Comparison of the Thermal and Photochemical Reaction Pathways of

Article pubs.acs.org/JPCC

Comparison of the Thermal and Photochemical Reaction Pathways of Melamine on TiO2 Yu-Chen Lin, Tzu-En Chien, Kun-Lin Li, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung University, No. 1 Ta Hsueh Road, Tainan, Taiwan 701, Republic of China S Supporting Information *

ABSTRACT: Fourier transform infrared spectroscopy was employed to study the thermal and photochemical reactions of melamine ((H2N)3(C3N3)) on TiO2. Also tested was the adsorption of urea, cyanamide (H2N−CN), dicyandiamide ((H2N)2CN−CN), and cyanuric acid ((OH)3(C3N3)) for identifying possible reaction intermediates. It was found that the thermal decomposition of melamine starts with N−H bond scission, possibly forming intermediates such as (H2N)2(C3N3)NH− and (H2N)(C3N3)(NH)2−. Further loss of hydrogen atoms and ring-opening from these intermediates lead to the formation of −NCO (isocyanate) and −N3 (azide) on the surface. The TiO2-mediated photochemical reaction of melamine proceeds via a different mechanism, forming dicyandiamide. These thermal and photochemical reaction pathways of melamine on TiO2 are reported for the first time. They are different from previous studies showing the processes of polymerization, and substitution of NH2 by OH to form cyanuric acid and urea.

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INTRODUCTION Melamine is used widely in commercial resins, adhesives, and plastics. In a recent food scandal, this nitrogen-rich compound was fraudulently added to feed and dairy products to increase their nitrogen content. Nitrogen is measured to calculate the apparent protein concentration in foodstuffs. However, melamine absorption by the human body may result in health problems, such as formation of kidney stones.1,2 Recently, melamine and its derivatives were used as a precursor to produce carbon nitride materials, e.g., C3N4.3−7 As shown in Scheme 1a, melamine can be prepared by mildly heating cyanamide (H 2 N−CN) or dicyandiamide ((H2N)2CN−CN).8,9 However, upon thermal treatment at a temperature higher than 300 °C, melamine starts to predominantly undergo deamination, instead of a ring-breaking process. Melam, melem, and melon are obtained via melamine condensation at 450 °C. Further loss of ammonia by prolonged heating at 550 °C can produce graphitic carbon nitrides.9−12 In the literature, reports regarding both melamine and TiO2 mainly cover the important aspects of surface modification, heteroatom incorporation for improving the material properties and applications, and degradation of melamine.13−18 TiO2 nanostructures modified with C3N4 or incorporated with carbon and nitrogen atoms from melamine decomposition show visible-light activities.13,14 Kisch’s group investigated the mechanism of urea-induced TiO2 modification to find out reaction intermediates and/or the products that are responsible for the formation of the visible-light-sensitive catalysts.12 They suggest that cyanamide is formed in the reaction of urea with TiO2. Cyanamide then trimerizes into melamine in the reaction © 2015 American Chemical Society

conditions and leads to polytriazine anchoring to the TiO2 surface.12 Krö cher’s group conducted the hydrolysis of melamine over TiO2 at a temperature higher than 150 °C and proposed the reaction process of melamine → ammeline → ammelide → cyanuric acid → CO2 + NH3 (Scheme 1b).17 Kiwi et al. observed a similar oxidation process from melamine to cyanuric acid during the UV irradiation of melamine over TiO2 in aqueous solutions containing H2O2.18 Furthermore, it has been demonstrated that melamine can be metabolized by bacteria through successive deamination and breakdown into CO2 and NH3, involving intermediates such as cyanuric acid and urea (Scheme 1c).19,20 On the basis of the possible reaction routes of melamine revealed in the previous studies,9−20 melamine molecules on TiO2 may undergo (1) condensation, eventually forming polytriazine and C3N4, (2) replacement of the NH2 groups by surface OH groups to form cyanuric acid, and/or (3) degradation to form urea, CO2, and NH3. TiO2 can serve as a heterogeneous catalyst promoting melamine reactions thermally or photochemically. However, no detailed mechanisms involving surface chemical processes of melamine on TiO2 have been reported to date. In the present research, we investigate the thermal and photochemical reactions of melamine on TiO2. Also tested was the adsorption of urea, cyanamide, dicyandiamide, and cyanuric acid to trace reaction intermediates and/or products. It is found that, in the reactions of melamine on TiO2, the replacement of the NH2 Received: December 9, 2014 Revised: March 5, 2015 Published: March 25, 2015 8645

