Langmuir 1991, 7, 947-954
947
Determination of the Actual Photocatalytic Rate of H202 Decomposition over Suspended TiO2. Fitting to the Langmuir-Hinshelwood Form Beat Jenny+and Pierre Pichat' URA au CNRS "Photocatalyse, Catalyse et Enuironnementn,Ecole Centrale de Lyon, BP 163, 69131 Ecully Cgdex, France Received June 12,1990 The kinetics of oxygen evolution from UV-illuminated acidic aqueous suspensions of a powder Ti02 sample containing nominal initial H202 concentrations, CO, in the range 10-6-10-1M has been studied by use of a gas chromatograph with a noise-optimized preamplifier. Steady-state rates were reached except at the lowest CO. The dark O2evolution was negligible. To determine the contributionof the photochemical decomposition of H202, the use of evolution rates measured in the absence of Ti02 was shown to be irrelevant. Therefore this contribution was derived, in an original manner, from the rates obtained with a Cr3+-dopedTi02 sample that has the same properties as the undoped specimen for the absorption and scattering of UV light but a negligible photocatalyticactivity as evidenced by other reactions. The steadystate photocatalytic rates thus calculated were numerically fitted to the Langmuir-Hinshelwood mechanism by iterative variations of the constants. According to this mechanism, an optimal surface coverage by H202 of ca. 0.4 was found to correspond to the maximal photocatalytic effect to background ratio q observed for co = ca. 1 mM. The value of q obviously depends on the wavelength range and type of TiO2; it was found equal to 25 in the region 300-400 nm for Ti02 Degussa P25,which illustrates the importance of the photocatalytic decomposition. The total 0 2 evolution, smaller than that expected from HzOz H20 + l / 2 0 2 , was thought consistent with the previously suggested internal hydroxylation of Ti02 by H2Oz.
-
Introduction The possible formation of H202 a t the surface of Ti02 electrodes during water photoelectrolysis has been proposed.lJ Peroxides can be photogenerated by various ways in the presence of oxygen and water at the surface of Ti02.1-14 Since oxygen evolution was never observed in attempts aiming at decomposing water over a number of Ti02 (or Sr TiOa) powder samples,l5J6 platinoid transition metals (mainly Pt and Rh) were deposited on them, generally as small particles, to catalyze the recombination of H atoms and to facilitate the separation of the photoproduced charges. Nevertheloss, no oxygen evolution was found over these metal/Ti02 ~ a m p l e s . ~ l This J ~ - ~was ~ t Present address: Laboratoriumfiir FestkGrperphysik, ETH,8093 Ztirich, Switzerland. (1) Salvador, P. J . Phys. Chem. 1984,88,3696. (2)Ferrer, I. J.; Muraki, H.; Salvador, P. J . Phys. Chem. 1986, 90,
2806.
(3) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979,83, 3146. (4) Van Damme, H.; Hall, W. K. J . Am. Chem. SOC.1979,101,4373. (5) Clechet, P.;Martelet,C.; Martin, J. R.; Olier, R. Electrochim. Acta 1979, 24, 457. (6) Muraki, H.;Saji, T.; Fujihira,M.; Oayagui,S.J.Electroana1.Chem. Interfacial Electrochem. 1984, 169, 319. (7) Yesodharan, E.; Yesodharan, S.;Gritzel, M. Sol. Energy Mater. 1984, 10, 287. (8) Kerchove,F. V.;Praet, A.; Gomes, W. P. J. Electrochem. SOC.1986, 132,2357. (9) Rives-Arnau, V., J. Electroanal, Chem. Interfacial Electrochem. 1986,190,279. (10) Gutierrez, C.; Salvador, P. J . Electrochem. SOC.1986, 133,924. (11) Gu, B.; Kiwi, J.; Gritzel, M. Nouu. J. Chim. 1985,9,539. J . Mol. Catal. 1987,39, 63. (12) Harbour, J. R.;Tromp,J.;Hair, M. L. Can. J. Chem. 1986,63,204. (13) Bickley, R. I.; Jayanty, R. K. M.; Vishwanathan, V.; Navio, J. A.
