Environ. Sci. Technol. 2008, 42, 6173–6178
Enhanced Sonocatalytic Degradation of Azo Dyes by Au/TiO2 YIFENG WANG, DAN ZHAO, WANHONG MA, CHUNCHENG CHEN, AND JINCAI ZHAO* Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China
Received January 17, 2008. Revised manuscript received May 7, 2008. Accepted May 8, 2008.
Au-loaded TiO2 (Au/TiO2) has been reported for the first time as a sonocatalyst. It was found that the catalyst Au/TiO2, with a low Au loading 0.5 wt % and under common and commercial frequency (40 kHz) ultrasonic irradiation, greatly accelerated both the discoloration and total organic carbon (TOC) removal of azo dyes such as orange II (Org II), ethyl orange (EO), and acid red G (ARG), as compared to bare TiO2 and nano-Au catalyst. About 80% TOC removal was achieved after complete discoloration of 2.5 × 10-4 M Org II. H2O2 and H2 formation as well as their accumulation was greatly enhanced due to Au loading on TiO2. Both oxidative and reductive degradation intermediates have been detected, and thus the mechanism involves both enhanced oxidation and enhanced reduction via the accelerated formation of active · OH and · H radicals due to Au loading on TiO2, which is supported by electron spin resonance (EPR) and other evidence. The study provides an admirable way to raise the efficiency of sonication and to treat azo dye-containing wastewaters with sonocatalytic processes.
Introduction Advanced oxidation processes (AOPs) such as UV/TiO2, radiolysis, UV/H2O2, Fenton/photo-Fenton, and sonocatalysis have been studied extensively for the degradation of a variety of environmentally hazardous pollutants (1–6), especially for the nonbiodegradable recalcitrant pollutants. Among them, sonocatalysis as a new and efficient method of degrading pollutants has attracted much attention despite its high cost for electrical consumption (5, 6). It is known that under intensive ultrasound irradiation water generates extremely active bubbles which subsequently collapse and cause high local temperature (>4000 K) and pressure (>1000 atm) (7). Water is homogenously split into · H and · OH radicals (eq 1), inducing a series of free radical reactions (eqs 2–8) within and out of the bubbles (8–10). US
H2O 98 H· + ·OH
(1)
·OH + ·OHfH2O2 [k2 ) 5.5 × 109 M-1 s-1] (ref 9) (2) H· + H·fH2 [k3 ) 1.4 × 1010 M-1 s-1] (ref 10)
(3)
H· + ·OHfH2O [k4 ) 2 × 1010 M-1 s-1] (ref 9)
(4)
* Corresponding author phone: 86-10-82616495; fax: 86-1082616495; e-mail:
[email protected]. 10.1021/es800168k CCC: $40.75
Published on Web 07/16/2008
2008 American Chemical Society
H· + O2f·HO2 [k5 ) 1.9 × 1010 M-1 s-1] (ref 9)
(5)
2·HO2fH2O2 + O2 [k6 ) 8.3 × 105 M-1 s-1] (ref 9) (6) H· + ·HO2fH2O2 [k7 ) 2 × 1010 M-1 s-1] (ref 10) (7) US
H2O2 98 2·OH
(8)
The coupling of ultrasound irradiation with other AOPs such as TiO2 photocatalysis, Fenton-like reactions, UV/ Fenton, or ozonolysis has attracted much interest (5, 6), and great progress has been made so far. There is usually a synergistic effect in the coupling systems on formation of active species. On the other hand, it is well-established that using a high-frequency ultrasound could enhance sonochemical reactions (11, 12). Therefore, most studies have been carried out by using high-frequency ultrasound such as 358 kHz to achieve higher rates. However, equipment with higher frequencies is usually much more expensive than those commercially available ultrasonic devices with lower frequencies such as 40 kHz. Production of textile dyestuffs in China takes up more than half of that of the world. In 2006, the total production of dyes in China was about 699 300 tons (13). Azo dyes account for up to 70% of all textile dyestuffs. The azo dyes are recalcitrant under typical usage conditions and are highly resistant to photolysis, O2, common acids and bases, and microbial attack, which is useful in textile applications but causes big problems in wastewater remediation and results in long-term pollution. Common methods such as biodegradation are impractical for abatement of such dyes (14). Moreover, in general, the primary degradation products of azo dyes are aromatic amines which are toxic, carcinogenic, and teratogenic (15, 16). Therefore, mineralization is more desirable than mere discoloration in view of environmental safety. Unfortunately, most discoloration methods are unable to