Mechanism of Photo-Oxidation of NH3 over TiO2 - American Chemical

Jun 29, 2007 - Selective catalytic oxidation of NH3 to give N2 under UV irradiation (photo-SCO) takes place at a low temperature over the TiO2 catalys...
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J. Phys. Chem. C 2007, 111, 11077-11085

11077

Mechanism of Photo-Oxidation of NH3 over TiO2: Fourier Transform Infrared Study of the Intermediate Species Seiji Yamazoe, Taro Okumura, Yutaka Hitomi, Tetsuya Shishido, and Tsunehiro Tanaka* Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed: April 5, 2007; In Final Form: April 26, 2007

Selective catalytic oxidation of NH3 to give N2 under UV irradiation (photo-SCO) takes place at a low temperature over the TiO2 catalyst. An active site for the photo-SCO is a Lewis acid site on TiO2, and adsorption of NH3 to the Lewis acid site is the first step in the photo-SCO. The adsorbed NH3 on the Lewis acid site is activated under UV irradiation to generate NH2 radical and Ti-OH (Brønsted acid site). The formed NH2 radical reacts with an oxygen anion radical species formed under UV irradiation resulting in the formation of NO. The NO is changed to surface NO2-, nitro, and NO3- (monodentate and bidentate) species by a reaction with O2. The generated monodentate and bidentate NO3- species, nitro species, and NO react with the NH2 radical to produce N2 selectively via nitrosoamide species. Additionally, NH4+ generated under the photoSCO also reacts with NOx (maybe NO3- species) and would produce N2 under photoirradiation. A small amount of N2O is produced when Ti3+ reacts with NO instead of O2.

Introduction It is important to remove ammonia from air or water in the view of the environmental preservation. Many chemical processes use reactants containing ammonia and/or produce ammonia. All these processes are annoyed with ammonia slip problem, which is to be solved urgently. Currently, ammonia can be eliminated by several ways such as biological treatment, thermal combustion, and catalytic oxidation. Selective catalytic oxidation (SCO) of ammonia to nitrogen is a potentially available method to reduce the ammonia pollution. SCO technology, attracts interest recently.1-6 Today, most researchers agree with the reaction stoichiometry in the typical SCO condition as7-12

4NH3 + 3O2 f 2N2 + 6H2O

(1)

The SCO process may also be utilized to remove unreacted ammonia in the selective catalytic reduction (SCR) of NOx with ammonia process. In this case, it is necessary to develop an SCO process operated at low temperatures (433 K). It is known that a photocatalytic reaction proceeds at ambient temperature and pressure.18-20 Nevertheless, the number of the reports relevant to the SCO reaction over photocatalyst is limited. Cant and Cole reported that photo-SCO took place in * To whom correspondence should be addressed. E-mail: tanakat@ moleng.kyoto-u.ac.jp. Tel: +81-75-383-2559. Fax: +81-75-383-2561.

a closed system over TiO2 although the activity was low.21 We have reported that photo-SCO of NH3 proceeds at room temperature over TiO2 photocatalyst in a fixed bed flow system and that 100% NH3 conversion and 84% N2 selectivity were achieved at gas hourly space velocity (GHSV) ) 25 000 h-1.22 On the basis of electron spin resonance (ESR) study, we clarified that NH2 radical and oxygen anion radical species produced over TiO2 by UV irradiation are the intermediate species of the photo-SCO. NH2 radical readily reacts with oxygen anion radical because both species are in the same electron-spin state. Moreover, the photo-SCO activity increased by increasing the amount of oxygen anion radical species as well as NH3 chemisorption. NH3 adsorbed on TiO2 traps a hole generated by photoexcitation to give NH2 radical. So far, some reaction mechanisms of the SCO have been proposed.12,15,23-25 For the SCO reaction over the metal oxide catalysts, two reaction mechanisms for oxidation of NH3 to N2 have been proposed. One is the pathway that hydrazine is formed as an intermediate. NH3 adsorbed on a catalyst gives rise to N2 through the decomposition of hydrazine produced by the dimerization of NH2 species that is generated by the oxydehydrogenation, as shown in eqs 2-5.

NH3 f NH2 + e- + H+

(2)

2NH2 f N2H4

(3)

N2H4 f N2 + 4H+ + 4e-

(4)

4H+ + O2 + 4e- f 2H2O

(5)

The other is composed of following two steps. The first step is oxidation of NH2 species generated by the subtraction of hydrogen from NH3 to NOx species by oxygen. In the second step, the produced NOx species is reduced by NH3 to N2 via nitrosoamide, as shown in eqs 6-9.

10.1021/jp0726790 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

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NH3 f NH2 + e- + H+

(6)

NH2 + O2 f NO + H2O

(7)

NH2 + NO f NH2NO

(8)

NH2NO f N2 + H2O

(9)

