Abstract
Thin dense Pd membranes for hydrogen filtration were deposited on modified porous stainless steel (PSS) tubes by an improved electroless plating technique. Three research parts are included. First, alumina oxide (Al2O¬) particles of two different sizes were subsequently used to modify the non-uniform pore distribution and the surface roughness of the PSS tubes. The principle of the modification was to use large Al2O3 particles (~10 μm) to fill larger pores on the surface, and leave the smaller pores intact. Small Al2O3 particles (~1 μm) were then used to further decrease the surface roughness. Moreover, instead of small Al2O3 particle, a layered double hydroxide (LDH) layer was chosen to reduce the surface roughness of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. Finally, the influence the support rotation rate ranging from 0 to 200 rpm exerted during the Pd deposition process was analysed, and the permeation of hydrogen flux through the membranes was investigated. First, the detailed manufacturing steps of the Al2O3 modification were investigated and optimized to achieve a continuous dense Pd membrane with a minimum thickness of 4.4 μm on the modified PSS tubes. The highest hydrogen permeance of the membrane (Pd/1 m Al2O3/10 m Al2O3/PSS) was 75.5 m3/m2 h bar0.5 at 500°C, with a selectivity coefficient (H2/He) of 1124 under a pressure difference of 8 bar. In comparison, the thickness and hydrogen permeance of a dense Pd membrane on unmodified PSS tubes were 31.5 μm and 15.3 m3/m2 h bar0.5, respectively, at 500°C. Moreover, the LDH layers successfully instead of small Al2O3 particles to reduce the surface roughness of the PSS and to be a good diffusion barrier layer. The results indicated the membrane (Pd/LDH/10 m Al2O3/PSS) with thickness of ~7.85 m had a hydrogen permeance up to 76.7 m3/m2 h bar0.5 and selectivity coefficient (H2/He) of 3817 at 400°C. Thermal cycling between room temperature and 400C was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 hours) of the membrane at 400C showed static results of H2 flux and H2/He selectivity over the 1500 hours test period. Finally, thin dense palladium (Pd) membranes (~ 5.0 m) were fabricated on rotating PSS supports using electroless plating. The influence the support rotation rate ranging from 0 to 200 rpm exerted during the Pd deposition process was analysed. The rate of Pd deposition increased as the support rotation rate increased during electroless Pd plating. Compared with conventional electroless plating methods, the proposed modified electroless plating using a support rotation technique yielded Pd membranes that had considerably more uniform and smooth surface morphology, which substantially enhanced the membrane stability. The membranes exhibited a hydrogen permeance as high as 78 m3/m2 h bar0.5 at 400°C. Moreover, the membranes were stable during long-term temperature cycling performed between room temperature and 400°C over a period of 350 hours. These results indicated that the electroless plating combined with support rotation process was a simple, effective method for preparing thin dense Pd membranes featuring high hydrogen permeation flux and high thermal durability.