Relationship between the mesoporous intermediate layer structure and the gas permeation property of an amorphous silica membrane synthesized by counter diffusion chemical vapor deposition

An amorphous silica membrane with an excellent hydrogen/nitrogen (H ₂/N₂) permselectivity of >10 000 and a He/H₂ permselectivity of 11 was successfully synthesized on a γ-alumina (γ-Al₂O₃)-coated α-alumina (α-Al ₂O₃) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H₂ permeance of >1.0 × 10^-7 mol·(m²·s·Pa)^-1 at ≥773 K. The dominant permeation mechanism for He and H₂ at 373-873 K was activated diffusion. On the other hand, that for CO₂, Ar, and N₂ at 373-673 K was a viscous flow. At ≥673 K, that for CO ₂, Ar, and N₂ was activated diffusion. H₂ permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a γ-Al₂O₃ mesoporous intermediate layer.