Thin Palladium Membranes Research Articles

Effects of different silica intermediate layers for hydrogen diffusion enhancement of palladium membranes applied to porous stainless steel support

Porous stainless steel (SUS) supports were modified with double intermediate layers, silicalite-1 and γ-alumina, to enhance the hydrogen diffusion of a thin palladium membrane. One of layers, silicalite-1, was prepared using the hydrothermal synthetic method on porous SUS supports. The differences in expansion/contraction behaviors caused by different thermal coefficients of expansion between silicalite-1 and the SUS resulted in a lowering of the durability of the membrane. Intermediates layers of mesoporous MCM-48 powders or commercial spherical non-porous silica particles were then applied to porous SUS supports via aspiration, γ-alumina was introduced by dip-coating, and the Pd membrane was subjected to electro-less plating. H2 permeance of the Pd membrane (membrane thickness: 11 μm) containing spherical silica particles was around 10 × 10−6 mol·m−2·s−1·Pa−1 at 600 °C, which was higher than that of the Pd membrane (membrane thickness: 7 μm) containing MCM-48. The durability of the Pd membrane containing spherical silica particles was higher than that of the version containing MCM-48 powders. These results suggest that commercial spherical non-porous silica particles will uniformly occupy the pores of the SUS tubes and enhance the H2 permeance and durability of the Pd membrane.

Indirect photo-electrochemical detection of carbohydrates with Pt@g-C3N4 immobilised into a polymer of intrinsic microporosity (PIM-1) and attached to a palladium hydrogen capture membrane

An “indirect” photo-electrochemical sensor is presented for the measurement of a mixture of analytes including reducing sugars (e.g. glucose, fructose) and non-reducing sugars (e.g. sucrose, trehalose). Its innovation relies on the use of a palladium film creating a two-compartment cell to separate the electrochemical and the photocatalytic processes. In this original way, the electrochemical detection is separated from the potential complex matrix of the analyte (i.e. colloids, salts, additives, etc.). Hydrogen is generated in the photocatalytic compartment by a Pt@g-C3N4 photocatalyst embedded into a hydrogen capture material composed of a polymer of intrinsic microporosity (PIM-1). The immobilised photocatalyst is deposited onto a thin palladium membrane, which allows rapid pure hydrogen diffusion, which is then monitored by chronopotentiometry (zero current) response in the electrochemical compartment. The concept is demonstrated herein for the analysis of sugar content in commercial soft drinks. There is no requirement for the analyte to be conducting with electrolyte or buffered. In this way, samples (biological or not) can be simply monitored by their exposition to blue LED light, opening the door to additional energy conversion and waste-to-energy applications.

Synthesis and gas transport parameters of membranes modified by star­shaped nanocrystalline palladium

Methods have been developed to modify the surface of Pd-23%Ag alloy films in order to increase the rate of hydrogen transfer with appearance of palladium coatings of the “nanostar” and “nanopore” types. The gas transport parameters of the membranes which surface is activated using the developed methods were investigated. The modification of surface of Pd-Ag films synthesized by star-like palladium nanocrystallites allows achieving a hydrogen flux density of up to 0.75 mmol / (s × m2) - that is 1.7 times more than in case of modification modified by “nanopore” type coating of sufficiently thin palladium membranes (< 10 µm) under low temperature (<100 °C) and pressure (<0.6 MPa).

Coating the porous Al2O3 substrate with a natural mineral of Nontronite-15A for fabrication of hydrogen-permeable palladium membranes

Substrate surface modification is a key pretreatment during fabrication of composite palladium membranes for hydrogen purification in hydrogen energy applications. The suspension of a natural porous material, Nontronite-15A mineral, without any organic additives was employed in dip-coating of the porous Al2O3 substrate. The Nontronite-15A mineral was characterized by SEM, XRD, TG−DSC and granulometry analysis. The surface and cross-section of the coated porous Al2O3 tubes were observed by SEM, and their pore size distribution and nitrogen flux were also measured. Palladium membranes were fabricated over the coated Al2O3 tubes by a suction-assisted electroless plating. The optimal loading amount of the Nontronite-15A mineral is just to fill in and level up the surface cavities of the Al2O3 substrate rather than to form an extra continuous layer. A thin and selective palladium membrane was successfully obtained, and its permeation performances were tested. The kinetic analyses on the hydrogen flux indicate that the hydrogen permeation behavior exhibits typical characteristics for most of the palladium membranes. During the stability test at 450 °C for 192 h, no membrane damage was detected, and the hydrogen flux increased slightly.

