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Selective photoreduction of nitrobenzene to aniline on TiO2 nanoparticles modified with amino acid
Journal of Hazardous Materials 178 (2010) 994–998
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Selective photoreduction of nitrobenzene to aniline on TiO2 nanoparticles modified with amino acid Heyong Huang a,b , Jiahong Zhou a,b , Hailong Liu b , Yanhuai Zhou c,∗ , Yuying Feng a,b,∗∗ a b c
Key Lab of Biofunctional Materials of Jiangsu Province, Analysis and Testing Center, Nanjing Normal University, Nanjing 210097, PR China Department of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, PR China Department of Physical Science and Technology, Nanjing Normal University, Nanjing 210097, PR China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 27 January 2010 Accepted 11 February 2010 Available online 18 February 2010 Keywords: TiO2 Photoreduction Amino acid Nitrobenzene Aniline
a b s t r a c t The photoreduction of nitrobenzene (NB) on TiO2 nanoparticles modified with asparagine (Asp), serine (Ser), phenylalanine (Phe) and tyrosine (Tyr), which were found to bind to TiO2 via carboxyl group, have been investigated under high-pressure mercury irradiation. Modification of TiO2 with Asp, Ser and Phe resulted in enhanced photocatalytic degradation rate of NB and high selective activity to aniline (AN) compared to using bare TiO2 . Furthermore, NB degradation followed a reductive approach over Asp, Ser, Phe-modified TiO2 whether in additional of methanol or not. The result indicates that modification of TiO2 with electron-donating groups is an effective way to enhance photoreduction of nitroaromatic compounds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalysis, using titanium dioxide (TiO2 ) as the photocatalyst of choice, is a useful technique for the degradation of both organic and inorganic contaminants [1–3]. The process, initiated by excitation of TiO2 with light energy greater than its band gap (3.2 eV) generates electron–hole (e− /h+ ) pairs that can be exploited in various process at the particle interface. Photogenerated carries migrate to the particle surface and participate in reduction and oxidation process. As such, the process is nonselective, some selectivity can be obtained by altering the charge on the surface of TiO2 [4–6]. Nitroaromatic compounds used widely as raw materials for munitions are notorious pollutants in water, and their detoxification is highly needed. Many treatment processes rely on an oxidative pathway due, in large part, to the high reactivity of the hydroxyl radical and the potential for complete mineralization of contaminants to nontoxic end products of carbon dioxide, nitrogen
∗ Corresponding author at: Department of Physical Science and Technology, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 85898170; fax: +86 25 85898170. ∗∗ Corresponding author at: Key Lab of Biofunctional Materials of Jiangsu Province, Analysis and Testing Center, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 85898170; fax: +86 25 85898170. E-mail addresses: [email protected] (Y. Zhou), [email protected] (Y. Feng). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.02.037
oxides, and water [7]. However, the nitrated compounds are resist to oxidative degradation, research has swayed towards the reduction of these compounds. On the other hand, from the perspective of organic synthesis, selective photocatalytic reductions can be expected because of the mild reducing power of excited electrons. Makarova et al. [8] have reported the oxidation of nitrobenzene (NB) using TiO2 photocatalyst. Brezova et al. [9] and Ferry et al. [4] reported that NB is selectively reduced to aniline (AN) using P-25 (Degussa), while the activity strongly depends on the kind of TiO2 . NB is commercially reduced with Sn/HCl, which results in a waste-disposal problem [10]. Recently that surface complexation of nanocrystalline TiO2 with some benzene derivatives [11], amino carboxylic acids [12,13] and arginine [8,14–17] resulted in enhanced reduction properties of photogenerated electrons. These works investigate surface modification of TiO2 particles with specific chelating agents that may enhance both TiO2 redox properties and adsorption of nitroaromatic compounds in order to develop a useful strategy for selective, photocatalytic removal of nitroaromatics from waste water. In this paper we report on a more integrated system for selective photocatalytic removal of NB to AN from waste streams (see Eq. (1)).
(1) Although the method is demonstrated hereby for NB, it can easily be extended to other nitroaromatic compounds in contaminated
H. Huang et al. / Journal of Hazardous Materials 178 (2010) 994–998
995
Extend C18 4.6 × 150 mm (Agilent); mobile phase H2 O–CH3 OH (1/1, v/v); flow rate = 1 mL/min; = 245 nm]. The aqueous solutions obtained upon filtration of the samples were extracted three times with CH2 Cl2 . The organic extracts were concentrated in conical vials under a stream of nitrogen and analyzed by GC/MS using a VARIAN 3800/2200 equipped with a XE-60 (25% cyanoethyl/75% dimethylpolysiloxane) 30 m × 0.25 mm capillary column. The oven temperature was programmed as follows: isothermal at 40 ◦ C for 3 min, from 40 ◦ C to 260 ◦ C at 10 ◦ C/min, and isothermal at 260 ◦ C for 1 min. The conversion of NB in the reaction process could be calculated by the following formula: Chart 1. Structures of the four types of amino acids.
