Separation of hydrogen from gas mixtures using supported platinum-group metal membranes

Supported metal membranes consisting of a thin layer of various precious me tals deposited in the pores of a tubular porous ceramic substrate were fabr icated using a CVD technique, where the corresponding Bis(acelyl-acetonato) complexes were selected as the CVD precursors. These membranes showed high H-2 fluxes comparable to the Pd composite membrane prepared by an electrol ess-plating technique as well as relatively high pennselectivities of H-2 o ver N-2. A possible gas transport mechanism of the CVD membranes may be the surface diffusion mechanism in which H-2 is selectively adsorbed on the me tal surface and diffuses at a relatively high rate. The practical performan ce for II, separation from a two-component mixed gas, especially a gas mixt ure including a gas strongly adsorbed on metal surface such as carbon monox ide (CO), was investigated. H-2 fluxes through the tested CVD non-Pd membra nes were drastically decreased. The poisoning effect of CO appeared in the following order: Rh > Pt > Ir > Ru, which is not completely consistent with the order of the heat of CO adsorption (Ir > Pt > Ph much greater than Ru) . The poisoning effect cannot be understood only from a thermodynamic viewp oint. Fortunately, the H-2 permeance of all tested membranes was almost tot ally restored after the CO was completely removed from the gas phase, indic ating that most of the CO was reversibly adsorbed on the metal surface at H -2 separation temperatures. (C) 2001 Elsevier Science B.V. All rights reser ved.