Promoting Effect of SiOx Monolayer Coverage of TiO2 on the

Promoting Effect of SiOx Monolayer Coverage of TiO2 on the Photoinduced .... photocatalyst: the role of MgO in photoinduced charge carrier separation...
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Langmuir 1998, 14, 2936-2939

Promoting Effect of SiOx Monolayer Coverage of TiO2 on the Photoinduced Oxidation of Cationic Surfactants Hiroaki Tada,*,† Yasuyuki Kubo,‡ Manabu Akazawa,‡ and Seishiro Ito‡ Environmental Science Research Institute, and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, 577-8502, Japan Received September 8, 1997. In Final Form: March 9, 1998 The effect of SiOx monolayer coverage on the rate of the TiO2 photoinduced oxidation of cetylpyridinium bromide (CPB) in aqueous solutions has been studied. The rate of the reaction increased ca. 3.4-fold with the surface modification in the 5-6 pH range. The promoting effect was emphasized as the initial concentration of CPB decreased. Adsorption experiments showed that the adsorption strength for CPB (β/M-1) increases from 3.7 × 103 (TiO2) to 2.25 × 104 (SiOx/TiO2), while the saturated amount of adsorption (Γs/mol m-2) is not so varied (Γs ∼ 3 × 10-6). A drastic change in the point of zero charge from 7.5 (TiO2) to 3.2 (SiOx/TiO2), which is near to the value of SiO2, was observed in titration experiments. A modified Langmuir-Hinshelwood model, where the rate of the surface reaction is assumed to be proportional to the coverage of CPB at the photostationary state that is determined by the balance between the rate of adsorption and the rate of decomposition, was proposed for dilute reaction systems. As the result of the kinetic analyses, the enhancement of the reaction with the SiOx monolayer coverage of TiO2 could be mainly attributed to the increase in the electrostatic attraction of cetylpyridinium ion (increase in β).

Introduction Increasing water contamination by organic and inorganic pollutants is now a serious problem, which is changing from a local one to a global one. A great deal of attention has been focused in recent years on the development of semiconductor photocatalysts because of their possible application to water decontamination.1,2 At present, TiO2 is regarded as the material most suitable for that purpose, owing to its powerful oxidation strength, high photostability in water, and nontoxicity. The essential subject to be resolved is the improvement in the efficiency of the photocatalytic oxidation.3 This achievement will guarantee the substitution of the technique for the usual UV-light/H2O2 system as a safer method, since the region where the photocatalytic oxidation takes place is restricted to the surface (or the vicinity of the surface).2 In principle, the photocatalytic oxidation of organics is induced by holes (h+) photogenerated in the valence band (VB) of TiO2 with the band gap excitation. It has been established in many reaction systems that heterogeneous photocatalytic oxidation obeys the Langmuir-Hinshelwood (LH) mechanism, which means that the degradation proceeds via quasi-bimolecular reaction between organic compounds adsorbed on the surface and charge carriers (or surface-trapped charge carriers) on the surface. Then three strategies can be envisaged in order to increase the reaction efficiency. First is to increase the rate of transport of charge carriers to the surface at the photostationary state. This is chiefly associated with bulk properties of TiO2 such as high crystallinity.4 Second is to increase the coverage of organic substrates at the photostationary state, which results from augmentation of the number of * To whom correspondence should be addressed: Telephone: +81-6-721-2332. Fax: +81-6-721-3384. E-mail: [email protected]. kindai.ac.jp. † Environmental Science Research Institute. ‡ Department of Applied Chemistry. (1) Photocatalytic Purification and Treatment of Water and Air; Ollis, F. D., Al-Ekabi, H., Eds.; Elsevier Science: Amsterdam, 1993. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Chem. Eng. News 1996, 29. (4) Tada, H.; Tanaka, M. Langmuir 1997, 13, 360.