Ramis et al. have reported that the reaction mechanism of SCO over CuO-TiO2-based catalysts is featured by eqs 2-5.25,26 Most researchers have insisted that the SCO reaction proceeds according to eqs 6-9 over various metal oxide catalysts.15,27-29 On the other hand, another mechanism of the photo-SCO has been proposed by Chuang and co-workers. They reported that NOx (x ) 1 or 2) species are formed on TiO2 surface in the reaction of NH3 with O2 under UV irradiation.24 They proposed that the formed NOx species react with NHx generated from NH3 under UV irradiation in photo-SCO. However, the identification of the surface NOx species is obscure. In addition, the formation of NHy on TiO2 surface and the reaction of NOx with NHy have not been confirmed. Furthermore, they have not checked the products (N2, N2O, and NO) in the photo-SCO. In our previous work, we have reported that the active species for the photo-SCO is NH2 radical and oxygen anion radical species and that the photo-SCO proceeds by the reaction of these radical species.22 However, the detailed mechanism of the photoSCO has not been clarified. Our aim of this study is to elucidate the detailed reaction mechanism of the photo-SCO over TiO2 by investigating the surface species on TiO2 under photoreaction by Fourier transform infrard (FT-IR) measurement. Experimental Methods Preparation Method of Catalyst. The TiO2 sample used in this study was kindly supplied by the Japan Catalysis Society (JRC-TIO-8). This sample contains 0.23 wt % SO4- and 0.3 wt % HCl. The sample was hydrated in distilled water for 2 h at 353 K and was evaporated at 353 K, followed by drying at 383 K overnight. The catalyst was calcined in dry air at 673 K for 3 h. The calcined sample was ground into powder whose size was 100 mesh. The crystal phase of the prepared TiO2 sample was anatase determined by X-ray diffraction (XRD). Catalytic Reaction. The photo-SCO was carried out in a conventional fixed bed flow system at an atmospheric pressure and at room temperature. The TiO2 sample was fixed with quartz wool and filled up in a quartz reactor that had flat facets (12 × 10 × 1 mm3). Before the reaction, the catalyst was pretreated at 673 K for 1 h with the flow of 10% O2 diluted with Ar at 50 mL/min. The typical reaction gas composition was 1000 ppm NH3 and 2% O2 diluted with Ar. A Perkin-Elmer PE300BF 300 W Xe lamp was used as a light source and the sample was irradiated from one side of the flat facets of the reactor. The product concentration of N2 and N2O were analyzed by SHIMADSU GC-8A TCD gas chromatograph with a MS-5A column for N2 detection and a Porapak Q for N2O. The quantity of produced NOx was determined by a Shimadzu NOA-7000 NOx analyzer. Fourier Transform Infrared Spectroscopy. The TiO2 sample (20 mg) was cast into a pellet (diameter ) 12 mm). The molded sample was introduced into an in situ IR cell equipped with BaF2 windows. This in situ IR cell can perform heating, introduction of substrates, photoirradiation, and recording spectra of the sample in situ. The sample was heated in air and evacuated for 30 min at 673 K, followed by treatment with

Figure 1. Products concentration of N2 (circle), N2O (square), and NO (triangle) and NH3 conversion (diamond) in the photo-SCO. NH3, 1000 ppm; O2, 2%; GHSV ) 50 000 h-1.

Figure 2. Light (UV ray) responsible for the photo-SCO over TiO2: N2, circle; N2O, square; NO, triangle. Reaction condition: NH3, 1000 ppm; O2, 2%; GHSV ) 50 000 h-1.

12.0 kPa O2 for 60 min and evacuation for 30 min at 673 K before records of FT-IR spectra. As a light source, a 250 W ultrahigh-pressure mercury lamp (Ushio Denki USH-250D) was used. FT-IR spectra of the sample before and under reaction were recorded with a Perkin-Elmer SPECTRUM ONE Fourier transform infrared spectrometer. The resolution of spectra was 4 cm-1 and the number of quantity survey was 16. Results Photo-SCO Reaction over TiO2. Figure 1 shows a result of photo-SCO reaction over JRC-TIO-8 at GHSV ) 50 000 h-1 in the conventional fixed bed flow system. N2 evolution rate increased gradually with irradiation time and attained a steady rate at 120 min. A 70% NH3 conversion and an 87% N2 selectivity were obtained. N2O and NO were detected as byproducts. We confirmed that the activity was constant over 72 h. Figure 2 shows a result of the responsibility of UV irradiation for the photo-SCO over TiO2. Upon UV irradiation, N2, N2O, and NO were produced gradually and the production rate became the steady rate at 120 min. After 120 min of UV irradiation, the illumination was stopped. The light being off (light-off) let the photo-SCO reaction stop (NH3 conversion was 0%). After 120 min of the light-off, UV irradiation was restarted. Photo-SCO reaction proceeded and the conversion jumped remarkably to the level of the steady rate of the first UV irradiation. Continuously, we carried out the light-off and lighton reaction, and the same result was obtained. This result exhibits that the UV irradiation is essential for the photo-SCO. Additionally, the induction period was observed only under the

Photo-Oxidation of NH3 over TiO2

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Figure 3. Production concentration of N2 (circle), N2O (square), and NO (triangle) in the photo-SCO before and after UV irradiation. NH3, 1000 ppm; O2, 2%; GHSV ) 50 000 h-1. Figure 5. FT-IR spectra of the adsorbed species on TiO2 in the photoreaction of NH3 with O2 in the range of 3800-2800 and 18001100 cm-1. (a) TiO2 after pretreatment, (b) after NH3 and O2 introduction and left for 30 min in the dark, (c) under photoirradiation for 5 min, (d) for 15 min, (e) for 30 min, (f) for 60 min, and (g) for 120 min.

Figure 4. Outlet concentration of N2 (circle), N2O (square), and NO (triangle) under varied experimental conditions. In the first 90 min, 1000 ppm NH3 flowed in the dark, and then the NH3 flow was switched to 2% O2 and UV irradiation was started. GHSV ) 50 000 h-1.

first UV irradiation. It is expected that the induction period is the time for equilibrium adsorption of reactant molecules or for the production of the intermediate of the photo-SCO. To examine this, reactants were passed into the catalyst bed in the dark for first 90 min and then the photoreaction was started. The result is shown in Figure 3. Although the photo-SCO did not proceed in the dark, the conversion rate and the selectivity became steady without an induction period. This result clearly shows that the induction period is attributed to the time for equilibrium adsorption of the reactants. It is known that NH3 is adsorbed on the acid site of TiO2.25,30,31 On the other hand, the adsorption of O2 on the TiO2 hardly occurred. These results strongly suggested that the NH3 adsorption to TiO2 catalyst is the first step in the photo-SCO. To clarify this, we carried out a couple of experiments. One was that 2% O2 (balanced with Ar) was streamed into the catalyst bed for 90 min in the dark. After that, the feed gas was switched to 1000 ppm NH3 and UV irradiation was started. In this case, N2, N2O and NO were not detected in the outlet gas even under the UV irradiation. On the contrary, when NH3 was initially passed, N2 and small amount of N2O and NO were observed under the photoreaction as shown in Figure 4. The initial N2 production amount was almost similar to that in the steady state of the photo-SCO in Figure 1 at the moment that the gas composition was switched and UV irradiation was on. The production rates of N2, N2O, and NO were gradually decreased with time on stream. The total amount of the formed N2, N2O, and NO were 49.1, 1.1, and 0.4 µmol, respectively. The total amount of the detected

