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2008, 112, 2239-2243 Published on Web 01/31/2008
Detection of HO2 Radicals in the Photocatalytic Oxidation of Methyl Ethyl Ketone Je´ roˆ me Thiebaud,† Alexander Parker,† Christa Fittschen,*,† Guillaume Vincent,‡ Orfan Zahraa,‡ and Paul-Marie Marquaire‡ 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 de Chimie Physique des Re´ actions (DCPR), Nancy-UniVersite´ , CNRS, 1 rue GrandVille, BP 20451, F-54001 Nancy France ReceiVed: December 3, 2007; In Final Form: January 10, 2008
The possibility of HO2 formation and its diffusion into the gas phase during the photocatalytic degradation of methy ethyl ketone (MEK) on TiO2 has been studied using the very sensitive and selective detection method of continuous wave cavity ring down spectroscopy (cw-CRDS), coupled to a continuous photolysis reactor. No HO2 radicals could be detected, given a detection limit of 3.2 × 109 cm-3.
Introduction Photocatalytic oxidation appears to be a promising process for the removal of volatile organic compounds (VOCs) or volatile odor compounds from polluted air.1 Generally, photocatalyzed reactions can be easily summarized by the mineralization of the organic compound as below2,3 semiconductor (TiO2)
organic pollutant + O2 9 8 hν g E g
CO2 + H2O + mineral acids Ideally, a semiconductor photocatalyst must be chemically inert, efficiently activated by UV and able to efficiently catalyze reactions. Titanium dioxide (TiO2) appears to be an ideal photocatalyst. Photonic activation of TiO2 with sufficient energy (hν g Eg) leads to a charge separation with an electron (e-cb) promoted to the conduction band (cb) and the generation of a positively charged hole (h+vb) in the valence band (vb). The anatase crystalline structure of TiO2, which is the most photoactive, possesses a band gap (Eg) equal to 3.20 eV which can be overcome with near-ultraviolet radiation at a wavelength of e387.7 nm. The main processes occurring on a semiconductor are illustrated in Figure 1. Photogeneration of Reactive Oxygen Species. The holes in the TiO2 valence band (vb) can react with adsorbed H2O (Dads) or surface titanol groups (>TiOH) to form hydroxyl radicals (OH•) as below
h+vb + H2Oads f OH•ads + H+ads Electrons in the conduction band (cb) on the catalyst surface can reduce molecular oxygen (Aads) to produce the superoxide anion (O2•-) via the following reaction: -
e
•-
cb
+ O2ads f O2
ads
* Corresponding author. E-mail:
[email protected]. † Universite ´ des Sciences et Technologies de Lille. ‡ Nancy-Universite ´.
10.1021/jp711388k CCC: $40.75
In the presence of adsorbed protons, the superoxide anion can produce hydroperoxyl radicals (HO2•) and then hydrogen peroxide (H2O2) which can decompose on the TiO2 surface either by UV radiation or by reaction with the superoxide anion to produce hydroxyl radicals
O2•-ads + H+ads f HO2•ads HO2•ads + e-cb + H+ads f H2O2ads H2O2ads + hν f 2OH•ads H2O2ads + O2•-ads f OH•ads + OH-ads + O2ads The secondary reactions with activated oxygen species in the photocatalytic process are summarized in Figure 2. The proposed mechanisms of photocatalytic degradation are mainly based on the photocatalytic generation of active oxygen species on TiO2 surfaces. However, recently4-6 it has been suggested that oxidation reactions could take place also in the gas phase. Aromatic and aliphatic substances were oxygenated and decomposed to CO2, probably by active oxygen species generated on the TiO2 surface and subsequently transported into the gas phase.5 In a previous work, a complete study of photocatalytic oxidation of methyl ethyl ketone (MEK) has been investigated in the gas phase using TiO2 Degussa P25. In this work, the very sensitive spectroscopic method of cw-CRDS (continuous wave cavity ring-down spectroscopy) has been employed in order to attempt to detect and quantify the possible diffusion of hydroperoxyl radicals (HO2•) into the gas phase during the photocatalytic process. Hydroxyl radicals (OH•), which are the most active, can be detected by LIF (laser-induced fluorescence) and will be the subject of a later paper. Experimental Details The continuous wave cavity ring-down spectrometer, which is schematically shown in Figure 4, is similar to the one described in our previous paper.7 However, the photolysis system was changed from a pulsed excimer laser to UV lamps in order to continuously irradiate the photocatalytic support © 2008 American Chemical Society
