Direct Detection of OH Radicals and Indirect Detection of H2O2 Molecules in the Gas Phasenear a TiO2 Photocatalyst Using LIF

Guillaume Vincent,‡ Alina Aluculesei,† Alexander Parker,† Christa Fittschen,*,† Orfan Zahraa,‡and Paul-Marie Marquaire‡

Physico-Chimie des Processus de Combustion et de l’Atmosphere (PC2A), CNRS UMR 8522, UniVersite desSciences et Technologies de Lille, F- 59655 VilleneuVe d’Ascq Cedex, France, and Departement de Chimie Physiquedes Reactions (DCPR), Nancy-UniVersite, CNRS, 1 rue GrandVille, BP 20451, F-54001 Nancy France

ReceiVed: March 27, 2008; ReVised Manuscript ReceiVed: May 5, 2008

The formation of OH radicals and its diffusion into the gas phase during the excitation of TiO2 in the presenceof H2O has been studied using the very sensitive and selective detection method of laser-induced fluorescence(LIF). The time-resolved evolution of the OH radical concentration has been observed at pressures between4 and 600 Torr and at varying distances between the photocatalytic surface and the detection volume. OHradicals have been detected even at the highest pressures, opening the assumption that gas-phase reactionswith OH radicals may very well be involved in the photocatalytic degradation of VOCs, even at atmosphericpressure. Interestingly, a second fluorescence signal peak has been observed at longer delays with respect tothe excitation pulse. The use of different fluorescence laser energies leads us to interpret this second peak asH2O2, released from the surface into the gas phase and detected by a two-photon process; in fact, we usefluorescence laser fluencies high enough to photolyze H2O2 molecules at the fluorescence-excitation wavelength(282 nm) and excite the generated OH radicals within the same laser pulse.

Introduction

Water and air purification by photocatalysis can be consideredas a major challenge for the years to come.1 Photocatalyticprocesses use a semiconductor photocatalyst, usually TiO2, asa slurry or deposited on a support. The semiconductor is exposedto near-UV light (<387 nm) and generates electron-hole pairse-/h+ separated by the valence band (VB) and the conductionband (CB). In the presence of air and humidity, electrons (e-CB)and holes (h+

VB) induce reduction and oxidation processesproducing reactive oxygenated species (ROS) such as OH•,HO2

•, H2O2, and 1O2,2,3 for example, through the followingreactions

TiO2 + hνf h++ e-

h++H2OfOH• +H+

O2 +H++ e-fHO2•

2OH•fH2O2

HO2• + e-+H+fH2O2

2HO2•fH2O2 +O2

O2 + e-fO2•-

H2O2 +O2•-fOH• +OH-+O2

On the basis of the above reactions, one can suggest that theROS are formed on the surface of the catalyst, although it iswell-known that the photocatalytic degradation occurs whenthese adsorbed species react with adsorbed organic compounds.

However, several authors have recently highlighted thediffusion of ROS into the gas phase.4–7,11 According to them,the ROS can diffuse far from the surface and induce oxidationreactions close to the photocatalyst surface in the gas phase.However, in a very recent attempt to detect HO2

• radicals bycw-CRDS,8 no radicals were observed during the photocatalyticdegradation of methyl ethyl ketone, with an estimated limit ofdetection of 3 × 109 molecule cm-3.

Recently, Murakami et al.4,5 reported evidence for the firstsuccessful detection of the presence of OH• radicals near thesurface of TiO2 by direct detection using LIF spectroscopy atvery low pressure (0.5 Torr). They have studied severalparameters such as the distance from the surface (5-8 mm),the gas nature (He, H2O, D2O, and O2), and the influence ofcalcination temperature on the OH-LIF intensity.

Furthermore, Tatsuma et al.6,7 have used indirect methods inorder to highlight the diffusion of ROS into the gas phase. Theyhave observed the degradation of different organic filmsseparated by an air gap (50 µm to 2.2 mm) from the TiO2

photocatalytic material.

