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OH Radicals and H2O2 Molecules in the Gas Phase near to TiO2 Surfaces Je´roˆme Thiebaud,†,§ Frederic The´venet,‡ and Christa Fittschen*,† Physico-Chimie des Processus de Combustion et de l’Atmosphe`re (PC2A), CNRS UMR 8522, UniVersite´ des Sciences et Technologies de Lille, F-59655 VilleneuVe d’Ascq Cedex, France, and De´partement Chimie et EnVironnement, Ecole des Mines de Douai, F-59001 Douai France ReceiVed: October 27, 2009; ReVised Manuscript ReceiVed: December 2, 2009
The formation of OH radicals and their diffusion into the gas phase after UV-excitation of TiO2 in the presence of H2O has been studied using the very sensitive and selective detection method of laserinduced fluorescence (LIF). The time-resolved evolution of the OH radical concentration has been observed at different pressures and at varying distances between the photocatalytic surface and the detection volume. H2O2 has been indirectly detected by LIF. The influence of O2, hydrocarbons, and excitation laser wavelength on the evolution of both species profiles has been studied in this work. The quantum yield for the formation of OH and H2O2 has been estimated by comparison with signals obtained after photolysis of H2O2 in the gas phase. Introduction Photocatalysis has found many applications since it was first described in 1972 by Fujishima and Honda.1 Water and air purification can be considered as a major challenge of the years to come and photocatalysis can play a major role in these fields.2 However, the detailed mechanism is still not well understood and new experimental approaches can help to obtain a more complete picture. This paper describes the application of laser photolysis/laser-induced fluorescence (LIF), an experimental technique widely used in gas phase chemistry but so far only rarely adapted to the study of photocatalytic processes. This technique permits the direct, in situ, and time-resolved investigation of the composition of the gas phase in the vicinity of a photocatalytic surface. Photocatalytic processes use a semiconductor photocatalyst, usually TiO2, as slurry or deposited on a support. The semiconductor, exposed to near UV light (λ < 387 nm), is able to generate electron-hole pairs (e-/h+). Those excitons can migrate into the semiconductor lattice and reach the surface. They are subsequently available for oxidation-reduction reactions. Heterogeneous reactions with the gas phase components (O2, H2O, hydrocarbons) lead to the formation of reactive oxygenated species (ROS) such as OH•, HO2•, H2O2 and 1O2,3,4 for example. Main reactions can be described as the following:
TiO2 + hν f h+ + e-
(R1)
h+ + H2O f OH• + H+
(R2)
O2 + H+ + e- f HO2•
(R3)
2OH• f H2O2
(R4)
* To whom correspondence should be addressed. E-mail: christa.fittschen@ univ-lille1.fr. † Universite´ des Sciences et Technologies de Lille. ‡ Ecole des Mines de Douai. § Current address: SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025.
HO2• + e- + H+ f H2O2
(R5)
2HO2• f H2O2 + O2
(R6)
O2 + e- f O2•-
(R7)
H2O2 + O2•- f OH• + OH- + O2
(R8)
OH• + HO2• f H2O + O2
(R9)
On the basis of the above reactions, one can suggest that the ROS are formed on the surface of the catalyst, although it is well-known that the photocatalytic degradation occurs when these adsorbed species react with adsorbed organic compounds.5 However, several authors have found evidence on the diffusion of ROS into the gas phase.6-10 According to them, ROS can diffuse far from the surface and induce remote oxidation reactions in the gas phase vicinity of the photocatalyst surface.11,12 However, in a recent attempt to detect HO2• radicals by cw-CRDS,13 no radicals were observed during the photocatalytic degradation of methyl ethyl ketone while the estimated detectable concentration was 3 × 109 molecule · cm-3. Murakami et al.7,8 reported evidence for the first successful direct detection of the presence of OH• radicals near the surface of TiO2 by direct detection using LIF spectroscopy at very low pressure (0.5 Torr). They studied several parameters such as the distance from the surface (5 to 8 mm), the gas nature (He, H2O, D2O, and O2) and the influence of calcination temperature on the OH-LIF intensity. Furthermore, Tatsuma et al.9,10 have used indirect methods in order to evidence the diffusion of ROS into the gas phase. They observed the degradation of different organic films separated by an air gap (50 µm to 2.2 mm) from the TiO2 photocatalytic material. Investigations with the colorimetric method performed by Kubo et al.12,14 on hydrogen peroxide formation by the exposure of TiO2 to UV radiation demonstrated the presence of H2O2 in gas flowing out of the photocatalyst.
