Investigation on the Hydrocarbon and Oxygen-Containing

Article pubs.acs.org/JPCC

Investigation on the Hydrocarbon and Oxygen-Containing Hydrocarbon Intermediates Leading to Crotonaldehyde Formation on TiO2 Jong-Liang Lin,* Po-Yuan Lin, and Ying-Chung Shih Department of Chemistry, National Cheng Kung University, No. 1 Ta Hsueh Road, Tainan, Taiwan 701, ROC ABSTRACT: Aldol condensation of CH3CHO, forming crotonaldehyde (2butenal, CH3CHCHCHO), readily occurs on TiO2 at 35 °C. At a higher coverage or at an elevated temperature, the crotonaldehyde can be oxidized to crotonate. Adsorption and thermal reactions of CH3CHBr2, BrCH2CH2Br, BrCH2CH2OH, and ClCH2CH2OH on TiO2 can produce crotonaldehyde, in contrast to CH2CHBr. CH3CHBr2 has the highest reactivity toward the crotonaldehyde formation among the halogenated compounds studied. The pathways of CH3CHBr2 + Ti−O−Ti → CH3CHO + 2Ti−Br and BrCH2CH2Br + Ti−O−Ti → Ti−O−CH2CH2Br + Ti−Br → CH3CHO +2 Ti−Br are proposed for the reactions of CH3CHBr2 and BrCH2CH2Br. The crotonaldehyde generated from the reactions of the four halogenated compounds on TiO2 has lower CO and CC stretching frequencies as compared to those of the crotonaldehyde directly from its adsorption on TiO2. This result is attributed to the presence of Br or Cl atoms near the crotonaldehyde adsorption sites and the change in the Ti ionic bonding environment. In addition, photoirradiation (325 nm) on ClCH2CH2OH on TiO2 can enhance the crotonaldehyde formation.

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INTRODUCTION

For halogenated hydrocarbons on TiO2, the decomposition processes may start with carbon−halogen bond activation. For example, methoxy and ethoxy species have been observed on TiO2 due to the C−I bond dissociation of methyl and ethyl iodides, respectively.23,24 Decomposition of dibromomethane on TiO2 can generate methoxy and formate species, possibly via the dioxymethylene intermediate from cleavage of both the C− Br bonds.25 In the oxidation of hexafluoropropene (CF2CF− CF3) on TiO2, a mechanism involving the surface intermediates of CF3COO and FCOO has been proposed, with the CF2 CF− moiety being considered to be the initial site for the oxidation.26 Multihalogenated and multifunctional molecules on TiO2 can have new chemical pathways due to multiple active sites in the molecules and generate new reaction intermediates, which are unlikely to be observed in the reactions of monofunctional molecules. In this research, we investigated the adsorption and reactions of CH2CHBr, CH3CHBr2, BrCH2CH2Br, BrCH2CH2OH, and ClCH2CH2OH on powdered TiO2, with additional photocatalytic reaction of ClCH2CH2OH on TiO2, exploring their possibilities for acetaldehyde and crotonaldehyde formation.

TiO2, due to its chemical stability, inexpensive availability, and photoactivity for a wide range of organic molecules, is widely used as a photocatalyst.1−4 For example, studies of TiO2 photocatalytic degradation of halogenated hydrocarbons, which are considered to be environmental pollutants, have been extensively conducted and reported.5−12 The origin for the TiO2 photocatalytic activities is the formation of electron− hole pairs by the bandgap excitation. On TiO2 surfaces, the adsorption and reaction behaviors of organic molecules are closely related to the chemical properties of the functional groups. For alcohols and carboxylic acids on TiO2, the dissociative adsorption occurs via the O−H bond scission to form alkoxy and carboxylate species.13−16 Formaldehyde is chemically adsorbed to be dioxymethylene (−OCH2O−) on TiO2 at 250 K.17 Upon heating to 300 K, the dioxymethylene intermediate undergoes disproportionation, resulting in the formation of methoxy (CH3O−) and formate (HCOO−) species, probably via a Cannizzaro-type mechanism. Formaldehyde on TiO2 can also be oxidized to formate.17 Acetaldehyde can dimerize, via aldol condensation, into crotonaldehyde on TiO2, 2CH3CHO → CH3CH CHCHO + H2O.18−21 It is found that adsorbed crotonaldehyde on TiO2 is readily replaced by acetaldehyde present in the gas phase.19 The aldolization of acetaldehyde can occur above 250 K, and 3-hydroxybutanal (CH3CH(OH)CH2CHO) has been considered as the surface reaction intermediate.21 Similarly through a aldol-condensation-like mechanism, TiO2 can catalyze acetone molecules to form mesityloxide ((CH3)2CCH−C(O)CH3).22 © XXXX American Chemical Society

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EXPERIMENTAL SECTION TiO2 powder (Degussa P25, ∼50 m2/g, anatase 70%, rutile 30%) was dispersed in water/acetone solution to form a uniform mixture, which was then sprayed onto a tungsten mesh Received: January 19, 2017 Revised: May 5, 2017 Published: May 6, 2017 A