Hydrogen Permeability of a Foil of Pd–Ag Alloy Modified with a Nanoporous Palladium Coating

Thin films with the composition Pd–23% Ag are obtained via magnetron sputtering. The magnetron sputtering of Pd–50% Zn films with subsequent diffusion annealing and etching of the active component is used to modify the surfaces of palladium–silver films to improve their hydrogen permeability. Modifying the surfaces of the resulting Pd–Ag films using a nanoporous palladium coating with a predominant distribution of particles ranging from 0 to 50 nm allows a hydrogen flux density of up to 0.4 mmol s−1 m−2 to be achieved for sufficiently thin palladium membranes (<10 μm) under conditions of low temperature (<90°C) and pressure (<0.6 MPa). Experimental evidence is gathered that under these conditions, the velocity of hydrogen transport is limited by dissociative–associative processes at membrane boundaries and can be greatly (by an order of magnitude) increased, due to acceleration of the limiting stage of the process via the formation of a palladium nanoporous coating on the film’s surface.

The Recovery of Hydrogen from Binary Gas Mixtures Using a Membrane Module Based on a Palladium Foil Taking into Account the Deactivation of the Membrane

The recovery of hydrogen from binary gas mixtures with active (deactivating the surface of the membrane) and passive impurities has been studied. Experiments have been carried out on a multifunctional membrane module, the operating area of which consists of gas chambers separated by a thin foil palladium membrane. A dimensionless coefficient of the deactivation of the surface of the membrane has been introduced. An analytical formula has been obtained for the hydrogen flux from binary mixtures at its preset pressures on the opposite sides of the membrane that takes into account adsorption–desorption, breaking to protons on the surface, the diffusion of the latter in the metal lattice, and the recombination of Н+. In the case when the condition are fulfilled, the Н2 flux can be calculated by the modifiable Sieverts equation. This equation and an assumption about the complete mixing of the mixtures in the chambers of the membrane module have made it possible to develop a theoretical model for the recovery of hydrogen from binary mixtures in the membrane module. An analytical dependence for the Н2 flux as a function of the fluxes of the mixtures at the inlet of the chambers, ratio of the pressures on the opposite sides of the membrane, initial composition of the hydrogen mixture, and the coefficient of deactivation has been found. Utilizing this dependence, a procedure for finding this coefficient using additional experiments has been proposed. As an example, the coefficients of deactivation for the products of steam reforming of methane (CO, СО2, СН4, and water vapor) have been calculated for a palladium membrane with a composition Pd–6%Ru. The theoretical model has been subjected to the experimental verification on Н2–5%CO and Н2–20%CO binary mixtures.

The role (or lack thereof) of nitrogen or ammonia adsorption-induced hydrogen flux inhibition on palladium membrane performance

The potential impact of nitrogen and ammonia exposure on hydrogen permeance through thin palladium membranes (1.3µm to 4.1 µm thick) fabricated by electroless plating was studied. Additionally, a robust approach is introduced to quantify the pressure exponent which accounts for contributions to Knudsen flow through defects present in very thin membranes. In sharp contrast to previously published results, no flux inhibition was observed due to nitrogen or ammonia exposure. Studies included 24h exposures to both pure gases and equimolar hydrogen/nitrogen or hydrogen/ammonia mixtures at trans-membrane pressures ranging up to 1.0MPa and temperatures of 598K to 773 K. One membrane did exhibit significant flux inhibition after helium exposure, but this was attributed to changes in surface microstructure associated with hydrogen departing the lattice. This apparent hydrogen flux inhibition behavior was permanently eliminated by air exposure which roughens the surface, and it is suggested that this surface structure mechanism is a more probable explanation for flux inhibition than adsorption of nitrogen–based species.