Investigations with the colorimetric method performed byKubo et al.9,10 on hydrogen peroxide formation by the exposureof TiO2 to UV have detected H2O2 in gas flowing out of thephotocatalyst.

* To whom correspondence should be addressed. E-mail: christa.fittschen@univ-lille1.fr.

‡ Nancy-Universite.† Universite des Sciences et Technologies de Lille.

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Published on Web 05/30/2008

2008, 112, 9115–9119

Park and Choi11 have highlighted the diffusion of OH• radicalsin the gas phase during the photocatalytic degradation of stearicacid. Moreover, they have also shown the remote photocatalyticoxidation mediated by active oxygen species diffusing throughan organic polymer membrane over surface-fluorinated TiO2.12

Lee and Choi13 have studied the photocatalytic oxidation of sootfilm deposited on TiO2. To conclude, they claimed the migrationof OH• radicals in all media (gas, liquid, and solid phases) duringa photocatalytic process.

In this study, the LIF technique has been employed in orderto detect OH• radicals close to the TiO2 surface in the gas phase.In the absence of oxygen and VOCs, several experimentalparameters have been investigated such as the distance fromthe catalyst surface (0-9 mm), pressure (4-600 Torr) andenergy of the fluorescence-excitation laser. Two main distinctpeaks have been observed; one is attributed to the diffused OH•

radicals, and the other corresponds to H2O2 in the gas phase, asillustrated in Figure 1.

Experimental Details

Experiments have been performed by laser photolysis/laser-induced fluorescence LIF, schematically shown in Figure 2.

The reaction cell is made of stainless steel and consists of acentral cross with an arm connected to each of the six openings.The main axis (hereafter denoted the x-axis) has a total lengthof 75 cm, while the two other axes are shorter (40 cm each);one short axis runs parallel to the table and is hereafter referredto as the y-axis, and the other runs vertical to the table and iscalled the z-axis. The photocatalytic medium used in thisexperiment was industrial titanium dioxide (Millennium PC 500,100% anatase) coated nonwoven paper, produced by Ahlstrom,the same as that described in our previous paper on the formationof HO2 radicals.8 A disk of this paper with a diameter of 2 cmwas fixed on a linear motion vacuum feedthrough (Caburn),connected to one of the openings of the y-axis, that is, the paperwas located parallel to the xz-plane and can be moved up to 5cm along the y-axis. The photocatalytic process was initiatedby an Excimer laser pulse (Lambda Physik LPX 202i), operatedat 248nm with an average pulse energy of 400 mJ/pulse. TheExcimer laser entered the reaction cell at the opposite openingof the y-axis through a quarz window (diameter of 2 cm) andwas aligned in order to directly strike the center of thephotocatalytic support.

The relative concentration of OH radicals was determinedfrom the integrated LIF intensity. The probe laser was afrequency-doubled dye laser (Quantel TDL 50, Rhodamin 590)

pumped by a frequency doubled YAG laser (Quantel YG 780).The probe beam was aligned in order to propagate parallel tothe center of the photocatalytic support, that is, along the x-axisof the cell. In order to get a good spatial and thus temporalresolution, the original beam (beam profile: 6 × 4 mm2, 3-10mJ/pulse) was focused through a 50 cm quartz lens into thecenter of the cell, that is, the beam diameter was minimal (wellbelow 1 mm) when passing the photocatalytic support. Theprobe beam passed through an attenuator (Newport ModelM-935-10) permitting easy variation of the pulse energy by 1order of magnitude. OH radicals were excited at 282 nm, andthe fluorescence was collected along the z-axis through twolenses and an interference filter (307 ( 5 nm fwhm). Thefluorescence 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 excitationpulses were obtained by way of a digital delay generator (EG&G9650), controlled by a PC. A typical decay consists of 20-50points at different delays between the two lasers; at each delay,the fluorescence is averaged over typically 30 laser shots. Allexperiments were performed at a repetition rate of 10 Hz. Theentire experiment was controlled by a Labview program.

