Developments for transactinide chemistry experiments behind the gas-filled separator TASCA

Topic of this thesis is the development of experiments behind the gas-filled separator TASCA(TransActinide Separator and Chemistry Apparatus) to study the chemical properties of the transactinide elements.rnIn the first part of the thesis, the electrodepositions of short-lived isotopes of ruthenium and osmium on gold electrodes were studied as model experiments for hassium. From literature it is known that the deposition potential of single atoms differs significantly from the potential predicted by the Nernst equation. This shift of the potential depends on the adsorption enthalpy of therndeposited element on the electrode material. If the adsorption on the electrode-material is favoured over the adsorption on a surface made of the same element as the deposited atom, the electrode potential is shifted to higher potentials. This phenomenon is called underpotential deposition.rnPossibilities to automatize an electro chemistry experiment behind the gas-filled separator were explored for later studies with transactinide elements.rnThe second part of this thesis is about the in-situ synthesis of transition-metal-carbonyl complexes with nuclear reaction products. Fission products of uranium-235 and californium-249 were produced at the TRIGA Mainz reactor and thermalized in a carbon-monoxide containing atmosphere. The formed volatile metal-carbonyl complexes could be transported in a gas-stream.rnFurthermore, short-lived isotopes of tungsten, rhenium, osmium, and iridium were synthesised at the linear accelerator UNILAC at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The recoiling fusion products were separated from the primary beam and the transfer products in the gas-filled separator TASCA. The fusion products were stopped in the focal plane of TASCA in a recoil transfer chamber. This chamber contained a carbon-monoxide – helium gas mixture. The formed metal-carbonyl complexes could be transported in a gas stream to various experimental setups. All synthesised carbonyl complexes were identified by nuclear decay spectroscopy. Some complexes were studied with isothermal chromatography or thermochromatography methods. The chromatograms were compared with Monte Carlo Simulations to determine the adsorption enthalpyrnon silicon dioxide and on gold. These simulations based on existing codes, that were modified for the different geometries of the chromatography channels. All observed adsorption enthalpies (on silcon oxide as well as on gold) are typical for physisorption. Additionally, the thermalstability of some of the carbonyl complexes was studied. This showed that at temperatures above 200 °C therncomplexes start to decompose.rnIt was demonstrated that carbonyl-complex chemistry is a suitable method to study rutherfordium, dubnium, seaborgium, bohrium, hassium, and meitnerium. Until now, only very simple, thermally stable compounds have been synthesized in the gas-phase chemistry of the transactindes. With the synthesis of transactinide-carbonyl complexes a new compound class would be discovered. Transactinide chemistry would reach the border between inorganic and metallorganic chemistry.rnFurthermore, the in-situ synthesised carbonyl complexes would allow nuclear spectroscopy studies under low background conditions making use of chemically prepared samples.

Using sulfur isotope fractionation to understand the atmospheric oxidation of SO 2

Sulfate aerosol plays an important but uncertain role in cloud formation and radiative forcing of the climate, and is also important for acid deposition and human health. The oxidation of SO2 to sulfate is a key reaction in determining the impact of sulfate in the environment through its effect on aerosol size distribution and composition. This thesis presents a laboratory investigation of sulfur isotope fractionation during SO2 oxidation by the most important gas-phase and heterogeneous pathways occurring in the atmosphere. The fractionation factors are then used to examine the role of sulfate formation in cloud processing of aerosol particles during the HCCT campaign in Thuringia, central Germany. The fractionation factor for the oxidation of SO2 by ·OH radicals was measured by reacting SO2 gas, with a known initial isotopic composition, with ·OH radicals generated from the photolysis of water at -25, 0, 19 and 40°C (Chapter 2). The product sulfate and the residual SO2 were collected as BaSO4 and the sulfur isotopic compositions measured with the Cameca NanoSIMS 50. The measured fractionation factor for 34S/32S during gas phase oxidation is αOH = (1.0089 ± 0.0007) − ((4 ± 5) × 10−5 )T (°C). Fractionation during oxidation by major aqueous pathways was measured by bubbling the SO2 gas through a solution of H2 O2

