Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (6): 717-724

The article information

Bing-hua Yao, Chao Peng, Yang-qing He, Wen Zhang, Yan Yu, Ting Zhang
姚秉华, 彭超, 何仰清, 张文, 于艳, 张亭
Preparation and Visible-Light Photocatalytic Activity of FeTPP-Cr-TiO2 Microspheres
FeTPP-Cr-TiO2微球的制备及其可见光催化性能
Chinese Journal of Chemical Physics, 2016, 29(6): 717-724
化学物理学报, 2016, 29(6): 717-724
http://dx.doi.org/10.1063/1674-0068/29/cjcp1605117

Article history

Received on: May 27, 2016
Accepted on: October 14, 2016
Preparation and Visible-Light Photocatalytic Activity of FeTPP-Cr-TiO2 Microspheres
Bing-hua Yaoa, Chao Penga,c, Yang-qing Hea, Wen Zhangb, Yan Yua, Ting Zhanga     
Dated: Received on May 27, 2016; Accepted on October 14, 2016
a. Department of Applied Chemistry, Xian University of Technology, Xi'an 710048, China;
b. Department of Civil Engineering, University of Arkansas, Fayetteville 72701, USA;
c. School of Materials Science and Engineering, Xian University of Technology, Xi'an 710048, China
*Author to whom correspondence should be addressed. Bing-hua Yao, E-mail:bhyao@xaut.edu.cn, Tel:+86-29-82066361; Wen Zhang, E-mail:wenzhang@uark.edu, Tel:+1-479-5756024
Abstract: Tetraphenyl-porphyrin iron (FeTPP) was chosen to sensitize Cr doped TiO2 (Cr-TiO2) nanoparticles, a novel multimodified photocatalyst FeTPP-Cr-TiO2 with excellent visiblelight photocatalytic activity was successfully synthesized. The FeTPP-Cr-TiO2 microspheres were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electronic microscopy, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectra and N2 adsorption-desorption isotherms. The photocatalytic activity of FeTPP-Cr-TiO2 was evaluated by degradations of methylene blue in aqueous solution under irradiation with Xe lamp (150 W). The results showed that the FeTPP-Cr-TiO2 multimodified photocatalyst was anatase phase with high specific surface area (74.7 m2/g), and exhibited higher photocatalytic degradation efficiency than Cr-TiO2 and FeTPP-TiO2. The photocatalytic degradations of three quinolone antibiotics (lomefloxacin, norfloxacin, and ofloxacin) were further estimated for the feasibility of practical application of catalyst in wastewater treatment. It is desirable that photodegradation of antibiotics with FeTPP-Cr-TiO2 achieved pretty high degradation rates and all followed the pseudo first-order reaction model, and the rate constants k of 3.02×10-2, 2.81×10-2, and 3.86×10-2 min-1 and the half-lifes t1/2 of 22.9, 24.6, and 17.9 min were achieved respectively.
Key words: Titanium dioxide    Chromium doping    Metalloporphyrin    Multimodified photocatalyst    Visible-light photocatalysis    Antibiotics    
Ⅰ. INTRODUCTION

TiO2 is the most widely investigated photocatalyst due to its high photo-activity, low cost, no toxicity, and good chemical and thermal stability [1-4]. Numerous reports have focused on the photocatalytic technology with TiO2 for degradation of the organic pollutants [5-7]. However, due to the wide band gap and fast recombination rate of photogenerated electron-hole (e--h+), the practical application of TiO2 in photocatalysis has been limited [8-10]. To overcome these disadvantages, many approaches have been developed, including doping of metal and non-metal elements, sensitization of dyes and coupling of other semiconductors with narrow band gap [11-16].

Doping TiO2 with transition metals, such as Cr, Ni, Fe and Cu, is one of the most promising methods for decreasing the band gap of TiO2 and extending its photoresponse to the visible light region [17-20]. Among these transition metals, it was found that Cr doping was particularly effective which could cause a distortion in the crystal lattice thereby introducing doping energy level with the band gap of TiO2 to extend its photoresponse. In addition, prominent improvement of the photocatalytic activity has been achieved by doping TiO2 with Cr [21-24]. Unfortunately, the capability of Cr doping to improve the photocatalytic activity of TiO2 is rather limited. Cr ions in Cr-TiO2 may also act as trapping centers in which excited electrons may recombine with holes, thus shortening the lifetime of the photogenerated charge carriers e--h+ [25, 26].

