Stability and hydrogen permeation behavior of supported platinum membranes in presence of hydrogen sulfide

Chemical stability for hydrogen sulfide of a platinum composite membrane, consisting of a platinum layer supported on a porous alumina tube by a CVD technique, was evaluated in comparison with a palladium composite membrane. These composite membranes gave high fluxes comparable to that of a reported palladium composite membrane prepared by an electroless-plating technique, as well as high ideal permselectivity for hydrogen over nitrogen, typically 240 for palladium and 210 for platinum at 773 K. When these composite membranes were in contact with gas stream including hydrogen sulfide, their hydrogen permeability declined rapidly. Many cracks were formed on the surface metallic layer of the palladium composite membrane, so that other gases besides hydrogen permeated mainly through the cracks formed. On the other hand, cracks were hardly formed for the platinum composite membrane. It was reported that the lattice constant of palladium was expanded from 0.39 to 0.65 nm by sulfidation of the metallic layer, but that of platinum was slightly changed from 0.39 to 0.35 nm. The difference in the expansion of lattice constant may affect structural change and rupture phenomena of these composite membranes. After the sulfurized platinum composite membrane was treated with pure oxygen flow, the hydrogen permeability was recovered up to 50% of that of the fresh membrane.     

Thermochemical decomposition of hydrogen sulfide by solar energy

The decomposition of hydrogen sulfide into hydrogen and sulfur using concentrated solar radiation was investigated. The effect of temperature and residence time on hydrogen yield was studied using alumina, nickel-molybdenum sulfide and cobalt molybdenum sulfide as catalysts. It was found that hydrogen yield of 13–14% can be obtained in a very short residence time (0.3 s) and at temperatures less than 800°C. The decomposition mechanism followed a second-order irreversible rate equation.     

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.     

Zeolite-templated sub-nanometer carbon nanotube arrays and membranes for hydrogen storage and separation

Adsorbents and membranes consisting of carbon nanotube (CNT) pores with diameters of molecular dimensions are highly desirable for hydrogen storage and selective, high-flux membrane separation. However, fabrication of such materials with precise pore sizes and monodispersity as well as evaluation of the mechanisms associated to adsorption and molecular transport are challenging. Herein, we grew aluminophsphate zeolites (CoAPO-5, AFI crystal structure) consisting of one-dimensional, monodisperse parallel pores with diameter of ～7 Å, and utilized them as templates to grow singe-walled CNTs (SWNTs) inside the pores. The resulting materials were examined as adsorbents and membranes for hydrogen storage and separation, respectively, using single-gas and real mixture feeds. Detailed mechanistic analysis and fundamental investigation of permeance and adsorption behavior of the resulting CNT-in-zeolite systems via combined adsorption, equilibrium, and kinetic studies were carried out. A superior gravimetric hydrogen uptake of 1.2 wt% at 35 °C and 1 bar was achieved in the case of the SWNTs grown in the cobalt-richer AFI host. Permeability measurements were performed on the respective Co(x)APO@SWNT membranes with the Co-richAPO@SWNT membrane exhibiting the highest permeance for all studied gases as a consequence of larger and more densely packed AFI crystals along with higher number of SWNT-filled pores, assets attributed to the higher Co catalyst content. Notably, the produced composite membranes exhibited gas permeability values that were two orders of magnitude higher than what predicted by the Knudsen mechanism.     

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

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

Recommended pre-operation cleanup procedures for hydrogen fueling station

The stainless steel (SS) tubing and in-line filters are found to be sources of particulates in hydrogen fuel from new hydrogen stations. The internal coating of fueling nozzle can be delaminated during fueling as another particulate source. Organic residues, acetone, heptanes, and C₄Cl₄F₆ isomers are found in new SS tubing, which also emits hydrogen sulfide and carbonyl sulfide. Nitrogen contained in new storage tanks, if not properly removed, can elevate the nitrogen concentration in hydrogen fuel. We find that high pressure hydrogen flow can remove particulates, sulfur compounds and residual organic compounds from SS tubing. However, in-line filters should be cleaned by sonication and nitrogen contained in new storage tank pumped away. It is recommended that internal coating of fueling nozzle or SS tubing should not contain oxygen in chemical composition.     