In Homogeneous and Heterogeneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; D. Reidel Publiehing Co.: Dordrecht, 1986; p 555. (14) Nakato,Y.;Tsumura,A.;Teubomura, H. J.PFLys.Chem. 1983,87,
2402. (16) Mille, A.; Porter, G. J . Chem. SOC.,Faraday, Tram. 1 1982, 78, 3659. (16) Borgarello, E.; Pelizzetti, E. Inorg. Chim. Acta 1984, 91, 296. (17) Pichat, P.; Disdier, J.; Mozzanega, M.-N.; Herrmann, J.-M. In
Proceedings of the 8th International Congress on Catalysis; Verlag Chemie: Dechema, 1984; Vol. 111, p 487.
ascribed to an accumulation of oxygen and/or peroxide species at the catalyst s ~ r f a c e . ~ ~These J " ~ ~species would be reduced and reoxidized so that the surface recombination becomes the major photoprocess. Oxidation of deposited rhodium was also considered as a further reason for the deactivation of the Rh/TiOz photocatalyst and the absence of peroxides in the liquid phase was explained by their decomposition at the Rh particles.18 However, the formation of peroxides in UV-illuminated alkaline aqueous suspensions of Pt/TiOz has later been shown and it was concluded that the oxidation product is probably a photostable titanium peroxo complex functioning as a recombination center." The formation of such species might also diminish or even suppress the photocatalytic activity of Ti02 in the decomposition of H202. On the other hand, the combination of Hz02 and UV light is used to oxidize water organic pollutantsm and, very recently, it has also been pointed out that the addition of H202 in UV-illuminated Ti02 aqueous suspensions accelerates the rate of degradation of some organic pollutants,21at least in the first stages. This new potentiality, as well as the other problems evoked in this introduction, prompted us to reexamine the kinetics of the decomposition of H202 in Ti02 aqueous suspension at various initial concentrations to complete the previous studies. Hydrogen peroxide, either gaseous or dissolved in water, has been reported to easily chemi(18) Munuera, G.; Soria,J.; Conesa, J. c.;Sanz, J.; Gonzalez-Elipe, A. R.; Navio, J. A.; Lopez-Molina, E. J.; Munoz, A.; Femandez, A.; Eepinoe, J. P. In Catalysis on the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier: Amsterdam, 1984; p 335. (19) Ah-Ichou, I.; Bianchi, D.; Formenti, M.; Teichner, 5. J. In Homogeneous and Heterogeneous Photocatalysis; Pelizzetti, E., Serpone, N., E&.; D. Reidel Publishing Co.: Dordrecht, 1986; p 433. (20) See, for example Ho, P. C. Enuiron. Sci. Technol. 1986,20,260. Glaze, W. H.; Kang, J.-W.; Chapin, D. H. Ozone Sci. Eng. 1987,9,335. Moza, P. N.; Fytianoa, K.; Samanidou, V.; KO&, F. Bull. Enuiron. Contam. Toxicol. 1988,41,678. Guittoneau, 5.;De Laat,J.; Dor6, M.;Duguet, J. P.; Bonnel, C. Reu. Sci. Eau 1988, 1, 35. (21) Tanaka, K.; Hisanaga, T.; Harada, K. New J . Chem. 1989,13,5. J . Photochem. Photobiol. A 1989,49,155. Augugliaro, V.; Davi, E.;Palmisano, L.; Schiavello, M.; Sclafani, A. Appl. Catal. 1990,65, 101.
