Article pubs.acs.org/Langmuir
Infrared Spectroscopic Study of the Adsorption Forms of Cyanuric Acid and Cyanuric Chloride on TiO2 Tzu-En Chien, Kun-Lin Li, Po-Yuan Lin, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung University, No. 1 Ta Hsueh Road, Tainan, Taiwan 701, ROC S Supporting Information *
ABSTRACT: Cyanuric acid is often found to be the end product in the hydrolysis of waste melamine and in the TiO2-mediated photocatalytic decomposition of s-triazine-containing compounds used as herbicides or dyes. The photocatalytically recalcitrant nature of cyanuric acid on TiO2 may be closely related to its adsorption properties, including the tautomeric forms present on the surfaces and their bonding structures, which remain to be determined. In this paper, we present the optimized adsorption structures of the four tautomeric isomers (triketo, diketo, monoketo, and triol) of cyanuric acid on a model rutile-TiO2(110) surface and their vibrational absorptions. Experimentally, the adsorption structures of cyanuric acid and chloride on powdered TiO2 are analyzed on the basis of the theoretically obtained, characteristic infrared information. Cyanuric acid on TiO2 at 35 °C exists in triketo and hydroxylated forms, but the diketo becomes the predominant form on the surface at 250 °C, being bonded to a titanium site via one of its carbonyl groups and with a N−H···O hydrogen bonding interaction. Hydroxylation of cyanuric chloride occurs as it is adsorbed on TiO2 at 35 °C. Upon being heated to 200 °C, the surface is mainly covered with the diketo form of cyanuric acid after the adsorption of cyanuric chloride.
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cyanuric acid can be dissociated into CO2 and NO3− ions under UV irradiation.1 Cyanuric acid has four tautomeric forms as shown in Scheme 1. Cyanuric acid is poorly soluble and weakly acidic in water.6 The triketo form (isocyanuric acid) predominates under neutral and acidic conditions. However, the levels of hydroxylated species in enolate structures gradually increase with increasing pH toward a basic environment.7 The triketo form is the tautomer present in the cyanuric acid crystal, with strong N− H···OC hydrogen bonding between the molecules.8 For an isolated cyanuric acid, the triketo is theoretically identified as the most stable form, in contrast to the opposite end of the triol.9 Infrared spectroscopy is a powerful technique and can be utilized to analyze the tautomeric structures of cyanuric acid. The previously measured and calculated infrared spectra of cyanuric acid have shown the carbonyl stretching mode (1700−
INTRODUCTION Cyanuric acid and chloride are cyclic compounds containing the s-triazine (C3N3) ring (Scheme 1) that can be used to disinfect water and, as a stabilizer, to minimize the consumption of chlorine in swimming pools.1,2 Cyanuric acid is frequently found to be the end product in the hydrolysis of waste melamine [(C3N3)(NH2)3]. The precipitates as the melamine− cyanuric acid complex may cause renal failure in humans. In the TiO2-mediated photocatalytic degradation of triazine ring-containing herbicides and dyes, the reactions are typically terminated at cyanuric acid.3 In the presence of F− in the aqueous TiO2 suspension at low pH, Jenks et al. found that the recalcitrant cyanuric acid was subjected to photodecomposition because of the formation of homogeneous hydroxy radicals.4,5 However, Serpone’s group insisted that cyanuric acid was not subjected to photodegradation even in the presence of F− and HO•.3 These inconsistent results may be due to the different reaction conditions used, leading to the variation in the adsorption state of cyanuric acid on the photomediator surfaces. The adsorption form of cyanuric acid on TiO2 remains an open question. In aqueous ZnO suspensions, © XXXX American Chemical Society
Received: April 7, 2016 Revised: May 12, 2016
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DOI: 10.1021/acs.langmuir.6b01334 Langmuir XXXX, XXX, XXX−XXX
