J. Phys. Chem. B 2004, 108, 18261-18268
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Adsorption and Surface Reactions of N(C2H5)3 on Powdered TiO2 Chen-Fu Lien, Yu-Feng Lin, Yi-Shiue Lin, Meng-Tso Chen, and Jong-Liang Lin* Department of Chemistry, National Cheng Kung UniVersity, 1, Ta Hsueh Road, Tainan, Taiwan, Republic of China ReceiVed: April 13, 2004; In Final Form: September 16, 2004
The adsorption, thermal stability, and photochemical reactions of N(C2H5)3 on powdered TiO2 have been studied by Fourier transform infrared spectroscopy (FTIR). N(C2H5)3 molecules are adsorbed molecularly on TiO2 at 35 °C and can be removed from the surface at 300 °C in a vacuum. In the presence of O2, N(C2H5)3 starts to decompose on TiO2 at a temperature of ∼215 °C. N(C2H5)3 on TiO2 is desorbed from the surface upon photoirradiation. In the presence of O2, the photodesorption process is suppressed and photoreactions of N(C2H5)3 are promoted. CO2(g), H2O(a), NCO(a), HCOO(a), CH3COO(a), and surface species containing NHx, CdO, C-N, or C-N-C groups are detected during N(C2H5)3 photodecomposition on TiO2. As H2O is added to the photoreaction system of N(C2H5)3 on TiO2 in O2, the amounts of NCO(a) and NHx detected during photoirradiation decrease, but with appearance of CdN groups. Most importantly, it is found that H2O participates in the HCOO(a) and CH3COO(a) formation.
Introduction Recently, heterogeneous photocatalysis by TiO2 has appeared promising for transforming organic pollutants into environmently innocuous species and has attracted much attention. TiO2 is a chemically stable substrate with a ∼3.2 eV band gap.1-4 As TiO2 absorbs photons with energies higher than its band gap, electron-hole pairs are generated, which are initiation species for photocatalytic reactions. Under UV irradiation, primary amines can react to form secondary amines in aqueous solutions containing a mixture of TiO2 and platinum black or can react to form N-alkylidene amines in the presence of suspended TiO2 in acetonitrile.5-7 However, adsorption and surface intermediates were not characterized in these studies. N(C2H5)3 is often used as a solvent in chemical synthesis, a corrosion inhibitor, a agent in curing and hardening of polymers, etc. N(C2H5)3 is highly toxic by ingestion and inhalation and is a strong irritant to tissue. Photooxidation of triethylamine on TiO2 thin film in the presence of O2, N2, or H2O has been investigated by Huang et al., focusing on the subjects of photoreaction rate as a function of concentrations of N(C2H5)3, O2, and H2O as well as on the deactivation mechanism after prolonged photoirradiation.8 The deactivation process was characterized by Fourier transform infrared spectroscopy (FTIR) and was attributed to the accumulation of surface intermediates formed during photoillumination. After photodegradation of N(C2H5)3 on TiO2 for a few hours, infrared bands at 1301, 1406, 1694, and 1724 cm-1 were observed and assigned to -C(O)OH groups. In addition, another band observed at 2208 cm-1 was assigned to N-Nd O.8 However, in this study the infrared bands were not wellresolved. This might lead to ambiguous band assignments, because correct identification of the surface intermediates is important in realizing the reaction pathways and the deactivation process (i.e., the bond breaking processes during photodegradation of N(C2H5)3). In the present study, we focus on the investigation of surface intermediates during photodegradation * Author to whom correspondence should be addressed. E-mail:
[email protected].
