A New Route for Degradation of Volatile Organic Compounds under

Feb 15, 2008 - Corresponding author phone and fax: (+86)591-83779256; e-mail: [email protected] (L.D.); [email protected] (F.X.). Cite this:Environ. Sci...
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Environ. Sci. Technol. 2008, 42, 2130–2135

A New Route for Degradation of Volatile Organic Compounds under Visible Light: Using the Bifunctional Photocatalyst Pt/TiO2-xNx in H2-O2 Atmosphere DANZHEN LI,* ZHIXIN CHEN, YILIN CHEN, WENJUAN LI, HANJIE HUANG, YUNHUI HE, AND XIANZHI FU* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou, 350002, P. R. China

Received October 6, 2007. Revised manuscript received December 20, 2007. Accepted December 31, 2007.

The bifunctional photocatalyst Pt/TiO2-xNx has been successfully prepared by wet impregnation. The properties of Pt/ TiO2-xNx have been investigated by diffuse reflectance spectra, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, a photoluminescence technique with terephthalic acid, and electric field induced surface photovoltage spectra. The photocatalytic activity of the sample was evaluated by the decomposition of volatile organic pollutants (VOCs) in a H2-O2 atmosphere under visible light irradiation. The results demonstrated that nitrogendoped and platinum-modified TiO2 in a H2-O2 atmosphere could enormously increase the quantum efficiency of the photocatalytic system with excellent photocatalytic activity and high catalytic stability. The increased quantum efficiency can be explained by enhanced separation efficiency of photogenerated electron–hole pairs, higher interface electron transfer rate, and an increased number of surface hydroxyl radicals in the photocatalytic process. A mechanism was proposed to elucidate the degradation of VOCs over Pt/TiO2-xNx in a H2-O2 atmosphere under visible light irradiation.

Introduction Degradation of volatile organic compounds (VOCs) in the presence of a semiconductor photocatalyst has been extensively studied as a potential method for reducing the severe atmospheric pollution all over the world. Recently, some intense research activities have been devoted to the synthesis of titania materials, which were doped with heteroatoms (1–7). Others have invested a lot of manpower and material resources for exploiting new nontitania photocatalysts (8–11). The goal is to develop effective photocatalysts that can work under visible light rather than ultraviolet (UV) light. In this way, sunlight and indoor light can be utilized more efficiently in the photocatalytic oxidation (PCO) process. However, the quantum efficiency of visible photocatalytic reaction was still very low. Therefore, it is quite difficult to degrade VOCs * Corresponding author phone and fax: (+86)591-83779256; e-mail: [email protected] (L.D.); [email protected] (F.X.). 2130

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under visible light irradiation. Herein, a new route for efficient degradation of VOCs under visible light irradiation has been exploited. One of the most promising and widely investigated materials in the visible photocatalysis was nitrogen-doped titanium dioxide, TiO2-xNx, which showed a high photocatalytic activity in various reactions performed under visible light irradiation (4–7). However, there are still two serious obstacles in the environmental application of photocatalyst TiO2-xNx for the degradation of VOCs. It suffers from both the limitation of low quantum efficiency and poor oxidative ability during the photocatalytic process of VOCs (12, 13). In our preview study, the photocatalytic activity of the Pt-modified TiO2 for the degradation of VOCs, such as benzene, was enhanced by 2 orders of magnitude by adding trace amount of hydrogen to the reaction system under UV light irradiation (14, 15). It is proved that a large number of · OH radicals will be produced when a trace H2-O2 atmosphere is added to the photocatalytic system. As known, the oxidative potential of · OH radicals ( · OH, H+/H2O, 2.72 V) (16) is high enough to attack many organic molecules. The · OH radicals play an important role in the mineralization mechanism of many hazardous chemical compounds in the photocatalytic reactions process (17). Moreover, the deposition of noble metals (e.g., Pt, Au, or Rh) on TiO2 particles could also effectively improve the activity of TiO2, because the noble metals, acting as a sink for photoinduced electrons, could promote the interfacial charge transferring processes in the composite systems and enhance the quantum efficiency of the photocatalytic system effectively (18–21). Therefore, a synergistic effect of TiO2-xNx, Pt, H2, and O2 was expected to achieve superior photoactivity toward VOCs degradation. In this paper, we incorporated the nitrogen-doped and platinum-modified TiO2 to obtain the bifunctional photocatalyst Pt/TiO2-xNx by wet impregnation of TiO2 xerogel. Hydrogen gas was introduced into the O2-rich photocatalytic system for improving the visible-light-driven performance.