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Aldrich, 99.998 atom %), and TiO2 powder (Degussa P25, ∼50 m2/g). The TiO2 used in this infrared spectroscopic study was supported on tungsten that was prepared by spraying a TiO2 aqueous suspension onto a tungsten fine mesh and then mounted inside the infrared cell for thermal treatment. The thermal process consisted of resistive heating to 450 °C in a vacuum and then keeping the sample at 350 °C for 30 min in the presence of O2. After the temperature of the supported TiO2 was decreased to 35 °C, the cell was evacuated. An infrared spectrum was recorded as the reference background. Different adsorption processes of cyanuric acid and chloride on TiO2 were conducted, according to their vapor pressures. The vapor of cyanuric chloride was introduced into the infrared cell, following the measurement of the background spectrum. However, because of the insufficient volatility of cyanuric acid, the high-temperature-treated TiO2/W sample was removed from the cell after the reference spectrum was recorded. A saturated aqueous solution of cyanuric acid was then sprayed onto the TiO2/W surface. Afterward, the TiO2/W sample with adsorbates was remounted inside the cell, followed by evacuation for ∼17 h to partially remove adsorbed water prior to temperature-dependent infrared measurements. The IR cell with two CaF2 windows for IR transmission down to ∼1000 cm−1 was connected to a gas manifold maintained by a turbomolecular pump at a base pressure of ∼1 × 10−7 Torr. Transmission IR spectra were recorded at 4 cm−1 resolution with a Bruker FTIR spectrometer (VECTOR 22) with a MCT detector.
Scheme 1. Structures of Cyanuric Chloride and the Four Tautomeric Isomers of Cyanuric Acid
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COMPUTATIONAL METHOD
The optimized chemisorption structures of cyanuric acid on TiO2, with rutile(110) as a model surface, were calculated in the framework of density functional theory (DFT) by using the DMol3 package, in which generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE) formulation was employed. A doublenumeric quality basis set with polarization functional, a Monkhorst− Pack k-point set at 4 × 3 × 2, and a supercell of 24 [TiO2] units with dimensions of 13.00 Å × 5.92 Å × 9.04 Å were used in this study. All slabs were separated by a vacuum space of 10.0 Å. The positions of the Ti and O atoms in the first and second layers of the slabs were allowed to vary for the optimized structure calculation.
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RESULTS AND DISCUSSION Calculated Infrared Absorptions of an Isolated Cyanuric Acid. In the theoretical study of the infrared absorptions of cyanuric acid, only the triketo and triol forms have been calculated previously.10,12 To obtain the trend in the infrared changes for the molecule as the CO groups are transformed in a stepwise fashion to C−OH groups (triketo → diketo → monoketo → triol), we have calculated the structures for the four tautomers and their vibrational frequencies and modes. Shown in Figure S1 are the theoretically obtained structures with bond lengths and angles for the four cyanuric acid forms. Our structural result is closely similar to those reported previously.15 For each tautomer of cyanuric acid, the calculated infrared frequencies of >1000 cm−1 and the corresponding vibrational modes are listed in Table 1. In the case of triketo, the absorptions in the range of 1000−3583 cm−1 can be divided into three parts: 1000−1406 cm−1 from ring stretching and/or N−H in-plane bending modes, 1754−1774 cm−1 mainly from CO stretching modes, and 3568−3583 cm−1 from N−H stretching vibrations. This infrared outcome agrees with the previous triketo vibrational calculations based on normal mode analysis or DFT.10,12 As a CO group is replaced by a C−OH group, together with formation of a C N bond in the triazine ring, to form the diketo species, the O− H stretching absorption is calculated to be 3665 cm−1. The absorption at 1633 cm−1, not observed in the triketo form, has two main contributions from the ring stretching associated with
1800 cm−1) and/or benzene-like triazine ring vibrations (1480−1590 cm−1).6,10−12 However, these infrared results were analyzed according to the triketo and triol forms only. The vibrational absorptions and modes of cyanuric acid in monoketo (diol) and diketo (monool) have not been reported. Adsorption on metal oxides plays an important role in heterogeneous processes, including photopromoted cyanuric acid degradation.3−5 It is well-known that acidic and basic sites are present on metal oxide surfaces that can determine the adsorption structures of cyanuric acid, like pH affects the presence of the tautomeric forms of cyanuric acid in aqueous solutions.13,14 We have investigated the adsorption state of cyanuric acid on TiO2 spectroscopically and theoretically, which provided important information that can improve our understanding of its photocatalytic reactivity.
