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Patterned TiO2/SnO2 Bilayer Type Photocatalyst. 2. Efficient Dehydrogenation of Methanol Tetsuro Kawahara,† Yasuhiro Konishi,‡ Hiroaki Tada,*,§ Noboru Tohge,⊥ and Seishiro Ito‡,§ Nippon Sheet Glass Co. Ltd., 1-7, 2-Chome, Kaigan, Minato-Ku Tokyo, 105-8552, Japan, Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, 577-8502, Japan, Molecular Engineering Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, 577-8502, Japan, and Department of Metallurgical Engineering, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Received February 27, 2001. In Final Form: July 2, 2001
Introduction At present, hydrogen is greatly expected as a clean energy source. There is a pressing need to develop inexpensive, onboard catalyst systems for converting natural gas, gasoline, or methanol to hydrogen.1 Copperbased catalysts usually used for dehydrogenation of methanol require operation at temperatures above 130 °C and yield a small amount of carbon monoxide.2 On the other hand, hydrogen can be obtained at room temperature from an aqueous solution of methanol without any detectable formation of carbon monoxide using a Pt-loaded TiO2 powder photocatalyst.3 However, without the aid of Pt, the activity is quite low. We have recently found that a patterned-TiO2/SnO2 bilayer type photocatalyst formed on glass substrates (pat-TiO2/SnO2) exhibits a high activity in the gas-phase oxidation of acetaldehyde, a model harmful compound in air.4 This paper describes the activity of pat-TiO2/SnO2 in the photocatalyzed dehydrogenation of methanol with a particular emphasis on the patterning effect of the TiO2 overlayer. Experimental Section Quartz plates and SnO2 film (580 ( 80 nm in thickness) coated soda-lime glass plates (conductivity ) 1.6 × 103 S cm-1, Nippon Sheet Glass Co.) were used as substrates. Patterned TiO2 films were formed on the SnO2 film coated glass plates by an organically modified sol-gel method (pat-TiO2/SnO2).5 If necessary, a second series of strips were assembled upon pat-TiO2/SnO2 orthogonally (crosspat-TiO2/SnO2). After an ultrathin film of Au had been formed by evaporation, the surface morphologies and cross sections of the TiO2 and SnO2 films were observed by a 3-D imaging surface structure analyzer (Zygo New View 100). X-ray * To whom correspondence should be addressed. Tel: +81-66721-2332. Fax: +81-6-6721-3384. E-mail:
[email protected]. kindai.ac.jp. † Nippon Sheet Glass Co. Ltd. ‡ Department of Applied Chemistry, Kinki University. § Molecular Engineering Institute, Kinki University. ⊥ Department of Metallurgical Engineering, Kinki University. (1) Thomas, J. M. Chem. Eng. News 1999, 101. (2) de Wild, P. J.; Verhaak, M. J. F. M. Catal. Today 2000, 60, 3. (3) Sakata, T.; Kawai, T. In Energy Resources through Photochemistry and Catalysis; Graetzel, M., Ed.; Academic Press: New York, 1983; pp 331. (4) Tada, H.; Hattori, A.; Tokihisa, Y.; Imai, K.; Tohge, N.; Ito, S. J. Phys. Chem. B 2000, 104, 4585. (5) Tohge, N.; Shinmou, K.; Minami, T. J. Sol.-Gel Sci. Technol. 1994, 2, 581.
