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In this study, we have reported a new strategy to prepare metal/ceramic membranes that can be used as an alternative to conventional high-cost palladium membranes. For the first time, a uniform Ni layer was deposited using the organic-inorganic activation method via the ELP technique. Although, alloying Pd–Ni resulted in a defect-free thin membrane layer (2 μm). The H 2 permeation flux of the composite membrane was 1.43 × 10−2 mol m−2 s−1 at 450 °C and TMP of 100 kPa with infinite H 2 /N 2 selectivity. Stage cut effect as a function of transmembrane pressure (TMP) was assessed in binary mixtures at 450 °C. The prepared composite membrane indicated perfect long-time permeance stability for 145 h at 450 °C and TMP of 100–300 kPa. The results showed that solution-diffusion is the dominant mechanism in the hydrogen transport through the composite membranes. [Display omitted] • A uniform Ni layer was deposited using the organic-inorganic activation method. • Knudsen diffusion governed hydrogen transport in the Ni composite membrane. • Alloying Pd–Ni results in a defect-free membrane layer with infinite H 2 /N 2 selectivity. • The Pd–Ni composite membrane showed perfect permeance stability over 145 h at 450 °C. • Solution-diffusion governed hydrogen transport in the Pd–Ni composite membrane. [ABSTRACT FROM AUTHOR]

Surface compositions play a predominant role in the efficiency and lifetime of membranes and catalysts. The surface composition can change during operation due to segregation, thus controlling and predicting the surface composition is essential. Computational modelling can aid in predicting alloy stability, along with designing surface alloys and near-surface alloys that can outperform existing materials. In this work, a computational model to predict surface segregation in ternary alloys is developed. The model, based on Miedema's semi-empirical model and Monte Carlo simulations, enables to predict long- and short-range ordering in the surface and subsurface layers. It is used to screen a vast range of alloy compositions to design a novel ternary Pd-based material for H 2 separation membranes. The addition of specific amounts of Cu and Zr to Pd is expected to reduce poisoning and enhance the permeability as compared to pure Pd. • A computational model to calculate surface segregation is developed. • Short- and long-range ordering effects in segregation can be predicted. • The model enables high throughput screening of surface segregation. • Pd–Cu–Zr is proposed for H 2 separation membranes with reduced poisoning and enhanced H 2 permeability. [ABSTRACT FROM AUTHOR]

Pd membranes can play an important role in H2 separation and purification for the development of sustainable and renewable energies. By supporting on porous substrates, Pd layer thickness can be reduced to several micrometers, thus improving the H2 permeance by several orders of magnitude. However, the supported thin Pd membranes are concomitant with pinhole formation due to either fabrication (e.g., electroless-plating) or thermal treatment, which exist as a remarkable challenge for its widespread applications. This study presents a novel and facile approach for self-repair of Pd membrane defects by immersing the stainless-steel supported Pd membranes in PdCl2 solution. Three membranes were deliberately selected with a low selectivity of 152–1687 (400 ​°C, 0.1Mpa), for which disproportionation reactions between Pd2+ and Fe/Cr/Ni at the defect sites spontaneously occur leading to the formation of Pd particles at the exact point of defects. This self-repair process can be enhanced when applying a high pressure of 30–50 ​bar in the PdCl2 solution for 30 ​min, by overcoming the capillary resistance and penetrating through the pinholes. Interestingly, densely distributed hillocks were observed on the membrane surface probably due to reduction of PdCl2 under following H2 treatment, thus increasing the H2 permeance with a higher effective surface area. The H2/N2 selectivity can be improved by more than one order of magnitude (in the best case from 1687 to 8768) and a long-term stability test of 300 ​h was achieved for the repaired membranes, corroborating the application potential of this approach. [ABSTRACT FROM AUTHOR]

In this study, the separation of hydrogen from gas mixtures using a palladium membrane coupled with a vacuum environment on the permeate side was studied experimentally. The gas mixtures composed of H 2 , N 2 , and CO 2 were used as the feed. Hydrogen permeation fluxes were measured with membrane operating temperature in the range of 320–380 °C, pressures on the retentate side in the range of 2–5 atm, and vacuum pressures on the permeate side in the range of 15–51 kPa. The Taguchi method was used to design the operating conditions for the experiments based on an orthogonal array. Using the measured H 2 permeation fluxes from the Taguchi approach, the stepwise regression analysis was also employed for establishing the prediction models of H 2 permeation flux, followed by the analysis of variance (ANOVA) to identify the significance and suitability of operating conditions. Based on both the Taguchi approach and ANOVA, the H 2 permeation flux was mostly affected by the gas mixture composition, followed by the retentate side pressure, the vacuum degree, and the membrane temperature. The predicted optimal operating conditions were the gas mixture with 75% H 2 and 25% N 2 , the membrane temperature of 320 °C, the retentate side pressure of 5 atm, and the vacuum degree of 51 kPa. Under these conditions, the H 2 permeation flux was 0.185 mol s−1 m−2. A second-order normalized regression model with a relative error of less than 7% was obtained based on the measured H 2 permeation flux. [Display omitted] • H 2 separation by a Pd membrane tube in vacuum environments is studied experimentally. • The H 2 permeation flux is mostly affected by the gas mixture composition. • The highest H 2 permeation flux in the experiments is 0.185 mol s−1 m−2. • The first-order regression model is not suitable to predict the H 2 permeation flux. • The second-order regression model predicts the H 2 flux with an error of less than 7%. [ABSTRACT FROM AUTHOR]

The equations for the concentration of metallic ions in the electroless plating solution have been derived. They are used to predict the effect of plating para- meters(i.e. pH, the concentration of ethylenediaminetetraacetic acid disodium salt (Na2EDTA) and plating temperature) on membrane composition in Pd‒Ag electroless plating theoretically. The experiments have shown that more palladium was deposited as the concentration of Na2EDTA and pH was increased, while the temperature was decreased. [ABSTRACT FROM AUTHOR]