DOI: 10.1021/jp5122307 J. Phys. Chem. C 2015, 119, 8645−8651

Article

The Journal of Physical Chemistry C Scheme 1. Previously Reported Reaction Pathways Related to Melamine

the cell was evacuated. An infrared spectrum was taken as reference background. The high-temperature-treated TiO2/W sample was removed from the cell after the reference spectrum was measured. An aqueous solution of melamine (0.01 M), cyanamide (0.01 M), dicyandiamide (0.01 M), urea (0.1 M), or cyanuric acid (0.05 M) was sprayed onto the TiO2/W surface. Afterward, the TiO2/W with adsorbates was remounted inside the cell, followed by evacuation for ∼17 h to remove adsorbed water before temperature-dependent infrared measurements. An IR cell with two CaF2 windows for IR transmission down to ∼1100 cm−1 was connected to a gas manifold maintained by a turbomolecular pump at a base pressure of ∼1 × 10−7 Torr. The pressure was monitored with a Baratron capacitance manometer and an ion gauge. In the photochemistry study, both the UV and IR beam were set 45° to the normal of the TiO2 sample. The UV light source was a combination of a Hg arc lamp (Oriel Corp.) operated at 350 W, a water filter, and a band-pass filter with a bandwidth of ∼100 nm centered at ∼320 nm (Oriel 5160). Transmission IR spectra were obtained at 4 cm−1 resolution by a Bruker FTIR spectrometer (VECTOR 22) with a MCT detector.

with OH, ring-condensation, and degradation to form urea do not occur. Instead, melamine decomposes thermally by N−H bond scission, with subsequent ring-opening leading to the formation of −NCO and −N3 on TiO2. However, as an interesting contrast, melamine photodissociates into dicyandiamide on TiO2, the dimer of cyanamide. Namely, melamine, as a trimer of cyanamide, breaks down into the monomer and/or dimer photochemically. These thermal and photodegradative pathways of melamine on TiO2 have never been reported previously. These interesting results are presented in this paper.

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EXPERIMENTAL SECTION The chemical reagents used in this study were melamine (Alfa Aesar, 99%), cyanamide (Sigma-Aldrich, 99%), dicyandiamide (Sigma-Aldrich, 99%), urea (Acros Organics, 99.5%), cyanuric acid (Sigma-Aldrich, 98%), 16O2 (Sigma-Aldrich, 99.998 atom %), and 18O2 (Sigma-Aldrich, 99 atom %). TiO2 powder (Degussa P25, ∼50 m2/g, anatase 70%, rutile 30%) or SiO2 powder (CAB-O-SIL M-5) was prepared by spraying the TiO2 or SiO2 aqueous suspension onto a tungsten fine mesh for support (denoted as TiO2/W or SiO2/W) and then mounted inside the infrared cell for thermal treatment. The thermal treatment consisted of resistive heating to 450 °C in a vacuum and then maintaining 350 °C in the presence of 16O2. After the temperature of the TiO2/W catalyst was decreased to 35 °C,

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RESULTS AND DISCUSSION To fully understand the chemical processes of melamine on TiO2, we also studied the adsorption systems of urea, 8646


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The Journal of Physical Chemistry C

