Thermal and chemical ionization in flames

Conclusions We have determined the rate constants of the potassium ionization process AA⁺+e^– in the flames of 2H₂+O₂+X (Ar, He) mixtures on the temperature interval 1500–2500° K. The activation energy of this process is close to the ionization potential of potassium (100 kcal).In our experiments the rate of ion formation in the front of a hydrogen flame seeded with potassium exceeded the purely thermal ionization rate by 0.5–2 orders. The presumed cause is recombination ionization of the potassium in the flame front, for example, K+O+OK⁺+O₂+e^–. This is confirmed by the intensification of ionization in the reaction zone in the presence of an excess of oxygen in homogeneous H₂-air and H₂–O₂–(He, Ar) mixtures with alkali impurities.At T=1700° K the recombination coefficient for electrons and potassium ions is close to 1·10^–8 cm³·sec^–1. For a more precise determination it is necessary to know the frequency of electron capture by molecules and atoms under the experimental conditions.Experiments on thermal ionization in turbulent flames confirm the earlier conclusion concerning the important role of mass transfer in the chemi-ionization of hydrocarbon flames.Fizika Goreniya i Vzryva, Vol. 6, No. 1, pp. 37–48, 1970     

Statistical data on thermal expansion in dinas refractories for coking ovens

Conclusions Statistical data have been obtained on the temperature dependence of the thermal expansion for dinas refractories for coking ovens in the range from 20 to 1400°C. The temperature coefficient of linear expansion does not vary monotonically as the temperature rises and has peaks near the temperatures of the phase transformations in tridimite, cristobalite, and quartz, at which the values attained are 50·10^–6, 80·10^–6 and 10·10^–6 K^–1 correspondingly. The averaged TCLE should be considered as standard reference ones. The thermalexpansion curves for dinas brick show hysteresis due to the phase transitions in silica.Translated from Ogneupory, No. 5, pp. 30–33, May, 1987.     

Preliminary study of cationic copolymerization of α-methylstyrene and isobutyl vinyl ether

Summary -Methyl styrène polymerization was carried out using t-BuCl/Et₂AlCl/CH₂Cl₂ system in the temperature range of 0° to –63.5°C. The effect of temperature on yield and molecular weights of Poly -Methyl styrene was determined. Based on Arrhenius plots, average activation energies of molecular weights were determined to be –9.2 ± 1.0 K cal/mole. (0° to 40°C) and 0 ± 0.5 K cal/mole (–40° to –63.5°C). These were postulated to reflect the molecular weight governing mechanisms such as transfer to monomer and termination respectively.     

On the kinetic mechanism of the reaction of NH2 with O2 in O-, H-, and N-containing flames. 1. Kinetic parameters of the NH2+O2=NHO+OH reaction

The reaction pathways for NH₂+O₂Products are considered on the basis of experimental data on the ignition of an NH₃/O₂/Ar mixture in reflected shock waves (p=1–10 atm, T=900–2160 K) and on the NH₃/O₂/Ar flame structure (p=35 torr, T=1050–2600 K) using a multistage kinetic mechanism. The rate constants of the NH₂+O₂=HNO+OH reaction, obtained from a comparison of experimental and calculated data, are reported (k=3·10¹¹ exp (–15,000/RT) cm³/(mole·s) at T1500–2160 L and k=3·10⁹ cm³/(mole·s) at T900–1400 K).Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 60–65, January–February, 1994.     

Redox chemistry of H2S oxidation by the British Gas Stretford Process Part III: Electrochemical behaviour of anthraquinone 2,7 disulphonate in alkaline electrolytes

