Metallic membranes for hydrogen separation

1 Metallic membranes for hydrogen separation D.A. Pacheco Tanaka,1 J.A. Medrano,2 J.L. Viviente Sole,1 Fausto Gallucci3 1

TECNALIA, Energy and Environment Division, San Sebastian-Donostia, Spain; 2Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands; 3Inorganic Membranes and Membrane Reactors, Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven, University of Technology (TU/e), Eindhoven, The Netherlands

Introduction Among the different membranes for the separation of hydrogen, palladium (Pd)-based membranes have the highest selectivity and permeation owning to their unique mechanism of permeation (Fig. 1.1). Hydrogen permeates following several steps: (a) hydrogen molecules are chemically adsorbed on Pd catalytic active sites on the high-pressure side of the membrane and split producing hydrogen atoms, (b) the atoms cross the membrane due to their difference in the partial pressure on both sides of the membrane, (c) in the other side, the atoms are catalytically recombined by Pd forming hydrogen molecules. Since these chemical reactions occur only between hydrogen and Pd, the membranes have specific permeation for hydrogen. The flux through the membrane follows the Sieverts law (Eq. 1.1), where the flux (J) depends on the difference of the square root (in the ideal case) of the hydrogen partial pressures in the outer (Po) and inner (Pi) sides of the membrane; D and S are the diffusivity and solubility of hydrogen, respectively, l is the membrane thickness, R, the ideal gas constant, T, the temperature, and Ea, the activation energy. The exponent is 0.5 but can vary depending on the composition, temperature, and selectivity of the membrane. The flux increases with the temperature and decreases with the thickness.

Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-818332-8.00001-6 © 2020 Elsevier Inc. All rights reserved.

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Chapter 1 Metallic membranes for hydrogen separation

Figure 1.1 Mechanism of permeation of hydrogen in Pd-based membranes.

 J ¼

 DS  e l

Ea= RT


Pon  Pin




(1.1)

Any other molecule that can interact with Pd on the surface can block the active sites for splitting the H2 molecule, as a consequence, the hydrogen permeation is reduced. This effect is more evident in thin and ultrathin membranes. The greatest barrier to exploit the Pd membrane technology for larger scale applications is the high cost and variability in the price of Pd [1]. The price increased from 5.8 US$ g1 in December 2008 to 49.7 in March 2018; at the same dates, the price of Pt was 25.9 and 27.0 US$ g1, respectively. Therefore, it is urgent to develop membranes with less expensive metals. Group V metals (V, Nb, and Ta) with the body-centered cubic (BCC) structure are promising alternative materials to Pd because of their lower costs (the price of vanadium is 0.15 US$ g1) and higher hydrogen permeabilities. However, thin Pd films are required to permeate hydrogen. When the membrane is operated at more than 400 C interdiffusion between Pd and V takes place, degrading the membrane. The main problem of the group 5 metals as membrane materials is their too high solubility to hydrogen, making the membranes prone to severe hydrogen embrittlement which can lead to the formation of pinholes and cracks.

Palladium membranes The first Pd-based membranes were produced from cold working foils; the advantage of this technique is that the composition

Chapter 1 Metallic membranes for hydrogen separation

of the membrane can be controlled. Thin Pd and Pd- alloy membranes tubes with thickness up to 50 mm have been produced [2]; the surface of the membranes are smooth and some irregularities in the production of the lamination can be present [3]. To avoid compressive mechanical stress, pretensioned springs and bellows are often used [4]. Self-standing PdeAg membranes are used commercially in the production of very high pure hydrogen for the electronic and LED processing plants; SAES’s Pure Gas Pd Hydrogen Purifiers can remove all impurities to below ppb levels (hydrogen purity 9 nines) starting from hydrogen 4 nines (99.99%) (www.saespuregas.com) Tokyo gas developed a palladium membrane reactor (PdMR) for the production of hydrogen from steam reforming of methane with a production capacity of 20 Nm3 h1 of 99.99% hydrogen purity [5], the reformer has 112 reactor tubes of 20 mm-thick Pd-rare earth alloy placed on a stainless steel support; the reactor was tested for more than 3000 h. Bredesen’s group [6], at SINTEF (Norway), reported a method of manufacturing freestanding Pd and Pd alloy membranes by a two-step method which involves: (A) producing a PdeAg layer by PVD magnetron sputtering on a substrate with a polished surface and (B) removing and transferring the film to a membrane support. The advantage of this technique is that binary and multicomponent membranes can be prepared, and their composition controlled. The structure of a Pd77Ag23 membrane was studied by XRD and TEM; it was observed that near the substrate, small grains of 12e18 nm were formed; as more material was deposited, some grains grow while others were terminated and covered; as result, the average grain size increases with the thickness [7]. The space between the grains can be considered defects, which will grow during long-term permeation at high temperature, reducing the selectivity of the membrane. Since freestanding membranes alone are not mechanically strong to resist high pressure, 10 mm-thick Pd77Ag23 membranes were supported on stainless steel plates having microchannel of 200 or 1000 mm width. Long-term operation, up to 50 days, at approximately 5 bars, and for temperatures lower than approximately 400 C, resulted in large deformative settling of the film into the microchannel support due to plastic deformation, resulting in membrane failure; the deformation was more extensive for the 1000 mm compared to the 200 mm channel. Very good stability over 1100 h at 450 C with nitrogen was obtained when 1 micron rating porous stainless steel was used [8].