Influence of the rotation rate of porous stainless steel tubes on electroless palladium deposition

Thin dense palladium (Pd) membranes (approximately 5μm) were fabricated on rotating porous stainless steel supports using electroless plating. During electroless Pd plating, the rate of Pd deposition increased as the support rotation rate increased. Compared with conventional electroless plating methods, the proposed modified electroless plating using a support rotation technique yielded Pd membranes that had considerably more uniform and smooth surface morphology, which substantially enhanced the membrane stability. The membranes exhibited a hydrogen permeance as high as 78m3/m2hbar0.5 (3.0×10−3mol/m2sPa0.5) and an ideal permselectivity exceeding 400 when the pressure difference was 4bar and the temperature was 400°C. Moreover, the membranes were stable during long-term temperature cycling performed between room temperature and 400°C over a period of 350h. These results indicated that the electroless plating combined with support rotation process was a simple, effective method for preparing thin dense Pd membranes featuring high hydrogen permeation flux and high thermal durability.

Development of thin palladium membranes supported on large porous 310L tubes for a steam reformer operated with gas-to-liquid fuel

Palladium membranes were prepared on large tubes (80mm diameter and 150mm length) of porous stainless steel supports (PSS) using a modified electroless plating technique. The morphology of the palladium layer was found to be depending on the container material of the coating apparatus. The use of PMMA resulted in compact palladium layers with smooth surfaces whereas PTFE led to inhomogeneous palladium coating with rough surface. Two different ceramic materials and coating methods were used to prepare an intermediate layer needed to prevent intermetallic diffusion between the palladium and the support at elevated temperatures. Wet powder spraying of TiO2 followed by sintering resulted in a smoother surface than atmospheric plasma spraying of YSZ, thus allowing for a thinner palladium coating. Pd/TiO2/PSS membranes showed about 4 times higher hydrogen permeances than Pd/YSZ/PSS membranes as a consequence of higher palladium thickness and lower porosity of the ceramic intermediate layer. The selectivity against nitrogen was comparable for both membranes. However, the YSZ intermediate layer showed better stability at elevated temperatures. Two membrane tubes were applied in the membrane reformer, which produced hydrogen successfully from a gas-to-liquid (GtL) fuel.

Repair of palladium membrane modules by metallic diffusion bonding

Thin palladium membranes are easy to form defects during the preparation and application processes. The inter-metallic diffusion offers a method to repair large membrane defects such as wrinkles and cracks by pasting a membrane patch on the defect area. A repair procedure based on the inter-metallic diffusion bonding is developed. A repair permeation factor is introduced to denote the influence of the repair on membrane module performance. The repair factor is not only a function of the membrane repair patch size, but also the flow hydrodynamics of the membrane module and the position of the repair patch on the membrane module. Repair of the defects on two membrane modules has been successfully demonstrated. The integrity of the modules after repair is confirmed. The measured repair permeation factors are very close to those predicted ones.

Thin palladium membranes supported on microstructured nickel for purification of reformate gases

Palladium membrane purification technology is a key unit operation to produce high purity hydrogen from a feedstock of hydrocarbon fuel. Palladium membrane technology has been investigated for many years, but designing affordable, leak free, and high flux membranes remains a challenge. Even more challenging are portable reforming applications where the membrane must integrate into fuel processing systems typically operating at much lower pressures than stationary industrial systems, in order to meet strict size, weight and rapid start-up requirements.In this work, a new supported palladium membrane composite structure is developed to meet the above challenges. Using microfabrication techniques, an ultra-thin palladium film supported on a microstructured nickel honeycomb is fabricated and characterized. The composite membrane is entirely metallic, highly selective and demonstrates flux rates of 0.47 mol of hydrogen per second per square meter of membrane area (140 standard cubic feet per hour per square) at 350 °C, using under 1 atm hydrogen partial pressure gradient driving force.