Electrochemical reduction of aqueous anthraquinone 2,7 disulphonate (AQ27DS) solutions at pH 9.3 were studied at Hg, Au and Pt electrodes. Cyclic voltammograms showed about 40 mV potential separation of the single pair of current peaks, precluding a simple one or two electron process. Charge measurements in controlled potential exhaustive reductions indicated a 2 mole^– per mol AQ27DS process leading to quinolate anions, whereas the partially reduced solution showed an EPR spectrum, indicating the presence of radical species, which, if produced directly, would involve only a 1 mole^– per mol AQ27DS process. U.v.-visible and EPR spectra of the deep red partially reduced AQ27DS solutions showed that both radical anions (AQ27DS _· ^– ) and fully reduced anthraquinolate were present; AQ27DS _· ^– radicals predominated at low conversions, while complete conversion produced AQ27DSH- ions. The AQ27DS diffusion coefficient was determined as 3.73 × 10^–10m²S^–1 from steady-state voltammetry at a gold rotating disc electrode.The results are congruous with a reduction mechanism involving an initial 2 mole^– per mol AQ27DS process to give anthraquinolate anions, from which electron transfer in solution to AQ27DS species produced AQ27DS - radical anions (a comproportionation). The comproportionation equilibrium constant was estimated as between 0.2 and 4.0 from cyclic voltammograms; together with a value of pK _a2(AQ27DSH₂) = 10.8 from the literature, this enabled the solution composition, and hence the major absorption spectral changes, to be predicted as a function of conversion. From a calculated potential-pH diagram, the AQ27DS reduction mechanism was predicted to involve disproportionation of the radical anions at pH 9.3 and two sequential one electron reductions at pH 9.3Nomenclature A electrode area (m²) - c concentration (mol m^–3) - D diffusion coefficient (m2 s^–1) - E ^° standard reversible electrode potential against SHE (V) - F Faraday constant 96 485C (mole^–)^–1 - HMDE hanging mercury drop electrode (–) - i current density (A m^–2) - i _L mass transport limited current density (A m^–2) - i _sp peak current density at HMDE (A m^–2) - IK, I _K EPR sensitivity factors, constants for a given geometry - mean mass transport rate constant (ms^–1) - l length of EPR cavity (m) - n number of moles of reactant - r radius of the EPR tube (m) - R gas constant (8.31441 J mol^–1 K^–1) - S EPR signal strength (–) - S ₀ EPR signal strength for one mole of spins within cavity (–) - t time (s) - T temperature (K) - V electrolyte volume (m³) - V _f volumetric flow rate (m3 s^–1) - X _e length of electrode (m) - z number of moles e^– per mole of reactant (–) Greek symbols molar absorptivity (m2 mol^–1) - Nernst diffusion layer thickness (m) - kinematic viscosity (m²s^–1) - potential sweep rate (V s^–1) - RDE rotation rate (s^–1)     

Behavior of Water and Oxygen Molecules on the Surface of Tin Dioxide Films

Polycrystalline films of compositions SnO₂, SnO_1.93F_0.07, 0.93SnO₂ · 0.07Sb₂O₃, and 0.97SnO₂ · 0.03CuO, which are of interest for use in gas-sensitive sensors, are prepared by the hydropyrolytic method from metal chlorides on a 22-XC ceramic substrate. The films prepared are studied using mass spectrometry, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) at different temperatures. The results of mass spectrometric investigations demonstrate that, except for water (18 amu), no compounds in the mass range 10–70 amu are desorbed from all the films. The differential thermal and thermogravimetric analyses have revealed that desorption of water molecules from the film surface occurs in the temperature range 50–125°C and the oxidation processes associated with the oxygen chemisorption proceed beginning from a temperature of 125°C.     

Hydrogen half-cells in molten salts. A potentiometric investigation of the systems (Pt or Au) H2O/H2, OH− in fused alkali nitrates

The potentiometric behaviour of the hydrogen electrodes (Pt or Au) H₂O-H₂, OH_–has been investigated in molten (Na_0·5, K_0·5)NO₃ at 503 K. In both cases the potential of the indifferent electrode could be expressed by the general equation [H₂O]/[H₂] [OH^–] which is different from the one expected on the basis of a Nernstian behaviour of the theoretical overall system 2H₂O+2e=H₂+2OH^–.The experimental findings are discussed in terms of mechanistic models involving the actual electrode surface and the standard potential for the theoretical (reversible) hydrogen electrode is calculated: =–·0V(versus Ag/Ag⁺ 0·07 M).     

Solid state ionics. High ionic conductivity solid in silver halide-silver sulphate system

The ionic conductivities and phase diagrams of AgI-Ag₂SO₄, AgCl-Ag₂SO₄ and AgBr-Ag₂SO₄ systems have been studied with the help of electrical conductivity, transport number, DTA and X-ray studies. In the AgI-Ag₂SO₄ system, the solid solution of -AgI with Ag₂SO₄ was quenched to room temperature to have high ionic conductivity of 5·0×10^–2 (ohm. cm)^–1 at room temperature and 1·3×10^–3 (ohm. cm)^–1 at –78°C. The quenched solid solution was stable at –20°C, but gradually decomposed to-AgI and-Ag₂SO₄ at room temperature. The transport number of silver ion in this solid solution was about 1·0. In the AgCl-Ag₂SO₄ and AgBr-Ag₂SO₄ systems, the electrical conductivity was of the order of 10^–4 to 10^–6 (ohm. cm)^–1 at room temperature.     