DOI: 10.1021/acs.jpcc.7b00599 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (1.5 × 2.2 cm2). After that, the TiO2 sample was mounted inside the IR cell for in situ FTIR spectroscopy. The W mesh was fully covered with ∼0.05 g of TiO2. The detailed cell structure with W support for TiO2 has been reported previously.2,27 The IR cell with two CaF2 windows for IR transmission down to ∼1000 cm−1 was connected to a gas manifold that was pumped by a turbomolecular pump with a base pressure of ∼1 × 10−7 Torr. The TiO2 sample in the cell was heated to 450 °C under vacuum for 24 h by resistive heating. The temperature of the TiO2 sample was measured by a K-type thermocouple spotwelded on the tungsten mesh. Before each run of the experiment, the TiO2 temperature was held at 450 °C in a vacuum for 2 h and at 350 °C for 0.5 h in the presence of 3.0 Torr O2 to remove possible organic contaminants on TiO2. After the heating treatment, the cell was evacuated. As the TiO2/W sample was cooled to 70 °C in a vacuum, 10 Torr of O2 was introduced into the cell to compensate for possible oxygen loss of the TiO2 caused by the heating treatment. When the TiO2 temperature reached 35 °C, the cell was evacuated again for gas or vapor dosing, and an infrared spectrum was taken as reference background. CH2 CHBr (98%, Aldrich), CH3CHBr2 (99%, Acros), BrCH2CH2Br (99%, Aldrich), BrCH2CH2OH (95%, Aldrich), ClCH2CH2OH (99%, Aldrich), and O2 (99.998%, Matheson) were used as received without further purification. To avoid possible contamination, the reagent was transferred to a sample tube as quickly as possible. Moreover, the air in the sample tube was removed by three cycles of freeze−pump−thaw with liquid nitrogen. The cell pressure was monitored with a Baratron capacitance manometer and an ion gauge. In the photochemistry study, both the UV and IR beams were 45° to the normal of the TiO2 sample. The UV light source used was a combination of a 350 W Hg arc lamp (Oriel Corp), a water filter, and a band-pass filter with a bandwidth of ∼100 nm centered at ∼325 nm. The photon power at the position of the TiO2 sample was ∼0.24 W/cm2. Infrared spectra were obtained with a 4 cm−1 resolution by a Bruker FTIR spectrometer with an MCT detector. The entire optical path was purged with CO2-free dry air. The spectra presented here have been ratioed against the TiO2 background spectrum.

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Crotonaldehyde Adsorption. First, a clean TiO2 surface at 35 °C was exposed to 0.5 Torr crotonaldehyde, with subsequent evacuation, and an infrared spectrum was taken (Figure 1a). After that, the TiO2 was successively exposed to

Figure 1. Infrared spectra of TiO2 measured after being in contact with 0.5 Torr of crotonaldehyde at 35 °C, followed by evacuation (a). (b) is the spectrum obtained after TiO2 is further exposed to 1.5 Torr of crotonaldehyde, with subsequent evacuation. All of the spectra were obtained at 35 °C in a vacuum.

1.5 Torr of the aldehyde molecules, and then another spectrum was taken under vacuum (Figure 1b), showing major infrared absorptions at 1163, 1375, 1422, 1451, 1549, 1592, 1636, 1659, 2876, 2917, 2928, 2973, and 3031 cm−1. Except for the 1549 cm−1, this set of peaks is also observed in Figure 1a. Table 1 compares the infrared frequencies we measured for the crotonaldehyde on TiO2 to those reported previously.19,21 In terms of the previous results,19,21 the labeled peaks in Figure 1 can be assigned as follows: 2917, 2928, 2973, and 3031 cm−1 to ν(CHx); 2876 cm−1 to resonance mode; 1659 cm−1 to ν(C O); 1592 and 1636 cm−1 to ν(CC); 1375, 1422, and 1451 cm−1 to δ(CH3); and 1163 cm−1 to ν(C−C). The two sharp features of ν(C−C) and ν(CO) can be used to identify crotonaldehyde adsorption on TiO2. As shown in Table 1, the 1549 cm−1 peak has not been observed in the cases of anatase and rutile at 313 K and of P25-TiO2 in the temperature range of 251−257 K. This peak is likely due to formation of carboxylate species on the TiO2 surface but is not clearly seen at the lower coverage of Figure 1a. Figure 2 shows the temperature-dependent infrared spectra measured after adsorption of crotonaldehyde (4.0 Torr) on 35 °C TiO2 and progressively heating the surface to the temperatures indicated under vacuum. All of the spectra were taken at 35 °C. As compared to the spectrum of Figure 1b, similar infrared absorptions are observed in the 35 °C spectrum

COMPUTATION METHOD

The optimized adsorption structure of CH3CHCHCHO on TiO2, with anatase (101) as a model surface, was calculated in the framework of density functional theory by using the DMol3 package, in which generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) formulation was employed. A double-numeric quality basis set with polarization functional, a Monkhorst−Pack k-point set at 2 × 3 × 1, and a supercell of 24 [TiO2] units with dimensions 7.55 × 10.89 × 9.35 Å3 were used in this study. All slabs were separated by a vacuum space of 12.0 Å. The Ti and O atoms in the first and second layers of the slabs were allowed to be varied in position for the optimized structure calculation.

