Kevin J. Hughes

Controlling Nanocrystal Superlattice Symmetry and Shape-Anisotropic Interactions through Variable Ligand Surface Coverage

The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order.

A unified approach to the reduced kinetic modeling of alkane combustion

A methodology for the generalized kinetic modelling of the combustion of alkanes is presented. By contrast to previous approaches to kinetic modelling of hydrocarbon oxidation, the reactions incorporated in the present model do not evolve from a specified fuel molecule. They are based directly on the numbers of primary, secondary or tertiary CH bonds formed selectively from the molecular structure of a single component fuel, or a mixture of components. The low-temperature oxidation of alkylperoxy radical isomerization the selective abstraction processes, is then represented by the modes of alkylperoxy radical isomerization which each type of alkyl radical can undergo. Competitive branching (via diperoxy species) and propagation reactions (via OH and HO2) are included in the scheme. The transition to the “high temperature” mechanism is made in a unified way to avoid a multiplicity of supplementary reactions as the complexity of the molecular fuel structure changes. The overall model comprises 41 species in 116 reactions, but this is applicable to a variety of alkane isomers and their mixtures without qualitative change. The model is tested against the ignition delays of a range of fuels measured in a rapid compression machine. Comparisons are made also with recent shock tube data. The simulation of pressure and temperature changes in two-stage ignition are demonstrated. Aspects of the kinetic structure of low temperature alkane oxidation are also discussed.

An investigation of important gas-phase reactions of nitrogenous species from the simulation of experimental measurements in combustion systems

Simulated results from a detailed elementary reaction mechanism for nitrogen-containing species in flames consisting of hydrogen, C1 or C2 fuels are presented, and compared with bulk experimental measurements of nitrogen-containing species in a variety of combustion systems including flow reactors, perfectly stirred reactors, and low pressure laminar flames. Sensitivity analysis has been employed to highlight the important reactions of nitrogenous species in each system. The rate coefficients for these reactions have been compared against the expressions used in three other recent reaction mechanisms: version 3.0 of the GRI mechanism, the mechanism of Glarborg, Miller and co-workers, and that of Dean and Bozzelli. Such comparisons indicate that there are still large discrepancies in the reaction mechanisms used to describe nitrogen chemistry in combustion systems. Reactions for which further measurements and evaluations are required are identified and the differences between the major mechanisms available are clearly demonstrated.

A global sensitivity study of cyclohexane oxidation under low temperature fuel-rich conditions using HDMR methods

This paper presents a global sensitivity analysis of simulations of low-temperature isothermal cyclohexane oxidation under fuel-rich conditions using the method of high-dimensional model representation (HDMR). The analysis is used to investigate the important features of the oxidation process, as well as possible factors underlying qualitative discrepancies between simulations and experiments. The particular feature of interest is the characteristic of quadratic autocatalysis, which is observed experimentally and leads to the maximum rate of reaction occurring at 50% consumption of the deficient reactant (oxygen), with the fuel consumption exerting only a weak dependence. The kinetic mechanisms tested do not exhibit this characteristic when simulating the experimental conditions. The models also exhibit shorter induction times than those observed in the experiment. The HDMR study demonstrates a sensitivity of these features to the A-factors of key reactions of the cyclohexylperoxy radical (C6H11OO). At the low temperatures studied here, these are peroxy–peroxy radical reactions rather than the isomerisation routes that have been the subject of other investigations at higher temperatures. The low temperature product channels for reactions of the cyclohexylperoxy radical are therefore an important area for future kinetic studies. The effects of wall reactions of peroxy and peroxide species were not found to outweigh the impact of the main A-factors, but including wall losses led to significant higher order interactions between input parameters. This constitutes an interesting and important area for further research.

Through-Plane Permeability for Untreated and PTFE-Treated Gas Diffusion Layers in Proton Exchange Membrane Fuel Cells

The through-plane permeability has been experimentally measured for untreated and polytetrafluoroethylene-treated (PTFE-treated) gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells. Contrary to what the literature has previously shown, the results suggest that there exists an optimum value for the wet-proofing agent, PTFE, at which the though-plane permeability of the GDL is a maximum. The analysis that is undertaken to investigate the effect of the air compressibility shows that there is an underestimation in the through-plane permeability of the tested GDLs by of the order of 9% if the density of the air is assumed to be constant. Also, a nondimensionalisation analysis shows that ignoring the non-Darcy effects at the maximum reported flow rates result in negligible errors for the untreated and treated GDLs.

