Adsorptive and Kinetic Properties on Photocatalytic Hydrogenation of

Article pubs.acs.org/JPCC

Adsorptive and Kinetic Properties on Photocatalytic Hydrogenation of Aromatic Ketones upon UV Irradiated Polycrystalline Titanium Dioxide: Differences between Acetophenone and Its Trifluoromethylated Derivative Shigeru Kohtani,*,† Eito Yoshioka,† Kenji Saito,‡,§ Akihiko Kudo,‡ and Hideto Miyabe*,† †

Department of Pharmacy, School of Pharmacy, Hyogo University of Health Sciences, 1-3-6, Minatojima, Chuo-ku, Kobe, 650-8530, Japan ‡ Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan S Supporting Information *

ABSTRACT: Adsorptive and kinetic properties on photocatalytic hydrogenation of acetophenone (AP) and 2,2,2-trifluoroacetophenone (TFAP) have been investigated upon UV irradiated P25 TiO2 in deaerated ethanol. Kinetic data of AP were analyzed by the Langmuir−Hinschelwood kinetic expression, −dCs/dt = kmaxKirrCs/(1 + AKirrCs), where kmax is the maximum rate and Kirr is the apparent adsorption constant under the irradiation. The hydrogenation of AP proceeded in the light-limited controlled manner, in which kmax increased with increasing light intensity. The hydrogenation of TFAP exhibited firstorder kinetics due to the predominant formation of ketal in ethanol. Electron transfer efficiency from conduction band and/or electron trap sites on TiO2 to AP or TFAP was also evaluated. In the case of AP, ca. 75% electrons trapped at relative shallow sites were able to transfer into AP, whereas ca. 25% ones trapped at deep sites remained on the TiO2 surface. For TFAP, all of the trapped electrons transferred into TFAP. Reaction models to account for these experimental results are proposed and discussed in detail.

1. INTRODUCTION Heterogeneous semiconductor photocatalysts have attracted much attention with respect to inducing characteristic organic transformations since the 1980s.1−10 Among those, photocatalytic reductions on titanium dioxide (TiO2) have been applied to limited organic reactions, for example, hydrogenations of alkenes and alkynes,11,12 aromatic nitro compounds,13−18 aldehydes,19,20 and ketones.21,22 In the reductive photocatalysis, electrons photogenerated in the conduction band (CB) or those trapped at surface defect sites on TiO2 can transfer into organic substrates adsorbed on TiO2 in the presence of appropriate sacrificial hole scavengers (water, alcohols, or amines, etc.) under deaerated conditions. Recently, we have reported that the polycrystalline P25 TiO2 powder exhibited the excellent photocatalytic activity to hydrogenate aromatic ketones into corresponding secondary alcohols under the combination of UV light irradiation and deaerated conditions in ethanol.22 The desired secondary alcohols were obtained with almost quantitative yields for 10 examples, e.g., from acetophenone (AP) to 1-phenylethanol (AP−OH) in 97% yield as depicted in Scheme 1. We have also found that most of the reaction rates depend on the reduction potentials (Ered) of substrates. However, contrary to our expectation, the hydrogenation rate for 2,2,2-trifluoroacetophenone (TFAP) © 2012 American Chemical Society

Scheme 1. TiO2-Photocatalyzed Hydrogenation of AP

having an electro-withdrawing trifluoromethyl group was much slower than that for AP, though Ered for TFAP is sufficiently positive compared to that for AP.22,23 This kinetic study has been undertaken to elucidate this unexpected and interesting behavior of TFAP. Ketones and aldehydes are two classes of compounds that possess a reactive carbonyl group, and therefore can react with excess alcohol to give ketals and acetals, respectively (Scheme 2). The mechanism of ketal and acetal formation involves equilibrium protonation, attack by alcohol, and then loss of a proton to give the neutral hemiketal (or hemiacetal). The hemiketal further undergoes protonation and loss of water to give an oxocarbonium ion, which is attacked by another mole of Received: June 7, 2012 Published: July 16, 2012 17705

dx.doi.org/10.1021/jp3056174 | J. Phys. Chem. C 2012, 116, 17705−17713

The Journal of Physical Chemistry C

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Scheme 2. Equilibrium between TFAP, Hemiketal, and Ketal Species in Ethanol

