Photocatalytic Oxidation of Aqueous Ammonia Using Atomic Single

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Photocatalytic Oxidation of Aqueous Ammonia Using Atomic Single Layer Graphitic‑C3N4 Hua Wang,†,‡ Yan Su,† Huanxin Zhao,† Hongtao Yu,† Shuo Chen,† Yaobin Zhang,† and Xie Quan*,† †

Key Laboratory of Industrial Ecology and Environment Engineering (Ministry of Education, China), Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ School of Fisheries and Life Science, Dalian Ocean University, Dalian 116023, China S Supporting Information *

ABSTRACT: Direct utilization of solar energy for photocatalytic removal of ammonia from water is a topic of strong interest. However, most of the photocatalysts with effective performance are solely metal-based semiconductors. Here, we report for the first time that a new type of atomic single layer graphitic-C3N4 (SL g-C3N4), a metal-free photocatalyst, has an excellent photocatalytic activity for total ammonia nitrogen (TAN) removal from water. The results demonstrated that over 80% of TAN (initial concentration 1.50 mg·L−1) could be removed in 6 h under Xe lamp irradiation (195 mW·cm−2). Furthermore, the SL g-C3N4 exhibited a higher photocatalytic activity in alkaline solution than that in neutral or acidic solutions. The investigation suggested that both photogenerated holes and hydroxyl radicals were involved the TAN photocatalytic oxidation process and that the major oxidation product was NO3−-N. In addition, SL g-C3N4 exhibited good photocatalytic stability in aqueous solution. This work highlights the appealing application of an inexpensive metal-free photocatalyst in aqueous ammonia treatment.

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INTRODUTION Ammonia, a typical environmental contamination, is often found in surface water and may originate from the drainage of industrial wastewater, the emissions of an agricultural nonpoint source, or the discharge of domestic sewage effluent. It is wellknown that ammonia is toxic to fish, even at low concentrations.1 It has been demonstrated that ammonia could diffuse through the epithelial membrane of fish and could then cross the blood-brain barrier to poison the central nervous system of fish.2 Long-term exposure to ammonia also significantly inhibits the growth of fish due to a decrease in food intake.3 Therefore, special attention must be paid to the removal of ammonia from water. Photocatalysis has been considered a potentially significant strategy for solar energy utilization.4 Among various semiconductor photocatalysts, TiO2 has been regarded as one of the most attractive candidates due to its promising photocatalytic performance, chemical inertness, photocorrosion resistance and nontoxicity.5−7 Photocatalytic removal of ammonia from water by TiO2 needs to be performed under UV light irradiation,8 whereas the UV light only accounts for approximately 4% of the solar radiation energy. To enhance solar light harvesting, efforts have been made to increase the quantity of photons absorbed by enlarging the optical response region.9−11 In addition, several other visible-light driven photocatalysts, such as Bi2Fe 4O9,12 Bi3NbO713 and Bi2WO6,14 have also been investigated. To date, however, most of the reported photocatalysts with high photocatalytic ability for ammonia treatment © XXXX American Chemical Society

under visible light irradiation are metal-based semiconductors. Considering that metals are relatively expensive materials with limited resources, alternative photocatalysts based on nonprecious metal-free materials are being actively pursued. Graphitic carbon nitride (g-C3N4), which is only composed of carbon and nitrogen, has been reported to be an attractive, inexpensive metal-free photocatalyst for hydrogen production from water splitting because of its reduction and oxidation levels both being located inside the bandgap.15 In addition, gC3N4 is fascinating because of its capability for potential applications, including the oxygen reduction reaction,16 selective organic synthesis,17 and organic pollutant degradation,18 from the viewpoint of solar energy conversion, environmental friendliness and cost effectiveness.19−22 Both theoretical calculations and experimental demonstrations have indicated that the two-dimensional g-C3N4 with ultrathin thickness on the nanometer and even subnanometer scale could exhibit a much higher photocatalytic ability than its bulk counterparts, which can generally be attributed to the high specific surface area, which increases the number of surface reaction sites, and to the enhancement of photogenerated charge carrier transfer capability by reducing the recombination probability.23 Most recently, a new type of atomic single layer Received: June 24, 2014 Revised: September 11, 2014 Accepted: September 18, 2014

