Study On Preparing Pd/Porous Stainless-Steel Composite Memebranes By Electroless Plating

Since 1980s, because of its good mechanical properties, thermal stability and uniquely hydrogen permeability, studies on preparation and application of palladium/palladium alloy composite membrane have been taken more attentions. Palladium has a huge hydrogen dissolution and permeability, and higher resistance to produce oxide film, compared with other metals in high temperature. So palladium is the most commonly material used for hydrogen transfer through membrane. Palladium and its alloy composite membranes have a widely potential applications in hydrogen involved reaction, separation and purification.The article mainly studies on the basic issues of preparing palladium membranes using electroless plating: developing a new method of activation;proposing a kinetic equation for modified electroless plating on porous stainless-steel supports;theoretically analyzing and experimentally evaluating the permeation performance of the membranes prepared. The details are given as follow:1. An improved activation technique was developed for preparing palladium/porous stainless steel (PSS) composite membranes. The technique utilized the reducibility of Fe²⁺ to reduce Pd²⁺ into Pd in activating bath, and the Pd-seeds were deposited on the surface of PSS. Effects of activating conditions on the electroless plating and film surface morphology were mainly investigated. Experimental results indicate that the activating time and temperature have significant effects on the distribution and assemble of Pd-seeds, as well as on the surface morphology of the palladium membrane. The optimal activating conditions obtained were: activating time 1 min, activating temperature 60℃. Using this activation technique, denser palladium membranes were obtained by electroless plating with a uniform rate.2. The kinetics of electroless plating for preparing Pd/Porous stainless-steel composite membranes was methodically studied. Effects of the plating conditions, such as temperature, palladium or hydrazine concentration, and so on, on the palladium deposition rate and surface morphology of the membrane were examined. The membrane surface morphology was characterized by SEM and the size growth of palladium grain with plating time was also investigated. Results indicate that the plating parameters play a vital role on both membrane deposition rate and structure of the Pd grain. And the average grain size increases monotonically with plating time. An empirical kinetic equation of electroless plating on porous stainless-steel supports was developed. This equation provides a tool to predict the palladium deposition rate and membrane thickness as a function of plating parameters.3. The performance of the membranes prepared by developed electroless plating wasinvestigated. The hydrogen solubility in Pd follows Sievert's law, and hydrogen diffusion through the bulk of the palladium membrane is the rate determining step. Whereas other gases only permeate through the pinholes or gaps in the Pd layer;the permeation can be quantitatively described as a combination of Knudsen diffusion and viscous flow. Both temperature and pressure difference crossover the membrane influences the H2/N2 permeabilities. The Pd membranes would exhibit as higher selectivity and H2 permeance in suitable conditions chosen carefully.