Quasicrystalline Catalyst for Steam-Reforming of Methanol
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Quasicrystalline Catalyst for Steam-Reforming of Methanol
A.P. Tsai1 and M. Yoshimura2 1. National Research Institute for Metals, Tsukuba 305-0047 and CREST JST, Japan. . Corporate Research Laboratory, Mitsubishi Gas Chemical Co, 22 Wadai, Tsukuba, Ibaraki 300-4247, Japan ABSTRACT
Steam reforming of methanol (CH3OH + H2O -> 3H2 + CO2) has been performed on a number of Al-Cubased stable quasicrystals and related alloys. Alloys leached in NaOH aqueous solution exhibit the highest activity for steam-reforming of methanol. Among the alloys, AlCuFe quasicrystal, after leaching treatment, reveals excellent activity. The production rate of H2 reaches 235 l/kgmin at 573K for steam reforming of methanol. The activity is due to Cu nanoparticles at the surfaces of quasicrystalline grains which are generated by leaching treatment. The quasicrystals have two advantages: one is their brittle nature, which allows them to be crushed efficiently; the other is the involvement of Fe, which suppresses the sintering of Cu particles.
INTRODUCTION
Discovered in 1984, icosahedral quasicrystals exhibit a forbidden five-fold diffraction pattern [1] with sharp diffraction peaks indicating the presence of a long-range order similar to that of crystals. The discovery has stimulated interactions between the fields of physics and materials science [2]. Intensive efforts have been dedicated to studies of alloy synthesis, structure and physical properties over the last decade. Over 100 binary, ternary and quarternary alloy systems have been found that contain quasicrystalline phases [3]. Due to lack of periodicity, most quasicrystals reveal some common characteristics in properties e.g., extremely high resistivity over a few thousands of mΩ cm, close to that of semiconductors [4,5]; hardness in the range 800-1000 Hv [6,7], which is comparable to silica; and low coefficients of friction, having same level as that of diamond [8,9]. Recently, quasicrystals are partly on the market in the form of various applications, and more new products are being developed. For example, those have been used as materials on coating, optical device and hydrogenation. A promising application of quasicrystals is catalysis. Since quasicrystals are equilibrium phases and stable up to high temperatures, they have potential to be used as catalysts at high temperature. Recently, the catalytic behavior of quasicrystalline Al-Pd-Mn has been investigated, relative to crystalline Al-Pd, pure Pd and pure Cu. Particles of size smaller than 200nm of each sample were mixed with MgO and calcinated at 775K for 5h[10]. In the methanol decomposition reaction the quasicrystalline catalyst achieved the highest amount of hydrogen gas generated andthe lowest reaction initiation temperature. Several quasicrystalline alloys containing Pd were tested and all found to be highly active toward methanol decomposition. It is reasonable that Al-Pd-Mn quasicrystal is a good catalyst for this reaction, since a considerable amount of Pd, which is active to hydrogen, is involved. Howev
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