Photocatalytic Oxidation of Aqueous Ammonia over Microwave

May 9, 2008 - Correspondence author phone: 886-2-33664406 ; fax: 886-2-23928830; e-mail:[email protected]., †. Graduate ... The performance of photoca...
2 downloads 10 Views 2MB Size
Download PDF
Environ. Sci. Technol. 2008, 42, 4507–4512

Photocatalytic Oxidation of Aqueous Ammonia over Microwave-Induced Titanate Nanotubes HSIN-HUNG OU,† CHING-HUI LIAO,† YA-HSUAN LIOU,‡ JIAN-HAO HONG,† A N D S H A N G - L I E N L O * ,† Research Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Road, Taipei, Taiwan 106, ROC, and Department of Geosciences, National Taiwan University, 245 Chou-Shan Rd., Taipei, Taiwan 106, ROC

Received December 21, 2007. Revised manuscript received February 18, 2008. Accepted March 31, 2008.

Characterizations of microwave-induced titanate nanotubes (NaxH2-xTi3O7, TNTs) were conducted by the determinations of specific surface area (SBET), X-ray diffraction (XRD), X-ray photoelectron spectroscopic (XPS), ionic coupled plasma-atomic emission spectrometry (ICP-AES), scanning electron microscopy/ energy dispersive X-ray (SEM/EDX), and high-resolution transmission electron microscopy (HR-TEM). The applied level of microwave irradiation during the fabrication process is responsible for both the intercalation intensity of Na atoms into TNTs and the type of crystallization phase within TNTs, which dominate the efficiency of photocatalytic NH3/NH4+. A pure TNT phase presents no powerful ability toward photocatalytic NH3/ NH4+, while the photocatalytic efficiency can be enhanced with the presence of a rutile phase within TNTs. In addition, the mixture of anatase and rutile phase within P25 TiO2 prefers forming NO3-, whereas TNTs yield higher NO2- amount. Regarding the effect of acid-washing treatment on TNTs, the acidtreated TNTs with enhanced ion exchangeability considerably improve the NH3/NH4+ degradation and NO2-/NO3- yields. This result is likely ascribed to the easy intercalation of NH3/ NH4+ into the structure of acid-washing TNTs so that the photocatalytic oxidation of intercalated NH3/NH4+ is not limited to the shielding effect resulting from the overload of TNTs.

Introduction Aqueous ammonia (NH3/NH4+, pKa ) 9.3 at 25 °C) is a major nitrogen-containing pollutant in wastewater. The presence of NH3/NH4+ in the aquatic body causes not only the eutrophication but also the reduction of chlorine disinfection efficiency during water treatment (1, 2). Although a variety of treatments, including biological nitrification, air stripping, break-point chlorination, and ion exchange, have been applied on the removal of NH3/NH4+ from water and wastewater, photocatalytic oxidation of NH3/NH4+ over TiO2 is still a promising candidate due to its inherent stability and high photocatalytic potential. Not much attention, however, has been paid to this regard in the past decades. * Correspondence author phone: 886-2-33664406; fax: 886-223928830; e-mail:[email protected]. † Graduate Institute of Environmental Engineering. ‡ Department of Geosciences. 10.1021/es703211u CCC: $40.75