Preparation of Pd Membrane Supported on Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Nanofiber Nonwoven Used in Hydrogen Separation

. In this paper, thin palladium (Pd) membranes, which are supported on alumina nanofiber nonwoven produced by electrospinning method, were prepared by electroless plating method. The influence of different pretreatment methods (iron wire activation) and reducing agent (sodium hypophosphite and hydrazine hydrate) on the structure of Pd membrane were studied. The morphology, surface element and crystal structure of the Pd membrane were investigated by SEM, EDS and XRD. The results showed that the compact Pd membrane was formed on the alumina nanofiber nonwoven when the pretreatment method of iron wire activation was adopted and the reducing agent of sodium hypophosphite was used, which will be applied widely in hydrogen separation area.

SOI-Supported Microdevice for Hydrogen Purification Using Palladium–Silver Membranes

High-purity hydrogen continues to receive attention as a promising energy source for fuel cells in portable power applications. On-demand hydrogen generation via fuel reforming offers a convenient alternative to hydrogen storage, but the concomitant CO generation is deleterious to the fuel cell catalyst. Of the possible hydrogen purification options, palladium membranes allow a compact design suitable for portable applications. We present a micromembrane device built in silicon-on-insulator wafers for hydrogen purification. The design imparts structural stability to a submicrometer-thick palladium-silver membrane, enabling hydrogen purification at higher pressures than were tolerated by previous devices with supported thin palladium membranes. The devices are manufactured using bulk micromachining techniques including photolithography, plasma, and wet etching. They are operated at pressures up to 2 atm with a correspondingly enhanced hydrogen flux. In particular, thin (200 nm) palladium-silver membrane yield high permeation rates of up to 50 mol/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s at 350°C . The different transport resistances controlling hydrogen permeation in the micromembrane system are evaluated.

Characteristic of Palladium Coated Tubular Alumina Composite Membrane for Hydrogen Separation

The palladium membrane has been developed for high temperature separation of hydrogen from other syngas molecules. In this study, tubular α-alumina substrate was used as a support for increase mechanical strength for thin palladium membrane. Prior depositing of palladium film, a dip-coating of palladium nuclei was performed to cover the substrate. Afterward, surface of activated support was modified with a thin intermediate layer for improving adhesion between support and Pd membrane. Electroless plating of dense palladium membrane was achieved from the plating bath containing EDTA stabilized palladium complex and hydrazine. The microstructural characteristics of palladium membranes were analyzed.

Design and synthesis of thin palladium membranes on porous metal substrate for hydrogen extraction

Membrane separation is regarded nowadays as a preferred method for production of purified hydrogen. Palladium (Pd) is an attractive membrane material due to its ability to dissociate molecular hydrogen into atoms. It is usually deposited on the porous substrate that can provide good mechanical support and reduce the thickness of the membrane for maximizing hydrogen permeability. Pd membrane used for hydrogen separation must be thin enough to increase hydrogen flux and reduce cost while remaining thick enough to retain adhesion, attrition resistance and mechanical integrity during high temperature cycles. In this paper, the progress of electroless deposition of Pd around the pore area at surface of porous stainless steel was recorded and a bridge structure that was formed during the membrane deposition around the pore area of the substrate was illustrated. After that, the porous substrate was modified using micro-or nano-size metal or metal oxide particles in order to reduce pore size in the substrate surface. The experimental results obtained from hydrogen permeation through the Pd membranes having the thickness from 400 nm to 18 μm built on both modified and original porous stainless steel substrates demonstrate that these thin membranes are solid and they can be used at the temperature of 550°C and hydrogen pressure difference of 3.447x105 Pa. The proposed processing will allow optimizing the design and fabrication of thin Pd membranes on different porous substrates for hydrogen separation.

In situ high-temperature X-ray diffraction study of thin palladium/α-alumina composite membranes and their hydrogen permeation properties

The changes in the crystal structure of a thin palladium membrane (3–4 μm in thickness) supported on a porous α-alumina substrate were investigated by in situ high-temperature X-ray diffraction analysis at temperatures of 30–850 °C in static air atmosphere. The diffraction peaks of palladium and α-alumina shifted to a lower scanning angle (2 θ) with an increase in temperature, which was attributed to the thermal expansion of the crystal lattice. The significant enhancement in the intensity of the diffraction peak corresponding to palladium above 820 °C strongly indicated the occurrence of grain growth in palladium; a well-crystallized palladium phase was formed due to the decrease in the gaps between the grain boundaries. Hydrogen permeation tests were performed on unannealed and annealed membranes; annealing was conducted in argon atmosphere at 850 °C. The annealed membranes had considerably higher hydrogen permeation flux than the unannealed ones; this was probably due to an increase in hydrogen diffusivity in the palladium membrane layer.