1234 cm−1 to the symmetric one, which is more sensitive to the strength of Lewis acid−base interaction resulting in the peak splitting.26−28 Theoretically, NH3 adsorption on various TiO2 model surfaces has been extensively investigated.29−35 These calculations point out that the most stable bonding form of NH3 molecules on TiO2 surfaces is the nitrogen atoms coordinated to the surface titanium ions. In this kind of chemisorption structure, there may be NH···O interaction leading to the slight changes in the bond angle and/or molecular symmetry of NH3.29−35 Moreover, according to the previous mode assignment for HNC(O)NH2 on TiO2, the other two peaks of 1457 and 1549 cm−1 observed in the 175 °C spectrum of Figure 1 are assigned to NCO and NCN vibrations, respectively. These modes are likely from the surface intermediates of HNC(O)NH and/or NC(O)NH due to further deprotonation of HNC(O)NH2. Cyanamide, Dicyandiamide, and Cyanuric Acid on TiO2. Previous studies regarding cyanamide and dicyandiamide focused on pyrolysis,8−11 hydrolysis,36,37 interaction with metal surfaces,38,39 and isomerization.40 For the latter case, transformation of cyanamide into carbodiimide (HNCNH) occurs on water−ice at low temperature (80 K).40 Carbodiimide is considered a condensing agent that can assemble amino acids into peptides. Figures 2 and 3 show the temperature-dependent

cyanamide, dicyandiamide, and cyanuric acid. These were prepared by spraying the aqueous solutions onto the hightemperature-treated TiO2 powder similar to the procedure of melamine. Urea on TiO2. Figure 1 shows the temperature-dependent infrared spectra of urea on TiO2. The 35 °C spectrum exhibits

Figure 1. Infrared spectra taken at the indicated temperatures in a vacuum after urea adsorption on TiO2.

five peaks at 1177, 1486, 1575, 1636, and 1648 cm−1. A similar infrared absorption pattern, including the peak positions and their relative intensities, has been observed in the previous systems of urea on V2O5−MoO3−TiO2 catalyst at 100 °C (1152, 1245, 1450, 1490, 1562, and 1652 cm−1) and on anatase TiO2 (Crystal Global DT-51) at 80 °C (1185, 1240, 1143, 1492, 1575, 1635, and 1655 cm−1).21,22 For the latter case, the reported infrared peaks are attributed to HNC(O)NH 2 adsorbed on the surface with HN bonded at one Ti site and/ or with HN and O bonded on two Ti sites.22 The HNC(O)NH2 intermediate is due to deprotonation of urea, which can occur on TiO2 at 35 °C as shown in Figure 1. The peaks of 1636 and 1648 cm−1 can be assigned to the NH2 bending mode, 1575 cm−1 to the NCO asymmetric stretching mode, 1486 cm−1 to the NCN bending mode, and 1177 cm−1 to the NH rocking mode.22 Upon raising the surface temperature for HNC(O)NH2-covered TiO2 (Figure 1), it is found that the deprotonated intermediate decomposes gradually and almost disappears at 200 °C. This is indicated by the considerable decrease in intensity of the 1648 cm−1 peak, but with enhanced absorption, as shown in the 175 °C spectrum, at 1190, 1234, 1457, 1549, 1610, and 2210 cm−1. The 2210 cm−1 peak is due to −NCO (isocyanate), which is a reaction product of urea decomposition on mixed oxide and anatase TiO2 and in the oxidation of CH3CN on TiO2.21−24 In our previous study of NH3 adsorption on TiO2 supported on W mesh, infrared peaks at 1186, 1240, and 1602 cm−1 were measured.25 In terms of both the closely matched peak positions and their relative intensities, the set of the peaks of 1190, 1234, and 1610 cm−1 is ascribed to NH3 generated from urea decomposition on the TiO2. Similar peak frequencies of NH3 adsorbed on anatase (TiO2) and CuO−TiO2 have been reported.26−28 NH3 can be adsorbed on different Lewis acid sites (Ti4+) of TiO2 surfaces. The 1610 cm−1 peak is assigned to asymmetric NH3 deformation mode and the peaks of 1190 and

Figure 2. Infrared spectra taken at the indicated temperatures under vacuum after cyanamide adsorption on TiO2.