Experimental Section Materials. All chemicals were of analytical purity, which can be available and used without further purification. The main chemical reagents were titanium tetraisopropoxide (Alfa, 99.95%), hexachloroplatinic acid (Alfa, 99.95%), benzene (Acros, spectrophotometric grade), and terephthalic acid (Acros, 99+ %). Ammonia–water and other reagents were analytical grade and were used without further treatment. All aqueous solutions were made by using deionized water. Preparation of Photocatalysts. Titanium dioxide was prepared by a modified sol–gel technique (22). Titanium tetraisopropoxide was hydrolyzed under acidic conditions, and the resulting suspension was then dialyzed to a pH of ca. 4. The particles were prepared by evaporating the dialyzed sol to form xerogel at 333 K and then grinding and sieving the xerogel to obtain particulates of the 0.21–0.25 mm sizes. The samples were impregnated with concentrated ammonia solution for 24 h and then thermally treated at different temperatures ranging from 473 to 773 K to form N-doped titanium dioxide, TiO2-xNx. Pt/TiO2-xNx particles were prepared by impregnating the N-doped titania catalysts with an aqueous H2PtCl6 solution (5.13 mol L-1). The initial ratio of Pt to TiO2 was fixed at 1 wt. %. The samples were dried in the air at 393 K for 5 h, heated at 573 K for 3 h, and then photoreduced by 30 wt. % formaldehyde solution, which was sacrificial agent. The resulting particles were washed with deionized water and were dried at 333 K in vacuum for 5 h. 10.1021/es702465g CCC: $40.75

 2008 American Chemical Society

Published on Web 02/15/2008

Characterization of Photocatalysts. UV–vis diffuse reflectance spectra (DRS) of samples were recorded by a spectrophotometer (Varian Carry 500 Scan) equipped with an integrating sphere attachment. X-ray diffraction (XRD) analysis was performed using a Bruker D8 Advance diffractometer with Cu KR radiation. The crystallite size of anatase was calculated from peak half-width by using Scherrer equation with corrections for instrumental line broadening. Specific surface area of the catalysts was determined by applying the Brunauer-Emmett-Teller (BET) method to the sorption of nitrogen at 77 K. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on a Tecnai F30 microscope. Carboncoated copper grid was used as the sample holder. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Kratos Axis Ultra spectrometer with Al KR radiation. The acceleration voltage of 12.5 kV and an emission current of 16 mA were used. The generation of · OH radicals was investigated by the method of photoluminescence with terephthalic acid (TA-PL) (23, 24). The · OH-trapping photoluminescence spectra was surveyed by an Edinburgh FL/ FS900 spectrophotometer. The electric field induced surface photovoltage spectra (EFISPS) were measured on an Edinburgh FL/FS900 spectrophotometer equipped with a lockin amplifier (Standford, SR830-DSP) synchronized with a light chopper (Standford, SR540). (The experimental details of TA-PL and EFISPS are present in the Supporting Information.) Tests of Photocatalytic Activity. Catalysts were packed into a fixed-bed rectangle quartz reactor (2.8 × 4.0 × 0.1 cm3) operated in a single-pass mode. The light resource was a 500 W Xe-arc lamp equipped with an IR-cutoff filter (λ < 800 nm) and an UV-cutoff filter (λ < 420 nm), to ensure it was visible light only (shown in Figure S1). The weight of catalysts was kept constant at 1.2 g in each photocatalytic test. In all cases, the illuminated area of catalysts was identical. Benzene (825 ppm v) was bubbled with pure oxygen (99.99%) and 7.8% hydrogen in nitrogen at a total flow-rate of 50 cm3 min-1. The ratio of H2/O2 was fixed at 0.02. The temperature of reactions was controlled at 303 ( 2 K by an air-cooling system. A flexible probe, type K thermocouple, was inserted into the photoreactor so its tip could be in contact with the catalysts. The effluent gas was analyzed by an online gas chromatograph (HP6890) equipped with a flame ionization detector (FID), a thermal conductivity detector (TCD), and a Porapak R column. The concentrations of organic pollutants and carbon dioxide were detected by the FID and TCD. The gas chromatograph was calibrated using standard reactants. Percent conversion and mineralization of reactants were calculated using eqS 1 and 2, %conversion ) [(C0 - C) ⁄ C0] × 100

(1)

[CO2]produced %mineralization ) × 100 x[CxHy]converted

(2)

where C0 is the initial concentration of the reactant and C is the concentration of the reactants after photocatalytic reaction.

