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Hydrogen permeation and stability in ultra-thin PdRu supported membranes
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Hydrogen permeation and stability in ultra-thin PdeRu supported membranes Jinxia Liu a,b,1, Stefano Bellini c,1, Niek C.A. de Nooijer d, Yu Sun e,f, David Alfredo Pacheco Tanaka g, Chunhua Tang a, Hui Li a,*, Fausto Gallucci d, Alessio Caravella c,** a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China c Department of Environmental and Chemical Engineering (DIATIC), University of Calabria, Via P. Bucci, Cubo 44A, Rende (CS), 87036, Italy d Inorganic Membranes and Membrane Reactors, Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, De Rondom 70, 5612 AP, Eindhoven, the Netherlands e Institute for International Collaboration, Hokkaido University, Sapporo, Hokkaido 060-0815, Japan f Department of Chemistry, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo, Hokkaido, 060-0810, Japan g Tecnalia Energy and Environment Division, Mikeletegi Pasealekua 2, 20009 San Sebastian-Donostia, Spain
article info
abstract
Article history:
In this paper, we report the performance of supported PdeRu membranes for possible
Received 10 December 2018
applications to hydrogen purification and/or production. For this purpose, we fabricated
Received in revised form
three ultra-thin a-alumina-supported membranes by combined plating techniques: a Pde
13 March 2019
Ag membrane (3 mm-thick ca.) and two PdeRu (1.8 mm-thick ca.). The former is set as a
Accepted 26 March 2019
benchmark for comparison. The membranes were characterised using different method-
Available online xxx
ologies: permeation tests, thermal treatment and SEM analysis. Preliminary leakage tests performed with nitrogen has revealed that the two PdeRu membranes, namely PdRu#1 and
Keywords:
PdRu#2, show a non-ideal (non-infinite) selectivity, which is relatively low for the former
PdeRu membranes
(around 830 at 400 C) and sufficiently high for the latter (2645 at 400 C). This indicates a
Hydrogen
relevant presence of defects in the PdRu#2 membrane, differently from what observed for
Surface tension
the PdeAg and PdRu#1 ones. The permeation tests show that the hydrogen permeating flux
Bubbles
is stable up to around 550 C, with an apparently unusual behaviour at higher temperatures
Thin-layer
(600 C), where we observe a slightly decrease of hydrogen flux with an increase of the
Purification
nitrogen one. Moreover, a peculiar bubble-shaped structure is observed in the metal layer of all membranes after usage by means of SEM image analysis. This is explained by considering the effect of the Pd-alloy grain surface energy, which tends to minimise the exposed surface area of the grain interface by creating sphere-like bubble in the lattice, similar to what occurs for soap bubbles in water. The above-mentioned decrease in
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (A. Caravella). 1 These authors contributed equally to this study. https://doi.org/10.1016/j.ijhydene.2019.03.212 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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hydrogen flux at 600 C is explained to be caused by the bubble formation, which pushes the alloy deeper in the support pores. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A strong effort has been conducted to find a reliable, cheap and environmental friendly method for H2 purification and production. In this sense, Pd-based membranes with high hydrogen permeance and selectivity have been identified as a promising technology thanks to their high hydrogen dissociation rate and fast permeation mechanism with a virtually infinite selectivity of hydrogen against the other gas species [1]. During the past two decades, the research on Pd-alloy membranes has led to a technology matureness that appears ready for up-scaling in applications involving operating temperatures less than 550 C [2,3]. At temperature above around 550 C, the ultra-thin metal layer deposed on appropriate porous supports can suffer of instability owing to defect formation on the surface induced, for example, by thermal cycles [4]. In order to preserve these membranes from sulphur poisoning and avoid undesired a-b phase transitions at low temperature as well as to enhance hydrogen permeability and thermal endurance at the same time, a number of studies have been made, developing optimised Pd-alloys with different materials and compositions [5e7]. A promising way to attain metal Pd-alloy membranes with satisfactory performance and robustness is to alloy Pd with Ru [8,9]. The mechanical, physical, electrical, kinetic and thermodynamic properties of the PdeRu system has been studying for a long time [10e16], providing a strong knowledge for the fabrication of high-performance membranes for hydrogen purification and/or supply [9,17e22] as well as for the development of metal membrane-integrated reactors [23e25]. Overall, PdeRu alloys with homogenous structure and arbitrary metallic ratio are highly desired for basic scientific research and commercial material design [26]. In 1966, PdeRu bimetallic membranes were already used for selective hydrogen diffusion in a US patent of the Engelhard Industries Corporation [27]. In this patent, PdeRu membranes with a Ru content between 1 and 10% prepared by annealing at high temperature, reporting that this type of membranes have higher tensile strength than the Pd ones after annealing. This characteristic was also confirmed by other research groups [28,29]. In 1995, Cabrera et al. [16] studied the hydrogen desorption kinetics on Pd and PdeRu foils, indicating that the Pde5%Ru alloy has lower hydrogen solubility and diffusivity than those of the pure Pd one. The reduction in hydrogen solubility was also investigated using a technique based on electro-sorption paired with a thermally-programmed desorption [30,31]. The overall conclusion from this and other studies is that the hydrogen solubilization/desolubilization process is strongly
dependent on the Ru composition and film thickness [26]. In particular, it was observed that the presence of Ru in the alloy changes the d-band of the Pd electronic structure, leading to a consequent change of the Pd properties. Anyway, it is generally difficult to mix more than around 15% due to thermodynamic miscibility gap [14]. Within a Ru-composition in PdeRu alloy ranging from 1 to 9.4 at%, the hydrogen permeability shows a maximum at around 4.5 at%. Furthermore, this alloy can be applied in processes involving relatively high temperature (up to 823 K), as it was found to be more thermostable than the PdeAg one, showing a long-term strength that was measured to be almost five times higher after operation for 1000 h [24]. Recently, Lee et al. showed the long-term stability of thin PdeRu membranes supported on PSS, carrying out permeation tests at 180 C for 1200 h without observing any trace of hydrogen embrittlement. In the same study, they also successfully showed the strong resistance of these membranes against HCl and SiHCl3 impurities, which indicates their great potential in applications to H2 separation from the off-gas of the solar cells production [9]. In this context, the aim of this work is to investigate the permeation performance of ultra-thin PdeRu membranes (around 1.8 mm thick), analysing in this way the effect of membrane thinness on the alloy stability and integrity. Indeed, the optimal alloy composition, providing both high permeability and mechanical strength for membranes with such a thin selective layer, is still unclear. In fact, at a Ru content less than the nominal optimal content for permeability (4.5%), these membranes could suffer from defects formation generated by local stresses due to the interactions between Pd and Ru atoms.
Methodology Samples preparation In this study, three membranes are considered for investigation: a PdeAg membrane and two PdeRu ones similar by composition and thickness. The former was fabricated at Tecnalia Research & Innovation by simultaneous electroless plating [35,36] on the outer side of a tubular a-alumina asymmetric support (average pore size of 100 nm) with external and internal diameters of 10 and 4 mm, respectively, provided by Rauschert Kloster Veilsdorf. The basic details about membrane geometry and composition are listed in Table 2. The PdeRu membranes were prepared at the Dalian Institute of Chemical Physics (DICP), where they were synthesized on pre-treated porous tubular asymmetric g-Al2O3/a-Al2O3
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 1 e Electroless plating baths composition. Components
[Pd(NH3)2]Cl2 RuCl3 NH3H2O (25e28%) HCl N2H4 EDTA$2Na Polyethyleneimine, PEI Temperature
Values Pd-plating
Ru-plating
2 g/L e 300 ml/L e 0.6 g/L 55 g/L e 62e64 C
e 10 mmol/L 1000 mmol/L 5 mmol/L 50 mmol/L 10 mg/L 60 C
Table 2 e Geometry and composition of the membranes used in this report. The “active” prefix is referred to the effective Pd length and area measurements taken after the sealing operation. The Pd-layer thicknesses were measured after membrane characterization with the Scanning Electron Microscopy (SEM). Geometry parameters
PdAg
PdRu#1
PdRu#2
Active surface length [mm] Diameter [mm] Active surface [cm2] Pd-layer thickness [mm] Composition wt.% ratio
88.0 10.2 28.7 3.4 PdeAg 95e5
102.2 10.2 32.7 1.8 PdeRu 98.7e1.3
101.2 10.2 32.5 1.7 PdeRu 99.2e0.8
supports of a 10.2 mm diameter and a 0.1 mm grade. The fabrication of PdeRu composite membranes followed a sequential plating process: first, the Pd layer was fabricated by a modified electroless plating method involving a preliminary support seeding [17], and then the Ru-layer was deposited by electroless plating on top of the Pd layer. The metal layers were eventually deposed on tubular 3 mm-thick a-alumina porous supports. Composition and operating conditions of the plating bath are listed in Table 1. Afterwards, membranes were cleaned several times with deionized water following the plating completion. Before permeation tests, all membranes were kept at a temperature of 400 C under argon for 48 h to favour the interdiffusion of ruthenium in the Pd lattice. Since the alloying procedure of Pd and Ru is critical due to the large difference
3
between their melting points (1555 C and 2334 C, respectively), we decided to reach a maximum Ru composition of around 1.3%, as a higher amount would have been affected the integrity of the ultra-thin metal layer. As for the PdeAg membrane, the low Ag content was selected to minimise the embrittlement.
Experimental setup The permeation tests were carried out at the laboratories of Eindhoven University of Technology (TU/e). Both membrane sides were sealed using Swagelok® fittings and graphite ferrules as described elsewhere [35]. Fig. 1 shows an example of membrane (PdeAg) before and after sealing, respectively. Prior to permeation tests, the sealed membranes were submerged in ethanol and then helium permeation flux was measured to detect any possible leaks. The results of the leakage tests are reported in Table 3. In particular, PdAg and PdRu#1 show an acceptable leakage level, whereas that of PdRu#2 is one order of magnitude higher. After the preliminary leakage tests, the membranes were placed in the permeation module (Fig. 2a), where up to five membranes can be tested simultaneously by appropriate valves placed on the top. In this way, the permeation through each membrane can be measured separately. The module was configured in order for the feed stream to flow up from the bottom of the shell, which has a diameter of 10 cm, whereas the permeate stream (high-purity hydrogen) passes through the membrane exiting the top. Flow rates were measured automatically using an interface software, by which temperature and pressure were also monitored at multiple points along the module.
