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Quantitative Detection of OH Radicals for Investigating the Reaction Mechanism of Various VisibleLight TiO Photocatalysts in Aqueous Suspension 2

Jie Zhang, and Yoshio Nosaka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3105166 • Publication Date (Web): 21 Dec 2012 Downloaded from http://pubs.acs.org on December 21, 2012

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The Journal of Physical Chemistry

Quantitative Detection of OH Radicals for Investigating the Reaction Mechanism of Various Visible-Light TiO2 Photocatalysts in Aqueous Suspension

Jie Zhang and Yoshio Nosaka*

.

Nagaoka University of Technology, Department of Materials Science and Technology, 1603-1 Kamitomioka, Nagaoka, 940-2188 Japan.

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ABSTRACT

The reaction mechanism of visible-light responsive photocatalysts was explored by analyzing OH radicals (•OH) quantitatively by means of a coumarin fluorescence probe method. The photocatalysts investigated were various modified TiO2, i.e., nitrogen doped, Pt-complex-deposited, Fe(III)-grafted, and Fe(III)-grafted Ru-doped TiO2. The formation rate of •OH was measured to calculate the •OH quantum yield from the absorbed intensity of 470 nm LED light. The highest quantum yield was obtained for Fe(III)-grafted Ru-doped TiO2. The •OH yield was increased on the addition of H2O2 for the Fe(III)-grafted TiO2, indicating that H2O2 is supposedly a reaction intermediate for producing •OH. The photocatalytic activity for each sample was obtained by measuring CO2 generation rate on the acetaldehyde decomposition in an aqueous suspension system and then it was compared with the •OH formation rate. Although the CO2 generation rate is positively correlated with the •OH formation rate for each photocatalyst, the values of CO2 generation were extremely larger than those of •OH. This finding indicates that the oxidation reaction takes place dominantly with surface trapped holes which probably exchange with the •OH in solution.

KEYWORDS.

fluorescence probe method,

coumarin,

nitrogen dope,

metal ion dope,

H2O2

.

2


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INTRODUCTION Photocatalytic reactions at the surface of TiO2 have been attracting much attention, because they can be applied to waste water treatment and environmental cleaning, which was realized as self-cleaning tiles, glasses, and windows.1-8 Among the many kinds of metal oxide semiconductors, TiO2 is the most promising photocatalyst owing to its high photocatalytic activity, chemical stability, no toxicity and commercial availability.3 However, only UV region in the light can be used for its photocatalytic reactions. Therefore, it is of great interest to make TiO2 visible-light-responsive without decrease of photocatalytic activity, and many kinds of modified TiO2 photocatalysts, such as doped TiO2,9-12 deposited TiO2,13,14 sensitized TiO215-18 and so on, have been developed up to now. In general, it has been reported that the TiO2 photocatalytic reactions proceed mainly by the contributions of active oxygen species, such as hydroxyl radical (•OH), 5-8,19

superoxide radical (•O2- ),20,21 and H2O2.21 Among them, •OH is an extremely

important species, being frequently assigned to the major reactant responsible for the photocatalytic oxidation of organic compounds.19 However, the mechanism of the photocatalytic reactions of modified TiO2 has not been experimentally evidenced so far. Detection of •OH formation may become a direct measure of the photocatalytic activity. But the high reactivity and short lifetime of •OH make the detection to be difficult. A fluorescence probe method, which has been developed in the field of radiation chemistry23 could be applied to the •OH in photocatalytic reactions.21,24-28 In our previous studies, the fluorescence method to detect •OH was compared with ESR spin trapping method for various TiO2 powders under UV irradiation.29 As a result, fluorescence probe method was shown to be a suitable method to detect •OH in solution, while spin trapping method detected different oxidant because the correlation between several kinds of TiO2 was not observed.29 The recent literature with the fluorescence probe method25,26 suggested the mechanism that •OH initiated the oxidation reaction under UV light excitation, because the photocatalytic activity showed good correlation

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with the •OH formation. However, the quantitative comparison has not been performed so far. Therefore, in the present study, we used the fluorescence probe method to quantitatively analyze the amount of •OH and investigated the contribution of •OH in the photocatalytic oxidation over the visible light responsive TiO2 particles. To clarify the difference in the reaction mechanisms of visible light responsive photocatalysts, the effect of H2O2 as the reaction intermediate was examined, besides the UV light irradiation was employed to investigate the dependence on the excitation wavelength. Finally, from the quantitative analysis of the •OH formation, we could conclude that the key species of visible-light photocatalytic activity was not the •OH which can be detected by the fluorescence probe method.

EXPERIMENTAL

Materials. All of the photocatalysts used in the present study were modified TiO2 photocatalysts. Platinum chloride complex-modified TiO2 (PtCl/TiO2) and Fe3+ oxide deposited TiO2 (FeO/TiO2), were prepared according to the literature.18 Nitrogen doped TiO2 (N-TiO2) and Fe(III) deposited TiO2 (Fe(III)/TiO2) were supplied as test samples for a research project by Showa Titanium Co., which were labeled as HP-N08 and HP-FT101, respectively. Fe(III)-deposited Ru-doped TiO2 (Fe(III)/Ru:TiO2) labeled as NPC-T01 was supplied by Mitsui Chemical Co. Ltd. The details in the preparation methods for Fe(III)/TiO2 and Fe(III)/Ru:TiO2 have been described in our previous report.30 As a reference, bare TiO2 sample powder (F1R) was supplied by Showa Titanium Co. These photocatalysts were made from TiO2 powders having mainly rutile phase except for N-TiO2 which was prepared by solution method with ammonia. Coumarin, umbelliferone (7-hydroxy coumarin) (Tokyo Kasei Co.), potassium chloride, potassium iodide, 30% hydrogen peroxide solution (Nacalai Tesque Co.) were used without further purification. Spectral measurements. The photo-absorption of the photocatalyst powders was

