Journal of Hazardous Materials 280 (2014) 531–535
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Enhanced photocatalytic ozonation of organics by g-C3 N4 under visible light irradiation Gaozu Liao ∗ , Dongyun Zhu, Laisheng Li ∗∗ , Bingyan Lan Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Higher Education Mega Center, Guangzhou 510006, China
h i g h l i g h t s • • • •
g-C3 N4 is employed as active catalyst in the photocatalytic ozonation system. The more negative conduction band of g-C3 N4 benefits the transfer of electrons. The synergistic effect between photocatalysis and ozonation is promoted by g-C3 N4 . Enhanced degradation of oxalic acid and biphenol A is achieved via g-C3 N4 /Vis/O3 .
a r t i c l e
i n f o
Article history: Received 5 June 2014 Received in revised form 23 July 2014 Accepted 20 August 2014 Available online 3 September 2014 Keywords: Carbon nitride Photocatalytic ozonation Synergistic effect Hydroxyl radical
a b s t r a c t Graphitic carbon nitride (g-C3 N4 ) was employed as the active photocatalyst in the photocatalytic ozonation coupling system in the present study. g-C3 N4 was prepared by directly heating thiourea in air at 550 ◦ C. XRD, FT-IR, UV–vis was used to characterize the structure and optical property. Oxalic acid and bisphenol A were selected as model substances for photocatalytic ozonation reactions to evaluate the catalytic ability of g-C3 N4 (g-C3 N4 /Vis/O3 ). The results showed that the degradation ratio of oxalic acid with g-C3 N4 /Vis/O3 was 65.2% higher than the sum of ratio when it was individually decomposed by g-C3 N4 /Vis and O3 . The TOC removal of biphenol A with g-C3 N4 /Vis/O3 was 2.17 times as great as the sum of the ratio when using g-C3 N4 /Vis and O3 . This improvement was attributed to the enhanced synergistic effect between photocatalysis and ozonation by g-C3 N4 . Under visible light irradiation, the photo-generated electrons produced on g-C3 N4 facilitated the electrons transfer owing to the more negative conduction band potential (−1.3 V versus NHE). It meant that the photo-generated electrons could be trapped by ozone and reaction with it more easily. Subsequently, the yield of hydroxyl radicals was improved so as to enhance the organics degradation efficiency. This work indicated that metal-free g-C3 N4 could be an excellent catalyst for mineralization of organic compounds in waste control. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor mediated photocatalysis is a promising method in wastewater treatment due to its availability of solar radiation and mineralization of various refractory compounds [1,2]. However, a major disadvantage of most semiconductor materials is the high degree of recombination between photogenerated charge carriers, which ultimately decreases the photocatalytic efficiency in the redox process [3,4]. To overcome this drawback, coupling photocatalysis with other technologies has been proposed, such
∗ Corresponding author. Tel.: +86 20 3931 0213; fax: +86 20 8470 0187. ∗∗ Corresponding author. Tel.: +86 20 3931 0185; fax: +86 20 8470 0187. E-mail addresses: [email protected] (G. Liao), [email protected] (L. Li). http://dx.doi.org/10.1016/j.jhazmat.2014.08.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
as UV/TiO2 /H2 O2 [5,6], TiO2 /plasma [7,8] and UV/TiO2 /microwave [9,10]. In the combined systems, photocatalytic ozonation (UV/TiO2 /O3 ) have already been reported as the most appropriate one for mineralization of organics [11–13]. When photocatalysis and ozonation treatments are carried out simultaneously, more hydroxyl radicals (• OH) radicals will be generated for mineralization of organics due to the synergistic effect. This synergistic effect is mainly ascribed to the reaction between photo-generated electrons and ozone [14,15]. Under UV light illumination, the dissolved ozone will get electrons from semiconductor and lead to formation of the ozonide radical (O3 •− ) due to the strong electron trapping effect. This reaction not only inhibits the recombination of photo-generated charges, but also improves the utilization efficiency of dissolved ozone remarkably. Consequently, the
532
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
generated ozonide radical (O3 •− ) further transform into • OH and lead to organics degradation. Among candidates for photocatalytic ozonation, TiO2 is the most widely used photocatalyst due to its non-toxicity, good stability and excellent photocatalytic activity [16]. However, the synergistic effect between photocatalysis and ozonation is somewhat limited by the band gap energy of TiO2 . On the one hand, the wide band gap (3.2 eV) of TiO2 limits the utilization of visible light which occupies a large part of the solar light [16]. Moreover, the availability of photo-generated electrons in the photocatalyst are restricted by the position of conduction band (CB, −0.2 V versus NHE). The reactivity in the presence of ozone is related to availability of electrons in the photocatalyst [17]. The • standard reduction potential for the reaction (O3 + e− → O3 − ) is about 1.6 V versus NHE [18]. The more negative conduction band • potential of photocatalyst than the reduction potential of O3 /O3 − will facilitate the electrons trapping by ozone. Therefore, photocatalyst that possesses visible light activity and more negative potential of CB could be employed as the active catalyst to enhance the synergistic effects for photocatalytic ozonation. Recently, graphitic carbon nitride (g-C3 N4 ) has been reported as a metal-free photocatalyst for contaminant removal under visible light irradiation [19,20]. g-C3 N4 is composed of carbon and nitrogen only, which is stable under light irradiation in solution with pH 0–14 due to the strong covalent bonds between carbon and nitride atoms. In addition, the optical band gap was determined to be 2.7 eV, and the conduction band (CB) potential of g-C3 N4 was reported to be −1.3 V versus NHE [21,22]. These properties imply that g-C3 N4 not only serve as an efficient visible light photocatalyst. Moreover, the photo-electrons of g-C3 N4 generated under visible light irradiation possess strong reduction capability. Accordingly, g-C3 N4 has exhibited promising potential in the field of hydrogen production [23,24], heavy metal reduction [25,26] and CO2 reduction [27,28]. In present study, g-C3 N4 was employed as the active photocatalyst in the photocatalytic ozonation coupling system (g-C3 N4 /Vis/O3 ) to achieve efficient mineralization of organic compounds under visible light irradiation. Compared with the conventional UV/TiO2 /O3 system, we expect that the high reduction potential of photo-generated electrons could facilitate the reaction with ozone, resulting in the improvement of utilization efficiency with ozone. Meanwhile, the rapid consumption of photogenerated electrons could benefit the separation of photogenerated charge. Both the two aspects enhanced the synergistic effects between photocatalysis and ozonation, so as to improve the degradation efficiency of organics. Oxalic acid and bisphenol A were used as model substances for photocatalytic ozonation reactions to evaluate the catalytic ability of g-C3 N4 under visible light irradiation.
separately. Then the precipitate was transferred to oven and dry at 80 ◦ C for 12 h.
