Preparation and performance of thin-layered PdAu/ceramic composite membranes

Preparation of 3–5 μm thick, hydrogen-selective PdAu layers via sequential electroless plating of Pd and Au onto ceramic microfiltration membranes was investigated employing a cyanide-free Au plating bath. The Au deposition rate was strongly dependent on bath temperature and alkalinity reaching an optimum at 333 K and pH 10. Homogenous alloying of the separate metal layers under atmospheric H₂ proved to be a protracted process and required approximately a week at 873 K for a PdAu layer as thin as 3 μm. After 300 h annealing at 823 K the 5 μm thick PdAu layer of a composite membrane still exhibited a Au gradient declining from 7.4 at.% at the top surface to 5.5 at.% at the support interface despite that the H₂ permeation rate had become stable. Nonetheless, the membrane exhibited a very high H₂ permeability of e.g. 1.3 × 10⁻⁸ mol m m⁻² s⁻¹ Pa^−0.5 at 673 K, but it decreased much faster with temperature below 573 K than above, likely due to a change from bulk H diffusion-controlled to H₂ adsorption or desorption-limited transport. The composite membrane withstood cycling between 523 and 723 K in H₂ well showing that differing thermal expansion of the joined metallic and ceramic materials stayed within the tolerance range up to 723 K.     

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.     

Hydrogen permeation behavior of Nb30Ti35Ni35−xCox (x = 0…35) alloys containing high fractions of eutectic

The hydrogen permeability of cast Nb₃₀Ti₃₅Ni_35−xCo_x (x = 0…35) alloys is found to increase with the Co content, induced by the tailored microstructures containing high fractions of eutectic and less primary phase. Among the cast alloys, Nb₃₀Ti₃₅Co₃₅ consisting entirely of eutectic exhibits the highest permeability, particularly 2.65 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 at 673 K. This permeability is further increased to 3.58 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 by aligning eutectic grains perpendicularly to the membrane surface using directional solidification. The high permeability is attributed to the high hydrogen solubility and diffusivity in alloys. Our work demonstrates that hydrogen permeable alloys containing high fractions of eutectic may exhibit high permeability by adequately optimizing morphology, volume fraction and alignment of the bcc-Nb phase in the eutectic as the pathways for hydrogen permeation.     

H2 production via water gas shift in a composite Pd membrane reactor prepared by the pore-plating method

In this work, H₂ production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A completely dense membrane with a Pd thickness of about 10.2 μm over oxidized porous stainless steel support has been prepared. Firstly, permeation measurements with pure gases (H₂ and N₂) and mixtures (H₂ with N_2, CO or CO₂) at four different temperatures (ranging from 350 to 450 °C) and trans-membrane pressure differences up to 2.5 bar have been carried out. The hydrogen permeance when feeding pure hydrogen is within the range 2.68–3.96·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5, while it decreases until 0.66–1.35·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide Fe–Cr catalyst by using a typical methane reformer outlet (dry basis: 70%H₂–18%CO–12%CO₂) and a stoichiometric H₂O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 °C to 400 °C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h⁻¹. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions.     

Improvement of hydrogen-separating performance by on-stream catalytic cracking of silane over hollow fiber MFI zeolite membrane

MFI zeolite membranes were synthesized on porous α-alumina hollow fibers by in-situ hydrothermal synthesis. The membranes were further modified for H₂ separation by on-stream catalytic cracking deposition of methyldiethoxysilane (MDES) in the zeolitic pores. The separation performance of the modified membranes was characterized by separation of H₂/CO₂ gas mixture at 500 °C. Activation of MFI zeolite membranes by air at 500 °C was found to promote catalytic cracking deposition of silane in the zeolitic pores effectively, which resulted in significant improvement of H₂-separating performance. The H₂/CO₂ separation factor of 45.6 with H₂ permeance of 1.0 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ was obtained at 500 °C for a modified hollow fiber MFI zeolite membrane. The as-made membranes showed good thermochemical stability for the separation of H₂/CO₂ gas mixture containing H₂O and H₂S, respectively.     