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characterized by a UV-Vis-NIR spectrophotometer (Shimadzu, UV-3150) equipped with an integrating sphere assembly (Shimadzu, ISR-3100). Fluorescence spectra were measured with a fluorescence spectrophotometer (Hitachi, Model 850) equipped with a PC recorder. An LED (light emitting diode) lamp emitting visible (470 nm) light (Seoul Semiconductor, B42180) was used as the excitation light source for the photocatalytic oxidation of acetaldehyde as well as the •OH detection with coumarin. For comparison, another LED lamp emitting UV (365 nm) light (NICHIA, NCCU033) was used for the experiments of •OH detection. The light intensities of the 470 nm and 365 nm LEDs through an 8 mm × 8 mm aperture were measured with a power meter (Advantest, TQ8210) to be 30.5 mW and 1.25 mW, respectively. Irradiance spectra of the LED light were measured with a fiber spectrometer (Avantes, AvaSpec-2048) calibrated with a standard light source (Avantes, AvaLight-HAL-CAL). Fluorescence probe method. The formation of •OH was measured by using a fluorescence probe method with coumarin (see, Scheme 1). Although terephthalic acid was used as the probe molecule in our previous reports,21,29 we used coumarin molecule in the present study because it contains no charges in the chemical structure and the product molecule (umbelliferone) is commonly available. Coumarin was already employed by the present authors as the •OH probe in the reaction of polymer electrolyte fuel cells.31, 32 Experimental procedure of the fluorescence probe method was as follows. Photocatalyst powder (15 mg) was suspended in a Pyrex cell (10 mm × 10 mm ×45 mm ) containing 3.5 mL of a 0.1 mM coumarin aqueous solution, and one side of the cell was irradiated with the LED beam through the 8 mm × 8 mm aperture. The suspension was stirred vigorously for 5 min before and during the light irradiation. After the irradiation, 0.5 g of KCl was added into the suspension, and then the suspension was kept in dark to precipitate the powder. After one day holding, the clear solution was taken out and its fluorescence spectrum was measured by the fluorescence spectrophotometer with the excitation wavelength at 332 nm. The fact that KCl did not

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affect the fluorescence intensity of umbelliferone in coumarin solution was confirmed in a separate experiment. In order to convert the fluorescence intensity into the concentration of umbelliferone, the fluorescence spectra of different concentrations of umbelliferone in 0.1 mM coumarin solution were measured by the fluorescence spectrophotometer as shown in Figure 1. The intensity of the spectrum was corrected by that of coumarin at 392 nm. The increase of the fluorescence intensities at 455 nm was proportional to the umbelliferone concentration up to 25 nM as shown in Supporting Information (Figure S1). By using this relationship, the concentration of umbelliferone generated in photocatalysis was calculated based on the fluorescence intensity of the solution. Calculation of •OH concentration. The amount of •OH was calculated from the concentration of umbelliferone based on the following deliberation. By irradiating γ-ray in an aqueous coumarin solution, several coumarin OH substitutes are formed due to the reaction with •OH.22 Among the products, only umbelliferone emits fluorescence and the yield is reported to be 14 nmol/J for 0.1 mM coumarin aqueous solution.22 The yield of •OH in the γ-ray irradiated water is reported to be 2.2 molecules / 100 eV 33 , which corresponds to 228 nmol/J. Based on the comparison between the yields of umbelliferone and •OH, it is calculated that 6.1% of •OH can be detected as umbelliferone by using 0.1 mM coumarin. Experiments with additives. Since H2O2 is a possible reaction intermediate, the effect of H2O2 on the •OH production was investigated. One hour before the irradiation, 50 µL of 10 mM H2O2 solution was added into the 3.5 mL of 0.1 mM coumarin aqueous solution, which corresponds to the H2O2 concentration of 0.14 mM in the test suspension. The suspension was irradiated with the LEDs and the same procedure followed as the evaluation of the •OH concentration. Addition of I- was also examined for investigating the rate constant of the reaction between coumarin and •OH by using a competitive reaction. Into the 3.5 mL of 0.1 mM coumarin aqueous solution containing the Fe(III)/Ru:TiO2 powder, KI of the

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concentrations ranging 5 – 20 nM was added. The irradiation time for the LEDs was fixed to 30 s. CO2 and O2 detection. The photocatalytic activity of the powders was examined for the decomposition of acetaldehyde by monitoring the amount of the final product, CO2. The concentration of CO2 was measured with an ion meter (TOA, IM-32P) with a CO2 electrode (TOA, CE2041). Experimental procedure is as follows. Photocatalyst suspension was prepared by adding photocatalyst powder (40 mg) into 94 mL of purified water in a 100 mL colorimetric bottle. This suspension was sonicated for 10 min and stirred vigorously for 1 hour under ambient condition to attain an equilibrium CO2 condition. Then, 40 µL of acetaldehyde liquid was added into this suspension in dark and the CO2 electrode was immersed into the suspension. The suspension was sealed with a silicon cap so as to occupy fully the bottle without space of air. The irradiation was carried out with the 470 nm LED used in the •OH fluorescence probe experiments. The suspension was stirred vigorously during the irradiation. The formation rate of CO2 was calculated from the increase of the CO2 concentration with the irradiation time up to 60 min. The measurement of dissolved oxygen was performed in a procedure similar to that of CO2.34 TiO2 powder (20 mg) was suspended with 47 mL of water in colorimetric bottle and 20 µL acetaldehyde was added. An oxygen meter (Mettler Toledo, MO128) was immersed into the suspension. The suspension was sealed with a silicon cap so as to fully occupy the bottle without space of air.

