The influence of the Ni deposition mode on the dispersion of
Ni2+ precursors was investigated in
the preparation of Ni/SiO2 catalysts. The coordination
sphere of Ni complexes was mainly studied by XAFS
spectroscopy in the initial step (dried samples) and after a vacuum
activating treatment at 700 °C. Four modes
of deposition were compared, two leading to supported silicate phases
(exchange with ammine ligands and
deposition−precipitation) and the other ones using the ethanediamine
ligand (exchange and impregnation modes)
which produced isolated Ni2+ precursors in electrostatic
interaction with the silica support. In this work,
EXAFS spectroscopy has been found to be a suitable technique to probe
metal−support interactions in the
first step of the preparation (dried samples). For samples
activated at 700 °C, this spectroscopy showed the
presence of several categories of atoms in the first (oxygen
backscatterers) and second (Ni and Si backscatterers)
shells. A distribution of long (d ≈ 2.04 Å) and
short (d ≈ 1.75 Å) Ni−O distances was found,
corresponding
to hexacoordinated (Ni2+
6c) and isolated
tricoordinated (Ni2+
3c) ions, respectively.
Modeling of the structure
of the Ni2+
3c site indicated a distorted site
with two short and one long Ni−O distance. The exchange
mode
with ethanediamine ligand led to isolated
Ni2+
3c ions which could then be photoreduced
into Ni+
3c ions with
the highest yield. Impregnation with ethanediamine Ni nitrate was
also found advantageous as it led after
thermal activation at 700 °C to NiO particles smaller than those
produced from impregnation with Ni nitrate
and therefore to smaller Ni particles after the subsequent reduction
step.
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