Effects of calcining conditions of kaolinite on pore structures of mesoporous materials prepared by the selective leaching of calcined kaolinite

This paper describes the effects of calcining conditions of kaolinite on pore structures of the porous materials obtained from the selective leaching of calcined kaolinite using KOH solution. Mesoporous -Al₂O₃ was the predominant crystalline phase in the samples calcined in the temperature range between 950°C and 1050°C for 24 h. The mean specific surface area of these samples was approximately 250 m² · g^–1 and the mean total pore volume was approximately 0.8 ml · g^–1. The pore size distribution curves of these samples showed a sharp peak at around 2–3 nm pore radius. This peak was sharper for the sample calcined at 1000°C for 24 h. On the other hand, the pore sizes of the sample calcined at 1100°C for 24 h increased abruptly to 10–20 nm and this change corresponded to the formation of mullite in the sample. The pore sizes of the samples calcined at 1100°C varied with calcining time. The specific surface area and total pore volume decreased, the longer the calcining time of the samples, and this was correlated with an increase in the amount of mullite in the samples.     

A study of molecular orientation in one-way drawn poly(ethylene terephthalate) films by means of polarized infra-red spectroscopy and birefringence measurements

Summary Polarized infrared spectroscopy and birefringence measurements have been used to study the changes in molecular orientation which occur on homogeneous hot (T=80°C) and cold (T=20°C) drawing of poly(ethylene terephthalate) (PET) sheets. Excellent agreement was obtained between the infrared orientation functions for absorption bands associated with 1507 cm^–1, 1580 cm^–1 and 1615 cm^–1, C-C stretching mode of the benzene ring, and average orientation functions obtained from optical birefringence. It was confirmed that the Gauche conformer of the ethylene glycol residue in the amorphous phase could be oriented perpendicularly to the draw direction (_p<4). The development of molecular orientation along the specimen axis in the necked sample confirms the entangled nature of the polymer chains in which the natural draw-ratio corresponds to the response of a network stretched up to its maximum of extensibility.     

Rheological and technological properties of titanium dioxide slips and the properties of the castings

Conclusions An investigation of the rheological and technological properties of titanium dioxide slips and slip castings demonstrated the presence of two castability regions. The properties of the slip and castings are optimal in the alkaline castability region (pH 8).A study of the influence of the firing temperature of the titanium dioxide starting material on the properties of the slip and castings showed that the properties are optimal when the TiO₂ had been fired at 1550° C.An analysis of the sintering of titanium dioxide slip castings showed that the properties of the sintered castings are optimal when the castings were produced from slip with pH 8 because in this case the density of the castings is at maximum.The strength of TiO₂ castings in the sintering process depends on their porosity and that of the sintered castings on the size of the crystals.Titanium dioxide fired at 1550°C yielded castings with open porosity 1.8–2.0%, apparent density 3.90–3.92 g/cm³, _b 1020–1050 kgf/cm², acid resistance 97.5%, specific inductive capacitance 90–100, and tangent of the dielectric loss angle at 20°C and 1 MHz 8·10^–4–9·10^–4 Translated from Ogneupory, No. 3, pp. 28–33, March, 1978.     

Preparation and electrochemical characteristics of a new Li-Mn-V-O system formed from heat-treatment of a MnO2, NH4VO3 and LiNO3 mixture

New quarternary oxides (Li₂O) _x · MnO₂ · yV₂O₅ (x = 0.125 0.25, y = 0.125 0.25), formed by heating mixtures of MnO₂, NH₄VO₃ and LiNO₃ at various Li/Mn and V/Mn atomic ratios and at different temperatures (300 400 °C in air, have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy. The quarternary oxide with x = 0.25 and y = 0.25 showed a discharge capacity of 220 A h (kg oxide)^–1 and an energy density of ca. 600 W h (kg oxide)^–1 at a current density of 0.20 mA cm^–1 in 1 m LiClO₄-propylene carbonate at 25 °C. When charge-discharge cycling with the (Li₂O) · MnO₂ · 0.25V₂O₅ electrode was performed at a constant capacity of 30 A h (kg oxide)^–1 and at a constant current density of 0.10 0.20 mA cm^–2, the electrode sustained over 100 cycles at a high mean discharge potential of ca. 3 V vs Li/Li⁺.This paper was originally presented at the 183rd meeting of the Electrochemical Society (Honolulu, Hawaii, 1993).     