alcohol, and then loss of a proton to afford the final product. It should be noted that the ketal (or acetal) formation is an equilibrium process. Guthrie reported that the hemiketal formation was predominant in methanol in the case of TFAP.24 Therefore, the photocatalytic hydrogenation of aromatic ketones in ethanol is expected to be influenced by the ketal formation, because a concentration ratio of keto/ hemiketal/ketal greatly affects the adsorptivity as well as the reactivity on the TiO2 surface. It is well-known that the white color of TiO2 photocatalyst powder changes into a blue-gray one when the TiO2 powder is sufficiently irradiated with UV light in the presence of a sacrificial hole scavenger (e.g., alcohols) and in the absence of an electron acceptor (e.g., under deaerated conditions). This color change is also observed during the photocatalytic hydrogenation of aromatic ketones.22 The blue-gray species with a broad absorption band in visible to infrared (IR) light regions have been assigned to the CB and surface trapped electrons (Ti3+) accumulated on the TiO2 surface induced by the UV irradiation.25−27 The electron accumulation on TiO2 has been investigated by means of electron paramagnetic resonance (ESR),28−32 photoacoustic,33,34 and diffuse reflectance IR or UV−vis spectroscopy.31,35−39 Furthermore, the reactivity of the accumulated electrons with electron acceptors (molecular oxygen,31,37,40,41 iodine,42 or methylviologen43) adsorbed on TiO2 has been examined using ESR31 and transient IR or UV−vis absorption measurements.37,40−43 However, little is known about the reactivity of the accumulated electrons with ketones and aldehydes upon the irradiated TiO2. It is therefore interesting to note the electron transfer efficiency from the Ti3+ sites to AP or TFAP adsorbed on TiO2. In this paper, we report the results of adsorptive and kinetic properties on the photocatalytic hydrogenation of AP and TFAP upon the UV irradiated TiO2 (P25) surface in deaerated ethanol. We have also examined the hemiketal and ketal formation from AP and TFAP in ethanol without the TiO2 photocatalyst, because the equilibrium process can affect the reductive photocatalysis as mentioned above. Further, we have evaluated electron transfer efficiencies of the electrons accumulated on TiO2 to AP or TFAP dissolved in ethanol by injection of AP or TFAP into a preirradiated TiO2 suspension in order to gain information about surface defect states formed on TiO2. Finally, we propose reaction models to account for the adsorptive and kinetic data on the photocatalytic hydrogenation of AP and TFAP.

received: AP (Nacalai Tesque, >98.5%), AP−OH (TCI, >98%), TFAP (Aldrich, 99%), and 1-phenyl-2,2,2-trifluoroethanol (TFAP−OH, Aldrich, 98%). The formation of hemiketal and ketal species from TFAP was followed by means of 19F-NMR spectroscopy (JEOL, ECX-400PKS, 400 MHz) in C2D5OD (Merck, 99%, deuteration degree >99%) at 305 K. Chemical shifts of the trifluoromethyl 19F peaks were reported in parts per million (ppm) relative to the hexafluorobenzene 19F peak. Normalized concentrations of hemiketal, ketal, and TFAP (keto form) in C2D5OD were calculated by integration of the area beneath the peaks. The transformation from TFAP into hemiketal and ketal in a mixed solvent of ethanol/CH3CN (19/1) was also observed using a UV−vis spectrophotometer (Shimadzu UV-2550). Changes in UV absorption spectra and a decay signal of absorbance at 256 nm were measured immediately after 20 times dilution of a stock solution of TFAP in CH3CN with ethanol. The starting concentration of TFAP was 0.522 mmol L−1. 2.2. Adsorption in Dark. The amounts of adsorption of substrate on P25 TiO 2 powders were determined by concentration changes (ΔC = C0 − Cs0) in ethanol, where C0 is various initial concentrations (0.4−40 mmol L−1) and Cs0 is the concentration attained in adsorptive equilibrium. To obtain the Cs0 values, 0.10 g of the TiO2 powders were put into 2 mL of the solution of AP in 4 mL amber glasses with screw caps and kept overnight in a shaking bath at 305 K. The suspended solution was centrifuged to remove the TiO2 powders, and then Cs0 in the supernatant solution was determined by gas chromatography/mass spectrometry (GC/MS) as described later. 2.3. Prolonged UV Irradiation Experiment. Irradiation experiments were carried out for a mixture of the substrate (initial concentration range: 1−20 mmol L−1) and TiO2 (0.10 g) in ethanol (30 mL). The solution was placed in a cylindrical glass cell (40 mm × 45 mm i.d.) and sealed with a rubber septum. Pure argon gas (>99.9%) was passed into the solution through the rubber septum for 30 min. The degassed solution was stirred in a water bath for 30 min to attain thermal equilibrium at 305 K in the dark, and then irradiated with UV light from a 300 W xenon arc lamp (ILC Technology, CERMAX LX300) through a dichroic mirror and a cutoff filter (Toshiba UV35). The incident light intensity was measured by the use of a thermopile sensor (Coherent 210). After appropriate irradiation time, a 0.2 mL aliquot of sample solution was withdrawn and centrifuged to remove the TiO2 powders. The concentration of the substrate and product in supernatant was determined by the GC/MS analysis. 2.4. Pre-UV Irradiation Experiment. TiO2 (0.10 g) in ethanol (30 mL) was placed in the same cylindrical glass cell and sealed with the rubber septum. The pure argon gas was passed into the solution through the rubber septum for more than 30 min to purge air gas dissolved in the solution. To the degassed solution was irradiated the UV light (1280 mW cm−2) from the same 300 W xenon arc lamp. After 2 h of irradiation, substrate (300 μmol) was injected into the solution using a

