Upon calcination at temperatures 800c only poorly

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Unformatted text preview: calcination at temperatures ≤ 800°C only poorly crystallised spinel phases and divalent metal oxide phases with periclase structure were observed next to a hydrotalcite-type phase, which is again attributed to partially surfacial reconstruction. Furthermore the crystallinity of all samples increased with increasing calcination temperature as determined by XRD line broadening (Tab. 10). The findings are in good agreement with the existing model concerning the structure of HT derived catalysts. Calcination at temperatures above 850°C results in the formation of well crystallised divalent metal oxide phases with periclase structure and spinel phases, whereas at temperatures below 850°C a poorly crystallised divalent metal oxide phase with periclase structure is formed in addition to an amorphous Al2O3 phase and a spinel phase at their interface [7, 53]. Tab. 10: Crystallite size determined by XRD line broadening Catalyst Crystallite size [nm] Ni10C700 6 Ni10C800 8 Ni10C900 14 Ni10C1000 25 83 3. Results and discussion - catalyst preparation and characterisation D D,S H S H D S S S S intensity [a.u.] Ni10C1000 Ni10C900 Ni10C800 Ni10C700 0 10 20 30 40 2Q 50 60 70 Fig. 14 XRD spectra of the catalyst precursor Ni10 calcined at 700, 800, 900 and 1000°C (D = divalent metal oxide phase with periclase structure, S = spinel phase, H = hydrotalcite-type phase) 80 3. Results and discussion - catalyst preparation and characterisation 84 The scanning electron microscopy (SEM) images recorded for the catalysts Ni0C900, Ni10C900, Ni30C900, Ni50C900 and Ni72C900 reveal a complex 3 dimensional structure (Fig. 15). Especially the images of Ni0C900, Ni30C00 and Ni50C900 show a rather flat. The craters through which the volatiles are supposed to escape during the calcination process are still visible in particular for the sample Ni50C900 [58]. Ni0C900 Ni50C900 Ni10C900 Ni50C900 Ni30C900 Ni72C900 Fig. 15 SEM images of the catalysts Ni0C900, Ni10C900, Ni30C900, Ni50C900 and Ni72C900 85 3. Results and discussion - catalyst prepar...
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