Al2O3 membranes by palladium deposition into the pores

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An improvement of the hydrogen permeability of C/Al2O3 membranes by palladium deposition into the pores Chan Wang, Jian Yu, Xiaojuan Hu, Yan Huang* State Key Laboratory of Materials-oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China

article info

abstract

Article history:

The development of compact hydrogen separator based on membrane technology is of key

Received 15 November 2012

importance for hydrogen energy utilization, and the Pd-modified carbon membranes with

Received in revised form

enhanced hydrogen permeability were investigated in this work. The C/Al2O3 membranes

26 December 2012

were prepared by coating and carbonization of polyfurfuryl alcohol, then the palladium

Accepted 1 January 2013

was introduced through impregnationeprecipitation and colloid impregnation methods

Available online 1 February 2013

with a PdCl2/HCl solution and a Pd(OH)2 colloid as the palladium resources, and the reduction was carried out with a N2H4 solution. The resulting Pd/C/Al2O3 membranes were

Keywords:

characterized by means of SEM, EDX, XRD, XPS and TEM, and their permeation perfor-

Hydrogen-permeable membrane

mances were tested with H2, CO2, N2 and CH4 at 25
C. Compared with the colloid

Carbon membrane

impregnation method, the impregnationeprecipitation is more effective in deposition of

Palladium deposition

palladium clusters inside of the carbon layer, and this kind of Pd/C/Al2O3 membranes

Impregnationeprecipitation

exhibits excellent hydrogen permeability and permselectivity. Best hydrogen permeance,

Colloid impregnation

1.9  107 mol/m2 s Pa, is observed at Pd/C ¼ 0.1 wt/wt, and the corresponding H2/N2, H2/ CO2 and H2/CH4 permselectivities are 275, 15 and 317, respectively. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Compactness of the hydrogen separators is of key importance for their use in hydrogen energy systems, and therefore the membrane approach becomes a most attractive choice against other hydrogen separation methods [1,2]. Among various kinds of hydrogen-permeable membranes [3,4], carbon ones have gained extensive attentions because of their excellent thermal and chemical stability. Apart from hydrogen, they can be utilized in separation of many other gases, and promising separation can be achieved even between two gases with close molecular size [5,6]. Currently, a challenging problem that hindered the wide application of the carbon membranes

remains to be the low hydrogen permeability, and many conventional stratagems to satisfy the membrane permeability often, unfortunately, decrease the membrane permselectivity [7]. One important solution is to support the carbon membrane on a porous substrate, so that the membrane thickness can be greatly reduced without causing too much difficulty in maintaining membrane integrity during fabrication. Besides, the introduction of porous materials, such as zeolite, silica and carbon nanotubes, into the carbon membrane matrix is also recognized to be an effective solution, and the mechanism relies on their molecular sieving ability and gas diffusivity [8e10]. However, the incorporation of these materials into the

* Corresponding author. Tel.: þ86 25 83172253. E-mail address: [email protected] (Y. Huang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.013

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carbon membrane matrix is often a complicated process, and some of them are quite expensive. Several researchers functionalized the carbon membranes by doping metal nanoparticles. Barsema et al. [11] found that silver nanoclusters may increase the carbon membrane flux by acting as a spacer, but the oxygen permeability can be specially enhanced because of the oxygen adsorption on silver. Yoda et al. [12] prepared the Pt- and Pd-doped carbon membranes by dispersion of metal compounds into the membrane precursor via “supercritical impregnation”, they did not tell the effects of the Pt-doping on membrane performance, and they claimed that the Pd-doping decreased both the hydrogen and the nitrogen permeability though increased the H2/N2 selectivity. Zhang et al. [13] prepared Ni-doped carbon membranes based on a porous ceramic substrate, and they found that the membrane performance behaves complicatedly but depends strongly on the nickel content. As reported in literature so far, the doping of metals have been done by dispersing metal compounds into the membrane precursor, therefore the metal clusters are spread homogeneously inside of the carbon matrix and firmly encapsulated by carbon. In this case, the metal clusters can influence the gas diffusion mainly by means of adsorption and blocking effects. In addition, the metal clusters have to go through the high-temperature carbonization conditions that are necessary for membrane formation, and the metal clusters will be sintered once they contact with each other. Hence, the final resulting metal particles are isolated. Being different from the literature approaches, this work attempts to introduce the palladium clusters directly into the pores of the carbon membrane, and the metal clusters may exchange the adsorbed gas molecules more easily, forming into a rapid hydrogen diffusion pathway. The particular selection of palladium as the metal cluster material is because the hydrogen can easily diffuse both along the palladium surface and through the palladium bulk, and indeed the carbon membranes with enhanced hydrogen permeability were achieved.