remove total organic carbon (TOC) efficiently. In this work, Au/TiO2 is employed for the first time as a sonocatalyst in sonocatalytic degradation of azo dyes. With a cheap, commercial, and low-frequency ultrasonic transducer (40 kHz), a great enhancement in the rates of both discoloration and TOC removal of azo dyes by Au/TiO2 has been observed as compared to bare TiO2. There have been two proposed pathways for sonolysis of organic compounds, · OH mediated oxidation and pyrolysis (17). Nonvolatile and hydrophilic organic compounds are difficult to enter the bubbles for pyrolysis and therefore usually undergo hydroxyl oxidative degradation (18, 19). It has been reported that addition of nanoparticles such as anatase and rutile TiO2 into aqueous solutions enhances degradation of organic compounds (20–23) via an oxidation pathway. On the other hand, little attention has been paid to sonocatalytic degradation of organic compounds via · H mediated reduction pathway although it has been involved in sonolysis of noble metal ions to form noble metal nanoparticles (24). In this article we carried out a series of experiments under Ar atmosphere or air atmosphere and demonstrate that both enhanced · OH mediated oxidation and enhanced · H mediated reduction play a significant role in simultaneous acceleration of degradation and mineralization of nonvolatile and hydrophilic azo dyes by Au/TiO2. This study provides a way to increase the efficiency of sonocatalysis and also helps to understand the mechanism of sonocatalytic degradation of azo dyes. VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Experimental Section TiO2 (Degussa P25, surface area 50 m2/g) was used as Au carrier. HAuCl4 was purchased from Sinopharm Chemical Reagent Co. Ltd. Orange II (Org II) (A. R.) and ethyl orange (EO) (A. R.) were purchased from Aldrich. Purified acid red G (ARG) was kindly supplied by Tianjin University, and the purity was confirmed by high-performance liquid chromatography (HPLC). The structures of the dyes are shown in Supporting Information Figure S1. Doubly distilled and deionized water (18.2 Ω with Milli-Q) was used throughout all the experimentation. Au/TiO2 was prepared by a photocatalytic reduction method (25), in which TiO2 is excited by UV light to generate electrons in the conduction band and holes in the valence band and HAuCl4 is readily reduced to Au0 by the photogenerated electrons on the surfaces of TiO2 when the holes are scavenged by electron donors. One gram of TiO2 P25 together with a calculated volume of HAuCl4 solution and 0.50 mL of methanol as a scavenger for photogenerated holes was dispersed in 100 mL of water in a Pyrex cylindrical bottle. The dispersion was sonicated for 15 min and then stirred in the dark for 30 min to achieve adsorption/desorption equilibrium. The final pH was ∼3.1. Then the dispersion was irradiated with a 500 W xenon lamp for 4 h for loading Au nanoparticles on the TiO2 surfaces. Afterward, the dispersion was filtered and the obtained catalyst was washed with water three times. It has been determined by UV-vis spectra that no detectable HAuCl4 remained in the filtrate or in the supernatant, evidencing the complete conversion of HAuCl4 to Au nanoparticles. The Au/TiO2 catalyst was characterized by HRTEM (Supporting Information Figure S5) which showed a uniform distribution of Au nanoparticles with average diameter 2.0 ( 0.5 nm on TiO2. The loaded quantity of Au on TiO2 was about 0.5 wt %. For comparison, Au nanoparticles (AuNP) were also prepared according to the literature (26). Sonocatalytic degradation was performed with a commercial 40 kHz transducer (UP2200H, Nanjing Panda Electronics Co. Ltd.) operating at 50 W. The average power introduced into the reactor was 0.045 W/cm3 according to the calorimetric method (27). Circulating water was used to maintain the temperature at 22-28 °C. Typically, 100 mL of dye solution (pH ) 3.5) along with 100 mg of catalyst was used during all degradation experiments in a 250 mL roundbottomed Pyrex flask fixed in the close vicinity of the transducer (Supporting Information Figure S2). For the experiments under Ar atmosphere, argon gas flow was maintained during all the course of reactions. Samples were centrifuged and filtered with 0.2 µm membranes (Whatman Co. Ltd.). The concentrations of the substrates were