Diffusion and equilibrium properties of hydrogen in palladium measured by the stripping potentiostatic method

Theory and experiments on the (non-equilibrium) stripping potentiostatic method for evaluation of the diffusion coefficient and desorption isotherms of hydrogen in α-Pd are discussed. Diffusion coefficient of hydrogen D can be evaluated either under transient conditions, from the anodic stripping current (I_a) or under steady-state condition, from the anodic charge corresponding to the stripping of hydrogen held initially in Pd membrane (Q_a). Following data are reported: D = (3.60±0.06) × 10⁻⁷ cm² s⁻¹ (transient) and D = 3.41 × 10⁻⁷ cm² s⁻ (steady-state) at 20°C in 0.1 N H₂SO₄. Equilibrium concentration and equivalent pressure of hydrogen at the entrance side of Pd membrane can be calculated from the steady-state permeation current (I^∞_a or the anodic stripping charge (Q_a) and its open-circuit potential just after stopping hydrogen generation, E⁰_c. Reliable Sieverts' constants (K_s) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: ΔH^o = −(2.6±0.1) kcal (mol H)⁻¹ and ΔS^o = −(14.5±0.4) cal K⁻¹ (mol H)⁻¹. These data agree well with those obtained in gas phase measurements.     

Modeling of hydrogen separation through porous YSZ hollow fiber‐supported graphene oxide membrane

In this work, hydrogen (H₂) permeation fluxes through 230 nm‐thick graphene oxide (GO) membrane deposited on porous YSZ hollow fiber were measured and correlated to an explicit H₂ permeation model. H₂ fluxes through such GO‐YSZ hollow fiber membrane increased from 4.83 × 10^?8 mol cm^?2 s^?1 to 2.11 × 10^?7 mol cm^?2 s^?1 with temperature rise from 20 to 100 °C. The activation energy of H₂ permeation was determined by the linear regression of the experimental data and was applied in the theoretical calculations. The model predictions fit well the temperature dependent and the argon sweep gas flow rate dependent H₂ fluxes data. Using the derived permeation model, the effects of vacuum pressure at lumen side and H₂ partial pressure at shell side, membrane area, and GO membrane film thickness on the membrane performance were simulated and discussed to provide insights for practical applications. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2711–2720, 2018     

Preparation of composite membranes with polyimides and poly (amide-imide)s skin via interfacial condensation for air separation

Composite membranes were prepared by the interfacial condensation of water-soluble diamines with an organic solvent (dichloromethane)-soluble dicarbo-methoxy terephthaloyl chloride or carbomethoxy terephthaloyl chloride on top of a porous aluminum oxide support. The morphology of skin on the composite membranes is different in the two different procedures. The polyimide composite membranes with 40-times coatings provide a high gas permeation rate of oxygen and good permselectivity [α(O₂/N₂)]. The composite membrane with the polyimides skin at 40-times coatings had a gas permeation rate of oxygen range from 83 × 10⁻⁵ to 130 × 10⁻⁵ cm³(STP) s⁻¹ cm⁻² cmHg⁻¹, and a permselectivity [α(O₂/N₂)] range of 3.57 to 5.60. The composite membrane with poly (amide-imide)s skin at 40-times coatings had a gas permeation rate of oxygen range from 102 × 10⁻⁵ to 146 × 10⁻⁵ cm³(STP) s⁻¹ cm⁻² cmHg⁻¹, and the permselectivity (α(O₂/N₂)) range from 3.20 to 4.96.     

钯复合膜的制备及其在氢气氮气分离中的应用

Thin palladium composite membranes were prepared by modified electroless plating method on a-alumina supports and a dense Pd/α-Al2O3 composite membrane with high hydrogen flux, good selectivity for hydrogen was obtained. It was tested in a single gas permeation system for hydrogen permeance and hydrogen selectivity over mtrogen. The hydrogen permeance of the corresponding membrane was ashigh as 2.45×10^-6mol·m^-2·s^-1.Pa^-1 and H2/N2 selectivityover700 at 623K and a pressure difference of 0.1MPa. The-main resistance of the composite membrane to H2 permeation lies in the aluminum ceramic support rather than the thin Pd layer.     

