γ-alumina graded membrane by electroless plating on nanoporous γ-alumina

γ-alumina graded membrane by electroless plating on nanoporous γ-alumina

Journal of Membrane Science 324 (2008) 181–187 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 324 (2008) 181–187

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Fabrication of a novel Pd/␥-alumina graded membrane by electroless plating on nanoporous ␥-alumina Masahiro Seshimo a,∗ , Minoru Ozawa a , Masato Sone b , Makoto Sakurai a , Hideo Kameyama a a b

Department of Chemical Engineering, Tokyo University of Agriculture & Technology, 2-24-16 Naka-Cho, Koganei-shi, Tokyo 184-8588, Japan Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

a r t i c l e

i n f o

Article history: Received 31 March 2008 Received in revised form 27 June 2008 Accepted 1 July 2008 Available online 10 July 2008 Keywords: Palladium Electroless plating ␥-Alumina Growth mechanism Composite

a b s t r a c t This study investigated a method of electroless plating to fabricate a novel functionally graded Pd/␥alumina/anodic alumina composite membrane. Electroless plating reactions were carried out on catalyzed anodic alumina surfaces and on catalyzed nanoporous ␥-alumina in an electroless plating solution. The reactions readily formed a smooth Pd membrane on the catalyzed nanoporous ␥-alumina, but failed to form a Pd membrane on the catalyzed anodic alumina. According to surface SEM images, assessment of the rate of Pd deposition into the nanoporous ␥-alumina, and cross-section analysis by EPMA, the Pd membrane grew into the nanoporous ␥-alumina in the direction of depth from 2 h up to 5 h during the electroless plating reaction, and then grew upward in the direction of thickness after 5 h. The agitation speed of the electroless plating bath influenced the growth of the Pd/␥-alumina graded layer from 2 to 5 h during the reaction, and also influenced the crystalline growth of the Pd. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The fuel cell has been widely studied as a clean energy device over the past several years. The polymer electrolyte fuel cell (PEFC) draws energy from high-purity hydrogen, a resource expected to be in growing demand in the future. Among the techniques feasible for high-efficiency hydrogen production, a membrane reactor with a hydrogen permselective membrane shows promise for the production of high-purity hydrogen [1–5]. The hydrogen permselective membrane is composed of mainly Pd or a Pd alloy membrane formed by a method such as electroplating [6], electroless plating [7–9], or chemical vapor deposition [10]. There is also a method that relies on a non-Pd hydrogen permselective membrane composed of mainly silica [11]. The membrane reactor with the hydrogen permselective membrane (Pd alloy membrane or silica membrane) can only be used with full effectiveness if the membrane is extremely thin and durable. Itoh et al. studied a Pd membrane deposited on the straight mesopores of anodic alumina by electroplating [12]. Volpe et al., on the other hand, studied Pd electroless deposition on anodic alumina [13]. Both groups obtained Pd membranes with good hydrogen permeation and separation from other gases, although the desired

∗ Corresponding author. Tel.: +81 42 388 7692. E-mail address: [email protected] (M. Seshimo). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.007

thicknesses of the Pd layers on the anodic alumina, i.e., about 5 ␮m, was not obtained. Kameyama et al. developed a method to fabricate nanoporous anodic alumina for application as a support for catalyst particles [14–16]. Porous anodic alumina serves as a highly dispersive support with excellent structural flexibility even when exposed to heat shock. To exploit the advantages of anodic alumina, we have proposed a method for forming a Pd membrane by Pd electroless plating in nano-sized anodic alumina pores, with hot water treatment (HWT) to produce a nanoporous ␥-alumina layer with improved adhesion [17]. In this research we propose a new method of Pd electroless plating via the induction of a plating reaction into pores for an improved hydrogen permselective membrane and improved support adhesion. We investigate the Pd formation into the nanoporous ␥-alumina pores during the electroless plating reaction by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). 2. Experimental 2.1. Preparation of the Pd/-alumina/anodic alumina composite membrane The process used to prepare the Pd/␥-alumina/anodic alumina composite membrane is shown in Fig. 1. This process was composed

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7 h in an electroless plating process maintained at 353 K, at plating bath agitation speeds of 10, 500, and 800 rpm. 2.2. Experimental apparatus and method Pd membrane surface images were obtained with a scanning electron microscope (FE-SEM, S-4500, Hitachi High-Tech Co.) and cross-section were analyzed with an electron probe microanalyzer (EPMA, JXA-8900, JEOL Ltd.). A precise cross-section of the Pd/␥-alumina/anodic alumina composite membrane was fabricated with a focused ion beam system (FIB, FB-2100, Hitachi High-Technologies Co. Ltd.) with scanning ion microscopy (SIM) so that the thickness of the plated film could be measured directly from the SIM image (20,000×) on the screen. The rate of Pd deposition on the nanoporous ␥-alumina was measured at hourly intervals based on the capacity of the increase of mass. Fig. 1. Preparation of the Pd/␥-alumina/anodic alumina composite membrane by electroless plating.

