| This work is devoted to the preparation and use of Pd-based catalytic membranes. The composite membrane form consisting of a Pd-based thin film supported on a porous substrate was considered for the purpose of commercial development of membrane reactors. Pd and Pd-Ag/porous stainless steel asymmetric membranes were prepared by the electroless plating technique using hydrazine as the reducing agent. The alloy membranes were obtained via a post-deposition thermal treatment in hydrogen. Hydrogen permeability through Pd membranes was measured. The deposited membranes were characterized by SEM, XRD, EDX, XPS, Auger spectrum as well as an electrochemical polarization method.; Measurements of hydrogen diffusivity and solubility through Pd-based membranes were performed using a volumetric method. XPS data revealed that hydrogen permeation results in a Pd surface segregation on the Pd-Ag membrane surface toward the high hydrogen pressure side, whereas silver segregation occurs on the opposite side. These phenomena were explained based on the so-called modern thermodynamic calculation of interface properties--first approximation (MTCIP-1A). It is concluded that the chemisorption of hydrogen induces palladium segregation on the Pd-Ag surface.; Simultaneous deposition of Pd and Ag was inhibited by the preferential deposition of silver. By predeposition of Pd, the codeposition could be achieved. XPS and polarization studies revealed that Pd(IV) species was electroinactive toward hydrazine anodic oxidation, while Pd(II) species could be reduced in the electroless hydrazine bath. A mechanism of Pd-Ag codeposition was proposed.; A membrane reactor made of stainless steel was designed to perform methane steam reforming. The methane conversion is significantly enhanced by partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membrane. For example, at a total pressure of 136 kPa, a temperature of 500{dollar}spcirc{dollar}C, a molar steam-to-methane ratio of 3 and in the presence of a commercial Ni/Al{dollar}rmsb2Osb3{dollar} catalyst together with continually pumping in the permeation side, a methane conversion twice as high as that in a non-membrane reactor was reached by using a Pd/SS membrane. These effects were examined under a variety of experimental conditions. A computer model of the membrane reactor designated as the kinetic permeation model was developed and utilized in predicting the effects of membrane separation on methane conversion and product composition. |
