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Hydrogen permeation in composite Pd-membranes prepared by conventional electroless plating and electroless pore-plating alternatives over ceramic and metallic supports
Abstract
Composite palladium membranes are attracting great attention for hydrogen separation and membrane reactor applications, due to the complete hydrogen selectivity with reasonably high permeability and suitable mechanical stability. These membranes are usually prepared by depositing a thin Pd layer over ceramic or metallic supports, which give to the system the necessary mechanical resistance. The Pd incorporation is generally made by electroless plating (ELP), although there is still no fully optimized and universally accepted method, and many researches are currently devoted to the generation of thin and homogeneous metallic coatings with good adhesion and resistance to real conditions. Among the many studies, very few compares directly the properties of the two different supports (metallic or ceramic) on the overall membrane structure and performance. In the present work, the permeation behavior of several Pd-composite membranes, prepared by conventional ELP and by a novel pore-plating method (ELP-PP) has been studied on both ceramic and metallic supports. The membrane prepared over a tubular ceramic support by conventional ELP shows a permeability in the range of 2.5–3.6·10⁻⁶ mol s⁻¹ bar−0.5 m, with nearly complete ideal selectivity and Pd thickness around 14 μm. With the alternative preparation method, ELP-PP, despite the lower Pd thickness, 8 μm, and also complete selectivity, lower hydrogen fluxes were observed with a permeability ranging from 5.8 to 8.5·10⁻⁷ mol s⁻¹ bar−0.5 m. This behavior can be explained by considering that the pore-plating method leads to a Pd deposition over the external surface of the support but also inside the pores, generating an effective Pd thickness higher than that obtained with the conventional ELP. In this manner, the real behavior of the membrane is equivalent to a conventional 33 μm thick palladium layer. Finally, these results are compared replacing the ceramic support by a metallic one (PSS), which led to an increase in the minimum thickness necessary to achieve a totally dense membrane (Pd thickness, 9 μm), and, consequently, to a reduction of the observed transmembrane flux. However, in this case a lower deposition of palladium inside the pores of the support is observed thus causing a lower resistance to the hydrogen permeation with respect to ceramic supported membranes.
... In this context, it seems to be widely approved the use of a porous support to reduce the thickness of the selective Pd layer and, hence, save costs at the same time that the permeation is increased [21,22]. Porous ceramic materials provide excellent surface properties for this purpose, i.e. low roughness, adequate porosity, and narrow pore size distribution with small pore mouths [23,24]. However, porous metallic supports offer a better fitting in most of current industrial devices, usually made of Hastelloy or stainless steel (SS), as well as ensuring a suitable thermal resistance due to the similar thermal expansion coefficient to that of palladium [24,25]. ...
... Porous ceramic materials provide excellent surface properties for this purpose, i.e. low roughness, adequate porosity, and narrow pore size distribution with small pore mouths [23,24]. However, porous metallic supports offer a better fitting in most of current industrial devices, usually made of Hastelloy or stainless steel (SS), as well as ensuring a suitable thermal resistance due to the similar thermal expansion coefficient to that of palladium [24,25]. In practice, many researchers combine both alternatives by using porous stainless steel (PSS) supports modified with diverse ceramic intermediate layers to improve the quality of the original PSS surface, while simultaneously preventing any possible metallic interdiffusion between SS and Pd layer [26e28]. ...
... Other interesting membranes are prepared by electroless pore-plating (ELP-PP), where the incorporation of palladium inside the pores of the support is aimed by feeding both metal source and reducing agent from opposite sides of the porous support. In this case, an external layer is also obtained [24,37] and the Pd incorporation between this external Pd layer and pores depends on i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 5 ( 2 0 2 0 ) 7 2 7 8 e7 2 8 9 the ELP-PP conditions [38]. In fact, it was reported that Pd location inside the PSS pores could be the responsible of additional permeation resistances [38,39]. ...
This work presents the use of doped CeO2 particles with palladium as intermediate barrier for the preparation of fully dense Pd films by Electroless Pore-Plating. The use of doped ceria particles instead of non-doped ones clearly helps to reduce the final palladium thickness required to prepare a fully dense membrane over porous stainless steel supports from 15 to 9 μm (average values by gravimetric analyses), thus saving around 40% of total palladium required in the process. Pure hydrogen permeation tests reveal a consequent increase in the H2 flux in the range 15–30%, depending on the operation mode. Thus, a H2 permeance of 6.26·10⁻⁴ mol m⁻² s⁻¹ Pa−0.5 at 400 °C and ΔP = 1 bar is reached, maintaining a really high H2/N2 ideal separation factor (≥10,000) and an activation energy within the typical range for these type of membranes, Ea = 13.1 kJ mol⁻¹. Permeation of binary H2/N2 gas mixtures and the effect of feeding the mixture from the inner or the outer side of the membrane have been also studied. A significant concentration-polarization effect was observed, being higher when the gas is fed from the inner to the outer side of the membrane. This effect becomes more relevant for the membrane prepared with doped CeO2, instead of raw CeO2, due to its lower Pd thickness and higher relative influence of the surface processes. However, it should be emphasized that higher H2 permeance values were obtained for the entire set of experiments when using the Pd-membranes containing doped ceria. Finally, long-term permeation tests for more than 850 h with pure gases at T = 400 °C and ΔP = 1 bar were also carried out, demonstrating a suitable mechanical stability of membranes at these operating conditions.
... The Pd deposition progress increases its thickness until the reactants are spent or the membrane is removed from the solution. However, a modification of this bare method, denoted as electroless pore-plating (ELP-PP), has been presented as a good alternative to ensure correct anchoring between both support and palladium film while the presence of defects during the preparation process is minimized [15,38,39]. Basically, the method consists of feeding both solutions (Pd source and reducing agent) from opposite sides of the support until the pores get completely sealed. ...
... After adjusting the optimal thickness for the CeO 2 intermediate layer, all modified supports were calcined at 500°C for 5 h in order to ensure their stability. Finally, the palladium deposition was carried out by ELP-PP following the experimental procedure detailed elsewhere [38][39][40] with a hydrazine concentration of 0.2 M as reducing agent. Diverse cycles were performed up to the membrane weight gain became negligible, indicative to a good sealing of pores with palladium. ...
... The first ones were incorporated partially covering the biggest pore mouths of the porous stainless-steel support, while the [16][17][18][19][20][21][22][23][24] palladium was deposited simultaneously into the pores and in the external surface due to the hydrazine pass through the larger pores, only partially covered. This particular morphology, inherent to the ELP-PP process if the support presents a certain pore size distribution, was extensively explained in previous works [38][39][40]45]. In this context, the deposition of palladium inside the pores expects to provide good anchoring for the H 2 -selective layer and, consequently, increase the mechanical resistance of the composite membrane. ...
This work presents the improvement of hydrogen permeance on electroless pore-plated Pd-composite membranes by the incorporation of ceria as intermediate barrier. This modification, in case of preparing a thick barrier, reduces both average pore size and external roughness of an oxidized Porous Stainless Steel (PSS) tube used as support. However, it also provokes a marked reduction of its permeance, turning more difficult the pass of the hydrazine through the modified support and, therefore, the palladium incorporation by electroless poreplating.
An optimization of this process leads to a Pd/CeO2/PSS composite membrane in which the initial roughness is halved, achieving a stable and selective Pd layer of around 15 μm. This composite membrane
exhibits a hydrogen permeance of 5.37 · 10−4 molm−2 s−1 Pa−0.5 at 400 °C, an ideal H2/N2 perm-selectivity ≥10,000 and an activation energy of 8.9 kJ mol−1. Moreover, the hydrogen flux increases around 400% with regard to previous results, in which no ceria was used as intermediate layer (measured range: 0.03–0.12 molm−2 s−1 versus 0.01–0.03 molm−2 s−1). This increase is derived from a high reduction, of around 30%, in the resistance to the permeation process when using electroless pore-plated membranes due to a lower penetration grade of the Pd external film into the support. In addition, it has been confirmed the successfully stability of the Pd membrane under thermal cycles and different operating conditions, including the
variation of permeate flux direction from the inner to the outer of the membrane, where the Pd-layer is placed, or vice versa
... Several technologies can be used to incorporate a thin film of the hydrogen selective metal, preferentially Pd or Pd-based alloy, onto a porous support. Cold-rolling [57][58][59], physical vapour deposition [60][61][62][63], chemical vapour deposition [64][65][66], electrochemical plating [67][68][69] and electroless plating can be mentioned [33,62,70,71]. The last option (electroless plating, or its acronym ELP) provides important advantages in terms of adherence and uniformity of deposits on both conducting and non-conducting surfaces with complex geometries. ...
... Thus, development of new ultrathin membranes without jeopardizing mechanical resistance and presence of defects is the main objective of many researchers in this field [59,60,77]. This goal is usually achieved by incorporating a thin Pd layer on the surface of a porous material that provides the required mechanical resistance to the supported membrane [71,[78][79][80]. This complex task is subject of numerous studies since many factors must be considered, i.e., the compatibility between support and selective layer, which strongly determines the mechanical resistance of the membrane due to cracks can be formed at high temperatures because of different expansion coefficients, as it will be discussed in detail later. ...
... Numerous porous materials, such as Vycor glass [81,82], sintered metals [71,78,83], a wide variety of ceramics [53,71,84,85] and even polymers [86][87][88], can be used as supporting materials for the H 2 -selective layer. The most relevant attributes of supports to be selected include porosity properties (mainly average porosity and pore sizes distribution), surface roughness and mechanical, chemical and thermal stabilities [89]. ...
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.