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Chapter 1 Metallic membranes for hydrogen separation

Supported Pd-based membranes In order to increase the hydrogen flux (at the same time reducing the cost by decreasing the number of membranes), and surpass the 2015 flux targets set by the US Department of Energy (w90 Nm3 m2 h1 at 1.4 bar pressure difference) [9], thin (<10 mm) and ultrathin (<2 mm) Pd-based membranes are required. Thin and ultrathin Pd-based membranes are in general supported on porous supports. When the Pd-based film is less than 5 mm thick, the cost of Pd is a small fraction of the support and module. The goal of developing supported Pd membranes is to obtain the highest permeance and selectivity and longterm stability (more than 10,000 h) under working conditions at low cost. The tubular configuration is preferred because it can be implemented into reactors with less sealings for the surface of the membrane, since sealing at high temperature is a big concern to produce hydrogen with high purity.

Fabrication of Pd/Pd alloy supported membranes Supported membranes are prepared by deposition of Pd (and other metals of the alloy) directly on the porous support. The size of the grains deposited is very important since it determines the volume of free space (defects) between the grains; large grains produce large defects; therefore, the size of the grain should be as small as possible and uniformly deposited in the whole membrane.

Chemical vapor deposition For membranes prepared by this technique, the support and an organometallic Pdþ2 (mainly Pd acetate) is introduced into a chamber. Applying vacuum and temperature, the Pd compound sublimates; on the surface of the support, Pdþ2 is reduced to Pd0; the deposition continues until a film is formed. The thickness of the membrane is controlled by the time of deposition, and the grain size formed will depend on the Pd precursor, sublimation conditions, and characteristics of the support. Itoh reported the preparation of 2e4 mm-thick Pd membranes on capillary alumina tubes; a membrane with H2/N2 permselectivity of more than 5000 at 300 C [10]. One advantage of this method is that wastewater is not produced. The drawbacks are that the equipment used are expensive and not all the Pd is deposited on the support and binary membranes are complicated to fabricate.

Chapter 1 Metallic membranes for hydrogen separation

Physical vapor deposition magnetron sputtering This process is similar to CVD, except that there is no reduction of Pd2þ because Pd0 is directly deposited on the support. During the deposition, Pd atoms are eroded from a “target” containing Pd by high-energy ion within a plasma; the liberated atoms travel through to the vacuum and are deposited on the porous substrate forming a film. An ultrathin PdAg membrane (z1 mm) was deposited by PVD on the nanoporous YSZ-gAl2O3 layer of an alumina tube; a good adhesion of the PdAg on the support was observed; however, a H2/N2 selectivity of only 10 was obtained; SEM image of the membrane surface showed PdAg grains of z0.1 mm with empty spaces between them [11]. PdAg films were deposited on ceramic supports of various pore sizes (Al2O3 (100 nm), ZrO2 (110 nm), ZrO2 (3 nm)); the N2 permeance of all the membranes were too high (w10% of the support). Increasing the heating power, bigger particles are deposited; therefore, the membranes prepared with the lowest heating power showed the lowest N2 permeance [12]. The selectivity of a Pd77Ag23 deposited on the ZrO2 (3 nm) was increased by the deposition of a very thin Pd layer by electroless plating (ELP); permeance of the 0.7e0.8 mm-thick Pd77Ag23 membrane at 400 C and 1 bar was 8  106 mol m2 s1 Pa1 (one of the highest reported for supported membranes) with H2/N2 permselectivity of 500 [13].

Electroless plating This method provides several advantages over the other methods such as the operational flexibility, simple equipment, cost performance, and applicability to nonconductive materials of any complicated shapes. ELP is an autocatalytic chemical reduction of Pdþ2 to Pd0 acting Pd0 as catalyst; using a chemical reductant. The reaction will take place only where Pd0 is present; therefore, the first step is the deposition of Pd0 on the porous substrate (seeds). The most common method is activation using SnCl2 and Pdþ2; however, on the support, Pd atoms are surrounded by colloidal particles of Snþ2 shielding the active Pd0 particles. An alternative procedure that eliminates tin contamination is the impregnation of the support with a solution of palladium acetate followed by reduction with hydrazine; with this method a uniform distribution of Pd with less than 20 nm is obtained, allowing the simultaneous deposition

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of Pd and Ag. This codeposition requires less temperature and shorter time to form the PdAg alloy [14]. The seeding step is very important for the preparation of dense Pd membranes; when Pd is not well distributed on the support, during the plating, big grains are formed, leaving empty spaces between them (defects). For the plating, the seeded support is introduced in a solution containing Pdþ2 salts, complexing agents (i.e., EDTA), ammonia, and hydrazine as reducing agent. During the reduction using hydrazine, N2 is produced; but, with other reductants, such as HPH2O2, hydrogen is released which can fragilize the membrane. The most common method for the preparation of PdAg, PdCu, and PdAu alloy membranes is the sequential ELP of Pd and Ag/ Cu/Au forming a bilayer membrane. For the formation of the alloy, the membrane must be treated at high temperature. The time and temperature for complete alloying depend on the metals and the thickness of the membrane; a 5 mm-thick PdAu membrane required more than 300 h at 550 C from the alloy [15]. Way [16] reported the complete annealing of a 5 mm-thick Au/Pd (10% Au) bilayer membrane in 9 h at 550 C and 3 MPa H2; whereas it took 120 h to alloy a similar membrane at 550 C and 0.1 MPa hydrogen. The activation energy (Ea) was used as a parameter to judge the extent of alloying; during annealing, the Ea changes until a minimum value is reached which does not change with additional annealing steps. During the annealing of bilayer membranes, defects can be produced due to the difference in the diffusion rates of the metal atoms (Kirkendall effect). During the annealing, stainless steel, Ni, Cr, and Fe particles of the reactor could be adhered on the Pd layer. The Kirkendall effect can also be responsible for the production of defects when metal particles, such as SUS-304, Ni, Cr, and Fe, coming from the reactors are adhered on the Pd layer [17].