Metalloporphyrins are excellent photosensitizers due to their large π-electron conjugate systems and high absorption coefficient within the visible-light region (in the 400-450 nm region of Soret band and in the 500-700 nm region of Q-bands) [27-30]. These properties of metalloporphyrins are very valuable for photocatalysis and attract great attentions for the sensitization of TiO2 [31-34]. As known, the mechanism of metalloporphyrins sensitized TiO2 is based on the transfer of electrons from the lowest unoccupied molecular orbital (LUMO) of excited metalloporphyrin molecules into the conduction band of TiO2 [35-37]. On the basis of the unique electron transfer mechanism, metalloporphyrin-TiO2 system can be introduced as a method to solve the above the recombination problem of photogenerated e--h+ from the doping energy levels [38, 39].

Obviously, the mechanism of dye sensitization to enhance the photocatalytic activity of TiO2 is completely different from that of metal doping. It is deduced that the combination of Cr doping and iron porphyrin sensitization may be an excellent approach to remarkably extend the photoresponse and significantly improve photocatalytic activity of TiO2. For this reason, a multimodified catalyst, Cr doped TiO2 nanoparticles (Cr-TiO2) sensitized by tetraphenyl-porphyrin iron (Ⅲ) (FeTPP) were prepared. The degradations of methylene blue (MB) were used to evaluate the photocatalytic activities of catalysts. For further investigating their feasibilities in practical application of wastewater treatment, photodegradations of quinolone antibiotics as target pollutants in aqueous solution were carried out simultaneously. Because antibiotics are an important class of water pollutants, the continuous input of antibiotics into the aquatic environment has posed adverse effects on the humans health and aquatic organisms. In comparison with insufficient removal of them from water by the conventional wastewater treatment, photocatalysis appears as one of the most promising technologies. Quinolones are the most widely used antibiotics in the treatment of bacterial infections and most frequently detected antibiotics in wastewater effluents. Herein, we examined the degradation of three representative fluoroquinolones, lomefloxacin, norfloxacin and ofloxacin by photocatalysts. To our best knowledge, this is the first time to employ multimodified photocatalysts FeTPP-Cr-TiO2 in the quinolone antibiotics degradation. Furthermore, the synergistic effect of Cr doping and FeTPP sensitization on the photocatalytic activity of TiO2 was also discussed.

Ⅱ. MATERIALS AND METHODS

Cr (NO3)3 and Ti (SO4)2 were AR, Tianjin Fuyu Fine Chemical Co., Ltd. FeTPP was obtained according to our previous work [34]. MB and methanol were supplied by China Sinopharm Chemical Reagent. Ethanol and trichloromethane were obtained from Tianjin Tianli Chemical Reagent Co. Ltd. Lomefloxacin (LFC, 98%) and ofloxaxin (OFC, 98%) were supplied by Wuhan Nuoan Pharmacy Co., Ltd and Shanghai J & K Chemical Reagent (China) Co. Ltd. Norfloxacin (NFC, 99%) was obtained from Shanxi Tanyuan pharmaceutical Co. Ltd. Ultrapure deionized water was used throughout the experiments. All reagents were used without further purification.

Benzaldehyde (5.4 mL) and propionic acid (150 mL) were dissolved into a three-necked flask equipped with reflux condenser and then heated to 135 ℃. Subsequently, a solution of pyrrole (3.4 mL) diluted with propionic acid (20 mL) was slowly added into the reaction solution dropwise over 60 min and refluxed for an additional 120 min. Following solvent removal under reduced pressure and concentration, the crude product was purified further by chromatography on silica gel column (200 mesh) with propionic acid/CHCl3 (3/2) as eluent. The fraction corresponding to the first colored band was collected, concentrated, and dried to give meso-tetraphenylporphyrin (TPP). TPP and FeCl3 (molar ratio of TPP to FeCl3 was 1/1.5) were dissolved in DMF (150 mL). The mixture was heated to 150 ℃ and refluxed with continuous stirring for 180 min. After removal of solvent under reduced pressure, the residue was then dissolved in ethanol and the insoluble impurities were removed by filtration. Finally, the extract was concentrated and dried to yield meso-tetraphenylporphyrin iron (FeTPP). The TPP and metallated product FeTPP were confirmed by UV-Vis spectroscopy.