Chemical Transformations in the Course of Solid-Phase Synthesis of Fibrous Alkali-Free Fluoroamphibole

This paper reports on the results of investigations into the possible chemical transformations occurring in the course of the solid-phase synthesis of fibrous alkali-free fluoroamphibole from synthetic magnesium silicate dihydrate (MSDH) MgO · SiO₂ · 2H₂O in the MSDH–MgF₂–NaCl model system in the concentration range of compositions corresponding to Mg-fluorocupfferite (Mg₇Si₈O₂₂F₂) at temperatures ranging from 60 to 1000°C. It is found that the formation of Mg-fluorocupfferite from MSDH under these conditions is a complex multistage chemical process. In the temperature range 60–700°C, dehydration, dehydroxylation of MSDH, and pyrohydrolysis of fluorides (MgF₂, SiF₄, etc.) proceed near the lower temperature boundary of the fluoroamphibole formation region (T 750°C). These processes result in the formation of compounds containing monomeric and simple polymeric silicon–oxygen anions [SiO₄]^4– and [SiO₃]^2– (forsterite, fluoronorbergite, enstatite, etc.). In the temperature range 850–900°C, the transformation of intermediate compounds leads to the formation of the fluoroamphibole structure. The main elements of this structure are silicon–oxygen anions of the [Si₄O₁₁]^6– type.     

Living carbocationic polymerization

The living continuous polymerization of isobutylene initiated by a bifunctional initiator, i.e., 2,4,4,6-tetramethyl-heptane-2,6-diacetate·BCl₃ complex, in CH₂Cl₂ and C₂H₅Cl diluent in the –12 to –20°C range is described. Experimental conditions have been found under which rather narrow molecular weight distribution ,-tert.-chloro-ended polyisobutylenes of theoretical M_n and I_eff=100% can be continuously prepared in a tubular reactor in a homogeneous system (in C₂H₅Cl at –12°C). System heterogeneity tends to increase the molecular weights, decrease the I_eff, and increase the ¯M_w/¯M_n.     

Solid-state ionics-solid electrolyte cells with copper ion conductors

A solid electrolyte cell has been developed using a high copper ion conductivity solid electrolyte, 7CuBr·C₆H₁₂N₄CH₃Br, a copper anode, and a chalcogen cathode. The open-circuit voltages of the cells with sulphur, selenium, and tellurium as cathode materials were 0·448, 0·373, and 0·258 V, respectively, at 25° C. These cells yielded a current of several tens of microamperes at room temperature and several milliamperes at 114° C without appreciable polarization. An energy density of 4.5 Wh kg^–1 at room temperature was evaluated from the weights of the electrolyte and electrode materials for the cell using a selenium cathode in the discharge current density range 60–150A cm^–2.     

Comparative analysis of mass transfer at vibrating electrodes

The application of oscillatory flows to electrochemical processes was found to increase the rate of mass transfer and improve the quality of deposit. Various mechanisms to which this phenomenon is attributable are discussed and expressions for the average rate of mass transfer, resulting thereof, are derived. Comparison with experimental data indicates that the stretched-film concept, although an oversimplification of the physical situation, is most successful in correlating the data.Nomenclature A Amplitude of oscillatory motion (cm) - c Concentration of the diffusing species (g mol cm^–3) - D Diffusivity (cm² s^–1) - F Frequency of oscillation (Hz) - k Instantaneous mass transfer coefficient (cm s^–1) - ¯k _vib Time-average vibratory mass transfer coefficient (cm s^–1) - L Length of active area (cm) - S Velocity gradient at solid-liquid interface (cm s^–1 cm^–1) - u Oscillatory velocity of fluid layers adjacent to the electrode (cm s^–1) - _u Rel Relative velocity between the electrode and the bulk of the fluid (cm s^–1) - v Relative velocity between the electrode and the fluid layers adjacent to it (cm s^–1) - W Width of active area (cm) - x Distance along the surface of the electrode (cm) - z Distance perpendicular to the surface of the electrode (cm) - Dimensionless distance=z(S/9Dx)^1/3 - Dimensionless distance=z ²/2 - Kinematic viscosity of the electroyte (cm² s^–1) - Angular frequency=2F     

Propane oxidative dehydrogenation on silica based oxide catalysts

The oxidative dehydrogenation of propane to propylene with molecular O₂ (PPD) has been investigated on commercial bare SiO₂ and medium loaded V₂O₅ and MoO₃ catalysts at 450–525°C. The direct relationship between the density of reduced sites (, 10¹⁶ s_r g _cat– ¹ ), evaluated in steady-state conditions by O₂ chemisorption, and the POD reaction rate proves the occurrence of a concerted reaction mechanism involving the activation of gas-phase O₂ on the reduced sites of the catalyst surface. A straight-line correlation between the activity of such silica based catalysts in POD and methane partial oxidation to formaldehyde (MPO) at 525°C has been disclosed.     

