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S. Ren et al. / Catalysis Communications 12 (2010) 132–136
obviously for the pretreated catalysts, especially for NiO/SBA-15 (20T)
and Ni/SBA-15 (20T) catalysts. This indicates that nickel oxide and
nickel metal particle sizes of the un-pretreated catalysts are larger
than that of the pretreated catalysts, suggesting that the dispersion of
nickel oxide and nickel metal can be enhanced by the pretreatment
with ammonia/water vapour.
In order to further explore the effect of ammonia/water vapour
pretreatment on the dispersion of the nickel species in NiO/SBA-15
samples, H2-TPR of different NiO/SBA-15 samples were measured as
shown in Fig. 2. It could be observed that the nickel species were
reduced at low temperature and high temperature separately. The
nickel species reduced at low temperature can be attributed to the
NiO un-interacted with support, and the nickel species reduced at
high temperature can be attributed to the NiO interacted strongly
with support to form a few surface layers of silicate-type compounds
[13]. It could be obviously observed that the ratios of peak area at high
temperature to low temperature in the pretreated NiO/SBA-15
samples, especially for NiO/SBA-15 (20T) sample, were greater than
the values of the un-pretreated NiO/SBA-15 samples. This demon-
strates that the amount of the nickel species interacted strongly with
support in NiO/SBA-15 sample can be improved by ammonia/water
vapour pretreatment. Generally the strong interaction between the
nickel species and support favors to enhance NiO dispersion.
Therefore the dispersion of NiO in NiO/SBA-15 sample is promoted
by ammonia/water vapour pretreatment, and it is accordance to the
result of XRD mentioned above.
Fig. 3 shows H2-TPD of different Ni/SBA-15 catalysts. Comparing
with the catalyst without the pretreatment (Fig. 3a, c), the area under
the TPD curve of the pretreated catalyst increased obviously
(Fig. 3b, d). It indicates that the pretreated catalyst consists of more
exposed surface Ni atoms. Based on the peak area of the H2-TPD
profiles and assuming the adsorption of one H atom per metal atom
[12], the H2 chemisorption uptake, surface area, dispersion and
particle size were estimated as shown in Table 1. It clearly shows that
the nickel metal particle sizes decrease (which is accordance with the
results calculated by the Scherrer equation from XRD data) and the
surface area and dispersion increase for the pretreated catalysts,
suggesting the dispersion of the catalyst being promoted by the
pretreatment of ammonia/water vapour.
d
c
b
a
100
200
300
400
500
°
Temperature ( C)
Fig. 3. H2-TPD of the different catalysts: (a) Ni/SBA-15 (10U); (b) Ni/SBA-15 (10T);
(c) Ni/SBA-15 (20U); (d) Ni/SBA-15 (20T).
3.2. Improvement of ammonia/water vapour pretreatment on the
hydrogenation activity
In order to explore the effects of the pretreatment by ammonia/
water vapour on the activity of the Ni/SBA-15 catalysts, the
hydrogenation reactions of naphthalene were carried out on the Ni/
SBA-15 catalysts. Table 2 shows the conversions of naphthalene and
selectivities of main products. The main products of the hydrogena-
tion were tetrain and decalin, and no hydrocracking product was
observed. It could be observed from Table 2 that the pretreated Ni/
SBA-15 catalysts had much higher catalytic activities for the
naphthalene conversion compared with those of un-pretreated
catalysts. For example, the naphthalene conversion increased from
72.0% of Ni/SBA-15 (10U) to 100.0% of Ni/SBA-15 (10T) when
naphthalene content in feed was 5.0 wt%. In the mean time the
selectivity of decalin also increased from 28.7% to 81.9%. Since decalin
is produced through deep hydrogenating of tetralin, the increased
selectivity of decalin is also correlated with the hydrogenating activity
of the catalyst. So the total hydrogenating activity of the catalyst
cannot be evaluated only using the conversion of naphthalene, and
the selectivity of decalin should be also considered. When naphtha-
lene content in feed increased from 5.0 wt% to 10.0 wt%, although
naphthalene conversions over the catalysts with and without
pretreatment obviously decreased, but selectivities of decalin on the
two catalysts were almost zero (0% for Ni/SBA-15 (10U) and 2.6% for
Ni/SBA-15 (10T)). In this case the hydrogenating activity of the
catalyst could be accurately evaluated only using naphthalene
conversion. Again, Table 2 clearly shows that when naphthalene
content in feed is 20 wt%, the selectivity of decalin on the catalyst with
or without pretreatment is almost zero, although the amounts of
nickel-loaded in the catalyst increase to 20 wt%. The naphthalene
conversion was 45.7% for the Ni/SBA-15 (20T) catalyst and it was
improved and approximated at a 100% compared with the un-
pretreated catalyst Ni/SBA-15 (20U) of 23.1%, indicating that the
catalytic activity of Ni/SBA-15 catalyst could be strongly enhanced by
ammonia/water vapour pretreatment.
d
c
b
a
3.3. Mechanism of the promotion of catalytic activity
In the case of zirconia or alumina, ammonia/water vapour is thought
as hydrolyzing agent to induce the internal hydrolysis process, which is
expected to precipitate zirconium hydroxide or aluminum hydroxide on
the walls of SBA-15 in a highly dispersed manner and the calcination of
theresulted composite material may leadtozirconia-coated or alumina-
coated SBA-15 [6,7]. The first possible reason for the promotion of
200
400
600
800
°
Temperature ( C)
Fig. 2. H2-TPR of different NiO/SBA-15 samples: (a) NiO/SBA-15 (10T); (b) NiO/SBA-15
(10U); (c) NiO/SBA-15 (20T); (d) NiO/SBA-15 (20U).