of an initial anodic oxidation, followed by pretreatment, electroless plating, and a second anodic oxidation. Anodic oxidation. An anodic oxidation reaction was induced on one side of an aluminum plate [14–16] for 30 min (Sample 1), or by immersion of an aluminum plate in a sulfuric acid solution for 45 min (Sample 2). The reactions were conducted at a voltage of 110 V and electric current density of 150 A/m2 using an electrode measuring 5.0 cm × 5.0 cm by 0.1 mm in thickness. Once prepared, the plates were treated by HWT for 1.5 h at a water temperature of 353 K to improve the BET-specific surface. Catalyzation. Pd and Sn particles were deposited onto the HWT anodic alumina as a nanoporous ␥-alumina surface simultaneously via a catalyzation reaction induced by immersing the nanoporous ␥-alumina in a PdCl2 /SnCl2 mixture solution for 1 min and then washing the plates with HCl. The PdCl2 /SnCl2 solution consisted of PdCl2 (100 mg/l), SnCl2 (10 mg/l), and HCl (87.5 g/l). After catalyzation, the plate surfaces were washed with sulfuric acid to remove the Sn particles from the deposited membrane of Pd. Pd plating. A pure Pd membrane was formed by Pd electroless plating. Pd electroless plating solutions consisting of PdCl2 (50 mg/l), HCOONa (100 mg/l), and NH2 CH2 CH2 NH2 (10 mg/l), respectively, were purchased from Okuno Chemical Industries Co. Ltd. The Pd plating reactions were conducted in a range from 1 to

3. Results and discussion 3.1. Pd electroless plating on anodic alumina and nanoporous -alumina via HWT Fig. 2 shows a nanoporous ␥-alumina surface after HWT (Fig. 2(a)) and an anodic alumina surface before HWT (Fig. 2(b)). The latter has a straight mesoporous structure. Kameyama et al. reported a network of interconnected pores of nanoporous ␥-alumina, most measuring about 4 nm, on an anodic alumina sample after HWT, and a hexagonally ordered arrangement of vertical nanopores on an anodic alumina sample untreated by HWT [14–16]. The nanoporous ␥-alumina was found to have a somewhat smoother porous structure than the anodic alumina sample untreated by HWT. This suggests that thin, uniformly connected, and flocculated aggregates of Pd can be formed on the smoother porous structure obtained by HWT for electroless plating. The surface SEM images in Fig. 3 were photographed after the catalyzation for the electroless plating. The former shows a catalyzed sample on nanoporous ␥-alumina; the latter, a catalyzed sample on anodic alumina. The Pd particles in Fig. 3(a) are dispersed on the nanoporous ␥-alumina surface, whereas those in Fig. 3(b) appear connected and flocculated on the anodic alumina surface under low magnification (4000×). This difference can be attributed to the excellent BET-specific surface area of nanoporous ␥-alumina compared to that of anodic alumina.

Fig. 2. Surface SEM images of (a) nanoporous ␥-alumina and (b) anodic alumina substrates.

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Fig. 3. SEM images of catalyzed (a) nanoporous ␥-alumina (30,000×) and (b) anodic alumina (4000×) surfaces.

The surface SEM images in Fig. 4 were photographed after 6 h of electroless plating. The nanoporous ␥-alumina surface in Fig. 4(a) was covered with Pd after 6 h of plating. In contrast, the anodic alumina surface shown in Fig. 4(b) was not covered with Pd at the same time point. Pd was deposited into the anodic alumina pores in the study by Volpe et al. [13], whereas in our study it was deposited on the anodic alumina surface. This result suggested that Pd particles are flocculated in the fashion shown in Fig. 3(b), and that Pd deposition occurs around the flocculated Pd particles on the anodic alumina surface with the straight mesoporous structure. If this is so, it would explain the effective and uniform formation of Pd on the nanoporous ␥-alumina via HWT (see Fig. 2(b)). 3.2. Analysis of the Pd deposition on a nanoporous -alumina surface over time The six surface SEM images shown in Fig. 5 were photographed at hourly intervals during the electroless plating reaction, from 2 h (Fig. 5(a)) up to 7 h (Fig. 5(f)). As the images clearly show, the Pd deposition onto the nanoporous ␥-alumina took place in two distinct steps. In the first step, from 2 to 4 h, the Pd was deposited into the nanoporous ␥-alumina pores and formed a covering over the nanoporous ␥-alumina surface. Next, once the Pd completely blanketed the pores, a granulate Pd was deposited over the surface with