10.1021/jp9102542 2010 American Chemical Society Published on Web 01/29/2010
OH Radicals and H2O2 Molecules in Gas Phase near TiO2
Figure 1. Schematic view of experimental setup. VF, vacuum motion feedthrough; PS, photocatalytic surface; P, prism; PD, photodiode; FC, flow controller; M, mirror; A, attenuator; L, lens; PMT, photomultiplier tube.
Park and Choi15 have highlighted the diffusion of OH• radicals in the gas phase during the photocatalytic degradation of stearic acid. Moreover, they have also shown the remote photocatalytic oxidation mediated by active oxygen species diffusing through organic polymer membrane over surface-fluorinated TiO2.16 Lee and Choi17 have studied the photocatalytic oxidation of soot film deposited on TiO2. To conclude, they claimed the migration of OH• radicals in all media (gas, liquid and solid phases) during a photocatalytic process. In an earlier letter,6 we have already reported the use of LIF technique in connection with laser photolysis for detection of OH• radicals close to the TiO2 surface in the gas phase. Both OH and H2O2 have been observed in the absence of oxygen and VOCs, even at elevated pressures. This paper presents the results of a more complete study of the same system; we have investigated the effect of O2 and VOC addition to the reaction, and we have also varied the excitation wavelength from 248 to 351 nm. Furthermore, we have attempted to calibrate the LIF signals to obtain absolute OH and H2O2 concentrations and estimate the quantum yield of the formation of these species in the gas phase.
J. Phys. Chem. C, Vol. 114, No. 7, 2010 3083 The relative concentration of OH radicals was determined from the integrated LIF intensity. The probe laser was a frequency doubled dye laser (Quantel TDL 50, Rhodamin 590) pumped by a frequency doubled YAG laser (Quantel YG 780). The probe beam is aligned in order to propagate parallel to the center of the photocatalytic support, i.e. along the x-axis of the cell. In order to get a good spatial and thus temporal resolution, the original beam (beam profile: 6 × 4 mm2, 3-10 mJ/pulse) was focused by a quartz lens (focal length of 50 cm) into the center of the cell, i.e. the beam diameter was minimal (much below 1 mm) when passing the photocatalytic support. The probe beam passed through an attenuator (Newport Model M-935-10) permitting easy variation of the pulse energy by 1 order of magnitude. OH radicals were excited at 282 nm and the fluorescence was collected along the z-axis through 2 lenses and an interference filter (307 ( 5 nm fwhm). The fluorescence signal was integrated with a boxcar averager (EG&G 4121B) and digitized and averaged in a computer. Different delays between the TiO2 activation and the excitation pulses were obtained by way of a digital delay generator (EG&G 9650), controlled by a PC. A typical decay consists of 20 - 50 points at different delays between the two lasers, at each delay the fluorescence is averaged over typically 30 laser shots. All experiments have been performed at a repetition rate of 10 Hz. The entire experiment is controlled by a set of Labview programs. He (4.5, Air Liquide) carrier gas and O2 (4.5, Air Liquide) were used without further purification, a part or the entire He flow was bubbled through distilled water at ambient temperature. Methylethylketone (MEK) was diluted in He and stored in a 20 L glass balloon. All gas flows were regulated with calibrated mass flowmeters (Tylan FC-260). At a total pressure of 5 Torr (1 Torr ) 133.3 Pa), a typical total flow rate of 60 cm3 min-1 STP leads to a gas flow velocity through the cell of 9 cm s-1 in the direction of the x-axis. Experiments were performed in the pressure range of 5-600 Torr of helium with the flow rates having been adapted to always have approximately the same relative H2O concentration as well as the same flow velocity. Results and Discussion
Experimental Details Experiments have been performed by LIF, schematically shown in Figure 1. The reaction cell is made of stainless steel and consists in a central cross with an arm connected to each of the six openings. The main axis (hereafter denoted the x-axis) has a total length of 75 cm, while the 2 other axes are shorter (40 cm each); one short axis runs parallel to the table and is hereafter referred to as the y-axis, the other runs vertical to the table and is called the z-axis. The photocatalytic medium used in this experiment was industrial titanium dioxide (Millennium PC 500, 100% anatase) coated nonwoven paper, produced by Ahlstrom, the same as described in our previous papers.6,13 A disk of this paper with a diameter of 2 cm was fixed on a linear motion vacuum feed through (Caburn), connected to one of the openings of the y-axis, that is, the paper is located parallel to the x-z-plane and can be moved up to 5 cm along the y-axis. The photocatalytic process is initiated by an Excimer laser pulse (Lambda Physik LPX 202i, beam size 3.5 × 1.2 cm2), operated either at 248 or 351 nm with an average pulse energy of 400 and 250 mJ/pulse, respectively and a pulse duration of around 10 ns. The Excimer radiation enters the reaction cell at the opposite opening of the y-axis through a quartz window (diameter 2 cm) and is aligned to directly strike the center of the photocatalytic support.
