Preparation of supported palladium membrane and separation of hydrogen

Palladium acetate was sublimed at a reduced pressure at 400°C., carried into the macropores of the porous wall of an α-alumina support tube and was decomposed there. A thin palladium membrane which was thus formed showed a hydrogen permeance of 10⁶ mol·m²·s¹.-Pa¹ and a hydrogen/nitrogen permselectivity higher than 1000. The membrane was stable against hydrogen embrittlement even when the permeation temperature was varied between 100 and 300°C., and it was stable to sulfur or chlorine. To test the ability of this system for the separation of hydrogen and deuterium, a palladium disk was used instead of the prepared membrane since a definite membrane thickness was necessary for calculation. When H₂ and D₂ permeated through the membrane independently, the H/D permselectivity was approximately 7 at 150–200°C under a feed side pressure of 0.4 MPa and a permeate side pressure of 0.1 MPa. When a mixture of H₂ and D₂ was fed, the H/D permselectivity was reduced to 1.2–1.6.     

Preparation of a tubular palladium membrane by photocatalytic deposition

A novel photocatalytic deposition method for the preparation of a thin tubular palladium membrane is presented in this paper. The membrane is prepared on a porous asymmetric TiO₂ support by photocatalytic reaction of palladium ion, followed by electroless plating. Gas permeation results show that the membrane exhibits increased hydrogen permeance with the increase of temperature. The hydrogen permeance and selectivity to nitrogen at 773 K are about 1.43×l0⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ and 17, respectively, when the pressure in the feed side is 0.1 MPa. The activation energy of hydrogen permeation is 11.06 kJ/mol at the temperature range of 573–773 K.     

Hydrogen recovery from cyclohexane as a chemical hydrogen carrier using a palladium membrane reactor

A palladium membrane reactor was applied to recover the hydrogen from cyclohexane as one of the promising chemical hydrogen carriers. The operation conditions of the palladium membrane reactor to obtain a higher hydrogen recovery were predicted by computer simulation. As a result, it was shown that the hydrogen recovery rate becomes higher as the pressure on the hydrogen permeation side is lowered below atmospheric pressure or as the reaction pressure increases. This was confirmed experimentally. As the perm-side pressure was lowered, the conversion as well as the hydrogen recovery rate at 573 K was found to increase. About 80% of the hydrogen contained in cyclohexane, depending on the operation condition was successfully recovered.     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

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.     

Preparation, performances and reaction mechanism for the synthesis of H2O2 from H2 and O2 based on palladium membranes

A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H₂O₂. Such TCM's are asymmetric -alumina mesoporous membranes supported on macroporous -alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition–precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1–10 μm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H₂O₂. Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250–300 ppm H₂O₂ were commonly achieved with both CAM's and AAM's after 6–7 h time on stream, whereas the decomposition rate was particularly high in the presence of H₂. Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H₂O₂ a smooth metal surface is needed.     

Hydrogen generation in a Pd membrane fuel processor: Productivity effects during methanol steam reforming

Performance analyses are carried out for the palladium membrane fuel processor for catalytic generation of high purity hydrogen. The reactor model includes detailed particle-scale multi-component diffusion, multiple reversible reactions, flow, and membrane transport. Using methanol steam reforming on Cu/ZnO/Al₂O₃ catalyst as the test reaction, a systematic examination of the effects of operating and reactor design parameters on key performance metrics is presented. Single particle simulations reveal a complex interplay between nonisobaric transport and the reversible reactions (methanol reforming and decomposition, and water-gas shift), which impact overall reactor performance. An analysis of characteristic times helps to identify four different productivity controlling regimes: (i) permeation control, encountered with thick membranes and/or insufficient membrane area; (ii) catalyst pore diffusion control encountered with diffusion of reacting species in larger particles; (iii) reaction control, encountered when intrinsic catalytic rates are too low because of inadequate activity or catalyst loading; and (iv) feed control, encountered when the limiting reactant feed rate is inadequate. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate space velocity, while the hydrogen utilization is a decreasing function of space velocity, implying a trade-off between productivity and hydrogen utilization. The locus of productivity maxima itself exhibits a maximum at an intermediate membrane surface to volume ratio, the specific value of which is dependent on the particle size, membrane thickness and reaction conditions. At moderate temperature and total pressure (, 10 bar), particles smaller than 2 mm diameter, Pd membranes with thickness less than , and membrane surface to volume ratio exceeding are needed to achieve viable productivity . A comparison between the packed-bed membrane reactor and conventional packed-bed reactor indicates a modest improvement in the conversion and productivity due to in situ hydrogen removal.     