nitrogen atom was 0.44 mmol g-cat-1, and this value approximately corresponds to the amount of the adsorbed NH3 on TiO2 (0.40 mmol g-cat-1).30 We have reported that the NH3 adsorbed on the acid site of TiO2 is oxidized to NH2 radical by trapping a hole generated by the photoexcitation of TiO2. The produced NH2 reacts with O2 anion radical species (O2- and O3- radical) generated from O2 on TiO2 under UV irradiation.22 These results indicate that the NH3 adsorbed on TiO2 reacts with O2 under UV irradiation to generate N2, N2O, and NO and that the adsorption of NH3 on TiO2 is the first step in the photo-SCO. FT-IR Spectra of the Reacted Species between NH3 and O2. To clarify the reaction mechanism of the photo-SCO, FTIR spectra of the TiO2 catalyst under photoreaction were recorded with the in situ cell. FT-IR spectra of the TiO2 before and under the photoreaction of NH3 with O2 in the ranges of 3800-2800 and 1800-1100 cm-1 are collected in Figure 5. Figure 5a shows the spectrum of the TiO2 catalyst after pretreatment with O2. A series of bands at 3719 and 3673 cm-1 are assigned to the stretching modes of the isolated surface hydroxyl groups. These are characteristic of anatase and the peak at 3673 and 3719 cm-1 are assigned to the bridged and terminal OH respectively.32-34 Four peaks appeared at 1126, 1153, 1275, and 1364 cm-1 because the TiO2 sample used here contains SO4 ion. The peaks at 1126, 1153, and 1275 cm-1 and the peak observed at 1364 cm-1 can be assigned to the bidentate SO4 ion and the asymmetric SdO-stretching mode of SO4 ion, respectively.35-37 After pretreatment, NH3 (33 µmol) and O2 (20 µmol) were introduced in the TiO2 sample. Figure 5b shows a spectrum of TiO2 exposed to NH3 and O2 for 30 min without UV irradiation. New peaks appeared at 3391, 3356, 3299, 3246, 3145, 1602, 1213, (broad and shoulder) and 1153 cm-1 by the introduction of NH3 and O2. The peaks due to SO4 ion disappeared, and the intensity of the peaks of the surface OH groups decreased. The disappearance of the peaks of SO4 ion is caused by the drastic blue shift by the strong interaction between an adsorbed NH3 and the surface SO4 ion.38,39 The shifted peaks were not shown due to the overlapping of skeletal vibration bands of TiO2. The decrease in the peaks due to surface OH groups indicates the weak interaction between an adsorbed

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Figure 6. FT-IR spectra of TiO2 under the photoreaction for 120 min with (a) 14NH3 + O2 and (b) 15NH3 + O2.

NH3 and surface OH groups. The peaks at 3391 and 3359 cm-1 are assigned to asymmetric- and symmetric-stretching frequencies of NH3 strongly adsorbed on Lewis acid sites, whereas the peaks at 3246 and 3145 cm-1 are assigned to the νs (NH3) split with the overtone of the asymmetric NH3 deformation.24,40,41 The peak at 3299 cm-1 is assigned to asymmetric-stretching frequency of NH3 weakly adsorbed on TiO2.24,40,41 In the lower wave number region, the peaks at 1153, 1213, and 1602 cm-1 are derived from symmetric (first two peaks) and asymmetric NH deformation vibration of NH3 adsorbed on Lewis acid sites.24,40,41 After NH3 and O2 were introduced, UV irradiation was carried out and FT-IR spectra of TiO2 under photoreaction are shown in Figure 5c-g. In the region of 2800-3600 cm-1, the intensity of a large and broad peak gradually increased as the photoreaction proceeds. This peak is assigned to molecular water. Additionally, the intensity of the peak at 1635 cm-1, which is assigned to the deformation frequency of H2O,42,43 gradually increased under UV irradiation. It is exhibited that H2O is generated by the reaction between NH3 and O2. A new peak due to surface hydroxyl species of TiO2 appeared at 3692 cm-1. The intensity of this peak increased under UV irradiation, indicating that the adsorbed NH3 is activated under UV irradiation. When the NH3 is adsorbed on the Lewis acid site, it is expected that NH3-Ti-O-Ti site is altered to TiNH2 (amide radical species) and a new surface OH species (TiOH) under irradiation.44 The NH2 radical reacts with oxygen anion radical species and some intermediates are generated. Although the OH groups on TiO2 combines with each other to form Ti-O-Ti, the increase in the intensity of newly formed OH species indicates that the NH2 species reacts with oxygen anion radical species smoothly and that the NH2 species is generated on the TiO2 surface as an intermediate. In the range of 1100-1800 cm-1, a weak peak at 1295 and a broad peak at 1430-1500 cm-1 appeared in 5 min photoreaction. After that, the peaks at 1300-1360, 1437, 1547, and 1632 cm-1 gradually grew under photoirradiation. It is known that the peaks derived from NHy and NOx species appear in the region of 1200-1800 cm-1.45-47 To identify these peaks, we carried out the same photoreaction with 15NH3. Figure 6 shows the FT-IR spectra of TiO2 under photoreaction for 120 min with (Figure 6a) 14NH3 + O2 and (Figure 6b) 15NH3 + O2. The peak at 1346 cm-1 did not shift, therefore, this peak is derived from a surface species except for nitrogen-contained species. We assigned this peak to an asymmetric SdO-stretching frequency of SO4 ion35-37 because the NH3 molecule that interacted with the SO4 ion is removed by the photo-SCO and the SO4 ion site is opened. It is known that the peak position of an asymmetric SdO-stretching frequency depends on its

Yamazoe et al.