Published on Web 05/09/2008

 2008 American Chemical Society

The dependence of NH3/NH4+ degradation over TiO2 on the solution pH has been revealed where NH3 is more likely to react with OH · than NH4+ is. A possible explanation was proposed by Bravo et al. (5). in terms of the electrostatic repulsion between NH4+ and TiO2 surface. But Zhu et al. (2). indicated that the pH-dependent equilibrium between NH3 and NH4+ was responsible for this phenomenon. In addition, Pollema et al. (6). demonstrated that the relationship between the formation of nitrite ions (NO2-)/nitrate ions (NO3-) and the pH values where high pH values favored the formation of NO3-, whereas NO2- was the principal product at lower pH condition. A similar conclusion was evidenced by the subsequent studies (1–4). The effect of the loading amount of TiO2 on the photocatalytic oxidation of NH3/NH4+ and on the yields of NO2- and NO3- had been studied by Zhu et al. (2), who indicated that the rate of photocatalytic NH3/NH4+ to NO3- was limited to the NH3/NH4+oxidation to NO2-. Meanwhile, Lee et al. (1) demonstrated that the presence of Pt particles not only enhanced the OH · production from N2O but also stabilized the intermediate NHx (x ) 0, 1, 2) species to facilitate the N2 formation. Since Kasuga et al. (7, 8) discovered titanate nanotubes (TNTs), TNTs have received a lot of attention due to their one-dimensional nanostructure and versatile applications, including solar cells, photocatalysis, and electroluminescent hybrid devices (9–13). Although the formation mechanism and chemical structure of TNTs are still ambiguous, the applications of TNTs and even their derivates (nanoparticles and nanorods) on photocatalysis have been emerging. Some researchers considered TNTs an effective photocatalyst for pollutants degradation (14, 15), whereas others came to the opposite conclusion (16, 17). In addition, the ion exchangeability of TNTs had also been confirmed by Sun and Li (9), who indicated that ammonia ions can be intercalated into the crystal lattice of TNTs. This specific attribute was also exploited to synthesize nanocomposites CdS/TNTs and subsequently used to evaluate the photocatalytic ability toward the degradation of methyl orange (11, 12). The aforementioned TNTs often require at least 20 h to achieve a perfect tube structure via the conventional hydrothermal process, in which the crystallization of TNTs depends on the applied temperature, treatment duration, and pressure (15, 18). So far, far few researchers have particularly attempted the rapid kinetics of TNT formation. To the best of our knowledge, Wu et al. (19) first discovered that TNTs can be obtained within the short fabrication duration (90 min) under the aid of microwave irradiation power (195 W). The effect of irradiation power on the characterization and the chemical composition of TNTs were also investigated in their following research (20). A similar result had also been revealed by Wang et al. (21) that the fabrication process was carried out under 750 W in a domestic microwave oven. However, the synthesis temperaturesa critical factor in synthesizing TNTssin their research cannot be maintained during the hydrothermal process. This implies the explicit effect of microwave irradiation on the characterization of TNTs cannot be investigated exclusively. Our recent research has examined the effect of microwave irradiation on the characterization of TNTs where the synthesis temperature can be maintained on the basis of the feedback of irradiation power. We have also found that the amount of intercalated Na within TNTs (NaxH2-xTi3O7) is subject to the applied level of microwave irradiation (10). Coupled with the present literature, the objective of this research was to evaluate the application of microwave-induced TNTs on the degradation of NH3/NH4+ VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4507

on the basis of its corresponding bifunctions of photocatalytic ability and ion exchangeability. The yields of NO2- and NO3were also determined to examine the degradation mechanism of NH3/NH4+ over microwave-induced TNTs. A possible pathway for the photocatalytic oxidation of NH3/NH4+ over microwave-induced TNTs was also suggested on the basis of demonstrated results.

Experimental Section Preparation of TNTs. The synthesis of microwave-induced TNTs has been described in the Supporting Information. HTNTs-70W, HTNTs-400W, and HTNTs-700W are the abbreviation of TNTs fabricated under different applied level of irradiation power and further received acid treatment by 0.5N HCl. By comparison, WTNTS-70W, WTNTs-400W, and WTNTs-700W are the TNTs samples without acid treatment. The morphology of TNTs is also demonstrated in Figure S1 of the Supporting Information. Characterization of TNTs. Characterization of TNTs included X-ray diffraction (XRD), specific surface areas (SBET), field-emission scanning electron microscopy (SEM), highresolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectrometry (ICP-AES). (See the Supporting Information for characterization of TNTs.) Photocatalytic Oxidation of NH3/NH4+ over TNTs. Photocatalytic oxidation of NH3/NH4+ over TNTs was conducted in a batch reactor. The initial concentration of NH3/NH4+ was prepared at (5.9 ( 0.3) × 10-4 M with the pH value controlled at the range of 10 ( 0.2, because the reaction of photocatalytic oxidation of NH3/NH4+ is very slow or negligible at pH values lower than 9 (2, 4–6). Other detailed information about the experimental procedure and the product analyses are available in the Supporting Information.