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

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

An ion-plasma technique for formation of anode-supported thin electrolyte films for IT-SOFC applications

This paper describes a preparation method and structural and electrochemical properties of a thin bilayer anode-electrolyte structure for a solid oxide fuel cell operating at intermediate temperatures (IT-SOFC). Thin anode-supported yttria-stabilized zirconia electrolyte films were prepared by reactive magnetron sputtering of a Zr-Y target in an Ar-O₂ atmosphere. Porous anode surfaces of IT-SOFCs were modified by a pulsed low-energy high-current electron beam prior to film deposition; the influence of this pretreatment on the performance of both the deposited films and a single cell was investigated. The optimal conditions of the pulsed electron beam pretreatment were obtained. For the electrolyte thickness about 2.5 μm and the value of gas permeability of the anode/electrolyte structure 1.01 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹, the maximum power density achieved for a single cell at 800 °C and 650 °C was found to be 620 and 220 mW cm⁻² in air, respectively.     

Pd-based membranes performance under hydrocarbon exposure for propane dehydrogenation processes: Experimental and modeling

In this work, a novel Pd–Ag double-skinned (DS-) membrane is used for the first time in conditions typical of propane dehydrogenation (PDH). This membrane presents a protective layer on top of the H₂-selective one, which acts as shield against chemical deactivation and mechanical erosion under reaction conditions. While the protective layer is already been proven as an efficient barrier against membrane erosion in fluidized beds, there is no validation yet under PDH reaction. The DS- membrane performance is compared with a conventional (C-) Pd–Ag membrane under alkane/alkene exposure, at 400–500 °C and 3 bar, to investigate whether the incorporation of the protective layer would be suited for H₂ separation in PDH systems, and if coking rate would be affected. The novel membrane shows a H₂ permeance of 2.28 × 10^?6 mol?m^?2 s^?1?Pa^?1 at 500 ?C and 4 bar of pressure difference, overcoming the performance of the conventional PdAg one (1.56x?10^?6 mol m^?2 s^?1?Pa^?1). Both membranes present a stable H₂ flux under alkane exposure, while deactivation occurs under exposure to alkenes. A model able to describe the H₂ flux through Pd-based membranes is presented to fit the experimental data and predict membrane performance. The model includes mass transfer limitations in the retentate and a corrective inhibition factor to account for the competitive adsorption of hydrocarbon species in the H₂ selective layer. The experimental results obtained under alkene exposure deviates from model predictions; this can be attributed to carbon deposition on the surface of the selective layer, as further detected on the DS-membrane by Scanning Electron Microscopy (SEM)/Energy Dispersive X-Ray Analysis (EDX), which is the main factor for membrane deactivation.     

Thermal stability and effect of typical water gas shift reactant composition on H2 permeability through a Pd-YSZ-PSS composite membrane

This work comprises a study of hydrogen separation with a composite Pd-YSZ-PSS membrane from mixtures of H₂, N₂, CO and CO₂, typical of a water gas shift reactor. The Pd layer is extended over a tubular porous stainless steel support (PSS) with an intermediate layer of yttria-stabilized-zirconia (YSZ). YSZ and Pd layers were incorporated over the PSS using Atmospheric Plasma Spraying and Electroless Plating techniques, respectively. The Pd and YSZ thickness values are 13.8 and 100 μm, respectively, and the Pd layer is fully dense. Permeation measurements with pure, binary and ternary gases at different temperatures (350–450 °C), trans-membrane pressures (0–2.5 bar) and gas composition have been carried out. Moreover, thermal stability of the membrane was also checked by repeating permeation measurements after several cycles of heating and cooling the system. Membrane hydrogen permeances were calculated using Sieverts' law, obtaining values in the range of 4·10⁻⁵–4·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5. The activation energy of the permeance was also calculated using Arrhenius' equation, obtaining a value of 16.4 kJ/mol. In spite of hydrogen selectivity being 100% for all experiments, the hydrogen permeability was affected by the composition of feed gas. Thus, a significant depletion in H₂ permeate flux was observed when other gases were in the mixture, especially CO, being also more or less significant depending on gas composition.     

