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Photocatalytic Decomposition of H2O2 on Different TiO2 Surfaces Along with the Concurrent Generation of HO2 Radicals Monitored Using Cavity Ring Down Spectroscopy Jaeseon Yi,† Chiheb Bahrini,‡ Coralie Schoemaecker,‡ Christa Fittschen,*,‡ and Wonyong Choi*,† †

School of Environmental Science & Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea ‡ Physico-Chimie des Processus de Combustion et de l’Atmosphère (PC2A), CNRS UMR 8522, Université Lille 1 Sciences et Technologies, F- 59655 Villeneuve d’Ascq Cedex, France ABSTRACT: Hydrogen peroxide (H2O2) is an important reactive oxygen species (ROS) involved in photocatalysis. To study the photocatalytic behavior of H2O2, the decomposition of H2O2 on illuminated TiO2 films was investigated using cavity ring down spectroscopy (CRDS). A mixture of H2O2 and O2 gas was flowed through a cavity reactor which contained a TiO2-coated plate. The removal of H2O2 and the accompanying production of HO2 radicals were monitored in the gas phase just above the TiO2 film which was irradiated by a UV light-emitting diode (LED) (375 nm). The TiO2 films tested in this study were mainly Degussa P25 TiO2 (DP), Aldrich anatase (AA), and Aldrich rutile (AR). The photocatalytic production of HO2 was observed only in the presence of H2O2, which indicates that the HO2 radicals were generated from the decomposition of H2O2, not from the photocatalytic reduction of O2. The direct photolysis of H2O2 in the absence of TiO2 was not observed at all under the present irradiation conditions. The degradation of H2O2 and the accompanying production of HO2 was not retarded at all in the absence of O2 (a common electron acceptor), which implies that H2O2 itself should serve as an electron acceptor. Although the HO2 radicals were originated from the decomposition of H2O2, the removal of H2O2 and the production of HO2 were not correlated. H2O2 could be rapidly degraded on illuminated DP with little production of HO2, whereas H2O2 was photodegraded much more slowly over AA and AR but with a marked production of HO2. On illuminated DP, the in situ generated HO2 radicals seem to be rapidly degraded with little chance of desorption into the gas phase, while those on AA and AR are long-lived enough that some desorb into the gas phase. This implies that the fate of HO2 radicals, which are universally involved in all photocatalytic reactions in the presence of O2, should be sensitively influenced by and dependent on the kind of TiO2. The photocatalytic decomposition of H2O2 with different TiO2 films was investigated with varying the experimental parameters such as light intensity, [H2O2], carrier gas composition (O2 vs N2), and alternative electron donor and acceptor (methanol, EDTA, silver ions). The result implications for photocatalytic mechanism and atmospheric chemistry are discussed.

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INTRODUCTION TiO2 photocatalysis has been established as an efficient method that can generate various reactive oxygen species (ROS) under ambient conditions. TiO2 photoactivates H2O and O2 to generate ROS like OH, HO2, O2−, H2O2, and O21, all of which were clearly confirmed for their production on illuminated TiO2.1−7 The oxidative power of the photogenerated ROS provides the basis on which TiO2 photocatalysis can be applied to the oxidation of various contaminants present in water and air.8−14 Among various ROS, H2O2 is the most stable species of which conversion is closely coupled with other ROS. It serves as a reservoir species of more reactive ROS as well as the precursor of other ROS. H2O2 can be photogenerated on TiO2 from either the reduction of O2 (eq 1) or the oxidation of H2O (eq 2). Likewise, H2O2 can be photodegraded on TiO2 either reductively (eq 3) or oxidatively (eq 4).15−21 In the production and decomposition of H2O2, hydroxyl radicals, superoxides, and hydroperoxyl radicals are involved as intermediates. Therefore, © 2012 American Chemical Society

understanding these reactions occurring on the irradiated surface of TiO2 should provide valuable information on the general mechanism of photocatalysis. O2 + 2ecb− + 2H+ → H 2O2

