Aluminizing and oxidation treatments on the porous stainless steel substrate for preparation of H2-permeable composite palladium membranes

Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al₂O₃-enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux.     

Toward low-cost Pd/ceramic composite membranes for hydrogen separation: A case study on reuse of the recycled porous Al2O3 substrates in membrane fabrication

The development of hydrogen energy systems has placed a high demand on hydrogen-permeable membranes as compact hydrogen separators and purifiers. Although Pd/Ceramic composite membranes are particularly effective in this role, the high cost of these membranes has greatly limited their applications; this high cost stems largely from the use of expensive substrate material. This problem may be solved by substrate recycling and the use of lower cost substrates. As a case study, we employed expensive asymmetric microporous Al₂O₃ and low-cost macroporous symmetric Al₂O₃ as membrane substrates (average pore sizes are 0.2 and 3.3 μm, respectively). The palladium membranes were fabricated by electroless plating, and substrate recycling was carried out by palladium dissolution with a hot HNO₃ solution. The functional surface layer of the microporous Al₂O₃ was damaged during substrate recycling, and the reuse of the substrate led to poor membrane selectivity. With the assistance of pencil coating as a facile and environmentally benign surface treatment, the macroporous Al₂O₃ can be successfully utilized. Furthermore, the macroporous Al₂O₃ can be also recycled and reused as membrane substrate, yielding highly permeable, selective and stable palladium membranes. Consequently, the substrate cost can be further decreased, and the applications of this kind of membranes would expand.     

In-situ repair of composite palladium membranes with macro defects: A case study

A new method was developed for repairing Pd/Al₂O₃ membranes with macro defects without the need of disassembling the membrane from the module. In order to target and fill the membrane defect automatically with solid particles, a TiO₂ powder was firstly tested by flowing high-pressure nitrogen as a carrier gas, followed by a heat treatment. A filter cake was found on the membrane defect but still porous. A glass powder was selected instead of TiO₂, and the membrane defect was successfully sealed by glazing. The in-situ repair of a waste commercial Pd/Al₂O₃ membrane separator was carried out with the glass powder, and the hydrogen flux and H₂/N₂ selectivity of the membrane separator at 450 °C under 100 kPa reached 12.6 m³m⁻²h⁻¹ and 1600, respectively.     

Silver coating on porous stainless steel substrate and preparation of H2-permeable palladium membranes

The H₂-permeable palladium membranes based on porous stainless steel (PSS) substrate are important for development of various hydrogen energy systems. To improve the surface of the PSS, a microporous silver layer was deposited successively by a coating with a suspension of silver powder in polyvinyl alcohol (PVA) solution, a heating under nitrogen at 500 °C for carbonization of PVA, an air treatment and a hydrogen reduction. The formation of carbon from PVA helps to maintain the porosity and integrity of the silver layer. After an activation of the resulting Ag/PSS surface through galvanic-cell reaction, palladium membranes with a thickness around 4 μm were successfully prepared by a suction-assisted electroless plating. SEM, EDS, metallography and porometry analyzes were conducted for material characterizations. The prepared Pd/Ag/PSS membrane is permeable and selective as compared with similar those reported in literature. The permeation tests were carried out at 350, 400, 450 and 500 °C for 48, 48, 48 and 60 h, respectively, and the membrane was found to be unstable at 500 °C due to the presence of pinholes. No significant intermetallic diffusion between the silver and palladium layers was observed.     

Fabrication,characterization, and application of palladium composite membrane on porous stainless steel substrate with NaY zeolite as an intermediate layer for hydrogen purification

Palladium composite membrane with excellent stability was successfully prepared using the electroless plating (ELP) route on a porous stainless steel (PSS) support for hydrogen separation. In order to modify the average pore size of PSS support and to prevent inter-metallic diffusion, the NaY zeolite layer was coated on the PSS support with the seeding and secondary growth method. A high-temperature membrane module was designed by Solid work software and fabricated from 316 L stainless steel with a knife-edge seal. The microstructures and morphologies of the samples were analyzed using XRD, BET, AFM, FESEM and EDX techniques. Permeation experiments were carried out with binary mixtures of H₂/N₂ with various ratios (90/10, 75/25 and 50/50) and pure H₂ and N₂ at different temperatures (350, 400 and 450 °C) and feed pressures (200–400 kPa). Hydrogen permeation tests showed that the membrane with a thickness of about 7 μm had a hydrogen permeance of 6.2 × 10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 with an ideal H₂/N₂ selectivity of 736, at 450 °C. In addition, the results of stability tests revealed that the membrane could remain stable during a long-term operation by varying temperature and feed gases.     