The evolution of the LIF signal has been studied after varying different parameters. We will present in what follows the results obtained for each parameter: - Influence of pressure and distance resumes and extends to other pressures what has already been presented with more details in an earlier paper.6 - Influence of excitation laser wavelength presents results obtain by changing the excitation wavelength from 248 to 351 nm, a wavelength where H2O2 does not absorb. - Influence of O2 on the LIF signal investigates the possible impact of (R3) on the formation of ROS. - Influence of VOC on the LIF signal has been studied by adding different amounts of methylethylketon, and a major change in the evolution of ROS in the gas phase has been obtained. - Calibration of OH and H2O2 signals has been performed to obtain the quantum yield for the formation of theses two species in the gas phase. Influence of Pressure and Distance. This paragraph gives only a short overview on the influence of pressure and distance on the shape of the signal and the reader is referred to a recent letter6 for more details. Figure 2 shows typical signals as obtained in this experiment. The LIF intensity is plotted as a function of delay between the excitation and the detection laser;
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Figure 2. OH-fluorescence signal at 6 and 26 Torr for two different distances between surface and probe volume. The inset shows a zoom of the first 1000 µs.
Thiebaud et al.
Figure 4. Plot of 1/tmax as a function of 1/distance for the signals from Figure 5a,b as well as for signals at 4.5 Torr (see ref J. Phys. Chem).
Figure 5. The slopes of the straight lines from Figure 6 as a function of 1/pressure (for better visibility the line including the result at 4.5 Torr is shown as zoom in the inset).
Figure 3. Influence of the distance between TiO2 surface and probe volume for different pressures (a) 400 Torr total pressure, (b) 100 Torr total pressure.
at low pressure and at short distances between photocatalytic support and detection volume, we observe two distinct fluorescence peaks, that is, the first peak at 0 µs and the second one at around 500 and 1500 µs for 6 and 26 Torr, respectively, for the example at 0 mm in Figure 2. The two peaks “wash together” with increased pressure and increased distance between surface and detection volume. In our earlier letter, we have shown that the first peak can be attributed to OH radicals diffusing from the surface into the gas phase, while the second peak has been attributed to H2O2, being formed on the surfaces and diffusing also into the gas phase. The increased delay observed with increasing pressure and distance has been attributed to diffusion; the higher the total gas pressure and the larger the distance, the longer it takes for OH and H2O2 to reach the detection volume. Figure 3a,b demonstrates this observation for 400 and 100 Torr total pressure, respectively. Here, 0 mm means the shortest feasible distance between surface and detection volume. Any shorter distance creates an important signal on the photomultiplier due to scattered light of the dye laser pulse when touching the TiO2 surface. At these pressures, the distinction between the two maxima becomes difficult, so it is not clear whether the signal is really due to a mixture of OH and H2O2, or if it is generated by OH or H2O2 alone. To
check if this behavior is coherent with the existence of two peaks, we have compared the peak of these high-pressure signals with the first peak of the signals obtained at 4.5 Torr, shown in ref 6. In Figure 4 are plotted the inverse delay 1/tmax (time necessary for the signal to reach the maximum) against the inverse distance; the linear relationship for each pressure is in good agreement with the hypothesis that the increased delay is due to diffusion. In the inset of Figure 4 are also shown the values from experiments at 4.5 Torr, presented in our earlier paper.6 The slope of these lines should be proportional to the diffusion coefficient, and in Figure 5 we have plotted these slopes as a function of the inverse pressure for all three series. A very good linearity can be seen for all three pressures, and as the values at 4.5 Torr is obtained for the delay of the first maximum, very distinct at this pressure from the second maximum, we are therefore confident that even though we do not see two distinct maxima at higher pressures, we still observe the same phenomena: a first release of OH radicals, followed by a delayed release of H2O2. The second maximum is also delayed with increasing pressure and distance. We think that this behavior can also be attributed to diffusion; however it is more difficult to do the same analysis as for the first peak because the second peak is less sharp even at short distances and low pressure. Influence of Excitation Laser Wavelength. In our experiments, an Excimer laser has been used for the photocatalytic surface excitation. In most experiments, we have used KrF-gas mixture, leading to a wavelength of 248 nm. H2O2 efficiently absorbs this wavelength (σ248 nm ) 9.1 × 10-20 cm2),18 leading through photolysis to the formation to two OH radicals. It would therefore be possible that the observed first peak does not originate in OH radicals generated by a photocatalytic process
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Figure 6. Comparison of two signals, obtained at 4.5 Torr and a distance of 0 mm, using two different wavelengths for the excitation of TiO2: 248 and 351 nm.
