Preparation and characterization of Pd–Cu composite membranes for hydrogen separation

https://doi.org/10.1016/S1385-8947(02)00106-7Get rights and content

Abstract

Pd–Cu composite membranes were made by successive electroless deposition of Pd and then Cu onto various tubular porous ceramic supports. Ceramic filters used as supports included symmetric α-alumina (nominal 200 nm in pore size), asymmetric zirconia on α-alumina (nominal 50 nm pore size), and asymmetric γ-alumina on α-alumina (nominal 5 nm pore size). The resulting metal/ceramic composite membranes were heat-treated between 350 and 700 °C for times ranging from 6 to 25 days to induce intermetallic diffusion and obtain homogeneous metal films. Pure gas permeability tests were conducted using hydrogen and nitrogen. For an 11 μm thick, 10 wt.% Cu film on a nominal 50 nm pore size asymmetric ultrafilter with zirconia top layer, the flux at 450 °C and 345 kPa H2 feed pressure was 0.8 mol/m2 s. The ideal hydrogen/nitrogen separation factor was 1150 at the same conditions. The thickness of the metallic film was progressively decreased from 28 μm down to 1–2 μm and the alloy concentration was increased to 30 wt.% Cu.

Structural factors related to the ceramic support and the metallic film chemical composition are shown to be responsible for the differences in membrane performance. Among the former are the support pore size, which controls the required metal film thickness to insure a leak-free membrane and the internal structure of the support (symmetric or asymmetric) which changes the mass transfer resistance. The support with the 200 nm pores required more Pd to plug the pores than the asymmetric membranes with smaller pore sizes, as was expected. However, leak-free films could not be deposited on the support with the smallest pore size (5 nm γ-alumina), presumably due to surface defects and/or a lack of adhesion between the metal film and the membrane surface.

Introduction

The hydrogen separating capability of Pd alloy membranes is well known. Applications include hydrogenation and dehydrogenation reactions [1] and recovery of hydrogen from petrochemical plant streams [2]. Recently, there has been interest in utilization of Pd membranes to separate hydrogen produced in hydrocarbon reforming and coal gasification for power generation in fuel cells. Such applications have the potential to reduce energy consumption, capital costs or the number of unit operations compared to conventional systems. To consume less Pd, thin films or foils of Pd on the order of microns in thickness are applied to porous substrates for mechanical strength. Porous glass, ceramic, stainless steel, and polymers are common supports [3]. Hydrogen permeable metals such as tantalum (Ta) with a Pd coating also function as effective hydrogen separators [2], [4].

Some very thin, permeable and permselective Pd composite membranes have been prepared by various research groups, although several hurdles inhibit commercial implementation of Pd membrane technology [5], [6], [7]. These problems include embrittlement and cracking due to the α→β Pd hydride phase transition, which occurs during temperature cycling [8] and poisoning or fouling due to the presence of sulfur or unsaturated carbon compounds in the operating stream [9], [10], [11]. Also, in order to be accepted by industry, membranes must have a lifecycle on the order of years under process conditions [12].

Alloys of Pd possess properties that may alleviate some of the shortcomings of pure Pd [13]. To begin with, the critical temperature for existence of the β phase hydride is lowered in alloys. This helps eliminate membrane rupture due to warping or cracking, a failure associated with temperature cycling. Many alloys are also more permeable to hydrogen than pure Pd including PdAg23 [14] (compositions in wt.%), PdCu40 [15], PdY7 [16], [17] and PdRu7 [18]. Alloys containing Au or Cu are more resistant to sulfur compounds [19], [20]. Ternary and higher alloys of Pd have been developed to impart high temperature operating capability [21].

The hydrogen permeability of Pd–Cu passes through a maximum around 40 wt.% Cu significantly reducing membrane cost (relative to pure Pd), and this alloy exhibits increased resistance to H2S [19], [22], [23], [24], [25], [26], [27]. Additionally, a b.c.c. alloy phase formed below 600 °C is credited with the increased permeability compared to pure Pd [26], [28], [29]. Like PdAg23, PdCu40 can withstand repeated temperature cycling with less distortion than pure Pd since at 40 wt.% Cu, the critical temperature for β-hydride phase formation is below room temperature [15], [30], [31], [32].