aqueous waste streams. The coordination sphere of the surface titanium atoms is incomplete and thus exhibits high affinity to oxygen-containing ligands to form chelating structures. Four amino acids Asparagine (Asp), Serine (Ser), Phenylalanine (Phe), Tyrosine (Tyr) were investigated to enhance photoreduction of NB. The structures of the four types of amino acids are shown in Chart 1. Surface complexation of TiO2 nanoparticles with these four amino acid were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). Chemical analysis of the degradation byproducts was performed using UV–vis spectroscopy, high-performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC/MS). 2. Experimental 2.1. Photocatalyst preparation The synthesis of Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 nanoparticles involved the following steps: (1) 4 mL of acetic acid and 4 mL of tetrabutyl titanate (C16 H36 O4 Ti) were dissolved into 20 mL of ethanol. (2) Appropriate amounts of HCl and water were dissolved in 10 mL ethanol. (3) Homogeneous solution prepared in step 2 was dropped into the step 1 mixture. After stirring for 3 h, a steady colloid solution was obtained. (4) The colloid was first dried at 100 ◦ C and calcined in a muffle at 450 ◦ C for 3 h. Then, anatase typed TiO2 sample can be obtained. (5) Modified TiO2 catalyst was achieved by soaking the prepared TiO2 for 24 h in ethanol of Asp solution at room temperature. Once Asp was anchored to TiO2 particles, it remained bound strongly. The Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 catalysts were prepared by the same method as described above. 2.2. Characterization DRIFT measurements were performed on a Nicolet NEXUS670 Fourier transform infrared spectrometer equipped with a SpectraTech Inc. (Stamford, CT) diffuse reflectance accessory. The resolution for these experiments was 4 cm−1 . Typically, 100 scans were averaged for each spectrum. 2.3. Photoreduction of nitrobenzene TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 or Tyr-TiO2 particles were suspended in a 1.6 × 10−4 mol/L solution (15 mL) of NB in a quartz reactor. After the suspension had been purged with N2 for 30 min, irradiation (ex > 300 nm) was carried out with a 400 W highpressure mercury lamp. The reactor was placed 15 cm in front of the lamp. N2 bubbling and magnetic stirring of the suspension were continued throughout the reaction. Product analysis was performed by both UV–vis spectroscopy and high-performance liquid chromatography [HPLC measurement conditions: column = Zorbox
Conversion of NB/% =
C0 − Cn × 100 C0
where Cn is the concentration of NB measured at time t, and C0 is the initial concentration of NB prior to reaction. The selectivity of AN in the reaction process could be calculated by the following formula: Selectivity of AN/% =
Ca × 100 C0 − Cn
where Ca is the concentration of AN measured at time t. 3. Result and discussion 3.1. Infrared spectra analysis of the catalysts Infrared spectra of bare TiO2 (curve a), asparagine-modified TiO2 (curve b) are shown in Fig. 1(A). The spectrum of bare TiO2 (curve a) contains a strong absorption band in the region of 3000 cm−1 due to the stretching of adsorbed water molecules that covers somewhat weak absorption of surface hydroxyl stretching vibrations at 3450 cm−1 and a band in the 1630 cm−1 region characteristic of adsorbed water bending. The –CO2 symmetric stretching band at 1428 cm−1 in asparagine is not observed in the spectrum of asparagine-modified TiO2 (curve b). Instead, a broad band at 1382 cm−1 characteristic of –CO2 symmetric vibrations in carboxylate salts (–COOM) [18] has appeared. The asymmetric –CO2 stretching band in carboxylate salts in the 1600 cm−1 region overlaps with the strong bending mode of adsorbed water molecules on TiO2 . We have observed similar changes in serine, phenylalanine and tyrosine modified TiO2 catalysts (see Fig. 1(B)–(D)). On the basis of these results obtained with infrared spectroscopy, we suggest that asparagine, serine, phenylalanine and tyrosine bind to the TiO2 surface via the carboxyl group and that most probably the binding is bidentate as was shown previously for other chelating agents such as cysteine [12], and alanine [13] on TiO2 . 3.2. Structural analysis of the catalysts The XRD patterns of TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and TyrTiO2 are shown in Fig. 2. It can be seen that there are little difference between curves a, b, c, d and e in shape and position of the diffraction peaks. The amount of amino acid is so small that no diffraction peaks are observed in curves b, c, d, e. The results imply that the crystalline phase of TiO2 has not been changed by the modification of Asp, Ser, Phe and Tyr. 3.3. Influences of surface modification on the adsorption capability The surface adsorption of organic pollutants on the catalyst was the key factor that affect the photocatalytic efficiency of heterogeneous photocatalysis. The adsorption capability of Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 was studied and compared with
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H. Huang et al. / Journal of Hazardous Materials 178 (2010) 994–998
Fig. 1. Infrared spectra of TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 .
that of bare TiO2 are shown in Fig. 3. It was found that the adsorption capability was greatly increased at all modified TiO2 . The stronger adsorption capability is attributed to hydrogen bonding, n– and – interaction between modified TiO2 and NB [19].