Figure 7. Plot of 1/tmax as a function of 1/distance for signals at 4.5 Torr at two different excitation wavelengths: 248 and 351 nm.
on the surface but rather in OH radicals generated by 248 nm photolysis of adsorbed H2O2. This H2O2 could have been formed in the precedent laser pulse and still stick to the surface when the next excitation pulse hits; we work at a repetition rate of 10 Hz, that is, the surface gets excited every 100 msec. We have therefore conducted a few experiments using another gas mixture, XeF, leading to a wavelength of 351 nm; at this wavelength, the absorption cross-section of H2O2 is negligible (σ248 nm ) 4 × 10-22 cm2)18 and TiO2 still absorbs. In Figure 6 are shown the LIF traces of two experiments at 4.5 Torr total pressure and 0.5 mm distance between surface and detection volume; the red dots show the common signal at 248 nm, and the black circles represent the experiment at 351 nm. The signal intensities have been normalized to the same height to better compare the shapes; the 351 nm needed to be multiplied by 1.7 to coincide with the 248 nm signal, but the excitation laser energies have been different also, around 400 mJ pulse-1 at 248 nm and 200 mJ pulse-1 at 351 nm. This is in very good agreement with the results of Murakami et al.8 They observed in similar experiments the same OH-LIF intensity for both excitation wavelengths, 266 and 355 nm. However, the most striking feature of the results in Figure 6 is the identical shape of both signals: the ratio between the first and the second peak is identical for both wavelengths. It can therefore be excluded that the first signal peak is generated by OH radicals originating from the photolysis of H2O2 molecules absorbed on the surface; in this case, we would not observe the first peak in the 351 nm experiment. This experiment also points to another conclusion: H2O2 is not very strongly adsorbed on this surface, as apparently all H2O2 formed at the surface has disappeared from the surface 100 msec later. Earlier experiments from Kubo and Tatsuma14 have already shown the release of H2O2 into the gas phase, and they have even proposed that OH radicals formed from the photolysis of gas-phase H2O2 is the active species responsible for the observed remote oxidation mechanism.12 Another interesting conclusion can be drawn from this experiment: varying the excitation energy by a factor of 2 has apparently no influence on the shape of the signal. This is in contradiction with the possibility that either the first OH-peak is due to important heating of the photocatalytic surface with a subsequent “explosive diffusion” of OH. In this case, one would expect that the ratio of the two peaks would change with changing energy. Also, the time to reach the maximum peak intensity does not change between both experiments, as can be seen from Figure 7; the behavior is the same for both wavelengths, even though the energy has changed by a factor of 2. However, it is possible that the laser energy is already saturating such kind of phenomena even at 351 nm (10 × 50 mJ cm-2 ) 0.5 W cm-2).
Figure 8. OH fluorescence signal as a function of O2 concentration. Total pressure was 9 Torr and distance between TiO2 surface and probe volume was 0.5 mm. (a) Raw signals, (b) same signals, but normalized to the first maximum.