Graphite coating on alumina substrate for the fabrication of hydrogen selective membranes

Development of composite membranes is a suitable alternative to improve the hydrogen flux through palladium membranes. The porous substrate should not represent a barrier to gas permeation, but the roughness of its surface should be sufficiently smooth for the deposition of a thin and defect-free metal layer. In this study, the performances of the modification of the outer surface of an asymmetric alumina hollow fibre substrate by the deposition of a graphite layer were evaluated. The roughness of the substrate outer surface was reduced from 120 to 37 nm after graphite coating. After graphite coating, the hydrogen permeance through the composite membrane produced with 2 Pd plating cycles was of 1.02 × 10^?3 mol s^?1 m^?2 kPa^?1 at 450 °C and with infinite H₂/N₂ selectivity. Similar hydrogen permeance was obtained with the composite membrane without graphite coating, also at infinite H₂/N₂ selectivity, but 3 Pd plating cycles were necessary. Thus, graphite coating on asymmetric alumina hollow fibres is a suitable alternative to reduce the required palladium amount to produce hydrogen selective membranes.     

Influence of the type of siliceous material used as intermediate layer in the preparation of hydrogen selective palladium composite membranes over a porous stainless steel support

In this work, several composite membranes were prepared by Pd electroless plating over modified porous stainless steel tubes (PSS). The influence of different siliceous materials used as intermediate layers was analyzed in their hydrogen permeation properties. The addition of three intermediate siliceous layers over the external surface of PSS (amorphous silica, silicalite-1 and HMS) was employed to reduce both roughness and pore size of the commercial PSS supports. These modifications allow the deposition of a thinner and continuous layer of palladium by electroless plating deposition. The technique used to prepare these silica layers on the porous stainless steel tubes is based on a controlled dip-coating process starting from the precursor gel of each silica material. The composite membranes were characterized by SEM, AFM, XRD and FT-IR. Moreover they were tested in a gas permeation set-up to determine the hydrogen and nitrogen permeability and selectivity. Roughness and porosity of original PSS supports were reduced after the incorporation of all types of silica layers, mainly for silicalite-1. As a consequence, the palladium deposition by electroless plating was clearly influenced by the feature of the intermediate layer incorporated. A defect free thin palladium layer with a thickness of ca. 5 μm over the support modified with silicalite-1 was obtained, showing a permeance of 1.423·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 and a complete ideal permselectivity of hydrogen.     

Fabrication of Pd/ceramic membranes for hydrogen separation based on low-cost macroporous ceramics with pencil coating

Increasing hydrogen energy utilization has greatly stimulated the development of the hydrogen-permeable palladium membrane, which is comprised of a thin layer of palladium or palladium alloy on a porous substrate. This work chose the low-cost macroporous Al₂O₃ as the substrate material, and the surface modification was carried out with a conventional 2B pencil, the lead of which is composed of graphite and clay. Based on the modified substrate, a highly permeable and selective Pd/pencil/Al₂O₃ composite membrane was successfully fabricated via electroless plating. The membrane was characterized by SEM (scanning electron microscopy), field-emission SEM and metallographic microscopy. The hydrogen flux and H₂/N₂ selectivity of the membrane (with a palladium thickness of 5 μm) under 1 bar at 723 K were 25 m³/(m² h) and 3700, respectively; the membrane was found to be stable during a time-on-stream of 330 h at 723 K.     

A comparative study of γAl2O3–C2H6O2 and γAl2O3–H2O near a vertical curved surface having porous medium

The present analysis reveals the impact of porous medium and mixed convection nano-fluid flow near an exponentially curved surface. For this purpose alumina is treated as nano-particles along with two base fluids such as $H_{2}O$ and $C_2{H}_6{O}_2$. Under different assumptions the mathematical model of energy and momentum equations are developed by using curvilinear coordinates. The PDEs are transformed into non-linear differential equations by using similarity transformation variables. The table of skin friction and Nusselt number are drawn by fluctuating the values of different parameters for both fluids. Moreover, the numerical solutions are generated in MATLAB by using Bvp4c. Influence of effective and without effective Pr numbers for an incompressible $\gamma A{l}_2{O}_3-C_2{H}_6{O}_2$ and $\gamma A{l}_2{O}_3-H_{2}O$ are investigated. We observe that the velocity profile increases for increasing values of curvature, mixed convection and volume fraction of nanoparticles for two different cases effective and with-out effective Prandtl numbers. Moreover, in case of effective $Pr$ number, the temperature profile decreases but for with-out effective $Pr$ number temperature profile increases. Furthermore, the graphs are drawn to check the behavior various parameters for effective and with-out effective Pr numbers.     