Preparation and characterization of polysulfone mixed matrix membrane incorporated with palladium nanoparticles in the inversed microemulsion for hydrogen separation

Polysulfone (PSf) membrane shows acceptable gas separation performance, but its application is limited by the “trade-off” between selectivity and permeability. In this study, PSf mixed matrix membranes (MMMs) incorporated with palladium (Pd) nanoparticles in the inversed microemulsion were proposed for hydrogen (H₂) separation. Pd nanoparticles can be kinetically stabilized and dispersed using electrostatic and/or steric forces of a stabilizer which is typically introduced during the formation of Pd nanoparticles in the inversed microemulsion. Pd nanoparticles were synthesized by loading (PdCl₂) into the polymeric matrix, polyethylene glycol (PEG) which acts as reducing agent and stabilizer. The dry–wet phase inversion method was applied for the preparation of asymmetric PSf MMMs. The effects of Pd (0–4 wt%) on the membrane characteristics and separation performance were studied. Experimental findings verified that the MMMs are able to achieved a high H₂/N₂ selectivity of 21.69 and a satisfactory H₂ permeance of 46.24 GPU due to the changes in membrane structure from fully developed finger-like structure to closed cell structure besides the growth of dense layer. However, the selectivity of H₂/CO₂ decreased due to the addition of PEG.     

Gas permeation characteristics of silica/alumina composite membrane prepared by chemical vapor deposition

Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane at 600–650 ‡C on a porous α-alumina tube with pore size of 110–180 nm or γ-alumina coated α-alumina tube with pore size of 6–8 nm.The forced cross-flow through the porous wall of the support was very effective in plugging macropores. The membranes formed on γ-alumina coated oc-alumina tube showed H₂ permeances much higher than the SiO₂ membranes formed on the α-alumina tube. This indicated that the γ-alumina film was effective in improving the H₂ permeance and H₂/N₂ selectivity. The permeation tests with CO₂, N₂, CH₄, C₃H₈ and i-C₄H₁₀ showed that a very small number of mesopores remained unplugged by the CVD. Permeation of hydrogen was explained by activated diffusion, and that of the other gases by Knudsen diffusion through the unplugged pores. Thus, the total permeance was composed of permeances due to the activated and Knudsen diffusion mechanisms. The contribution of Knudsen diffusion pores decreased to 0.02 when the γ-alumina film was modified at 650 ‡C until P_fe=50 Pa.     

Hydrogen generation and purification in a composite Pd hollow fiber membrane reactor: Experiments and modeling

“Pd nanopore” composite membranes are a novel class of H₂ permselective membranes in which a thin layer of Pd is grown within the pores of a supported nanoporous layer. In this work, Pd nanopore membranes and conventional Pd top-layer membranes were used in the generation of high-purity H₂ from the catalytic decomposition of anhydrous NH₃. An effective 4 μm thick Pd nanopore membrane and 13 μm thick Pd top-layer membrane were synthesized on 2 mm O.D. α-Al₂O₃ hollow fibers. The permeation features of the membranes were determined and the membranes were then employed in a single fiber packed-bed membrane reactor in which Ni-catalyzed NH₃ decomposition served as the test reaction, with conditions spanning a range of conditions (500–600 °C; 3–5 bar total retentate pressure; 60–1200 scc/h g cat space velocity). The NH₃ conversions in both the PBMRs were approximately 10% higher than in a packed-bed reactor (PBR) under similar conditions. The increase in conversion with the PBMR was attributed to the removal of H₂, which has an inhibitory effect on the forward kinetics of the reaction as per the Temkin-Pyzhev type rate mechanism. Reactor productivities in the range of 2 mol/s m³ (at 85% H₂ utilization) to 7 mol/s m³ (at 50% H₂ utilization) were obtained. The permeate stream purity exceeded 99.2% H₂. A two-dimensional pseudo-homogeneous model was successfully used to simulate the experimental results and to interpret the findings. Permeation and kinetic parameters were estimated in permeation and PBR experiments, respectively. Without any data fitting the PBMR model predictions demonstrated very good agreement with experimental trends. Together with an analysis of the characteristic times, the model determined that transverse transport of hydrogen in the catalyst bed limited PBMR performance. The model was used to determine the rate limiting step and to suggest ways in which the reactor productivities could be further improved.     

An integrated purification and production of hydrogen with a palladium membrane-catalytic reactor

In this paper, we presented an integrated production and purification process of hydrogen by the use of a defect-free palladium membrane. Hydrogen could be purified from a variety of mixtures providing the purity of 3–7 N depending on the feeding stream. The permeation parameters are accurately predicted by a separation model as established. The membrane is prepared by electroless plating and is stable among 300–400°C. Using an active catalyst, the rate of steam reforming of methanol was found to be significantly faster than that without a membrane module. In the steam reforming of methane, the reaction temperature was lowered to 500°C to achieve a conversion of 45%, which is 15% higher than the thermodynamic equilibrium conversion.     