Fabrication,permeation, and corrosion stability measurements of silica membranes for HI decomposition in the thermochemical iodine–sulfur process

In this study, a corrosion-stable silica membrane was developed to be used in H₂ purification during the hydrogen iodide decomposition (2HI → H₂ + I₂), which is a new application of the silica membranes. From a practical perspective, the membrane separation length was enlarged up to 400 mm and one end of the membrane tubes was closed to avoid any thermal variation along the membrane length and sealing issues. The silica membranes consisted of a three-layer structure comprising a porous α-Al₂O₃ ceramic support, an intermediate layer, and a top silica layer. The intermediate layer was composed of γ-Al₂O₃ or silica, and the top silica layer that is H₂ selective was prepared via counter-diffusion chemical vapor deposition of a hexyltrimethoxysilane.To the best of our knowledge, this is the first report of 400-mm-long closed-end silica membranes supported on Si-formed α-Al₂O₃ tubes produced via chemical vapor deposition method. A 400-mm-long closed-end membrane using a Si-formed α-Al₂O₃ tube exhibited a higher H₂/SF₆ selectivity of 1240 but lower H₂ permeance of 1.4 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹ with compared with the membrane using a γ-Al₂O₃-formed α-Al₂O₃ tube (907 and 5.6 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹, respectively). The membrane using the Si-formed α-Al₂O₃ tube was more stable in corrosive HI gas than a membrane with a γ-Al₂O₃-formed α-Al₂O₃ tube after 300 h of stability tests. In conclusion, the developed silica membranes using the Si-formed α-Al₂O₃ tubes seem suitable for membrane reactors that produce H₂ on large scale using HI decomposition in the thermochemical iodine–sulfur process.     

Equilibrium shift of methylcyclohexane dehydrogenation in a thermally stable organosilica membrane reactor for high-purity hydrogen production

A high-performance organosilica membrane was prepared via sol–gel processing for use in methylcyclohexane (MCH) dehydrogenation to produce high-purity hydrogen. The membrane showed a high H₂ permeance of 1.29 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, with extremely high H₂/C₃H₈ and H₂/SF₆ selectivities of 6680 and 48,900, respectively, at 200 °C. The extraction of hydrogen from the membrane reactor led to the MCH conversion higher than the thermodynamic equilibrium, with almost pure hydrogen obtained in the permeate stream without considering the effect of carrier gas and sweep gas in the membrane reactor, and the organosilica membrane reactor was very stable under the reaction conditions employed.     

Hydrogen permeation through a Pd/Ta composite membrane with a HfN intermediate layer

Dense hafnium nitride (HfN) layers were prepared between Pd protection films and a Ta substrate in a composite hydrogen separation membrane to prevent a reaction between the Pd and the substrate at high temperatures. No significant reduction in hydrogen permeation rate was observed for the membrane with 50-nm-thick HfN layers at 873 K through at least 35 h, whereas the specimen without HfN layers rapidly deteriorated within 5 h. Hydrogen permeability of the former specimen was 4 × 10⁻⁹ mol m⁻¹ s⁻¹ Pa^−0.5 at 873 K at steady state. This value was smaller than the initial permeability of Pd-covered Ta before deterioration by an order of magnitude. The measurements of pressure–composition isotherms by using a HfN powder specimen showed that the hydrogen solubility in HfN was sufficiently high and comparable with the solubility in Ta. Therefore, the low permeability observed with the HfN intermediate layers was ascribed to low hydrogen diffusivity in HfN.     

Highly H2 permeable SAPO-34 membranes by steam-assisted conversion seeding

An effective steam-assisted conversion (SAC) seeding method was proposed for the growth of a thin and high-quality SAPO-34 membrane on a low-cost and coarse macroporous α-Al₂O₃ tubular support. This seeding technique was composed of depositing the seeds-containing paste on a support and transforming the paste into continuous and compact seed layer by the SAC process. The paste could serve as the binder to prevent small seeds penetrating into the support. With the aid of the perfect seed layer, a high-quality SAPO-34 membrane with the thickness about 4 μm was synthesized by secondary growth. The as-synthesized membrane exhibited a high H₂ permeance of 6.96 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at room temperature, with H₂/CO₂, H₂/N₂, H₂/CH₄, H₂/C₂H₆, H₂/C₃H₈, H₂/n-C₄H₁₀ and H₂/i-C₄H₁₀ ideal selectivities of 1.83, 7.58, 14.80, 18.24, 26.51, 40.15 and 53.02, respectively.     

