Hydrogen permeation characteristics and stability of Ni-doped silica membranes in steam at high temperature
Introduction
Hydrogen is widely used in various fields of industries, such as chemical, steel, oil refining and petro-chemical industries and is also expected as a clean alternative energy source, anticipating enormous demand for hydrogen in the near future. Among various hydrogen sources, about 50% of world's hydrogen is produced from fossil fuels, such as natural gas and naphtha by means of steam reforming reactions, which are thermodynamically well understood to favorably proceed at higher temperatures (>800 °C) and lower pressures. If H2 can be selectively removed from the reforming reactor through H2-selective membranes, the thermodynamic equilibrium can be shifted to the product side, resulting in a higher conversion of CH4 to H2 even at lower temperatures (∼500 °C). Some researchers already confirmed experimentally the phenomena by continuous removal of H2 from the reaction system through inorganic membranes [1], [2], [3]. The important things in the membrane application to the reactors are the membrane characteristics or hydrogen permeance, selectivity and stability in steam at high temperatures.
In the last two decades, H2-separation membranes have been developed using various materials, such as palladium and its alloys, silica and alumina, etc. Palladium membranes formed on porous alumina supports by the electro-less plating techniques and the MOCVD methods were reported to show high H2-permeselectivity at 300–500 °C [4], [5], but they have some disadvantages, such as degradation of H2-separation performance, for example, in the presence of hydrocarbons at high temperatures. In order to improve the stability of Pd membranes against the chemical poisoning and the mechanical load Pd alloys (Pd–Ag, Pd–Ni, Pd–Nb) were employed for metal membranes [6], [7], still leaving some problems in their stability at high temperatures besides the high cost of Pd. Ceramic materials, on the other hand, can be hopefully expected as the membrane materials of high stability at high temperatures. Silica is one of the most attractive materials for H2-separation membranes because of its amorphous structures with pores (or spaces) smaller than 1 nm in a wide temperature range below around 1000 °C. Dense silica membranes prepared by CVD or by the sol–gel techniques were found to show some excellent separation performance for hydrogen and helium in dry conditions in a wide temperature range, 50–600 °C [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Their stability against water or water vapor, however, is rather poor at high temperatures or even at room temperatures [10], [11], [12], [13], [14], [15]. In order to improve the stability of silica membranes in steam, inorganic oxides, such as TiO2, ZrO2, Fe2O3, Al2O3, NiO, etc. were added to silica [19], [20], [21], [22], [23]. Of these metal-doped membranes, Ni-doped silica membranes showed relatively high H2-permence and high stability against water vapor at 35–300 °C [21], [22], suggesting the effectiveness of the addition of nickel oxides to silica for the membrane stability against steam at higher temperatures.
In order to examine the stability of composite silica membranes in steam at higher temperatures at around 500 °C several kinds of Ni-doped silica membranes were fabricated in this work by the sol–gel techniques under various conditions of Ni contents and firing temperatures. The stability and gas permeation characteristics of the Ni-doped silica membranes were examined by observing gas permeation characteristics (N2, H2, He and H2O) after exposed to steam at 500 °C.
Section snippets
Preparation of colloidal sols
Ni-doped silica colloidal sols of various molar ratios (Si/Ni = 4/1, 3/1, 2/1, 1/1) were prepared by hydrolysis and condensation reactions of tetraethoxysilane (TEOS) in ethanol with specific amounts of Ni(NO3)2·6H2O and water, as reported previously [21], [22], [23]. The preparation procedures of Ni-doped silica colloidal sols of Si/Ni = 2/1, for example, are as follows. A specific amount of TEOS (10 g, for example) was added to ethanol (50 g) with Ni(NO3)2·6H2O (6.98 g), and the solution was stirred
Membrane characterization
Fig. 2 shows a SEM photo of typical cross-section of a Ni-doped silica membrane (Si/Ni = 2/1). A thin Ni-doped silica layer effective for the selective H2-permeation (see Section 3.2) can be seen on a α-alumina particle–silica–zirconia intermediate layer, covering completely the roughness (bumps) of the surface. The thickness of this active separation layer is clearly less than 0.5 μm. The improved methods in the work are quite effective to make a thin separation layer without any appreciable
Conclusion
The sol–gel techniques were applied to fabricate Ni-doped silica membranes of various Ni contents (Si/Ni = 4/1-1/1) for separation of H2 at high temperatures. And H2-selective permeation characteristics and hydrothermal stability of the membranes were tested in steam at 500 °C to show that hydrothermal treatments of the membranes, or firing in steamed atmosphere (steam: 90 kPa) at 650 °C before exposed to H2, were quite effective to prevent further densification of Ni-doped amorphous silica networks
Acknowledgements
This work was performed as a part of the R&D Project for High Efficiency Hydrogen Production/Separation System using Ceramic Membrane carried out by the New Energy and Industrial Technology Development Organization. The authors show their cordial acknowledgement for the financial supports.
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