RESULTS Photons absorbed by photocatalysts. Diffuse reflectance spectra for the various modified TiO2 were measured. Figure 2 shows the absorption spectra as the complement of the reflectance (1-R) and the irradiance spectra of LED used in the present study. As shown in Figure 2(e), N-doped TiO2 (N-TiO2) has largest absorption in the visible light region. Fe(III)-deposited TiO2 (c) showed small absorption around 470 nm comparable

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to that of the bare TiO2 (d). This absorption is attributed to the interfacial charge transfer (IFCT)14 which means the electron transfer from the valence band of TiO2 to the Fe(III) grafted at the surface of TiO2. The rutile TiO2 (d) shows very small absorption in this wavelength region. The amount of light power absorbed in the photocatalysts was calculated by integrating the irradiance spectra of LED (Figures 2(g) and 2(h)) over the wavelength after multiplying the each absorption spectrum. The absorbed light intensities thus calculated for the 470 nm and 365 nm irradiations were listed in Tables 1 and 2, respectively.

Quantum yield of •OH. Figure 3 shows the amount of umbelliferone produced by the visible light (470 nm) irradiation on modified TiO2 photocatalysts as a function of the irradiation time. All experiments showed almost linear increase in the plots. The slope of the line was divided by 0.061 to calculate the rate of •OH generation and the values obtained for each photocatalyst were listed in Table 1. The effect of H2O2 on the formation of umbelliferone for various photocatalysts was shown in Figure 4 and the rates of •OH generation were also listed in Table 1. The quantum yields of •OH were calculated from the rate of •OH generation divided by the photo excitation rate that was calculated from the light power absorbed in 3.5 mL solution. The obtained •OH quantum yields on visible light (470 nm) irradiation without and with H2O2 were shown in Table 1. It should be mentioned that the bare TiO2 showed a very high quantum yield under the irradiation of 470 nm. Although the generation rate of •OH radical was very small, the absorbed light power was also very small. Then the quantum yield became similar value to that on the UV irradiation as described below. Since OH formation process with H2O2 reduction was added for Ru/Fe-TiO2, the quantum yield of OH was not larger than that of bare TiO2. This observation indicates that doping of Ru ions and deposition of Fe ions made Ru/Fe-TiO2 have higher recombination than bare TiO2. Accordingly, for Ru/Fe-TiO2 the yield of effective valence band holes to become

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trapped holes significantly decreased. In the similar way, the quantum yield of •OH generation on the UV light (365 nm) irradiation was calculated from the measurements of umbelliferone formation and the obtained value was listed in Table 2. The low yield for PtCl/TiO2 may be caused by the decay of •OH due to the reaction with PtCl complex deposited on TiO2 surface. The fact that the •OH yield for Fe(III)/TiO2 is larger than that for bare TiO2 is likely explained by that he UV-light induced conduction-band electrons are effectively trapped by Fe3+ ions. In contrast, the •OH yield for N-TiO2 and FeO/TiO2 was decreased, indicating that the recombination of UV-light induced electron-hole pairs was increased by the doping of N and Fe ions, respectively. In the presence of H2O2, there were almost no changes in the •OH quantum yields for these modified TiO2, while in the case of the visible light irradiation the •OH yield was increased as shown in Table 1. Therefore, in the case of UV light excitation, H2O2 is not involved in the formation mechanism of •OH. This observation in the suspension system is different from that in gas phase,35 where OH radicals were detected by laser induced fluorescence method and found to be formed by the reduction of O2 via H2O2.

Rate of the reaction between •OH and coumarin. In order to investigate the reaction between •OH and coumarin (eq (1)), the competitive reaction was examine for the reaction between •OH and I- (eq (2)).

•OH

+

•OH

+

coumarin I-

-(ku) -(kI)

umbelliferone HIO

(1) (2)

Based on the set of competitive reactions in homogeneous solution, the rate of umbelliferone formation, ru , in the presence of I- is given by the following equation.

ru

=

ru0× ku [coumarin]/( ku [coumarin]+ kI [I-])

(3)

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where ru0 represents the ru in the absence of I-, and [coumarin] and [I-] are the concentrations of coumarin and I- in solution. Since the concentration of umbelliferone as the product increased linearly with the irradiation time even in the presence of I-, the concentration at a given time of the irradiation should be proportional to the reaction rate ru. Therefore, following equation is obtained

1/[umbelliferone] = 1/[umbelliferone]0 + (kI /ku[coumarin] [umbelliferone]0)×[I-]

(4)

Where [umbelliferone]0 represents the concentration of umbelliferone produced in the absence of I- at the given irradiation time. Therefore, the plot of 1/[umbelliferone] against [I-] should have a linear line and the ratio of kI/ku can be obtained from the slope and intersection of the plot. For the both 470 nm and 365 nm irradiations on Fe(III)/Ru:TiO2, experimental data were plotted in Figure 5. Figure 5 shows that the data lay on each line, indicating that the relationship of eq (4) is held. Therefore, I- ions must locate in homogeneous state, because if I- is adsorbed on the TiO2 surface, the linear correlation based on the reactions (1) and (2) cannot be obtained. The slope of the line for the 470 nm irradiation shown in Figure 5 was 40 nM-1mM-1 with the intersection of 0.55 nM-1. By adopting the values of [coumarin] = 0.1 mM and kI = 1.1×1010 M-1s-1,36,37 the value of ku was calculated to be ku = 1.5×109 M-1s-1 . The value of ku , which is the rate constant of the umbelliferone formation, is not available in literature, but the reaction rate constant of •OH with coumarin, kc, has been reported36, 38 to be 2×109 M-1s-1. Since the reaction of •OH with coumarin forms 3OH-, 4OH-, 5OH-, 6OH- and 8OH-coumarin in addition to 7OH-coumarin (umbelliferone), ku can be estimated from the total rate constant, kc , by multiplying the yield ratio of the 7OH derivative, 16/56, deduced from the literature.22 Thus, ku = 5.7 ×108 M-1s-1 in

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homogeneous solution could be estimated. The value of ku obtained in the present photocatalytic reaction was 1.5×109 M-1s-1 , which was 2.6 times larger than that in aqueous solution. In the case of the UV light (365 nm) irradiation, the value of ku obtained from the plot shown in Figure 5 was ku =2.9×109 M-1s-1, which was five times larger than that expected in homogeneous solution. The increases in the rate constant for both cases may indicate that the effective coumarin concentration to detect •OH is somewhat larger than that in homogeneous solution. Because •OH was produced by photocatalytic reaction, this observation suggests that the •OH detected by coumarin locates near the photocatalyst surface than in solution bulk.