Effect of the porous support on the preparation of defect-free Pd membranes The choice of the support is very important to prepare thin and ultrathin Pd-based membranes. The surface on which the Pd layer will be deposited should be smooth, with uniform pore size distribution, adequate pore size, and should not react with the Pd layer at hydrogen permeation conditions. The supports can be classified into ceramic and metallic.

Chapter 1 Metallic membranes for hydrogen separation

Ceramic supports The porous supports are prepared from spherical ceramic particles with uniform size, which will determine the size of the pore. Alumina supports from 70 to 800 nm pore size and porosity of 40%e55% are commercialized by Inopor (www.inopor.com). In order to reduce the resistance of the passage of gas through the support, asymmetric supports have been developed. In these supports, the pore size decreases gradually toward the surface where the Pd layer is deposited. The disadvantage of the porous ceramic membranes is that they are not very strong mechanically, and they are very difficult to be assembled into metallic reactors because of their difference in the thermal expansion coefficient. In order to facilitate the manipulation and deposition of the PdAg layer, Pacheco and Llosa [18,19] conditioned the porous supports by joining them with dense alumina tubes using a glass paste. To these supports, a thin PdeAg membrane was deposited by the method of simultaneous ELP [14]. Nitrogen permeation showed the presence of leaks in the interface of the glass and the Pd layer (Fig. 1.2Aacc) [20] caused by the poor adhesion of Pd film on the smooth glass and their differences in their thermal expansion coefficient. Covering the interfaces, the nitrogen permeance was reduced considerably. The glass part was removed and the Pd membrane was sealed using Swagelok connectors and graphite gaskets; one extreme was closed and the other connected to a dense metallic tube which can be easily assembled to metallic reactors [21]. Five PdAg membranes were prepared by the simultaneous deposition method (4e5 mm thick, 13%e15% Ag) onto asymmetric alumina supports (10/7 mm outer/inner diameters, pore size of the top layer 100 nm) and sealed with Swagelok connectors and graphite gaskets (Fig. 1.2B) [22]. The membranes showed very high hydrogen fluxes (3.9$106 mol m2 s1 Pa1 at 400 C and 1 atm pressure difference) and 21,000 H2/N2 selectivity which

Figure 1.2 (A) Leaks in the interface between the glass and the PdAg layer, (B) membranes with Swagelok connectors [22].

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are beyond the DOE 2015 membrane performance targets. The five membranes module was used in long-term (900 h) demonstration unit with a capacity of production of 1Nm3 of hydrogen from high-temperature (400e450 C) water gas shift (WGS) PdMR under bubbling fluidization (FB). The results showed high hydrogen recovery factors producing hydrogen with less than 10 ppm of CO [22]. A membrane similar to those used above was used for WGS using a packed bed (PB) configuration; the membrane performance was measured before and after catalyst packing and also after reaction for 2100 h of total operation; the membrane reactor showed very good performance in terms of both CO conversion and hydrogen recovery at 400 C and 4 bar [23]. Ultrathin PdAg (0.46e1.3 mm) membranes were prepared by the method and supports described above; the thickness of the PdAg film was controlled by the time used for the plating. A membrane plated for 1h (1.3 m thick) was tested for 1000 h at 400 C and 1 atm pressure difference; the membrane showed stable hydrogen permeance (w9.0e9.4$106 mol m2 s1 Pa1) and 1900 H2/N2 selectivity [24]. However, a similar 4 mm-thick PdAg membrane showed nitrogen leak after being tested at 600 C for 7 days [21]. The effect of the support wall thickness and the number of Swagelok connectors in the nitrogen leak on the membranes is shown in Fig. 1.3. The PdAg layers were deposited on

Figure 1.3 Relative N2 permeation change with the time during hydrogen permeation test at 500 C for PdAg membranes on alumina support with various outer and inner diameters (O/I) and with one or two Swagelok connectors [25].

Chapter 1 Metallic membranes for hydrogen separation

100 nm asymmetric alumina tubes by the codeposition ELP method. For the membranes having two Swagelok connectors, higher nitrogen leak was observed for the membrane with thinner wall support (1.5 against 3.5 mm). For the 10/7 membrane, the maximum torque that could be applied before being broken was 6 Nm; but, for the 14/7 12 Nm could be applied. For the membrane 14/7 with only one connector, a very small variation of nitrogen leak was observed. These observations indicate that at 500 C, the leaks are mainly originated in the sealings [25].