It can be mentioned that Murakami et al.8 have also observed OH radicals with a similar quantum yield in similar experiments using laser pulse at either 266 or 355 nm as excitation source although they used roughly 100 times less laser power than in our experiments. Further investigations under more realistic conditions, especially lower photon fluxes, are planed. Influence of O2 on the LIF Signal. Next, the influence of O2 on the LIF signal has been investigated. O2 is known to scavenge e- by photosorbing on UV-activated TiO2 surface as described by (R7). This reaction leads to the formation of the highly reactive •O2- species. The reaction of surface available electrons with O2 indirectly decreases the e-/h+ recombination hindering the photocatalytic process. This might subsequently lead to an increased concentration of OH• and H2O2 which are formed by reaction with h+. Figure 8a shows the LIF-signals obtained at a total pressure of 9 Torr and various O2 concentrations. In the experiment represented by the black dots, no O2 was added to the reaction cell; however some O2 due to a small leak, especially in the water bubbler, could not be completely be excluded. The green signal represents an experiment where 3 Torr of He has been replaced by O2, and the red dots represent
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Figure 9. Influence of methylethylketone on the evolution of the LIF signal at 4.5 Torr total pressure and a distance of 0.5 mm.
Figure 10. Photolysis of 3 × 1015 cm-3 H2O2. OH concentration has been calculated to of 6 × 1013 cm-3 from 400 mJ/laser pulse.
an experiment in 9 Torr O2. As can be seen, the signal intensity increases slightly after addition of 3 Torr O2, while it decreases in 9 Torr O2. Unfortunately, LIF is not an absolute technique, and quenching phenomena have to be taken into account before comparing signal intensities obtained under different experimental conditions. Oxygen is known to be a very efficient quencher for OH fluorescence (kquenching ) 1.35 × 10-10 cm3 s-1),19 owing that even though the LIF signal increases only slightly after addition of 3 Torr O2, we estimate that the OH and H2O2-concentration increases significantly after addition of 3 Torr O2. Replacing the total He by O2, the LIF signal clearly decreases; we believe that this can be attributed to an increase in fluorescence quenching with the OH and H2O2 concentration being comparable to the experiment with 3 Torr O2. To evaluate the temporal evolution of the signal, we have plotted in Figure 8b the same signals, but here the maximum LIF intensity of the first peak has been normalized for all three conditions. No influence of O2-concentration on the rise of the signal is visible, while the decrease, and with this the delay to reach the second maximum, is more and more delayed with increased O2concentration. This again can be attributed to a slower diffusion of OH and H2O2 in O2 compared to Helium. The second peak, attributed to H2O2, seems to reach the same height for all conditions, even though one would expect a decrease with increased diffusion time (see ref 6), it is therefore probable that the increase in H2O2 concentration is more pronounced than the increase in OH concentration. More experiments using a better adapted technique to obtain quantitative results are needed; absorption techniques would be much better suited for this kind of study, as their signals depend less on the reaction conditions. Influence of VOC on the LIF Signal. We have then studied the influence of hydrocarbons on the signal shape. Figure 9 shows signals obtained at the shortest possible distance in 9 Torr total pressure. The black triangles represent signals in pure He, and the typical double-peak signal is obtained. After adding small amounts of methyl ethyl ketone (MEK), the signal shape changes completely; it is a striking observation that only one peak is visible anymore. This peak appears at a delay in-between the former two peaks. Unfortunately, our LIF technique cannot distinguish selectively if the remaining signal is due to OH or H2O2; the measurement of the absolute signal height as a function of the laser energy is much less precise than the measurement of the ratio of the two peak-intensities. However, OH is reacting rapidly even in the gas phase with OH radicals. It is therefore highly probable that OH radicals would rather react with MEK than appearing at a longer delay in the observation volume. Raillard et al.20 report that the influence of water vapor on photocatalytic oxidation of MEK highly depends on relative humidity (RH). Nevertheless they notice that under 30% RH the photocatalytic reaction rate is increased.
