- Email: [email protected]
Destruction of toluene by ozone-enhanced photocatalysis: Performance and mechanism
Applied Catalysis B: Environmental 102 (2011) 449–453
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Destruction of toluene by ozone-enhanced photocatalysis: Performance and mechanism Haibao Huang ∗ , Weibin Li Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 8 December 2010 Accepted 11 December 2010 Available online 17 December 2010 Keywords: Ozone-enhanced photocatalysis Toluene destruction Mechanism Intermediate products Destruction pathway
a b s t r a c t The enhanced performance of the ozone-enhanced photocatalysis process (O3 -PCO) on toluene destruction was evaluated and its mechanism was investigated. The toluene removal efficiency (TRE) and stability of O3 -PCO was compared with that of conventional PCO. The gaseous intermediates of toluene oxidation in various processes were analyzed by GC–MS. The formation pathways of main oxidants and their role in toluene destruction were studied. Results indicated that the TRE of O3 -PCO was 8 times higher than that of PCO because more oxidants were generated besides hydroxyl radicals (OH• ). The generation rate and amount of OH• was remarkably increased in the O3 -PCO process which contained more pathways (such as UV/O3 and O3 /TiO2 ) to produce OH• besides UV/TiO2 . The GC–MS results showed that the types and amounts of byproducts were dramatically reduced due to the formation of more oxidants and complete oxidation, leading to remarkable enhancement of the photocatalyst’s durability in the O3 -PCO process. Its mechanism was greatly different from the conventional PCO. Both OH• and active oxygen (O• ) played a key role in the O3 -PCO process. Based on the intermediates and the main oxidants, possible pathways of toluene degradation were proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are common air pollutants that cause great harm to the environment and human health [1,2]. Photocatalytic oxidation (PCO) is one of the fastest developed and most widely used technologies for VOCs control. Many studies have been conducted on the VOC destruction by PCO in the past decades [3–8]. However, its application has been greatly prohibited by the deactivation of photocatalysts [9–12], recombination of electron–hole pair [13,14] and low efficiency [9,15]. Therefore, many efforts have been made to avoid the deactivation of photocatalysts and improve the PCO rate [7,12,16]. Recently, ozone-enhanced photocatalytic oxidation (O3 -PCO) process has attracted much attention for its strong ability to improve PCO performance on VOC removal [2,9,17,18]. The recent studies are mainly focused on how to enhance efficiency and to optimize operating parameters, whereas, little attention has been paid to the investigation of intermediates of VOCs oxidation and the reasons why removal efficiency of pollutants and durability of photocatalyst was enhanced, and how pollutants were destructed in the O3 -PCO process. The mechanism in the O3 -PCO process is not well understood presently. It is much more complicated than that in the PCO pro-
∗ Corresponding author. Tel.: +86 755 26036390; fax: +86 755 26036390. E-mail address: [email protected] (H. Huang). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.12.025
cess, which involves the dominant strong oxidant of OH• [2,19–21]. In previous works, VOCs destruction was mainly attributed to the UV/TiO2 and UV/O3 process and OH• was considered as the main oxidants in the O3 -PCO process [2,9,17,22]. However, the roles of both the O3 /TiO2 process and the active oxygen (O• ) are not properly addressed. These issues represent challenges for a better comprehension of O3 -PCO process and its commercialization. The present work addressed these questions. We investigated the enhanced destruction of toluene, and its mechanism based on the intermediates and main oxidants in the O3 -PCO process. PCO and O3 -PCO were compared in terms of toluene degradation. The multiple sub-processes in the O3 -PCO system were also studied individually. To the best of our knowledge, the intermediates analysis and the study of the mechanism of toluene destruction have never been conducted in the O3 -PCO process. Intermediates are useful to assess the risk of PCO for VOC oxidation [23] as well as to study the mechanism. The proposed mechanism provided a way for better comprehension and application of the O3 -PCO process. 2. Experimental A honeycomb continuous-flow type reactor was used. It is a cubic container made of acryl glass with size of 30 cm × 15 cm × 10 cm (length × width × height). The photocatalyst mesh was fixed in the centre of the reactor. UV irradiation was provided by two low pressure mercury UV lamps (254 nm,
450
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
Philips) with maximum output of approximately 4 W. Toluene was chosen as a representative VOCs because it was one of the main indoor air pollutants as well as the major industrial emissions. The air flow rate, initial toluene and O3 concentration, and humidity of inlet gas were 2 L/min, 45 ppmv, 370 ppmv and 1 wt%, respectively. Toluene in the air stream was analyzed on-line by a gas chromatograph (GC-2010, Shimadzu) equipped with a FID and O3 was analyzed by iodimetry method (CJ/T3028.2-94). The gaseous intermediates in the outlet gas were collected by an absorption bottle filled with HPLC grade methanol (Dima Technology, USA) for 1 h after the reaction reached equilibrium. The absorption solution was analyzed by a Shimadzu GCMS-QP2010 instrument equipped with an EI ion source (electron energy 70 eV) and an electron multiplier detector enabling recording of ions from m/z equal to 10–300. A ␥-Al2 O3 /nickel foam supported TiO2 catalyst was prepared by impregnation method [24]. Prior to loading, the nickel foam was washed in 2% Na2 CO3 boiled solution and 2% oxalic acid boiled solution successively for 10 min in order to remove the dust and increase the surface area. The weight of ␥-Al2 O3 /nickel foam hybrid support and loaded TiO2 is 3.6 g and 1.0 g, respectively. The ␥Al2 O3 /nickel foam supported photocatalyst has low pressure drop and high surface-area-to-volume ratio. It possesses a pentagonal framework, and there were a lot of small holes on the surface of the nickel foam. Such 3D structure can improve the molecular transport of reactants and products. It also allows the light penetrate easily into the inner body of the catalyst to enhance the utilization efficiency of light in the photocatalytic reaction. 3. Results and discussion 3.1. Comparison between PCO and O3 -PCO Tests on toluene destruction in the PCO, O3 -PCO and subprocesses (e.g. O3 /TiO2 and UV/O3 ) were conducted under the same conditions. The TRE of different processes was compared, as shown in Fig. 1. It can be found that direct destruction of toluene by UV or O3 alone can be excluded. Toluene can hardly be decomposed directly by them due to the aromatic ring of toluene. Only a maximum of 12% TRE is achieved in the PCO process while it reached 96% in the O3 -PCO process, 8-fold that of PCO. The introduction of O3 into the PCO system could significantly improve the TRE. The TRE of O3 -PCO is much higher than the sum of TRE of PCO and O3 alone. Thus, the combination PCO with O3 exhibited an excellent synergistic effect. Fig. 1 further suggests that the photocatalyst tend to be deactivated in the UV/TiO2 process after operation for a
Fig. 2. Ozone removal efficiency in different processes under the same conditions.