measured either by a Hitachi U-3010 spectrometer or by an HPLC system (Shimadzu Co.) equipped with a UV-vis detector (SPD-20A at UV/230nm), a binary pump (LC-20A), and a C18 column (Diamonsil). The mobile phase consisted of 0.01 M ammonium acetate (A) and acetonitrile (B) at a total flow rate 1.0 mL/min; controlled mixing of two phases was performed with a linear gradient procedure, i.e., the percentage of phase B was raised from 0% to 25% in the first 6 min, and raised further to 40% in the next 15 min, and kept constant for 20 min more. To measure the adsorbed quantity of Org II on Au/TiO2, 80 mL of water (pH ) 3.5) together with 100 mg of catalyst was sonicated for 15 min, and then 20 mL of 2.5 × 10-4 M Org II (pH ) 3.5) was added to obtain 5.0 × 10-5 M Org II. The dispersion was stirred in the dark for another 30 min to achieve adsorption/desorption equilibrium. The adsorbed quantity of Org II was calculated by measuring the Org II concentration remained in the solution. Mineralization of the dyes was analyzed by a TOC analyzer (Appollo 9000, Tekmar Dohrmann Co.). Identification of intermediates was performed with an HPLC/quadrupole 6174
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FIGURE 1. (A) Time evolution of Org II (5.0 × 10-5 M, pH ) 3.5) under Ar atmosphere under sonication (a) with Au/TiO2 (1 g/L), (b) with TiO2 (1 g/L), and (c) without catalyst. (B) UV-vis spectral changes corresponding to curve a. time-of-flight mass spectrometer (HPLC/Q-TOF-MS) (Waters Co., U.S.A.) operating at negative electrospray ionization (ESI-) mode. The concentration of SO42- was determined using an ion chromatograph (Dionex DX-120). Spin-trapping electron spin resonance (EPR) experiments were performed with a Bruker model E500 spectrometer: centerfield ) 3480 G, sweep width ) 150.0 G, microfrequency ) 9.77 GHz, power ) 12.6 mW. To minimize experimental errors, the same quartz capillary tube was used for all EPR measurements. EPR spectral simulations were carried out by the WinSim program (28). Further, H2O2 concentration was determined by the spectrophotometric DPD method described elsewhere (29). H2 production was measured by a thermal conductivity detector (TCD) gas chromatograph (Techcomp GC7890) equipped with a molecular sieve 5A column (3 mm × 3 m). Triiodide (I3-) concentration was measured by its maximum absorbance at 352 nm.
Results and Discussion Sonocatalytic Degradation of Org II under Different Conditions. Figure 1A shows the sonocatalytic destruction of Org II in aqueous solutions in the absence and presence of catalysts under Ar atmosphere. Almost no degradation was observed in the absence of catalyst (curve c). Since Org II is a nonvolatile compound and is highly soluble in water, it can hardly enter the bubbles and does not favor ultrasonic degradation (18, 19). This is the reason that Org II was resistant to sonication. Both TiO2 and Au/TiO2 could adsorb the dye as shown in Figure 1A before the reaction (about 20%). However, TiO2 showed little effect on sonolysis of Org II under the present conditions (curve b), while a rapid sonocatalytic degradation of Org II with a pseudo-first-order kinetic constant k ) 0.0178 min-1 was observed in the presence of Au/TiO2 (curve a). The absorption band at λmax ) 484.5 nm corresponding to azo conjugate absorption and λmax ) 228, 262, and 311 nm corresponding to those of aromatic rings diminished simultaneously (Figure 1B), indicating complete destruction of the chromophore structure of Org II. The Au/ TiO2 catalyst was also very efficient for the destruction of Org II at higher concentrations. The rate constants were k ) 0.0106
TABLE 1. Pseudo-First-Order Kinetic Constants for Degradation of Org IIa no. 1 2 3 4 5 6 7 8 9 10 11
a
conditions 5.0 × 10-5 M Org II 5.0 × 10-5 M Org II TiO2 5.0 × 10-5 M Org II AuNP 5.0 × 10-5 M Org II Au/TiO2 2.5 × 10-4 M Org II Au/TiO2 1.0 × 10-3 M Org II Au/TiO2 5.0 × 10-5 M Org II 5.0 × 10-5 M Org II TiO2 5.0 × 10-5 M Org II Au/TiO2 5.0 × 10-5 M Org II Au/TiO2 + t-BuOH (0.1% v/v) 5.0 × 10-5 M Org II Au/TiO2 + t-BuOH (0.1% v/v)
atmosphere
kapp, min-1
+
Ar Ar
very slow very slow
+
Ar
0.0077 ( 0.0008
+
Ar
0.0178 ( 0.0014
+
Ar
0.0105 ( 0.0004
+
Ar
0.0014 ( 0.0001
+
air air
0.0004 ( 0.0001 0.0024 ( 0.0001
+
air
0.0302 ( 0.0017
+
air
0.0013 ( 0.0001
+
Ar
0.0108 ( 0.0013
Note: catalyst ) 1 g/L, pH ) 3.5.
