Langmuir 1999, 15, 3699-3702
3699
Promoting Effect of MgOx Submonolayer Coverage of TiO2 on the Photoinduced Oxidation of Anionic Surfactants Hiroaki Tada,*,† Miwako Yamamoto,‡ and Seishiro Ito†,‡ Environmental Science Research Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Received December 2, 1998. In Final Form: April 2, 1999 It has been revealed that magnesium acetylacetonate ([MgII(acac)2]) irreversibly adsorbs on TiO2 from an EtOH and n-hexane mixed solution via a one-to-one stoichiometric ligand exchange of acac- for Tis-O-, whereas it hardly adsorbs on MgO. The successive oxidation of the resultant surface complex (MgII(acac)(OTis)) formed a MgOx submonolayer on the surface of TiO2. Six-time repetition of the adsorption-oxidation process achieved an approximate MgOx monolayer owing to the selective adsorption of [MgII(acac)2] on the portion of naked TiO2. The rate of the TiO2 photoinduced oxidation of sodium dodecylbenzenesulfonate increased with the MgOx submonolayer coverage (θ = 0.5) by a factor of 1.3.
Introduction
Experimental Section
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 To achieve this in dilute reaction systems, it is of great importance to increase the rate of adsorption. We have recently reported that surface modification of TiO2 with a SiOx monolayer remarkably enhances the photoinduced oxidation of cationic substrates including cetylpyridinium bromide4 and Rhodamine 6G5 in neutral aqueous media. On the basis of the adsorption and kinetic data analyses, accelerated adsorption followed by an efficient surface oxidation was revealed to be responsible for the findings. Both the actions could be attributed respectively to the electrostatic attraction and repulsion of the surface (Sis-O-) for the cationic substrates and the counteranions, because the SiOx monolayer coating shifts the point of zero charge from 7.5 to 3.2.4 However, the opposite effect is predicted for anionic substrates. This is the first report on the effect of the MgOx monolayer coverage of TiO2 on the photoinduced oxidation of anionic substrates in neutral aqueous media. It has been indicated that the reaction is enhanced in a submonolayer region.
Magnesium acetylacetonate ([MgII(acac)2], >98%, Tokyo Kasei) was adsorbed on 1 g of anatase TiO2 particles (A-100, Ishihara Sangyo Co., BET surface area S ) 8.1 m2 g-1) or 1 g of crystalline MgO particles (>99.0%, Kanto Chemicals, S ) 4.0 m2 g-1) from 50 mL of a 1.60 × 10-3 M solution in EtOH-n-hexane (3:17 v/v) by stirring the suspensions at 25 ( 1 °C for 24 h. After the particles recovered by centrifugation had been dried in vacuo at room temperature overnight, they were calcined at 500 °C for 1 h in air in an electric oven. The amount of Mg loaded was controlled by repeating the adsorption-oxidation process; the sample obtained after n-time cycles of the adsorption-oxidation process was denoted as MgOx(n)/TiO2. The MgOx was dissolved by treating the particles with HNO3, and the aqueous Mg2+ solution was subjected to induced coupled plasma spectroscopy (ICP-1000, Shimadzu) for its quantification. Also, a SiOx monolayer was formed on TiO2 in the same way as previously reported;6 the sample was denoted as SiOx/TiO2. The adsorption process of [MgII(acac)2] on TiO2 particles was examined by following the change of the electronic absorption spectrum with adsorption time. The absorption spectra were recorded on a Hitachi U-400 spectrophotometer in the 200-800 nm range. After the exposure of TiO2 or MgO particles to solutions of [MgII(acac)2], the spectrophotometric analyses of the supernatants were carried out. The concentrations of [MgII(acac)2] and acacH liberated during the adsorption process were determined from the absorbances at 283.5 nm (max ) 1.29 × 104 M-1 cm-1) and 270 nm (max ) 1.38 × 104 M-1 cm-1), respectively. Diffuse reflectance Fourier transform infrared (DRIFT) spectra of the particles diluted 2.0 wt % with KBr (spectroscopic grade, >99.9%, Nacalai Tesque) were obtained with a Perkin-Elmer 1760-X FT-IR spectrometer equipped with a diffuse reflectance