Synthesis and Characterization of Ni-doped Silica Membranes Prepared Using a Hybrid Sol–Gel/CVD Method

Abstract

Improving the hydrogen permeation flux, while maintaining high hydrogen selectivity, is important for reducing membrane areas and enhancing membrane reactor (MR) performance. We have attempted to enhance the hydrogen permeation through the addition of nickel, which will improve both hydrogen adsorption and pore size control. The silica layer was deposited with high selectivity, using an extended counter-diffusion CVD method.Hydrogen is widely used in chemical and petroleum industries and has been predicted to be a clean fuel for polymer electrolyte fuel cells (PEFCs) in the future. More than the half the hydrogen worldwide is produced from the steam reforming (SR) of natural gas, whose main feedstock is methane.1 In this reaction, by-product gases, such as carbon dioxide, carbon monoxide, and methane, in addition to hydrogen, make up the reforming gas. Therefore, much research has been focused on separation technologies, such as membrane separation. Of the primary membrane materials, silica has attracted much attention, owing to its high heat stability and low production cost.Recently silica-based membranes, which have high hydrogen permeance and high selectivity, have been prepared using a hybrid processing method that combines the solgel and CVD methods.2 In this study, a Ni-doped silica membrane is prepared using a hybrid method involving the solgel and high-pressure CVD methods. Ni-doped silica layers were prepared on ¡-alumina support with a £-alumina interlayer, using the sol gel method. We have attempted to enhance hydrogen permeation through the addition of nickel, which is expected to improve the hydrogen adsorption and pore size control. Silica membranes with a high selectivity were subsequently prepared using counter-diffusion CVD, under high-pressure conditions at the membrane side, where tetramethylorthosilicate (TMOS) is supplied.An ¡-alumina tube (10 mm o.d., 6 mm i.d., 35 mm length, Noritake Co., Ltd.) with a £-alumina layer was used as the membrane substrate. The porous ¡-alumina tube (35 mm length) was joined to a dense ¡-alumina tube (10 mm o.d., 6 mm i.d., 250 mm length) and an ¡-alumina disk (12 mm diameter, 2 mm thickness) with a glass sealant. The £-alumina interlayers were prepared by dipping ¡-alumina tubes into a 0.6 mol L ¹1 boehmite (£-AlOOH) sol, containing 1.5 wt % poly(vinyl alcohol). The tube was dried at room temperature for 3 h and then calcined at 600°C for 3 h, at a heating and cooling rate of 1°C min ¹1. Tetraethoxysilane (20 g, Wako) was added after all the nickel(II) nitrate hexahydrate (Ni/Si mol ratio = 0.5, 0.25, and 0) had dissolved in ethanol. 1 M HNO 3 (7.5 mL) was added dropwise to the mixture with gentle stirring at room temperature over a period of 15 min.3,4 The temperature was then increased to 70°C and the mixture was heated at reflux for 150 min. The resulting solution was diluted to 0.1 mol L ¹1 in ethanol. The membrane substrate was dipped into the nickelsilica solution for 10 s. Finally, the membrane was dried at room temperature for 3 h and calcined at 873 ...