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EXPERIMENTAL SECTION
The chemical reagents used in this study were cyanuric acid (SigmaAldrich, 98%) and cyanuric chloride (Alfa Aesar, 98%), O2 (SigmaB
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Table 1. Calculated Infrared Frequencies (cm−1) and Vibrational Modes of an Isolated Cyanuric Acid in Four Different Tautomeric Formsa triketo frequency
a
diketo mode
frequency
monoketo mode
frequency
1000
δ(N−H), νR
1030
νR, δ(N−H), δ(O−H)
1065
1179 1337 1363 1368 1401 1406 1754
νR δ(N−H) δ(N−H) δ(N−H), νR νR, δ(N−H) νR, δ(N−H) ν(CO), δ(N−H)
1148 1204 1297 1339 1345 1508 1633
δ(O−H), νR, δ(N−H) δ(O−H), δ(N−H), ν(C−O) δ(N−H), δ(O−H) δ(N−H), νR δ(N−H), δ(O−H) δ(N−H), νR, δ(O−H), ν(C−O) νR, δ(O−H), δ(N−H)
1146 1192 1247 1293 1440 1482 1537
1757 1774 3568 3582 3583
ν(CO), δ(N−H) ν(CO) ν(N−H) ν(N−H) ν(N−H)
1711 1763 3560 3563 3665
ν(CO), δ(N−H) ν(CO), δ(N−H) ν(N−H) ν(N−H) ν(O−H)
1621 1739 3549 3662 3675
mode δ(O−H), δ(N−H), ν(C− O), νR δ(O−H), νR, δ(N−H) δ(O−H), δ(N−H) δ(O−H) δ(N−H), δ(O−H) ν(C−O), δ(O−H) δ(O−H), νR, δ(N−H) ν(C−O), δ(N−H), δ(O− H), νR νR, δ(O−H) ν(CO), δ(N−H) ν(N−H) ν(O−H) ν(O−H)
triol frequency
mode
1052
δ(O−H), νR, ν(C−O)
1056 1210 1224 1227 1344 1416 1452
δ(O−H), δ(O−H) δ(O−H) δ(O−H) δ(O−H), δ(O−H), δ(O−H),
1489 1563 1601 3669 3673 3681
ν(C−O) δ(O−H), νR δ(O−H), νR ν(O−H) ν(O−H) ν(O−H)
νR, ν(C−O)
νR ν(C−O), νR ν(C−O), νR
Legend: ν, stretching vibration; δ, bending vibration; νR, ring stretching vibration.