To prepare H2-permeable palladium composite membranes, a novel carbon-doped microporous TiO2 intermediate layer was introduced to modify the surface of macroporous Al2O3 substrates. The Pd/TiO2–C/Al2O3 membrane was prepared via electroless plating, and thereafter, carbon residue in the intermediate layer was removed by calcination in air, yielding a Pd/TiO2/Al2O3 membrane. Experimental results indicate that the carbon residue shrinks the pore size of the intermediate layer and facilitates a decrease of membrane defects. Additionally, carbon removal induces a higher effective membrane area at the permeate side, which enhances hydrogen permeability. Furthermore, the apparent activation energy (EA) and stability of the as-prepared Pd/TiO2/Al2O3 membrane were investigated. [ABSTRACT FROM AUTHOR]

Abstract High temperature H 2 -selective membranes can be applied as the membrane reactor for pure hydrogen production by catalytic reforming of alcohols or hydrocarbons. Conventional Pd-based membranes are limited for this purpose due to the low thermal stability, significant hydrogen embrittlement, quick poisoning by other impurity species and high material cost. In this work, metallic nickel (Ni) hollow fiber membranes with thin wall thickness and optimal microstructure were fabricated by the dry-wet spinning and sintering technique, and employed for H 2 separation from the model reformate mixtures containing CO 2 , CO, H 2 O and H 2 S at elevated temperatures up to 1000 °C. The prepared Ni hollow fiber membranes possess 100% H 2 -permselectivity, only allowing for the hydrogen in the reformate mixtures to permeate through under experimental conditions. In the presence of CO, CO 2 and H 2 O (vapor), the hydrogen recovery from reformate mixtures may be noticeably influenced due to the water gas shift reaction (WGS: CO + H 2 O ↔ CO 2 +H 2). Multiple cycling operation and long-term tests were conducted, indicating that the Ni hollow fiber membranes have good cycling operation performance and high resistance to CO, CO 2 , H 2 O and H 2 S poisoning at high temperatures. The excellent thermal and chemical stability as well as the high permeation performance make the Ni hollow fiber membranes great potentials in advanced applications such as the portable hydrogen sources or the large-scale hydrogen production from coal gasification. Highlights • Compact Ni hollow fiber membranes are fabricated by a dry-wet phase-inversion technique. • Pure H 2 separation from reformate mixtures is achieved using the Ni hollow fiber membrane. • The Ni membranes exhibit excellent thermal and chemical stability in reformate gases. • H 2 recovery is highly dependent on the CO and CO 2 concentrations due to the WGS reaction. [ABSTRACT FROM AUTHOR]

Supported, thin-layered Pd membranes are promising tools for pre-combustion carbon capture because they can deliver simultaneously high-pressure CO 2 and H 2 streams as needed for sequestration and power generation, respectively. Moreover, the diluent required for the gas turbine fuel can be advantageously added as sweep gas in the membrane separator to boost H 2 permeation. A composite membrane has been prepared with Pd layers on both sides of a porous ceramic tube to eliminate the mass transfer resistance resulting from sweep gas diffusion into the support of conventional Pd/ceramic membranes. The H 2 permeability of the Pd/ceramic/Pd membrane (1.01 ×10 −8 mol m −1 s −1 Pa −0.5 at 773 K) is similar to that of a supported single Pd layer membrane but its H 2 /N 2 selectivity (18033) is enhanced by an order of magnitude at 773 K and Δ P  = 500 kPa. Intriguingly, the selectivity of the double Pd layer membrane improves with increasing pressure difference Δ P while that of Pd/ceramic membranes declines. This unusual pressure dependence originates from the elevated pressure within the ceramic of the double Pd layer membrane. The sweep gas-induced mass transfer resistance grows rapidly with increasing permeate pressure and sweep gas amount attenuating the single gas H 2 flux of the Pd/ceramic membrane by 67% at 1 MPa permeate pressure. This resistance remains significant under CO 2 capture conditions albeit concentration polarization on the feed side has an even stronger permeation-retarding effect in that situation. The CO 2 capture rate and purity benefit greatly from the enhanced high-pressure H 2 selectivity of Pd/ceramic/Pd membranes which renders them attractive tools for any kind of H 2 separation task. [ABSTRACT FROM AUTHOR]

The permeation characteristics of palladium pore filled (PF) membranes have been investigated with gas permeation and structural characterization of the membranes. PF membranes have been prepared by filling with Pd the nanoporous γ-Al 2 O 3 /YSZ (or pure YSZ) layer supported onto porous α-Al 2 O 3 and ZrO 2 . The number of nanoporous layers and the applied vacuum level during the electroless plating process have been studied. Gas permeation properties of the PF membranes have been determined in a temperature range of 300–550 °C. The measured hydrogen permeances have been found to be lower than previously reported for similar membranes. It has been found that the hydrogen fluxes do not depend on the thickness of the nanoporous layers (γ-Al 2 O 3 /YSZ or pure YSZ) or on the vacuum pump employed for filling with Pd. The physicochemical characterization performed showed that the palladium deposited does not form a percolated network across the mesoporous layer(s), leading to low hydrogen permeances and thus low H 2 /N 2 perm-selectivities. [ABSTRACT FROM AUTHOR]

