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.
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 & 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
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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