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Enhanced Remote Photocatalytic Oxidation on Surface-Fluorinated TiO2 Jong Sung Park and Wonyong Choi* School of Environmental Science and Engineering and Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Korea Received August 2, 2004. In Final Form: October 4, 2004 The mobile nature of active oxygen species generated on the UV-illuminated TiO2 surface is now wellrecognized. Surface oxidants not only migrate two-dimensionally but also desorb from the surface to be air-borne oxidants. The remote photocatalytic oxidation (PCO) of stearic acids over the surface-fluorinated TiO2 (F-TiO2) film was carried out in the ambient air to study the effects of fluorination on the desorption of oxidants from the surface. The F-TiO2 film was faced to a stearic acid-coated glass plate separated by a small gap (typically 30 µm), and the photocatalytic degradation of the stearic acid was monitored by Fourier transform infrared measurement or gas-chromatographic CO2 production analysis. Remote photocatalytic degradation of stearic acids was markedly faster with F-TiO2 than with the pure TiO2 film, which indicates that the generation of air-borne oxidants is enhanced over the F-TiO2 surface. The remote PCO activity was higher with a higher surface fluoride concentration, higher UV intensity, and smaller gap. The remote photocatalytic activity of F-TiO2 was maximal at a relative humidity of 50% and did not show any sign of deactivation with repeated reactions. The production of CO2 that evolved as a result of the remote PCO of stearic acids was enhanced when H2O2 vapor was present but was strongly inhibited in the presence of ammonia gas that should scavenge OH radicals. Judging from various evidences, the air-borne oxidants in remote PCO are most likely OH radicals and the surface fluorination of TiO2 seems to facilitate the desorption of OH radicals.
Introduction TiO2 has demonstrated successful performances as an environmental photocatalyst that mineralizes a wide range of organic pollutants.1-4 The strong oxidizing power of TiO2 photocatalyst has been ascribed to highly oxidative valence band (VB) holes (+2.7 V versus the normal hydrogen electrode at pH 7) and various oxygen-containing radical species (e.g., •OH, O2-, HO2•) that are generated on the UV-illuminated TiO2 surface. The traditional view on the mechanistic behavior of oxidant radicals involved in photocatalysis was that they remain on the surface and react with substrates that are preadsorbed or diffusing onto the surface. However, recent findings from several independent studies showed that this model, that TiO2 photocatalytic oxidation (PCO) takes place strictly on the surface, is not completely true though it is largely valid in understanding most photocatalytic reactions.5-8 Tatsuma et al.5 first reported a case of remote PCO in which the test substrate (organic dye or polymer film) on a glass plate that was faced toward a TiO2-coated glass * Corresponding author. E-mail:
[email protected]. Fax: +8254-279-8299. (1) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Peral, J.; Domenech, X.; Ollis, D. F. J. Chem. Technol. Biotechnol. 1997, 70, 117. (4) (a) Choi, W.; Hong, S. J.; Chang, Y.-S.; Cho, Y. Environ. Sci. Technol. 2000, 34, 4810. (b) Lee, H.; Choi, W. Environ. Sci. Technol. 2002, 36, 3872. (c) Lee, J.; Park, H.; Choi, W. Environ. Sci. Technol. 2002, 36, 5462. (d) Choi, W.; Lee, J.; Kim, S.; Hwang, S.; Lee, M. C.; Lee, T. K. J. Ind. Eng. Chem. 2003, 9, 96. (5) (a) Tatsuma, T.; Tachibana, S.; Miwa, T.; Tryk, D. A.; Fujishima, A. J. Phys. Chem. B 1999, 103, 8033. (b) Tatsuma, T.; Tachibana, S.; Fujishima, A. J. Phys. Chem. B 2001, 105, 6987. (c) Tatsuma, T.; Kubo, W.; Fujishima, A. Langmuir 2002, 18, 9632. (6) Cho, S.; Choi, W. J. Photochem. Photobiol., A 2001, 143, 221. (7) Haick, H.; Paz, Y. J. Phys. Chem. B 2001, 105, 3045. (8) Lee, M. C.; Choi, W. J. Phys. Chem. B 2002, 106, 11818.