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RESULTS In our study, reactions of CH 3 CHBr 2 , BrCH 2 CH 2 Br, BrCH2CH2OH, and ClCH2CH2OH on TiO2 have been found to generate crotonaldehyde on the surface, and its formation temperature may vary with the precursor molecules. Therefore, we present the result of crotonaldehyde adsorption and the temperature effect first. B


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The Journal of Physical Chemistry C

3030 cm−1) are similar in terms of the peak positions and their relative intensities, indicating that the CH3CHCHCH− moiety of the crotonaldehyde retains in the heating process. In the range of 1000−1750 cm−1, the 35 and 50 °C spectra show the same absorption behavior, which gradually varies from 50 to 300 °C. The prominent changes include the significant intensity loss of the ν(CO) (1656 cm−1) and ν(C−C) (1163 cm−1). Meanwhile, two broad bands at ∼1417 and 1531 cm−1 grow in the 300 °C spectrum, which could be attributed to symmetric and antisymmetric COO stretching modes of the carboxylate group (νs(COO) and νas(COO)). It is proposed that crotonaldehyde is thermally converted to crotonate (CH3CHCH−COO) on TiO2 between 50 and 300 °C. This chemical change can explain the temperature-dependent infrared absorption patterns, such as the large drop in the carbonyl (1656 cm−1) absorption, the enhanced carboxylate absorptions (∼1417 and 1531 cm−1), and the basically intact CHx stretching vibrations. CH2CHBr Adsorption. The possibility of forming crotonaldehyde from vinyl bromide reaction on TiO2 has been explored in this study. Dissociation of the C−Br bond of CH2CHBr on TiO2 may have a chance to form surface enolate (CH2CH−O−), which can be transformed into CH3CHO, finally leading to crotonaldehyde formation.28 Figure 3a shows the temperature-dependent infrared spectra of TiO2 obtained after being in contact with 0.4 Torr of CH2 CHBr at 35 °C, followed by cell evacuation and temperature increase. All of the spectra were obtained at 35 °C in a vacuum.

Table 1. Comparison of the Infrared Frequencies (cm−1) of Crotonaldehyde on TiO2a anataseb (313 K)

rutileb (313 K)

1165 1394

1156 1406

1636 1686 2745 2845 2920

1602 1656 2740 2830 2905

2954

2950

3038

3036

P25c (251−257 K) 1148 1376 1442

1656 2822 2857 2931 2941 2968 2977 3017

mode ν(C−C) δs(CH3) δas(CH3)

ν(CC) ν(CO) ν(C−H) resonance νs(CH3) νas(CH3) ν(C−H) sp2 ν(CC−H)

P25d (308 K) 1163 1375 1422 1451 1549 1592 1636 1659 2876 2917 2928 2973 3031

νs: symmetric stretching; νas: antisymmetric stretching; δs: symmetric bending; δas: antisymmetric bending. bRef 19. cRef 21. dThis work (Degussa, P25-TiO2). a

Figure 2. Infrared spectra measured after adsorption of crotonaldehyde (4.0 Torr) on TiO2 (35 °C), followed by progressively heating the surface to the temperatures indicated in a vacuum. As the TiO2 was heated to the preset temperature, the heating power was shut off immediately. All of the spectra were obtained at 35 °C in a vacuum. Figure 3. Temperature-dependent infrared spectra of TiO2 measured after adsorption of CH2CHBr at 35 °C, followed by progressively heating the surface to the temperatures indicated under vacuum (a). As the TiO2 was heated to the preset temperature, the heating power was shut off immediately. All of the spectra in (a) were taken at 35 °C in a vacuum. (b) are the temperature-dependent infrared spectra of TiO2 in the presence of 25.0 Torr of CH2CHBr. The TiO2 was maintained at the indicated temperatures for 1 min, and infrared spectra were taken in this time slot.

of Figure 2, but with stronger peak intensities, showing more crotonaldehyde molecules are adsorbed on the surface. The slight frequency changes for some peaks in the two spectra could be due to a small variation in the interaction between the adsorbed molecules. Moreover, the 1548 cm−1 peak is relatively enhanced in the 35 °C spectrum of Figure 2. From 35 to 300 °C, the infrared peaks of CHx stretching vibrations (2876− C


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measured at 35 °C. In the 35 °C spectrum, the peaks at 1041, 1168, 1266, 1381, and 1441 cm−1 are similar to those found for gaseous CH3CHBr2 reported previously (Table 3);30 therefore,

The CH2CHBr on TiO2 mainly absorbs at 1244, 1258, 1369, 1583, and 1601 cm−1, which are listed in Table 2 and compared Table 2. Infrared Frequencies (cm−1) of CH2CHBr on TiO2 and Mode Assignmenta CH2CHClb (vapor)

a

mode

1281 1377

δ(CCH), δ(CH2), ρ(CH2) δ(CH2), δ(CCH)

1614

ν(CC)

Table 3. Comparison of the Infrared Frequencies (cm−1) of CH3CHBr2a

CH2CHBr/TiO2, 35 °C 1244 1258 1369 1583 1601

δ: bending; ρ: rocking; ν: stretching. bRef 29.