period of time. The TRE of PCO starts dropping after the reaction for 35 min. It has been reported that the accumulation of intermediates generated from PCO on the surface of photocatalyst and the blocking of active sites resulted in the deactivation of photocatalysts [3,9–11,25,26]. Nevertheless, this phenomenon was not observed in the O3 -PCO process. It should be noted that the addition of O3 can also remarkably improve the durability of photocatalyst. As known, O3 -PCO process consisted of multiple sub-processes, including PCO (UV/TiO2 ), ozone photolysis (UV/O3 ), catalytic ozonation (O3 /TiO2 ) and their synergetic processes due to interaction. As shown in Fig. 1, the stable TRE in the UV/O3 and O3 /TiO2 process was 89.5% and 86.5%, respectively, under the same conditions. However, the contribution of UV/TiO2 was not significant in the O3 -PCO process. There were both heterogeneous catalysis (e.g. UV/TiO2 and O3 /TiO2 ) and O3 /UV gas-phase reaction in the O3 -PCO process. O3 is the precursor of main oxidant in the O3 -PCO process. The ozone removal efficiency (ORE) of different processes was shown in Fig. 2. The ORE of different processes is as following: O3 -PCO > O3 /TiO2 > UV/O3 . The ORE is 97.3% in the O3 -PCO process, 84.6% in the O3 /TiO2 process and 67.3% in the UV/O3 process. The higher ORE in the O3 -PCO process is understandable because O3 is mainly consumed through four pathways: destruction by photolysis, catalytic destruction, acting as electron acceptor and scavenger to OH• . Zhang et al. [22] did not take the first pathway into account to the O3 destruction. However, catalytic destruction played an important role in O3 destruction, achieving an ORE of 84.6%, as shown in Fig. 2. It can be found from Figs. 1 and 2, the TRE of UV/O3 is greater than that of O3 /TiO2 , although the ORE of the former is lower than that of the latter. The toluene oxidation in the UV/O3 and O3 /TiO2 process is attributed to homogeneous photochemical reaction and heterogeneous catalysis reaction, respectively. The mixture of O3 and toluene during reaction period is probably more uniform in homogeneous reaction when compared with that in heterogeneous catalysis reaction. Some O• reacted with O3 (O3 + O• → 2O2 ) on the surface of photocatalysts, which possibly caused the loss of active oxygen atom. The TRE was greatly influenced by the ORE, but not entirely dependent on the latter. 3.2. Intermediate products analysis
Fig. 1. Destruction of toluene in different processes under the same conditions.
Intermediate products are often used to assess the risk of VOC oxidation by PCO. They are also helpful to study the mechanism of VOC destruction. Some research has been conducted to detect the intermediates of toluene oxidation in the PCO process, and found that benzaldehyde, benzyl alcohol, benzoic acid and phenol were the main by-products [21,23,27]. However, intermediates have never been identified in the O3 -PCO process.
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453 Table 1 Intermediates and their relative abundance identified by GC–MS in different processes. Intermediates
Benzaldehyde Benzoic acid Formic acid Acetic acid Benzyl alcohol
Relative abundance, ×106 UV/TiO2
O3 -PCO
UV/O3
O3 /TiO2
0.5 – 2.7 3.9 –
0.45 0.25 – – –
0.6 0.5 7 9 0.2
0.35 – – – –
Table 1 summarizes the gaseous intermediates and their relative abundance identified by GC–MS in the PCO, O3 -PCO and its sub-processes. Benzaldehyde, formic acid and acetic acid were the main intermediates of toluene PCO, which was consistent with the results of the previous study [5,23,28]. Benzoic acid was not detected in the gaseous intermediates because it was mainly adsorbed on the surface of photocatalysts, which was confirmed by Mo et al. [23]. The intermediates generated from toluene oxidation resulted in the quick deactivation of photocatalysts in the PCO process, as shown in Fig. 1. Compared with the PCO process, both types and quantity of gaseous intermediates were greatly reduced in the toluene destruction by O3 -PCO. Only benzaldehyde and benzoic acid were detected in the O3 -PCO. The addition of O3 in the PCO process could facilitate the mineralization, leading to the formation of fewer intermediates. This further confirmed that PCO contributed little to toluene oxidation and can partly explained why the durability was remarkably enhanced in the O3 -PCO process, as shown in Fig. 1. Benzaldehyde, benzoic acid, formic acid, acetic acid and benzyl alcohol were detected in the UV/O3 process, and its quantity was the largest among the four processes, as shown in Table 1. This could be attributed to the fact that toluene oxidation by UV/O3 happened in the gas-phase and gaseous intermediates were directly discharged without being trapped by the photocatalyst mesh. It has been found that the byproducts, such as benzoic acid, benzaldehyde and benzyl alcohol, were on the surface of TiO2 [2,19–21]. The gaseous intermediates in the O3 -PCO system mainly came from UV/O3 process because little toluene was oxidized by PCO and few intermediates were formed in the O3 /TiO2 process. It was also confirmed that benzoic acid could only be yielded in both UV/O3
451
and O3 -PCO processes. In the O3 -PCO reaction, there are sufficient strong oxidants formed, leading to the more complete oxidation of toluene. In addition, the trace intermediates from the UV/O3 process can be trapped by the photocatalyst mesh and further oxidized by strong oxidants. Accordingly, intermediates such as formic acid and acetic acid may not be found in the O3 -PCO process. As shown in Table 1, the types and quantity of intermediates formed in the O3 /TiO2 and O3 -PCO process was smaller than the other processes, which indicated that O3 /TiO2 played an important role among O3 -PCO. Because if most toluene was oxidized by UV/O3 or PCO, the types and quantity of intermediates would greatly be increased, this disagreed with the GC–MS results in Table 1. 3.3. Mechanism The mechanism of toluene O3 -PCO is much more complicated than that of PCO. In the PCO process, OH• , derived from the oxidation of adsorbed water or adsorbed OH−1 , is the primary strong oxidant [19,21]. PCO reaction is typically governed by the generation of the OH• [2,9,17,22]. However, there are multiple subprocesses, including UV/TiO2 , UV/O3 , O3 /TiO2 and other processes due to interaction, in the O3 -PCO process. There are two crucial issues on the mechanism of toluene O3 -PCO: (1) What are the dominant oxidants? (2) How is toluene destructed? 