plate but was separated by a small gap could be oxidized despite the absence of direct contact with the TiO2 surface. They proposed that PCO is carried out remotely by photogenerated air-borne oxidants that should desorb from the illuminated TiO2 surface. Cho and Choi6 reported that PCO of the TiO2-blended PVC film developed cavities around the imbedded TiO2 particles, and the cavity boundary was well-separated by 1-2 µm from the particle boundary, which was also attributed to diffusing air-borne oxidant radicals. Haick and Paz7 observed a remote photodegradation of self-assembled monolayers of aliphatic chains anchored to the inert silicon surface although the chains were located as far as 20 µm away from the TiO2 microdomains. Lee and Choi8 observed a similar phenomenon in a PCO experiment of carbon soot on TiO2 film. The soot film deposited alongside the TiO2 film could be degraded with developing a gap between the edges of soot and TiO2 domains: the gap distance continuously increased with UV illumination up to 80 µm. This phenomenon of remote PCO of soot was further confirmed by Lee et al.9 who provided visible images taken by a digital camera. On the basis of these evidences, it is now well accepted that the oxidant species generated on illuminated TiO2 both diffuses laterally on the surface and desorbs from the surface into the air. The mobility of oxidant species should be influenced by the surface property of TiO2. Recently, surface fluorinated TiO2 (F-TiO2) has been investigated as a new surface modification method.10-12 F-TiO2 can be prepared by a simple ligand exchange between surface hydroxyl groups on TiO2 and fluoride anions. (9) Lee, S.-K.; McIntyre, S.; Mills, A. J. Photochem. Photobiol., A 2004, 162, 203. (10) (a) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Langmuir 2000, 16, 2632. (b) Minero, C.; Mariella, G.; Maurino, V.; Vione, D.; Pelizzetti, E. Langmuir 2000, 16, 8964. (11) Vohra, M. S.; Kim, S.; Choi, W. J. Photochem. Photobiol., A 2003, 160, 55. (12) Park, H.; Choi, W. J. Phys. Chem. B 2004, 108, 4086.
10.1021/la048051n CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004
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tTisOH + F- T tTisF + OH-
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pKF ) 6.2 (1)
Minero et al.10 reported that the PCO of phenol in the aqueous TiO2 suspension was markedly enhanced in the presence of fluoride and ascribed this to the enhanced generation of mobile free OH radicals (reaction 2), whereas most OH radicals generated on the naked TiO2 surface prefer to remain adsorbed (reaction 3).
tTisF+H2O + hvb+ f tTisF+ •OHfree(aq) + H+ (2) tTisOH + hvb+ f tTisOH•+
(3)
Choi and co-workers confirmed this behavior in related studies and concluded that substrates that react mainly through an OH radical-mediated pathway are more rapidly degraded in the F-TiO2 suspension but the holemediated degradation paths are slower because of the hindered adsorption of substrates on F-TiO2.11-13 In general, the PCO reactions taking place on F-TiO2 seem to be more homogeneous-like in their mechanistic nature because of the enhanced mobility of active oxidants involved. This study was motivated by an assumption that the remote PCO can be enhanced on F-TiO2. Although the surface fluoride-enhanced effect was not observed in the gas-phase photocatalytic reactions,14 the enhanced desorption of oxidants from the F-TiO2 surface into the air might be observed in a controlled experiment (reaction 4).