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Figure 1. Processes occurring on a semiconductor particle (i.e., TiO2): (a) electron-hole generation; (b) oxidation of an adsorbed-electron donor (Dads); (c) reduction of an adsorbed-electron acceptor (Aads); (d and e) electron-hole recombination at surface and in bulk, respectively.3
Figure 2. Secondary reactions with activated oxygen species in the photocatalytic process.18
TiO2. The photocatalytic medium used in this experiment was industrial titanium dioxide (Millennium PC 500, 100% anatase) coated non-woven paper, produced by Ahlstro¨m. The effect of the incident light irradiance on the photocatalytic degradation of MEK was investigated. From Figure 3, we can show that the MEK conversion (X), for an initial concentration of 500 ppm(v), increases dramatically from 40 to 90% as the incident light irradiance enhances from 0.12 to 2.53 mW/cm2. Therefore, this TiO2 coated non-woven paper shows good photocatalytic efficiency in order to highlight the possible HO2 diffusion from the TiO2 surface. The photolysis box detailed in Figure 4 is a 48 cm × 37 cm × 90 cm wooden box lined with aluminum foil to reflect UV light. Three 80 cm long Hg lamps are fixed to the ceiling of the box, each one providing 40 W of radiation centered at a wavelength of 350 nm thus allowing the photonic activation of the catalyst. Two fans are present to cool the unit, while a thermistor monitors the internal temperature. The glass cell, positioned 20 cm below the lamps, is 78 cm long with an outer diameter of 2.4 cm, and has an outlet at each end. Each end of the cell connects to an aluminum block housing the cavity mirror mounts, with o-rings providing the vacuum seal. These blocks are easily removable, which allows the introduction of the TiO2
Figure 3. Effect of incident light irradiance on the MEK conversion. Regular conditions used were: total volume flow rate, Qv ) 300 mL/ min; relative humidity, RH ) 10%; photoreactor temperature, TR ) 30 °C; initial concentration, C0 ) 500 ppm(v); oxygen content, air (20 vol% O2); apparent surface of Ahlstro¨m photocatalytic support, S ) 380 cm2.
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J. Phys. Chem. C, Vol. 112, No. 7, 2008 2241
Figure 5. Absorption spectrum of HO2 radicals obtained from continuous photolysis of Cl2 in the presence of CH3OH and O2: [HO2] ) 4.3 × 1011 cm-3
ment can be defined as the minimum possible absorption coefficient Rmin that can be detected by the spectrometer following9 Figure 4. Schematic of the experimental setup. OI, optical isolator; AOM, acousto-optic modulator; APD, avalanche effect photodiode; L, lens; M, mirror.
into the cell. The photocatalytic support is located on the internal wall of tubular glass cell. The HO2 detection is at the glass cell center, where the distance between the photocatalytic support and the detection volume is around 1 cm. The HO2 detection in the gas phase is based on our previous work:7 we have shown that a sensitive and selective detection of HO2 radicals can be achieved by cw-CRDS in the nearinfrared range. More recently, we have published the absorption spectrum of HO2 between 6604 and 6696 cm-1 including the absorption cross-sections of the most intense lines in this range.8 The strongest line was observed at 6638.20 cm-1 and it exhibits a cross-section of σ6638.20 cm-1 ) 2.72 × 10-19 cm2 at 50 Torr. The present work was carried out at this wavelength, at the center of the strongest line, in order to obtain a detection limit as low as possible. The radiation is emitted by a distributed feed-back (DFB) diode laser (Fitel-Furukawa) providing around 20 mW at wavelengths around 1506 nm via a fiber optic cable. As shown in Figure 4, the beam passes through an optical isolator (OI) to prevent feed-back to the diode and an acoustooptic modulator (AOM) to switch off quickly the radiation entering the cavity. A set of lenses enabled the near-infrared radiation to match the fundamental transverse mode TEM00 of the cavity and to limit higher order modes. This gives a clean comb-like