Electroless plating synthesis, characterization and permeation properties of Pd–Cu membranes supported on ZrO2 modified porous stainless steel

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Abstract

Thin Pd–Cu membranes were prepared by electroless plating technique on porous stainless steel (PSS) disks coated with a mesoporous palladium impregnated zirconia intermediate layer. This intermediate layer provides seeds for electroless plating growth of Pd–Cu film during synthesis and serves as an inter-metallic diffusion barrier that improves membrane stability for practical application. XPS analyses showed that the average surface compositions of the two membranes were respectively Pd84Cu16 and Pd46Cu54 (at.%). XRD analyses indicated that deposited Pd84Cu16 alloy film contained only α-fcc (face-centred cubic) phase structure whereas Pd46Cu54 alloy film contained a mixture of α-fcc and β-bcc phases. The 10 μm thick Pd46Cu54/ZrO2-PSS membrane exhibited an infinite separation factor for H2 over N2, with H2 permeance of 1.1 × 10−7 mol/m2 s Pa at 753 K. The activation energies for hydrogen permeation and hydrogen pressure exponents are respectively 14.5 kJ/mol and 0.6 for Pd84Cu16/ZrO2-PSS membrane and 15.4 kJ/mol and 1 for Pd46Cu54/ZrO2-PSS composite membrane. The different permeation properties for the two membranes are discussed in terms of different permeation mechanisms associated with membrane thickness, structure, surface composition and morphology.

Introduction

The interest in palladium alloy membranes has recently resurged because of the prospective of the hydrogen economy. Metal membranes are expected to play a key role in hydrogen production (as membrane reactors or separators) [1], [2], [3], [4]. Compared to the pure palladium membrane, Pd-alloyed membranes have higher permselectivity and permeation rate, better resistance to hydrogen embrittlement and poisoning by gaseous impurities (H2S, CO and H2O, etc.) [5], [6], [7], and the advantages of lower material costs. Generally, palladium alloy composite membranes are prepared as a thin, dense layer on a porous support. Porous glass [8], [9], ceramics [10], [11], [12], [13] and stainless steel (PSS) [9], [11], [14], [15], [16], [17] have been employed as the supports. A variety of techniques are available to deposit these metallic films, including electroless plating [4], [8], [9], magnetron sputtering [13], [18], chemical vapor deposition (CVD) [18] and electroless plating [17]. Especially, the modified electroless plating technique has a number of advantages over other preparation methods. These include uniformity of deposits on complex shapes, hardness of the deposits, low preparation cost, and simplicity in the equipment required for membrane synthesis.

Majority of the work related to Pd-based alloy membranes was on Pd–Ag alloy. For instance, Lin and coworkers [18] have deposited ultrathin (0.1–1.5 μm) 75%Pd–25%Ag films onto porous ceramic substrates consisting of a macroporous α-Al2O3 disk coated with a sol–gel derived, mesoporous γ-Al2O3 top layer using the methods of MOCVD and magnetron sputtering. Pd–Ag alloy films have also been prepared and studied by other research groups utilizing the methods of sequential electroless plating or co-depositing followed by annealing [7], [16], [19]. However, Pd–Ag alloys are quite expensive and prone to poisoning by sulfur-containing gases. Recently, composite membranes made from alloys of palladium and copper have received more attention [4], [17], [20], [21], [22], [23] because Pd–Cu alloy system offers more desirable properties and lower cost than Pd–Ag alloy.

Pd–Cu alloy is structurally different from Pd–Ag alloy. Pd–Ag forms a disordered face-centered cubic (fcc) structure at all composition. In contract, Pd–Cu alloy can be present either as the disordered fcc structure (α-phase) or ordered body-center cubic (bcc) CsCl structure (β-phase) at temperatures below 873 K [24], as illustrated in Fig. 1. The lattice parameter of the representative alloy with 52.7 at.% Cu was found as a0 = 3.76 × 10−10 m (3.76 Å) in the α-phase and a0 = 2.97 × 10−10 m (2.97 Å) in the β-phase [25]. The alloys with about 55 to 62 at.% Cu consist of the bcc structure only (after proper annealing treatment) [24]. Annealing temperatures for a given alloy composition has a significant effect on the alloy phase [25].

Howard et al. [21] recently studied hydrogen permeation through thick (100 μm) Pd–Cu alloy foils with different compositions and phase structures. They found that variation of phase structure has a strong effect on hydrogen permeance of Pd–Cu membranes. For instance, compared to Pd60Cu40 (wt.%) alloy (bcc structure) the Pd80Cu20 (wt.%) alloy (fcc structure) is less permeable at 623 K, but more permeable above 840 K. In 623–1173 K, the Pd80Cu20 (wt.%) alloy displays a higher hydrogen permeability than Pd53Cu47 (wt.%) alloy (both being in the bcc and fcc structure below and above 873 K). The higher hydrogen permeability for Pd60Cu40 (wt.%) alloy is due to higher mobility of hydrogen atoms within the bcc structure relative to the fcc continuous solid solution. On the other hand, the activation energy for diffusion of hydrogen in the β-phase (bcc structure) is smaller than that in the α-phase (fcc structure), although the bcc ordered structure of β-phase is more dense than the fcc structure of the α-phase [25].

