Preparation of palladium membrane on Pd/silicalite-1 zeolite particles modified macroporous alumina substrate for hydrogen separation

A palladium composite membrane was successfully fabricated by electroless plating on a macroporous alumina tube. Pd/silicalite-1 zeolite particles were employed to reduce the pore size of the alumina support and improve its surface roughness. Moreover, the Pd⁰ existed in the Sil-1 particle can avoid the time consuming sensitization and activation steps for palladium seeding. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) analysis were conducted for analyzing the detailed microstructure of the palladium composite membrane. The hydrogen permeation performance of the resulting palladium membrane was investigated at temperatures of 623 K, 673 K, 723 K and 773 K. The hydrogen permeance of 1.95 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with an H₂/N₂ ideal selectivity of 1165 for the palladium membrane was obtained at 773 K. Furthermore, the resulting palladium membrane was stable for a long-term operation of 15 days at 673 K.     

Transient dynamic of hydrogen permeation through a palladium membrane

Transient dynamic of hydrogen permeation through a palladium membrane is studied in the present study. Three different pressure differences between the two sides of the membrane are considered; they are 3, 5 and 8 atm. The experimental results indicate that the variation in the hydrogen permeation process is notable at the selected pressure differences. When the pressure difference is relatively low (i.e. 3 atm), the hydrogen permeation process proceeds from a time-lag period, then to a concave up period and eventually to a concave down period. Therefore, the transient hydrogen permeation is characterized by a three-stage mass transfer process. When the pressure difference is increased to 5 atm, the time-lag period disappears, thereby evolving the three-stage mass transfer process into a two-stage one. However, the concave up period withers significantly. Once the pressure difference is as high as 8 atm, the transient hydrogen permeation is completely characterized by a concave down curve, yielding a single-stage mass transfer process. A quasi-steady state of hydrogen permeation is defined to evaluate the period of the transient mass transfer process. It suggests that, within the investigated conditions of operation, the time required for hydrogen permeation to reach the steady value is around or over 1 h. For the low pressure difference cases, the transient period is especially long, resulting from the time-lag characteristic. Once the hydrogen permeation is in the steady state, over 80% of hydrogen can be recovered from the membrane.     

Preparation of palladium membrane by bio-membrane assisted electroless plating for hydrogen separation

Palladium membrane was prepared on the inner surface of alumina tube by bio-membrane assisted electroless plating combined with osmosis method (BELP). In this preparation technique, an egg-shell film not only served as a semipermeable membrane to form osmotic system for preparing palladium membrane, but also acted as a protection layer to prevent the contamination of the palladium membrane from the osmotic solution. Moreover, the plating solution was circulated through the tube side to promote the mass transfer on the solid–liquid interface between the plating surface and the solution. The detailed depositing process of the palladium membrane was studied by scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDXS). Both long term operation and temperature cycling test carried out for hydrogen and nitrogen permeation confirmed that the palladium membrane was stable.     

Devlopment of palladium/ceramic membranes for hydrogen separation

This paper presents the major objectives involved in the development of a thin-layer palladium/ceramic composite membranes. These are (a) electroless plating of palladium on ceramic substrate, (b) characterization of palladium/ceramic composite formed, (c) evaluation of selectivity of the composite membranes for hydrogen separation. Commercially available ceramic was used as substrate for deposition of hydrogen selective layers. The substrate was coated with a thin palladium layer by electroless plating. The plating technique allowed to vary the thickness by depositing multiple metal layers. The details of the plating procedures and formulations of the plating solutions are presented. The palladium/ceramic composite membranes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and measuring the thickness of the coated film by a weight-gain method. The palladium film thicknesses were determined to be between 2 μm and 5 μm. Sorption performances of composite membranes were evaluated using temperature swing sorption process (TSSP). A gases mixture, provided from biomass gasification process, containing about 40-50% vol. H₂ and numerous other gases such as CO, CO₂, CH₄, hydrocarbons C₂-C₁₀, was used for the tests. In a first step, at first temperature (5-10 °C), the palladium/ceramic composites sorb only hydrogen from mixture and form hydride, all other components leave the sorbent. Then, subsequently, in a second step, energy is added to the sorbent, thus bringing it to a second temperature (105-120 °C) and hydrogen is desorbed while the pressure is reduced. The hydrogen obtained in desorption step is of a high purity (>99.5% vol). The results obtained show that this kind of composite membranes have certain separation selectivity for hydrogen and can have good industrial applications.     