Photocatalytic activity.

Decomposition of acetaldehyde was used to evaluate

the photocatalytic activity on 470 nm excitation in the present study. The overall decomposition reaction can be represented by eq (5)

CH3CHO + 5/2O2

2CO2 + 2 H2O

(5)

The concentration of generated CO2 as the product of the photocatalytic decomposition is shown in Figure 6. If the oxidation proceeds stepwise via CH3COOH, HCHO, and HCOOH, four photons may be used to complete eq (5). Namely, two photons are used to produce one CO2 molecule. In Figure 6, the line having the largest slope (k=0.39 ppm/min) indicates the maximum CO2 generation rate calculated for Fe(III)/Ru:TiO2 by assuming that all of the absorbed photons were used to decompose acetaldehyde. Since the observed slope was 0.3 ppm/min, the quantum yield of photocatalytic decomposition for this particular photocatalyst was calculated to be 77%. This value is larger than that (22%) reported for the decomposition of 2-propanol with Fe(III)-grafted TiO2 of the highest activity.14 Besides the formation of CO2, the consumption of oxygen was measured in the identical reaction condition. Figure 7 shows the time profile of dissolved oxygen during

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the oxidation of AA. The consumption rate of the dissolved oxygen was listed in Table 1. It was very fast for Fe-modified TiO2 (FeO/TiO2, Fe(III)/TiO2, and Fe(III)/Ru:TiO2) indicating that O2 is consumed by the reaction with Fe ions, as will be discussed later.

DISCUSSION Excitation properties of various modified TiO2. Figure 8 shows the suggested energy levels for several kinds of TiO2 photocatalysts modified for visible light response. Modification for visible light response may be classified into four types; (a) photosensitization type, (b) interfacial charge transfer (IFCT) type, (c) cation doping type, and (d) anion doping type. In Figure 8, on the UV-excitation for photocatalysts (b) and (c) the reduction process at the conduction band takes place to a certain extent similarly to the case of bare TiO2 (e). However, this process was not shown in the figure to avoid the complexity to compare them with photosensitization type (a). Pt-chloride-complex modified TiO2 (PtCl/TiO2) is classified to the photosensitization type as shown in Figure 8a, in which visible light is absorbed by the deposited chromophore (PtCl) and the oxidized PtCl plays a role of oxidant for reactant.17,18 However, our recent ESR study at 77 K showed the formation of trapped holes, suggesting the presence of the excitation of electrons in the TiO2 valence band to the oxidized PtCl.39 Fe(III)-grafted TiO2 (Fe(III)/TiO2) absorbs visible light by IFCT from the TiO2valence band to the grafted metal ions (Figure 8b).14 Although the photo absorption of IFCT is small, the direct excitation causes an effective electron transfer to the surface metal ions and then recombination of the separated charges is expected to be minor. Fe3+ compound deposited TiO2 (FeO/TiO2) may have both character of the photosensitization type (a) and the IFCT type (b). Doping of metal ions such as Ru4+ forms an acceptor level to which valence band electrons can be excited with photo absorption (Figure 8c). Since one electron reduction of O2 into •O2- by the doped level is thermodynamically difficult, some cocatalyst such as Fe3+ to promote two electron reduction may be indispensable.14,30 Nitrogen doped TiO2 (N-TiO2) is a representative

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TiO2 photocatalyst modified for visible light response, which is classified to the anion doping type (Figure 8d), and the photoexcited electrons have the potential to reduce O2 directly into •O2-.40 The consumption of dissolved oxygen was very fast for Fe(III)-containing photocatalysts as shown in Figure 6 and listed in Table 1. This result is accordant with the reaction mechanism shown in Figure 8. The electrons in the valence band were excited to the Fe3+ level by the IFCT absorption, and the reduced Fe2+ plays an important role in the oxygen reduction. As a result, in these reactions, the oxygen was not only used in the decomposition of AA, but also used to accept the electrons from the reduced Fe3+. On UV-light excitation the absorption mainly takes place by inter band excitation of TiO2 even in the case of Fe(III)-grafted Ru-doped TiO2.. Therefore, the electrons in the valance band of TiO2 would be excited to the conduction band of TiO2, and then they would transfer to the Fe3+ level to make reduction reactions. At the same time the oxidize reaction would occur at the valance band of TiO2. On the other hand, when Fe(III)/Ru:TiO2 is irradiated by visible light, the absorption mainly takes place between valence band and doped Ru level. The presence of the absorption (see, Figure 2f) at the longer wavelength (>600 nm) suggests the electron excitation from Ru dope level to the conduction band of TiO2 and then transfer to the Fe3+ level. The electrons in the valance band of TiO2 are excited to the Ru dope level as well. In other words, two step excitation of valence band electrons to the conduction band takes place as will be discussed below.

The formation process of •OH by visible light. In general, the photocatalytic formation of •OH under UV light irradiation has been represented by the oxidation of water by photo-induced valence band holes (h+).