New constraints on the gas age-ice age difference along the EPICA ice cores, 0-50 kyr

Gas is trapped in polar ice sheets at ~50–120 m below the surface and is therefore younger than the surrounding ice. Firn densification models are used to evaluate this ice age-gas age difference (Δage) in the past. However, such models need to be validated by data, in particular for periods colder than present day on the East Antarctic plateau. Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We also use the structured 10Be peak, occurring 41 kyr before present (BP) and due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the Δage during this event. Our results seem to reveal an overestimate of the Δage by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and temperature scenarios do not entirely resolve this discrepancy. Although the exact reasons for the Δage overestimate at the two EPICA sites remain unknown at this stage, we conclude that current densification model simulations have deficits under glacial climatic conditions. Whatever the cause of the Δage overestimate, our finding suggests that the phase relationship between CO2 and EDC temperature previously inferred for the start of the last deglaciation (lag of CO2 by 800±600 yr) seems to be overestimated.

Two-Phase Flow In Microchannels: Morphology And Interface Phenomena

The existence and morphology, as well as the dynamics of micro-scale gas-liquid interfaces is investigated numerically and experimentally. These studies can be used to assess liquid management issues in microsystems such as PEMFC gas flow channels, and are meant to open new research perspectives in two-phase flow, particularly in film deposition on non-wetting surfaces. For example the critical plug volume data can be used to deliver desired length plugs, or to determine the plug formation frequency. The dynamics of gas-liquid interfaces, of interest for applications involving small passages (e.g. heat exchangers, phase separators and filtration systems), was investigated using high-speed microscopy - a method that also proved useful for the study of film deposition processes. The existence limit for a liquid plug forming in a mixed wetting channel is determined by numerical simulations using Surface Evolver. The plug model simulate actual conditions in the gas flow channels of PEM fuel cells, the wetting of the gas diffusion layer (GDL) side of the channel being different from the wetting of the bipolar plate walls. The minimum plug volume, denoted as critical volume is computed for a series of GDL and bipolar plate wetting properties. Critical volume data is meant to assist in the water management of PEMFC, when corroborated with experimental data. The effect of cross section geometry is assessed by computing the critical volume in square and trapezoidal channels. Droplet simulations show that water can be passively removed from the GDL surface towards the bipolar plate if we take advantage on differing wetting properties between the two surfaces, to possibly avoid the gas transport blockage through the GDL. High speed microscopy was employed in two-phase and film deposition experiments with water in round and square capillary tubes. Periodic interface destabilization was observed and the existence of compression waves in the gas phase is discussed by taking into consideration a naturally occurring convergent-divergent nozzle formed by the flowing liquid phase. The effect of channel geometry and wetting properties was investigated through two-phase water-air flow in square and round microchannels, having three static contact angles of 20, 80 and 105 degrees. Four different flow regimes are observed for a fixed flow rate, this being thought to be caused by the wetting behavior of liquid flowing in the corners as well as the liquid film stability. Film deposition experiments in wetting and non-wetting round microchannels show that a thicker film is deposited for wetting conditions departing from the ideal 0 degrees contact angle. A film thickness dependence with the contact angle theta as well as the Capillary number, in the form h_R ~ Ca^(2/3)/ cos(theta) is inferred from scaling arguments, for contact angles smaller than 36 degrees. Non-wetting film deposition experiments reveal that a film significantly thicker than the wetting Bretherton film is deposited. A hydraulic jump occurs if critical conditions are met, as given by a proposed nondimensional parameter similar to the Froude number. Film thickness correlations are also found by matching the measured and the proposed velocity derived in the shock theory. The surface wetting as well as the presence of the shock cause morphological changes in the Taylor bubble flow.