(1)

2H 2O + 2h vb+ → H 2O2 + 2H+

(2)

H 2O2 + 2ecb− + 2H+ → 2H 2O

(3)

H 2O2 + 2h vb+ → O2 + 2H+

(4)

The production of H2O2 as a reaction intermediate has been frequently monitored in various photocatalytic reaction systems.1,3,7 However, its intrinsic reactivity on the illuminated Received: February 12, 2012 Revised: April 21, 2012 Published: April 23, 2012 10090

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Figure 1. Experimental setup. DL, diode laser; OI, optical isolator; BS, beam splitter; AOM, acousto-optical modulator; APD, Avalanche Photo Diode.

decomposition of H2O2 may generate HO and HO2 radicals, their production accompanied by the degradation of H2O2 was monitored right above the illuminated TiO2 surface. Our first attempt of using cw-CRDS for the selective and sensitive detection of HO2 radicals could not detect HO2 radicals during the photocatalytic oxidation of methyl ethyl ketone, possibly because of the use of a not well-adapted reactor.2a Then, the continued study with an improvement in the cw-CRDS reactor design successfully detected the formation of HO2 radicals from the photocatalytic degradation of H2O2 above a TiO2 surface,2b which demonstrated that HO2 radicals can migrate several tens of millimeters into the gas phase even at pressures as high as 230 Torr. This work further investigated the photocatalytic decomposition of H2O2 on several different TiO2 surfaces along with the concurrent monitoring of HO2 radicals using cwCRDS. The decomposition behavior was found highly dependent on the kind of TiO2 films, and its implications for the photocatalytic mechanism are discussed.

surface of TiO2 has not been well studied. Understanding the photocatalytic behavior of H2O2 on TiO2 is critical in deciphering the mechanism of TiO2 photocatalysis because it is a key intermediate. On the other hand, H2O2 is also an important trace gas species in the atmosphere and acts as a precursor for OH radicals and a reservoir of HO2 radicals. H2O2 is highly soluble in water, and an important loss process in the atmosphere is its uptake into water droplets. In a recent field campaign, the impact of mineral dust on the atmospheric trace gas composition has been studied. Model calculations of this field study gave evidence that H2O2 is also efficiently lost on mineral dust;22 however, experimental data regarding the uptake of H2O2 on dust particles had been hardly available. Recent laboratory studies investigated the heterogeneous uptake and loss processes of H2O2 on different surfaces (either synthetic particles23,24 or authentic dust particles25). Pradhan et al.23 employed submicrometer TiO2 particles for the uptake of H2O2 as a proxy for real dust particles since dust can include a variable amount of TiO2. All the above studies23,24 observed a heterogeneous loss of H2O2 in the dark condition, with the uptake coefficients depending on the relative humidity. Considering that the efficiency of heterogeneous reactions on authentic dust particles containing photoactive components can be enhanced under light irradiation,26−28 the photocatalytic reaction of H2O2 on TiO2 surfaces under solar irradiation should have atmospheric implications as well. In this study, the photocatalytic decomposition of H2O2 on TiO2 was investigated using continuous wave cavity ring down spectroscopy (cw-CRDS),29 a spectroscopic technique that has been rarely employed in the study of heterogeneous reactions. CRDS has a high sensitivity and selectivity for the detection of small gaseous species and enables the in situ monitoring of the removal not only of H2O2 molecules2 but also of HO2 radicals30,31 that are possibly emitted from the illuminated surface of TiO2. It is now well established that ROS generated on TiO2 can desorb into the gas phase and subsequently initiate the photocatalytic oxidation of organic substrates in a remote distance from the surface.32−35 Since the photocatalytic

