A silica membrane was formed by chemical vapor deposition using tetraethylorthosilicate in macropores of an α-alumina tube or a γ-alumina film coated on the α-alumina tube. The reactant was evacuated through the porous wall, and silica was deposited in the macropores at 600-700°C. When the silica membrane was formed in a γ-alumina film coated on the α-alumina tube, hydrogen permeance at a permeation temperature of 600°C was 3 × 10-7 mol m-2 s-1 Pa-1, which was one order of magnitude higher than that of a membrane formed directly on the α-alumina tube. H2/N2 selectivity determined from the permeance of each component was 100-1000. To separate hydrogen selectively from abundant steam, however, a higher permselectivity was required. The membrane formed in the γ-alumina film at 650°C showed a hydrogen permeance of 3 × 10-8 mol m-2 s-1 Pa-1 and an H2/H2O selectivity of 7.6 at 400°C.
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