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H. Song et al. / Catalysis Communications 59 (2015) 61–64
FTIR measurements were carried out with a Spectrum GX Fourier trans-
form infrared spectrometer. H2 temperature-programmed reduction
(H2-TPR) was carried out on a Builder PCA-1200 instrument.
3.3. XRD
As shown in Fig. 2, the diffraction peaks of the tetragonal ZrO2 at
2θ = 30.2, 34.7, 35.1, 50.0 and 60.5° are detected in all samples, while
no diffraction peaks assign to Ni and La or related compounds can be
observed, implying that no or very little crystalline Ni and La or related
compounds are formed in these samples. The crystalline phase struc-
tures of ZrO2 were kept unchanged after modification by rare earth La
or Ni, indicating that Ni and La species were dispersed on the surface
of catalysts. Mean crystallite sizes are shown in Table 1. All catalysts
except SZA exhibit much smaller crystallite size of tetragonal ZrO2,
and the crystallite sizes decrease in the order: SZA N Ni–SZA N La–Ni–
SZA N La–SZA. This reveals that the modification of metals (La and/or
Ni) leads to a decrease in the crystallite size of tetragonal ZrO2.
2.3. Catalytic test
The isomerization of n-pentane was performed in a fixed-bed
flow reactor. Prior to the reaction, the catalyst was activated. A dose of
n-pentane was passed over the 5 g of activated catalyst under the
following reaction conditions: a reaction pressure of 2.0 MPa, a molar
H2/n-pentane ratio of 4:1, and a weight hourly space velocity of
1.0 h−1. The products were analyzed by an online FL9790 gas chromato-
graph equipped with a FID detector.
3.4. H2-TPR
3. Results and discussion
As shown in Fig. 3, for SZA, only one peak attributed to the reduction
of sulfate ions is observed around 655 °C, while a wider and bigger peak
is exhibited around 685 °C for La–SZA. The addition of La leads to the
apparent increase in the peak intensity, which indicates that La can
lead to a better dispersion of sulfate ions on the surface of the catalyst
[10]; as a result, more acid sites are provided for La–SZA, and the results
of sulfur content (Table 1, column 7) also illustrate that La–SZA pos-
sesses more sulfur species than SZA. The shift of the reduction peak of
sulfate ions for La–SZA to higher temperature shows a strong interac-
tion exists between La and S species. No appearance reduction peaks
of La2O3 are observed. This is understandable because the La2O3 is hard-
ly reduced at the temperature below 1000 °C [11].
For Ni–SZA and La–Ni–SZA, an overlapping peak is detected. Gener-
ally, the reduction peak of Ni oxide to metallic state is around 320 °C
[12]. However, the reduction peak of Ni oxide to metallic state is detect-
ed at temperature around 510 °C for Ni–SZA and around 520 °C for La–
Ni–SZA. These shifts are owing to the strong interaction of Ni oxide with
support [13]. The reduction peaks at 542 °C for Ni–SZA and 532 °C for
La–Ni–SZA are assigned to the reduction of sulfate ions. The strong in-
teraction between Ni and S is responsible to the shift of the reduction
of sulfate ions to lower temperature; in other words, the addition of
Ni improves the redox performance of the catalysts.
3.1. Textural properties
The surface area of SZA is 92.3 m2 g−1 and shows an increase to
138.6 m2 g−1 for La–SZA, accompanying by an increase in the pore vol-
ume from 0.101 mL g−1 to 0.163 mL g−1, indicating some structural
changes caused by the addition of La (Table 1). It suggests that most
La species are located on the external surface of SZA. By contrast, Ni–
SZA (95.1 m2 g−1, 0.089 mL g−1) exhibits slightly higher specific surface
area and lower pore volume in comparison with the SZA. This is possibly
because of plugging or blocking the pores by Ni species. The surface area
of La–Ni–SZA catalyst is 98.4 m2 g−1, which is much lower than that of
La–SZA, showing that the surface area decreases significantly with addi-
tion of Ni. However, the surface area of the La–Ni–SZA is still higher than
that of SZA owing to the effect of La. The increased surface area of
catalysts would facilitate the dispersion of active metal and acid sites;
therefore, it is no doubt that the increment of textural properties in
the La–Ni–SZA is beneficial to the catalytic activity. In addition the N2
adsorption–desorption isotherms and pore diameter distribution were
measured. The N2 adsorption–desorption isotherms of support and cat-
alysts are type IV with hysteresis loops, indicating the present of some
mesoporous. The pore size distributions exhibit only one modal peak
for each sample.
The reduction of Ni oxide for La–Ni–SZA catalyst starts at around
450 °C, which is earlier than that of Ni–SZA; this probably results from
the interaction between La and Ni species. In addition, the reduction
peak intensity of sulfate ions for La–Ni–SZA is higher compared with
Ni–SZA; as a result, more acid sites are provided for La–Ni–SZA, which
would lead to a better catalytic activity for isomerization.
3.2. TEM
Fig. 1(A) reveals that Ni particles over Ni–SZA were agglomerated
with the average particle size approximately 12.1 nm. With addition
of La (Fig. 1(B)), the agglomeration of Ni particles was less severe and
the average particle size decreased to about 9.1 nm, indicating a better
dispersion of Ni particles. The observation is in consistent with the
results reported by Tatiana et al. [9]. The Ni content on surface of Ni–
SZA (0.7 wt.%) calculated from XPS (Table 1, column 6) shows that
La–Ni–SZA catalyst (1.35 wt.%) possesses much higher Ni content.
3.5. Acid properties
FTIR of various catalysts were recorded. As shown in Fig. 4, all sam-
ples exhibit the bands at 1182 and 1071 cm−1 assigning to the symmet-
ric O\S\O stretching mode of bidentate persulfate ions coordinated to
the metal ion. The band at 1278 cm−1 corresponds to the antisymmetric
O_S_O stretching frequency of persulfates ions bonded to ZrO2. The
characteristic vibration peaks of S_O and S\O bonds confirm the exis-
tence of solid superacid structure over all the prepared samples [14].
The partially ionic nature of the S_O bond is responsible for the
Brønsted acid sites in SZA samples [15]. Among the samples, the La–
Ni–SZA catalyst possesses the highest intensity of S_O vibration
band at 1278 cm−1; in addition, the bands at 1071 and 1182 cm−1
corresponding to S\O vibration are the sharpest. The sulfuric group
bands mentioned above intensity and their splitting extent reflect
the proportion of active sites linked to the catalyst. For the La–Ni–SZA
catalyst, both the intensity of the three bands and the extent of
their splitting are the highest, indicating La could strengthen the inter-
action between persulfate ions and support; as a result, the strongest
superacid is formed over La–Ni–SZA catalyst. The sulfur content
Table 1
The physical properties of the support and catalysts.
Ni content b Sulfur
a
Sample
SBET
Vp
dp
Dc
(m2 g–1
)
(cm3 g–1
)
(nm) (nm) wt. %
content b
wt. %
SZA
La–SZA
Ni–SZA
92.3
138.6
95.1
0.101
0.163
0.089
0.123
0.112
4.370 14.3
–
1.36
1.96
2.63
3.00
2.48
4.714
3.726
4.997
5.041
7.6
9.7
7.7
9.5
–
0.70
1.35
–
La–Ni–SZA
Spent La–Ni–SZA
98.4
89.2
a
Calculated from the Dc = Kλ/βcos(θ) (Debye–Scherrer's equation) based on the
tetragonal ZrO2.
b
The surface sulfur and Ni contents are analyzed by X-ray photoelectron spectroscopy
(XPS).