Results and Discussion Figure S2 of the Supporting Information shows the results of photodegradation of ethylene on nitrogen-doped TiO2 calcined at different temperatures under visible light irradiation. The samples of TiO2-xNx-series displayed different photodegradation ability of ethylene. The nitrogen-doped TiO2 calcined at 673 K exhibited the best conversion (30%) of ethylene and the highest yield of CO2 (132 ppm). Hence, we selected 673 K as the typical research temperature for further study. Table S1 of the Supporting Information exhibits the crystal structure and specific surface area of samples. It shows that

FIGURE 1. The DRS spectra of TiO2, TiO2-xNx, and Pt/TiO2-xNx.

FIGURE 2. The HRTEM image of Pt/TiO2-xNx. TiO2, TiO2-xNx, and Pt/TiO2-xNx possessed virtually identical textural properties. The dominant crystal phase for these samples was anatase, with an average crystal size of 8.5 ( 1 nm. No diffraction peak for N or Pt dopant was detected in the investigation of XRD. It indicated that the amount of nitrogen was little and that the platinum was highly dispersed on the catalysts. The specific surface area of samples was almost the same. These results agreed well with the literature (25). The specific surface area and the crystal properties of samples remained almost unaltered after loading with a little platinum. Figure 1 shows the DRS spectra of TiO2, TiO2-xNx and Pt/TiO2-xNx. The curve of TiO2-xNx displays an enhanced optical absorption throughout the detected region. It could be caused by the substitution of oxygen by nitrogen in TiO2 and imported the subband-gap levels in their band gap. This result was consistent with the ref. 5. Furthermore, the Pt/ TiO2-xNx presented an added-optical absorption. This phenomenon was due to the presence of Pt on TiO2-xNx, in agreement with the black color of the powder. To obtain information about the structure of the sample, TEM observation of the Pt/TiO2-xNx composite was carried out. From the HRTEM image (Figure 2), the uniform lattice fringes of titania can be observed over an entire primary particle. The lattice fringes of d ) 0.351 and 0.227 nm in Figure 2 well matched with the crystallographic planes of anatase TiO2 (101) and Pt (111), respectively. The energy dispersive X-ray spectroscopy (EDX) indicated that the photocatalyst was composed of titanium, platinum, and oxygen (Figure S3, the copper signal was due to the carboncoated copper grid used as a sample holder), and the TEM EDX mapping image of Pt/TiO2-xNx also suggested that Pt was highly dispersed on the surface of TiO2-xNx (Figure S4). VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. The XPS spectra of Pt 4f and N2p of Pt/TiO2-xNx, and TiO2 taken as a reference.

FIGURE 4. Effects of different reaction atmospheres on the photodegradation of benzene over Pt/TiO2-xNx irradiated by visible light (H2/O2 ratio: 0.02). XPS analysis was carried out to determine the surface chemical composition of the catalysts and the valence states of various elements present therein. As shown in Figure 3, the core lines of Pt4f for Pt/TiO2-xNx were fixed at 70.7 and 74.0 eV respectively, with a peak area ratio of 4:3, which were attributed to the Pt4f7/2 and Pt4f5/2 of Pt(0) (26). It suggests that the oxidation state of Pt on the sample was Pt(0). The XPS measurement also confirmed the doping of N in TiO2. Compared to TiO2, the analysis of the N2p core line (399.6 eV) of the Pt/TiO2-xNx sample clearly indicates the N-O structure of TiO2-xNx (6). The percentage of N was ca. 0.78%, autocalculated by the instrument. It indicated that the nitrogen-doped titania was successfully synthesized by wet impregnation. Figure S5 of the Supporting Information shows the results of photodegradation of ethylene on TiO2-xNx and Pt/TiO2-xNx under visible light irradiation. The sample of Pt/TiO2-xNx gained higher visible photocatalytic activity than that of TiO2-xNx. The conversions of both photocatalysts were stable, 30.6% for TiO2-xNx and 46.0% for Pt/TiO2-xNx. But the mineralizations of two samples were gradually stable. After the test run (about 4 h), they were stabilized by 94% for TiO2-xNx and 100% for Pt/TiO2-xNx, respectively. The higher activity of Pt/TiO2-xNx may be due to the Schottkybarrier formed at the surface between Pt and TiO2-xNx. The platinum acted as a sink for photoinduced electrons, promoted the interfacial charge-transfer processes, and the separation of photogenerated carriers in the systems (15). The photocatalytic conversion of benzene on Pt/TiO2-xNx in different atmospheres under visible light irradiation is shown in Figure 4. In a pure H2 atmosphere, the conversion of benzene was about zero under visible light irradiation. It was anticipated that the photocatalytic oxidation of benzene usually required oxygen molecules and that the thermal catalysis hydrogenation of benzene occurred at high tem2132