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EXPERIMENTAL SECTION Reactor Setup and Measurements for CRDS. Experiments were performed in a reactor designed for in situ analysis of the gas phase above the photocatalyst surface, using cwCRDS. Details of the experimental setup have been presented in a recent work,2b and a brief description is given here. A major improvement in the reactor (different from the previous work2b) was to employ a light-emitting diode (LED) array as the UV irradiation source instead of black light lamps. The schematic view of the setup is shown in Figure 1: the reactor consisted of a rectangular cell made of aluminum with inner dimensions of 68 cm × 8.5 cm × 4 cm (total volume of 2300 cm3). Photocatalyst-coated glass plates (arranged in series with each plate size of 7.5 cm × 2.5 cm: total size 60 cm × 2.5 cm) were placed horizontally at the bottom of the reactor, and the vertical position of the photocatalyst surface was adjusted (using two motion feedthroughs) at 5 mm below the optical pathway. The upper side of the reactor was closed by two quartz windows (33 cm × 3.4 cm each, total 66 cm long) through which UV irradiation passed. An array of 3 times 60 10091

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the bottom of the reactor. Each TiO2 powder was mixed with a binder of 50% carbowax (polyethylene glycol, PEG) aqueous solution to obtain a paste with 20 wt % TiO2. The mixed paste was cast on the glass plate with two tracks of one layer of Scotch Magic Tape. After the paste was dried under air, the attached tapes were removed from the glass plate, and then the TiO2 paste-coated glass plate was heated at 450 °C for 1 h to burn out the organics in the TiO2 film. DP is the most popular photocatalyst which has been employed in numerous studies of water and air purification because of its high activity. In general, DP exhibits high photocatalytic activities for a variety of substrates compared with other commercial TiO2 samples.41 It consists of mixed crystalline phases (usually an 8:2 mixture of anatase and rutile) with a reasonably high surface area (∼50 m2/g). HBK consists of a pure anatase phase with higher surface area (348 m2/g). AA and AR are of single phase (anatase and rutile) with much lower surface area (9.2 and 1.9 m2/g), respectively. To investigate the effects of an additional electron acceptor (or donor) on the photocatalytic decomposition of H2O2, the surface of TiO2(DP)-coated glass plates was adsorbed with Ag+ (electron acceptor) or ethylenediaminetetraacetic acid (EDTA, electron donor). The aqueous solution of 100 mM silver nitrate (Sigma-Aldrich) or 50 mM EDTA (Sigma-Aldrich) was prepared with deionized water. The DP-coated glass plate was soaked in AgNO3 or EDTA solution for 30 min in the dark to allow the adsorption equilibrium and then dried under air. The effect of methanol vapor on the photocatalytic generation of HO2 radicals from the decomposition of H2O2 was also investigated with AA and AR films.

LEDs (Nitride Semiconductors, N375-5LRO) was used as a light source and positioned roughly 7 cm above the photocatalyst surface. Each LED has emission centered at 375 nm (fwhm 20 nm) with a maximal output power of 14 mW, and the irradiation flux at the photocatalyst surface was estimated to be about 1.2 mW cm−2, which corresponds to around 2 × 1015 photons cm−2 s−1. The detection of HO2 radicals in the gas phase was carried out on the basis of our previous work30,31 where a sensitive and selective detection of HO2 radicals could be achieved in the near-infrared range by cw-CRDS: the strongest absorption of the 2ν1-band at 6638.20 cm−1 exhibited a cross-section of σ = 2.72 × 10−19 cm2 at 50 Torr He. The present work was therefore carried out at this wavenumber to obtain a detection limit as low as possible. The pressure broadening coefficients, needed for calculating the absorption cross sections at different pressures, were measured for several absorption lines36 and recently confirmed by Tang et al.37 At typical pressures applied in this work (5 Torr N2), this leads to an absorption crosssection of 3.9 × 10−19 cm2. H2O2 was also detected and quantified by cw-CRDS using an absorption line at 6639.89 cm−1. In a recent work, we determined the absorption cross section at 10 Torr He (σ(6639.88 cm−1) = 3.14 × 10−22 cm2) using a combination of time-resolved laser-induced flourescence (LIF) and cw-CRDS.38,39 The concentration of a monitored species [A], produced or consumed in the photocatalytic process, can be obtained by measuring the ring down time (for details see Bahrini et al.2b) of the empty cavity before turning on the UV LEDs (τ0) and the ring down time after turning on the UV LEDs (τ) α = [A] × σ =