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dx.doi.org/10.1021/es503073z | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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(AFM Agilent Pico Plus), and the UV-vis diffused reflectance spectrum of the sample was recorded on a Shimadzu UV-2450 spectrophotometer. In addition, to compare the photocatalytic performance with SL g-C3N4, N-doped TiO2 (N-TiO2) and Bi2WO6 were also prepared using the methods described by Cong et al.26 and Zhang et al.,27 respectively. Photocatalytic Removal of Ammonia. The photocatalytic removal of aqueous ammonia was performed in a cylindrical quartz container (volume of 100 mL). A highpressure Xe short arc lamp (CHF-XM35−500W, Beijing Changtuo Sci-tech Co. Ltd., China) was used as a light source, providing an incident light with an intensity of 195 mW·cm−2, which was measured by a radiometer (model FZ-A, Photoelectric Instrument Factory Beijing Normal University, China). All of the experiments were performed at room temperature (approximately 25 °C) with a magnetic stirrer at a constant speed. Considering the critical concentration of ammonia that might induce the toxic effect to fish in neutral solutions, here, the initial concentration of total ammonia nitrogen (TAN) was set at 1.50 mg·L−1. This solution was prepared by diluting stock ammonium chloride solution with ultrapure water, and the pH of solution was adjusted with 2 M HCl or NaOH. At certain time intervals of light irradiation, a volume of 1 mL of the reaction solution was sampled and immediately filtered to remove the photocatalyst for analysis. The concentrations of TAN, nitrite-nitrogen (NO2−-N), and nitrate-nitrogen (NO3−N) were determined colorimetrically using a Technicon AutoAnalyzer II system (Bran + Luebbe, Buffalo Grove, IL). The •OH was detected using an electron spin resonance spectrometer (ESR, Bruker Elexsys A200, Germany) using 5,5dimethyl-1-pyrroline- N-oxide (DMPO) as the spin-trap reagent. To investigate the effects of scavengers on the photocatalysis process of TAN, tert-butyl alcohol (t-BuOH) or ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na) was added to the TAN solution and adjusted the chemical scavengers’ concentration at 1 mM.

g-C3N4 (SL g-C3N4) was successfully fabricated with an excellent photocatalytic activity for the degradation of organic pollutants under visible light irradiation.24 It is important to note that for the ultrathin g-C3N4, the position of the conduction band (CB) is −1.3 V (vs NHE, pH 7), and that of the valence band (VB) is 1.4 V.25 This range of bandgap fully covers the corresponding redox levels of ammonia with the result that oxidation of ammonia can thermodynamically proceed on the SL g-C3N4 (Figure 1).

Figure 1. Scheme of the bandgap structure of g-C3N4 and the solution redox levels for oxidation of ammonia. All of the energies are referred to the electrochemical energy scale (normal hydrogen electrode, NHE). The band positions and redox levels are adjusted for pH 7.

Light of energy greater than the bandgap of the SL g-C3N4 generally leads to the formation of electron−hole pairs (eq 1). SL g − C3N4 + hv → e− + h+

(1)

Valence band holes (h+) have been demonstrated to be powerful oxidants,7 whereas conduction band electrons (e−) can act as reductants to reduce O2 for the formation of hydroxyl radicals (•OH), with the following steps in the sequence: O2 + 2e− + 2H+ → H 2O2

(2)

H 2O2 + e− → •OH + OH−

(3)

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RESULTS AND DISCUSSION Characterization of SL g-C3N4. Bulk polymeric g-C3N4, similar to graphite, is composed of horizontal stacked carbon nitride layers by van der Waals interactions (SI Figure S1). The obtained SL g-C3N4, as illustrated in the TEM image in SI Figure S2, displays a layer structure with some chiffon-like wrinkles, and the characteristic XRD peak (SI Figure S3) at 27.5 is attributed to the g-C3N4.19 The AFM image in SI Figure S4 shows the size of the SL g-C3N4 is from approximately 2 to 5 μm, and the typical thickness of this layer is approximately 0.5 nm (SI Figure S5). Considering the theoretical issues in distance calculation between each layer of g-C3N4 (0.326 nm),12 the measured thickness of the SL g-C3N4 is thinner than that of the sum of the double layer spacing, which demonstrated that here, an atomic single layer g-C3N4 was obtained. It can be observed from SI Figure S6 that the absorption edge of SL g-C3N4 is approximately 415 nm, which corresponds to a bandgap of 3.0 eV. Compared with the bandgap of both bulk g-C3N4 and an ultrathin g-C3N4 nanosheet (thickness of 2.5 nm), which are approximately 2.7 eV,28 the larger bandgap by 0.3 eV of the SL g-C3N4 is attributed to the quantum confinement effect of the atomic single layer, which shifts the conduction and valence band edges in opposite directions. Photocatalytic Removal of TAN with SL g-C3N4. The experiments used to verify the effect of the light source on the