Gas composition of the main volatile elements in protoplanetary discs and its implication for planet formation

Context. Direct observations of gaseous exoplanets reveal that their gas envelope has a higher C/O ratio than that of the host star (e.g., Wasp 12-b). This has been explained by considering that the gas phase of the disc could be inhomogeneous, exceeding the stellar C/O ratio in regions where these planets formed; but few studies have considered the drift of the gas and planet migration. Aims. We aim to derive the gas composition in planets through planet formation to evaluate if the formation of giant planets with an enriched C/O ratio is possible. The study focusses on the effects of different processes on the C/O ratio, such as the disc evolution, the drift of gas, and planet migration. Methods. We used our previous models for computing the chemical composition, together with a planet formation model, to which we added the composition and drift of the gas phase of the disc, which is composed of the main volatile species H2O, CO, CO2, NH3, N2, CH3OH, CH4, and H2S, H2 and He. The study focusses on the region where ice lines are present and influence the C/O ratio of the planets. Results. Modelling shows that the condensation of volatile species as a function of radial distance allows for C/O enrichment in specific parts of the protoplanetary disc of up to four times the solar value. This leads to the formation of planets that can be enriched in C/O in their envelope up to three times the solar value. Planet migration, gas phase evolution and disc irradiation enables the evolution of the initial C/O ratio that decreases in the outer part of the disc and increases in the inner part of the disc. The total C/O ratio of the planets is governed by the contribution of ices accreted, suggesting that high C/O ratios measured in planetary atmospheres are indicative of a lack of exchange of material between the core of a planet and its envelope or an observational bias. It also suggests that the observed C/O ratio is not representative of the total C/O ratio of the planet.

Maps and acoustic profiles of free gas distribution, Western Baltic Sea and Kattgat-Skagerrak area

A shallow gas depth-contour map covering the Skagerrak-western Baltic Sea region has been constructed using a relatively dense grid of existing shallow seismic lines. The digital map is stored as an ESRI shape file in order to facilitate comparison with other data from the region. Free gas usually occurs in mud and sandy mud but is observed only when sediment thickness exceeds a certain threshold value, depending on the water depth of the area in question. Gassy sediments exist at all water depths from approx. 20 m in the coastal waters of the Kattegat to 360 m in the Skagerrak. In spite of the large difference in water depths, the depth of free gas below seabed varies only little within the region, indicating a relatively fast movement of methane in the gas phase towards the seabed compared to the rate of diffusion of dissolved methane. Seeps of old microbial methane occur in the northern Kattegat where a relatively thin cover of sandy sediments exists over shallow, glacially deformed Pleistocene marine sediments. Previous estimates of total methane escape from the area may be correct but the extrapolation of local methane seepage rate data to much larger areas on the continental shelf is probably not justified. Preliminary data on porewater chemistry were compared with the free gas depth contours in the Aarhus Bay area, which occasionally suffers from oxygen deficiency, in order to examine if acoustic gas mapping may be used for monitoring the condition of the bay.

Carbon and deuterium isotopes and C ratios in gas hydrates of ODP Leg 204 sites

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%-40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%-15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%-5%) at greater depths (30-200 meters below seafloor [mbsf]) resulting from the buildup of in situ-generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30-50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2-4 mmol/m**3/yr, and integrated net rates are 20-50 mmol/m**2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m**3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of <0.1 mmol/m**3/yr at depths >100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25-80 mmol/m**2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.