Results and discussion Permeation tests Long-term hydrogen permeation tests (530 h) were performed within a temperature range of 400e550 C at a pressure difference of 2 bar for both hydrogen and nitrogen tests keeping the permeate pressure at the atmospheric value. Afterwards, membranes were further tested at 600 C for additional 185 h (for a total time of 715 h) for thermal resistance checking
Fig. 1 e Pictures of a membrane (a) before and (b) after sealing. Example of the PdeAg membrane. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 3 e Leakage test results of membranes with He as leakage detector at PFeed ¼ 2 bar and room temperature. He vol. flow [ml min1]
Membrane conditions
PdAg Dry (in air) Bottom submerged Bottom þ Middle submerged Full submerged
0.171 0.163 0.021 0.020
PdRu#1 0.140 0.031 0.028 0.005
PdRu#2 0.371 0.346 0.120 0.080
Apparent He permeating flux [mmol m2 s1] PdAg 9.93 9.47 1.22 1.16
4
10 104 104 104
PdRu#1
PdRu#2
4
19.0 104 17.7 104 6.15 104 4.10 104
7.14 1.58 1.43 0.26
10 104 104 104
Fig. 2 e Pictures of (a) permeation module and (b) conceptual drawing.
(Fig. 3). In particular, first, membranes were tested at 400 C for 165 h, then at 500 for 30 h, successively at 550 for 335 h and finally at 600 C. Based on the trends shown in Fig. 3a, we can distinguish the behaviour below 600 C and that at 600 C. In the former case, it can be observed that, at each temperature, the hydrogen flux increases progressively up to reaching a steady value, most likely corresponding to the completion of the alloying process. Overall, all the three membranes show similar behaviour up to 550 C, with the extent of hydrogen flux that increases according to the following order: PdAg < PdRu#1 < PdRu#2. As regards the nitrogen flux at the same conditions (below 600 C), which is evaluated from 400 C to check the defect extent in runtime, it reaches a steady value at each temperature after a continuous increase, similar to what observed for the hydrogen flux. This indicates that the defect extent
increases with increasing temperature for the three membranes, even though the nitrogen flux of the PdAg sample is one order of magnitude lower than that of PdRu#1 and two orders lower than that of PdRu#2 due to also the lower membrane thickness of the PdeAg membrane. The situation is different at 600 C, where a peculiar observation is made analysing the combined behaviour of hydrogen and nitrogen fluxes. In fact, for the PdAg and PdRu#1 membranes, the nitrogen flux is found to increase continuously, as also observed in a previous work [37], whereas the hydrogen flux decreases with time. In general, the permeation can decrease at high temperature for the strong interaction between palladium and alumina support [38e40]. Furthermore, the continuous increase of nitrogen flux with time could be due to sintering of Pd grains, which lead to the formation of defects. Also PdRu#2 shows this trend, but not in the whole time range where the
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 3 e Hydrogen permeation flux at different temperatures for each membrane a) in pure-H2 conditions and b) in pure N2 conditions. PFeed ¼ 2 bar, PPerm ¼ 1 bar.
temperature of 600 C is set. This can be explained with the higher amount of defects present on the surface of PdRu#2 with respect to the other two samples. A more detailed explanation for this behaviour can be provided analysing the trend of membrane selectivity, which is defined for our purposes as the ratio of the single-gas hydrogen and nitrogen fluxes (Eq. (1)). The selectivity values are reported in Table 4. S¼
JH2 JN2 At the same conditions
(1)
As expected from the results shown in Fig. 3, the most selective membrane among the investigated ones was found to be the conventional PdeAg membrane, followed by the PdRu#1 and PdRu#2 at all temperature values considered.
Table 4 e Selectivity evaluated based on Eq. (1) at different temperatures using the H2 and N2 fluxes taken at the end of each data set. 885. Selectivity [] T [ C] 400 500 550 600
PdAg
PdRu#1
PdRu#2
>56544 >68725 4948 547
2465 2611 547 16
830 486 204 18
Beside the PdeAg membrane, which shows a virtually infinite selectivity, the PdRu#1 one shows an acceptable value up to 500 C. For the PdRu#2 sample, some problems must have been occurred already during preparation probably due to a non-uniform seeding on the surface, as the selectivity value is relatively low for dense metal membranes already at 400 C. It is notable that selectivity increases from 450 to 500 C, which is explained by considering that the resistance offered by the metal layer to the hydrogen transport lowers with temperature thanks to the fact that the internal diffusion is an activated process. Therefore, in conditions of relatively high temperature, the hydrogen transport is favoured with respect to the nitrogen transport through defects and porous support, which is a non-activated process. However, this is not valid for sufficiently high defect level, as the hydrogen transport preferentially occurs through defects instead of through the metal layer. This causes selectivity to tend to the Knudsen theoretical value or, for a more seriously damaged membrane, to the viscous flow one, which is a non-selective transport mechanism. This is just what occurs for the PdeRu membranes at 600 C in the last part of the time range, where the presence of pinholes becomes so serious that both hydrogen and nitrogen fluxes increase with time. The defect extent, which dramatically increases at temperature above around 550 C, is caused by the coalescence of smaller defects inducing larger ones. Beside the PdeAg membrane, considering the thinness of our PdeRu membranes, the stability and the mechanical
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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strength of the PdRu#1 membrane are satisfactory up to 550 C, whereas the PdRu#2 one has started to be severely damaged above 500 C. Using the steady values of flux at each temperature reported in Fig. 3 along with additional data at different pressure values and 450 C, we evaluated the membrane permeability based on Sieverts’ law (Eq. (2)) as a function of temperature (Eq. (3)). For this purpose, the hydrogen flux is reported as a function of the square root of the hydrogen pressure difference (Fig. 4), whose trend is found to be linear with a good approximation (R2 ranging from 0.9930 to 0.9960). Due to the poor selectivity obtained at 600 C, such a temperature was not considered in this analysis. The related results are shown in an Arrhenius-type plot (Fig. 5). JH2 ¼
d
f pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pH2 ;Feed pH2 ;Perm
Mem
(2)
The corresponding calculated parameters, which assume similar values as those reported for PdeAg membranes in other literature works [41e43], are listed in Table 5.
Ea f ¼ f0 exp RT
(3)
In order to analyse in a more detail the permeation behaviour and the effect of the possible influence of support and defects, the values of the pressure exponent (n) appearing in the empirical Sieverts law (Eq. (4)) are calculated along with the corresponding permeabilities at various temperatures (Table 7). JH2 ¼
fðnÞ n p pnH2 ;Perm dMem H2 ;Feed
(4)
Table 6 reports permeability and selectivity data about several membranes of the literature compared to the PdeRu ones prepared in this work. Specifically, we can observe that only the membranes prepared by Gade et al. (2009) [29] are comparable for thickness with ours, although the testing time used by those Authors is around 1/tenth less than that used in
Fig. 5 e Activation energy lines obtained by logarithmic rearrangement of the permeability Arrhenius-type equation.
Table 5 e Derived pre-exponential factor f0 and activation energy Ea. Arrhenius factors Ea f0 R2
1
[kJ mol ] [109 mol s1 m1 Pa0.5] []
PdAg
PdRu#1
PdRu#2
8.3 33.6 0.9989
9.7 33.5 0.9988
9.4 28.9 0.9988
our work, which does not allow making a completely fair comparison. As reported in several previous works, significant information about the mass transport mechanisms determining the overall permeation process can be withdrawn only in pure hydrogen conditions [44], like in the present case. In
Fig. 4 e Hydrogen permeation flux as a function of Sieverts' driving force at different temperatures. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 6 e Comparison with some literature PdeRu membranes in terms of permeability and selectivity. Sample
Composition PdeRu [wt%]
PdRu#1
98.7e1.3
PdRu#2 CSM 493H PdeRu Pd-1 Pd-2 Pd-3 Multilayer Ru/Pd
99.2e0.8 99.7e0.3 99.8e0.2 95.5e4.5 95.0e5.0 90.0e10.0 11.4/88.6
Conditions 500 C PFeed Max ¼ 500 C PFeed Max ¼ 550 C PFeed Max ¼ 580 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼
Permeability [109 mol s1 m1 Pa0.5]
Selectivity
Refs
1.8
7.41
2611
This Work
1.7
6.70
486
This Work
6.0
12.6
1860
[4]
5.0
13.7
>200
[8]
1.9
12.0
18200
[29]
2.9
14.2
1850
[29]
4.5
15.4
2780
[29]
6.8/0.085
23.0
∞
[20]
Thickness [mm]
200kPa 200kPa 840kPa 2700kPa 137:9kPa 137:9kPa 137:9kPa 200kPa
Table 7 e Pressure exponent and related parameters as functions of temperature for each sample. Temperature
550 C
500 C
450 C
400 C
Parameters n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 []
Pan
Pan
Pan
Pan
particular, all the exponent values lay within the range 0.64e0.69, which means that there are some phenomena making membranes deviate from the ideal behaviour (corresponding to n ¼ 0.5). In the considered operating conditions, the possible phenomena able to induce such a discrepancy in the metal alloys are i) the non-ideal diffusion in the metal lattice, ii) Knudsen and/or viscous flow within possible pinholes and iii) resistance of the gas permeation by the porous support. The non-ideal diffusion makes the pressure exponent decrease with increasing temperature, whereas the last two transport phenomena cause the opposite behaviour [45e47]. Therefore, to check which of them is dominant, we show the trend of pressure exponent with temperature (Fig. 6). As for the PdeAg membrane, we observe a regularly increasing trend, which indicates that, although the variability of n is relatively narrow, the mass transport in the support and/or possible defects is playing an appreciable role. It must be underlined that it is not generally possible to distinguish the influence of the transport through the membrane defects from that of the transport through the support from the value of the pressure exponent only. The former, however, can be estimated from the previously calculated selectivity value.
