Dinitrotoluene (DNT) is an intermediate product in the manufacture of 2,4,6-trinitrotoluene and a precursor of toluene diisocyanate used for the preparation of polyurethane foams. Both 2,4-DNT and 2,6-DNT are priority pollutants due to their toxicity and possible carcinogencity. Therefore, the degradation of DNT in wastewater has gained much attention. In the study of Chen et al. [1, 2, 3], Fenton’s reagent, O3, and UV/O3 were used to mineralize DNT isomers in spent acid from a toluene nitration process. Several researchers [4, 5, 6, 7] have focused on the degradation of nitroaromatic compounds by UV/Fenton’s reagent or a UV/H2O2 technique. The development of an effective and inexpensive decontamination technology for explosive wastewaters is of continuing interest.
The photocatalytic degradation of DNT has been considered an emerging promising method. It can decompose most organic pollutants and mineralize them to CO2, H2O, and other small inorganic molecules completely. Feng and coworkers [8, 9, 10] reported that as a photocatalyst, POMs/TiO2 (POM: polyoxometalate) exhibited excellent activity in photocatalyzing the decomposition of various organic contaminants, including dyes, pesticides, explosives, and other inorganic pollutants. The catalytic studies further indicated that the activity of the catalyst correlated well with the surface area and porosity. Therefore, one can try to increase efficiency by increasing the specific surface area and porosity of the catalyst by introducing a structure directing agent.
In the present work, a new route is demonstrated for preparing H3PW12O40/TiO2 and H4SiW12O40/TiO2 using P123 as a structure directing agent. The catalysts have excellent surface physicochemical properties. The average particle size became smaller and the pore volume became larger, and the selective adsorption of organic compounds was increased. All these factors enhanced the activity of the photocatalyst. The photocatalysts demonstrated the validity and universal applicability of P123 and could provide a new direction for the photocatalytic degradation of explosive wastewater.
H3PW12O40/TiO2 and H4SiW12O40/TiO2 were synthesized by the combination of sol-gel synthesis, template self-assembly, and the solvothermal treatment method. An amount of P123 (EO20PO70EO20, EO = CH2CH2O, PO = CH2(CH3)CHO, MW 5800, Sigma-Aldrich) was dissolved in 22.5 mL pure ethanol (99.7%, Beijing Chemical Works, China) with vigorous stirring. After the complete dissolution of the P123, 15 mL tetrabutyl titanate (Ti(OBu)4, Sinopharm Chemical Reagent Co., Ltd., China) was added as a titanium precursor. To this solution, 12 mol/L HCl (Beijing Chemical Works, China) was added to adjust pH value to 1.00±0.01, then 0.75 g H3PW12O40 or H4SiW12O40 (Sinopharm) dissolved into 1 mL double distilled water was dropwise added into the mixed solution. After stirring for 2 h, another 22.5 mL ethanol was added to reduce the acidity to make hydrolysis favorable. Finally, 2 mL distilled water was added for hydrolysis (molar ratio of Ti(OBu)4 to distilled water was 1:4 to ensure the complete hydrolysis of Ti(OBu)4). The resulting acidic solution was stirred at room temperature until a gel was obtained. The gel was transferred into a hydrothermal reactor and heated to 120 °C for 2 h at a heating rate of 2 °C/min, then warmed to 150 °C for 2 h with the same rate of 2 °C/min. Subsequently, clearing, separating, drying, and milling were performed to obtain the powder. Finally, the powders were thermally treated at 400 °C for 5 h with a heating rate of 5 °C/min to remove P123. The obtained H3PW12O40/TiO2 and H4SiW12O40/TiO2 catalysts were denoted H3PW12O40/TiO2(P) and H4SiW12O40 /TiO2(P), respectively. For comparison, H3PW12O40/TiO2 and H4SiW12O40/TiO2 were also prepared by the same method without using P123.
X-ray diffraction patterns were obtained with a Netherlands PaNalytical X'Pert Pro-MPD diffractometer with Cu Kα irradiation (λ = 0.154 nm) operating at 40 kV and 40 mA. The structure of the samples was characterized by Raman spectroscopy using a Renishaw Invia used at 633 nm wavelength. The surface area of the composite catalysts was determined by a Nova 2200e automatic specific surface and porosity analyzer, which used the BET equation and adsorption data in the relative pressure (p/p0) range from 0.05 to 0.3. The average pore diameter and pore volume were obtained by the BJH model at the relative pressure of 0.976 (the catalysts were outgassed under vacuum at 200 °C overnight). The shape, size and size distribution of the catalysts were observed by scanning electron microscope (SEM, Hitachi S-4800N) with an acceleration voltage of 5 kV. The ultraviolet-visible (UV-Vis) spectra of the samples over a range of 190-600 nm were recorded with a Shimadzu UV-3600 ultraviolet-visible spectrophotometer.
The photocatalytic activity of the composite catalysts was evaluated by monitoring the degradation of DNT in a suspension system. The DNT explosive wastewater was provided by an ordnance factory and contained 2,4-DNT, 2,6-DNT, and 2,3-DNT. The concentration of total nitrocompound was 400 mg/L, and the chemical oxygen demand (COD) was 3100 mg/L. The initial pH of the wastewater was 7.8. The DNT wastewater was diluted with 10 times amount of water before the process of degradation.