This behavior is consistent with the generally proposed mechanism for photocatalytic oxidation of ketones, H-abstraction from R-carbon by HO• (Vincent et al.21). The subsequent β-scission of the aliphatic chain leads to the formation of alkyl radicals which highly react with HO• to produce alcohols. MEK photocatalytic oxidation can be considered as a highly HO• consuming process. We therefore believe that the remaining signal is due to H2O2. The signal intensity seems to increase and possibly the mechanism of H2O2 formation has changed in the presence of MEK also because the delay has become much shorter. However, much more detailed investigations using other analytical techniques, for example, correlating the photoconductivity with selective measurements of ROS in the gas phase, will be necessary to better understand the mechanism induced by addition of VOCs. Calibration of OH and H2O2 Signals. We have attempted to calibrate our LIF signals to estimate a quantum yield for the formation of OH radicals and H2O2 molecules in the gas phase. To do so, we have photolyzed H2O2 molecules in the gas phase and have measured the obtained signal intensities. Stable but unknown concentrations of H2O2 in the reaction cell can be obtained by bubbling a small fraction of the He-flow through a H2O2-solution. It is well-known22,23 that the photolysis of H2O2 is leading to the formation of 2 OH radicals
H2O2 + hν248 nm f 2OH These OH radicals will subsequently react with the remaining excess H2O2 (only a few percent of H2O2 will be photolyzed at each laser pulse under our conditions)
OH + H2O2 f HO2 + H2O The rate constant for this reaction is well-known (k ) 1.7 × 10-12 cm3 s-1),24 we will therefore be able to calculate the H2O2 concentration, present in the reaction cell, from the decay rate of OH radicals. Losses of OH radicals due to diffusion out of the detection volume or due to reaction with impurities have been neglected, as we know from former experiments, that these losses are on the order of 100 s-1 or less, much slower than the decays measured in the presence of H2O2 (several 1000 s-1). From the employed photolysis energy and the well-known absorption cross section we can then calculate the OH concentration, present at short delays after the laser pulse. Figure 10 shows such a kinetic OH decay, following the photolysis of H2O2. The signal before the photolysis pulse is indicated as 0, which is typical for laser photolysis experiments; in general,
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Figure 12. Signal for calibration: distance 0.5 mm, p ) 4.5 Torr.
Figure 11. (a) Scan of the fluorescence excitation line at two different delays between photolysis and excitation: 5 and 1000 µs. (b) Integration of the fluorescence intensity for both scans: I5 µs ) 108, I1000 µs ) 55.
some stray light is always detected from the fluorescence excitation laser, that is, there is a signal even in the absence of any OH radical or with a fluorescence excitation wavelength off an OH-absorption line. This background signal is taken into account by measuring the LIF signal just before the laser pulse and is subsequently deduced from each LIF signal, bringing evidently to 0 the signal just before the photolysis laser pulse. In our case, however, we do not only observe some possible stray light, but also the signal due to the photolysis of H2O2 with subsequent excitation of OH radicals, that is, the process at the origin of the second peak. This is also the reason why the LIF signal decays below zero at longer reaction times; due to the photolysis and also the subsequent reaction of OH with H2O2, the H2O2 concentration after the photolysis laser pulse, and especially at longer reaction times, is lower than just before the laser pulse. As a consequence, subtracting the background signal generated by the higher H2O2 concentration before the photolysis pulse brings the signal below zero. We can precisely determine the signal intensity for a known H2O2 concentration by measuring the signal intensity just before the photolysis pulse, and the signal intensity for the known OH concentration by measuring the signal intensity at a very short delay after the photolysis pulse. We can then compare these calibrated signal intensities to signal intensities obtained under similar reaction conditions (especially H2O and O2 concentrations for comparable quenching effects) in the photocatalysis experiments. This will enable us to estimate the OH and H2O2 concentration in the gas phase above the photocatalytic surface. OH absorption lines are very narrow, and it is challenging to keep the laser emission precisely at the wavelength corresponding to the peak of the absorption line. To minimize the possible uncertainty due to a drift in the excitation laser wavelength between the calibration experiment and the photocatalytic experiment, we have not simply compared the signal heights, but we have scanned the full fluorescence excitation line and integrated the line strength. This is shown in Figure 11 for the H2O2 photolysis signal from Figure 10; the red dots in Figure 11a correspond to the absorption line at 50 µs before the
Figure 13. Scan of the fluorescence intensity at a very short delay and at 500 µs, integrated intensity is I5 µs ) 15.9, I500 µs ) 13.3.
