Oxygen diffusion in nanostructured perovskites

Nonstoichiometric perovskite-related oxides (such as ferrites and cobaltites, etc.) are characterized by fast oxygen transport at ambient temperatures, which relates to the microstructural texturing of these materials, consisting wholly of nanoscale microdomains. We have developed an inhomogeneous diffusion model to describe the kinetics of oxygen incorporation into nanostructured oxides. Nanodomain boundaries are assumed to be the high diffusivity paths for oxygen transport whereas diffusion into the domains proceeds much slower. Using Laplace transform methods, an exact solution is found for a ramped stepwise potential, allowing fitting of the experimental data to theoretical curves (in Laplace transforms). A further model generalization is considered by introducing additional parameters for the size distribution of domains and particles. The model has been applied for qualitative evaluation of oxygen diffusion parameters from the data on wet electrochemical oxidation of nano-structured perovskite SrCo_0.5Fe_0.2Ta_0.3O_{3-y} samples.Comment: Submitted for ICCMR-7 conference (Italy). Latex (elsart.cls), 15 pages, 7 figure

Deep Oxidation of Fluorinated Hydrocarbons in Molten Catalysts

The oxidation of fluorine-containing organic substances: fluorocarbon liquid M-1, fluorinated alcohol H(CF2)8CH2OH, and powder polytetrafluoroethylene with air has been studied in melts: NaOH; 43 mol.% LiCl В - 33 mol.% NaCl - 24 mol.% KCl (eutectic mixture); (LiCl-NaCl-KCl)eutec. + 10 mass.% V2O5; (LiCl-NaCl-KCl) eutec. + 15 mass.% V2O5; 56 mol.% Na2CO3 - 44 mol.% K2CO3 (eutectic), (Na2CO3 K2CO3)eutect. + 15 mass.% V2O5, and K3V5O14. The compositions of the melts have been examined by GC, DTA, chemical analysis and XRD, and they have been shown to change during the reaction, depending on the composition and partial pressure of the gaseous products over the melt surface. The alkali metal chloride melt containing 15 mass.% V2O5 has been found to be most stable to the action of fluorine compounds. Possibility of deep oxidation of fluorine-containing organic substances in melts based on hydroxides, carbonates and chlorides of alkali metals doped with oxides of vanadium has been proved. The process of deep oxidation of fluorinated hydrocarbons is accompanied by formation of an equilibrium mixture containing hydroxides, carbonates, chlorides and fluorides of alkali metals, as well as their vanadates, if V2O5 additive is used. The relative amounts of these substances in molten systems are determined by the partial pressure of oxygen, CO2 and water vapor

Catalytic Oxidation of Volatile Organic Compounds in Industrial Off-Gases

Processes and apparatuses for catalytic oxidation of VOCs in industrial off-gases are described, including steady state and unsteady state processes, a combined adsorption-catalytic process and an advanced method of ozone induced oxidation for low concentrated exhausts. On the basis of research and development works a series of catalytic incinerators, operating in steady state and unsteady state mode, of various capacity were designed, constructed and tested in the purification of ventilation air and off-gases from VOCs. The principles of operation of different types of catalytic incinerators and possible areas of application are discussed. For VOC concentrations 150-1000 mg/m3 unsteady state catalytic incinerators of KART type should be used, for concentrations 1000-3000 mg/m3 steady state KROT apparatuses are recommended, and for concentrations over 3000 mg/m3 up to 7000 mg/m3 installations TKM-250. It is shown that for the purification of low concentrated gases with the content of organic vapors below 150 mg/m3 adsorptioncatalytic method or catalytic oxidation with ozone in the installation OKA-3000 are most effective. Main kinetic dependencies of the ozone induced oxidation of toluene and acetone over copper oxide catalyst are given and discussed. It is shown that the efficiency of this method of VOCs removal is based on low operation temperature 313-343 K, by contrast to conventional catalytic incineration by air requiring preliminary heating of the gases to 523-573 K. A special consideration is given to adsorptive damping as an efficient method for leveling the VOCs concentrations in the real industrial exhausts directed to the catalytic treatment. The use of adsorptive dampers filled with carbon allows elimination of large deviations of pollutant concentrations in the gas entering the catalyst bed, thus increasing the VOCs removal efficiency from average values. For calculations of adsorptive dampers, an equation describing the profiles of VOC concentrations in gas phase along the length of the adsorbate bed in the damper was derived

Development of Fe-based Catalysts for Purification of Coke Oven Gases

Fe-based catalysts of different geometry are developed for the purification of coke oven gases: bulk, supported on alumina and supported on alumina silicate monoliths. Adsorption and decomposition of H2S on the catalysts developed are studied. Influence of active component content, type of support material and modification by Mn and Mo on the catalyst activity in de-H2S process is elucidated. Supported monolith catalysts show superior activity over bulk and supported spherical catalysts in H2S decomposition reaction and demonstrate stable operation in ammonia decomposition process during 2 hours at 900 °C giving 100% ammonia conversion

Method for control of the thickness of mesoporous titania films for applications in catalytic microreactors

Mesoporous titania films with a long-order structure were obtained on silicon and metal substrates. Control of the thickness of the titania film was achieved by multiple spin-coating. Successful control of the film thickness in the range of 300 to 1000 nm is considered an important step toward application in microstructured reactors, as well as for basic research on model catalysts. After doping the Ti sol with gold nanoparticles, the resulting catalysts were tested in the hydrogenation of citral. The reaction occurred in the kinetic regime at a coating thickness of 300 nm, while thicker films demonstrated severe internal diffusion limitations

Method for control of the thickness of mesoporous titania films for applications in catalytic microreactors

Mesoporous titania films with a long-order structure were obtained on silicon and metal substrates. Control of the thickness of the titania film was achieved by multiple spin-coating. Successful control of the film thickness in the range of 300 to 1000 nm is considered an important step toward application in microstructured reactors, as well as for basic research on model catalysts. After doping the Ti sol with gold nanoparticles, the resulting catalysts were tested in the hydrogenation of citral. The reaction occurred in the kinetic regime at a coating thickness of 300 nm, while thicker films demonstrated severe internal diffusion limitations

Synthesis and characterization of mesoporous silica thin films as a catalyst support on a titanium substrate

Mesoporous silica films with a thickness of 500–900 nm were synthesized on a titanium substrate by the evaporation-induced self-assembly method (with 900–1200 rpm for 90 s) using cetyltrimethylammonium bromide (CTAB) as structure-directing agent and tetraethyl orthosilicate as the silica source. Prior to coating deposition, the titanium substrate was oxidized to increase the surface roughness up to 500 nm and to produce a thin titania layer. Just before the synthesis, the titania layer was made super hydrophilic by an UV treatment for 2 h to provide a better adhesion of the silica film to the substrate. Films with hexagonal and cubic mesostructures with a uniform pore size of 2.8 nm and a surface area of 1080 m2/g were obtained and characterized by different methods. An alternative approach for surfactant removal by gradual heating up to 250 °C in vacuum was applied. Complete removal of CTAB from the as-synthesized silica films was confirmed by infrared spectroscopy

Highly dispersed supported Pt-Sn/TiO2 catalysts derived from mixed metal cluster precursors for selective hydrogenation of [alpha],[beta]-unsaturated aldehydes

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