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Unformatted text preview: catalytic activity of Ni. However, the sintering due to the nitric oxides has been suggested to be avoidable by the fast removal of the nitric oxide gases by a stream of air. Fine crushing of the sample further assists the fast removal during calcination [4, 59, 66]. It was also pointed out that under such conditions also water is quickly removed. Water also leads to sintering of NiO particles. Schaper et al. also reported that the effect of Ni particle sintering due to the evolution of nitric oxide gases can be avoided by carrying out the calcination in a flow of hydrogen by which the nitric oxides should be reduced to nitrogen or ammonia [66]. Also adsorbed species can influence the catalytic behaviour. For example catalysts derived from HTs prepared from sodium containing solutions can contain Na salts due to insufficient washing of the HTs after coprecipitation. Na ions are a poison for steam reforming and methanisation catalysts [1, 4, 59]. The HT derived mixed metal oxides can be doped with any additional metal ions by impregnation methods [1, 25]. For metal ions that are not suitable for the preparation or the intercalation of HTs this is the only way of their introduction to the catalyst. The ‘memory effect’ can be activated by contact with water and even more by aqueous solutions containing anions. Therefore impregnation is best performed in anhydrous solvents such as anhydrous ethanol, which are able to dissolve many metal salts but do not activate the ‘memory effect’ [67]. For many technical applications a good mechanical stability of the catalysts is crucial. The mechanical stability of the mixed metal oxides derived from HT precursors can be improved by using a support. Silicates, hydrated aluminium, aluminium-, zirconiumand titanium oxide as well as graphite are suitable supports [1]. Two different procedures have been used for the preparation of supported HTs [1, 5, 6, 66]. Most 22 1. Introduction ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff commonly the HTs are directl...
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This document was uploaded on 10/07/2013.

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