The amount of the spinel phase formed generally

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Unformatted text preview: ination temperature. Therefore the property of the ‘memory effect’ is strongly temperature dependent and great care must be taken for keeping the calcination temperature as low as possible [25, 55, 56]. 1.2.2. Thermal decomposition During the heating of HTs three different temperature regions can be found [57]: 100 - 250°C: Water in the interlayer is lost reversibly. The morphology of the HT is maintained. Lost water can be reabsorbed from the air. 250 - 850°C: The brucite-type sheets are dehydroxylated resulting in the formation of a mixed metal oxide. Intercalated anions that are thermally easily destructible, like carbonate and nitrate, are also decomposed. The sheet structure is maintained during the dehydroxylation, no delamination is taking place. Steam and volatiles escape through small and fairly regularly spaced holes in the crystal surface. It appears that the interlaminar forces in the HTs are sufficiently strong even during the heating to prevent intersistal venting, which would result in the delamination of the sheet structure [58]. Investigations concerning a Mg/Al HT calcined at 300°C revealed that a 1. Introduction 16 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff metastable phase is formed, which becomes obscure in a tight sealed silica gel filled desiccator and easily reconstructs to a HT in a humid atmosphere [57]. At temperatures above 500°C the HTs are generally completely transformed into mixed metal oxides. These mixed metal oxides also have the ‘memory effect’. The XRD of the mixed metal oxides reveals generally only the presence of a divalent metal oxide phase of low crystallinity. Studies concerning the structure revealed 3 separate phases [7, 53]. A M(II)O-phase, a M(II)O doped M(III)2O3 phase and a spinel-type phase at their interface, decorating the M(II)O phase. The decoration of the M(II)O phase by the spinel phase is believed to be responsible for the high stability of the materials against heat, steam, sintering and reduction of the M(II)O phase as shown for Ni/Al catalysts derived from HT precursors [6, 7, 53, 59]. With increasing calcination temperature increases the amount of th...
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