Synthesis of hydrogen permselective silicon dioxide, titanium dioxide, aluminum oxide, and boron oxide membranes from the chloride precursors

Abstract

Hydrogen-permselective membranes were synthesized by chemical vapor deposition of SiO_2, TiO_2, Al_2O_3, and B_2O_3 layers within the pores of Vycor tubes. The deposition involved reaction of SiCl_4, etc., with water at 100-800 °C depending on the chloride and the reaction geometry. Permselective SiO_2 layers could be formed in either the one-sided or the two-sided (opposing reactants) geometry, while deposition of TiO_2 and Al_2O_3 layers was achieved only in the two-sided geometry. The permeation coefficients at 450 °C were 0.3 and 0.1 cm^3/(cm^2•min•atm) for SiO_2 membranes produced in the one-sided and two-sided geometries, respectively. The H_2:N_2 permeation ratios were 1000-5000. The TiO_2 and Al_2O_3 membranes had somewhat lower permeation coefficients and H_2:N_2 ratios. The membranes were characterized by scanning electron microscopy (SEM) and electron microprobe analysis (EMA). A reaction mechanism is suggested involving heterogeneous reactions of chloride and water molecules with hydroxyl and chloride groups in the growing deposit layer.