3.3.1. Formation of dominant oxidants In previous works, VOCs destruction was mainly attributed to the UV/TiO2 and UV/O3 process and OH• was considered as the main oxidants in the O3 -PCO process [17,22]. However, the importance of O3 /TiO2 process and the role of O• were not properly addressed. As shown in Fig. 1, the TRE of UV/TiO2 was only 12%, whereas, it was greatly increased to 86.5% and 89.5% in the O3 /TiO2 and UV/O3 process, respectively. PCO contributed less than 12% and it was far less important than O3 /TiO2 and UV/O3 process. O3 /TiO2 , in which O• is one of the dominant oxidants, played an important role in the toluene oxidation and they should not be excluded from the mechanism of toluene oxidation in the O3 -PCO process. In the O3 -PCO process, the oxidation of toluene not only happened on the surface of photocatalyst (e.g. UV/TiO2 and O3 /TiO2 ) but also occurred in the bulk of the gas-phase (e.g. UV/O3 ). Both
Table 2 Possible pathways of oxidants formation in different processes. Process UV/TiO2
UV/O3
O3 /TiO2
Reaction steps
Stoichiometry −
TiO2 + h → TiO2 + e + h h+ + H2 O → OH• + H+ h+ + OH− → OH• O2 + e− → O• − 2O2 • − + 2H2 O → H2 O2 + 2OH− + O2 H2 O2 + e− → OH• + OH− O3 + h → O• + O2 O• + H2 O → 2OH•
TiO2
O3 + ∗ −→O• + O2 TiO2
O3 -PCO
+
(R1) (R2) (R3) (R4) (R5) (R6) (R8) (R9)
(R12)
O• + H2 O−→2OH• Formation of OH• : (1) UV/TiO2 : R1–R6 (2) O3 /electron–hole pairs:
(R13)
h+ + H2 O → OH• + H+ O 3 + e − → O3 • − H+ + O3 • − → HO3 • HO3 • → O2 + OH• (3) UV/O3 : R8, R9 (4) O3 /Ti O2 : R12, R13 Formation of O• : R8, R12
(R14) (R15) (R16) (R17)
−
+
Remarks •
3e + 3h + 2H2 O + O2 → 4OH
O3 → O• + O2 O3 + H2 O → 2OH• + O2
(R7)
(R10) (R11)
R10 R11
Zhang et al. [2], Yu et al. [9], Huang et al. [17,22]
Yu, et al. [2], Huang, et al. [9], Komoma, et al. [29]
(* represents active sites on TiO2 surface)
R7 (2)–(4): (e− +h+ ) or h or TiO2
O2 + H2 O −−−−−−−−−−−−−−−→ 2OH• + O2
h or TiO2
O3 −−−−−−→O • + O2
(R18)
(R19)
452
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
OH• and O• are the main dominant strong oxidants. In the O3 -PCO process, more oxidants were generated, resulting in the enhanced removal of toluene. Table 2 summarizes the possible reaction pathways of oxidants formation in different processes. OH• acted as the primary oxidizer for organic compounds oxidation in the PCO reaction. The possible pathways of TiO2 /UV for the OH• formation are described as R1–R6 in Table 2 and the detailed pathway of OH• formation was described in the previous study [2]. According to the stoichiometry, one oxygen molecule will react with three electron–hole pairs to generate four OH• . There were more pathways to generate OH• besides UVirradiated electron–hole pairs in the O3 -PCO process. Abundant OH• could also be produced in the UV/O3 , O3 /TiO2 and O3 /electron–hole pairs processes by the pathways R8–R9, R12–R13 and R14–R17, respectively, as shown in Table 2. O3 can efficiently be decomposed into O• by UV irradiation in the UV/O3 process and by catalytic destruction in the O3 /TiO2 process. O• can further react with H2 O to generate OH• . These reaction steps are described as R8–R9 and R12–R13 in Table 2, and the stoichiometry of UV/O3 and O3 /TiO2 reaction for OH• was described as reaction (R18). In accordance with this stoichiometry, one O3 molecule could generate two OH• and one O2 molecule. When O3 was used as the oxidants instead of O2 in the photocatalysis, both the generation rate and amount of OH• radicals were greatly enhanced. The excited electrons on the conduction band of TiO2 were captured by O3 more easily than by O2 due to the higher electron affinity of O3 . The electron affinity of O3 was relatively high (2.1 eV) compared with that of O2 (0.44 eV) [2]. It has been reported that O3 , a stronger oxidant than oxygen, is more easily reduced by photogenerated conduction electrons from TiO2 . This can improve the output of OH• through reducing the recombination rate of photogenerated electron/hole pairs [30–32]. Such introduced OH• radicals could finally improve the toluene oxidation rates.
In brief, the enhancement of O3 on photocatalysis was mainly due to four reasons: (1) more oxidant species (e.g. O• ) besides OH• ; (2) more pathways to form OH• besides electron–hole pairs; (3) reduction of the recombination of electron–hole pair due to the higher electron affinity of O3 ; (4) higher ratio of “(OH• product)/(electron–hole pairs consumption)” of O3 pathway compared to that of O2 pathway. 3.3.2. Pathways of toluene destruction The dissociation energy of the C–H bond in methyl is 3.7 eV. Since the value was smaller than the bond energies of C–H in aromatic ring (4.3 eV), C–C in methyl (4.4 eV), C–C in aromatic ring (5.0–5.3 eV) and C C in aromatic ring (5.5 eV) [33]. The primary pathway of toluene oxidation was the H-abstraction from the methyl group by OH• or O• . Some mechanism on the toluene PCO has been proposed in recent years [1,23,27]. However, they cannot well demonstrate the O3 -PCO process because the mechanism of O3 -PCO is greatly different from that of PCO. Just as mentioned above, there are two strong oxidants (OH• and O• ), resulting in two pathways to toluene destruction in the O3 -PCO process. (1) OH• as oxidant OH• was abundantly formed in the UV/TiO2 , O3 /TiO2 , and UV/O3 and O3 /electron–hole pair processes, as described in Table 2. Some mechanism has been proposed for the PCO process based on the speculation that OH• was main oxidant [5,23,27]. However, the proposed mechanism cannot fully explain the destruction of toluene due to the absence of convincing evidence and it is very complicated due to inclusion of many minor pathways. The primary pathway of toluene oxidation by OH• was the H-abstraction from the methyl group, resulting in the production of a benzyl radical and then the formation of benzyl alcohol OH
CH2OH
Ring Opening
(Benzyl alcohol)
O
COOH
CH2
CH3
H 3C
OH OH
OH
OH
O
OH
CHO
OH
OH
(Acetic acid)
Ring Opening
(Benzoic acid)
OH
OH OH
H
(Formic acid)
CO2 H2O
CO
(Toluene) OH (Benzaldehyde)
Ring Opening
(a) OH• as oxidant +O Ring Opening CHO
CH3
O
COOH H3C
OH
(Acetic acid) +O
+O - 2H (Toluene)
(Benzaldehyde)
O
+O Ring Opening
(Benzoic acid)
H
+O OH
(Formic acid) CO
(b) O• as oxidant Fig. 3. Proposed degradation pathways of toluene by: (a) OH• and (b) O• .