Preparation of Pd alloys has been accomplished in the past by casting and rolling or induction melting followed by cold working into a foil or tube [33], [34], [35]. Composite Pd alloy membranes have been fabricated by sputtering, CVD, electroplating, and electroless plating. Much recent work has involved the use of electroless plating to make Pd–Ag [36], [37], [38], [39], Pd–Ni [40] and Pd–Au alloy membranes [41].

Pd–Ag alloy films have been fabricated by several research groups using sequential metal deposition followed by annealing [39], [42], [43]. To obtain a homogeneous alloy film from two distinct metal layers in a reasonable amount of time, a high enough temperature must be utilized to promote complete intermetallic diffusion [38]. Shu et al. [37] annealed a codeposited Pd–Ag film for 150 min at 400 °C and a sequentially deposited Pd–Ag film for 5 h at 700 °C, while Kikuchi [39] annealed a sequentially deposited Pd–Cu film between 300 and 540 °C. Kikuchi and Uemiya [44] deposited Pd–Ag alloy films by sequential electroless plating, followed by heat treatment at temperatures of 800–1300 °C. Uemiya et al. [38] found that Pd–Ag films deposited by sequential electroless deposition require high temperatures (>800 °C) to produce a homogeneous film. Sakai et al. [43] and Kawae et al. [45] annealed Pd–Ag at 900 °C for 2 and 12 h, respectively.

Lin and coworkers [46], [47], [48] have deposited Pd–Ag films onto asymmetric γ-alumina supports (3 nm pores) using magnetron sputtering. A membrane with a 177 nm PdAg18 film exhibited hydrogen/helium permselectivity of 3845 at 300 °C [48]. An observation they made was that the beginning surface roughness of the support was a critical parameter in obtaining a defect-free and adherent membrane [46].

Deposition under an osmotic pressure gradient by conducting electroless plating with a more concentrated solution on the opposite side of the porous support has been found to produce thinner Pd films that are more impenetrable to permeation of gases other than hydrogen [49], [50]. Several desirable features of applying this technique to the manufacture of the Pd–Cu membranes include smaller grain-size, reduction in porosity, surface homogeneity and densification of the plated film.

Most Pd–Cu alloy membrane work has been carried out using foils [15], [17], [26], [51]. Apparently, the only group to previously fabricate Pd–Cu alloy films on porous supports by electroless deposition of Pd and then Cu with subsequent annealing (500 °C for 12 h) was Kikuchi and coworkers [38], [39]. The objective of the present work was to fabricate a thin and hydrogen selective Pd–Cu composite membrane in a similar fashion while reducing the thickness of the metal film. Three different types of tubular, porous ceramic membranes with progressively smaller pore sizes were used as supports for the thin Pd–Cu films. The effect of film thickness and composition on annealing conditions and hydrogen permeability was also investigated.

Section snippets

Support specifications

Tubular, porous alumina microfilters with nominal 0.2 μm pore size, were procured from Golden Technologies Company (now Coors Tech Oak Ridge, Oak Ridge, Tennessee). Designated GTC998, these tubes have an OD of 9 mm, an ID of 6.1 mm, and are fabricated from 99.8% pure α-alumina. Fig. 1 is a scanning electron microscope (SEM) image of the GTC998 cross-section. Asymmetric ceramic membranes consisting of a sol–gel zirconia selective layer on top of several porous support layers composed of α-alumina

Annealing

The objective of this part of the work was to prove that alloys of Pd and Cu could be prepared in situ by heating successively electrolessly deposited layers of each metal. The literature provides wide support to this hypothesis [27], [38], [39]. From the Pd–Cu phase diagram, Pd and Cu are miscible over the entire range of compositions, so their intermixing should take place with relative ease [56].