Results and Discussion XRD Patterns and Na/Ti Ratio of TNTs. Figure 1a demonstrates the XRD patterns of P25 TiO2 and TNTs synthesized under different irradiation power. It is natural that P25 TiO2 shows a mixture of anatase and rutile phases. TNTs are preferentially assigned for NaxH2-xTi3O7, of which the XRD patterns are consistent with the results reported by previous researchers (22–25). No anatase phase is present, but identifiable rutile phase peaks can be observed. This phenomenon is corresponding to the anatase phase being the preferred phase in synthesizing TNTs (15, 26, 27). Regardless of the effect of acid-washing on TNTs, the intensity of the rutile phase became relatively weak with increasing the applied level of irradiation power during the fabrication process (Figure 1b). The cases of WTNTs-700W and HTNTs700W with the unapparent rutile phase suggest that the TNT phase dominates in these samples. The result also reflects that the relatively stable rutile phase can be transformed to TNT phase under the presence of strong irradiation power. In addition, the intensity of the rutile phase within HTNTs is relatively higher than that within WTNTs, which suggests acid-washing on TNTs can improve the crystallization of the rutile phase. A similar phenomenon had also been revealed by Tsai and Teng (28), who indicated that the intensity of anatase was improved with decreasing pH values. They also ascribed this result to shrinking of the titanate layer into the anatase phase under the acidity condition because of the similar zigzag configuration. An increase in the ratio of peak [211] to peak [110] in XRD patterns suggests more Na atoms intercalated into the structure of TNTs, which resulted in an increase in Na/H ratio within TNTs (22–24). Similar XRD patterns concerning the effect of acid-washing on TNTs had also been demonstrated by Teng’s group although they tended to assign TNTs 4508

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008

FIGURE 1. (a) XRD patterns of P25 TiO2, HTNTs-70W, HTNTs-400W, and HTNTs-700W. (b) XRD patterns of WTNTs-70W, WTNTs-400W, WTNTs-700W, HTNTs-70W, HTNTs400W, and HTNTs-700W. for NaxH2-xTi2O5(H2O) (15, 28, 29). Figure 1b shows the intercalated Na atoms within TNTs-70W are easily replaced by H atoms during acid-washing treatment. In other words, O-Na bonds within the TNT structure can be strengthened under the assistance of strong microwave irradiation. This means most of the intercalated Na atoms are still retained within the TNTs-700W structure even after acid-washing treatment. Determinations of ICP-AES and EDX on TNTs can support this result other than the rough analysis via XRD patterns (Table S1 of the Supporting Information). Another similar trend was also observed as HTNTs were sintered at 700 °C, where HTNTs-70W, -400W, and -700W had the corresponding Na/Ti ratios of 0, 0.326, and 0.518, respectively. This indicates the intercalation of Na atoms into the structure of TNTs instead of weak adsorption on the surface of TNTs. Effect of Applied Microwave Irradiation and AcidWashing on the Ion Exchangeability of TNTs. Figure 2presents the dependence of ion exchangeability of HTNTs on the applied irradiation power during the fabrication process. With intention to more detailed investigations on the effect of irradiation power, WTNTs were also studied to eliminate the influence caused by the acid-washing. Regardless of the effect of acid-washing on TNTs, the exchange capability of TNTs decreases with an increase in applied microwave irradiation power utilized in the fabrication of TNTs (Figure 2). This phenomenon is corresponding to the aforementioned result that strong microwave irradiation intensifies the intercalation of Na atoms within the TNT structure even though TNTs receive the acid-washing treatment. In addition, the influence of SBET of TNTs on the amount

FIGURE 2. Normalized concentration of remaining NH3/NH4+ and the exchange capacity of TNTs as a function of the applied irradiation power during the fabrication process. The experimental conditions were [NH3/NH4+]ini ) (5.9 ( 0.3) × 10-4 M, pH ) 10 ( 0.2, and air-saturated. (The ordinate scale refers to the concentrations of NH3/NH4+ after 6 h reaction normalized with respect to the initial NH3/NH4+ concentration.) of ion-exchanged NH3/NH4+ is not apparent, which also supports that the exchange potential of TNTs is indeed limited to the intensity of Na intercalation within TNTs rather than the SBET of TNTs (Table S1 of the Supporting Information). Briefly, the potential of ion exchangeability is subject to the intercalation amount of H atoms within TNTs, which is dominated by the applied level of microwave irradiation during the fabrication process. According to the analysis of thermal gravity, Sun and Li (9) have confirmed the ion exchangeability of TNTs synthesized via conventional hydrothermal treatment. The fabrication of CdS/TNTs reported by Hodos et al. (11) was also based on this specific property. As to why HTNTs presented a better ability toward NH3/ NH4+ degradation than WTNTs did, it can be attributed to the following potential factor. Neutral NH3 molecules are accessible to be adsorbed on the surface of HTNTs which behave as a weak Brønsted acid due to more intercalated H atoms (23). The adsorbed NH3 are likely transformed to NH4+ resulting from the contribution by Brøntsed acid. Comparing with WTNTs, the relatively high amounts of H atoms within HTNTs also provide more exchange sites for NH4+ to be exchanged. XPS analysis indirectly support this inference that the concentrations of Na in atomic ratio for WTNTs70W and HTNTs-70W are 9.1 and 3.3%, respectively. Further description and evidence will be demonstrated in the following section. Photocatalytic Oxidation of Agueous Ammonia over Microwave-Induced TNTs. As shown in Table 1, the presence of TNTs leveled off the degradation performance of NH3/ NH4+. This phenomenon can be explained in terms of the enhancing heterogeneous reaction due to the presence of TNT catalysts. In other words, the homogeneous reaction of 36% NH3/NH4+ degradation caused by the direct UV photolysis will be suppressed if TNTs participate in the reaction of photocatalytic NH3/NH4+. This result is consistent with what Zhu et al. (2) presented, that the heterogeneous reaction increased in importance with the presence of TiO2. Regarding the effect of homogeneous reaction, only 5% degradation of NH3/NH4+ was observed by Bonsen et al. (4), whereas Zhu et al. (2). reported that at 55%. Such a difference could be ascribed to the different irradiation source utilized in the photocatalytic reaction. On the basis of the aforementioned results, TNTs-70W was chosen as a representative catalyst to examine the feasibility of TNTs on photocatalytic oxidation of NH3/NH4+. Figure 3 demonstrates the optimum loading amount of TNTs70W falling at 0.8 g L-1. In the case of WTNTs-70W, the overload of the loading amount results in a dramatic decrease