attachment (Spectra Tech, Inc.). TiO2 particles with a high surface area (ST-01, Ishihara Sangyo Co., S ) 392 m2 g-1) were used only in preparation of the samples for the DRIFT analyses to enhance the signals. The measurements were performed in the range of 4000-400 cm-1 at a resolution of 4 cm-1 with 200 coadded scans (reference ) KBr). The spectra were recorded after the Kubelka-Munk transformation by an interfaced computer. Rates of MgOx(n)/TiO2 (0e n e 6) and SiOx/TiO2 photoinduced oxidation of sodium dodecylbenzenesulfonate (DBS, >98%, Tokyo Kasei) were determined directly with DBS solutions (1.0 × 10-4 M) at 30 ( 1 °C and pH = 7. A slurry of 50 mL of distilled H2O (air saturated) and 0.05 g of catalyst was prepared in a photochemical reaction vessel. Irradiation of the suspension (λ
* To whom correspondence should be addressed. Telephone: +81-6-6721-2332, Fax: +81-6-6721-3384. E-mail: h-tada@ apsrv.apch.kindai.ac.jp. † Environmental Science Research Institute, Kinki University. ‡ Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University. (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) Bolton, J. R. Chem. Eng. News 1996, 29. (4) Tada, H.; Kubo, Y.; Akazawa, M.; Ito, S. Langmuir 1998, 14, 2936. (5) Tada, H.; Akazawa, M.; Kubo, Y.; Ito, S. J. Phys. Chem. B 1998, 102, 6360.
(6) Tada, H. Langmuir 1995, 11, 3281.
10.1021/la9816712 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999
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Figure 1. (A) Electronic absorption spectra of a 1.62 × 10-3 M [MgII(acac)2] solution before (a) and after (b) adsorption on TiO2 at 25 ( 1 °C. (B) Absorption spectra of a 1.62 × 10-3 M [MgII(acac)2] solution before (a) and after (d) adsorption on MgO at 25 ( 1 °C. > 300 nm, with the light intensity integrated from 320 to 400 nm ) 0.6 mW cm-2) was started just after dispersion of the catalyst into the solution. After 10 mL of aliquots were periodically removed and centrifuged, the absorption spectra of the supernatants were measured. The concentration of the remaining DBS was determined from the absorbance at 240 nm (max ) 6.8 × 103 M-1 cm-1). In each case, the concentration of DBS decreased exponentially with respect to illumination time (t) and the apparent first-order rate constant (k/min-1) was calculated from the equation of k ) (1/t) ln(C0/C); C0 and C are the concentrations of DBS at t ) 0 and t ) t, respectively. An optimized structure of [MgII(acac)2] was obtained from the molecular orbital calculations using Zerner’s intermediate neglect of differential overlap program (ZINDO). The solvent effect was taken into account by the conductor-like screening method.
Results and Discussion Adsorption of [MgII(acac)2] on TiO2 and MgO was studied at 25 ( 1 °C. In the adsorption on TiO2, the adsorption amount (Γ/mol g-1) is equal to the total of the initial content in the solution (n0/mol), reaching a constant of 5.0 × 10-5 mol g-1 at n0 > 5.0 × 10-5 mol. The area occupied by one complex (σ) in the saturated state was estimated to be 0.28 nm2 complex-1. This value was in good agreement with the cross-sectional area of the acac- ligand in the [MgII(acac)2] complex structurally optimized by the calculation using ZINDO (∼0.25 nm2). On the other hand, Γ for MgO was below 2.0 × 10-6 mol g-1. These facts indicate that strong and irreversible adsorption of [MgII(acac)2] takes place on TiO2, while the interaction between the complex and the MgO surface is very weak. Figure 1A shows electronic absorption spectra of a 1.62 × 10-3 M [MgII(acac)2] solution in EtOH-n-hexane (3:17 v/v) before (a) and after (b) adsorption on TiO2 at 25 ( 1 °C. After adsorption, the π f π* absorption peak of the acac- ligand remarkably weakens concurrently with its blue shift from 283.5 to 282 nm. Noticeable is the appearance of a shoulder at ca. 280 nm. GC-MAS analyses detected acetylacetone (acacH; m/z ) 43, 85, 100) from
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Figure 2. DRIFT spectra of [MgII(acac)2] (a), MgII(acac)(OTis) - TiO2 (b), and MgII(acac)(OTis) heated at 500 °C - TiO2 (c).