Behaviour of a potassium electrode pre-treated with water vapour or gaseous oxygen

The electrochemical behaviour of K electrodes in highly dried 0.5 M KPF₆ in propylene carbonate (<0.01 ppm H₂O) was studied. The electrode surface was chemically modified before contacting the electrolyte either by O₂-free H₂O vapour or by dry gaseous O₂; the exposure amounted to 1.2×10⁹ and 2.7×10⁸ langmuirs, respectively (ie1.6×10⁵ and 3.5×10⁴ Pa s). Long-term dependences of the open-circuit potential on the time and interrupted galvanostatic E-t curves proved the existence of firmly adsorbed or chemisorbed passivating layers formed by reaction of K with H₂O vapour or O₂. Their influence on the electrochemical characteristics was apparent even after passage of an anodic charge of 50 mAh cm⁻² at 1 mA cm⁻² current density.     

Thermal Isomerization of Azulene. Single-Pulse Shock Tube Investigation

The thermal isomerization of azulene was studied behind reflected shocks in a pressurized driver single-pulse shock tube. The temperature range covered was 1050–1400 K at overall densities of ∼2.5 × 10⁻⁵ mol/cm³. The main reaction of azulene under these conditions is a unimolecular isomerization to naphthalene, but it also isomerizes, although at a much lower rate, to another isomer. The suggested “tetracyclic triene” intermediate structure for the azulene-naphthalene isomerization can lead also to transition states that can describe isomerizations to 1-methylene-1H-indene and 1, 2,3-metheno-1H-indene,2,3-dihydro. Small quantities of C₂H₂, C₄H₂, C₆H₆, and C₆H₅-C≡CH were also found in the post-shock samples, particularly at high temperatures. The Arrhenius parameters of the two high pressure limit rate constants for the isomerization processes are: azulene → naphthalene, k₁ = 10^12.93 exp(–62.8 × 10³/RT) s⁻¹ azulene → second isomer, k₂ = 10^12.42 exp(–69.5 × 10³/RT) s⁻¹ A discussion of the mechanism for these isomerization processes is presented.     

Branched-pore model applied to the adsorption of basic dyes on carbon

The branched-pore adsorption model, expressed by an external mass transfer coefficient k_f, a solid diffusivity D_s, a lumped micropore diffusion rate parameter k_b, and the fraction of macropores f, describes kinetic data from initial contact of adsorbent-adsorbate to the long-term ( > 24 hours) adsorption stages with reasonable accuracy.In this work the model is applied for three basic dye systems, namely Basic Red 22, Basic Yellow 21 and Basic Blue 69, all on carbon. A single value of each parameter describes each dye system. The k_f values are 0.18 × 10⁻²±28%, 0.3 × 10⁻²±17% and 0.2 × 10⁻² ± 20% cm s⁻¹, the D_s values are 0.33 × 10⁻⁹ 21%, 0.72 × 10⁻⁹ ± 9% and 0.72 × 10⁻⁹ ± 9% cm² s⁻¹, the k_b values are 0.65 × 10⁻⁶ ± 7.7%, 1.8 × 10⁻⁶ 0.2 × 10⁻⁶ 1% s⁻¹, while the f values are 0.55 ± 9%, 0.60 ± 10 % and 0.18 ± 11%, each for Basic Red 22, Basic Yellow 21 and Basic Blue 69 respectively.The model is based on the internal structure of the carbon particle being divided into a macropore and a micropore region. The latter has an upper-bound capacity of 241, 245 and 656 mg g⁻¹ for Basic Red 22, Basic Yellow 21 and Basic Blue 69 respectively. A sensitivity analysis for each parameter has been carried out.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

Electrochemical analysis of trace amounts of water in propylene carbonate electrolytes

Cyclic voltammetry and limiting currents to an rde have been used for the determination of the water in the 0.3–20 ppm range, in PC electrolytes. Carefully dried solutions yielded voltammetric peaks proportional to the amount of water added. In KPF₆ electrolytes, the cathodic current corresponding to the reduction of water was at 0.48 V, while in KAlCl₄ solutions the reduction occurred already starting at 2.8 V, both relative to a K/K⁺ reference. At these potentials, the current to an rde showed Levich type dependence on rpm and was proportional to the concentration of water in the range 0.5–20 ppm. The calculated diffusion coefficients of water are higher than expected: 25°C, D_H2O = 8.5±2 × 10⁻⁶ cm² s⁻¹ in 0.5 M KPF₆, while in 0.25 M KAlCl₄D_H2O, 10±2 × 10⁻⁶ cm² s⁻¹.     

