Development and characterization of silica-based membranes for hydrogen separation

Abstract

The method of chemical vapor deposition (CVD) in the counter current configuration was employed in the present study for the development of composite silica membranes. The experiments were carried out in a horizontal CVD reactor under controlled temperature conditions and at various reaction times and differential pressures across the substrate sides. Tetraethylorthosilicate (TEOS) and ozone were used as deposition precursors. Two types of substrates were employed: a porous Vycor tube and an alumina (γ-Al₂O₃) nanofiltration (NF) tube. Measurements with a novel mercury intrusion technique showed that significant reduction of the initial pore size of the γ-Al₂O₃ substrates was achieved, which reached 76% in the cases of extended silica deposition. Additionally, by appropriately interpreting the Knudsen type O₂ permeance results, acquired during the CVD treatment of Vycor tubes, a pore radius reduction even down to the 30% of the initial value was concluded. The permeance of Η₂ and other gases (Ηe, Ν₂, Αr, CO₂) on the developed membranes was measured in a home-made apparatus. The separation capability of the composite membranes was determined by calculating the selectivity of hydrogen over helium, nitrogen, argon and carbon dioxide.