infrared spectra of cyanamide and dicyandiamide on TiO2, respectively. The infrared bands observed in the 35 °C spectrum of cyanamide on TiO2 are listed in Table 1 and compared to previously reported infrared absorptions of cyanamide at gas and solid states and in Ar matrix.41,42 However, discrepancies exist, including a much lower CN stretching frequency and extra peaks of 1173, 1632, and 1646 cm−1 that cannot be attributed to cyanamide. Interestingly, the infrared absorption pattern between 1150 and 1750 cm−1 observed in the 35 °C spectrum of cyanamide closely resemble that of urea (the 35 °C spectrum in Figure 1), which is due to −HNC(O)NH2. The formation of this intermediate indicates that hydrolysis of cyanamide at functional CN can occur on TiO2 at 35 °C. Broad absorption at ∼2200 cm−1 in the cyanamide 35 °C spectrum is also observed in the adsorption of dicyandiamide. As shown in Figure 2, further raising the surface temperature leads to the generation of NH3 (1197, 1240, and 8647


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responsible for the bands at 1178 and 1469 cm−1. Heating the dicyandiamide/TiO2 adsorption system produces NH3 (1194, 1240, and 1613 cm−1), −NCO (2211 cm−1), and −N3 (2010 cm−1). The latter species with a wavenumber of 2015 cm−1 has been reported in the reaction NH3 over TiO2.25 It is not surprising that azide is formed in the thermal decomposition of dicyandiamide due to its high nitrogen content. Note that the broad absorption appearing at ∼2200 cm−1 in the 35 °C spectrum of cyanamide matches the CN stretching frequency of dicyandiamide, suggesting the occurrence of a dimerization pathway for cyanamide on TiO2. Figure S1, Supporting Information, shows the infrared absorption spectra for the cyanuric acid on TiO2. The three main absorptions are located at 1559, 1645, and 1806 cm−1. The latter peak is due to carbonyl stretching mode. Since these spectra are only used to judge whether cyanuric acid is generated in the reaction of melamine on TiO2, no further analysis is provided. Thermal Decomposition of Melamine on TiO2. Figure 4a shows the infrared spectrum taken after deposition of Figure 3. Infrared spectra taken at the indicated temperatures in a vacuum after dicyandiamide adsorption on TiO2.

Table 1. Comparison of the Infrared Frequencies (cm−1) of Cyanamide (H2N−CN) on TiO2 (35 °C)

gasa phase

solidb film

in Ara matrix

mode

2208 2178

2270

2265 2239 2220

2267

ν(CN)

1595

1580

1589

δ(NH2)

1060

ν(C−N)

1646 1632 1570 1173

1055 a

b

Reference 41. Reference 42.

1613 cm−1) and −NCO (2213 cm−1). In the case of dicyandiamide (Figure 3), the peaks observed in the 35 °C spectrum are included in Table 2 for comparison to the infrared frequencies of dicyandiamide molecules in solid or film on zinc.38,43 Existence of dicyandiamide on TiO2 can account for the measured bands of 1512−2218 cm−1. However, the possibility of minor hydrolysis, forming (H2N)2CN−C(O)NH−, and/or N−H bond scission, forming −HN(H2N)C N−CN, cannot be completely ruled out and may be

Figure 4. Infrared spectra taken at the indicated temperatures in a vacuum after melamine adsorption on TiO2.

melamine on TiO2, followed by evacuation at 35 °C. The peaks located at 1176, 1422, 1460, 1556, and 1622 cm−1 are found to match the characteristic infrared absorptions of melamine in gas and solid states reported previously (Table 3).2,44,45 However, adsorption of melamine on TiO2 leads to a very broad hydrogen-bonding band in the range of 2500−3700 cm−1 without showing isolated NH2 stretching peaks (Figure S2, Supporting Information). Heating the surface to 200 °C causes several spectral changes, as compared to 35 °C: (1) diminution of the 1622 cm−1 peak, (2) peak shifts of 1176 cm−1 → 1190 and 1556 cm−1 → 1545 cm−1, (3) relative intensity of the peaks at 1422 and 1460 cm−1, and (4) largely decreased H-bonding band (Figure S2, Supporting Information). These changes could be due to desorption of residual water and the decrease of surface OH groups. H2O on TiO2 shows a strong band at ∼1620 cm−1 for its bending mode and can be completely removed at 200 °C under vacuum (Figure S3, Supporting Information). From 200 to 250 °C, the small peak at 1190