The He (4.5) (Air-Liquide) carrier gas was used withoutfurther purification, and a part or the entire He flow was bubbledthrough distilled water. The gas flows were regulated withcalibrated mass flowmeters (Tylan FC-260). At a total pressureof 5 Torr, a typical total flow rate of 60 cm3 min-1 STP leadsto a gas flow velocity through the cell of 9 cm s-1 in thedirection of the x-axis. Experiments were performed in thepressure range of 5-600 Torr of helium, with the flow rateshaving been adapted in order to always have approximately thesame relative H2O concentration as well as the same flowvelocity.

Results and Discussion

LIF profiles have been observed for three different series ofexperimental conditions.

1. At a fixed low pressure (4.5 Torr), we have studied thetime-resolved LIF signals as a function of the distance betweenthe photocatalytic support and the detection volume. (Figure3a and 3b).

2. LIF signals have been detected at a fixed distance betweenthe photocatalytic support and the detection volume (1 mm) atpressures between 6 and 554 Torr (Figures 4 to 6)

3. LIF profiles have been studied at a fixed pressure (4.5 Torr)and distance (0 mm) at two different fluorescence-excitationlaser fluencies (Figure 7).

1. LIF Signals at 4.5 Torr of Total Pressure As a Functionof the Distance between Photocatalytic Support and ProbeVolume. Typical OH radical concentration time profiles areshown in Figure 3; the x-axis represents the delay between the248 nm excimer laser pulse and the 282 nm dye laser pulse,and the y-axis shows the fluorescence intensity in arbitrary units.This fluorescence intensity is proportional to the OH radicalconcentration for a given wavelength, temperature, and bath gasconcentration. Each data point is the average of 30 laser pulses(3 s at 10 Hz repetition rate). Curves are shown for differentdistances between the photocatalytic support and the probevolume; the green dots are given as 0 mm, which means thatthis is the shortest feasible distance; any shorter distance createdan important parasite signal on the photomultiplier due to strongdiffusion of the dye laser pulse when touching the TiO2 surface.

As can be clearly seen in Figure 3a and b (the same signalsare shown, in Figure 3a, up to 5000 µs, and in Figure 3b,

Figure 1. Schematic representation of OH radicals and H2O2 diffusioninto the gas phase during a photocatalytic process.

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zoomed in to 100 µs), two distinct maxima are observed forshort distances between the photocatalytic support and the probevolume. While this behavior is still clearly visible at 2 mm (bluediamonds), already at a distance of 3 mm, the difference betweenthe two maxima can only vaguely be discerned. The concentra-tion time profile at “0 mm” can be regarded as the time-resolvedrelease of OH radicals from the surface into the gas phase, whilethe other signals at longer distances are the convolution of thistime-resolved behavior and the diffusion into the probe volume.It is therefore probable that the diffusion is responsible for thedelayed maxima of the signals at longer distances. Also, theintensity of the maximum OH concentration decreases withincreasing distance, which can also be attributed to diffusion;OH radicals generated on the surface will diffuse into all threedirections and hence will be diluted.

The nature and behavior of the “second peak”, that is, asecond increase of the fluorescence intensity at longer delayswith respect to the TiO2 excitation, clearly distinct at shortdistances, is less obvious. In fact, to our knowledge, such anevolution of the OH fluorescence signal with time has neverbeen observed before. We have interpreted this signal as H2O2

molecules (see point 3 for further explanation), and again, thesignal at “0 mm” can be interpreted as the time-resolved releaseof H2O2 into the gas phase, while the signal at longer distanceswill be the convolution with diffusion. The release of H2O2

seems to be finished at around 500 µs; the decrease of the signalthereafter is probably due to diffusion. In a first approximation,one can interpret the time constant of H2O2 release and/orbuildup as the reaction time necessary for the formation of H2O2

on the surface by heterogeneous recombination of OH radicals.2. OH Profiles at a Fixed Distance As a Function of the

Total Pressure. In another experiment, we have measured theOH concentration time profile at a fixed distance (1 mm) atdifferent pressures. In Figure 4 are shown the results of fourexperiments between 6 and 554 Torr, and in Figure 5, the samegraphs are shown using the appropriate y-axis for the differentsignal intensities. Again, two distinct maxima can be clearlyseen at the two lowest pressures; at the higher pressures, thesetwo maxima are probably convoluted due to diffusion. Theintensity of the signal decreases strongly with increasingpressure. The reason is not necessarily a decrease in the OHconcentration but can also be due to increased quenching ofthe OH fluorescence with increasing pressure. No further

Figure 2. Schematic view of the 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.