Heterogeneous catalysts for environmentally sustainable reactions

This PhD work deals with problems of synthetic organic chemistry with particular attention to the development of environmentally friendly processes. In particular, new synthetic strategies have been studied based on the use of low cost heterogeneous catalysts, non-toxic reagents and mild operating conditions that do not involve, when possible, the use of solvents. The catalysts examined are both basic and acids, commercial or prepared by hetereogenization of homogeneous catalysts synthesized by tethering or impregnation. In particular it will be discussed the catalytic activity of oxides (Al2O3 and TiO2), supported sulphonic acids and hydrotalcites for the reactions of selective monoesterificazion of dicarboxylic acids, dehydrogenation of butane in gas phase, esterification of levulinic acid, Friedel-Craft acylations, C-C and C-P coupling. The use of these materials has allowed the development of simple processes with low environmental impact. The operating conditions are in fact mild and reaction times short. The selectivity for the desired products is in all reported cases very high and the catalysts can be recycled maintaining their optimum performances.

Synthesis of high surface area TiO2 nanoparticles by mild acid treatment with HCl or HI for photocatalytic propene oxidation

Nanostructured TiO2 photocatalysts with small crystalline sizes have been synthesized by sol-gel using the amphiphilic triblock copolymer Pluronic P123 as template. A new synthesis route, based on the treatment of TiO2 xerogels with acid-ethanol mixtures in two different steps, synthesis and extraction-crystallization, has been investigated, analyzing two acids, hydrochloric and hydriodic acid. As reference, samples have also been prepared by extraction-crystallization in ethanol, being these TiO2 materials amorphous and presenting higher porosities. The prepared materials present different degrees of crystallinity depending on the experimental conditions used. In general, these materials exhibit high surface areas, with an important contribution of microporosity and mesoporosity, and with very small size anatase crystals, ranging from 5 to 7 nm. The activity of the obtained photocatalysts has been assessed in the oxidation of propene in gas phase at low concentration (100 ppmv) under a UVA lamp with 365 nm wavelength. In the conditions studied, these photocatalysts show different activities in the oxidation of propene which do not depend on their surface areas, but on their crystallinity and band gap energies, being sample prepared with HCl both during synthesis and in extraction-crystallizations steps, the most active one, with superior performance than Evonik P25.

Isotopic study of ceria-catalyzed soot oxidation in the presence of NOx

The ceria-catalyzed soot oxidation mechanism has been studied by a pulse technique with labeled O2 in the absence and presence of NO, using ceria–soot mixtures prepared in the loose contact mode. In the absence of soot, the ceria-catalyzed oxidation of NO to NO2 takes place with ceria oxygen and not with gas-phase O2. However, the oxygen exchange process between gas-phase O2 and ceria oxygen (to yield back O2, but with oxygen atoms coming from ceria) prevailed with regard to the ceria-catalyzed oxidation of NO to NO2. Gas-phase O2 did not react directly with soot when pulsed to a soot–ceria loose contact mixture. Instead, ceria oxygen is transferred to soot (this step does not require gas-phase molecular oxygen to be present), and gas-phase O2 fills up the vacancies created on the oxide in a further step. The transfer of oxygen between ceria and soot occurred directly in the absence of NO. However, in the presence of NO, NO2 is expected to be additionally generated by ceria oxygen oxidation, which also reacts with soot. The main reaction products of the ceria-catalyzed soot oxidation reaction with NO/O2 were CO2 and NO. Additionally, evidence of the reduction of NOx to N2 was found.

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe2O3-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H-2, N-2, and CO2 or CO and CO2 controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 degrees C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO(2) = 1 center dot 10(-8) atm at 1200 degrees C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 degrees C to 1400 degrees C at pO(2) = 1 center dot 10(-6) atm and at 1200 degrees C at pO(2) values ranging from 1 center dot 10(-4) to 1 center dot 10(-8) atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.

Gas-liquid-solid flow modelling in a bubble column

An alternative approach to the modelling of solid-liquid and gas-liquid-solid flows for a 5:1 height to width aspect ratio bubble column is presented here. A modified transport equation for the volume fraction of a dispersed phase has been developed for the investigation of turbulent buoyancy driven flows (Chem. Eng. Proc., in press). In this study, a modified transport equation has been employed for discrete phase motion considering both solid-liquid and gas-liquid-solid flows. The modelling of the three-phase flow in a bubble column was achieved in the following case: injecting a slug of solid particles into the column for 10 s at a velocity of 0.1 m s-1 and then the gas phase flow was initiated with a superficial gas velocity of 0.02 cm s-1. © 2003 Elsevier B.V. All rights reserved.