Figure 7. FT-IR spectra of NH3 adsorbed on TiO2 (a) before and after (b) after UV irradiation. (a) TiO2 was exposed to NH3 (33 µmol) for 30 min and then was evacuated for 30 min. (b) UV irradiation time: 30 min.

covalent double bond character.35-37 The peak shift of SO4 ion from 1364 to 1346 cm-1 would be caused by the delicate reduction of the bond order of SdO. The peaks at 1170, 1437, 1547, and 1605 cm-1 slightly shifted to 1165, 1433, 1542, and 1601 cm-1, respectively. These peaks are derived from NHy species. The peaks at 1170 and 1605 cm-1 are assigned to symmetric and asymmetric NH deformation vibration of NH3 adsorbed on Lewis acid sites.24,40,41 It is reported that the asymmetric deformation mode of the NH4+ cation (NH3 adsorbed on Brønsted acid site) is observed in the range of 1420-1460 cm-1. 24,25,40,41 Therefore, the peak at 1437 cm-1 is assigned to NH4+ ion adsorbed on Brønsted acid site, which is the OH groups that formed under photoreaction. The peak at 1547 cm-1 is assigned to the NH2-bending vibration.41,48 The peaks at 1295, 1325 (shoulder), and 1484 cm-1 shifted to 1273, 1300, and 1462 cm-1, respectively. The peaks at 1295 cm-1 are assigned to monodentate NO3- species (the peak at 1500 cm-1 was hidden by the large peak at 1484 cm-1).34,47,49 The shoulder peak at 1325 cm-1 is assigned to the symmetricstretching frequency of nitro species.34,45,49 The peak at 1484 cm-1 is attributed to the asymmetric-stretching frequency of unidentate NO2-.34, 49 It has been demonstrated that NH2 radical species is generated on the TiO2 catalyst under UV irradiation and reacts with oxygen anion radical species.22,50 Our results indicate that NH2 species are formed by the activation of NH3 adsorbed on TiO2 under UV irradiation and continuously NOx species are generated by the reaction of NH2 with O2. FT-IR Spectra of the Reactivity of NH3 Adsorbed on TiO2. Figure 7 represents the effect of UV irradiation on the NH3 species adsorbed on TiO2. The FT-IR spectrum of the NH3 species adsorbed on TiO2 before UV irradiation is shown in Figure 7a. This spectrum is almost similar to Figure 5b except for the OH region (360-380 cm-1). The peaks at 1153, 1213, and 1602 cm-1 are assigned to symmetric (first two peaks) and asymmetric NH deformation vibration of NH3 adsorbed on TiO2.24,40,41 The peaks at 3391 and 3359 cm-1 are assigned to asymmetric and symmetric N-H-stretching frequencies of NH3, whereas, the peaks at 3246 and 3145 cm-1 are assigned to the νs(NH3) split with the overtone of the asymmetric NH3 deformation.24,40,41 The peak at 3198 cm-1 is assigned to asymmetric-stretching frequency of NH3 adsorbed on TiO2.24,40,41,51 The shape of the peaks of OH groups of TiO2 in the region of 3600-3800 cm-1 was different from that of Figure 5b and similar to that of pretreated TiO2 (Figure 5a), indicating that the adsorbed NH3 on the surface OH species is easily

Photo-Oxidation of NH3 over TiO2

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Figure 9. FT-IR spectra of TiO2 under UV irradiation for 60 min (a) with adsorbed 14NH3 + O2 and (b) with adsorbed 15NH3 + O2.

Figure 8. FT-IR spectra of the adsorbed species on TiO2 in the photoreaction of adsorbed NH3 with O2 in the range of 3800-2800 and 1800-1100 cm-1. (a) adsorbed NH3 on TiO2, (b) under photoirradiation for 5 min (c) for 15 min, (d) for 60 min, and (e) for 140 min.

removed by evacuation, and the bridged and terminal OH (at 3673 and 3719 cm-1, respectively) are generated again. In other words, the acidity of OH groups is much weaker. The spectrum after UV irradiation was almost identical with that before UV irradiation. Ramis et al. have proposed that the hydrazine species is generated by the reaction between NH2 species formed on CuO/TiO2 surface and is decomposed to N2.25,26 In fact, they observed hydrazine and NH2 species by FT-IR. However, we could not observe the peaks of not only the hydrazine but also the NH2 species by means of FT-IR. This suggests that the coupling of NH2 species (eq 3) and the decomposition of hydrazine (eq 4) are quite fast. However, the FT-IR spectra shown in Figure 7 indicate that the amount of NH3 adsorbed on TiO2 did not change under UV irradiation. Therefore, it is clear that the coupling of NH2 species to hydrazine has hardly occurred. We conclude that the reaction path via hydrazine (eqs 2-5) is not a main path for the photo-SCO. FT-IR Spectra of the Reaction between NH3 Adsorbed on TiO2 and O2. Figure 8a shows a FT-IR spectrum of TiO2 which was exposed to O2 (20 µmol) after NH3 was adsorbed on TiO2. This spectrum is almost similar to that of the adsorbed NH3 on TiO2 as shown in Figure 7a, indicating that the adsorbed NH3 is not influenced by the exposure to O2. Figure 8b-e shows FT-IR spectra of TiO2 under UV irradiation for 5, 15, 60, and 140 min, respectively. All peaks assigned to adsorbed NH3 (1153, 1213, 1602, 3145, 3198, 3246, 3356, and 3391 cm-1) were decreased, whereas a broad peak in the region of 28003600 cm-1 and a peak at 1626 cm-1, which were assigned to H2O,42,43 gradually grew as the photoreaction time. It is revealed that H2O is generated by the reaction between adsorbed NH3 and O2. Moreover, a new peak assigned to the stretching vibration of OH appeared at 3692 cm-1. This peak would be due to a newly formed surface OH species (Ti-OH) from the NH3-Ti-O-Ti site under UV irradiation. In the region of 1200-1600 cm-1, many peaks appeared by the photoreaction. Figure 9 shows FT-IR spectra of TiO2 under photoreaction between (Figure 9a) adsorbed 14NH3 + O2, and (Figure 9b) adsorbed 15NH3 + O2. The small peak at 1439 cm-1 shifted to 1434 cm-1 and was assigned to NH4 ion.24,40,41 The peaks appeared at 1293 and 1497 cm-1 due to monodentate NO3- 34,49 were shifted to 1276 and 1468 cm-1, respectively. The peaks at 1254, 1564, and 1582 cm-1 due to bidentate NO334,45,47,49 were shifted to 1224 (shoulder), 1547, and 1564 cm-1,