and 0.0014 min-1 for initial [Org II] ) 2.5 × 10-4 and 1.0 × 10-3 M, respectively (Table 1, entries 5 and 6). The turnover number (TON: molecules of Org II degraded/atoms of surface-loaded Au) reached 39 for initial [Org II] ) 1.0 × 10-3 M, indicating that the degradation was a catalytic reaction. The catalyst remained the high sonocatalytic activity after three repeated experiments (Supporting Information Figure S3). Moreover, no apparent change was observed in color or shape of the Au/TiO2 catalyst by transmission electron microscopy (TEM) even after 12 h of sonocatalytic reaction (Supporting Information Figures S4 and S5) that shows a high stability of Au/TiO2 catalyst under current usage conditions. The degradation of Org II followed the pseudo-first-order kinetic model under different conditions. The kinetic constants obtained have been illustrated in Table 1. As reported in the literature (20–23), TiO2 nanoparticles in aqueous solutions could accelerate sonolysis of organic compounds. However, the sonocatalytic degradation of Org II in the presence of bare TiO2 was very slow under the present conditions (Table 1, entries 2 and 8). The low sonocatalytic activity of TiO2 could be greatly enhanced by loading Au nanoparticles on its surface (Figure 1A, curve a and Table 1, entries 4 and 9). As blank experiments, we further examined the sonocatalytic degradation of Org II with AuNP (Table 1, entry 3). The degradation was slower with AuNP than Au/ TiO2 showing a synergistic effect between the two components. Moreover, Au/TiO2 has many advantages over AuNP like simple preparation, high stability, and easier manipulation. Similar results were obtained in the sonocatalytic degradation of Org II under air atmosphere (Table 1, entries 7-9) elucidating that Au/TiO2 is also an effective sonocatalyst under air atmosphere. The activity of Au/TiO2 was further examined by sonocatalytic degradation of the other two azo dyes, EO and ARG (Supporting Information Figures S6 and S7). In both of the cases, Au/TiO2 displayed a significantly enhanced activity on sonocatalysis, in comparison to bare TiO2. Mineralization and Intermediates. In typical photocatalytic and other AOPs degradation of organic compounds, TOC removal usually takes a long time after the disappearance of the substrates (30). The fast and simultaneous removal of substrates and TOC is attractive. With the help of Au/TiO2,
FIGURE 2. TOC removal during sonocatalytic degradation of Org II (2.5 × 10-4 M, pH ) 3.5) with Au/TiO2 (1 g/L) (a) under Ar atmosphere, (b) under air atmosphere, (c) with bare TiO2 (1 g/L) under air atmosphere.
FIGURE 3. HPLC chromatogram at 4 h of sonocatalytic degradation of Org II (2.5 × 10-4 M, pH ) 3.5) under air atmosphere in the presence of Au/TiO2 (1 g/L). The structures of the numbered peaks refer to Table 2. sonication rapidly reduced TOC by about 80% under Ar atmosphere after discoloration of Org II at 9 h and by about 65% under air atmosphere at 14 h (Figure 2). In contrast, with bare TiO2, discoloration and mineralization of Org II took place at a much slower rate, and TOC was reduced by 67% after discoloration at 36 h. As detected by ion chromatography, the sulfonic group of Org II was sonocatalytically converted to SO42- ion (Supporting Information Figure S8). The degradative intermediates of Org II under air atmosphere were analyzed by HPLC/TOF-MS in ESI- mode as shown in Figure 3 and Table 2. A series of Org II/ · OH adducts of 6, 7, 9, and 11, Org II/ · H adducts of 8, as well as Org II/ · H + · OH adducts of 4 and 5 were detected. Therefore, Org II underwent both reduction by · H and oxidation by · OH, that is to say, both · H and · OH play a role in the sonocatalytic degradation of Org II (see also Supporting Information Scheme S1). The intermediates under Ar atmosphere were similar to those under air atmosphere, whereas the sonocatalytic reaction time profiles and the relative concentrations were somewhat different. Thus, the sonocatalytic degradation under Ar atmosphere is also a coexisting process of both oxidation and reduction although the proportion of them differs from that under air atmosphere. EPR Detection of Free Radicals. According to