FIGURE 3. Degradation of NH3/NH4+, the yields of NO2- and NO3- as a function of the loading amount of (a) WTNTs-70W and (b) HTNTs-70W. The experimental conditions were [NH3/ NH4+]ini ) (5.9 ( 0.3) × 10-4 M, pH ) 10 ( 0.2, air-saturated, and reaction time ) 6 h. either in the NH3/NH4+ degradation or in the NO2-/NO3yields (Figure 3a). Comparatively, despite the overload of HTNTs-70W also making an inhibitory effect on the photocatalytic oxidation of NH3/NH4+, no dramatic decrease was observed in the yields of NO2- and NO3- (Figure 3b). This reveals that the shielding effect caused by the overload of the loading amount is more obvious for WTNTs-70W than for HTNTs-70W. The result can be explained on the basis of the aforementioned result that HTNTs-70W can provide excellent ability toward NH3/NH4+ intercalation. Therefore, partly intercalated NH3/NH4+ can be degraded once HTNTs-70W accepts the incident UV so that the reduction of photocatalytic efficiency caused by the shielding effect is trivial. Naturally, the intercalation amount of NH3/NH4+ into HTNTs-70W increases with increasingly loading amounts, whereas there is no such a phenomenon in the case of WTNTs-70W (Figure S2 of the Supporting Information). Unlike the specific behavior provided by HTNTs-70W, the overload of WTNTs70W just caused an inhibitory effect on the photocatalytic oxidation of NH3/NH4+. At the condition of optimum loading amount, the degraded NH3/NH4+ in the case of HTNTs-70W is 38.2%, while that of WTNTs-70W is 25.8%, suggesting the effect of acid-washing improves the photocatalytic potential of TNTs. The conclusion is supported by the NO3- yield, where HTNTs70W led to the NO3- yield of 13.0%, whereas WTNTs-70W caused that in 9.6% (Table 1). This is because Zhu et al. (2) have assigned the formation of NO3- in the photocatalytic oxidation of NH3/NH4+ to the heterogeneous photochemical reaction, which can be considered as an indicator of photocatalytic efficiency. A similar phenomenon can be observed on the basis of the yields of NO3- over WTNTs700W and HTNTs-700W (Table 1). As to why the effect of acid-washing improves the efficiency of photocatalytic NH3/ NH4+, it can be attributed to the following two origins: (1) VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4509

TABLE 1. Degradation of NH3/NH4+, Yields of NO2- and NO3-, and Mass Recoveries as N Atom for Photocatalytic Oxidation of NH3/NH4+ over Different Catalysts after 6 h Reaction (Uncertainties, 95% Confidence Intervals) samples

degraded NH3/NH4+ (%)

NO2- yieldb (%)

NO3- yieldc (%)

mass recovery as N atomd (%)