... In this context, new routes from waste materials are being widely studied in the last years, considering both solid matter such as biomass [11], plastics [11], tea [12] or industrial municipal wastes [13] and liquids, in example olive mill wastewater [14]. In most of these cases, hydrogen purification is required for particular applications, i.e., combustion engines, turbines or fuel cell systems and Pd-based membranes are suggested as an attractive technology [15][16][17] to be used as independent separator [18,19] or in a membrane reactor configuration [17,20]. These membranes are usually incorporated onto a substrate to reduce the film thickness and simultaneously maintain a suitable mechanical stability [21][22][23]. ...
... Only a few works propose the use of relatively thick unsupported films [24,25]. A wide variety of materials has been suggested for the preparation of supported Pd-based membranes, highlighting the porous metals [23,26] and the ceramic ones [18,27]. The properties of supports have a clear influence on the membrane performance, despite an ideal solution is not adopted. ...
... The properties of supports have a clear influence on the membrane performance, despite an ideal solution is not adopted. In general, ceramic supports with a smooth surface and narrow pore sizes provide a better surface for the membrane preparation [28][29][30] but the limited mechanical resistance and diverse thermal expansion coefficient to that of palladium jeopardize the membrane lifespan [18]. On the other hand, metallic supports present an exceptional mechanical resistance that ensures good sealing and suitable integration in conventional stainless steel devices [31,32], although the rougher surface with relatively large pore sizes hinders the preparation of a thin and free-defect Pd film [31,33]. ...
Hydrogen is considered as a real alternative for improving the current energy scenario in the near future and separation processes are a crucial step for the economy of the process in both centralized and distributed production systems. In this context, Pd-based composite membranes appear as an attractive technology trying to reduce the Pd thickness by modifying the commercial supports, mainly formed by metals to fit properly in conventional industrial devices. In most cases, a final calcination step is required and hence, the metallic support can be oxidized. This work analyzes in detail the properties of intermediate layers generated by in-situ oxidation of tubular PSS supports as a crucial step for the preparation of Pd/PSS membranes. The oxidation temperature determines the modification of original morphology and permeability by increasing the presence of mixed iron-chromium oxides as temperature rises. A compromise solution need to be adopted in order to reduce the average pore mouth size and the external roughness, while maintaining a high permeation capacity. Temperature of 600 °C lets to reduce the average pore size from 3.5 to 2.1 μm or from 4.5 to 2.3 μm in case of using PSS supports with 0.1 or 0.2 μm porous media grades, respectively but maintaining a hydrogen permeation beyond targets of United States of America Department of Energy (US DOE). Lower temperatures provoke an insufficient surface modification, while greater values derive in a drastic reduction of permeability. In these conditions, two composite membranes were prepared by ELP-PP, obtaining 14.7 and 18.0 μm thick palladium layers in case of modifying PSS tubes of 0.1 or 0.2 μm media grades, respectively. In both cases, the composite Pd membranes exhibited a hydrogen perm-selectivity greater than 2000 with permeances ranged from 2.83 to 5.84·10−4 mol m−2 s−1 Pa−0.5 and activation energies of around 13–14 kJ mol−1.
... The most common inorganic substrate materials are ceramic oxides [1,2], and dense Pd-based membranes are often made as thin dense layers on a tube [3] and flat disc [4] porous ceramic oxide substrate that offers mechanical support. Pd membranes deposited on aluminum oxide (Al 2 O 3 ) substrates using the electroless plating approach [5][6][7] are viewed as the most promising technique for practical applications [6][7][8][9][10][11]. α-Al 2 O 3 hollow fibers are a typical substrate for the production of Pd membranes [1,2,8,[12][13][14]. Orakwe et al. [15] and Terra et al. [16] used an electroless deposition technique to produce thinner Pd layers on 77% alumina and 33% titania tubular ceramic oxide substrates and asymmetric alumina hollow fiber substrates, respectively. ...
... The most common inorganic substrate materials are ceramic oxides [1,2], and dense Pd-based membranes are often made as thin dense layers on a tube [3] and flat disc [4] porous ceramic oxide substrate that offers mechanical support. Pd membranes deposited on aluminum oxide (Al 2 O 3 ) substrates using the electroless plating approach [5][6][7] are viewed as the most promising technique for practical applications [6][7][8][9][10][11]. α-Al 2 O 3 hollow fibers are a typical substrate for the production of Pd membranes [1,2,8,[12][13][14]. ...
The phase inversion procedure was used to prepare Al2O3 ceramic hollow fiber substrates (AlCHFS) utilizing the dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAC), and 1-methyl-2-pyrrolidone (NMP) aprotic solvents. Different aprotic solvent and non-solvent (water) interactions were used to calibrate different ceramic oxide substrates. AlCHFS with an asymmetrical structure consisting of a finger-like structure on the lumen side and a pore structure on the shell side was obtained using an aprotic solvent/non-solvent pair with a high total Hildebrand solubility (δt). As the aprotic strength was reduced, the finger-like structure on the shell sides became more prominent. Hydrogen permeation studies conducted between 350°C and 450°C reveal that the Pd-coated AlCHFS produced from DMAC aprotic solvent has the maximum hydrogen flux (~0.24 mol m⁻² s⁻¹). The activation energy for the thermally activated hydrogen transport process through the Pd-coated AlCHFSs is determined to be around 11.06–14.61 kJ mol⁻¹ in the temperature range of 350°C to 450°C, and it increases linearly with increasing surface porosity of ceramic oxide substrate.
... However, independently of the commercial quality, their original surface is rougher and presents larger pore mouths in comparison with ceramic supports. Therefore, the subsequent deposition of an ultra-thin but fully dense palladium film becomes more difficult [14], and many researchers opt to modify these metal supports by incorporating inert inorganic particles that partially cover the biggest original pores and smooth the surface before palladium deposition, forming an intermediate layer between the support and the H 2 -selective film [15]. Moreover, this additional layer also serves to prevent any intermetallic diffusion from steel to palladium during operation at high temperatures [16]. ...
... Besides the intrinsic difficulties in characterizing these samples through non-destructive analyses at a lab-scale, the mechanical resistance of the resulting composite material could play a critical role. In this context, the palladium incorporation onto modified PSS supports through the electroless pore-plating (ELP-PP) technique developed by Alique et al. [32] demonstrated an excellent adherence to the porous substrate under diverse operating conditions and long-term permeation analyses, even in the case of generating tensile stresses due to the particular permeation flux direction [14,33,34]. In this manner, the excellent mechanical resistance of composite membranes prepared by ELP-PP was observed independently of collecting the permeate flux from the porous support side or the contrary one, where the Pd-film is incorporated. ...
Pd-membranes are interesting in multiple ultra-pure hydrogen production processes, although they can suffer inhibition by certain species or abrasion under fluidization conditions in membrane reactors, thus requiring additional protective layers to ensure long and stable operation. The ability to incorporate intermediate and palladium films with enough adherence on both external and internal surfaces of tubular porous supports becomes crucial to minimize their complexity and cost. This study addresses the incorporation of CeO2 and Pd films onto the internal side of PSS tubes for applications in which further protection could be required. The membranes so prepared, with a Pd-thickness around 12–15 μm, show an excellent mechanical resistance and similar performance to those prepared on the external surface. A good fit to Sieverts’ law with an H2-permeance of 4.571 × 10−3 mol m−2 s−1 Pa−0.5 at 400 °C, activation energy around 15.031 kJ mol−1, and complete ideal perm-selectivity was observed. The permeate fluxes reached in H2 mixtures with N2, He, or CO2 decreased with dilution and temperature due to the inherent concentration-polarization. The presence of CO in mixtures provoked a higher decrease because of a further inhibition effect. However, the original flux was completely recovered after feeding again with pure hydrogen, maintaining stable operation for at least 1000 h.
... In the last decades, Pd-based supported membranes have been the focus of attention in hydrogen purification due to their relatively high permeability and theoretically infinite hydrogen selectivity [1]. This is due to pure Pd and Pd-based alloys are well-known to reversibly absorb and transport hydrogen [2]. ...
... This is due to pure Pd and Pd-based alloys are well-known to reversibly absorb and transport hydrogen [2]. Among all the membrane synthesis methods reported so far (including electroless plating, magnetron sputtering, chemical vapor deposition), electroless-plating is one of the most widely used techniques [1,3,4]. It presents several advantages over other techniques, e.g. ...
PdAu and PdAuAg alloy membranes were synthesized on top of YSZ-modified planar porous stainless steel supports by electroless deposition. A uniform, continuous and smooth coverage was observed after three YSZ deposition calcination cycles, with neither uncovered zones nor detachment after calcination at 673 K. Through this modification procedure, we could obtain thinner and more robust PdAu and PdAuAg alloy membranes. These membranes showed higher hydrogen permeance during the evaluations (623-723 K) and high thermal stability during the long-term stability test. The PdAu and PdAuAg membranes exhibited a good performance during the exposure to CO/H2O steam mixture. Contrary to the PdAu membrane, where no significant change on the atomic surface composition was observed, silver segregation was evidenced on the PdAuAg membrane after exposure to CO/H2O steam mixture.
... Most researchers stand up for preparing composite membranes in which a thin Pd or Pd-alloy film is stacked onto porous supports [12], usually formed by agglomeration and sintering of alumina or stainless steel (SS) particles [13]. The support choice determines the final properties of the H 2 -selective Pd films in terms of morphology, homogeneity, adherence, and thickness [14]. ...
... In general, ceramic supports make the preparation of ultrathin Pd layers easier due to their lower external roughness and high-controlled pore distribution with sizes up to a few nanometers. However, these supports show important concerns related to their resistance against thermal cycles and to be fitted in a membrane reactor device [13]. On the contrary, the similar thermal expansion coefficient between Pd and SS guarantees a suitable membrane performance and sealing in most of the typical industrial devices. ...