tTisF + H2Oad + hvb+ f tTisF + •OH(air-borne) + H+ (4) We investigated the remote PCO of stearic acids (SAs) over F-TiO2 and bare TiO2 films under the ambient air condition. The results showed that the remote PCO was indeed enhanced when the TiO2 surface was fluorinated. Experimental Section Materials, Sample Preparation, and Characterization. The photocatalyst used was Degussa P-25 TiO2, a mixture of anatase and rutile (8:2). TiO2 film was deposited on a Pyrex glass plate [Corning; surface area of 3.24 cm2 (1.8 cm × 1.8 cm); thickness 1.0 mm]. A suspension (5 wt %) of TiO2 was prepared in distilled water, stirred for 2 h, and sonicated for 30 min. An aliquot of the TiO2 suspension was dropped onto the glass plate, which was dried under air and then calcined at 400 °C for 30 min. The coating procedure was repeated five times. Sodium fluoride (NaF) solutions of 0.2, 1, 10, and 50 mM were prepared, and the pH of the NaF solution was adjusted to 3.5 using HClO4. The TiO2-coated glass plate was soaked in the NaF solution for 30 min to fluorinate the TiO2 surface (via reaction 1) and then dried under air. The surface atomic compositions of TiO2 and F-TiO2 films were determined by X-ray photoelectron spectroscopy (XPS: Kratos XSAM 800 pci) using the Mg KR line (1253.6 eV) as an excitation source. The spectra were taken for each sample after Ar+ (3 keV) sputter cleaning. Surface charging was minimized by spraying low energy electrons over the sample surface using a neutralizer gun. Binding energy spectra were recorded in the regions of C(1s), Ti(2p), O(1s), and F(1s). The binding energies of all peaks were referenced to the Ti(2p) line (454.1 eV). On the other hand, to investigate how the surface fluorination affects the surface OH groups and water adsorption on TiO2, the Fourier transform infrared (FT-IR) spectra of F-TiO2 and TiO2 powders diluted in KBr pellets (TiO2/KBr ) 1:50 by weight) were also obtained using a FT-IR spectrometer (Bomem MB104). (13) Ryu, J.; Choi, W. Environ. Sci. Technol. 2004, 38, 2928. (14) Lewandowski, M.; Ollis, D. F. J. Catal. 2003, 217, 38.
Figure 1. Schematic illustration of the experimental setups for (a) the remote PCO of SA and (b) the control experiment. SA [CH3(CH2)16COOH, Aldrich] was used as a target substrate. The SA substrate was coated on the same kind of the Pyrex glass plate as the TiO2-coated one. For most experiments, a SA-coated glass plate was prepared by dropping 10 µL of 5 mM SA solution in methanol (or 30 µL of 12.5 mM SA for CO2 production measurement) on a glass plate and evaporating the solvent. The initial coating density of SA on the glass plate was typically 3 × 1016 SA molecules/cm2. Remote PCO Experiments. The SA-coated glass plate and the TiO2-coated plate were faced to each other and held together but separated by a small intervening gap (30-150 µm) as illustrated in Figure 1a. The gap distance was varied by using different spacers such as paper (30, 100 µm) and cover glass (150 µm). A black-light UV lamp (10 W: Sankyo Denki) irradiated the sample from the TiO2 side. The UV intensity was 1.5 mW/ cm2. A control experiment was carried out with reversing the TiO2-coated glass plate upside down as shown in Figure 1b. In the remote PCO experiments for detecting CO2 generated from the SA degradation, the sandwiched glass plate unit was placed in a gastight glass cell (volume, 75 cm3), and the same UV lamp was used. The atmospheric conditions in the glass cell were controlled. The relative humidity (RH) in the glass cell was adjusted by flowing the dry air (RH 0%) and water-saturated air (RH 100%) with a specific mixing ratio. When NH3 or H2O2 vapor was introduced, dry air passed through 0.5 M NH4Cl solution (pH 11) or 30 wt % H2O2 solution in a temperature-controlled saturator and then filled the glass cell. Vapor concentrations of NH3 and H2O2 were calculated from the Henry’s law constants obtained from the literature.15,16 After purging the glass cell with a specific air for about 1 h, the reactor was sealed from the ambient air. A FT-IR spectrometer (Bomem MB104) monitored the IR spectral absorbance of SA on the glass plate in the region of the C-H stretching band (2800-3000 cm-1) whose intensity was quantified by integrating the peak area. The IR beam transmitted through the SA-coated glass plate. The bare Pyrex glass plate had no IR absorption bands in the C-H stretching region, and the FT-IR spectra of the SA-coated glass plate were referenced against the bare glass plate. The spectra were taken at specific time intervals: 0, 3, 15, and 18 h irradiation. The CO2 generated from the remote PCO of SA was quantified using a gas chromatograph (GC Hewlett-Packard 6890) that was equipped with a flame ionization detector, a Porapak N column (Agilent Technologies), and a CO2 methanizer (HP G2747A).
Results and Discussion Enhanced Remote PCO on F-TiO2. Figure 2a compares the time-dependent profiles of SA degradation as a result of remote PCO with TiO2 and F-TiO2 under ambient air. Figure 2b shows that the intensity of the C-H stretching band of SA gradually decreases with irradiation time. Direct photolysis of SA in the absence of TiO2 (control) was not observed at all. The remote PCO (15) Clegg, S. L.; Brimblecombe, P. J. Phys. Chem. 1989, 93, 7237. (16) O’Sullivan, D. W.; Lee, M.; Noone, B. C.; Heikes, B. G. J. Phys. Chem. 1996, 100, 3241.