Palladium nanoparticle binding in functionalized track etched PET membrane for hydrogen gas separation

In this work, track-etched poly (ethylene terephthalate) (PET) membranes having different pore sizes were functionalized by the carboxylic groups and the amino groups. Palladium (Pd) nanoparticles of average diameter 5 nm were synthesized chemically and deposited onto pore walls as well as on the surface of these pristine and functionalized membranes. Effect of Pd nanoparticles binding on these membranes were explored and aminated membrane were found to bind more Pd nanoparticles due to its affinity. The morphology of these composite membranes is characterized by Scanning Electron Microscope (SEM) for confirmation of Pd nanoparticle deposition on pore wall as well as on the surface. Gas permeability of functionalized and non-functionalized membranes for hydrogen and carbon dioxide has been examined. From the gas permeability data of hydrogen (H₂) and carbon dioxide (CO₂) gases, it was observed that these membranes have higher permeability for H₂ as compared with CO₂. Due to absorption of hydrogen by Pd nanoparticles selectivity of H₂ over CO₂ was found higher as compared to without Pd embedded membranes. Such type of membranes can be used to develop hydrogen selective nanofilters for purification/separation technology.     

PdAg/alumina membranes prepared by high power impulse magnetron sputtering for hydrogen separation

The development of hydrogen purification membranes that meet market demands such as high purity, dynamic hydrogen production even at small scale, and reduced costs is still an open question. With this view, the present study aims at developing, for the first time, a method based on high power impulse magnetron sputtering for the deposition of Pd77Ag23 (wt%) films onto porous alumina substrates to achieve composite membranes with high hydrogen permeability and stability. This technique allows the deposition of films also on complex geometries and can be easily scaled up, thus making this technology a potential candidate for preparing high performing membranes. Membranes made by stable and porous alumina supports and metallic, dense and crystalline Pd₇₇Ag₂₃ layers, from 3.5 μm to 17 μm thick, have been prepared and tested. The membranes showed good hydrogen permeability values, showing flux values up to a maximum of 0.62 mol_H2 m^?2 s^?1 at 450 °C and ΔP of 300 kPa. The resistance to hydrogen embrittlement and the chemical inertness to syngas were also demonstrated.     

Preparation and characterization of palladium ceramic alumina membrane for hydrogen permeation

In this study, a tubular palladium membrane has been prepared by an electroless plating method using palladium II chloride as a precursor with the intent of not having a completely dense film since its application does not require high hydrogen selectivity. The support used was a 15 nm pore sized tubular ceramic alumina material that comprised of 77% alumina and 33% titania. It has dimensions of 7 mm inner and 10 mm outer diameters respectively. The catalyst was deposited on the outside tube surface using the electroless deposition process. The membrane was morphologically characterized using scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDXA) and liquid nitrogen adsorption/desorption analysis (BET) to study the shape and nature of the palladium plating on the membrane. The catalytic membrane was then inserted into a tubular stainless-steel holder which was wrapped in heating tapes so as to enable the heating of the membrane in the reactor. The gases used for permeation tests comprised H₂, N₂, O₂ and He. Permeation tests were out at 573 K and at pressure range between 0.05 and 1 barg. The results showed that hydrogen displayed a higher permeation when compared to other gases that permeated through the membrane and its diffusion is also thought to include solution diffusion through the dense portions of the palladium in addition to Knudsen, convective and molecular sieving mechanisms occurring through cracks and voids along the grain boundaries. While high hydrogen selectivity is critically important in connection with hydrogen purification for fuel cells and in catalytic membrane reactors used to increase the yield of thermodynamically limited reactions such as methane steam reforming and water–gas shift reactions whereby the effective and selective removal of the H₂ produced from the reaction zone shifts the equilibrium, it is not so important in situations in which the membrane has catalytic activity such that it is possible to carryout the reaction in situations where the premixed reactants are forced-through the membrane on which the catalysts is attached. This type of catalytically active membranes is novel and has not been tested in gas-solid-liquid reactions and liquid-solid reactions before. With such a reactor configuration, it is possible to achieve good feed stream distribution and an optimal usage of the catalytic material. The preparation and characterization of such membrane catalysts has gained increased interest in the process industries because it can be adapted to carryout the chemical reactions if one of the reactants is present in low concentration and an optimal reactant distribution results in a better utilization of the active catalytic material. However, there are concerns in terms of the high cost of palladium membranes and research on how to fabricate membranes with a very low content of the palladium catalyst is still ongoing. Work is currently underway to deploy the Pd/Al₂O₃ membrane catalysts for the deoxygenating of water for downhole injection for pressure maintenance and in process applications.     