H2-selective composite membranes, particularly those based on palladium films deposited onto porous stainless-steel supports, represent a promising technology to be practically included in both independent devices and membrane reactors. To reach thin H2-selective films and hence high permeance values, the use of a wide variety of intermediate layers is usually adopted in the literature. However, an agreement about the best solution is not found up to now. In this context, the current study presents the use of Ordered Mesoporous Ceria (OMC) particles as intermediate layer for the improvement of permeation properties of Electroless Pore-Plated (ELP-PP) Pd-composite membranes. OMC was obtained by nanocasting from SBA-15 as temporary template and cerium nitrate (III) hexahydrate as metal precursor. Resultant OMC particles have around 100 nm spherical diameter, an average pore-size diameter of 10–12 nm, and a total BET surface of around 134 m2/g. This material was next deposited onto the external surface of tubular Porous Stainless-Steel (PSS) supports by vacuum-assisted dip-coating (VA-DC) to form an intermediate layer that makes the preparation of a defect-free and thin Pd-film easier. This procedure allows the preparation of Pd composite membranes (OMC-Pd) with Pd thicknesses around 10 μm, H2 permeances of 1.03·10−3 mol m−2 s−1 Pa−0.5 at 400 °C, and high ideal selectivity αH2/N2 ≥ 24,000. It should be noted that H2 permeance has been increased up to 6 times in comparison with other ELP-PP membranes without any intermediate layer and 2 times in contrast to membranes containing dense CeO2 particles instead of the mesoporous ones for the intermediate layer. Moreover, Pd-membranes so prepared (OMC-Pd) have shown excellent mechanical resistance in a wide variety of operating conditions such as temperature, pressure, and permeate flux direction, maintaining a high H2-selectivity without delamination or peeling. These properties were also maintained in case of feeding different H2/N2 mixtures, where the concentration-polarization effect seems to stabilize for lower H2 concentration values in the feed stream, not being noticeably influenced by temperature.
... In that sense, the minimum thickness required to prepare an electroless plated Pd membrane on porous support was indicated by Mardilovich et al. [36] to be about threefold as the average size of the largest pores. Despite Vycor glass being one of the initial porous supports used by electroless plating for Pd [37], the use in most of the science-based publications in this field is ceramic materials [38,39], sintered porous metals [38,40] or intermetallic compounds [41] J o u r n a l P r e -p r o o f Therefore, this study tailors the effect of porosity to describe the atomistic interaction and separation mechanism between H 2 /CH 4 gas molecules and Pd nanoporous membranes using molecular dynamics. our research focuses on the fundamental systems of the molecular mass, molecular kinetic and gas interaction with the palladium membrane surface at nanopore border to investigate the H 2 /CH 4 separation efficiency through engineered porosity of 0.1% to 2.2% on the surface of Pd membrane. ...
... In that sense, the minimum thickness required to prepare an electroless plated Pd membrane on porous support was indicated by Mardilovich et al. [36] to be about threefold as the average size of the largest pores. Despite Vycor glass being one of the initial porous supports used by electroless plating for Pd [37], the use in most of the science-based publications in this field is ceramic materials [38,39], sintered porous metals [38,40] or intermetallic compounds [41] J o u r n a l P r e -p r o o f Therefore, this study tailors the effect of porosity to describe the atomistic interaction and separation mechanism between H 2 /CH 4 gas molecules and Pd nanoporous membranes using molecular dynamics. our research focuses on the fundamental systems of the molecular mass, molecular kinetic and gas interaction with the palladium membrane surface at nanopore border to investigate the H 2 /CH 4 separation efficiency through engineered porosity of 0.1% to 2.2% on the surface of Pd membrane. ...
We conducted molecular dynamic (MD) calculations to explore the efficiency of H2/CH4 separation through nanoporous palladium membrane. A palladium membrane with engineered-porosity of 0.1% to 2.2% is used in our model of gas component separation from a mixture. We use computations of molecular dynamics to measure many trajectories of the molecules and thereby collect low statistical uncertainty projections of the gas flow rates. Our simulations demonstrate that high porosity palladium membranes are permeable to both gasses. As the porosity decreases, the permeability of larger molecules greatly reduced, which contributes to an exclusion effect of molecular size for a range of porosity that can permit smaller molecules. This implies that the determined porosity can achieve high selectivity in the separation of gas molecules while the desired gas molecules exhibiting high permeability. We also found that external driving force has a good effect on hydrogen permeation mechanism through a membrane. The gas flux of hydrogen levels increases as the pressure difference increases. The real mechanism of hydrogen permeability can also be visualized through our simulation, showing time dependence of flux, selectivity and pressure dependence. This work is expected to provide the framework for the development of an energy-efficient palladium-based gas separation system.
... ELP based on autocatalysis homogenous deposition of metal on the target surface [25]. Thin metal film deposits on the substrate without using any external source such as electricity, electrodes by cause of this reduces the operating cost. ...
... Trying to address this problem, the ELP-PP technique investigated to reduce generated defects on the membrane. This technique aims to, incorporates palladium nanoparticles inside the pores of support material and prepared a completely dense membrane [25]. Permeability and selectivity of hydrogen also improve ( Table 2). ...
In recent decades, hydrogen has gained renewed and growing interest all over the world as a high-quality and renewable energy carrier, mostly due to advances in fuel cells and environmental issues including climate change. This review explains the hydrogen separation membranes with various fabrication methods as well as different membrane supports, and it also covers a different type of membrane for hydrogen separation. In recent, the researcher placed an interest in the study of the hydrogen selectivity and permeability of the membranes. It concluded that the composite palladium membrane developed by electroless pore plating fabrication technique on porous stainless steel has found a promising application for hydrogen-selective membranes. The use of the palladium membranes in industries has some associated problems such as high cost of pure Pd-based membrane, membrane fouling, and durability of the membrane.
... Chen et al. [66] Electroless plating is very similar to electroplating which involves the use of reducing agents rather than the electrical current in the deposition process. Therefore, it is possible to extend the choice of substrates with non-conductive materials such as porous Al2O3 [69,70]. Chen et al. [66] compared Pd membranes produced via both electroplating and electroless deposition. ...
... Metallic separation membranes basically fall into three groups; face-centered cubic (f.c.c.), body-centered cubic (b.c.c.) and amorphous membranes. Among them, Pd and its alloys [56,69,[149][150][151][152][153], i.e. f.c.c. based membranes are attractive due to their high hydrogen permeability and selectivity at operating conditions. ...
... Therefore, hydrogen has to diffuse through a thicker effective distance in the Pd bulk after being dissociated onto its surface. Alique et al. [54] described a similar behavior for ELP-PP membranes prepared onto asymmetric alumina supports, in which they exhibited permeate H 2 fluxes equivalent to other thicker membranes due to a greater infiltration into the pores of the substrate. ...
This work addresses the use of TiO2-based particles as an intermediate layer for reaching fully dense Pd-membranes by Electroless Pore-Plating for long-time hydrogen separation. Two different intermediate layers formed by raw and Pd-doped TiO2 particles were considered. The estimated Pd-thickness of the composite membrane was reduced in half when the ceramic particles were doped with Pd nuclei before their incorporation onto the porous support by vacuum-assisted dip-coating. The real thickness of the top Pd-film was even lower (around 3 μm), as evidenced by the cross-section SEM images. However, a certain amount of palladium penetrates in some points of the porous structure of the support up to 50 μm in depth. In this manner, despite saving a noticeable amount of palladium during the membrane fabrication, lower H2-permeance was found while permeating pure hydrogen from the inner to the outer surface of the membrane at 400 °C (3.55·10⁻⁴ against 4.59·10⁻⁴ mol m⁻² s⁻¹ Pa−0.5). Certain concentration-polarization was found in the case of feeding binary H2–N2 mixtures for all the conditions, especially in the case of reaching the porous support before the Pd-film during the permeation process. Similarly, the effect of using sweep gas is more significant when applied on the side where the Pd-film is placed. Besides, both membranes showed good mechanical stability for around 200 h, obtaining a complete H2/N2 ideal separation factor for the entire set of experiments. At this point, this value decreased up to around 400 for the membrane prepared with raw TiO2 particles as intermediate layer (TiO2/Pd). At the same time, complete selectivity was maintained up to 1000 h in case of using doped TiO2 particles (Pd–TiO2/Pd). However, a specific decrease in the H2-permeate flux was found while operating at 450 °C due to a possible alloy between palladium and titanium that is not formed at a lower temperature (400 °C). Therefore, Pd–TiO2/Pd membranes prepared by Electroless Pore-Plating could be very attractive to be used under stable operation in either independent separators or membrane reactors in which moderate temperatures are required.
... On the contrary, the stainless steel supports offer high mechanical resistance to handle and a thermal expansion coefficient very close to that of the palladium film, thus ensuring the lifespan of composite-membranes despite suffering thermal stress. However, their morphological surface properties are less favorable to reach ultra-thin Pd-film without further modifications (Alique et al., 2016(Alique et al., , 2018b. In this context, each author bet on one particular alternative based on their particular interests. ...