Optimization analysis of hydrogen separation from an H2/CO2 gas mixture via a palladium membrane with a vacuum using response surface methodology

This study uses a palladium membrane to separate hydrogen from an H₂/CO₂ (90/10 vol%) gas mixture. Three different operating parameters of temperature (320–380 °C), total pressure difference (2–3.5 atm), and vacuum degree (15–49 kPa) on hydrogen are taken into account, and the experiments are designed utilizing a central composite design (CCD). Analysis of variance (ANOVA) is also used to analyze the importance and suitability of the operating factors. Both the H₂ flux and CO₂ (impurity) concentration on the permeate side are the targets in this study. The ANOVA results indicate that the influences of the three factors on the H₂ flux follow the order of vacuum degree, temperature, and total pressure difference. However, for CO₂ transport across the membrane, the parameters rank as total pressure difference > vacuum degree > temperature. The predictions of the maximum H₂ flux and minimum CO₂ concentration by the response surface methodology are close to those by experiments. The maximum H₂ flux is 0.2163 mol s^?1 m^?2, occurring at 380 °C, 3.5 atm total pressure difference, and 49 kPa vacuum degree. Meanwhile, the minimum CO₂ concentration in the permeate stream is t 643.58 ppm with the operations of 320 °C, 2 atm total pressure difference, and 15 kPa vacuum degree. The operation with a vacuum can significantly intensify H₂ permeation, but it also facilitates CO₂ diffusion across the Pd membrane. Therefore, a compromise between the H₂ flux and the impurity in the treated gas should be taken into account, depending on the requirement of the gas product.     

Tuning thermal expansion behavior and surface roughness of tubular Al2O3 substrates for fabricating high-performance carbon molecular sieving membranes for H2 separation

Tubular alumina substrates have been widely used as supporting membranes in gas separation. Owing to the demand for supporting membranes with a dense or ultra-micropore texture, the quality/quantity control of substrates is required to prevent the formation of defects due to rough surfaces, high curvature, and high difference in thermal expansion between the polymer precursor and the alumina substrate. This study proposes a new strategy to modify the pore texture, surface properties, and thermal expansion coefficient of the substrate by filling it with TiO₂ nanoparticles and using the grinding/polishing method. The effect of CMS preparation conditions, including coating cycles and pyrolysis temperature on the microstructure of the carbon matrix is also discussed. A tubular CMS membrane with excellent permselectivity toward H₂/CO, H₂/N₂, and O₂/N₂ (77.52, 162.94, and 13.87, respectively), and permeabilities of 55.9 barrer and 6.49 barrer are obtained for H₂ and O₂, respectively.     

Oscillatory slip flow of Fe3O4 and Al2O3 nanoparticles in a vertical porous channel using Darcy's law with thermal radiation

The heat transfer phenomena and oscillatory flow of an electrically conducting viscous nanofluid (NF) in a channel with porous walls and saturated porous media exposed to the thermal radiation are studied. The nanoparticles (NPs) Fe₃O₄ and Al₂O₃ are taken with water as base fluid along with nonuniform temperature and velocity slip at the wall of channel (y′ = 0). The basic laws of momentum and energy conservation are converted into the dimensionless system of the partial differential equations (PDEs) using similarity variables. Closed‐form solutions of these coupled PDEs are constructed for all values of time by taking the oscillatory pressure gradient. The physical insight of involved parameters on the fluid velocity, temperature profile, heat transfer rate, and surface friction is studied and analyzed graphically. It is noted from this study that the fluid velocity shows a decreasing behavior with the volume fraction of NPs. Furthermore, the amplitude of the oscillatory motion in case of skin friction decreases for a large magnetic field.     