Copper separation from nitrate/nitric acid media using Acorga M5640 extractant: Part II. Supported liquid membrane study

The facilitated transport of copper(II) from nitrate/nitric acid media through a flat-sheet supported liquid membrane (FSSLM) is investigated, using the commercially available oxime Acorga M5640 as ionophore, as a function of hydrodynamic conditions, concentration of copper (7.9×10⁻⁵ to 1.3×10⁻³ M) and H⁺ (pH 1.0–2.0) and ionic strength in the feed solution, carrier concentration (5–40% v/v) in the membrane and support characteristics. The performance of the system is also compared using various diluents for the organic phase and against other available oxime extractants (MOC-55TD, LIX 860 and LIX 622). A model is presented that describes the transport mechanism, consisting of diffusion through a feed side aqueous diffusion layer, a fast interfacial chemical reaction, and diffusion of carrier and its metal complex through the organic membrane. The organic membrane diffusional resistance (Δ_o) and aqueous diffusional resistance (Δ_a) were calculated from the proposed model, and their values were 7.6×10⁶ and 273 s/cm, respectively. It was observed that the copper flux across the membrane tends to reach a plateau at high concentration of copper or a low concentration of H⁺ owing to carrier saturation within the membrane, and leads to a diffusion-controlled process. The values of the apparent diffusion coefficient (D_o^a) and limiting metal flux (J_lim) were calculated from the limiting conditions and found to be 2.0×10⁻⁸ cm²/s and 2.3×10⁻¹¹ mol/cm² s, respectively. The values of the bulk diffusion coefficient (D_o,b) and diffusion coefficient (D_o) calculated from the model were 5.9×10⁻⁹ and 1.6×10⁻⁹ cm²/s, respectively. The polymeric microporous solid support, Durapore GVHP 04700, was selected throughout the study as it gave the best performance.     

Aluminium alloys for aluminium primary cell

Aluminium has not widely been used for anode material for primary cells, despite its attractive properties. Aluminium loses a great part of the potential when anodically polarized, and is easily coroded in aqueous solution with hydrogen evolved. Aluminium alloys derived from high purity aluminium (99.999%) were prepared for application to primary dry cell. Small amounts of the following elements were added to aluminium: Zn, Sn, Bi, In, Mg. The electrode potential, the anode Faradaic efficiency and the rate of corrosion of the alloy specimen sheet (10 × 10 × 0.5) were measured in 1 M AlCl₃ aqueous solution. The rate of corrosion significantly increased after anodic polarization (anodic dissolution). The electrode potentials at open circuit and anodic polarization (10 mA cm⁻² shifted remarkably in the negative direction by alloying. The anode Faradaic efficiency and the rate of corrosion depended upon the alloy composition. The alloy, Al-3.0 Zn-0.13 Sn-0.01 Ga-0.07 Bi (% w/w), gave the best results. The anode efficiency amounted to 99.7% as compared with 87.4% for 99.999% Al. The average rate of corrosion was reduced to 0.03 mg cm⁻² h⁻¹ (short term) and 0.01 mg cm⁻² h⁻¹ (long term). These electrochemical properties will improve the characteristics of aluminium dry cells.     

Gasification of carbon by CO2: A transient kinetics experiment

A transient kinetic technique was used to measure the intrinsic rate constant k₂ of the reaction C(O) → CO + C_f, where C_f is an available site and C(O) is an occupied site. It was found that k₂ = 10^{11.6 ± 2.3} exp[ −(225 000 ± 39000)/RT]min⁻¹. Two systems were studied, one using an uncatalysed carbon, the other a Ca-catalysed carbon. The gasification rates for these two systems differed by a factor of 100, yet they yielded the same k₂. This strongly supports the contention that Ca catalyses the system by increasing the number of active sites.     