Palladium based membranes and membrane reactors for hydrogen production and purification: An overview of research activities at Tecnalia and TU/e

In this paper, the main achievements of several European research projects on Pd based membranes and Pd membrane reactors for hydrogen production are reported. Pd-based membranes have received an increasing interest for separation and purification of hydrogen. In addition, the integration of such membranes in membrane reactors has been widely studied for enhancing the efficiency of several dehydrogenation reactions. The integration of reaction and separation in one multifunctional reactor allows obtaining higher conversion degrees, smaller reactor volumes and higher efficiencies compared with conventional systems. In the last decade, much thinner dense Pd-based membranes have been produced that can be used in membrane reactors. However, the thinner the membranes the higher the flux and the higher the effect of concentration polarization in packed bed membrane reactors. A reactor concept that can circumvent (or at least strongly reduce) concentration polarization is the fluidized bed membrane reactor configuration, which improves the heat transfer as well. Tecnalia and TU/e are involved in several European projects that are related to development of fluidized bed membrane reactors for hydrogen production using thin Pd-based (<5 μm) supported membranes for different application: In DEMCAMER project a water gas shift (WGS) membrane reactor was developed for high purity hydrogen production. ReforCELL aims at developing a high efficient heat and power micro-cogeneration system (m-CHP) using a methane reforming fluidized membrane reactor. The main objective of FERRET is the development of a flexible natural gas membrane reformer directly linked to the fuel processor of the micro-CHP system. FluidCELL aims the Proof-of-Concept of a m-CHP system for decentralized off-grid using a bioethanol reforming membrane reactor. BIONICO aims at applying membrane reactors for biogas conversion to hydrogen. The fluidized bed system allows operating at a virtually uniform temperature which is beneficial in terms of both membrane stability and durability and for the reaction selectivity and yield.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Effect of mesoporous silica modification on the structure of hybrid carbon membrane for hydrogen separation

An SBA-15/carbon molecular sieve (CMS) composite membrane, using polyetherimide as a precursor and mesoporous silica as filler, was fabricated for hydrogen separation. The effect of mesoporous SBA-15 on the gas transport properties of the composite membrane was evaluated. The permeability and selectivity coefficients of H₂, CO₂, O₂, N₂, and CH₄ were estimated for the pure CMS and SBA-15/CMS composite membranes at a feed pressure of 2-7 atm for 30 °C. The SBA-15/CMS composite membrane had a gas permeability higher than that of the pure CMS membrane, whereas its selectivity was the same. The permeability was found to be independent of pressure; this indicates that the gases are transported through the membrane by a molecular sieve mechanism. The membranes appeared to have a more microporous structure when the mesoporous silica SBA-15 was incorporated. These results concur with the hypothesis that SBA-15 improves gas diffusivity by increasing pore volume.     