Table 2. Comparison of the Infrared Frequencies (cm−1) of Dicyandiamide ((H2N)2CN−CN) on TiO2 (35 °C)

filma on Zn

solida state

mode

2218 2182 1691 1624 1556 1512 1469

2211 2169 1666 1647 1576 1510

2208 2165 1658 1639 1585 1508

νa(N−CN)

1257

1254

νs(N−CN)

1098

1096

ρ(NH2)

δ(NH2) νa(N−C−N) νa(N−CN)

1178

a

References 38 and 43. 8648


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300 °C, but with additional CO2 of 2364 cm−1. In contrast to the continuous growth of the azide peak with increasing temperature, the isocyanate intensity of 2206 cm−1 remains about the same between 300 and 375 °C, which can be attributed to isocyanate decomposition in O2 to form CO2. On the basis of the temperature when −NCO and −N3 starts to appear, it is concluded that the effect of O2 on melamine thermal decomposition on TiO2 may not be large. Photochemical Reaction of Melamine on TiO2. Figure 5 shows the time-dependent infrared spectra of melamine on

Table 3. Comparison of the Infrared Frequencies (cm−1) of Melamine on TiO2 (35 °C)

solida film

1622 1556

1660 1643 1626 1583 1565

1460

1549 1531 1469

1526 1465

1422

1434

1432

1190

1194

1175

1170

1176 a

powderb

modeb

1646 1626

δ(NH2)

1567

ν(C−N) + δ(NH2)

gasc phase

modec

1598

δ(NH2) + ring str

1556

NCN bend + ring def

1440

ring and sidechain CN str

ν(C−N) + δ(NH2)

ring def + ρ(NH2)

Reference 44. bReference 2. cReference 45.

cm−1 disappears and the ∼1620 cm−1 peak continues to decrease but the intensities at 1419, 1459, and 1545 cm−1 from ring vibrations remaining about the same. Because the 1190 and 1620 cm−1 peaks are due to NH2 rocking and bending modes, respectively, their decrease indicates a chemical change in the NH2 groups of melamine on TiO2. Together with the unchanged ring vibration modes, we propose that thermal decomposition of melamine on TiO2 starts with N−H bond breakage, possibly forming the intermediates as shown in Scheme 2a, with intact ring structure. These intermediates Figure 5. Comparison of infrared spectra taken before and after the indicated photoirradiation times of melamine adsorbed on TiO2 in the closed cell with three different conditions, without O2 or in the presence of 10 Torr of 16O2 or 18O2.

Scheme 2. Degradation of Melamine on TiO2

TiO2 exposed to UV light for three conditions: without oxygen, in the presence of 16O2, and in the presence of 18O2. It is found that two new peaks appear at ∼2170 and ∼2210 cm−1 in the melamine photodecomposition. Moreover, O2 enhances the peak intensities and no isotope shift occurs. The latter result strongly suggests that the species responsible for the bands of 2170 and 2210 cm−1 contain no oxygen. The broad absorption near 2200 cm−1 is similar to those found in the adsorption of cyanamide and dicyandiamide on TiO2 (Figures 2 and 3). This result indicates that melamine on TiO2 is photodecomposed into cyanamide and/or dicyandiamide (Scheme 2b). The cyanamide can then dimerize to form dicyandiamide. To check whether the photoreaction of melamine on TiO2 is a surfacemediated or direct decomposition process, we performed a photochemical study of melamine on SiO2 in 16O2, with the result shown in Figure S5, Supporting Information. The unchanged infrared features of melamine after 180 min photoirradiation clearly show the decomposition of melamine on TiO2 under UV exposure is through TiO2 excitation, i.e., a TiO2 surface-mediated process. UV light absorption by TiO2 can cause band-to-band excitation, producing electron−hole pairs. The photochemical decomposition of melamine on TiO2 in the present study may be initiated by hole-related species, for example, h+, surface O−, or OḢ . The presence of oxygen, enhancing the formation of cyanamide- and/or dicyandiamidelike species, could be due to lifetime elongation of hole species,