2.

Experimental

2.1.

Preparation

Some porous Al2O3 tubes (o.d., 13 mm; i.d., 8 mm; length, 70 mm) which mean and largest pore sizes are ca. 2.7 and 7.4 mm, respectively, were employed as the membrane substrate, and their surface was modified by solegel method [14]. They were dip-coated with the polyfurfuryl alcohol, which was synthesized by polymerization of furfuryl alcohol with oxalic acid as catalyst [15]. The coated Al2O3 tubes were then dried at 80
C for 12 h and heated under argon atmosphere at 700
C for 4 h with a temperature ramp of 1
C/min, followed by a natural cooling to room temperature. The resulting C/Al2O3 membranes were further subjected to palladium doping in the following two ways:

2.1.1.

Impregnationeprecipitation

The C/Al2O3 membranes were sealed with rubber stoppers and immersed in a solution of 50 mL/L hydrochloric acid (38 wt.%) and 4 g/L PdCl2. After a certain time, the membranes

were successively soaked in a NaOH solution (1 mol/L) for 10 min and in a N2H4 solution (2 mol/L, 30
C) for 20 min. After cleaning with deionized water, the membranes were dried in a vacuum oven at 80
C for 3 h and then stored in a desiccator. For convenience of description, the resulting membranes were denoted as Pd/C/Al2O3eIP.

2.1.2.

Colloid impregnation

A NaOH (1 mol/L) solution was added dropwise into the PdCl2 solution (PdCl2, 4 g/L; HCl, 50 mL/L) at 30
C under stirring until the pH value reached 8e9, and a dark-brown Pd(OH)2 colloid was obtained. The C/Al2O3 membranes were successively immersed in the Pd(OH)2 colloid for 15 min and in the N2H4 solution (2 mol/L; 30
C) for 10 min, and the other posttreatments were the same as the those in the impregnationeprecipitation method. The resulting membranes were denoted as Pd/C/Al2O3eCI.

2.2.

Characterization

The pore size of the Al2O3 substrate was measured with GaoQ PSDA-20 porometer by bubble-point method. The membrane morphologies were observed by scanning electron microscopy (SEM) with an FEI Quanta-200 system, and the line-scan of X-ray energy dispersion (EDX) was carried out to detect the palladium dispersion profile along the membrane crosssection. X-ray diffractometer (XRD) analysis was conducted using a Bruker D8 advanced diffractometer with Cu Ka radiation of 1.54  A wavelength, and the tubular specimens were placed in parallel to X-ray window [16]. X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG ESCALAB MKII spectrometer with Mg Ka excitation (1253.6 eV), and the binding energy was calibrated with the C 1s band at 284.6 eV. The following sequence of spectra was recorded: C 1s, survey spectrum, Pd 3d and C 1s, where the C 1s was recorded for two times to check the instrument stability and the possible sample degradation during analyses. Transmission electron microscopy (TEM) analyses were done on Hitachi H-800 to study the dispersion of Pd clusters in carbon membranes. The gas permeability tests were carried out with H2, CO2, N2 and CH4 at 25
C following single-gas mode. Each membrane was assembled in a home-made compact module [15], giving an effective membrane length of 4 cm. The pressure at the retentate side was 1 bar, while the permeate pressure was always ambient. The membrane selectivity between two gases is defined as the ratio of their fluxes.

3.