4.32 g Ti (SO4)2 and 0.22 g Cr (NO3)3 (molar ratio 1:0.05) were dissolved in 30 mL of deionized water. Then the mixture solution was transferred into a Teflon-lined stainless steel autoclave (50 mL). The hydrothermal reaction was carried out at 180 ℃ for 16 h. The precipitates were filterd by ultrafiltration membrane (Φ=0.22), and washed three times with deionized water. The filter cakes were dried in the vacuum oven at 60 ℃. The Cr-TiO2 samples were thus obtained. In addition, pure TiO2 was also prepared by the similar procedure without the addition of Cr (NO3)3. After that, 20 mg FeTPP was dissolved in 100 mL of ethanol. An appropriate amount of Cr-TiO2 was added to the FeTPP solution with ultrasonic dispersion for 30 min, then the suspension was transferred into a 250 mL three-necked flask with stirring at 80 ℃, and refluxed for 120 min in an oil bath, subsequently filterd. The resulting solids were washed three times with ethanol, dried under vacuum to obtain the FeTPP-Cr-TiO2 photocatalyst. The FeTPP-TiO2 was prepared by the similar procedure with the raw material of pure TiO2.

The Fourier transform infrared (FT-IR) spectra were performed with a Shimadzu FT-IR spectrometer (Japan 8900) with the reference of KBr. Scanning electron microscopy (SEM) profiles were obtained on a Tescan SEM (Czech VEGA 3 SBH) equipped with an EDX spectrometer. The phase compositions of catalysts were characterized by powder X-ray diffraction (XRD) with Shimadzu X-ray diffractometer (Japan 7000S) at tube current of 30 mA, tube voltage of 40 kV, and scanning speed of 10°/min. The UV-Visible diffuse reflectance spectra (UV-Vis DRS) were determined by double-beam UV-Vis diffuse reflectance spectrophotometer (China TU-1901) with BaSO4 as reference. X-ray photoelectron spectroscopy (XPS) were collected by a Kratos spectrometer (Japan AXIS-ULTRA). The Brunauer-Emmett-Teller (BET) surface area and pore volume was determined by the N2 physisorption at -196 ℃ using a surface area analyzer (China JW-BK122W). The UV-Vis absorption spectra over a range of 200-800 nm wavelength were measured by a Mapada UV-Vis spectrophotometer (China UV-3200 PCS).

Self-made photocatalytic reaction device was used for the evaluation of photocatalytic activity of catalysts. The device consists of a light source, the sample tube (100 mL quartz tube of length 22.0 cm, diameter 2.0 cm, 10 cm away from the light source), a cold trap, an air bubbler, as well as several additional accessories. In a degradation process, 50 mg of catalyst and 50 mL of MB (10 mg/L) or antibiotic aqueous solution (LFC of 25 mg/L, NFC of 25 mg/L, OFC of 25 mg/L) were added to the sample tube. The irradiation was performed with 150 W Xe lamp with an emission wavelength ranging from 290 nm to 800 nm, and a 1% NaNO2 solution was circulated through the cooling jacket to filter out the UV emission of the lamp below 400 nm. The air tube was inserted into the bottom of sample tube, maintaining a controlled air flow at 3 L/min to achieve the suspended catalyst in the degradation solution. The suspension was kept in dark for 30 min to reach the adsorption equilibrium prior to irradiation (preliminary results indicated that adsorption equilibrium could be quickly reached less than 30 min), and then the suspension was withdrawn at regular time intervals (20 min) and high-speed centrifuged for 10 min. The absorbance of the supernatant from centrifugation was measured at the characteristic absorption peaks of MB and antibiotic solutions. According to the relationship between the absorbance and concentration, the degradation rate D was calculated using the equation:

$ D = \frac{{{A_0}-{A_{\text{t}}}}}{{{A_0}}} \times 100\% $ (1)

Where A0 is the initial absorbance of solution, At is the absorbance of solution at different times, D is used to evaluate the photocatalytic activity of catalysts.