2. EXPERIMENTAL SECTION 2.1. Materials. Polycrystalline TiO2 powder (Degussa P25, specific surface area: 50 m2g−1) was purchased from Japan Aerosil and used as received. A ratio of anatase/rutile in TiO2 was estimated to be ca. 9/1 by powder X-ray diffraction (Rigaku, Miniflex, Cu Kα).22 Special grade ethanol (>99.5%) was purchased from Nacalai Tesque and used without further purification. The following organic reagents were used as 17706



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reacts with ethanol to produce hemiketal and/or ketal species, as illustrated in Scheme 2. The lifetime of TFAP in ethanol is estimated to be a few ten minutes as indicated in the inset in Figure 1. To gain further insight into the transformation of TFAP into hemiketal and ketal forms in ethanol, 19F-NMR signals originated from the trifluoromethyl group were measured in deuterated ethanol (C2D5OD). Figure 2a and b are 19F-NMR

microcylinge through the rubber septum in the dark. After the mixed solution was quickly shaken, a 0.2 mL aliquot was immediately withdrawn, centrifuged, and analyzed by the GC/ MS instrument. The same procedures were carried out at appropriate time intervals after the injection of substrate. 2.5. Quantitative Analysis Using Gas Chromatography/Mass Spectrometry (GC/MS). The amounts of substrate (AP or TFAP) and product (AP−OH or TFAP− OH) in solution upon the adsorption, the prolonged UV irradiation, and the pre-UV irradiation experiments were quantitatively analyzed using the GC/MS instrument (Shimadzu GC17A/QCMS-QP5050A) equipped with an auto injector (Shimadzu AOC-20i/20s) and a capillary column (GL Science, InertCap WAX, 30 m × 0.25 mm i.d., film thickness 0.25 μm). Helium carrier gas was employed with a linear velocity of 33 cm s−1. A 1 μL sample solution was injected with a slit ratio of 100:1 at an injector temperature of 503 K. The column temperature was kept at 343 K for the first 2 min and ramped at a rate of 10 K min−1 to 433 K. The mass spectrometer was set to single ion monitoring mode at the m/z values of molecular ions for electron impact ionization with a source temperature of 503 K. Retention times for AP, AP−OH, TFAP, and TFAP−OH were observed at 10.38, 12.26, 4.63, and 14.04 min, respectively.

3. RESULTS AND DISCUSSION 3.1. Transformation of TFAP into Hemiketal and Ketal Forms in Ethanol. Figure 1 shows UV absorption spectra of

Figure 2. 19F-NMR spectra recorded at (a) 0.16 and (b) 3 h after preparation of TFAP sample in C2D5OD at 305 K. Chemical shifts of the trifluoromethyl 19F peaks were reported in ppm relative to the hexafluorobenzene 19F peak. Peaks at 79.1, 80.2, and 91.6 ppm can be assigned to hemiketal, ketal, and TFAP, respectively.

spectra recorded at 0.16 and 3 h after preparation of the TFAP sample in C2D5OD. Peaks at 79.1, 80.2, and 91.6 ppm can be assigned to hemiketal, ketal, and TFAP (keto form), respectively. From the peak areas, time profiles of the decrease in TFAP and the formation of hemiketal and ketal species can be illustrated in normalized concentration, as shown in Figure 3. While TFAP decreases, hemiketal increases to reach the maximum at ca. 0.5 h and then gradually decreases, and ketal rapidly increases to approach an asymptotic value. Finally, the equilibrium is attained after 8 h where the normalized concentrations of TFAP, hemiketal, and ketal were 0.038, 0.032, and 0.930, respectively. The equilibrium constant K1 ([hemiketal]/[TFAP][C2D5OD]) at the first step is estimated to be about 0.05 L mol−1, whereas K2 ([ketal]/[hemiketal][C2D5OD]) at the second step is ca. 1.7 L mol−1. This means that the rates of forward (k+1 = k1[EtOH]) and backward (k−1) reactions at the first step are almost equal, while the rate of the forward (k+2 = k2[EtOH]) reaction is much larger than that of the backward (k−2) reaction at the second step. Thus, TFAP dissolved in ethanol mostly transforms into the ketal species, and only a few percent of TFAP remains in the original keto form. The hemiketal form is

Figure 1. UV absorption spectra of AP (black) in ethanol and TFAP (red) in the mixed solvent of ethanol/CH3CN (19/1). The orange line indicates the absorption spectrum of TFAP immediately after dilution of the CH3CN stock solution with ethanol. The inset is the decay signal of absorbance of TFAP at 256 nm.