Corrosion behavior analyses of metallic membranes in hydrogen iodide environment for iodine-sulfur thermochemical cycle of hydrogen production

HI decomposition in Iodine-Sulfur (IS) thermochemical process for hydrogen production is one of the critical steps, which suffers from low equilibrium conversion as well as highly corrosive environment. Corrosion-resistant metal membrane reactor is proposed to be a process intensification tool, which can enable efficient HI decomposition by enhancing the equilibrium conversion value. Here we report corrosion resistance studies on tantalum, niobium and palladium membranes, along with their comparative evaluation. Thin layer each of tantalum, palladium and niobium was coated on tubular alumina support of length 250 mm and 10 mm OD using DC sputter deposition technique. Small pieces of the coated tubes were subject to immersion coupon tests in HI-water environment (57 wt% HI in water) at a temperature of 125–130 °C under reflux environment, and simulated HI decomposition environment at 450 °C. The unexposed and exposed cut pieces were analyzed using scanning electron microscope (SEM), energy dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS). The extent of leaching of metal into liquid HI was quantified using inductively coupled plasma-mass spectrometer (ICP-MS). Findings confirmed that tantalum is the most resistant membrane material in HI environment (liquid and gas) followed by niobium and palladium.     

Effect of aluminium acetyl acetonate on the hydrogen and nitrogen permeation of carbon molecular sieves membranes

With a growing interest in hydrogen as energy carrier, the efficient purification of hydrogen from gaseous mixtures is very important. This paper addresses the separation of hydrogen using Carbon Molecular Sieves Membranes (CMSM), which show an attractive combination of high permeability, selectivity and stability. Supported CMSM containing various amounts of aluminium have been prepared from novolac and aluminium acetyl acetonate (Al(acac)₃) as carbon and alumina precursors. The thickness of the CMSM layers depend on the content of Al(acac)₃ in the dipping solution, which also has influence in the pore size and pore size distribution of the membranes. The permeation properties of the membranes against the Al content in the membrane follows a volcano shape, where the membrane containing 4 wt (%) of Al(acac)₃ has the best properties and was stable during 720 h for hydrogen at 150 °C and 6 bar pressure difference. All the CMSM have permeation properties well above the Robeson Upper limit.     

Design and performance of asymmetric supported membranes for oxygen and hydrogen separation

Producing syngas and hydrogen from biofuels is a promising technology in the modern energy. In this work results of authors’ research aimed at design of supported membranes for oxygen and hydrogen separation are reviewed. Nanocomposites were deposited as thin layers on Ni–Al foam substrates. Oxygen separation membranes were tested in CH₄ selective oxidation/oxi-dry reforming. The hydrogen separation membranes were tested in C₂H₅OH steam reforming. High oxygen/hydrogen fluxes were demonstrated. For oxygen separation membranes syngas yield and methane conversion increase with temperature and contact time. For reactor with hydrogen separation membrane a good performance in ethanol steam reforming was obtained. Hydrogen permeation increases with ethanol inlet concentration, then a slight decrease is observed. The results of tests demonstrated the oxygen/hydrogen permeability promising for the practical application, high catalytic performance and a good thermochemical stability.     

Development of hydrogen selective microporous PVDF membrane

Hydrogen is a sustainable clean and green energy source used to eliminate the problem of greenhouse effect. In the present work, the feasibility of gas permeability in separation of H₂ from CO₂ and N₂ have been examined using polyvinylidene fluoride (PVDF) membranes synthesised in our laboratory by the phase inversion process. Effect of various non-solvent additives, such as lithium chloride (LiCl) and Tetraethoxysilane (TEOS) in the PVDF dope solution, have been studied. The resulted asymmetric flat sheet microporous hydrophobic membrane, shows higher hydrogen permeability and selectivity over other gases (CO₂ & N₂). It has been observed that the MT5 membrane has shown the highest selectivity for hydrogen in comparison to CO₂ and N₂. The highest value of selectivity was obtained as 4.8 and 3.7 in case of H₂/CO₂ and H₂/N₂ respectively. The permeability of membrane has been obtained in the range of 2.3–4.2 mega barrer. SEM analysis is used for the investigation of membrane surface morphology.     

