A density functional theory study of the a-olefin selectivity in Fischer-Tropsch synthesis

We studied the alpha-olefin selectivity in Fischer-Tropsch (FT) synthesis using density functional theory (131717) calculations. We calculated the relevant elementary steps from C-2 to C-6 species. Our results showed that the barriers of hydrogenation and dehydrogenation reactions were constant with different chain lengths, and the chemisorption energies of alpha-olefins from DFT calculations also were very similar, except for C-2 species. A simple expression of the paraffin/olefin ratio was obtained based on a kinetic model. Combining the expression of the paraffin/olefin ratio and our calculation results, experimental findings are satisfactorily explained. We found that the physical origin of the chain length dependence of paraffin/olefin ratio is the chain length dependence of both the van der Waals interaction between adsorbed alpha-olefins and metal surfaces and the entropy difference between adsorbed and gaseous alpha-olefins, and that the greater chemisorption energy of ethylene is the main reason for the abnormal ethane/ethylene ratio. (c) 2008 Elsevier Inc. All rights reserved.

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.

A modified commercial DRIFTS cell for kinetically relevant operando studies of heterogeneous catalytic reactions

This paper discusses a number of checks that should be carried out to ensure that the kinetic and spectroscopic measurements made using a DRIFTS cell are meaningful. The observations reported here demonstrate how an appropriately modified commercial DRIFTS cell can provide pertinent kinetic information about both gaseous products and the related surface intermediates. The oxidation of CO with 02 was used as a test to assess the catalyst bed bypass by the reaction mixture. Full CO conversion was obtained after the light-off temperature in the case of the modified cell, contrary to the case of the original cell, for which 80% of the reaction mixture bypassed the catalyst bed. The water-gas shift reaction over a Pt/CeO2 catalyst was used as a model reaction to further characterize the behavior of the cell under reaction conditions. The catalyst bed was shown not to be a dead-zone and was purged in essentially the same time as that needed to purge the cell. The reaction chamber globally operated in a quasi plug-flow mode and the gas composition in the thin catalyst bed appears to be homogeneous when operated under differential conditions. The production of the gas-phase reaction product CO2 could be simultaneously followed both by mass spectrometry and DRIFTS, both techniques leading to identical results. Various IR bands integration methods were discussed to allow a precise and accurate determination of the surface concentration of adsorbates during isotopic exchange. (c) 2008 Elsevier B.V. All rights reserved.

Mineralogy of selected geological deposits from the United Kingdom and the Republic of Ireland as possible capping materials for low-level radioactive waste disposal facilities

Solid low-level radioactive waste (LLW) is currently being disposed at a number of facilities in the United Kingdom (UK). The safety of these facilities relies to some extent on the use of engineered barriers, such as a cap, to isolate the waste and protect the environment. Generally, the material used as the barrier layer within such a cap should be of low permeability and it should retain this property over long timescales (beyond a few decades normally required for facilities containing non-radioactive wastes). The objective of this research is to determine the mineralogy of selected geological deposits from the UK and Ireland as part of a larger project to examine their suitability as a capping material, particularly on LLW sites. Mineral transformations, as a result of future climate change, may impact on the long-term performance of the cap and even the disposal facility. X-ray diffraction (XRD) was carried-out on the sand, silt and clay fractions of the London Clay, Belfast Upper Boulder Clay, Irish Glacial Till, Belfast Sleech, and Ampthill Clay geological deposits. Minerals were present that could pose both positive and negative effects on the long-term performance of the cap. Smectite, which has a high shrink swell potential, may produce cracks in London Clay, Belfast Upper Boulder Clay and Ampthill Clay capping material during dry, hotter periods as a possible consequence of future climate change; thus, resulting in higher permeability. Ampthill Clay and Belfast Sleech had elevated amounts of organic matter (OM) at 5.93% and 5.88%, respectively, which may also contribute to cracking. Over time, this OM may decompose and result in increased permeability. Gypsum (CaSO4) in the silt and sand fractions of Ampthill Clay may reduce the impact of erosion during wetter periods if it is incorporated into the upper portion of the cap. There are potential negative effects from the acidity created by the weathering of pyrite (FeS2) present in the silt and sand fractions of Belfast Sleech and Ampthill Clay that could impede the growth of grasses used to stabilize the surface of the capping material if this material is used as part of the vegetative soil layer. Additionally, acidic waters generated from pyrite weathering could negatively impact the lower lying capping layers and the disposal facility in general. However, the calcium carbonate (CaCO3) present in the silt and sand fractions of these deposits, and dolomite (CaMg(CO3)2) in Belfast Sleech, may counter act the acidity.