undergo further hydrogen loss and ring-opening, resulting in the formation of −NCO (2206 cm−1) and −N3 (2014 cm−1) as shown in the 325 °C spectrum. In Figure 4, no infrared absorptions similar to those of urea, cyanamide and dicyandiamide on TiO2 were observed. The −NCO peak from melamine decomposition appears at higher temperature than that from urea, cyanamide, or dicyandiamide. Therefore, it is concluded that no urea-, cyanamide-, or dicyandiamiderelated species are generated in the thermal reaction of melamine on TiO2. Even if they are involved in the melamine decomposition pathway on TiO2, their contribution must be minor. Furthermore, no reaction product of ammeline, ammelide, or cyanuric acid (Figure S1, Supporting Information) is generated, because the characteristic bands of ∼1700− 1800 cm−1 and/or ∼1375 cm−1 were not detected.46 No gC3N4 material, which has a relatively strong ∼1320 cm−1 band, was formed either.3−6 We also investigated thermal decomposition of melamine on TiO2 in the closed cell with 5 Torr of O2 initially. The infrared spectral result is shown in Figure S4, Supporting Information. Similarly, −NCO and −N3 appear at 8649


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(8) Pankratov, V. A.; Chesnokova, A. E. Polycyclotrimerisation of Cyanamides. Russ. Chem. Rev. 1989, 58, 1528−1548. (9) Jürgens, B.; Irran, E.; Senker, J.; Kroll, P.; Müller, H.; Schnick, W. Melem (2,5,8-Triamino-tri-s-triazine), An Important Intermediate during Condensation of Melamine Rings to Graphitic Carbon Nitride: Synthesis, Structure Determination by X-ray Powder Diffractometry, Solid-State NMR, and Theoretical Studies. J. Am. Chem. Soc. 2003, 125, 10288−10300. (10) Costa, L.; Camino, G. Thermal Behavior of Melamine. J. Therm. Anal. 1988, 34, 423−429. (11) Lotsch, B. V.; Schnick, W. New Light on an Old Story: Formation of Melam during Thermal Condensation of Melamine. Chem.Eur. J. 2007, 13, 4956−4968. (12) Mitoraj, D.; Kisch, H. On the Mechanism of Urea-Induced Titania Modification. Chem.Eur. J. 2010, 16, 261−269. (13) Wang, J.; Zhang, W.-D. Modification of TiO2 Nanorod Arrays by Graphite-Like C3N4 with High Visible Light Photoelectrochemical Activity. Electrochim. Acta 2012, 71, 10−16. (14) Wang, J.; Huang, B.; Wang, Z.; Qin, X.; Zhang, X. Synthesis and Characterization of C,N-Doped TiO2 Nanotubes/Nanorods with Visible-Light Activity. Rare Metals 2011, 30, 161−165. (15) 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. (16) Neville, E. M.; Mattle, M. J.; Loughrey, D.; Rajesh, B.; Rahman, M.; Don MacElroy, J. M.; Sullivan, J. A.; Thampi, K. R. Carbon-Doped TiO2 and Carbon, Tungsten-Codoped TiO2 through Sol−Gel Processes in the Presence of Melamine Borate: Reflections through Photocatalysis. J. Phys. Chem. C 2012, 116, 16511−16521. (17) Bernhard, A. M.; Peitz, D.; Elsener, M.; Wokaun, A.; Kröcher, O. Hydrolysis and Thermolysis of Urea and Its Decomposition Byproducts Biuret, Cyanuric Acid and Melamine over Anatase TiO2. Appl. Catal., B 2012, 115−116, 129−137. (18) Bozzi, A.; Dhananjeyan, M.; Guasaquillo, I.; Parra, S.; Pulgarin, C.; Weins, C.; Kiwi, J. Evolution of Toxicity during Melamine Photocatalysis with TiO2 Suspensions. J. Photochem. Photobiol., A 2004, 162, 179−185. (19) Jutzi, K.; Cook, A. M.; Hutter, R. The Degradative Pathway of the S-Triazine Melamine. The Steps to Ring Cleavage. Biochem. J. 1982, 208, 679−684. (20) Shelton, D. R.; Karns, J. S.; McCarty, G. W.; Durham, D. R. Metabolism of Melamine by Klebsiella Terragena. Appl. Environ. Microbiol. 