Visible-Light-Induced Photocatalytic Oxidation of Polycyclic Aromatic

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Visible-Light-Induced Photocatalytic Oxidation of Polycyclic Aromatic Hydrocarbons over Tantalum Oxynitride Photocatalysts J I A H U I K O U , †,‡,§ Z H A O S H E N G L I , †,‡,§ Y U P E N G Y U A N , †,‡,§ H A I T A O Z H A N G , †,‡,§ Y I N G W A N G , †,§ A N D Z H I G A N G Z O U * ,†,§ Eco-materials and Renewable Energy Research Center (ERERC), Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P. R. China, and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

Received October 20, 2008. Revised manuscript received February 24, 2009. Accepted February 24, 2009.

The photooxidations of five typical polycyclic aromatic hydrocarbons (PAHs) were investigated by using tantalum oxynitride and Pt-tantalum oxynitride as visible light-driven photocatalysts. The electron paramagnetic resonance spintrap technique and hydrogen peroxide test strip were used to monitor active species formed in these photocatalytic systems. Moreover, the participations of HO•, O2-• anions, and holes were further examined by adding their scavengers t-butanol, benzoquinone, and iodine anions, respectively. The reaction intermediates were analyzed by gas chromatography-mass spectrometer (GC-MS). The results show that tantalum oxynitride exhibits good photocatalytic activity for the PAHs photodegradation and the activity is greatly promoted by loading cocatalyst Pt. After 6 h visible light irradiation, phenanthrene, anthracene, benzo[a]anthracene, and acenaphthene can be completely oxidized over Pt-tantalum oxynitride. Under UV light irradiation, the photodegradation rate of PHE over Pt-tantalum oxynitride is 8 times faster than that over titanium dioxide (P25, 80% anatase, 20% rutile). Oxygen plays a crucial role on the photooxidations of PAHs. t-Butanol and benzoquinone almost have no effect on PAHs photodegradations, which indicate that HO• and O2-• anions play a negligible role on the photodegradations of PAHs. However, the presence of iodide anions significantly inhibits these degradation reactions, implying the crucial effect of holes on the photocatalytic systems. The PAHs degradations could therefore be attributed to the formation of holes in these systems. Based on the GC-MS analysis, the possible photooxidation pathways of PAHs were also proposed.

Introduction Polycyclic aromatic hydrocarbons (PAHs), a group of aromatic compounds, have been known as toxic and important pollutants in ecosystem. They can be produced in fossil fuels burning, coke oven, forest, and agricultural fires, metal * Corresponding author phone: +86-25-83686630; fax: +86-2583686632; e-mail: [email protected]. † ERERC. ‡ Department of Materials Science and Engineering. § National Laboratory of Solid State Microstructures. 10.1021/es802940a CCC: $40.75

Published on Web 03/19/2009

 2009 American Chemical Society

processing facilities, hydrocarbon production, and so on. PAHs exhibit high stability and low water solubility due to the delocalization of π electron, leading to their accumulation in food chains. Therefore, the degradation of PAHs is a current focus of research in environment science (1-4). In recent years, one of the most attractive methods for PAHs degradation is photocatalysis from the point of view of solar energy utilization (5-8). To date, most of the studies are dominated by TiO2 (9) because of its stability and relatively high activity. However, the band gap of TiO2 is so large that it can only utilize less than 5% of solar energy. As a result, current interests are directed toward the application of photocatalysts that function under visible light irradiation to efficiently utilize sunlight. Metal oxynitrides with valence bands composed of hybridized N2p and O2p orbitals is one promising candidate of visible-light-responsive photocatalysts. Tantalum oxynitride (TaON) was reported to absorb light of the wavelength up to ca. 530 nm (10). It was found to be a highly active visible-light-responsive photocatalyst for water splitting (11-14). Furthermore, Pt-modified TaON showed superior photocatalytic activity for the oxidation of methanol, and its activity was 19.3 times higher than that of TiO2 (P25, 80% anatase, 20% rutile) under simulated sunlight (15). Hence, TaON might be an efficient photocatalyst to eliminate PAHs. However, to our knowledge, the photooxidation of PAHs over TaON and Pt-TaON photocatalyst under visible light irradiation has not yet been reported. Moreover, it remains unclear how the photooxidation of organic compounds proceeds over TaON up to now. Among all types of chemistry, reaction mechanism is considered to be one of the most fundamental points to be clarified. It is thus desirable to investigate the photooxidation of PAHs using TaON as a photocatalyst not only for PAHs decomposition but also for the study of photocatalytic property of TaON. In this study, five PAHs including phenanthrene (PHE), anthracene (ANT), acenaphthene (ACE), benzo[a]anthracene (BaA), and pyrene (PY) were chosen to be photocatalytically degraded over TaON and Pt-TaON. The electron paramagnetic resonance (EPR) spin-trap technique and hydrogen peroxide test strip were used to monitor active species in these photocatalytic systems. The effects of radicals and hole scavengers on the photocatalytic degradation of PAHs were examined. The intermediates and photodegraded products were analyzed by gas chromatography-mass spectrometer (GC-MS). The reaction pathways were proposed based on the experimental results. This paper presents a highly effective method to degrade PAHs under visible light irradiation and provides an experimental basis for predicting reactivity of much larger PAHs having similar structural features and physical properties. In addition, investigations of the PAHs degradation mechanisms will assist in understanding and predicting the fates of PAHs at similar interfaces.