PdAg
PdRu#1
PdRu#2
0.663 1.08 109 0.9967 0.657 1.09 109 0.9961 0.649 1.111 109 0.9961 0.641 1.12 109 0.9973
0.645 1.01 109 0.9980 0.641 0.98 109 0.9967 0.680 0.532 109 0.9971 0.666 0.56 109 0.9962
0.648 0.96 109 0.9976 0.645 0.93 109 0.9975 0.690 0.465 109 0.9974 0.675 0.50 109 0.9968
As for both the PdeRu samples, they show very similar irregular trends having a maximum and a minimum. Although such an irregularity within 400e550 C can be attributed to the experimental error due to the low-pressure range considered, the unexpected behaviour could be differently explained as follows: up to 450 C, the transport through support and defects plays a significant role, providing an appreciable resistance causing the pressure exponent to increase with temperature tending to the theoretical Knudsen value (n ¼ 1). Between 450 and 500 C, the thermal effect could make grains enucleate, inducing a higher relative resistance in the metal layer with respect to the resistance provided by defects and porous support, this making the pressure exponent to decrease with temperature. Above around 500 C, the transport in through the metal layer becomes faster than that in the defects and support and, thus, n starts increasing again with temperature.
XRD and SEM characterization The last series of experiments involves the characterization of fresh and used membranes, i.e., before and after the experiments, respectively.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 8 e XRD pattern for PdRu#1 membrane. Fig. 6 e Pressure exponent as a function of temperature for each sample.
The XRD results for each membrane are shown below in Figs. 7e9. In particular, for the PdeAg membrane, the presence of some double peaks in the spectrum of the fresh membrane is observed, which is an indication o that the membrane is still not perfectly alloyed. During the permeation experiments under hydrogen, the alloy is formed as reflected in the sharp single peak of the spectrum of the used membrane. The gradual increase in the hydrogen permeation observed in Fig. 5 is an indication of the alloying process. Regarding the PdRu#2, the XRD spectrum shows a consistent presence of corundum, the crystalline form of a-alumina, which arises from the support. In all cases, the XRD diagrams show higher peaks for the spectrum of the used membranes with respect to the fresh ones for all membranes.
Fig. 7 e XRD pattern for PdeAg membrane.
Table 8 reports the grain size as estimated using the Scherrer equation (Eq. (5)). L¼
Kl Dð2wÞcosðwÞ
(5)
where L is the grain size, K is a shape factor taken equal to the unity, q is the diffraction angle (in radians), l is the wavelength and D(2q) is the full width at half maximum (also indicated as FWHM in the literature). Figs. 10e12 show the three pairs of fresh/used samples. Furthermore, a peculiar observation is made concerning the internal structure of the metal layer, which shows a sort of bubble-shaped cavities after usage. A similar phenomenon is reported in the paper of Wassie et al. (2018), who carried out reforming and partial oxidation reactions for hydrogen production and CO2 capture [48]. The interesting fact to note is that in this case such a stress appears to produce not much cracks, but rather bubbles, whose sphericity indicates that the induced stress is locally isotropic. If we consider the internal grain redistribution as a process
Fig. 9 e XRD pattern for PdRu#2 membrane.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 8 e Grain size for the three membranes estimated by means of Eq. (5). l ¼ 0.1541 nm. Sample
(1 1 1)
(2 0 0)
2q [deg] Grain Size (L) 2q [deg] Grain Size PdAg Fresh PdAg Used PdRu #1 Fresh PdRu #1 Used PdRu #2 Fresh PdRu #2 Used
39.32 40.24 40.06 40.3 41.1 41.22
17 66 69 75 34 43
46.66 46.78 46.64 46.92 47.70 47.84
12 51 42 52 31 40
tending to minimise the interface energy interactions, the formation of a distributed bubble-shaped structure is the most efficient way for the lattice to relax the local stresses. In fact, at the same level of stress, a higher number of bubbles allows a more efficient stress redistribution within the lattice volume, also considering that sphere is the geometrical shape that has the smallest external area at a certain volume. From this point of view, this phenomenon is similar to that happening for soap bubbles in water, where the surface
9
tension locally minimises the surface area inducing a spherical shape. The picture of the membrane structures depicted in Figs. 10e12 before and after testing presents a possible explanation of the reason why the permeating flux decreases with time at 600 C. In particular, during hydrogen permeation, the presence of hydrogen atoms allows the formation of hydride phases [50], whose interphase starts enucleating within the metal lattice generating plastic deformations that eventually lead to sphere-like bubble shapes (Figs. 10be12b). Since permeation has a preferential direction, we actually think that such bubbles are not formed by the permeation itself but rather by the hydrogen absorption, which occurs also at zero driving force in hydrogen atmosphere. At a sufficiently high temperature, the so-described enucleation mechanism becomes significant and, thus, the bubble growth pushes the metal layer into the pores of the support, making in this way the adhesion stronger and densifying the metal-support interface. This occurrence causes the flux to decrease with time up to reach a possible minimum or a steady trend. Specifically, for the PdAg sample, neither the minimum nor the steady trend are found in the
Fig. 10 e Pictures of the PdeAg membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one.
Fig. 11 e Pictures of the PdRu#1 membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 12 e Pictures of the PdRu#2 membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one.
time range considered, as the level of defects is so low that it is not detectable. On the contrary, for the PdeRu membranes, the flux reaches a steady trend in the case of the PdRu#1 sample and a minimum for the PdRu#2 one. The difference between the two cases is that in the former the defect level keeps constant, whereas in the latter the defect level increases with time, this being stated by the poor selectivity values at 600 C reported in Table 7. The fact that the bubble shapes are poorly visible in the SEM picture of the PdRu#2 sample could indicate that the major presence of defects could have a certain influence on the bubble formation. However, a further deeper investigation about this aspect is required and no clear conclusion can be withdrawn at the moment. The presence of the bubble-shaped cavities is also related to a more practical issue related to the membrane thickness evaluation. In fact, at the same metal amount, the nominal thickness increases as the bubbles grows up during permeation tests. The cross-section analysis is accompanied with a surface analysis to check the defect degree for
both fresh and used samples, whose SEM images are shown in Figs. 13e15. The first membrane analysed is the PdeAg one, which shows an increase of crystal size during tests. This is a further confirmation of what depicted in the previous XRD patterns. Furthermore, some pinholes of about 800 nm of diameter (visible as little black stains) are formed during experiments. The surface of the fresh membrane changed to a more platelike one due to an increase in the crystal domain size by coalescence growth [49]. Similar behaviour is observed in the PdeRu membranes. Concerning the PdRu#1, a consistent number of pinholes is noticed, and this is the cause of the very high N2 fluxes recorded in the final nitrogen permeation tests. As discussed above, this is in close relationship with the low thickness of PdeRu separation layer. The PdRu#2 seems to have a surface morphology different from the others, at maybe this could be e along with some trouble occurred during the tubular membrane preparation e one of the cause of the low selectivity seen during the N2 tests, and this refers not only to the tests carried
Fig. 13 e SEM images of the PdAg membrane for (a) the fresh sample and (b) the used one.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 14 e SEM images of the PdRu#1 membrane for (a) fresh sample and (b) used one. The red circles underline the major defects (pinholes) on the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 15 e SEM images of the PdRu#2 membrane for (a) the fresh sample and (b) the used one. The red circles underline the major defects (pinholes) on the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
out after the high temperature experiments, but also to the previous ones, which show a N2 flux four times higher than the “twin” PdRu#1.
Conclusions In this work, one supported PdeAg membrane (ca. 3.5 mm) and two PdeRu ones. (ca. 1.8 mm), here named PdAg, PdRu#1 and PdRu#2, respectively, were prepared and characterised. The membrane behaviour to hydrogen permeation was studied under pure-hydrogen and pure-nitrogen conditions at different temperatures, performing also a long-term test (around 750 h). These tests allowed us to evaluate membrane permeability along with the behaviour of the pressure exponent with temperature, from which the transport
phenomena occurring during overall permeation process were identified. The high-temperature experiments at 550 C highlighted the good thermal resistance of all prepared membranes, whereas the PdeRu ones showed a certain vulnerability at higher temperature (600 C), which severely affects their selectivity. Strangely, at this temperature, the hydrogen flux was found to slightly decrease with respect to 550 C, whereas the flux of nitrogen was observed to increase. This was explained by considering the effect of alloy grains redistribution along with the strong interaction of the extremely thin membrane layer with the porous support. Furthermore, a peculiar observation was made regarding the internal structure of the metal layer, showing bubble-like cavities after usage. The reason for the formation of such a structure was explained by considering that the grain surface interaction energy tends to minimise the surface area by
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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creating spherical shapes analogously to what occurs for the surface tension inducing a spherical shape to soap bubbles in water. The decrease in hydrogen flux observed at 600 C was attributed to the bubble formation, which tends to push the metal alloy deeper in the pores of the support.