Pd-based membranes are attracting great attention to reach ultra-pure hydrogen in independent separators or combined with catalysts in membrane reactors. Many advances have been proposed for their fabrication over the last few years, reaching relatively thin Pd-films onto porous substrates with high permeation capacities and mechanical stability, although their commercialization and penetration in the industry are still scarce. At this point, it is important to complete all these technological advances with data about related economic and environmental implications during their fabrication to detect possible bottlenecks and select the best strategy. In this context, the current study presents for the first time a life cycle assessment focused on the preparation of Pd-based composite-membranes by Electroless Pore-Plating (ELP-PP). Two different types of composite membranes supported onto porous stainless steel tubes are analyzed, including or not an additional CeO2 intermediate layer between the support and the Pd-film. Precise experimental data of the fabrication process at laboratory-scale were considered to account for both materials and energy requirements. Thereafter, the environmental impacts were estimated through ReCiPe methodology by using the software Simapro 8.5. The results evidence that climate change (CC), human toxicity (HT), acidification (AC), freshwater ecotoxicity (FWE), metal depletion (MD) and fossil fuel resources depletion (FD) are the most relevant environmental impacts generated during the manufacturing of the Pd-based membrane. Under this perspective, palladium deposition appears as the manufacturing step with the highest impacts. It can be explained by the metal consumption and the high-energy consumption required for deposition cycles. Thus, the electricity mix of the country where the factory is located is critical to minimize the environmental impacts. For this reason, European countries are expected to be the most favorable ones for membrane fabrication. Finally, comparing both membrane types (with or without a CeO2 intermediate layer), it can be stated that the incorporation of the ceramic layer noticeably reduces the necessary amount of Pd to reach a fully dense membrane and therefore the associated environmental impacts.
... Finally, substrate activation with both Pd nuclei and Pd deposition was carried out based on the well-established electroless pore-plating alternative [12,17,33]. Accordingly, the solutions containing the Pd source and the reducing agent were fed from opposite sides of the support with the aim that they will be present and reactive just in the pores or surrounding areas [30,34,35]. ...
Hydrogen promotion as a clean energy vector could provide an efficient strategy for realizing real decarbonization of the current energy system. Purification steps are usually required in most H2-production processes, providing the use of Pd-based membranes, particularly those supported on porous stainless steel (PSS), important advantages against other alternatives. In this work, new composite membranes were prepared by modifying PSS supports with graphite, as an intermediate layer, before incorporating a palladium film by electroless pore-plating. Fully dense Pd layers were reached, with an estimated thickness of around 17 µm. Permeation measurements were carried out in two different modes: H2 permeation from the inner to the outer side of the membrane (in-out) and in the opposite way (out-in). H2 permeances between 3.24 × 10 −4 and 4.33 × 10 −4 mol m−2 s−1 Pa−0.5 with α H2/N2 ≥ 10,000 were reached at 350-450 • C when permeating from the outer to the inner surface. Despite a general linear trend between permeating H 2 fluxes and pressures, the predicted intercept in (0,0) by the Sieverts' law was missed due to the partial Pd infiltration inside the pores. H2-permeances progressively decreased up to around 33% for binary H2-N2 mixtures containing 40 vol% N2 due to concentration-polarization phenomena. Finally, the good performance of these membranes was maintained after reversing the direction of the permeate flux. This fact practically demonstrates an adequate mechanical resistance despite generating tensile stress on the Pd layer during operation, which is not accomplished in other Pd membranes.
... Electroless plating was used to deposit the Pd membranes on porous ceramic/Ti-Al alloy supports (Alique et al., 2016). Prior to the preparation of the palladium membranes, the surface of the Ti-Al alloy supports was activated and sensitized to seed Pd nuclei (Keuler et al., 2002). ...
High stability Pd/ceramic/Ti-Al alloy composite membranes were prepared by electroless plating. Ceramic membranes fabricated by an in situ oxidation method were used as an inter-diffusion barrier between the Pd layer and the Ti-Al alloy support of the membranes to prevent intermetallic diffusion. The stabilities of the ceramic membranes at high temperatures in an H2 atmosphere were investigated. The permeation performances and stabilities of the Pd/ceramic/Ti-Al alloy composite membranes were also studied. The results showed that the thickness, pore size, and microstructure of the ceramic membranes did not change significantly after the treatment in an H2 atmosphere at high temperatures, indicating that the ceramic membranes prepared by the in situ oxidation method were stable in an H2 atmosphere at high temperatures. The thickness of the Pd layer was ~13 μm. The hydrogen permeability and H2/N2 selectivity of the Pd composite membranes at 773 K were 2.13 × 10−3 mol m−2 s−1 Pa−0.5 and 600, respectively. In addition, the H2 flux, N2 flux, and H2/N2 selectivity of the composite membranes remained nearly constant over three heat cycles (under the same conditions), indicating that the structures of the Pd/ceramic/Ti-Al alloy composite membranes were stable.
... Considering these potential benefits as well as the large amount of previous experience on designing dense metal membranes modules for hydrogen purification [36][37][38], the research group headed by Tosti also investigated the exploitation of OMW for hydrogen production in palladium membrane reactors [24,25,39]. Palladium-based dense membranes are a widespread alternative for hydrogen separation in independent equipment or coupled with a catalytic chemical reaction in a membrane reactor at moderate-high temperatures with an extremely elevated perm-selectivity, up to infinite for completely free-defect systems [33,[40][41][42]. The hydrogen permeation through these membranes can be divided in different steps, including transport in the gas phase, adsorption on the membrane surface, hydrogen dissociation, diffusion through the bulk metal membrane, hydrogen recombination, and desorption on the contrary membrane side. ...
Olive mill wastewater (OMW) presents high environmental impact due to the fact of its elevated organic load and toxicity, especially in Mediterranean countries. Its valorization for simultaneous pollutants degradation and green energy production is receiving great attention, mainly via steam reforming for hydrogen generation. Following previous works, the present research goes into detail about OMW valorization, particularly investigating for the first time the potential benefits of OMW–bioethanol mixtures co-reforming for ultra-pure hydrogen production in Pd-membrane reactors. In this manner, the typical large dilution of OMW and, hence, excess water can be used as a reactant for obtaining additional hydrogen from ethanol. Fresh OMW was previously conditioned by filtration and distillation processes, analyzing later the effect of pressure (1–5 bar), oxidizing conditions (N2 or air as carrier gas), gas hourly space velocity (150–1500 h−1), and alcohol concentration on the co-reforming process (5–10% v/v). In all cases, the exploitation of OMW as a source of environmentally friendly hydrogen was demonstrated, obtaining up to 30 NmL·min−1 of pure H2 at the most favorable experimental conditions. In the membrane reactor, higher pressures up to 5 bar promoted both total H2 production and pure H2 recovery due to the increase in the permeate flux despite the negative effect on reforming thermodynamics. The increase of ethanol concentration also provoked a positive effect, although not in a proportional relation. Thus, a greater effect was obtained for the increase from 5% to 7.5% v/v in comparison to the additional improvement up to 10% v/v. On the contrary, the use of oxidative conditions slightly decreased the hydrogen production rate, while the effect of gas hourly space velocity needs to be carefully analyzed due to the contrary effect on potential total H2 generation and pure H2 recovery.
... However, temperature is limited by the thermal stability of the H 2 -selective membrane. Pd-based membranes are prepared onto supporting materials and experimentally they are used in the typical range of 400-550 • C to prevent possible damages on the composite structure, although it is expected to resist slightly higher temperatures [33,34]. In this manner, it is also possible to find several works in which these membranes operate at temperatures up to 650 • C with satisfactory results in terms of mechanical stability [35][36][37][38]. ...
Hydrogen, as an energy carrier, can take the main role in the transition to a new energy model based on renewable sources. However, its application in the transport sector is limited by its difficult storage and the lack of infrastructure for its distribution. On-board H2 production is proposed as a possible solution to these problems, especially in the case of considering renewable feedstocks such as bio-ethanol or bio-methane. This work addresses a first approach for analyzing the viability of these alternatives by using Pd-membrane reactors in polymer electrolyte membrane fuel cell (PEM-FC) vehicles. It has been demonstrated that the use of Pd-based membrane reactors enhances hydrogen productivity and provides enough pure hydrogen to feed the PEM-FC requirements in one single step. Both alternatives seem to be feasible, although the methane-based on-board hydrogen production offers some additional advantages. For this case, it is possible to generate 1.82 kmol h−1 of pure H2 to feed the PEM-FC while minimizing the CO2 emissions to 71 g CO2/100 km. This value would be under the future emissions limits proposed by the European Union (EU) for year 2020. In this case, the operating conditions of the on-board reformer are T = 650 °C, Pret = 10 bar and H2O/CH4 = 2.25, requiring 1 kg of catalyst load and a membrane area of 1.76 m2.
... When the activated ZTA particles were immersed in the electroless nickel plating solution, the deposition mechanism could be explained by the following features: The oxygen atoms of the reducing agent H 2 PO 2 were replaced during activated treatment in the process of electroless plating. H 2 originates from the hypophosphite reducing agent [20,21]. These characteristics ensure that nickel deposition is always accompanied with H 2 evolution. ...
With the aim to effectively improve the interface between ZrO2 toughened Al2O3 (ZTA) particles and metal matrix, nickel was deposited on the surface of ZTA particles by electroless plating method. Formation mechanism of nickel coating and effects of the solution pH, loading capacity of ZTA particles and temperature on the nickel deposition were investigated. Microstructures, thickness and element distributions of nickel coating were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results showed that the nickel was successfully deposited on the surface of ZTA particles by electroless plating without noticeable defects. The process of electroless nickel plating could be explained by combination of atomic hydrogen and electrochemistry theories. The interfacial nucleation of nickel is easier to form than spontaneous nucleation in the solution. Deposited Nickel has priority on the surface of ZTA particles comparing to that in solution. The optimal conditions to coat nickel on the surface of ZTA particles are: solution pH 4.7–4.8, loading capacity 15–20 g/L, and electroless plating temperature 85 °C. The ZTA particle reinforced iron matrix composites prepared by powder metallurgy could have better interfacial bonding between ZTA particle and iron matrix because of the nickel coating on the surface of ZTA particle. Nickel diffuses into the iron matrix during the sintering preparation of composite materials. The interface between ZTA particle and iron matrix presents the evidence of non-chemical bonding.
