The ability to reconstruct also depends on the metal

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Unformatted text preview: tal composition of the HT precursors. Mg/Al mixed metal oxides derived from HT precursors by calcination at temperatures lower than that required for the formation of a separate spinel readily rehydrate to reconstruct the HT structure, whereas their Ni/Al counterparts calcined at the same temperatures are even stable in hot aqueous NaOH solution at atmospheric pressure. Their reconstruction was only observed upon the energetic stirring of their aqueous suspensions at 250°C at 4 MPa during 12 hours [7, 53]. It must also be pointed out that during the thermal evolution HTs containing variable valence metal ions can lead to the occurrence of other phases than those discussed above. Infra red (IR) investigations concerning Ni/Cr calcined at 450°C in air showed the presence of NiCrO4 and NiO. The same HT sample calcined at the same temperature, but in vacuum instead of air exhibited no evidence for the oxidation of Cr(III) to Cr(VI) [8, 60]. However NiO and NiCr2O4 were the final product of the calcination performed in air and in vacuum. The temperature for the formation of the final product was slightly lower for the sample calcined in air (550°C) than in vacuum 18 1. Introduction ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff (600°C). For Ni/Al HTs the formation of the final product, spinel and divalent metal oxide phase, was only detected after calcination ≥ 900°C, showing the great influence of the nature of the trivalent anion on the thermal evolution of the HTs [4, 8, 33, 60]. The calcination temperature has a significant effect on the surface area of the metal oxide systems derived from HT phases (Fig. 2). Their surface area increases upon the reversible loss of intersistal water and the dehydroxylation of the brucite-type metal hydroxide layers up to the point were the HTs are completely transformed into mixed metal oxides. Above this point a steady decrease of the surface area with increasing calcination temperature is observed. The decrease of the surface area may be d...
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