Moreover, other characteristic peaks of CAL appeared at distances of
10.4 Å (2θ = 8.495°), which characterize the main peak of palygorskite
at 6.36 Å (2θ = 13.913°), 5.4 Å (2θ = 16.402°), 4.47 Å (2θ = 19.846°),
3.68 Å (2θ = 24.165°), and 3.18 Å (2θ = 28.037°). The indexing of
these peaks confirmed the presence of palygoskrite clay in the raw
materials [25].
In addition, we observe more intense calcite peak compared to that
of palygoskrite. This indicates that this material is mostly composed of
calcite carbonate and slight contents of palygorskite clay. For TiO 2 ,
XRD patterns exhibit strong diffraction peaks at 25° and 48° indicating
that TiO 2 was mostly in the anatase phase. Peaks of the rutile phase are
also at 31.08° observed but with very low intensity [26,27]. Our results
indicate that XRD patterns of CAL5, CAL15 and CAL30 samples show no
change in the structure, but rather just a diminution in characteristic
peak intensity. Besides, the characteristic peaks of TiO 2 supported on
CAL are not observed probably due to the screen effect and/or a high
TiO 2 distribution under very small particles configuration. This behavior could be investigated by the SEM-EDX analysis.
3.2. ATR-FTIR spectroscopy
The ATR spectra of synthesized TiO 2 , CAL and CAL/TiO 2 systems
are shown in Fig. 2. The spectra indicate two strong bands positioned at
1429.61 and 872.60 cm
–1
, which correspond to CeO bond of calcite
material. Bands between 400 and 500 cm
−1 are attributed to elongation modes of the CaeO bond of calcite. However, peaks located near
1100, 800, and 471 cm
−1 can be assigned to the asymmetric and
symmetric stretching vibrations and bending vibrations of SieOeSi
characteristic of palygorskite present in the raw materials [28,29].
Bands at around 3400 and 1600 cm
–1 are assigned to the extending
vibration of hydroxyl groups and water molecules spectra [19,29]. It is
noteworthy mentioning that the intensity of all CAL/TiO 2 bands was
weakened after adding TiO 2 . This lessening was found to be more significant when the amount of raw material mass decreased.
The peaks at 400–600 cm
−1 can also be related to TieO and
TieOeTi of TiO 2 anatase phase [29,30,33]. Our findings are in agreement with the study involved a palygoskrite coated with TiO 2 [31]. On
the other hand, the weak absorption peak observed around
900–930 cm
−1 may result from SieOeTi linkages [32]. This very weak
peak at the spectra of prepared catalysts is probably due to the small
number of these bonds because of the poor aluminosilicates sites in the
raw material. This result will be confirmed hereafter by EDX analysis.
Thus, ATR-FTIR corroborate our earlier XRD observations and stresses
that the support “CAL” is not significantly affected by the heat treatment (480 °C) involved during the preparation process.
3.3. Porosity and specific surface area (SSA) studies
The adsorption–desorption isotherms of nitrogen and pore size
distributions for TiO 2 , CAL, and CAL/TiO 2 systems are presented in
Fig. 3. The BET isotherm of the raw material CAL matched the type IV
isotherm, where the surface area was 26.82 m
2 . g
−1 and pore-size distribution was 9.5 nm. These results are similar to those obtained by
Chen et al. [26]. The BET isotherm of elaborated TiO 2 and CAL/TiO 2
system matched also as type IV. It can be envisaged that the introduction of TiO 2 into the raw material can lead to SSA decrease of the
CAL/TiO 2 system due to the pore clogging effect. However, the SSA
values of the samples increased drastically with augmenting the TiO 2
content. In fact, the SSA of CAL was 26.82 m
2 . g
−1 while that of CAL5,
CAL15 and CAL30 were 30.73, 55.46 and 115.73 m
2
. g
−1
, respectively.
This important trend may be related to the SSA of TiO 2
(130.33 m
2 . g
−1
) which is considered as a mesoporous material. The
SSA of the CAL/TiO 2 increased as well as TiO 2 amount.
In contrast, the pore sizes diminished gradually with the increase of
TiO 2 content, which lead to believe on the pore clogging. Considering
the different aspects, the coexistence of micropores and mesopores are
highly envisaged, and the presence of TiO 2 particles within the pore of
CAL and in the same time at the surface of CAL particles is evidenced.
The parameters of pore structure calculated from the N 2 adsorption–desorption isotherms are given in Table 1.
3.4. SEM and EDX analysis
The morphologies of TiO 2 , CAL and CAL/TiO 2 system are displayed
in Fig. 4. For CAL, a cubic-shaped crystal can clearly be distinguished
(Fig. 4a) and is attributed to calcite crystals (< 10 μm) with a surface
that appears smoother than the powder that represents the raw material; it is like a clay and other minerals that constitute the raw material.
Fig. 4b and c give the images of CAL5 and CAL15. TiO 2 particles can be
Fig. 2. ATR spectra of TiO 2 , CAL and CAL/TiO 2 system.
Fig. 3. The nitrogen adsorption–desorption isotherms of TiO 2 , CAL and CAL/
TiO 2 system.
Table 1
Specific surface area, pore size and total pore volume of CAL, TiO 2 and CAL/
TiO 2 system.