a progressively larger grain size as the electroless plating reaction proceeded. In other words, the Pd deposition on the nanoporous ␥alumina changed from a needle-shaped deposition to a spheroidal deposition at some point between 4 and 5 h during the electroless plating reaction. Fig. 6 shows the change of the rate of Pd deposition onto the nanoporous ␥-alumina over time during the electroless plating reaction. As the graph indicates, no increase in the mass of the sample was observed until the electroless plating reaction had proceeded for 2 h. This might be the time required to induce the electroless plating reaction by our method. The rate of Pd deposition increased only slightly from 2 to 4 h, then rose sharply between 4 and 5 h. The results shown in Figs. 5 and 6 suggest that the low Pd deposition rate from 2 to 4 h relative to that from 4 h onwards resulted from the limited range of Pd deposition inside the nanoporous ␥-alumina pores. On this basis, we can speculate that the rate of Pd deposition from 2 to 4 h may be influenced by the penetration of the electroless plating solutions into the nanoporous ␥-alumina pores. The rapidly increasing rate of Pd deposition just afterwards, from 4 to 5 h, was attributable to the deposition and fusing of the granulate Pd over the nanoporous ␥-alumina. We describe the Pd deposition into the nanoporous ␥-alumina pores as a “Pd/␥-alumina graded layer.” After 5 h of the Pd electroless

Fig. 4. SEM images of Pd membrane plated on (a) nanoporous ␥-alumina and (b) anodic alumina after 6 h of electroless plating.

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Fig. 5. SEM observation of the surface at hourly intervals during the electroless plating reaction (SEM, 15,000×): (a) 2 h, (b) 3 h, (c) 4 h, (d) 5 h, (e) 6 h, and (f) 7 h.

plating, the rate of the Pd deposition surpassed the rate observed from 2 to 4 h. We can therefore conclude that the Pd deposition inside the nanoporous ␥-alumina pores switched to spheroidal deposition on the nanoporous ␥-alumina surface after 5 h, as shown in Fig. 5(d)–(f). Thus, the membrane obtained had a three-layer structure consisting of a Pd layer, a Pd/␥-alumina graded layer, and an anodic alumina layer, from the surface inward, as shown

Fig. 6. Change of the Pd deposition rate as the electroless plating reaction proceeded.

in Fig. 7. A composite membrane of Pd/ceramic/porous stainless steel tested by Li et al. exhibited improved durability, as the ceramic layers were effective in preventing interdiffusion between the Pd membrane and porous stainless steel [18]. In our study, the Pd/␥alumina/anodic alumina composite membrane between the Pd and ␥-alumina may have provided a buffer against the stress generated by the difference between the thermal expansion coefficients of the Pd and ␥-alumina. With such an effect, the Pd/␥-alumina/anodic alumina composite membrane could be expected to improve the durability. Figs. 8, 9 and 10 show the results of cross-section EPMA analyses of Pd membranes plated on nanoporous ␥-alumina via plating reactions conducted for 4, 5, and 7 h, respectively. The Pd/␥-alumina graded layer in the figures appears as a piling of peaks for Pd and oxygen of alumina. After 4 h of electroless plating, a Pd/␥-alumina graded layer of about 2.0 ␮m in thickness was obtained. At 5 and 7 h, the membrane thickness had increased to about 2.5 ␮m. The thickness of the Pd/␥-alumina graded layer at 5 h was greater than that at 4 h. The nanoporous ␥-alumina pore penetration of the Pd electroless plating solution was apparently still underway during this 60-min period. In comparison between 5 and 7 h, however, no change in the thickness of the Pd/␥-alumina graded layer could be observed. Thus, our results confirmed that the pure Pd membrane on the nanoporous ␥-alumina surface was adequately thickened by 7 h. The results of the electroless plating reaction observed between 2 and 5 h indicate a deposition of Pd with nanoporous ␥-alumina pores and the formation of a Pd/␥-alumina graded layer. After 5 h of electroless plating, the Pd was clearly deposited in a vertical

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Fig. 7. Pd/␥-alumina/anodic alumina composite membrane cross-section SEM image of a three-layer structure consisting of a Pd layer, Pd/␥-alumina graded layer, and anodic alumina layer.

Fig. 9. EPMA cross-section analyses of the surface after 5 h of electroless plating.

Fig. 8. EPMA cross-section analyses of the surface after 4 h of electroless plating.

Fig. 10. EPMA cross-section analyses of the surface after 7 h of electroless plating.

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Fig. 11. Thickness of the pure Pd membrane on the surface of the nanoporous ␥alumina and Pd/␥-alumina graded layer.