CO2 H2O
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
and/or benzaldehyde. As shown in Table 1, benzaldehyde was the only intermediate found in all processes OH• as dominant oxidant, which confirmed the methyl group oxidation was the main pathway of toluene oxidation by OH• . Benzyl alcohol and benzaldehyde was attacked by an OH• leading to benzoic acid. Benzaldehyde, benzoic acid and benzyl alcohol were further attacked by OH• , resulting in the opening of the aromatic ring, as proposed by Sleiman et al. [27] and Frankcombe and Smith [34]. The compounds generated after the ring opening were substances with small molecular mass, such as formic acid, acetic acid, CO, etc. The reaction was proceeded by a series of oxidation step by OH• attack, finally leading to the formation of harmless CO2 and H2 O. The aromatic rings of benzaldehyde and benzyl alcohol can also be directly attacked and opened by OH• , which agreed with the earlier study [27,35]. The reaction pathways proposed in this study are a little different from that proposed by Guo et al. [1] and Sleiman et al. [27], in which the formation of benzoic acid was the necessary pathway of aromatic rings opening. Fig. 3a presents possible degradation pathways of toluene by OH• . (2) O• as oxidant O• is commonly regarded as the main oxidant in the catalytic ozonation [36]. The primary pathway of toluene oxidation by O• is the abstraction of two hydrogen atoms from the methyl group, directly resulting in the production of benzaldehyde, without the formation of benzyl alcohol. Benzyl alcohol was not found in this study, which confirmed the proposed pathway. The solid intermediate on the TiO2 was not detected in this study, whereas, Einaga and Futamura [36] found intermediates such as formic acid, acetic acid, on the surface of catalysts in the catalytic ozonation process. Benzaldehyde was further attacked by an O• leading to benzoic acid or the direct opening of the aromatic ring. The subsequent reactions were similar to that of OH• as oxidant. The aromatic ring was possibly opened via benzaldehyde, which was confirmed by the result that benzoic acid was not detected in the O3 /TiO2 process in this study. Fig. 3b presents possible degradation pathways of toluene by O• . Compared with toluene oxidation by OH• , O• as oxidant had shorter steps and less byproducts. Benzaldehyde, formic acid, acetic acid, benzoic acid were plentifully present in the PCO and UV/O3 process, in which OH• is the main oxidant. However, only benzaldehyde was formed in the O3 /TiO2 process, in which O• was the dominant oxidant. Compared with the PCO process, more oxidants formed in the O3 -PCO process and fewer intermediates formed in the O3 /TiO2 sub-process resulted in complete oxidation of toluene and great reduction of byproducts in the O3 -PCO process, as shown in Table 1. The pathways of toluene oxidation by O• and OH• are inseparable and cooperative due to their coexisting. The oxidation of methyl is very fast by both OH• and O• and the opening of benzene rings is the rate-determining step. The proposed mechanism needs further investigation to confirm it.
453
ity of photocatalyst was greatly improved in the O3 -PCO process, compared with that of PCO. The types and amounts of gaseous byproducts also were decreased. The enhancement effect was mainly attributed to more oxidants formed besides OH• , and more pathways and higher rates of OH• generation. Both OH• and O• were the dominant oxidants for toluene oxidation in the O3 -PCO process. Based on the intermediates and main oxidants, possible pathways of toluene degradation in the O3 -PCO process were proposed. Acknowledgments Professor Dennis Leung is kindly acknowledged for his constant and stimulating discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
4. Conclusions
[34] [35]
The combination of O3 with photocatalysis efficiently enhanced the toluene destruction process. The TRE as well as the durabil-
[36]
T. Guo, Z. Bai, C. Wu, T. Zhu, Appl. Catal. B: Environ. 79 (2008) 171–178. K.P. Yu, G.W.M. Lee, Appl. Catal. B: Environ. 75 (2007) 29–38. M.C. Blount, J.L. Falconer, J. Catal. 200 (2001) 21–33. G.S. Li, D.Q. Zhang, J.C. Yu, Environ. Sci. Technol. 43 (2009) 7079–7085. O. D’Hennezel, P. Pichat, D.F. Ollis, J. Photochem. Photobiol. A: Chem. 118 (1998) 197–204. T. Noguchi, A. Fujishima, Environ. Sci. Technol. 32 (1998) 3831–3833. Y. Luo, L. Zou, E. Hu, Environ. Technol. 27 (2006) 359–366. H.P. Kuo, C.T. Wu, R.C. Hsu, Powder Technol. 195 (2009) 50–56. X. Huang, J. Yuan, J. Shi, W. Shangguan, J. Hazard. Mater. 191 (2009) 827–832. E. Piera, J.A. Ayllon, X. Domenech, J. Peral. Catal. Today 76 (2002) 259–270. J. Peral, D.F. Ollis, J. Mol. Catal. A: Chem. 115 (1997) 347–354. C. Belver, M.J. Lopez-Munoz, J.M. Coronado, J. Soria, Appl. Catal. B: Environ. 46 (2003) 497–509. L. Rideh, A. Wehrer, D. Ronze, A. Zoulalian, Ind. Eng. Chem. Res. 36 (1997) 4712–4718. S. Peng, Y. Li, F. Jiang, G. Lu, S. Li, Chem. Phys. Lett. 398 (2004) 235–239. C.H. Ao, S.C. Lee, J.Z. Yu, J.H. Xu, Appl. Catal. B: Environ. 54 (2004) 41–50. L. Zou, Y.G. Luo, M. Hooper, E. Hu, Chem. Eng. Process. 45 (2006) 959–964. P.Y. Zhang, J. Liu, J. Photochem. Photobiol. A: Chem. 167 (2004) 87–94. Y.S. Shen, Y. Ku, Chemosphere 46 (2002) 101–107. D.-R. Park, J. Zhang, K. Ikeue, H. Yamashita, M. Anpo, J. Catal. 185 (1999) 114–119. V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano, M. Schiavello, Appl. Catal. B: Environ. 20 (1999) 15–27. J. Zhao, X.D. Yang, Build. Environ. 38 (2003) 645–654. P.Y. Zhang, F.Y. Liang, G. Yu, Q. Chen, W.P. Zhu, J. Photochem. Photobiol. A: Chem. 156 (2003) 189–194. J. Mo, Y. Zhang, Q. Xu, Y. Zhu, J.J. Lamson, R. Zhao, Appl. Catal. B: Environ. 89 (2009) 570–576. H. Huang, D. Ye, D. Leung, J. Environ. Eng. 1 (2010) 195. M. Lewandowski, D.F. Ollis, Appl. Catal. B: Environ. 45 (2003) 223–238. L.X. Cao, Z. Gao, S.L. Suib, J. Catal. 220 (2000) 269–1269. M. Sleiman, P. Conchon, C. Ferronato, J.-M. Chovelon, Appl. Catal. B: Environ. 86 (2009) 159–165. Y. Irokawa, T. Morikawa, K. Aoki, S. Kosaka, T. Ohwaki, Y. Taga, Phys. Chem. Chem. Phys. 8 (2006) 1116–1121. M. Kogoma, Y. Miki, K. Tanaka, K. Takahashi, Plasma Process. Polym. 3 (2006) 727–733. B. Kasprzyk-Hordern, M. Ziolek, J. Nawrocki, Appl. Catal. B: Environ. 46 (2003) 639–669. M.M. Ye, Z.L. Chen, X.W. Liu, Y. Ben, J.M. Shen, J. Hazard. Mater. 167 (2009) 1021–1027. V. Augugliaro, M. Litter, L. Palmisano, J. Soria, J. Photochem. Photobiol. C: Photochem. Rev. 7 (2006) 127–144. A. Ogata, D. Ito, K. Mizuno, S. Kushiyama, A. Gal, T. Yamamoto, Appl. Catal. A: Gen. 236 (2002) 9–15. T.J. Frankcombe, S.C. Smith, J. Phys. Chem. A 111 (2007) 3686–3690. J. Van Durme, J. Dewulf, W. Sysmans, C. Leys, H. Van Langenhove, Chemosphere 68 (2007) 1821–1829. H. Einaga, S. Futamura, J. Catal. 227 (2004) 304–312.