adsorption sites and/or their adsorption strength. Third, the high specific surface area of TiO2, contributing to both the factors, must lead to a significant increase in the rate of the reaction.5 Particularly in dilute reaction systems, the second and third factors become of more importance. It has recently been reported that SiOx monolayers are formed on the surface of TiO2 by a method consisting of chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and subsequent irradiation (λ > 300 nm) in the air.6,7 The SiOx monolayer coverage permits the interfacial charge transfer between photoexcited TiO2 and adsorbates through the tunneling effect. Langmuir’s “principles of independent surface action”8,9 make us expect modification of the adsorption properties of TiO2 by the monolayer coverage, with maintenance of its strong oxidizing ability. This is the first report on the effect of the SiOx monolayer coverage on the TiO2 photoinduced oxidation of organics in aqueous media. A remarkable increase in the reaction rate with the surface treatment was discussed on the basis of a modified LH mechanism presented for dilute reaction systems. Experimental Section 1,3,5,7-Tetramethylcyclotetrasiloxane (200 mL) (TMCTS, >98%, Shin-Etsu Chem.) was allowed to react with 1 g of TiO2 particles (A-100, Ishihara Sangyo Co., anatase, average particle diameter ) 0.15 µm, BET surface area ) 8.1 m2 g-1) placed in a vacuum chamber under 0.6 ( 0.1 Torr at 80 °C for 0.5 h. Then the temperature was raised to 100 °C, evacuating for an additional 0.5 h to remove the physisorbed TMCTS. After the particles obtained (TMCTS/TiO2) had been floated on the surface of 100 mL of aerated H2O, they were irradiated under magnetic stirring with a 400-W high-pressure mercury arc (H-400P, Toshiba) for 1 h. The light intensity integrated between 320 and 400 nm impinging on the sample corresponds to an energy flux of 6.0 mW cm-2 (I). The particles were recovered with centrifugation and dried in a vacuum desiccator at room temperature (SiOx/ TiO2, BET surface area ) 6.8 m2 g-1). (5) Dagan, G.; Tomkiewics, M. J. Phys. Chem. 1993, 97, 12651. (6) Tada, H. Langmuir 1995, 11, 3281. (7) Tada, H. Langmuir 1996, 12, 966. (8) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (9) Langmuir, I. J. Am. Chem. Soc. 1929, 6, 451.

S0743-7463(97)01015-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998

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Rates of TiO2 (or SiOx/TiO2) photoinduced decomposition of cetylpyridinium bromide (CPB, Tokyo Kasei) were determined directly with CPB solutions (ca. 2.2 × 10-4 M) at 21 ( 1 °C. A slurry of 50 mL of distilled H2O (air saturated) and 0.05 g of catalyst was prepared in a photochemical reaction vessel. Steadystate irradiations were carried out using the same UV-light source as in the preparation of SiOx/TiO2 (I ) 0.6 mW cm-2) for the suspensions, which had been stirred for 1 h in the dark to attain the adsorption equilibrium. Ten-milliliter aliquots were periodically removed and centrifuged. The concentration of CPB was determined from the absorbance of the peak maximum at 258 nm (max ) 3.84 × 103 M-1 cm-1) on a Hitachi U-400 spectrophotometer. Adsorption isotherms were obtained in a similar way by exposing the catalysts to solutions with different concentrations of CPB in the absence of irradiation followed by centrifugation and spectrophotometric analysis of the CPB remaining in the solutions. Diffuse reflectance Fourier transform infrared (DRIFT) spectra of the samples were obtained with a Perkin Elmer 1760-X FT-IR spectrometer equipped with a diffuse reflectance attachment (Spectra Tech, Inc.). In order to check the photostability of the interfacial Ti-O-Si bonds in H2O, the SiOx/TiO2 particles, which were recovered after three cycles of centrifugation and washing with 150 mL of distilled H2O, were subjected to the measurements. TiO2 particles with a greater surface area (P-25, Degussa, BET surface area ) 41 m2 g-1) were used only in preparation of the samples for the DRIFT analyses to enhance the signal intensities. Prior to the measurements, the samples were dried in a vacuum desiccator at 150 °C. The spectra were recorded in the range 4000-400 cm-1 at a resolution of 4 cm-1 with 200 coadded scans using KBr (optical grade, >99.9%, Nacalai Tesque, Inc.) as a reference.