Ⅲ. Results and discussion A. XRD analysis

XRD analysis was carried out to investigate the crystal identity of the catalysts and effects of Cr doping and FeTPP sensitization on the crystal structure of TiO2. As shown in Fig. 1(a), the major diffraction peaks of Cr-TiO2, FeTPP-TiO2, and FeTPP-Cr-TiO2 had similar values to that of anatase TiO2 [32], which indicates that the doping and sensitization has no influence on the configuration of TiO2, except for the small change in the position of peak at 25.3° (shown in Fig. 1(b)). It is found that the position of the peak at 25.3° of Cr-TiO2, FeTPP-TiO2, and FeTPP-Cr-TiO2 shifted to higher 2θ values, as compared to TiO2. The position shift indicates a slight decrease in the spacing value between the lattice planes which might be caused by the insertion of Cr in the lattice [40]. It suggests that Cr successfully entered into the crystal structure of TiO2. Because the Cr3+ ion possesses smaller ion radius compared to Ti4+ ion, so there certainly occurred the lattice distortion and defect in Cr-TiO2 crystal, which acts as doping level with the band gap of TiO2 [26].

FIG. 1 (a) XRD patterns of pure TiO2 and catalysts and (b) Amplification of the peak at 25.3° in the spectra of catalysts.
B. FT-IR analysis

Figure 2 shows the FT-IR spectra of FeTPP, TiO2, Cr-TiO2, FeTPP-TiO2 and FeTPP-Cr-TiO2. In the spectra of TiO2 and Cr-TiO2, the absorption peak around 3430 cm-1 was associated with the stretching vibrations of hydroxyl groups on TiO2 surface, the stretching vibration band of Ti-O-Ti appeared at 490-700 cm-1 [41]. As shown in the spectrum of FeTPP, these strong peaks at 1070 and 1000 cm-1 which were characteristics of C-N and Fe-N stretching vibrations could be observed [42]. After FeTPP sensitized, the intensity of these peaks greatly reduced and position shifted to 1128 and 1054 cm-1 in the spectra of FeTPP-TiO2 and FeTPP-Cr-TiO2. The main characteristic vibrations attributed to the C=C and C-C around 1692 and 1400 cm-1, C-H around 2925 cm-1 of FeTPP could be observed, which indicates that the TiO2 and Cr-TiO2 sensitized by FeTPP have been successfully achieved [43]. In addition, C-N stretching and Fe-N vibration blue-shifted to 1128 and 1054 cm-1 in spectra of FeTPP-TiO2 and FeTPP-Cr-TiO2. These information indicated that there existed a weak interaction between FeTPP molecule and surface of TiO2/Cr-TiO2, and the interaction can be considered as physical adsorption [16].

FIG. 2 FT-IR spectra of FeTPP, TiO2, Cr-TiO2, FeTPP-TiO2, and FeTPP-Cr-TiO2.
C. SEM and EDX analysis

As shown in Fig. 3, the microstructures of FeTPP-Cr-TiO2 appeared loose and irregular spheres with rough surface. The average size of these nanoparticles diameter estimated from the SEM image is about 20 nm. The morphology of image implies that all the Cr-TiO2 nanoparticles were coated by the FeTPP and formed an organic surface. The organic surface was a compatible substrate to contact MB molecule and organic target compound (such as quinolone antibiotics in this series of experiments), beneficial to enhance adsorption of target compounds prominently. In order to further confirm the doping of Cr with TiO2, EDX spectrum analysis was employed, as seen from Fig. 4. The presence of element Cr with a weight percentage of 0.07% (ratio of atom) indicates the achievement of doping Cr into the TiO2 crystal lattice. And no presence of element Fe could be attributed to the too low quantity.

