Porous Al 2 O 3 /Al catalyst supports fabricated by an Al(OH) 3 /Al mixture and the effect of agglomerates
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Yoshio Sakka Fine Particle Processing Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan
Yutaka Kagawa Composite Materials Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan; and Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan (Received 26 September 2004; accepted 3 December 2004)
Porous Al2O3/Al catalyst supports were fabricated using a mixture of Al(OH)3 and Al powders, followed by pressureless sintering at a temperature of 600 °C in vacuum. Different pressures were used to prepare green compacts. High compaction pressure led to a high surface area and good mechanical and electrical properties for the sintered specimens. However, when the Al content in the sintered specimen exceeded a definite value, high compaction pressure decreased the surface area abruptly. Scanning electron microscopy observations revealed that agglomeration in the starting mixture has a significant effect on the microstructure of the sintered specimens. High compaction pressure greatly eliminated the agglomerates and led to a uniform microstructure for the sintered specimens. However, when the Al content in the starting mixture was too high, Al particles in the compacts prepared by the high pressure were largely sintered due to the high compact density so that most of the pores were closed. The present study indicates that a suitable compaction pressure is critical to obtaining superior Al2O3/Al supports.
I. INTRODUCTION
Transitional Al 2 O 3 phases such as ␥–, ␦–, and –Al2O3 are widely used as catalyst supports due to their high surface area. Usually, the mechanical properties of pure transitional Al2O3 supports are very poor. However, in some cases, good mechanical properties are needed for catalyst supports, for example, the supports used in the automobile for the removal of NOx in the combustion exhaust gas. The catalytic abatement of NOx in exhaust emission requires catalyst supports with high attrition resistance.1 Recently, Al2O3/Al catalyst supports have received considerable attention because the existence of Al enhances the mechanical properties and the electrical conductivity. The high electrical conductivity implies the high thermal conductivity for metal matrix composites,2
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0077 672
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 04 Apr 2015
which is favorable for the local thermal diffusion and transfer if the catalytic reaction is exothermic.3 At the same time, the oxidation products of metal Al are transitional Al2O3 phases. Different porous Al2O3/Al catalyst supports were obtained by different methods. For example, the anodization of an Al monolith produced an adherent, porous and thermally and mechanically resistant Al2O3 layer, which is suitable for catalytic processes for which the temperature requirements are not very high.4 The oxidation of Al powder under hydrothermal conditions foll
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