Strong interaction between palladium and membrane support The term strong metalesupport interaction (SMSI) is generally used to summarize changes in catalytic activity and selectivity when metals supported on reducible oxides are treated by a high-temperature reduction process. SMSI involves the encapsulation of the metal surface followed by the partial reduction of the support oxide in reducing conditions (450e500 C) [26]. It is known for the strong interaction between Pd and ZnO, and TiO2 and FeOx. Studying the production of hydrogen from WGS fluidized bed reaction using Pd membrane rectors at 400 C, it was observed that when TiO2 was used as catalyst support, the hydrogen permeation reduced dramatically due to the strong interaction between the Pd layer and TiO2; this effect was not produced when the support was alumina [20]. Okasaki et al. [27] reported SMSI between alumina support and Pd during hydrogen permeation testing at elevated temperatures; at 650 C an appreciable decrease of permeation occurs, at 850 C almost no flux was observed. The reduction of the hydron permeation was attributed to the formation of the less hydrogen permeable palladiume aluminum alloy via reduction of alumina under hydrogen atmosphere; the migration of aluminum to the membrane top surface also obstructs the dissociative adsorption of hydrogen molecules. When YSZ was used as a support, the hydrogen flux was constant for 336 h as shown in Fig. 1.4 [28]. Rapid and severe decrease of the hydrogen permeation of a Pd membrane supported on porous MgO/MgAl2O4 at 700 C was observed in the product of SMSI of magnesium with Pd; in a control test, when the membrane was soaked in hydrogen at the same temperature (without permeation), it resulted in only very slight or no reduction of Mg; the reduction was enhanced by the atomic hydrogen present in the fluxing membrane [29].

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Figure 1.4 Reduction of the hydrogen flux of Pd membranes supported on YSZ and alumina at 100 kPa as a function of time. Adapted from J. Okazaki, T. Ikeda, D.A. Pacheco Tanaka, M.A. Llosa Tanco, Y. Wakui, K. Sato, F. Mizukami, T.M. Suzuki, Importance of the support material in thin palladium composite membranes for steady hydrogen permeation at elevated temperatures, Phys. Chem. Chem. Phys. 11 (2009) 8632. https://doi.org/10.1039/b909401f.

Metallic support Metallic supports are mechanically stronger and can be easily assembled into metallic reactors than ceramics. However, they have low porosity, their pores are bigger with no uniform pore size distribution, the surface is rough, at less than 450 C SMSI with Pd is produced, and, at this moment, they are much more expensive than ceramics. The roughness, pore size, and SMSI can be reduced by deposition of a ceramic porous layer. The ceramics more commonly used are ZrO2, YSZ, Al2O3 and have been deposited by dip-coating, sputtering, and atmospheric plasma deposition; however, the ceramic layer reduces the gas permeation and the porosity of the support. Tecnalia (Spain) and TU/e (The Netherlands) reported the preparation and characterization of a thin (4e5 mm) PdAg membrane, prepared by the simultaneous ELP deposition, supported on porous Hastelloy X tube. First, the roughness and the pore size of the surface support were reduced by ground and reactive steps. Then, a ceramic layer of composite alumina-YSZ was deposited by dip-coating of the tube in a suspension of the oxides; finally, to increase the adhesion of the ceramic to the metal support, it was sintered at 750 C [30]. The hydrogen permeation of a membrane was tested for a long time; after 1250 h at 400 C, the membrane

Chapter 1 Metallic membranes for hydrogen separation

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Figure 1.5 Long-term hydrogen permeation and H2/N2 selectivity of a PdAg membrane supported on porous Hastelloy X. (A) at 400 C [30], (B) from 500 to 600 C [31], (C) a supported PdAg membrane showing the compatibility of the PdAg layer with the metallic support. Figures A and B adapted from E. Fernandez, A. Helmi, J.A. Medrano, K. Coenen, A. Arratibel, J. Melendez, N.C.A. de Nooijer, V. Spallina, J.L. Viviente, J. Zuiga, M. van Sint Annaland, D.A. Pacheco Tanaka, F. Gallucci, Palladium based membranes and membrane reactors for hydrogen production and purification: an overview of research activities at Tecnalia and TU/e, Int. J. Hydrogen Energy 42 (2017) 13763e13776. https://doi.org/10.1016/j. ijhydene.2017.03.067.

showed hydrogen permeance of 0.9$106 mol m2 s1 Pa1 at 1 atm and very high H2/N2 selectivity (>150,000) (Fig. 1.5A) In contrast to ceramic supports (Fig. 1.2A), in the metallic support the PdeAg layer is covering the interface between the porous and the nonporous sections (Fig. 1.5C) and no leaks were produced during 1250 h test. Another membrane prepared in the same way was tested between 500 and 600 C [31]. Over 550 C a slight decrease of hydrogen permeation is observed probably for the SMSI interaction between alumina and Pd; the nitrogen leak was extremely low for almost 800 h which resulted in an ideal H2/N2 permselectivity higher than 200,000. However, after 795 h of test (at 600 C), some nitrogen permeated through the membrane, resulting in a decrease of selectivity to a value of 2650 (Fig. 1.5B).

Pd aeb phase transition Pd hydrides could contain two crystalline phases, a and b, with different unit cell sizes of 3.89 Å and 4.02 Å, below the critical

Chapter 1 Metallic membranes for hydrogen separation

temperature for the aeb phase transition. The coexistence of both face-centered cubic phases produces distortion of the metal lattice and dislocations leading to destruction of the membrane structure (hydrogen embrittlement). The temperature of embrittlement can be decreased by alloying Pd with other metals. PdAg is the most studied binary alloy [33]; it has the advantage that it can form alloys in all proportions; moreover, the addition of silver increases hydrogen permeability up to 70% for an alloy with 23% Ag. Hydrogen permeability for PdeAg alloy membranes with various Ag content was examined at the temperature range of 100e300 C (Fig. 1.6) [34]. For the membrane with 5% Ag a peak of hydrogen flux was observed at around 150e170 C. As the Ag content increases, the intensity of the peak decreases, shifting to lower temperatures; at 20% silver, only a shoulder is observed. This behavior is related to the a to b Pd hydride transition; the increase of permeation is related to the increase of the crystal sizes going from a to b phase. The lattice size of a and b phases became closer with increase of Ag content; at around 24%, they are almost similar, therefore the embrittlement is suppressed. PdCu alloys have received great attention because of the reduction of cost by replacing Pd with the cheaper metal. However, PdCu has a complex alloy phase diagram consisting of more than one phase structure; hydrogen permeability has a maximum