2.3. Characterization of g-C3 N4 The structural information for the samples was measured by Fourier transform spectrophotometer (FT-IR, Nicolet 6700) with KBr as the reference sample. The crystal structure of the samples was investigated using an X-ray diffractometer (XRD, BRUKER D8 ADVANCE) with Cu-Ka radiation. The optical absorption properties of the samples were investigated through diffuse reflectance spectra (DRS) using a UV–vis spectrophotometer (U-3010, HITACHI, Japan).
2.4. Photocatalytic ozonation experiment A 1 L glass tubular photoreactor (h = 400 mm, ˚in = 85 mm) was employed for the photocatalytic ozonation experiments. A volume of 1 L of simulated wastewater (the initial concentration of oxalic acid solution and biphenol A was 10 mg L−1 ) and 0.5 g of catalyst powder were placed in the reactor. A high pressure xenon long-arc lamp (GXH500W, Beijing NBET Technology Co., Ltd) served as the visible light source. The lamp was jacketed by a quartz thimble filled with flowing and thermostatted aqueous NaNO2 solution (1 M) between the lamp and the reaction chamber as a filter to block UV light ( < 400 nm). Ozone was produced from ozone generator (CF5G, made in China). Ozonized oxygen was continuously bubbled into the solution through a porous glass plate and flowed upward in the annular section. The flow rate of oxygen was 1 L min−1 , and the dosage of ozone was 500 mg h−1 . Samples were taken at intervals and filtered with 0.45 m microfilters to collect the filtrate. Na2 S2 O3 solution was used to stop the continuous ozonation reaction in the sample. The reaction temperature was remained at 25 ◦ C in all experiments.
2.5. Analytical methods The concentration of oxalic acid was determined by high performance liquid chromatography (Shimadzu, LC10A HPLC) with a UV detector (SPD-10AV) at 254 nm. A Diamonsil 5U C18 column (5l m, 250 mm × 4.6 mm, Dikma technologies) was used, and the analysis was carried out with an aqueous solution containing 0.018 mol/L potassium dihydrogen phosphate and 0.0025 mol/L tetrabutyl ammonium hydrogen sulfate at pH 2.5–2.7 was used as mobile phase. Flow rate was 1.0 mL/min at 25 ◦ C. Total organic carbon (TOC) was determined by a Shimadzu TOC 5000 analyzer.
2. Experiment 3. Results and discussion 2.1. Chemicals 3.1. Crystal structure of g-C3 N4 All of the reagents (analytical grade purity) were purchased from Guangzhou Chemical Reagents Factory and used without purification. Deionized water was used to prepare solutions.
2.2. Preparation of g-C3 N4 The polymeric g-C3 N4 was prepared by a facile method according to the method reported by Dong et al. [29]. In a typical synthesis, 20 g of urea powder was put into an alumina crucible with a cover, and then heated to 550 ◦ C in a muffle furnace for 2 h with a heating rate of 15 ◦ C/min. After cooling to room temperature, the resultant yellow powder was washed with ethanol and water for 2 times,
The X-ray diffraction patterns (XRD) of g-C3 N4 is presented in Fig. 1. Two peaks are present for g-C3 N4 . The peak at 13.0◦ corresponds to in-plane structural packing motif of tri-s-triazine units, which is indexed as (1 0 0) peak. The distance is calculated as d = 0.675 nm. The peak at 27.50 corresponds to interlayer stacking of aromatic segments with distance of 0.326 nm, which is indexed as (0 0 2) peak of the stacking of the conjugated aromatic system. Fig. 2 shows the FT-IR spectra of g-C3 N4 . The peak at 801 cm−1 is the characteristic absorption peak of triazine units. The peaks in the 1200–1650 cm−1 range correspond to the typical stretching modes of C N heterocycles. Additionally, the characteristic stretching vibration modes of NH at around 3300 cm−1 is observed.
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
533
Fig. 3. UV–vis DRS of g-C3 N4 .
Fig. 1. XRD patterns of g-C3 N4 .
3.2. Optical absorption The optical absorption of the as-prepared g-C3 N4 samples are measured using UV–vis diffuses reflectance spectra (DRS). As presented in Fig. 3, the band gap absorption edge of g-C3 N4 was around 450 nm. The band gap energy of g-C3 N4 estimate with the Kubelka–Munk function is about 2.7 eV, which is in good agreement with the values reported in the literature [19,29]. 3.3. Photocatalytic ozonation experiment Oxalic acid has been identified as one of the most common final oxidation products from organic compounds degradation for ozonation. The catalytic activity of g-C3 N4 was ascertained by monitoring the degradation of oxalic acid (10 mg L−1 ). The results were presented in Fig. 4. As shown in Fig. 4, the degradation ratio of oxalic acid was 9.6% for direct photolysis under visible light irradiation and 19.0% for ozonation alone, respectively. The presence of visible light or g-C3 N4 in ozonation slightly increased oxalic acid degradation efficiency. 19.1% and 20.8% of oxalic acid was removed using Vis/O3 and g-C3 N4 /O3 . By comparison, 29.4% of oxalic acid was decomposed during 120 min using g-C3 N4 under visible light irradiation. When photocatalysis and ozonation were performed
Fig. 2. FT-IR spectrum of g-C3 N4 .