UV light only P25 TiO2 WTNTs-70W HTNTs-70W WTNTs-700W HTNTs-700W

36 53 (2)a 25.8 ( 2.0 (29)a 38.2 ( 3.9 (36)a 20.0 ( 1.7 (7)a 31.3 ( 1.5 (21)a

9.1 0.87 28.9 ( 0.2 42.0 ( 0.6 23.5 ( 0.7 26.6 ( 0.5

0 38.4 9.6 ( 0.6 13.0 ( 0.8 5.3 ( 0.3 9.7 ( 0.5

79.0 86.3 94.0 90.5 96.0 86.9

a The values in the parentheses are the degraded NH3/NH4+ over TNTs without the presence of UV light. [NH3]T,O × 100. c [NO3-]6h/[NH3]T,O × 100. d (14/46[NO2-]6h +14/62 [NO3-]6h + 14/17[NH3]6h) × 100/14/17[NH3]T,O.

b

[NO2-]6h/

FIGURE 4. X-ray photoelectron spectra (XPS) for (a) N 1s region of WTNTs-70W after reaction, (b) N 1s region of HTNTs-70W after reaction, (c) Ti 2p region of HTNTs-70W before and after reaction, and (d) O 1s region of HTNTs-70W before and after reaction. improved intensity of the rutile phase (Figure 1b); The rutile phase in the TiO2 system is also an efficient candidate responsible for the photocatalytic oxidation of NH3/NH4+, and the presence of the rutile phase within TNTs constitutes the effect of interparticles electron transfer, which aids in inhibiting the recombination of irradiated electrons and holes (31, 32): (2) enhanced intercalation sites toward NH3/NH4+ (the protonation process enables the NH4+ to easily diffuse into the layered TNTs structure; the intercalated NH4+ within TNTs can avoid the shielding effect and efficiently be degraded via heterogeneous photochemical reaction). The behavior of NH3/NH4+ within TNTs caused by the effect of the protonation process can be evidenced by XPS determinations. N 1s regions for WTNTs-70W and for HTNTs70W after reaction are demonstrated in Figure 4a,b, respectively. On deconvolution, it could be seen that WTNTs-70W after reaction presents a broad peak consisting of three discernible peaks at 401.1, 399.6, and 397.5 eV, whereas four peaks appear at 404.4, 401.6, 399.0, and 397.4 eV in the case of HTNTs-70W after reaction. For the N 1s region, previous studies have reported the typical binding energy of nitrogen in NH3 and NH4+ falling at the range of 399.1 ( 0.5 and 401.2 ( 0.5, respectively (33–38). Therefore, N 1s core lines for WTNTs-70W (399.6 eV) and HTNTs-70W (399.0 eV) after reaction were preferentially assigned for NH3, while the binding energy of 401.1 eV for WTNTs-70W and 401.6 eV for HTNTs-70W were attributed to the response of NH4+. The 4510

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008

contribution of NH4+ to the N 1s region of HTNTs-70W (31.7%) after reaction is larger than that of WTNTs-70W (10.6%), which again confirms the aid of transformation from NH3 to NH4+ by acid-washing treatment. In addition, the binding energy of 397.0-397.8 is normally indicative of the Ti-N bond (33, 39, 40). Binding energies of 397.5 eV in WTNTs-70W and 397.4 eV in HTNTs-70W after reaction suggest the doping of N atoms into the TNT structure after photocatalytic reaction. An unidentified binding energy of 404.4 eV for HTNTs-70W after reaction lays higher than the 403.8 eV of NaNO2 and lower than the 404.5 eV of the N-Na bond, which would be attributed to the binding energy of N atom in the environment of Na-N-O (33, 36, 38). For the case of Ti 2p for HTNTs-70W after reaction, there is a slight shift in the Ti4+ binding energy toward the lower region due to the reduced oxidation state (Figure 4c). The formation of the reduced oxidation state implies the reaction between the TNTs matrix and NH3/NH4+ during photocatalytic reaction. As mentioned in the above result, doping of N atom into the TNT structure is a possible pathway because the binding energy of Ti-N is lower than that of Ti-O due to the lower electronegativity of the N atom (39–41). Regarding the O 1s region demonstrated in Figure 4d, curve fitting indicated the presence of a shoulder peak (∼532.0 eV) in addition to a dominant peak (∼530.0 eV), which is principally assigned for the surface-adsorbed OH (Ti-OH) and crystal lattice oxygen (Ti4+-O), respectively (41, 42). The percent

SCHEME 1. Hypothetical Scheme Related to the Photocatalytic Oxidation of NH3/NH4+ over TNT Structure

oxidation of NH3/NH4+ adsorbed on the surface of TNTs (pathway 5). In summary, microwave-induced TNTs not only can be obtained within a shorter synthesis time but also constitute the well-defined behavior in photocatalytic capability and ion exchangeability. Although microwaveinduced TNTs have no powerful ability in photocatalysis and ion exchange, they can still be considered as a potential material in some applications due to the corresponding bifunctions. Some modifications (27) about how to enhance the crystallization of TNTs for improving photocatalytic ability are under progress in our group.