FIG. 3 SEM image of FeTPP-Cr-TiO2.
FIG. 4 EDX spectrum of FeTPP-Cr-TiO2.
D. XPS analysis

XPS analysis was used to investigate the surface chemical states and composition change of TiO2 and FeTPP-Cr-TiO2. Figure 5(a) compares the XPS spectra of TiO2 and FeTPP-Cr-TiO2 in the O 1s, Ti 2p and C 1s peaks. From the analysis of O 1s and Ti 2p, there were slight shifts towards lower binding energy after the sensitization of FeTPP, which was probably due to the weak interactions between FeTPP molecules and Cr-TiO2 surface [44]. Compared to the C 1s peak (mainly comes from the adventitious carbon-based contaminant) of TiO2, it is found that there was obvious enhancement of C 1s peak intensity in FeTPP-Cr-TiO2, which was probably due to the massive amounts of adsorbed FeTPP molecules. Moreover, Fig. 5(b) and (c) show high-resolution XPS spectra of Cr 2p and Fe 2p in FeTPP-Cr-TiO2, the peaks located at 579.32 and 565.61 eV could be attributed to the Cr 2p [45]. And the peaks located at 712 and 725 eV could be attributed to the Fe 2p in the FeTPP-Cr-TiO2 composite, indicating FeTPP molecules adsorbed on the Cr-TiO2 surface. These XPS results further confirm the presence of Cr3+ in TiO2 crystal structure and adsorption of FeTPP molecule on the nanoparticle surface.

FIG. 5 XPS spectra of (a) TiO2 and FeTPP-Cr-TiO2. High-resolution XPS spectra of (b) Cr2p and (c) Fe2p.
E. UV-Vis DRS analysis

Optical property of a semiconductor is one of the important factors for its photocatalytic activity. Figure 6 exhibits a comparison of UV-Vis DRS spectra of TiO2, Cr-TiO2, FeTPP-TiO2 and FeTPP-Cr-TiO2. Obviously, for TiO2, it is clearly seen that the absorption band is mainly in the range of 200-400 nm. From the spectrum of Cr-TiO2, a stronger absorption in the visible region could be observed in comparison with TiO2, which was attributed to the Cr doping. While for both FeTPP-TiO2 and FeTPP-Cr-TiO2, the existence of the feature peaks of the Soret and Q bands indicates that the FeTPP was successfully senstitized onto the surface of TiO2 and Cr-TiO2 nanoparticles. In addition, the presence of the Soret and Q bands in spectrum of FeTPP-Cr-TiO2 also suggests that its visible absorption was obviously improved.

FIG. 6 UV-Vis DRS spectra of TiO2 and catalysts.
F. BET analysis

N2 physical adsorption-desorption studies were conducted to determine the specific surface area and the pore structure of the catalysts. N2 adsorption-desorption isotherms of TiO2, Cr-TiO2, FeTPP-TiO2 and FeTPP-Cr-TiO2 are shown in Fig. 7. As shown in N2 adsorption-desorption isotherms, all catalysts exhibited typical IUPAC type Ⅲ pattern with H3-type hysteresis loop, which is the major characteristic of a mesoporous material (mesoporous size between 2 and 12 nm) according to the classification of IUPAC [43]. It is evident that with the same relative pressure FeTPP-Cr-TiO2 had the biggest surface area and adsorption quantity. The specific surface areas of TiO2, Cr-TiO2, FeTPP-TiO2 and FeTPP-Cr-TiO2 were 61.5, 65.6, 62.3, and 74.7 m2/g, respectively. Furthermore, a sharp increase in the adsorption curve of FeTPP-Cr-TiO2 at a high relative pressure (P/P0) in the range of 0.80-0.90 implies a capillary condensation of N2 molecules inside the mesoporous, as well as the well-uniform pore size distributions [32]. These increases in the specific surface areas, pore volumes and pore sizes were due to the formation of mesoporous structure, which could be possibly attributed to the adsorption of FeTPP molecules on the Cr-TiO2 surface. Figure 8 shows the pore size distributions calculated from the desorption branch of the isotherms. The FeTPP-Cr-TiO2 possessed very narrow pore size distributions entirely locating in the mesoporous region, and these pore sizes and pore volumes increased from 8.73 nm, 0.147 cm3/g (TiO2) to 26.2 nm, 0.487 cm3/g (FeTPP-Cr-TiO2) respectively. Table Ⅰ shows the summary of the specific surface areas, average pore diameters and pore volumes of different catalysts. The results show that the specific surface area, pore size, and pore volume of FeTPP-Cr-TiO2 increased after doping and sensitization.