H2 O

+ h+

•OH

+

H+

(6)

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As shown in Table 2, for bare TiO2 under the UV irradiation, the quantum yield obtained in the present study was as small as 0.36 %. It is suggested that in the oxidation process of water on rutile surface the main intermediate is adsorbed H2O2 (eq (7)). 41

2H2O

+ 2 h+

H2O2(ad)

+

2H+

(7)

Since the addition of H2O2 caused no significant change in the •OH formation under the UV irradiation (see, Table 2), •OH was not formed by the reduction of H2O2 with photoinduced conduction band electrons as stated above. Therefore, the formation process of •OH under UV irradiation is apparently described by eq (6). As shown in Table 1, in the case of visible light irradiation, the addition of H2O2 increased the •OH yield, except for N-TiO2. This increase indicates that •OH was produced via the reduction of H2O2 on the visible-light irradiation. For N-TiO2, the fact that the yield of •OH was not changed by the addition of H2O2 indicates that the oxidation of water (eq (6)) by the N-doped level is only the process of •OH formation, Although the quantum yield was extremely small to be 0.0013%. The observation37 of the stable formation of H2O2 by the reduction of O2 for N-TiO2 supports the inertness of this photocatalyst in the reduction of H2O2. A large quantum yield of •OH for the bare TiO2 indicates that the small absorption of visible light is caused by the interband excitation. Namely, for bare TiO2, the visible-light absorption is small but it excites electrons in the valence band to the conduction band. For PtCl/TiO2, a significant increase in the •OH yield by the addition of H2O2 indicates the •OH formation from H2O2. Since the reduction of Pt4+ to Pt3+ was observed on the visible-light irradiation,39 •OH may be produced by the reduction of H2O2 with the Pt3+ species. For other visible light responsive photocatalysts, owing to the contained Fe3+ ions, the reduction of H2O2 takes place by reduced metal ions, Fe2+, as follows.

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H2 O2

+

Fe2+

•OH

+

OH-

+

Fe3+

(8)

This reaction is commonly known as Fenton reaction42 for producing •OH radicals. Therefore, Fe(III)-grafted TiO2 photocatalysts, Fe(III)/TiO2, showed a higher quantum yield of •OH under visible light irradiation, because the electrons excited by the IFCT absorption directly reduce Fe3+ into Fe2+, as shown in Figure 8b.13,30 Furthermore, in the case of metal doped TiO2, Fe(III)/Ru:TiO2 (see, Figure 8c), electrons in doped level reduces the grafted Fe(III) as experimentally shown in our previous report.30 Since such electron transfers to the Fe(III) ions occurred in addition to the direct IFCT excitation, the higher yield of •OH was obtained for Fe(III)/Ru:TiO2 than Fe(III)/TiO2 as shown in Table 1

Relationship between •OH formation and photocatalytic activity. In the present study, photocatalytic activity was measured by the amount of CO2 generated by the decomposition of acetaldehyde (AA) on 470-nm light irradiation. The relationship between the generation rates of CO2 and •OH for the modified TiO2 was shown in Figure 9. The generation rate of CO2 increased with the increase of the •OH formation for each photocatalyst except for (a) PtCl/TiO2, indicating a certain correlation between photocatalytic activity and the •OH formation. Although the correlation has been reported for the TiO2 photocatalyst under UV light irradiation so far,25,26 in the present research the correlation was shown for visible-light responsive TiO2 photocatalysts for the first time. The correlation in Figure 9 seems to show that the photocatalytic decomposition takes place with the generated •OH, similarly to the cases reported for UV photocatalysts. However, the rate of CO2 generation was significantly larger (about 103 times) than that of the •OH formation as realized in the scales of Figure 9. This observation clearly shows that AA was not oxidized via the •OH generated in solution. The CO2 formation rate is quite small against the OH radical formation rate for the

15



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Page 16 of 36

sample a, e and b as shown in Fig.9. It seems to have a threshold for the CO2 formation. Since CO2 is the final product of acetaldehyde decomposition after some steps of decomposition process which is caused by the trapped holes, some threshold of the amount of trapped holes for the CO2 formation may exist. Then, the amount of OH radicals, which are probably in rapid equilibration with the trapped holes, shows also threshold to produce CO2 in Figure 9. As for the photocatalytic decomposition mechanism of AA, the abstraction of aldehyde proton to form CH3CO• radical and then the addition of O2 to the radical has been suggested based on the ESR observation of CH3CO3• radical as the unstable reaction intermediate.43 Thus, the surface oxidation of AA may be the rate determining step for the decomposition. Therefore, Figure 9 indicates that some correlation exists between the ability of surface oxidation and the amount of •OH in solution. However, as evidenced experimentally in the present study, •OH was formed from the reduction of H2O2 with the assist of Fe2+. Therefore, only reliable explanation of the excellent relationship between the photocatalytic activity and the •OH formation is that the generated •OH was trapped on TiO2 surface, as have been suggested by pulse radiolysis experiments.44 In the report, the •OH produced by electron-beam irradiation in TiO2 suspension was found to become surface trapped holes.44

•OH

•OH



+ Ti4+

•O-



+

H+

(pKa=11.9)

Ti4+O•-

+

H+

(9)

(∆G ≈-39kJ/mol)

(10)

Although the proton dissociation constant of •OH (eq (9)) in homogeneous solution is reported to be pKa=11.9,45 in the state adsorbed on TiO2 , the pKa of •OH shift to pKa = 2.8 because charge density shifts to the lattice oxygen.44 In other words, at neutral pH, •OH is adsorbed dissociatively in the form of trapped hole as shown by eq (10). The trapped holes could not react with coumarin to produce umbelliferone but with organic

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reactant (RH in Figure 8) such as AA. The redox potential of the trapped holes was experimentally estimated to be ca. 1.5 V (V vs. NHE).44 Since the redox potential of •OH is 1.90 V,46 adsorption of •OH on TiO2 in aqueous solution (eq (10)) is an exoenergetic process by about 0.4 eV or 39 kJ/mol. Thus, the •OH in solution becomes the surface trapped hole by the adsorption, because proton is released owing to the shift of the pKa associated with the adsorption. This consideration is supported by the report of an ESR study where the signal for the surface trapped holes of hydrated TiO2 did not show the feature of the ESR signal of •OH.47 Therefore, we could conclude that •OH in the bulk solution is equilibrated with trapped holes (eq (10)), but the equilibrium was significantly shifted to the surface trapped holes. Thus, due to taking the equilibrium between trapped holes at the surface and free •OH in solution, the excellent correlation between photocatalytic reactivity and the formation rate of •OH in Figure 9 could be explained.