respectively. The peaks at 1325 and 1409 cm-1 shifted to 1299 and 1397 (shoulder) cm-1. These peaks are assigned to the symmetric- and asymmetric-stretching frequency of NO2- (nitro) species.34,45,49 These results indicate that the adsorbed NH3 reacts with O2 and generates NOx species. Additionally, the amount of NOx species increased as the amount of NH3 adsorbed on TiO2 decreased (see Figure 8e). The changes of these surface NH3 and NOx species suggested that the photo-SCO takes place by the reaction of NOx with NH3 under UV irradiation. We have reported that selective catalytic reduction of NO with NH3 under UV irradiation (photo-SCR) proceeds by the reaction between the NH2 species generated from NH3 adsorbed on the TiO2 surface under UV irradiation and NO.44,50 In the case of the photo-SCR, NH2, NO2-, and NO3- ions, and NH2NO (this species is an intermediate of the photo-SCR), species were observed on the TiO2 surface under the photo-SCR reaction.44 The similar adsorbed species such as NH2, NO2-, and NO3ions were obtained in the photo-SCO. Therefore, it seems that the photo-SCO proceeds via NH2NO species that is generated by the reaction of NH2 species and NOx species. FT-IR Spectra of the Reaction between the Adsorbed NOx Species and NH3. As shown in Figure 8, it is suggested that surface NOx species, which is formed by the reaction between NH3 and O2, reacts with NH3. To clear whether NOx species react with NH3 or not, we recorded FT-IR spectra of TiO2 under reaction between NOx species on TiO2 surface and NH3. Hadjiivanov and Kno¨zinger have reported that some NOx species are formed on TiO2 by the co-adsorption of NO and O2, and these species exist on TiO2 stably under evacuation.45 Figure 10a shows a FT-IR spectrum of TiO2 after NO (10 µmol) and O2 (10 µmol) introduction, followed by 30 min evacuation. This spectrum is consistent with that reported by Hadjiivanov and Kno¨zinger. The peaks at 1223 and 1628 cm-1 is assigned to the stretching vibration of the bridged NO3-. The peaks at 1243, 1555, and 1581 cm-1 and the peaks at 1300 and 1500 cm-1 are assigned to bidentate and monodentates NO3-, respectively. The peaks appeared at 1350 and 1616 cm-1 are attributed to adsorbed NO2. These results demonstrate the formation of the NOx species including monodentate and bidentate NO3- on TiO2 surface by the co-adsorption of NO and O2. After the introduction of NH3 (20 µmol) for 30 min in the dark, the spectrum extremely changed as shown in Figure 10b. The peaks of surface OH groups (3676 and 3719 cm-1), adsorbed NO2 (1350 and 1616 cm-1), bridged NO3- (1223 and 1628 cm-1), and bidentate NO3- (1243, 1555, and 1581 cm-1) species disappeared whereas the peaks assigned to NH3 appeared at 3352, 3245, 3150, 1602, 1219, and 1189 cm-1. By means of Q-mass analysis, it is shown that the NOx

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Figure 10. FT-IR spectra of the surface species on TiO2 (a) after NO and O2 co-adsorption, followed by evacuation for 30 min, (b) after NH3 introduction, (c) after evacuation for 30 min, (d) photoirradiation for 30 min, (e) for 60 min, and (f) for 120 min.

Figure 11. FT-IR spectra of surface species on TiO2 after introduction of 14NH3 to (a) 14NO + O2 coadsorbed TiO2 and (b) 15NO + O2 coadsorbed TiO2.

species were not detected in the gas phase after NH3 introduction. Therefore, the disappearance of adsorbed NO2, bridged NO3-, and bidentate NO3- species results from the reaction with NH3 or the rearrangement to the another NOx species. In the region of 2600-3500 cm-1, a broad peak overlapped with the peaks of NH3 appeared. This peak would be assigned to H2O (discussed below). In the range of 1100-1700 cm-1, the intensities of peaks due to monodentate NO3- at 1300 and 1500 cm-1 (the peak at 1500 cm-1 may be hidden by the broad peak at 1400-1520 cm-1) increased and new peaks appeared. To assign these peaks, we carried out the isotopic experiment. In the case of Figure 11a, 14NH3 was introduced after coadsorption of 14NO and O2 on TiO2. In Figure 11b, 14NH3 was introduced after coadsorption of 15NO and O2 on TiO2. The peaks at 1273, 1300, 1323, 1374, 1468, and 1552 cm-1 due to NOx species adsorbed on TiO2 were shifted to 1242, 1272, 1299, 1356, 1441,

Yamazoe et al. and 1524 cm-1, respectively. We decided that the peak at 1300 cm-1 is assigned to monodentate NO3-. The peaks at 1323 and 1374 cm-1 are assigned to the nitrite (nitro) and nitrate (NO3 free ion) species.34,45,47,49 On the other hand, the peaks at 1273, 1468, and 1552 cm-1 in Figure 10b (Figure 11a) disappeared and the peaks at 1263 and 1578 cm-1, assigned to bidentate NO3- species, appeared after evacuation (Figure 10c). It can be seen that this drastic change is caused by the removal of NH3 in the gas phase by evacuation. This fact gives us an idea that the peaks that appeared at 1273, 1468, and 1552 cm-1 are the NOx species that weakly interact with NH3 in the presence of gas-phase NH3, and the bidentate NO3- species is formed by the removal of NH3 interacted with NOx species. Hadjiivanov et al. reported that N-O-H-bending mode of HNO3 species appears at 1460 cm-1.34 Therefore, we considered that the peaks at 1273, 1468, and 1552 cm-1 in Figure 11a are assigned to the bidentate NO3- species that interacted with H of adsorbed NH3. A peak at 1445 cm-1 is assigned to NH4+ and is formed by the adsorption of NH3 on Brønsted acid site (discussed below). Additionally, a small peak was observed at 1151 cm-1 in Figure 11a. There are several reports that N-N-stretching vibration is observed in the range of 1090-1290 cm-1.25,26,34,52 The isotopic experiment showed that this peak that appeared at 1151 cm-1 in Figure 11a shifted to 1142 cm-1. This suggests that the reaction between adsorbed 15NOx and 14NH3 proceeds. This information exhibits that the peak at 1151 cm-1 in Figure 11a is derived from N-N stretching vibration, whose bond is formed by the reaction between the adsorbed NOx and NH3. Moreover, a small peak was observed at 1559 cm-1 in Figure 11b. This peak was assigned to the 14NH2-bending vibration (the peak attributed to NH2-bending vibration was hidden in the large peak at 1552 cm-1 and was assigned to the bidentate NO3- species that interacted with NH3 in Figure 11a).41,48 The formation of NH2 species indicates that the introduced NH3 is activated to NH2 species by the reaction with adsorbed NOx species in the dark because NH3 is not activated to NH2 species in the absence of adsorbed NOx species on TiO2 (see Figure 7b). Ramis et al. reported that the peaks of the NH2-bending vibration and NO-stretching vibration of NH2NO species appear at 1555 and 1460 cm-1, respectively.41,48 In our case, it is unknown whether the peak was assigned to the NO-stretching vibration of NH2NO species because a large and broad peak was observed around 1468 cm-1. However, the peaks of the NH2 species and the N-N bond are consistent with the nitrogen of adsorbed NOx and NH3, indicating the formation of NH2NO species. Furthermore, 14N15N was detected in the gas phase by mass analysis under the conditions of Figure 11b. This result demonstrates that a part of the adsorbed NOx species reacts with NH3 and that N2 is produced via NH2NO species as an intermediate. Figure 10 shows the change of FT-IR spectra of TiO2 under the UV irradiation. The peak intensities of adsorbed NH3 on Lewis acid site (3352, 3245, 3150, 1602, and 1200 cm-1), monodentate NO3- (1300 and 1500 cm-1), bidentate NO3(1263 and 1578 cm-1), and nitro species (1327 cm-1) diminished gradually under UV irradiation. On the other hand, the peaks of terminal OH (3719 cm-1) and bridged OH (3680 cm-1) and a broad peak of H2O (2800-3600 cm-1) increased. Moreover, mass analysis indicated that 14N15N was produced mainly by the reaction of adsorbed 15NOx species with adsorbed 14NH3 under UV irradiation. These results showed that the adsorbed NH3 reacts with adsorbed NOx species to produce N2 and H2O. A part of H2O contributes to the formation of surface OH groups.