1997, 63, 2832−2835. (21) Larrubia, M. A.; Ramis, G.; Busca, G. An FT-IR Study of the Adsorption of Urea and Ammonia over V2O5-MoO3-TiO2 SCR Catalysts. Appl. Catal., B 2000, 27, L145−L151. (22) Bernhard, A. M.; Czekaj, I.; Elsener, M.; Kröcher, O. Adsorption and Catalytic Thermolysis of Gaseous Urea on Anatase TiO2 Studied by HPLC Analysis, DRIFT Spectroscopy and DFT Calculations. Appl. Catal., B 2013, 134−135, 316−323. (23) Zhuang, J.; Rusu, C. N.; Yates, J. T., Jr. Adsorption and Photooxidation of CH3CN on TiO2. J. Phys. Chem. B 1999, 103, 6957−6967. (24) Chuang, C.-C.; Wu, W.-C.; Lee, M.-X.; Lin, J.-L. Adsorption and Photochemistry of CH3CN and CH3CONH2 on Powdered TiO2. Phys. Chem. Chem. Phys. 2000, 2, 3877−3882. (25) Chuang, C.-C.; Shiu, J.-S.; Lin, J.-L. Interaction of Hydrazine and Ammonia with TiO2. Phys. Chem. Chem. Phys. 2000, 2, 2629− 2633. (26) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. FT-IR Characterization of the Surface Acidity of Different Titanium Oxide Anatase Preparations. Appl. Catal. 1985, 14, 245−260. (27) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Fourier Transform Infrared Study of the Adsorption and Coadsorption of Nitric Oxide, Nitrogen Dioxide and Ammonia on TiO2 Anatase. Appl. Catal. 1990, 64, 243−257.

because of the role of O2 in capturing the photogenerated electrons.

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CONCLUSION Melamine is subjected to thermal and photochemical decomposition on TiO2, but through different reaction mechanisms. Melamine thermal degradation starts with N−H bond scission, possibly forming the intermediates, such as (H2N)2(C3N3)NH− and (H2N)(C3N3)(NH)2−. Continuous hydrogen loss and ring rupture result in the formation of surface ioscyanate (−NCO) and azide (−N3). The photodegradation of melamine is a TiO2 surface-mediated process, which does not occur on SiO2. Melamine decomposes into cyanamide and/or dicyandiamide on TiO2 under photoirradiation. No ammeline, ammelide, or cyanuric acid products were detected in the reactions of melamine on TiO2.

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ASSOCIATED CONTENT

S Supporting Information *

Infrared spectra of cyanuric acid on TiO2 and infrared results for melamine adsorption on TiO2 (2500−4000 cm−1), water adsorption on TiO2, melamine thermal reaction on TiO2 in O2, and melamine on SiO2 under UV exposure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 886-6-2740552. Phone: 886-6-2757575 ext. 65326. Email: [email protected]. Author Contributions

The first and second authors have an equivalent contribution to this article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology of the Republic of China under contract no. NSC 101-2113-M-006-005-MY3.

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REFERENCES

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