of adsorbed N(C2H5)3 on TiO2 in 16O2 or 18O2 by using FTIR. The photoreaction products and intermediates are examined, based on the characteristic functional absorptions and oxygen isotopic shifts, revealing versatile photoreaction pathways of N(C2H5)3 on TiO2. Experimental Section The sample preparation of TiO2 powder supported on a tungsten fine mesh (∼6 cm2) has been described previously.9,10 In brief, TiO2 powder (Degussa P25, ∼50 m2/g, anatase 70%, rutile 30%) was dispersed in a water/acetone solution to form a uniform mixture, which was then sprayed onto a tungsten mesh. Uniform deposition of TiO2 powder sample on tungsten grid via the slurry method has been demonstrated by scanning electron microscopy.9 Tungsten reactions have been shown to be prevented by the TiO2 coating of uniform thickness over the holes and the metal grid. The TiO2/W sample was mounted inside the IR cell for simultaneous photochemistry and FTIR spectroscopy. The IR cell with two CaF2 windows for IR transmission down to 1000 cm-1 was connected to a gas manifold, which was pumped by a 60 L/s 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 spot-welded on the tungsten mesh. Before each run of the experiment, the TiO2 sample was heated to 450 °C in a vacuum for 2 h. After the sample was heated, 10 Torr of O2 was introduced into the cell as the sample was cooled to 70 °C. When the TiO2 temperature reached 35 °C, the cell was evacuated for gas dosing. The TiO2 surface after the above treatment still possessed residual isolated hydroxyl groups.11 16O (99.998%, Matheson) and 18O (99 atom %, Isotec) were 2 2 used as received in compressed states. N(C2H5)3 (99.9%, Tedia) was purified by several cycles of freeze-pump-thaw. Pressure was monitored with a Baratron capacitance manometer and an ion gauge. In the photochemistry study, both the UV and IR beams were set 45° to the normal of the TiO2 sample. The UV light source used was a combination of a Hg arc lamp (Oriel
10.1021/jp0483728 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/02/2004
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Figure 1. (a) Infrared spectrum of a TiO2 surface exposed to ∼0.15 Torr N(C2H5)3 and then evacuated at 35 °C. (b) Infrared spectrum of a TiO2 surface exposed to N(C2H5)3 and H2O at 35 °C. The two spectra were recorded with 50 scans.
Corp), a water filter, and a band-pass filter with a bandwidth of ∼100 nm centered at 400 nm (Oriel 51670). The power at the position of the TiO2 sample was 140 mW/cm2 measured in the air by a power meter (Molectron, PM10V1). The UV absorption of N(C2H5)3 for the wavelengths used in the present study was negligible.12 Infrared spectra were obtained with a 4 cm-1 resolution by a Bruker FTIR spectrometer with a MCT detector. The entire optical path was purged with CO2-free dry air. The spectra presented here have been rationalized against a clean TiO2 spectrum, providing the metal oxide background. In the study of photooxidation, the photoirradiation time started to count as the UV lamp was turned on. It took 40-50 s to reach the full power. Results N(C2H5)3 Adsorption and Effect of Coadsorbed H2O on TiO2. Figure 1a shows the infrared spectrum of a TiO2 surface after being in contact with ∼0.15 Torr of N(C2H5)3, followed by evacuation at 35 °C. In this spectrum, infrared bands appear at 1007, 1042, 1083, 1157, 1181, 1310, 1359, 1382, 1395, 1454, 2881, 2944, and 2973 cm-1. Table 1 compares the infrared absorptions of N(C2H5)3 in liquid phase and on TiO2. The similarity in the absorption frequencies in both cases indicates that N(C2H5)3 is adsorbed molecularly on TiO2 at 35 °C. There is other circumstantial evidence that further supports that N(C2H5)3 remains intact on TiO2 at 35 °C. In the previous studies of dissociative adsorption of CH3I and C2H5I on TiO2, adsorbed CH3O(a) with characteristic CH3 stretching bands at
Lien et al.
Figure 2. Infrared spectra of a TiO2 surface exposed to ∼0.4 Torr N(C2H5)3 and then evacuated at the indicated temperatures for 1 min. All of the spectra were recorded at 35 °C with 50 scans.
∼2830 and 2930 cm-1 and C2H5O(a) with characteristic C-O stretching bands at 1073 and 1120 cm-1 were generated from the C-I bond dissociation of CH3I and C2H5I, respectively.16,17 If the C-C or C-N bonds of N(C2H5)3 dissociate on TiO2, then CH3O(a) or C2H5O(a) is expected to be generated. However, there is no spectroscopic evidence for the formation of the two species in Figure 1a. If the C-H bonds of N(C2H5)3 dissociate, then the surface OH groups are expected to increase. However, the surface OH groups decrease upon N(C2H5)3 adsorption instead, as shown by the negative bands between 3600 and 3800 cm-1. The TiO2 surface used in the present study possesses surface OH and Lewis acid Ti4+ sites on which N(C2H5)3 can be bound, just like the case of NH3 adsorption on TiO2.18 Adsorbed N(C2H5)3 can interact with the surface OH groups through hydrogen bonding and interact with Ti4+ through Lewis baseacid attraction. The effect of addition of H2O on the N(C2H5)3 adsorption was also investigated. This study may have important implications in TiO2-catalyzed N(C2H5)3 photoreactions in aqueous solutions. Figure 1b shows the infrared spectrum of a TiO2 surface adsorbed with N(C2H5)3 and H2O. The surface was prepared by making a N(C2H5)3-adsorbed TiO2 using the procedure for Figure 1a and then exposing it to ∼0.2 Torr H2O followed by evacuation. In comparison to Figure 1a, the N(C2H5)3 bands decrease in intensity due to the H2O adsorption revealed by the 1635 cm-1 band, indicating that H2O molecules compete for adsorption sites on the TiO2 surface and replace some of the preadsorbed N(C2H5)3 molecules. However, absorptions at 1202, 1359, 1476, and 2846 cm-1 are largely enhanced. This effect is likely due to interactions between adsorbed H2O and N(C2H5)3 molecules via hydrogen bonding.
