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H.R. Mahmoud / Journal of Alloys and Compounds 687 (2016) 954e963
steady-state tests were conducted isothermally every 25 ꢁC from
150 ꢁC to 400 ꢁC and the gas products were analyzed by a flame
ionization detection (FID) of Perkin-Elmer Auto System XL Gas
Chromatograph (GC) equipped with a packed column (10% squa-
lane supported on chromosorb, 4 m). The temperature of the de-
tector was 250 ꢁC and the column temperature was programmed at
60 ꢁC.
the energy needed for the formation of a new solid phase [30]. In
addition to, the cationic surfactants in solution can surround the
nanoparticles via their positive ends to prevent these particles from
being agglomerated and therefore decreasing the particle size of
the nanoparticles [31]. Energy dispersive spectroscopy (EDS) was
performed to further confirm the composition of the prepared
nanomaterials. The EDS results of CZ-HC0.1 nanomaterial (Fig. 2E)
proved the presence of chromium, zirconium and oxygen elements.
The analytical results from EDS were virtually identical or very
close to the nominal wt% of Cr2O3eZrO2 binary oxide catalysts
(Fig. 2E inset).
3. Results and discussion
3.1. Structural characterization (XRD, HR-TEM, FT-IR, BET)
FT-IR is a technique that can be used to obtain information on
the chemical structures of materials. Fig. 3 shows the FT-IR spectra
of the as-prepared mesoporous CZ-H213, CZ-HC0.1, CZ-HT0.1 and
CZ-HS0.1 nanomaterials. It could be found that all prepared mes-
oporous catalysts showed broad and strong absorption bands in the
range 3700e3000 cmꢂ1, centered at 3430 cmꢂ1 which character-
izes the stretching vibrations of surface hydroxyl groups [17]. And
that the bands at 1630 cmꢂ1 can be assigned to the bend stretching
vibration of adsorbed water [17]. In addition, weak absorption
bands centered at 1400 cmꢂ1 were ascribed to the MeOeM
deformation vibrations [32]. Moreover, the FT-IR spectra shows two
The XRD patterns of 0.25Cr2O3e0.75ZrO2 binary oxide catalysts
synthesized using SDS, CTAB and Triton X-100 as surfactants with
two different contents were displayed in Fig. 1. According to the
results, no characteristic peaks were observed, meaning that all
prepared nanomaterials were amorphous materials. Several studies
have reported that zirconia-based binary metal oxides at higher
ZrO2 content calcined at 500 ꢁC were an amorphous material [28]. It
has been reported that the mesoporous 0.25Cr2O3e0.75ZrO2 cata-
lyst which was hydrothermally synthesized previously [27] at
210 ꢁC for 3 h (CZ-H213) without surfactant was totally amorphous
material. It has been reported that ZrO2 may inhibit the formation
of well crystalline particles, i.e., binary oxides mostly exist in the
form of microcrystals [29].
The effects of surfactants on the morphologies produced in
hydrothermal synthesis of different nanomaterials were studied.
Fig. 2(AeD) shows the high-resolution TEM (HR-TEM) images of the
CZ-H213, CZ-HC0.1, CZ-HT0.1 and CZ-HS0.1 nanomaterials,
respectively. It can be seen from Fig. 2 that the particles of all cat-
alysts have nearly spherical shapes with weak agglomeration. The
average diameters of CZ-H213, CZ-HC0.1, CZ-HT0.1 and CZ-HS0.1
nanomaterials are 1.5 nm, 0.9 nm, 1.3 nm and 2.1 nm, respec-
tively. Furthermore, the average particle size measurements clearly
indicated that the cationic surfactant, CTAB, produced the smallest
particle sizes of 0.25Cr2O3e0.75ZrO2 nanomaterials. On the other
hand, the anionic surfactant, SDS, gave the largest sizes of
0.25Cr2O3e0.75ZrO2 nanoparticles. Notably, the average particle
size of the 0.25Cr2O3e0.75ZrO2 catalyst prepared by hydrothermal
method without surfactant, CZ-H213, (1.5 nm) is higher than the
catalysts prepared with cationic and non-ionic surfactants. The
surfactants can be divided into two major categories including ionic
and non-ionic surfactants. These surfactants affect the morphol-
ogies of the synthesized nanoparticles. In fact for the reaction
system in the presence of surfactants, the surface tension of solu-
tion is reduced due to the existence of surfactant, which reduces
weak characteristic absorption bands at 1030 cmꢂ1 and 960 cmꢂ1
,
which were attributed to anti-symmetric stretching vibration of
MeOeM and vibration of M ꢂ OH groups, respectively [33].
Furthermore, the infrared vibration bands around 660 cmꢂ1 and
460 cmꢂ1 were corresponded to the stretching vibration of M ꢂ O
[34]. Notably, in the FT-IR spectra of the as-prepared surfactant-
assisted catalysts, no typical adsorption bands in the region of
2900e2800 cmꢂ1, corresponding to the stretching modes of the
surfactant hydrocarbon chain [35]. Furthermore, another doublet
bands correspond to asymmetric vibrations of SO2 at 1249 cmꢂ1
and 1219 cmꢂ1 of SDS surfactant were absent [36]. On the basis of
FT-IR analyses, it can be concluded that the different three surfac-
tants (CTAB, Triton X-100 and SDS) were completely removed from
the as-prepared nanomaterials.
Some studies found that improving the texture properties of the
catalyst was beneficial for improving the catalytic activity. The N2
adsorptionedesorption isotherms technique was used to deter-
mine the surface area and type of porosity for synthesized nano-
materials and the isotherms are shown in Fig. 4A. The pore size
distribution was measured using BJH method and is shown in
Fig. 4B. Furthermore, the textural characteristics for all nano-
materials were tabulated in Table 1. It can be seen in Fig. 4A that, all
isotherms are of Type IV, which indicates that the materials are
mesoporous in nature. The shape of isotherms for CZ-H213, CZ-
HS0.1 and CZ-HC0.6 corresponds to H2-type hysteresis loop
nevertheless the shape of isotherms for CZ-HC0.1, CZ-HT0.1 and CZ-
HT0.6 corresponds to H3-type hysteresis loop [23]. The H2-type
hysteresis was believed to be associated with ink-bottle-like
pores, often generated by agglomerates of spherical particles of
non-uniform size and arrangements [37]. Furthermore, the H3-
type hysteresis is generally observed for aggregates of plate-like
particles giving rise to slit-shaped pores [38].
As can be seen in Table 1, the sequence of surface areas of the
nanomaterials synthesized using SDS, CTAB and Triton X-100 as
surfactants with two different contents was as follows: CZ-HT0.1
(646.4 m2/g) > CZ-HC0.1 (588.4 m2/g) > CZ-HC0.6 (569.9 m2/
g) > CZ-HT0.6 (557.6 m2/g) > CZ-H213 (526.6 m2/g) > CZ-HS0.1
(511.6 m2/g). In addition to these mentioned above, it could be
concluded that the mesoporous catalysts prepared with non-ionic
and cationic surfactants, have the highest surface area and pore
volume. Nevertheless, the mesoporous catalysts prepared with
anionic surfactant or without surfactant showed a decrease in BET
surface area. Furthermore, increasing the surfactant content
Fig. 1. XRD patterns of 0.25Cr2O3e0.75ZrO2 nanomaterials synthesized in the absence
and presence of surfactants (SDS, CTAB and Triton X-100).