This paper reports the preparation and performance characterization of new PdAg supported membranes with a porous protecting layer to protect the membrane surface from particles in a fluidized bed membrane reactor. Supported membranes with a selective layer of 1 µm and a protective layer have been prepared. Outstanding H 2 permeance (5·10 −6 mol m −2 s −1 Pa −1 ) and H 2 /N 2 perm-selectivity (over 25,000) were measured at 400 °C and 1 bar of pressure difference. One membrane has been tested for more than 750 h in the presence of fluidized glass beads showing a decay in the perm-selectivity to approximately 5000, mainly due to sealing leakage. However, the protective layer was removed during this long-term test. Another membrane has been tested for more than 2000 h in a fluidized bed membrane reactor with a Rh reforming catalyst supported on promoted alumina in the bubbling fluidization regime. During tests with binary mixtures mass transfer limitations toward the membrane were observed due to large H 2 permeance of the membranes. [ABSTRACT FROM AUTHOR]

In the last years, hydrogen has been considered as a promising energy vector for the oncoming modification of the current energy sector, mainly based on fossil fuels. Hydrogen can be produced from water with no significant pollutant emissions but in the nearest future its production from different hydrocarbon raw materials by thermochemical processes seems to be more feasible. In any case, a mixture of gaseous compounds containing hydrogen is produced, so a further purification step is needed to purify the hydrogen up to required levels accordingly to the final application, i.e., PEM fuel cells. In this mean, membrane technology is one of the available separation options, providing an efficient solution at reasonable cost. Particularly, dense palladium-based membranes have been proposed as an ideal chance in hydrogen purification due to the nearly complete hydrogen selectivity (ideally 100%), high thermal stability and mechanical resistance. Moreover, these membranes can be used in a membrane reactor, offering the possibility to combine both the chemical reaction for hydrogen production and the purification step in a unique device. There are many papers in the literature regarding the preparation of Pd-based membranes, trying to improve the properties of these materials in terms of permeability, thermal and mechanical resistance, poisoning and cost-efficiency. In this review, the most relevant advances in the preparation of supported Pd-based membranes for hydrogen production in recent years are presented. The work is mainly focused in the incorporation of the hydrogen selective layer (palladium or palladium-based alloy) by the electroless plating, since it is one of the most promising alternatives for a real industrial application of these membranes. The information is organized in different sections including: (i) a general introduction; (ii) raw commercial and modified membrane supports; (iii) metal deposition insights by electroless-plating; (iv) trends in preparation of Pd-based alloys, and, finally; (v) some essential concluding remarks in addition to futures perspectives. [ABSTRACT FROM AUTHOR]

The manufacturing of ultrathin Pd based hydrogen separation membranes with high permeation performance has great potential in industrial applications. However, there usually exists a trade-off between reduced thickness and high hydrogen permselectivity which are regarded as a pair of contradictory requirements. In this study, a novel technique of confined and in-situ zeolite synthesis for defects reparation is proposed to improve the hydrogen permselectivity of preformed Pd membranes without further increasing their thickness. Two technology routes are developed for different defect dimensions. Zeolites can only grow inside large defects in one route, which is suitable for repairing broken membranes. In the other route, a thin zeolite layer is formed on Pd membrane surface, covering small defects for further enhancing selectivity. The purity of permeate hydrogen is enhanced from 99.884 to 99.994% after zeolite reparation. High purity hydrogen (99.986%) can be separated from 75% H 2 /N 2 mixture with a hydrogen recovery of 91% by the repaired membrane at 500 °C and 0.4 MPa. [ABSTRACT FROM AUTHOR]

Surface topography and hydrogen permeation properties of Porous Stainless Steel (PSS) substrates for thin films deposition of Pd-based hydrogen separation membrane were investigated. Hydrogen permeance through the as received PSS substrates demonstrated a wide range, despite a similar average surface pore size of ~15 µm determined by SEM and confocal laser microscopy analyses. The surface pores of the PSS substrates were modified by impregnation of varying amounts of tungsten (W) powder. Maximum hydrogen flux reduction of 28% suggested that W has a limited effect on the hydrogen permeation through the PSS substrate. Therefore, it appears that hydrogen transport through PSS substrates is mainly controlled by the substrate geometrical factor ( ε τ ), that is the ratio of the porosity to tortuosity. In addition, tungsten was shown to inhibit the iron inter-diffusion between the PSS substrate and the deposited Pd 60 Cu 40 film at temperature as high as 800 °C. Thus, tungsten layer also serves as an effective inter-diffusion barrier. The variation in the permeance between the nominally similar PSS substrates indicates the importance to independently assess the hydrogen transport characteristics of each of the components in a composite membrane. [ABSTRACT FROM AUTHOR]

The increasing demand in compact hydrogen separators greatly stimulated the investigation and utilization of composite palladium membranes. Porous stainless steel (PSS) tubes were chosen as substrate material in this study, and a novel process of carbon-assisted solid-state sintering was introduced to modify the PSS surface with a TiO2layer. A Pd/TiO2/PSS membrane with a Pd thickness of 6 µm was successfully fabricated via electroless plating. Scanning electron microscopy (SEM), metallographic microscopy, X-ray diffraction and pore-size analyses were performed for material characterizations. As measured by H2/N2single-gas testing, the fabricated Pd/TiO2/PSS membrane is permeable and selective to hydrogen, and it was stable during a time-on-stream of 100 h under 450°C. [ABSTRACT FROM AUTHOR]

Palladium (Pd) membranes on porous stainless steel (PSS) were prepared by electroless plating. An intermediate layer of NaX nanozeolite was employed to modify the pore size of the PSS support and improve the hydrogen permeability. NaX nanozeolites were synthesized using convectional hydrothermal method. The NaX intermediate layer was coated by vacuum-assisted method on the PSS surface. The structure and morphology of the prepared membranes were characterized using SEM, XPS and XRD analysis. Permeation measurements were conducted using H 2 , N 2 and mixture of H 2 /N 2 at different temperatures (350–450 °C), and trans-membrane pressures (4–8 bar). The results indicated that the permeability and hydrogen selectivity were increased by enhancement in temperature and pressure. The hydrogen fluxes in H 2 /N 2 mixtures were lower than those corresponding to pure hydrogen at the same partial pressures and temperatures. The obtained results suggests that the use of NaX nanozolite is an effective intermediate layer in Pd/PSS membranes for hydrogen separation. [ABSTRACT FROM AUTHOR]