RL ⎛ 1 1⎞ ⎜ − ⎟ c ⎝τ τ0 ⎠

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RESULTS AND DISCUSSION The photocatalytic decomposition of H2O2 on TiO2 may depend on various parameters such as the adsorption site, catalyst surface area, light intensity, [H 2O2 ], and the composition of carrier gas. In particular, it is well-known that the photocatalytic activity is highly specific to the kind of target substrates or TiO2 samples.41 By comparing different TiO2 samples (DP, AA, AR, and HBK), this study investigated how the photocatalytic decomposition of H2O2 behaves differently depending on the TiO2 samples. This may provide a clue about the different photocatalytic activities observed among various TiO2 samples. Before investigating the photocatalytic degradation of H2O2, various control experiments were performed in the CRD cell with or without the TiO2 sample. The direct photolysis of H2O2 in the absence of TiO2 was not observed at all under the present irradiation condition as can be seen in Figure 2a (A, violet), and H2O2 was not degraded on the surface of TiO2 in the dark. The dark control experiments shown in Figure 2a were carried out with a bare glass plate (B, green) and a DPcoated plate (C, red) with monitoring the time profiles of [H2O2] upon closing and reopening the flow of H2O2. The decay and rise profiles of [H2O2] in the dark were not much influenced by the presence and absence of DP film because the DP film was pre-equilibrated with the adsorption of H2O2. Therefore, the decay and rise time in the dark condition reflect the flow characteristics in the CRD cell. The decay and rise profiles were also compared upon turning on and off the UV irradiation over the DP film (D, blue). The photocatalytic decay of H2O2 was slightly faster than the dark flow-off decay (upon closing the flow). However, it is interesting to note that the H2O2 recovery over DP film (D, blue) after turning off the

(5)

where α is the absorption coefficient; σ is the absorption cross section; RL is the ratio between the cavity length L (i.e., the distance between the two cavity mirrors) to the length LA over which the absorber is present; and c is the speed of light. Knowing the absorption cross section σ, one can obtain the concentration [A] of the target molecule. The present work was carried out at a pressure around 5 Torr N2 or O2. Typical ring down time in the empty cavity was 300 μs, representing a mirror reflectivity of 99.9991%. Under these conditions, we estimated a minimum detectable concentration of [HO2]min = 3.3 × 109 cm−3 at 5 Torr synthetic air (equal to a decrease of 3 μs in τ) after averaging ring down signals over 1 s; the minimum detectable concentration of [H2O2]min = 4 × 1012 cm−3. The cell was operated in a quasi-static way; slow gas flow was admitted through calibrated flow meters. A gas flow (varying between 1 and 20 STP cm3 min−1) of pure O2 or N2 was purged through an aqueous solution of 50% H2O2 and then diluted with 80 STP cm3 min−1 of synthetic air before entering the cell. A typical residence time was around 10 s under the flow condition. Photocatalyst Sample Preparation. Several commercial titanium dioxide (TiO2) samples were selected as a photocatalyst in this study, and they were Degussa P25 TiO2 (DP), Aldrich anatase (AA), Aldrich rutile (AR), and Hombikat UV100 (HBK). All of the TiO2 samples were used without further purification. A glass plate (7.5 cm × 2.5 cm) was coated with TiO2 film according to the Dr. Blade method described in the literature,40 and several coated plates were placed in series at 10092

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Figure 3. Time profiles of the photocatalytic degradation of H2O2 (a) and the accompanying production of HO2 radicals (b) over the UVirradiated film of PT for different irradiation intensities. Total pressure was 4 Torr, with total flow 82 ccm min−1 STP O2 where 2 ccm min−1 was bubbled through 50% H2O2 solution. I0 = max LED power.