AP in ethanol and TFAP in the mixed solvent of ethanol/ CH3CN (19/1). The absorption spectrum of AP with maximum wavelength at 241 nm (molar extinction coefficient: ε = 11900 L mol−1 cm−1) and 279 nm (ε = 950 L mol−1 cm−1) is consistent with that reported previously,44 and does not change with time. This result indicates that AP is stable in ethanol in the dark. On the other hand, the absorption spectrum of TFAP immediately after dilution of the CH3CN stock solution with ethanol is depicted by the orange line in Figure 1. This spectral shape with maximum wavelength at 256 nm is very similar to that of AP (ca. 15 nm red-shifted). However, this absorption spectrum of TFAP finally changed to the red line with time. This spectral change suggests that TFAP 17707



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g−1, the density of adsorption sites on this powder is estimated to be ca. 10−7 mol m−2 for AP. The value of Kad for AP in ethanol is about 2 orders of magnitude smaller than that in water45 because of sufficient solubility of AP in ethanol. On the other hand, the amount of adsorption of TFAP is too small to estimate in our experiment. This is mainly due to the predominant formation of ketal that must be less adsorptive onto the TiO2 surface as discussed below. The most plausible adsorption model of aromatic ketones should be an interaction between surface Ti sites on TiO2 and the lone pair electrons on the carbonyl oxygen as reported for other carbonyl compounds.46,47 Surface hydroxyl groups on TiO2 strongly interact with ethanol through the hydrogen bonds so that the carbonyl group of AP as well as TFAP cannot interact with the surface hydroxyl groups. Henderson has proposed the major adsorption state of acetone on rutile TiO2 (110) single crystal on the basis of high-resolution electron energy-loss spectroscopy, in which acetone presumably binds at Ti4+ sites via the lone pair on the oxygen atom of acetone.46 Rekoske and Barteau have also reported that adsorption of acetaldehyde on rutile and anatase TiO2 involves a strong interaction between the surface cation (Ti4+) sites and the carbonyl oxygen atom, causing a significant shift in the location of the CO vibrational mode to lower frequencies on FI-IR spectrometry, whereas no interaction of carbonyl oxygen atom with surface hydroxyl groups on TiO2 was observed.47 On the basis of this adsorption model, the ketal species from TFAP cannot interact with the surface Ti4+ sites on TiO2 because of the lack of the carbonyl group. The reasons for the lesser adsorptivity of TFAP onto the TiO2 surface may also be caused by (a) decreasing electron density of the lone pair on the carbonyl oxygen atom of TFAP by the strong electron withdrawing effect of the trifluoromethyl group and (b) increasing hydrophobicity with the fluorine substitution. Reason (a) was supported by our ab initio molecular orbital calculations. The potential energy on the oxygen atom of AP is calculated to be −196 kJ/mol, while that of TFAP is −157 kJ/mol, suggesting that electron density is reduced by the trifluoromethyl group (see Figure S1 in the Supporting Information). For reason (b), the hydrophobicity of AP and TFAP can be evaluated by the octanol−water partition coefficient (P). The values of log P are 1.63 and 2.15 for AP and TFAP, respectively.48 Thus, the value of P for TFAP is about 3.3 times larger than that of P for AP, indicating that the hydrophobicity of TFAP becomes greater than that of AP by the trifluoromethyl substitution. The increased hydrophobicity of TFAP should become a great disadvantage for adsorption onto the hydrophilic TiO2 surface. 3.3. Photocatalytic Hydrogenation during the Prolonged UV Irradiation. We have recently reported the effect of an alcoholic hole scavenger on the TiO2-catalyzed hydrogenation of AP.22 The reactions were tested by using three alcohols of methanol, ethanol, and propan-2-ol as the solvent and scavenger. Among those, the rate of hydrogenation in ethanol was the fastest of all. Therefore, we focused on the reaction in ethanol in this study. We have also reported that the hydrogenation of AP proceeded almost quantitatively in ethanol. The formation of acetaldehyde was simultaneously observed during the irradiation. Acetaldehyde was produced in the oxidation of ethanol by holes generated in the valence band or those trapped at the surface sites on TiO2. The overall reaction is illustrated in Scheme 1. On the other hand, the spectral irradiance curves at three different light intensities are