Results and discussion

According to the literature on metal-doped carbon membranes, it is often believed that the metal particles should be homogeneously dispersed throughout the carbon matrix so that the gas diffusion can be improved, and the metal compound must be introduced into the precursor of the carbon membranes [17]. The metallic clusters are often formed insitu during high-temperature treatment in carbon membrane preparation. On one hand, this will lead to an encapsulation of the metal clusters by the carbon matrix and a consequent decrease in their availability for gas adsorption;

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Fig. 1 e SEM and EDX results of the C/Al2O3 and Pd/C/Al2O3 (Pd/C [ 0.05 wt/wt) membranes. (a), (b) and (c): SEM views on the surface of the fresh C/Al2O3 membrane, the Pd/C/Al2O3eIP and the Pd/C/Al2O3eCI membranes; (d) and (e): Cross-section views of Pd/C/Al2O3eIP and Pd/C/Al2O3eCI; (f) EDX results of Pd/C/Al2O3eIP and Pd/C/Al2O3eCI.

on the other hand, the carbonization temperature that is necessary for carbon membrane formation is always very high, and the metal clusters will be unavoidably sintered. Since the gas diffusion mainly takes place through the pores of the carbon membranes, and only the metal clusters in the pore channels can promote the gas diffusion effectively. In alternative to creating the metal clusters during carbon membrane preparation, this work introduces them thereafter. The Pd-modified carbon membranes Pd/C/Al2O3eIP and Pd/C/Al2O3eCI were prepared by means of impregnationeprecipitation and colloid impregnation, respectively. Our preliminary experiments indicated that the loading amount of palladium depends strongly on the soaking time of the carbon membrane in the impregnation liquids, and thus the palladium content can be controlled. In this work, a series of Pd/C/ Al2O3 membranes with different palladium content were prepared, and the content of palladium may be defined as the ratio of the palladium weight vs. the carbon weight. Both

weights were measured according to the weight gain. The SEM results of the fresh and the Pd-modified carbon membranes (Pd/C ¼ 0.05 wt/wt) are demonstrated in Fig. 1. The fresh carbon membrane looks highly smooth, homogeneous and defect-free. Particles with a size of 0.1e0.2 mm are seen on the Pd/C/Al2O3 membranes, but more of them are on Pd/C/ Al2O3eCI, and then its palladium concentration inside of the carbon layer will be relatively lower because the overall content of palladium is the same for the two membranes. The cross-section observations of Pd/C/Al2O3eIP and Pd/C/ Al2O3eCI are displayed in Fig. 1(d) and (e). The carbon layers are continuous and well anchored on the Al2O3 substrate, and the thickness of the both carbon membranes is about 7 mm. The “AB” lines marked in Fig. 1(d) and (e) indicate the positions for line-scan EDX analysis on elements Pd, C, O and Al, and the molar content profile of Pd is plotted in Fig. 1(f). With the increase in the analyzing depth of the carbon layer, the palladium concentration in Pd/C/Al2O3eIP tends to decrease

Fig. 2 e XPS spectra of Pd/C/Al2O3eIP (below) and Pd/C/Al2O3eCI (above) membranes (Pd/C [ 0.05 wt/wt).

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Fig. 3 e XRD patterns of (a) Al2O3 substrate, (b) fresh C/Al2O3 membrane, (c) Pd/C/Al2O3eCI and (d) Pd/C/ Al2O3eIP (Pd/C [ 0.05 wt/wt).

slightly, while that in Pd/C/Al2O3eCI decreases rapidly, and most of the palladium appears only at the surface and nearsurface. These results are well consistent with the observations in Fig. 1(b) and (c). It can be concluded that the palladium clusters of Pd/C/Al2O3eCI mainly stay on the surface and have been poorly dispersed into the carbon membrane, while the palladium distribution in Pd/C/Al2O3eIP is more homogeneous. Such a great difference relates closely to the preparation method. Compared between the impregnationeprecipitation and the colloid impregnation processes, the palladium precursors are Pd2þ and Pd(OH)2, respectively, and the former can diffuse easily into the membrane pores while the latter cannot because of its larger molecular size. The XPS analysis was carried out directly on the surface of Pd/C/Al2O3eIP and Pd/C/Al2O3eCI (Pd/C ¼ 0.05 wt/wt), and the

