Chemical vapor deposition of permselective oxide membranes for hydrogen separation

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Membranes have recently emerged as energy-efficient means of gas separation. Oxide membranes that are effective for hydrogen separation at elevated temperatures (400-800°C) were developed. The membranes have high [...] selectivities along with permeances that are comparable to those of conventional polymeric membranes operating at ambient temperature. Hydrogen-permselective membranes were synthesized by chemical vapor deposition of [...], [...], [...], and [...] layers within the pores of Vycor tubes. The deposition was carried out by continuous flow of [...], etc., and water in either the one-sided or the two-sided (opposing-reactants) geometry at 100-800°C. Permselective [...] layers could be formed in both geometries, while deposition of [...] and [...] layers was achieved only in the two-sided geometry. The permeation coefficients at 450°C were 0.3 and 0.1 [...] for [...] membranes produced in the one-sided geometry and two-sided geometries, respectively. The [...] permeation ratios were 500-5000. The [...] and [...] membranes had somewhat lower permeation coefficients and [...] ratios. The [...] membranes prepared in the opposing-reactants geometry were characterized by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Examining tube cross section containing [...] deposit revealed that the membrane consisted of a thin region (approximately 1 [...] thick) of totally plugged pores and a broad region (100 [...]) of partially filled pores. The spatial density distribution of [...] in the tube cross section showed an asymmetric profile located close to the side of chloride flow indicating heterogeneous mechanism of the deposition reaction. The [...] permeance of the membranes decreased by 50-75%, and the selectivity diminished upon prolonged hydrothermal treatment. A mechanism of the deposition process is suggested involving heterogeneous reactions of gaseous chloride and surface hydroxyl groups and of water vapor with surface chloride groups on the growing deposit layer. The mechanism and kinetics of [...] deposition were investigated by thermogravimetric analysis. The reactions were decoupled by alternating reactions of [...] and [...] with the surface of Vycor glass. The results are interpreted by postulating two types of silanol groups Si-OH on the Vycor surface, paired and isolated. Reaction of [...] with paired silanols proceeds by competing steps which control the total extent of silylation. Upon reaction with [...], all surface Si-Cl groups are hydrolyzed regenerating Si-OH. Simultaneous silanol-silanol and silanol-surface chloride condensations eliminate [...] or HCl to form new Si-O-Si bridges with final product silica. A kinetic model incorporating the silylation, hydrolysis and condensation steps describes well the TGA data. The reaction of the chlorosilanes with surface hydroxyls occurs very rapidly at temperatures above [...] and causes significant axial concentration gradients in the one-sided deposition, resulting in nonuniform deposit layer along the length of the tube. A new technique that eliminated the axial concentration gradients was used to deposit thin [...] layers on the surface of porous Vycor glass. Alternating vapor phase reaction with SiC14 and H2O was used to prepare membranes having [...] permeance of 0.3-0.4 [...] and [...] selectivity of 500-1000 at [...] 600°C. The [...] dosage at each silylation cycle, the concentration of initial surface -OH groups, and the reaction temperature influence significantly the deposit layer thickness. The membranes were stable to long hydrothermal treatments with the [...] permeance decreasing by about 20% and the selectivity increasing to more than 2000. The membrane properties after hydrothermal treatment are superior to those of membranes prepared by one-sided (steady flow) deposition due to thinner and more uniform deposit layers. A simple model incorporating diffusion and surface reaction was used to study the effect of various parameters on the formation of the deposit layer