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perature (27). When the experiment was conducted in pure O2 atmosphere, the benzene conversion was initially 1%. However, it decreased quickly to zero after PCO for 0.5 h. Meanwhile, no CO2 was detected. This indicated that only a small amount of benzene was partially oxidized at the beginning and that formed intermediates might occupy the active sites and deactivate the catalyst. Interestingly, when the feeding gas contained both H2 and O2, the conversion of benzene increased from 0 to 18%. It was accompanied by the production of a large amount of CO2. The CO2 yield was calibrated to be 712 ppm, corresponding to a mineralization ratio of 80%. In addition, during this measurement, the gas at the entrance of photocatalytic reactor was desiccative (relative humidity, RH < 1%), and the RH at the end was about 75%. With the proceeding of reaction, the RH became higher than 75%, the sample became wet, and a small amount of water could be collected by a condenser behind the photocatalyst reactor. Two reasons may lead to the formation of water. One is the deep mineralization of benzene, accompanied with the produced CO2. The other is the consumption of · OH radical by the excess H2 (14), because the quantity of the produced water is more than that calculated by the mineralization of benzene. As known, moderate humidity may play an important role in the photocatalytic system, which can hinder the catalyst from carbon accumulation (28, 29). The production of the water may also contribute to the enhancement of the performance of Pt/TiO2-xNx in the H2-O2 photooxidation system. Figure S6 shows the long-time reaction for the Pt/TiO2-xNx photocatalyst toward benzene decomposition in a H2-O2 atmosphere under visible light irradiation. The results demonstrated that the high photocatalytic performance of Pt/TiO2-xNx for the benzene oxidation was effectively maintained at about 40 h of testing, and the conversion and mineralization of benzene were 18 and 80%, respectively. As a comparison, measurements were also taken for Pt/TiO2 under visible light irradiation and for Pt/TiO2-xNx in the dark, both operated in a H2–O2 atmosphere. As expected, no degradation of benzene was detected for the Pt/TiO2 and the Pt/TiO2-xNx samples. In conclusion, nitrogen-doped in the photocatalyst was one of the most important reasons for the photocatalytic reactivity of Pt/TiO2-xNx under visible light irradiation, and it is believed that the synergistic effect of TiO2-xNx, Pt, H2, and O2 was indispensable for Pt/ TiO2-xNx to achieved the superior activity and durability. The photocatalytic activity of Pt/TiO2-xNx toward some other VOCs (toluene, ethylbenzene, cyclohexane, and acetone) has been also investigated. Figure S7 indicated that Pt/TiO2-xNx exhibited excellent photocatalytic performance for the degradation of other VOCs in a H2-O2 atmosphere under visible light irradiation. The conversion of the reactants and the production of CO2 were measured at steady state after PCO for 6 h. It seems that if an alkyl (e.g., methyl or ethyl) was substituted onto the aromatic ring, pollutants should be easier to decompose as compared with benzene. The photocatalytic conversions of toluene and ethylbenzene on Pt/TiO2-xNx were 20 and 23%, respectively, accompanied by a similar mineralization of 84%. As the aromatic rings with alkyl groups (methyl or ethyl) may have higher electron density on the conjugate ring, these reactant molecules can be easily bound on Pt particles and then attacked by surfaceactive oxygen species. Contrarily, with a decrease of the electron density of the cyclic six-ring, the conversion of cyclohexane was relatively low (5.3%), as compared with benzene. A similar phenomenon was observed in the previous study (15), which means the reaction mechanisms were parallel with the degradation of VOCs on Pt/TiO2 in a H2-O2 atmosphere under UV light, that is to say, the degradation of VOCs may be mainly due to the attack of photogenerated