Fig. 4 and Table 1 shows the result for the conversion of nitrobenzene and the selective reduction of NB over different TiO2 -based photocatalysts in the presence of CH3 OH. Ag-TiO2 was adopted to
compare the photoreduction abilities of amino acid modified TiO2 . From Fig. 4, we can find that the selectivity to AN was the highest (>95%) on the Asp-TiO2 with high conversion (>90%) of NB. It is suggest that Asp-TiO2 is more active and selective than other photocatalysts under this condition. Actually, while 13% of NB is converted in the presence of TiO2 with merely AN, adding just AgTiO2 increases the NB conversion to 86% with 70% selectivity to AN. Ser-TiO2 brings similar conversion and selectivity to Ag-TiO2 , adding Phe-TiO2 decrease the NB conversion to 69% but higher selectivity (95%). However, there is only 20% conversion observed over Try-TiO2 with no selectivity to AN.
Fig. 2. X-ray diffraction patterns of TiO2 nanoparticles (a), Asp-TiO2 (b), Ser-TiO2 (c), Phe-TiO2 (d) and Tyr-TiO2 (e).
Fig. 3. Adsorption of NB on TiO2 nanoparticles (a), Asp-TiO2 (b), Ser-TiO2 (c), PheTiO2 (d) and Tyr-TiO2 (e).
3.4. Influences of surface modification on the selective reduction of nitrobenzene
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Table 1 Photocatalytic performance of different catalysts for the NB reduction to AN in the presence of CH3 OH. Catalysts
Ag-TiO2
Asp-TiO2
Ser-TiO2
Phe-TiO2
Tyr-TiO2
TiO2
NB conversion/% AN selectivity/%
86 70
90 97
89 70
69 95
20 –
13 –
Fig. 4. NB (1.6 × 10−4 mol/L), TiO2 -based catalyst (15 mg), CH3 OH (40 L), 298 K, 1 h.
Fig. 5. NB (1.6 × 10−4 mol/L), TiO2 -based catalyst (15 mg), 298 K, 1 h.
Fig. 5 and Table 2 shows the result for the conversion of nitrobenzene and the selective reduction of NB over different TiO2 -based photocatalysts in the absence of CH3 OH. From Fig. 5 we can find that the selectivity to AN was the low (14%) on the Ag-TiO2 with conversion to 72% of NB. However, the selectivity over different amino acid modified TiO2 catalysts vary in short range from 89% to 96% with Asp/TiO2 > Ser/TiO2 > Phe/TiO2 , indicating that the reaction is influenced by the surface of TiO2 and slightly influenced by the nature of TiO2 . Conversions vary in an large range from 59% to 90% with Asp/TiO2 > Ser/TiO2 > Phe/TiO2 . Obviously, Asp/TiO2 is the most active and selective catalysts neither under condition of CH3 OH or not. Table 2 Photocatalytic performance of different catalysts for the NB reduction to AN in the absence of CH3 OH. Catalysts
Ag-TiO2
Asp-TiO2
Ser-TiO2
Phe-TiO2
NB conversion/% AN selectivity/%
20 14
90 96
70 95
59 89
Noticeably, in Ag/TiO2 system, when CH3 OH is added, the reaction proceeds with high conversion and selectivity. The loading of Ag on TiO2 enhanced both the adsorption of NB and the separation of electron-hole pairs. On the other hand, CH3 OH is well-known to act as a hole scavenger to accelerate TiO2 photocatalytic reactions. It has been reported that CH3 OH exerts a current doubling effect on the TiO2 surface in the reduction process [17]. However, in Asp/TiO2 , Ser/TiO2 , Phe/TiO2 system whether CH3 OH is added, the reaction remains with high conversion and selectivity. Especially, Asp/TiO2 brings the NB conversion to >90% with >95% selectivity. These support the conclusion that only electrons, but not • CH2 OH, are responsible for photoredution of NB over TiO2 [4]. Upon the conditions of TiO2 , merely AN were produced. This result indicates that reduction of NB by photogenerated electrons at TiO2 surface is kinetically hindered. Therefore, surface modification to bring NB close to the TiO2 surface can enhance the kinetics and the yield of NB degradation. Different amino acids were taken into modify the TiO2 for the selective reduction of NB to AN. Similar positive effect was found for modified with Asp, Ser and Phe, without addition of CH3 OH, similar phenomena of NB reduction was observed just with a smaller yield. Due to strong electron-donating properties, Asp, Ser and Phe act as a hole trap to prevent electron/hole recombination, they can provide stable surface layer with reduction degradation pathway for NB. Asp, Ser and Phe also improves the coupling between NB and TiO2 , and electrons from the conduction band of TiO2 can be transferred to NB without significant activation energy. The current doubling agents can be used to convert holes into electrons and increase the yield of photocatalytic degradation. However, the modification of TiO2 by Try brings low conversion and selectivity, it is may be Try form – donor–acceptor complexes that can’t efficient prevent electron/hole recombination. The existence of both reduction and oxidation pathways reduces the possibilities for the optimization of a catalyst. The photocatalytic stability of Asp-TiO2 nanocomposites was performed with the concentration of NB (1.6 × 10−4 mol/L), catalyst dosage (1 g/L) and irradiation time (1 h) for each cycling run. The regeneration of the photocatalyst was done by filtrating the suspension to remove the bulk solution, and drying at 60 ◦ C for 3 h. The recovered Asp-TiO2 nanocomposite were reused in the next cycle. The results show that after 10 successive cycles under the irradiation, the nitrobenzene conversion is 75% and aniline selectivity is still 80%, indicating that Asp-TiO2 nanocomposites possess excellent photocatalytic stability. 