CONCLUSIONS

The •OH formation on various kinds of TiO2 modified for visible light response was analyzed quantitatively in aqueous suspension by using the coumarin fluorescence probe method. Since •OH was increased in the presence of H2O2 for the Fe(III)-grafted TiO2, H2O2 seems to be a reaction intermediate to produce •OH. The photocatalytic activity was estimated by the rate of CO2 generation associated with acetaldehyde decomposition in the same reaction system and then it was compared with the •OH formation rate. Although the CO2 generation rates of the photocatalysts were positively correlated with those of the •OH formation, the formation rates of CO2 were extremely larger (103 times) than those of •OH for each photocatalyst. This experimental result suggests that •OH in the bulk solution is equilibrated with trapped holes (eq (10)), but the equilibrium is significantly shifted to the surface trapped holes. This finding indicates that the oxidation reaction dominantly takes place at the photocatalyst surface

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Page 18 of 36

with the trapped holes. The highest photocatalytic activity in the suspension system was obtained for Fe(III)-deposited Ru-doped TiO2 whose reaction mechanism was shown in Figure 10. Based on the ESR and chemiluminescence experiments in our previous study,30 conduction band electrons are formed by two step excitation with visible light irradiation and O2 is reduced to H2O2. The grafted Fe3+ is reduced by Ru3+ or by IFCT and then the formed Fe2+ produces •OH from H2O2. In the present study, we concluded that the •OH produced in solution is adsorbed on the TiO2 surface to form trapped holes which could oxidize organic compounds, such as acetaldehyde, leading to CO2.

ACKNOWLEDGMENTS The authors thank Mr. Hiroshi Suizu and Dr. Hideyuki Nagai of Mitsui Chemical Co. Ltd., Dr. Yasuhiro Hosogi and Dr. Yasushi Kuroda of Showa Titanium Co., Ltd., for supplying the samples. and Dr. Junya Sato of Ishihara Sangyo Co. Ltd for suggestions of sample preparation. This work was performed under the management of the Project to Create Photocatalyst Industry for Recycling-Oriented Society, supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

Supporting Information Available

The relationship between fluorescence intensity and the concentration of unbelliferone in the presece of 0.1 mM coumarine has been shown in Figure S1 in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author Yoshio Nosaka e-mail; [email protected] Phone/Fax; +81-258-47-9315

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References (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69-96. (2) Kaneko, M.; Ohkura, I., Eds. Photocatalysis; Kodansha-Springer: Tokyo, 2002. (3) Fujishima, A; Zhang, X; Tryk, D. Surf. Sci. Rep. 2008, 63, 515-582. (4) Henderson, M. A. Surf. Sci. Rep. 2011, 66, 185-297. (5) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33−177. (6) Ryu, J.; Choi, W. Environ. Sci. Technol. 2008, 42, 294−300. (7) Herrmann, J.- M. J. Photochem. Photobiol. A 2010, 216, 85−93. (8) Teoh, W.Y.; Scott, J. A.; Amal, R. J. Phys. Chem. Lett. 2012, 3, 629−639. (9) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269-271. (10) Nosaka, Y.; Matsushita, M.; Nishino, J.; Nosaka, A. Y. Sci. Tech. Adv. Mater. 2005, 6, 143-148. (11) Rengifo-Herrera, J. A.; Pierzchała, K.; Sienkiewicz, A.; Forro, L.; Kiwi, J.; Moser, J. E.; Pulgarin, C. J. Phys. Chem. C 2010, 114, 2717–2723. (12) Choi, J.; Park, H.; Hoffmann, M. R. J. Phys. Chem. C 2010, 114, 783-792. (13) Irie, H.; Kamiya, K.; Shibanuma, T.; Miura, S.; Tryk, D. A.; Yokoyama, T.; Hashimoto, K. J. Phys. Chem. C 2009, 113, 10761-10766. (14) Yu, H.; Irie, H.; Shimodaira, Y.; Hosogi, Y.; Kuroda, Y.; Miyauchi, M.; Hashimoto, K. J. Phys. Chem. C 2010, 114, 16481-16487. (15) Vinodgopal, K.; Wynkoop, D. E.; Kamat, P. V. Environ. Sci. Technol. 1996, 30, 1660-1666. (16) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1998, 102, 5845- 5851. (17) Burgeth, G.; Kisch, H. Coord. Chem. Rev. 2002, 230, 41-47. (18) Ishibai, Y.; Sato, J.; Akita, S.; Nishikawa, T.; Miyagishi, S. J. Photochem. Photobiol.

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A 2007, 188, 106–111; Patent, JP WO2007/125998 A1. (19) Turchi, C. S.; Ollis, D. F. J. Cat. 1990, 122, 178-192. (20) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261-5267. (21) Hirakawa, T.; Nosaka, Y. Langmuir 2002, 18, 3247-3254. (22) Louit, G.; Foley, S.; Cabillac, J.; Coffigny, H.; Taran, F.; Valleix, A.; Renault, J. P.; Pin, S. Rad. Phys. Chem. 2005, 72, 119-124. (23) Ashawa, S. C.; Kini, U. R.; Madhvanath, U. Int. J. Appl. Radiat. Isotope, 1979, 30, 7-10. (24) Ishibashi, K.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Electrochem. Commun. 2000, 2, 207-210. (25) Guan, H.; Zhu, L.; Zhou, H.; Tang, H. Anal. Chim. Acta, 2008, 608, 73-78. (26) Xiang, Q; Yu, J.; Wong, P. K. J. Colloid Interf. Sci. 2011, 357 ,163-167. (27) Czili, H.; Horvath, A. Appl. Catal. B 2008, 81, 295-302. (28) Yu, J.; Qi, L,; Jaroniec, M. J. Phys. Chem. 2010, 114, 13118-13125. (29) (a) Hirakawa, T.; Yawata, K.; Nosaka, Y. Appl. Catal., A. 2007, 325, 105–111; (b) Nosaka, Y.; Komori, S.; Yawata, K.; Hirakawa, T.; Nosaka, A. Y. Phys. Chem.Chem. Phys. 2003, 5, 4731- 4735. (30) Nishikawa, M.; Mitani, Y.; Nosaka, Y. J. Phys. Chem. C 2012, 116, 14900-14907. (31) Nosaka, Y.; Ohtaka, K.; Kitazawa, M.; Kishioka, S.; Nosaka, A. Y. Electrochem. Solid-State Lett. 2009, 12, B14- B17. (32) Ohguri, N.; Nosaka, A. Y.; Nosaka, Y. Electrochem. Solid-State Lett. 2009, 12, B94- B96. (33) Dainton, F. S., Watt, W. S. Nature, 1962, 195, 1294-1296. (34) Hirakawa, T.; Daimon, T.; Kitazawa, M.; Ohguri, N.; Koga, C.; Negishi, N.; Matsuzawa, S.; Nosaka, Y. J. Photochem. Photobiol. A. Chem. 2007, 190, 58-68. (35) Murakami, Y.; Endo, K.; Ohta, I.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem, C 2007, 111, 11339-11346. (36) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref.