Effects of preparation parameters on CO2/N2 gas permselectivity of polyether thin film composite membrane

In this work, ether oxide (EO)-based multilayer composite membranes were prepared via interfacial polymerization (IP) of trimesoyl chloride (TMC) and polyetheramine (PEA) on polydimethylsiloxane precoated polysulfone support membrane. The effects of preparation parameters, such as monomer concentrations, reaction time, and heat-treatment temperature on the membrane performance were investigated. The optimal preparation parameters have been concluded. The results showed the increasing monomers concentration of both PEA and TMC can lead to the decrease of CO₂ permeance and increase of CO₂/N₂ selectivity. The optimal monomers concentration was found. When monomer concentrations are higher than the optimal values, the CO₂ permeance decreases continually while CO₂/N₂ selectivity only shows a very limited improvement with the further increase of monomers concentration. The reaction time has similar effects on membrane performance as the monomers concentration. The effect of heat-treatment temperature was also studied. With the increasing heat-treatment temperature, the CO₂ permeance shows a decrease tendency, while the CO₂/N₂ selectivity shows a maximum at 80 °C. When PEA is 0.013 mol L⁻¹, TMC is 0.020 mol L⁻¹, reaction time is 3 min, and heat-treatment temperature is 80 °C, the optimum preparation conditions are achieved with CO₂ permeance of 378.3 gas permeation unit (GPU) and CO₂/N₂ selectivity of 51.7 at 0.03 MPa. This work may help to design and fabricate gas separation membranes with desired performance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47755.     

Preparation and modification of thin film composite membrane using a bulky dianhydride monomer

One of the most effective methods to modify thin film composite (TFC) membranes is changing the chemistry of top selective layer by different monomers and different monomer concentrations. Herein, we report the preparation of modified TFC membranes using a pyromellitic dianhydride (PMDA) mixed with organic phase (trimesoyl chloride) and meta phenylene diamine (MPD). By manipulating the PMDA amount in organic phase, the structures and chemical compositions of polyamide selective layer could be modified. It was realized that the presence of PMDA could result in a modified membrane with higher surface roughness, less dense selective layer, more surface charge density, and better hydrophilic properties and consequently less fouling. The optimum PMDA concentration was found 0.05 wt%, such that the obtained membrane had 35.6 L m⁻² h⁻¹ pure water flux, about 1.6-fold higher than the reference membrane with similar salt rejection. Fouling intensity for the reference membrane was 38.1%, while for the modified membranes it decreased to 16.7%.     

The study of hydrogen electrosorption in layered nickel foam/palladium/carbon nanofibers composite electrodes

In the present work, the process of hydrogen electrosorption occurring in alkaline KOH solution on the nickel foam/palladium/carbon nanofibers (Ni/Pd/CNF) composite electrodes is examined. The layered Ni/Pd/CNF electrodes were prepared by a two-step method consisting of chemical deposition of a thin layer of palladium on the nickel foam support to form Ni/Pd electrode followed by coating the palladium layer with carbon nanofibers layer by means of the CVD method. The scanning electron microscope was used for studying the morphology of both the palladium and carbon layer. The process of hydrogen sorption/desorption into/from Ni/Pd as well as Ni/Pd/CNF electrode was examined using the cyclic voltammetry method. The amount of hydrogen stored in both types of composite electrodes was shown to increase on lowering the potential of hydrogen sorption. The mechanism of the anodic desorption of hydrogen changes depending on whether or not CNF layer is present on the Pd surface. The anodic peak corresponding to the removal of hydrogen from palladium is lower for Ni/Pd/CNF electrode as compared to that measured for Ni/Pd one due to a partial screening of the Pd surface area by CNF layer. The important feature of Ni/Pd/CNF electrode is anodic peak appearing on voltammetric curves at potential ca. 0.4 V more positive than the peak corresponding to hydrogen desorption from palladium. The obtained results showed that upon storing the hydrogen saturated Ni/Pd/CNF electrode at open circuit potential, diffusion of hydrogen from carbon to palladium phase occurs due to interaction between carbon fibers and Pd sites on the nickel foam support.     