Acknowledgements A. Caravella has received funding through the “Programma Per Giovani Ricercatori «Rita Levi Montalcini»” granted by the e della Ricerca, “Ministero dell’Istruzione, dell’Universita MIUR” (Grant no. PGR12BV33A), which is gratefully acknowledged. H. Li has received financial support from the 100-Talent Project of CAS, National Natural Science Foundation of China (Grant No. 21676265; 51501177; 21306183), and The Ministry of Science and Technology (MOST) of the People's Republic of China (Grant No. 2016YFE0118300).
references
~ o M, Sa nchez-Herva s JM. Pd-based [1] Conde JJ, Maron membranes for hydrogen separation: review of alloying elements and their influence on membrane properties. Separ Purif Rev 2017;46:152e77. [2] Brunetti A, Caravella A, Fernandez E, Pacheco Tanaka DA, Gallucci F, Drioli E, Curcio E, Viviente JL, Barbieri G. Syngas upgrading in a membrane reactor with thin Pd-alloy supported membrane. Int J Hydrogen Energy 2015;40:10883e93. [3] Peters TA, Rørvik PM, Sunde TO, Stange M, Roness F, Reinertsen TR, Ræder JH, Larring Y, Bredesen R. Palladium (Pd) membranes as key enabling technology for precombustion CO2 capture and hydrogen production. Energy Procedia 2017;114:37e45. [4] El Hawa HWA, Paglieri SN, Morris CC, Harale A, Way JD. Identification of thermally stable Pd-alloy composite membranes for high temperature applications. J Membr Sci 2014;466:151e60. [5] Adams BD, Chen A. The role of palladium in a hydrogen economy. Mater Today 2011;14:282e9. [6] Bredesen R, Peters TA, Tim B, Dittmeyer R. Pd-based membranes in hydrogen production for fuel cells. In: Gallucci F, van Sint Annaland M, editors. Process intensification for sustainable energy conversion. John Wiley & Sons; 2015. p. 209e41. [7] Li H, Caravella A, Xu H. Recent progress in Pd-based composite membranes. J Mater Chem A 2016;4:14069e94. [8] El Hawa HWA, Paglieri SN, Morris CC, Harale A, Way JD. Application of a Pd-Ru composite membrane to hydrogen production in a high temperature membrane reactor. Separ Purif Technol 2015;147:388e97. [9] Lee SM, Xu N, Su Kim S, Li A, Grace JR, Lim CJ, Boyd T, Ryi SK, Susdorf A, Schaadt A. Palladium/ruthenium composite membrane for hydrogen separation from the off-gas of solar cell production via chemical vapor deposition. J Membr Sci 2017;541:1e8. [10] Darling AS, Yorke JM. The ruthenium-palladium system, Plat. Met Rev 1960;4:104e10. [11] Gryaznov VM, Mishchenko AP, Polyakova VP, Roshan NR, Savitskij EM, Smirnov VS, Khrapova EV, Shimulis VI. Palladium-ruthenium alloys as membrane catalysts. Dokl Akad Nauk SSSR 1973;211:624e7.
[12] Skakunova EV, Ermilova MM, Gryaznov VM. Hydrogenolysis of propane on a palladium-ruthenium alloy membrane catalyst. Bull Acad Sci USSR, Div Chem Sci 1988;37:858e61. [13] Kleykamp H. Constitution and thermodynamics of the MoRu, Mo-Pd, Ru-Pd and Mo-Ru-Pd systems. J Nucl Mater 1989;167:49e63. [14] Tripathi SN, Bharadwaj SR, Dharwadkar SR. The Pd-Ru system (palladium-ruthenium). J Phase Equilibria 1993;14:638e42. [15] Gurler R. A computer assessment of the Ru-Rh and Ru-Pd systems. J Alloy Comp 1993;191:31e5. [16] Cabrera AL, Morales E, Armor JN. Kinetics of hydrogen desorption from palladium and ruthenium-palladium foils. J Mater Res 1995;10:779e85. [17] Zhang J, Xu H, Li W. High-purity COx-free H2 generation from NH3 via the ultra-permeable and highly selective Pd membranes. J Membr Sci 2006;277:85e93. [18] Wu D, Kusada K, Kitagawa H. Recent progress in the structure control of Pd-Ru bimetallic nanomaterials. Sci Technol Adv Mater 2016;17:583e96. [19] Xu N, Kim SS, Li A, Grace JR, Lim CJ, Boyd T. Preparation and characterization of palladiumeruthenium composite membrane on alumina-modified PSS substrate. Can J Chem Eng 2014;92:1041e7. [20] Ryi SK, Li A, Lim CJ, Grace JR. Novel non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation. Int J Hydrogen Energy 2011;36:9335e40. [21] Xu N, Kim SS, Li A, Grace JR, Lim CJ, Boyd T. Novel electroless plating of ruthenium for fabrication of palladium-ruthenium composite membrane on PSS substrate and its characterization. J. Membr. Sci. Res. 2015;1:73e8. [22] Uemiya S, Kajiwara M, Kojima T. Composite membranes of group VIII metal supported on porous alumina. AIChE J 1997;43:2715e23. [23] Nam SW, Yoon SP, Ha HY, Hong SA, Maganyuk AP. Methane steam reforming in a Pd-Ru membrane reactor. Kor J Chem Eng 2000;17:288e91. [24] M Gryaznov V, Orekhova NV. Reactors with metal and metalcontaining membranes. In: Cybulski Andrej, Moulijn Jacob A, editors. Structured catalysts and reactors. CRC Press; 2005. p. 435e62. ISBN: 9780824723439. [25] Burkhanov GS, Gorina NB, Kolchugina NB, Roshan NR. Palladium-based alloy membranes for separation of high purity hydrogen from hydrogen-containing gas mixtures. Platin Met Rev 2011;55:3e12. [26] Wu D, Kusada K, Kitagawa H. Recent progress in the structure control of Pd-Ru bimetallic nanomaterials. Sci Technol Adv Mater 2016;17:583e96. [27] Colin JGE. Engelhard Industries, inc., hydrogen diffusion process. United States patent US 1966 Mar 8;3(238):700. [28] Cabrera A, Hasen J, Schuller IK. Structural changes induced by hydrogen absorption in palladium and palladiumruthenium alloys. Appl Phys Lett 1995;66:1216e8. [29] Gade SK, Keeling MK, Davidson AP, Hatlevik O, Way JD. Palladium-ruthenium membranes for hydrogen separation fabricated by electroless co-deposition. Int J Hydrogen Energy 2009;34:6484e91. [30] Ferrari P, Diaz-Droguett DE, Rojas S, Cabrera AL. Inhibition of hydrogen absorption in bulk Pd by the formation of RuePd surface alloy. Thin Solid Films 2014;550:732e7. [31] Hubkowska K, Koss U, Łukaszewski M, Czerwinski A. Hydrogen electrosorption into Pd-rich Pd-Ru alloys. J Electroanal Chem 2013;704:10e8. [35] Fernandez E, Helmi A, Coenen K, Melendez J, Viviente JL, Pacheco Tanaka DA, van Sint Annaland M, Gallucci F. Development of thin Pd-Ag supported membranes for fluidized bed membrane reactors including WGS related gases. Int J Hydrogen Energy 2015;40:3506e19.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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[36] Pacheco Tanaka DA, Llosa Tanco MA, Niwa S, Wakui Y, Mizukami F, Namba T, Suzuki TM. Preparation of palladium and silver alloy membrane on a porous a-alumina tube via simultaneous electroless plating. J Membr Sci 2005;247:21e7. [37] Peters TA, Tucho WM, Ramachandran A, Stange M, Walmsley JC, Holmestad R, Borg A, Bredesen R. Thin Pd-23% Ag/stainless steel composite membranes: long-term stability, life-time estimation and post-process characterisation. J Membr Sci 2009;326:572e81. [38] Okazaki J, Ikeda T, Pacheco Tanaka DA, Llosa Tanco MA, Wakui Y, Sato K, Mizukami F, Suzuki TM. Strong interaction at the palladium/alumina interface of membrane during hydrogen permeation at elevated temperature. Chem Lett 2008;37:1004e5. [39] Okazaki J, Ikeda T, Pacheco Tanaka DA, Sato K, Suzuki TM, Mizukami Fujio. An investigation of thermal stability of thin palladiumesilver alloy membranes for high temperature hydrogen separation. J Membr Sci 2011;366:212e9. [40] Okazaki J, Ikeda T, Pacheco Tanaka DA, Llosa Tanco MA, Wakui Y, Sato K, Mizukami F, Suzuki TM. Importance of the support material in thin palladium composite membranes for steady hydrogen permeation at elevated temperatures. Phys Chem Chem Phys 2009;11:8632e8. [41] Shu J, Bongondo BE, Grandjean BP, Adnot A, Kaliaguine S. Surface segregation of Pd-Ag membranes upon hydrogen permeation. Surf Sci 1993;291:129e38. [42] Serra E, Kemali M, Ross DK, Perujo A. Hydrogen and deuterium in Pd-25 pct Ag alloy: permeation, diffusion, solubilization, and surface reaction. Met. Mater. Trans. A 1998;29:1023e8. [43] Nguyen TH, Mori S, Suzuki M. Hydrogen permeance and the effect of H2O and CO on the permeability of Pd0.75Ag0.25 membranes under gas-driven permeation and plasmadriven permeation. Chem Eng J 2009;155:55e61.
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[44] Caravella A, Scura F, Barbieri G, Drioli E. Sieverts law empirical exponent for Pd-based membranes: critical analysis in pure H2 permeation. J Phys Chem B 2010;114:6033e47. [45] Caravella A, Hara N, Negishi H, Hara S. Quantitative contribution of non-ideal permeability under diffusioncontrolled hydrogen permeation through Pd-membranes. Int J Hydrogen Energy 2014;39:4676e82. [46] Caravella A, Hara S, Drioli E, Barbieri G. Sieverts law pressure exponent for hydrogen permeation through Pd-based membranes: coupled influence of non-ideal diffusion and multicomponent external mass transfer. Int J Hydrogen Energy 2013;38:16229e44. [47] Caravella A, Hara S, Sun Y, Drioli E, Barbieri G. Coupled influence of non-ideal diffusion and multilayer asymmetric porous supports on Sieverts law pressure exponent for hydrogen permeation in composite Pd-based membranes. Int J Hydrogen Energy 2014;39:2201e14. [48] Wassie SA, Medrano JA, Zaabout A, Cloete S, Melendez J, Pacheco Tanaka DA, Amini S, van Sint Annaland M, Gallucci F. Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: proof-of-Concept. Int J Hydrogen Energy 2018;43:6177e90. [49] Okazaki J, Ikeda T, Pacheco Tanaka DA, Suzuki TM, Mizukami F. In situ high-temperature X-ray diffraction study of thin palladium/a-alumina composite membranes and their hydrogen permeation properties. J Membr Sci 2009;335:126e32. [50] Peters TA, Carvalho PA, van Wees JF, Overbeek JP, Sagvolden E, van Berkel FPF, Løvvik OM, Bredesen R. Leakage evolution and atomic-scale changes in Pd-based membranes induced by long-term hydrogen permeation. J Membr Sci 2018;563:398e404.