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Data, 1988, 17, 513-886. (37) Jayson, G. G.; Parsons, B. J.; Swallow, A. J. J. Chem. Soc. Faraday Trans. 1 1973, 69, 1597-1607. (38) Gopakumar, K.; Kini, U. R.; Ashawa, S. C.; Bhandari, N. S.; Krishnan, G. U.; Krishnan, D. Radiat. Effect, 1977, 32, 199-203. (39) Nishikawa, M.; Sakamoto, H.; Nosaka, Y. J. Phys. Chem. A 2012, 116, 9674-9679. (40) Hirakawa, T.; Nosaka, Y. J. Phys. Chem. C 2008, 112, 15818-15823. (41) Imanishi, A.; Okamura, T.; Ohashi, N.; Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2007, 129, 11569-11578. (42) Walling, C. Acc. Chem. Res. 1975, 8, 125-131. (43) Jenkins, C. A.; Murphy, D. M. J. Phys. Chem. B 1999, 103, 1019-1026. (44) Lawless, D.; Serpone, N.; Meisel. D, J. Phys. Chem. 1991, 95, 5166-5170. (45) Bard, A. J., Parsons, R., Jordan, J., Eds. Standard Potentials in Aqueous Solution; Marcel Dekker: New York, 1985. (46) Wardam, P. J. Phys. Chem. Ref. Data, 1988, 18, 1637-1755. (47) Micic, O. I.; Zhang, Y.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C. J. Phys. Chem. 1993, 97, 1211-1283.

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Figure captions. Figure 1. Fluorescence spectra of 0-25 nM umbelliferone in 0.1 mM coumarin solution. Excitation wavelength was 332 nm. Figure 2. Absorption spectra of various modified TiO2 photocatalysts. R; reflectance. Figure 3. The concentration of umbelliferone generated under 470 nm irradiation in 3.5 mL aqueous suspension of various modified TiO2 photocatalysts (15 mg) containing 0.1 mM coumarin was plotted as a function of the irradiation time. Figure 4. The concentration of umbelliferone generated under 470-nm irradiation in the presence of 0.14 mM H2O2. The other experimental conditions were the same as Figure 3. Figure 5. The relationship between 1/[umbelliferone] and [I-]. Fe(III)/Ru:TiO2 was irradiated by 470 nm and 365 nm LED for 30s. The other experimental conditions were the same as Figure 3 Figure 6. The concentration of CO2 generated by the photocatalytic decomposition of acetaldehyde with modified TiO2 was plotted as a function of the irradiation time of 470-nm LED. Figure 7. The concentration of dissolved oxygen in the acetaldehyde solution suspended with modified TiO2 was plotted as a function of the irradiation time of 470 nm LED Figure 8. Proposed mechanism for various visible-light-responsive TiO2 photocatalysts; (a) PtCl/TiO2, (b) Fe(III)/TiO2, (c) Fe(III)/Ru:TiO2, (d) N-TiO2, and (e) bare TiO2. Figure 9. Relationship between the formation rates of CO2 and OH radicals under the irradiation of 470 nm LED. The CO2 formation rate is a measure of the photocatalytic reaction rate in the acetaldehyde decomposition in aqueous suspension system. (a) PtCl/TiO2, (b) Fe(III)/TiO2, (c) Fe(III)/Ru:TiO2, (d) N-TiO2, (e) bare TiO2. and (f) FeO/TiO2 Figure 10 Schematic illustration of reaction mechanism of Fe(III)-grafted Ru-doped TiO2 photocatalyst based on the detection of OH radical and CO2. IFCT; interfacial charge transfer, h+tr; surface trapped hole.

22


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. The •OH quantum yield and photocatalytic activity for various modified TiO2 photocatalysts in aqueous suspension under the irradiation of 470-nm LED.

photocatalysts

rate of ·OH Absorbed generation light (nM/s) Without With (mW) H2O2 H2O2

Without H2O2

With H2O2

Rate of CO2 generation from AA (ppm/min)

Quantum yield of ·OH (x10-2 %)

Rate of DO decrease with AA (ppm/min)

PtCl/TiO2

2.43

0.04

0.86

0.16

3.19

0.002

0.037

FeO/TiO2

2.73

0.19

0.71

0.72

2.40

0.016

0.20

Fe(III)/TiO2

0.25

0.11

0.40

4.1

14.1

0.003

0.11

Fe(III)/Ru:TiO2

3.50

1.42

4.5

3.6

21.9

0.296

1.23

N-TiO2

8.84

0.14

0.04

0.14

0.04

0.008

0.016

Bare TiO2

0.03

0.09

0.23

27

67.9

0.001

0.023

N-TiO2; nitrogen doped TiO2. PtCl/TiO2; platinum chloride-complex deposited TiO2. FeO/TiO2; iron compound deposited TiO2. Fe(III)/TiO2; Fe(III) grafted TiO2. Fe(III)/Ru:TiO; Fe(III)-grafted Ru-doped TiO2. AA; acetaldehyde. DO; dissolved oxygen.