Figure 3. Time profiles of the decrease in TFAP (red circles: ○) and the formation of hemiketal (blue triangles: △) and ketal (black squares: □) species in C2D5OD at 305 K. The normalized concentrations were calculated by integration of the area beneath the peaks on 19F-NMR spectra recorded at several times after preparation of TFAP sample in C2D5OD.

less stable than the ketal form. However, Guthrie has previously reported that the ketal formation from TFAP in methanol would be extremely slow compared to the hemiketal one.24 Although the discrepancy is still unclear, the 19F peak resolution between ketal and hemiketal in the previous NMR spectrometer in the 1970s could be incomplete, thereby overlapping the two peaks possibly.

Figure 4. Adsorption isotherm of AP on P25 TiO2 at 305 K. The inset plots the reciprocal of the adsorption amount n versus the reciprocal of concentration CAP0.

3.2. Adsorption on TiO2 Surface in Dark. Figure 4 shows the adsorption isotherm of AP in ethanol fitted by the Langmuir equation: n=

nmax K adCAP0 1 + K adCAP0

(1)

where n is the molar amount of adsorption on the TiO2 powder in mol g−1, nmax is the maximum adsorption in mol g−1, Kad is the adsorption constant in L mol−1, and CAP0 is the concentration of AP in equilibrium. The plots of the reciprocal of n versus the reciprocal of CAP0 (the inset in Figure 4) indicate that the adsorption is well analyzed by the Langmuir equation (eq 1). The fitting is well done with nmax = 5.1 ± 0.9 μmol g−1 and Kad = 44 ± 12 L mol−1. The obtained nmax value is roughly consistent with that in aqueous medium (1.25 μmol g−1).45 Since the specific surface area of P25 TiO2 powder is ca. 50 m2 17708



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shown in Figure S2 in the Supporting Information along with the diffuse reflectance spectrum of P25 TiO2 powder in the Supporting Information. The excitation wavelength (>350 nm) was chosen to avoid direct photolysis of AP and TFAP (see Figure 1 and Figure S2, Supporting Information). Time profiles for several initial concentrations (CAP0) of AP under the medium light intensity (790 mW cm−2) are shown in Figure 5. Decays in the AP concentration (Figure 5a) and

−

k K Cs dCs = max irr dt 1 + K irrCs

(2)

listed in Table 1. The maximum rate of reaction kmax increases with increasing light intensity, whereas the apparent adsorption Table 1. Best Fitting Parameters for the L−H Kinetic Expression (2) at Different Light Intensities on the Photocatalytic Hydrogenation of APa light intensity (mW cm−2)

kobs (10−3 mol L−1 h−1)

Kirr (L mol−1)

500 790 1280

1.4 ± 0.1 2.0 ± 0.1 2.7 ± 0.1

630 ± 30 780 ± 90 670 ± 90

a

Equation 2: v0 = kmaxKirrCAP0/(1 + KirrCAP0), where v0 is the initial reaction rate, kmax is the maximum value of reaction rate, Kirr is the adsorption constant under the irradiation, and CAP0 is the initial concentration of acetophenone in equilibrium.

constant Kirr is distributed in the range 630−780 L mol−1 with experimental errors (about 10% more or less). It is noteworthy that these Kirr values are about 1 order of magnitude larger than Kad (44 L mol−1) in the dark. Similar results have been reported for photocatalytic oxidation of benzoic acid derivatives,49 phenols,50−52 and AP;45 there is a discrepancy between Kad obtained from adsorption equilibrium experiments in the dark and Kirr obtained from kinetic experiments. Cunningham and Al-Sayyed have measured a dark adsorption isotherm on TiO2 for three benzoic acid derivatives and first found that the Kad values were significantly smaller than the values of Kirr obtained from the oxidative kinetic data.49 Although the reason for the discrepancy has still been unclear, an UV-induced enhancement of adsorption onto the TiO2 surface could appear, which could resemble the UV-induced superhydrophilicity of TiO2.26,53 Time profiles for TFAP under the medium light intensity (790 mW cm−2) are shown for decays of TFAP in Figure 7a

Figure 5. Time profiles of (a) decay of AP and (b) formation of AP− OH in ethanol at 305 K under the irradiation conditions (>350 nm, 790 mW cm−2).

growths in the AP−OH one (Figure 5b) indicate that the hydrogenation reaction proceeds quantitatively for all samples. The time dependence shows a linear relationship between CAP and irradiation time in the high AP concentration region, suggesting that this reaction proceeds in the zero-order rate dependence at the high AP concentration. However, the slopes of time profiles become smaller with decreasing CAP. This implies that the kinetic behavior varies from zero to first order depending on the concentration of AP. Figure 6 depicts the

Figure 6. Dependencies of initial rate (v0) of hydrogenation at initial concentration (CAP0) of AP under three different light intensities (green circle ○: 500 mW cm−2, blue square □: 790 mW cm−2, red triangle △: 1280 mW cm−2). These data were fitted by eq 2: v0 = kmaxKirrCAP0/(1 + KirrCAP0). The best fitting parameters are listed in Table 1.