Figure 2. Different control experiments are shown in (a): A, photolysis control experiment with plain glass (violet); B, dark experiment on plain glass with closing H2O2 flow (green); C, dark experiment with closing H2O2 flow on PT (red); and D, photocatalytic experiment with PT (blue). Photocatalytic experiments with different substrates are shown in (b): PT, AA, AR, and HBK showing the different photocatalytic activities and recovering behavior after turning off the light. I0 = 100% for all experiments. Initial H2O2 concentration was (3 ± 1) × 1013 cm−3 for all experiments.

light was markedly slower than the dark rise over DP film after reopening the flow (C, red). This implies that the preirradiated surface of DP film has some residual activity for H2O2 removal even after the light was turned off. This residual activities observed after the light-off were also markedly different among different TiO2 samples as shown in Figure 2b. HBK film exhibited an outstanding residual activity in the dark period. This seems to imply that some active species on the surface of TiO2 persist long even after the light-off and influence the uptake and degradation of H2O2. It has been demonstrated that the trapped electrons and trapped holes on the surface of TiO2 have an unusually long lifetime extending to hours after UV irradiation.42,43 Such long-lived electrons and holes can react with H 2O2 even after the light-off, and their surface concentration seems to vary depending on the kind of TiO2. Figure 3 shows the time profiles of the photocatalytic degradation of H2O2 (Figure 3a) and the accompanying production of HO2 radicals (Figure 3b) over the UV-irradiated film of DP. Figures 4 and 5 show the same for the AA and AR film, respectively. The photocatalytic degradation experiments were carried out with varying the intensity of the UV-LED irradiation. It is noted that the removal of H2O2 and the accompanying production of HO2 radicals were not correlated at all, although HO2 radicals are generated mainly from the decomposition of H2O2. H2O2 could be rapidly degraded on illuminated DP with little production of HO2 (Figure 3). On the other hand, H2O2 was photodegraded much more slowly over AA and AR but with a marked production of HO2 (Figures 4 and 5). With AA and AR, both the removal of H2O2 and the production of HO2 were enhanced under higher light intensity. However, over the irradiated DP film, the higher

Figure 4. Time profiles of the photocatalytic degradation of H2O2 (a) and the accompanying production of HO2 radicals (b) over the UVirradiated film of AA, under the same conditions as Figure 2.

light intensity increased the removal rate of H2O2, but not the production of HO2 at all. This observation implies that the photocatalytic decomposition of H2O2 on DP proceeds through a mechanistic pathway different from that on AA and AR. The primary reaction steps that might be involved in the decomposition of H2O2 are proposed as follows. H 2O2 (g) → H 2O2 (ad) 10093

(6)

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Figure 5. Time profiles of the photocatalytic degradation of H2O2 (a) and the accompanying production of HO2 radicals (b) over the UVirradiated film of AR, under the same conditions as Figure 2.

H 2O2 (ad) + ·OH/h+ → HO2 ·(ad) + H 2O/H+

(7)

HO2 ·(ad) + ·OH/h+ → O2 (ad) + H 2O/H+

(8)

HO2 ·(ad) + HO2 ·(ad) → H 2O2 (ad) + O2 (ad)

(9)

O2 (ad) + e− → O2−(ad)

(10)

O2 (ad) + e− + H 2O → HO2 ·(ad) + HO−(ad)

(11)

HO2 ·(ad) + e− + H+ → H 2O2 (ad)

(12)

H 2O2 (ad) + e− → ·OH(ad) + HO−(ad)

(13)

HO2 ·(ad) → HO2 ·(g)

(14)

HO·(ad) → HO·(g)

(15)

H 2O2 (ad) → H 2O2 (g)

(16)

O2 (ad) → O2 (g)

(17)

Figure 6. Production of HO2 from the irradiated AR film at different initial [H2O2]0. A, 3.10 × 1013 cm−3; B, 6.36 × 1013 cm−3; C, 1.20 × 1014 cm−3. Total pressure: 4 Torr. Total flow: 82 ccm min−1 STP O2. H2O2 concentration was varied by varying the portion of O2 bubbling through H2O2, and light intensity was maximum LED power.

