Characterization of a highly selective hydrogen permeable silica membrane

The permeability properties of a new type of silica membrane for the small gas molecules CO2, CO, Ne, CH4, He, and H-2 are presented. The new membrane, denoted as Nanosil, has unusually high permeance for H-2, but also allows passage of He and to a smaller extent Ne, while excluding all other molecules. The membrane is formed by the decomposition of a silica precursor (tetraethyl orthosilicate) onto a Vycor glass substrate. Nitrogen physisorption isotherms of the Vycor glass substrate indicate that it is a microporous solid with slit-like pores of 3.6 nm diameter, that remains unchanged after the silica deposition. Atomic force microscopy (AFM) shows that the Vycor substrate is made up of rectangular plate-like elements of size 90 nm x 30 nm. Between the plates are found rectangular features of 4 nm breadth which are likely to be the pore mouths. The deposited silica forms a thin layer on top of these plates so as to erase fine structures and increase the average feature size to 110 nm x 50 nm. (C) 2001 Kluwer Academic Publishers.