Design and Synthesis of Thin Palladium Membranes for Hydrogen Separation

The progress of electroless deposition of palladium around the pore area at surface of porous stainless steel was recorded in order to understand membrane formation and to control the membrane quality. A bridge structure is formed during the membrane formation around the pore area of the substrate. The porous substrate was modified to be smooth using micro-or nano-size metal or metal oxide particles in order to make sure that palladium membrane is strongly supported by the substrate and as the result the membrane thickness can be further reduced. The experimental results obtained from hydrogen permeation through the palladium membranes having the thickness from 400 nm to 18 μm demonstrate that these thin membranes are solid and they can be used at the temperature of 550°C and hydrogen pressure difference of 350 kPa. The proposed processing will allow optimizing the design and fabrication of thin palladium membranes for hydrogen separation.

Preparation of thin palladium membranes on a porous support with rough surface

The integrity of thin composite palladium membranes is influenced by the surface roughness of the porous support. Supports with smooth surface and small pore size are expensive as they are composed of several layers with decreasing pore size which require multiple successive energy and time consuming sintering steps. In addition, smooth surfaces may cause poor membrane adhesion. It is therefore of interest to develop methods for preparation of thin defect-free palladium membranes over supports with rough surfaces. Porous stainless steel tubes coated by atmospheric plasma spraying with a porous layer of yttria-stabilized zirconia (YSZ) were used in this work as a support for palladium composite membranes. The YSZ layer served as a barrier against intermetallic diffusion between the palladium membrane and the metallic support. Three different techniques to create the membranes were compared: magnetron sputtering did not result in sufficiently dense films. Atmospheric plasma spraying produced relatively thick continuous films, but with some residual open porosity. Electroless plating gave the densest layers. Yet rather thick layers were required to limit the number of defects, which is associated with high cost and low hydrogen flux. Activation of the support surface by metal organic chemical vapor deposition of palladium instead of the conventional sensitization and activation pretreatment based on successive immersion in SnCl 2 and PdCl 2 solutions allowed to reduce the membrane thickness without compromising its integrity. The resulting membrane showed significantly higher permselectivity but at the same time decreased hydrogen permeability.

Characterization of the adhesion of thin palladium membranes supported on tubular porous ceramics

Thin palladium membranes supported on porous materials, i.e. composite membranes are excellent hydrogen separators and purifiers, and the adhesion of the palladium layer is a key factor for membrane stability. To determine the adhesion of composite palladium membranes, particularly tubular membranes, the methodological studies are quite few in literature. This work employed five methods to characterize the adhesion of typical tubular composite palladium membranes: cross-cut test, thermal-shock test, hydrogen embrittling test, pull-off test and the pressure tolerating test, the first three of which are qualitative and the others are quantitative. These methods except for the thermal-shock test can successfully distinguish the adhesion difference of the tested membrane specimen. It can be concluded that a single method may give biased or wrong results, and only the testing with different methods together can provide all-round information. It was confirmed that the porous support with rougher surface and larger pores favors the adhesion of the palladium membrane.

Ceramic Supported Capillary Pd Membranes for Hydrogen Separation: Potential and Present Limitations

AbstractComposite ceramic capillaries coated with thin palladium membranes are developed for the production of CO‐free hydrogen for PEM fuel cells, via alcohol steam reforming. The composite membranes are tested for pure H2 and N2, as well as for synthetic reformate gas. The aim is to develop a heat‐integrated compact membrane reformer for decentralized hydrogen production. In this context, a deep knowledge of the performance, behavior, and necessary treatment of the composite palladium membranes plays a decisive role in process design. The current contribution focuses on the main hurdles met while attempting to exploit the potential of ceramic supported capillary palladium membranes.