Adsorption and Photoadsorption States of Benzene Derivatives on

Jul 6, 2011 - Division of Environmental Research, EcoTopia Science. Institute, Nagoya University, Nagoya, 464-8603, Japan. Adsorption states of molecu...
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Adsorption and Photoadsorption States of Benzene Derivatives on Titanium Oxide Studied by NMR Hayato Yuzawa,†,‡ Masanori Aoki,† Hideaki Itoh,§ and Hisao Yoshida*,† †

Department of Applied Chemistry, Graduate School of Engineering and §Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan ABSTRACT: Solid-state NMR spectroscopy revealed the adsorption state of benzene derivatives on TiO2. All of the carbons of adsorbed benzene derivatives exhibited each chemical shift higher than the corresponding carbons of the molecules in the CCl4 solution. This result indicates that the electrons of the adsorbed molecules were withdrawn by TiO2. For the adsorbed benzene, only one narrow signal was observed, indicating that the benzene ring faced parallel to the TiO2 surface and was mobile. On the other hand, for the adsorbed benzene derivatives, the change of the chemical shift was different from carbon to carbon, showing that these benzene rings were inclined at various angles to the TiO2 surface. In addition, the chemical shift was further changed by photoirradiation only for the adsorbed anisole and phenol on the TiO2, indicating that they further changed in the angle and in the interaction strength. These results also demonstrated that the NMR is an efficient analytical method to reveal the adsorption states. SECTION: Surfaces, Interfaces, Catalysis

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dsorption states of molecules or reaction intermediates on the surface are of wide concern because they often greatly influence their physical and chemical properties such as adsorption/desorption properties, reactivity in catalysis, and so on. Thus, various analytical methods such as FT-IR,1 5 STM,6,7 XPS, AES, LEED,8 UPS,9 and theoretical calculation (e.g., DFT calculation,10 12 Monte Carlo simulation,13 etc.) have been developed to clarify the state of adsorbed species on the surface. Solid-state NMR is also a useful analytical method to observe the adsorbed species on the surface of the solid materials such as zeolite,14 17 silica,18,19 alumina,19 layered compounds,20 22 and so on because the chemical shift can elucidate the chemical structure of the adsorbed species, and the width of the signals reflects the mobility of the adsorbed species and the interaction between the adsorbate and the solid surface. In addition, it is known that the chemical shift for the molecules is varied with adsorption and indicative of the interaction strength between the adsorbate and the solid surface.20 However, NMR spectroscopy has been scarcely used to elucidate the details of the adsorption state such as the direction of the adsorbates. In the present study, we systematically investigated the change of the NMR chemical shifts for various benzene derivatives with adsorption on the surface of TiO2 and found that the change of the chemical shift gave information about the state of the adsorbed species in detail. Figure 1 shows 13C CP MAS NMR spectra for benzene derivatives (phenol, anisole, toluene, benzene, chlorobenzene, and nitrobenzene) adsorbed on TiO2. In the case of the adsorbed benzene, only a sharp peak at 130.2 ppm was observed (Figure 1d). The width of this signal was narrow, similar to the one for benzene in the CCl4 solution in the literature.23 Similarly, sharp signals were also observed for the adsorbed toluene and chlorobenzene r 2011 American Chemical Society

(Figure 1c and e). The half-widths of the signals were 0.5 ppm for the adsorbed benzene and 0.6 0.7 ppm for the adsorbed toluene and chlorobenzene. These sharp signals suggest that the molecules such as benzene, toluene, and chlorobenzene would be weakly adsorbed on the TiO2 surface in fast motion such as rotation. On the other hand, in the cases of the adsorbed phenol, anisole and nitrobenzene, broader signals were observed (Figure 1a, b, and f), that is, their half-widths were 0.6 2.3 ppm for the adsorbed phenol, 0.8 1.8 ppm for the adsorbed anisole, and 0.9 4.1 ppm for the adsorbed nitrobenzene. These broader signals would be derived from their strong interaction with the TiO2 surface, where the mobility of these adsorbates should be more restricted than the benzene, toluene, and chlorobenzene on the TiO2 surface.15 In 13C CP MAS spectra, the signal intensity cannot quantitatively reflect the number of the chemically equivalent carbons because the signal intensity varies with the cross-polarization by the excitation energy transfer from 13C to 1H.24 The chemical shifts of the obtained signals in each adsorbate on the TiO2 surface were roughly similar to, but slightly different from, those for the molecules in the CCl4 solution.23 Table 1 shows the difference (Δx) of the 13C chemical shift for the adsorbates on the TiO2 surface from that for the molecules in the CCl4 solution. The Δx value reflects the change of the electron density for each carbon because the chemical shift reflects the shielding effect of the magnetic field depending on the electron density. Thus, the positive Δx value means that the electrons of the corresponding carbon atom in the adsorbates were more Received: May 9, 2011 Accepted: July 6, 2011 Published: July 06, 2011 1868