Stability and activity of a co-precipitated Mg promoted Ni/Al2O3 catalyst for supercritical water gasification of biomass

We have investigated the stability and activity of a co-precipitated Mg promoted Ni/Al₂O₃ catalyst (Ni-Mg-Al) for supercritical water gasification (SCWG) of various biomass model compounds and real biomass. Phase stability and activity recovery of the Ni-Mg-Al catalyst were first compared with a catalyst prepared by impregnation method. It was found that the co-participated catalyst showed higher activity recoveries than the impregnated catalyst due to the stable Ni crystal size. Then, effects of SCWG variables including heating up rate, gasification temperature, catalyst loading amount and feedstock concentration, on the non-catalytic and catalytic gas yields and gasification efficiencies of glucose and phenol were evaluated. Results demonstrated that the presence of sufficient amount of Ni catalyst could realize complete carbon gasification of different organics, including phenol and real biomass. Catalyzed by Ni, CH₄ was the more favored produced gas at 400–500 °C while H₂ yields were more abundant at 500–600 °C. Without catalyst, carbon gasification efficiencies of SCWG of different feedstock were in the order: glycerol > glucose > cellulose ≈ corncob ≈ poplar leaf ≈ sawdust > phenol, while those catalyzed by Ni were in the order: glycerol ≈ glucose ≈ cellulose ≈ phenol > corncob ≈ poplar leaf ≈ sawdust, illustrating that the co-precipitated NiMgAl catalyst is more active on catalyzing the gasification of water-soluble organics than real biomass.     

Optimization of a fluidized bed reactor for methane decomposition over Fe/Al2O3 catalysts: Activity and regeneration studies

Catalytic methane decomposition was investigated over 40 wt% Fe/Al₂O₃ catalyst in fluidized bed reactor (FLBR). After optimization of FLBR conditions in terms of catalyst bulk density, particle size, minimum fluidization velocity, and the catalyst bed height, the catalyst activity and stability tests were conducted by comparison with a fixed bed reactor (FBR). Although a similar stable methane conversion was obtained over both reactors, the pressure drop during 35 min operation of FBR was 9 times higher than that of FLBR, which indicated the possibility of continuous operation of methane decomposition process over FLBR. Further, the influence of the space velocity, feed dilution and regeneration on catalysts reactivity was studied in FLBR to conclude that a reaction condition of 12 L/g_cat∙h, feed of 20%H₂–80%CH₄ and CO₂-regeneration of deactivated catalysts may be favourable for operating methane decomposition in FLBR continually and effectively to provide stable hydrogen.     

Glycerol conversion in the experimental study of catalytic hydrolysis of triglycerides for fatty acids production using Ni or Pd on Al2O3 or SiO2

In many reactions to produce biodiesel the glycerol represents 10 wt% of the total products therefore it is important to find an adequate destination of such byproduct. The catalytic hydrolysis of triglycerides is another way to produce biodiesel from fatty raw materials. This work shows the catalytic hydrolysis of triglycerides (soybean oil and tallow) with nickel (NiO) or palladium (PdO) catalysts supported on Al₂O₃ or SiO₂. The results showed the direct conversion of in situ generated glycerol into hydrogen (H₂) and carbon dioxide (CO₂). The glycerol conversion was evaluated through the capacity to hydrogenate the unsaturated fatty acids leading to the formation of stearic acid (saturated compound). The catalyst, temperature and time were varied and evaluated in the experiments utilizing the two different kinds of raw materials (beef tallow and soybean oil). Selectivity and statistical planning studies were performed to optimize the formation of stearic acid as it is linked to the hydrogenation of unsaturated compounds by hydrogen generated from the glycerol liquid reforming.     

Cobalt nanoparticles supported on alumina nanofibers (Co/Al2O3): Cost effective catalytic system for the hydrolysis of methylamine borane

Amongst different amine-borane derivatives, methylamine-borane (CH₃NH₂BH₃) seems to be one of the capable aspirants in the storing of hydrogen attributable to its high hydrogen capacity, stability and aptitude to generate hydrogen through its catalytic hydrolysis reaction under ambient conditions. In this research paper, we report that cobalt nanoparticles supported on alumina nanofibers (Co/Al₂O₃) are acting as active nanocatalyst for catalytic hydrolysis of methylamine-borane. Co/Al₂O₃ nanocatalyst was fabricated by double-solvent method followed with wet-chemical reduction, and was characterized by utilizing various spectroscopic methods and imaging techniques. The results gathered from these analyses showed that the formation Al₂O₃ nanofibers supported cobalt(0) nanoparticles with a mean diameter of 3.9 ± 1.2 nm. The catalytic feat of these cobalt nanoparticles was scrutinized in the catalytic hydrolysis of methylamine-borane by considering their activity and durability performances. They achieve releasing of 3.0 equivalent of H₂ via methylamine-borane hydrolysis at room temperature (initial TOF = 297 mol H₂/mol metal × h). Along with activity the catalytic durability of Co/Al₂O₃ was also studied by carrying out recyclability tests and it was found that these supported cobalt nanoparticles have good durability during the course of the catalytic recycles so that Co/Al₂O₃ preserves almost its innate activity at 5th catalytic recycle. The studies presented here also contains kinetic investigation of Co/Al₂O₃ catalyzed methylamine borane hydrolysis depending on the temperature, cobalt and methylamine borane concentrations, which were used to define rate expression and the activation energy of the catalytic reaction.     