4. Conclusion In conclusion, the effects of surface modification of TiO2 with specific amino acid on photocatalytic degradation of NB was investigated in order to design a selective and effective catalyst for removal of nitroaromatic compounds from contaminated waste streams. Modification of the TiO2 surface with Asp resulted in enhanced NB adsorption and photodecomposition compared to unmodified TiO2 . NB degradation followed a reductive pathway over Asp-modified TiO2 and was enhanced upon addition of methanol. Asp improved the coupling between NB and TiO2 and facilitated the transfer of photogenerated electrons from the TiO2 conduction band to the adsorbed NB. These results demonstrate that the surface modifiers with good affinity to TiO2 surface
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and good electron-donating properties significantly enhanced photoreduction process. Due to strong electron-donating properties, electrons from the conduction of TiO2 can be easily transferred to NB and resulted in enhanced reduction of NB to AN. The study could provide a novel method for treating wastewater containing nitroaromatic compounds. Acknowledgments This work was supported by the Natural Science Foundation of China (Grant No. 20603018) and Science Foundation of Jiangsu (Grant No. BM2007132), China. We gratefully acknowledge the anonymous reviewers whose comments helped to improve the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jhazmat.2010.02.037. References [1] D.S. Bhatkhande, V.G. Pangarkar, Photocatalytic degradation of nitrobenzene using titanium dioxide and concentrated solar radiation: chemical effects and scaleup, Water Res. 37 (2003) 1223–1230. [2] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [3] T.E. Doll, F.H. Frimmel, Removal of selected persistent organic pollutants by heterogeneous photocatalysis in water, Catal. Today 101 (2005) 195–202. [4] J.L. Ferry, W.H. Glaze, Photocatalytic reduction of nitro organics over illuminated titanium dioxide: role of the TiO2 surface, Langmuir 14 (1998) 3551–3557. [5] H. Tada, K. Teranishi, Y. Inubushi, S. Ito, TiO2 photocatalytic reduction of bis(2dipyridyl) disulfide to 2-mercaptopyridine by H2 O: incorporation effect of nanometer-sized Ag particles, Chem. Commun. 21 (1998) 2345–2346.
[6] A. Maldotti, L. Andreotti, A. Molinari, S. Tollari, A. Penoni, S. Cenini, Photochemical and photocatalytic reduction of nitrobenzene in the presence of cyclohexene, J. Photochem. Photobiol. A 133 (2000) 129–133. [7] P. Bose, W.H. Glaze, D.S. Maddox, Degradation of RDX by various advanced oxidation processes. I. Reaction rates, Water Res. 32 (1998) 997–1004. [8] O.V. Makarova, T. Rajh, M.C. Thurnauer, Surface modification of TiO2 nanoparticles for photochemical reduction of nitrobenzene, Environ. Sci. Technol. 34 (2000) 4797–4806. [9] V. Brezova, A. Blazkova, I. Surina, B. Havlinova, Solvent effect on the photocatalytic reduction of 4-nitrophenol in titanium dioxide suspensions, J. Photochem. Photobiol. A 107 (1997) 233–240. [10] B. Elvers, S. Hawkins, G. Schulz (Eds.), Ullmann’s Encyclopedia of Industrial Chemistry, A 17, 5th ed., Wiley-VCH, Weinheim, 1991, p. 416. [11] J. Moser, S. Punchihewa, P.P. Infelta, M. Gratzel, Surface complexation of colloidal semiconductors strongly enhances interfacial electron-transfer rates, Langmuir 7 (1991) 3012. [12] T. Rajh, A.E. Ostafin, O.I. Micic, D.M. Tiede, M.C. Thurnauer, Surface modification of small particle TiO2 colloids with cysteine for enhanced photochemical reduction: an EPR study, J. Phys. Chem. 100 (1996) 4538–4546. [13] T. Rajh, L.X. Chen, O. Poluektov, M.C. Thurnauer, Improving optical and charge separation properties of nanocrystalline TiO2 by surface modification with Vitamin C, J. Phys. Chem. 103 (1999) 3515–3523. [14] L.R. Skubal, N.K. Meshkov, Reduction and removal of mercury from water using arginine-modified TiO2 , J. Photochem. Photobiol. A 148 (2002) 211–214. [15] W.Y. Ahn, S.A. Sheeley, T. Rajh, D.M. Cropek, Photocatalytic reduction of 4-nitrophenol with arginine-modified titanium dioxide nanoparticles, Appl. Catal. B 74 (2007) 103–110. [16] D. Cropek, P.A. Kemme, O.V. Makarova, L.X. Chen, T. Rajh, Selective photocatalytic decomposition of nitrobenzene using surface modified TiO2 nanoparticles, J. Phys. Chem. C 112 (2008) 8311–8318. [17] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, New York, 1991. [18] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [19] Y.M. Xu, C.H. Langford, UV- or visible-light-induced degradation of X3 B on TiO2 nanoparticles: the influence of adsorption, Langmuir 17 (2000) 897–902.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Selective photoreduction of nitrobenzene to aniline on TiO2 nanoparticles modified with amino acid Heyong Huang a,b , Jiahong Zhou a,b , Hailong Liu b , Yanhuai Zhou c,∗ , Yuying Feng a,b,∗∗ a b c
