The electrolyte separation behaviour of a supported bi-layered ceramic membrane is investigated experimentally and the measured ion retentions are compared with the predictions of a site-binding transport model with no adjustable parameters. Due to the difference in iso-electric point between its two separating layers, the bi-layered system is expected to perform better over a large pH range compared with a membrane with only one type of selective layer. The separating layers in the membrane are a microporous silica and a mesoporous ␥-alumina (pore sizes of 0.8 and 2 nm, respectively) and their retention is studied for a binary electrolyte solution of NaCl at 1 mol/m 3 for pH values between 4 and 10. Because of its smaller pores and high charge, the silica layer mainly determines the membrane retention at neutral and alkaline pH, while the ␥-alumina layer has a significant impact on the NaCl retention at 4 < pH < 5. The model predictions are in good agreement with the experimental data for Na + at 4 < pH < 9 and for Cl − at the whole pH range. For a pH of 4, the predicted chloride retention is lower than the sodium retention while the experimental data show the opposite effect.
An intermediate surfactant-templated silica (STS) layer is applied between the supporting mesoporous γ -Al 2 O 3 and the amorphous microporous silica overlayer resulting in dual-layered microporous silica membranes for gas separation applications that show improved values for both hydrogen flux and selectivity. Determination of thickness and porosity of as-deposited membrane layers by spectroscopic ellipsometry reveals that the STS layer is present as a distinctive layer of ∼20 nm thickness, with penetration up to a depth of ∼70 nm into the underlying γ -Al 2 O 3 support layer, whose thickness and porosity are determined to be 1.3 µm and 50%, respectively.
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