Permeation properties of hydrogen and water vapor through porous silica membranes at high temperatures

Abstract

Silica and cobalt-doped silica membranes that showed a high permeance of 1.8 Â 10 À7 mol m À2 s À1 Pa À1 and a H 2 /N 2 permeance ratio of $730, with excellent hydrothermal stability under steam pressure of 300 kPa, were successfully prepared. The permeation mechanism of gas molecules, focusing particularly on hydrogen and water vapor, was investigated in the 300-500 C range and is discussed based on the activation energy of permeation and the selectivity of gaseous molecules. The activation energy of H 2 permeation correlated well with the permeance ratio of He/H 2 for porous silica membranes prepared by sol-gel processing, chemical vapor deposition (CVD), and vitreous glasses, indicating that similar amorphous silica network structures were formed. The permeance ratios of H 2 /H 2 O were found to range from 5 to 40, that is, hydrogen (kinetic diameter: 0.289 nm) was always more permeable than water (0.265 nm).

Development of Metal-doped Silica Membranes for Increased Hydrothermal Stability and Their Applications to Membrane Reactors for Steam Reforming of Methane

“…This result suggested that the Co doped into the silica existed as not only Co3O4 crystalline powder, but also as noncrystalline compounds, such as extremely fine particles that are undetectable by XRD or as Co composites with silica. XRD peaks of Co-doped silica powders fired at 19) in air, and at 600 in steamed air, had approximately the same peak intensities, suggesting that Codoped silica had good hydrothermal stability. The effect of Co composition on XRD pattern revealed that the Co3O4 peak intensities increased with the Co content in the range of from 0 to 30 Co mol%; at Co atomic composition greater than 30 mol%, the peak intensity remained relatively constant.…”

“…Figure 5 shows the TEM cross-sectional image of the top layer with 10 μm depth of a Co-doped silica membrane, which was observed using a TOPCON (Model EM002b, Tokyo, Japan) 19) . The sample was prepared using a focused ion beam (FIB) and tungsten was deposited on the surface of the top layer.…”

“…・Pa -1 ) with a H2/N2 permeance ratio of 730 19) . Figure 5 shows the TEM cross-sectional image of the top layer with 10 μm depth of a Co-doped silica membrane, which was observed using a TOPCON (Model EM002b, Tokyo, Japan) 19) .…”

“…The sample was prepared using a focused ion beam (FIB) and tungsten was deposited on the surface of the top layer. The Co- 19) doped silica membrane showed a composite membrane structure, consisting of an α-alumina substrate that was prepared using α-alumina particles of several μms in size (pore size: 1 μm), an intermediate layer prepared using α-alumina fine particles (200 nm in diameter), a SiO2 _ ZrO2 layer, and a Co-doped silica top layer for hydrogen separation according to the preparation procedure. It should be noted that the hydrothermal stability of an intermediate layer is as important as that of silica for hydrogen separation, and improved hydrothermally stability of γ-alumina intermediate layers were prepared by doping metal such as lanthanum into boehmite sols 28)～30) or using phosphorus 29) .…”

“…Among the metals, Ni was the first reported to be effective in improving hydrothermal stability 11),17) . We also reported improved hydrothermal stability by doping Co into silica with different Si/Co compositions by sol-gel processing, and applied this method to hydrogen separation 18), 19) . The Co-doped silica with improved hydro thermal stability was also investigated by other research groups based on permeation experiments 20) .…”