Key Lab of Biofunctional Materials of Jiangsu Province, Analysis and Testing Center, Nanjing Normal University, Nanjing 210097, PR China Department of Chemistry and Environmental Science, Nanjing Normal University, Nanjing 210097, PR China Department of Physical Science and Technology, Nanjing Normal University, Nanjing 210097, PR China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 27 January 2010 Accepted 11 February 2010 Available online 18 February 2010 Keywords: TiO2 Photoreduction Amino acid Nitrobenzene Aniline
a b s t r a c t The photoreduction of nitrobenzene (NB) on TiO2 nanoparticles modified with asparagine (Asp), serine (Ser), phenylalanine (Phe) and tyrosine (Tyr), which were found to bind to TiO2 via carboxyl group, have been investigated under high-pressure mercury irradiation. Modification of TiO2 with Asp, Ser and Phe resulted in enhanced photocatalytic degradation rate of NB and high selective activity to aniline (AN) compared to using bare TiO2 . Furthermore, NB degradation followed a reductive approach over Asp, Ser, Phe-modified TiO2 whether in additional of methanol or not. The result indicates that modification of TiO2 with electron-donating groups is an effective way to enhance photoreduction of nitroaromatic compounds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Heterogeneous photocatalysis, using titanium dioxide (TiO2 ) as the photocatalyst of choice, is a useful technique for the degradation of both organic and inorganic contaminants [1–3]. The process, initiated by excitation of TiO2 with light energy greater than its band gap (3.2 eV) generates electron–hole (e− /h+ ) pairs that can be exploited in various process at the particle interface. Photogenerated carries migrate to the particle surface and participate in reduction and oxidation process. As such, the process is nonselective, some selectivity can be obtained by altering the charge on the surface of TiO2 [4–6]. Nitroaromatic compounds used widely as raw materials for munitions are notorious pollutants in water, and their detoxification is highly needed. Many treatment processes rely on an oxidative pathway due, in large part, to the high reactivity of the hydroxyl radical and the potential for complete mineralization of contaminants to nontoxic end products of carbon dioxide, nitrogen
∗ Corresponding author at: Department of Physical Science and Technology, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 85898170; fax: +86 25 85898170. ∗∗ Corresponding author at: Key Lab of Biofunctional Materials of Jiangsu Province, Analysis and Testing Center, Nanjing Normal University, Nanjing 210097, PR China. Tel.: +86 25 85898170; fax: +86 25 85898170. E-mail addresses: [email protected] (Y. Zhou), [email protected] (Y. Feng). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.02.037
oxides, and water [7]. However, the nitrated compounds are resist to oxidative degradation, research has swayed towards the reduction of these compounds. On the other hand, from the perspective of organic synthesis, selective photocatalytic reductions can be expected because of the mild reducing power of excited electrons. Makarova et al. [8] have reported the oxidation of nitrobenzene (NB) using TiO2 photocatalyst. Brezova et al. [9] and Ferry et al. [4] reported that NB is selectively reduced to aniline (AN) using P-25 (Degussa), while the activity strongly depends on the kind of TiO2 . NB is commercially reduced with Sn/HCl, which results in a waste-disposal problem [10]. Recently that surface complexation of nanocrystalline TiO2 with some benzene derivatives [11], amino carboxylic acids [12,13] and arginine [8,14–17] resulted in enhanced reduction properties of photogenerated electrons. These works investigate surface modification of TiO2 particles with specific chelating agents that may enhance both TiO2 redox properties and adsorption of nitroaromatic compounds in order to develop a useful strategy for selective, photocatalytic removal of nitroaromatics from waste water. In this paper we report on a more integrated system for selective photocatalytic removal of NB to AN from waste streams (see Eq. (1)).