FIGURE 5. · OH-trapping photoluminescence spectra of Pt/ TiO2-xNx under visible light irradiation (420 nm < λ < 800 nm) in a solution of terephthalic acid at room temperature (Excitation (Ex) at 312 nm, Em at 426 nm), (a) H2-O2 and (b) O2. Insert: Plot of the induced fluorescence intensity at 426 nm against illumination time. active oxygen species on surfaces in the photoreaction process. For comparison, the results measured in a pure O2 atmosphere showed that no any organic reactant (toluene, ethylbenzene, and cyclohexane) was oxidized on Pt/TiO2-xNx and that CO2 was not detected. This suggests that it may contribute to the low oxidative ability of hole in the N2p impurity state of Pt/TiO2-xNx under visible light irradiation (13). In addition, acetone was chosen in this study because it was detected as an intermediate of benzene photocatalytic degradation as reported by Sitkiewitz and Heller (30). The result of investigation demonstrated that acetone could be photodegraded on Pt/TiO2-xNx in a pure O2 atmosphere. The conversion and mineralization were 9.4 and 9.6%, respectively. Similarly, these values increased, respectively, to 23.5 and 63.5% when the photocatalytic atmosphere was H2-O2. Figure 5 shows the · OH-trapping photoluminescence spectra of Pt/TiO2-xNx in a terephthalic acid solution at room temperature under visible light irradiation. It is evident that some active radicals were photogenerated on Pt/TiO2-xNx in O2 and H2-O2 atmospheres under visible light irradiation. When the system contains both H2 and O2, the photoluminescence emission peak of 2-hydroxy terephthalic acid (Emisssion (Em), 426 nm) was continuously enhanced (Figure 5a). However, when Pt/TiO2-xNx was exposed to a pure O2 atmosphere, the fluorescence emission peak intensity of TAOH was much weaker than that in a H2-O2 atmosphere (Figure 5b). The insert was the plot of the induced fluorescence intensity at 426 nm against illumination time, and the linear slope of the H2-O2 system was much steeper than the O2 system. In addition, the photoluminescence emission peak of TAOH can not be detected either in the H2-O2-Pt/TiO2-xNx system (in dark) or the H2-O2-Pt/TiO2 system (under visible light irradiation). From the above discussion, we can conclude that the large amount of · OH was generated in the H2-O2Pt/TiO2-xNx photocatalytic system. The superior visible photocatalytic activity of Pt/TiO2-xNx was the synergistic effect of TiO2-xNx, Pt, H2, and O2. All of them play the important roles in improving the visible light photocatalytic activity during the experiment. Figure 6a displayed the EFISPS spectra of TiO2, TiO2-xNx, and Pt/TiO2-xNx in an O2 atmosphere with positive potential (+2 V). It indicated that the suface photovoltage (SPV) response gradually increased in the range from 330 to 375 nm by the sequence of TiO2-xNx < TiO2 < Pt/TiO2-xNx, but the sequence in the visible region was TiO2 < TiO2-xNx < Pt/TiO2-xNx. There may be several reasons for these phenomena. First, nitrogen doped in TiO2, which would serve as charge recombination centers and promote the recom-

FIGURE 6. (a) EFISPS of TiO2, TiO2-xNx, and Pt/TiO2-xNx in an O2 atmosphere with a positive potential (+2 V); (b) EFISPS of Pt/ TiO2-xNx in O2 and H2-O2 atmospheres with a positive potential (+2 V).

SCHEME 1. Proposed mechanism for the visible light photodegradation of VOCs on a Pt/TiO2-xNx catalyst in the coexistence of H2 and O2