Therefore, the holes or •OH required for the oxidation of ammonia are provided either by reaction 1 or 3, respectively. However, to the best of our knowledge, no prior work regarding the application of SL g-C3N4 as a metal-free photocatalyst for ammonia removal from water has been reported to date. Here, we seek to explore, for the first time, the photocatalytic ability of SL g-C3N4 for aqueous ammonia treatment under Xe lamp irradiation. In addition, a mechanism for ammonia oxidation over SL g-C3N4 is suggested. This strategy may provide a very interesting opportunity for further application of metal-free SL g-C3N4 as a solar-light-driven photocatalyst in aqueous ammonia treatment.

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MATERIALS AND METHODS Preparation of SL g-C3N4. The SL g-C3N4 was prepared according to our previous report.24 Briefly, melamine was used as the source material and calcined at 520 °C for 4 h with a heating rate of 5 °C min−1. Then, the received bulk g-C3N4 was milled into powder with a mortar followed by an annealing process at 550 °C for 3 h to produce a light yellow powder of gC3N4 nanosheets (NS g-C3N4). After that, the NS g-C3N4 was dispersed in isopropanol, and the mixture was sonicated for 8 h. Lastly, the product was separated by centrifugation and then dried in vacuum at 50 °C for 6 h. The morphology of SL gC3N4 was characterized using an atomic force microscopy B



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concentration of 0.30 g·L−1 led to a reduction of the photocatalytic performance. This finding is reasonable because the introduction of a large amount of photocatalysts could induce a significant decrease of light penetration. A suitable concentration of SL g-C3N4 is crucial for optimizing the photocatalytic reaction. Therefore, all subsequent experiments were conducted using 0.20 g·L−1 SL g-C3N4. Aqueous ammonia exists in both ionized ammonium (NH4+) and un-ionized ammonia (NH3); these two forms are in equilibrium in water, and the relative proportion of each type is mainly dependent on the pH, salinity and temperature of the water. The pH of natural water is affected by atmospheric carbon dioxide dissolving and the activities of aquatic organisms. The respiration and photosynthesis of aquatic organisms can cause the water to become acidic with pH levels lower than 7 and to become alkaline with pH levels greater than 9, respectively. Therefore, to clarify the effect of pH on SL gC3N4 photocatalytic performance, we evaluated the pH dependence of the TAN removal at initial pH values of 5, 7, 9, and 11. The effect of pH on TAN (1.50 mg·L−1) photocatalytic removal with 0.20 g·L−1 SL g-C3N4 under Xe lamp irradiation (195 mW·cm−2) is shown in Figure 4. The

SL g-C3N4 photocatalytic performance were performed with a 500-W Xe lamp. Figure 2 reveals that the TAN concentration

Figure 2. Photocatalytic removal of 1.50 mg·L−1 TAN under Xe lamp irradiation (195 mW·cm−2) with or without a 400 nm cutoff filter.

was nearly unchanged in the dark, indicating that the adsorption of TAN by SL g-C3N4 can be neglected. In contrast, the concentration of TAN decreased significantly under irradiation with the full output of the Xe lamp, demonstrating that the SL g-C3N4 exhibits a good photocatalytic activity for ammonia treatment. In addition, the performance of SL g-C3N4 for TAN removal was also evaluated in the visible light region (Xe lamp with a 400 nm cutoff filter), and it can also be observed from Figure 2 that although the TAN decrease tendency is lower than that of under the full output of Xe lamp irradiation, the TAN concentration also continually decreased with time under visible light irradiation. This result is consistent with the SL g-C3N4 diffuse reflectance spectrum (SI Figure S6) analysis, for which the maximum absorption wavelength is 415 nm; wavelengths ranging from 400 to 415 nm could be utilized by SL g-C3N4. Figure 3 shows the effects of SL g-C3N4 dosage on photocatalytic performance under Xe lamp irradiation (195