Figure 7. Time profiles of (a) decay of TFAP and (b) formation of TFAP−OH in ethanol at 305 K under the irradiation condition (>350 nm, 790 mW cm−2). The decay time profiles were well fitted by the first-order rate form, CT = CT0e−kt, where k = 0.122, 0.118, 0.124, 0.120, and 0.139 h−1 for the five initial concentrations of CT0 = 19.7, 11.1, 6.2, 3.6, and 2.0 mol L−1, respectively.

initial reaction rates v0 (the slopes in Figure 5a) vs CAP0 under three independent light intensities. The P25 TiO2 powders can absorb the incident light in the 350−410 nm region in proportion to the light intensity, as shown in Figure S2 in the Supporting Information. v0 increases to attain the maximum values asymptotically with increasing CAP0, and the maximum value increases with increasing light intensity. These data are well analyzed by the Langmuir−Hinschelwood (L−H) kinetic expression (eq 2). The best fitting parameters are

and for growths in the production of TFAP−OH in Figure 7b. The total concentration of TFAP (CT = CTFAP + Chemi + Cketal) in the solution was measured in the GC/MS analysis, where CTFAP, Chemi, and Cketal are the concentrations of the TFAP (keto form), hemiketal, and ketal forms, respectively. Interestingly, contrary to the case of AP, these time profiles are well fitted by the first-order rate kinetics (CT = CT0e−kt) for all samples. As shown in Figure 8, the initial rates (v0 = kCT0) vs 17709



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Figure 8. Dependencies of initial rate (v0) of hydrogenation at initial concentration (CT0) of TFAP under three different light intensities (green circle ○: 500 mW cm−2, blue square □: 790 mW cm−2, red triangle △: 1280 mW cm−2).

Figure 9. Time evolutions of the amount of AP−OH (○) and TFAP− OH (●) after 300 μmol injection of substrate (AP or TFAP) into the 2 h preirradiated TiO2 suspension at 305 K.

CT0 indicate the linear relationship; i.e., the hydrogenation of TFAP proceeds in the first-order rate law. This is consistent with the rate expression of −dCs/dt = kmaxKirrCs under the condition of KirrCs ≪ 1. Moreover, the rate shows less dependence on the incident light intensity as depicted in Figure 8, which is similar to the photocatalytic behavior under the mass transport-limited condition.54 3.4. Electron Transfer Efficiency on the Preirradiated TiO2 Surface. The electrons accumulated on the P25 TiO2 surface are quite stable in the deaerated ethanol. Figure S3 in the Supporting Information shows that the blue-gray color of the preirradiated TiO2 suspension remained for a few days in the absence of substrates. This extremely long lifetime of the accumulated electrons is attributable to the excellent hole scavenging ability of ethanol on the TiO2 surface, which leads to prevent the recombination with the surface trapped holes. In the presence of substrate, the color of TiO2 rapidly changed after the injection of AP as well as TFAP. The change from blue-gray to white completed within 3 h by the injection of TFAP. Thus, almost all of the accumulated electrons disappeared within 3 h in the reduction of TFAP. On the other hand, in the case of AP, a part of the blue-gray species on TiO2 was remarkably stable even after 50 h, as shown in Figure S3 in the Supporting Information, which may be due to the deep electron trap states (Tidt3+) formed on the P25 TiO2 powder as discussed later. Figure 9 illustrates time evolutions of the amount of AP−OH and TFAP−OH after the injection of AP or TFAP into the sufficient preirradiated TiO2 suspension, respectively. The amount of TFAP−OH production rapidly grows up, and reaches 5.4 μmol within 1 h. On the other hand, the time evolution of AP−OH consists of the fast component within 0.5 h and the slow one after 0.5 h which increases gradually to reach 4.1 μmol. The amount of AP−OH production is finally ca. 25% smaller than that of TFAP−OH production. Thus, in the reduction of AP, the accumulated electrons still remained on the TiO2 surface for a long time after the injection of AP. The amount of remaining Tidt3+ was estimated to be ca. 25% by the difference between the amounts of TFAP−OH (5.4 μmol) and AP−OH (4.1 μmol). This preirradiation experiment clearly demonstrated that the accumulated electrons on TiO2 actually reduce AP as well as TFAP. Interestingly, all of the accumulated electrons can reduce TFAP, whereas only ca. 75% of those can take part in the reduction of AP. The residual 25% electrons could not react