eq 1, sonolysis of water generates short-lived · OH and · H radicals (8), which can form longer-lived adducts with DMPO to be detected by EPR. As shown in Figure 4, the EPR signal of 1:2:2:1 quartets is assigned to DMPO-OH (curves c and d) which has an R-N and a β-H (aN )14.9 G, aH )14.7 G) as simulated by the WinSim program (curve e), whereas the 1:1:2:1:2:1:2:1:1 spectrum of curve f is the simulation of DMPO-H which has an R-N and two equivalent β-H (aN ) 16.5 G, aH ) 22.5 G). Since the quartet peaks of DMPO-OH overlap with the spectrum of DMPO-H, only a nonuple spectrum could be observed when DMPO-OH coexists with DMPO-H as simulated using the WinSim program (curve g). Sonolysis of water under Ar atmosphere either with Au/ TiO2 or with bare TiO2 (curves a and b, respectively) generated a nonuple spectrum which has been identified as the overlap signal of DMPO-OH and DMPO-H. Thus, we can infer that both Au/TiO2 and bare TiO2 follow the same mechanism for enhancing sonocatalysis and Org II underwent the same VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Identified Intermediates in Sonocatalytic Degradation of Org IIa
FIGURE 5. Time evolution of H2O2 in water under sonication under Ar atmosphere (a) with Au/TiO2, (b) with TiO2, and (c) without catalyst. pH ) 3.5, catalyst ) 1 g/L.
a Note: peaks 13 and 14 are ghost peaks due to the eluent gradient and present in every chromatograph.
than the bare TiO2 system, indicating Au/TiO2 is also the more effective sonocatalyst than bare TiO2 under air atmosphere. This result is in good agreement with the result that Org II degradation was faster over Au/TiO2 than over bare TiO2 under air atmosphere. No DMPO-H was detected under air atmospheres because O2 competes with DMPO to capture · H and form · HO2 (eq 5) while the lifetime of the DMPO-OOH adduct is too short to be detected in water. To confirm this point, DMPO-OOH was indeed observed (aN ) 13.8, aH ) 10.6) in water/methanol solutions both with Au/TiO2 and with bare TiO2 under air atmosphere (Supporting Information Figure S9) because DMPO-OOH is more stable in water/methanol solvent and hence could be detected by EPR. Although we could not observe DMPO-H adduct under air atmosphere, both Org II/ · H adducts and Org II/ · H + · OH adducts were actually formed under air atmosphere, indicating reduction of Org II by · H was not completely quenched by O2. H2O2 Generation during the Degradation of Org II. Both oxidative · OH and reductive · H contribute to H2O2 formation (eqs 2, 5, 6, and 7). However, under Ar atmosphere, since no O2 participates in the reactions (reactions of eqs 5–7 are not involved), H2O2 accumulation followed eq 9: d[H2O2]/dt ) k2[·OH]2 - k8[H2O2]
FIGURE 4. EPR spectra of spin-trapped radicals obtained by sonolysis of water containing DMPO: (a) with Au/TiO2 and (b) bare TiO2 under Ar atmosphere for 6.5 min, (c) with Au/TiO2 and (d) bare TiO2 under air atmosphere for 4 min, (e) simulation of DMPO-OH, (f) simulation of DMPO-H, (g) simulation of (a). [DMPO] ) 4 mM, pH ) 3.5, catalyst ) 1 g/L. degradative pathway as earlier mentioned. However, many more · OH and · H radicals were produced with Au/TiO2 than with bare TiO2, implying the function of Au on TiO2 is that it accelerates significantly decomposition of water (eq 1) to generate · OH and · H radicals. Therefore, it should be that both enhanced oxidation and enhanced reduction play a role in simultaneous acceleration of degradation and mineralization of azo dyes by Au/TiO2. On the other hand, under air atmosphere, only DMPO-OH was detected in the presence of Au/TiO2 or bare TiO2 (curves c and d, respectively). The Au/TiO2 system generated more · OH radicals 6176
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(9)