Fig. 4. Degradation of oxalic acid by different processes: (ozone dose: 500 mg h−1 ; flow rate of oxygen: 1.0 L min−1 ; catalyst dose: 0.5 mg L−1 ; initial concentration of oxalic acid solution: 10 mg L−1 ; temperature: 25 ◦ C).
simultaneously, almost 80.0% of oxalic acid was removed using gC3 N4 . The decomposition efficiency of oxalic acid was 4.21 and 2.72 times as large as that with O3 and g-C3 N4 /Vis, respectively. Moreover, it was found to be 65.2% higher than the sum of the ratio when oxalic acid was individually degraded by the two methods (labeled as g-C3 N4 /Vis + O3 , in Fig. 5), which indicates the presence of synergistic effect for photocatalysis and ozonation. In order to confirm
Fig. 5. Comparison of oxalic acid removal by different processes.
534
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
Fig. 6. Oxalic acids removal efficiency in 120 min by g-C3 N4 photocatalytic ozonation reaction for four cycles.
the catalytic mechanism in the reaction, the oxidative species in the catalytic process were detected through radical trapping experiments. Tert-butanol (TBA) and triethanolamine (TEOA) were employed as the radical and hole scavenger, separately. In the presence of TBA or TEOA, the photocatalytic ozonation of oxalic acid with g-C3 N4 was performed. The results were shown in Fig. 5. During 120 min, oxalic acid removal efficiency reached 80.0% without TBA and TEOA. However, only 46.2% and 66.5% of oxalic acid was removed with the presence of 5 mg L−1 TBA and TEOA, separately. It means that both radical and hole contributed to the degradation of oxalic acid. But the photocatalytic ozonation of oxalic acid was inhibited more obviously by the addition of TBA, which suggested that • OH play a dominant role in the photocatalytic ozonation of oxalic acid. In order to investigate the stability of g-C3 N4 in photocatalytic ozonation system, g-C3 N4 was further recycled for oxalic acids degradation. The results were displayed in Fig. 6. After four consecutive experiments, the catalytic ability of g-C3 N4 did not decrease obviously. Oxalic acids removal efficiency slightly decreased from 80.1% to 76.3% and kept stable after being reused for one times, indicating that the g-C3 N4 possesses excellent catalytic activity and stability. In additional to oxalic acid removal, biphenol A was chosen as another representative model pollutant to evaluate the catalytic performance of g-C3 N4 . Fig. 7 showed the TOC removal efficiency of biphenol A as a function of reaction time. The direct photolysis of biphenol A served as control. During 120 min illumination, the removal of TOC under direct visible light irradiation was near zero. The performance was improved slightly in the presence of g-C3 N4 . About 7.1% of TOC was removed in 120 min. This was ascribed to the photocatalytic capability of g-C3 N4 under visible light irradiation. The TOC removal efficiency of biphenol A is 32.2% for ozonation alone. The presence of visible light or g-C3 N4 in ozonation slightly changed biphenol A conversion. The TOC removal efficiency was 32.5% and 27.3% using Vis/O3 and g-C3 N4 /O3 , respectively. For comparison, the performance of g-C3 N4 under visible light photocatalytic ozonation was investigated. During 120 min, 85.2% of TOC was removed by photocatalytic ozonation with g-C3 N4 , which was 12.0 and 2.65 times as great as that using g-C3 N4 /Vis and O3 , respectively. Moreover, The TOC removal of biphenol A with gC3 N4 /Vis/O3 was 2.17 times as great as the sum of the ratio when using g-C3 N4 /Vis and O3 . This enhancement was attributed to the synergistic effect of photocatalysis and ozonation.
Fig. 7. TOC removal of bisphenol A degradation by different processes (ozone dose: 500 mg h−1 ; flow rate of oxygen: 1.0 L min−1 ; catalyst dose: 0.5 mg L−1 ; initial concentration of biphenol A solution: 10 mg L−1 ; temperature: 25 ◦ C).
3.4. Discussion of mechanism of the photocatalytic ozonation with g-C3 N4 For the photocatalysis and ozonation combined system, hydroxyl radical is generally considered to be mainly responsible for the organic decomposition [16]. In present study, g-C3 N4 showed excellent catalytic performance for the degradation of organic when photocatalysis and ozonation were conducted simultaneously. Based on the results of characterization and catalytic experiment, the efficient catalytic activity could be ascribed to the enhanced synergistic effects between photocatalysis and ozonation with g-C3 N4 . A scheme for the proposed mechanism was shown in Fig. 8. Under visible light irradiation, g-C3 N4 absorbed visible light and was excited. The photo-generated electrons transferred from the valence band to the conduction band. These photo-generated electrons of g-C3 N4 possessed of more availability than other semiconductor (e.g. TiO2 ) owing to the potential of conduction band (CB) were −1.3 V versus NHE, which facilitated electron trapping by ozone. O3 + e− → O3 •−
•−
(1)
+
+ H → HO3 •
(2)
HO3 • → O2 + HO•
(3)
O3
Fig. 8. Proposed mechanism of photocatalytic ozonation for g-C3 N4 .