Experimental Section Material Preparation and Characterization. The TaON sample was prepared by heating Ta2O5 powder under flowing NH3 (20 mL · min-1) at 850 °C for 15 h (15). Pt (0.5 wt%) was loaded on TaON by the photocatalytic reduction of H2PtCl6 in methanol aqueous solution under full arc light irradiation for 8 h. The crystal structures of prepared samples were detected by an X-ray diffractometer (XRD, Rigaku Ultima III, Japan). The morphology and dispersion of deposited Pt particles were examined by transmission electron microscopy (TEM, FEI Tecnai 20, U.S.). The optical absorption spectra ofTaONandPt-TaONwereobtainedbyusinganultraviolet-visible spectrophotometer (UV-vis, Shimadzu UV-2550, Japan). The VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effects of irradiation time on the conversion of PAHs over TaON or Pt-TaON under visible light irradiation. (a) PHE and ANT; (b) BaA, ACE, and PY.

FIGURE 2. EPR spectral changes of the DMPO-O2-• adducts generated in the suspended liquid of TaON under light irradiation. concentration of PAHs before and after adsorption/desorption equilibrium were determined by a spectrofluorometer (Varian Cary Eclipse, U.S.), using an excitation wavelength of 344 nm. Photocatalytic Reactions. The irradiation system consisted of a 300 W Xe arc lamp, a cutoff filter, a 100 mL quartz reaction cell, and a water cooler trough (preventing the thermal catalytic effect). In a typical experiment, 0.003 g of PAH was first dissolved in the solvent consisted of 30 mL of water and 30 mL of acetone. Then the solution of PAH was added into the quartz cell containing a certain amount of photocatalyst. The suspension was magnetically stirred in the dark for 2 h before irradiation to ensure the establishment of an adsorption/desorption equilibrium of PAHs on the surface of photocatalyst. Subsequently, the quartz cell was immersed in an ice-water mixture, followed by irradiating with a 300 W Xe arc lamp equipped with a cutoff filter (λ ) 420 nm). After reaction, the slurry of reaction mixture was taken out and centrifuged to remove photocatalyst. Afterward, products were extracted by dichloromethane. The dichloromethane layer dried by anhydrous sodium sulfate was quantified by GC (Agilent 6890N, USA) and identified using GC-MS (Agilent 6890N/5973I, U.S.). The stability experiments of Pt-TaON were also conducted. The original Pt-TaON and PHE dose was 0.3 and 0.003 g, respectively. In this experiment, the conversion of PHE was analyzed by disposal all of the reaction remainder, using the 2920

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FIGURE 3. Total ion chromatogram and mass spectra of peaks c and f obtained from the PHE photodegradation over Pt-TaON after 2 h visible light irradiation. disposal method mentioned above. Then, the recycled PtTaON was used in the next run of the experiment. At the beginning of each run, PHE was added into the reactor to keep the original PHE dose of 0.003 g. The same experiments were performed for four runs, and the irradiation time of each run was 2 h. To ascertain radicals in this system, the DMPO-free-radicals were detected at room temperature by a Bruker EPR 10/12 spectrometer. In the experiment of determining active species, the amount of t-butanol, KI, and benzoquinone (BQ) were all 2 × 10-4 mol. In addition, the existence of hydrogen peroxide was examined by hydrogen peroxide test strip in the photocatalytic reaction solution. Details of experimental conditions, including chemicals as well as the operation conditions of GC-MS and EPR, are given in the Supporting Information (SI).