Materials
Specific surface area
[m
2
.g]
Pore size
[nm]
Total pore volume
[cm
3 .g]
TiO 2
130.33
7.8
0.272666
CAL
26.82
9.5
0.065370
CAL5
30.73
7.7
0.061122
CAL15
55.46
5.2
0.074056
CAL30
115.73
4.4
0.134058
N. Belhouchet et al.
Journal of Photochemistry & Photobiology A: Chemistry 372 (2019) 196–205
198
10.4 Å (2θ = 8.495°), which characterize the main peak of palygorskite
at 6.36 Å (2θ = 13.913°), 5.4 Å (2θ = 16.402°), 4.47 Å (2θ = 19.846°),
3.68 Å (2θ = 24.165°), and 3.18 Å (2θ = 28.037°). The indexing of
these peaks confirmed the presence of palygoskrite clay in the raw
materials [25].
In addition, we observe more intense calcite peak compared to that
of palygoskrite. This indicates that this material is mostly composed of
calcite carbonate and slight contents of palygorskite clay. For TiO 2 ,
XRD patterns exhibit strong diffraction peaks at 25° and 48° indicating
that TiO 2 was mostly in the anatase phase. Peaks of the rutile phase are
also at 31.08° observed but with very low intensity [26,27]. Our results
indicate that XRD patterns of CAL5, CAL15 and CAL30 samples show no
change in the structure, but rather just a diminution in characteristic
peak intensity. Besides, the characteristic peaks of TiO 2 supported on
CAL are not observed probably due to the screen effect and/or a high
TiO 2 distribution under very small particles configuration. This behavior could be investigated by the SEM-EDX analysis.
3.2. ATR-FTIR spectroscopy
The ATR spectra of synthesized TiO 2 , CAL and CAL/TiO 2 systems
are shown in Fig. 2. The spectra indicate two strong bands positioned at
1429.61 and 872.60 cm
–1
, which correspond to CeO bond of calcite
material. Bands between 400 and 500 cm
−1 are attributed to elongation modes of the CaeO bond of calcite. However, peaks located near
1100, 800, and 471 cm
−1 can be assigned to the asymmetric and
symmetric stretching vibrations and bending vibrations of SieOeSi
characteristic of palygorskite present in the raw materials [28,29].
Bands at around 3400 and 1600 cm
–1 are assigned to the extending
vibration of hydroxyl groups and water molecules spectra [19,29]. It is
noteworthy mentioning that the intensity of all CAL/TiO 2 bands was
weakened after adding TiO 2 . This lessening was found to be more significant when the amount of raw material mass decreased.
The peaks at 400–600 cm
−1 can also be related to TieO and
TieOeTi of TiO 2 anatase phase [29,30,33]. Our findings are in agreement with the study involved a palygoskrite coated with TiO 2 [31]. On
the other hand, the weak absorption peak observed around
900–930 cm
−1 may result from SieOeTi linkages [32]. This very weak
peak at the spectra of prepared catalysts is probably due to the small
number of these bonds because of the poor aluminosilicates sites in the
raw material. This result will be confirmed hereafter by EDX analysis.
Thus, ATR-FTIR corroborate our earlier XRD observations and stresses
that the support “CAL” is not significantly affected by the heat treatment (480 °C) involved during the preparation process.
3.3. Porosity and specific surface area (SSA) studies
The adsorption–desorption isotherms of nitrogen and pore size
distributions for TiO 2 , CAL, and CAL/TiO 2 systems are presented in
Fig. 3. The BET isotherm of the raw material CAL matched the type IV
isotherm, where the surface area was 26.82 m
2 . g
−1 and pore-size distribution was 9.5 nm. These results are similar to those obtained by
Chen et al. [26]. The BET isotherm of elaborated TiO 2 and CAL/TiO 2
system matched also as type IV. It can be envisaged that the introduction of TiO 2 into the raw material can lead to SSA decrease of the
CAL/TiO 2 system due to the pore clogging effect. However, the SSA
values of the samples increased drastically with augmenting the TiO 2
content. In fact, the SSA of CAL was 26.82 m
2 . g
−1 while that of CAL5,
CAL15 and CAL30 were 30.73, 55.46 and 115.73 m
2
. g
−1
, respectively.
This important trend may be related to the SSA of TiO 2
(130.33 m
2 . g
−1
) which is considered as a mesoporous material. The
SSA of the CAL/TiO 2 increased as well as TiO 2 amount.
In contrast, the pore sizes diminished gradually with the increase of
TiO 2 content, which lead to believe on the pore clogging. Considering
the different aspects, the coexistence of micropores and mesopores are
highly envisaged, and the presence of TiO 2 particles within the pore of
CAL and in the same time at the surface of CAL particles is evidenced.
The parameters of pore structure calculated from the N 2 adsorption–desorption isotherms are given in Table 1.
3.4. SEM and EDX analysis
The morphologies of TiO 2 , CAL and CAL/TiO 2 system are displayed
in Fig. 4. For CAL, a cubic-shaped crystal can clearly be distinguished
(Fig. 4a) and is attributed to calcite crystals (< 10 μm) with a surface
that appears smoother than the powder that represents the raw material; it is like a clay and other minerals that constitute the raw material.
Fig. 4b and c give the images of CAL5 and CAL15. TiO 2 particles can be
Fig. 2. ATR spectra of TiO 2 , CAL and CAL/TiO 2 system.
Fig. 3. The nitrogen adsorption–desorption isotherms of TiO 2 , CAL and CAL/
TiO 2 system.
Table 1
Specific surface area, pore size and total pore volume of CAL, TiO 2 and CAL/
TiO 2 system.
Materials
Specific surface area
[m
2
.g]
Pore size
[nm]
Total pore volume
[cm
3 .g]
TiO 2
130.33
7.8
0.272666
CAL
26.82
9.5
0.065370
CAL5
30.73
7.7
0.061122
CAL15
55.46
5.2
0.074056
CAL30
115.73
4.4
0.134058
N. Belhouchet et al.
Journal of Photochemistry & Photobiology A: Chemistry 372 (2019) 196–205
198