Oxidation on Surface-Fluorinated TiO2
Figure 2. (a) Remote photocatalytic degradation of SA as a function of UV irradiation time. The degradation of SA was monitored by the IR absorption of the C-H stretching band. In preparing F-TiO2, 10 mM NaF solution (pH 3.5) was used. Other experimental conditions were gap distance, 30 µm; UV intensity, 1.5 mW/cm2. (b) FT-IR spectra in the region of the C-H stretching band of SA that were measured at various stages of the remote PCO with F-TiO2 are shown.
Figure 3. Repeated runs of the remote PCO of SA over the same F-TiO2 film. The SA-coated glass plate was replaced by a new one after each run. Other experimental conditions were the same as those of Figure 2.
with F-TiO2 was much faster than with pure TiO2, which indicates that more air-borne oxidants are generated over F-TiO2. The apparent quantum efficiencies (the number of SA molecules degraded divided by the number of incident photons) of the remote PCO of SA with F-TiO2 and TiO2 were calculated to be 1.3 × 10-4 and 3.0 × 10-5, respectively. To investigate if surface fluorides are depleted with the irradiation time, the remote PCO of SA was repeated several times using the same F-TiO2 film as shown in Figure 3. The SA-coated glass plate was replaced by a new one after each run. The remote PCO activity of F-TiO2 did not show any sign of decrease with repeated uses, which indicates that the surface fluoride species is stable and not depleted under irradiation. Effects of Experimental Parameters. Figure 4a shows the effect of the surface fluoride concentration on the remote PCO. When the TiO2 film was fluorinated in
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Figure 4. (a) Effect of the NaF concentration used in the preparation of F-TiO2 on the remote photocatalytic activity. (b) Comparison of the XPS F(1s) peak intensity of F-TiO2 films prepared at [NaF] ) 0.2, 1, 10, and 50 mM. The inset is an XPS survey spectrum of F-TiO2 (400-800 eV).
higher [NaF] solution, the resulting F-TiO2 exhibited a higher remote PCO activity. The XPS analysis of the F-TiO2 films confirmed that the surface fluoride concentration [i.e., the F(1s) peak intensity] increased with the concentration of NaF used in the preparation step (Figure 4b). The binding energy (BE) of the F(1s) peak (BE ) 684.3 eV) corresponded to that of F- adsorbed on TiO2, and no sign of F- ions in the lattice (BE ) 688.5 eV) was detected.17 The remote PCO activity is clearly proportional to the surface fluoride concentration. The gap distance also influenced the remote PCO activity of F-TiO2. Figure 5a clearly shows that the degradation of SA was faster with narrowing the gap. This is consistent with a previous result5a obtained with pure TiO2 film and supports that some reactive species are transported through the air. When the F-TiO2 film was in direct contact with the SA-coated glass (0 µm in Figure 5a), the degradation rate was the fastest. The effect of light intensity is shown in Figure 5b. When decreasing the UV intensity from 1.5 to 0.75 mW/cm2, the SA degradation rate was markedly retarded, which agrees with a previous observation5c that the remote PCO of octadecyltriethoxysilane with pure TiO2 film was faster under higher UV intensity. Although it is taken for granted that more oxidants are generated at higher light intensity, higher surface concentration of photooxidants does not mean necessarily that more photooxidants should desorb from F-TiO2. The result of Figure 5b shows that their desorption from the F-TiO2 surface is also enhanced at higher light intensity. It should be noted that the remote PCO rate with F-TiO2 under 0.75 mW/cm2 UV was slightly faster than that with pure TiO2 under 1.5 mW/cm2 UV despite the fact that fewer photooxidants should be generated on F-TiO2 than on the pure TiO2 surface. This (17) Yu, J. C.; Yu, J.; Ho, W.; Jiang, Z.; Zhang, L. Chem. Mater. 2002, 14, 3808.
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Figure 6. Remote PCO (expressed in terms of the CO2 generation rate) of SA in various atmospheres. (A-C) Effect of humidity; (D, E) effect of the addition of H2O2 vapor in the presence or absence of the photocatalyst; and (F) effect of the addition of NH3 vapor.