Acknowledgments The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 95-2221-E-002-143-MY3.

Supporting Information Available

contribution to the total peak from the component of Ti-OH increased dramatically for HTNTs-70W after reaction as a result of the increasing amount of chemisorbed water on the TNT surface. WTNTs-700W (0.8 g L-1) with a pure TNT crystallization was used to survey the photocatalytic potential of the TNT phase for NH3/NH4+ degradation. NO3- yields over WTNTs700W (a dominant TNT phase) and HTNTs-700W (mixed phases of TNT and rutile phases) are 5.3 and 9.6%, respectively, suggesting TNTs have no powerful photocatalytic ability toward photocatalytic NH3/NH4+ while in comparison with P25 TiO2 which caused NO3- yield at 38.4%. Meanwhile, all TNTs catalysts produced the comparative yields of NO2and NO3-, whereas P25 TiO2 resulted in an overwhelming majority in the yield of NO3- (Table 1). Figure 1a demonstrated this result can be ascribed to the mixture of anatase and rutile phases within P25 TiO2, because Bonsen et al. (4) indicated similar results by evaluating the crystal phases of various commercially available TiO2 powders in the photocatalytic oxidation of NH3/NH4+. Despite P25 TiO2 prevailing over TNTs both in the degradation efficiency of NH3/NH4+ and the yield of NO3-, the best results with respect to NO2formation were found with TNT catalysts (Table 1). Mechanism of the Photocatalytic Oxidation of NH3/NH4+ over TNTs. Either in the case of WTNTs-70W or HTNTs70W, the utilization of UV radiation made trivial effect on NH3/NH4+ degradation. Also, no NO2- and NO3- can be detected without the presence of UV light (Table 1). This reveals the presence of UV light promotes the formation of NO2- and NO3- rather than the NH3/NH4+ degradation. This observation concludes that partly intercalated NH3/NH4+ within TNTs could be transformed to NO2- and NO3-, indicating an alternative pathway in forming NO2- and NO3besides the common photocatalytic oxidation of aqueous NH3/NH4+. In addition, the formation proceeding of NO2and NO3- suggested their formation kinetics follow the firstorder reaction (Figure S4 of the Supporting Information). Therefore, the mechanism of photocatalytic oxidation of NH3/ NH4+ over microwave-induced TNTs can be demonstrated in Scheme 1. With a focus on HTNTs, NH3/NH4+ rapidly approach the surface of TNTs due to the electrostatic attraction, as demonstrated in pathway 1. Pathway 2 shows that NH3 on the TNT surface is transformed into NH4+ because of the surface acidity of HTNTs. The resulting NH4+ is exchanged into the structure of TNTs, as shown in pathway 3. Partly intercalated NH3/NH4+were oxidized via photocatalytic process and then principally transformed into NO2and NO3-, as demonstrated in pathway 4. Simultaneously, NO2- and NO3- were also formed from the photocatalytic