Small pieces of GTC Pd–Cu Membrane were heated to 600 °C for 12 h under helium and analyzed

Economics

The driving force towards thinner palladium–copper layers is twofold in nature. First, from a mass transport standpoint, thin metal films present less resistance to flow, giving higher flux as a result, therefore enhancing productivity. Second, less palladium is needed to produce the same amount of hydrogen per unit length. This last point is also addressed with the alloying of palladium with inexpensive copper. Both of these factors contribute enormously to the overall economics of the

Conclusions

The objective of this work was to make ceramic-supported Pd–Cu metal membranes for high temperature hydrogen separation. It was shown that the deposition of Pd and Cu using electroless plating followed by annealing to alloy the metals is a viable way of forming a thin homogenous and defect-free metal film capable of selectively separating hydrogen at temperatures between 350 and 700 °C.

Controlling the deposition time during the sequential electroless plating was found to be important to tailor

Acknowledgements

This work was supported by the US Department of Energy (DOE), University Coal Research Program under Grant no. DE-FG2699–FT 40585. The US Department of Energy (DOE) sponsored the portion of this work performed by Los Alamos National Laboratory. The authors are grateful to Prof. S. Uemiya of Gifu University for the generous donation of glass sealing powder. The authors also want to recognize Ms. Mary Anne Alvin and her colleagues from Siemens Westinghouse Power Corp. for her aid obtaining some

References (61)

  • V. Gryaznov

    Catal. Today

    (1999)
  • N.M. Peachey et al.

    J. Membr. Sci.

    (1996)
  • J.N. Armor

    Catal. Today

    (1995)
  • G. Saracco et al.

    Chem. Eng. Sci.

    (1999)
  • J.K. Ali et al.

    Int. J. Hydrogen Energy

    (1994)
  • J.N. Armor

    J. Membr. Sci.

    (1998)
  • F.N. Berseneva et al.

    Int. J. Hydrogen Energy

    (1993)
  • T.B. Flanagan et al.

    Solid State Commun.

    (1975)
  • D. Fisher et al.

    J. Solid State Chem.

    (1977)
  • J. Shu et al.

    J. Membr. Sci.

    (1993)
  • S. Uemiya et al.

    J. Membr. Sci.

    (1991)
  • J.N. Keuler et al.

    Nucl. Instr. Meth. Phys. Res. B

    (1999)
  • E. Kikuchi et al.

    Gas Sep. Purif.

    (1991)
  • V. Jayaraman et al.

    J. Membr. Sci.

    (1995)
  • G. Xomeritakis et al.

    J. Membr. Sci.

    (1997)
  • B. McCool et al.

    J. Membr. Sci.

    (1999)
  • K.L. Yeung et al.

    Catal. Today

    (1995)
  • M. Kajiwara et al.

    Catal. Today

    (2000)
  • A. Criscuoli et al.

    J. Membr. Sci.

    (2001)
  • S. Roy et al.

    Int. J. Hydrogen Energy

    (1998)
  • R.E. Buxbaum et al.

    Ind. Eng. Chem. Res.

    (1996)
  • V.M. Gryaznov et al.

    Polym. Sci.

    (1993)
  • S. Uemiya

    Sep. Pur. Meth.

    (1999)
  • J.B. Hunter

    Platinum Met. Rev.

    (1960)
  • J.P. Collins et al.

    Ind. Eng. Chem. Res.

    (1996)
  • H.C. Foley et al.

    ACS Symp. Ser.

    (1993)
  • G.J. Grashoff et al.

    Platinum Met. Rev.

    (1983)
  • J.B. Hunter, US Patent 2,773,561...
  • D.L. McKinley, US Patent 3,439,474...
  • D.T. Hughes et al.

    J. Less-common Met.

    (1978)
  • Cited by (0)

    1

    Present address: NREL, 1617 Cole Blvd., Mail Stop 1633, Golden, CO 80401, USA.

    View full text