The experimental nickel load determined by AA was between 7
and 9 wt% in all of the samples used in this work (Table 1). After
impregnation and calcination at 500 C, the final specific surface
areas of samples NS and NA were similar to those of the corresponding
supports (Table 1). The specific surface area (Sg) of NS
was about 20 times higher than that one of NA. Besides, after
coprecipitation and thermal decomposition, NMA and NZA mixed
oxides have similar Sg to the one of NS (Table 1).
The XRD patterns of oxide precursors are shown in Fig. 1. Only a
single polycrystalline phase of NiO (JCPDS 22-1189) was formed in
the case of NS sample after calcination. In addition, the amorphous
halo of SiO2, due to the scattering of the Si–O–Si structure, was observed
at 2h < 30. In the case of NA sample, two well defined polycrystalline
phases were observed: one characteristic of the low
surface a-Al2O3 (JCPDS 46-1212) and another one corresponding
to NiO (JCPDS 22-1189). Instead, broad diffraction lines were observed
for the NMA mixed oxide in the range 2h = 30–70. These
lines could be assigned to small NiO crystallites. However, similar
XRD patterns were obtained for MgAl2O4 spinels synthesized at
temperatures lower or equal to 600 C [33]. The X-ray diffractograms
for these mixed oxides showed that the diffraction line of
highest intensity corresponds to the (4 0 0) plane family
(2h = 44), instead of the (311) plane family (2h = 39), which is
similar to the X-ray diffactogram obtained for NMA sample in this
Tabl