the CN and O−H bending modes. The absorption at 1508 cm−1 is also related to CN stretching in the ring, but with a smaller vibrational amplitude compared to that of the absorption at 1633 cm−1. The other ring stretching modes or the vibrations involving the changes in the in-plane C−N−H and C−O−H bond angles fall in the range of 1030−1345 cm−1. For the monoketo tautomer, the absorptions at 1537 and 1621 cm−1 have a contribution from CN stretching vibration, similar to the case of the diketo. The CO (1739 cm−1), N−H (3549 cm−1), and O−H (3662 and 3675 cm−1) stretching frequencies are at typical positions, without being largely changed, but we did find that the highest CO stretching frequency gradually decreases from the triketo to the monoketo form (1774 cm−1 → 1763 cm−1 → 1738 cm−1). In the triol form with a fully delocalized π system, the three O−H stretching modes are in the range of 3669−3681 cm−1. The absorptions at 1563 and 1601 cm−1 are assigned to O−H bending and ring stretching modes, respectively. The highest frequency regarding the ring stretching vibrations decreases from the diketo to the triol form (1633 cm−1 → 1621 cm−1 → 1601 cm−1). Calculated Adsorption Structures of Cyanuric Acid on Rutile-TiO2(110) and Their Infrared Absorptions. We have also theoretically investigated the adsorption structures of a cyanuric acid molecule on rutile-TiO2(110) and their vibrational absorptions to assist in identifying the cyanuric acid tautomers on powdered TiO2 using infrared spectroscopy. Figure 1 shows the optimized adsorption structures of the four cyanuric acid tautomers, with the bond lengths and angles listed in Figure S2. The triketo species is adsorbed on the model surface, with one of its carbonyl groups being attached to a 5fold-coordinated Ti site (a Lewis acid site).16 The distance between the carbonyl oxygen and the surface titanium atom is 2.223 Å. Because of the CO···Ti interaction, the bond angle at the carbonyl carbon in the ring increases to 115.5°, larger than those (112.6° and 113.8°) of the other two carbonyls, and the CO bond length (1.235 Å) of the carbonyl is greater than those of the others, by ∼0.017 Å. Note that the calculated carbonyl bond angles and lengths for an isolated triketo form are 113.0° and 1.219 Å, respectively (Figure S1). In contrast, the two C−N bond lengths (1.375 and 1.381 Å) of the
Figure 1. Theoretically predicted adsorption structures of the four tautomers of cyanuric acid on a TiO2(110)-rutile model surface.
carbonyl attached to the surface are shorter than the other four C−N bonds in the ring (1.392−1.403 Å). In this tilt adsorption geometry of the triketo isomer, one of the N−H groups next to the carbonyl at the titanium site is close to a 2-fold-coordinated O site, with a short NH···O distance of 2.255 Å indicating the formation of hydrogen bonds.17 The calculated infrared absorptions for the adsorbed triketo form are listed in Table 2, which has lower N−H stretching frequencies, as compared to those of a free triketo molecule. In particular, for the NH group directly interacting with surface oxygen, its N−H stretching mode is significantly red-shifted to 3338 cm−1. Similarly, the CO stretching vibration of the carbonyl group at the Ti site is moved downward to 1734 cm−1, in contrast to the vibration at 1754−1774 cm−1 for a free triketo form (Table 1). However, the other two CO groups for the adsorbed triketo isomer have higher frequencies of 1799 and 1812 cm−1. The diketo is predicted to be adsorbed at a titanium site through one of its carbonyl groups and has a hydrogen bonding interaction, with a −CO···Ti bond length of 2.159 Å and a C
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δ(O−H), νR, ν(C−O) δ(O−H), νR, ν(C−O) δ(O−H), νR δ(O−H) δ(O−H), νR δ(O−H) ν(C−O), δ(O−H), νR ν(C−O), δ(O−H), νR δ(O−H), νR ν(C−O), δ(O−H), νR ν(C−O), νR, δ(O−H) ν(O−H) ν(O−H) ν(O−H) 1049 1162 1228 1266 1304 1408 1462 1586 1626 1803 2785 3447 3578 1198 1310 1330 1358 1371 1377 1734 1799 1812 3338 3505 3518
Legend: ν, stretching vibration; δ, bending vibration; νR, ring stretching vibration. a
frequency