Figure 5. (a) Effect of the gap distance on the remote photocatalytic degradation rate of SA ([NaF] ) 10 mM, UV intensity 1.5 mW/cm2). (b) Effect of the UV intensity on the remote photocatalytic degradation rate of SA ([NaF] ) 10 mM, gap distance 30 µm).
also supports that the oxidant desorption from the photocatalyst should be enhanced when the surface is fluorinated. Consideration on Air-Borne Photooxidants in Remote PCO. Now the fact that some active oxidants are desorbed from the illuminated photocatalyst surface into the gas phase to initial remote PCO is undoubted. This study newly found that the surface fluorination of TiO2 enhances the remote PCO by facilitating the desorption of oxidants. However, what oxidant species is responsible for remote PCO reactions is not clearly identified. Reactive oxygen species such as •OH, •O2H, H2O2, and 1O2 have been considered as candidates.5-8 The most probable species is the hydroxyl radical (•OH), which has been assumed as the diffusing oxidant in previous studies. Because the remote PCO has been successfully applied to various organic and inorganic substrates such as dye,5a polymer,5b saturated alkyl chain monolayer,5c,7 carbon soot,8 silicon carbide,18 and copper,5c only •OH is capable enough to oxidize such diverse materials. The • OH is a ubiquitous oxidant present in the ambient air, and its average steady-state concentration in the air is known to be about 106 radicals cm-3.19 Although •OH is (18) Ishikawa, Y.; Matsumoto, Y.; Nishida, Y.; Taniguchi, S.; Watanabe, J. J. Am. Chem. Soc. 2003, 125, 6558. (19) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; Wiley & Sons: New York, 1998; Chapter 5.
a transient species, the average lifetime of a single OH radical in an ambient atmosphere is estimated to be around 0.5 s.19 Considering that typical gas diffusion coefficients at the atmospheric condition range in 0.10.2 cm2 s-1,20 the lifetime of 0.5 s seems to be long enough to allow an OH radical to migrate up to 100 µm in the present experimental system. Because the water molecule adsorbed on the TiO2 surface serves as a precursor of the OH radical, the surface water concentration should influence the remote PCO activity. The surface of F-TiO2 might be rather hydrophobic and, hence, the adsorption of water molecules on F-TiO2 could be reduced. In such a case, the enhanced remote PCO on F-TiO2 through the OH radical desorption (reaction 4) is not likely to occur. To check out this possibility, the FT-IR spectra of pure TiO2 and F-TiO2 powders were compared in the OH stretching region (3000-3700 cm-1). The intensity of the broad OH band was reduced by only 10-15% upon the surface fluorination (spectra not shown). It seems that the amount of physisorbed water molecules on the surface of TiO2 under the ambient condition was not significantly influenced by the surface fluorination. As a next step, the effect of water vapor on the remote PCO was investigated and compared between the pure TiO2 and the F-TiO2 systems. Figure 6 shows that the CO2 production rates measured under different water vapor concentrations (i.e., RH 0, 50, and 100%) were consistently higher with F-TiO2 than with pure TiO2. However, it should be noted that the fluoride enhancement effect was outstanding at RH 50% but relatively minor at RH 0 and 100%. At RH 0%, not enough H2O molecules are available for reaction 4; hence, the remote PCO on F-TiO2 cannot be much enhanced. The in situ production of water vapor from the remote PCO of SA may increase RH with time, but it turned out to be too small to change RH significantly. The H2O production corresponding to the observed CO2 generation for 10 h of PCO was estimated to increase RH only by 1.5% in a closed reactor. With increasing RH up to 100%, the remote PCO rate with pure TiO2 continuously increased but that with F-TiO2 was maximal at RH 50% and reduced at 100% RH. Moisture condensation between the gap at RH 100% may (20) CRC Handbook of Chemistry and Physics, 78th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, 1997.