Palladium composite membrane fabricated on rough porous alumina tube without intermediate layer for hydrogen separation

A thin palladium composite membrane without any modified layer was successfully obtained on a rough porous alumina substrate. Prior to the fabrication of palladium membrane, a poly(vinyl) alcohol (PVA) layer was first coated onto the porous substrate by dip-coating technique to improve its surface roughness and pore size. After deposition of palladium membrane on the PVA modified substrate, the polymer layer can be completely removed from the composite membrane by heat treatment. The microstructure of the palladium composite membrane was characterized in detail using SEM, EDXS and XRD analysis. Permeation measurements were carried out using H₂ and N₂ at temperatures of 623 K, 673 K, 723 K and 773 K. The results indicated that the hydrogen permeation flux of 0.238 mol m^?2 s^?1 with H₂ separation factor α(H₂/N₂) of 956 for the as-prepared palladium membrane was obtained at 773 K and 100 kPa. Furthermore, the good membrane stability was proven during the total operation time of 160 h at the temperature range of 623 K–773 K and gas exchange cycles of 30 between hydrogen and nitrogen at 723 K.     

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.     

Preparation, testing and modelling of a hydrogen selective Pd/YSZ/SS composite membrane

A palladium selective tubular membrane has been prepared to separate and purify hydrogen. The membrane consists of a composite material, formed by different layers: a stainless steel support (thickness of 1.9 mm), an yttria-stabilized zirconia interphase (thickness of 50 μm) prepared by Atmospheric Plasma Spraying and a palladium layer (thickness of 27.7 μm) prepared by Electroless Plating. The permeation properties of the membrane have been tested at different operating conditions: retentate pressure (1-5 bar), temperature (350-450 °C) and hydrogen molar fraction of feed gas (0.7-1). At 400 °C, a permeability of 1.1 × 10⁻⁸ mol/(s m Pa^0.5) and a complete selectivity to hydrogen were obtained. The complete retention of nitrogen was maintained for all tested experiment conditions, with both single and mixtures of gases, ensuring 100% purity in the hydrogen permeate flux.A rigorous model considering all the resistances involved in the hydrogen transport has been applied for evaluating the relative importance of the different resistances, concluding that the transport through the palladium layer is the controlling one. In the same way, a model considering the axial variations of hydrogen concentration because of the cylindrical geometry of the experimental device has been applied to the fitting of the experimental data. The best fitting results have been obtained considering Sieverts’-law dependences of the permeation on the hydrogen partial pressure.     

Toward effective membranes for hydrogen separation: Multichannel composite palladium membranes

Composite palladium membranes can be used as a hydrogen separator because of their excellent permeability and permselectivity. The total membrane area in a hydrogen separator must be reasonably large for industrial use, and it is important that each membrane provides a large enough area. Such a demand can be well met by introducing multichannel composite membranes. In this work, a commercially available microporous ceramic filter with 19 channels was used as a membrane substrate, and the diameter of each channel was 4 mm. A uniform thin palladium layer was fabricated inside the narrow channels by using an electroless plating method, and the resulting membranes were highly permeable and selective. This membrane concept provides a high surface-to-volume ratio without causing significant pressure loss, making the hydrogen separator compact and capable. However, special attention should be paid to cleaning the membrane after electroless plating.     

Impact of vacuum operation on hydrogen permeation through a palladium membrane tube

Palladium (Pd) membranes are characterized by their high permselectivity to hydrogen and easy operation, and are promising devises for separating hydrogen from hydrogen-rich gases. The membranes are normally operated with atmospheric pressure at the permeate side. Instead of this common operation, hydrogen permeation through a Pd membrane under vacuum operation at the permeate side is investigated and compared with that under normal operation. In this study, two membrane operating temperatures (320 and 380 °C), four H₂ partial pressure differences (2, 3, 4, and 5 atm) across the membrane, and four feed gases are considered. The results suggest that the vacuum operation can efficiently intensify the H₂ permeation rate. The improvement in H₂ permeation rate due to the vacuum operation can be increased up to 136%. The positive effect of the vacuum operation is especially pronounced when the gas mixtures are used as the feed gases, stemming from the effective attenuation of the concentration polarization. An increase in membrane temperature raises the H₂ permeation rate, but its influence in enhancing the H₂ permeation rate with the vacuum operation is not as significant as that without the vacuum one. It is found that the retardation effect of impurities on the mass transfer is always ranked as CO > CO₂ > N₂, regardless of with/without vacuum operation.     