An alternative pathway of HO2 radical generation could be the photocatalytic reduction of O2 (reaction 11),21 which is possible even in the absence of H2O2. However, the photogeneration of HO2 radicals was observed only in the presence of H2O2, not at all in the presence of O2 and water vapor only. Figure 6 shows that the production of HO2 from the irradiated AR film increased with increasing [H2O2]0. This indicates that the observed HO2 radicals were generated from the decomposition of H2O2, not from the photocatalytic reduction of O2. To test the possible role of O2 in the photocatalytic decomposition of H2O2, the photocatalytic experiment was also carried out in the absence of O2 (N2 carrier gas), which was a little different from that in the presence of O2 (see Figure 7). The photogeneration of HO2 radicals was not affected at all by the presence and absence of O2 in the carrier gas. This reconfirms that the generation of HO2 radicals is originated from the decomposition of H2O2,

Figure 7. Time profiles of the photocatalytic degradation of H2O2 (a) and the accompanying production of HO2 radicals (b) over the UVirradiated film of AR with O2 or N2 flow. Total pressure was 4 Torr. Total flow was 82 ccm min−1 STP O2 or N2 where 2 ccm min−1 was bubbled through 50% H2O2 solution. Initial H2O2 concentration was 3 × 1013 cm−3 for all experiments. Light intensity was 0.33I0 where I0 is max LED power.

not the photoreduction of O2. In general, the photocatalytic degradation reactions require the presence of efficient electron acceptors; otherwise, the fast recombination between electrons and holes inhibits the interfacial charge transfer and overall 10094

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Figure 8. Photocatalytic production of HO2 in the absence and presence of methanol for both AA (a) and AR (b). Total pressure was 4 Torr, and total flow was 82 ccm min−1 STP O2. Initial H2O2 concentration was 2.5 × 1013 cm−3 for all experiments. Initial methanol concentration was 6.3 × 1013 cm−3. Light intensity was 0.33I0 where I0 is max LED power.

the surface of DP film. Figure 9a shows that the decomposition of H2O2 was retarded in the presence of preadsorbed EDTA

photocatalysis. The dioxygen molecules usually serve as an electron acceptor, and therefore most photocatalytic reactions do not occur in the absence of O2.13,44 The fact that the decomposition of H2O2 was not retarded at all even in the absence of O2 implies that H2O2 itself should serve as an electron acceptor as in reaction 13. Otherwise, the rapid recombination between holes and electrons would have prevented the degradation of H2O2 in the absence of O2. As a result, the photocatalytic decomposition of H2O2 should proceed in a unique way as H2O2 plays the dual role of hole and electron scavenger (i.e., the combination of reaction 7 and reaction 13). The initial electron or hole transfer to H2O2 generates HO2 or HO,16 which may further react on the surface or desorb into the gas phase (reactions 14 and 15). The fact that OH radicals desorb from the TiO2 surface into the gas phase has been clearly confirmed by single molecule imaging using fluorescence microscopy45,46 as well as laser-induced fluorescence spectroscopy.1,3−5 Even though OH radicals have already been detected using cw-CRDS in the near-infrared region,47 the absorption line strengths are rather small, and the desorption of OH radicals from the irradiated TiO2 surface could not be detected at all by CRDS in this study. The desorption of HO2 radicals from the photocatalytic decomposition of H2O2 on TiO2 was also observed in the previous CRDS investigation.2b The initial reaction between adsorbed H2O2 and the hole (or OH radical) (reaction 7) was investigated in the presence of methanol vapor that should serve as an efficient hole scavenger. Figure 8 shows that the photocatalytic production of HO2 was not changed at all by the presence of methanol for both AA (Figure 8a) and AR (Figure 8b). The photocatalytic oxidation of methanol should induce the additional generation of HO2 through the following reactions.48 CH3OH + ·OH/h+ → CH3O· + H 2O/H+

(18)