bination of photogenerated e- and h+, result in a fall of the UV SPV signal. The nitrogen-doped TiO2 could also lower the absorption edge energy, so the SPV response of TiO2-xNx was higher than TiO2 in the visible region. Second, platinum loaded in the catalyst TiO2-xNx surface, which would form the Schottky-barrier in the interface, acted as a sink for photoinduced electrons and promoted the separation of photogenerated carriers, so the SPV response of Pt/TiO2-xNx was the highest in both the UV and the visible region. Correspondingly, the Pt/TiO2-xNx gained the highest photocatalytic reactivity of VOCs under visible and UV light irradiation. Figure 6b displayed the EFISPS of Pt/TiO2-xNx in O2 and H2-O2 atmospheres with positive potential (+2 V). Obviously, an increasing SPV response was obtained by adding trace H2 into the O2-Pt/TiO2-xNx system in the visible region. The result suggested that the separating efficiency of photogenerated electron–hole pairs in the H2-O2-Pt/TiO2-xNx system was higher than that in the O2-Pt/TiO2-xNx system. This may, in principle, contribute to the high photocatalytic activity of Pt/TiO2-xNx for decomposing persistent organic pollutants in a H2-O2 atmosphere. It was not surprising that the higher separation efficiency was obtained by adding trace H2 into the O2-rich atmosphere, because the · OH radical formed in the H2-O2 system had a higher electron-affinity than that of the O adatom produced in the O2 system ( · OH/ -OH: 1.828 V, O/O-: 1.478 V) (31). On the basis of the above experimental results, the photocatalytic processes in the H2-O2-Pt/TiO2-xNx reaction system were proposed and elucidated in Scheme 1 and eqs 3-7 . These processes may involve several steps: (a) the electron–hole pairs were generated in the body of TiO2-xNx under the visible light irradiation [eq 3]. (b) Oxygen (O2) and hydrogen (H2) dissociatively adsorbed onto Pt particles to give surface O (Os) and surface H (Hs) adatoms. The resulting VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Os adatom captured a photogenerated electron from the Pt particles, producing an Os- ion [eq 4]. The Hs adatom migrated from Pt to the interface of Pt|TiO2-xNx (32), then served as a trapping agent for the hole, and generated a surface proton (Hs+) [eq 5]. (c) The Os- ion could also reacted with the surface proton (Hs+) to produce the · OH radicals [eq 6]. These three processes were believed to generate a large number of · OH groups, which was proven by the TA-PL spectra and by the study of photocatalytic reactivity. Because the oxidative degradation of organic compounds such as benzene was initiated by · OH radicals (33), the generation of · OH radicals was a key factor of the reaction of the photo-oxidation of VOCs [eq 7]. In addtion, under the nonillumination condition, any TAOH fluorescence peak is not examined in the TA-PL experiment, and the photocatalysis activity test also confirmed that the TiO2-xNx cannot oxidize VOCs. In summary, we can conclude that the · OH radicals play the most important role in the photooxidation of VOCs. TiO2-xNx (Vis.-irradiated) f e-+h+

(3)

Os - Pt + e- f Os-

(4)

+

+

Hs - Pt + h f Hs

(5)

Os- + Hs+ f · OH

(6)

·OH+VOCs f ··· f H2O+CO2

(7)

On second thoughts, it was noted that the interfacial electron transfer on the photocatalyst surface was a ratedetermining step during the photocatalytic reactions (34). Normally, the interfacial electron transfer was accomplished by capturing the electron at surface-adsorbed O2 on the TiO2 or TiO2-xNxphotocatalyst (34), but the Pt/TiO2-xNx photocatalyst captured the electron by surface-adsorbed Os. Obviously, the electron-affinity of the O adatom (O/O-, 1.478 V) was much higher than that of O2 (O2/O2-, 0.43 V) (31). Therefore, the O adatom induced photocatalytic reactivity of ethylene with Pt/TiO2-xNx was much faster than that of TiO2 in the O2 atmosphere. The oxidation ability of oxidant (e.g., · OH) was stronger than that of Os to the photodegradation of stable organic pollutants. In the H2-O2-Pt/TiO2-xNx system, a large number of · OH radicals would be generated. They could promote the photodegradation of stable organic pollutants. Furthermore, the electron-affinity of · OH radicals ( · OH/-OH, 1.828 V) (31) was much higher than that of the O adatom (O/O-, 1.478 V) and of O2 (O2/ O2-, 0.43 V) (31). The photogenerated electron of the conduction band would be easily captured by the frontier, which increased the interfacial electron-transfer rate and gained the high quantum efficiencies of the photoctalytic reaction. This may thus contribute to the high photocatalytic activity and exceptional catalytic stability of Pt/TiO2-xNx. In conclusion, the bifunctional photocatalyst Pt/TiO2-xNx has been successfully prepared. Superior photocatalytic activity and exceptional catalytic stability of Pt/TiO2-xNx for decomposing benzene had been obtained under visible light in a H2-O2 atmosphere. The H2-O2-Pt/TiO2-xNx photocatalytic system can also be successfully applied to decompose some other persistent VOCs such as toluene, ethylbenzene, cyclohexane, and so on. Combined substitutional nitrogen with platinum-modified titanium dioxide can enormously increase the quantum efficiency of the photocatalytic system toward VOCs under visible light irradiation in a H2-O2 atmosphere. It provides a new route for degradation of volatile organic compounds under visible light.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (20537010, 20677010, 20473018, and 20573020), an “863” project from the MOST of China 2134

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(2006AA03Z340), National Basic Research Program of China (973 Program, 2007CB613306), and the Natural Science Foundation of Fujian, China (2003F004, 2005HZ1007).