Figure 3. OH fluorescence signal at 4.5 Torr of total pressure as afunction of delay between the photolysis and the excitation laser, fordifferent distances between the photocatalytic surface and the detectionvolume; (a) delay(max) ) 5000 µs; (b) zoomed in on the first 100 µs.

Figure 4. OH fluorescence signal as a function of delay between thephotolysis and excitation lasers at a distance of 1 mm between thephotocatalytic surface and the detection volume for four differentpressures between 6 and 554 Torr. The inset shows a zoom of the graphfrom 0 to 1000 µs.

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experiments have been performed to quantify this point. Themost interesting aspect of this series is the fact that, even atpressures up to 554 Torr, OH radicals have been observed inthe gas phase. This means that it is highly probable thatexperiments on the photocatalytic degradation of VOCs atatmospheric pressures are at least partly due to reaction withOH radicals in the gas phase. In order to check if the peaks at

higher pressure are due to OH radicals in the gas phase and notto H2O2, we have plotted in Figure 6 the delay of the peakmaximum as a function of the pressure; a very good linearrelationship as expected for a diffusion-controlled process isobserved for the four pressures. We are thus confident that thesignal observed at the highest pressure is at least partly due toOH radicals in the gas phase.

We have shown the diffusion in the gas phase of both OH•

radicals and H2O2 during a photocatalytic process. From ourpoint of view, the OH• radical is the most active species in theremote photocatalytic oxidation, as suggested by Murakami etal.4,5 and Park and Choi.11,12 It can be noticed that a germicidelamp (254 nm) could be used in order to improve thephotocatalytic efficiency by the photolysis of H2O2, which ispresent in the gas phase.

OH• radicals appear to be a strong oxidant for volatile organiccompounds (VOCs) using the same radical mechanisms likethose in atmospheric chemistry and combustion. In all cases,OH• radicals react with an organic molecule by metathesis(hydrogen transfer): OH• + RH f H2O + R• f ... It can benoticed that H2O2 is inactive (without photolysis), but thisspecies can react with OH• radicals and other radicals (R•,HO2

•, ...) and should therefore been taken into account in theremote photocatalytic oxidation mechanism.

3. OH Profiles at Fixed Distance and Pressure As aFunction of Excitation Laser Energy. In order to verify ourhypothesis that the second fluorescence signal peak at longerdelays, always clearly observed at low pressures or shortdistances, is due to H2O2, we have performed experiments at afixed pressure and a fixed distance but at varying excitation laserenergies. Without oxygen in the reaction cell, the productionof H2O2 is due to the recombination of OH radicals at TiO2

surfaces. In fact, it is well-known that H2O2 absorbs at 282 nm,the wavelength used in our experiment for the excitation of theOH fluorescence. H2O2 will be photolyzed into two OH radicals,which, in principle, can be excited within the same laser pulse.This OH fluorescence is the result of a two-photon process, andits intensity depends thus on the square of the laser fluence.

Figure 5. The same signals as those in Figure 3, represented for better visibility in four different graphs, using different y-axes adjusted to thecorresponding signal intensities. The inset in the upper left graph shows a zoom of the signal from 0 to 100 µs.

Figure 6. Plot of 1/tmax as a function of 1/p for the signals from Figures4 and 5. The inset shows a zoom of the graph from 0 to 0.02 Torr-1.

Figure 7. OH fluorescence signal for two different excitation laserenergies. The signals have been normalized to match within the first100 µsec (shown as the zoom in the inset).