... Recently, the ELP-PP technique is also applied onto ceramic substrates, evidencing the possibility to generate an ultrathin selective layer by this technique over a support with vary small pores and a narrow pore size distribution, although the presence of palladium in both internal pores and external surface is still presented [130]. Other interesting alternative is proposed by Pacheco Tanaka et al. [40,131,132] that consists on the preparation of pore-filled membranes by the incorporation of the selective metal between two ceramic layers in a sandwich-type structure. ...
Coatings and thin film materials are employed in many different industrial fields for decades, mainly for protective purposes. This large experience provokes that, currently, a wide variety of technologies for preparation and characterization of these materials are available. Particularly, focusing on energy and environmental applications, three main film types can be distinguished: (1) materials with catalytic activity for hydrogen production, (2) membranes for hydrogen separation or CO2 capture, and (3) coatings for some specific fuel cell components. Membranes are especially relevant for hydrogen separation from other gases after the production unit or combining both production and separation steps in a unique equipment, the membrane reactor. The last case represents a significant advance in terms of process intensification, increasing the hydrogen production rate with a high purity and saving costs. In the last years, the relevance of these membrane materials has significantly increased, as can be denoted by the large number of published manuscripts in indexed scientific journals of high impact. In this context, this chapter summarizes the main advances in thin film membranes towards energy and environmental applications, including both preparation strategies and the most common characterization techniques. The production of all these thin films, independently of the particular application, can be carried out by different physical-chemical alternatives such as Sol–Gel methods, Electrodeposition, Electroless Plating, Physical Vapor Deposition, Chemical Vapor Deposition, Atomic Layer Deposition, or Molecular Beam Epitaxy, achieving thicknesses ranged from the nanometer scale to some microns. Each technique presents advantages and disadvantages that have to be taken into account for final applications. Moreover, the structure of the film should also be considered, being possible to distinguish amorphous or crystalline materials. All these films, independently of the composition, structure, or production technique, are usually prepared over a substrate material. Thus, the original coating surface properties can affect in a significant grade to the final properties of the film and many researchers focus their efforts on studying these effects and developing new strategies to improve the final quality of films in terms of homogeneity, thickness reduction, thermal and mechanical resistance, and adherence.
... The PdeAg alloy with silver content around 25 wt.% exhibits the best hydrogen permeability and mechanical strength, with heavily reduced embrittlement effects [10]. Therefore, PdeAg alloys with this composition are now commercially available for the manufacturing of membrane devices used for both hydrogen separation (permeators/diffusers) and hydrogen production (membrane reactors) [11,12]. The further optimization of the Pdmembrane devices requires a deep understanding of the chemicalephysical properties of the hydrogenated Pd-alloys in the range of the operating conditions typical of the membrane processes (temperature up to 400e450 C and pressure up to 10e20 bar). ...
Experiments of deuterium absorption/desorption were performed on a foil of Pd77Ag23 (wt%) alloy in the 78–196 °C temperature and 1–4 bar pressure ranges under the neutron beam at ILL (Grenoble). Powder diffraction patterns were collected on the D1B diffractometer (λ = 1.2871 Å) on the Pd0.772Ag0.228Dν deuterated alloy, and the face-centred-cubic structure was Rietveld-refined locating the D atom in the octahedral site with a variable occupancy ν (=D/M ratio). The results of ν(T) curves at different p(D2) pressures are discussed in comparison with literature data on H and D absorption into Pdsingle bondAg alloys from thermodynamic measurements. A negative deviation from predictions of Sieverts' law is shown by the dependence of D occupancy on pressure. This effect relates the behaviour of the activity coefficient γD to D–D and D–M interactions in the solid solution phase.
The electroless cobalt plating modification of ZTAp (Zirconia toughened alumina particle) surface and the impact-abrasive wear behavior of ZTAp and NbCp(Niobium carbide particle) reinforced Fe60 matrix composites (ZTAp-NbCp/Fe60) were investigated. Cobalt was deposited on the ZTAp surface by electroless plating (modified ZTAp) to strengthen the interface bonding between ZTAp and matrix. Both composites reinforced with modified ZTAp and unmodified ZTAp were prepared through vacuum sintering, and their impact-abrasive wear performance testing under different impact energy were carried out with MLD-10 impact-abrasive wear tester. The results showed that the optimal conditions to coat cobalt on the ZTAp surface included of: initial pH value of plating solution was 9, the concentration of NaH2PO2.H2O, and C4H4Na2O6.2H2O was 0.25 mol/L and 0.8 mol/L respectively. The impact-abrasive wear-resistant performance of the modified ZTAp reinforced Fe60 matrix composite was significantly enhanced by more than 35% than the unmodified ZTAp reinforced composite under high energy impact condition. Cobalt and liquid iron had good wettability, and the interface strength significantly increased. During the sintering process, cobalt diffused at the interface, thus leading to a significant binding force between ZTAp and the iron matrix interface. This study suggested that the modified ZTAp could have a better and longer protective effect than unmodified ZTAp on the matrix.
A phase-inversion approach was used to manufacture Al2O3 hollow fibre supports, which were then sintered at 1723 K. The electroless plating technique is developed to prepare palladium-coated Al2O3 hollow fibre membranes for hydrogen separation. Three different scaling-up configurations were produced and tested: single membrane, membrane unit obtained by assembling three membranes, and advanced membrane module obtained by assembling twelve replaceable membranes. The hydrogen flux was investigated under vacuum and without vacuum using a feed gas of pure H2 (100%) and a binary feed gas mixture of H2 (80%) and CO2 (20%) at different feed gas pressures (100-800 kPa), feed gas rate (0.2-6.0 L min⁻¹), and temperature (673-723 K). The hydrogen flux increases from 0.2162 mol m⁻² s⁻¹ (feed gas pressure=600 kPa, feed gas rate=0.2 L min⁻¹) to 0.4487 mol m⁻² s⁻¹ (feed gas pressure=800 kPa, feed gas rate=6.0 L min⁻¹) under the binary gas mixture at 723 K by switching from a single to the advanced membrane module, while the hydrogen purity remains above 97.5% throughout the experiment. Some aspects about the scalability of palladium-coated Al2O3 hollow fibre membranes for hydrogen separation are discussed.
The circular economy is presented as a cutting-edge perspective to maintain economic development while preserving the environmental quality for future generations. This strategy aims to reduce wastes and residual products as much as possible by their valorization into new valuable products or energy, thus contributing to avoiding depletion of natural resources. This perspective is especially interesting by considering the increasing levels of anthropogenic CO2 emissions during last decades and their direct influence on global warming. The current chapter goes into deep on this concern, analyzing the simultaneous production of valuable energy resources and food-grade CO2. The purity of both compounds is a key factor for the intended applications and selective membranes could provide important advantages against other alternatives. After some general concerns about CO2 emissions, capture processes, and biogas production, two general strategies are distinguished: (1) separation of CH4 and CO2 directly coming from biogas and (2) biogas valorization into green-H2 and further purification. In this context, a general overview of both strategies is pointed out to describe current technological approaches and typically required operating conditions. Moreover, dedicated sections for the most relevant insights reached for a wide variety of CO2 and H2-selective membranes have been addressed.
Understanding the effects of thin film electroless deposition parameters at nanoscale is crucial for complete understanding and control of the thin film deposition process. In this study, we investigated and optimized the effect of PdCl2 precursor concentration on the nanoscale surface morphology of electroless deposited Pd thin film. The FESEM characterization of plain substrates showed that the dominant features of plain alumina substrates were terraces, steps and bumpy microstructures and the final surface morphologies of the deposited Pd thin film was strongly dependant on the surface morphologies of the substrate. FESEM characterization results of seeding technique displayed a thin film of Pd nanoparticles on the surface of the alumina substrate. The number of times that the seeding process was carried out was optimized at five seeding times using hydrazine as a reducing agent. FESEM characterization revealed that the nanoscale surface morphology of the Pd thin film was strongly dependent on PdCl2 precursor concentration. Three types of secondary nanoscale surface morphologies formed were nanorods, nanoflakes and flower-like Pd nanostructures at various concentrations. The nanoflake surface density was strongly dependent on PdCl2 precursor concentration. Results of this research provided a foundation and method to tailor the nanoscale surface morphology to the specific requirement of surface dependent processes or reactions. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.
Hydrogen production from residual biomass and wastes is a sustainable approach for reducing their final accumulation in landfills and simultaneously a very promising alternative for the energy recovery. Most developed technologies to produce H2 from residual biomass and wastes are reviewed in this chapter focusing on the separation/purification of the produced hydrogen. Suitability of both thermochemical and biological technologies for hydrogen production is described, and examples of industrial processes are included. Basics of hydrogen separation/purification with membranes are detailed, and suitable separation technologies for the purification of hydrogen produced from biomass and waste conversion are presented focusing on the most recent advances in Pd-based membranes. The use of membrane reactors in which the traditional chemical reaction is combined to the continuous extraction of the main product with high purity, in this case hydrogen, is particularly interesting, being also addressed the most recent developments in this field.
