Membrane reactor application to hydrogen production

Selective removal of hydrogen in a membrane reactor enables the hydrogen production by steam reforming at lower reaction temperatures than conventional processes. We invented a composite membrane consisting of thin palladium layer deposited on the outer surface of porous ceramics by electroless-plating. The palladium layer could completely cover the surface, so that only hydrogen could permeate through the membrane with a 100% selectivity. By use of this kind of membrane, Tokyo Gas and Mitsubishi Heavy Industries have developed a membrane reformer applicable to the polymer electrolyte fuel cell system. Our current approach is the application of a CVD technique to deposit non-Pd metals, such as Pt, to overcome the limitations caused by Pd membranes, such as hydrogen embrittlement. The permeability and permeation selectivity of these metal-ceramic composite CVD membranes were investigated in a comparison with electroless-plating palladium membranes, as well as the performance in membrane reactors applied to steam reforming of methane. The permeation of hydrogen through the CVD membranes is not based on the solution-diffusion transport mechanism but on the surface diffusion mechanism. Although the CVD membranes gave higher conversion of methane than thermodynamic equilibrium, their performance became similar to electroless-plating membranes, only when the membranes showed high H-2/N-2 separation factors. (C) 2000 Elsevier Science B.V. All rights reserved.