Results and Discussion In our previous paper, it was demonstrated that TMCTS molecules chemisorb on the surface of TiO2 particles via Ti-O-Si bonds.7 Figure 1a shows the difference DRIFT spectrum of TiO2 particles (4000-2000 cm-1) before and after adsorption of a TMCTS monolayer from a gas phase. A negative peak due to the stretching vibration of the surface-isolated TiO-H groups (ν(TisO-H)) is observed centered at 3670 cm-1. Several positive peaks appear at 2972, 2860, and 2171 cm-1, which can be assigned to the antisymmetric (νas(CH3)) and symmetric (νs(CH3)) stretching vibrations of CH3 groups and the stretching vibration of Si-H groups (ν(Si-H)), respectively.10 TiO2 particles treated with TMCTS (TMCTS/TiO2) sank and dispersed into H2O after irradiation (λ > 300 nm), while they thoroughly floated on the surface of H2O before it. A similar photosinking phenomenon was observed for the TiO2 particles treated with a silane coupling agent by the other authors.11 Figure 1b shows the DRIFT spectrum of TiO2 subtracted from that of TMCTS/TiO2 irradiated in aerated H2O for 1 h. The three absorption peaks of νas(CH3), νs(CH3), and ν(Si-H) completely disappear. On the other hand, a new broad peak assignable to the stretching vibration of the hydrogen-bonded Si-OH groups (ν(SihyO-H)) taking part in hydrogen bonds between adjacent SiO-H groups and/or interparticles appears at 3580 cm-1, while the negative ν(TisO-H) peak remains after irradiation in H2O. These data indicate that both the Si-CH3 and Si-H groups are transformed into Sis-OH groups by the TiO2 photoinduced oxidation in the aqueous solution just as in the gas phase, although Fujishima et al. assumed that all the Ti-O-Si bonds cleave in the photosinking process.11 The resistance to photocleavage of the Si-O-Ti bonds encourages the (10) Smith, A. L. Analysis of Silicones; Wiley-Interscience: New York, 1974. (11) Fujishima, A.; Kato, T.; Maekawa, E.; Honda, K. Denki Kagaku 1986, 54, 153.

Figure 1. Difference DRIFT spectra; a, TMCTS/TiO2 - TiO2; (b) SiOx/TiO2 - TiO2 (irradiation time ) 1 h).

application of the TiO2 particles covered with a SiOx monolayer (SiOx/TiO2) as a photocatalyst for the oxidation of organics in aqueous solutions. Figure 2 shows adsorption isotherms of CPB from aqueous solutions on TiO2 (a) and SiOx/TiO2 (b) particles below the critical micelle concentration (cmc ) 9 × 10-4 M).12 The SiOx/TiO2 system (b) demonstrates Langmuirtype behavior, while the TiO2 system (a) follows Henrytype behavior below the equilibrium concentration (Ceq ) 3 × 10-4 M). A significant increase in the amount of CPB adsorbed (Γ/mol m-2) with the SiOx monolayer coverage is observed in the range Ceq < 1 × 10-3 M. According to the Langmuir model, Γ can be expressed by the equation Γ ) ΓsβCeq/(1 + βCeq), where Γs is the saturation amount of adsorption and β is K1/a1 (K1 ) the equilibrium constant for adsorption process; a1 ) the activity of H2O in the solution). In the SiOx/TiO2 system (b), the plot of Ceq/Γ vs Ceq is linear, as shown in the inset (R ) 0.991). From the slope and the intercept at Ceq ) 0, the adsorption parameters Γs and β (M-1) for SiOx/TiO2 (Γs′, β′) were determined to be 2.96 × 10-6 and 2.25 × 104. On the other hand, in the TiO2 system (a), the constancy of Ceq/Γ (Ceq < 3 × 10-4 M) exhibits Henry-type adsorption behavior where Γ can be written as ΓsβCeq. Since the value of Γ for TiO2 (Γ) is equal to that for SiOx/TiO2 (Γ′) at Ceq ) 2.25 × 10-4 M, the equation Γs′β′Ceq/(1 + β′Ceq) ) ΓsβCeq is (12) Handbook of Surfactants; Takahashi, K., Namba, Y., Koike, K., Kobayashi, M., Eds.; Kogaku Toshyo Co.: Tokyo, 1968.