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Destruction of toluene by ozone-enhanced photocatalysis: Performance and mechanism Haibao Huang ∗ , Weibin Li Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 8 December 2010 Accepted 11 December 2010 Available online 17 December 2010 Keywords: Ozone-enhanced photocatalysis Toluene destruction Mechanism Intermediate products Destruction pathway
a b s t r a c t The enhanced performance of the ozone-enhanced photocatalysis process (O3 -PCO) on toluene destruction was evaluated and its mechanism was investigated. The toluene removal efficiency (TRE) and stability of O3 -PCO was compared with that of conventional PCO. The gaseous intermediates of toluene oxidation in various processes were analyzed by GC–MS. The formation pathways of main oxidants and their role in toluene destruction were studied. Results indicated that the TRE of O3 -PCO was 8 times higher than that of PCO because more oxidants were generated besides hydroxyl radicals (OH• ). The generation rate and amount of OH• was remarkably increased in the O3 -PCO process which contained more pathways (such as UV/O3 and O3 /TiO2 ) to produce OH• besides UV/TiO2 . The GC–MS results showed that the types and amounts of byproducts were dramatically reduced due to the formation of more oxidants and complete oxidation, leading to remarkable enhancement of the photocatalyst’s durability in the O3 -PCO process. Its mechanism was greatly different from the conventional PCO. Both OH• and active oxygen (O• ) played a key role in the O3 -PCO process. Based on the intermediates and the main oxidants, possible pathways of toluene degradation were proposed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are common air pollutants that cause great harm to the environment and human health [1,2]. Photocatalytic oxidation (PCO) is one of the fastest developed and most widely used technologies for VOCs control. Many studies have been conducted on the VOC destruction by PCO in the past decades [3–8]. However, its application has been greatly prohibited by the deactivation of photocatalysts [9–12], recombination of electron–hole pair [13,14] and low efficiency [9,15]. Therefore, many efforts have been made to avoid the deactivation of photocatalysts and improve the PCO rate [7,12,16]. Recently, ozone-enhanced photocatalytic oxidation (O3 -PCO) process has attracted much attention for its strong ability to improve PCO performance on VOC removal [2,9,17,18]. The recent studies are mainly focused on how to enhance efficiency and to optimize operating parameters, whereas, little attention has been paid to the investigation of intermediates of VOCs oxidation and the reasons why removal efficiency of pollutants and durability of photocatalyst was enhanced, and how pollutants were destructed in the O3 -PCO process. The mechanism in the O3 -PCO process is not well understood presently. It is much more complicated than that in the PCO pro-
∗ Corresponding author. Tel.: +86 755 26036390; fax: +86 755 26036390. E-mail address: [email protected] (H. Huang). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.12.025
cess, which involves the dominant strong oxidant of OH• [2,19–21]. In previous works, VOCs destruction was mainly attributed to the UV/TiO2 and UV/O3 process and OH• was considered as the main oxidants in the O3 -PCO process [2,9,17,22]. However, the roles of both the O3 /TiO2 process and the active oxygen (O• ) are not properly addressed. These issues represent challenges for a better comprehension of O3 -PCO process and its commercialization. The present work addressed these questions. We investigated the enhanced destruction of toluene, and its mechanism based on the intermediates and main oxidants in the O3 -PCO process. PCO and O3 -PCO were compared in terms of toluene degradation. The multiple sub-processes in the O3 -PCO system were also studied individually. To the best of our knowledge, the intermediates analysis and the study of the mechanism of toluene destruction have never been conducted in the O3 -PCO process. Intermediates are useful to assess the risk of PCO for VOC oxidation [23] as well as to study the mechanism. The proposed mechanism provided a way for better comprehension and application of the O3 -PCO process. 2. Experimental A honeycomb continuous-flow type reactor was used. It is a cubic container made of acryl glass with size of 30 cm × 15 cm × 10 cm (length × width × height). The photocatalyst mesh was fixed in the centre of the reactor. UV irradiation was provided by two low pressure mercury UV lamps (254 nm,
450
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
Philips) with maximum output of approximately 4 W. Toluene was chosen as a representative VOCs because it was one of the main indoor air pollutants as well as the major industrial emissions. The air flow rate, initial toluene and O3 concentration, and humidity of inlet gas were 2 L/min, 45 ppmv, 370 ppmv and 1 wt%, respectively. Toluene in the air stream was analyzed on-line by a gas chromatograph (GC-2010, Shimadzu) equipped with a FID and O3 was analyzed by iodimetry method (CJ/T3028.2-94). The gaseous intermediates in the outlet gas were collected by an absorption bottle filled with HPLC grade methanol (Dima Technology, USA) for 1 h after the reaction reached equilibrium. The absorption solution was analyzed by a Shimadzu GCMS-QP2010 instrument equipped with an EI ion source (electron energy 70 eV) and an electron multiplier detector enabling recording of ions from m/z equal to 10–300. A ␥-Al2 O3 /nickel foam supported TiO2 catalyst was prepared by impregnation method [24]. Prior to loading, the nickel foam was washed in 2% Na2 CO3 boiled solution and 2% oxalic acid boiled solution successively for 10 min in order to remove the dust and increase the surface area. The weight of ␥-Al2 O3 /nickel foam hybrid support and loaded TiO2 is 3.6 g and 1.0 g, respectively. The ␥Al2 O3 /nickel foam supported photocatalyst has low pressure drop and high surface-area-to-volume ratio. It possesses a pentagonal framework, and there were a lot of small holes on the surface of the nickel foam. Such 3D structure can improve the molecular transport of reactants and products. It also allows the light penetrate easily into the inner body of the catalyst to enhance the utilization efficiency of light in the photocatalytic reaction. 3. Results and discussion 3.1. Comparison between PCO and O3 -PCO Tests on toluene destruction in the PCO, O3 -PCO and subprocesses (e.g. O3 /TiO2 and UV/O3 ) were conducted under the same conditions. The TRE of different processes was compared, as shown in Fig. 1. It can be found that direct destruction of toluene by UV or O3 alone can be excluded. Toluene can hardly be decomposed directly by them due to the aromatic ring of toluene. Only a maximum of 12% TRE is achieved in the PCO process while it reached 96% in the O3 -PCO process, 8-fold that of PCO. The introduction of O3 into the PCO system could significantly improve the TRE. The TRE of O3 -PCO is much higher than the sum of TRE of PCO and O3 alone. Thus, the combination PCO with O3 exhibited an excellent synergistic effect. Fig. 1 further suggests that the photocatalyst tend to be deactivated in the UV/TiO2 process after operation for a
Fig. 2. Ozone removal efficiency in different processes under the same conditions.