CH3CHBr2b (vapor)

mode

CH3CHBr2/TiO2, 35 °C

1045 1070 1172 1260 1383 1443

ρ(CH3), δ(CH) ρ(CH3), ν(C−C) δ(CH), ρ(CH3) δ(CH), ν(C−C) δs(CH3) δas(CH3)

1041 1168 1266 1381 1441

ρ: rocking; δs: symmetric bending; δas: antisymmetric bending. bRef 30.

a

to the frequencies of vinyl chloride in the gas state.29 Increasing the temperature to 125 °C causes the decrease of the CH2 CHBr, but without leaving notable peaks near 1163 and 1659 cm−1 (as shown in Figure 1) that can be ascribed to crotonaldehyde. Figure 3b shows the infrared spectra of TiO2 taken at the temperatures indicated in 25.0 Torr of CH2 CHBr. There is still no sign of crotonaldehyde formation even in the presence of gaseous CH2CHBr and at a higher temperature of 180 °C. It is concluded that CH2CHBr on TiO2 does not react to generate aldehyde in the heating process. The main absorptions in Figure 3b are from CH2 CHBr in the adsorbed phase (e.g., 1600 cm−1) and the gas phase (e.g., 1610 cm−1). CH3CHBr2 Adsorption. Figure 4 shows the infrared spectrum of TiO2 obtained after being in contact with 1.5 Torr of CH3CHBr2 at 35 °C, followed by evacuation and surface heating to 50, 100, and 125 °C. All of the spectra were

these peaks are attributed to adsorbed CH3CHBr2. Besides CH3CHBr2 on TiO2, other smaller peaks appear at 1625, 1680, and 1695 cm−1. As the temperature is increased to 50 °C, the set of peaks belonging to CH3CHBr2 decreases in intensity, together with slightly enhanced absorptions between 1580 and 1720 cm−1. The largely reduced CH3CHBr2 peaks in the 100 °C spectrum indicate that the amount of adsorbed CH3CHBr2 diminishes considerably. This result leads to further growths of the 1680 and 1695 cm−1 peaks and a relatively strong absorption at 1635 cm−1 with a 1619 cm−1 shoulder. In addition to these changes, a peak near 1180 cm−1 is formed, but overlapping with the reduced 1168 cm−1 peak. From 100 to 125 °C, all of the CH3CHBr2 peaks become very small, and the peaks at 1680 and 1695 cm−1 decrease in intensity. At 125 °C, the absorptions at 1180, 1619, and 1635 cm−1 continuously grow and become the main peaks. This set of the peaks (1180, 1619, and 1635 cm−1) is attributed to adsorbed crotonaldehyde, as compared to its infrared absorptions on TiO2 shown in Figure 1. However, due to the presence of Br atoms on the surface in the transformation from CH3CHBr2 to crotonaldehyde, the ν(CO) and ν(CC) are red-shifted to 1635 and 1619 cm−1, instead of the original 1659 and 1636 cm−1. The frequencies of ν(CO) and ν(CC) of the adsorbed crotonaldehyde are sensitive to the TiO2 crystal phase and TiO2 surface state.19,21 The 1680 and 1695 cm−1 peaks observed in Figure 4 are assigned to red-shifted CO stretching mode and are likely from adsorbed acetaldehyde, which can be formed from CH3CHBr2 reaction on TiO2 and further dimerizes into crotonaldehyde. Furthermore, no broad band near 1549 cm−1 to show carboxylate formation is observed in the 100 or 125 °C spectrum of Figure 4, being different from the case of crotonaldehyde adsorption (Figure 2). This could be due to the presence of Br atoms on the surface that prevent the oxidation of crotonaldehyde to crotonate. BrCH2CH2Br Adsorption. Shown in Figure 5a are the temperature-dependent TiO2 infrared spectra measured after being exposed to 2.0 Torr of BrCH2CH2Br at 35 °C, with subsequent cell evacuation and surface-temperature increase. Five main peaks appear at 1193, 1247, 1281, 1420, and 1442 cm−1 in the 35 °C spectrum of Figure 5a. These peaks are listed in Table 4 and are shown to be similar to the infrared absorptions of BrCH2CH2Br in the liquid state,31,32 which are assigned to CH2 scissoring or wagging modes. Moreover, both gauche and trans BrCH2CH2Br molecules are present on the TiO2 surface at 35 °C. Upon heating to 100 °C, the peaks of

Figure 4. Infrared spectra of TiO2 measured after adsorption of CH3CHBr2 at 35 °C, followed by progressively heating the surface to the temperatures indicated in a vacuum. As the TiO2 was heated to the preset temperature, the heating power was shut off immediately. All of the spectra were obtained at 35 °C in a vacuum. D