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Hydrogen permeation and stability in ultra-thin PdeRu supported membranes Jinxia Liu a,b,1, Stefano Bellini c,1, Niek C.A. de Nooijer d, Yu Sun e,f, David Alfredo Pacheco Tanaka g, Chunhua Tang a, Hui Li a,*, Fausto Gallucci d, Alessio Caravella c,** a
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China c Department of Environmental and Chemical Engineering (DIATIC), University of Calabria, Via P. Bucci, Cubo 44A, Rende (CS), 87036, Italy d Inorganic Membranes and Membrane Reactors, Sustainable Process Engineering, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, De Rondom 70, 5612 AP, Eindhoven, the Netherlands e Institute for International Collaboration, Hokkaido University, Sapporo, Hokkaido 060-0815, Japan f Department of Chemistry, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo, Hokkaido, 060-0810, Japan g Tecnalia Energy and Environment Division, Mikeletegi Pasealekua 2, 20009 San Sebastian-Donostia, Spain
article info
abstract
Article history:
In this paper, we report the performance of supported PdeRu membranes for possible
Received 10 December 2018
applications to hydrogen purification and/or production. For this purpose, we fabricated
Received in revised form
three ultra-thin a-alumina-supported membranes by combined plating techniques: a Pde
13 March 2019
Ag membrane (3 mm-thick ca.) and two PdeRu (1.8 mm-thick ca.). The former is set as a
Accepted 26 March 2019
benchmark for comparison. The membranes were characterised using different method-
Available online xxx
ologies: permeation tests, thermal treatment and SEM analysis. Preliminary leakage tests performed with nitrogen has revealed that the two PdeRu membranes, namely PdRu#1 and
Keywords:
PdRu#2, show a non-ideal (non-infinite) selectivity, which is relatively low for the former
PdeRu membranes
(around 830 at 400 C) and sufficiently high for the latter (2645 at 400 C). This indicates a
Hydrogen
relevant presence of defects in the PdRu#2 membrane, differently from what observed for
Surface tension
the PdeAg and PdRu#1 ones. The permeation tests show that the hydrogen permeating flux
Bubbles
is stable up to around 550 C, with an apparently unusual behaviour at higher temperatures
Thin-layer
(600 C), where we observe a slightly decrease of hydrogen flux with an increase of the
Purification
nitrogen one. Moreover, a peculiar bubble-shaped structure is observed in the metal layer of all membranes after usage by means of SEM image analysis. This is explained by considering the effect of the Pd-alloy grain surface energy, which tends to minimise the exposed surface area of the grain interface by creating sphere-like bubble in the lattice, similar to what occurs for soap bubbles in water. The above-mentioned decrease in
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (A. Caravella). 1 These authors contributed equally to this study. https://doi.org/10.1016/j.ijhydene.2019.03.212 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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hydrogen flux at 600 C is explained to be caused by the bubble formation, which pushes the alloy deeper in the support pores. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A strong effort has been conducted to find a reliable, cheap and environmental friendly method for H2 purification and production. In this sense, Pd-based membranes with high hydrogen permeance and selectivity have been identified as a promising technology thanks to their high hydrogen dissociation rate and fast permeation mechanism with a virtually infinite selectivity of hydrogen against the other gas species [1]. During the past two decades, the research on Pd-alloy membranes has led to a technology matureness that appears ready for up-scaling in applications involving operating temperatures less than 550 C [2,3]. At temperature above around 550 C, the ultra-thin metal layer deposed on appropriate porous supports can suffer of instability owing to defect formation on the surface induced, for example, by thermal cycles [4]. In order to preserve these membranes from sulphur poisoning and avoid undesired a-b phase transitions at low temperature as well as to enhance hydrogen permeability and thermal endurance at the same time, a number of studies have been made, developing optimised Pd-alloys with different materials and compositions [5e7]. A promising way to attain metal Pd-alloy membranes with satisfactory performance and robustness is to alloy Pd with Ru [8,9]. The mechanical, physical, electrical, kinetic and thermodynamic properties of the PdeRu system has been studying for a long time [10e16], providing a strong knowledge for the fabrication of high-performance membranes for hydrogen purification and/or supply [9,17e22] as well as for the development of metal membrane-integrated reactors [23e25]. Overall, PdeRu alloys with homogenous structure and arbitrary metallic ratio are highly desired for basic scientific research and commercial material design [26]. In 1966, PdeRu bimetallic membranes were already used for selective hydrogen diffusion in a US patent of the Engelhard Industries Corporation [27]. In this patent, PdeRu membranes with a Ru content between 1 and 10% prepared by annealing at high temperature, reporting that this type of membranes have higher tensile strength than the Pd ones after annealing. This characteristic was also confirmed by other research groups [28,29]. In 1995, Cabrera et al. [16] studied the hydrogen desorption kinetics on Pd and PdeRu foils, indicating that the Pde5%Ru alloy has lower hydrogen solubility and diffusivity than those of the pure Pd one. The reduction in hydrogen solubility was also investigated using a technique based on electro-sorption paired with a thermally-programmed desorption [30,31]. The overall conclusion from this and other studies is that the hydrogen solubilization/desolubilization process is strongly
dependent on the Ru composition and film thickness [26]. In particular, it was observed that the presence of Ru in the alloy changes the d-band of the Pd electronic structure, leading to a consequent change of the Pd properties. Anyway, it is generally difficult to mix more than around 15% due to thermodynamic miscibility gap [14]. Within a Ru-composition in PdeRu alloy ranging from 1 to 9.4 at%, the hydrogen permeability shows a maximum at around 4.5 at%. Furthermore, this alloy can be applied in processes involving relatively high temperature (up to 823 K), as it was found to be more thermostable than the PdeAg one, showing a long-term strength that was measured to be almost five times higher after operation for 1000 h [24]. Recently, Lee et al. showed the long-term stability of thin PdeRu membranes supported on PSS, carrying out permeation tests at 180 C for 1200 h without observing any trace of hydrogen embrittlement. In the same study, they also successfully showed the strong resistance of these membranes against HCl and SiHCl3 impurities, which indicates their great potential in applications to H2 separation from the off-gas of the solar cells production [9]. In this context, the aim of this work is to investigate the permeation performance of ultra-thin PdeRu membranes (around 1.8 mm thick), analysing in this way the effect of membrane thinness on the alloy stability and integrity. Indeed, the optimal alloy composition, providing both high permeability and mechanical strength for membranes with such a thin selective layer, is still unclear. In fact, at a Ru content less than the nominal optimal content for permeability (4.5%), these membranes could suffer from defects formation generated by local stresses due to the interactions between Pd and Ru atoms.
Methodology Samples preparation In this study, three membranes are considered for investigation: a PdeAg membrane and two PdeRu ones similar by composition and thickness. The former was fabricated at Tecnalia Research & Innovation by simultaneous electroless plating [35,36] on the outer side of a tubular a-alumina asymmetric support (average pore size of 100 nm) with external and internal diameters of 10 and 4 mm, respectively, provided by Rauschert Kloster Veilsdorf. The basic details about membrane geometry and composition are listed in Table 2. The PdeRu membranes were prepared at the Dalian Institute of Chemical Physics (DICP), where they were synthesized on pre-treated porous tubular asymmetric g-Al2O3/a-Al2O3
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 1 e Electroless plating baths composition. Components
[Pd(NH3)2]Cl2 RuCl3 NH3H2O (25e28%) HCl N2H4 EDTA$2Na Polyethyleneimine, PEI Temperature
Values Pd-plating
Ru-plating
2 g/L e 300 ml/L e 0.6 g/L 55 g/L e 62e64 C
e 10 mmol/L 1000 mmol/L 5 mmol/L 50 mmol/L 10 mg/L 60 C
Table 2 e Geometry and composition of the membranes used in this report. The “active” prefix is referred to the effective Pd length and area measurements taken after the sealing operation. The Pd-layer thicknesses were measured after membrane characterization with the Scanning Electron Microscopy (SEM). Geometry parameters
PdAg
PdRu#1
PdRu#2
Active surface length [mm] Diameter [mm] Active surface [cm2] Pd-layer thickness [mm] Composition wt.% ratio
88.0 10.2 28.7 3.4 PdeAg 95e5
102.2 10.2 32.7 1.8 PdeRu 98.7e1.3
101.2 10.2 32.5 1.7 PdeRu 99.2e0.8
supports of a 10.2 mm diameter and a 0.1 mm grade. The fabrication of PdeRu composite membranes followed a sequential plating process: first, the Pd layer was fabricated by a modified electroless plating method involving a preliminary support seeding [17], and then the Ru-layer was deposited by electroless plating on top of the Pd layer. The metal layers were eventually deposed on tubular 3 mm-thick a-alumina porous supports. Composition and operating conditions of the plating bath are listed in Table 1. Afterwards, membranes were cleaned several times with deionized water following the plating completion. Before permeation tests, all membranes were kept at a temperature of 400 C under argon for 48 h to favour the interdiffusion of ruthenium in the Pd lattice. Since the alloying procedure of Pd and Ru is critical due to the large difference
3
between their melting points (1555 C and 2334 C, respectively), we decided to reach a maximum Ru composition of around 1.3%, as a higher amount would have been affected the integrity of the ultra-thin metal layer. As for the PdeAg membrane, the low Ag content was selected to minimise the embrittlement.
Experimental setup The permeation tests were carried out at the laboratories of Eindhoven University of Technology (TU/e). Both membrane sides were sealed using Swagelok® fittings and graphite ferrules as described elsewhere [35]. Fig. 1 shows an example of membrane (PdeAg) before and after sealing, respectively. Prior to permeation tests, the sealed membranes were submerged in ethanol and then helium permeation flux was measured to detect any possible leaks. The results of the leakage tests are reported in Table 3. In particular, PdAg and PdRu#1 show an acceptable leakage level, whereas that of PdRu#2 is one order of magnitude higher. After the preliminary leakage tests, the membranes were placed in the permeation module (Fig. 2a), where up to five membranes can be tested simultaneously by appropriate valves placed on the top. In this way, the permeation through each membrane can be measured separately. The module was configured in order for the feed stream to flow up from the bottom of the shell, which has a diameter of 10 cm, whereas the permeate stream (high-purity hydrogen) passes through the membrane exiting the top. Flow rates were measured automatically using an interface software, by which temperature and pressure were also monitored at multiple points along the module.