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Figure 2. Adsorption isotherms of CPB from aqueous solutions on TiO2 (a) and SiOx/TiO2 particles (b) at 25 °C; plots of CeqΓ-1 vs Ceq for TiO2 (a) and SiOx/TiO2 particles (b). 0.5 g of particles was dispersed into the CPB solutions (50 mL) with different concentrations.

Figure 3. Titration curves for an acid solution consisting of 100 mL of 0.1 M aqueous NaCl and 5 mL of 0.01 M aqueous HCl with 0.01 M aqueous NaOH in the absence (r) and presence (a, 1 g of TiO2; b, 1 g of SiOx/TiO2) of particles.

valid for this concentration. Taking into account Γs ∼ Γs′, β was estimated to be 3.7 × 103 M-1 from the equation β ) β′/(1 + β′Ceq) by substituting Ceq ) 2.25 × 10-4 M and β′ ) 2.25 × 104 M-1. These results indicate that the SiOx monolayer coverage of TiO2 increases the adsorption strength for CPB by a factor of as much as 6, while Γs is not so varied (∼3 × 10-6 mol m-2). The number of CPB molecules adsorbed per unit surface area (σ0-1/molecules nm-2) was calculated from Γs to be 1.6 for SiOx/TiO2. This value is in good agreement with that for vertical orientation (σ0 ) 0.45 nm2) reported for CPB adsorption on glassy carbon.13 Figure 3 shows titration curves for an acid solution consisting of 100 mL of 0.1 M aqueous NaCl and 5 mL of a 0.01 M aqueous HCl with 0.01 M aqueous NaOH. In the absence of particles (r), a typical acid-base titration curve is obtained, where the pH value sharply changes from 4 to 10 between 4 and 6 mL of the aqueous NaOH (13) Dong, S.; Zhu, Y.; Cheng, G. Langmuir 1991, 7, 389.

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Figure 4. Time profiles of CPB decomposition upon illumination (λ > 300 nm) at 22 ( 1 °C; without catalysts (r1) or illumination (r2); in the presence of TiO2 (a, pH ) 5.8 ( 0.2) and SiOx/TiO2 particles (b, pH ) 5.20 ( 0.06 ).

