Fabrication And Characterization Of Palladium Composite Membranes By Chemical Fluid Deposition From Supercritical Fluids

Because of their excellent permeability, perm-selectivity and chemical andthermal stability as well as mechanical stability, Palladium-based compositemembranes have attracted wide attention in membrane technology. Additionally, aseries of bi-functional catalysts can be obtained by dispersing palladium nanoparticleson the surface of crystal grains or into the boundaries in zeolite or molecular sievefilms, which is constituted by an acid matrix and a metallic function in porousenvironments. This can get product in a single-step process instead of multi-processeslittle byproduct, which may play an important role in industry.The paper discussed the principle and influence factors of hydrogen permeation,preparation methods of palladium-based membranes and their applications. Based onthe characteristics of supercritical fluid which exhibit densities like those of a liquidsolvent while retaining the transport properties of a gas, a novel preparation techniqueunder supercritical conditions was emphasized. Chemial fluid deposition fromsupcritical fluid is a novel approach to metal deposition that involves the chemicalreduction of organometallic compounds in supercritical dioxide to deposit Palladiuminto the pores of different substrates.Palladium were deposited into the pores or on the surface of Al₂O₃ substratethrough opposing reactants chemical fluid deposition (ORCFD) in a continuousprocess combined with an organometallic palladium as the precursor with ethanol asthe reducing agent which may avoid hydrogen embrittlement and be less dangerous.By changing diffusion time or activing the surface of substrates, Pd film or Pdparticles were found on the surface or within the substrates; the contents of Palladiumincreased through prolonging reaction time. The microstructure within the porousα-alumina support is improved by annealing. A study of the surface seeding/activationstep were used, in which the substrate was pre-seeded with a catalytically activematerial and then deposited palladium from supercritical carbon dioxide solution.Material microstructure and surface morphology vary significantly between theseeded and unseeded substrates surface.Pd metals were deposited into pores of different substrates using this method. Using silica as the stabilizing agent, the defect-free silica-zirconia membrane wasfabrated with good thermal stability, which makes the silica-zirconia membrane agood choice as the intermediate layer for zeolite and Pd-based membranes. Pdnano-particles were also deposited into the matrics of molecular sieve using thismethod, which could be used as bi-functional catalysts for intensification ofmulti-step reactions.The research results from gas permeation through the Pd /Al₂O₃,Pd/ZrO2/Al₂O₃,Pd/silicliate-1/Al₂O₃ and Pd/SAPO-5/Al₂O₃ showed that the structure, surfacecompositions and permeation temperatures play important roles on hydrogen transportbehavior. Furthermore, the values of hydrogen pressure exponent and permeationenergy are important parameters which can explain the mechanisms of hydrogentransport.