the supernatant separated from the particles by centrifugation subsequent. The absorption peak in spectrum (b) can be deconvoluted into the two components of [MgII(acac)2] (c1; λmax ) 283.5 nm) and acacH (c2; λmax ) 270 nm) using a Lorentzian. The inset shows the plot of the amount of [MgII(acac)2] adsorbed vs that of acacH liberated, giving a straight line with a slope of ca. 1.0. Figure 1B shows an absorption spectra of a 1.62 × 10-3 M [MgII(acac)2] solution before (a) and after (d) adsorption on MgO at 25 ( 1 °C. No spectral change other than a very slight decrease in the absorbance is observed. Figure 2 shows DRIFT spectra of [MgII(acac)2] (a), TiO2 adsorbed with the complex - TiO2 (b) and TiO2 adsorbed with the complex after the heating at 500 °C - TiO2 (c). Three peaks at 1603, 1523, and 1406 cm-1 in difference spectrum (b) can be assigned to the combination of ν(CC) + ν(C-O), the combination of ν(C-O) + ν(C-C), and δd(CH3), respectively.7 The peak positions are near those of the corresponding peaks in spectrum (a) (1613, 1515, and 1418 cm-1). There was no absorption peak in the 1350-1700 cm-1 range of the spectrum of the TiO2 particles that contained preadsorbed acacH and then were dried in vacuo. The possibility of the re-coordination of the liberated acacH to coordinatively unsaturated surface Ti4+ ions can be excluded. These findings strongly suggest the coordination of the remaining acac- ligands to Mg2+ ions, i.e., the formation of a mixed ligand surface complex (MgII(acac)(O-Tis)). All the absorption peaks in spectrum b disappear when the sample was heated at 500 °C (difference spectrum c). Clearly, the chemisorbed complexes are oxidized to be transformed into an oxide on the surface (MgOx). The results of detailed characterization of the MgOx layer by X-ray photoelectron spectroscopy will be reported in a subsequent paper. (7) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley-Interscience, New York, 1986.
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Langmuir, Vol. 15, No. 11, 1999 3701 Scheme 1. Proposed Mechanism of a MgOx Monolayer Formation
Figure 3. Coverage of the MgOx layer (θ) and the S value of the particles as a function of n; θ was calculated from Γ assuming that the area occupied by one Mg2+ ion in MgOx layer at θ ) 1 is approximate to that in a MgO crystal (0.88 nm2/Mg2+ ion).
Figure 3 shows the coverage of MgOx layer (θ) and the S value of the particles as a function of n. It should be noted that the increment of θ per cycle decreases as n increases (curve a). The value of S gradually increases with increasing n (0-4), decreasing at n ) 6 (curve b). Preheating of TiO2 particles at 500 °C did not change the Γ or θ (cross point c); this indicates that the decrease in
the density of Tis-OH groups with the heating (e 500 °C) hardly affects the amount of adsorption. Then, the trend in curve a can be explained on the basis of the assumption that the adsorption occurs on the portion of naked TiO2 in preference to the portion already covered with MgOx. The similarity of the adsorptivity between the MgOx layer and MgO crystal may lead to this selective adsorption. From the results above, it can be concluded that [MgII(acac)2] irreversibly adsorbs on TiO2 via the ligand exchange of acac- for TisO-. This ligand exchange proceeds stoichiometrically with a ratio of [acacH]/[MgII(acac)2]ad = 1.0 (process I in Scheme 1). A similar mechanism was presented for the adsorption of [VIVO(acac)2]8 and [CrIII(acac)3]9 on SiO2. Rao et al. reported that [MgII(acac)2]‚ 2H2O adsorbs on γ-Al2O3 without the loss of acac- ligands.10 The adsorption amount and the adsorption mechanism of [MgII(acac)2] are suggested to be strongly dependent on the kind of supports. A MgOx submonolayer is generated as the result of the successive oxidation of the acac--ligand surviving after adsorption (process II). The selective adsorption of [MgII(acac)2] on the surface of naked TiO2 (8) Voort, P. V. D.; White, M. G.; Vansant, E. F. Langmuir 1998, 14, 106. (9) Haukka, S.; Lakomaa, E.-L.; Suntola, T. Appl. Surf. Sci. 1994, 75, 220. (10) Rao, S. N. R.; Waddell, E.; Mitchell, M. B.; White, M. G. J. Catal. 1996, 163, 176.