transmission spectrum, with each mode being separated by a free spectral range FSR ) c/2L where c is the speed of light and L the cavity length. The cavity is composed of two concave mirror with a separation of L ) 78 cm giving a FSR ) 192 MHz. In order to match the laser frequency to one of the cavity TEM00 modes, one of the mirrors is mounted on a piezo transducer to sweep the transmission comb on a range larger than the FSR. In this way, the cavity resonates twice per modulation period, each sweep leading to a rapid rise of the light transmitted through the cavity. This optical signal is detected by an avalanche effect photodiode (APD) connected to the trigger circuit: when the signal intensity reaches the trigger level, the AOM modulator is switched and the ring-down signal is recorded by a digital oscilloscope (TDS 5052, Tektronix). This approach allows clean and reproducible exponential decays to be obtained. The ring-down time in the empty resonator was τ0 ) 55 µs at 6638.20 cm-1. The spectrometer measures the absorption coefficient R through
R ) [HO2] × σ6638.20 cm-1 ) L/(c.d) × (1/τ - 1/τ0) where [HO2] is the concentration of HO2 integrated over the absorption pathway d. The limiting sensitivity of a CRD instru-
Rmin ) L/(c.d.τ0) × ∆τmin/τ0 where ∆τmin is the minimum detectable change in the cavity ring-down time. By fitting individual decays, we measured ∆τmin/τ0 ) 1.5%. If the radicals are considered to be confined between the inlet and outlet of the cell, i.e., 60 cm, and τ0 ) 55 µs, we calculate Rmin ) 1.2 × 10-8 cm-1. The cavity length modulation frequency used during this work was around 60 Hz, leading to an acquisition rate of 120 decays/s for perfect triggering settings, i.e., each sweep through resonance gives a ring-down event. Assuming a square-root improvement of the signal-to-noise ratio with increased signal-averaging time, we calculate Rmin ) 1.1 × 10-9 cm-1 Hz-1/2. The present work was carried out at a wavenumber of 6638.20 cm-1 and a pressure of 25 Torr, leading to an absorption cross-section σ ) 3.4 × 10-19 cm2 molecule-1.8 We calculate a minimum detectable concentration of HO2 in these conditions of [HO2]min ) 3.2 × 109 cm-3 for a 1-Hz detection bandwidth. Results and Discussion In order to test the continuous photolysis setup coupled to the detection by cw-CRDS, HO2 radicals were generated by photolysis of Cl2 in the presence of CH3OH and O2
CH3OH + Cl f CH2OH CH2OH + O2 f CH2O + HO2 In our earlier work,8 the most intense absorption line was found to be at 6638.20 cm-1. This part of the spectrum has been measured again with this new setup in order to check the photolysis efficiency. The spectrum shown in Figure 5 has been obtained by photolyzing 4 × 1015 cm-3 Cl2 in 25 Torr O2 and trace amounts of CH3OH, leading to a steady-state concentration of [HO2] ) 4.3 × 1011 cm-3. The “zigzag” visible in the baseline of the spectrum is not random noise, but depends on the precise alignment: when light exiting the cavity is back reflected from some optical surfaces (e.g., lenses, fiber output, and detector), etaloning effects can appear on the baseline.10 After this test, a strip of around 380 cm2 of TiO2 coated nonwoven papers was rolled up inside the glass cell, covering nearly the entire wall of the reactor, and soaked completely with methyl ethyl ketone (MEK). Initial attempts to cover the reactor walls with dry TiO2 tissue and use only a saturated flow of MEK were not successful as when the cell was placed under vacuum a cloud of TiO2 was produced that contaminated the cavity mirrors. In addition the total flow of 50 cm3 min-1 O2 was bubbled through a solution of MEK contained in a saturator. A total pressure of 25 Torr was maintained in the cell and,
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Letters cm3s-1), but OH radicals will react, in spite of a slower rate constant15 (1.04 × 10-12 cm3s-1), predominantly with MEK due to its much higher concentration. The reactivity of HO2 radicals at room temperature with saturated hydrocarbons is much too low to induce a noteworthy loss of HO2 radicals. (d) It is also well-known16 that HO2 forms a complex with formaldehyde (CH2O)
CH2O + HO2 T O2CH2OH Figure 6. Typical signal obtained: each dot is the average of 120 ring-down events.