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Figure 1. 13C CP MAS NMR spectra of (a) phenol, (b) anisole, (c) toluene, (d) benzene, (e) chlorobenzene, and (f) nitrobenzene adsorbed on TiO2 surface. Cx stands for the position of the carbon in the adsorbates to show the assignment of each signal.

Table 1.

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C Chemical Shifts for Benzene Derivatives Adsorbed on TiO2 and Those in the CCl4 Solution

a

Cx stands for the position of the carbon shown in Figure 1. b For the benzene derivatives adsorbed on the TiO2 surface. c For the benzene derivatives in the CCl4 solution. The data were from ref 23. d Defined by the following equation: Δx = (chemical shift of Cx for the benzene derivative adsorbed on TiO2) (chemical shift of Cx for the benzene derivative in the CCl4 solution). e EDG = electron-donating group. f EWG = electron-withdrawing group.

withdrawn by the TiO2 surface compared to those in the CCl4 solution, while the negative Δx value means the opposite case. As a result, the Δx values for all of the carbons in the adsorbates showed positive values, which indicates that the electrons of the benzene derivatives used in this study were withdrawn by the TiO2 surface more or less. In the case of the adsorbed benzene, the Δx value was 1.7 ppm. All of the Δx values for the adsorbed toluene and chlorobenzene were similar to that for benzene compared to the other substituted benzenes, and the distributions of the Δx values were relatively narrow, such as 1.6 2.1 and 1.2 1.8 ppm, respectively. On the other hand, the adsorbates having more electron-donating or -withdrawing groups (EDG or EWG), that is, the adsorbed phenol, anisole, and nitrobenzene, exhibited a broader range of Δx values than that for the adsorbed

benzene, such as 1.1 3.9, 1.5 3.8, and 1.1 2.9 ppm, respectively. It is noted that the order of the Δx values was characteristic for each adsorbate. For the phenol, the largest Δx value was Δ1, which was for the carbon bonding to the EDG (C1), and the values decreased with distance from the carbon in the following order: Δ1 > Δ2 > Δ3 > Δ4. For the anisole, the carbon of the substituent showed the largest value (Δ5), which was followed by the values for the carbons at C2 and C4 positions (Δ2 > Δ4 > Δ3 > Δ1). For the nitrobenzene, the larger value was observed for the carbons farther from the EWG (Δ4 > Δ3 > Δ2 > Δ1). Because the Δx value reflects the strength in the electron withdrawing by the TiO2 surface, the Δx value would be usable as an index of the strength in the interaction between the carbon of the adsorbate and the TiO2 surface.20 Thus, through the analysis 1869

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Figure 2. Proposed adsorption state of (a) benzene, (b) chlorobenzene, (c) toluene, (d) phenol, (e) anisole, and (f) nitrobenzene on TiO2 surface.