Synthesis of microwave-induced TNTs, characterization of TNTs, operating conditions for batch reaction of photocatalytic NH3/NH4+, analysis methods for determination of NH3/NH4+, NO2-, and NO3-, effect of applied level of microwave irradiation on the Na/Ti ratio and SBET of TNTs (Table S1), morphology of HTNTs-700W (Figure S1), degradation proceeding of NH3/NH4+ as a function of loading amounts of WTNTs-70W and HTNTs-70W (Figure S2), variation of surface charge of HTNTs-70W and WTNTs-70W with pH (Figure S3), and photocatalytic conversion of NH3/ NH4+ over HTNTs-70W and WTNTS-70W (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Lee, J.; Park, H.; Choi, W. Selective photocatalytic oxidation of NH3 to N2 on platinized TiO2 in water. Environ. Sci. Technol. 2002, 36, 5462–5468. (2) Zhu, X.; Castleberry, S. R.; Nanny, M. A.; Butler, E. C. Effect of pH and catalyst concentration on photocatalytic oxidation of aqueous ammonia and nitrite in titanium dioxide suspensions. Environ. Sci. Technol. 2005, 39, 3784–3791. (3) Wang, A.; Edwards, J. G.; Davies, J. A. Photooxidation of aqueous ammonia with titania-based heterogeneous catalysis. Sol. Energy 1994, 52, 459–466. (4) Bonsen, E. M.; Schroeter, S. S.; Jacobs, H.; Broekaert, J. A. C. Photocatalytic degradation of ammonia with TiO2 as photocatalyst in the laboratory and under the use of solar radiation. Chemosphere 1997, 35, 1431–1445. (5) Bravo, A.; Garcia, J.; Dome`nech, X.; Peral, J. Some aspects of the photocatalytic oxidation of ammonia ion by titanium dioxide. J. Chem. Res. 1993, 376–377. (6) Pollema, C. H.; Milosavljevic, E. B.; Hendrix, J. L.; Solujic, L.; Nelson, J. H. Photocatalytic oxidation of aqueous ammonia (ammonium ion) to nitrite or nitrate at TiO2 particles. Monatsh. Chem. 1992, 123, 333–339. (7) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of titanium oxide nanotube. Langmuir 1998, 14, 3160–3163. (8) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Titania nanotubes prepared by chemical processing. Adv. Mater. 1999, 11, 1307–1311. (9) Sun, X.; Li, Yi. Synthesis and characterization of ion-exchangeable titanate nanotubes. Chem. Eur. J. 2003, 9, 2229–2238. (10) Ou, H. H.; Lo, S. L.; Liou, Y. H. Microwave-induced titanate nanotubes and the corresponding behaviour after thermal treatment. Nanotechnology 2007, 18, 175702–175707. ´ .; Ko´nya, Z.; (11) Hodos, M.; Horv´ath, E.; Haspel, H.; Kukovecz, A Kiricsi, I. Photosenstization of ion-exchangeable titanate nanotubes by CdS nanoparticles. Chem. Phys. Lett. 2004, 399, 512– 515. ´ .; Hodos, M.; Ko´nya, Z.; Kiricsi, I. Complex-assisted (12) Kukovecz, A one-step synthesis of ion-exchangeable titanate nanotubes decorated with CdS nanoparticles. Chem. Phys. Lett. 2005, 411, 445–449. (13) Tsai, C. C.; Nian, J. N.; Teng, H. H. Mesoporous nanotube aggregates obtained from hydrothermally treating TiO2 with NaOH. Appl. Surf. Sci. 2006, 253, 1898–1902. VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

4511

(14) Nakahira, A.; Kato, W.; Tamai, M.; Isshiki, T.; Nishio, K. Synthesis of nanotube from a layered H2Ti4O9(H2O) in a hydrothermal treatment using various titania sources. J. Mater. Sci. 2004, 39, 4239–4245. (15) Tsai, C. C.; Teng, H. Regulation of the physical characteristics of titania nanotube aggregates synthesized from hydrothermal treatment. Chem. Mater. 2004, 16, 4352–4358. (16) Sˇtengl, V.; Bakardjieva, S.; Sˇubrt, J.; Vec¸ernı´kova´, E.; Szatmary, L.; Klementova´; M.; Balek, V. Sodium titanate nanorods: Preparation, microstructure characterization and photocatalytic activity. Appl. Catal., B 2006, 63, 20–30. (17) Zhang, S.; Li, W.; Jin, Z.; Yang, J.; Zhang, J.; Du, Z.; Zhang, Z. Study on ESR and inter-related properties of vacummdehydrated nanotube titanic acid. J. Solid State Chem. 2004, 11, 1365–1371. (18) Poudel, B.; Wang, W. Z.; Dames, C.; Huang, J. Y.; Kunwar, S.; Wang, D. Z.; Banerjee, D.; Chen, G.; Ren, Z. F. Formaiton of crystallized titania nanotubes and their transformation into nanowires. Nanotechnology 2005, 16, 1935–1940. (19) Wu, X.; Jiang, Q. Z.; Ma, Z. F.; Fu, M.; Shangguan, W. F. Synthesis of titania nanotubes by microwave irradiation. Solid State Commun. 2005, 136, 513–517. (20) Wu, X.; Jiang, Q. Z.; Ma, Z. F.; Shangguan, W. F. Synthesis of titania nanotubes by microwave method. Chin. J. Inorg. Chem. 2006, 22, 341–345. (21) Wang, Y. A.; Yang, J.; Zhang, J.; Liu, H.; Zhang, Z. Microwaveassisted preparation of titanate nanotubes. Chem. Lett. 2005, 34, 1168–1169. (22) Chen, Q.; Zhou, W.; Du, G.; Peng, L. M. Tritanate nanotubes made via a single alkali treatment. Adv. Mater. 2002, 14, 1208– 1211. (23) Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. The structure of tritinate nanotubes. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 587–593. (24) Morgado, E., Jr.; de Abreu, M. A. S.; Pravia, O. R. C.; Marinkovic, B. A.; Jardim, P. M.; Rizzo, F. C.; Arau ´ jo, A. S. A study on the structure and thermal stability of titanate nanotubes as a function of sodium content. Solid State Sci. 2006, 8, 888–899. (25) Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, J. T. S.; Zhou, W. Formation, structure, and stability of titanate nanotubes and their proton conductivity. J. Phys. Chem. B 2005, 109, 5439– 5444. (26) Seo, D. S.; Kim, J. K.; Kim, H. Preparation of nanotube-shaped TiO2 powder. J. Cryst. Growth 2001, 229, 428–432. (27) Khan, M. A.; Jung, H. T.; Yang, O. B. Synthesis and characterization of ultrahigh crystalline TiO2 nanotubes. J. Phys. Chem. B 2006, 110, 6626–6630. (28) Tsai, C. C.; Teng, H. Structure features of nanotubes synthesized from NaOH treatment on TiO2 with different post-treatment. Chem. Mater. 2006, 18, 367–373.