photolysis pulse, leading to an integrated line strength of 50 au (arbitrary units), as shown in Figure 11b, that is, this signal has been generated by 3.0 × 1015 cm-3 H2O2 molecules. The black dots in Figure 11, obtained at 5 µs, correspond to a line strength of 105 au, that is, representing the signal generated by the sum of 6 × 1013 cm-3 OH radicals and 2.94 × 1015 cm-3 H2O2 molecules. We have then measured in the same way the line strengths presented in Figure 13 at the maxima of both peaks of a photocatalytic signal, that is, at 5 µs (17 au) and 500 µs (13 au) in the example of Figure 12. These line strengths can now be converted into absolute concentrations by comparing them with the above obtained calibration factors. We find in the example from Figure 12 for OH, 1.8 × 1013 cm-3 ((6 × 1013/(105-50) × 17), and for H2O2, 1.8 × 1014 cm-3 (3 × 1015/ 50 × 13). These numbers have to be compared by the number of incident photons; the average excitation laser fluence was around 1017 photons cm-2 pulse-1. The concentrations have been measured in a volume and have to be transformed into total concentrations generated per laser pulse. To do so, we assume that the concentration profile is uniform for 2 mm above the surface. This leads to the following absolute concentrations: [OH] ) 3.6 × 1012 radicals pulse-1 and [H2O2] ) 1.6 × 1014 molecules pulse-1. Even though we are aware that the assumption can only be a rough guess, we think that the order of magnitude of this estimation is meaningful. The obtained concentrations can now be converted into quantum yields: φOH ) 3.6 × 10-5 and φH2O2 ) 1.6 × 10-3. These values can be compared to the few literature values available so far. The most recent determination for the quantum yield of OH-radicals comes from Murakami et al.;8 they have compared the OH-LIF signal obtained after irradiation of TiO2powder with a 266 nm laser pulse with the OH signal obtained from the photolysis of a known concentration of HNO3 under the same conditions. They estimated the quantum yield to be 5 × 10-5, which is in excellent agreement with our results. A more indirect determination of the quantum yield has been done by Tatsuma et al.;10 they have measured the yield of CO2 from
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remote oxidation of polystyrene and polyethylene and have obtained quantum yields of 20 × 10-5 and 3 × 10-5 for both components. This quantum yields can be compared to OHquantum yields if one supposes that OH-radicals are the reactive species and that for each CO2 molecule formed one OH-radicals is necessary. Quantum yields for gas phase H2O2 have already been published by Kubo et al.;14,25 absolute values for H2O2 were determined by bubbling the gas after passing above the photocatalytic surface through a scavenger solution. They estimated a quantum yield for the H2O2 release into the gas phase of only 1.4 and 1.8 × 10-7, which is 4 orders of magnitude lower than our values. They state that with their method some H2O2 might be photolyzed before reaching the scavenger solution; each photolyzed H2O2 would furthermore destroy up to two other H2O2 molecules by reaction with OH. They also mention that possibly not 100% of the H2O2 might be scavenged by the solution. Our results of course bear also some possible errors; the reaction conditions between calibration and photocatalytic experiment are not exactly the same, leading to different fluorescence quenching efficiencies between calibration and photocatalytic experiment. However, we estimate that the accuracy is better than a factor of 2. Also, we cannot be sure that no OH radicals are present at 500 µs in the example shown in Figure 12, contributing to the LIF signal; this would lead to an overestimation of H2O2, but again, looking at the decay of the first peak, we do not think that this error source is very important, especially as we are seeking 4 orders of magnitude. On the other hand, the experimental conditions in both experiments are very different, especially the photon density; the laser excitation used in our experiments generates on a very short time scale high concentrations of e-/h+ pairs and with this other reactive species, therefore recombination reactions will be enhanced, which could lead to an increase in H2O2 through (R4). However, a high photon flux would also lead to an increased recombination of h+ and e-, which would lead to an overall decrease in photocatalytic efficiency and therefore in the quantum yield. Also, an increased recombination of OH radicals should in turn lead to a decrease in the quantum yield of OH radicals; however, our observed quantum yield for OH radicals is already 200 times higher than the quantum yield for H2O2, observed by Kubo et al.14 Interestingly, our quantum