period of time. The TRE of PCO starts dropping after the reaction for 35 min. It has been reported that the accumulation of intermediates generated from PCO on the surface of photocatalyst and the blocking of active sites resulted in the deactivation of photocatalysts [3,9–11,25,26]. Nevertheless, this phenomenon was not observed in the O3 -PCO process. It should be noted that the addition of O3 can also remarkably improve the durability of photocatalyst. As known, O3 -PCO process consisted of multiple sub-processes, including PCO (UV/TiO2 ), ozone photolysis (UV/O3 ), catalytic ozonation (O3 /TiO2 ) and their synergetic processes due to interaction. As shown in Fig. 1, the stable TRE in the UV/O3 and O3 /TiO2 process was 89.5% and 86.5%, respectively, under the same conditions. However, the contribution of UV/TiO2 was not significant in the O3 -PCO process. There were both heterogeneous catalysis (e.g. UV/TiO2 and O3 /TiO2 ) and O3 /UV gas-phase reaction in the O3 -PCO process. O3 is the precursor of main oxidant in the O3 -PCO process. The ozone removal efficiency (ORE) of different processes was shown in Fig. 2. The ORE of different processes is as following: O3 -PCO > O3 /TiO2 > UV/O3 . The ORE is 97.3% in the O3 -PCO process, 84.6% in the O3 /TiO2 process and 67.3% in the UV/O3 process. The higher ORE in the O3 -PCO process is understandable because O3 is mainly consumed through four pathways: destruction by photolysis, catalytic destruction, acting as electron acceptor and scavenger to OH• . Zhang et al. [22] did not take the first pathway into account to the O3 destruction. However, catalytic destruction played an important role in O3 destruction, achieving an ORE of 84.6%, as shown in Fig. 2. It can be found from Figs. 1 and 2, the TRE of UV/O3 is greater than that of O3 /TiO2 , although the ORE of the former is lower than that of the latter. The toluene oxidation in the UV/O3 and O3 /TiO2 process is attributed to homogeneous photochemical reaction and heterogeneous catalysis reaction, respectively. The mixture of O3 and toluene during reaction period is probably more uniform in homogeneous reaction when compared with that in heterogeneous catalysis reaction. Some O• reacted with O3 (O3 + O• → 2O2 ) on the surface of photocatalysts, which possibly caused the loss of active oxygen atom. The TRE was greatly influenced by the ORE, but not entirely dependent on the latter. 3.2. Intermediate products analysis
Fig. 1. Destruction of toluene in different processes under the same conditions.
Intermediate products are often used to assess the risk of VOC oxidation by PCO. They are also helpful to study the mechanism of VOC destruction. Some research has been conducted to detect the intermediates of toluene oxidation in the PCO process, and found that benzaldehyde, benzyl alcohol, benzoic acid and phenol were the main by-products [21,23,27]. However, intermediates have never been identified in the O3 -PCO process.
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453 Table 1 Intermediates and their relative abundance identified by GC–MS in different processes. Intermediates
Benzaldehyde Benzoic acid Formic acid Acetic acid Benzyl alcohol
Relative abundance, ×106 UV/TiO2
O3 -PCO
UV/O3
O3 /TiO2
0.5 – 2.7 3.9 –
0.45 0.25 – – –
0.6 0.5 7 9 0.2
0.35 – – – –
Table 1 summarizes the gaseous intermediates and their relative abundance identified by GC–MS in the PCO, O3 -PCO and its sub-processes. Benzaldehyde, formic acid and acetic acid were the main intermediates of toluene PCO, which was consistent with the results of the previous study [5,23,28]. Benzoic acid was not detected in the gaseous intermediates because it was mainly adsorbed on the surface of photocatalysts, which was confirmed by Mo et al. [23]. The intermediates generated from toluene oxidation resulted in the quick deactivation of photocatalysts in the PCO process, as shown in Fig. 1. Compared with the PCO process, both types and quantity of gaseous intermediates were greatly reduced in the toluene destruction by O3 -PCO. Only benzaldehyde and benzoic acid were detected in the O3 -PCO. The addition of O3 in the PCO process could facilitate the mineralization, leading to the formation of fewer intermediates. This further confirmed that PCO contributed little to toluene oxidation and can partly explained why the durability was remarkably enhanced in the O3 -PCO process, as shown in Fig. 1. Benzaldehyde, benzoic acid, formic acid, acetic acid and benzyl alcohol were detected in the UV/O3 process, and its quantity was the largest among the four processes, as shown in Table 1. This could be attributed to the fact that toluene oxidation by UV/O3 happened in the gas-phase and gaseous intermediates were directly discharged without being trapped by the photocatalyst mesh. It has been found that the byproducts, such as benzoic acid, benzaldehyde and benzyl alcohol, were on the surface of TiO2 [2,19–21]. The gaseous intermediates in the O3 -PCO system mainly came from UV/O3 process because little toluene was oxidized by PCO and few intermediates were formed in the O3 /TiO2 process. It was also confirmed that benzoic acid could only be yielded in both UV/O3
451
and O3 -PCO processes. In the O3 -PCO reaction, there are sufficient strong oxidants formed, leading to the more complete oxidation of toluene. In addition, the trace intermediates from the UV/O3 process can be trapped by the photocatalyst mesh and further oxidized by strong oxidants. Accordingly, intermediates such as formic acid and acetic acid may not be found in the O3 -PCO process. As shown in Table 1, the types and quantity of intermediates formed in the O3 /TiO2 and O3 -PCO process was smaller than the other processes, which indicated that O3 /TiO2 played an important role among O3 -PCO. Because if most toluene was oxidized by UV/O3 or PCO, the types and quantity of intermediates would greatly be increased, this disagreed with the GC–MS results in Table 1. 3.3. Mechanism The mechanism of toluene O3 -PCO is much more complicated than that of PCO. In the PCO process, OH• , derived from the oxidation of adsorbed water or adsorbed OH−1 , is the primary strong oxidant [19,21]. PCO reaction is typically governed by the generation of the OH• [2,9,17,22]. However, there are multiple subprocesses, including UV/TiO2 , UV/O3 , O3 /TiO2 and other processes due to interaction, in the O3 -PCO process. There are two crucial issues on the mechanism of toluene O3 -PCO: (1) What are the dominant oxidants? (2) How is toluene destructed? 