Catalytic Properties of Ni 3 Al Foils for Hydrogen Production
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0980-II07-09
Catalytic Properties of Ni3Al Foils for Hydrogen Production Toshiyuki Hirano National Institute for Materials Science, Tsukuba, 305-0047, Japan ABSTRACT We have successfully developed thin foils of boron-free Ni3Al (below 100 µm in thickness) by cold rolling, and recently found that the foils exhibit high catalytic activity for methanol decomposition. A little has been known about catalytic activity in Ni3Al. Even more interestingly, the high catalytic activity appears on flat foils whose surface area is very low. This paper provides a review of the characteristic features of the catalytic properties investigated in my group. Methanol was effectively decomposed into H2 and CO over the foils above 713 K. The production rates of H2 and CO increased with an increase of time during the initial period of reaction, indicating that the Ni3Al foils were spontaneously activated under the reaction conditions. Surface analyses revealed that fine Ni particles dispersed on carbon nanofibers formed on the foils during the reaction. The high catalytic performance of the foils can be attributed to the spontaneous formation of this nanostructure during the reaction. INTRODUCTION Ni3Al has attracted interest as a promising high-temperature structural material because of its excellent high-temperature strength and oxidation/corrosion resistance, and thus many studies have been focused on the mechanical properties [1]. However, since the high-temperature creep strength was found to be less than that of Ni3Al/Ni solid solution two-phase alloys [2], it gets less attractive as a simple structural material which bears load at high temperatures. New ideas to develop the potential of Ni3Al are required. Ni3Al is known to suffer from grain boundary brittleness. The brittleness can be overcome by microalloying with boron [3,4], but even all the beneficial effects, the ductility is not sufficient enough to heavily cold-roll into thin foil form [5]. Recently, we for the first time have successfully fabricated thin foils of boron-free Ni3Al with 23 µm in thickness by cold rolling of single crystals (Figure 1) [6-9]. Since the foils have good formability after heat treatment [10], it is possible to assemble them into honeycomb structure, which provides a potential application to high-temperature chemical process, for example a catalyst support of automotive exhaust gas converter [11]. Recently we found that Ni3Al exhibits high catalytic activity in a powder form and selectivity for methanol decomposition into H2 and CO (CH3OH→2H2+CO) after alkali-leaching treatment, showing a potential of Ni3Al as catalysts [12,13]. More interestingly, we found similar high catalytic properties in the cold-rolled Ni3Al foils without alkali-leaching treatment [14,15]. Because of their very low surface area, it is quite unusual that flat foils exhibit high catalytic activity. In commercial heterogeneous catalysts active metal is in a fine particle form and dispersed on porous supports in order to achieve large surface area of active metal. The surface
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