2.1 XRD results2
The XRD pattern of the as-prepared pure MMT, TiO2 and 30%TiO2/MMT catalysts are shown in Fig. 1. For pure montmorillonite, the diffraction peak at 2θ = 19.77°,21.85° and 26.76° attribute to the Si-O-Si diffraction peak of Na-montmorillonite, the Si-O-Al and the (101) of quartz, respectively. The original montmorillonite sample (MMT) shows a series of reflections at 2θ = 18.64° with d003 = 0.448 nm and 2θ = 19.77° with d100 = 0.449 nm, respectively. According to the JCPDS card (NO. 00-003-0010), it is corresponding to Na-montmorillonite (Hongjuan Sun 2015). The diffraction peaks of TiO2 at 2θ = 25.2°, 38.1°, 47.7° and 54.4° correspond to those of anatase (101), (004), (105) and (204), respectively. At the same time, there is a rutile characteristic diffraction peak (2θ = 27.43°) for pure TiO2(600 ℃, 4 h) and 30%TiO2/MMT (700 ℃, 4 h) which suggests that the montmorillonite as a carrier can increase TiO2 phase transition temperature, and make it still anatase phase after calcined at 600 ℃. The positions of the characteristic diffraction peaks of montmorillonite in the 30%TiO2/MMT composite calcined at different temperatures has no obvious change, and the peak strength is weaker. It can be seen that the introduction of TiO2 do not change the structure of montmorillonite. The diffraction peaks of TiO2/MMT at 2θ = 26.76° and 31.02° are obviously weakened or even disappeared compared with the original montmorillonite, which indicates that the original Si-O-Al bond was destroyed, and a new Si-O-Ti bond is constructed after replaced the aluminum ion in the interlayer (Lila Djouadi 2018). The average crystal size of the particles can be estimated from the widths of TiO2/MMT composites reflections by using the Scherrer formula, d = 0.89·λ/(β·cosθ), where λ (1.5413 Å) is the wavelength, θ is the Bragg angle (°), d is the average crystallite size (nm) and β is the full width at half-maximum. The calculated data of the five TiO2-based catalysts are followed in Table 2.
Table 2
Crystalline size of TiO2 in different samples
Sample
|
d/nm
|
TiO2(600℃,4h)
|
22.51
|
30%TiO2/MMT (400℃,4h)
|
6.61
|
30%TiO2/MMT (500℃,4h)
|
7.68
|
30%TiO2/MMT (600℃,4h)
|
10.66
|
30%TiO2/MMT (700℃,4h)
|
29.50
|
It can be seen from Table.2, The grain size of TiO2 reduces from 22.51 nm to 10.66 nm for pure TiO2 and 30%TiO2/MMT calcined at 600℃ for 4h, indicating that the particle size of TiO2 pillared montmorillonite decreased effectively. And the crystalline size of TiO2 in 30% TiO2/MMT composites calcined at different temperatures also gradually increased with the temperature.
2.2 SEM and TEM
Figure 2 shows SEM images of raw montmorillonite (a) and TEM images of pure TiO2 (b) and 30% TiO2/MMT (c) composites calcined at 600 ℃ for 4h. The raw montmorillonite shows larger particle aggregates with a flower-like layer structure, the flaky structure is relatively close. For the pure TiO2, the particles is relatively uniform, with some agglomeration, which may be related to the higher calcination temperature, the particle size is about 19 nm. For the 30% TiO2/MMT (600℃, 4h) composite, it can be observed the larger platelets and layer structure of montmorillonite clearly. Since the TiO2 nanoparticles were intercalated, the layer space was enlarged and the structural layers of montmorillonite were separated (R. Djellabi 2014 ). Although the montmorillonite layers were still largely parallel, they lost long-range order in the c direction after TiO2 intercalation. Which is consistent with the disappearance of the (001) reflection in the XRD patterns(Ping Zhang 2009). The lattice fringe image of the original montmorillonite sample shows that the structural layers are orderly with a layer space of 2.84 nm. And there are a lot of small aggregates, which are probably broken platelets and agglomerates of TiO2 crystallites caused by the hydrolysis of surface Ti(OC4H9)4 (Boualem Damardji 2008). TiO2 particle size is about 10 nm by calculation, which is consistent with the data calculated by Scherrer formula in XRD. At the same time, TiO2 pillared montmorillonite decreases its particle size from 19 nm to 10 nm, while the layered structure of montmorillonite remains unchanged, which also make TiO2/MMT composites keep good adsorption efficiency of montmorillonite, and then enhance the phenol degradation activity of TiO2/MMT.