1052 1078 1209 1241 1267 1348 1437 1457 1543 1546 1583 2987 3183 3662 δ(N−H), νR, δ(O−H) δ(O−H), δ(N−H), νR δ(O−H), δ(N−H), νR δ(O−H), δ(N−H), νR δ(O−H), δ(N−H), νR δ(N−H), δ(O−H) δ(O−H), νR ν(C−O), δ(O−H), νR, δ(N−H) δ(N−H), δ(O−H), ν(C−O), νR νR, δ(O−H), δ(N−H), ν(C−O) ν(CO), δ(N−H) ν(N−H) ν(O−H) ν(O−H)
mode
NH···O bond length of 1.545 Å (Figure 1). The strong adsorbate−surface interaction of the NH···O group increases the N−H bond length to 1.078 Å as compared to another 1.020 Å N−H bond (Figure S1), resulting in a relatively low N−H stretching frequency of 2785 cm−1 (Table 2). The optimized surface diketo form has a CN bond length of 1.299 Å. In particular, the stretching vibration of the CO group attached to the surface is considerably coupled to the ring stretching and H-bending modes, with frequencies of 1586 and 1626 cm−1. In contrast, another CO group has a frequency of 1803 cm−1. In the case of the monoketo on TiO2(110), the carbonyl group is also theoretically predicted to be bonded at a Ti site with a −CO···Ti bond length of 2.173 Å. However, no NH···O or OH···O hydrogen bonding effect occurs between the monoketo and the surface. The softened CO stretching mode coupled with N−H bending appears at 1671 cm−1. In general, the optimized surface tri-, di-, and monoketo molecules, as compared to their free, isolated forms, have lower O−H and N−H stretching frequencies. As shown in Figure 1 and Table 2, the triol, without any carbonyl, interacts with the TiO2(110) surface oxygen ions through two OH groups, with two hydrogen bond lengths of 1.598 and 1.683 Å, resulting in two strongly softened O−H stretching modes at 2987 and 3185 cm−1. These theoretically obtained adsorption structures and their vibrational frequencies would be a useful guide for assisting in the interpretation of the possible cyanuric acid forms on powdered TiO2. As shown in Table 2, the adsorbed tirketo has no frequencies in the range of ∼1500−1650 cm−1, in contrast to the other three tautomers. Only the triketo and diketo isomers on TiO2(110) possibly have a CO stretching frequency higher than 1700 cm−1. Because the adsorbed triketo has three CO stretching modes covering the frequency range of 1734−1812 cm−1, a broad absorption or multiple peaks for CO stretching vibrations appearing in experimental studies of adsorption of cyanuric acid on TiO2 may indicate the presence of the triketo form on the surface. On the other hand, an observed carbonyl peak at ∼1800 cm−1 together with peaks in the range of ∼1550−1650 cm−1 can be attributed to the diketo form. The calculated surface monoketo has only a downshifted CO stretching peak at ∼1650 cm−1, lower than 1700 cm−1, and the adsorbed triol form has no absorptions between ∼1600 and 1800 cm−1. Adsorption of Cyanuric Acid and Cyanuric Chloride on Powdered TiO2. Adsorption of cyanuric acid was conducted by spraying a saturated cyanuric acid aqueous solution onto a high-temperature-treated TiO2 supported on tungsten fine mesh. Figure 2a shows the infrared spectrum measured at 35 °C for the cyanuric acid on TiO2 in a vacuum. The bands observed are listed in Table 3 and are compared to those from the previous infrared studies of cyanuric acid in the solid state and in an Ar matrix.8,10 First, the multiple peaks at 1700, 1754, and 1799 cm−1 in Figure 2a indicate the presence of the triketo and/or diketo molecules according to the assignments listed in Table 2. The relatively strong peak of 1584 cm−1 does not belong to the triketo but is instead from the diketo, monoketo, and/or triol form. Therefore, not just one type of cyanuric acid tautomer exists on the TiO2 surface at 35 °C. The peaks in the range of 1000−1500 cm−1 are due to ring stretching and/or N−H and O−H bending modes of cyanuric acid in different forms. Besides, a small amount of residual water is observed, with a relatively weak shoulder at 1622 cm−1. The broad, enhanced background absorption
1003 1054 1169 1195 1257 1298 1445 1476 1544 1569 1671 3450 3566 3576
frequency mode frequency mode
νR νR, δ(N−H) δ(N−H) δ(N−H), νR δ(N−H) δ(N−H), νR ν(CO), δ(N−H) ν(CO), δ(N−H) ν(CO), δ(N−H) ν(N−H) ν(N−H) ν(N−H)
frequency