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BrCH2CH2Br has less reactivity toward crotonaldehyde formation on TiO2, as compared to CH3CHBr2. Figure 5b shows the time-dependent infrared spectra of TiO2 at 140 °C and in contact with 2.0 Torr of BrCH2CH2Br in the closed cell. In the 0 min spectrum, the infrared absorptions of BrCH2CH2Br, including both adsorbed and gaseous phases, appear at 1189, 1247, 1281, 1421, and 1443 cm−1. Due to the similar absorption frequencies, the peak positions for the BrCH2CH2Br in the gas phase and on the surface cannot be differentiated. Along with the increase of the reaction time, the peaks of the dibrominated ethane gradually decrease and almost vanish at 20 min. Parallel to this change is the gradually enhanced formation of adsorbed crotonaldehyde with its characteristic peaks of 1183, 1589, 1608, and 1634 cm−1. In addition, the carbonyl absorption at 1674 cm−1 could be due to strongly bonded CH3CHO or weakly bonded crotonaldehyde. However, no peaks at 1074, 1114, 1129, and 1165 cm−1 as those shown in the 150 °C spectrum of Figure 5a are measured. Probably, the species responsible for these peaks are relatively unstable and promptly react on the surface to form crotonaldehyde in the experimental condition used for obtaining the spectra in Figure 5b. BrCH2CH2OH Adsorption. Figure 6 shows the temperature-dependent infrared spectra measured after adsorption of

Figure 5. Infrared spectra of TiO2 measured after adsorption of BrCH2CH2Br at 35 °C, followed by progressively heating the surface to the temperatures indicated under vacuum. (a) All of the spectra in (a) were obtained at the temperatures pointed out in a vacuum. The TiO2 was maintained at the indicated temperatures for 1 min, and infrared spectra were taken in this time slot. (b) Infrared spectra of TiO2, at 140 °C and in the presence of 2.0 Torr of BrCH CH Br, measured after the contact times indicated.

Table 4. Comparison of the Infrared Frequencies (cm−1) of BrCH2CH2Bra

a

BrCH2CH2Brb (liquid)

modec

BrCH2CH2Br/TiO2, 35 °C

1185 1244 1276 1419 1437

ω(CH2), t ω(CH2), g ω(CH2), g δ(CH2), g δ(CH2), t

1193 1247 1281 1420 1442

t: trans; g: gauche; δ: scissoring; ω: wagging. bRef 31. cRef 32.

BrCH2CH2Br decrease in intensity, but the formation of crotonaldehyde is not clearly seen. This result reveals that a part of the surface BrCH2CH2Br molecules desorbs from the TiO2 at 100 °C. Raising the temperature to 150 °C causes a further decrease of BrCH2CH2Br on TiO2 and the appearance of the peaks (1180, 1593, 1620, and 1633 cm−1) belonging to crotonaldehyde. In addition, the absorptions of 1074, 1114, 1129, and 1165 cm−1 are also enhanced in the 150 °C spectrum, and they can be assigned to C−C and/or C−O stretching vibrations, such as the infrared absorptions of ethoxy groups on TiO2.33 It is suggested that intermediates containing C−C and C−O bonds, such as −CH 2 CH 2 OH or BrCH2CH2O−, are generated on the surface and eventually are turned into crotonaldehyde. This point is supported in the adsorption of BrCH 2 CH 2 OH on TiO 2 shown later.

Figure 6. Infrared spectra of TiO2 measured after adsorption of BrCH2CH2OH at 35 °C, followed by progressively heating the surface to the temperatures indicated under vacuum. All of the spectra were measured at the temperatures pointed out in a vacuum. The TiO2 was maintained at the indicated temperatures for 1 min, and infrared spectra were taken in this time slot.

BrCH2CH2OH (2.0 Torr) on TiO2 at 35 °C, with subsequent cell evacuation. In the 35 °C spectrum, the infrared peaks are located at 1013, 1072, 1104, 1179, 1218, 1276, 1363, 1424, and 1441 cm−1. The previous adsorption study of ethanol on TiO2 at 35 °C has shown the coexistence of ethanol itself and the ethoxy group from the O−H bond cleavage.33 Besides, it has already been shown in Figure 5 that BrCH2CH2Br is basically E


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The Journal of Physical Chemistry C Table 5. Comparison of the Infrared Frequencies (cm−1) of BrCH2CH2OH and ClCH2CH2OHa

a

BrCH2CH2OHb (vapor)

modeb

BrCH2CH2OH/TiO2, 35 °C

ClCH2CH2OHc (liquid)

1003 1047 1083 1157, 1189 1211, 1224 1254−1269 1349, 1355 1375, 1382 1418

ν(CC) ν(CO) ν(CO) δ(COH) tw(CH2), g ω(CH2), t tw(CH2), t ω(CH2), g δ(CH2)

1013 1072 1104 1179 1218 1276 1363

1039 1072

ρ(CH2) ν(CO)

1198 1254 1288 1349 1387 1431 1456

tw(CH2) combination ω(CH2) δ(OH) ω(CH2) δ(CH2) δ(CH2)

1424 1441

modec

ClCH2CH2OH/TiO2, 35 °C 1042 1076 1120 1194 1251 1296 1366 1399 1432 1447

t: trans; g: gauche; ν: stretching; δ: bending; tw: twisting; ω: wagging. bRef 34. cRef 35.