added, reaching approximately 7 upon addition of 5 mL, as expected. In the presence of 1 g of the TiO2 particles (a), the variation of pH becomes gentle over the whole range of the volume of aqueous NaOH. In the presence of 1 g of the SiOx/TiO2 particles (b), the variation range of pH further decreases and the solution exhibited acidity even after 10 mL of the aqueous NaOH was added. From the pH values at the intersections of curves r, a and curves r, b, the points of zero charge (pzc) for TiO2 and SiOx/TiO2 were determined to be 7.5 and 3.2, respectively. The pzc of the TiO2 particles is to some extent larger than the reported values (5-7);14-16 however, it is known to be affected by the preparation method and the thermal history. The fact that the pzc of the SiOx/TiO2 particles is near to that of SiO2 (ca. 1.9)17 is consistent with the conclusion that a SiOx monolayer is formed by liquidphase oxidation of the TMCTS monolayer.18 Noticeably, the surface Bro¨nsted acidity is drastically increased with coverage of the SiOx monolayer, which also means that the surface has a greater negative charge density as compared to bare TiO2 in aqueous solutions. The increase in the adsorption strength for CPB can be explained in terms of an increase in the electrostatic interaction of the surface and dissociated CPB molecules (CP+). TiO2 and SiOx/TiO2 particles have an intense absorption band below 385 nm due to the band gap transition. On the other hand, CPB has an absorption band (λmax ) 258 nm) above 230 nm. Since the light (λ < 300 nm) was cut off by the Pyrex glass of the reaction vessel, TiO2 and SiOx/TiO2 were selectively excited by irradiation in the reaction system. Figure 4 shows time profiles of CPB decomposition upon illumination. Without either TiO2 (or SiOx/TiO2) particles (r1) or illumination (r2), CPB hardly (14) Parks, C. A. Chem. Rev. 1965, 15, 177. (15) Yoon, R. H.; Salman, T.; Donnay, G. J. Colloid Interface Sci. 1979, 70, 483. (16) Mpandou, A.; Siffert, B. J. Colloid Interface Sci. 1984, 102, 138. (17) Odenbrand, C. V. I.; Bransin, J. G. M.; Busca, G. J. Catal. 1992, 135, 505. (18) For plane TiO2 samples, X-ray photoelectron spectroscopic measurements confirmed the formation of the uniform SiOx monolayer (see ref 6). Although the coverage of the SiOx monolayer on TiO2 particles could not be determined, the drastic shift of the pzc with the surface treatment strongly suggests that the particles are uniformly covered with the film.

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Figure 5. Initial rates of CPB removal from the solution (v0) as a function of initial concentration (C0) for TiO2 (a) and SiOx/ TiO2 (b) systems. In these experiments, irradiation was started just after dispersing the photocatalysts into reaction solutions at 12 ( 1 °C. The value of v0 was determined from the decrease in the concentration of CPB after 10 min of irradiation.

decomposes. In the presence of TiO2 (a), the concentration of CPB slowly decreases with increasing irradiation time (t). In the presence of SiOx/TiO2 particles (b), the rate remarkably increases. These facts indicate that the decomposition of CPB is induced by photoexcitation of TiO2 (or SiOx/TiO2). It is conceivable that coverage of TiO2 with insulating monolayers decreases the rate of the interfacial charge transfer between the surface and the adsorbates. The rate of the TiO2 photoinduced oxidation of TMCTS in the gas phase was previously confirmed to be decreased by the SiOx monolayer coating.6 Why does the rate of the CPB decomposition in the aqueous solutions significantly increase with the SiOx monolayer coverage? The LH kinetic treatments for gas-solid and liquid-solid heterogeneous reactions are commonly used to rationalize the mechanisms of the photocatalytic reactions. In these cases, the rate of reaction (v) will obey the form v ) kθ, where k is the apparent rate constant and θ is the coverage of the reactant. Although the equilibrium value of θ in the dark (θd) is approximately θ,19-21 it is more strict that the value at the photostationary state (θp) substitutes for θd. This would be particularly true for the cases where the rate of adsorption is smaller than that of the surface reaction in a low concentration range of substrates. Under these circumstances, the equilibrium population of the adsorbed species cannot be maintained because of their depletion through photolysis. In the present analysis, we assumed that CP+ adsorbing at the rate (va) ka(1 - θp)C is oxidized on the surface at the rate (v) kθp before desorbing at the photostationary state (ka is the rate constant of adsorption). Since θp is determined by the balance of va and v (θp ) kaC/(k + kaC)), the rate equation v ) -dC/dt ) kkaC/(k + kaC) is obtained. For a dilute reaction system (k . kaC), ln C0/C ) k′t (k′ ) kak) holds. As shown in the inset, plots of ln (C0/C) vs t provide straight lines (a, R ) 0.926; b, R ) 0.998), whose slopes yield the (19) Ohtani, B.; Okugawa, Y.; Nishimoto, S.-i.; Kagiya, T.J. Phys. Chem. 1987, 91, 3550. (20) Matthews, R. W. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1291. (21) Cunningham, J.; Al-Sayyed, G. J. Chem. Soc., Faraday Trans. 1990, 86, 3935.