Formation of Ammonia during the NO-H-2 Reaction over Pt/ZrO2

The mechanism for the formation of NH3 during the NO-H-2 reaction over Pt/ZrO2 was studied. Steady-state isotopic transient kinetic analysis was carried out with isotopic switching from (NO)-N-15-D-2 to (NO)-N-14-D-2, and the results revealed that formation of N-2 and N2O was associated with linearly adsorbed NO on the Pt surface, whereas ammonia formation was associated with NDx species adsorbed on ZrO2. The adsorbed NHx species were not observed on the surface of ZrO2 during NH3 adsorption. From transient kinetic experiments, the formation rates of NHx species and of gaseous NH3 agreed with each other, suggesting that the NHx species on ZrO2 was an ammonia intermediate. The NDx species did not react with D-2 directly, but H-D exchange occurred easily. The addition of H2O to the NO-H-2 feed gas enhanced the formation of NH3. In situ diffuse reflectance spectra and transient kinetic analysis revealed that H2O enhanced the conversion of NHx species to NH3.

Thermochemistry of Ionic Liquid-Catalyzed Reactions. Experimental and Theoretical Study of Chemical Equilibria of Isomerization and Transalkylation of tert-Butylbenzenes

The chemical equilibrium of mutual interconversions of tert-butylbenzenes was studied in the temperature range 286 to 423 K using chloroaluminate ionic liquids as a catalyst. Enthalpies of five reactions of isomerization and transalkylation of tert-butylbenzenes were obtained from temperature dependences of the corresponding equilibrium constants in the liquid phase. Molar enthalpies of vaporization of methyl-tert-butylbenzenes and 1,4-ditert-butylbenzene were obtained by the transpiration method and were used for a recalculation of enthalpies of reactions and equilibrium constants into the gaseous phase. Using these experimental results, ab initio methods (B3LYP and G3MP2) have been tested for prediction thermodynamic functions of the five reactions under study successfully. Thermochemical investigations of tert-butyl benzenes available in the literature combined with experimental results have helped to resolve contradictions in the available thermochemical data for tert-butylbenzene and to recommend consistent and reliable enthalpies of formation for this compound in the liquid and the gaseous state.

SN 2008S: an electron-capture SN from a super-AGB progenitor?

We present comprehensive photometric and spectroscopic observations of the faint transient SN 2008S discovered in the nearby galaxy NGC 6946. SN 2008S exhibited slow photometric evolution and almost no spectral variability during the first nine months, implying a long photon diffusion time and a high-density circumstellar medium. Its bolometric luminosity (similar or equal to 10(41) erg s(-1) at peak) is low with respect to most core-collapse supernovae but is comparable to the faintest Type II-P events. Our quasi-bolometric light curve extends to 300 d and shows a tail phase decay rate consistent with that of Co-56. We propose that this is evidence for an explosion and formation of Ni-56 (0.0014 +/- 0.0003 M-circle dot). Spectra of SN 2008S show intense emission lines of H alpha, [Ca II] doublet and Ca II near-infrared (NIR) triplet, all without obvious P-Cygni absorption troughs. The large mid-infrared (MIR) flux detected shortly after explosion can be explained by a light echo from pre-existing dust. The late NIR flux excess is plausibly due to a combination of warm newly formed ejecta dust together with shock-heated dust in the circumstellar environment. We reassess the progenitor object detected previously in Spitzer archive images, supplementing this discussion with a model of the MIR spectral energy distribution. This supports the idea of a dusty, optically thick shell around SN 2008S with an inner radius of nearly 90 AU and outer radius of 450 AU, and an inferred heating source of 3000 K. The luminosity of the central star is L similar or equal to 10(4.6) L-circle dot. All the nearby progenitor dust was likely evaporated in the explosion leaving only the much older dust lying further out in the circumstellar environment. The combination of our long-term multiwavelength monitoring data and the evidence from the progenitor analysis leads us to support the scenario of a weak electron-capture supernova explosion in a super-asymptotic giant branch progenitor star (of initial mass 6-8 M-circle dot) embedded within a thick circumstellar gaseous envelope. We suggest that all of main properties of the electron-capture SN phenomenon are observed in SN 2008S and future observations may allow a definitive answer.