Results and discussion Permeation tests Long-term hydrogen permeation tests (530 h) were performed within a temperature range of 400e550 C at a pressure difference of 2 bar for both hydrogen and nitrogen tests keeping the permeate pressure at the atmospheric value. Afterwards, membranes were further tested at 600 C for additional 185 h (for a total time of 715 h) for thermal resistance checking
Fig. 1 e Pictures of a membrane (a) before and (b) after sealing. Example of the PdeAg membrane. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 3 e Leakage test results of membranes with He as leakage detector at PFeed ¼ 2 bar and room temperature. He vol. flow [ml min1]
Membrane conditions
PdAg Dry (in air) Bottom submerged Bottom þ Middle submerged Full submerged
0.171 0.163 0.021 0.020
PdRu#1 0.140 0.031 0.028 0.005
PdRu#2 0.371 0.346 0.120 0.080
Apparent He permeating flux [mmol m2 s1] PdAg 9.93 9.47 1.22 1.16
4
10 104 104 104
PdRu#1
PdRu#2
4
19.0 104 17.7 104 6.15 104 4.10 104
7.14 1.58 1.43 0.26
10 104 104 104
Fig. 2 e Pictures of (a) permeation module and (b) conceptual drawing.
(Fig. 3). In particular, first, membranes were tested at 400 C for 165 h, then at 500 for 30 h, successively at 550 for 335 h and finally at 600 C. Based on the trends shown in Fig. 3a, we can distinguish the behaviour below 600 C and that at 600 C. In the former case, it can be observed that, at each temperature, the hydrogen flux increases progressively up to reaching a steady value, most likely corresponding to the completion of the alloying process. Overall, all the three membranes show similar behaviour up to 550 C, with the extent of hydrogen flux that increases according to the following order: PdAg < PdRu#1 < PdRu#2. As regards the nitrogen flux at the same conditions (below 600 C), which is evaluated from 400 C to check the defect extent in runtime, it reaches a steady value at each temperature after a continuous increase, similar to what observed for the hydrogen flux. This indicates that the defect extent
increases with increasing temperature for the three membranes, even though the nitrogen flux of the PdAg sample is one order of magnitude lower than that of PdRu#1 and two orders lower than that of PdRu#2 due to also the lower membrane thickness of the PdeAg membrane. The situation is different at 600 C, where a peculiar observation is made analysing the combined behaviour of hydrogen and nitrogen fluxes. In fact, for the PdAg and PdRu#1 membranes, the nitrogen flux is found to increase continuously, as also observed in a previous work [37], whereas the hydrogen flux decreases with time. In general, the permeation can decrease at high temperature for the strong interaction between palladium and alumina support [38e40]. Furthermore, the continuous increase of nitrogen flux with time could be due to sintering of Pd grains, which lead to the formation of defects. Also PdRu#2 shows this trend, but not in the whole time range where the
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 3 e Hydrogen permeation flux at different temperatures for each membrane a) in pure-H2 conditions and b) in pure N2 conditions. PFeed ¼ 2 bar, PPerm ¼ 1 bar.
temperature of 600 C is set. This can be explained with the higher amount of defects present on the surface of PdRu#2 with respect to the other two samples. A more detailed explanation for this behaviour can be provided analysing the trend of membrane selectivity, which is defined for our purposes as the ratio of the single-gas hydrogen and nitrogen fluxes (Eq. (1)). The selectivity values are reported in Table 4. S¼
JH2 JN2 At the same conditions
(1)
As expected from the results shown in Fig. 3, the most selective membrane among the investigated ones was found to be the conventional PdeAg membrane, followed by the PdRu#1 and PdRu#2 at all temperature values considered.
Table 4 e Selectivity evaluated based on Eq. (1) at different temperatures using the H2 and N2 fluxes taken at the end of each data set. 885. Selectivity [] T [ C] 400 500 550 600
PdAg
PdRu#1
PdRu#2
>56544 >68725 4948 547
2465 2611 547 16
830 486 204 18
Beside the PdeAg membrane, which shows a virtually infinite selectivity, the PdRu#1 one shows an acceptable value up to 500 C. For the PdRu#2 sample, some problems must have been occurred already during preparation probably due to a non-uniform seeding on the surface, as the selectivity value is relatively low for dense metal membranes already at 400 C. It is notable that selectivity increases from 450 to 500 C, which is explained by considering that the resistance offered by the metal layer to the hydrogen transport lowers with temperature thanks to the fact that the internal diffusion is an activated process. Therefore, in conditions of relatively high temperature, the hydrogen transport is favoured with respect to the nitrogen transport through defects and porous support, which is a non-activated process. However, this is not valid for sufficiently high defect level, as the hydrogen transport preferentially occurs through defects instead of through the metal layer. This causes selectivity to tend to the Knudsen theoretical value or, for a more seriously damaged membrane, to the viscous flow one, which is a non-selective transport mechanism. This is just what occurs for the PdeRu membranes at 600 C in the last part of the time range, where the presence of pinholes becomes so serious that both hydrogen and nitrogen fluxes increase with time. The defect extent, which dramatically increases at temperature above around 550 C, is caused by the coalescence of smaller defects inducing larger ones. Beside the PdeAg membrane, considering the thinness of our PdeRu membranes, the stability and the mechanical
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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strength of the PdRu#1 membrane are satisfactory up to 550 C, whereas the PdRu#2 one has started to be severely damaged above 500 C. Using the steady values of flux at each temperature reported in Fig. 3 along with additional data at different pressure values and 450 C, we evaluated the membrane permeability based on Sieverts’ law (Eq. (2)) as a function of temperature (Eq. (3)). For this purpose, the hydrogen flux is reported as a function of the square root of the hydrogen pressure difference (Fig. 4), whose trend is found to be linear with a good approximation (R2 ranging from 0.9930 to 0.9960). Due to the poor selectivity obtained at 600 C, such a temperature was not considered in this analysis. The related results are shown in an Arrhenius-type plot (Fig. 5). JH2 ¼
d
f pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pH2 ;Feed pH2 ;Perm
Mem
(2)
The corresponding calculated parameters, which assume similar values as those reported for PdeAg membranes in other literature works [41e43], are listed in Table 5.
Ea f ¼ f0 exp RT
(3)
In order to analyse in a more detail the permeation behaviour and the effect of the possible influence of support and defects, the values of the pressure exponent (n) appearing in the empirical Sieverts law (Eq. (4)) are calculated along with the corresponding permeabilities at various temperatures (Table 7). JH2 ¼
fðnÞ n p pnH2 ;Perm dMem H2 ;Feed
(4)
Table 6 reports permeability and selectivity data about several membranes of the literature compared to the PdeRu ones prepared in this work. Specifically, we can observe that only the membranes prepared by Gade et al. (2009) [29] are comparable for thickness with ours, although the testing time used by those Authors is around 1/tenth less than that used in
Fig. 5 e Activation energy lines obtained by logarithmic rearrangement of the permeability Arrhenius-type equation.
Table 5 e Derived pre-exponential factor f0 and activation energy Ea. Arrhenius factors Ea f0 R2
1
[kJ mol ] [109 mol s1 m1 Pa0.5] []
PdAg
PdRu#1
PdRu#2
8.3 33.6 0.9989
9.7 33.5 0.9988
9.4 28.9 0.9988
our work, which does not allow making a completely fair comparison. As reported in several previous works, significant information about the mass transport mechanisms determining the overall permeation process can be withdrawn only in pure hydrogen conditions [44], like in the present case. In
Fig. 4 e Hydrogen permeation flux as a function of Sieverts' driving force at different temperatures. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Table 6 e Comparison with some literature PdeRu membranes in terms of permeability and selectivity. Sample
Composition PdeRu [wt%]
PdRu#1
98.7e1.3
PdRu#2 CSM 493H PdeRu Pd-1 Pd-2 Pd-3 Multilayer Ru/Pd
99.2e0.8 99.7e0.3 99.8e0.2 95.5e4.5 95.0e5.0 90.0e10.0 11.4/88.6
Conditions 500 C PFeed Max ¼ 500 C PFeed Max ¼ 550 C PFeed Max ¼ 580 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼ 500 C PFeed Max ¼
Permeability [109 mol s1 m1 Pa0.5]
Selectivity
Refs
1.8
7.41
2611
This Work
1.7
6.70
486
This Work
6.0
12.6
1860
[4]
5.0
13.7
>200
[8]
1.9
12.0
18200
[29]
2.9
14.2
1850
[29]
4.5
15.4
2780
[29]
6.8/0.085
23.0
∞
[20]
Thickness [mm]
200kPa 200kPa 840kPa 2700kPa 137:9kPa 137:9kPa 137:9kPa 200kPa
Table 7 e Pressure exponent and related parameters as functions of temperature for each sample. Temperature
550 C
500 C
450 C
400 C
Parameters n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 [] n [] f, mol m1 s1 R2 []
Pan
Pan
Pan
Pan
particular, all the exponent values lay within the range 0.64e0.69, which means that there are some phenomena making membranes deviate from the ideal behaviour (corresponding to n ¼ 0.5). In the considered operating conditions, the possible phenomena able to induce such a discrepancy in the metal alloys are i) the non-ideal diffusion in the metal lattice, ii) Knudsen and/or viscous flow within possible pinholes and iii) resistance of the gas permeation by the porous support. The non-ideal diffusion makes the pressure exponent decrease with increasing temperature, whereas the last two transport phenomena cause the opposite behaviour [45e47]. Therefore, to check which of them is dominant, we show the trend of pressure exponent with temperature (Fig. 6). As for the PdeAg membrane, we observe a regularly increasing trend, which indicates that, although the variability of n is relatively narrow, the mass transport in the support and/or possible defects is playing an appreciable role. It must be underlined that it is not generally possible to distinguish the influence of the transport through the membrane defects from that of the transport through the support from the value of the pressure exponent only. The former, however, can be estimated from the previously calculated selectivity value.