2.3 FT-IR characterization
Figure 3 presents FT-IR spectra of different samples of TiO2, MMT and TiO2/MMT dried at 110℃ and calcined at 600 ℃ for 4h, respectively. For the purified MMT, the broad band at 3422 cm− 1 is due to the stretching vibration of the hydroxyl group and the interlayer water molecules. And the weak peak at 3624 cm− 1 is attributed to -OH stretching vibration of Al-O-H. Bands at 1645 cm− 1 and 1468 cm− 1 are caused by the bending vibration of the hydroxyl group and the interlayer water molecules. The vibration peak at 1079 cm− 1 is caused by the antisymmetric stretching vibration of Si-O-Si in montmorillonite, which caused by the strong hydration of Ca, Mg and Al plasma strengthens the hydrogen bond between the surface of montmorillonite layer and water, weakens and disappears the band of Si-O bond, resulting in the changing of Si-O and Si-O-Si into a single peak (Ke Chen 2001). Bands at 845 cm− 1 and 794 cm− 1 attribute to the bending vibration peak of Mg2+-OH and the hydroxyl vibration of MgAl-OH, respectively. And also bands at 521 cm− 1, 474 cm− 1 and 916 cm− 1 are suggested to the bending vibration of Si-O-Al, Si-O-Si and octahedral Al-O(OH)-Al in montmorillonite independently. The broad peak at 587 cm− 1 is attributed to Ti-O bond vibration, which indicates that TiO2 is amorphous. After TiO2 modified MMT, interlayer water and hydroxyl increased, which restricted the vibration of structural hydroxyl Al-OH. The peak at 3621 cm− 1 lost, and the two peaks at 1021 and 917 cm− 1 appeared, it shows that the composition of the interlayer of montmorillonite changed after TiO2 was introduced, and the in-plane stretching vibration peaks of Ti-O-C and Si-O-Si generated (Lingling Yuan 2011; Chihiro Ooka et al. 2003 ). Also at 796 cm− 1, the absorption of the hydroxyl vibration peak of MgAl-OH weakened, which may be related to the removal of hydroxyl as octahedron Ti hydrated ions into the interlayer. At the same time, the strength of 521 cm− 1 peak in the composite is obviously weakened and widened, which is the superposition of Ti-O and Si-O absorption peaks.
The results show that the strength of Si-O bond is also affected by titanium in lay. For the 30% TiO2/MMT calcined at 600℃ for 2h, 3422 cm− 1 and 916 cm− 1 disappear, while 1468 cm− 1 peak strength decreases, indicating that interlayer water lost. The vibration peak of Ti-O becomes narrow at 600 cm− 1, indicating the TiO2 crystal phase formed. At the same time, the 1070 cm− 1 peak of 30% TiO2/MMT changed into a single peak, which seems that the basic framework of MMT remained unchanged and remained the original structure. For the 30%TiO2/MMT calcined at different temperatures, it can be seen that the peaks at 3625 cm− 1, 1030 cm− 1 and 845 cm− 1 gradually disappear, and the peaks of 3431 and 1648 cm− 1 gradually decrease, which is caused by the evaporation of interlayer water (V. Makrigianni 2015).
2.4 UV-ViS
Figure 4 shows diffuse reflectance UV-Vis spectra of 30% TiO2/MMT calcined from 400–700℃. It can be seen from the Fig. 4 that the absorption band of pure TiO2 at 200–400 nm belongs to the electron coordinated by the titanium coordinated oxygen atom to the empty orbit of the central titanium atom, the characteristic absorption formed by OB2p→TiB3dB charge transfer, in which the tetrahedron at 248 nm. The characteristic peak of coordination titanium, the absorption peak around 337 nm was the absorption peak of octahedral coordination titanium(Aydin Hassani 2015). The peaks of 30% TiO2/MMT composites calcined at different temperatures around 433 and 504 nm are Fe-O absorption peaks of Fe2O3, because of the used montmorillonite content of 2.8% Fe2O3. At the same time, 30% TiO2/MMT composites calcined at different temperatures have a strong light absorption performance. The value Eg was deliberate from the UV–visible pattern of the titled TiO2/MMT by extrapolating the line through the k axis and it is exposed in Fig. 4. The wavelength maxima had 409.6 nm (pure TiO2), 427.1 nm (30%TiO2/MMT, 400℃), 434.7 nm (30%TiO2/MMT, 500℃), 418.4nm (30%TiO2/MMT, 600℃), and 451.5nm (30%TiO2/MMT, 700℃), respectively. Since the photoabsorption performance of the photocatalyst is positively related to the photocatalytic degradation activity of phenol, it was confirmed that the 30% TiO2/MMT(600℃,2h) prepared by the sol-gel method has the best photocatalytic activity for phenol degradation.