Modeling of transient hydrogen permeation process across a palladium membrane

Transient mass transfer processes of hydrogen permeating through a Pd membrane are modeled to aid in predicting the hydrogen transport behavior. The model is established in terms of the quasi-steady time and the steady permeation rate. Meanwhile, four important parameters are considered; they are the permeation lag time, the initial permeation rate, the concave up period and the concave down period. A unit step function is embedded in the model to account for the effect of the hydrogen permeation lag at a lower pressure difference. Corresponding to the lower, the moderate and the higher pressure differences (i.e. 3, 5 and 8 atm), though the hydrogen permeation undergoes a three-stage, a two-stage and a one-stage processes, respectively, these processes can be predicted well by an arc tangential function. By introducing an adjusting parameter in the arc tangential function, there exists an optimal value of the adjusting parameter when the pressure difference is lower. In regard to the moderate and higher pressure differences, the predictions agree with experiments well if the adjusting parameter is sufficiently large. Physically, the unit step function is used to account for the controlling mechanisms of hydrogen diffusion toward the membrane and the spillover of the hydrogen across the membrane. The initial jump parameter represents the rapid response of the initial hydrogen permeation. The adjusting parameter can be used to describe the relative importance of the concave up and the concave down periods.     

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H₂ flux of 28–36 m³/(m² h) and H₂/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H₂ flux (∼30 m³/(m² h)) and H₂/He selectivity (∼2000) over the 1500 h test period.     

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.     

Effect of synthesis conditions on performance of a hydrogen selective nano-composite ceramic membrane

A hydrogen-selective nano-composite ceramic membrane was prepared by depositing a dense layer composed of SiO₂ and Al₂O₃ on top of a graded multilayer substrate using co-current chemical vapor deposition (CVD) method. The multilayer substrate was made by dip-coating a macroporous α-alumina tubular support by a series of boehmite solutions to get a graded structure. Using DLS analysis, it was concluded that decreasing hydrolysis time and increasing acid concentration lead to smaller particle size of boehmite sols. XRD analysis was carried out to investigate the structure of intermediate layer and an optimized calcination temperature of 973 K was obtained. SEM images indicated the formation of a graded membrane with a porous intermediate layer having a thickness of about 2 μm and a dense top selective layer with a thickness of 80–100 nm. Permeation tests showed that H₂ permeance flux decreased from 5 × 10⁻⁵ mol m⁻² s⁻¹ Pa⁻¹ for a fresh substrate to 6.30 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ after 6 h of deposition, but H₂ selectivity over N₂ increased considerably from 5.6 to 203.     

Stability of pore-plated membranes for hydrogen production in fluidized-bed membrane reactors

Pd-based membranes prepared by pore-plating technique have been investigated for the first time under fluidization conditions. A palladium thickness around 20 μm was achieved onto an oxidized porous stainless steel support. The stability of the membranes has been assessed for more than 1300 h in gas separation mode (no catalyst) and other additional 200 h to continuous fluidization conditions. Permeances in the order of 5·10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ have been obtained for temperatures in a range between 375 and 500 °C. During fluidization, a small decrease in permeance is observed, as consequence of the increased external (bed-to-wall) mass transfer resistances. Moreover, water gas shift (WGS) reaction cases have been carried out in a fluidized bed membrane reactor. It has been confirmed that the selective H₂ separation through the membranes resulted in CO conversions beyond the thermodynamic equilibrium (of conventional systems), showing the benefits of membrane reactors in chemical conversions.     

The effect of mixture gas on hydrogen permeation through a palladium membrane: Experimental study and theoretical approach

Hydrogen permeation through a palladium membrane has been measured in the presence of several gases, such as CO, _N2${\mathrm{N}}_2$, _CO2${\mathrm{CO}}_2$, and Ar, both in the feed side and in the shell side of the (membrane) module. It has been found that CO molecules, remarkably inhibit hydrogen permeation. In particular, in the presence of carbon monoxide the permeation decreases with two different slopes: (I) for low CO concentrations, the hydrogen permeation decreases quickly (surface effects), whereas (II) for higher ones it decreases smoothly (dilute effect). Permeation of hydrogen, in the presence of the other gases, i.e. _N2${\mathrm{N}}_2$, _CO2${\mathrm{CO}}_2$ and Ar, always decreases with the same slope (dilute effect). In order to describe the CO inhibition, a theoretical investigation has been proposed. In particular, the framework of the Density Functional Theory has been used. CO and _N2${\mathrm{N}}_2$ Density Functional full optimisations on palladium clusters show that CO and _N2${\mathrm{N}}_2$ molecules present two minima on the cluster surfaces with bond lengths of 2.0 and 3.8 Å, respectively. The CO minima are much stable than _N2${\mathrm{N}}_2$ minima, resulting in a surface effect on the hydrogen permeation through the membrane.     