The reactor system consists of a xenon lamp, a set of quartz beakers, and a magnetic stirrer. The main emission wavelength range of the xenon lamp (PLS-SXE300CUV, 300 W) was 250-380 nm. The sample was irradiated with a light intensity of 100-200 mW/cm2.
The photocatalytic experiments were carried out by adding an appropriate amount of catalyst (0.04 g H3PW12O40/TiO2(P) or 0.03 g H4SiW12O40/TiO2(P)) into 50 mL DNT solution with the initial concentration of 40 mg/L. The pH value of the reaction solution was adjusted to 2.00±0.01 by adding HClO4. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to assure adsorption equilibrium. After different irradiation times, some wastewater was sampled from the reactor and then filtered by a microporous membrane to remove catalyst particles before analysis. The concentrations of total nitrocompounds were measured by the reducing-azo spectrophotometry method.
The phase structure of the catalysts was investigated by XRD analysis. The average crystallite sizes were determined using the Debye-Scherrer formula D = kλ/βcosθ, where k is a Scherrer constant (k = 0.89 for a spherical particle), λ is the wavelength of the Cu Kα irradiation, β is the full width at half maximum, and θ is the diffraction angle [11].
Figure 1 shows the XRD patterns of H3PW12O40/TiO2, H4SiW12O40/TiO2, H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P). All the samples showed characteristic peaks at 25.3° (101), 37.8° (004), 48.1° (200), 54.0° (105), and 62.7° (211), indicating the presence of the anatase phase. These four catalysts showed similar peaks, which suggested that the crystal structure was unchanged by the template P123 added. As shown in Table 1, the average crystal sizes of H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P) decreased to 15.5 and 7.8 nm, respectively. The small crystal size facilitates fast charge migration from the bulk to the surface, which leads to better catalytic activity.
Raman spectra of H3PW12O40/TiO2(P) and H4SiW12O40/ TiO2(P) were shown in Fig. 2. The peaks at 399.1, 519.5, and 640.3 cm-1 indicated the presence of the anatase phase [12]. According to Li et al. [13], the characteristic Raman peaks of the Keggin unit are in the range of 1100-900 cm-1, where the peaks at 1009.5, 993.9, and 912.4 cm-1 corresponded to the P-O, W=O, and W-O-W bonds, respectively. For the composite H3PW12O40/TiO2(P), the peaks corresponding to the W=O, W-O-W, and P-O bonds vibrations were broadened, and only one wide peak was observed at 1100-900 cm-1. That is, there were Raman shifts due to a strong interaction between the Keggin unit and the TiO2 network, which led to the decrease in the symmetry of either TiO2 or H3PW12O40 [14]. Similar results are found with H4SiW12O40/TiO2(P). Thus the Raman results confirmed the structure integrity of H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P).
The specific surface area is one of the most important properties of materials, which affects its photocatalytic activity. Figure 3 shows the N2 adsorption-desorption isotherms and the BJH pore size distribution of the catalysts. Table 2 lists the pore size, pore volume, and specific surface area of the samples.
The adsorption behavior is typical of mesoporous materials with capillary condensation in the mesoporous channels [15, 16]. These organized pore structures are extremely useful in photocatalysis as they provide efficient transport pathways for reactant and product molecules [17, 18]. The different samples display similar isotherms but have different specific surface area and pore volume. As shown in Table 2, the specific surface area and pore volume of the samples increased with the addition of the template P123. The catalyst H3PW12O40/TiO2(P) exhibited the highest specific surface area of 252.2 m2/g, which was 1.42 times larger than that of H3PW12O40/TiO2. Similarly, the specific surface area of H4SiW12O40/TiO2(P) was increased to 250.0 from 179.3 m2/g.
The SEM images shown in Fig. 4 provided visualization of the textural properties of samples and indicated that the four kinds of catalysts were made up of regular spherical particles with sizes less than 20 nm. It showed that the addition of the structure directing agent P123 did not significantly affect the particle morphology, but the particle sizes were obviously decreased, which corroborated the increase of specific surface area (Table 2).
The UV-Vis spectra of H3PW12O40/TiO2, H4SiW12O40/TiO2, H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P) are shown in Figure 5.
The composite catalysts exhibited broad and strong absorption in the range from 200 to 400 nm, and the adsorption bands showed an obvious red shift compared with pure TiO2. The results indicated that the charge transfer band was shifted to a higher wavelength and the band gap of the composite materials became narrower after the introduction of H3PW12O40 and H4SiW12O40.
The absorption spectrum of pure TiO2 was from the charge transfer from the O 2p to Ti 3d orbitals. However, the charge transfer from O 2p to a new orbital constructed from the hybridization of Ti 3d and W 5d orbitals contributed to the absorption spectra of the composite catalysts [14, 19]. This is the most important factor in the narrowing of the band gap of the composites. Therefore, the Keggin unit can efficiently improve the electronic properties of TiO2 in the composites.