Preparation of a novel bimodal catalytic membrane reactor and its application to ammonia decomposition for COx-free hydrogen production

A novel bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a silica separation layer was proposed. The catalytic activity of the support was successfully improved due to enhanced Ru dispersion by the increased specific surface area for the γ-Al₂O₃/α-Al₂O₃ bimodal structure. The silica separation layer was prepared via a sol–gel process, showing a H₂ permeance of 2.6 × 10⁻⁷ mol Pa⁻¹ m⁻² s⁻¹, with H₂/NH₃ and H₂/N₂ permeance ratios of 120 and 180 at 500 °C. The BCMR was applied to NH₃ decomposition for CO_x-free hydrogen production. When the reaction was carried out with a NH₃ feed flow rate of 40 ml min⁻¹ at 450 °C and the reaction pressure was increased from 0.1 to 0.3 MPa, NH₃ conversion decreased from 50.8 to 35.5% without H₂ extraction mainly due to the increased H₂ inhibition effect. With H₂ extraction, however, NH₃ conversion increased from 68.8 to 74.4% due to the enhanced driving force for H₂ permeation through the membrane.     

The influence of the CO inhibition effect on the estimation of the H2 purification unit surface

The CO inhibition effect on H₂ permeance through commercial Pd-based membranes was analysed by means of permeation measurements at different CO compositions (0–30% molar) and temperatures (593–723 K) with the aim to determine the increase of the membrane area in order to compensate the H₂ flux reduction owing to the CO inhibition effect. The permeance of H₂ fed with carbon monoxide was observed to decrease with respect to the case of pure hydrogen. At 647 K the H₂ permeance of a pure feed of 316 μmol m⁻² s⁻¹ Pa^−0.5 reduces progressively until 275 μmol m⁻² s⁻¹ Pa^−0.5 when 15% or more of CO is present in the system, until it reaches a plateau at 20%. The inhibition effect occurring when CO is present in the feed stream reduces with the progressive temperature increase; the reduction of the permeance decreases exponentially by 23% at 593 K and by 3% at 723 K with 10% of CO. The inhibition effect is seen to be reversible. An H₂ flux profile in a Sieverts' plot shows the effect produced by the increase of the CO composition along the Pd-based membrane length. The H₂ flux profile allows the area of a Pd-based membrane to be evaluated in order to have the same permeate flow rate of H₂ when it is fed with CO or as a pure stream. Moreover, a qualitative comparison between the H₂ flux profiles and a previously proposed model has been carried out.     

Enhanced hydrogen permeance through graphene oxide membrane deposited on asymmetric spinel hollow fiber substrate

Membranes of graphene oxide (GO) present suitable application for hydrogen (H₂) purification. The deposition of a selective and high permeable GO membrane on a proper substrate is still a challenge. Here we applied the vacuum-assisted method to deposit a GO layer on asymmetric spinel (MgAl₂O₄) hollow fibers. The synthetized GO showed a nanosheeted morphological structure and a relative high degree of oxidation. The hollow fibers were produced with dolomite and alumina as the ceramic starting material and showed the desired asymmetric pore size distribution, in addition to suitable bending strength, 54.88 ± 4.25 MPa, and average surface roughness, 180 ± 8.2 nm. A continuous GO layer of 1.7 ± 0.2 μm was deposited onto the fiber outer surface. The composite MgAl₂O₄/GO membrane presented H₂ permeance of 8.2 ± 0.0 × 10^?7 mol s^?1 m^?2 Pa^?1 at room temperature (approximately 25 °C) and 0.3 MPa of transmembrane pressure. Ideal hydrogen/nitrogen and hydrogen/carbon dioxide selectivity values were of 3.3 ± 0.0 and 11.4 ± 0.1, respectively.     