Results and Discussion XRD, TEM, and UV-vis Analysis. The XRD patterns of the prepared samples are shown in Figure S1 of the SI. TaON and Pt-TaON samples are well-crystallized and all of the diffraction peaks can be assigned to TaON (16). No Pt phase

TABLE 1. Structures of the PAHs under Study and the Corresponding Products

is observed in the XRD pattern of Pt-TaON due to the low Pt amount. However, Pt particles can be found in the TEM image of Pt-TaON (SI Figure S2). Spherical or hemispherical Pt particles with the diameter of 1-3 nm are successfully deposited onto the surface of TaON. SI Figure S3 displays the UV-vis absorption spectra of TaON and Pt-TaON. The absorption band-edge of TaON is at about 500 nm, corresponding to the band gap energy of 2.5 eV (17). No obvious change of absorption edge could be observed after Pt loading. Photodegradation of PAHs. Figure 1 illustrates the time courses for the photodegradation of PAHs over TaON and Pt-TaON. The conversions of PAHs increase with the prolonging of irradiation time. After loading Pt cocatalyst, the activity of TaON can be greatly promoted, and the photooxidation rates of the five PAHs over Pt-TaON are about 10 times higher than those over TaON. PHE, BaA, ACE, and ANT can be oxidated completely over Pt-TaON after 2, 3, 4, and 6 h, respectively. According to the UV-vis absorption spectra (SI Figure S3), the absorption edges of TaON and Pt-TaON are essentially the same. Therefore, the difference in the activity of TaON and Pt-TaON could not be attributed to the distinction of their light absorption. The reason of improvement of photocatalytic activity will be discussed in detail later. It was reported that the conversion of PHE over BiVO4 was only 8% after 8 h visible light irradiation (1). The present results imply that TaON has superior activity for the PHE photodegradation than BiVO4 under visible light irradiation. Under UV light irradiation, PHE photooxidations were investigated over Pt-TaON and P25 (SI Figure S4). PHE could be completely converted after 60 min irradiation over Pt-TaON. However, the conversion of PHE over P25 is only 12.5% in the same condition. The results show that the activity of Pt-TaON is about 8 times higher than P25 for PHE

photodegradation. The durability results of Pt-TaON for PHE photooxidation are shown in SI Figure S5. The conversions of PHE keep at 100% in the first three cycles, and become 98.5% in the fourth run, suggesting the excellent stability of Pt-TaON. Furthermore, the XRD patterns also imply the fine stability of Pt-TaON (SI Figure S6). In addition, it should be noted that the reaction rate of PHE is faster than ANT over TaON and Pt-TaON. However, several reported results indicated that ANT was more active than PHE in some other reactions, such as Fenton reaction (18) and BiVO4 photocatalysis system (1). In the aforementioned two reactions, HO• radical was determined as the main active species. The present facts imply that the photooxidations of PAHs in the system of TaON and Pt-TaON are possible independent of HO• radicals. In the present of HO• radical scavenger t-butanol, the photodegradation of PHE was investigated. It is found that t-butanol has no effect on the photodegradations of PHE, which confirm HO• radical is not the active species in both system of TaON and Pt-TaON. To investigate the photocatalytic role of TaON and PtTaON, several experiments were carried out. Degradation experiments over TaON and Pt-TaON were first preformed in the dark to demonstrate the effect of light. The five PAHs can not be oxidized even after 6 h without light. The results imply that the oxidation reactions of the PAHs are driven by light. The absorption percentage of PHE, ANT, ACE, BaA, and PY over Pt-TaON is 5.2, 4.5, 4.4, 4.7, and 3.8%, respectively. The facts indicate that the absorbed PAHs only account small parts. Also, no direct photolysis of the PAHs is observed under visible light irradiation of 6 h in the absence of a catalyst, except that the conversion of BaA is 3.5%. These results suggest that the photodegradation of the PAHs result from the photocatalytic role of TaON or Pt-TaON. In addition, the VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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SCHEME 2. Possible Pathway of ANT Photodegradation over Pt-TaON under Visible Light Irradiation

FIGURE 4. Total ion chromatogram and mass spectrum of peak l obtained from the ANT photodegradation over Pt-TaON after 2 h visible light irradiation.