–0.5

300

250

200

T (°C) 150

100

–1.0 –1.5 In (H2 flux)

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–2.0 –2.5

Ag 5%

–3.0

Ag 10% Ag 20%

–3.5 –4.0 1.6

1.8

2.0

2.2 2.4 1/T × 10–3 (K–1)

2.6

2.8

Figure 1.6 Arrhenius plot of hydrogen flux for palladiumesilver alloy membranes under the 200 kPa pressure difference [34].

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Chapter 1 Metallic membranes for hydrogen separation

of around 60 wt% Pd in the bcc phase; within this region, small deviations of 2e3 wt% of Pd from the optimal concentration result in enormous decrease in hydrogen permeability by 40%e60% when compared to the peak value [35].

Effect of the temperature on the supported Pd-based membranes Pd delamination Supports with smooth surface and small pore size allow the deposition of ultrathin membranes; however, during hydrogen permeation and thermal cycling, the membrane can be delaminated mainly due to the difference in their thermal expansion coefficient. More stable membranes are obtained, lowering the thickness of the selective layer; this is the case of g-alumina supports (pore size diameter 2e10 nm) where the thickness of the membrane could be less than 1 mm depending on the working temperature. The hydrogen flux of these ultrathin membranes is not as high as expected because of the resistance of the gases to pass through nanometric size pores of the g-alumina. To improve the adhesion of the Pd layer, YSZ was added to g-alumina to decrease their difference of thermal expansion. A composite g-alumina-YSZ (50/50) nanoporous layer was deposited on a symmetric capillary a-alumina (200 nm pore size) and treated at 1050 C for 20 min; the heat treatment produced the sintering of YSZ, increasing the pore size of the pores; the nitrogen permeance of the composite support was similar to the capillary tube indicating the g-alumina-YSZ layer low resistance to the passage of gases. To the treated support, an ultrathin PdeAg membrane (0.9 mm) was deposited; the hydrogen permeance of the membrane was 7.8$106 mol m2 s1 Pa1 with H2/N2 permselectivity of 4700 at 1 atm pressure difference and 600 C (Fig. 1.7) [36].

Formation of defects at temperatures lower than Tammann temperature Pd is deposited on the support as particles; the size of the particles depends on the deposition method and conditions. To decrease the volume of free space, particles should be as small and uniform as possible. Melting of materials depends on their particle size; the melting point of nanomaterials decreases sharply as the particle reaches critical diameter (for Pd < 20e30 nm depending on the shape [37]) because of their larger surface to

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Figure 1.7 Ultrathin PdeAg layer on an asymmetric capillary tube.

volume ratio. At around the Tammann temperature (half of the melting temperature), the atoms or clusters of Pd acquire enough energy to start sintering. During hydrogen permeation, as the temperature increases, the smallest particles start to sinter first forming bigger particles, leaving behind cavities which will be filled with hydrogen. The pressure of hydrogen will produce plastic deformation on the PdH walls forming spherical bubbles. Increasing the temperature and time, larger particles and bubbles will be formed [38]. Postmortem SEM analysis of the surface of a PdeAg membrane after hydrogen permeation test at 600 C for 7 days is presented in Fig. 1.8A; it can be observed that bubbles were formed in the membrane, some of which collapsed producing pinholes [21]. Bubbles were also observed on a PdAg membrane supported on porous metallic support after membrane-assisted gas switching reforming (MA-GSR) test as shown in Fig. 1.8B [39].

Figure 1.8 Bubbles formed in PdAg membranes (A) after hydrogen permeation test at 600 C for 7 days [21], (B) after MA-GSR test [39].

Chapter 1 Metallic membranes for hydrogen separation

Heat treatment in air During Pd deposition and membrane storage, contaminants can be present on the surface of the membrane which will inhibit the dissociation and recombination of hydrogen, reducing considerably the hydrogen permeation. Air treatment is commonly applied to remove organic contaminants from the surface. After air treatment, an increase of hydrogen permeation is observed; very often the nitrogen permeation also increases, declining the H2 selectivity. Depending on the pressure and temperature, four types of oxide phases are formed [40]: (A) chemisorbed phase, (B) a surface oxide of monoatomic thickness with composition Pd5O4 on Pd(111), (C) a surface Pd oxide with more than a monolayer thickness, and (D) PdO with tetragonal structure. In situ TEM SAED studies of the oxidation of Pd showed that until 300 C and 300 mbar O2, no PdO was present. At higher temperatures, it was observed fast transition of Pd to larger crystals of PdO at the grain boundaries and defects. The oxidation of Pd to PdO is accompanied by a volume change. Hydrogen reduction of large areas of PdO (nonepitaxial oxide) resulted in the formation of small Pd islands having an orientation not related to the Pd before the oxidation, but partly random. Crack and void formation (5e50 nm) was observed after the reduction of PdO grains [40]. The formation of the small islands produces surface roughening, increasing the active sites for the adsorption of hydrogen; however, the generation of defects reduces the selectivity of the membrane. In situ ESEM images of a 4 mm Pd membrane [41] showed after air treatment at 350 C, the presence of few pinholes from dozens to 300 nm. When treated at 650 C for 20 min, visible grains are observed on the surface, increasing the number of pinholes. Then, after reduction with hydrogen at 650 C, coalescence of Pd grains and removal of pinholes were observed. The membrane treated with 50% steam/N2 mixture at 400 C for 2 h remained leak free with a significant increase of the hydrogen permeation. The operation conditions (temperature, time, and oxygen concentration) should be carefully controlled during activation of Pd membranes. Okasaki et al. [42] reported in situ high temperature XRD analysis of a 3e4 mm-thick Pd membrane at temperatures of 20e850 C in air. The formation of a t-PdO phase was observed above 450 C, but, at 820 C the oxygen atoms were removed from the Pd crystal lattice. Between 800 and 850 C a remarkable increase in the grain size of palladium was observed due to rapid grain growth of the Pd