Anodic evolution of oxygen on ruthenium in acidic solutions

Electrochemical properties of ruthenium, particularly the anodic evolution of oxygen and anodic dissolution of ruthenium have been investigated by means of polarization measurements and product analyses. The electrode surface gradually colours black during oxygen evolution. This is due to the accumulation of hydrous RuO₂ resulting from decomposition of corrosion product. The black oxide film suppresses the ruthenium dissolution rate and the current efficiency for the dissolution reaction is less than 6% at a cd below 0.2 A/cm² in 1 N H₂SO₄, where the anodic evolution of oxygen is predominant. The overall current for oxygen evolution is expressed by i = nFka^?2_H exp (2FE/RT) The probable mechanism of oxygen evolution on the ruthenium anode under the Langmuir conditions of intermediate adsorption is

     

The Oxidation of Coated SOFC Interconnects in Fuel Side Environments

This study focuses on examining the effect of PVD coatings on the oxidation performance of interconnects in fuel (anode) side environments. A Fe‐22Cr ferritic steel was coated with (i) Ce 10 nm (ii) La 10 nm and (iii) Co 600 nm. The samples were exposed at 850 °C in Ar‐5% H₂‐3% H₂O in a tubular furnace over 500 h. Additionally, the effect of a pre‐oxidation step was investigated by exposure in air prior to the simulated fuel gas environment. Chemical analysis on the samples was subsequently performed with SEM/EDX and XRD. It was established that the Ce and La coatings brought about a factor 2–3 reduction (k_p values of 2.16 × 10⁻¹⁴ ± 3.6 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹ for the La 10 nm coated steel compared to 7.72 × 10⁻¹⁴ ± 5.86 × 10⁻¹⁵ g² cm⁻⁴ s⁻¹ for the uncoated steel) in the oxidation rate while the Co coating disintegrated into metallic islands in and on the thermally grown oxide after exposure. Additionally, the La coating resulted in the formation of a continuous perovskite layer by reaction with the thermally grown oxide.     

Ionic mass transfer rate of CuSO4 in electrodialysis

Ionic mass transfer rate of copper ion in the ion exchange membrane electrodialysis at limiting current density has been studied using a planar flow electrodialyzer consisted of a cation exchange membrane and an anion exchange membrane. Their effective area is 4 × 5 cm². The effects of flow rate, viscosity of electrodialysate solution and thickness of electrodialyzer to the ionic mass transfer rate have been studied. An empirical correlation equation (Nu)_exp. = 4.57 × Re^0.333 Sc^0.307 (de/L)^O.33 is obtained. It fitted well with the theoretical performance equation (NU)_theor.= 3.70 × (Re · Sc · de/L), which is derived from the Nernst-Planck Equation based on the assumption that mass transfer in the concentration boundary layer in the desalting compartment is controlling.     

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation

An ultrathin La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm³/cm² s (5.77 cm³/cm² min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm³/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9-11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).     

Hydrogen Atom Attack on Fluorotoluenes: Rates of Fluorine Displacement

Rates of hydrogen atom attack on o-fluorotoluene (o-FTOL) and m-fluorotoluene (m-FTOL) at temperatures of 988–1144 K and pressures of 2–2.5 bar have been determined in a single-pulse shock tube study. Hydrogen atoms, generated from the decomposition of hexamethylethane, were allowed to react with the substrates and the characteristic products observed. Rate constants for two reaction channels, displacement of fluorine or methyl, were determined relative to displacement of methyl from 1, 3,5-trimethylbenzene (135TMB). Evidence is presented that abstraction of F is unimportant over the studied temperature range. With k(H + 135TMB → m-xylene + CH₃) = 6.7 × 10¹³ exp(–3255/T) cm³ mol⁻¹s⁻¹, the following rate expressions have been derived: k(H + o-FTOL → C₆H₅CH₃ + F) = 8.38 × 10¹³ exp(–6041/T) cm³ mol⁻¹s⁻¹; (1012–1142 K) k(H + o-FTOL → C₆H₅F + CH₃) = 2.37 × 10¹³ exp(–2938/T) cm³ mol⁻¹s⁻¹; (988–1142 K) k(H + m-FTOL → C₆H₅CH₃ + F) = 1.33 × 10¹⁴ exp(–6810/T) cm³ mol⁻¹s⁻¹; (1046–1144 K) k(H + m-FTOL → C₆H₅F + CH3) = 2.04 × 10¹³ exp(–3104/T) cm³ mol⁻¹s⁻¹; (1008–1144 K) Uncertainties in the relative rate constants are estimated to be factors of about 1.1, while the above absolute values have estimated expanded uncertainties of about a factor of 1.4 in rate, 10 kJ mol⁻¹ in the activation energy, and a factor of 3 in the A-factor. The present data are compared with relevant literature data. From our data and the thermochemistry, a model of the elementary steps comprising displacement of F is developed. On the basis of the model fit to our data, rate constants for the addition of atomic fluorine to toluene at 1100 K are derived. Rate expressions for fluorination reactions of toluene are also determined. The significance of the present results is discussed in the context of the formation of fluorinated byproducts in high-temperature systems.     