CH3O· + O2 → HCHO + HO2 ·

(19)

Figure 9. Time profiles of the photocatalytic degradation of H2O2 (a) and the accompanying production of HO2 radicals (b) over the UVirradiated film of bare PT (red), Ag+-adsorbed PT (blue), and EDTAadsorbed PT (green). Total pressure was 4 Torr, and total flow was 82 ccm min−1 STP O2. Initial H2O2 concentration was 1.6 × 1014 cm−3 for all experiments. Each coated photocatalytic plate (15 cm × 2.5 cm) was placed in a reactor. Light intensity was 0.33I0 where I0 is max LED power.

but enhanced in the presence of preadsorbed Ag+ ions. This supports that the decomposition of H2O2 is initiated mainly by its reaction with holes (reaction 7) and not by the reaction with electrons (reaction 13). In the presence of preadsorbed EDTA that should play the dual role of a competing adsorbate and a hole scavenger, H2O2 should be less adsorbed and less degraded. On the contrary, the electron scavenging by silver ions should increase the lifetime of holes with enhancing reaction 7, although the presence of preadsorbed Ag+ may hinder the adsorption of H2O2 to some extent. Silver ions are a strong electron acceptor [E0(Ag+/Ag0) = 0.80 VNHE].49 The net effect of silver ion addition on DP was to accelerate the

No methanol effect on the production of HO2 indicates that holes and OH radicals are preferentially scavenged by adsorbed H2O2. Although methanol is a strong hole scavenger, it is a much weaker adsorbate than H2O2 on the surface of TiO2. The adsorbed H2O2 molecules scavenge holes more efficiently than methanol. As a result, reaction 18 cannot compete with reaction 7. The photocatalytic decomposition of H2O2 was further investigated in the presence of an alternative hole scavenger (EDTA) or electron scavenger (Ag+) that was preadsorbed on 10095

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degradation of H2O2 along with the enhanced production of HO2 radicals. On the other hand, the production of HO2 was slightly enhanced in the presence of EDTA despite that EDTA retarded the decomposition of H2O2. As shown in Figures 3−5, the decomposition of H2O2 was not directly correlated with the production of HO2. A key factor seems to be the fate of HO2 radicals: either to desorb into the gas phase (reaction 14) or to further react with hole (reaction 8). The presence of preadsorbed EDTA as a hole scavenger should hinder not only reaction 7 but also reaction 8. That is, the presence of EDTA can hinder the further decomposition of HO2 radicals, which may induce more HO2 to desorb into the gas phase as shown in Figure 9b. Under such conditions, hindered decomposition of H2O2 does not necessarily lead to the reduced production of HO2 radicals. The observation that the appearance of HO2 radicals in the gas phase was observed only with specific TiO2 samples implies that the production of intermediate HO2 radicals and their subsequent degradation sensitively depend on the kind of TiO2. On illuminated DP, the in situ generated HO2 radicals seem to be rapidly degraded with little chance of desorption into the gas phase, while those on AA and AR are long-lived enough that some desorb into the air. DP is generally more active than other commercial TiO2 samples for a wide range of photocatalytic reactions.41 The comparison among Figures 3−5 shows that the photocatalytic activity for H2O2 degradation decreases in the order of DP > AA > AR, while the activity for HO2 generation decreases in the order of AA > AR > DP. It seems that the photocatalytic production of HO2 radicals is optimized with a photocatalyst (AA) having an intermediate level of activity for H2O2 degradation. When the photocatalytic activity is too low, the production of HO2 radicals is limited by the slow decomposition of H2O2 as in the case of AR. On the other hand, when the photocatalytic activity is high enough, the HO2 radicals are immediately decomposed as soon as they are generated on the catalyst surface, which appears to be the case of DP. As a result, AA, which has an activity in between DP and AR, yields the highest concentration of HO2 radicals in the gas phase. The HO2 radical is a key intermediate of general photocatalytic reactions, and its fate should be critical in controlling the overall photocatalytic mechanism. They are usually generated through the reaction of O2 with a CB electron (reaction 11). Once they are formed, they may react with either a VB hole or a CB electron (reactions 8 and 12), which should determine the overall mechanistic pathway. The present observation that the fate of HO2 (generated from the decomposition of H2O2) can be different among several TiO2 films implies that the general photocatalytic reaction mechanism involving HO2 radicals can be also different depending on the kind of TiO2. The mechanistic difference may change the fate of HO2 radicals (i.e., the relative contribution among the disproportionation, oxidation, reduction, and desorption of HO2 radicals). Whether HO2 leads to the formation of H2O2 (reactions 9 and 12) or not influences the overall mechanism of photocatalysis, which could be different among various TiO2 samples. For example, the HO2 radical is an important intermediate in the production of atmospheric H2O2, to which the heterogeneous photocatalytic process on the atmospheric aerosols (containing TiO2 component) may contribute.15 The findings in this work imply that the photocatalytic production of H2O2 on atmospheric aerosols, if any, could be different depending on the kind of TiO2

