Effect of CVD parameters on hydrogen permeation properties in a nano-composite SiO2–Al2O3 membrane

Tubular ceramic membranes were synthesized by depositing a dense layer of silica–alumina on top of a multilayer substrate using co-current chemical vapor deposition (CVD) method. The multilayer substrate was prepared by coating with a series of bohemite sols with certain particle sizes. Cross-sectional and surface images obtained from high resolution FESEM showed that the intermediate layer had a thickness of about 1 μm and the top selective layer was uniform and dense with a thickness of less than 100 nm. Permeance tests, which were carried out with H2, CO2, N2 and CH4 at a high temperature range of 923–1073 K, indicated that gas permeation took place via different mechanisms through different layers of the membranes. During the deposition time, permeance values were reduced and after 6 h, H2 permeation decreased from 5×10−5 mol m−2 s−1 Pa−1 to 6.3×10−7 mol m−2 s−1 Pa−1, but high selectivities of 105, 203 and 573 were obtained at 1073 K for H2 over CO2, N2 and CH4, respectively. Hydrothermal stability tests at 1073 K and in the presence of 30 mol% steam showed that using 10 mol% alumina precursor (ATSB) in the CVD method improved membrane stability, as after 96 h of exposure to humidity, H2 permeance flux of silica–alumina membrane with ATSB/TEOS molar ratio of 0.1 was 1.9 times more than that of a pure silica membrane.