(1) Although the method is demonstrated hereby for NB, it can easily be extended to other nitroaromatic compounds in contaminated
H. Huang et al. / Journal of Hazardous Materials 178 (2010) 994–998
995
Extend C18 4.6 × 150 mm (Agilent); mobile phase H2 O–CH3 OH (1/1, v/v); flow rate = 1 mL/min; = 245 nm]. The aqueous solutions obtained upon filtration of the samples were extracted three times with CH2 Cl2 . The organic extracts were concentrated in conical vials under a stream of nitrogen and analyzed by GC/MS using a VARIAN 3800/2200 equipped with a XE-60 (25% cyanoethyl/75% dimethylpolysiloxane) 30 m × 0.25 mm capillary column. The oven temperature was programmed as follows: isothermal at 40 ◦ C for 3 min, from 40 ◦ C to 260 ◦ C at 10 ◦ C/min, and isothermal at 260 ◦ C for 1 min. The conversion of NB in the reaction process could be calculated by the following formula: Chart 1. Structures of the four types of amino acids.
aqueous waste streams. The coordination sphere of the surface titanium atoms is incomplete and thus exhibits high affinity to oxygen-containing ligands to form chelating structures. Four amino acids Asparagine (Asp), Serine (Ser), Phenylalanine (Phe), Tyrosine (Tyr) were investigated to enhance photoreduction of NB. The structures of the four types of amino acids are shown in Chart 1. Surface complexation of TiO2 nanoparticles with these four amino acid were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). Chemical analysis of the degradation byproducts was performed using UV–vis spectroscopy, high-performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GC/MS). 2. Experimental 2.1. Photocatalyst preparation The synthesis of Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 nanoparticles involved the following steps: (1) 4 mL of acetic acid and 4 mL of tetrabutyl titanate (C16 H36 O4 Ti) were dissolved into 20 mL of ethanol. (2) Appropriate amounts of HCl and water were dissolved in 10 mL ethanol. (3) Homogeneous solution prepared in step 2 was dropped into the step 1 mixture. After stirring for 3 h, a steady colloid solution was obtained. (4) The colloid was first dried at 100 ◦ C and calcined in a muffle at 450 ◦ C for 3 h. Then, anatase typed TiO2 sample can be obtained. (5) Modified TiO2 catalyst was achieved by soaking the prepared TiO2 for 24 h in ethanol of Asp solution at room temperature. Once Asp was anchored to TiO2 particles, it remained bound strongly. The Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 catalysts were prepared by the same method as described above. 2.2. Characterization DRIFT measurements were performed on a Nicolet NEXUS670 Fourier transform infrared spectrometer equipped with a SpectraTech Inc. (Stamford, CT) diffuse reflectance accessory. The resolution for these experiments was 4 cm−1 . Typically, 100 scans were averaged for each spectrum. 2.3. Photoreduction of nitrobenzene TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 or Tyr-TiO2 particles were suspended in a 1.6 × 10−4 mol/L solution (15 mL) of NB in a quartz reactor. After the suspension had been purged with N2 for 30 min, irradiation (ex > 300 nm) was carried out with a 400 W highpressure mercury lamp. The reactor was placed 15 cm in front of the lamp. N2 bubbling and magnetic stirring of the suspension were continued throughout the reaction. Product analysis was performed by both UV–vis spectroscopy and high-performance liquid chromatography [HPLC measurement conditions: column = Zorbox
Conversion of NB/% =
C0 − Cn × 100 C0
where Cn is the concentration of NB measured at time t, and C0 is the initial concentration of NB prior to reaction. The selectivity of AN in the reaction process could be calculated by the following formula: Selectivity of AN/% =
Ca × 100 C0 − Cn