Thus, the rate of H2O2 accumulation decreases with increasing the H2O2 concentration and becomes 0 when [H2O2] ) k2[ · OH]2/k8 (Figure 5). Sonolysis of water in the absence of catalyst generated a little H2O2 as reported previously (5, 6), whereas adding TiO2 could increase its yield (curves c and b, respectively). Further, the concentration of H2O2 increased to about 4-fold with Au/TiO2 as compared to bare TiO2. Thus, we can infer from the equation of [ · OH] ) (k8[H2O2]/k2)1/2 that Au loading on TiO2 can greatly enhance · OH formation, which is consistent with the EPR results. Enhanced Oxidation and Reduction Reactions by Au/ TiO2 under Sonication. Formation of H2 was detected under sonication under Ar atmosphere (Figure 6A), and the reactions involve eqs 1 and 3. It was found that sonolysis of water could produce H2 at a very slow rate (data no shown) while addition of TiO2 accelerated limitedly the rate (curve b). However, Au/TiO2 could greatly accelerate the rate of hydrogen evolution (curve e). Similar to H2O2 formation discussed above, the quicker H2 is produced, the higher concentration of · H should be in aqueous solution, and the same conclusion could be drawn that Au loading on TiO2 significantly enhanced water splitting to produce · H radical. Since · H radical is a highly active reductant and can reduce many species, as exemplified by reports on sonolysis of noble metal ions to form noble metal nanoparticles (24), the enrichment of · H radical should accelerate reduction reactions. As shown in Figure 6A, H2 evolution was slowed down in Org II solutions, and with increasing the Org II concentration, H2 yield decreased (curves e, d, and c). Therefore, parallel
This study also provides helpful details for a deep understanding of the sonocatalytic mechanism for degradation of azo dyes.
Acknowledgments Thisworkwassupportedbythe973Project(No.2007CB613306), NFSC (Nos. 20537010, 20777076, and 50436040), and CAS.
Supporting Information Available Structures of dyes, experimental setup, photographs, and TEM characterization of Au/TiO2, repetitive experiments, SO42- evolution, degradation of EO and ARG, degradation pathways of Org II, and EPR spectra of DMPO-OOH. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited
FIGURE 6. (A) Sonocatalytic H2 evolution under Ar atmosphere: (a) in TiO2/Org II (5.0 × 10-5 M) system, (b) in TiO2/H2O system, (c) in Au/TiO2/Org II (2.5 × 10-4 M) system, (d) in Au/TiO2/Org II (5.0 × 10-5 M) system, (e) in Au/TiO2/H2O system. (B) Ι3formation in the sonocatalytic oxidation of I- ([I-]0 ) 0.200 M, pH ) 3.5) under air atmosphere: (a) with bare TiO2 and (b) with Au/TiO2. Catalyst ) 1 g/L. to the results obtained with HPLC-MS, it was certainly found that the reactions between the dye and · H occurred more efficiently in the presence of Au/TiO2 than that of the bare TiO2. The oxidative power was testified by I- oxidation as evidenced in Figure 6B. Since both E 0· OH/H2O ) 2.8 V and 0 ) EH ) 1.763 V are more positive than 2O⁄H2O 0 EI 3-⁄I - ) 0.536 V, I- is readily oxidized to I3- in the presence of · OH or H2O2. Sonocatalytic I3- formation with Au/TiO2 (curve b) was much faster than with bare TiO2 (curve a). Therefore, Au/TiO2 under sonication generated more oxidative species than TiO2, in agreement with the EPR results and the H2O2 formation results. To further confirm the role of · OH radicals in the present study, we examined the influence of 1-butanol, a known · OH radical scavenger, on the sonocatalytic degradation of Org II. As mentioned above, the · OH concentration is relative closely to both the oxidation processes and the H2O2 formation (eq 2). As shown in Supporting Information Figure S10, H2O2 production was greatly suppressed by addition of 1-butanol. Sonocatalytic degradation of Org II in the presence of Au/TiO2 under Ar atmosphere was also slowed down upon addition of 1-butanol (Table 1, entry 11), indicating that · OH radicals play an important role in the degradation of Org II. On the other hand, under air atmosphere the degradation of Org II became very slow in the presence of 1-butanol (Table 1, entry 10). Therefore, under this condition, · OH radicals play the more important part in the degradation of Org II because the role of · H mediated reduction was suppressed by O2, consistent with the EPR results that no DMPO-H was detected under air atmosphere. Our results show that Au/TiO2 can be used as an effective sonocatalyst to significantly enhance degradation and mineralization of azo dyes. The mechanism involves both the enhanced oxidation and reduction by · OH and · H radicals.
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