results are shown in Fig. 2. Besides of Pd and C signals, that of Al was also seen from Fig. 2 (Left). Since the detection depth of XPS is only several nanometers, while the carbon layer thickness is up to 7 mm, the Al signal might originate from the contamination by the Al2O3 substrate material during membrane breaking for sampling. Compared with Pd/C/Al2O3eIP, the Pd/C/Al2O3eCI exhibits much stronger Pd 3d and weaker C1s responses. These findings coincide with the observations in Fig. 1(b) and (c). For both carbon membranes, Fig. 2 (Middle) is the typical signals of pure carbon, and Fig. 2 (Right) can be ascribed to the responses of Pd0, which 3d 5/2 binding energy is around 335.5 eV [18,19]. XRD results of the fresh and the Pd-modified carbon membranes (Pd/C ¼ 0.05 wt/wt) are shown in Fig. 3, and the XRD pattern of Al2O3 substrate was also given for reference. Despite of the existence of the carbon layer, the Al2O3 signals still appear in XRD pattern of the fresh C/Al2O3 membrane, and this is no surprising because of the super penetrability of the X-ray through carbon. However, the Al2O3 signals become quite weak in the patterns of the Pd-modified carbon membranes, and this is due to the fact that the metallic palladium particles can effectively block the penetration of X-ray. For all the three patterns, a peak appears around 2q ¼ 24
, which is the characteristic XRD response of carbon membrane and can be attributed to (002) crystalline plane of carbon. For the two Pd-modified carbon membranes, the peaks around 2q ¼ 40.04, 46.48 and 68.04
can be ascribed to Pd (111), Pd (200) and Pd (220) responses of the face-centered cubic (f.c.c.) crystalline of Pd0 (JCPDS 46-1043) [20]. Comparison between Fig. 3(c) and (d) indicates that the palladium XRD peaks of Pd/C/Al2O3eIP are significantly broader than those of Pd/C/Al2O3eCI, and therefore the average particle size of the palladium cluster in Pd/C/ Al2O3eIP will be smaller than that in Pd/C/Al2O3eCI according to Scherer theory. The XRD responses of palladium are contributed not only by the palladium particles on the membrane surface but also by those inside of the carbon layer. For Pd/C/ Al2O3eIP, most of its palladium clusters are dispersed inside of the carbon layer, and they will be extremely small because the pore size of the carbon membranes is usually below 1 nm [6,21]. In case of Pd/C/Al2O3eCI, there is no significant XRD broadening effect, indicating most of its palladium clusters

Table 1 e Comparison between the composite carbon membranes in this work and those reported in literature. Membrane

C/Al2O3 Pd/C/Al2O3eIPa Pd/C/Al2O3eCIa C/Al2O3 C/Al2O3 C/Al2O3 SBA-15/C/Al2O3 C/Resin C/SS316 a Pd/C ¼ 0.05 wt/wt. b Not mentioned.

Permselectivity

Permeance (109 mol/m2 s Pa)

Ref.

H2

CO2

N2

CH4

H2/N2

H2/CO2

H2/CH4

41 144 55 1.0 17 32 25 1.7 2.6

10 12 11 2.2 3.4 11 12 0.17 0.19

0.92 0.64 0.83 0.4 0.44 0.96 0.63 0.055 0.12

0.41 0.60 0.46 eb 0.37 1.9 0.48 0.058 0.004

45 225 66 2.3 39 33 39 31 21

4.1 12 5.0 0.5 5.1 2.8 2.1 9.9 14

100 240 120 eb 46 16 51 29 640

This work This work This work [13] [25] [25] [26] [27] [28]

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Table 2 e Effects of the palladium content on the permeation performances of the Pd/C/Al2O3eIP membranes. Pd/C (wt/wt) 0.05 0.1 0.2 0.4

Permeance (109 mol/m2 s Pa)