assuming saturation of the O2 flow, led to an absolute concentration of around 1 × 1017 cm-3 MEK and 6.5 × 1017 cm-3 of O2. Under these conditions the residence time within the cell is around 5 s. The ring-down time as well as the wavelength were continuously monitored during the experiment. The wavenumber was then fixed to 6638.20 cm-1, i.e., the maximum of the absorption line (Figure 5), and the baseline observed for a few minutes. Once the baseline was perfectly stable (Figure 6), the UV lamps were turned on: a very small change in the ring down time from 55.01 ( 0.03 to 54.97 ( 0.03 µs, corresponding to [HO2] ) 1.3 × 109 cm-3, has been observed. After switching off the lamps again, the ring down time returned to its initial value (left part of Figure 6). However, the same decrease in ring down time has been detected after changing the wavelength of the diode laser to 6638.15 cm-1 in order to be off the absorption line (from 54.75 ( 0.04 to 54.70 ( 0.03 µs, the given ring-down times are the averages of all data points from Figure 6 with their statistical errors). These small changes in the ring down time are thus not due to HO2 radicals in the gas phase, but have been attributed to the temperature change caused by the UV lamps. We can thus determine that less than 1.3 × 109 cm-3 HO2 radicals are present in the center of the reactor under our experimental conditions. Some conditions could lead to a decrease of the detected HO2 radical concentration compared to potentially formed radicals at the surface: (a) Radicals have to diffuse from the surface into the detection volume, located 1 cm from each other. The diffusion coefficient of HO2 radicals in He has been measured11 as D ) 430 cm2Torr s-1, from which the diffusion coefficient in air has been calculated as D ) 107 cm2 Torr s-1 by the same authors. The mixing time tm necessary to reduce the concentration gradient to less than 5% of the initial value can be estimated using the following equation:12,13
r02 tm ) 5D Under our conditions tm can be calculated to be 0.02 s, orders of magnitude shorter than the average residence time of 5 s. Therefore, HO2 radicals formed at the surface and released into the gas phase have sufficient time to reach the detection volume. (b) HO2 radicals can also react in the gas phase. From the well know rate constant for the self-reaction,14 the half-life for an initial HO2 concentration slightly above our detection limit is calculated to be 30 s, 6 times longer than the residence time. Self-reaction is thus negligible under our conditions; if HO2 radicals are formed at the surface and released into the gas phase at a concentration above our detection limit, they will therefore not self-react before reaching the detection volume. (c) Other possible gas-phase reactions should not play any role either: the reaction with OH radicals, possibly formed and released as well into the gas phase, is very rapid14 (1.1 × 10-10
It is thought that the analogue of this reaction with other aldehydes or ketones is thermodynamically less favorable;17 therefore, we can calculate, using the equilibrium constant for CH2O, an upper limit of 2% of HO2 being scavenged in a complex with MEK. In order to quantify an upper limit of the HO2 yield from the absence of any HO2, it is necessary to know the exact fluence available within the reactor. Unfortunately, we do not have this information, but we have shown in preliminary tests using the photolysis of Cl2/CH3OH/O2 that the fluence is sufficient to generate HO2 radicals with steady-state concentration 2 orders of magnitude above the detection limit. From this consideration we conclude, that HO2 radials are not released into the gas phase in noteworthy concentrations during the photocatalytic degradation of MEK. Accordingly, HO2 radicals do not appear to be the main radicals that contribute to the degradation of organic compounds in the gas phase for several possible reasons: (1) There is no HO2 production by photonic activated TiO2 surfaces. (2) HO2 radicals are produced but without diffusion into the gas phase. (3) There is formation and HO2 diffusion into the gas phase, but at very low concentrations only (below our detection limit), thus the possible MEK degradation through gas-phase reaction with HO2 will be very minor. Anyway, HO2 radicals are not very reactive compared with OH radicals, which are probably involved in the remote oxidation. 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. A.P. thanks the EC for financial support within the project Marie-Curie EST-CT-2005-020659. References and Notes (1) Peral, J.; Ollis, D. F. J. Mol. Catal. A 1997, 115, 347-354. (2) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A 1997, 108, 1-35. (3) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (4) Tatsuma, T.; Tachibana, S. I.; Miwa, T.; Tryk, D. A.; Fujishima, A. J. Phys. Chem. B 1999, 103, 8033-8035. (5) Tatsuma, T.; Tachibana, S. I.; Fujishima, A. J. Phys. Chem. B 2001, 105, 6987-6992. (6) Tatsuma, T.; Kubo, W.; Fujishima, A. Langmuir 2002, 18, 96329634. (7) Thiebaud, J.; Fittschen, C. Appl. Phys. B: Lasers Opt. 2006, 85, 383-389. (8) Thiebaud, J.; Crunaire, S.; Fittschen, C. J. Phys. Chem. A 2007, 111, 6959-6966. (9) Mazurenka, M.; Orr-Ewing, A. J.; Peverall, R.; Ritchie, G. A. D. Annu. Rep. Prog. Chem., Sect. C 2005, 101, 100-142. (10) Macko, P.; Romanini, D.; Mikhailenko, S. N.; Naumenko, O. V.; Kassi, S.; Jenouvrier, A.; Tyuterev, G.; Campargue, A. J. Mol. Spectrosc. 2004, 227, 90-108. (11) Ivanov, A. V.; Trakhtenberg, S.; Bertram, A. K.; Gershenzon, Y. M.; Molina, M. J. J. Phys. Chem. A 2007. (12) Taylor, G. Proc. R. Soc. London. Ser. A 1953, 219, 186-203. (13) Keyser, L. F. J. Phys. Chem. 1984, 88, 4750-4758.
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J. Phys. Chem. C, Vol. 112, No. 7, 2008 2243 (16) Veyret, B.; Lesclaux, R.; Rayez, M. T.; Rayez, J. C.; Cox, R. A.; Moortgat, G. K. J. Phys. Chem. 1989, 93, 2368-2374. (17) Benson, S. W. Int. J. Chem. Kinet. 2001, 33, 509-512. (18) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69-96.