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reduce the remote PCO activity, but there is no reason it should happen only on F-TiO2 and not on the pure TiO2 film. We did not observe any visible condensation of water between the two glass plates at RH 100%. From the fact that TiO2 and F-TiO2 showed different dependences on the RH in their remote PCO, we speculate that the presence of water adlayers on the photocatalyst surface may influence the desorption of oxidants differently depending on whether the TiO2 surface is fluorinated or not. Further studies are required to address this issue. Although the reason is not clear at this moment, it should be kept in mind that the fluoride-enhanced effect in the remote PCO is optimized at an intermediate level of RH. If the air-borne •OH is responsible for the remote PCO, it should be inhibited in the presence of gaseous scavengers of OH radicals. For example, Tatsuma et al.5a observed that the remote bleaching of methylene blue over a TiO2 film was significantly suppressed in the presence of ethanol vapor that should scavenge most air-borne OH radicals. However, other oxidants such as 1O2 and •O2H may also react with ethanol. To assess the possible role of OH radicals in the remote PCO, the effect of the presence of an OH radical precursor, H2O2, was investigated. As shown in Figure 6D, when 66 ppmv H2O2 vapor was introduced into the reactor cell, the CO2 generation rate increased. However, without TiO2 (control), no CO2 generation was detected in the presence of H2O2 (Figure 6E). This indicates that additional OH radicals were generated on the F-TiO2 (or TiO2) surface and subsequently desorbed into the gas phase through a reaction with the conduction band electrons (reaction 5).
H2O2(ad) + ecb- f OH- + •OH(g)
photocatalyst surface plays a major role in the remote PCO process.
H2O2(g) + hν f 2•OH(g)
They proposed a double excitation mechanism (meaning excitation of both TiO2 and H2O2), which involves excitation of diffusing species (i.e., H2O2) in the vicinity of the substrate surface. However, this mechanism cannot be supported in the present case because no CO2 generation was observed in the control experiment (Figure 6E) in which H2O2 and the SA-coated glass plate only were employed. Because we used a black light lamp that emits mostly UV A (centered around λ ∼ 365 nm) and the Pyrex plate itself cuts off wavelengths shorter than 300 nm, the direct photolysis of H2O2 should be negligible under this irradiation. The effect of OH radical scavengers on the remote PCO was also investigated. Because we monitored the CO2 generation from the SA degradation, organic scavengers such as alcohols whose reaction with OH radicals also generates CO2 could not be used. Ammonia gas was used as an OH radical scavenger (reaction 8) in this study.
NH3(g) + •OH(g) f H2O + NH2 (k298 ) 1.6 × 10-13 cm3/molecule·s)21 (8) (The lifetime of •OH due to this reaction alone is 28 µs in the presence of 0.9% NH3.) As shown in Figure 6F, the rate of CO2 production from the remote PCO was drastically reduced in the presence of 0.9% NH3. This also supports the role of OH radicals in the remote PCO.
(5)
At the same time, H2O2 in the gas phase should scavenge air-borne •OH (reaction 6) to reduce the remote PCO rate.
H2O2(g) + •OH(g) f H2O + •O2H (k298 ) 1.7 × 10-12 cm3/molecule·s)21 (6) (The lifetime of •OH due to this reaction alone is 0.4 ms in the presence of 66 ppmv H2O2.) Therefore, the net positive effect of H2O2 on the remote PCO implies that reaction 5 outweighed reaction 6 in the present experimental condition. On the other hand, Kubo et al.22 recently suggested that the photolysis of gaseous H2O2 (reaction 7) that is photogenerated on and subsequently desorbed from the (21) DeMore, W. B.; et al. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; JPL publication 94-26; NASA JPL, 1994. (22) Kubo, W.; Tatsuma, T.; Fujishima, A.; Kobayashi, H. J. Phys. Chem. B 2004, 108, 3005.
(7)
Conclusions This study clearly demonstrated that the surface fluorination of TiO2 enhances the remote PCO activity without depleting the surface fluorides during irradiation. The surface fluorination seems to facilitate the desorption of oxidants from the illuminated TiO 2 surface. The remote PCO activity was higher with higher surface fluoride concentration, higher UV intensity, and a smaller gap. By measuring and comparing the rates of CO2 generation from the remote PCO of SA in various atmospheres, we concluded that the air-borne oxidants that are primarily responsible for the remote PCO are OH radicals. From a practical point of view, F-TiO2 is certainly better than pure TiO2 when the remote PCO process is utilized. Acknowledgment. This work was supported by KOSEF through the Center for Integrated Molecular Systems (CIMS) and partly by the Brain Korea 21 project. A helpful suggestion from Dr. Soo-Keun Lee is much appreciated. LA048051N