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membrane reducing the grain boundaries in the metal reducing the number of pinholes in the membrane. The MA-GSR concept integrates the ultrapure hydrogen production with CO2 capture in a single fluidized bed reactor containing catalytically active oxygen carrier particles. In a first stage, the methane steam reforming reaction is performed, and the hydrogen produced is used to reduce the catalyst (the active form of the catalyst); the excess hydrogen is removed by Pd membranes. In a second stage, air is introduced to the reactor; the catalyst is oxidized producing at the same time the heat required for the reaction. The MA-GSR reaction was carried out at 508 C using a PdAg membrane supported on a porous metallic support. The ideal H2/N2 selectivity at the end of the work was only 150, which is much lower than the initial value of 11,000. Postmortem membrane characterization revealed defects in the membrane selective layer as a consequence of the frequent exposure to thermal cycles with alternating oxidative and reducing atmospheres [39]. During the oxidation, defects are formed, which remain after reduction. Postmortem of the tested membrane showed the presence of many defects in the form of bubbles (Fig. 1.8B).

Heat treatment at temperatures higher than Tamman temperature (>640 C) Way [43] reported that the pinholes generated in Pd-based membranes prepared by ELP during heating a temperatures greater than 400 C can be eliminated by annealing the membrane at much higher temperatures (>640 C) under hydrogen. The heat process densifies the microstructure of the membranes by eliminating interparticles grain boundaries through coalescence; SEM analysis of a sample annealed at 700 C for 3 days under hydrogen shows much denser microstructure than a similar sample annealed at 550 C for 168 h. The heat-treated membranes showed improved thermal stability when the membrane was operated at lower temperatures. Zhue at.al. reported in situ ESEM studies of supported Pd membrane under Ar atmosphere. It was shown that at 650 C almost all the previously existing spikes and scratch marks faded away making the surface smoother [41]. Okasaki et al. [28] reported the improvement of the hydrogen permeation, by approximately 14%, when a 3-4 -thick Pd membrane supported on alumina support was heated at 850 C for 12 h in Ar; despite the high temperature and time of exposition, SMSI was not observed. The increase in hydrogen permeance

Chapter 1 Metallic membranes for hydrogen separation

Figure 1.9 SEM images of the surface of a supported Pd membrane (A) before and (B) after heat treatment at 850 C for 12 h in Ar [28].

was assigned to the higher crystallinity of the Pd layer after the heat treatment. The nitrogen leak of the membrane increased only slightly during the annealing, probably due to leaks in the interface between the Pd layer and the glass in a similar way to that shown in Fig. 1.2A. The morphology of the surface before and after heat treatment is shown in Fig. 1.9. It can be observed an increase on the crystal domain size by coalescence growth forming a mosaic type structure; similar structure was observed for a PdAg membrane after hydrogen permeation for 2650 h at 550 C. Postmortem analysis of the permeated side of the membrane showed that it was irregular with higher roughness indicating the penetration of the Pd membrane into the pores of the alumina support. Similar diffusion of Pd into ZrO2 was observed by Zhu et al. [41].

Effect of other gases in the hydrogen permeation The presence of other gases could decrease the hydrogen permeation due to (A) reduction of the partial pressure of hydrogen product of the dilution, (B) concentration polarization, and (C) adsorption of the other species on the surface [44] which can inhibit (e.g., CO) or poison (e.g., H2S) the hydrogen adsorption sites of the membrane surface influencing the whole hydrogen dissociation path.

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Chapter 1 Metallic membranes for hydrogen separation

Concentration polarization The hydrogen permeation flux for pure hydrogen and various H2/ N2 binary mixtures at 400 C of a 1.29 mm-thick PdAg supported membrane in function of the difference of the square root of the partial pressure of hydrogen is displayed in Fig. 1.10A [24]. Pure hydrogen follows the Sieverts law (linear dependence (Eq. 1.1), but, as the concentration of hydrogen in the binary mixture decreases, the fluxes are not linear, showing a downward convexity toward the pressure axis. Since using partial pressure, the effect of the dilution is already considered and the fact that nitrogen is inert, the mass transfer limitation from the bulk to the surface of the membrane (concentration polarization) is the most probable explanation of this phenomenon; it becomes more important in case of high-flux highly selective membranes. Similar behavior was observed in a Pd0.8 Ag 0.2 2.5 mm-thick supported membrane [45].