with AP and remained at the deep trap sites. It has been revealed experimentally by Ohtani et al. that electronic energy of the shallow and deep Ti states is located just below the CB edge of TiO2 in the ranges of 0−0.35 V for anatase and 0−0.25 V for rutile.43 The distribution of the surface Ti3+ states within 0−0.2 V is predominant so that electrons accumulated in these shallow trap states participate in the reduction of AP. However, the residual electrons accumulated at the deeper Ti sites (more than 0.2 V below the CB edge) remained on the TiO2 surface. Recently, Deskins et al. have calculated relative energies of the Ti3+ states formed in the (110)-terminated rutile TiO2 surface by means of the density functional theory (DFT + U method).55,56 They have modeled the formation of Ti3+ at various Ti sites, for example, the five coordinate Ti55 and O vacancies.56 The calculation for the five coordinate Ti in the presence of surface hydroxyls indicated that the deep Ti3+ sites occur in the second Ti layer from the surface or under the five coodinated Ti rows.55 Electrons trapped at these Ti sites can remain on the TiO2 surface. 3.5. Reaction Models on the TiO2 Surface. On the basis of the observations about the ketal formation in ethanol, the adsorptive property, and the electron transfer efficiency of AP and TFAP upon the TiO2 surface, schematic models on the earlier stages of the photocatalytic hydrogenation for AP and TFAP can be illustrated in Figures 10 and 11, respectively. The common behavior of the photogenerated electrons and holes in the photocatalytic hydrogenation of AP and TFAP can be depicted as shown in the steps below. Step 0. Electron−hole pair generation in TiO2: TiO2 + hν → TiO2

(e− + h+)

Step 1. Trapping CB electrons at the deep defect Ti site: Tidt 4 + + e− → Tidt 3 +

Step 2. Trapping VB holes at terminal Ti−OH sites: Ti−OH + h+ → Ti − OHs•+

(surface trapped hole)

Step 3. Reduction of adsorbed substrate with CB electrons at reduction sites: e− + Sad → Sad•− 17710



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OHs•+ generated on the TiO2 surface can be about nanosecond or subnanosecond time scale in ethanol (ca. 1 ns) because of the fast reaction of Ti−OHs•+ with ethanol adsorbed on the TiO2 surface (step 5).59 On the other hand, the interband electron−hole recombination (e− + h+ → hv or heat) may be neglected, because the free hole h+ may be rapidly trapped to the surface terminal Ti−OH sites in step 2. The rate-determining step for AP can be ascribed to the electron transfer process (steps 3 and 4) on the UV irradiated TiO2 surface (Figure 10). This reaction mechanism is supported by the fact that the reaction rates mostly depend on the Ered values in the photocatalytic hydrogenation of aromatic ketones22 and the observation that the maximum rate of reaction, kmax, almost linearly increases with increasing light intensity; i.e., the hydrogenation of AP proceeds in a lightlimited controlled manner. On the other hand, a hydrogen transfer mechanism,61 involving proton reduction with e− to generate hydrogen atoms on TiO2 which could be responsible for the hydrogenation of aromatic ketones, should be ruled out because hydrogen evolution accompanied with this reaction was hardly detected in the actual photocatalytic hydrogenation of AP.62 The difference in the electron transfer efficiency between AP (Figure 10) and TFAP (Figure 11) on the TiO2 surface can be evaluated by the preirradiation experiment. In this condition, a sufficient concentration of substrate (ca. 10 mmol L−1) on the TiO2 surface can be attained immediately after the injection of both AP and TFAP. Even for TFAP giving the ketal form, TFAP survives as the keto form for several ten minutes, as shown in Figures 1 and 3. Therefore, electron transfer efficiency from CB and the trap sites on TiO2 to the substrate can be properly evaluated. The results in Figure 9 indicate that the reduction rate for AP was slower than that for TFAP on the TiO2 surface and ca. 25% electrons trapped at the deep states did not participate in the photocatalytic reduction of AP. The model for AP is depicted in Figure 10. On the contrary, for TFAP, the electron transfer rate is faster and all of the CB and the trapped electrons transfer into TFAP efficiently (Figure 11). This result is consistent with the order of the Ered values of AP (−2.13 V) and TFAP (−1.59 V) vs SCE in CH3CN.22,23 However, under the prolonged irradiation condition, the hydrogenation rate for TFAP (Figure 7) was slower than that for AP (Figure 5), which was contrary to the expectation from the Ered values. This is attributed to the very low concentration of the TFAP keto form (a few percent, as indicated in Figure 3)

Figure 10. Schematic illustration on the earlier stage of the photocatalytic hydrogenation of AP.