yield for OH radicals is the same order of magnitude than the quantum yield for remote oxidation, obtained by Tatsuma et al.10 Nevertheless, 4 orders of magnitude is a very important disagreement and more experiments are thus needed under various reaction conditions, using different and more selective detection methods for H2O2 to understand this disagreement. Conclusion We have presented in this work the direct detection of OH radicals and the indirect detection of H2O2 molecules in the gas phase by laser-induced fluorescence after UV-irradiation of a TiO2 surface. Experiments have been performed not only as a function of the distance between the photocatalytic surface and the detection volume, but also as a function of the total pressure; the evolution of the profiles can be satisfactorily explained by diffusion phenomena. Also, the excitation laser wavelength has been changed from 248 to 351 nm to verify that the LIF signal does not originate in the photolysis of adsorbed H2O2 by the excitation laser, but is linked to the formation of OH on the surface and its subsequent diffusion into the gas phase. O2 has
Thiebaud et al. been added in various concentrations, and it seems possible that the concentration of the released OH radicals increased. Important changes in the shape of the signal have been observed after addition of MEK as a proxy for hydrocarbons, interpreted as the complete disappearance of OH radicals in the gas phase and a faster appearance of H2O2. However, more detailed work using other detection methods is needed to fully understand the reasons for these changes and possibly get a new view from another perspective onto the photocatalytic oxidation mechanism. Experiments have been performed to estimate the quantum yield of OH and H2O2 formation. The quantum yield for OH radicals is in perfect agreement with values found in the literature, while the quantum yield for H2O2 has been found higher, in complete disagreement with the literature. More selective experiments will be necessary to understand this disagreement. Acknowledgment. Financial support by the Re´gion Nord/ Pas de Calais within the framework of IRENI, by the CNRS and the European funds for Regional Economic Development FEDER are acknowledged. The authors thank A. Aluculesei for help in the execution of some experiments. The authors are grateful to Paul-Marie Marquaire for having brought the idea of radical detection over photocatalytic surfaces to the authors. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem.Photobiol., C 2000, 1, 1–21. (3) Peral, J.; Ollis, D. F. J. Mol. Catal A: Chem. 1997, 115, 347–354. (4) Mills, A.; Hunte, S. L. J. Photochem.Photobiol., A 1997, 108, 1– 35. (5) Shiraishi, Y.; Hirai, T. J. Photochem.Photobiol., C 2008, 9, 157– 170. (6) Vincent, G.; Aluculesei, A.; Parker, A.; Fittschen, C.; Zahraa, O.; Marquaire, P.-M. J. Phys. Chem. C 2008, 112, 9115–9119. (7) Murakami, Y.; Kenji, E.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem. B 2006, 110, 16808–16811. (8) Murakami, Y.; Endo, K.; Ohta, I.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem. C 2007, 111, 11339–11346. (9) Tatsuma, T.; Tachibana, S. I.; Miwa, T.; Tryk, D. A.; Fujishima, A. J. Phys. Chem. B 1999, 103, 8033–8035. (10) Tatsuma, T.; Tachibana, S. I.; Fujishima, A. J. Phys. Chem. B 2001, 105, 6987–6992. (11) Tatsuma, T.; Tachibana, S.-i.; Fujishima, A. J. Phys. Chem. B 2001, 105, 6987–6992. (12) Kubo, W.; Tatsuma, T. J. Am. Chem. Soc. 2006, 128, 16034–16035. (13) Thiebaud, J.; Parker, A.; Fittschen, C.; Vincent, G.; Zahraa, O.; Marquaire, P.-M. J. Phys. Chem. C 2008, 112, 2239–2243. (14) Kubo, W.; Tatsuma, T. Anal. Sci. 2004, 20, 591–593. (15) Park, J. S.; Choi, W. Langmuir 2004, 20, 11523–11527. (16) Park, J. S.; Choi, W. Chem. Lett. 2005, 34, 1630–1631. (17) Lee, M. C.; Choi, W. J. Phys. Chem. B 2002, 106, 11818–11822. (18) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. JPL Publ. 1997, 97-4, 1–266. (19) Wysong, I. J.; Jeffries, J. B.; Crosley, D. R. J. Chem. Phys. 1990, 92, 5218–5222. (20) Raillard, C.; He´quet, V.; Cloirec, P. L.; Legrand, J. Appl. Catal., B: 2005, 59, 213–220. (21) Vincent, G.; Marquaire, P. M.; Zahraa, O. J. Photochem. Photobiol., A 2008, 197, 177–189. (22) Thiebaud, J.; Aluculesei, A.; Fittschen, C. J. Chem. Phys. 2007, 126, 186101. (23) Vaghjiani, G. L.; Ravishankara, A. R. J. Chem. Phys. 1990, 92, 996–1003. (24) Sander, S. P.; Orkin, V. L.; Kurylo, M. J.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Finlayson-Pitts, B. J.; Molina, M. J.; Friedl, R. R.; Ravishankara, A. R.; Moortgat, G. K.; Keller-Rudek, H.; Wine, P. H. JPL Publ. 2006, 06-2. (25) Kubo, W.; Tatsuma, T.; Fujishima, A.; Kobayashi, H. J. Phys. Chem. B 2004, 108, 3005–3009.
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