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apparent rate constants (k′) 7.3 × 10-4 min-1 for the TiO2 system and 2.5 × 10-3 min-1 for the SiOx/TiO2 system. The 3.4-fold increase in k′ with the SiOx monolayer coating is mainly attributable to the increase in ka, resulting from the augment in β due to the electrostatic interaction between CP+ and the surface (Sis-O-). The modified LH kinetic analysis reasonably explains the fact that the rate of the photodecomposition of CPB increases irrespective of invariance in Γs. Figure 5 shows the initial rate of CPB removal from the solution (v0) as a function of the initial concentration (C0). In both cases (a, TiO2; b, SiOx/TiO2), v0 increases monotonically with increasing C0. In the range C0 < 3 × 10-4 M, the v0 value for SiOx/TiO2 is greater than that for bare TiO2, while they are comparable in the higher concentration range. This finding strongly supports the conclusion that the increase in ka is the main factor for the promotion effect in the low concentration range, where a significant depletion of CP+ adsorbed occurs during irradiation (the diffusion-controlled region). For electrostatic reasons, h+ will react preferably with negatively charge entities present on the surface.22 Then, the higher negative surface charge density of the treated surface may facilitate the actual photooxidation of CP+ adsorbed. However, this effect seems to be less likely than a change in the binding rate, because the v0 value of SiOx/TiO2 is almost equal to that of TiO2 in the high concentration range (the surface reaction-controlled region). The significant increase in the rate of decomposition with the SiOx monolayer coating deserves noting, because cationic surfactants are generally difficult to decompose photocatalytically with TiO2.23 The photostability of the Ti-O-Si bonds in H2O also seems to be responsible for the fact that the decomposition rate is kept constant during irradiation in the SiOx/TiO2 system. Mixed TiO2/SiO2 materials were reported by Anderson et al. to improve the photocatalytic activity in the oxidation of rhodamin-6G.24,25 From comparison of the mixed TiO2/SiO2 photocatalyst with the SiOx/TiO2 one, it can be claimed that the latter has a feature that the effective reaction area per unit weight is maintained at the same level of the original TiO2 particles. In conclusion, a SiOx monolayer, which is stable against UV irradiation, was successfully formed on the surface of TiO2 by the photoinduced oxidation of chemisorbed TMCTS in H2O. The rate of the TiO2 photoinduced oxidation of CPB was remarkably increased with the SiOx monolayer coating. This finding was rationalized chiefly in terms of the increase in the electrostatic attraction of CP+ to the surface on the basis of a modified LH kinetic model. Although the detailed reaction mechanism deserves scrutiny, the present results strongly indicate that the oxide monolayer coating of TiO2 is effective in increasing the efficiency of the photocatalytic liquid-phase oxidation of some organic pollutants in the pH region of natural water. Acknowledgment. The authors express sincere gratitude to Ishihara Techno Co. for the gift of the TiO2 particles (A-100) and Dr. M. Iwasaki of Kinki University for valuable comments. LA971015M (22) Herrmann, J.-M.; Pichat, P. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1138. (23) Hidaka, H.; Yamada, S.; Suenaga, S.; Zhao, J.; Serpone, N.; Pelizzetti, E. J. Mol. Catal. 1990, 59, 279. (24) Anderson, C.; Bard, A. J. J. Phys. Chem. 1995, 99, 9882. (25) Anderson, C.; Bard, A. J. J. Phys. Chem. B 1997, 101, 2611.