Fabrication of Hydrogen-separating Composite Membranes for Separation-integrated Reaction Processes and Their Hydrogen Permeation Behavior

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The fabrication of a defect-free Pd-Ag alloy membrane using a porous Al_2O_3 plate support (pore size, 0.06 μm) was tried using developed supercritical electroplating technique for the purpose of the suppression of H_2 embrittlement as well as the improvement of H_2 permeance. A thin and defect-free Pd-Ag alloy film was formed using a plating bath consisting of PdCl_2 (0.1 mol L^1), AgNO_3 (0.005 mol L_1), KBr (4.0 mol L_1), KNO_2 (0.2 mol L_4), H_3BO_3 (0.49 mol L_1), C_2H_5NO_2 (0.1 mol L_1), and a suitable surfactant at 12 MPa with the coexistence of supercritical CO_2. The average thickness and Ag content of the resulting membrane were 2.2 μm and 20 wt%, respectively. The H2 permeation coefficient at 673 K was 3.0 x 10^(-9) mol m^(-1) s^(-1) Pa^(-0.5), which was larger than the reported value of a pure palladium film.A new electroplating reactor with the outside circulation of the mixture of supercritical CO_2 and plating solution was designed for the application to a cylindrical support. The resulting membrane was completely defect-free and had the desirable alloy composition of Pd 77 wt% and Ag 23 wt%, and the thickness of 1.5 μm. Unfortunately, the H_2 permeation coefficient was decreased compared with that of the composite membrane using the planar support. It may be related to the increased resistance of H_2 permeation in the pores of the support. The addition of Ru to the Pd-Ag alloy film gave about 1.2 times higher H_2 permeance at 723 K.Additionally, the supercritical plating technique was also applied to Pd electroless plating, which should be conducted under acidic condition. A developed electroless plating bath includes phosphinic acid, phosphonate, and trimethylamine borane as the reducing agent. It was experimentally confirmed that the generation of cracks due to the FI2 embrittlement was suppressed by the coexistence of supercritical CO_2.