δ(O−H), δ(N−H), νR δ(O−H), δ(N−H) δ(O−H), νR, δ(N−H) δ(N−H), νR, δ(O−H) δ(N−H), δ(O−H) δ(N−H) δ(N−H), νR ν(CO), νR, δ(N−H), δ(O−H) ν(CO), νR, δ(N−H), δ(O−H) ν(CO), δ(N−H) ν(N−H) ν(N−H) ν(O−H)
triol monoketo diketo triketo
Table 2. Calculated Infrared Frequencies (cm−1) and Vibrational Modes of Adsorbed Cyanuric Acid in Four Different Tautomeric Forms on TiO2(110)a
mode
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3456 cm−1 in this range. As the surface is heated to 100 °C (Figure 2b), the spectrum shows an increased absorption forming a sharp peak at 1803 cm−1. At 200 °C, the magnitude of this 1803 cm−1 peak continues to grow, in contrast to the largely decreased peak at 1700 cm−1 (Figure 2c). The increased resolvability in the range of ∼1700−1800 cm−1 reveals that the relative concentration of the triketo form is decreased. In addition, other changes also occur as compared to the 35 and 100 °C spectra, including an enhanced absorption at 1648 cm−1, a decrease in peak width at 1453 cm−1, and disappearance of the small peaks at 1401 and 1420 cm−1. These overall results indicate that the relative amount of the cyanuric acid tautomeric species present on the surface changes with an increase in temperature. In the range of 2500−3750 cm−1 in Figure 2c, the infrared absorptions appear at 3451 and 3654 cm−1, with a broad hydrogen bonding feature. The former could be due to the N−H stretching mode and the latter due to OH groups including surface hydroxyls. From 200 to 250 °C, the magnitude of the 1700 cm−1 peak is further decreased and becomes a weak shoulder of the 1648 cm−1 peak (Figure 2d). At 250 °C, the absorption peaks of cyanuric acid on TiO2 appear at 1193, 1329, 1349, 1451, 1561, 1648, 1806, 3448, and 3651 cm−1. Cyanuric acid has been proposed to be an intermediate, with a broad infrared shoulder at 1740 cm−1, in the reaction of urea over TiO 2 at 150 and 200 °C. 18 In the previous thermogravimetric analysis of cyanuric acid, this compound sublimates, with a small amount of decomposition between 250 and 275 °C.19 At 400 °C, cyanuric acid transforms into HNCO.6 Hydrolysis of cyanuric acid over Al2O3 takes place at a temperature of >250 °C.20 Over TiO2, cyanuric acid thermally dissociates to form HNCO (>250 °C) in the presence of O2 and hydrolytically to form NH3 and CO2 (>200 °C) in a stream containing O2 and H2O.21 In this study, the adsorption of cyanuric chloride was performed by introducing its vapor into the infrared cell containing powdered TiO2 at 35 °C. Shown in Figure 3a is the 35 °C spectrum taken under vacuum after cyanuric chloride adsorption, with the bands of 1269, 1304, 1493, 1534, 1560, and 1620 cm−1. In terms of the previous vibrational study of cyanuric chloride in solutions or in the solid state (Table 4), the vibrations at 1269 and 1493 cm−1 may be related to cyanuric chloride itself, due to the C−N stretching and ring deforming modes, respectively.22,23 However, cyanuric chloride cannot account for the strong peaks at 1534 and 1560 cm−1. As shown by the calculated infrared frequencies of cyanuric acid in monoketo and triol forms on TiO2(110) (Table 2), the hydroxylated s-triazine rings have peaks in the range of ∼1540− 1585 cm−1 due to ring stretching and N−H and O−H bending modes. Therefore, the peaks at 1534 and 1560 cm−1 are likely from hydroxylated cyanuric chloride generated by the interaction of chloride with surface OH groups and/or residual water. The spectral features do not change much at 100 °C (Figure 3b). When the sample is heated to 150 °C in a vacuum, the 1269 and 1493 cm−1 peaks almost disappear, but a new, small peak is generated at 1807 cm−1 (Figure 3c). At 200 °C, this peak grows and the doublet feature of 1534 and 1560 cm−1 no longer exists. Figure 3d shows three main peaks at 1552, 1645, and 1807 cm−1 and a small absorption at 1451 cm−1. This 200 °C spectral pattern, including peak positions, shapes, and relative intensities, closely resembles that of cyanuric acid on TiO2 at 250 °C (Figure 2d and Table 4). The 1807 and 1552 cm−1 vibrations are indicative of CO-
Figure 2. Infrared spectra taken at the indicated temperatures after adsorption of cyanuric acid on TiO2.