of the Δx values, the state of the benzene derivatives adsorbed on the TiO2 surface can be discussed. In the case of the benzene, the Δx value was equal for all of the carbons (Table 1). This result clearly means that all of the carbons in the adsorbed benzene equally interacted with the TiO2 surface, probably with the cation such as Ti4+ or H+. In addition, as described above, the sharp signal width for the benzene would be derived from the fast motion of the benzene adsorbed on the TiO2 surface, such as rotation. Thus, it is suggested that the aromatic ring would be parallel to the TiO2 surface in fast motion, as shown in Figure 2a. The obtained adsorption state of benzene from the present NMR analysis is consistent with that from the reported FT-IR studies,1,2 although the motion of the molecule on the surface was unclear in these previous studies. Δx values and line widths similar to those of the adsorbed benzene were obtained in all of the carbons of the adsorbed chlorobenzene and toluene. This suggests that the orientation and the motion of the benzene ring for the chlorobenzene and toluene on the TiO2 surface would be similar to those for the benzene. These results are not conflicting with those from the FT-IR study.1 However, the Δx values were slightly different from each other. In the case of the adsorbed chlorobenzene, the Δ1 and Δ2 values are smaller, while the Δ3 and Δ4 values are larger than the Δ1 value for the adsorbed benzene, that is, the order of the Δx values was Δ1 < Δ2 < Δ1 (benzene) < Δ3 = Δ4. This means that the carbon farther from the substituent would interact a little more strongly with the TiO2 surface. Thus, the adsorbed state of the chlorobenzene is expected to be slightly inclined in the fast motion on the TiO2 surface, as shown in Figure 2b. In the case of toluene, because the order of the Δx values was opposite from tendency for the chlorobenzene (Δ1 > Δ2 = Δ1 (benzene) > Δ3 = Δ4), the carbon nearer to the substituent would interact a little more strongly with the TiO2 surface. Thus, the adsorbed state of the toluene is expected to be inclined oppositely to the chlorobenzene on the TiO2 surface, as shown in Figure 2c. However, the Δ5 value for the methyl group in the toluene was smaller than the Δ1 value for the carbon next to the methyl group. This might be because the Δ5 value for the methyl carbon (sp3 carbon) was influenced differently from the Δx (x = 1 4) values for the aromatic carbons (sp2 carbons) by the interaction with the TiO2 surface. On the other hand, largely positive Δx values were obtained in the other benzene derivatives having stronger electron-donating or -withdrawing substituents such as phenol, anisole, and nitrobenzene. From the order of the Δx values for the phenol (Δ1 > Δ2 > Δ3 > Δ4), the carbons that are closer to the substituent would interact more strongly with the TiO2 surface (C1 >

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C2 > C3 > C4). From the molecular geometry, it is suggested that the hydroxyl group, probably the oxygen atom, interacted with the TiO2 surface more strongly than the aromatic carbons, as shown in Figure 2d. In such a situation, the electron density of the carbons that are closer to the O atom in the phenol would be more largely withdrawn by the inductive effect of the adsorption through the oxygen atom. In addition, the farthest C4 carbon exhibited a Δx value lower than that for the adsorbed benzene, indicating that this part would be very far from the surface. This adsorption state can be suggested, as shown in Figure 2d, which is consistent with the reported one from FT-IR spectroscopy.5 Although the chemical structure of anisole was similar to that of phenol, the adsorption state of anisole would be different from that of phenol because the order of the Δx values for the carbons in the aromatic ring of the adsorbed anisole was clearly different from that for the adsorbed phenol (Table 1). Though the Δx values in the aromatic ring for the adsorbed anisole seem to be irregularly arranged (Δ2 > Δ4 > Δ3 > Δ1), the order was the same as that of the Mulliken populations (C2 > C4 > C3 > C1) of the anisole molecule before the adsorption, which were calculated by using MOPAC 200925 (Table 2). This means that the carbon originally having higher electron density would interact more strongly with the TiO2 surface. Thus, it is suggested that the aromatic ring in anisole would be adsorbed almost in parallel to the TiO2 surface. In the case of the other adsorbates, there is no clear relationship between the order of the Mulliken population in aromatic carbons and that of the Δx values (Table 2). Because the substituent strongly influences the chemical shift of the carbons in the aromatic ring, the interaction between the substituent and the surface would provide large changes in each chemical shift. On the other hand, the largest Δx value for the adsorbed anisole was 3.8 for the C5 carbon, which is the methyl of the substituent. The Δ5 value for the adsorbed anisole was similar to the Δ1 value for the aromatic ring in the adsorbed phenol. It is noticed that both the C5 carbon of anisole and the C1 carbon of phenol are directly linked to the O atom by a single bond. Thus, the large Δ5 value for the adsorbed anisole would not result from a direct interaction with the TiO2 surface but from an inductive effect of the adsorption through the O atom, as discussed above. Therefore, the adsorption state of anisole is expected to be as depicted in Figure 2e, which shows that both the O atom and the aromatic ring in anisole interacted with the TiO2 surface. The motion of the adsorbed anisole in this form would be much restricted, which accounts for the broad line width. The difference in the adsorption state between phenol and anisole would originate from the electron distribution of each molecule. In the case of the adsorbed nitrobenzene, an opposite tendency (Δ4 > Δ3 > Δ2 > Δ1) to that for the adsorbed phenol was obtained. Thus, it is proposed that the plane of the aromatic ring of the adsorbed nitrobenzene on the TiO2 surface would be oppositely tilted, that is, the side with the substituent would be relatively far apart from the TiO2 surface, as shown in Figure 2f. This proposal is different from the conclusion derived from FTIR spectroscopy in the literature,2 that is, nitrobenzene would adsorb perpendicular to the surface in end-on geometry through the interaction between the nitro group and the TiO2 surface. On the other hand, in another report, it was suggested that there are two adsorption modes; one is the bidentate bonding of the nitro group to the surface Ti4+ sites, and another is the monodentate bonding between the nitro group and surface hydroxy groups (OdN+ O 3 3 3 H Osurface).3 Although both adsorption states might provide the end-on geometry, the latter (monodentate) 1870