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
Thus the photo-generated electrons could transfer to ozone and reduce it more easily (step 1). More hydroxyl radicals would be generated via the electron transfer and further reaction (steps 2 and 3). On the other hand, the recombination of photo-generated holes and electrons was prevented, which lead to abundant holes left on the valence band of g-C3 N4 . Both the two aspects as stated above promoted the synergistic effects between photocatalysis and ozonation effectively. Consequently, the enhanced degradation of organics (oxalic acid, biphenol A) was achieved. 4. Conclusions In summary, remarkable enhancement of photocatalytic ozonation capability under visible light irradiation was achieved by g-C3 N4 . This enhanced catalytic ability mainly resulted from the enhanced synergistic effects between photocatalysis and ozonation owing to the facilitated the photo-generated electrons trapping by ozone. It is believed that g-C3 N4 provides valuable knowledge for the development of highly efficient catalyst for phtotocatalytic ozonation. Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 21207042), the Natural Science Foundation of Guangdong Province (S2011040005987). References [1] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [4] A. Wold, Photocatalytic properties of titanium dioxide (TiO2 ), Chem. Mater. 5 (1993) 280–283. [5] C. Pablos, J. Marugán, R. van Grieken, E. Serrano, Emerging micropollutant oxidation during disinfection processes using UV-C, UV-C/H2 O2 , UV-A/TiO2 and UV-A/TiO2 /H2 O2 , Water Res. 47 (2013) 1237–1245. [6] L.D. Sánchez, S.F.M. Taxt-Lamolle, E.O. Hole, TiO2 suspension exposed to H2 O2 in ambient light or darkness: degradation of methylene blue and EPR evidence for radical oxygen species, Appl. Catal. B: Environ. 142–143 (2013) 662–667. [7] T.C. Wang, N. Lu, J. Li, Y. Wu, Plasma-TiO2 catalytic method for high-efficiency remediation of p-nitrophenol contaminated soil in pulsed discharge, Environ. Sci. Technol. 45 (2011) 9301–9307. [8] H.J. Wang, J. Li, X. Quan, Y. Wu, G.F. Li, F.Z. Wang, Formation of hydrogen peroxide and degradation of phenol in synergistic system of pulsed corona discharge combined with TiO2 photocatalysis, J. Hazard. Mater. 141 (2007) 336–343. [9] Z.H. Ai, P. Yang, X.H. Lu, Degradation of 4-chlorophenol by a microwave assisted photocatalysis method, J. Hazard. Mater. 124 (2005) 147–152.
535
[10] S. Horikoshi, H. Hidaka, N. Serpone, Hydroxyl radicals in microwave photocatalysis. Enhanced formation of • OH radicals probed by ESR techniques in microwave-assisted photocatalysis in aqueous TiO2 dispersions, Chem. Phys. Lett. 376 (2003) 475–480. [11] L.S. Li, W.P. Zhu, P.Y. Zhang, Z.Y. Chen, W.Y. Han, Photocatalytic oxidation and ozonation of catechol over carbon-black-modified nano-TiO2 thin films supported on Al sheet, Water Res. 37 (2003) 3646–3651. [12] L.S. Li, W.P. Zhu, L. Chen, P.Y. Zhang, Z.Y. Chen, Photocatalytic ozonation of dibutyl phthalate over TiO2 film, J. Photocem. Photobiol. A: Chem. 175 (2005) 172–177. [13] L. SaÂnchez, J. Peral, X. DomeÁnech, Aniline degradation by combined photocatalysis and ozonation, Appl. Catal. B: Environ. 19 (1998) 59–65. [14] R.R. Giri, H. Ozaki, T. Ishida, Synergy of ozonation and photocatalysis to mineralize low concentration 2, 4-dichlorophenoxyacetic acid in aqueous solution, Chemosphere 66 (2007) 1610–1617. [15] M. Mehrjouei, S. Müller, D. Möller, Degradation of oxalic acid in a photocatalytic ozonation system by means of Pilkington ActiveTM glass, J. Photocem. Photobiol. A: Chem. 217 (2011) 417–424. [16] T.E. Agustina, H.M. Ang, V.K. Vareek, A review of synergistic effect of photocatalysis and ozonation on wastewater treatment, J. Photocem. Photobiol. C: Photochem. Rev. 6 (2005) 264–273. [17] M.D. Hernandez-Alonso, J.M. Coronado, A.J. Maira, J. Soria, V. Loddo, V. Augugliaro, Ozone enhanced activity of aqueous titanium dioxide suspensions for photocatalytic oxidation of free cyanide ions, Appl. Catal. B: Environ. 39 (2002) 257–267. • [18] W.H. Koppenol, The reduction potential of the couple O3 /O3 − , FEBS Lett. 140 (1982) 169–172. [19] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [20] G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, G.Q. Lu, H.M. Ming, Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3 N4 , J. Am. Chem. Soc. 132 (2010) 11642–11648. [21] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.O. Müller, R. Scholl, J.M. Carlson, Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts, J. Mater. Chem. 18 (2008) 4893–4908. [22] F.Z. Su, S.C. Mathew, G. Liner, X.Z. Fu, M. Antonietti, S. Belcher, X.C. Wang, mpgC3 N4 -catalyzed selective oxidation of alcohols using O2 and visible Light, J. Am. Chem. Soc. 132 (2010) 16299–16301. [23] X.C. Wang, K. Maeda, X.F. Chen, K. Takanabe, K. Domen, Y.D. Hour, X.Z. Fu, M. Antonietti, Polymer semiconductors for artificial photosynthesis: hydrogen evolution by malodorous graphitic carbon nitride with visible light, J. Am. Chem. Soc. 131 (2009) 1680–1681. [24] J.H. Zhang, G.G. Zhang, X.F. Chen, S. Lin, L. Mailman, G. Dogleg, G. Liner, M. Antonietti, S. Belcher, X.C. Wang, Co-monomer control of carbon nitride semiconductors to optimise hydrogen evolution with visible light, Anew. Chem. Int. Ed. 124 (2012) 3237–3241. [25] G.H. Dong, L.Z. Zhang, Synthesis and enhanced Cr(VI) photo reduction property of format anion containing graphitic carbon nitride, J. Phys. Chem. C 117 (2013) 4062–4068. [26] X.F. Hue, H.H. Jail, F. Chang, Y.M. Lou, Simultaneous photocatalytic Cr(VI) reduction and 2,4,6-TCP oxidation over g-C3 N4 under visible light irradiation, Catalo. Today 224 (2014) 34–40. [27] F. Goettmann, A. Thoma, M. Antonietti, Metal-free activation of CO2 by malodorous graphitic carbon nitride, Anew. Chem. Int. Edit. 46 (2007) 2717–2720. [28] M. Ansari, B. Min, Y. Mo, S. Park, CO2 activation and promotional effect in the oxidation of cyclic olefins over malodorous carbon nitrides, Green Chem. 13 (2011) 1416–1421. [29] F. Dong, L.W. Wu, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, Efficient synthesis of polymeric g-C3 N4 layered materials as novel efficient visible light driven photocatalysts, J. Mater. Chem. 21 (2011) 15171–15174.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Enhanced photocatalytic ozonation of organics by g-C3 N4 under visible light irradiation Gaozu Liao ∗ , Dongyun Zhu, Laisheng Li ∗∗ , Bingyan Lan Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of Chemistry & Environment, South China Normal University, Higher Education Mega Center, Guangzhou 510006, China
h i g h l i g h t s • • • •
g-C3 N4 is employed as active catalyst in the photocatalytic ozonation system. The more negative conduction band of g-C3 N4 benefits the transfer of electrons. The synergistic effect between photocatalysis and ozonation is promoted by g-C3 N4 . Enhanced degradation of oxalic acid and biphenol A is achieved via g-C3 N4 /Vis/O3 .
