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ABSTRACT: An automated flow injection analysis system for H <sub>2</sub> O <sub>2</sub> detection in the liquid phase, based on the photometric determination of the complex between hydrogen peroxide and a titanium (IV) reagent is presented. The system was tested for monitoring the H <sub>2</sub> O <sub>2</sub> concentration in a lab-scale experimental setup for H <sub>2</sub> O <sub>2</sub> direct synthesis. In its final configuration, the developed system allows a reliable and reproducible determination of H <sub>2</sub> O <sub>2</sub> for a broad concentration interval between 1.02×10<sup>-3</sup> mol l <sup>-1</sup> and 1.16 mol l <sup>-1</sup> .
Review of Scientific Instruments 06/2009; · 1.37 Impact Factor
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ABSTRACT: The interaction of water vapour with mineral dust and soot surfaces has been studied in the temperature range 203 K < T < 298 K using a Knudsen cell reactor. For the uptake of water on mineral dust an initial uptake coefficient of gamma(ini) = (6.3 +/- 0.7) x 10(-2) independent of temperature has been determined. In contrast the desorption rate has been found to be strongly temperature dependent with desorption rate constants decreasing from 1 x 10(-3) at 265 K to 1 x 10(-4) at 223 K. In addition, relatively high surface coverages have been determined from which an adsorption enthalpy of -40 kJ mol(-1) is inferred. For the uptake of water on soot the initial uptake coefficient has been found to be independent of temperature with a value of gamma(ini) = (4.7 +/- 0.2) x 10(-2), similar to the case of mineral dust. However, the corresponding desorption rate constants have been found to be three orders of magnitude larger than for mineral dust. Consistent with this finding, low surface coverages with an adsorption enthalpy of -27 kJ mol(-1) have been derived. A comparison of the uptake kinetics and adsorption enthalpies of water on mineral dust and soot leads to the conclusion that water is much stronger interacting with mineral dust than with soot. In terms of a hydrophilicity concept the results suggest, that mineral dust may be regarded as hydrophilic whereas soot is hydrophobic and that fundamental kinetic and thermochemical quantities may be related to that concept.
Faraday Discussions 02/2005; 130:437-51; discussion 491-517, 519-24. · 5.00 Impact Factor
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ABSTRACT: A continuous process for safe direct synthesis of H2O2 from H2 and O2 over Pd-catalysts in a gas/liquid membrane contactor is proposed. As a prerequisite for the process a method was developed for coating of Pd based catalysts into the fine porous layer on the inside of asymmetric tubular membranes. The method reached uniform distribution and high dispersion of the Pd nano particles with ca. 11 nm size for a range of membrane geometries and lengths and a variety of membrane materials. For the selection of the best catalytic system additional experiments with supported catalysts were performed. They were prepared using the same materials and coating method as for the manufacture of the catalytic membranes. The principle of the membrane contactor was verified with 10 cm long single channel membranes with stable operation of the continuous system for more than 10 h at differential pressures up to 4.5 bar in methanol. Further, the role of key process parameters such as solvent type, system pressure and flow regime were assessed. Productivities up to 1.7 molH2O2 gpd−1 h−1 (6.1 molH2O2 m−2 h−1 related to the geometric surface area of the membrane) with a selectivity of 83% towards H2O2 were achieved.
Chemical Engineering Journal.
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Aneta Pashkova
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ABSTRACT: The chemistry of atmospheric aerosol surfaces is currently not well understood. Important remaining questions concern the modification of the aerosol surfaces during ageing, the removal of trace gases from the gas phase and the processing in the hydrological cycle. Mineral dust and soot have been chosen as typical heterogeneous surfaces, which are ubiquitous in the atmosphere, and the role of which is still not very well understood. In this work these surfaces have been characterized in terms of two important issues:
Hydrophilicity of atmospheric surfaces: Hydrophilicity is a critical but difficult to quantify parameter, characterizing the possibility of aerosol particles to act as cloud/ice condensation nuclei. Therefore, adsorption and desorption rates together with the amount of water adsorbed, are important parameters in the assessment of the hydrophilicity. Regarding this issue we have studied the interaction of mineral dust and kerosene soot surfaces with water vapour in the temperature interval 203 K < T < 298 K using a Knudsen cell reactor. A comparison of the uptake kinetics and adsorption enthalpies of water on mineral dust and soot leads to the conclusion that water is interacting with mineral dust much stronger than with soot. In terms of the hydrophilicity concept the results suggest, that mineral dust may be regarded as hydrophilic whereas soot is hydrophobic and that fundamental kinetic and thermo-chemical parameters may be used to quantify this concept.
Reactivity of atmospheric surfaces: We have investigated two different types of soot samples (kerosene and ethene) and their behaviour towards NO2 and SO2 (as typical atmospheric pollutants) in a wide temperature interval between 180 K < T < 670 K. It has been found that under these conditions SO2 undergoes only reversible adsorption/desorption on the surface, whereas for temperatures above 300 K NO2 adsorbs irreversibly on the surface and reacts further leading to the formation of NO and HONO as gas phase products with strongly temperature dependent yields. Complementary, for temperatures above 470 K, the formation of CO2 as gas phase product is observed. For the mechanistic interpretation of the reaction we have found that NO2 adsorbs irreversibly on the surface, followed by further formation of HONO or reduction to NO (depending on temperature). Additionally, due to the reduction to NO, a new oxidised surface site is formed at higher temperatures, which releases CO2 into the gas phase, indicating the ability of NO2 to oxidize the soot sample followed by the consumption of soot.