3.3.1. Formation of dominant oxidants In previous works, VOCs destruction was mainly attributed to the UV/TiO2 and UV/O3 process and OH• was considered as the main oxidants in the O3 -PCO process [17,22]. However, the importance of O3 /TiO2 process and the role of O• were not properly addressed. As shown in Fig. 1, the TRE of UV/TiO2 was only 12%, whereas, it was greatly increased to 86.5% and 89.5% in the O3 /TiO2 and UV/O3 process, respectively. PCO contributed less than 12% and it was far less important than O3 /TiO2 and UV/O3 process. O3 /TiO2 , in which O• is one of the dominant oxidants, played an important role in the toluene oxidation and they should not be excluded from the mechanism of toluene oxidation in the O3 -PCO process. In the O3 -PCO process, the oxidation of toluene not only happened on the surface of photocatalyst (e.g. UV/TiO2 and O3 /TiO2 ) but also occurred in the bulk of the gas-phase (e.g. UV/O3 ). Both
Table 2 Possible pathways of oxidants formation in different processes. Process UV/TiO2
UV/O3
O3 /TiO2
Reaction steps
Stoichiometry −
TiO2 + h → TiO2 + e + h h+ + H2 O → OH• + H+ h+ + OH− → OH• O2 + e− → O• − 2O2 • − + 2H2 O → H2 O2 + 2OH− + O2 H2 O2 + e− → OH• + OH− O3 + h → O• + O2 O• + H2 O → 2OH•
TiO2
O3 + ∗ −→O• + O2 TiO2
O3 -PCO
+
(R1) (R2) (R3) (R4) (R5) (R6) (R8) (R9)
(R12)
O• + H2 O−→2OH• Formation of OH• : (1) UV/TiO2 : R1–R6 (2) O3 /electron–hole pairs:
(R13)
h+ + H2 O → OH• + H+ O 3 + e − → O3 • − H+ + O3 • − → HO3 • HO3 • → O2 + OH• (3) UV/O3 : R8, R9 (4) O3 /Ti O2 : R12, R13 Formation of O• : R8, R12
(R14) (R15) (R16) (R17)
−
+
Remarks •
3e + 3h + 2H2 O + O2 → 4OH
O3 → O• + O2 O3 + H2 O → 2OH• + O2
(R7)
(R10) (R11)
R10 R11
Zhang et al. [2], Yu et al. [9], Huang et al. [17,22]
Yu, et al. [2], Huang, et al. [9], Komoma, et al. [29]
(* represents active sites on TiO2 surface)
R7 (2)–(4): (e− +h+ ) or h or TiO2
O2 + H2 O −−−−−−−−−−−−−−−→ 2OH• + O2
h or TiO2
O3 −−−−−−→O • + O2
(R18)
(R19)
452
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
OH• and O• are the main dominant strong oxidants. In the O3 -PCO process, more oxidants were generated, resulting in the enhanced removal of toluene. Table 2 summarizes the possible reaction pathways of oxidants formation in different processes. OH• acted as the primary oxidizer for organic compounds oxidation in the PCO reaction. The possible pathways of TiO2 /UV for the OH• formation are described as R1–R6 in Table 2 and the detailed pathway of OH• formation was described in the previous study [2]. According to the stoichiometry, one oxygen molecule will react with three electron–hole pairs to generate four OH• . There were more pathways to generate OH• besides UVirradiated electron–hole pairs in the O3 -PCO process. Abundant OH• could also be produced in the UV/O3 , O3 /TiO2 and O3 /electron–hole pairs processes by the pathways R8–R9, R12–R13 and R14–R17, respectively, as shown in Table 2. O3 can efficiently be decomposed into O• by UV irradiation in the UV/O3 process and by catalytic destruction in the O3 /TiO2 process. O• can further react with H2 O to generate OH• . These reaction steps are described as R8–R9 and R12–R13 in Table 2, and the stoichiometry of UV/O3 and O3 /TiO2 reaction for OH• was described as reaction (R18). In accordance with this stoichiometry, one O3 molecule could generate two OH• and one O2 molecule. When O3 was used as the oxidants instead of O2 in the photocatalysis, both the generation rate and amount of OH• radicals were greatly enhanced. The excited electrons on the conduction band of TiO2 were captured by O3 more easily than by O2 due to the higher electron affinity of O3 . The electron affinity of O3 was relatively high (2.1 eV) compared with that of O2 (0.44 eV) [2]. It has been reported that O3 , a stronger oxidant than oxygen, is more easily reduced by photogenerated conduction electrons from TiO2 . This can improve the output of OH• through reducing the recombination rate of photogenerated electron/hole pairs [30–32]. Such introduced OH• radicals could finally improve the toluene oxidation rates.
In brief, the enhancement of O3 on photocatalysis was mainly due to four reasons: (1) more oxidant species (e.g. O• ) besides OH• ; (2) more pathways to form OH• besides electron–hole pairs; (3) reduction of the recombination of electron–hole pair due to the higher electron affinity of O3 ; (4) higher ratio of “(OH• product)/(electron–hole pairs consumption)” of O3 pathway compared to that of O2 pathway. 3.3.2. Pathways of toluene destruction The dissociation energy of the C–H bond in methyl is 3.7 eV. Since the value was smaller than the bond energies of C–H in aromatic ring (4.3 eV), C–C in methyl (4.4 eV), C–C in aromatic ring (5.0–5.3 eV) and C C in aromatic ring (5.5 eV) [33]. The primary pathway of toluene oxidation was the H-abstraction from the methyl group by OH• or O• . Some mechanism on the toluene PCO has been proposed in recent years [1,23,27]. However, they cannot well demonstrate the O3 -PCO process because the mechanism of O3 -PCO is greatly different from that of PCO. Just as mentioned above, there are two strong oxidants (OH• and O• ), resulting in two pathways to toluene destruction in the O3 -PCO process. (1) OH• as oxidant OH• was abundantly formed in the UV/TiO2 , O3 /TiO2 , and UV/O3 and O3 /electron–hole pair processes, as described in Table 2. Some mechanism has been proposed for the PCO process based on the speculation that OH• was main oxidant [5,23,27]. However, the proposed mechanism cannot fully explain the destruction of toluene due to the absence of convincing evidence and it is very complicated due to inclusion of many minor pathways. The primary pathway of toluene oxidation by OH• was the H-abstraction from the methyl group, resulting in the production of a benzyl radical and then the formation of benzyl alcohol OH
CH2OH
Ring Opening
(Benzyl alcohol)
O
COOH
CH2
CH3
H 3C
OH OH
OH
OH
O
OH
CHO
OH
OH
(Acetic acid)
Ring Opening
(Benzoic acid)
OH
OH OH
H
(Formic acid)
CO2 H2O
CO
(Toluene) OH (Benzaldehyde)
Ring Opening
(a) OH• as oxidant +O Ring Opening CHO
CH3
O
COOH H3C
OH
(Acetic acid) +O
+O - 2H (Toluene)
(Benzaldehyde)
O
+O Ring Opening
(Benzoic acid)
H
+O OH
(Formic acid) CO
(b) O• as oxidant Fig. 3. Proposed degradation pathways of toluene by: (a) OH• and (b) O• .
