Preparation of H2 Permselective Silica Membranes by Alternating Reactant Vapor Deposition

Thin SiO_2 layers were deposited on the surface of porous Vycor glass by alternating vapor phase reaction with SiCl_4 and H_2O. The membranes prepared by this technique had a H_2 permeance of 0.3-0.4 cm^3(STP)/(min•cm^2•atm) and a H_2:N_2 selectivity of 500-1000 at 600 °C. The SiCl_4 dosage at each silylation cycle, the concentration of initial surface OH groups, and the reaction temperature significantly influence the deposit layer thickness. After two weeks of heating at 550 °C under 3 atm of water vapor, the membrane H_2 permeance decreased by about 20%, and the selectivity increased to more than 2000. The membrane properties after this hydrothermal treatment are superior to those of membranes prepared earlier by one-sided (steady flow) deposition. A simple model incorporating diffusion and surface reaction was used to study the effect of various parameters on the formation of the deposit layer.