Hydrogen recovery from a H₂-H₂O-HBr mixture utilizing silica-based membranes at elevated temperatures. 1. Preparation of H₂O- and H₂-selective membranes

Amorphous silica-based membranes were prepared for use in recovering hydrogen at elevated temperatures from a H₂-H₂I-HBr mixture produced by a thermochemical water decomposition process (the UT-3 process). The silica-based membranes, which were formed on porous tubes by chemical vapor deposition of tetraethoxysilane at 650 °C with a forced cross-flow through the porous wall, exhibited a H₂ or H₂O selectivity, which was related to the preparation and posttreatment conditions. The microporous silica membrane formed on a macroporous α-alumina tube and heat-treated at 900 °C in an inert atmosphere showed a H₂ permeance of (3-4) × 10^-7 mol m^-2 s^-1 Pa^-1 and a H₂/H₂O selectivity of 7-15 at permeation temperatures of 200-400 °C. The microporous silica membrane which was formed on a mesoporous γ-alumina layer coated on the α-alumina tube and then calcined in air showed a H₂O permeance of the order of 10^-6 mol m^-2 s^-1 Pa^-1 and a H₂O/H₂ selectivity of 12-25 at 200-400 °C. This membrane rejected HBr at a H₂O/HBr selectivity of 300-1000.