Ceramic supported thin Pd–Cu membranes with different alloy compositions were synthesized by a few research groups. Roa and Way [4] prepared Pd–Cu alloy composite membranes by successive electroless deposition of Pd and Cu onto various tubular porous ceramic supports. They found that the metallic film chemical composition had a strong influence on the membrane performance. For example, (∼1.5 μm) Pd60Cu40 (wt.%) (dominating bcc structure) alloy composite membrane showed higher H2 permeance (0.12 mol/m2 s Pa0.5) than other membranes tested with different alloy compositions. They attributed this difference to the higher H2 diffusivity in the β-phase (bcc structure). They also reported hydrogen permeation with pressure exponent of 0.515 and suggested that the H-atom diffusion in the bulk metal was the rate-controlling step [4]. In contract, a thin (∼2 μm thickness) Pd63Cu37 (wt.%) (in fcc structure) alloy composite membranes with a diffusion barrier (SiO2) were fabricated on Ni-modified porous stainless steel support by Nam and Lee [17] using vacuum electrodeposition. H2 permeance and H2/N2 selectivity were, respectively, 8.48 × 10−6 mol/m2 s Pa and 70,000 at 723 K for the membrane with the hydrogen permeance pressure exponent of 1. They believed that the surface reaction was rate-limiting in hydrogen transport. Both studies did not give activation energy data for hydrogen permeation through thin Pd–Cu membranes. Ma et al. [22] synthesized Pd–Cu alloy film on in situ oxidized porous stainless steel support by electroless plating technique. The work was focused on the microstructure of the interfaces between the metal film, oxide layer and porous stainless steel support. Hydrogen permeance data were reported recently [23].

The studies summarized above represent initial efforts but significant advances in making thin Pd–Cu membranes for hydrogen separation. However, from the divergent results described above, more work is required to improve synthesis and structure of supported thin Pd–Cu membranes and to understand the relationship between the membrane composition and structure to hydrogen permeation properties of Pd–Cu membranes. In this paper, we report electroless plating synthesis of Pd–Cu alloy films on porous stainless steel support modified with Pd seeded mesoporous ZrO2 layer. The new structure is advantageous over Pd–Cu on ceramic support because Pd seeded ZrO2 layer not only facilitates electroless-plating growth of Pd–Cu film but also provides a diffusion barrier layer between the Pd–Cu film and the stainless steel support. Comprehensive hydrogen permeation data for the Pd–Cu membranes were measured and used to elaborate the structural and permeation properties for the Pd–Cu membranes.

Section snippets

Preparation of Pd–Cu/ZrO2–PSS membranes

A Pd doped colloidal zirconia sol was prepared by the sol–gel method. Zirconyl chloride octahydrate (ZrOCl2·8H2O) and the organic alkali hexamethylenete tramine (C6H12N4) were dissolved in deionized water respectively at room temperature. Then the solution of the organic alkali hexamethylenete tramine was slowly added to the ZrOCl2·8H2O solution while stirring, resulting in a stable zirconia sol at pH 3–4. Finally small amounts of 5 wt.%polyvinyl alcohol (PVA) solution and 5 mM PdCl2 solution

Results

Two groups of Pd–Cu membranes with different Cu/Pd ratios were prepared in this work. Typical compositions for each group of the membranes were respectively Pd84Cu16 and Pd46Cu54 (atomic compositions, confirmed by XPS, respectively as to be discussed later). It is worth mentioning several noticeable matters involved in plating palladium and copper. First, it was very important to keep electroplating bath at high pH value (∼11) during Pd deposition process. Although plating rate was initially

Conclusions

The thin defect-free Pd–Cu/ZrO2–PSS composite membranes can be prepared by sequential electroless plating of palladium and copper on zirconia-coated porous stainless steel (PSS, 0.2 μm graded) disk. The ultrathin intermediate layer of zirconia is introduced to modify PSS surface and to serve as a diffusion barrier between Pd–Cu and the PSS support. The homogeneous Pd–Cu alloy films can be obtained by in situ annealing of the sequentially deposited palladium and copper layers. The composite

Acknowledgements

This work was supported by Cheung Kong Scholar Programme, NSFC (Grant No. 50228203, 20076033) and Tianjin University 985 Project.

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