PdAg
PdRu#1
PdRu#2
0.663 1.08 109 0.9967 0.657 1.09 109 0.9961 0.649 1.111 109 0.9961 0.641 1.12 109 0.9973
0.645 1.01 109 0.9980 0.641 0.98 109 0.9967 0.680 0.532 109 0.9971 0.666 0.56 109 0.9962
0.648 0.96 109 0.9976 0.645 0.93 109 0.9975 0.690 0.465 109 0.9974 0.675 0.50 109 0.9968
As for both the PdeRu samples, they show very similar irregular trends having a maximum and a minimum. Although such an irregularity within 400e550 C can be attributed to the experimental error due to the low-pressure range considered, the unexpected behaviour could be differently explained as follows: up to 450 C, the transport through support and defects plays a significant role, providing an appreciable resistance causing the pressure exponent to increase with temperature tending to the theoretical Knudsen value (n ¼ 1). Between 450 and 500 C, the thermal effect could make grains enucleate, inducing a higher relative resistance in the metal layer with respect to the resistance provided by defects and porous support, this making the pressure exponent to decrease with temperature. Above around 500 C, the transport in through the metal layer becomes faster than that in the defects and support and, thus, n starts increasing again with temperature.
XRD and SEM characterization The last series of experiments involves the characterization of fresh and used membranes, i.e., before and after the experiments, respectively.
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Fig. 8 e XRD pattern for PdRu#1 membrane. Fig. 6 e Pressure exponent as a function of temperature for each sample.
The XRD results for each membrane are shown below in Figs. 7e9. In particular, for the PdeAg membrane, the presence of some double peaks in the spectrum of the fresh membrane is observed, which is an indication o that the membrane is still not perfectly alloyed. During the permeation experiments under hydrogen, the alloy is formed as reflected in the sharp single peak of the spectrum of the used membrane. The gradual increase in the hydrogen permeation observed in Fig. 5 is an indication of the alloying process. Regarding the PdRu#2, the XRD spectrum shows a consistent presence of corundum, the crystalline form of a-alumina, which arises from the support. In all cases, the XRD diagrams show higher peaks for the spectrum of the used membranes with respect to the fresh ones for all membranes.
Fig. 7 e XRD pattern for PdeAg membrane.
Table 8 reports the grain size as estimated using the Scherrer equation (Eq. (5)). L¼
Kl Dð2wÞcosðwÞ
(5)
where L is the grain size, K is a shape factor taken equal to the unity, q is the diffraction angle (in radians), l is the wavelength and D(2q) is the full width at half maximum (also indicated as FWHM in the literature). Figs. 10e12 show the three pairs of fresh/used samples. Furthermore, a peculiar observation is made concerning the internal structure of the metal layer, which shows a sort of bubble-shaped cavities after usage. A similar phenomenon is reported in the paper of Wassie et al. (2018), who carried out reforming and partial oxidation reactions for hydrogen production and CO2 capture [48]. The interesting fact to note is that in this case such a stress appears to produce not much cracks, but rather bubbles, whose sphericity indicates that the induced stress is locally isotropic. If we consider the internal grain redistribution as a process
Fig. 9 e XRD pattern for PdRu#2 membrane.
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Table 8 e Grain size for the three membranes estimated by means of Eq. (5). l ¼ 0.1541 nm. Sample
(1 1 1)
(2 0 0)
2q [deg] Grain Size (L) 2q [deg] Grain Size PdAg Fresh PdAg Used PdRu #1 Fresh PdRu #1 Used PdRu #2 Fresh PdRu #2 Used
39.32 40.24 40.06 40.3 41.1 41.22
17 66 69 75 34 43
46.66 46.78 46.64 46.92 47.70 47.84
12 51 42 52 31 40
tending to minimise the interface energy interactions, the formation of a distributed bubble-shaped structure is the most efficient way for the lattice to relax the local stresses. In fact, at the same level of stress, a higher number of bubbles allows a more efficient stress redistribution within the lattice volume, also considering that sphere is the geometrical shape that has the smallest external area at a certain volume. From this point of view, this phenomenon is similar to that happening for soap bubbles in water, where the surface
9
tension locally minimises the surface area inducing a spherical shape. The picture of the membrane structures depicted in Figs. 10e12 before and after testing presents a possible explanation of the reason why the permeating flux decreases with time at 600 C. In particular, during hydrogen permeation, the presence of hydrogen atoms allows the formation of hydride phases [50], whose interphase starts enucleating within the metal lattice generating plastic deformations that eventually lead to sphere-like bubble shapes (Figs. 10be12b). Since permeation has a preferential direction, we actually think that such bubbles are not formed by the permeation itself but rather by the hydrogen absorption, which occurs also at zero driving force in hydrogen atmosphere. At a sufficiently high temperature, the so-described enucleation mechanism becomes significant and, thus, the bubble growth pushes the metal layer into the pores of the support, making in this way the adhesion stronger and densifying the metal-support interface. This occurrence causes the flux to decrease with time up to reach a possible minimum or a steady trend. Specifically, for the PdAg sample, neither the minimum nor the steady trend are found in the
Fig. 10 e Pictures of the PdeAg membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one.
Fig. 11 e Pictures of the PdRu#1 membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one. Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 12 e Pictures of the PdRu#2 membrane made by SEM showing the cross section of (a) a fresh sample and (b) a used one.
time range considered, as the level of defects is so low that it is not detectable. On the contrary, for the PdeRu membranes, the flux reaches a steady trend in the case of the PdRu#1 sample and a minimum for the PdRu#2 one. The difference between the two cases is that in the former the defect level keeps constant, whereas in the latter the defect level increases with time, this being stated by the poor selectivity values at 600 C reported in Table 7. The fact that the bubble shapes are poorly visible in the SEM picture of the PdRu#2 sample could indicate that the major presence of defects could have a certain influence on the bubble formation. However, a further deeper investigation about this aspect is required and no clear conclusion can be withdrawn at the moment. The presence of the bubble-shaped cavities is also related to a more practical issue related to the membrane thickness evaluation. In fact, at the same metal amount, the nominal thickness increases as the bubbles grows up during permeation tests. The cross-section analysis is accompanied with a surface analysis to check the defect degree for
both fresh and used samples, whose SEM images are shown in Figs. 13e15. The first membrane analysed is the PdeAg one, which shows an increase of crystal size during tests. This is a further confirmation of what depicted in the previous XRD patterns. Furthermore, some pinholes of about 800 nm of diameter (visible as little black stains) are formed during experiments. The surface of the fresh membrane changed to a more platelike one due to an increase in the crystal domain size by coalescence growth [49]. Similar behaviour is observed in the PdeRu membranes. Concerning the PdRu#1, a consistent number of pinholes is noticed, and this is the cause of the very high N2 fluxes recorded in the final nitrogen permeation tests. As discussed above, this is in close relationship with the low thickness of PdeRu separation layer. The PdRu#2 seems to have a surface morphology different from the others, at maybe this could be e along with some trouble occurred during the tubular membrane preparation e one of the cause of the low selectivity seen during the N2 tests, and this refers not only to the tests carried
Fig. 13 e SEM images of the PdAg membrane for (a) the fresh sample and (b) the used one.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
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Fig. 14 e SEM images of the PdRu#1 membrane for (a) fresh sample and (b) used one. The red circles underline the major defects (pinholes) on the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 15 e SEM images of the PdRu#2 membrane for (a) the fresh sample and (b) the used one. The red circles underline the major defects (pinholes) on the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
out after the high temperature experiments, but also to the previous ones, which show a N2 flux four times higher than the “twin” PdRu#1.
Conclusions In this work, one supported PdeAg membrane (ca. 3.5 mm) and two PdeRu ones. (ca. 1.8 mm), here named PdAg, PdRu#1 and PdRu#2, respectively, were prepared and characterised. The membrane behaviour to hydrogen permeation was studied under pure-hydrogen and pure-nitrogen conditions at different temperatures, performing also a long-term test (around 750 h). These tests allowed us to evaluate membrane permeability along with the behaviour of the pressure exponent with temperature, from which the transport
phenomena occurring during overall permeation process were identified. The high-temperature experiments at 550 C highlighted the good thermal resistance of all prepared membranes, whereas the PdeRu ones showed a certain vulnerability at higher temperature (600 C), which severely affects their selectivity. Strangely, at this temperature, the hydrogen flux was found to slightly decrease with respect to 550 C, whereas the flux of nitrogen was observed to increase. This was explained by considering the effect of alloy grains redistribution along with the strong interaction of the extremely thin membrane layer with the porous support. Furthermore, a peculiar observation was made regarding the internal structure of the metal layer, showing bubble-like cavities after usage. The reason for the formation of such a structure was explained by considering that the grain surface interaction energy tends to minimise the surface area by
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creating spherical shapes analogously to what occurs for the surface tension inducing a spherical shape to soap bubbles in water. The decrease in hydrogen flux observed at 600 C was attributed to the bubble formation, which tends to push the metal alloy deeper in the pores of the support.