contained in the aerosols. With all the complexities considered, the photocatalytic behavior of the uptake, degradation, and production of H2O2 on the irradiated surface of TiO2 (or mineral aerosols) should be investigated in a careful way. It should be noted that each kind of TiO2 catalyst should be tested individually, and the photocatalytic reactivity measured with a specific TiO2 sample may not be generalized to other TiO2 samples.

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CONCLUSIONS The photocatalytic interconversion between HO2 and H2O2 is important in understanding the general photocatalytic mechanism because they are ubiquitous in various environmental media of water and air. H2O2 is in situ generated and degraded on the illuminated photocatalyst surface and is closely coupled with the fate of OH radicals and superoxides. To study the photocatalytic behavior of H2O2, the decomposition of H2O2 on illuminated TiO2 films was investigated using CRDS, which enabled an in situ monitoring of HO2 radicals as well as H2O2 over the photocatalyst film. The photocatalytic degradation of H2O2 should be initiated by its reaction with VB holes but was not retarded at all in the absence of O2, which implies that H2O2 itself should serve as an electron acceptor as well as an electron donor. The photocatalytic degradation of H2O2 was strongly dependent on the kind of TiO2. H2O2 was rapidly degraded on DP but much more slowly over AA and AR. As a result of H2O2 decomposition, the HO2 radical was produced as an intermediate, but its production was not directly correlated with the photodegradation of H2O2. DP photodegraded H2O2 with the fastest rate but generated little HO2 radicals. On the other hand, AA and AR photodegraded H2O2 much more slowly, but a marked production of HO2 was accompanied. Whether the in situ generated HO2 radicals on the photocatalyst surface are further degraded or desorbed into the gas phase determines the overall photocatalytic behavior, which appears to depend on the kind of TiO2. The fact that the photocatalytic activity of TiO2 widely varies depending on the kind of TiO2 sample is well-known but is not well explained.41 The present study of H2O2 degradation accompanied with the generation of HO2 shows an example of how the photocatalytic activity is specific to the kind of TiO2. In general, the photocatalytic activity depending on the kind of TiO2 or test substrates can be ascribed to the complexity in the photocatalytic reaction mechanism. Therefore, the quantitative assessment of the specific photocatalytic activity is meaningful only when the mechanism is fully understood. Because the relation between the photocatalytic activity and the mechanism is not fully understood, more thorough investigation on this issue should be needed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: christa.fi[email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the French Ministry of Foreign and European Affaires and Korean National Research Foundation (NRF) for financial support through the STAR program. W. Choi appreciates the funding from the KOSEF NRL program (No. R0A-2008-000-20068-0) and the KOSEF EPB center 10096

dx.doi.org/10.1021/jp301405e | J. Phys. Chem. C 2012, 116, 10090−10097

The Journal of Physical Chemistry C

Article

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(Grant No. R11-2008-052-02002). J. Yi thanks the French Embassy in Seoul for financial support through a Blaise Pascal grant.

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