diffraction (XRD) measurements were performed on a Rigaku Rotaflex RTP 300 RC. The crystallinity of TiO2 was evaluated by the average size of the crystallites (d101/nm), which was determined from Scherrer’s equation of d101 ) Lλ/b cos θ (λ ) 0.154 nm, θ ) 12.65°, L ) 0.9, and b ) the half-width of the diffraction peak from the (101) plane of anatase).6 Photocatalysts were placed at the bottom of a Pyrex vessel (volume ) 35 mL, inside diameter ) 16 mm) with TiO2 positioned upward, and then a 1 M aqueous solution of CH3OH (10 mL) was poured into it. After deaeration by Ar-bubbling for 30 min, irradiation (λex > 300 nm) was carried out with a 400 W high-pressure mercury arc (H-400 P, Toshiba); the light intensity integrated from 320 to 400 nm was ca. 14.3 mW cm-2. H2 and CO2 evolved in a closed system were quantified by gas chromatography (Shimadzu GC8A; tcd column SHINCARBON T (6 m × 2 mm)) as a function of illumination time. The analysis conditions are as follows: for H2 analysis, the carrier gas was Ar (350 kPa) and both the injection temperature and the column temperature were 50 °C; and for CO2 analysis, the carrier gas was Ar (450 kPa), the injection temperature was 150 °C, and the column temperature was 200 °C. For evaluation of photocatalytic activity, a TiO2 film uniformly coated on quartz (TiO2/quartz) was used as a standard.6
Results and Discussion A 3-D imaging surface structure of crosspat-TiO2/SnO2 (Figure 1, top) demonstrates that cross-strips of the TiO2 films 1 mm in width are regularly formed in a 1 mm pitch on an SnO2 film coated glass plate. The regions classified by green, light green, and yellow correspond to the bare SnO2 underlayer, the TiO2 monolayer (TiO2(1)), and the TiO2 bilayer (TiO2(2)), respectively. The surface topology analyses (bottom) along the lines indicated in the center panel show that the step between TiO2(1) and TiO2(2) and the step between SnO2 and TiO2(1) are ca. 60 and 75 nm, respectively. X-ray diffraction patterns of TiO2/quartz and TiO2/SnO2 indicated that the TiO2 films have an anatase crystal form. Analyses of these data yielded the average crystal sizes of 13.2 ( 0.5 nm for the former and 12.6 ( 0.5 nm for the latter. Evidently, the TiO2 film on the SnO2 film coated glass has good anatase crystallinity. This is a result of the SnO2 layer’s complete inhibition of the Na+ ion diffusion from the soda-lime glass substrate.7 Figure 2 shows time courses of H2 evolution upon irradiation (λex > 300 nm) of a 1 M CH3OH aqueous solution in the presence of SnO2 (a), TiO2/quartz (b), and TiO2/ SnO2 (c). Curve d is the time dependence of the ratio of the amount of H2 in system c to that in system b (H2(c)/ H2(b)). Both TiO2 and irradiation were necessary for the generation of H2, and SnO2 alone hardly gives activity (a).8 Previously, a very low activity of SnO2 particles (25 m2 g-1) was confirmed for the photocatalyzed oxidation of 1,4-pentanediol.9 In this study, the activity might not be observed because of the smaller surface area of the SnO2 film.10 In systems b and c, the quantity of H2 monotonically increases with increasing irradiation time (tp). As shown in curve d, their activity ratio, H2(c)/H2(b), is almost constant at 0.30 ( 0.03 in the 1.5-3 h range of tp, whereas this trend is reversed as far as oxidation processes are (6) Hattori, A.; Shimoda, K.; Tada, H.; Ito, S. Langmuir 1999, 15, 5422. (7) Tada, H.; Tanaka, M. Langmuir 1997, 13, 360. (8) Cao et al. have recently reported that SnO2 particles with a diameter of 5 nm are photocatalytically active for 1-butene oxidation whereas SnO2 particles with a diameter of 22 nm have no activity. This phenomenon was attributed to the quantum size effect: Cao, L.; Spiess, F. J.; Hung, A.; Suib, S.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. J. Phys. Chem. B 1999, 103, 2912. (9) Fox, M. A.; Ogawa, H.; Pichat, P. J. Org. Chem. 1999, 54, 3847. (10) The apparent surface area of the SnO2 film is smaller than that of the SnO2 particles used in ref 9 by a factor of ca. 3000.
10.1021/la010307r CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001
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Figure 1. Top: a 3-D imaging surface structure of crosspat-TiO2/SnO2. Bottom: the surface topology analyses along the scanning lines indicated (center).
concerned.11 Figure 3 shows the patterning effect of the TiO2 overlayer on the activity in the photocatalyzed dehydrogenation of CH3OH: a, TiO2/SnO2; b, pat-TiO2/ SnO2; c, crosspat-TiO2/SnO2; d, H2(b)/H2(a); e, H2(c)/H2(a). The TiO2 patterning increases the activity by a factor of ca. 15 (d), and the cross-patterning attains an as much as 60-fold activity (e). The formation of CO2 was also confirmed by gas chromatography. Thus, this overall reaction can formally be written as eq 1, being an uphill reaction with a standard Gibbs free energy change (∆G°298K) of +8.8 kJ mol-1. TiO2
CH3OH(l) + H2O(l)9 8 hν CO2(g) + 3H2(g)
∆G°298K ) +8.8 kJ mol-1 (1)
A plausible mechanism on the TiO2 photocatalyzed dehydrogenation of CH3OH is presented in Scheme 1. Hole-electron pairs (h+‚‚‚e-) are generated in TiO2 by its interband transition (step 1, rate ) Iφa). Most of them are (11) Hattori, A.; Tokihisa, Y.; Tada, H.; Ito, S. J. Electrochem. Soc. 2000, 147, 2279.
