Characteristics of the H2-permselective inorganic membranes synthesized by chemical vapor deposition

Abstract

Hydrogen-permselective membranes were synthesized by chemical vapeor deposition of $SiO_2$, $TiO_2$, and $SiO_2$-$TiO_2$ layers within the pores of Vycor glass tubes at 200$\sim$800$^\circ$C, using the precursors such as $SiCl_4$, tetraethylortho silicate (TEOS) and tetraisopropyl titanate (TPT). The thin films of each oxide produced were characterized for their gas permeabilities, microstructures and stabilities. The silica membranes were also examined as catalytic reactors for dehydrogenation of cyclohexane and $CH_4$ steam reforming. The property of each membrane was very different from one another depending on the type of oxides deposited, type of precursors, composition of the film and the deposition conditions such as deposition temperature, the concentration of reactants and flow rate of the sweep gas. Thin $SiO_2$ films were grown on porous Vycor glass tubes by hydrolysis of $SiCl_4$ or pyrolysis of TEOS at temperatures of 200$\sim$800$^\circ$C. The typical $SiO_2$-membrane had a $H_2$-permeation coefficient of 0.20 cm$^3$/min-cm$^2$-atm and a selectivity for $H_2$ over $N_2$ of 2000 at 600$^\circ$C, while the unmodified porous Vycor glass had a $H_2$-permeation coefficient of 0.410 cm$^3$/min-cm$^2$-atm and a selectivity of 3.72. The high selectivity for $H_2$ indicates that permeation of hydrogen occurs through a temperature-activated diffusion mechanism. The properties of the $SiO_2$ films obtained from TEOS were greatly affected by the deposition ambients, deposition temperature and the deposition geometry. Among others, the effect of oxygen concentration in the reactants appeared to be very profound. In the presence of oxygen, very stable and highly permeselective membranes were synthesized at tempertures above 300$^\circ$C regardless of the reactant concentration and deposition time, while in the absence of oxygen the films were very labile and their stabilities were strongly affected by the deposition conditions. The $SiO_2$ films produced from T...