a r t i c l e
i n f o
Article history: Received 5 June 2014 Received in revised form 23 July 2014 Accepted 20 August 2014 Available online 3 September 2014 Keywords: Carbon nitride Photocatalytic ozonation Synergistic effect Hydroxyl radical
a b s t r a c t Graphitic carbon nitride (g-C3 N4 ) was employed as the active photocatalyst in the photocatalytic ozonation coupling system in the present study. g-C3 N4 was prepared by directly heating thiourea in air at 550 ◦ C. XRD, FT-IR, UV–vis was used to characterize the structure and optical property. Oxalic acid and bisphenol A were selected as model substances for photocatalytic ozonation reactions to evaluate the catalytic ability of g-C3 N4 (g-C3 N4 /Vis/O3 ). The results showed that the degradation ratio of oxalic acid with g-C3 N4 /Vis/O3 was 65.2% higher than the sum of ratio when it was individually decomposed by g-C3 N4 /Vis and O3 . The TOC removal of biphenol A with g-C3 N4 /Vis/O3 was 2.17 times as great as the sum of the ratio when using g-C3 N4 /Vis and O3 . This improvement was attributed to the enhanced synergistic effect between photocatalysis and ozonation by g-C3 N4 . Under visible light irradiation, the photo-generated electrons produced on g-C3 N4 facilitated the electrons transfer owing to the more negative conduction band potential (−1.3 V versus NHE). It meant that the photo-generated electrons could be trapped by ozone and reaction with it more easily. Subsequently, the yield of hydroxyl radicals was improved so as to enhance the organics degradation efficiency. This work indicated that metal-free g-C3 N4 could be an excellent catalyst for mineralization of organic compounds in waste control. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor mediated photocatalysis is a promising method in wastewater treatment due to its availability of solar radiation and mineralization of various refractory compounds [1,2]. However, a major disadvantage of most semiconductor materials is the high degree of recombination between photogenerated charge carriers, which ultimately decreases the photocatalytic efficiency in the redox process [3,4]. To overcome this drawback, coupling photocatalysis with other technologies has been proposed, such
∗ Corresponding author. Tel.: +86 20 3931 0213; fax: +86 20 8470 0187. ∗∗ Corresponding author. Tel.: +86 20 3931 0185; fax: +86 20 8470 0187. E-mail addresses: [email protected] (G. Liao), [email protected] (L. Li). http://dx.doi.org/10.1016/j.jhazmat.2014.08.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
as UV/TiO2 /H2 O2 [5,6], TiO2 /plasma [7,8] and UV/TiO2 /microwave [9,10]. In the combined systems, photocatalytic ozonation (UV/TiO2 /O3 ) have already been reported as the most appropriate one for mineralization of organics [11–13]. When photocatalysis and ozonation treatments are carried out simultaneously, more hydroxyl radicals (• OH) radicals will be generated for mineralization of organics due to the synergistic effect. This synergistic effect is mainly ascribed to the reaction between photo-generated electrons and ozone [14,15]. Under UV light illumination, the dissolved ozone will get electrons from semiconductor and lead to formation of the ozonide radical (O3 •− ) due to the strong electron trapping effect. This reaction not only inhibits the recombination of photo-generated charges, but also improves the utilization efficiency of dissolved ozone remarkably. Consequently, the
532
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
generated ozonide radical (O3 •− ) further transform into • OH and lead to organics degradation. Among candidates for photocatalytic ozonation, TiO2 is the most widely used photocatalyst due to its non-toxicity, good stability and excellent photocatalytic activity [16]. However, the synergistic effect between photocatalysis and ozonation is somewhat limited by the band gap energy of TiO2 . On the one hand, the wide band gap (3.2 eV) of TiO2 limits the utilization of visible light which occupies a large part of the solar light [16]. Moreover, the availability of photo-generated electrons in the photocatalyst are restricted by the position of conduction band (CB, −0.2 V versus NHE). The reactivity in the presence of ozone is related to availability of electrons in the photocatalyst [17]. The • standard reduction potential for the reaction (O3 + e− → O3 − ) is about 1.6 V versus NHE [18]. The more negative conduction band • potential of photocatalyst than the reduction potential of O3 /O3 − will facilitate the electrons trapping by ozone. Therefore, photocatalyst that possesses visible light activity and more negative potential of CB could be employed as the active catalyst to enhance the synergistic effects for photocatalytic ozonation. Recently, graphitic carbon nitride (g-C3 N4 ) has been reported as a metal-free photocatalyst for contaminant removal under visible light irradiation [19,20]. g-C3 N4 is composed of carbon and nitrogen only, which is stable under light irradiation in solution with pH 0–14 due to the strong covalent bonds between carbon and nitride atoms. In addition, the optical band gap was determined to be 2.7 eV, and the conduction band (CB) potential of g-C3 N4 was reported to be −1.3 V versus NHE [21,22]. These properties imply that g-C3 N4 not only serve as an efficient visible light photocatalyst. Moreover, the photo-electrons of g-C3 N4 generated under visible light irradiation possess strong reduction capability. Accordingly, g-C3 N4 has exhibited promising potential in the field of hydrogen production [23,24], heavy metal reduction [25,26] and CO2 reduction [27,28]. In present study, g-C3 N4 was employed as the active photocatalyst in the photocatalytic ozonation coupling system (g-C3 N4 /Vis/O3 ) to achieve efficient mineralization of organic compounds under visible light irradiation. Compared with the conventional UV/TiO2 /O3 system, we expect that the high reduction potential of photo-generated electrons could facilitate the reaction with ozone, resulting in the improvement of utilization efficiency with ozone. Meanwhile, the rapid consumption of photogenerated electrons could benefit the separation of photogenerated charge. Both the two aspects enhanced the synergistic effects between photocatalysis and ozonation, so as to improve the degradation efficiency of organics. Oxalic acid and bisphenol A were used as model substances for photocatalytic ozonation reactions to evaluate the catalytic ability of g-C3 N4 under visible light irradiation.
separately. Then the precipitate was transferred to oven and dry at 80 ◦ C for 12 h.