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Table 2. Mulliken Population of Benzene Derivatives and the Change of Chemical Shift (Δx) with Adsorption

a Cx stands for the position of the carbon shown in Figure 1. b EDG = electron-donating group. c Mulliken population calculated by MOPAC 2009.25 This value was calculated based on the molecular structure before the adsorption. d See the footnote (d) in Table 1. e EWG = electron-withdrawing group.

Figure 3. 13C CP MAS NMR spectra of (a) phenol, (b) anisole, (c) toluene, (d) benzene, (e) chlorobenzene, and (f) nitrobenzene adsorbed on TiO2 surface, which were photoirradiated for 10 min in the presence of the gaseous molecules. Cx stands for the position of the carbon in the adsorbates to show the assignment of each signal.

would allow more flexibility in the benzene ring orientation. In the present case, the major nitrobenzene molecules would adsorb in the latter way, and the benzene ring could interact with the surface, as illustrated in Figure 2f. Figure 3 shows the 13C CP MAS NMR spectra for the benzene derivatives adsorbed on the TiO2 surface after photoirradiation for 10 min. These spectra show similar signals to those before photoirradiation for all of the adsorbates except for the phenol and the anisole. Because no peaks other than those assigned to the adsorbates were observed in all of the spectra, no photoreactions of the adsorbates would occur upon photoirradiation.

Table 3 shows the change of the chemical shift by photoirradiation (Δx photo). For the adsorbed toluene, benzene, chlorobenzene, and nitrobenzene, the Δx photo values were almost negligible for all of the carbons. On the other hand, both positive and negative Δx photo values were observed for the adsorbed phenol, and only positive Δx photo values were observed for the adsorbed anisole. Thus, it is suggested that the adsorption states of the phenol and anisole on the TiO2 surface were changed by photoirradiation. The order of the positive and negative Δx photo values for the adsorbed phenol was Δ1 photo > Δ2 photo > 0 > Δ3 photo > Δ4 photo. The increases of the Δ1 photo and Δ2 photo values elucidate 1871

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Table 3. Effect of Photoirradiation after Adsorption on the Chemical Shift

a Δx photo = (chemical shift of Cx for the adsorbed benzene derivatives after the photoirradiation) (chemical shift of Cx for the adsorbed benzene derivatives before the photoirradiation). b Cx stands for the position of the carbon shown in Figure 3. c EDG = electron-donating group. d EWG = electron-withdrawing group.

suggested, as mentioned above. Further, the change of the adsorption state by photoirradiation could also be discussed. The results mentioned above also confirm that 13C CP MAS NMR spectroscopy can provide detailed information for the analysis of the adsorption state of these organic compounds. In the present study, the amount of adsorbed molecules was not controlled. Because the adsorption state should depend on the loading amount of the molecules on the surface, further systematic studies using the present method are desired to clarify the adsorption state totally.