The demand for clean and green energy has raised the consumption of hydrogen continuously during the last years. Hydrogen is most economically produced in large scale systems by methane steam reforming followed by pressure swing adsorption (PSA). However, with a rising demand for small-scale production of hydrogen, and as down-scaling to smaller PSA-systems ( < 500 Nm3/h H2) is not economic, a substantial demand for hydrogen generation using palladium membranes has emerged. Porous tubes made of an oxide dispersion strengthened powder metallurgy Fe-Cr alloy (trade name ITM) constitute the backbone for the thin solid Pd films. The tubes provide mechanical and chemical long-term stability in atmospheres with hydrogen- and carbon-species at operation temperatures up to 600°C. A porous ceramic diffusion barrier layer (DBL) is deposited between the ITM-backbone and the Pd thin-film to avoid Pd diffusion into the Fe-Cr substrate and thereby ensure long-term integrity of the system. The Pd thin-film with a thickness < 10 μm is applied onto the DBL by a proprietary coating technology. This paper describes the production route of a tube/diffusion-barrier-layer/Pd-membrane system, its structure and permeation properties. [ABSTRACT FROM AUTHOR]

Layered alloy membranes, with sub-micron Pd catalyst layers over a highly permeable vanadium alloy core, provide a low-cost alternative to supported Pd-alloy membranes. Despite the minimal Pd consumption, Pd still comprises a significant fraction of the overall membrane costs, and further cost reductions can be achieved by replacing Pd with suitable alternative catalytic layers. Ni is an obvious candidate, exhibiting high catalytic activity for a range of reactions while being relatively inexpensive, but its instability in syngas renders it unsuitable as a feed-side catalyst. The permeate surface of an alloy membrane is exposed to H 2 only during operation, making Ni of interest as a permeate-side catalyst. Asymmetric Pd/V/Ni alloy membranes have been fabricated with varying Ni thickness, and hydrogen permeance has been examined over a wide range of pressures and temperatures. Hydrogen permeance increases with decreasing Ni thickness, down to a limiting thickness of 150 nm, beyond which permeance degraded due to incomplete Ni coverage. The permeance was 65% that of a symmetrical Pd/V/Pd membrane with 500 nm Pd layers. The cost-effectiveness is dependent on raw Pd price and manufacturing costs. [ABSTRACT FROM AUTHOR]

High purity hydrogen has many applications one of which is in the hydrogen fuel cell industry. Hydrogen can be easily produced from water electrolysis; however, the most economical method is steam reforming of methane. This delivers a mixture of gaseous compounds from which hydrogen can be extracted. Besides various techniques such as pressure swing adsorption and cryogenic distillation, dense metal membranes offer an energy efficient and highly selective method for separating hydrogen from a hot gas mixture achieving high purity levels. This review article covers the fundamentals of hydrogen selective membranes for both the porous and dense kind. An in-depth look at dense and porous membranes is taken to establish their current development and a comparison is drawn between both types showing that dense metal membranes have the best hydrogen flux and selectivity. A variety of commercial dense metal membranes are compared revealing the Group V elements such as vanadium (V), niobium (Nb) and tantalum (Ta) to have the highest hydrogen permeability. A major limitation with these metals is their tendency to form a stable oxide layer under ambient conditions. Palladium (Pd) does not suffer this problem at typical membrane operating conditions and with relatively high hydrogen permeability is a suitable alternative as a dense metal membrane. Over the years it has been discovered that alloying Pd with elements such as silver (Ag), yttrium (Y) and copper (Cu) results in marked improvements in hydrogen permeability, mechanical durability and in some cases resistance to contamination by sulphur containing compounds. Nevertheless, there are still opportunities to improve the performance of the existing commercial Pd-based membranes by investigating the endless scope of unexplored Pd binary and ternary alloys. [ABSTRACT FROM AUTHOR]

Despite the fastest transcrystalline transport of hydrogen in V, the application of pure V for the membranes permeable to hydrogen is problematic because of too large hydrogen solubility. The latter prevents from working at pressures of practical interest and obtaining a relevant permeation flux. To get a membrane material with optimal hydrogen solubility the substitutional V–Pd alloys with the content of Pd from 5 to 18.8 at% were studied with respect to hydrogen solution, diffusion and permeation in the pressure range from 0.02 to 1.2 MPa at 350 and 400 °C. Though Pd itself is a good occluder of hydrogen, the alloying of V with Pd suppressed the hydrogen solubility stronger than that is known with any other alloying elements while the hydrogen diffusivity was only slightly reduced. Hydrogen permeability was examined with plane and tubular membranes of the V–Pd alloys with thickness from 60 to 160 μm and surface area from 10 to 40 cm 2 . Both sides of the membranes were plated with Pd. Hydrogen permeability of the V–Pd alloys significantly exceeded that of Pd. The permeation flux density reached 0.7 scc/(cm 2 s) at a tolerable concentration of solute hydrogen when feed and permeate pressures of hydrogen were in the range of practical interest: ≈1 and 0.1 MPa respectively. [ABSTRACT FROM AUTHOR]

The increasing applications of hydrogen energy greatly promote the development of composite palladium membranes, which are ideal for hydrogen purification because of their high permeability, excellent permselectivity and easy modulation. However, these membranes are still associated with problems such as high cost and limited operating life. This work investigates membrane recycling and reuse as a solution to these problems. Pd/Al 2 O 3 membranes were prepared by electroless plating. The recycling of palladium was achieved by treating it with HNO 3 and HCl–H 2 O 2 agents. Adequate results were achieved using an agent composed of 2 mol/L HCl and 1 mol/L H 2 O 2 . The recycled palladium can be utilized in a new plating bath, while the Al 2 O 3 substrate remaining after palladium recycling can be reused as the substrate material for the preparation of new membranes. The surface morphology, thickness and permeation performances of the recycled membranes, as characterized by scanning electron microscopy, metallographic microscopy and permeation tests, are similar to those of the original palladium membranes. It can be concluded that the recycling and reuse of the Pd/Al 2 O 3 membranes are successful and may provide a solution to the current key problems associated with the application of composite palladium membranes. [ABSTRACT FROM AUTHOR]