FIG. 7 N2 adsorption-desorption isotherms of TiO2, Cr-TiO2, FeTPP-TiO2, and FeTPP-Cr-TiO2.
FIG. 8 Corresponding pore size distribution curves of TiO2, Cr-TiO2, FeTPP-TiO2, and FeTPP-Cr-TiO2.
Table Ⅰ Specific surface areas SBET, average pore sizes d and pore volumes V of different catalysts.
Ⅳ. PHOTOCATALYTIC ACTIVITIES A. Photocatalytic degradation of MB

The photocatalytic activities of catalysts were evaluated by measuring the degradation rate of MB under visible light irradiation. Figure 9 shows UV-Vis absorption spectra of MB aqueous solution in the presence of FeTPP-Cr-TiO2. It is obvious that the characteristic absorption peak of MB at 665 nm decreased sharply and disappeared absolutely in 120 min, and the degradation rate of MB reached 97.6%. The photocatalytic degradation rates of different catalysts are shown in Table Ⅱ. Owing to the no response to visible light, TiO2 could only degrade about 49.1% of MB in 120 min. On the contrary, the Cr-TiO2 (72.5%) exhibited better photocatalytic activity for the degradation of MB than TiO2, which indicates that Cr doping could greatly improve the photocatalytic activity of TiO2. So, FeTPP-Cr-TiO2 exhibited excellent photocatalytic activity.

FIG. 9 UV-Vis absorption spectra of MB in the presence of FeTPP-Cr-TiO2.
Table Ⅱ Kinetic parameters of photocatalytic degradation reactions.

In summary, the excellent photocatalytic activity of FeTPP-Cr-TiO2 can be attributed to the multi-modification and synergistic effect of two kinds of modified methods. First of all, a larger number of FeTPP adsorbed on the surface of Cr-TiO2 lead to the production of more photogenerated electrons and holes [46]. Moreover, or other factor is that Cr-doping introduces doping levels, the doping levels in Cr-TiO2 act as electron traps which can capture the photogenerated electrons and play a significant role in inhibiting the recombination rate of photogenerated electron-hole pairs, thus enhancing the photocatalytic activity of TiO2 [47].

In order to better compare the effect of catalysts on the MB degradation, the apparent rate constant (k) was calculated, the kinetics of MB degradation was quantitatively studied by Langmuir-Hinshelwood first-order model as expressed by ln (c0/ct)=-kt. This model is generally used for the photocatalytic degradation process, where c0 and ct are the concentration of initial and t time, k can be obtained from the slope of the curve of ln (c0/ct versus t. From Fig. 10, the ln (c0/ct values of MB degradation were very good linear with respect to the degradation times t for all catalysts. This shows that the degradation of MB followed the first order kinetics model, the kinetic parameters for photocatalytic degradation of different catalysts are listed in Table Ⅱ. As seen in Table Ⅱ, the kFeTPP-Cr-TiO2 value was the largest (0.0613 min-1) compared to that of others. In addition, the half-life t1/2 value of MB degradation with FeTPP-Cr-TiO2 was 7.42 min, which was about 4.4 times faster than that observed for TiO2 (t1/2=32.7 min). The photocatalytic activity of catalysts increased in the order of TiO2, FeTPP-TiO2, Cr-TiO2, and FeTPP-Cr-TiO2.

FIG. 10 Photocatalytic degradation kinetics curves of MB by catalysts.
B. Photocatalytic degradation of antibiotics

The photocatalytic degradations of three quinolone antibiotics (LFC, NFC and OFC) in aqueous solution were investigated by different catalysts. As shown in Fig. 11(a), (b), and (c), the ln (c0/ct values of LFC, NFC and OFC degradation were very good linear with respect to the reaction times t for all catalysts. As can be seen from Table Ⅲ, the photocatalytic degradations of LFC, NFC and OFC fitted well with first-order kinetic model; the degradation rate constants kFeTPP-Cr-TiO2 were 3.02×10-2, 2.81×10-2, and 3.86×10-2 min-1 and the half-lifes t1/2 of antibiotics were 22.9, 24.6, and 17.9 min, respectively. The rate constants k and t1/2 of LFC, NFC and OFC for FeTPP-Cr-TiO2 increased to 6-15 times as compared to that of TiO2. The UV-Vis absorption spectra of LFC, NFC and OFC degradation solution by FeTPP-Cr-TiO2 (shown in Fig. 11(d), (e) and (f) showed that the characteristic absorption peaks of LFC, NFC and OFC at 285, 279 and 293 nm decreased sharply with the increase of reaction time, and disappeared completely after 120 min respectively. The degradation rates of these antibiotics were up to 98.7%, 98.4% and 97.5% respectively, which were much higher than that of others. This comparison indicates that the synergistic effect of Cr doping with FeTPP sensitization played an important role in the improvement of TiO2 photocatalytic performance.