2.5 Photocatalytic activity
2.5.1 Effect of TiO2 loading amount on TiO2/MMT
Figure 5 shows the phenol removal efficiency under UV light over MMT, TiO2 and different loading of TiO2 on TiO2/MMT calcined at 600℃ for 4h. The results showed that the photolysis of phenol in the absence of photocatalyst is only 1.5%, which can be neglected. And the phenol degradation is 11.5% over MMT which shows that MMT has a certain adsorption capacity for phenol solution, but the adsorption activity was very low. It has 58.5% photocatalysis activity over pure TiO2. Also, for different amount loading of TiO2/MMT, the results indicated that the photo degradation efficiency of phenol increased with TiO2 content until 30%TiO2, then decreased after more TiO2 loading. Because TiO2 was the active center of the photocatalytic reaction, when its loading is less, the phenol degradation is low. When the loading of TiO2 was overdose, photo-generated electrons and holes were generated under light conditions, and the photo-generated electrons and holes recombine to form a composite band, which affects the formation of hydroxyl active groups, reduces the utilization of ultraviolet light, causes degradation efficiency reduced. It was concluded that the photocatalytic activity of 30% TiO2/MMT composites has the best phenol degradation performance under ultraviolet light.
Theoretically, the adsorption and degradation activity of TiO2/MMT composites on phenol was the sum of the phenol adsorption on montmorillonite and photodegradation over pure TiO2. However, it can be seen from the Fig. 5, the phenol adsorption rate of montmorillonite to 11.5% and the degradation rate of TiO2 was 58.8% after 240 minutes under UV light, the sum of the two was only 70.3%, which is significantly lower than the 30%TiO2/MMT composite (89.75%). On the one hand, the loading of montmorillonite may increase the crystalline phase transition temperature of TiO2 to a certain extent; it still maintains the anatase phase structure at 600℃. And also increases the degree of TiO2 dispersion and makes its particles smaller, which contributed to improving the phenol degradation over TiO2 /MMT composites(Hongjuan Sun 2015).
2.4.2 Effect of calcined temperature
Figure 6 shows the phenol photocatalytic degradation over 30%TiO2/MMT composites calcined at different temperatures (from 300 to 700℃). For the 30%TiO2/MMT composites calcined from 300℃ to 600℃, it can be seen that the degradation rate gradually increased with roasting temperature. When calcined at 600℃, the phenol degradation rate reached 89.75%, but is only 15.08% when the roasting temperature is 700℃, decreased significantly. Therefore, in this catalytic reaction system, the optimal calcination temperature of 30% TiO2 /MMT photocatalytic composite is 600℃.
2.5.2 The effect of the initial pH
The solution pH is one of the important parameters that can markedly affect the photocatalytic process. In the present work, the effect of the initial pH on the phenol photocatalytic degradation efficiency was studied in the pH range of 2–12 (Fig. 7). In this experiment, acetic acid and ammonia are usually used to adjust the pH value of the solution. The results indicated that the degree of photodegradation efficiency increased with the pH values from 2 to 6 and then decreased. It can be seen from Fig. 7 that the photocatalytic performance of the reaction solution under slightly acidic conditions was significantly higher than that obtained under slightly alkaline conditions, the pH value will change the interfacial charge of the TiO2/MMT, thereby changing the dispersion of the particles in the solution, and affecting the adsorption behavior of the montmorillonite matrix on the catalyst surface. In the photocatalyst reaction, the electrons were excited from the valence band to the conduction band, forming electrons (e−) and holes (h+). The highly active holes consume OH− in water during the reaction and oxidize them to have strong oxidative active๒OH, then e− and h+ undergo redox reactions with substances dispersed in water; As the pH value increases, the concentration of OH− in the solution sharply increases, then the concentration of · OH increased significantly, the conduction electrons reacted with the adsorbed O2 to form O2−, which could also generate · OH with H2O, so the degradation efficiency increased with the pH value. However, too high a concentration of · OH will cause a decrease in photocatalytic activity (Chunquan Li 2021). Therefore, the photocatalytic effect was better under suitable pH conditions, which has a high photocatalytic degradation rate under acidic conditions.