N(C2H5)3 Reactions on Powdered TiO2
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TABLE 1: Comparison of Infrared Frequencies (cm-1) of N(C2H5)3a
a
ν, stretching; F, rocking; ω, wagging; tw, twisting; δ, bending.
Thermal Stability and Decomposition of N(C2H5)3 on TiO2. Figure 2 shows the infrared spectra of N(C2H5)3-covered TiO2 after heating the surface to the indicated temperatures for 1 min in a vacuum. All of the spectra in Figure 2 were taken at 35 °C. It is found that the N(C2H5)3 bands start to decrease in intensity at ∼100 °C and almost disappear after 250 °C. No new bands are observed while the N(C2H5)3 decreases with temperature, suggesting that the N(C2H5)3 desorbs due to the surface annealing above 100 °C. If N(C2H5)3 on TiO2 decomposes at the elevated temperatures, then its amount cannot be large. Figure 3 shows the infrared spectra of TiO2, initially in contact with a mixture of ∼25 Torr N(C2H5)3 and ∼25 Torr O2 in a closed cell, taken during heating the surface to the temperatures indicated. The heating rate was ∼2 °C/s. Infrared absorptions are enhanced between 1500 and 1800 cm-1 after increasing the surface temperature higher than 215 °C, suggesting that N(C2H5)3 starts to decompose on TiO2 at ∼215 °C in the presence of O2. However, due to the broad, unresolved absorption feature in this frequency region, no further analysis is possible for intermediate identification, except the product of CO2(g) at 2349 cm-1 which appears at a temperature higher than 300 °C. Photoreactions of N(C2H5)3 were studied under several different conditions, including photoirradiation of a TiO2 surface covered with (1) N(C2H5)3, (2) N(C2H5)3 and H2O, (3) N(C2H5)3 in the presence of O2, and (4) N(C2H5)3 and H2O in the presence of O2. To investigate the role of H2O and correctly assign the infrared bands observed during N(C2H5)3 photodecomposition on TiO2, 16O2 and 18O2 were used. In these experiments, it was found that the N(C2H5)3 photoconsumption rate may vary for different TiO2 samples. However, similar infrared bands were observed in the spectra recorded during photodecomposition of N(C2H5)3.
Photoirradiation of N(C2H5)3 on TiO2. Figure 4 shows the infrared spectra taken before and after the indicated times during photoirradiation of a TiO2 surface covered with N(C2H5)3 in a closed cell. The N(C2H5)3 bands decrease in intensity with increasing photoirradiation time. After 10 min, the bands between 1100 and 1600 cm-1 disappear, and those between 2750 and 3050 cm-1 also decrease significantly. But, along with the decrease of N(C2H5)3, no new bands are observed, suggesting the desorption of adsorbed N(C2H5)3 upon photoirradiation. Note that the TiO2 surface temperature was increased to ∼56 °C during the photoillumination. However, the disappearance of the adsorbed N(C2H5)3 during photoirradiation cannot be explained by the thermal effect. A thermal control experiment was carried out by holding a TiO2 surface covered with N(C2H5)3 at a temperature of 56 °C for 30 min but without photoillumination. It was found that the bands of N(C2H5)3 did not decrease in intensity after 30 min of surface annealing. Photoirradiation of a TiO2 covered with N(C2H5)3 and H2O was carried out as well. Upon photoirradiation, the N(C2H5)3 decreases without forming new bands, but the N(C2H5)3 desorption rate is lower, compared to the case in the absence of H2O. The integrated band intensity between 2750 and 3050 cm-1 only decreases by ∼25% after 30 min of irradiation. Photoirradiation of N(C2H5)3 on TiO2 in the Presence of O2. Figure 5 shows the infrared spectra taken after the indicated times during photoirradiation of a TiO2 surface covered with N(C2H5)3 initially in 10 Torr of 16O2 in a closed cell. It is found that the band intensities of the adsorbed N(C2H5)3 decrease with increasing photoirradiation time. After 30 min, there is ∼30% of the adsorbed N(C2H5)3 consumed. As a contrast, adsorbed N(C2H5)3 is barely detectable after 30 min of photoirradiation in the absence of O2 as shown in Figure 4. In the presence of O2, the decrease of N(C2H5)3 on TiO2 induced by photoirra-
18264 J. Phys. Chem. B, Vol. 108, No. 47, 2004
Figure 3. Development of infrared spectra of a mixture of ∼25 Torr N(C2H5)3 and ∼25 Torr O2 over a TiO2 surface heated at 2 °C/s. All of the spectra were taken at the indicated temperatures with five scans.