The role of effectiveness factor on the modeling of methanol steam reforming over CuO/ZnO/Al2O3 catalyst in a multi-tubular reactor

A pseudo-homogeneous model for the methanol steam reforming process was developed based on reaction kinetics over a CuO/ZnO/Al₂O₃ catalyst and non-adiabatic heat and mass transfer performances in a co-current packed-bed reactor. A Thiele modulus method and an intraparticle distribution method were applied for predicting the effectiveness factors for main reactions and providing insights into the diffusion-reaction process in a cylindrical catalyst pellet. The results of both methods are validated and show good agreements with the experimental data, but the intraparticle distribution method provides better predictions. Results indicate that increases in catalyst size and bulk fluid temperature amplify the impact of intraparticle diffusion limitations, showing a decrease in effectiveness factors. To satisfy the requirements of a high temperature polymer electrolyte membrane fuel cell stack, the optimized operating conditions, which bring the methanol and CO concentrations to less than 1% vol in the reformate stream, are determined based on the simulation results.     

A numerical approach for MHD Al2O3–TiO2/H2O hybrid nanofluids over a stretching cylinder under the impact of shape factor

In this research, the heat transfer and magnetohydrodynamic stagnation point flow of a (Al₂O₃–TiO₂/H₂O) hybrid nanofluid past a stretching cylinder under the impact of heat generation, nonlinear thermal radiation, and nanoparticles shape factor has been analyzed using the Runge‐Kutta‐Fehlberg fifth order numerically method. The impact of changing diverse parameters, such as nanoparticles shape factor, named hexahedron and lamina, on temperature and velocity profiles and induced magnetic field, has been explored. The main motivation of this article is using hybrid nanoparticles to improve heat transfer. The novel findings of the current research illustrate that the Lorentz force produced by increasing magnetic field parameter () causes a decline in velocity profile; also increasing solar radiation, shape factor and the use of hybrid nanoparticles caused increment in the temperature profile. Furthermore, the lamina nanoparticle shape has more impact on Nusselt number () compared with hexahedron‐shaped nanoparticle.     

Effects of the Caputo fractional derivatives on convective flow in wavy vented enclosures filled with a porous medium using Al2O3‐Cu hybrid nanofluids

This paper studies the effect of fractional derivatives on the fractional convective flow of hybrid nanofluids in a wavy enclosure that has inlet and outlet parts near the left wall and is filled with a porous medium. The Caputo definition of the fractional derivatives is applied on the partial differential equations governing flow. The complex shape is mapped to a rectangular domain using appropriate transformations. The finite difference method is used to solve the resulting system. The results showed that an increase in order of the fractional derivatives causes a low activity of the fluid flow and a reduction in the rate of heat transfer. Also, an increase in the nanoparticles volume fractions reduces the activity of the fluid flow and, as a result, the rate of heat transfer is diminished. An enhancement in fluid motion and rate of the heat transfer is obtained by increasing the amplitude of the wavy wall.     

Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis

A commercial FAA3-50 membrane was investigated as a solid polymer electrolyte in an alkaline water electrolysis. An improved chemical treatment based on alkaline KOH solution was carried out. A limited degradation of the functional groups was observed allowing to maintain a good anion conductivity approaching 55 mS cm-1 at 100 °C. Thermal stability up to 200 °C was assessed by thermal analysis.A specific membrane-electrodes assembly based on FAA3-50 anionic membrane and NiMn₂O₄ anode catalyst was developed and investigated in a single cell for water electrolysis at a moderate temperature (50 °C).Performance stability was assessed by a potential cycling-based durability test for 1000 h by varying the cell potential between 1 and 1.8 V for the FAA3-50 and NiMn₂O₄ based-MEA.According to this evaluation, both the FAA3-50 membrane and the NiMn₂O₄ catalyst appear sufficiently stable for electrolysis operation under mild operating temperatures.