Fig. 12. Relationship between the agitation speed and thicknesses of the Pd/␥alumina graded layer and Pd over the nanoporous ␥-alumina surface.

direction, forming a Pd membrane over the nanoporous ␥-alumina surface. Fig. 11 charts the change in the Pd thickness over time, calculated from the data on the Pd deposition rate shown in Fig. 6. The Pd thickness is the thickness of the pure Pd membrane that appeared on the surface of the nanoporous ␥-alumina and Pd/␥-alumina graded layer. As the figure shows, the Pd thicknesses of both samples increased almost rectilinearly, and the increase from 2 to 5 h was exponential. Given that the Pd was deposited within only a limited range of pores, this pattern of increase indicates that the electroless plating solution penetrated the nanoporous ␥-alumina pores in a controlled fashion. The thickness of the Pd deposition in the vertical direction over the nanoporous ␥-alumina surface increased linearly after 5 h. The penetration of the electroless plating solutions into the nanoporous ␥-alumina pores was an important factor for the formation of the Pd/␥-alumina graded layer. Noting this, we tried to control the growth of the graded layer by changing the agitation speed of the plating bath from 10 to 800 rpm. Fig. 12 charts the relationship between the agitation speed and thickness of the Pd/␥-alumina graded layer. The total Pd membrane thickness on

nanoporous ␥-alumina was the summation of the pure Pd layer and Pd/␥-alumina graded layer. As Fig. 12 illustrates, the thickness of the Pd/␥-alumina graded layer increased at the higher agitation speed. The pure Pd on the nanoporous ␥-alumina surface, meanwhile, became thinner. Thus, we could attribute the Pd deposition in the direction of depth to the increase in the penetration of the nanoporous ␥-alumina pores by the electroless plating solution as the higher agitation speed. An agitation speed of about 300 rpm appears to be optimum for Pd consumption and for adhesion between the Pd and the nanoporous ␥-alumina. Fig. 13 shows surface SEM images after Pd electroless plating for 6 h, at different agitation speeds of 10 rpm (Fig. 13(a)) and 500 rpm (Fig. 13(b)). The Pd membrane formed by three-dimensional aggregation and fusion of spheroidal Pd deposition on the nanoporous ␥-alumina at the agitation speed of 500 rpm (Fig. 13(b)) has a smoother surface than the membrane formed at the agitation of 10 rpm (Fig. 13(a)). Thus, the agitation speed of the electroless plating bath apparently influenced the Pd membrane surface formation. The results shown in Figs. 12 and 13 suggest that a high agitation speed in the electroless plating bath may inhibit Pd deposition in the vertical direction and promote Pd deposition into the nanoporous ␥-alumina pores.

Fig. 13. SEM images of Pd plated on a nanoporous ␥-alumina surface after 6 h of electroless plating at agitation speeds of (a) 10 rpm and (b) 500 rpm in the plating bath.

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Better surface conditions and thicker Pd/␥-alumina graded layers formed in electroless plating baths at high agitation speeds are expected to improve hydrogen permeability and membrane durability. Our group will soon be reporting the results of measurements of the hydrogen permeability and membrane durability. 4. Concluding remarks In this study we fabricated a novel Pd/␥-alumina/anodic alumina composite membrane by electroless plating on nanoporous ␥-alumina. By studying the Pd/␥-alumina graded layer formation on the nanoporous ␥-alumina, we sought to elucidate the mechanisms of catalyst nucleation and membrane growth. The Pd was deposited into the nanoporous ␥-alumina pores from 2 to 5 h during the electroless plating, thus forming the Pd/␥-alumina graded layer. From 5 h onwards into the reaction, the Pd granulate was deposited over the Pd/␥-alumina graded layer, forming a pure Pd membrane. The agitation speed of the electroless plating bath significantly influenced the formation of the Pd/␥-alumina graded layer into nanoporous ␥-alumina pores. At a high agitation speed, the Pd grew upward in the direction of depth ascendancy, promoting the growth of the Pd/␥-alumina graded layer. At a low agitation speed, the Pd grew upward in the direction of thickness ascendancy and a pure Pd membrane grew on the surface of the nanoporous ␥alumina. These results suggest that the method we propose allows us to improve the adhesion of the Pd/␥-alumina graded layer by controlling the thickness of the layer to an optimal level. The agitation speed of the electroless plating bath also influenced the Pd deposition. When grown under an increased agitation speed, the Pd membrane had a smoother surface. Through the experiments in this study, we established a method for growing a Pd/␥-alumina graded layer into nanoporous ␥-alumina. The Pd/␥-alumina graded layer between the Pd and ␥-alumina layers provided a buffer against the stress generated by the difference between the thermal expansion coefficients of the Pd and ␥-alumina. The Pd/␥-alumina/anodic alumina composite membrane can thus be expected to improve durability. References [1] J. Tong, Y. Matsumura, Effect of catalytic activity on methane steam reforming in hydrogen-permeable membrane reactor, Appl. Catal. A 286 (2005) 226.

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