Figure 4. Effect of pH on TAN (1.50 mg·L−1) photocatalytic removal with 0.20 g·L−1 SL g-C3N4 under Xe lamp irradiation (195 mW·cm−2).

control experiments conducted in the dark without the photocatalyst revealed that the concentration of TAN was nearly unchanged at pH 5, 7, 9, and even at 11 within 6 h, indicating that the volatilization of TAN can be neglected under present experimental conditions. The photocatalytic removal of TAN was observed to the pseudo-first-order kinetics by the linear transform ln(C0/Ct) = Kt (C0 is the initial concentration of TAN, Ct is the concentration of TAN at time t, and K is a kinetic constant). Under the given experimental conditions, the kinetic constant of TAN removal at pH 11 (0.300 h−1) was 1.8 times that of pH 9 (0.169 h−1), approximately 3 times that of pH 7 (0.101 h−1), and nearly 8.3 times that of the value for pH 5 (0.036 h−1). This indicated that the SL g-C3N4 exhibited more effective photocatalytic activity in alkaline solution than in neutral or acidic solutions. This result is consistent with the previous reports that a high pH value would be beneficial for ammonia photocatalytic removal from water.29−31 In other words, the photocatalytic oxidation of ammonia is the decisive approach of the TAN removal, and pH is an important factor influencing the rate of TAN photocatalytic oxidation. The variation of pH during the photocatalytic process of TAN was also examined to obtain further insight into the effect of pH on TAN removal with SL g-C3N4. Figure 5 shows the

Figure 3. Effects of SL g-C3N4 dosage on photocatalytic treatment of 100 mL of TAN (1.50 mg·L−1) solution under Xe lamp irradiation (195 mW·cm−2).

mW·cm−2). Compared with the control experiment without the photocatalyst, the dosage of SL g-C3N4 exhibited a significant effect on the TAN removal efficiency. Even with a small amount of SL g-C3N4 (0.05 g·L−1), the TAN removal rate was noticeably increased. When the concentration of SL g-C3N4 was 0.20 g·L−1, the TAN removal rate reached the highest value. A further increase in the amount of SL g-C3N4 at a C



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conversion. In addition, the calculated mass recoveries of nitrogen atoms in water indicated that the direct quantitative measurement of the N-mass of TAN, NO2−-N, and NO3−-N in the aqueous solution was in good agreement with the total nitrogen mass balance. For comparison, the performances of NS g-C3N4, bulk gC3N4, N-TiO2 and Bi2WO6 were also evaluated under the same experimental conditions. The photocatalytic removal of TAN with different photocatalysts is shown in Figure 7. Under Xe

Figure 5. Correlation between pH (initial value of 11.0) variation and TAN (initial concentration of 1.50 mg·L−1) removal in 100 mL of solution with 0.20 g·L−1 SL g-C3N4 under Xe lamp irradiation (195 mW·cm−2).

correlation between the TAN (initial concentration of 1.50 mg· L−1) removal and the variation of pH (initial value of 11.0) in a photocatalytic process with 0.20 g·L−1 SL g-C3N4 under Xe lamp irradiation (195 mW·cm−2). Over 80% TAN was removed in 6 h, which was accompanied by a slight decrease in the pH value from 11.0 to 10.4. This result suggested that the pH value of solution was also affected by the variation of the amount of TAN during the photocatalytic process. The reason for this finding was that the removal of TAN reduces the concentration of aqueous ammonia, which can cause the original alkaline water to become slightly acidic. To trace the inorganic N atoms variation in water, the concentrations of NO2−-N and NO3−-N were measured during the TAN photocatalytic process. As can be seen in Figure 6,

Figure 7. Comparison of the photocatalytic performance of samples of SL g-C3N4, NS g-C3N4, bulk g-C3N4, N-TiO2, and BiWO4 for TAN removal with 0.20 g·L−1 photocatalyst at a pH value of 11under Xe lamp irradiation (195 mW·cm−2).

