons tested, only one of the adsorbents showed an exception to this behavior, and a good relation was observed between Methylene Blue capacity and activated carbon performance. The high capacity of cationic dyes in comparison to anionic dyes was also evident in the results obtained by a preliminary kinetic study carried out on the selected systems. Surface net charge of activated carbon and the nature of attractions between the molecules were suggested to be one of the reasons attributed for this behavior.