CO inhibition The CO inhibition on the hydrogen permeation in ultrathin 0.78 mm PdAg supported membrane was studied by measuring the permeation of gas mixtures containing 60% hydrogen and increasing amounts of CO (from 0% to 15%) adding nitrogen as a third gas in the mixture. In Fig. 1.10B is observed that as the amount of CO increases, the hydrogen flux decreases by 13% and 15% at 400 C for a hydrogen feed content of 60% and 80%, respectively, when feeding 15% CO content. On the other hand,

Figure 1.10 Hydrogen flux as a function of the square root of the partial pressure difference of hydrogen (A) hydrogen/nitrogen binary mixture and (B) hydrogen nitrogen and CO ternary mixture [24].

Chapter 1 Metallic membranes for hydrogen separation

the reduction in the hydrogen permeance at 450 C for the same CO contents are 9% and 13%, respectively; this confirms that at high temperatures the CO poisoning effect is lower [24]. The CO inhibition effect is reduced as the thickness of the membrane increases [20]. Between 350 and 450 C, the effect on inhibition of methane [24] and water [22] are negligible. Due to its structure, CO2 is considered to have negligible inhibition; however, the 1.29 mm-thick Pd0.93 Ag 0.072 described above shows some inhibition probably due to the interaction of the quadrupole moment of CO2 with Pd. It could be deduced that the order of inhibition is CO >> CO2 > H2O > N2, CH4.

H2S poisoning When Pd membranes are exposed to very few ppm of H2S, the hydrogen permeation is reduced drastically. The loss of hydrogen permeance is the result of the adsorption of H2S on the Pd surface, decreasing the number of active sites for adsorption of hydrogen (Eq. 1.2); each adsorbed sulfur could block 4e13 sites for hydrogen adsorption [46,47]. The adsorbed H2S on Pd is unstable and its catalytic decomposition in hydrogen and S is a facile process (Eq. 1.3) [48] The introduction of strong repulsive interactions form HeS and S can result in further blocking of hydrogen dissociation sites [48,49]. Pd(s) þ H2S (g) 4 Pd e H2S(s) (physisorption)

(1.2)

Pd e H2S(s) 4 PdS(s) þ H2(s) (Chemisorption)

(1.3)

After the incorporation of S in the membrane, PdS, Pd16S7, and Pd4S phases can exist [50] which depend on the concentration of H2S and the temperature. The formation of Pd4S is the most probable as it has the lowest free energy of formation [50]. The permeability of Pd4S is approximately 20 times less than pure palladium [51]. The formation of Pd4S causes an irreversible reduction of permeance and a failure in the membrane due to the formation of defects. Membranes consisting of alloys of PdeCu and PdeAu have received attention because they have some sulfur resistance. A PdCu self-supported 45.8 mm-thick fcc PdeCu membrane did not show a decline of hydrogen permeance when 5e39 ppmv H2S was added into a simulated syngas feed at temperatures between 400 and 500 C [3]. PdeAu membranes show better resistance to bulk sulfidation as compared to the PdeCu. However,

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Chapter 1 Metallic membranes for hydrogen separation

PdAu composition and the fabrication method play an important role in determining the H2S inhibition and stability of Pd alloy membranes [52].

The effect of fluidization on the membrane De Nooijer et al. [35] have studied the effect of the fluidization of Rh/Al2O3 particles (average particle size of 180 mm) on the nitrogen and hydrogen permeation at various temperatures on a supported PdAg membrane. The hydrogen and nitrogen permeation were stable at 400 C for 850 h of fluidization. Afterward, the temperature was increased to 500 C and tested for another 300 h; the hydrogen permeation increased and remained constant during the experiment; however, a constant increase of nitrogen with the time was observed (4.3  1011 mol m2 s1 Pa1 h1). Postmortem SEM image of the membrane shows that the surface looks like fish scales with very small pinholes product of the plastic deformation of the PdAg-hydride layer by the collision of the particles during fluidization at 500 C (Fig. 1.11). In order to protect the Pd surface from damages during fluidization and strong interaction with catalysts in membrane reactors, Pacheco Tanaka et al. developed a “Pore fill” membrane where the nanopores of a ceramic support is filled with Pd particles which is below a nanoporous ceramic protecting layer (Fig. 1.12A), [53,54]. Recently, Arratibel et al. developed an attrition resistant membrane for fluidized bed membrane reactor; in this membrane, a nanoporous Al2O3-YSZ protective layer is

Figure 1.11 SEM images of a PdAg membrane after hydrogen permeation for 1150 h under fluidization at temperatures up to 500 C [35]. Pd membranes are resistant to attrition.

Chapter 1 Metallic membranes for hydrogen separation

Figure 1.12 Pd membranes with a nanoporous protecting layer. (A) double skin, (B) pore fill.

deposited on a thin PdeAg (“double-skin” membrane) (Fig. 1.12B). A “double-skin” membrane prepared on a metallic support was immersed in a bubbling bed of alumina particles containing 0.5% of rhodium (particle size 100e300 mm) and tested at 400 C and 500 C for 115 and 500 h, respectively; a hydrogen permeance of 1.55  106 mol m2 s1 Pa1 and virtually infinite hydrogen selectivity was obtained during the entire test [55].