Exploration of ceramic supports to be used in membrane reactors for hydrogen production and separation

The objective of this study is to develop ceramic supports employed in the preparation of catalytic membrane reactor using less catalyst and lower temperature and enabling H₂ production and separation together through the dry reforming of methane. For this reason, nine different ceramic supports are fabricated by using three different types of activated alumina (acidic, basic and neutral) and three different Si/Al molar ratios at two different calcination temperatures in order to investigate the surface acidity and basicity effect of supports to be used with impregnated Rh catalyst for rising the activity to CH₄, CO₂ and sensitivity to coke formation encountered with Ni-based catalysts. Subsequently, the effect of those variations on supports is determined by using XRD, SEM and BET instruments, in addition to this, H₂ permeability test of five supports having high BET surface areas is also performed with using constant volume-variable pressure technique at the temperatures of between 25 and 250 °C. Acidic alumina sintered at 600 °C and containing Si/Al ratio of 0.648 represented the highest hydrogen permeability with higher activity, whereas, neutral alumina calcined at the temperature of 600 °C having Si/Al ratio of 0.555 gave the highest activity with the lower hydrogen permeability, while basic alumina sintered at the temperature of 600 °C and including Si/Al ratio of 0.648 imparted lower activity with higher hydrogen permeability.     

Study on a catalytic membrane reactor for hydrogen production from ethanol steam reforming

Ethanol steam reforming in a membrane reactor with catalytic membranes was investigated to achieve important aims in one process, such as improvement in ethanol conversion and hydrogen yield, high hydrogen recovery and CO reduction. In order to confirm the efficiency of reaction and CO reduction, an ethanol reforming-catalytic membrane reactor with water–gas shift reaction (ECRW) in the permeate side was compared with a conventional reactor (CR) and an ethanol reforming-catalytic membrane reactor (ECR). In comparison with the CR, ethanol conversion improvement of 11.9–19% and high hydrogen recovery of 78–87% were observed in the temperature range of 300–600 °C in the ECRW. Compared with CR and ECR, the hydrogen yield of ECRW increased up to 38% and 30%, respectively. Particularly, the ECRW showed higher hydrogen yield at high temperature, because Pt/Degussa P25 loaded in the permeate side showed catalytic activity for the methane steam reforming as well as WGS reaction. Moreover, CO concentration was reduced under 1% by the WGS reaction in the permeate side in the temperature range of 300–500 °C.     

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

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

Capacity development of Pd doped Si2BN nanotube for hydrogen storage

Using first principles calculations employing density functional theory (DFT) we have investigated the electronic properties of Si₂BN nanotubes having 10 Å and 15 Å (armchair and zigzag) diameters. The obtained electronic band structures reveal all structures to be metallic in nature. The partial density of states (DOS) shows that the contributions of 3p state of Si is most prominent in the conduction band and majority contribution of the valence band is due to the 2p state of N atoms for all nanotubes. H₂ storage in Pd doped Si₂BN nanotubes (Si₂BN@Pd) was investigated up to the maximum limit. Our calculations have uncovered the possibility of storing 2H₂ and 3H_2, respectively, inside, and outside of the Si₂BN@Pd in each unit cell. When diameter increases from 10 Å to 15 Å the static dielectric constant, ε₁(0), upsurges in value from 6.9 to 10.7. The variation in n(ω) for the 10 Å diameter nanotube, with and without H₂ molecules, indicates no significant changes after the sensing of H₂ molecules, while in the 15 Å diameter nanotube, the n(0) value in the near infra-red (NIR) region is found to decrease and shows a further decrease if we store more H₂. We have also calculated the adsorption energies for the Si₂BN@Pd and Si₂BN@PdH₂ structures (for 10 Å and 15 Å diameters). Our theoretical approach has revealed that the Si₂BN nanotube can efficiently store hydrogen and we propose efficient ways for hydrogen storage for future improvements.