Treatment of metal‐containing wastewaters with a novel extractive membrane reactor using sulfate‐reducing bacteria

This work reports a novel system for the treatment of acidic metal‐containing wastewaters, the Extractive Membrane Bioreactor–Sulfate‐Reducing Bacteria (EMB‐SRB) system. In this system, hydrogen sulfide is produced in the bioreactor by the sulfate‐reducing bacteria, transfers through a dense phase membrane, and precipitates metal ions in the wastewater. The non‐porous membrane prevents the SRB from having direct contact with the toxic metals, extremes of pH, or high salinity in the wastewater. Silicone rubber, which is permeable to H₂S but virtually impermeable to ionic species in the system, was used as a membrane. The rate of mass transfer of H₂S across the membrane was studied and found to be well described by a resistances‐in‐series model. k_ov values vary in the range 5 × 10⁻⁶ –10 × 10⁻⁶ ms⁻¹ 1 depending on the membrane thickness. A continuous EMB–SRB system was operated and more than 90% (w/v) of the Zn²⁺ present in a wastewater was removed. A film of metal precipitate was found to build up on the inside (wastewater) side of the membrane, and became the dominant resistance contributing to the overall mass transfer coefficient during operation. © 2001 Society of Chemical Industry     

The interaction of hydrogen sulfide with lead- and barium–cadmium–zinc-stabilized poly(vinyl chloride)

Poly(vinyl chloride) polymers stabilized with tribasic lead sulfate discolor upon exposure to hydrogen sulfide gas as a result of lead sulfide formation. The discoloration occurs for samples in both cord and sheet forms and is shown to be a function of total H₂S exposure, reaching a limiting value that is determined by the amount of lead stabilizer used in the polymer formulation. The permeation and diffusion constants for H₂S through PVC stabilized with tribasic lead sulfate and with a liquid Ba–Cd–Zn formulation are found to be P_Pb = (6.0 ± 0.2) × 10^?9, P_BaCdZn = (5.2 ± 0.2) × 10^?9 (both in cm³ gas?cm film/cm² area?sec?cm Hg), D_Pb = (1.3 ± 0.2) × 10^?7 cm²/sec, and D_BaCdZn = (6.4 ± 0.6) × 10^?8 cm²/sec, all measured at 21°C. The stabilizing efficiencies of the formulations were assessed by HCl evolution measurements, which show that exposure to H₂S decreases the initial polymer stability for both Pb-stabilized and Ba–Ca–Zn-stabilized formulations. Protection of stabilized PVC formulations from diffusing hydrogen sulfide is thus advisable for long-term stability as well as for color integrity.     

High performance BaCe0.8Y0.2O3−a (BCY) hollow fibre membranes for hydrogen permeation

In this work, BaCe_0.8Y_0.2O_3−α (BCY) perovskite hollow fibre membranes were fabricated by a phase inversion and sintering method. BCY powder was prepared by the sol–gel technique using ethylenediaminetetraacetic acid (EDTA) and citric acid as the complexing agents. Gel calcination was carried out at high temperature to form the desired crystal structure. The qualified BCY hollow fibre membranes could not be achieved even the sintering was carried out at temperatures up to 1550^oC due to the poor densification behavior of the BCY material. The addition of sintering aid (1 wt% Co₂O₃) inside BCY powder as the membrane starting material significantly improved the densification process, leading to the formation of gas-tight BCY hollow fibres. The optimum sintering temperature of BCY hollow fibre membrane was 1400 °C to achieve the best mechanical strength. H₂ permeation through the BCY hollow fibre membranes was carried out between 700 and 1050 °C using 25% H₂–He mixture as feed gas and N₂ as sweep gas, respectively. For comparison purpose, the disk-shaped BCY membrane with a thickness of 1 mm was also prepared. The measured H₂ permeation flux through the BCY hollow fibres reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C strikingly contrasting to the low values of less than 0.01 mL cm⁻² min⁻¹ from the disk-shaped membrane. After the permeation test, the microstructure of BCY hollow fibre membrane was still maintained well without signals of membrane disintegration or peeling off.