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Figure 4. Apparent rate constants (k/min-1) for the MgOx(n)/TiO2 (a) and SiOx/TiO2 (b) photoinduced oxidation of DBS at 30 ( 1 °C and pH = 7 as a function of n.
would enable the formation of the MgOx monolayer by the adsorption-oxidation cycle at n = 6 (process III). Figure 4 shows the k values for the MgOx(n)/TiO2 (a) and SiOx/TiO2 (b) photoinduced oxidation of DBS at 30 ( 1 °C and pH = 7. The presence of MgOx(n)/TiO2 or SiOx/ TiO2 was required before DBS could be decomposed by irradiation indicates that this is a TiO2 photoinduced reaction. The first study on the TiO2 photocatalytic oxidation of DBS by Hidaka et al. indicated the formation of CO2 and SO32- as final products.11 The k value decreases with the SiOx monolayer coverage by a factor of 4.5. A drastic shift of the point of zero charge (pzc) from 7.5 to 3.2 with the surface modification4 leads to an interpretation that the deactivation results from the depletion of the adsorption due to the electrostatic repulsion between DBS- and the surface (Sis-O-). On the other hand, in the MgOx(n)/TiO2 system, the k value increases with increasing n, going through a maximum, which is ca. 1.3 times as great as the value for TiO2, at n ) 2 (θ = 0.5). This finding is important in that the loading of promoters of
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common use including Pt and RuO2 reduces the activity of TiO2 in the photoinduced oxidation of DBS.11 The pzc was increased to 8.7 with the MgOx submonolayer coverage (θ = 0.5), which should increase the adsorption rate due to the electrostatic attraction between DBS- and the surface (Mgs-OH2+). When the k value normalized to the surface area (k′ ) k/S) is compared, the enhancing factor (k′(n ) 2)/k′(n ) 0)) decreases to 1.1. Since it is larger than 1.0, the MgOx monolayer does not act only as a textural promoter. At n g 2, the suppression of the interfacial charge transfer by the electrically insulating MgOx layer seems to overwhelm the acceleration of adsorption, resulting in the decrease in k.12 Since the MgOx layer is optically transparent at λ > 300 nm, the optimal coverage of ca. 0.5 is thought to be established by the balance of the two factors, i.e., the acceleration of the adsorption and the suppression of the interfacial charge transfer. In conclusion, it has been found that [MgII(acac)2] irreversibly adsorbs on TiO2 from an EtOH and n-hexane mixed solution via a one-to-one stoichiometric ligand exchange on Tis-O- sites. The successive oxidation of the remaining acac- ligands yielded a MgOx submonolayer on the surface of TiO2. The repetition of the adsorptionoxidation process formed the MgOx monolayer at n = 6 owing to the selective adsorption of [MgII(acac)2] on the portion of naked TiO2. The rate of the TiO2 photoinduced oxidation of DBS was revealed to increase with the MgOx submonolayer coverage (θ = 0.5) by a factor of 1.3. Although the detailed mechanisms of the adsorption and the oxidation deserve further scrutiny, the present results demonstrate that MgOx monolayer coverage of TiO2 enhances the photoinduced oxidation of anionic substrates in neutral aqueous media. 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. LA9816712
(11) Hidaka, H.; Kubota, H.; Graetzel, M.; Pelizzetti, E.; Serpone, N. J. Photochem. 1986, 35, 219.
(12) Tada, H. Langmuir 1996, 12, 966.