Origin of rate bistability in Mn–O/Al2O3 catalysts for carbon monoxide oxidation: role of the Jahn–Teller effect

Peculiarities in catalytic activity in carbon monoxide oxidation as well as some structure, electronic and magnetic properties of the three oxide catalysts, Mn³⁺–O/Al₂O₃ (1), Mn³⁺–O–Fe/Al₂O₃ (Mn-substituted spinel, 2) and -Fe₂O₃/Al₂O₃ (3), were studied by kinetic measurements and by Mössbauer spectroscopy. The catalysts 1 and 2 showed a kinetic bistability with a sharp transition towards more reactive state at 200°C (ignition point). In contrast, for catalyst 3, at 200–250°C, the behavior of reaction rate against temperature did not display noticeable hysteresis. On cooling the catalysts 1 and 2, extinction was observed at about 170 and 120°C, respectively, i.e., at 30–80°C lower than the corresponding ignition points. Proximity of activation energy for the high and low activity (15–19 kJ/mol) for both Mn-containing catalysts suggests an increase in the number of active sites at high temperature with no changes in the reaction mechanism. The considerable difference between Mn-containing catalysts 1, 2 and Fe-containing catalyst 3 may be caused by Jahn–Teller (JT) type distortions of the oxygen polyhedron around Mn³⁺. A significant spontaneous axial bond stretching within the local polyhedron seems to diminish Mn–O binding energy, facilitate the participation of surface oxygen species, O_S, in the oxidation of CO by a redox mechanism and promote oxygen vacancies at the surface that would cause considerable effect on the activity. An increase in the width of the counterclockwise hysteresis loop for the catalyst 2 compared to the catalyst 1 indicates that clusters of mixed spinel provide more active sites and more labile O_S species than clusters of the binary Mn oxide.     

Double layer capacity of graphite in cryolite-alumina melts and surface area changes by electrolyte consumption of graphite and baked carbon

The double layer (d.l.) capacity of pyrolytic graphite in cryolite-alumina melts at 1010°C was found to exhibit a minimum of 20F cm^–2 at 0·9 V positive to the aluminium electrode. The d.l. capacity attained a plateau of 60F cm^–2 at 1·1–1·4 V, while it rose steeply at potentials below 0·7 V. During electrolytic consumption involving CO₂ evolution the d.l. capacity of pyrolytic graphite remained unaffected, while that of baked carbon was changed, reflecting changes in surface area. At low current densities (cds) the surface area increased substantially and the surface was noticeably roughened, while the opposite behaviour was observed at above 2.5 A cm^–2.     

Effect of sodium and potassium sulphates on zinc electrowinning

In order to evaluate the intrinsic effect of high concentrations of sodium and potassium sulphates in zinc electrowinning solutions, measurements of coulombic efficiency were carried out under mass transfer-controlled conditions in synthetic solutions of very high purity. A solution composition of 1 mol dm^–3 ZnSO₄+1.5 mol dm^–3 H₂SO₄ was employed with and without additions of 0.5 mol dm^–3 Na₂SO₄ and/or 0.25 mol dm^–3 K₂SO₄. With temperature and current density similar to plant practice (37° C, 650 A m^–2) and electrode rotation rates of 10 and 45 s^–1, the coulombic efficiency for three successive batch tests (200 mg zinc) increased by an average of 1.2% (from an average of 96.0%) for additions of 0.5 mol dm^–3 Na₂SO₄+0.25 mol dm^–3 K₂SO₄. The results were evaluated in terms of available theories, solution purity and predicted changes in solution composition (zinc and hydrogen ion activities) and physical properties following additions of Na₂SO₄/K₂SO₄. It was concluded that in the plant situation the increase in coulombic efficiency would probably be offset by an increase in cell voltage of about 2%, the net effect on power efficiency being a decrease of about 1%. The zinc deposit morphology and preferred orientation were also studied. The addition of sodium and/or potassium sulphate to the solution resulted in rougher, darker zinc deposits, a slight grain refining effect, and a change from random to predominantly basal (002), (004) crystal orientation (at 45 s^–1).