molecularly adsorbed on TiO2 at 35 °C; i.e., the C−Br bonds remain intact. Therefore, the observed peaks in the 35 °C spectrum of Figure 6 are attributed to both BrCH2CH2OH and BrCH2CH2O− and are similar to the infrared absorptions of BrCH2CH2OH vapor, as shown in Table 5.34 BrCH2CH2OH and BrCH2CH2O− on TiO2 are expected to have close infrared frequencies. Several spectral changes occur from 35 to 150 °C in Figure 6. Crotonaldehyde peaks (1593, 1615, and 1639 cm−1) and possibly acetaldehyde (1678 and 1692 cm−1) appear at 100 °C and increase with the temperature. As a contrast, the infrared absorptions from BrCH2CH2O diminish in intensity. Among them, the 1104 cm−1 peak has a relatively high decrement. This peak could be due to the C−O stretching mode of BrCH2CH2OH, which can undergo O−H bond dissociation in response to the temperature increase. It is noticed that the infrared pattern between 1000 and 1150 cm−1 for both the BrCH2CH2Br (Figure 5) and BrCH2CH2OH on TiO2 at 150 °C are very similar, in terms of the peak positions (∼1070, 1110, and 1128 cm−1) and their relative intensities. Therefore, it is proposed that BrCH2CH2O− is formed as an intermediate in the reaction of BrCH2CH2Br on TiO2 and exists at 150 °C. ClCH2CH2OH Adsorption. Figure 7 shows the temperature-dependent infrared spectra measured after adsorption of ClCH2CH2OH (2.0 Torr) on TiO2 at 35 °C, followed by cell evacuation. The peaks observed in the 35 °C spectrum are similar to those of ClCH2CH2OH in the liquid state (Table 5).35 Just as the case of BrCH2CH2OH/TiO2, it is proposed that both ClCH2CH2OH and ClCH2CH2O− are present on the surface after ClCH2CH2OH adsorption on the 35 °C TiO2. Crotonaldehyde (1176, 1608, 1621, and 1647 cm−1) forms at 100 °C and largely increases at 150 °C at the sacrifice of the ClCH2CH2OH. The reaction pathway of ClCH2CH2OH on TiO2 is similar to that of its brominated analog. Enhanced Crotonaldehyde Formation from ClCH2CH2OH on TiO2 under Photoirradiation. Figure 8a shows the infrared spectra obtained at the times indicated during the photoirradiation of ClCH2CH2OH adsorbed on TiO2 at 325 nm under vacuum. Because the surface temperature was raised to 70 °C in the photoirradition process, a thermal control experiment for ClCH2CH2OH adsorbed on TiO2 at this temperature was carried out, with the infrared spectra shown in Figure 8b for comparison. In both Figure 8a and 8b, crotonaldehyde is generated (1173, 1620, and 1642 cm−1), and its amount increases with time from the reactions of ClCH2CH2OH on TiO2. However, the 325 nm irradiation promotes the product formation in the early stage and results in more crotonaldehyde at 180 min, as compared to the case at 70

Figure 7. Infrared spectra of TiO2 measured after adsorption of ClCH2CH2OH at 35 °C, followed by progressively heating the surface to the temperatures indicated under vacuum. All of the spectra were measured at the temperatures pointed out in a vacuum. The TiO2 was maintained at the indicated temperatures for 1 min, and infrared spectra were taken in this time slot.

°C without photoillumination. Besides, the photon exposure also causes a stronger absorption at 1690 cm−1, possibly from adsorbed acetaldehyde. The relative changes in the concentrations of crotonaldehyde with time for ClCH2CH2OH/TiO2 during the irradiation at 325 nm and during the surface annealing at 70 °C are shown in Figure 9. The peak intensity at 1642 cm−1 is used to represent the relative amount of crotonaldehyde on TiO2. The ClCH2CH2OH/TiO2 held at 70 °C results in linearly increased crotonaldehyde. However, the photoillumination of the ClCH2CH2OH/TiO2 largely enhances the crotonaldehyde formation, especially at the early stage. It is clear that the irradiation causes the photocatalytic reaction of ClCH2CH2OH on TiO2 to produce crotonaldehyde. F


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Figure 10. Calculated adsorption structures of crotonaldehyde on anatase TiO2(101) (a) and on Cl-substituted anatase TiO (101), with replacement of a bridging oxygen atom by a Cl atom (b).