A cermet composite consisting of palladium and BaCe 0.4 Zr0. 4 Gd 0.1 Dy 0.1 O 3− x (BCZGD) is fabricated by mixing palladium and BCZGD powders in a ball mill, followed by pressing and sintering at 1450 °C for 24 h in air. The Pd–BCZGD cermet demonstrates impressive hydrogen permeation flux in a mixture of hydrogen and carbon dioxide at elevated temperature, in which the palladium plays the predominant role of facile transport in the hydrogen atoms whereas the BCZGD provides channels for proton conduction. Material characterization including scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermo-gravimetric analysis (TGA) are performed. XRD patterns indicate pure phases of fcc palladium and perovskite BCZGD. SEM images and element mapping suggest a homogeneous mixture of cermet without noticeable defect and phase segregation. TGA results confirm stability of the cermet against carbon dioxide without chemical decomposition. The hydrogen permeation flux is determined via a gas chromatography from 400 to 700 °C at various hydrogen concentration gradients. We record a hydrogen flux of 1.25 cm 3 min −1 cm −2 in 50% hydrogen and 50% carbon dioxide at 700 °C, with a selectivity of H 2 /CO 2 approaching infinity. [ABSTRACT FROM AUTHOR]

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V- x Pd alloys ( x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–10 6 ) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10 −4 to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys. [ABSTRACT FROM AUTHOR]

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 Al 2 O 3 -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. [ABSTRACT FROM AUTHOR]

Thin palladium membranes were fabricated on macroporous α-Al2O3 tubes by electroless plating. The silicalite-1 (Sil-1) zeolite serving as intermediate and diffusion barrier layer was introduced to modify the surface roughness and pore size of the porous substrate and prevent the atomic interdiffusions of the metal elements between Pd layer and the support. The Pd composite membranes were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA), revealing that morphology and structure of the Sil-1 layer significantly influence the Pd membrane preparation. Single-gas permeation tests were carried out with gas H2 and N2 to determine the permeation performance of the membranes. The resulting membrane exhibited long-term stability under hydrogen permeation. [ABSTRACT FROM AUTHOR]

A new Ru/Pd/Al2O3/PSS layered membrane was fabricated by electroless plating of palladium and ruthenium for hydrogen separation. Pd and Ru were successively deposited on a pretreated PSS substrate using palladium and ruthenium plating baths at 333 and 358 K, respectively. EDX, XRD and XPS results confirmed the successful plating of ruthenium. Surface analysis showed that the palladium and ruthenium layers did not alloy significantly at temperatures of 723, 823 and 923 K over 100 h. The hydrogen permeation performance of the Pd-Ru composite membrane followed Sieverts' law, suggesting that diffusion of atomic hydrogen through the dense metal layer was rate-limiting. The addition of ruthenium had a negligible effect on hydrogen permeability when the Ru weight content was ≤8 wt%. However, when the Ru content was increased to 11 wt%, H2 permeability decreased by 10 %. The Pd-Ru membrane exhibited good chemical stability and high H2/N2 selectivity in long-term permeation testing. [ABSTRACT FROM AUTHOR]

The composite membranes based on group 5 metals are capable of H2 separation with high speed and infinite selectivity. Hydrogen transport through Pd–V–Pd composite membranes of planar and tubular shape was investigated in the pressure range of (1×10−8–6×10−1)MPa at 400°C. Due to the wide pressure range both the diffusion rate limited regime of hydrogen permeation and the regime limited by the H2 molecule dissociation were observed in one experiment. The density of flux permeating through the 100µm-membrane reached 2.4scc/(cm2 s) that seems to be one of highest ever achieved in the membranes based on group 5 metals as well as in any other unsupported metallic membranes. The concentration of dissolved hydrogen reached 0.42 but no noticeable reduction in hydrogen diffusivity was found. The constant value of sticking probability of H2 molecules (4×10−4) that was observed over full range of pressures used in this study indicates that the surface saturation has insignificant influence on the kinetics of boundary processes. The 100-fold cycling of hydrogen pressure from 0 to 0.6MPa did not result in any loss of tubular membrane integrity and change of its shape despite the fact that the ductile-to-brittle transition occurred in each cycle. The operation with samples of planar and tubular shape demonstrated that V-based composite membranes can be used at least for laboratory studies. Further work to improve the long-term stability is necessary for their practical applications. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: This study investigated the effect of gases such as CO2, N2, H2O on hydrogen permeation through a Pd-based membrane −0.012 m2 – in a bench-scale reactor. Different mixtures were chosen of H2/CO2, H2/N2/CO2 and H2/H2O/CO2 at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H2 loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H2 and the H2/CO2 mixtures. The best operating conditions to separate hydrogen from the mixtures were identified. [Copyright &y& Elsevier]