Figure 6 illustrates the degradation rate of DNT by photocatalytic degradation using H3PW12O40/TiO2(P) as the catalyst.
As shown in Fig. 6(a), the organic compounds were effectively degraded. The total degradation rate of DNT was as high as 95% after 4 h of irradiation under the optimum conditions. In order to determine the optimum dosage of P123, different dosage of P123 were added in the preparation of H3PW12O40/ TiO2. The results (Fig. 6(b)) clearly indicated that the degradation rate obviously increased with increasing P123 amount from 0.5 to 2.0 g, but only slightly changed with the use of 2.0 to 5.0 g, which meant that the optimum dosage of P123 was 2.0 g for H3PW12O40/TiO2.
Figure 7 compares the degradation rate of DNT over H4SiW12O40/TiO2(P) catalyst. It shows that although DNT was effectively degraded with all the catalysts, the degradation rate was obviously higher when the amount of P123 used was 1.0 g, which means the optimum dosage of P123 for H4SiW12O40/TiO2 was 1.0 g.
As mentioned above, the optimum addition amount of P123 for H3PW12O40/TiO2 and H4SiW12O40/TiO2 were 2.0 and 1.0 g, respectively, which was most beneficial for increasing the specific surface area and improving the pore structure of the composites.
Photolysis and photocatalytic degradation processes are often modeled as first order kinetics [20]. Hence, the degradation kinetics of DNT was fitted using the first order kinetics model lnC0/Ct = kt, where k is the reaction rate constant (mg/(L·min)), t is the illumination time (min), C0 is the initial DNT concentration (mg/L), and Ct is the DNT concentration at irradiation time t (mg/L).
The photocatalytic activity of H3PW12O40/TiO2, H4SiW12O40/ TiO2, H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P) is compared in Fig. 8. The photocatalytic activity of commercial Degussa P25 was also investigated for comparison. The rate constants and reaction half-time of the different catalysts are given in Table 3.
The experimental data were fitted well with the formula, showing that DNT photocatalytic degradation follows first order kinetics. The data indicate that 39% DNT was removed after 4 h of irradiation without a catalyst, indicating some direct photolysis occurred under UV light. However, the conversion of DNT is considerably increased with a catalyst. The results showed that all the composite catalysts exhibited significantly higher photocatalytic activity than commercial Degussa P25. The rate constant reaches 0.0094 mg/(L·min) over H4SiW12O40/TiO2(P) catalyst, 1.8 times higher than H4SiW12O40/TiO2, and the reaction half time was decreased from 2.2 to 1.2 h. Compared with H3PW12O40/TiO2, H3PW12O40/ TiO2(P) displayed much higher photocatalytic activity with the degradation rate of DNT reaching 95% after 4 h irradiation. The reaction rate constant of H3PW12O40/TiO2(P) was 1.9 times higher than that of H3PW12O40/TiO2, and the reaction half time was almost reduced to half.
These results illustrated that the rate constants for the degradation of DNT were made significantly higher by the adding of a suitable amount of template. The excellent catalytic performance of H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P) was attributed to their smaller crystal size and higher surface area. The small crystal size facilitated fast charge migration from the bulk to the surface, and the large surface area provided more active sites for heterogeneous photocatalysis, which gave higher photocatalytic activity [21, 22, 23]. In addition, with the directing function of P123 in the aqueous solution, H3PW12O40/ TiO2(P) and H4SiW12O40/TiO2(P) exhibited a dual function for the degradation of the organics [24, 25]. Their mesoporous structures led to a faster internal diffusion rate than in H3PW12O40/TiO2 and H4SiW12O40/TiO2, which is advantageous for the separation of the electron and hole [26].
Therefore, it was concluded that the photocatalyst activity depends on both the specific surface area and the particle pore structure. The structure directing agent P123 plays an important role in the preparing of mesoporous H3PW12O40/TiO2 and H4SiW12O40/TiO2.
H3PW12O40/TiO2(P) was chosen to evaluate the stability and reusability of the composite. This is important from the view of industrial application. The catalyst was separated, washed, and dried after the reaction and was used for the next run. The catalytic reaction was repeated three times.
As shown in Fig. 9, H3PW12O40/TiO2(P) maintained its activity after it was reused three times, suggesting that the composite catalyst exhibited high stability. Therefore, the new route that combines sol-gel synthesis, template self-assembly, and a solvothermal treatment is suitable for preparing high stable photoctalyst.
A new route using the combination of sol-gel synthesis and solvothermal treatment was demonstrated for preparing four kinds of composite catalysts. The structure directing agent P123 plays an important role in preparing mesoporous H3PW12O40/TiO2 and H4SiW12O40/TiO2, which possessed higher specific surface area, and smaller crystal size. Photocatalytic tests showed that H3PW12O40/TiO2(P) and H4SiW12O40/TiO2(P) exhibited higher photocatalytic activities for DNT degradation. Using H3PW12O40/TiO2(P) as the catalyst, the degradation rate of DNT reached 95% after 4 h of irradiation and maintained after three cycles.
2014, Vol. 35 Issue (2): 168-174
PDF (757KB)
2014, Vol. 35 Issue (2): 168-174
PDF (757KB)