Modified electroless plating technique for preparation of palladium composite membranes

[摘要] An increased demand for hydrogen in recent years has led to a revival of interest in methods forhydrogen separation and purification. Palladium (Pd) and palladium composite membranes havetherefore received growing attention largely due to their unique permselectivity for hydrogen andgood mechanical and thermal stability.Previous research on Pd composite membranes by Keuler (2000) in the Department of ProcessEngineering at the University of Stellenbosch has shown that some assumptions which he madeduring characterisation procedures needed further investigation, such as the assumptions about theinfluence of support membranes on preparation of Pd composite membranes, method of precleaningbefore pretreatment, vacuum applied during electroless plating, and heat treatment afterelectroless plating. In this study, Pd composite membranes (with Pd film thickness of 1.7 μm ~ 4μm) were prepared on the inside layer (claimed pore diameter of 200 nm) of α-alumina ceramicsupport membrane tubes, consisting of three layers with varying pore diameters from inside to theoutside layer, via a modified electroless plating technique (with a gauge vacuum of 20 kPa appliedon the shell side of the plating reactor). Bubble point tests and bubble point screening tests wereperformed on the support membranes before the electroless plating to investigate the influence ofthe substrates characteristics on the preparation of the Pd composite membranes. It was found thatPd composite membranes with a better permselectivity can be prepared on a support membrane thatcontains smaller pore sizes and a smoother surface.The surface pretreatment step was modified to provide a uniform Pd surface for Pd electrolessplating. The membrane was first rinsed in PdCl2 solution for 15 min using a stirrer at a stirringspeed 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 intodistilled water 10 times. These procedures were repeated 4 times. In addition, by using a newmethod of assessment for heat treatment (i.e. cutting the Pd composite membranes into two piecesand then exposing them to two different heating methods), the most effective heat treatment methodcould be identified without the influences of the substrates or the plating technique. The preferableprocedures was to anneal the Pd composite membrane in N2 for 5 h from 20°C to 320°C, and thenoxidize it in air for 2 h at 320°C, followed by annealing it in N2 for 130 min from 320°C to 450°Cand then in H2 for 3 h at 450°C. Finally the membrane was cooled down in N2 to 350°C and held atthis 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 Pdcomposite membrane. More detailed characterisations of the Pd composite membranes wereperformed by membrane permselectivity tests (from 350°C to 550 ◦C) using either H2 or N2 insingle 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/nitrogenpermselectivity of ≥ 150 were achieved in this study. The permselectivities of the heat treatedmembranes 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 andcompact Pd layers were obtained.

[发布机构] Stellenbosch University