2.3. Characterization of g-C3 N4 The structural information for the samples was measured by Fourier transform spectrophotometer (FT-IR, Nicolet 6700) with KBr as the reference sample. The crystal structure of the samples was investigated using an X-ray diffractometer (XRD, BRUKER D8 ADVANCE) with Cu-Ka radiation. The optical absorption properties of the samples were investigated through diffuse reflectance spectra (DRS) using a UV–vis spectrophotometer (U-3010, HITACHI, Japan).
2.4. Photocatalytic ozonation experiment A 1 L glass tubular photoreactor (h = 400 mm, ˚in = 85 mm) was employed for the photocatalytic ozonation experiments. A volume of 1 L of simulated wastewater (the initial concentration of oxalic acid solution and biphenol A was 10 mg L−1 ) and 0.5 g of catalyst powder were placed in the reactor. A high pressure xenon long-arc lamp (GXH500W, Beijing NBET Technology Co., Ltd) served as the visible light source. The lamp was jacketed by a quartz thimble filled with flowing and thermostatted aqueous NaNO2 solution (1 M) between the lamp and the reaction chamber as a filter to block UV light ( < 400 nm). Ozone was produced from ozone generator (CF5G, made in China). Ozonized oxygen was continuously bubbled into the solution through a porous glass plate and flowed upward in the annular section. The flow rate of oxygen was 1 L min−1 , and the dosage of ozone was 500 mg h−1 . Samples were taken at intervals and filtered with 0.45 m microfilters to collect the filtrate. Na2 S2 O3 solution was used to stop the continuous ozonation reaction in the sample. The reaction temperature was remained at 25 ◦ C in all experiments.
2.5. Analytical methods The concentration of oxalic acid was determined by high performance liquid chromatography (Shimadzu, LC10A HPLC) with a UV detector (SPD-10AV) at 254 nm. A Diamonsil 5U C18 column (5l m, 250 mm × 4.6 mm, Dikma technologies) was used, and the analysis was carried out with an aqueous solution containing 0.018 mol/L potassium dihydrogen phosphate and 0.0025 mol/L tetrabutyl ammonium hydrogen sulfate at pH 2.5–2.7 was used as mobile phase. Flow rate was 1.0 mL/min at 25 ◦ C. Total organic carbon (TOC) was determined by a Shimadzu TOC 5000 analyzer.
2. Experiment 3. Results and discussion 2.1. Chemicals 3.1. Crystal structure of g-C3 N4 All of the reagents (analytical grade purity) were purchased from Guangzhou Chemical Reagents Factory and used without purification. Deionized water was used to prepare solutions.
2.2. Preparation of g-C3 N4 The polymeric g-C3 N4 was prepared by a facile method according to the method reported by Dong et al. [29]. In a typical synthesis, 20 g of urea powder was put into an alumina crucible with a cover, and then heated to 550 ◦ C in a muffle furnace for 2 h with a heating rate of 15 ◦ C/min. After cooling to room temperature, the resultant yellow powder was washed with ethanol and water for 2 times,
The X-ray diffraction patterns (XRD) of g-C3 N4 is presented in Fig. 1. Two peaks are present for g-C3 N4 . The peak at 13.0◦ corresponds to in-plane structural packing motif of tri-s-triazine units, which is indexed as (1 0 0) peak. The distance is calculated as d = 0.675 nm. The peak at 27.50 corresponds to interlayer stacking of aromatic segments with distance of 0.326 nm, which is indexed as (0 0 2) peak of the stacking of the conjugated aromatic system. Fig. 2 shows the FT-IR spectra of g-C3 N4 . The peak at 801 cm−1 is the characteristic absorption peak of triazine units. The peaks in the 1200–1650 cm−1 range correspond to the typical stretching modes of C N heterocycles. Additionally, the characteristic stretching vibration modes of NH at around 3300 cm−1 is observed.
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
533
Fig. 3. UV–vis DRS of g-C3 N4 .
Fig. 1. XRD patterns of g-C3 N4 .
3.2. Optical absorption The optical absorption of the as-prepared g-C3 N4 samples are measured using UV–vis diffuses reflectance spectra (DRS). As presented in Fig. 3, the band gap absorption edge of g-C3 N4 was around 450 nm. The band gap energy of g-C3 N4 estimate with the Kubelka–Munk function is about 2.7 eV, which is in good agreement with the values reported in the literature [19,29]. 3.3. Photocatalytic ozonation experiment Oxalic acid has been identified as one of the most common final oxidation products from organic compounds degradation for ozonation. The catalytic activity of g-C3 N4 was ascertained by monitoring the degradation of oxalic acid (10 mg L−1 ). The results were presented in Fig. 4. As shown in Fig. 4, the degradation ratio of oxalic acid was 9.6% for direct photolysis under visible light irradiation and 19.0% for ozonation alone, respectively. The presence of visible light or g-C3 N4 in ozonation slightly increased oxalic acid degradation efficiency. 19.1% and 20.8% of oxalic acid was removed using Vis/O3 and g-C3 N4 /O3 . By comparison, 29.4% of oxalic acid was decomposed during 120 min using g-C3 N4 under visible light irradiation. When photocatalysis and ozonation were performed
Fig. 2. FT-IR spectrum of g-C3 N4 .