diation is significantly suppressed. Furthermore, new bands form, even in the early stage of 2 min of photoirradiation. In the 180-min spectrum, the infrared bands not belonging to N(C2H5)3 appear at 1123, 1166, 1359, 1382, 1452, 1566, 1627, 1727, 2207, 2349, 3228, and 3253 cm-1. Among them, the 2349 cm-1 band is attributed to gaseous CO2. After 180 min of irradiation followed by evacuation, the CO2 band is no more observed, but the other bands are still present, indicating that they belong to surface species. To correctly identify these surface species, 18O2 was used in the photoreaction study of N(C2H5)3 on TiO2 as well. Figure 6 shows the infrared spectra of N(C2H5)3(a) on TiO2, initially in 10 Torr of 18O2 in a closed cell, taken after the indicated times during 180 min of photoirradiation. In the infrared analysis for Figures 5 and 6, the band assignments are based on characteristic frequencies of functional groups and oxygen isotopic shifts in frequency. In comparison to the bands found in the 16O2 case, any red-shifted bands observed in Figure 6 suggest that the responsible species possess 18O. As an example, the 2349 cm-1 C16O2 band in the 16O2 case is shifted to 2331 cm-1 and attributed to C18O2. Other red-shifted bands include 1315 and 1335 cm-1 shifted from 1359 cm-1, 1418 from 1452 cm-1, 1549 from 1566 cm-1, 1620 from 1627 cm-1, 1702 from 1727 cm-1, and 2197 from 2207 cm-1. The three bands at 1166, 3228, and 3253 cm-1 are not shifted toward lower frequencies by using 18O . Therefore, these bands should be assigned to functional 2 groups not containing oxygen. The 3228 and 3253 cm-1 bands are assigned to NHx stretching modes, and the 1166 cm-1 to C-N or C-N-C stretching modes.19 In the previous study of the photodegradation of N(C2H5)3 on TiO2, Huang et al. observed an infrared band at 2208 cm-1 and attributed it to N-NdO species.8 However, the band at 2207 cm-1 observed
Lien et al.
Figure 4. Infrared spectra taken after the indicated times during UV irradiation of N(C2H5)3 adsorbed on TiO2 in a closed cell. The N(C2H5)3covered TiO2 surface was prepared by exposing a clean TiO2 surface to ∼0.15 Torr N(C2H5)3, followed by evacuation. All of the spectra were recorded with five scans.
in Figure 5 in our study is not attributed to N-NdO, but to a NCO(a) (isocyanate) species instead, based on the following two reasons. First, in the recent study of the photooxidation of CH3CN on TiO2 in 16O2, an infrared band at 2204 cm-1 was observed and attributed to NCO(a), with the N atom bonded to a surface Ti4+ site, by isotopic study.20 Our observation of the shift from 2207 cm-1 in 16O2 to 2197 cm-1 in 18O2 is consistent in that the CN frequency of NC16O(a) is higher than that of NC18O(a) on TiO2 by a ∼8-10 cm-1.20 Second, in the previous photooxidation study of NH3 on TiO2, although gaseous N2O at 2224 cm-1 was found, no band at ∼2207 cm-1 was detected using a similar photoreaction condition.21 The 1727 cm-1 band in Figure 5 for N(C2H5)3 photodecomposition in 16O2 is assigned to a carbonyl stretching vibration, because it is shifted to 1702 cm-1 in the 18O2 case.19 The strong band at 1625 cm-1 in Figure 5 is slightly shifted to 1620 cm-1 in 18O2. These two frequencies are close to the gaseous H2O bending absorption, and it is known that the bending frequency of H216O(g) is ∼6.5 cm-1 higher than that of H218O(g), therefore the 1625 and 1620 cm-1 bands are attributed to adsorbed H216O and H218O, respectively.22 The strong band at 1359 cm-1 in the 16O2 case is split and shifted to 1315 and 1335 cm-1 in 18O2. The 1566 cm-1 is shifted to 1549 cm-1. Previously in the study of dissociative adsorption of formic acid on TiO2, two infrared bands at 1371 and 1557 cm-1 were observed and assigned to symmetric and antisymmetric -C16O16O- stretching of adsorbed formate groups. The 1359 and 1566 cm-1 bands in Figure 5 are close to the formate absorptions. Besides, oxygen isotopic infrared studies of HCOONa have shown that the symmetric stretching vibrations of -C16O16O-, -C16O18O-, and -C18O18O- absorb at 1340, 1315, and 1297 cm-1 respectively.23 The symmetric formate
N(C2H5)3 Reactions on Powdered TiO2
Figure 5. Infrared spectra taken after the indicated times during UV irradiation of N(C2H5)3 adsorbed on TiO2 initially in 10 Torr 16O2 in a closed cell. All of the spectra were recorded with five scans.