Supporting Information Available The experimental details of ethylene degradation, TA-PL and EFISPS, transmittance of the combined light filters (Figure S1), calcined temperature selected (Figure S2), crystal structure and specific surface area of samples (Table S1), EDX image of Pt/TiO2-xNx (Figure S3), TEM EDX mapping image of Pt/TiO2-xNx (Figure S4), photodegradations of ethylene on TiO2-xNx and Pt/TiO2-xNx (Figure S5), long-time reaction toward benzene decomposition over Pt/TiO2-xNx (Figure S6), and photooxidation of different pollutants over Pt/TiO2-xNx (Figure S7), These materials are available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Kim, S.; Hwang, S. J.; Choi, W. Y. Visible light active platinumion-doped TiO2 photocatalyst. J. Phys. Chem. B 2005, 109, 24260– 24267. (2) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002, 297, 2243–2245. (3) Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities under visible light. Appl. Catal., A 2004, 265, 115–121. (4) Sato, S. Photocatalytic activity of NOx-doped TiO2 in the visiblelight region. Chem. Phys. Lett. 1986, 123, 126–128. (5) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. (6) Suda, Y.; Kawasaki, H.; Ueda, T.; Ohshima, T. Preparation of high quality nitrogen doped TiO2 thin film as a photocatalyst using a pulsed laser deposition method. Thin Solid Films 2004, 453–54, 162–166. (7) Rane, K. S.; Mhalsiker, R.; Yin, S.; Sato, T.; Cho, K.; Dunbar, E.; Cho, Kuk.; Biswas, P. Visible light — sensitive yellow TiO2-xNx and Fe-N co-doped Ti1-yFeyO2-xNx anatase photocatalysts. J. Solid State Chem. 2006, 179, 3033–3044. (8) Zou, Z. G.; Ye, J. H.; Arakawa, H. Photophysical and photocatalytic properties of InMO4 (M ) Nb5+, Ta5+) under visible light irradiation. Mater. Res. Bull. 2001, 36, 1185–1193. (9) Zhang, C.; Zhu, Y. F. Synthesis of square Bi2WO6 nanoplates as high-activity visible-light-driven photocatalysts. Chem. Mater. 2005, 17, 3537–3545. (10) Li, Z. H.; Dong, T. T.; Zhang, Y. F.; Wu, L.; Li, J. Q.; Wang, X. X.; Fu, X. Z. Studies on In(OH)ySz solid solutions: syntheses, characterizations, electronic structure, and visible-light-driven photocatalytic activities. J. Phys. Chem. C 2007, 111, 4727–4733. (11) Hou, Y. D.; Wang, X. C.; Wu, L.; Ding, Z. X.; Fu, X. Z. Efficient decomposition of benzene over a beta-Ga2O3 photocatalyst under ambient conditions. Environ. Sci. Technol. 2006, 40, 5799– 5803. (12) Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-concentration dependence on photocatalytic activity of TiO2-xNx powders. J. Phys. Chem.B 2003, 107, 5483–5486. (13) Mrowetz, M.; Balcerski, W.; Colussi, A. J.; Hoffmann, M. R. Oxidative power of nitrogen-doped TiO2 photocatalysts under visible illumination. J. Phys. Chem.B 2004, 108, 17269–17273. (14) Chen, Y. L.; Li, D. Z.; Wang, X. C.; Wang, X. X.; Fu, X. Z. H2-O2 atm increases the activity of Pt/TiO2 for benzene photocatalytic oxidation by two orders of magnitude. Chem. Comm. 2004, 2304–2305. (15) Chen, Y. L.; Li, D. Z.; Wang, X. C.; Wu, L.; Wang, X. X.; Fu, X. Z. Promoting effects of H2 on photooxidation of volatile organic pollutants over Pt/TiO2. New J. Chem. 2005, 29, 1514–1519. (16) Schwarz, H. A.; Dodson, R. W. Equilibrium between hydroxyl radicals and thallium(II) and the oxidation potential of OH(aq). J. Phys. Chem. 1984, 88, 3643–3647. (17) Kochany, J.; Bolton, J. R. Mechanism of photodegradation of aqueous organic pollutants 0.2. measurement of the primary rate constants for reaction of · OH radicals with benzene and some halobenzenes using an EPR spin-trapping method following the photolysis of H2O2. Environ. Sci. Technol. 1992, 26, 262–265. (18) Baba, R.; Nakabayashi, S.; Fujishima, A.; Honda, K. Investigation of the mechanism of hydrogen evolution during photocatalytic