Table 3. Comparison of the Cyanuric Acid Vibrational Frequencies (cm−1) solida
3210 3060 1800
1710 1470 1425
in Arb matrix
on TiO2c (35 °C)
3458 3456 3438
3456
1778 1772 1765 1752 1742 1722 1717 1451 1447 1409 1395 1393 1389
1065
3213 3078 1799 1754 1700
1584 1466 1420 1401 1372 1336 1177 1057
1016 1012 a
From ref 8. bFrom ref 10. cFrom this work.
between ∼2500 and 3750 cm−1 is due to hydrogen bonding between the adsorbate and surface and between the adsorbates containing NH and OH groups. Several softened N−H or O− H stretching vibrations can be observed at 3078, 3213, and E
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Cyanuric acid has four tautomeric forms; however, its adsorption structures on metal oxides have not been explored previously, in spite of the reported thermal and hydrolytic decomposition of cyanuric acid over TiO2 and Al2O3.20,21 We provide theoretical and experimental evidence showing that the triketo form is present on TiO2 at 35 °C but that the diketo form becomes the predominant surface species at 250 °C. Cyanuric chloride is also transformed into the diketo-cyanuric acid on TiO2 at 200 °C. Our studies emphasize the adsorption of cyanuric acid and chloride. This subject is essential and important for fully understanding the surface processes regarding photocatalytic, hydrolytic, and thermal reactions of cyanuric acid, cyanuric chloride, and s-triazine-containing compounds on TiO2. In our theoretical studies, the rutile(110) surface was employed because it is the most stable surface of the most stable TiO2 polymorph. This surface contains 2-fold-coordinated O sites (O2C, bridging surface O atoms) and 5-foldcoordinated Ti sites (Ti5C), which are active centers for binding cyanuric acid molecules, in addition to 3-fold-coordinated O atoms (O3C). In our experimental work, the P25-TiO2 powder consists of both the rutile and anatase particles. The (101) facet is the most frequently observed facet for the anatase-TiO2, which also contains O2C and Ti5C surface atoms, with additional O3C and Ti6C sites. Previous studies have shown similar adsorption properties for the rutile(110) and anatase(101) surfaces.24−30 For example, the calculation for water adsorption on anatase(101) reveals that the O atom of H2O is located above the Ti5C site and the H2O forms H-bonds with the bridging oxygens.25 On rutile(110), the water O is also directly bonded to the Ti5C site, with a single H-bond with the bridging O.26 A similar adsorption geometry of bi-isonicotinic acid on rutile(110) and anatase(101) has been reported.27,28 The acid molecule bonds to the surfaces via the two deprotonated carboxyl groups at Ti5C sites. In addition, the aromatic planes are tilted in this adsorption geometry, with an angle of ∼25° relative to the surface normal of the rutile surface, but is slightly changed to 20 ± 3° on the anatase surface, as evidenced by near-edge X-ray absorption fine structure measurements.27,28 Moreover, the photoemission and scanning tunneling studies point out that, on rutile(110) and anatase(101), both
Figure 3. Infrared spectra taken at the indicated temperatures after adsorption of cyanuric chloride on TiO2.