Figure 4. Proposed changes of adsorption state of (a) phenol and (b) anisole by photoirradiation.

that the interaction with the TiO2 surface through the O atom of the hydroxyl group became stronger, while the decreases of the Δ3 photo and Δ4 photo values indicate that the interaction between the benzene ring and the TiO2 surface became weaker by photoirradiation. Thus, it is proposed that the benzene ring came to stand at a higher angle with the TiO2 surface by photoirradiation (Figure 4a). This is consistent with the report that TiO2 adsorbed a larger amount of phenol upon photoirradiation in an aqueous phenol solution.26 On the other hand, for the adsorbed anisole, the positive Δx photo values for all of the carbons indicate that the interaction between the adsorbed anisole and the TiO2 surface was enhanced by photoirradiation (Figure 4b), although the detailed mechanism of this photoadsorption was unclear. In conclusion, the adsorption states of the benzene derivatives on the TiO2 surface were clarified by using 13C CP MAS NMR spectroscopy. When the benzene derivatives were adsorbed on the TiO2 surface, the electrons of adsorbed molecules were withdrawn by the TiO2 surface. This phenomenon could be observed as the change (Δx) of chemical shift, which was to the lower magnetic field (1.1 3.9 ppm) compared to the benzene derivatives in the CCl4 solution. The larger Δx value would originate from the stronger interaction between the carbon (Cx) and the TiO2 surface. It is noted that this method can clearly show the change of the electron density of each carbon atom in the adsorbed molecule, while FT-IR and UV vis spectroscopies can only show the information about specific moieties. In addition, the line width of the NMR signal shows the motion of the molecule on the surface. From such information, the detailed adsorption states of benzene derivatives on the TiO2 surface were clearly

’ EXPERIMENTAL SECTION The TiO2 sample was donated from the Catalysis Society of Japan as JRC-TIO-8 (anatase, BET specific surface area: 338 m2/g). The employed adsorbates, phenol (Wako), anisole, toluene, benzene, chlorobenzene, and nitrobenzene (Kishida chemical), were purchased and used as received. The TiO2 powders adsorbing benzene or benzene derivatives were prepared in a closed system connected with a vacuum line. The TiO2 sample (0.5 g) was spread on the bottom of the quartz cell and was photoirradiated by a 300 W Xe lamp, which emitted both UV and visible light, from outside of the bottom for 1 h in the presence of air at room temperature, followed by evacuation at 473 K for 2 h to clean up the surface. After cooling down to room temperature, the vapor of the adsorbate was introduced to the cell to be adsorbed by the TiO2, and the system was kept at room temperature for 6 h to establish the adsorption equilibrium. For the preparation of the photoadsorbed sample, the adsorbed sample was photoirradiated for 10 min by the Xe lamp equipped with an optical band-pass filter to limit the light wavelength to 365 ( 20 nm in the presence of the adsorbate molecules in the gas phase. The 13C CP MAS NMR spectra were recorded on a Bruker 300 MHz NMR spectrometer. The prepared sample was transferred into a zirconia rotor (7 mm in diameter) at room temperature under atmospheric conditions, and then, the rotor was sealed. The rotation frequency of the zirconia rotor was 5.0 kHz. The π/2 pulse width, the 1H 13C contact time, and the recycle delay were 4.5 μs, 2.0 ms and 4.0 s, respectively. The accumulated number was 5000 times. The chemical shift was calibrated by the signal for the carbonyl carbon in glycine powder (176.03 ppm). In the present study, each reproducibility error of the obtained signal was less than (0.1 ppm through all of the measurements. 1872

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’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-52-789-4609. Fax: +81-52-789-3178. E-mail: yoshidah@ apchem.nagoya-u.ac.jp. Notes ‡

Research Fellow of the Japan Society of Division for the Promotion of Science.

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