Acknowledgements A. Caravella has received funding through the “Programma Per Giovani Ricercatori «Rita Levi Montalcini»” granted by the e della Ricerca, “Ministero dell’Istruzione, dell’Universita MIUR” (Grant no. PGR12BV33A), which is gratefully acknowledged. H. Li has received financial support from the 100-Talent Project of CAS, National Natural Science Foundation of China (Grant No. 21676265; 51501177; 21306183), and The Ministry of Science and Technology (MOST) of the People's Republic of China (Grant No. 2016YFE0118300).
references
~ o M, Sa nchez-Herva s JM. Pd-based [1] Conde JJ, Maron membranes for hydrogen separation: review of alloying elements and their influence on membrane properties. Separ Purif Rev 2017;46:152e77. [2] Brunetti A, Caravella A, Fernandez E, Pacheco Tanaka DA, Gallucci F, Drioli E, Curcio E, Viviente JL, Barbieri G. Syngas upgrading in a membrane reactor with thin Pd-alloy supported membrane. Int J Hydrogen Energy 2015;40:10883e93. [3] Peters TA, Rørvik PM, Sunde TO, Stange M, Roness F, Reinertsen TR, Ræder JH, Larring Y, Bredesen R. Palladium (Pd) membranes as key enabling technology for precombustion CO2 capture and hydrogen production. Energy Procedia 2017;114:37e45. [4] El Hawa HWA, Paglieri SN, Morris CC, Harale A, Way JD. Identification of thermally stable Pd-alloy composite membranes for high temperature applications. J Membr Sci 2014;466:151e60. [5] Adams BD, Chen A. The role of palladium in a hydrogen economy. Mater Today 2011;14:282e9. [6] Bredesen R, Peters TA, Tim B, Dittmeyer R. Pd-based membranes in hydrogen production for fuel cells. In: Gallucci F, van Sint Annaland M, editors. Process intensification for sustainable energy conversion. John Wiley & Sons; 2015. p. 209e41. [7] Li H, Caravella A, Xu H. Recent progress in Pd-based composite membranes. J Mater Chem A 2016;4:14069e94. [8] El Hawa HWA, Paglieri SN, Morris CC, Harale A, Way JD. Application of a Pd-Ru composite membrane to hydrogen production in a high temperature membrane reactor. Separ Purif Technol 2015;147:388e97. [9] Lee SM, Xu N, Su Kim S, Li A, Grace JR, Lim CJ, Boyd T, Ryi SK, Susdorf A, Schaadt A. Palladium/ruthenium composite membrane for hydrogen separation from the off-gas of solar cell production via chemical vapor deposition. J Membr Sci 2017;541:1e8. [10] Darling AS, Yorke JM. The ruthenium-palladium system, Plat. Met Rev 1960;4:104e10. [11] Gryaznov VM, Mishchenko AP, Polyakova VP, Roshan NR, Savitskij EM, Smirnov VS, Khrapova EV, Shimulis VI. Palladium-ruthenium alloys as membrane catalysts. Dokl Akad Nauk SSSR 1973;211:624e7.
[12] Skakunova EV, Ermilova MM, Gryaznov VM. Hydrogenolysis of propane on a palladium-ruthenium alloy membrane catalyst. Bull Acad Sci USSR, Div Chem Sci 1988;37:858e61. [13] Kleykamp H. Constitution and thermodynamics of the MoRu, Mo-Pd, Ru-Pd and Mo-Ru-Pd systems. J Nucl Mater 1989;167:49e63. [14] Tripathi SN, Bharadwaj SR, Dharwadkar SR. The Pd-Ru system (palladium-ruthenium). J Phase Equilibria 1993;14:638e42. [15] Gurler R. A computer assessment of the Ru-Rh and Ru-Pd systems. J Alloy Comp 1993;191:31e5. [16] Cabrera AL, Morales E, Armor JN. Kinetics of hydrogen desorption from palladium and ruthenium-palladium foils. J Mater Res 1995;10:779e85. [17] Zhang J, Xu H, Li W. High-purity COx-free H2 generation from NH3 via the ultra-permeable and highly selective Pd membranes. J Membr Sci 2006;277:85e93. [18] Wu D, Kusada K, Kitagawa H. Recent progress in the structure control of Pd-Ru bimetallic nanomaterials. Sci Technol Adv Mater 2016;17:583e96. [19] Xu N, Kim SS, Li A, Grace JR, Lim CJ, Boyd T. Preparation and characterization of palladiumeruthenium composite membrane on alumina-modified PSS substrate. Can J Chem Eng 2014;92:1041e7. [20] Ryi SK, Li A, Lim CJ, Grace JR. Novel non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation. Int J Hydrogen Energy 2011;36:9335e40. [21] Xu N, Kim SS, Li A, Grace JR, Lim CJ, Boyd T. Novel electroless plating of ruthenium for fabrication of palladium-ruthenium composite membrane on PSS substrate and its characterization. J. Membr. Sci. Res. 2015;1:73e8. [22] Uemiya S, Kajiwara M, Kojima T. Composite membranes of group VIII metal supported on porous alumina. AIChE J 1997;43:2715e23. [23] Nam SW, Yoon SP, Ha HY, Hong SA, Maganyuk AP. Methane steam reforming in a Pd-Ru membrane reactor. Kor J Chem Eng 2000;17:288e91. [24] M Gryaznov V, Orekhova NV. Reactors with metal and metalcontaining membranes. In: Cybulski Andrej, Moulijn Jacob A, editors. Structured catalysts and reactors. CRC Press; 2005. p. 435e62. ISBN: 9780824723439. [25] Burkhanov GS, Gorina NB, Kolchugina NB, Roshan NR. Palladium-based alloy membranes for separation of high purity hydrogen from hydrogen-containing gas mixtures. Platin Met Rev 2011;55:3e12. [26] Wu D, Kusada K, Kitagawa H. Recent progress in the structure control of Pd-Ru bimetallic nanomaterials. Sci Technol Adv Mater 2016;17:583e96. [27] Colin JGE. Engelhard Industries, inc., hydrogen diffusion process. United States patent US 1966 Mar 8;3(238):700. [28] Cabrera A, Hasen J, Schuller IK. Structural changes induced by hydrogen absorption in palladium and palladiumruthenium alloys. Appl Phys Lett 1995;66:1216e8. [29] Gade SK, Keeling MK, Davidson AP, Hatlevik O, Way JD. Palladium-ruthenium membranes for hydrogen separation fabricated by electroless co-deposition. Int J Hydrogen Energy 2009;34:6484e91. [30] Ferrari P, Diaz-Droguett DE, Rojas S, Cabrera AL. Inhibition of hydrogen absorption in bulk Pd by the formation of RuePd surface alloy. Thin Solid Films 2014;550:732e7. [31] Hubkowska K, Koss U, Łukaszewski M, Czerwinski A. Hydrogen electrosorption into Pd-rich Pd-Ru alloys. J Electroanal Chem 2013;704:10e8. [35] Fernandez E, Helmi A, Coenen K, Melendez J, Viviente JL, Pacheco Tanaka DA, van Sint Annaland M, Gallucci F. Development of thin Pd-Ag supported membranes for fluidized bed membrane reactors including WGS related gases. Int J Hydrogen Energy 2015;40:3506e19.
Please cite this article as: Liu J et al., Hydrogen permeation and stability in ultra-thin PdeRu supported membranes, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.212
international journal of hydrogen energy xxx (xxxx) xxx
[36] Pacheco Tanaka DA, Llosa Tanco MA, Niwa S, Wakui Y, Mizukami F, Namba T, Suzuki TM. Preparation of palladium and silver alloy membrane on a porous a-alumina tube via simultaneous electroless plating. J Membr Sci 2005;247:21e7. [37] Peters TA, Tucho WM, Ramachandran A, Stange M, Walmsley JC, Holmestad R, Borg A, Bredesen R. Thin Pd-23% Ag/stainless steel composite membranes: long-term stability, life-time estimation and post-process characterisation. J Membr Sci 2009;326:572e81. [38] Okazaki J, Ikeda T, Pacheco Tanaka DA, Llosa Tanco MA, Wakui Y, Sato K, Mizukami F, Suzuki TM. Strong interaction at the palladium/alumina interface of membrane during hydrogen permeation at elevated temperature. Chem Lett 2008;37:1004e5. [39] Okazaki J, Ikeda T, Pacheco Tanaka DA, Sato K, Suzuki TM, Mizukami Fujio. An investigation of thermal stability of thin palladiumesilver alloy membranes for high temperature hydrogen separation. J Membr Sci 2011;366:212e9. [40] Okazaki J, Ikeda T, Pacheco Tanaka DA, Llosa Tanco MA, Wakui Y, Sato K, Mizukami F, Suzuki TM. Importance of the support material in thin palladium composite membranes for steady hydrogen permeation at elevated temperatures. Phys Chem Chem Phys 2009;11:8632e8. [41] Shu J, Bongondo BE, Grandjean BP, Adnot A, Kaliaguine S. Surface segregation of Pd-Ag membranes upon hydrogen permeation. Surf Sci 1993;291:129e38. [42] Serra E, Kemali M, Ross DK, Perujo A. Hydrogen and deuterium in Pd-25 pct Ag alloy: permeation, diffusion, solubilization, and surface reaction. Met. Mater. Trans. A 1998;29:1023e8. [43] Nguyen TH, Mori S, Suzuki M. Hydrogen permeance and the effect of H2O and CO on the permeability of Pd0.75Ag0.25 membranes under gas-driven permeation and plasmadriven permeation. Chem Eng J 2009;155:55e61.
13
[44] Caravella A, Scura F, Barbieri G, Drioli E. Sieverts law empirical exponent for Pd-based membranes: critical analysis in pure H2 permeation. J Phys Chem B 2010;114:6033e47. [45] Caravella A, Hara N, Negishi H, Hara S. Quantitative contribution of non-ideal permeability under diffusioncontrolled hydrogen permeation through Pd-membranes. Int J Hydrogen Energy 2014;39:4676e82. [46] Caravella A, Hara S, Drioli E, Barbieri G. Sieverts law pressure exponent for hydrogen permeation through Pd-based membranes: coupled influence of non-ideal diffusion and multicomponent external mass transfer. Int J Hydrogen Energy 2013;38:16229e44. [47] Caravella A, Hara S, Sun Y, Drioli E, Barbieri G. Coupled influence of non-ideal diffusion and multilayer asymmetric porous supports on Sieverts law pressure exponent for hydrogen permeation in composite Pd-based membranes. Int J Hydrogen Energy 2014;39:2201e14. [48] Wassie SA, Medrano JA, Zaabout A, Cloete S, Melendez J, Pacheco Tanaka DA, Amini S, van Sint Annaland M, Gallucci F. Hydrogen production with integrated CO2 capture in a membrane assisted gas switching reforming reactor: proof-of-Concept. Int J Hydrogen Energy 2018;43:6177e90. [49] Okazaki J, Ikeda T, Pacheco Tanaka DA, Suzuki TM, Mizukami F. In situ high-temperature X-ray diffraction study of thin palladium/a-alumina composite membranes and their hydrogen permeation properties. J Membr Sci 2009;335:126e32. [50] Peters TA, Carvalho PA, van Wees JF, Overbeek JP, Sagvolden E, van Berkel FPF, Løvvik OM, Bredesen R. Leakage evolution and atomic-scale changes in Pd-based membranes induced by long-term hydrogen permeation. J Membr Sci 2018;563:398e404.
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