Separation of hydrogen adsorption and absorption on Pd thin films

Hydrogen sorption at Pd films of 20-80 nm deposited on a polycrystalline gold electrode was studied in sulfuric and perchloric acid. Assuming that the hydrogen adsorption does not vary with the Pd films thickness, hydrogen adsorption/absorption charges in Pd were separated in the two contributions in the hydrogen-poor α-Pd-H phase. The results are compared to those obtained at Pd monolayers on Au(1 1 1). The adsorption on polycrystalline Pd begins at potentials more negative than on 0.8 ML Pd on Au(1 1 1) and is not much affected by the nature of anion (sulfate or perchlorate), contrary to the thin layers on Au(1 1 1). The absorption charge in α-PdH phase in the potential range of 0.08-0.15 V was found to be similar to that at a 25 μm Pd foil in this potential range while at more positive potentials it is larger. In the presence of crystal violet which adsorbs at the electrode surface it was found that some residual H adsorption exists. There is more hydrogen absorbed in Pd in the presence of crystal violet in the hydrogen-poor α phase but in the hydrogen-rich β phase the amount of hydrogen is the same.     

Selective recovery of palladium from used aqua regia by hollow fiber supported with liquid membrane

This research study was aimed at recovering palladium from used aqua regia by means of a hollow fiber thoroughly supported with liquid membrane. The liquid membrane, consisting of two extractants—thioridazine HCl and oleic acid-solubilized in chloroform—was used to coat the rmcroporous hollow fiber throughout. Sodium nitrite, a stripping agent, which was fed through the shell side, flowed counter-currently with the feed solution fed via the tube side. The following factors were investigated: the concentrations of the two extractants and of sodium nitrite, the pH of used aqua regia, the flow rates of both the feed and stripping solution, and the number of runs in the hollow fiber module. It was found that after a 30-mmute operation, 29.10% of palladium ion was optimally recovered at 0.0005-M thioridazine and 0.05-M oleic acid. With reference to the precious metals recovered, the following order was recorded: Pd(II)>Pt(IV)>Cu(II)>Au(III). It was observed that synergistic extraction could be gained at the concentration level of the extradants, regulated in the experiment. The liquid membrane system had long-term stability and even after the third run, it could still recover palladium up to 65%.     

Permeability and permselectivity of polyphenylenediamine films synthesized at a palladium disk electrode

Three phenylenediamine isomers (including ortho-, meta- and para-derivatives) were electrochemically polymerized on palladium disk electrodes. The permeability and permselectivity of polyphenylenediamine films for hydrogen peroxide, ascorbic acid, uric acid, acetaminophen, and cysteine were compared. The resulting polyphenylenediamine electrodes showed rapid electrochemical responses to hydrogen peroxide and good anti-fouling ability to possible interferences. Experimental results provide useful information for the fabrication of membrane-based enzyme electrodes.     

Preparation of a water-permselective composite membrane by the concentrated emulsion method: Its swelling and permselectivity characteristics

A water-permselective composite membrane was prepared by the concentrated emulsion polymerization method. A large volume fraction of an aqueous sodium acrylate solution was dispersed in a small amount of divinyl benzene. Each of the two phases contained a suitable initiator and the continuous phase contained an appropriate surfactant. The concentrated emulsion thus obtained has the appearance and behavior of a gel. The gel was sandwiched between tow glass plates and subjected to polymerization via heating at 45°C for 24 h. The resulting membrane was dried and further employed in several kinds of experiments. The swelling of the membrane in water depends on the pH of water and can be as large as 86. At low pH values, the swelling was very small. The permeation rate of a water-ethanol mixture was in the range of 96–560 g/m² h and decreased with increasing alcohol concentration and increasing poly(sodium acrylate) fraction in the membrane. The permselectivity varied between 32 and 235, increasing with increasing poly(sodium acrylate) fraction in the membrane and with increasing or decreasing ethanol concentration (depending upon the composition of the membrane). The activation energy for pervaporation varied between 6.58 and 8.14 kcal/mol, depending upon the composition of the feed. The permselectivity decreased slightly with increasing temperature.     

Electrochemical study of a hydrogen diffusion anode-membrane assembly for membrane electrolysis

The membrane electrode assembly (MEA) studied was constituted with a gas diffusion electrode (E-TEK) impregnated with Nafion^® solution which was assembled with a Nafion^® 117 cation exchange membrane under heat and pressure. The MEA was used as anode in a membrane electrolysis (ME) cell with the objective to regenerate HCl and NaOH from NaCl. Current efficiency for hydrogen oxidation was determined and its value is 100%, which indicates that the only reaction occurring at the anode is the oxidation of hydrogen. Current-potential curves, recorded in different conditions, showed a linear variation in the range 0-3000 A m⁻² when hydrochloric acid concentration is below 2 mol dm⁻³. In this case, the overvoltage was shown to be mainly due to the ohmic drop in the membrane and in the layer where Nafion^® impregnation was performed. MEA overvoltage necessary to reach 3000 A m⁻² current density was about 0.12 V. For high HCl concentration (6-8 mol dm⁻³), the MEA overvoltage increased sharply with current density due to the adsorption of chloride anions on platinum catalyst.