This paper presents for the first time relevant insights on the influence of tuning porous stainless steel (PSS) supports for the preparation of metallic membranes by the Electroless Pore-Plated (ELP-PP) method. Two types of oxides, used as intermediate layer between the support and the Pd layer, were deposited on raw PSS supports for reducing porosity and surface roughness. Both independent and combined routes were considered: (i) Fe2O3-Cr2O3 oxides obtained by direct calcination of the metallic support and (ii) dense and porous SiO2 deposited by dip-coating (DC) technique. Membranes morphology (porosity and surface homogeneity) was characterized by SEM and EDX analysis and gravimetric analysis was used to calculate the total amount of Pd deposited on the external selective layer. Preliminary screening tests suggested that the combined route (calcination followed by DC of SiO2) did not show evident beneficial effects for the preparation of Pd films by ELP-PP. Based on those results, four tubular membranes were fabricated using independent routes: one by calcination and other three by DC, including 1, 2 and 3 layers of SiO2. The first one, including a Fe2O3-Cr2O3 oxides intermediate layer, maintained its mechanical integrity when tested up to ΔP = 4 bar. On the other hand, it was observed that the use of SiO2 intermediate layers noticeably affects the ELP-PP method. Extensive characterization of fresh and used samples has been performed in order to understand the influence of the oxides in the final preparation of a continuous and thin layer of Pd. Results obtained evidenced that the most adequate sample was the support prepared with 2 layers of SiO2 and studies continue to improve membrane stability and to reduce thickness.
Pd-based membranes prepared by pore-plating technique have been investigated for the first time under fluidization conditions. A palladium thickness around 20 μm was achieved onto an oxidized porous stainless steel support. The stability of the membranes has been assessed for more than 1300 h in gas separation mode (no catalyst) and other additional 200 h to continuous fluidization conditions. Permeances in the order of 5·10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ have been obtained for temperatures in a range between 375 and 500 °C. During fluidization, a small decrease in permeance is observed, as consequence of the increased external (bed-to-wall) mass transfer resistances. Moreover, water gas shift (WGS) reaction cases have been carried out in a fluidized bed membrane reactor. It has been confirmed that the selective H2 separation through the membranes resulted in CO conversions beyond the thermodynamic equilibrium (of conventional systems), showing the benefits of membrane reactors in chemical conversions.
This research introduces an unabridged two-dimensional mathematical model for H2 recovery from an ammonia plant's purge stream through two-stage industrial polyimide hollow fiber (PIHF) membrane modules. According to validated results, the effect of operational and geometrical parameters beside flow patterns was investigated on H2 recovery potential at first and second stages of PIHF membrane modules. H2 recovery at first and second separation stages decreased from 5.887% to 4.82% and from 99.57% to 99.05%, respectively when the feed mass flow rate increased from 0.3847 to 0.5014 kg/s at constant pressure of 97.5 barg. However, a reverse trend between H2 cumulation and feed pressure was observed at constant feed mass flow rate for each separation steps. As fiber length increased from 2 m to 3.06 m, the H2 recovery percentage grew up at each of those separation stages at fixed operational conditions. Counter-current flow pattern showed higher H2 recovery in comparison with co-current flow arrangement at the same operation conditions due to the higher partial pressure difference. Moreover, a concentration polarization index (CPI) was also described to demonstrate the scope of polarization. Evaluations depicted that CPI enhanced with increasing the feed pressure and serious concentration polarization arose at higher feed pressures.
Palladium composite membrane with excellent stability was successfully prepared using the electroless plating (ELP) route on a porous stainless steel (PSS) support for hydrogen separation. In order to modify the average pore size of PSS support and to prevent inter-metallic diffusion, the NaY zeolite layer was coated on the PSS support with the seeding and secondary growth method. A high-temperature membrane module was designed by Solid work software and fabricated from 316 L stainless steel with a knife-edge seal. The microstructures and morphologies of the samples were analyzed using XRD, BET, AFM, FESEM and EDX techniques. Permeation experiments were carried out with binary mixtures of H2/N2 with various ratios (90/10, 75/25 and 50/50) and pure H2 and N2 at different temperatures (350, 400 and 450 °C) and feed pressures (200–400 kPa). Hydrogen permeation tests showed that the membrane with a thickness of about 7 μm had a hydrogen permeance of 6.2 × 10⁻⁴ mol m⁻² s⁻¹ Pa−0.5 with an ideal H2/N2 selectivity of 736, at 450 °C. In addition, the results of stability tests revealed that the membrane could remain stable during a long-term operation by varying temperature and feed gases.
The novel paradigm of distributed energy production foresees the production of hydrogen from methane and biomasses in small plants, which may take advantage from membrane-based processes. By means of a modeling approach, this paper compares the energy efficiency of two membrane-based processes to produce H2 from methane steam reforming. The two-step process (TS) envisages a high temperature classical reactor and a following WGS stage in a membrane reactor, while the alternative process uses a simple packed-bed membrane reactor (MR). Both processes show a general increase of H2 production and energy efficiency with the pressure and a maximum energy efficiency for S/C of 4, while the increase of the space velocity reduces the performances of the MR. The results show that the TS process performs better than the studied MR and that the maximum energy efficiency of both processes is between 30 and 40%. A comparison with the literature shows that the TS process may achieve similar performances respect to an intensified MR.
An investigation was carried out into Nb-Pd-Ti ternary system to determine possible body-centered cubic
(b.c.c.) membranes that can be used for hydrogen separation. A library of thin films was produced covering
the greater portion of Nb-Pd-Ti ternary diagram using a combinatorial approach. The library was screened
both structurally and in terms of a reactivity index defined as the ratio of the resistivity measured in the
films under hydrogen and argon. The study showed that a substantial portion of compositional field
stretching from Nb to Ti yield thin films with b.c.c. structure. The evaluation based on the reactivity index
showed a narrow region close to Nb corner as possible compositions for separation membranes. The b.c.c.
field was also screened with regard to the lattice volume so as to identify regions of acceptable hydrogen
solubility. The superposition of two maps; one reactivity index and the other lattice volume yield a field
32 < Nb < 41, 27 < Pd < 44, 20 < Ti < 38 as possible compositions for separation membranes.
Palladium coated porous alumina ceramic membrane tube was obtained using a combination of sol-gel process and electroless plating technique. The thickness, structure and composition of palladium-alumina composite membrane were analyzed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and atomic force microscopy (AFM). Palladium particle size was 6.18 to 7.64 nm. Palladium membrane with thickness of approximately 301.5 to 815.1 nm was formed at the outer surface of the alumina layer. EDX data confirmed the formation of palladium-alumina membrane containing 45% of palladium. From this research it shows the combination of sol-gel process and electroless plating technique with one-time coating can produce a homogeneous and smoother palladium nano layer film on alumina substrate.
There are currently a number of fundamental sustainability challenges which impact upon a variety of social, economic and environmental domains. One of these is energy supply where significant uncertainties exist in how best to tackle the multi-dimensional problems being raised in each of these areas. Because of this uncertainty, normative approaches to the development of low-carbon technological innovations will always contested. Q methodology, an established form of discourse analysis, was therefore undertaken in order to explore the competing visions and perspectives of experts working in the radical technological field of hydrogen production from waste. This is the first time that the perspectives of expert stakeholders involved in a technological innovation system (TIS) for hydrogen have been investigated using Q methodology. These contributors revealed limited awareness of each other. From this, we conclude that improvements need to be made to policies aimed at boosting the networks being used by the broad community of hydrogen-from-waste professionals. In particular, efforts should be centred on the skills required to manage the dissemination of communication regarding successful innovation.
Cu/Pd alloys were deposited onto Si(100) and SiO2 (fused silica) substrates by MOCVD from PdL2�CuL2, (L¼2-methoxy-2,6,6-trimethylheptane-3,5-dionate),a new single source bimetallic precursor. Deposi-
ion was performed at 10 Torr in a temperature range between 200°C and 350°C and was assisted by vacuum ultraviolet (VUV) irradiation of the precursor vapor from an excimer Xe-lamp. It was shown that
the elementaland phase composition of the films can be controlled by varying the deposition temperature and by stimulating by VUV the precursor decomposition. The bulk compositional properties
of the obtained films confirmed the feasibility of proposed approach and precursor to prepare Pd alloy membrane materials by the CVD method.
Thin and thermally stable Pd membrane can be successfully coated on "defect-free" alumina sol/γ-Al2O3 interlayer with a controlled pore-mouth size of ca. 0.08 μm obtained by modified sol-gel method on macroporous α-Al2O3 support. The "modified" bubble-point method and home-developed "modified" liquid-liquid displacement method were used to check the size and amount of defects (>3 μm), and characterize pore-mouth size distribution of interlayers, respectively. The modified sol-gel method shows superiority in smoothening out defects and bumps compared to conventional suspended particles sintering method as the incorporation of alumina sol-gel particles can significantly improve the adhesion and dispersal uniformity of γ-Al2O3 particles. The synthesized Pd composite membranes of 4.5 μm thickness exhibit high hydrogen permeance and selectivity compared to similar studies. In addition, the good membrane stability was verified by the long-term operation under hydrogen permeation conditions. This can be mainly ascribed to the formation of defect-free and smooth interlayer which effectively suppress the shear stress between Pd layer and intermediate layer when enduring thermal cycles and hydrogen adsorption and desorption cycles.
The present work focuses on the study of a metallic supported Pd-Ag membrane for high temperature applications with a particular attention to long-term stability. In this work, a metallic supported thin-film Pd-Ag membrane has been tested for more than 800 h and sustained hydrogen perm-selectivities higher than 200000 have been measured. Furthermore, it has been demonstrated that there is no interaction of the membrane with the Ni/CaAl2O4 reforming catalyst particles, thus resulting in a constant permeance in the fluidized bed membrane reactor mode. The membrane has been tested under steam and autothermal reforming of methane conditions and the membrane performance has been quantified in terms of the hydrogen recovery and separation factors demonstrating a good reactor performance accomplishing an enhancement in the process efficiency by in-situ selective H2 separation. A decrease in ideal perm-selectivity has been observed at high temperatures (600 °C). Small defects at the Pd/Ag surface as a result of interaction of the Pd/Ag later with the metallic support have been observed in after test membrane characterization, which provides appreciated information for the improvement in the performance and production of future membranes.