TiO2 at 180 °C (Figure 3). For the other halogenated compounds studied in this research, crotonaldehyde is clearly observed on TiO2 below 180 °C. The C(sp2)−Br bond of CH2CHBr is stronger than C(sp3)−Br bonds of CH3CHBr2, BrCH2CH2Br, and BrCH2CH2OH and can be less reactive on TiO2. Although crotonaldehyde (CH3CHCHCHO) with 4 carbon atoms and 6 hydrogen atoms is a possible product from the coupling reaction of CH2CHBr molecules on TiO2, via C−Br bond dissociation and replacement of the Br atoms with TiO2 surface oxygens, the reaction pathway is not observed. For the dibrominated compounds (CH3CHBr2 (Figure 4) and BrCH2CH2Br (Figure 5)) on TiO2, CH3CHBr2 has a higher reactivity toward crotonaldehyde formation. Reactions of these two brominated compounds on TiO2 to form the carbonyl-containing crotonaldehyde must involve cleavage of the C−Br bonds and exchange between the bromine atoms and TiO2 surface oxygens. It is proposed that CH3CHO is formed in the reaction of CH3CHBr2 on TiO2, via the following pathway, CH3CHBr2 + Ti−O−Ti → CH3CHO + 2Ti−Br. The CH3CHO molecules can promptly condense into crotonaldehyde on the surface. For BrCH2CH2Br/TiO2, the reaction may proceed via BrCH2CH2Br + Ti−O−Ti → Ti−O−CH2CH2Br + Ti−Br → CH3CHO + 2Ti−Br. Formation of Ti− OCH2CH2Br is proposed as the first step involving C−Br bond dissociation in the reaction of BrCH2CH2Br on TiO2 because it is found that both BrCH2CH2Br and BrCH2CH2OH on TiO2 at 150 °C have similar infrared absorptions between 1000 and 1150 cm−1 (Figure 5 and Figure 6). The transition from −OCH2CH2Br to CH3CHO occurs through a combination of dissociation of the C−Br and Ti−O bonds, formation of Ti−Br and CO bonds, and H transfer. The less reactivity of BrCH2CH2Br for crotonaldehyde formation on TiO2, with respect to CH3CHBr2, could be due to the H-transfer as ratelimiting step and/or a lower C−Br dissociation rate. As for the role of residual surface OH groups in the reactions of CH3CHBr2 and BrCH2CH2Br, the concentration of OH on the TiO2 is small because it has been heated to 450 °C under vacuum for 24 h.36 The surface OH groups cannot play a predominant role in the crotonaldehyde formation. Note that dimerization of CH3CHO on TiO2 forms CH3CHCHCHO and H2O. H2O adsorbed on TiO2 (P25) has the bending vibration in the range of 1620−1635 cm−1 depending on the coverage.37 Therefore, the H2O absorption may contribute to the strongest ν(CO) band (≥1634 cm−1) of the CH3CH CHCHO from CH3CHBr2, BrCH2CH2Br, BrCH2CH2OH, and

Figure 8. Infrared spectra obtained at the times indicated during the illumination of ClCH2CH2OH/TiO2 at 325 nm in a vacuum (a). Infrared spectra obtained at the times indicated with ClCH2CH2OH/ TiO2 held at 70 °C under vacuum as a thermal control experiment (b).

Figure 9. Comparison of the relative amounts of CH3CHCHCHO in the photoirradiation of ClCH2CH2OH/TiO2 at 325 nm and in the annealing of ClCH2CH2OH/TiO2 at 70 °C.

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DISCUSSION The experimental results have shown that adsorption and reactions of CH3CHBr2, BrCH2CH2Br, BrCH2CH2OH, and ClCH2CH2OH on TiO2 can generate crotonaldehyde, although its formation temperature may not be the same. CH2CHBr on TiO2 is not the case; no sign of crotonaldehyde formation is found even in the presence of CH2CHBr vapor over the G


Article

The Journal of Physical Chemistry C Table 6. Comparison of the Calculated Infrared Frequencies (cm−1) of Crotonaldehyde on TiO2 CH3CHCHCHO/TiO2

CH3CHCHCHO + Cl/TiO2

freq.

mode

freq.

mode

1004 1022 1130 1267 1306 1339 1366 1426 1451 1579 1673 2886 2945 3000 3108 3124 3140

δ(CH3), δ(C3−H) δ(C2−H), δ(C3−H), δ(CH3) ν(C1−H) δ(C3−H), δ(C2−H), δ(C1−H) δ(C2−H), δ(C3−H) δ(C1−H) δ(CH3) δ(CH3) δ(CH3) ν(CC) ν(CO) ν(C1−H) νs(CH3) νas(CH3) ν(C3−H), ν(C4−H) ν(C3−H), ν(C4−H) ν(C2−H)

1010 1028 1123 1254 1289 1337 1369 1430 1450 1542 1638 2897 2960 3010 3082 3134 3143

δ(C2−H), δ(C3−H), δ(CH3) δ(C3−H), δ(CH3) ν(C3−C4), δ(CH3) δ(C3−H), δ(C3−H), δ(C1−H) δ(C2−H), δ(C3−H) δ(C1−H) δ(CH3) δ(CH3) δ(CH3) ν(CC) ν(CO) ν(C1−H) νs(CH3) νas(CH3) ν(C4−H) ν(C2−H) ν(C3−H)