where Ca is the concentration of AN measured at time t. 3. Result and discussion 3.1. Infrared spectra analysis of the catalysts Infrared spectra of bare TiO2 (curve a), asparagine-modified TiO2 (curve b) are shown in Fig. 1(A). The spectrum of bare TiO2 (curve a) contains a strong absorption band in the region of 3000 cm−1 due to the stretching of adsorbed water molecules that covers somewhat weak absorption of surface hydroxyl stretching vibrations at 3450 cm−1 and a band in the 1630 cm−1 region characteristic of adsorbed water bending. The –CO2 symmetric stretching band at 1428 cm−1 in asparagine is not observed in the spectrum of asparagine-modified TiO2 (curve b). Instead, a broad band at 1382 cm−1 characteristic of –CO2 symmetric vibrations in carboxylate salts (–COOM) [18] has appeared. The asymmetric –CO2 stretching band in carboxylate salts in the 1600 cm−1 region overlaps with the strong bending mode of adsorbed water molecules on TiO2 . We have observed similar changes in serine, phenylalanine and tyrosine modified TiO2 catalysts (see Fig. 1(B)–(D)). On the basis of these results obtained with infrared spectroscopy, we suggest that asparagine, serine, phenylalanine and tyrosine bind to the TiO2 surface via the carboxyl group and that most probably the binding is bidentate as was shown previously for other chelating agents such as cysteine [12], and alanine [13] on TiO2 . 3.2. Structural analysis of the catalysts The XRD patterns of TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and TyrTiO2 are shown in Fig. 2. It can be seen that there are little difference between curves a, b, c, d and e in shape and position of the diffraction peaks. The amount of amino acid is so small that no diffraction peaks are observed in curves b, c, d, e. The results imply that the crystalline phase of TiO2 has not been changed by the modification of Asp, Ser, Phe and Tyr. 3.3. Influences of surface modification on the adsorption capability The surface adsorption of organic pollutants on the catalyst was the key factor that affect the photocatalytic efficiency of heterogeneous photocatalysis. The adsorption capability of Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 was studied and compared with
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Fig. 1. Infrared spectra of TiO2 , Asp-TiO2 , Ser-TiO2 , Phe-TiO2 and Tyr-TiO2 .
that of bare TiO2 are shown in Fig. 3. It was found that the adsorption capability was greatly increased at all modified TiO2 . The stronger adsorption capability is attributed to hydrogen bonding, n– and – interaction between modified TiO2 and NB [19].
Fig. 4 and Table 1 shows the result for the conversion of nitrobenzene and the selective reduction of NB over different TiO2 -based photocatalysts in the presence of CH3 OH. Ag-TiO2 was adopted to
compare the photoreduction abilities of amino acid modified TiO2 . From Fig. 4, we can find that the selectivity to AN was the highest (>95%) on the Asp-TiO2 with high conversion (>90%) of NB. It is suggest that Asp-TiO2 is more active and selective than other photocatalysts under this condition. Actually, while 13% of NB is converted in the presence of TiO2 with merely AN, adding just AgTiO2 increases the NB conversion to 86% with 70% selectivity to AN. Ser-TiO2 brings similar conversion and selectivity to Ag-TiO2 , adding Phe-TiO2 decrease the NB conversion to 69% but higher selectivity (95%). However, there is only 20% conversion observed over Try-TiO2 with no selectivity to AN.
Fig. 2. X-ray diffraction patterns of TiO2 nanoparticles (a), Asp-TiO2 (b), Ser-TiO2 (c), Phe-TiO2 (d) and Tyr-TiO2 (e).
Fig. 3. Adsorption of NB on TiO2 nanoparticles (a), Asp-TiO2 (b), Ser-TiO2 (c), PheTiO2 (d) and Tyr-TiO2 (e).
3.4. Influences of surface modification on the selective reduction of nitrobenzene
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Table 1 Photocatalytic performance of different catalysts for the NB reduction to AN in the presence of CH3 OH. Catalysts
Ag-TiO2
Asp-TiO2
Ser-TiO2
Phe-TiO2
Tyr-TiO2
TiO2
NB conversion/% AN selectivity/%
86 70
90 97
89 70
69 95
20 –
13 –
Fig. 4. NB (1.6 × 10−4 mol/L), TiO2 -based catalyst (15 mg), CH3 OH (40 L), 298 K, 1 h.
Fig. 5. NB (1.6 × 10−4 mol/L), TiO2 -based catalyst (15 mg), 298 K, 1 h.