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Page 24 of 36

Table 2. The •OH generation rates and quantum yields for various modified TiO2 photocatalysts under the irradiation of 365 nm LED. Rate of ·OH generation (nM/s) Without With H2 O2 H2O2

Quantum yield of ·OH (x10-2 %) Without With H2O2 H2O2

photocatalysts

Absorbed light (mW)

PtCl/TiO2

0.72

0.31

0.28

4.9

4.6

FeO/TiO2

0.75

1.70

1.63

26

25

Fe(III)/TiO2

0.69

4.27

3.73

71

62

Fe(III)/Ru:TiO2

0.67

2.78

2.16

48

37

N-TiO2

0.79

1.69

1.52

25

22

Bare TiO2

0.64

1.98

1.96

36

35

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Scheme 1 Reaction for detecting OH radicals in solution with coumarin.

O

O

+ ・OH

Coumarin

O

HO

O

+ Others (6.1%) Umbelliferone

25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fluorescence intensity / a.u.


Page 26 of 36

400 350

25 nM

300

10 nM

250

5 nM

200 150

0 nM

100 50 0 350

400

450

500

550

600

wavelength / nm

Figure 1. Fluorescence spectra of 0-25 nM umbelliferone in 0.1 mM coumarin solution. Excitation wavelength was 332 nm.

26


Page 27 of 36

100

2.0

60

g e

40

PtCl/TiO2 FeO/TiO2

1.6

Fe(Ⅲ)/TiO2 Bare TiO2 N-TiO2

1.2

Fe(Ⅲ)/Ru:TiO2 470 LED 365 LED

0.8

b a

20

f

h

c,d

0 350

0.4

Irradiance （mW/nm)

a b c d e f g h

80

(1-R) %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

450

0.0 500

550

600

wavelength / nm

650

700

750

800

Figure 2. Absorption spectra of various modified TiO2 photocatalysts. R; reflectance.

27



7 6

[umbelliferone] / nM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

Fe(Ⅲ)/Ru:TiO2

5

FeO/TiO2 N-TiO2

4

Fe(Ⅲ)/TiO2 PtCl/TiO2

3

Bare TiO2

2 1 0 0

20

40

60

80

100

120

irradiation time / s Figure 3. The concentration of umbelliferone generated under 470 nm irradiation in 3.5 mL aqueous suspension of various modified TiO2 photocatalysts (15 mg) containing 0.1 mM coumarin was plotted as a function of the irradiation time.

28


Page 29 of 36

20 Fe(Ⅲ)/Ru:TiO2 PtCl/TiO2

[umbelliferone ] / nM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

FeO/TiO2 Fe(Ⅲ)/TiO2 Bare TiO2

10

N-TiO2

5

0 0

20

40

60

irradiation time / s

80

100

120

Figure 4. The concentration of umbelliferone generated under 470-nm irradiation in the presence of 0.14 mM H2O2. The other experimental conditions were the same as Figure 3.

29



2

1/[umbelliferone] / nM-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

1.5

1

470 nm 365 nm

0.5

0 0

0.005

0.01

[I-]

0.015

0.02

0.025

/ mM

Figure 5. The relationship between 1/[umbelliferone] and [I-]. Fe(III)/Ru:TiO2 was irradiated by 470 nm and 365 nm LED for 30s. The other experimental conditions were the same as Figure 3

30


Page 31 of 36

7

generated CO2 / ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe(Ⅲ)/Ru:TiO2

6

FeO/TiO2

5

N-TiO2

4

Fe(Ⅲ)/TiO2 ◆ PtCl/TiO2

3

Bare TiO2

2 1 0

0

10

20

30

40

50

60

irradiation time / min

Figure 6. The concentration of CO2 generated by the photocatalytic decomposition of acetaldehyde with modified TiO2 was plotted as a function of the irradiation time of 470-nm LED.

31



8 7

dissoived oxygen / ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

6

N-TiO2

5

Bare TiO2

4

PtCl/TiO2

3

Fe(Ⅲ)/TiO2 FeO/TiO2

2

Fe(Ⅲ)/Ru:TiO2

1 0

0

20

40

60

80

irradiation time / min

Figure 7. The concentration of dissolved oxygen in the acetaldehyde solution suspended with modified TiO2 was plotted as a function of the irradiation time of 470 nm LED

32


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Figure 8. Proposed mechanism for various visible-light-responsive TiO2 photocatalysts; (a) PtCl/TiO2, (b) Fe(III)/TiO2, (c) Fe(III)/Ru:TiO2, (d) N-TiO2, and (e) bare TiO2.

33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CO2 formation rate (10-9 mol/s)


Page 34 of 36

12 c

10 8 1.2 0.8 f

0.4 a

0

0

e

0.2

d b

0.4

0.6

4

5

6

OH radical formation rate (10-12 mol/s) Figure 9. Relationship between the formation rates of CO2 and OH radicals under the irradiation of 470 nm LED. The CO2 formation rate is a measure of the photocatalytic reaction rate in the acetaldehyde decomposition in aqueous suspension system. (a) PtCl/TiO2, (b) Fe(III)/TiO2, (c) Fe(III)/Ru:TiO2, (d) N-TiO2, (e)bare TiO2. and (f) FeO/TiO2

34


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O2

CB

e-

Ru４+/3+

H2O2

Fe３+/Fe2＋

・OH h+ｔｒ h+ h+ VB

CO2 CH3CHO

Fe(Ⅲ)/Ru:TiO2 Figure 10. Schematic illustration of reaction mechanism of Fe(III)-grafted Ru-doped TiO2 photocatalyst based on the detection of OH radical and CO2. IFCT; interfacial charge transfer, h+tr; surface trapped hole.

35



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Page 36 of 36

TOC Graphical abstract

O2

CB

e-

Ru４+/3+

H2O2

Fe３+/Fe2＋

・OH h+ｔｒ h+ h+ VB

CO2 CH3CHO

Fe(Ⅲ)/Ru:TiO2

36


Quantitative Detection of OH Radicals for ... - ACS Publications

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