Step 4. Reduction of adsorbed substrate with electrons trapped at the Tidt3+ sites: Tidt 3 + + Sad → Sad•−

Step 5. Oxidation of adsorbed ethanol by the trapped hole (Ti− OHs•+) at oxidation sites: Ti − OHs•+ + EtOHad → Ti − OH + EtOHad•+

Step 6. Recombination of e− with Ti−OHs•+: e− + Ti − OHs•+ → Ti − OH

Step 7. Recombination of Tidt3+ with Ti−OHs•+: Tidt 3 + + Ti − OHs•+ → Tidt 4 + + Ti − OH

Furube and co-workers have investigated the charge carrier dynamics under weak excitation conditions for nanocrystalline TiO2 samples in femtosecond to microsecond time scales.57−59 The results should be compatible with our actual photocatalytic reactions under usual UV irradiation conditions.60 Furube et al. observed the e− and h+ pair generation within 100 fs (step 0) and the electron migration between CB and shallow trap sites in equilibrium. These electrons finally relaxed to deep trap sites with a 500 ps time constant (step 1); meanwhile, h+ was rapidly trapped to the surface terminal Ti−OH sites within 100 fs to afford Ti−OHs•+ (step 2).57,58 Moreover, the lifetime of Ti−

Figure 11. Schematic illustration on the earlier stage of the photocatalytic hydrogenation of TFAP. 17711



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ACKNOWLEDGMENTS The authors are grateful to Ms. Emi Cho and Ms. Yuna Kamoi for their helpful experimental assistance. This work was supported by Grant-in-Aid for Scientific Research (No. 21590052, No. 22590026, and No. 24590067) from the Japan Society for the Promotion of Science.

on the TiO2 surface caused by the predominant ketal formation in ethanol. Therefore, in the photocatalytic hydrogenation of TFAP, the transformation of ketal to hemiketal (k−2) can be the rate-determining step, as illustrated in Figure 11. This is the reason why the reaction of TFAP was less dependent on the incident light intensity. Thus, the discrepancy between the preirradiation and the prolonged irradiation results for TFAP can be well explained.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 - Potential energy maps of AP and TFAP calculated using the ab initio MO method (HF/6-311G*). Figure S2 Diffuse reflectance spectrum of the P25 TiO2 powder and spectral irradiance curves for Xe arc lamp through a cutoff filter at different light intensities. Figure S3 - Color changes in the absence of substrates and after injection of AP or TFAP into the suspensions of P25 TiO2 powder dispersed in deaerated ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS Adsorptive and kinetic behaviors on the reductive photocatalysis of acetophenone (AP) and its fluorinated derivative, 2,2,2-trifluoroacetophenone (TFAP), have been examined upon the UV irradiated TiO2 (P25) surface in deaerated ethanol. The reaction models proposed in the schematic illustrations of Figures 10 and 11 well explain the photocatalytic hydrogenation of AP and TFAP on TiO2, respectively. In summary, our findings are described as below: (1) The photocatalytic hydrogenation of AP and TFAP to the corresponding secondary alcohols occurs quantitatively regardless of the initial concentration of the substrates. (2) The kinetic data of AP are well analyzed by the Langmuir− Hinschelwood expression, whereas those of TFAP follow the first-order rate law. (3) The formation of ketal and hemiketal from TFAP in ethanol greatly affects the kinetics on the photocatalysis. (4) The apparent adsorption constant of AP under the irradiation is 1 order of magnitude larger than the adsorption constant in the dark. (5) The accumulated CB electrons or those trapped at surface defect sites on TiO2 actually reduce AP as well as TFAP. All of the accumulated electrons can reduce TFAP, whereas only ca. 75% of those can take part in the reduction of AP. The residual 25% electrons cannot react with AP and remain at the deep trap sites. To our knowledge, such observations may be for the first time, and will provide good insight into the information about surface defect states on TiO2. Therefore, particular attention should be directed toward the interactions between surface trapped electrons and other aromatic ketones or other compounds adsorbed on TiO2 in the future direction.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-78-304-3158. Fax: +81-78-304-2858. E-mail: [email protected] (S.K.); [email protected] (H.M.). Present Address §

Office for Development of Young Researchers, Research Planning and Promotion Division, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan.

Notes

The authors declare no competing financial interest. 17712



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Adsorptive and Kinetic Properties on Photocatalytic Hydrogenation of

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