Hydrogen permeation properties through composite membranes of platinum supported on porous alumina

The hydrogen permeation mechanism through a supported platinum membrane pre pared by chemical vapor deposition (CVD) was investigated and compared with an electroless-plating platinum membrane. A CVD membrane while maintaining an excellent hydrogen permselectivity provided a 10(3)-10(4) times higher hydrogen permeation coefficient than an electroless-plating platinum membra ne. This large difference in the hydrogen permeation coefficient resulted f rom the hydrogen transport phenomena, namely the surface diffusion mechanis m for the CVD membrane and solution-diffusion mechanism for an electroless- plating membrane. Furthermore, the relationship between the structure of th e metal layer and hydrogen permeance was also discussed for the CVD membran e. The membranes with smaller platinum grains gave higher hydrogen fluxes b ecause the effective platinum area for the hydrogen surface diffusion incre ased with a decrease in grain size. (C) 2000 Elsevier Science B.V. All righ ts reserved.