The reflections observed correspond to the structures

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Unformatted text preview: ell crystallised solids of hydrotalcite-type structure were obtained. The reflections observed correspond to the structures of hydrotalcite (Mg6Al2(OH)16CO3· 4H2O) and takovite (Ni6Al2(OH)16CO3· 4H2O), the Ni derivative of hydrotalcite, the latter found only in Ni72, the pure Ni/Al compound and Ni50, the solid with a high Ni/Mg ratio (~5/2). 75 3. Results and discussion - catalyst preparation and characterisation H,T T H,T H,T H,T H,T H,T H,T Ni72 Ni50 intensity [a.u.] Ni30 Ni15 Ni10 Ni5 Ni0 0 10 20 30 40 2Q 50 60 Fig. 10 XRD spectra of the coprecipitated solids (H = hydrotalcite, T = takovite) 70 80 3. Results and discussion - catalyst preparation and characterisation 76 3.1.3. Analysis of the thermal degradation However, as only crystalline phases can be observed by XRD, the presence of amorphous side phases cannot be excluded. Such phases can generally be detected in hydrotalcite-type solids by the study of the thermal degradation process using TG and DSC. The TG profiles with the corresponding DTA curves are presented in Figure 11 and the DSC thermograms in Figure 12. Two endothermic weight losses were observed during the calcination of all samples at about 140 and 360°C. The temperature of the second weight loss increased with decreasing Ni/Mg ratio of the solids from 310°C for Ni72 to 420 for Ni0. Depending on the sample composition, the first weight loss accounted for about 8-15% mass loss and the second one for about 15-35%. The temperature, the amount of lost weight and the shape of the curves are typical for HT materials. On the basis of the on-line analyses of the volatiles by MS and the literature (see chapter 1.2.2.), the first weight loss should be attributed to the loss of the interlayer water, the latter to the elimination of the carbonate and nitrate anions in the interlayer and the dehydroxylation of the metal hydroxide. The results show that the thermal stability of the hydrotalcite-type solids decreases with increasing substitution of Mg2+ by Ni2+. The analysis of the structure and the thermal degradation process proofs that coprecipitation yielded well crystallised solids of hydrotalcite-type structure their compositio...
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This document was uploaded on 10/07/2013.

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