stretching is red-shifted by 25 cm-1 as one 16O atom is replaced by 18O and by 43 cm-1 as both of the 16O atoms are replaced. For the antisymmetric stretching vibrations of the -COO- of HCOONa, -C16O16O- and -C18O18O- absorb at 1607 and 1587 cm-1 with a 20 cm-1 difference. Therefore, the 1315, 1335, and 1549 cm-1 bands observed in the 18O2 case are attributed to HC16O18O(a) and HC18O18O(a). In Figure 5, the 1382 cm-1 also belongs to HC16O16O(a) and is assigned to a C-H bending mode. In Figure 6, the 1454 cm-1 band of adsorbed N(C2H5)3 significantly decreases in intensity after 180 min of photoirradiation, but with formation of a new band at 1418 cm-1. As a contrast in the case of 16O2, the absorption at ∼1450 cm-1 becomes broader and is still relatively strong after 180 min of photoirradiation. Furthermore, there is no band formed at 1418 cm-1 in Figure 5. This comparative result reveals the a band at ∼1450 cm-1 is formed in the photodecomposition of N(C2H5)3 on TiO2 in 16O2 and is shifted to 1418 cm-1 as 16O2 is replaced by 18O2. The 1452 cm-1 band in the 180 min spectrum of Figure 5 is assigned to the -COO- symmetric stretching vibration of adsorbed CH3COO. This assignment is supported by the previous infrared study of dissociative adsorption of acetic acid on TiO2. The -COO- symmetric and antisymmetric stretching vibrations of CH3COO(a) on TiO2 absorb at 1456 and 1532 cm-1, respectively.24 The -COO- antisymmetric stretching of CH3COO(a) in Figure 5 is not resolved due to overlapping with the strong 1566 cm-1 HCOO(a) band. Figure 7 shows the relative concentrations of N(C2H5)3 and its photodecomposition products of HCOO(a), CH3COO(a), H2O(a), NCO(a), and CO2(g) as a function of photoirradiation time in the photodecomposition of adsorbed N(C2H5)3 on TiO2 in 10 Torr 16O2 (Figure 5). The amounts of H2O(a), HCOO(a), and CH3COO(a) are estimated by curve fitting the broad feature between 1500
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Figure 6. Infrared spectra taken after the indicated times during UV irradiation of N(C2H5)3 adsorbed on TiO2 initially in 10 Torr 18O2 in a closed cell. All of the spectra were recorded with five scans.
and 1800 cm-1 in Figure 5 using three bands at 1627 cm-1 for water, 1566 cm-1 for formate, and 1535 cm-1 for acetate.24-26 The amount of N(C2H5)3 is estimated by the integrated area between 2800 and 3100 cm-1 in Figure 5, with the contributions from HCOO(a) and CH3COO(a) subtracted. Separate experiments of the photoirradiation of adsorbed N(C2H5)3 on TiO2 in 10 Torr O2 using band-pass filters at 365 ( 5 and 435 ( 5 nm were also carried out. The power at the position of the TiO2 sample was 9.2 mW/cm2 for the 365 nm filter and was 25.5 mW for the 435 nm one. It was found that 365 nm (3.4 eV) light can generate NCO(a), the characteristic product of N(C2H5)3 photodecomposition, but 435 nm (2.9 eV) cannot. These comparative results show that N(C2H5)3 photodecomposition on TiO2 is mediated by TiO2 band gap excitation. Direct 400 nm light absorption by N(C2H5)3 to cause its decomposition is unlikely to occur, because gaseous N(C2H5)3 absorbs lights at wavelengths
