Preparation And Characterization Of Palladium Membranes On Porous 316L Stainless Steel Modified By Ceramic

Palladium membrane has excellent selective permeability to hydrogen.It can be widely used in the fields of catalysis,dehydrogenation,hydrogenation,hydrogen separation and purification.The conventional palladium membrane preparation technology is mainly based on cold-rolling method,and the obtained palladium membrane has a large thickness,a high cost,and a low hydrogen permeation efficiency.In recent years,porous materials supported palladium membranes have received extensive attention from researchers.The thickness of palladium membrane was greatly reduced,and the hydrogen permeation efficiency was remarkably improved.Porous metal used as support for palladium membrane has advantages of excellent sealing performance and high mechanical strength.It is a research hotspot of palladium membrane support.This paper proposes the preparation of titania ceramic barrier on porous 316L stainless steel tube surface by high temperature sintering method.The pore size of the porous metal support was reduced after ceramic barrier preparation,which is favorable for the preparation of the dense continuous palladium membrane.In addition,the ceramic barrier can effectively isolate the direct contact between the porous 316L stainless steel tube and the palladium membrane to prevent element inter-diffusion at high temperatures,thereby improving the high temperature stability of the palladium membrane module.In order to obtain a ceramic barrier layer with high bonding strength and uniform pore size distribution,different ceramics preparing method and sintering temperature were compared.And a uniform titania layer was prepared on the surface of porous 316L stainless steel by dip-coating method and sintered.On this basis,a palladium membrane and a palladium-silver composite membrane were prepared on the surface of titania modified porous stainless steel by electroless plating.And the palladium-silver membrane was heat-treated to alloy.X-ray diffraction(XRD)and scanning electron microscopy-energy spectroscopy(SEM-EDS)were used to study the structure,morphology and composition distribution of palladium membrane.The hydrogen permeation of palladium membrane was tested,and the permeation activation energy of hydrogen on the surface of palladium membrane was investigated to evaluate the performance of palladium membrane or palladium silver membrane.Access to research results are as follows:(1)A stainless steel layer and a titania barrier were prepared by dip-coating and sintered onto porous 316L stainless steel tube.The effects of dispersant,binder and sintering atmosphere on the microstructure of titania layer were studied.The pore size of porous 316L stainless steel tube decrease from 5?m to 2.44?m after stainless steel layer preparing.On this basis,a titania layer was prepared on the surface of the stainless steel layer coated porous 316L stainless steel tube by titania slurry dip-coating method.The titania powder was ball-milled by adding 0.5wt%dispersant and 3wt%binder.The prepared suspension was stable,and the particles were uniformly dispersed,which was favorable for the formation of a uniform titania layer.The effects of vacuum and hydrogen atmosphere on the microstructure of sintered titania layer were compared.The microstructure of hydrogen sintered titania layer at 1000? was uniform and compact,and the bonding strength with porous 316L stainless steel carrier was good.The surface of the 316L stainless steel modified by the titania ceramic layer was flat and uniform with the average pore size of 1.1 ?m.(2)A palladium membrane was prepared by electroless plating onto the titania modified porous 316L stainless steel tube.Studies have shown that the distribution of palladium particles on the surface of the porous support after activation by the traditional sensitization-activation method is not uniform,which is not conducive to the uniformity of the layer of electroless palladium.Meanwhile,the palladium particles on the surface of the support after activation of palladium acetate coating are fine and uniform.A dense palladium membrane was deposited on the titania modified porous 316L stainless steel tube by electroless plating method at 35 ? and pH of 11.5.And the palladium membrane has good bonding strength with the porous tube.The hydrogen permeability of palladium membrane was characterized.The hydrogen permeation of palladium membrane is 1.58×10-3mol·m-2·s-1 Pa-0.5 at 450 ? and the H2/N2 selectivity of palladium membrane was 1700 at pressure difference of 0.5MPa.The permeation of hydrogen into the palladium membrane is a bulk diffusion control process and the hydrogen permeation activation energy of the membrane was 15.46kJ·mol-1.The composition of the palladium membrane after the hydrogen permeation test was analyzed with no inter-diffusion between palladium membrane and porous stainless steel,indicating that the titania layer has an excellent diffusion barrier effect.(3)On the basis of the preparation of Pd membrane,a Pd-Ag composite membrane was deposited by stepwise electroless plating onto the porous 316L stainless steel tube,and it was alloyed by heat treatment under hydrogen atmosphere.Sodium citrate is used as a reducing agent,and Na2S2O3 is added as a surfactant to obtain a silver film having a smooth surface and a fine grain.The hydrogen permeation test of the palladium-silver membrane heat-treated at 500 ? under hydrogen atmosphere was carried out.The segregation of silver on the surface of the alloy film leads to a decrease in the adsorption activity of hydrogen on the surface of the palladium membrane,which significantly affects the hydrogen permeability of the palladium silver film.Aiming at this problem,the heat treatment temperature of palladium-silver membrane was raised to 550?.Silver was completely dissolved in palladium,and palladium crystal lattice expanded after heat treatment,which shows palladium and silver formed a solid solution alloy.