FIG. 11 Photocatalytic degradation kinetics curves of (a) LFC, (b) NFC, and (c) OFC by catalysts. The UV-Vis absorption spectra of (d) LFC, (e) NFC, and (f) OFC aqueous solution by FeTPP-Cr-TiO2.
Table Ⅲ Degradation kinetics parameters for LFC, NFC and OFC with FeTPP-Cr-TiO2 and TiO2 using the firstorder kinetic model (k is in unit of 102min-1, t1/2 is in unit of min).
Ⅴ. CONCLUSION

In order to further improve the visible response of TiO2, the Cr-TiO2 nanoparticles were sensitized by metalloporphyrin FeTPP on the basis of Cr-doping TiO2. The results shows that the FeTPP-Cr-TiO2 exhibited higher visible-light photocatalytic activity than both of Cr-TiO2 and FeTPP-TiO2 under visible light irradiation, which can be attributed to the synergistic effect of FeTPP sensitization and Cr-doping to TiO2. This synergistic effect resulted in the decrease of TiO2 band gap and shifted the absorption edge of TiO2 toward visible-light region. In comparison with Cr-TiO2 and FeTPP-TiO2, the multimodified FeTPP-Cr-TiO2 possessed excellent photocatalytic performance on the degradation of MB. Further studies exhibit that three quinolone antibiotics, lomefloxacin, norfloxacin and ofloxacin in aqueous solution were remarkably degraded by FeTPP-Cr-TiO2 under irradiation of visible light. Additionally, it is found that the degradation processes of three antibiotics all followed a pseudo first order kinetics, and the rate constant k and half-life t1/2 were evaluated. In conclusion, the synergistic effect between the FeTPP sensitization and Cr-doping significantly enhanced the photocatalytic activity and provided a promising way for modification of TiO2 catalyst.

Ⅵ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21276208), the International Science Technology Cooperation Program of China (No.2015DFR50350), the Research Fund of Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources (No.2014SKY-WK003), the Special Research Fund of Shaanxi Provincial Department of Education of China (No.15JK1862), and the Research Fund for Innovation Doctoral Thesis of Xi'an University of Technology (No.310-11202J304 and No.310-252071508).

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FeTPP-Cr-TiO2微球的制备及其可见光催化性能
姚秉华a, 彭超a,c, 何仰清a, 张文b, 于艳a, 张亭a     
a. 西安理工大学应用化学系, 西安 710048;
b. 美国阿肯色大学土木工程系, 费耶特维尔 72701;
c. 西安理工大学材料科学与工程学院, 西安 710048
摘要: 通过四苯基铁卟啉敏化Cr掺杂TiO2微球成功合成了一种复合改性可见光催化剂FeTPP-Cr-TiO2.利用XRD、FT-IR、SEM、XPS、UV-Vis和N2吸附-脱附等温线等技术对其结构和形貌进行了表征.以亚甲基蓝为目标降解物,在150 W氙灯的辐照下,考察了其可见光催化活性.结果表明FeTPP-Cr-TiO2呈锐钛矿相,表面积为74.7 m2/g.与Cr-TiO2和FeTPP-TiO2相比,FeTPP-Cr-TiO2表现出很好的可见光降解性能.以三种喹诺酮类抗生素(洛美沙星、诺氟沙星和氧氟沙星)为实验对象,考察了FeTPP-Cr-TiO2降解水体中抗生素的可行性,对三种抗生素均有很高的降解率,降解过程符合一级动力学模型,反应动力学常数分别为3.02×10-2、2.81×10-2和3.86×10-2min-1,半衰期为22.9、24.6及17.9 min.
关键词: TiO2    Cr掺杂    金属卟啉    多改性光催化剂    可见光催化    抗生素