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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 . 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’ . 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 . 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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