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The interpretation of the two maxima observed in this work isthat the first peak is due to OH radicals released into the gasphase (i.e., a one-photon process), while the second peak is dueto H2O2 released into the gas phase (i.e., a two-photon process).Under these conditions, the ratio of the intensity of the two peaksshould depend on the dye laser fluence; the intensity of the firstmaximum will decrease linearly with energy, while the intensityof the second maxima will decrease with the square of theenergy. The OH fluorescence profiles from two experiments (4.5Torr, 0 mm above the surface) are shown in Figure 7; the laserfluence has been decreased by roughly a factor of 5 in theexperiment represented by the green dots. The signals have beennormalized to match each other in the first 100 µs. It can beclearly seen that the ratio of the intensities is strongly dependenton the laser fluence. We are thus highly confident that the secondfluorescence peak is due to H2O2 in the gas phase.

In a recent paper on the direct detection of OH radicals,4,5

the existence of the second peak, due to H2O2, was notmentioned. The authors did not state the energy used for thefluorescence-excitation, but the fluence used in this work isprobably much higher due to the focusing of the excitation laserbeam to a spot size well below 1 mm (Murakami et al.4,5 statea probe beam diameter of 2 mm).

Conclusion

We have presented in this work the direct detection of OHradicals and the indirect detection of H2O2 molecules in the gasphase by laser-induced fluorescence after UV irradiation of aTiO2 surface. Experiments have been performed not only as afunction of the distance between the photocatalytic surface andthe detection volume but also as a function of the total pressureand the excitation laser energy. For the first time, OH radicalshave been detected in the gas phase even at high pressure, closeto atmospheric conditions, leading to the assumption that the

photocatalytic degradation of VOCs might be at least partiallydue to a gas-phase reaction of the VOCs with OH radicals.Furthermore, also for the first time, the time-resolved releaseof H2O2 molecules into the gas phase has been detectedindirectly by LIF; due to the high laser fluence, H2O2 moleculesare photolyzed at 282 nm, and the generated OH radicals aredetected by LIF.

Acknowledgment. Financial support by the Region Nord/Pas de Calais within the framework of IRENI, by the CNRS,and the European funds for Regional Economic DevelopmentFEDER are acknowledged. A.A. and A.P. thank the EC forfinancial support within the project Marie-Curie EST-CT-2005-020659.

References and Notes

(1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol.,C 2000, 1, 1–21.

(2) Peral, J.; Ollis, D. F. J. Mol. Catalysis A: Chem. 1997, 115, 347–354.

(3) Mills, A.; Hunte, S. L. J. Photochem. Photobiol., A 1997, 108, 1–35.

(4) Murakami, Y.; Kenji, E.; Nosaka, A. Y.; Nosaka, Y. J. Phys. Chem.B 2006, 110, 16808–16811.

(5) Murakami, Y.; Endo, K.; Ohta, I.; Nosaka, A. Y.; Nosaka, Y. J.Phys. Chem. C 2007, 111, 11339–11346.

(6) Tatsuma, T.; Tachibana, S. I.; Miwa, T.; Tryk, D. A.; Fujishima,A. J. Phys. Chem. B 1999, 103, 8033–8035.

(7) Tatsuma, T.; Tachibana, S. I.; Fujishima, A. J. Phys. Chem. B 2001,105, 6987–6992.

(8) Thiebaud, J.; Parker, A.; Fittschen, C.; Vincent, G.; Zahraa, O.;Marquaire, P.-M. J. Phys. Chem. C 2008, 112, 2239–2243.

(9) Kubo, W.; Tatsuma, T. Anal. Sci. 2004, 20, 591–593.(10) Kubo, W.; Tatsuma, T. J. Am. Chem. Soc. 2006, 128, 16034–16035.(11) Park, J. S.; Choi, W. Langmuir 2004, 20, 11523–11527.(12) Park, J. S.; Choi, W. Chem. Lett. 2005, 34, 1630–1631.(13) Lee, M. C.; Choi, W. J. Phys. Chem. B 2002, 106, 11818–11822.

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