The present work is focused on the investigation of the performance and long-term stability of two composite palladium membranes under different operating conditions. One membrane (Pd/porous stainless steel (PSS)) is characterized by a ~10 μm-thick palladium layer on a porous stainless steel substrate, which is pretreated by means of surface modification and oxidation; the other membrane (Pd/Al2O3) is constituted by a ~7 μm-thick palladium layer on an asymmetric microporous Al2O3 substrate. The operating temperature and pressure ranges, used for studying the performance of these two kinds of membranes, are 350-450 °C and 200-800 kPa, respectively. The H2 permeances and the H2/N2 selectivities of both membranes were investigated and compared with literature data. At 400 °C and 200 kPa as pressure difference, Pd/PSS and Pd/Al2O3 membranes exhibited an H2/N2 ideal selectivity equal to 11700 and 6200, respectively, showing stability for 600 h. Thereafter, H2/N2 selectivity of both membranes progressively decreased and after around 2000 h, dropped dramatically to 55 and 310 for the Pd/PSS and Pd/Al2O3 membranes, respectively. As evidenced by Scanning Electron Microscope (SEM) analyses, the pinholes appear on the whole surface of the Pd/PSS membrane and this is probably due to release of sulphur from the graphite seal rings. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work was to study separation of hydrogen from the main components of syngas, i.e. synthesis gas from gasification of e.g. coal, biomass or wastes. Experiments were performed in a bench-scale facility designed to treat up to 2m3 h−1 (STP) at 873K, and 1.2MPa. A Pd-based membrane (O.D.=0.0254m; L =0.15m) from CRI Catalyst Co., US was used. Firstly the membrane was tested in pure hydrogen experiments. Permeation was determined as a function of pressure and temperature. An increase of pressure resulted in an increase of permeation. An increase of temperature also enhanced permeation. The presence of nitrogen or carbon dioxide resulted in a decrease of hydrogen permeation. Carbon dioxide inhibited hydrogen permeation to a slightly higher extent than nitrogen, and in addition the reverse water–gas-shift WGS reaction was found to be taking place. Steam also led to a decrease of hydrogen permeation. The effect of carbon monoxide was also analysed. At 723K, it did not modify hydrogen permeation, regardless carbon monoxide content in the feed gas. A strong inhibition was noticed, however, on decreasing operating temperature. Production of a pure hydrogen permeate stream was achievable even when a mixture similar in composition to a synthetic unconverted shift gas was fed to the reactor. However, hydrogen recovery was smaller. [Copyright &y& Elsevier]

Abstract: The performance of selective hydrogen permeation of different Pd-containing (Pd thickness in the range 11–30μm) structured membranes has been studied in this work. Pd was deposited over tubular porous stainless steel supports by the novel electroless pore-plating method. The permeation properties of the membranes have been tested at different operating conditions: retentate pressure (1–4bar), temperature (623–723k) and hydrogen molar fraction of feed gas (0.7–1). A complete selectivity to hydrogen was observed for all tested conditions, ensuring 100% purity in the hydrogen permeate flux. Permeances in the range of 2.3–6.4×10−4 mol/m2 sPa0.5 were obtained, maintaining a good mechanical stability of the composite system. A mathematical model considering the different mass transfer resistances in series of the composite membrane, e.g. Pd active layer, porous support and gas phase film layer, has been proposed to describe the permeation of hydrogen. The model also considers the axial variations of hydrogen concentration that takes place in tubular membranes. A set of experimental data has been used to fit the empirical parameters of the model, using another set of experiments for validating the proposed model. [Copyright &y& Elsevier]

Abstract: Fabrication of thermally stable, low cost hydrogen separation membrane for pre-combustion process was studied. A novel composite membrane with Pd layer encapsulated in a porous α-alumina substrate (Pd/Al2O3(PF)) was successfully prepared. Pd was observed only in the porous substrate matrix at a depth of 10μm from the top surface. This membrane showed extremely high H2 permselectivity (αH2/N2 > 10,000) at 773K and good stability. [Copyright &y& Elsevier]

Abstract: Palladium–platinum (Pd–Pt) alloy membranes have been fabricated by sequential, electroless deposition onto porous yttria-stablized zirconia supports manufactured by Praxair, Inc. Membranes were synthesized with thicknesses of 4–12μm that contained up to 27wt% Pt. Pd–Pt alloy membranes had lower pure-gas hydrogen flux compared with pure Pd membranes of equal thickness. However, when tested at 673K under an identical synthetic water–gas shift feed gas mixture composed of H2, H2O, CO2, and CO at 689.5kPa total pressure, the Pd–Pt alloy membranes had over 25% higher hydrogen fluxes than a pure Pd membrane of similar thickness. Membrane films were analyzed after testing with SEM, EDS, and ICP-AES to corroborate membrane thickness and alloy compositions estimated by mass gain. SEM thickness estimates on membrane cross sections were similar to those estimated by mass gain. ICP-AES analysis was performed on two membranes and confirmed the composition estimated by gravimetric analysis. In five of six membranes the film composition estimated by gravimetric analysis was consistent with the surface composition estimated by EDS which indicated that the membranes had been adequately annealed. [Copyright &y& Elsevier]

Abstract: 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. A completely dense Pd membrane with a thickness in the range 11–20 μm directly over tubular porous stainless steel tubes with a high roughness has been achieved. The permeation properties of the membranes have been tested at different operating conditions for pure feed gases: retentate pressure (1–4 bar) and temperature (350–450 °C). All membranes present good permeance reproducibility after several thermal cycles and a complete hydrogen ideal selectivity, since complete retention of nitrogen is maintained for all tested experiment conditions, ensuring 100% purity in the hydrogen permeate flux. The permeance of both membranes is maintained in the range of 1–3·10−4 mol m−2 s−1 Pa−0.5. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The hydrogen permeation characteristics of alloy membranes based on Pd-coated V90Al10 alloy membrane have been investigated in the pressure range 1–3 atm under pure hydrogen and hydrogen–carbon dioxide gas mixture at 450 °C. Hydrogen permeation experiments have been confirmed that hydrogen flux was 21.1 ml/min/cm2 for a Pd-coated V90Al10 alloy membrane (thickness: 0.5 mm) using pure hydrogen as the feed gas. It has been found that Pd-coated V90Al10 alloy membranes exhibit good resistance to hydrogen embrittlement in pure hydrogen atmosphere. After different hydrogen permeation flux tests under different pressure condition in presence of hydrogen–carbon dioxide gas mixture, the characteristics of the Pd-coated V90Al10 alloy membranes were examined by ex-situ analysis techniques. The loss of cell performance observed in the presence of hydrogen–carbon dioxide gas mixture is mainly attributed to both physical and chemical degradations of membrane, which led to structural changes in the Pd-coated V90Al10 alloy membrane. [Copyright &y& Elsevier]