Figure 2. Time courses of H2 evolution upon irradiation (λex > 300 nm) to a 1 M aqueous solution of CH3OH in the presence of SnO2 (a), TiO2/quartz (b), and TiO2/SnO2 (c): curve d is the time dependence of the ratio of the amount of H2 in system c to that in system b (H2(c)/H2(b)). The apparent surface areas (σ) are as follows: (a) σ(SnO2) ) 1.6 cm2; (b) σ(TiO2) ) 1.6 cm2; (c) σ(TiO2) ) 1.6 cm2.
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Figure 3. TiO2-patterning effect on the activity for the photocatalyzed dehydrogenation of CH3OH: (a) TiO2/SnO2; (b) pat-TiO2/SnO2; (c) crosspat-TiO2/SnO2; (d) x ) b; (e), x ) c. The apparent surface areas (σ) are as follows: (a) σ(TiO2) ) 1.6 cm2; (b) σ(TiO2) ) 1.6 cm2 and σ(SnO2) ) 1.8 cm2; (c) σ(SnO2) ) 0.96 cm2, σ(TiO2(1)) ) 1.6 cm2, and σ(TiO2(2)) ) 0.8 cm2. Scheme 1. Proposed Mechanism on the TiO2 Photocatalytic Dehydrogenation of CH3OH
Notes
Figure 4. Plots of ln n(H2) vs ln t for various photocatalyst systems: (a) TiO2/SnO2; (b) TiO2/quartz; (c) pat-TiO2/SnO2; (d) crosspat-TiO2/SnO2. Table 1. Kinetic Parameters for the Photocatalytic Dehydrogenation of CH3OH photocatalyst
log(Iφaφr)
log kH2
log Rra
nonpat-TiO2/quartz pat-TiO2/SnO2 crosspat-TiO2/SnO2
-1.83 -1.16 -0.56
1.65 0.84 0.17
0 0.17 1.3
a R expresses the relative rate with respect to that for nonpatr TiO2/quartz.
ln n(H2) ) 3 ln t + ln[4(Iφaφr)2kH2/3] ln n(H2) ) ln t + ln(Iφaφr)
lost by recombination (step 2, rate constant ) krec). A part of the holes escaping from the recombination oxidizes CH3OH adsorbed at the surface, giving ‚CH2OH and H+ (step 3, rate constant ) kox1). The ‚CH2OH radical has an enough potential (-0.98 V vs standard hydrogen electrode) to inject another electron into the conduction band (cb) of TiO2 (step 4, rate constant ) kox2).10 In parallel with step 3, the oxidation of H2O occurs to yield ‚OH (step 3′, not shown in Scheme 1). The HCHO or its hydrate (H2C(OH)2)12 is likely to be oxidized successively to CO2 by the holes and the ‚OH radicals. Kawai and Sakata confirmed the formation of HCHO and HCOOH by mass spectrometry in the dehydrogenation of CH3OH using a Pt-loaded TiO2 powder as a photocatalyst.13,14 The H+ is reduced by the excited electrons (step 5, rate constant ) kred), and H2 is produced as a result of the coupling of H‚ (step 6, rate constant ) kH2). The molar ratio of H2/CO2 in system c at 1 e tp e 3 h was 30 ( 3, that is, about 1 order of magnitude greater as anticipated by eq 1. This suggests that the rate of step 3′ is much smaller than that of step 3, allowing us to omit step 3′ in Scheme 1. By assuming the reaction mechanism in Scheme 1, one can derive rate equations 2 and 3 for the H2 generation at the initial stage and the steady state, respectively (see Appendix). (12) Lilie, V. J.; Beck, G.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1971, 75, 458. (13) March, J. Advanced Organic Chemistry Reactions, Mechanisms, and Structure, 4th ed.; John Wiley & Sons: New York, 1992; p 882. (14) Kawai, T.; Sakata, T. J. Chem. Soc., Chem. Commun. 1980, 694.
