Synthesis, characterization, and gas permeation properties of a hydrogen permeable silica membrane supported on porous alumina

A highly hydrogen permselective composite silica membrane was obtained by depositing a thin silica layer on a porous alumina support by the chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) at 873K in inert gas at atmospheric pressure. The silica/alumina membrane showed a high hydrogen permeance (∼10^-7molm^-2s^-1Pa^-1) with selectivity over CH₄, CO, and CO₂ in excess of 1000 at 873K. Cross-sectional and surface images of the membranes obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the silica film deposited on the γ-Al₂O₃ support was uniform with a thickness of 20-30nm. This indicated that the high temperature thermal decomposition of tetraethylorthosilicate (TEOS) used in this work was excellent in controlling the uniformity and thickness of the silica film formed on the porous alumina support. On the fresh alumina support the permeance of gases (He, H₂, CH₄, CO, and CO ₂) decreased with temperature and molecular weight in agreement with a Knudsen transport mechanism. However, on the silica/alumina membrane the permeation of H₂ and He was activated and increased with temperature. The transport mechanism for the small gas molecules (H₂ and He) through the silica membrane was analyzed using a permeation mechanism which involves the jumping of the diffusing molecules between adjacent solubility sites. The model analysis indicated that the structure of the silica layer is more open than that of vitreous silica glass, with larger interconnecting passageways and low activation energies for permeation, allowing for easier gas diffusion.