Electroless platinum and palladium deposition over alumina coated stainless steel plates and their catalytic activity for hydrogen mitigation in severe nuclear accidents

This work focused on platinum and palladium-based autocatalytic plates, which used to remove hydrogen. The six sandblasted stainless steel plates were coated with platinum and palladium metals using the electroless coating method. The three plates A-01, A-02, A-03 were first coated with alumina using the sol-gel dip method, and after that, different ratios of platinum and palladium were deposited on them. The three other plates, B-01, B-02, B-03 (without alumina coated), were coated directly with different ratios of platinum and palladium. The platinum and palladium ratio used for coating these plates were Pt 80%: Pd 20%; Pt 90%: Pd 10%, and Pt 70%: Pd 30. The coating of these plates was characterized using Scanning Electron Microscopy, X-rays Fluorescence Spectroscopy, and their catalytic efficiency was measured by the Passive Autocatalytic Recombiner testing rig method. It was found that the plates coated with alumina using two dipping cycles are suitable for different coating of mixed metals by electroless coating method compared to three dipping cycles. It was observed that the alumina-coated catalytic plates (A-01, A-02, A-03) exhibited excellent catalytic efficiency as compared to those without alumina-coated plates (B-01, B-02, B-03). Furthermore, the catalytic efficiency of A-03 and B-03 is higher than other plates. It was also found that the catalytic efficiency of A-03 is higher than B-03. The best coating ratio is Pt 90%: Pd10%, and alumina-coated plates give excellent results.     

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.     

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

A Pd membrane has been prepared by electroless plating on the surface of a porous TiO₂ tube in this work. The mean thickness of the resulting Pd membrane on the modified tube was 13 µm. The hydrogen permeation flux was as high as 0.16 mol m^?2 s^?1 with a pressure difference of 108.75 Pa at 648 K. H₂ permeances were 1.6 × 10^?7–6.4 × 10^?7/molm^?2 s^?1 Pa^?1/at 580–650 K. The separation factor of H₂/N₂ was over 1,000. Measurements of the temperature coefficient for hydrogen permeation through the membrane gave a value of 9.49 kJ mol^?1 in good agreement with previous reports. The results showed that the prepared membranes can be used in membrane reactors for H₂ permeation in the dehydrogenation of methylcyclohexane.     

Hydrogen permeation enhancement in a Pd membrane tube system under various vacuum degrees

Hydrogen purification using palladium (Pd) membrane technology has been seen as a potential solution for producing pure hydrogen form hydrogen-rich gas. Compared to traditional practices of operating the permeate side of the membrane at atmospheric pressure, in this study, a vacuum is applied. The effects of various vacuum degrees applied to the permeate side of the Pd membrane are investigated and compared to the results under normal operation without a vacuum. The feed gas used for experiments consists of a mixture of hydrogen (70 vol%) and nitrogen (30 vol%). Three membrane operating temperatures (320, 350, and 380 °C), four pressure differences (2, 3, 4, and 5 atm) across the membrane, and four vacuum degrees (−15, −30, −45, and −53 kPa) applied to the permeate side are considered. For the three operating temperatures, the best improvements in the performance of hydrogen permeation are at 320 and 350 °C when a −53 kPa vacuum is applied, resulting in 79.4% and 79.1% improvements, respectively, compared to normal operations. Increasing temperatures leads to an increase in H₂ permeation both with and without a vacuum; however, best performances of H₂ permeation are observed in cases without a vacuum.     

New synthesis method of Pd membranes over tubular PSS supports via “pore-plating” for hydrogen separation processes

A new synthesis method to prepare Pd membranes by novelty modified electroless plating over tubular porous stainless steel supports (PSS) has been developed. This new pore plating method basically consists on feeding both plating solution and reducing agent from opposite sides of support, allowing the preparation of totally hydrogen selective membranes with a significantly lower Pd consumption than the corresponding to the conventional electroless plating procedure. In the latter, both reducing agent and plating solution are added simultaneously in one side of the PSS support. This new plating method has been applied over raw commercial PSS supports and air calcined supports in order to generate a Fe–Cr oxide intermediate layer.