Fig. 4. Degradation of oxalic acid by different processes: (ozone dose: 500 mg h−1 ; flow rate of oxygen: 1.0 L min−1 ; catalyst dose: 0.5 mg L−1 ; initial concentration of oxalic acid solution: 10 mg L−1 ; temperature: 25 ◦ C).
simultaneously, almost 80.0% of oxalic acid was removed using gC3 N4 . The decomposition efficiency of oxalic acid was 4.21 and 2.72 times as large as that with O3 and g-C3 N4 /Vis, respectively. Moreover, it was found to be 65.2% higher than the sum of the ratio when oxalic acid was individually degraded by the two methods (labeled as g-C3 N4 /Vis + O3 , in Fig. 5), which indicates the presence of synergistic effect for photocatalysis and ozonation. In order to confirm
Fig. 5. Comparison of oxalic acid removal by different processes.
534
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
Fig. 6. Oxalic acids removal efficiency in 120 min by g-C3 N4 photocatalytic ozonation reaction for four cycles.
the catalytic mechanism in the reaction, the oxidative species in the catalytic process were detected through radical trapping experiments. Tert-butanol (TBA) and triethanolamine (TEOA) were employed as the radical and hole scavenger, separately. In the presence of TBA or TEOA, the photocatalytic ozonation of oxalic acid with g-C3 N4 was performed. The results were shown in Fig. 5. During 120 min, oxalic acid removal efficiency reached 80.0% without TBA and TEOA. However, only 46.2% and 66.5% of oxalic acid was removed with the presence of 5 mg L−1 TBA and TEOA, separately. It means that both radical and hole contributed to the degradation of oxalic acid. But the photocatalytic ozonation of oxalic acid was inhibited more obviously by the addition of TBA, which suggested that • OH play a dominant role in the photocatalytic ozonation of oxalic acid. In order to investigate the stability of g-C3 N4 in photocatalytic ozonation system, g-C3 N4 was further recycled for oxalic acids degradation. The results were displayed in Fig. 6. After four consecutive experiments, the catalytic ability of g-C3 N4 did not decrease obviously. Oxalic acids removal efficiency slightly decreased from 80.1% to 76.3% and kept stable after being reused for one times, indicating that the g-C3 N4 possesses excellent catalytic activity and stability. In additional to oxalic acid removal, biphenol A was chosen as another representative model pollutant to evaluate the catalytic performance of g-C3 N4 . Fig. 7 showed the TOC removal efficiency of biphenol A as a function of reaction time. The direct photolysis of biphenol A served as control. During 120 min illumination, the removal of TOC under direct visible light irradiation was near zero. The performance was improved slightly in the presence of g-C3 N4 . About 7.1% of TOC was removed in 120 min. This was ascribed to the photocatalytic capability of g-C3 N4 under visible light irradiation. The TOC removal efficiency of biphenol A is 32.2% for ozonation alone. The presence of visible light or g-C3 N4 in ozonation slightly changed biphenol A conversion. The TOC removal efficiency was 32.5% and 27.3% using Vis/O3 and g-C3 N4 /O3 , respectively. For comparison, the performance of g-C3 N4 under visible light photocatalytic ozonation was investigated. During 120 min, 85.2% of TOC was removed by photocatalytic ozonation with g-C3 N4 , which was 12.0 and 2.65 times as great as that using g-C3 N4 /Vis and O3 , respectively. Moreover, The TOC removal of biphenol A with gC3 N4 /Vis/O3 was 2.17 times as great as the sum of the ratio when using g-C3 N4 /Vis and O3 . This enhancement was attributed to the synergistic effect of photocatalysis and ozonation.
Fig. 7. TOC removal of bisphenol A degradation by different processes (ozone dose: 500 mg h−1 ; flow rate of oxygen: 1.0 L min−1 ; catalyst dose: 0.5 mg L−1 ; initial concentration of biphenol A solution: 10 mg L−1 ; temperature: 25 ◦ C).
3.4. Discussion of mechanism of the photocatalytic ozonation with g-C3 N4 For the photocatalysis and ozonation combined system, hydroxyl radical is generally considered to be mainly responsible for the organic decomposition [16]. In present study, g-C3 N4 showed excellent catalytic performance for the degradation of organic when photocatalysis and ozonation were conducted simultaneously. Based on the results of characterization and catalytic experiment, the efficient catalytic activity could be ascribed to the enhanced synergistic effects between photocatalysis and ozonation with g-C3 N4 . A scheme for the proposed mechanism was shown in Fig. 8. Under visible light irradiation, g-C3 N4 absorbed visible light and was excited. The photo-generated electrons transferred from the valence band to the conduction band. These photo-generated electrons of g-C3 N4 possessed of more availability than other semiconductor (e.g. TiO2 ) owing to the potential of conduction band (CB) were −1.3 V versus NHE, which facilitated electron trapping by ozone. O3 + e− → O3 •−
•−
(1)
+
+ H → HO3 •
(2)
HO3 • → O2 + HO•
(3)
O3
Fig. 8. Proposed mechanism of photocatalytic ozonation for g-C3 N4 .