Photo-Oxidation of NH3 over TiO2

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11083 NO3- are formed by the reaction with O2, as shown by eqs 13 and 14.

Discussion Reaction Mechanism of the Photo-SCO over TiO2. In the photo-SCO reaction, the induction period was observed in Figures 1 and 2. We confirmed that this induction period is due to the adsorption of NH3 on TiO2 in Figures 3 and 4. Additionally, Figure 4 showed that the total amount of the generated nitrogen atom was almost equal to the amount of adsorbed NH3 on acid site (Lewis acid site, see Figure 5b) of TiO2. These results indicate that the Lewis acid site on TiO2 is an active site for the photo-SCO and that the first step of the photo-SCO reaction is the adsorption of NH3 on the Lewis acid site. The UV irradiation is necessary for the photo-SCO reaction because the photo-SCO does not proceed in the dark. Additionally, judging from the results of Figures 5 and 8, the adsorbed NH3 on the Lewis acid site reacts with O2 or NH3 only under UV irradiation. Therefore, the photoactivation of the adsorbed NH3 and/or O2 is essential for the photo-SCO as the second step. We have reported that the adsorbed NH3 on TiO2 is activated to NH2 radical by trapping a hole under UV irradiation (eq 10). The formed NH2 radical reacts with oxygen anion radical generated on TiO2 under UV irradiation.22 Moreover, we demonstrated by ESR that the oxygen anion radical does not react with NH3 in the dark.22 The NH2 species was detected by FT-IR under photo-SCO. These results indicate that the NH2 radical is generated from the adsorbed NH3 on TiO2 under UV irradiation and is an active species for the photo-SCO.

NH3ads + h+ f NH2‚ + H+

(10)

Two pathways of the NH3 oxidation to N2 in the presence of O2 over metal oxide catalyst have been proposed in the literature. One is the direct reaction of two NH2 to N2 via hydrazine (NH2NH2) expressed by eqs 2-5.25,26 The other is a two-step reaction composed of the oxidation of NH3 to NOx and the reduction of NOx to N2 by NH3 via NH2NO species, expressed by eqs 6-9.15,27-29 In each case, the NH2 species is contained in the active species. Therefore, it seems that the formed NH2 radical reacts with NH2 radical or O2 in the photo-SCO. If the reaction of NH2 radical with another NH2 radical has occurred, hydrazine would be detected by FT-IR. In fact, Chuang et al. have reported that hydrazine exists stably on TiO2 surface at room temperature.24 However, the result in Figure 7 demonstrates that hydrazine is not generated on the TiO2 surface and that all the peaks hardly changed in the photoreaction of adsorbed NH3 without O2. The isotopic experiments exhibit that the NH2 radical selectively reacts with NOx species to generate N2. Additionally, hydrazine could not be detected on TiO2 surface under the photoreaction of NH3 with O2 in Figures 5 and 8. These results exclude the possibility that the photo-SCO takes place by the pathway via hydrazine. In our previous study, the NH2 radical reacts with O2 anion radical, which is generated by trapping an electron (eq 11).22 Furthermore, NO was produced in the photo-SCO reaction in Figure 1 and NO2- and NO3- species were detected on the TiO2 surface in the initial stage of the photo-SCO as shown in Figure 5c. From these results, we have no doubt whatsoever that NO- is produced by the reaction of NH2 radical with oxygen anion radical, as shown by eq 12, and continuously NO2- and

O2 + e- f O2‚-

(11)

NH2‚ + O2‚- f NO- + H2O

(12)

2NO- + O2 f 2NO2-

(13)

NO- + O2 f NO3-

(14)