lamp irradiation for 6 h, more than 80% of TAN were removed in the presence of SL g-C3N4, whereas the concentration of TAN was only decreased by 64%, 33%, 36% and 47% for NS gC3N4, bulk g-C3N4, N-TiO2 and Bi2WO6, respectively. As we know, the photogenerated carrier separation is a crucial factor during the photocatalytic process. Obviously, the thickness of the SL g-C3N4 (0.5 nm) is thinner than that of NS g-C3N4 (2.5 nm). Comparing to NS g-C3N4 or bulk g-C3N4, SL g-C3N4 with thinner thickness could shorten the transfer distance for the photogenerated carriers arriving to the surface, resulting in the enhancement of photocatalytic activity owing to the improved separation efficiency of photogenerated carriers. In addition, it can be found from SI Table S1 that the specific surface area of SL g-C3N4 reaches 380 m2·g−1, whereas the specific surface area of bulk g-C3N4 and NS g-C3N4 are 15 and 320 m2·g−1, respectively. The higher specific surface area of SL g-C3N4 is most likely another key factor contributing to the good performance. Because the photocatalytic oxidation of ammonia is a surface reaction, a large specific surface area could provide more sites for ammonia oxidation and contribute to the excellent photocatalytic activity. The stability of SL g-C3N4 was also evaluated, and a cyclic photocatalytic experiment was performed using 20 mg SL gC3N4 in 100 mL of TAN solution with an initial concentration of approximately 1.5 mg·L−1. As illustrated in Figure 8, after six cycles, the photocatalyst did not exhibit any obvious loss of activity, and the SL g-C3N4 described here presented a high stability in TAN elimination. In the photocatalytic process, the photogenerated holes in the valence band and the photogenerated electrons from the conduction band could move to the catalyst surface to participate in the surface reaction. It has been reported that the photocatalytic oxidation of ammonia is a series of holemediated reactions;33 however, other researches consider that

Figure 6. Variations of TAN, NO2−-N and NO3−-N concentrations during the photocatalytic process with 0.20 g·L−1 SL g-C3N4 at pH 11 under Xe lamp irradiation (195 mW·cm−2).

with decreasing TAN concentration, other inorganic nitrogen forms of both NO3−-N and NO2−-N appeared followed by the concentration of these two forms increasing. When the TAN concentration decreased from 1.50 mg·L−1 to approximately 0.20 mg·L−1 in 6 h, the concentration of NO3−-N reached 1.20 mg·L−1. In addition, it is important to note that the product of NO2−-N only accumulated to less than 0.10 mg·L−1, which would be beneficial for environmental protection because NO2−-N is a well-known toxic contaminate that threatens aquatic animals’ survival and growth.32 The yield of NO2−-N was clearly lower than that of NO3−-N, demonstrating that the NO3−-N was the major product for the TAN photocatalytic D



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could make the charge separation more efficient and inhibit the probability of electron−hole recombination. To further confirm the species of oxidative radicals in the TAN solution, an ESR spin-trapping technique was performed using DMPO as a spin-trap reagent.37 The ESR spectra of SL gC3N4 at a TAN concentration of 1.50 mg·L−1 under Xe lamp irradiation revealed that labeled quartet peaks with a 1:2:2:1 intensity (aN = 14.6 G, aH = 14.8 G) appeared, which could be attributed to the DMPO−OH adduct,38 whereas no ESR signals were observed in the dark (SI Figure S7). This result demonstrates that the oxidative radicals of •OH existed in the present experimental conditions. Based on the results described above, a mechanism for the TAN photocatalytic oxidation using SL g-C3N4 is proposed (SI Figure S8). Under Xe lamp irradiation, light of energy greater than the bandgap of the SL g-C3N4 creates electron−hole pairs via reaction 1. The VB electrons of SL g-C3N4 are excited to the CB and then reduced the dissolved oxygen to form •OH via chain reactions (eqs 2 and 3). Meanwhile, the holes in the VB migrate to the SL g-C3N4 surface. Both •OH and holes could oxidize NH3 with two different reaction patterns. Based on previous findings,8,9,29,32,39,40 the suggested mechanism and conceptual reaction pathway for NH3 oxidation with •OH could be described as

Figure 8. Repeated photocatalytic oxidation of TAN over 0.20 g·L−1 SL g-C3N4.