SCHEME 1. Possible Pathway of PHE Photodegradation over Pt-TaON under Visible Light Irradiation

the oxygen free radicals generated in the present irradiation system. As Figure 2 shows, six characteristic peaks of the DMPO-O2-• adducts are observed after light irradiation (22-24). However, no signal is detected in the dark. These evidence show that the generation of O2-• anion in the photocatalysis system of TaON is implicated in light irradiation. However, no signal of O2-• anions is detected in the system of Pt-TaON which is more active than TaON. O2-• anions are produced by the reduction of oxygen molecules adsorbed on the catalyst surface. It is well-known that multielectron reduction of molecular oxygen readily proceeds on the surface of platinum, which produce H2O2 or H2O (9, 25). Therefore, it is expected that the multielectron reduction, which does not produce radical species of oxygen, take place over the Pt-TaON photocatalyst. It can be concluded that the O2-• anions are not the active species in Pt-TaON system, but it possibly plays an important role in the TaON system. BQ has the potential to trap superoxide anions by an electron transfer mechanism (eq 1) (26). -· BQ + O-· 2 f BQ + O2

experiment excluded O2 was carried out to examine the effect of O2. Even after 6 h irradiation, no PAH is converted at all. The facts indicate that O2 plays an important role in the oxidation of the PAHs. As described previously, the five PAHs could not be oxidized after 6 h without light irradiation. It also implies that O2 could not directly oxidize these PAHs. Therefore, other active species should exist in these reaction systems. Study of Active Species. Photoinduced oxide radicals, hydrogen peroxide, and hole were considered as the main oxidizing species in semiconductor photocatalytic processes (19). The EPR spin-trap technique, which is useful for detecting radical species (20, 21), was employed to determine 2922

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(1)

Experiments in the presence of BQ (O2-• anions radical scavengers) were therefore carried out to examine the role of O2-• anions in the system of TaON and Pt-TaON. The negligible effect of BQ implies that O2-• anions are not the active species in the two systems. In addition, no characteristic peak of DMPO-HO• adducts is found, which is consistent with the speculation mentioned above. Consequently, it can be also concluded that the active species in the photocatalytic system of TaON is different from that of the classical photocatalyst TiO2 (23, 27, 28). Furthermore, the existence of hydrogen peroxide in the photocatalytic reactions was examined by hydrogen peroxide test strip. However, hydrogen peroxide is detected neither in the system of TaON nor Pt-TaON. The minimum detectability of this hydrogen peroxide test strip is 1 mg · L-1 of the hydrogen peroxide concentration. Even if hydrogen peroxide exists in these reaction systems, its concentration should be less than 1 mg · L-1. As a result, experiments in the presence of 1 mg · L-1 hydrogen peroxide were carried out without photocatalyst. The compared experiments show that hydrogen peroxide with concentration of 1 mg · L-1 has no influence on the conversion of the PAHs under visible light irradiation. Photoinduced hydrogen peroxide was therefore not the active species in these systems. According to the results mentioned above, it can be speculated that the photoinduced holes may be the main

active species in the TaON and Pt-TaON systems. Iodide has been used as a hole scavenger to study the effect of holes on the photocatalytic reaction, and the reactions between holes and iodine anions are shown in eqs 2-4 (26, 29, 30). h++I- f I•

(2)

I•+I- f I•2

(3)

h+ + I•2 f I2

(4)

Iodine anions were therefore added to the TaON and PtTaON systems to examine the participation of oxidative holes. In the presence of iodine anions, PHE could not convert even after 8 h irradiation. These evidence suggest that holes are the main active species for both TaON and Pt-TaON. For the Pt-TaON, Pt possibly serves as an efficient electron trap preventing electron-hole recombination in TaON (31), and thus evidently improves photocatalytic activity of TaON. Pathway of PAHs Oxidation. Table 1 summarizes the structure of PAHs under study and the corresponding reaction products. Figure 3 displays the total ion chromatogram of PHE photooxidation over Pt-TaON after 2 h visible light irradiation and the corresponding mass spectra of peaks c and f. The mass spectra of other main peaks are shown in SI Figure S7. Though the conversion of PHE over TaON is different from that over Pt-TaON, similar products are detected in the two systems. It is interesting to find peaks c and f, which have the maximum m/z value of 212, in the intermediates of the photodegradation of PHE. According to the standard spectra in NIST 02 library data, the peak f can be assigned to compound 7 (9,10-dihydro-9,10-dihydroxyphenanthrene, similarity: 90%). Similarly, the peak c can be tentatively assigned to compound 4 (1,4-dihydro-1,4-dihydroxyphenanthrene) by analyzing the fragmentation pattern of mass spectra. It is necessary to point out that compounds 4 and 7 are two hydrogenated-oxidation products of PHE. Thus, the oxidation of PHE probably is accompanied with hydrogenation. Similar to PHE, the compound with the maximum m/z value of 212 (peak l, Figure 4) is also detected in the products of ANT photooxidation. The product could be tentatively assigned to compound 13 (9,10-dihydro-9,10dihydroxyanthrance), based on the identification of the mass spectra and the comparison with other known compounds from spectra library. The results further confirm that the hydrogenation of PAHs possibly occur on the surface of TaON as well as oxidation. To clarify the hydrogenation reaction, D2O was used instead of H2O in the photodegradation reaction of PAHs. Unfortunately, both PHE and ANT can not be oxidated after 6 h in the D2O solution, possibly owing to the difference of chemical properties between H2O and D2O (32). It can be concluded from the quantitative analysis of the intermediates of PHE degradation that the yields of products 2 to 9 increase at the beginning of the photodegradation process, and then decrease gradually after 2 h irradiation, probably resulting from their further degradation. After 4 h irradiation, all of them drop to a very low amount. According to the quantitative analysis, a possible oxidation mechanism of PHE over TaON or Pt-TaON under visible light irradiation is proposed, as Scheme 1 illustrates. The primary step is the photoinduced generation of holes and active H over TaON or Pt-TaON. The resulting holes attack PHE to produce PHE+•, as shown in eq 5. h+ + PHE f PHE+·

