Hydrogen Purification using Ultra-Thin Palladium Films Supported on Porous Anodic Alumina Membranes
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1023-JJ09-02
Hydrogen Purification using Ultra-Thin Palladium Films Supported on Porous Anodic Alumina Membranes Alexander Kirchner, Ian W.M. Brown, Mark E. Bowden, and Tim Kemmitt MacDiarmid Institute for Advanced Materials and Nanotechnology, Industrial Research, PO Box 31-310, Lower Hutt, New Zealand ABSTRACT Nanostructured anodic alumina membranes have been utilized as high-temperature stable supports for 150 nm thick continuous palladium films. The palladium has been deposited by vacuum evaporation onto the rotating substrate. The thermal stability of the resulting compound membranes has been demonstrated for temperatures up to 700∫C under a reducing atmosphere. Hydrogen permeation has been measured up to 280∫C, where the permeability has a value of 2.5∑10-7 mol m-2 s-1 Pa-1. At the same time the selectivity factor over carbon dioxide is at least 33. INTRODUCTION Technologies employing hydrogen as an energy carrier are developing rapidly. The efficient conversion of fossil fuels into hydrogen by steam reforming [1] as well as its application in polymer electrolyte fuel cells call for economical high-flux hydrogen filters able to operate above 700∫C. Palladium and some of its alloys possess high permeability for hydrogen but are impervious to other gases [2]. To reduce the amount of noble metal needed and increase the hydrogen permeability a variety of composite membranes consisting of a thin active metal film and a porous support structure have been fabricated [3-5]. Porous anodic alumina (PAA) with its straight, parallel and very uniformly sized pores is an especially attractive support structure [6,7]. Its stability at high temperatures differs depending on the preparation method, particularly the selection of acid electrolyte. Previously we have shown by thermal analysis that PAA prepared in sulfuric acid is a suitable substrate material able to withstand 800∫C without deformation [8]. Here we focus on the deposition of an ultra-thin palladium film onto PAA, its thermal stability and gas permeation properties. EXPERIMENTAL PAA membranes were prepared by anodizing annealed high-purity aluminum foil (Alfa Aesar, 99.99%, 0.25 mm thick, degreased) in 0.3 M H2SO4 electrolyte. The use of a clamp-on cell as described by Lee et al [9] facilitated the growth of a circular oxide disc (9 mm diameter). The specimens were anodized at 7∫C and a constant potential of 25.0 V versus a platinum cathode for at least 12 h. Afterwards excess metal was dissolved in a saturated iodine-methanol solution at 50∫C. Subsequently the PAA discs were cleaned in methanol, pore-opened in 5% H3PO4 at 30∫C for 60 min and rinsed in DI water. Specimens were characterised by scanning electron microscopy (SEM) using a JEOL 6500F microscope. The permeation rates of several gases were measured for pressure gradients up to 1 bar across the membrane using a soap film flow meter.
Metal deposition onto the membraneís basal face (formerly in contact with aluminum) was carried out in two steps. First an intermediate layer of 40 ≈ titanium was deposite
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