at t f 0 at t f ∞
(2) (3)
where n(H2) is the mole number of H2 generated at tp ) t, and φr () kox1[CH3OHad]/(krec + kox1[CH3OHad])) expresses the reaction efficiency. First, the apparent induction period in Figure 3 can be explained by eq 2, which suggests that the amount of H2 generated is proportional to the third power in time at the initial stage of the reaction. Figure 4 shows the plots of ln n(H2) versus ln t for various photocatalyst systems: a, TiO2/SnO2; b, TiO2/quartz; c, pat-TiO2/SnO2; d, crosspatTiO2/SnO2. As expected from eqs 2 and 3, the plot for each system gives two straight lines having slopes of approximately 3 and 1 at the initial stage and the steady state of the reaction, respectively. In system a, the amount of H2 was too small to be determined at tp < 0.75 h. The kinetic parameters of kH2 and Iφaφr can be evaluated from the intercepts of the two straight lines in the same photocatalyst system. Table 1 summarizes the kinetic parameters for TiO2/quartz, pat-TiO2/SnO2, and crosspatTiO2/SnO2 and the relative activity (Rr) with respective to the activity of TiO2/quartz. The increase in activity, TiO2/quartz < pat-TiO2/SnO2 < crosspat-TiO2/SnO2, could be caused by the increase(s) in the rate of h+‚‚‚egeneration (Iφa) and/or φr, because the kH2 decreases in that order. Labeling and visualization of reduction sites in pat-TiO2/SnO2 by Ag particles have previously verified the long-range charge separation due to the electron transfer from TiO2 to SnO2.4 The thickness and area of the TiO2 film of TiO2/quartz are equal to those of patTiO2/SnO2, that is, Iφa(TiO2/quartz) = Iφa(pat-TiO2/SnO2). Thus, the higher activity of the latter can mainly be attributed to the increase in φr. Further, the rise in Iφa with increasing area and thickness of the TiO2 film explains the activity of crosspat-TiO2/SnO2 exceeding that of pat-TiO2/SnO2. We have shown that UV-light irradiation of a (cross)pat-TiO2/SnO2 bilayer type photocatalyst leads to efficient
Notes
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H2 generation from an aqueous solution of CH3OH at room temperature. From the kinetic considerations, the conclusion has been drawn that the high photocatalytic activity arises from the increase in the reaction efficiency due to the electron transfer from TiO2 to SnO2. This study may present a guide for designing inexpensive and highly active onboard photocatalysts for converting CH3OH to H2. Acknowledgment. The authors express sincere gratitude to Mr. Kazutoshi Adachi (Technology Research Institute of Osaka Prefecture) for the 3-D imaging surface structure analyses and Dr. Mitsunobu Iwasaki (Kinki University) for helpful comments.
Integration of eq A2 in the ranges of 0 e x e x and 0 e t e t yields eq A3.
x ) K[2/{1 + exp(-2aKt} - 1]
At the initial stage (t f 0) and the steady state (t f ∞) of the reaction, x is approximately equal to aK2t and K, respectively. Thus, eqs A4 and A5 are obtained for each case.
n(H2) ) a3K4t3/6
Appendix According to Scheme 1, the rate of H2 generation is given by eq A1, where n(H2) is the mole number of H2 after irradiation of t h.
d n(H2)/dt ) kH2 [H·]
2
(A1)
The application of the steady-state approximation to each intermediate (h+‚‚‚e-, ‚CH2OH, H+, and H‚) leads to eq A2.
dx/dt ) -a(x2 - K2) where x ) [H‚], a ) 2kH2, and K2 ) 2Iφaφr/a.
(A2)
(A3)
n(H2) ) aK2t/2
at t f 0
(A4)
at t f ∞
(A5)
The replacement of a and K with kH2 and Iφaφr followed by a rearrangement gives eqs A6 and A7 for both the limiting cases.
ln n(H2) ) 3 ln t + ln[4(Iφaφr)2kH2/3] ln n(H2) ) ln t + ln(Iφaφr) LA010307R
at t f 0 (A6) at t f ∞
(A7)