The production of NO2- and NO3- species on TiO2 surface easily occurs in the presence of O2 because only NO2- and NO3- species are generated on TiO2 surface by the co-adsorption of NO and O2 in Figure 10a. Therefore, NO2- and NO3- species were mainly detected on the surface by FT-IR. In the initial stage of the photoreaction of adsorbed NH3 with O2 (Figure 8b-e), the amount of generated NOx species was quite small in spite of the remarkable decrease in those of adsorbed NH3. Then, the amount of NOx species increased drastically as well as the adsorbed NH3 decreased. When a large amount of adsorbed NH3 is present on the TiO2 surface, NOx species generated by the reaction between adsorbed NH3 and O2 can react with other unreacted NH3 adsorbed on TiO2 immediately. On the other hand, when the amount of adsorbed NH3 is small, it is difficult for the formed NOx to react with NH3. Consequently, NOx species is accumulated on the TiO2 surface. Additionally, we have demonstrated that the formed NH2 radical selectively reacts with NO in the presence of O2 and N2 was produced via NH2NO as an intermediate over TiO2 under UV irradiation.44 Therefore, it is assumed that the formed NOx species reacts with NH2 radical (and/or NH3) to N2. We observed the reaction of NOx species on TiO2 surface with NH3 in Figure 10. The adsorbed NO2, bridged NO3-, and bidentate NO3- species on TiO2 were changed to other species by the introduction of NH3 without UV irradiation (Figure 10b), instead of the increase in the monodentate NO3- and the formation of the HNO3-like species. The HNO3-like species would be bidentate NO3- species interacted with NH3 because the bidentate NO3- species appeared at the same time as the disappearance of HNO3-like species by the evacuation of NH3 in Figure 10c. The bidentate NO3- species also increased by the evacuation after NH3 introduction (Figure 10a,c). The adsorption of NH3 on TiO2 is stronger than that of NOx species because NOx species is not adsorbed on TiO2 where NH3 is already adsorbed although NH3 is adsorbed on TiO2 where NOx species exists.44 The increase in the monodentate and the bidentate NO3- species would be the rearrangement of the bridged NO3- species to the monodentate and the bidentate NO3- species by the adsorption of NH3. The HNO3-like species would be formed by the interaction of the existing and formed bidentate NO3- species with NH3. Furthermore, N-N-stretching vibration and NH2 deformation vibration of ON-NH2 species were observed in Figure 10b, and N2 was formed in the gas phase. The isotopic experiment showed that the formed N2 was produced by the reaction of the surface NOx species and NH3. On the other hand, the monodentate NO3-, HNO3-like species, and nitro species with the exception of adsorbed NO2, which disappeared by the introduction of NH3, were not changed for more than 1 h after the NH3 introduction. These results indicate that the introduced NH3 reacts with adsorbed NO2 to give N2 via NH2NO species without UV irradiation. Sanchez-Escribano et al. reported that NO2 is more active than NO and NH3 reacts with NO2 to give N2 over H-ZSM-5 catalyst at room temperature.53 They proposed that NH4+ formed by the NH3 adsorption

11084 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Yamazoe et al.

on Brønsted acid site reacts with NO2 to produce N2 and H2O via NH4NO2 intermediate. In our case, the same reaction would take place on the Lewis acid site of the TiO2 surface as shown in eq 15 because pretreated TiO2 has no Brønsted acid site on the surface (as we demonstrated in Figure 5b) and the formation of NH2 species were observed. NO2 ads + NH3 f NH2NO + OH

(15)

NO2 extracts the H atom from NH3 to produce the NH2 species and the formed NH2 species reacts with NO to give the NH2NO species. The NH2NO species is decomposed to N2 and H2O. The generated OH species becomes the adsorption site (Brønsted acid site) of NH3, and NH4+ is generated by the adsorption of NH3 on formed OH species. In fact, NH4+ was observed at 1461 cm-1 in Figure 10b. The remained NO2- (nitro species) and NO3- species (monodentae and bidentate species) on TiO2 surface react with adsorbed NH3 to N2 under UV irradiation because the peaks of both NOx species and adsorbed NH3 were diminished as shown in Figure 10d-f. This indicates that the NH2 radical generated from adsorbed NH3 under UV irradiation would react with NOx species. When 15NOx and 14NH3 were used, 14N15N was mainly obtained by mass measurement. It is clearly indicated that N2 was produced by the reaction of NH2 radical with nitro species and monodentate and bidentate NO3- species in this condition (eqs 16 and 17). Ti4+ is reduced to Ti3+ by trapping an electron generated by photoexcitation at the same time of the formation of NH2 radical under UV irradiation.50 The reduced Ti3+ is oxidized to Ti4+ by the reaction of O2 without photoirradiation.44 The formed O2 and/or surface oxygen species in eqs 16 and 17 would also reoxidize the reduced Ti3+ site to Ti4+ with the formation of H2O as shown in eq 18. A part of O2 and/or surface oxygen species reacts with NH2 radical to NO. The formation of 14N14N is derived from the reaction of 14NO formed from 14NH with 14NH radical (eq 19). However, the reaction of 143 2 NO with 14NH2 radical is a minor path because the amount of 14N production was much lower than that of 14N15N. 2 NH2 + NO3 ads f (NH2NO3) f N2 + H2O + O2 (or 2Oads) (16) NH2 + NO2 ads f (NH2NO2) f N2O + H2O + Oads (17) Ti3+ + 1/4O2 (or 1/2O) + H+ f Ti4+ + 1/2H2O

(18)

NH2 + NO f (NH2NO) f N2 + H2O

(19)

Several researchers agree that the intermediate is the nitrosoamide species (NHyNOx) in the reaction of adsorbed NH3 (NH4+) with NOx to N2.15,27-29 Nitrosoamide species was observed in the reaction of NH3 with adsorbed NO2 species in Figure 10b. Therefore the intermediate in the reaction of eqs 16, 17 and 19 would be nitrosoamide species. Additionally, the peak of NH4+ in Figure 10c disappeared under photoreaction in Figure 10d. The NO3- (NO2- is not an active species for the reaction of NH4+) could combine with NH4+ to NH4NO3 under UV irradiation on TiO2.44 Formed NH4NO3 is decomposed to N2 as shown in eq 20. The same reaction occurs in the present case. NH4+ + NO3- f NH4NO3 f N2 + 2H2O + Oads

(20)

In the case of the reaction of NH3 with O2, NOx species (the bidentate and monodentate NO3- and nitro species) were generated on the TiO2 surface as shown in Figures 5 and 8. NO was produced in the gas phase under photo-SCO. These