4512

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008

(29) Nian, J. N.; Teng, H. Hydrothermal synthesis of single-crystalline anatase TiO2 nanorods with nanotubes as the precursor. J. Phys. Chem. B 2006, 110, 4193–4198. (30) Yoshida, R.; Suzuki, Y.; Yoshikawa, S. Effects of synthetic conditions and heat-treatment on the structure of partially ionexchanged titanate nanotubes. Mater. Chem. Phys. 2005, 91, 409–416. (31) Serpone, N.; Maruthanuthu, P.; Pichat, P.; Pelizzatti, E.; Hidaka, H. Exploiting the interparticle electron transfer process in the photocatalysed oxidation of phenol, 2-chlorophenol, and pentachlorophenol: Chemical evidence for electron and hole transfer between coupled semiconductors. J. Photochem. Photobiol., A 1995, 85, 247–255. (32) Ou, H. H.; Lo, S. L.; Wu, C. H. Exploiting the interparticle electron transfer process in the photocatalytic oxidation of 4-chlorophenol. J. Hazard. Mater. 2006, 137, 1362–1370. (33) http://www.lasurface.com/database/elementxps.php. http:// srdata.nist.gov/xps/. (34) Swartz, W. E.; Alfonso, R. A. N(1s) photoelectron spectra of transition metal biguanide complexes. J. Electron Spectrosc. Relat. Phenom. 1974, 4, 351–354. (35) Burger, K.; Tschismarov, F.; Ebel, H. XPS/ESCA applied to quickfrozen solution I.-A study of nitrogen compound in aqueous solution. J. Electron Spectrosc. Relat. Phenom. 1977, 10, 461– 465. (36) Datta, M.; Mathieu, H. J.; Landolt, D. Characterization of transpassive films on nickel by sputter profiling and angle resolved AES/XPS. Appl. Surf. Sci. 1984, 18, 299–314. (37) Larkins, F. P.; Lubenfeld, A. The auger spectrum of solid ammonia. J. Electron Spectrosc. Relat. Phenom. 1979, 15, 137– 144. (38) Hendrickson, D. N.; Hollander, J. M.; Jolly, W. L. Nitrogen 1s electron binding energies. Correlations with molecular orbital calculated nitrogen charges. Inorg. Chem. 1969, 8, 2642–2648. (39) Wang, J.; Zhu, W.; Zhang, Y.; Liu, S. An efficient two-step technology for nitrogen-doped titanium dioxide synthesizing: Visible-light-induced photodecomposition of methylene blue. J. Phys. Chem. C 2007, 111, 1010–1014. (40) Sathish, M.; Viswanathan, B.; Viswanath, R. P. Characterization and photocatalytic activity of N-doped TiO2 prepared by thermal decomposition of Ti-melamine complex. Appl. Catal., B 2007, 74, 307–312. (41) Yu, J. C.; Yu, J.; Zhao, J. Enhanced photocatalytic activity of mesoporous and ordinary TiO2 thin films by sulfuric acid treatment. Appl. Catal., B 2002, 36, 31–43. (42) Ou, H. H.; Lo, S. H. Effect of Pt/Pd-doped TiO2 on the photocatalytic degradation of trichloroethylene. J. Mol. Catal. A: Chem. 2007, 275, 200–205.

ES703211U