(5)

PHE+• react with O2 and active H species, leading to the formation of products 4, 7, and 8. Then products 7 and 8 are oxidized to form compound 9, which is one of the main

products of PHE photodegradation. Product 9 can be further destructed to yield products 2, 3, 5, and 6. Product 2 is another main product of PHE photodegradation. At last, these intermediates are further oxidized to small molecules, such as CO2. Figure 4 displays the total ion chromatogram along with the mass spectrum of peak l from ANT photooxidation over Pt-TaON after 2 h visible light irradiation. The mass spectra of other products are displayed in SI Figure S8. The intermediates of ANT photooxidation over TaON are similar to Pt-TaON. According to the GC-MS analysis, compound 12 (m/z ) 208) is identified as the main product of ANT photooxidation over both TaON and Pt-TaON. A pathway of ANT degradation over TaON or Pt-TaON under visible light irradiation is proposed and shown in Scheme 2. Holes and active H species are first produced under light irradiation. Then, the active species ANT+• are produced similar to PHE+•. In succession, the most active positions (9 and 10 positions (33)) of ANT+• are attacked by O2 and active H species, resulting in the formation of products 13, 14, and 15. Subsequent reactions of compounds 13 and 15 with O2-• anions directly produce compound 12. Compound 11, which derived from compound 14, is also further converted to compound 12. Similar to PHE, ANT is finally degraded to small organic molecules or CO2. The photooxidation products of ACE, BaA, and PY were also analyzed by GC-MS. The total ion chromatograms obtained from the PAHs photooxidation over Pt-TaON and the corresponding mass spectra of main products are shown in SI Figures S9-S11. According to the GC-MS analysis, the possible photooxidation pathways of ACE, BaA, and PY are proposed and shown in SI Schemes S1, S2, and S3, respectively. In the oxidation process of ACE, the 1 position is first oxidated to produce compound 17, which is one of the main degradation products of ACE. Further oxidation of compound 17 leads to the formation of product 18, which is another main product. Subsequently, small organic molecules or CO2 are produced, owing to the further photodegradation of these intermediates. The initial reaction of BaA occurred at the 7 and 12 positions or at the 5 and 6 positions, resulting in the formation of products 20 and 21, respectively. The yield of product 21 is very low, whereas a large amount of compound 20 is produced. Upon further oxidation of these compounds, small organic molecules or CO2 are produced. In the photooxidation of PY, two kinds of pyrenequinone (m/z ) 232) are detected as the main products, which could be tentatively assigned to compounds 23 and 24 (3, 34). The oxidation of 1, 6 positions and 1, 8 positions of PY can result in the formation of compounds 23 and 24, respectively. The yields of pyrenequinones increase before 6 h, and then decrease with the prolonging of time. Finally, pyrenequinones are further oxidated to produce small organic molecules or CO2 via intermediates 25 and 26.

Acknowledgments Financial support from the National Natural Science Foundation of China (Nos. 50732004 and 20773064), the National High Technology Research and Development Program of China (No. 2006AA05Z113), the Science and Technology Research Program of the Ministry of Education (MOE) of China (No. 307012), and the National Basic Research Program of China (973 Program, 2007CB613301, 2007CB613305) is gratefully acknowledged.

Supporting Information Available Auxiliary information on experimental procedures, characterization of samples, GC-MS data, and reaction mechanisms. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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