2.5.2 The effect of 30%TiO2/MMT (600℃,4h) dosage
Catalyst dosage is an important parameter in heterogeneous photocatalytic reaction. In order to determine the effect of catalyst dosage on the phenol photodegradation, experiments were carried out by varying the amount of 30%TiO2/MMT from 0.1 to 0.4 g (Fig. 8). Increasing the amount of photocatalyst from 0.1 g to 0.2 g resulted in increasing the photodegradation efficiency from 42.25–89.96% at the reaction time of 240 min, respectively. By adding the catalyst dosage, promoted the production of reactive species. However, more catalyst dosage would also induce greater aggregation of the catalysts and added the turbidity of the solution, reduce the degree of light penetration through the solution and thereby leading to a reduction in the phenol degradation efficiency. Therefore, 0.2 g 30%TiO2/MMT photocatalysts is the best amount for phenol photocatalytic reaction(Shivatharsiny Rasalingama 2014).
2.5.3 High performance liquid chromatography -(HPLC) analysis
HPLC was employed to identify the degradation products in order to evaluate the catalytic activities over the pure TiO2 (600°C, 4 h) and 30% TiO2/MMT (600°C, 4 h) photocatalysts. Under our experimental conditions, benzoquinone (BQ), hydroquinone (HQ), maleic acid (MA), and fumaric acid (FA) have been identified as intermediates. It should be noted that the four compounds comprising were produced in significant amounts during photocatalytic degradation with the two photocatalysts. For the pure TiO2, the phenol concentration gradually decreased with the reaction time. It found that the four materials (HQ, BQ, MA and FA) are the main aromatic compounds produced by phenol conversion. The highest concentration of the four materials are detected at 210 min and then decreased gradually, also the MA and FA keep the higher concentration than BQ and HQ, which mightbe caused by the BQ and HQ can turn into MA and FA or the phenol itself will be further converted into maleic acid and fumaric acid, and the residual phenol concentration was 27.17 mg/L after reaction for 420 min. During the phenol photocatalytic degradation over TiO2, phenol is gradually degraded in the progress of the reaction, and most of it is directly mineralized into CO2 and water (Asma Turki 2014).
For the 30% TiO2/MMT catalyst, with the extension of the photocatalytic reaction time, the phenol concentration in the residual solution decreased significantly. After 420 min reaction, the residual phenol concentration was 9.36 mg/L. Total benzoquinone (HQ), hydroquinone (BQ), maleic acid and fumaric acid (FA + MA) produced by the photocatalytic degradation system of 30%TiO2/MMT. Compared with the pure TiO2 system, 30%TiO2/MMT had more phenol photo-degradation efficiency, and the total amount of intermediate products detected is larger than that in the TiO2 system. The larger amount of HQ + BQ is due to the higher selectivity of 30%TiO2/MMT in the phenol conversion. The loading of montmorillonite makes๒OH produced by TiO2 easier to attack the para position of phenol. Hydroquinone is produced and further converted into p-benzoquinone. The amount of butadionic acid (FA + MA) detected in the reaction system of 30%TiO2/MMT for phenol degradation was higher than that of pure TiO2. Considering the production of HQ + BQ and FA + MA amount produced in this system, their concentration increased first and then decreased. It is speculated that the reaction course of the montmorillonite loading system may be the rapid phenol mineralization to produce CO2 + H2O. The amount of acid produced was small, or is rapidly converted to CO2 under these catalytic conditions (Xiaobo Wang 2014).
The conversion of phenol calculated by liquid chromatography data was 45.7% (TiO2) and 81.9% (30%TiO2/MMT), respectively. It was found that the sum of the carbon content of the intermediate substances was much lower than the degradation rate caused by phenol. It is possible that a large amount of carbon was converted into carbon dioxide, leaving the reaction system. In addition, it can be seen from Fig. 10 that almost all the intermediate products of phenol degradation are converted into small molecular substances. With reference to high-performance liquid reaction data and a large amount of literature, it is concluded that under this experimental condition, the reaction mechanism of phenol degradation may be like this:
The good photocatalytic activity of the 30%TiO2/MMT composites can be ascribed to the following reasons. The photocatalytic mechanism of 30%TiO2/MMT composites is showed in Fig. 10. TiO2 in the interlayer of MMT could increase the surface area of TiO2/MMT composites,and adding the adsorption efficiency of phenol༌thus promoting the photodegradation efficiency of 30%TiO2/MMT. Secondly, TiO2 loaded on the surface of MMT and improved its disperse, also enlarged the amount of exposed activates for reactions. When the composite photocatalyst is irradiated by sunlight, the electrons (e−) in the valence band (VB) of TiO2 will be excited by energy and transfer to the conduction band (CB) to generate photogenerated electrons, which will generate photogenerated holes (hv+), Photogenerated holes and electrons will combine with water and dissolved oxygen in water to form superoxide radical (· O2 −) and hydroxyl radical (· OH). The two radicals can oxidize phenol to HQ + HQ, and then convert into FA + MA, and finally mineralize into small molecules of H2O and CO2.