This paper reports the preparation and characterization of thin-film (4-5μm thick) Pd-Ag metallic supported membranes for high temperature applications. Various thin film membranes have been prepared by depositing a ceramic interdiffusion barrier layer prior to the simultaneous Pd-Ag electroless plating deposition. Two deposition techniques for ceramic layers (made of zirconia and alumina) have been evaluated: Atmospheric Plasma Spraying and dip coating of a powder suspension. Initially, the prepared ceramic layers have been characterized for nitrogen permeation at room temperature and surface roughness for the selection of the appropriate type of ceramic layer. The most promising membranes have been tested at 400-600°C for single gas permeation (H2 and N2), and have shown extremely high H2/N2 permselectivities (>200,000).
In on-site hydrogen stations, hydrogen production systems using steam methane reforming and pressure swing adsorption are commonly adopted. However, these systems are too expensive and large for commercial uses. We have earlier developed a compact system of hydrogen production from natural gas using a ceramic-supported Pd membrane module. This system can simultaneously produce and separate pure hydrogen in a single reactor. However, one of the biggest inhibitory factors for hydrogen permeation across a Pd membrane is concentration polarization. Herein, we investigated the effects of linear velocity of feedstock gas on the system's hydrogen production performance, and found that the performance increased on increasing the linear velocity of feedstock gas. This can be explained by the fact that the boundary layer of flow on the membrane became thin, thus decreasing the concentration polarization. We also confirmed the effect of linear velocity on the hydrogen production performance by computer fluid dynamics simulation.
The permeability of a 0.175 mm thick Pd-Ag tubular membrane to pure H2 and binary mixtures of H2/CO or H2/CO2 was studied. The tests were performed in a wide range of temperature (523-723 K) and pressure (200-800 kPa). Pure H2-permeation through a dense metal membrane is described by the Sieverts' law. However, it was already found that the H2 permeation does not follow the Sieverts' law when other components are present in the feed and namely CO or CO2. In this work, it is proposed a new permeation model based on the Sieverts' law considering: i) the mass transfer resistance due to the surface effects and ii) the barrier effect due to the presence of either CO or CO2. The model was successfully validated against experimental data of hydrogen permeation for binary (H2/CO and H2/CO2) experiments for every working temperature and pressure. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
This work addresses the combinatorial plating characteristics of dense Pd/Ni/porous stainless steel (PSS) composite membranes in comparison with Pd/PSS membranes. While Pd/PSS membranes were fabricated using 0.1 pin nominal pore size PSS supports, Pd/Ni/PSS membranes were fabricated using 0.5 and 0.1 pm nominal pore size PSS supports. Both Ni and Pd films were deposited using an identified novel electroless plating process that characterizes the optimal utilization of surfactant, sonication and reducing agent contacting pattern in Pd electroless plating baths. It was observed that the combinatorial plating characteristics for Pd/Ni/PSS membranes were significantly different and poorer in comparison with those obtained for the Pd/PSS membranes. In summary, it has been inferred that the introduction of nickel interdiffusion barrier was not fruitful to reduce the critical thickness of dense Pd film without jeopardizing upon the pore densification. (C) 2014 Published by Elsevier B.V.
The integrity and performance of Pd-based membranes depends heavily on the properties of the substrate. This study presents the development of a new planar Ni-based porous support for Pd composite membranes. Ni powder was mixed with a 0.1 wt% of Ti and Cu, which have different melting temperatures, and was later uniaxially pressed in a metal mold to make substrate. The pressed substrates were heat treated at 1223 K in a hydrogen environment for 2 h. The pore size and its uniformity were determined using a mercury porosimeter. Adding 0.1 wt% of Ti sufficiently prevents sintering of metal and results in a good pore size distribution. With the aid of a pretreatment, i.e., polishing, etching and alumina coating, and electroless plating, a 9.3-μm-thick Pd membrane could be fabricated on the Ni–Ti substrate, exhibiting a H2 permeation flux of ∼0.1445 mol m−2 s−1 with selectivity (H2/He) of ∼1600 at 723 K and a pressure difference of 100 kPa.
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/Al2O3 membranes were prepared by electroless plating. The recycling of palladium was achieved by treating it with HNO3 and HCl–H2O2 agents. Adequate results were achieved using an agent composed of 2 mol/L HCl and 1 mol/L H2O2. The recycled palladium can be utilized in a new plating bath, while the Al2O3 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/Al2O3 membranes are successful and may provide a solution to the current key problems associated with the application of composite palladium membranes.
A laboratory reactor equipped with a Pd-composite membrane prepared by ELP “poreplating”
method (Pd thickness of 10.2 mm) has been used for performing the water gas shift
reaction (WGSR). Reaction experiments were carried out with and without the membrane
at different operating conditions: H2O/CO ratio (1e3), temperature (350e400 �C) and GHSV
(4000e5500 h�1). In all cases, CO conversion was found to be higher when using the
membrane to separate hydrogen. The membrane maintained the integrity with complete
selectivity to H2. The membrane reactor has been modelled using a 2D mathematical
model, capable of modelling the non-ideal flow pattern formed in this type of reactors. The
model predicts the experimental CO conversion with an accuracy of ±10%. The proposed
model was used as a tool in the scale-up of a membrane reactor for the wateregas-shift
reaction (feed: 100 m3/h synthesis gas), designed to achieve high CO conversion (>99%) and
hydrogen recovery (>99.5%). The permeation of hydrogen through the membrane was
found to be ruled by mass transfer in the membrane support and palladium layer
Thin dense palladium (Pd) membranes (approximately 5 mu m) 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 pet meance as high as 78 m(3)/m(2) h bar(0.5) (3.0 x 10(-3) Mol/M-2 s Pa-0.5) and an ideal permselectivity exceeding 400 when the pressure difference was 4 bar and the temperature was 400 degrees C. Moreover, the membranes were stable during long-term temperature cycling performed between room temperature and 400 degrees C over a period of 350 h. 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.
The exact conditions for deposition of pure palladium and pure silver membranes as well as simultaneous, co-deposition of palladium and silver have been identi®ed for a hydrazine-based plating bath containing both palladium and silver precursors. The electroless plating kinetics have been determined for co-deposition of palladium and silver from the mixed plating bath. A mathematical model has been developed to predict the ®lm thickness, plating rate and composition pro®le as a function of plating parameters (i.e., reactant concentrations and time). The evolution of the microstructure and composition of the ®lm during electroless plating has also been investigated. A hydrogen permeation study has demonstrated that the Pd±Ag alloy membrane has superior performance compared to a pure palladium membrane of similar thickness. In addition, the formation of a single-phase alloy results in substantial enhancement in the hydrogen permeation rate. # 1999 Elsevier Science B.V. All rights reserved.
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 mm over oxidized porous
stainless steel support has been prepared. Firstly, permeation measurements with pure
gases (H2 and N2) and mixtures (H2 with N2, 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 2.68e3.96$10�4 mol m�2 s�1 Pa�0.5, while it decreases until 0.66e1.35$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 FeeCr catalyst by using a typical methane reformer outlet (dry basis: 70%H2e18%COe12%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
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.
The performanceofselectivehydrogenpermeationofdifferentPd-containing(Pdthicknessintherange
11–30 μm) structuredmembraneshasbeenstudiedinthiswork.Pdwasdepositedovertubularporous
stainless steelsupportsbythenovelelectrolesspore-platingmethod.Thepermeationpropertiesofthe
membranes havebeentestedatdifferentoperatingconditions:retentatepressure(1–4 bar),temperature
(623–723k)andhydrogenmolarfractionoffeedgas(0.7–1). Acompleteselectivitytohydrogenwas
observed foralltestedconditions,ensuring100%purityinthehydrogenpermeate flux. Permeancesin
the rangeof2.3–6.4�10�4 mol/m2 s Pa0.5 wereobtained,maintainingagoodmechanicalstabilityofthe
composite system.
A mathematicalmodelconsideringthedifferentmasstransferresistancesinseriesofthecomposite
membrane, e.g.Pdactivelayer,poroussupportandgasphase film layer,hasbeenproposedtodescribe
the permeationofhydrogen.Themodelalsoconsiderstheaxialvariationsofhydrogenconcentration
that takesplaceintubularmembranes.Asetofexperimentaldatahasbeenusedto fit theempirical
parametersofthemodel,usinganothersetofexperimentsforvalidatingtheproposedmodel.
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.
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.
The present work was focused on the preparation of palladium alloy membranes and the effect of properties of ceramic support on the composited membrane morphology. Palladium-base membrane is known to have high selectivity and stability for hydrogen separation. In order to increase hydrogen permeation and separation factor, the membrane must be thinner and defect-free. Palladium membrane supported on a porous alumina prepared by electroless plating is the promising method to provide good hydrogen permeability. The alumina tube substrate was pre-seeded by immersing in the palladium acetate solution and followed by reduction in the alkaline hydrazine solution. After that, the deposition of palladium membrane could be achieved from the plating bath containing ethylenediamine tetraacetic acid (EDTA) stabilized palladium complex and hydrazine. The morphology of palladium film was observed to progress as a function of plating time and a dense layer membrane was available after plating for 3 h. The porosity of ceramic support exhibited an effect on the microstructure of deposited film such that the support with low porosity tended to achieve a defect free palladium membrane.
This review highlights various aspects of current palladium membrane research and serves as a comprehensive bibliography covering palladium membrane preparation methods and applications. There are many promising uses for palladium membranes, although widespread use of the available technologies is constrained primarily by the high cost of palladium, lack of durability due to hydrogen embrittlement, and susceptibility to fouling. Various researchers in the field are tackling these problems and fabricating thinner palladium alloy composite membranes that better withstand contaminantion and thermal cycling. What has been accomplished to address these issues and the directions presently being explored are discussed.
Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773K and at pressure differences up to 6bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd–Ag layer (20wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts’ law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance.
Hydrogen transport in Pd-based supported membranes was described by means of a model considering several elementary steps of the permeation process, improving what done by Ward and Dao [1999. Model of hydrogen permeation behavior in palladium membranes. Journal of Membrane Science 153 (2), 211–231] for self-supported membranes. The model includes the external mass transfer in the multicomponent gaseous phases on both sides of the membrane, described by the Stefan–Maxwell equations. The transport of the multicomponent mixture in the multilayered porous support was also considered and described by means of the dusty gas model, which takes into account Knudsen, Poiseuille and ordinary diffusion. The diffusion in the Pd-alloy layer is modeled by the irreversible thermodynamics theory, taking the hydrogen chemical potential as the driving force of the diffusion in the metallic bulk. The interfacial phenomena (adsorption, desorption, transition from Pd-based surface to Pd-based bulk and vice-versa) were described by the same expressions used by Ward and Dao (1999). Thicknesses of 1 and 10μm are considered for the Pd-alloy layer. The asymmetric support consists of five layers, each one characterized by a specific thickness and mean pore diameter. The model separates the permeation steps and consequently their influence, quantifying the relative resistances offered by each of them. Comparison with some experimental data in several conditions in the literature shows a good agreement. The developed tool is able to describe hydrogen transport through a supported Pd-based membrane, recognizing the rate-determining steps (e.g., diffusion in the metallic bulk or in the porous support) involved in the permeation.
A theoretical model is proposed to describe hydrogen permeation in palladium and silver–palladium membranes in presence of a non-inert gas as CO; it is known indeed that hydrogen flux through palladium-based membranes drastically decreases when H2 is fed in mixtures containing carbon monoxide due to the interaction of the latter gas with the membrane surface. To model this process, the adsorption step of the well-known approach suggested by Ward and Dao has been suitably modified, since it must be considered as a competitive adsorption of the different non-inert molecules on the metal interface. In particular, the competitive adsorption of CO and H2 has been examined accounting for the spectrum of information available for CO adsorption on palladium, as well as for hydrogen in palladium and palladium–silver alloys. A validation of the model proposed has been performed through a comparison between several literature data and model calculations, over a rather broad range of operating conditions. A quite good agreement was obtained in the different cases; the model, thus, can be profitably used for predictive purposes.
Hydrogen permeation experiments were performed to evaluate the influence of water vapor on hydrogen permeability in 80–20% by weight Pd–Ag membranes of 2.5 μm thickness. In particular, hydrogen flux was measured in pure hydrogen permeation tests as well as in experiments with binary mixtures containing also nitrogen or water vapor, that were performed at temperatures ranging from 473 to 723 K and at a transmembrane pressure differences up to about 3 bar. The membranes, supplied by NGK Insulator Ltd., Japan, showed a very high hydrogen permeance and lifetime, as well as virtually infinite selectivity (exceeding 10000 for H2–N2 mixtures). The experiments in hydrogen–nitrogen mixtures were carried out at different temperatures, hydrogen concentrations and feed flow rates and confirmed the existence of a non-negligible concentration polarization phenomenon in the experimental module. The gas phase mass transport and the module fluid dynamics were thus analyzed and the dimensionless numbers characterizing these processes were evaluated at the different operative conditions; a linear correlation was found to hold between Sherwood and Péclet numbers. Interestingly, the hydrogen permeate fluxes measured with feeds containing H2–H2O mixtures resulted always lower than those obtained for the nitrogen–hydrogen mixtures performed at the same hydrogen mole fraction and operative conditions: in particular, the hydrogen flux depletion increased with decreasing temperature and/or increasing the concentration of water vapor. All the experimental evidences suggest a clear interaction between water vapor and metallic layer, causing a lower hydrogen adsorption capacity of the membrane surface. That phenomenon is reversible, since the original permeance of the membrane was restored once the water vapor was removed from the feed, and is apparently due to a competitive H2–H2O adsorption on the Pd–Ag surface. The hydrogen flux depletion was then modeled by considering the simultaneous effects of gas phase resistance and competitive adsorption on the surface, obtaining a rather good agreement between experimental data and calculated results.
This review describes palladium and palladium alloy membranes for hydrogen separation prepared by different fabrication methods and using different membrane supports. Several correlations of structure and function for those membranes are provided based on mechanistic considerations of permeance along with structural properties and membrane morphologies. Particular attraction is placed in analysis of the hydrogen permeance and selectivity of membranes reported in recent papers. Composite palladium membranes prepared by the electroless plating technique deposited on alumina substrates are found to be the most promising for practical applications. It is concluded that the prospects for the use of palladium membranes in industrial applications are improving due to extensive research addressing current problems such as durability, hydrogen embrittlement, fouling by hydrocarbons or hydrosulfide compounds, and the high cost of palladium.Graphical abstractA review is presented of Pd and Pd-alloy membranes concentrating on the reporting of general correlations found in permeance behavior versus physical properties. The major methods of preparation are described and contrasted.View high quality image (178K)Highlights► Pd-based membrane systems for H2 separation in bulk and thin film form are reviewed. ► Concentration is placed on correlations between properties and performance. ► Electroless plating (ELP), chemical vapor deposition (CVD) and electroplating (EPD).
The hydrogen permeation and stability of tubular palladium alloy (Pd–23%Ag) composite membranes have been investigated at elevated temperatures and pressures. In our analysis we differentiate between dilution of hydrogen by other gas components, hydrogen depletion along the membrane length, concentration polarization adjacent to the membrane surface, and effects due to surface adsorption, on the hydrogen flux. A maximum H2 flux of 1223 mL cm−2 min−1 or 8.4 mol m−2 s−1 was obtained at 400 °C and 26 bar hydrogen feed pressure, corresponding to a permeance of 6.4 × 10−3 mol m−2 s−1 Pa−0.5. A good linear relationship was found between hydrogen flux and pressure as predicted for rate controlling bulk diffusion. In a mixture of 50% H2 + 50% N2 a maximum H2 flux of 230 mL cm−2 min−1 and separation factor of 1400 were achieved at 26 bar. The large reduction in hydrogen flux is mainly caused by the build-up of a hydrogen-depleted concentration polarization layer adjacent to the membrane due to insufficient mass transport in the gas phase. Substituting N2 with CO2 results in further reduction of flux, but not as large as for CO where adsorption prevail as the dominating flow controlling factor. In WGS conditions (57.5% H2, 18.7% CO2, 3.8% CO, 1.2% CH4 and 18.7% steam), a H2 permeance of 1.1 × 10−3 mol m−2 s−1 Pa−0.5 was found at 400 °C and 26 bar feed pressure. Operating the membrane for 500 h under various conditions (WGS and H2 + N2 mixtures) at 26 bars indicated no membrane failure, but a small decrease in flux. A peculiar flux inhibiting effect of long term exposure to high concentration of N2 was observed. The membrane surface was deformed and expanded after operation, mainly following the topography of the macroporous support.
The permeability of hydrogen selective Pd-based membranes was tested in different experimental conditions. The membranes were obtained by depositing palladium–silver films onto ceramic porous supports, with film composition of about 20 wt% of silver and thicknesses of about 2.5 μm. Their permeance was measured at 400 °C at total trans-membrane pressures between 0.2 and 6 bar, using pure feeds of H2 and N2, as well as H2/N2 and H2/CO mixtures; the temperature dependence of permeability was investigated using pure H2 feeds at 300, 400 and 500 °C. The membranes exhibit a very attractive behavior, maintaining a virtually infinite selectivity throughout the testing, with permeance values among the highest values reported in literature for similar membranes. Permeation of pure hydrogen accurately follows Sieverts’ law and confirms the presence of a chemisorption–dissociation–diffusion mechanism, characterised by the transport of atomic hydrogen through the Pd–Ag layer as the limiting step. In the case of H2/N2 mixtures, the high membrane permeance originates also significant concentration polarization phenomena resulting in apparent deviations from Sieverts’ behavior; the presence of CO in the feed may reduce hydrogen permeability even by 75%, although this effect is shown to be fully reversible after a subsequent air treatment at 400 °C. The temperature dependence of the membrane permeability is of Arrhenius type, with an activation energy of about 17 kJ/mol, that is, close to what is reported for Pd–Ag membranes following Sieverts’ behavior.
Thin (less than 2 μm thickness) and dense palladium/nickel (Pd/Ni) alloy composite membrane has been fabricated on mesoporous stainless steel (SUS) support modified with submicron nickel powder and CuCN solution. The deposition layer was made by the vacuum electrodeposition technique employing Pd/Ni complex reagent. The deposition layer depended on the experimental condition, especially the current density having an important effect on thickness, composition, and microstructure of deposited film. The composition and phase structures of the alloy film were studied by energy dispersive electroscopic analysis (EDS) and X-ray diffraction (XRD); Alloy films prepared with lower current density resulted in smaller grain size and higher Pd content. Also, composite membranes prepared at lower current density showed higher hydrogen permeance and greater hydrogen/nitrogen selectivities. For the current density of 6.5 mA/cm2, the hydrogen permeance was 2.0×10−2 cm3/cm2 cm Hg s and hydrogen/nitrogen (H2/N2) selectivity was 3000 at 723 K.
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.
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 SnCl2 and PdCl2 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.
Cited By (since 1996): 2, Export Date: 18 September 2011, Source: Scopus, CODEN: IJHED, doi: 10.1016/j.ijhydene.2010.03.062
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