photocatalytic routes. CH3OH on TiO2 has been shown to be oxidized to CH2O under UV irradiation.39 The oxidation process is initiated by photogenerated holes. On the other end, the ClCH2 group may undergo C−Cl bond scission as it receives a photogenerated electron, via the so-called dissociative electron attachment.40 Calculated Adsorption Structures of Crotonaldehyde on Anatase−TiO2(101) and Their Infrared Absorptions. We have theoretically investigated the adsorption structure of the crotonaldehyde molecule on the anatase−TiO2(101) model surface and its vibrational absorptions and the effect due to the replacement of a bridging oxygen by a chlorine atom. Figure 10a shows the optimized adsorption structure of crotonaldehyde on the TiO2(101), with the CC group near a 5-foldcoordinated Ti site. The lengths from the two carbon atoms to the titanium ion are 3.660 and 4.155 Å, respectively. The calculated structure has the CO and CC stretching frequencies at 1673 and 1579 cm−1, as shown in Table 6. Figure 10b shows the adsorption structure of crotonaldehyde on the TiO2(101) but with a bridging O being substituted by Cl, which creates a titanium site only surrounded by four oxygen ions. The surface crotonaldehyde molecule is bound near this titanium site. In this adsorption structure, the distances from the two carbon atoms of the CC group to the titanium are 3.508 and 3.918 Å, smaller than those in the case of Figure 10a. The shorter C−Ti lengths suggest a stronger interaction between the adsorbed crotonaldehyde on the chlorinated TiO2 surface, leading to red-shifted CO and CC stretching modes (1638 and 1542 cm−1 (Table 6)) for the adsorbed crotonaldehyde. This theoretical outcome is consistent with the experimental result, for example, Figures 1 and 7.

ClCH2CH2OH on TiO2. However, because both the H2O and CH3CHCHCHO come from the same reaction, the ν(C O) band can be used to monitor the extent of the crotonaldehyde formation. It is found that the crotonaldehyde formed from the reactions of CH3CHBr2 and BrCH2CH2Br on TiO2 has lower CC and CO stretching frequencies (∼1620 and 1635 cm−1 (Figures 4 and 5)), as compared to those (1636 and 1659 cm−1) from crotonaldehyde adsorption (Figure 1). These infrared changes reflect the difference in the local environments around the crotonaldehyde adsorption sites, i.e., Ti−Br or Ti− O−Ti. Moreover, the oxidation step from crotonaldehyde to crotonate (∼1550 cm −1 ) found in the adsorption of crotonaldehyde (Figures 1 and 2) is largely suppressed in the reactions of CH3CHBr2 and BrCH2CH2OH (Figures 4 and 6). This result can be explained by the formation of Ti−Br groups in the dissociation of these two brominated ethanes on TiO2 which prevents the process of CH3CHCHCHO + Ti−O−Ti → CH 3 CHCH−C−(O−Ti) 2 . In the reactions of BrCH2CH2Br, BrCH2CH2OH, and ClCH2CH2OH on TiO2, it is found that BrCH2CH2Br has the lowest reactivity to form crotonaldehyde, as indicated by the 100 °C spectra of Figures 5, 6, and 7. This result indicates that cleavage of the first C−Br bond of BrCH2CH2Br on TiO2, together with cleavage of the surface Ti−O−Ti, plays an important role in the crotonaldehyde formation. That is, BrCH2CH2Br + Ti−O−Ti → TiOCH2CH2Br + Ti−Br is likely to be the rate-limiting step for the crotonaldehyde formation. In the photoirradiation of ClCH2CH2OH on TiO2 at 325 nm, crotonaldehyde formation is enhanced (Figures 8 and 9). The functional groups of C−Cl and C−OH are transparent to the light used in this study.38 Therefore, the enhanced crotonaldehyde formation is not due to direct ClCH2CH2OH photoreaction but is mediated by TiO2. The origin of the TiO2 photocatalytic reaction is the band-to-band transition, forming photogenerated electron−hole pairs. The TiO2-mediated promotion for the crotonaldehyde generation could occur in the steps of C−Cl bond scission and/or formation of the carbonyl group involving oxidation and H-transfer, eventually leading to CH3CHO. The ClCH2 and CH2OH moieties of the adsorbed ClCH2CH2OH may have different TiO2-mediated

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CONCLUSION

There is no evidence for crotonaldehyde formation in the reaction of CH2CHBr on TiO2. However, adsorption and thermal reactions of CH3CHBr2, BrCH2CH2Br, BrCH2CH2OH, and ClCH2CH2OH on TiO2 can produce crotonaldehyde, which is considered to be originated from CH3CHO aldol condensation. The reactions of CH3CHBr2 and H


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BrCH2CH2Br on TiO2, generating CH3CHO, are proposed as follows CH3CHBr2 + Ti−O−Ti → CH3CHO + 2Ti−Br

and BrCH 2CH 2Br + Ti−O−Ti → Ti−O−CH 2CH 2Br + Ti−Br → CH3CHO + 2Ti−Br

It is also found that photoirradition of ClCH2CH2OH on TiO2 at 325 nm can enhance crotonaldehyde formation. This TiO2mediated photocatalysis may promote the reaction steps of C− Cl bond activation and/or formation of the carbonyl group involving H-transfer in the ClCH2CH2OH reaction to form CH3CHO.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 886-6-2740552. E-mail: [email protected]. Phone: 886-6-2757575 ext. 65326. ORCID

Jong-Liang Lin: 0000-0002-1276-5479 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 1052113-M-006-002).

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