G. Liao et al. / Journal of Hazardous Materials 280 (2014) 531–535
Thus the photo-generated electrons could transfer to ozone and reduce it more easily (step 1). More hydroxyl radicals would be generated via the electron transfer and further reaction (steps 2 and 3). On the other hand, the recombination of photo-generated holes and electrons was prevented, which lead to abundant holes left on the valence band of g-C3 N4 . Both the two aspects as stated above promoted the synergistic effects between photocatalysis and ozonation effectively. Consequently, the enhanced degradation of organics (oxalic acid, biphenol A) was achieved. 4. Conclusions In summary, remarkable enhancement of photocatalytic ozonation capability under visible light irradiation was achieved by g-C3 N4 . This enhanced catalytic ability mainly resulted from the enhanced synergistic effects between photocatalysis and ozonation owing to the facilitated the photo-generated electrons trapping by ozone. It is believed that g-C3 N4 provides valuable knowledge for the development of highly efficient catalyst for phtotocatalytic ozonation. Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 21207042), the Natural Science Foundation of Guangdong Province (S2011040005987). References [1] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [3] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [4] A. Wold, Photocatalytic properties of titanium dioxide (TiO2 ), Chem. Mater. 5 (1993) 280–283. [5] C. Pablos, J. Marugán, R. van Grieken, E. Serrano, Emerging micropollutant oxidation during disinfection processes using UV-C, UV-C/H2 O2 , UV-A/TiO2 and UV-A/TiO2 /H2 O2 , Water Res. 47 (2013) 1237–1245. [6] L.D. Sánchez, S.F.M. Taxt-Lamolle, E.O. Hole, TiO2 suspension exposed to H2 O2 in ambient light or darkness: degradation of methylene blue and EPR evidence for radical oxygen species, Appl. Catal. B: Environ. 142–143 (2013) 662–667. [7] T.C. Wang, N. Lu, J. Li, Y. Wu, Plasma-TiO2 catalytic method for high-efficiency remediation of p-nitrophenol contaminated soil in pulsed discharge, Environ. Sci. Technol. 45 (2011) 9301–9307. [8] H.J. Wang, J. Li, X. Quan, Y. Wu, G.F. Li, F.Z. Wang, Formation of hydrogen peroxide and degradation of phenol in synergistic system of pulsed corona discharge combined with TiO2 photocatalysis, J. Hazard. Mater. 141 (2007) 336–343. [9] Z.H. Ai, P. Yang, X.H. Lu, Degradation of 4-chlorophenol by a microwave assisted photocatalysis method, J. Hazard. Mater. 124 (2005) 147–152.
535
[10] S. Horikoshi, H. Hidaka, N. Serpone, Hydroxyl radicals in microwave photocatalysis. Enhanced formation of • OH radicals probed by ESR techniques in microwave-assisted photocatalysis in aqueous TiO2 dispersions, Chem. Phys. Lett. 376 (2003) 475–480. [11] L.S. Li, W.P. Zhu, P.Y. Zhang, Z.Y. Chen, W.Y. Han, Photocatalytic oxidation and ozonation of catechol over carbon-black-modified nano-TiO2 thin films supported on Al sheet, Water Res. 37 (2003) 3646–3651. [12] L.S. Li, W.P. Zhu, L. Chen, P.Y. Zhang, Z.Y. Chen, Photocatalytic ozonation of dibutyl phthalate over TiO2 film, J. Photocem. Photobiol. A: Chem. 175 (2005) 172–177. [13] L. SaÂnchez, J. Peral, X. DomeÁnech, Aniline degradation by combined photocatalysis and ozonation, Appl. Catal. B: Environ. 19 (1998) 59–65. [14] R.R. Giri, H. Ozaki, T. Ishida, Synergy of ozonation and photocatalysis to mineralize low concentration 2, 4-dichlorophenoxyacetic acid in aqueous solution, Chemosphere 66 (2007) 1610–1617. [15] M. Mehrjouei, S. Müller, D. Möller, Degradation of oxalic acid in a photocatalytic ozonation system by means of Pilkington ActiveTM glass, J. Photocem. Photobiol. A: Chem. 217 (2011) 417–424. [16] T.E. Agustina, H.M. Ang, V.K. Vareek, A review of synergistic effect of photocatalysis and ozonation on wastewater treatment, J. Photocem. Photobiol. C: Photochem. Rev. 6 (2005) 264–273. [17] M.D. Hernandez-Alonso, J.M. Coronado, A.J. Maira, J. Soria, V. Loddo, V. Augugliaro, Ozone enhanced activity of aqueous titanium dioxide suspensions for photocatalytic oxidation of free cyanide ions, Appl. Catal. B: Environ. 39 (2002) 257–267. • [18] W.H. Koppenol, The reduction potential of the couple O3 /O3 − , FEBS Lett. 140 (1982) 169–172. [19] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [20] G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, G.Q. Lu, H.M. Ming, Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3 N4 , J. Am. Chem. Soc. 132 (2010) 11642–11648. [21] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.O. Müller, R. Scholl, J.M. Carlson, Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts, J. Mater. Chem. 18 (2008) 4893–4908. [22] F.Z. Su, S.C. Mathew, G. Liner, X.Z. Fu, M. Antonietti, S. Belcher, X.C. Wang, mpgC3 N4 -catalyzed selective oxidation of alcohols using O2 and visible Light, J. Am. Chem. Soc. 132 (2010) 16299–16301. [23] X.C. Wang, K. Maeda, X.F. Chen, K. Takanabe, K. Domen, Y.D. Hour, X.Z. Fu, M. Antonietti, Polymer semiconductors for artificial photosynthesis: hydrogen evolution by malodorous graphitic carbon nitride with visible light, J. Am. Chem. Soc. 131 (2009) 1680–1681. [24] J.H. Zhang, G.G. Zhang, X.F. Chen, S. Lin, L. Mailman, G. Dogleg, G. Liner, M. Antonietti, S. Belcher, X.C. Wang, Co-monomer control of carbon nitride semiconductors to optimise hydrogen evolution with visible light, Anew. Chem. Int. Ed. 124 (2012) 3237–3241. [25] G.H. Dong, L.Z. Zhang, Synthesis and enhanced Cr(VI) photo reduction property of format anion containing graphitic carbon nitride, J. Phys. Chem. C 117 (2013) 4062–4068. [26] X.F. Hue, H.H. Jail, F. Chang, Y.M. Lou, Simultaneous photocatalytic Cr(VI) reduction and 2,4,6-TCP oxidation over g-C3 N4 under visible light irradiation, Catalo. Today 224 (2014) 34–40. [27] F. Goettmann, A. Thoma, M. Antonietti, Metal-free activation of CO2 by malodorous graphitic carbon nitride, Anew. Chem. Int. Edit. 46 (2007) 2717–2720. [28] M. Ansari, B. Min, Y. Mo, S. Park, CO2 activation and promotional effect in the oxidation of cyclic olefins over malodorous carbon nitrides, Green Chem. 13 (2011) 1416–1421. [29] F. Dong, L.W. Wu, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, Efficient synthesis of polymeric g-C3 N4 layered materials as novel efficient visible light driven photocatalysts, J. Mater. Chem. 21 (2011) 15171–15174.