(19)

(20)

(21)

(22)

(23)

(24)

(25) (26)

water decomposition on metal-loaded semiconductor powders. J. Phys. Chem. 1985, 89, 1902–1905. Domen, K.; Sakata, Y.; Kudo, A.; Maruya, K.; Onishi, T. The photocatalytic activity of a platinized titanium-dioxide catalyst supported over silica. Bull. Chem. Soc. Jpn. 1988, 61, 359–362. Anpo, M.; Chiba, K.; Tomonari, M.; Coluccia, S.; Che, M.; Fox, M. A. Photocatalysis on native and platinum-loaded TiO2 and ZnO catalysts—Origin of different reactivities on wet and dry metal-oxides. Bull. Chem. Soc. Jpn. 1991, 64, 543–551. Einaga, H.; Futamura, S.; Ibusuki, T. Improvement of catalyst durability in benzene photooxidation by rhodium deposition on TiO2. Chem. Lett. 2001, 30, 582–583. Fu, X. Z.; Zeltner, W. A.; Yang, Q.; Anderson, M. A. Catalytic hydrolysis of dichlorodifluoromethane (CFC-12) on sol-gelderived titania unmodified and modified with H2SO4. J. Catal. 1997, 168, 482–490. Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Quantum yields of active oxidative species formed on TiO2 photocatalyst. J. Photochem. Photobiol. A 2000, 134, 139–142. Hirakawa, T.; Nosaka, Y. Properties of O2-• and OH• formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions. Langmuir 2002, 18, 3247– 3254. Fu, X. Z.; Zeltner, W. A.; Anderson, M. A. The gas-phase photocatalytic mineralization of benzene on porous titaniabased catalysts. Appl. Catal. B 1995, 6, 209–224. Briggs D.; Seah M. P. Pratical Surface Analysis: Auger and X-ray Photoelectron Spectroscopy, Second edition; John Willey and Sons: New York, 1992; Vol 1.

(27) Kazantsev, R. V.; Gaidai, N. A.; Nekrasov, N. V.; Tenchev, K.; Petrov, L.; Lapidus, A. L. Kinetics of benzene and toluene hydrogenation on a Pt/TiO2 catalyst. Kinet. Catal. 2003, 44, 529– 535. (28) Keller, V.; Bernhardt, P.; Garin, F. Photocatalytic oxidation of butyl acetate in vapor phase on TiO2, Pt/TiO2 and WO3/TiO2 catalysts. J. Catal. 2003, 215, 129–138. (29) Hernandez-Alonso, M. D.; Coronado, J. M.; Bachiller-Baeza, B.; Fernandez-Garcia, M.; Soria, J. Influence of structural and surface characteristics of Ti1-xZrxO2 nanoparticles on the photocatalytic degradation of methylcyclohexane in the gas phase. Chem. Mater. 2007, 19, 4283–4291. (30) Sitkiewitz, S.; Heller, A. Photocatalytic oxidation of benzene and stearic acid on sol-gel derived TiO2 thin films attached to glass. New J. Chem. 1996, 20, 233–241. (31) Berry, R. S.; Mackie, J. C.; Taylor, R. L.; Lynch, R. Spin-orbit coupling and electron-affinity determinations from radiative capture of electrons by oxygen atoms. J. Chem. Phys. 1965, 43, 3067–3074. (32) Benvenutti, E. V.; Franken, L.; Moro, C. C.; Davanzo, C. U. FTIR study of hydrogen and carbon monoxide adsorption on Pt/ TiO2, Pt/ZrO2, and Pt/Al2O3. Langmuir 1999, 15, 8140–8146. (33) Ghigo, G.; Tonachini, G. From benzene to muconaldehyde: theoretical mechanistic investigation on some tropospheric oxidation channels. J. Am. Chem. Soc. 1999, 121, 8366–8372. (34) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69–96.

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