Modified electroless plating technique for preparation of palladium composite membranes
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2005.
An increased demand for hydrogen in recent years has led to a revival of interest in methods for hydrogen separation and purification. Palladium (Pd) and palladium composite membranes have therefore received growing attention largely due to their unique permselectivity for hydrogen and good mechanical and thermal stability. Previous research on Pd composite membranes by Keuler (2000) in the Department of Process Engineering at the University of Stellenbosch has shown that some assumptions which he made during characterisation procedures needed further investigation, such as the assumptions about the influence of support membranes on preparation of Pd composite membranes, method of precleaning before pretreatment, vacuum applied during electroless plating, and heat treatment after electroless plating. In this study, Pd composite membranes (with Pd film thickness of 1.7 μm ~ 4 μm) were prepared on the inside layer (claimed pore diameter of 200 nm) of α-alumina ceramic support membrane tubes, consisting of three layers with varying pore diameters from inside to the outside layer, via a modified electroless plating technique (with a gauge vacuum of 20 kPa applied on the shell side of the plating reactor). Bubble point tests and bubble point screening tests were performed on the support membranes before the electroless plating to investigate the influence of the substrates characteristics on the preparation of the Pd composite membranes. It was found that Pd composite membranes with a better permselectivity can be prepared on a support membrane that contains smaller pore sizes and a smoother surface. The surface pretreatment step was modified to provide a uniform Pd surface for Pd electroless plating. The membrane was first rinsed in PdCl2 solution for 15 min using a stirrer at a stirring speed of 1300 rpm, and was then dipped into distilled water 10 times (1-2 second each). Subsequently, the membrane was rinsed in SnCl2 solution for 15 min, and was then dipped into distilled water 10 times. These procedures were repeated 4 times. In addition, by using a new method of assessment for heat treatment (i.e. cutting the Pd composite membranes into two pieces and then exposing them to two different heating methods), the most effective heat treatment method could be identified without the influences of the substrates or the plating technique. The preferable procedures was to anneal the Pd composite membrane in N2 for 5 h from 20°C to 320°C, and then oxidize it in air for 2 h at 320°C, followed by annealing it in N2 for 130 min from 320°C to 450°C and then in H2 for 3 h at 450°C. Finally the membrane was cooled down in N2 to 350°C and held at this temperature for 30 min. Additional oxidation in air for more than 10 hours changes the structure of the Pd films. PdO then forms and decreases the H2 permeation through the Pd composite membrane. More detailed characterisations of the Pd composite membranes were performed by membrane permselectivity tests (from 350°C to 550 ◦C) using either H2 or N2 in single gas test, membrane morphology and structure analysis using scanning electron microscopy (SEM), energy dispersive detectors (EDS), atomic force microscopy (AFM), Brunauer-Emmett- Teller (BET) and X-ray diffraction (XRD) analysis. Hydrogen permeability between 4.5-12 μmol/(m2.Pa.s) and an average hydrogen/nitrogen permselectivity of ≥ 150 were achieved in this study. The permselectivities of the heat treated membranes were superior to Keuler’s membranes, which had an average permselectivity of ≥ 100. AFM and BET analysis showed that dense and smooth Pd films with smaller Pd crystals sizes and compact Pd layers were obtained.