Nonpalladium membranes Group 5 metals such as V, Nb, and Ta with the BCC structure are promising alternative materials because of their lower cost (the price of vanadium is 0.15 US$ g1) and higher hydrogen permeability (20e100 times than Pd) mainly attributed to their very high hydrogen solubility and exothermicity [56]. Group 5 metals have poor catalytic activity for hydrogen and are easily oxidized; therefore, a thin Pd coating is typically deposited on both sides of the membrane to prevent oxidation and promote hydrogen dissociation/association, forming a sandwich-like membrane [57]. However, when the membrane is operated at more than 400 C interdiffusion between Pd and V takes place degrading the membrane. The main problem of the group 5 metals as membrane materials is their too high solubility of hydrogen making the membranes prone to severe hydrogen embrittlement which can lead to pinhole and cracks. As the hydrogen solubility decreases with the temperature (exothermic reaction), the solubility can be decreased by increasing the temperature of permeation; however, the temperature cannot be higher than 400 C because of Pd interdiffusion. The resistance to embrittlement is improved by reducing the hydrogen solubility alloying with 3d transition metals (e.g., Ni, Co, Fe, Cu and Mo, Zr, etc.); the problem of the alloys is their difficulty to produce thin-walled tubes, as result, these alloy membranes are typically thick, flat, and small [58].

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Chapter 1 Metallic membranes for hydrogen separation

If the Pd layer is very thin, it should not decrease significantly the high hydrogen permeation of the group 5 metal membrane. D. Cooney prepared V, Nb, and Ta membranes with 100 nm Pd films on both sides, the hydrogen permeability at 500 C obtained was very close to the theoretical permeability, the permeances of these membranes were 3 times greater than the industry standard 25 mm Pd75Ag25 alloy self-supported membrane despite their thicknesses of 50e100 mm; however, these membranes failed rapidly due to Pd interdiffusion and oxidation [59]. Dolan group [60] prepared 75 mm-long tubular vanadium membranes (9.54 mm outer diameter 0.28 mm thick) coated on both surfaces with 0.5 mm-thick Pd. The membrane showed at 300 C a permeance of around 1.2$106 mol m2 s1 Pa1. In order to decrease the amount of Pd, the inner Pd coating was replaced by Ni (Pd/V/Ni); a V membrane 200 mm thick was coated with 0.5 mm Pd in one side and in the other side Ni with various thicknesses. It was observed that the hydrogen permeance increases with decreasing Ni thickness, down to a limiting thickness of 150 nm beyond which permeance degraded due to incomplete Ni coverage; the permeance of this membrane is 65% of a symmetrical Pd/V/Pd membrane with 0.5 mm layers [61] because of the much lower catalytic activity of Ni. A way to avoid the interdiffusion is to physically separate the Pd film from the group V layer by a hydrogen permeable barrier which allows the membranes to operate at higher temperatures. The addition of a Pd catalyst layer on top of Mo2C showed hydrogen permeability of 2$108 mol m1 s1 Pa0.5 at 500 C, for V-based membranes, at higher temperatures Pd was removed from the Mo2C surface [62]. When applied on vanadium foils, TiC was a highly active catalytic coating enabling permeation of ultrapure hydrogen with fluxes up to 0.71 mol m2 s1 at 10 bar and 650 C; but, competitive adsorption in mixed gas environments inhibited hydrogen flux through TiC/V membranes. Addition of thin (50e100 nm) Pd films to the TiC surface provided chemical resistance and improved permeation at 500 C. V membranes with TiC thicknesses <20 nm but 23 nm-thick coatings demonstrated stable flux of 2  107 for 30 h at 600 C and 5 bar pressure difference [63]. Hydrogen permeability of air-treated V-based alloy membranes without Pd coating has been reported by Nakamura [64]. Recently, Wolden et al. [65] reported that nanocrystalline V2O3.(OOO1) surfaces were formed after short air exposure followed by hydrogen reduction at 550 C having adsorption energies for hydrogen dissociation/recombination operating via spillover

Chapter 1 Metallic membranes for hydrogen separation

mechanism; the activity was comparable to those of known hydrogen activation catalysts; a 50 mm-thick V membrane showed a stable hydrogen permeance of 4  107 mol m2 s1 Pa1 for more than 120 h at 550 C.

Conclusions and future trends Pd membranes have high potential for the purification and production of hydrogen. Self-supported membranes are thick, producing very high purity hydrogen. To reduce cost, thin and ultrathin Pd-based supported membranes are being developed; the permeation performance of these membranes is above the US Department of Energy targets for 2015; as the amount of Pd in these membranes is small, the main cost of the production of the membranes are the supports, especially for metallic supports. Supported membranes were tested for around 2000 h; however, permeation tests for more than 10,000 h are still required. At more than 500 C the membranes are not very stable for long time since pinholes appear, reducing the membrane selectivity. Under fluidization conditions, at high temperatures, the membranes suffer of plastic deformation and pinholes appear; Pd membranes protected with a porous ceramic layer showed that they are stable at these conditions. Non-Pd membranes have higher hydrogen diffusion than Pd, but still they require very thin Pd membranes to be active; even so, their performance is still lower than supported Pd-based membranes.

List of acronyms BCC CVD DOE ELP ESEM FB LED MA-GSR MR PB PVD SEM SMSI TEM WGS XRD YSZ

Body-centered cubic Chemical vapor deposition Department of Energy Electroless plating Environmental Scanning Electron Microscope Fluidized bed reactor Light-emitting diode Membrane-assisted gas switching reforming Membrane reactor Packed bed Physical vapor deposition Scanning electron microscope Strong metalesupport interaction Transmission electron microscope Water gas shift X-ray diffraction Yttria stabilized zirconia

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Chapter 1 Metallic membranes for hydrogen separation

List of symbols D Ea J l P R S T

Diffusivity in the membrane Apparent activation energy Membrane Flux Membrane thickness Pressure, partial pressure Ideal gas law constant Solubility in the membrane Temperature

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Chapter 1 Metallic membranes for hydrogen separation

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Chapter 1 Metallic membranes for hydrogen separation

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