ZnO nanofiber skeleton induced robust zeolitic imidazolate framework membranes for gas separation

Rationally designing compact metal-organic framework (MOF) membrane is highly desired but challenging. Herein, we proposed a ZnO nanofiber skeleton induced zeolitic imidazolate framework (ZIF) membrane inspired by the reinforced concrete structure. In this process, the ZnO nanofiber skeleton was employed as active anchor sites to assist the heteroepitaxial growth of continuous membranes, like the reinforcing steel in structure. The formed ZIFs particles were tightly embedded in the skeleton like the concrete. With this approach, highly compact Co-based ZIF-9 membrane and Zn-based ZIF-8 membrane were successfully achieved and exhibit effective H₂ separation performance. For ZIF-9 membrane, the H₂ permeance and the ideal selectivity of H₂/CO₂ are 2.19 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and 15.3, respectively. For ZIF-8 membrane, the H₂ permeance and the ideal selectivity of H₂/CH₄ are 2.26 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ and 9.7, respectively. More importantly, benefiting from the novel structure, the membrane showed a highly robust architecture under different pressures, good durability against rubbing, and separation stability of 100 h. This strategy provides a new approach toward other compact and robust MOF membrane.     

Rapid thermal processing of tubular cobalt oxide silica membranes

This work is the first demonstration of rapid thermal processing techniques as applied to metal oxide/silica membranes on tubular geometries. A procedure was developed which combined fast sol–gel synthesis, rapid calcination steps and a thermal annealing stage to reduce the membrane fabrication time by more than two-thirds, from a conventional process taking seven or more days to less than two. A significant aspect of this major development was the use of a pre-hydrolysed silica precursor ethyl silicate 40 (ES40), instead of the generally preferred tetraethyl orthosilicate (TEOS) which eliminated the need for the researcher specific and time consuming sol–gel reaction stages prior to membrane fabrication. As a result, modified-silica membranes containing cobalt oxides could be directly calcined at 600 °C, instead of conventional thermal process which require slow ramping rates of ≤1 °C min⁻¹ to avoid cracking. As-prepared membranes delivered H₂ permeances of 5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 450 °C and H₂/N₂ permselectivities of 54. The RTP techniques demonstrated in this work greatly reduced the production time and should both allow researchers to significantly increase their productivity and ultimately reduce the barriers for deployment of inorganic membranes into industrial applications.     

Synthesis of thin amine-functionalized MIL-53 membrane with high hydrogen permeability

A thin amine-functionalized MIL-53 membrane with high permeability of hydrogen was successfully prepared on a porous α-Al₂O₃ support by using the secondary growth method. Seeded α-Al₂O₃ supports were prepared by a dip-coating technique. In contrast, a discontinuous membrane was obtained by using unseeded support under the same synthesis conditions, implying that the seeds play the key role in the formation of compact membranes. The resulting compact membranes were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM) and single gas permeation testing. Results showed that the thickness of the as-prepared membrane was around 2 ∼ 4 μm. Hydrogen permeance of the as-prepared membrane reached a remarkable value of 1.5 × 10⁻⁵ mol m⁻² s⁻¹·Pa⁻¹ at room temperature under a 0.1 MPa pressure drop. The ideal H₂/CO₂ selectivity was found to be 4.4. In addition, the influence of seeding solution on the membrane performance was investigated. We found that the membrane permeance decreased and the ideal selectivity increased when the seeding solution content was increased.     

Effective water splitting using N-doped TiO2 films: Role of preferred orientation on hydrogen production

N-doped TiO₂ film with preferred (211) orientation, deposited by RF magnetron sputtering, was investigated for the water-splitting hydrogen production. It is found that the preferred crystal growth orientation of the films can be controlled by N₂ flow rate during the deposition. The results reveal that not only the N-doping, but also the preferred orientation (i.e., large percentage of exposed (211) facet), can effectively enhance the activity of TiO₂-based photocatalyst. With the increase of exposed (211) facets, the hydrogen production rates of N-doped TiO₂ films rise from 760 μmol H₂ h⁻¹ m⁻² to 4500 μmol H₂ h⁻¹ m⁻², indicating that high performance of TiO₂-based photocatalyst can be achieved by controlling the preferred orientation of the films.