Glycerol valorization:dehydration to acrolein over silica-supported niobia catalysts

The catalytic dehydration of glycerol to acrolein is investigated over silica-supported niobia catalysts in a continuous fixed-bed gas-phase reactor. Various supported niobia catalysts are prepared and characterized using surface analysis and spectroscopic methods (XRD, UV-Vis, XPS, N2 adsorption), as well as with ammonia adsorption microcalorimetry. Good results are obtained with initial glycerol conversions of over 70% and with 50-70% selectivity to acrolein. We investigate the influence of changing the catalyst acid strength by varying the niobia content and catalyst calcination temperature. Glycerol conversion and acrolein selectivity depend on the surface acid strength. Catalyst deactivation by coking is also observed, but simple oxidative treatment in air restores the activity of the catalysts completely. © The Author(s) 2010.

Phenol methylation over nanoparticulate Co2FeO4 inverse spinel catalysts:the effect of morphology on catalytic performance

A series of CoFe2O4 nanoparticles have been prepared via co-precipitation and controlled thermal sintering, with tunable diameters spanning 7–50 nm. XRD confirms that the inverse spinel structure is adopted by all samples, while XPS shows their surface compositions depend on calcination temperature and associated particle size. Small (<20 nm) particles expose Fe3+ enriched surfaces, whereas larger (∼50 nm) particles formed at higher temperatures possess Co:Fe surface compositions close to the expected 1:2 bulk ratio. A model is proposed in which smaller crystallites expose predominately (1 1 1) facets, preferentially terminated in tetrahedral Fe3+ surface sites, while sintering favours (1 1 0) and (1 0 0) facets and Co:Fe surface compositions closer to the bulk inverse spinel phase. All materials were active towards the gas-phase methylation of phenol to o-cresol at temperatures as low as 300 °C. Under these conditions, materials calcined at 450 and 750 °C exhibit o-cresol selectivities of ∼90% and 80%, respectively. Increasing either particle size or reaction temperature promotes methanol decomposition and the evolution of gaseous reductants (principally CO and H2), which may play a role in CoFe2O4 reduction and the concomitant respective dehydroxylation of phenol to benzene. The degree of methanol decomposition, and consequent H2 or CO evolution, appears to correlate with surface Co2+ content: larger CoFe2O4 nanoparticles have more Co rich surfaces and are more active towards methanol decomposition than their smaller counterparts. Reduction of the inverse spinel surface thus switches catalysis from the regio- and chemo-selective methylation of phenol to o-cresol, towards methanol decomposition and phenol dehydroxylation to benzene. At 300 °C sub-20 nm CoFe2O4 nanoparticles are less active for methanol decomposition and become less susceptible to reduction than their 50 nm counterparts, favouring a high selectivity towards methylation.

A fast XPS study of propene decomposition over clean and sulphated Pt{111}

The thermal decomposition of propene over clean and sulphate precovered Pt{111} has been followed by Fast XPS. The saturation propene coverage over the clean surface is 0.21 mL at 90 K. Propene is stable up to 200 K, above which molecular desorption and dehydrogenation result in the formation of a stable propylidyne intermediate adlayer at 300 K. Propylidyne decomposes above 400 K eventually forming graphitic carbon above 800 K. Preadsorbed surface sulphate promotes room temperature propene combustion associated with the decomposition of a thermally unstable alkyl--sulphate complex. Propylidyne also forms as on clean Pt{111}, but is less reactive, its decomposition above 450 K triggering partial oxidation with residual surface oxygen to liberate gas phase CO.