and OH-containing s-triazine, which is considered to be the cyanuric acid in diketo form, according to the calculated infrared result (Table 2). Table 4 shows the similarity for the experimental frequencies of the adsorbed cyanuric acid (250 °C) and cyanuric chloride (200 °C) on powdered TiO2 and theoretically predicted infrared absorptions of diketo-cyanuric acid on TiO2(110). Besides, the 1645 cm−1 peak originates from the carbonyl of the diketo form that directly attaches to a surface titanium Lewis acid site, leading to a softened CO stretching mode. The solubility of cyanuric chloride in water is poor; however, it has been shown that cyanuric chloride can be hydrolyzed to cyanuric acid.6
Table 4. Comparison of the Infrared Absorptions (cm−1) of Cyanuric Acid and Cyanuric Chloride cyanurica chloride (solid)
1500
modeb
ring deformation
cyanuric chloride/ TiO2, 35 °C 1620 1560 1534 1493
cyanuric chloride/ TiO2, 200 °C
cyanuric acid/ TiO2, 250 °C
calculatedc cyanuric acid on TiO2(110)
mode
1807 1645 1552
1806 1648 1561
1803 1626 1586
ν(CO), δ(N−H) ν(CO), νR, δ(N−H), δ(O−H) ν(CO), νR, δ(N−H), δ(O−H)
1451
1451
1462 1408
δ(N−H), νR δ(N−H)
1304 1266 1228 1162 1049
δ(N−H), δ(N−H), δ(O−H), δ(O−H), δ(O−H),
1349 1329
1262 849 794 a
ν(CN)
1304 1269
1193
δ(O−H) νR, δ(O−H) νR, δ(N−H) δ(N−H) δ(N−H),νR
ν(C−Cl) ω(C−Cl)
From ref 22. bFrom ref 23. cDiketo form. F
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azobenzene and aniline form phenyl imide bonded to the surface Ti5C sites.29,30 It is believed that cyanuric acid on rutile(110) and anatase(101), the two most abundant facets of the TiO2 crystal phases used in our experiments, would have similar adsorption geometry and infrared absorption behavior. The 250 °C spectrum of Figure 2 and the 200 °C spectrum of Figure 3, showing a very similar infrared absorption pattern, are attributed to the diketo-cyanuric acid. However, as shown by the peak intensities in both spectra, the surface concentration for obtaining the former spectrum is ∼5 times that for the latter. This result indicates that the interactions of the adsorbates, such as H-bonds if they exist, are not significant to cause a large change in the adsorption geometry of the diketo form and therefore its infrared frequencies. Note that no water adsorption (∼1620 cm−1) on the TiO2 (Figures 2 and 3) is found at ≥200 °C. The 100 °C spectrum of Figure 2 has a contribution from the diketo form with a carbonyl stretching peak at 1803 cm−1, very close to the peak located at 1806 cm−1 in the 250 °C spectrum. The 100 °C TiO2 surface still contains residual water with a H-bonding environment; however, it does not affect the carbonyl frequency much. In the previous theoretical study, it was found that the presence of a water molecule could reduce the barrier height in the tautomerization of cyanuric acid.9
CONCLUSIONS In this study, we present (1) the theoretically obtained infrared absorptions of the four tautomeric forms of isolated cyanuric acid and (2) the optimized bonding structures of these four isomers on a model rutile-TiO2(110) surface and their characteristic infrared absorptions. The triol form is predicted to be adsorbed with O−H···O (surface) hydrogen bonding interaction; however, the adsorption for the other three tautomers involves a carbonyl group attaching to a surface titanium site. The triketo and diketo forms also have an adsorbate−surface (NH···O) hydrogen bonding interaction. Experimentally, cyanuric acid on TiO2 at 250 °C and cyanuric chloride on TiO2 at 200 °C have the same infrared absorption behavior, which is attributed to the adsorbed diketo isomer of cyanuric acid. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01334. Calculated structures for the four tautomers of cyanuric acid (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*Fax: 886-6-2740552. E-mail:
[email protected]. Phone: 886-6-2757575, ext. 65326. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the Ministry of Science and Technology of the Republic of China (MOST 1042113-M-006-016). G
DOI: 10.1021/acs.langmuir.6b01334 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.6b01334 Langmuir XXXX, XXX, XXX−XXX
