Dr gnter geismar and from 15th august 1997 to 8th

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Unformatted text preview: supervisor Prof. Dr. Günter Geismar and from 15th August 1997 to 8th June 2001 at the Faculty of Chemistry, Jagiellonian University, Krakow, Poland, supervisor Prof. Dr. Roman Dziembaj. Abstract Anionic clays of hydrotalcite-type structure of the approximate formula [NixMg72-xAl28(OH)200][(NO3)y(CO3)(28-y)/2]· 50H2O (x = 0, 5, 10, 15, 30, 50, 72 and 11 ≤ y ≤ 16) were synthesised by coprecipitation at low supersaturation at pH 8 ± 0.2. Their composition and structure was determined by elemental analysis, XRD, TG-MS and DSC. Their calcination at temperature • ƒ& LQ D IORZ RI DLU UHVXOWHG LQ WKH WUDQVIRUPDWLRQ LQWR WKH FRUUHVSRQGLQJ VROLGV of well crystallised divalent metal oxide phases with periclase structure and spinel phases. At lower temperatures poorly crystallised divalent metal oxides and amorphous Al2O3 phases with poorly crystallised spinel at their interfaces were formed. The degree of crystallinity increased with increasing calcination temperature. Catalysts with a lower nickel content exhibited remarkably high surface areas. This suggests that the fast removal of the gases formed during the calcination process combined with a slow heating rate had a beneficial influence on the catalysts’ preparation. Temperature programmed reduction/oxidation of Ni2+/Ni0 and high temperature XRD studies of the catalysts calcined at 900°C showed that their reduction takes place in two distinct steps. First the Ni2+ ions in the divalent metal oxide phase are reduced followed at higher temperatures by the Ni2+ ions in the spinel phase. The catalysts with a lower nickel content were more difficult to reduce but exhibited higher nickel dispersion. These results are attributed to the substitution of Mg2+ by Ni2+ leading to the formation of a NiO-MgO solid solution upon calcination, which stabilises the Ni2+ ions against reduction and sintering. The formed metallic nickel particles are not stabilised by the MgO matrix against the consecutive oxidation. The reduction temperature increases slightly wit...
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This document was uploaded on 10/07/2013.

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