CO2 H2O
H. Huang, W. Li / Applied Catalysis B: Environmental 102 (2011) 449–453
and/or benzaldehyde. As shown in Table 1, benzaldehyde was the only intermediate found in all processes OH• as dominant oxidant, which confirmed the methyl group oxidation was the main pathway of toluene oxidation by OH• . Benzyl alcohol and benzaldehyde was attacked by an OH• leading to benzoic acid. Benzaldehyde, benzoic acid and benzyl alcohol were further attacked by OH• , resulting in the opening of the aromatic ring, as proposed by Sleiman et al. [27] and Frankcombe and Smith [34]. The compounds generated after the ring opening were substances with small molecular mass, such as formic acid, acetic acid, CO, etc. The reaction was proceeded by a series of oxidation step by OH• attack, finally leading to the formation of harmless CO2 and H2 O. The aromatic rings of benzaldehyde and benzyl alcohol can also be directly attacked and opened by OH• , which agreed with the earlier study [27,35]. The reaction pathways proposed in this study are a little different from that proposed by Guo et al. [1] and Sleiman et al. [27], in which the formation of benzoic acid was the necessary pathway of aromatic rings opening. Fig. 3a presents possible degradation pathways of toluene by OH• . (2) O• as oxidant O• is commonly regarded as the main oxidant in the catalytic ozonation [36]. The primary pathway of toluene oxidation by O• is the abstraction of two hydrogen atoms from the methyl group, directly resulting in the production of benzaldehyde, without the formation of benzyl alcohol. Benzyl alcohol was not found in this study, which confirmed the proposed pathway. The solid intermediate on the TiO2 was not detected in this study, whereas, Einaga and Futamura [36] found intermediates such as formic acid, acetic acid, on the surface of catalysts in the catalytic ozonation process. Benzaldehyde was further attacked by an O• leading to benzoic acid or the direct opening of the aromatic ring. The subsequent reactions were similar to that of OH• as oxidant. The aromatic ring was possibly opened via benzaldehyde, which was confirmed by the result that benzoic acid was not detected in the O3 /TiO2 process in this study. Fig. 3b presents possible degradation pathways of toluene by O• . Compared with toluene oxidation by OH• , O• as oxidant had shorter steps and less byproducts. Benzaldehyde, formic acid, acetic acid, benzoic acid were plentifully present in the PCO and UV/O3 process, in which OH• is the main oxidant. However, only benzaldehyde was formed in the O3 /TiO2 process, in which O• was the dominant oxidant. Compared with the PCO process, more oxidants formed in the O3 -PCO process and fewer intermediates formed in the O3 /TiO2 sub-process resulted in complete oxidation of toluene and great reduction of byproducts in the O3 -PCO process, as shown in Table 1. The pathways of toluene oxidation by O• and OH• are inseparable and cooperative due to their coexisting. The oxidation of methyl is very fast by both OH• and O• and the opening of benzene rings is the rate-determining step. The proposed mechanism needs further investigation to confirm it.
453
ity of photocatalyst was greatly improved in the O3 -PCO process, compared with that of PCO. The types and amounts of gaseous byproducts also were decreased. The enhancement effect was mainly attributed to more oxidants formed besides OH• , and more pathways and higher rates of OH• generation. Both OH• and O• were the dominant oxidants for toluene oxidation in the O3 -PCO process. Based on the intermediates and main oxidants, possible pathways of toluene degradation in the O3 -PCO process were proposed. Acknowledgments Professor Dennis Leung is kindly acknowledged for his constant and stimulating discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
4. Conclusions
[34] [35]
The combination of O3 with photocatalysis efficiently enhanced the toluene destruction process. The TRE as well as the durabil-
[36]
T. Guo, Z. Bai, C. Wu, T. Zhu, Appl. Catal. B: Environ. 79 (2008) 171–178. K.P. Yu, G.W.M. Lee, Appl. Catal. B: Environ. 75 (2007) 29–38. M.C. Blount, J.L. Falconer, J. Catal. 200 (2001) 21–33. G.S. Li, D.Q. Zhang, J.C. Yu, Environ. Sci. Technol. 43 (2009) 7079–7085. O. D’Hennezel, P. Pichat, D.F. Ollis, J. Photochem. Photobiol. A: Chem. 118 (1998) 197–204. T. Noguchi, A. Fujishima, Environ. Sci. Technol. 32 (1998) 3831–3833. Y. Luo, L. Zou, E. Hu, Environ. Technol. 27 (2006) 359–366. H.P. Kuo, C.T. Wu, R.C. Hsu, Powder Technol. 195 (2009) 50–56. X. Huang, J. Yuan, J. Shi, W. Shangguan, J. Hazard. Mater. 191 (2009) 827–832. E. Piera, J.A. Ayllon, X. Domenech, J. Peral. Catal. Today 76 (2002) 259–270. J. Peral, D.F. Ollis, J. Mol. Catal. A: Chem. 115 (1997) 347–354. C. Belver, M.J. Lopez-Munoz, J.M. Coronado, J. Soria, Appl. Catal. B: Environ. 46 (2003) 497–509. L. Rideh, A. Wehrer, D. Ronze, A. Zoulalian, Ind. Eng. Chem. Res. 36 (1997) 4712–4718. S. Peng, Y. Li, F. Jiang, G. Lu, S. Li, Chem. Phys. Lett. 398 (2004) 235–239. C.H. Ao, S.C. Lee, J.Z. Yu, J.H. Xu, Appl. Catal. B: Environ. 54 (2004) 41–50. L. Zou, Y.G. Luo, M. Hooper, E. Hu, Chem. Eng. Process. 45 (2006) 959–964. P.Y. Zhang, J. Liu, J. Photochem. Photobiol. A: Chem. 167 (2004) 87–94. Y.S. Shen, Y. Ku, Chemosphere 46 (2002) 101–107. D.-R. Park, J. Zhang, K. Ikeue, H. Yamashita, M. Anpo, J. Catal. 185 (1999) 114–119. V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano, M. Schiavello, Appl. Catal. B: Environ. 20 (1999) 15–27. J. Zhao, X.D. Yang, Build. Environ. 38 (2003) 645–654. P.Y. Zhang, F.Y. Liang, G. Yu, Q. Chen, W.P. Zhu, J. Photochem. Photobiol. A: Chem. 156 (2003) 189–194. J. Mo, Y. Zhang, Q. Xu, Y. Zhu, J.J. Lamson, R. Zhao, Appl. Catal. B: Environ. 89 (2009) 570–576. H. Huang, D. Ye, D. Leung, J. Environ. Eng. 1 (2010) 195. M. Lewandowski, D.F. Ollis, Appl. Catal. B: Environ. 45 (2003) 223–238. L.X. Cao, Z. Gao, S.L. Suib, J. Catal. 220 (2000) 269–1269. M. Sleiman, P. Conchon, C. Ferronato, J.-M. Chovelon, Appl. Catal. B: Environ. 86 (2009) 159–165. Y. Irokawa, T. Morikawa, K. Aoki, S. Kosaka, T. Ohwaki, Y. Taga, Phys. Chem. Chem. Phys. 8 (2006) 1116–1121. M. Kogoma, Y. Miki, K. Tanaka, K. Takahashi, Plasma Process. Polym. 3 (2006) 727–733. B. Kasprzyk-Hordern, M. Ziolek, J. Nawrocki, Appl. Catal. B: Environ. 46 (2003) 639–669. M.M. Ye, Z.L. Chen, X.W. Liu, Y. Ben, J.M. Shen, J. Hazard. Mater. 167 (2009) 1021–1027. V. Augugliaro, M. Litter, L. Palmisano, J. Soria, J. Photochem. Photobiol. C: Photochem. Rev. 7 (2006) 127–144. A. Ogata, D. Ito, K. Mizuno, S. Kushiyama, A. Gal, T. Yamamoto, Appl. Catal. A: Gen. 236 (2002) 9–15. T.J. Frankcombe, S.C. Smith, J. Phys. Chem. A 111 (2007) 3686–3690. J. Van Durme, J. Dewulf, W. Sysmans, C. Leys, H. Van Langenhove, Chemosphere 68 (2007) 1821–1829. H. Einaga, S. Futamura, J. Catal. 227 (2004) 304–312.