Permselectivity

H2

CO2

N2

CH4

H2/N2

H2/CO2

H2/CH4

144 190 180 177

12 13 12 11

0.64 0.69 0.63 0.60

0.60 0.60 0.51 0.47

225 275 286 295

12 15 15 16

240 317 353 377

must be on the surface of carbon layer, and this well agrees with the SEM findings in Fig. 1(b) and (c). The permeation performances of the fresh and the Pdmodified C/Al2O3 membranes (Pd/C ¼ 0.05 wt/wt) were tested with H2, CO2, N2 and CH4 at 25
C, and the results are listed in Table 1. In general, the gas permeances are in the following order: H2 > CO2 > N2 > CH4, which is reverse to the order of their kinetic molecular diameter, indicating that the gas diffusion should be mainly governed by molecular sieving mechanism [22,23]. Compared with the fresh C/Al2O3 membrane, both Pd/C/Al2O3eIP and Pd/C/Al2O3eCI show a higher hydrogen permeance and similar permeances of CO2, N2 and CH4. Particularly, the Pd/C/Al2O3eIP reveals the best hydrogen permeance and permselectivity. It was well known that the hydrogen molecules can easily adsorb and dissociate on the

palladium surface. Moreover, the hydrogen atoms can be also dissolved into the palladium bulk, and they can easily diffuse with the concentration gradient as the driving force [24]. Hence, the improved hydrogen permeability of the Pdmodified carbon membranes should be attributed to the enhanced hydrogen diffusion by palladium clusters. Namely, the better dispersion of palladium clusters in the carbon layer of Pd/C/Al2O3eIP resulted in a better hydrogen permeance, and the palladium clusters created a rapid hydrogen diffusion pathway. Table 1 also lists the permeation data of some composite carbon membranes reported in literature. It can be found that the hydrogen permeance and permselectivity of Pd/C/Al2O3eIP are superior. The effects of the palladium content (Pd/C ¼ 0.05, 0.1, 0.2, 0.4 wt/wt) on the permeation performances of the Pd/C/

Fig. 4 e TEM images of the Pd/C/Al2O3eIP membranes with Pd/C ratio (wt/wt) of (a) 0.05, (b) 0.1, (c) 0.2 and (d) 0.4.

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Al2O3eIP membranes were further studied, and the results are listed in Table 2. With the increase of the Pd/C ratio, the hydrogen permeance increases up to a maximum, 1.9  107 mol/m2 s Pa at Pd/C ¼ 0.1, and then decreases very slightly, while the permeances of CO2, N2 and CH4 are almost insensitive to the Pd/C ratio. Therefore, the best Pd/C weight ratio is 0.1, and the corresponding H2/N2, H2/CO2 and H2/CH4 permselectivities are 275, 15 and 317, respectively. To directly view the palladium nanoclusters, the TEM analysis was performed, and the results are displayed in Fig. 4. In all of the four Pd/C/Al2O3eIP samples, the palladium clusters are uniformly dispersed, and the increasing Pd/C ratio increases the concentration of palladium clusters in the carbon layer. The average particle size of palladium clusters is 0.5e0.7 nm. However, due to the limitation of the TEM magnificence, some smaller palladium clusters might be unseen, and perhaps they have played a key role in hydrogen permeation. The palladium clusters shown by TEM is much smaller than those shown by SEM on the membrane surface (cf. Fig. 1(b) and (c)), and this is because the migration and agglomeration of palladium nuclei have been limited inside the pores of the carbon membrane.

[4]

[5]

[6]

[7] [8]

[9]

[10]

[11]

4.

Conclusions

The Pd-modification of the carbon membranes can be carried out by post-treatment, and impregnationeprecipitation is found to be an effective way, which can well disperse the palladium clusters inside of the carbon layer. The resulting Pd/ C/Al2O3 membranes exhibit promoted hydrogen permeability and permselectivity, which are found to be better than the composite carbon membranes reported in literature so far. The increase in palladium content may improve the hydrogen permeability but such an improvement is not infinite.

[12]

[13]

[14]

[15]

Acknowledgments [16]

We are grateful to the financial support by the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province, China (09KJA530003). Wang C. thanks the Graduate Student Research Innovation Foundation of the Higher Education Institutions of Jiangsu Province, China (No. CXZZ11_0348).

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