Experimental and modelling study of sulfur and nitrogen doped premixed methane flames at low pressure

Laser-induced fluorescence (LIF) has been used to observe NS and NO in methane/oxygen/argon laminar flames at low pressure doped with ammonia and sulfur dioxide. NS profiles as a function of height above the burner have been measured for rich flames. The effect of adding various amounts of sulfur dioxide on the observed NO in the burnt gas region has been investigated for a variety of stoichiometries. The experimental measurements have been compared with PREMIX simulations using a detailed elementary reaction mechanism for nitrogen- and sulfur-containing species in a methane flame. Sensitivity analysis has been employed to highlight the important reactions for NS, NO and SO2. The results demonstrate significant uncertainties in currently best available rate data for important reactions involving sulfur-containing species.

Direct measurements of the peroxy—hydroperoxy radical isomerisation, a key step in hydrocarbon combustion

Abstract The rate coefficient for an alkyl peroxy (RO 2 ) → alkyl hydroperoxy radical isomerisation reaction has been measured directly for the first time (with R = neo-pentyl). The alkyl radical was generated by the 248 nm pulsed photolysis of neo-pentyl iodide and the OH product of the peroxy radical reaction sequence was detected by laser-induced fluorescence. The time constant for the growth of OH varies with [O 2 ], as the rate determining step in the reaction sequence changes. This variation permits the rate coefficients for peroxy radical isomerisation ( k 3 ) and decomposition ( k −2 ) to be determined. At 700 K, k 3 =1.24 × 10 3 s −1 and k −2 =9.1 × 10 3 s −1 . The value obtained for k 3 is more than an order of magnitude smaller than that obtained by Baldwin, Hisham and Walker using indirect methods. It is demonstrated that the present experimental results are compatible with those of Baldwin, Hisham and Walker and the discrepancy arises from their use of an inaccurate estimate of the equilibrium constant for formation of the peroxy radical.

Through-plane permeability for untreated and PTFE-treated gas diffusion layers in proton exchange membrane fuel cells

The through-plane permeability has been experimentally measured for untreated and PTFE-treated gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells. Contrary to what the literature has previously shown, the results suggest that there exists an optimum value for the wet-proofing agent, PTFE, at which the though-plane permeability of the GDL is a maximum. The analysis that is undertaken to investigate the effect of the air compressibility shows that there is an underestimation in the through-plane permeability of the tested GDLs by of the order of 9% if the density of the air is assumed to be constant. Also, a non-dimensionlisation analysis shows that ignoring the non-Darcy effects at the maximum reported flow rates results in negligible errors for the untreated and treated GDLs.Copyright

Direct measurements of the neopentyl peroxy-hydroperoxy radical isomerisation over the temperature range 660-750 K

The rate constant for the isomerisation reaction neo-C 5 H 11 O 2 →C 5 H 10 OOH ( k 3 ) has been determined directly over the temperature range 660–750 K. neo-C 5 H 11 I was photolysed at 248 nm using a KrF laser in the presence of O 2 and He. The alkyl radical generated in the photolysis reacts with O 2 to form the peroxy radical which then isomerises to the hydroperoxy radical. Subsequent rapid reactions lead to the generation of OH, which was detected by laser induced fluorescence as a function of time. At high [O 2 ] the time constant, λ + , for the build up of OH tends to − k 3 . As [O 2 ] decreases, earlier reactions in the peroxy radical chain become important and analysis of the [O 2 ] dependence of λ + allows both k 3 and k 2 , the rate constant for the peroxy radical decomposition, to be determined. Data analysis shows that the results are fully compatible with the steady-state measurements of Baldwin et al except that values for k 3 a factor of over ten lower than their values are obtained. The discrepancy is shown to be due to errors in the equilibrium constant, K 2 , they used for the (R2) reaction. C 5 H 11 + O 2 ⇌ C 5 H 11 O 2 An Arrhenius analysis gives ( k 3 / s 1 ) = 10 12.2 − ˙ 0.77 exp { − ( 1.48 ± 0.12 ) × 10 4 K / T } The measurements of k −2 were combined with literature data for k 2 and calculated values of ΔS 2 8 to give ΔH 2 8 (298)=142±6kJ mol −1 for the neo-C 5 H 11 + O 2 ⇌ C 5 H 11 O 2 equilibrium, in satisfactory agreement with group additivity values.

Fitness Diversity Based Adaptive Memetic Algorithm for solving inverse problems of chemical kinetics

This paper proposes the fitness diversity based adaptive memetic algorithm (FIDAMA) for solving the problem of the inverse type consisting of retrieving chemical kinetics reaction rate coefficients in the generalised Arrhenius form based on the observed concentrations in a given range of temperatures of a limited set of species which describe the reaction mechanism. FIDAMA consists of the evolutionary framework and three local searchers adaptively governed by a novel fitness diversity based measure. Moreover, a certain simplification of the decision space was carried out without any deterioration in the result obtained. The numerical results preseted show the superiority of FIDAMA compared to the other published computational intelligence methods.