NOx species react with NH2 radical to form N2 via nitrosoamide intermediate according to eqs 16, 17 and 19. Additionally, NH4+ formed under photo-SCO also reacts with NO3- species to N2 via nitrosoamide species (eq 20). N2O as a byproduct was produced in the photo-SCO reaction. N2O is formed in the oxidation of the reduced Ti3+ to Ti4+ with NO under UV irradiation.44 Therefore, the production of N2O in the photoSCO occurs by the reaction of the generated NO, instead of O2, with the reduced Ti3+. To summarize, the reaction mechanism for the photo-SCO over the TiO2 catalyst is expressed as follows: 1. The active site for the photo-SCO is the Lewis acid site on TiO2, and the adsorption of NH3 to the Lewis acid site is the first step in the photo-SCO. 2. The adsorbed NH3 on Ti-O-Ti (Lewis acid site) is activated to the NH2 radical, and Ti-OH is generated under UV irradiation. 3. The NH2 radical reacts with the oxygen anion radical species, which are formed under UV irradiation, to form NO. The formed NO is changed to surface NO2- (nitro) and NO3- (monodentate and bidentate) species by the reaction with O2. 4. The monodentate and bidentate NO3- species, nitro species, and NO react with NH2 radical formed under UV irradiation, and N2 is selectively produced via nitrosoamide species (NH2NO3, NH2NO2, and NH2NO). 5. N2O formed in the photo-SCO is produced in the oxidation of Ti3+ with NO instead of O2. 6. The formed NH4+ also reacts with NOx (maybe NO3species) and would be produce N2 under photoirradiation. 7. Adsorbed NO2 reacts with NH3 to N2 via NH2NO without UV irradiation. There are two proposed reaction mechanisms for the SCO over the metal oxide catalyst. Ramis et al. proposed that the two NH2 formed from adsorbed NH3 produce N2 via hydrazine over the CuO-TiO2 catalyst.25,26 On the other hand, several authors proposed the inner SCO of NO with NH3 reaction mechanism. NHy (y ) 2 or 4) generated from NH3 reacts with O2 and NOx species (x ) 1, 2, and 3). The formed NOx species reacts with other NHy to N2 via nitrosoamide species as an intermediate. In our case, the NH2 radical, which is formed from NH3 adsorbed on Lewis acid site under UV irradiation, reacts with oxygen anion radical to NOx species (x ) 1, 2, and 3). The formed NOx species are active species and react with the NH2 radical under UV irradiation to form N2 via nitrosoamide species, indicated by the results of the FT-IR study. Therefore, we conclude that the mechanism of the photo-SCO is the inner SCR via nitrosoamide species as well as the later reaction mechanism proposed for the thermal SCO. Conclusion NH3 photo-oxidation to N2 could take place over TiO2 in the presence of O2 under UV irradiation. The first step of the photoSCO is the adsorption of NH3 on Lewis acid site of TiO2. The Lewis acid site on TiO2 is an active site for the photo-SCO, and the adsorbed NH3 is converted to NH2 radical by trapping a hole, which is formed by the photoexcitation of TiO2 catalyst under UV irradiation. O2 is transformed to oxygen anion radical species under UV irradiation. The NH2 radical reacts with the oxygen anion radical species to give the surface NOx (x ) 2 and 3) species and NO would be also formed in this reaction step. The formed NOx species (adsorbed nitro, monodentate NO3-, and bidentate NO3- species) could react with the NH2 radical under UV irradiation (at the same time, Ti3+ is formed by Ti4+ trapping an electron) and the NH2NOx species is

Photo-Oxidation of NH3 over TiO2 generated as an intermediate. On the other hand, adsorbed NO2, which is formed on the TiO2 surface by the co-adsorption of NO with O2, is more active than other surface NOx species and reacts with NH3 to produce the NH2NO intermediate without UV irradiation. These NH2NOx (x ) 1, 2, and 3) species formed as an intermediate are decomposed to N2. The reduced Ti3+ is reoxidized by O2 without photoirradiation. When NO is employed as an oxidant instead of O2, N2O is produced in the reoxidation of Ti3+ to Ti4+ with NO under UV irradiation. References and Notes (1) Long, R.; Yang, R. Chem. Commun. 2000, 1651. (2) Yang, M.; Wu, C.; Zhang, C.; He, H. Catal. Today 2004, 90, 263. (3) Gang, L.; Anderson, B.; van Grondelle, J.; van Santen, R. Appl. Catal., B 2003, 40, 101. (4) Slavinskaya, E.; Veniaminov, S.; Notte, P.; Ivanova, A.; Boronin, A.; Chesalov, Y.; Polukhina, I.; Noskov, A. J. Catal. 2004, 222, 129. (5) Gang, L.; Anderson, B.; van Grondelle, J.; van Santen, R. Catal. Today 2000, 61, 179. (6) Long, R.; Yang, R. J. Catal. 2002, 207, 158. (7) Mccabe, R. W.; Pignet, T.; Schmidt, L. D. J. Catal. 1974, 32, 114. (8) Gland, J. L.; Korchak, V. N. J. Catal. 1978, 53, 9. (9) van den Broek, A. C. M.; van Grondelle, J.; van Santen, R. A. J. Catal. 1999, 185, 297. (10) Amblard, M.; Burch, R.; Southward, B. W. L. Catal. Today 2000, 59, 365. (11) Lietti, L.; Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Catal. Today 2000, 61, 187. (12) Carabineiro, S. A. C.; Matveev, A. V.; Gorodetskii, V. V.; Nieuwenhuys, B. E. Surf. Sci. 2004, 555, 83. (13) Li, Y.; Armor, J. N. Appl. Catal., B 1997, 13, 131. (14) Gang, L.; van Grondelle, J.; Anderson, B.; van Santen, R. J. Catal. 1999, 186, 100. (15) Curtin, T.; O’Regan, F.; Deconinck, C.; Knuttle, N.; Hodnett, B. Catal. Today 2000, 55, 189. (16) Sazonova, N.; Simakov, A.; Nikoro, T.; Barannik, G.; Lyakhova, V.; Zheivot, V.; Ismagilov, Z.; Veringa, H. React. Kinet. Catal. Lett. 1996, 57, 71. (17) Dannevang, F. U. S. Patent 1996, 5, 137. (18) Wang, K. H.; Hsieh, Y. H.; Chou, M. Y.; Chang, C. Y. Appl. Catal., B 1999, 21, 1. (19) Li, X. Z.; Li, F. B.; Yang, C. L.; Ge, W. K. J. Photochem. Photobiol., A. 2001, 141, 209. (20) Farhadi, S.; Afshari, M.; Maleki, M.; Babazadeh, Z. Terahedron Lett. 2005, 46, 8483. (21) Cant, N.; Cole, J. R. J. Catal. 1992, 134, 317. (22) Yamazoe, S.; Okumura, T.; Tanaka, T. Catal. Today 2007, 120, 220.

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