Abstract: Dense Pd/TiO2/Ti–Al alloy composite membranes on porous Ti–Al alloy supports were prepared by electroless plating. The TiO2 membrane was applied as an inter-diffusion barrier between the Pd layer and the Ti–Al alloy support to prevent intermetallic diffusion. Electron probe microanalysis (EPMA) and EDX cross-sectional line scans showed that the intermediate layer was effective as the diffusion barrier for Pd membranes on Ti–Al alloy supports after 40h of treatment at 973K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14μm. The hydrogen permeance through the Pd composite membrane was 1.07×10−3 molm−2 s−1 Pa−0.5 at 773K. The permeance and selectivity of the Pd composite membrane were measured at different temperatures and pressures. The permeance tests showed that the Pd/TiO2/Ti–Al alloy composite membranes have high H2/N2 selectivity. In addition, heat cycles indicated that the structure of the Pd composite membranes was stable. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 Pa0.5) 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. [Copyright &y& Elsevier]

Abstract: The composite membranes based on Group 5 metals are capable of H2 separation with the high speed and infinite selectivity. The chemical and thermal stability are critical issues for the application of such membranes in the field of hydrogen energy. In order to understand the degradation mechanisms, the H2 permeation through composite Pd2μm–Nb100μm–Pd2μm membranes was investigated in a very wide pressure range: (10−5–104) Pa. At higher pressures the surface contaminations only moderately decreased the permeation. However the permeation experiments at lower pressures demonstrated that an orders of magnitude change in the probability of H2 molecule dissociative sticking is actually hidden behind this relatively moderate effect. The membranes poisoned by the surface contaminations could be recovered by their exposure to O2 at (300–400) °C. Heating at temperature higher than 500 °C resulted in the irreversible decrease of permeation and in the pronounced change of permeation behavior at the variation of H2 pressure. An extremely high permeation was observed at lower pressures at the clean surface of Pd coating. That allows developing an effective membrane pump for hydrogen isotopes. [Copyright &y& Elsevier]

Abstract: An efficient palladium membrane was employed for hydrogen separation in lab-scale membrane reactor during the process of catalytic dehydrogenation of ethylbenzene to styrene. Compared with the conventional fixed bed reactor, the effects of reaction conditions including pressure, space velocity and temperatures on the ethylbenzene conversion and selectivity to styrene in the membrane reactor were investigated. It was found that the efficient removal of hydrogen in the membrane reactor could significantly increase the ethylbenzene conversion without at the expense of styrene selectivity. Under the optimal reaction conditions, 11.3% enhancement in yield to styrene was obtained in the membrane reactor. [Copyright &y& Elsevier]

Abstract: Ultra-thin Pd composite membranes with a thickness of 1μm were prepared by a novel electric-field assisted activation technique followed by electroless deposition of Pd on a hollow-fiber α-alumina support. The novel activation method places Pd precursors and a reducing agent on opposite sides of a porous substrate and uses an electric field to cause migration of Pd ions to the outer surface where they are reduced to form seeds in high density in a narrow spatial region. The resulting membranes showed a high hydrogen permeance in the range of 4.0–5.0×10−6 molm−2 s−1 Pa−1 and stable H2/N2 selectivity of 3000–9000 during stability tests for 150h at 733K with H2 flow. The formation of the thin, defect-less and robust Pd layer can be ascribed to the evenly distributed Pd seeds on the support layer and the enhanced bonding between the Pd layer and the support layer resulting from the strong anchoring of the Pd seeds onto the support in the new activation step. [Copyright &y& Elsevier]

Abstract: This paper reports the synthesis and characterization of TiO2 membranes impregnated with elemental palladium (Pd–TiO2) for hydrogen separation at elevated temperatures and pressures. The membranes were prepared by sol–gel method where the elemental Pd was introduced through sol-mixing of titania sol and Pd precursor solution. The Pd–TiO2 membranes were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), thermogravimetric analysis (TG/DTA), N2 adsorb/desorption analysis, and single permeation test of H2 and N2. The XRD analysis revealed that the presence of Pd nanoparticles has retarded the crystallization and anatase-rutile phase transition. The TEM images indicated that the Pd–TiO2 membranes are composed of non-uniform particle size as compared to that of TiO2 membranes alone. For four times dip-coating, the membrane thickness was measured to be approximately 634nm. By impregnating small concentration of Pd, it significantly enlarged the pore size of the original TiO2 membrane structure besides increasing its surface area and pore volume. However, a reverse trend was observed when the Pd concentration was increased further. Single permeance of H2 and N2 for both TiO2 and Pd–TiO2 membranes showed that the transport mechanism to be dominated by Knudsen diffusion and Poiseuille flow. The ideal selectivity of H2/N2 crossing through the Pd–TiO2 membrane was calculated and found to be less than the theoretical value of Knudsen diffusion based separation. The selectivity of Pd–TiO2 membrane increased with the temperature increment, whereas the same trend was not observed for TiO2 membrane. These results show that, there were other transport mechanisms involved apart from Knudsen diffusion and Poiseuille flow. [Copyright &y& Elsevier]