Fig. 5 and Table 2 shows the result for the conversion of nitrobenzene and the selective reduction of NB over different TiO2 -based photocatalysts in the absence of CH3 OH. From Fig. 5 we can find that the selectivity to AN was the low (14%) on the Ag-TiO2 with conversion to 72% of NB. However, the selectivity over different amino acid modified TiO2 catalysts vary in short range from 89% to 96% with Asp/TiO2 > Ser/TiO2 > Phe/TiO2 , indicating that the reaction is influenced by the surface of TiO2 and slightly influenced by the nature of TiO2 . Conversions vary in an large range from 59% to 90% with Asp/TiO2 > Ser/TiO2 > Phe/TiO2 . Obviously, Asp/TiO2 is the most active and selective catalysts neither under condition of CH3 OH or not. Table 2 Photocatalytic performance of different catalysts for the NB reduction to AN in the absence of CH3 OH. Catalysts
Ag-TiO2
Asp-TiO2
Ser-TiO2
Phe-TiO2
NB conversion/% AN selectivity/%
20 14
90 96
70 95
59 89
Noticeably, in Ag/TiO2 system, when CH3 OH is added, the reaction proceeds with high conversion and selectivity. The loading of Ag on TiO2 enhanced both the adsorption of NB and the separation of electron-hole pairs. On the other hand, CH3 OH is well-known to act as a hole scavenger to accelerate TiO2 photocatalytic reactions. It has been reported that CH3 OH exerts a current doubling effect on the TiO2 surface in the reduction process [17]. However, in Asp/TiO2 , Ser/TiO2 , Phe/TiO2 system whether CH3 OH is added, the reaction remains with high conversion and selectivity. Especially, Asp/TiO2 brings the NB conversion to >90% with >95% selectivity. These support the conclusion that only electrons, but not • CH2 OH, are responsible for photoredution of NB over TiO2 [4]. Upon the conditions of TiO2 , merely AN were produced. This result indicates that reduction of NB by photogenerated electrons at TiO2 surface is kinetically hindered. Therefore, surface modification to bring NB close to the TiO2 surface can enhance the kinetics and the yield of NB degradation. Different amino acids were taken into modify the TiO2 for the selective reduction of NB to AN. Similar positive effect was found for modified with Asp, Ser and Phe, without addition of CH3 OH, similar phenomena of NB reduction was observed just with a smaller yield. Due to strong electron-donating properties, Asp, Ser and Phe act as a hole trap to prevent electron/hole recombination, they can provide stable surface layer with reduction degradation pathway for NB. Asp, Ser and Phe also improves the coupling between NB and TiO2 , and electrons from the conduction band of TiO2 can be transferred to NB without significant activation energy. The current doubling agents can be used to convert holes into electrons and increase the yield of photocatalytic degradation. However, the modification of TiO2 by Try brings low conversion and selectivity, it is may be Try form – donor–acceptor complexes that can’t efficient prevent electron/hole recombination. The existence of both reduction and oxidation pathways reduces the possibilities for the optimization of a catalyst. The photocatalytic stability of Asp-TiO2 nanocomposites was performed with the concentration of NB (1.6 × 10−4 mol/L), catalyst dosage (1 g/L) and irradiation time (1 h) for each cycling run. The regeneration of the photocatalyst was done by filtrating the suspension to remove the bulk solution, and drying at 60 ◦ C for 3 h. The recovered Asp-TiO2 nanocomposite were reused in the next cycle. The results show that after 10 successive cycles under the irradiation, the nitrobenzene conversion is 75% and aniline selectivity is still 80%, indicating that Asp-TiO2 nanocomposites possess excellent photocatalytic stability. 4. Conclusion In conclusion, the effects of surface modification of TiO2 with specific amino acid on photocatalytic degradation of NB was investigated in order to design a selective and effective catalyst for removal of nitroaromatic compounds from contaminated waste streams. Modification of the TiO2 surface with Asp resulted in enhanced NB adsorption and photodecomposition compared to unmodified TiO2 . NB degradation followed a reductive pathway over Asp-modified TiO2 and was enhanced upon addition of methanol. Asp improved the coupling between NB and TiO2 and facilitated the transfer of photogenerated electrons from the TiO2 conduction band to the adsorbed NB. These results demonstrate that the surface modifiers with good affinity to TiO2 surface
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and good electron-donating properties significantly enhanced photoreduction process. Due to strong electron-donating properties, electrons from the conduction of TiO2 can be easily transferred to NB and resulted in enhanced reduction of NB to AN. The study could provide a novel method for treating wastewater containing nitroaromatic compounds. Acknowledgments This work was supported by the Natural Science Foundation of China (Grant No. 20603018) and Science Foundation of Jiangsu (Grant No. BM2007132), China. We gratefully acknowledge the anonymous reviewers whose comments helped to improve the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jhazmat.2010.02.037. References [1] D.S. Bhatkhande, V.G. Pangarkar, Photocatalytic degradation of nitrobenzene using titanium dioxide and concentrated solar radiation: chemical effects and scaleup, Water Res. 37 (2003) 1223–1230. [2] O. Carp, C.L. Huisman, A. Reller, Photoinduced reactivity of titanium dioxide, Prog. Solid State Chem. 32 (2004) 33–177. [3] T.E. Doll, F.H. Frimmel, Removal of selected persistent organic pollutants by heterogeneous photocatalysis in water, Catal. Today 101 (2005) 195–202. [4] J.L. Ferry, W.H. Glaze, Photocatalytic reduction of nitro organics over illuminated titanium dioxide: role of the TiO2 surface, Langmuir 14 (1998) 3551–3557. [5] H. Tada, K. Teranishi, Y. Inubushi, S. Ito, TiO2 photocatalytic reduction of bis(2dipyridyl) disulfide to 2-mercaptopyridine by H2 O: incorporation effect of nanometer-sized Ag particles, Chem. Commun. 21 (1998) 2345–2346.
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