Ultraviolet and extreme-ultraviolet line ratio diagnostics for O IV.

Aims: We generate theoretical ultraviolet and extreme-ultraviolet emission line ratios for O IV and show their strong versatility as electron temperature and density diagnostics for astrophysical plasmas.
Methods: Recent fully relativistic calculations of radiative rates and electron impact excitation cross sections for O IV, supplemented with earlier data for A-values and proton excitation rates, are used to derive theoretical O IV line intensity ratios for a wide range of electron temperatures and densities.
Results: Diagnostic line ratios involving ultraviolet or extreme-ultraviolet transitions in O IV are presented, that are applicable to a wide variety of astrophysical plasmas ranging from low density gaseous nebulae to the densest solar and stellar flares. Comparisons with observational data, where available, show good agreement between theory and experiment, providing support for the accuracy of the diagnostics. However, diagnostics are also presented involving lines that are blended in existing astronomical spectra, in the hope this might encourage further observational studies at higher spectral resolution.

Solvent strength of ionic liquid/CO2 mixtures

Previously we have shown that organic solutes can be extracted from ionic liquids (ILs) with supercritical CO2 and that ILs can be induced to separate from organic and aqueous mixtures by applying gaseous CO2 pressure. Thus, we are interested in the solvent strength of IL/CO2 mixtures. Here we use 4-nitroaniline, N,N-diethyl-4-nitroaniline and Reichardt's dye 33 to determine the Kamlet-Taft parameters for four different imidazolium based ILs and their mixtures with CO2 at 25 and 40degreesC. The effect of temperature and carbon dioxide concentration on these parameters was determined. The polarizability parameter depends weakly on the CO2 concentration. However, the hydrogen bond donating ability and the hydrogen bond accepting ability are virtually independent Of CO2 pressure. The results indicate that the strong interactions between ILs and probe molecules are not influenced by CO2.

Nebular and Auroral Emission Lines of [Ar IV] in the Optical Spectra of Planetary Nebulae

Recent R-matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams (R₁, R₂), (R_1, R₃), and (R_1, R₄), where K₁ = I(4711 Å)/I(4740 Å), R₂ = I(7238 Å)/I(4711 + 4740 Å), R₃ = I(7263 Å)/I(4711 + 4740 Å), and R₄ = I(7171 Å)/I(4711 + 4740 Å), for a range of electron temperatures (T_e = 5000-20,000 K) and electron densities (N_e = 10-10⁶ cm^-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of T_e and N_e from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R_1, R₃) and (R_1, R₄), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of T_e and N_e estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 Å line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R₂ are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 Å line with the strong C II transition at 7236 Å. 

Thermochemistry of Ionic Liquid-Catalyzed Reactions: Theoretical and Experimental Study of the Beckmann Rearrangement-Kinetic or Thermodynamic Control?

A thermodynamic analysis of the experimental conditions of the Beckmann rearrangement reaction of oximes into amides has been undertaken to examine whether the reaction is under thermodynamic or kinetic control. To answer this question, the thermodynamic properties of the typical Beckmann rearrangement reactions in the ideal gaseous state-cyclohexanone oxime to caprolactam and 2-butanone oxime to N-methylpropanarnide-were studied by using the quantum mechanical method. Gibbs energy and equilibrium constants of the Beckmann rearrangement have been assessed in the gaseous and the liquid phases. Results of the thermodynamic analysis have shown that Beckmann rearrange ments are kinetically controlled. Thus, a search for possible active ionic liquid based catalysts for the mild reaction conditions has been performed.