Abstract: Hafnium nitride (HfN) was chosen as a material for non-porous intermediate layer to improve the high temperature stability of Pd–Ta composite membranes for hydrogen separation. A layer of dense HfN (70 nm) was prepared between Ta substrate and thin Pd film (300 nm), and the high temperature stability of Pd coating was examined by hydrogen absorption experiments at 573 K after the heat treatments at 873 and 973 K. The HfN layer showed obvious hydrogen permeability, though the permeation rate in HfN appeared to be smaller than that in Pd and Ta. In addition, the degradation in coating effects of Pd at elevated temperatures was substantially retarded by HfN layer. Such improved stability was ascribed to retardation of open porosity development in Pd films and interdiffusion between Pd and Ta. It was concluded that HfN is a potential candidate material for intermediate layer to improve high temperature stability of Pd-group 5 metal composite hydrogen separation membranes. [Copyright &y& Elsevier]

Abstract: Nanostructured ceramic membranes with ultrathin coatings of palladium metal have been demonstrated to separate hydrogen gas from a gas mixture containing nitrogen with 10% carbon dioxide and 10% hydrogen at temperatures up to 550 °C. The mechanically robust and thermally durable membranes were fabricated using a combination of conventional and high-efficiency anodisation processes on high purity aluminium foils. A pH-neutral plating solution has also been developed to enable electroless deposition of palladium metal on templates which were normally prone to chemical corrosion in strong acid or base environment. Activation and thus seeding of palladium nuclei on the surface of the template were essential to ensure uniform and fast deposition, and the thickness of the metal film was controlled by time of deposition. The palladium coated membranes showed improved hydrogen selectivity with increased temperature as well as after prolonged exposure to hydrogen, demonstrating excellent potential for gas separation technologies. [ABSTRACT FROM AUTHOR]

Abstract: Thin Pd membranes for hydrogen filtration were deposited on modified porous stainless steel (PSS) tubes using an electroless plating technique. Alumina oxide (Al2O3) particles of two different sizes were subsequently used to modify the non-uniform pore distribution and the surface roughness of the PSS tubes. The principle of the modification was to use large Al2O3 particles (∼10 μm) to fill larger pores on the surface, and leave the smaller pores intact. Small Al2O3 particles (∼1 μm) were then used to further decrease the surface roughness. The detailed manufacturing steps of the Al2O3 modification were investigated and optimized to achieve a continuous dense Pd membrane with a minimum thickness of 4.4 μm on the modified PSS tubes. The highest hydrogen permeance of the membrane was 2.94 × 10−3 mol/m2-s-kPa0.5 at 773 K, with a selectivity coefficient (H2/He) of 1124 under a pressure difference of 800 kPa. In comparison, the thickness and hydrogen permeance of a dense Pd membrane on unmodified PSS tubes were 31.5 μm and 5.97 × 10−4 mol/m2-s-kPa0.5, respectively, at 773 K under an 800 kPa pressure difference. The stability of the membranes at high temperatures was also investigated. The hydrogen permeation flux at 773 K was stable during a test period of 500 h. These results demonstrate that the two-step method modifies the surface of PSS tubes in a relatively simple way and results in thin, dense Pd membranes with high hydrogen permeance and good thermal stability. [Copyright &y& Elsevier]

Abstract: The effect of CO and CO2 on the performance and stability of Pd–Ag thin film membranes prepared by electroless plating deposition (EPD) was investigated, observing the presence of dissociation to carbon and oxygen which slowly diffuse in the membrane influencing also H2 permeability. The effect of the two carbon oxides was investigated both separately and combined in the 400–450 °C temperature range over long-term cumulative experiments (up to over 350 h) on a membrane that already worked for over 350 h in H2 or H2–N2 mixtures. An increase of the H2 permeation flux was observed feeding only CO2 in the range 10–20%. This effect was interpreted as deriving from the facilitated H2 flux caused from oxygen diffusion (deriving from CO2 dissociation) in the membrane. CO induces instead a partial inhibition on the H2 flux deriving from the negative effect of CO competitive chemisorption as well as C diffusion in the membrane, which overcome the positive effect associated to oxygen diffusion in the membrane. Carbon and oxygen diffuse through the membrane with a rate two order of magnitude lower than hydrogen, and recombinate at the permeate side forming CO, CO2 and CH4 which amount increases with time-on-stream. The effect is reversible and not associated with the creation of cracks or defects in the membrane, as supported by leak tests. [Copyright &y& Elsevier]

Abstract: Our aim is to survey the state-of-the-art in the development of palladium and palladium alloy membranes for hydrogen separations and hydrogen production. At the time this paper was written, several research groups have developed palladium and/or palladium alloy membranes with pure hydrogen permeances approaching 0.01mol/m2 sPa0.5. This includes our work to prepare Pd and Pd alloy composite membranes on stainless steel porous substrates with zirconia diffusion barriers. A 0.93μm thick pure Pd composite membrane on a Pall AccuSep® stainless steel substrate achieved a pure hydrogen flux of 1.3mol/m2 s at 400°C and 1.38bar differential pressure. The hydrogen flux values we have measured compare favorably to the U.S. DOE 2010 pure hydrogen flux performance target of 0.96mol/m2 s=250ft3(STP)/ft2 h at conditions of 400°C and a differential pressure of 20psi or 1.38bar. Pd alloy composite membranes containing Au and/or Ag show higher hydrogen flux than pure Pd membranes, as expected from the literature. At these same DOE test conditions of 400°C and 1.38bar, the pure hydrogen flux for a 2.3μm thick, Pd95Au5 composite membrane was 1.01mol/m2 s. Approximately the same pure hydrogen flux of 0.97mol/m2 s was measured for a thicker, 4.6μm, Pd80Ag20 composite membrane at 400°C and 1.38bar, consistent with the higher permeability of the Pd–Ag alloy compared to pure Pd and Pd–Au. Ideal H2/N2 separation factors of these composite membranes ranged from 337 to over 80,000. [Copyright &y& Elsevier]