•OH is the dominant reactive radical species for ammonia oxidation.34 According to a previous report, the photogenerated holes in g-C3N4 cannot oxidize hydroxyl ion (OH−) to form •OH because the standard redox potential of g-C3N4 is more negative than the standard redox potentials of •OH/OH−;35 however, •OH may be derived from the photogenerated electrons from the conduction band of g-C3N4. Therefore, in this study, the possible pathways for ammonia photocatalytic oxidation to nitrite and nitrate might involve a direct oxidation process induced by the photogenerated holes and/or by the oxidation of •OH that formed from the photogenerated electrons. To further investigate the effects of •OH and holes during the TAN photocatalytic process, radical trapping experiments were performed using t-BuOH (•OH scavengers) and EDTA2Na (holes scavengers), respectively.36 Figure 9 shows the

NH3 + •OH → •NH 2 + H+

(4)

•NH 2 + •OH → NH 2OH

(5)

NH 2OH + •OH → •NHOH + H+

(6)

The NH3 adsorbed on the surface of SL g-C3N4 is oxidized into •NH2 by •OH, which is a prerequisite species to the formation of NH2OH, and then NH2OH is transformed into •NHOH. However, oxidation of NH3 can also be preceded by the participation of holes via corresponding reactions: NH3 + h+ → •NH 2 + H+

(7)

•NH 2 + OH− → NH 2OH

(8)

NH 2OH + h+ → •NHOH + H+

(9)

Hence, the •NHOH yielded in eqs 6 and 9 appears to be the most important intermediate for NO2− and NO3− production as follows: •NHOH + O2 → •O2 NHOH −

−

•O2 NHOH + OH → NO2 + H 2O + •OH

Figure 9. Effects of scavengers on the TAN photocatalytic process with 0.20 g·L−1 SL g-C3N4 at an initial pH value of 11 under Xe lamp irradiation (195 mW·cm−2).

−

variation of TAN concentrations with the addition of scavengers under the same experimental conditions. Compared with the TAN photocatalytic process without scavengers, the presence of t-BuOH and EDTA-2Na both reduced the photocatalytic activity of SL g-C3N4, indicating that •OH and holes both participate in the TAN oxidation. In addition, the effect of feeding pure oxygen for the TAN photocatalytic process was also tested. It is apparent that pure oxygen in water could enhance the TAN removal. The role of the oxygen in enhancing the TAN photocatalytic oxidation might involve the photogenerated electrons from the conduction band of SL gC3N4 being captured by the oxygen that originally adsorbed on the catalyst surface to promote the formation of •OH and

(10) (11)

NO2 + •OH → HONO2

(12)

HONO2 → NO3− + H+

(13)

Followed by reactions 11−13, both •NHOH and O2 combine to generate •O2NHOH, and then •O2NHOH decays to yield NO2−; last, further oxidation of NO2− will lead to the formation of NO3−. In summary, a high efficiency of the photocatalytic aqueous TAN oxidation under Xe lamp irradiation has been achieved over the metal-free SL g-C3N4 photocatalyst. Compared with bulk g-C3N4 and g-C3N4 nanosheets, SL g-C3N4 with a thickness of approximately 0.5 nm can dramatically improve the photocatalytic activity for TAN removal. The enhanced photocatalytic activity of SL g-C3N4 was attributed to the unique features of the atomic single layer, which improve the E



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separation of photogenerated electron−hole pairs. The results suggested that the VB holes and •OH derived from the electroreduction of dissolved oxygen with electrons via chain reactions were the main reactive species for NH3 oxidation. This work opens a new avenue for the development of a metalfree photocatalysts in TAN treatment.

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ASSOCIATED CONTENT

S Supporting Information *

The structure, TEM image, XRD spectrum, AFM image, and UV−vis diffuse reflectance spectrum of SL g-C3N4 (Figures S1−S6), the ESR spectra of TAN with SL g-C3N4 (Figures S7), a schematic illustration of the TAN oxidation mechanism (Figures S8), and the specific surface area of different photocatalysts (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-411-84706263; fax: +86-411-84706263; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the support of the National Basic Research Program of China (2011CB936002), the National Nature Science Foundation of China (NSFC-JST 21261140334, 21107010, 21107019) and China Postdoctoral Science Foundation (2012M520631). This work was also supported by the Natural Science Foundation of Liaoning province of China (2014020149), the Major Subject of the Committee of Science and Technology of Liaoning province of China (2011203005) and the Public Science and Technology Research Funds Projects of Ocean (2012418025).

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Photocatalytic Oxidation of Aqueous Ammonia Using Atomic Single

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