WASP-21b: a hot-Saturn exoplanet transiting a thick disc star

We report the discovery of WASP-21b, a new transiting exoplanet discovered by the Wide Angle Search for Planets (WASP) Consortium and established and characterized with the FIES, SOPHIE, CORALIE and HARPS fiber-fed echelle spectrographs. A 4.3-d period, 1.1% transit depth and 3.4-h duration are derived for WASP-21b using SuperWASP-North and high precision photometric observations at the Liverpool Telescope. Simultaneous fitting to the photometric and radial velocity data with a Markov Chain Monte Carlo procedure leads to a planet in the mass regime of Saturn. With a radius of 1.07 RJup and mass of 0.30 MJup, WASP-21b has a density close to 0.24 ?Jup corresponding to the distribution peak at low density of transiting gaseous giant planets. With a host star metallicity [Fe/H] of -0.46, WASP-21b strengthens the correlation between planetary density and host star metallicity for the five known Saturn-like transiting planets. Furthermore there are clear indications that WASP-21b is the first transiting planet belonging to the thick disc.

Spatial Allocation of Material Flow Analysis: A Geographic Information System Application of Material Flow Analysis in Ireland

This study concerns the spatial allocation of material flows, with emphasis on construction material in the Irish housing sector. It addresses some of the key issues concerning anthropogenic impact on the environment through spatial temporal visualisation of the flow of materials, wastes and emissions at different spatial levels. This is presented in the form of a spatial model, Spatial Allocation of Material Flow Analysis (SAMFA), which enables the simulation of construction material flows and associated energy use. SAMFA parallels the Island Limits project (EPA funded under 2004-SD-MS-22-M2), which aimed to create a material flow analysis of the Irish economy classified by industrial sector. SAMFA further develops this by attempting to establish the material flows at the subnational geographical scale that could be used in the development of local authority (LA) sustainability strategies and spatial planning frameworks by highlighting the cumulative environmental impacts of the development of the built environment. By drawing on the idea of planning support systems, SAMFA also aims to provide a cross-disciplinary, integrative medium for involving stakeholders in strategies for a sustainable built environment and, as such, would help illustrate the sustainability consequences of alternative The pilot run of the model in Kildare has shown that the model can be successfully calibrated and applied to develop alternative material flows and energy-use scenarios at the ED level. This has been demonstrated through the development of an integrated and a business-as-usual scenario, with the former integrating a range of potential material efficiency and energysaving policy options and the latter replicating conditions that best describe the current trend. Their comparison shows that the former is better than the latter in terms of both material and energy use. This report also identifies a number of potential areas of future research and areas of broader application. This includes improving the accuracy of the SAMFA model (e.g. by establishing actual life expectancy of buildings in the Irish context through field surveys) and the extension of the model to other Irish counties. This would establish SAMFA as a valuable predicting and monitoring tool that is capable of integrating national and local spatial planning objectives with actual environmental impacts. Furthermore, should the model prove successful at this level, it then has the potential to transfer the modelling approach to other areas of the built environment, such as commercial development and other key contributors of greenhouse emissions. The ultimate aim is to develop a meta-model for predicting the consequences of consumption patterns at the local scale. This therefore offers the possibility of creating critical links between socio technical systems with the most important challenge of all the limitations of the biophysical environment.

Investigating the mechanism of the H 2-assisted selective catalytic reduction (SCR) of NOx with octane using fast cycling transient in situ DRIFTS-MS analysis

A mechanistic study of the H-2-assisted Selective Catalytic Reduction (SCR) of NOx with octane as reductant over a Ag/Al2O3 catalyst was carried out using a modified DRIFTS cell coupled to a mass spectrometer Using fast transient cycling switching of H-2 with a time resolution of a few seconds It was possible to differentiate potential reaction intermediates from other moieties that are clearly spectator species Using such a periodic operation mode effects were uncovered that are normally hidden in conventional transient studies which typically consist of a single transient In experiments based on a single transient addition of H-2 to or removal of H-2 from the SCR feed it was found that the changes in the concentrations of gaseous species (products and reactants) were not matched by changes at comparable timescales of the concentration of surface species observed by IR This observation indicates that the majority of sur face species observed by DRIFTS under steady-state reaction conditions are spectators In contrast under fast cycling experimental conditions It was found that a surface isocyanate species had a temporal response that matched that of N-15(2) This suggests that some of the isocyanate species observed by infrared spectroscopy could be important intermediates in the hydrogen-assisted SCR reaction although it is emphasised that this may be dependent on the way in which the infrared spectra are obtained It is concluded that the use of fast transient cycling switching techniques may provide useful mechanistic information under certain circumstances.