0 1991 American Chemical Society
Jenny and Pichat
948 Langmuir, Vol. 7, No. 5, 1991
sorb on Ti02 powders giving rise to a yellow surface compound.22 The surface coverage by H2Oz molecules was found to depend on the initial degree of hydroxylation of Ti02.23 The same sites were suggested to be involved in the adsorption of HzO and H202 on dehydroxylated Ti02 from the gas p h a ~ e , 2and ~ , ~it~was proposed that HzO2 forms Ti4+ peroxo c0mplexes.~3From measurements of the photouptake of oxygen dissolved in neutral aqueous Ti02 suspensions, the generation and incorporation of Hz02 into the solid with the formation of a gellike surface was tentatively proposed.12 This internal hydroxylation, which would lead to the corrosion of Ti02 in alkaline solutions, is supported by X-ray diffraction and X-ray photoelectron spectraSz5 Since H202 is thermally and photochemically unstable, assessing the contributions of these phenomena is needed to measure the real photocatalytic rate of decomposition in the presence of UV-excited TiO2. To our knowledge, this has never been made adequately and it is one of the purposes of this investigation. As we found irrelevant the usual procedure of performing blank experiments without photocatalyst to determine the photochemical contribution, we propose and discuss anew approach. In addition, this study includes procedures necessary to evaluate and improve the accuracy of chromatographic measurements of 0 2 evolution from liquid water. Results obtained over a large range of initial H202 concentrations are presented. Their fitting to the Langmuir-Hinshelwood mechanism, by numerical optimization, is critically examined.
Experimental Section 1. Photoreactor. A vacuum-tight Pyrex cylindrical photoreactor (vertical axis, 5 cm in diameter, 320 cm3)was used. The suspension (120cms) was maintained at 288 K and stirred by a glass-sealed magnet (500rpm). A Philips HPK 125-Wmercuryarc lamp equipped with a circulating-water cuvette (2.5cm thick) was situated 15 cm above the fluid level. Optical windows were made of fused silica, with the exception of the cell entrancewindow (3.5 mm thick Pyrex). Viton O-rings on brillant glass surfaces ensured the vacuum-tightness. Pressure measurements were made by use of either a Bourdon, a mercury manometer, or a U-shaped manometer containing butanoic acid dibutyl ester ClsHnO, (density 1.04, low vapor pressure). Pressure changes down to 5 Pa (1 Torr = 133.33Pa) were observable. However, caution had to be taken for absolute pressure measurements, since, within several days, the apparent vapor pressure increased,presumably as a result of the dissolution of small amounts of water. 2. Gas Chromatography Analysis. 2.1. Apparatus and Procedure. Oxygen pressures were measured by means of a catharometer gas chromatograph (Intersmat IGC 120 MB) equipped with a 50/80 mesh, 2 m long, Porapak Q column, a 80/100 mesh, 1 m long, Porapak R column, both at 363 K, and a 50/80 mesh, 3 m long molecular-sieve column at room temperature. These columns were in series with the catharometer filaments just before and after the molecular-sievecolumn. The carrier gas was helium (flowrate 50cm3/min). A noise-optimized preamplifier in the electrometer circuit allowed one to reach the detection limit due to the noise of the filament. With a sampling loop of 1.0 cm3heated at 363 K, this limit was ca. 0.3 Pa, Le. 0.03 pmol of 02 in the cell. At the same time, the system permitted controlling the N2, Con, and HzO pressures. ~
~~~
(22) Boonstra, A. H.; Mutsaers, C. A. H. A. J.Phys. Chem. 1975, 79, 1940. (23) Munuere, G.;Gonzalez-Elipe, A. R.; Soria, J.; Sanz, J. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1535. (24) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Chem. SOC.,Faraday Trans. 1 1979, 75, 736. (25) Munuera, G.; Gonzalez-Elipe, A. R.; Espinos, J. P.; Navio, A. J. Mol. Struct. 1986, 143, 227. Espinos, J. P.; Fernandez, A.; GonzalezElipe, A. R.; Munuera, G. In Book of Abstracts, 6th International Conference on Photochemical Conversion and Storage of Solar Energy, Paris, 1986, p (2-83 and p D-133.
Stainless steel and Pyrex glass tubes of 2 and 5 mm inner diameter, respectively, linked the manometers and the gas chromatograph to the cell. Those between the cell and the gas chromatograph were heated to 363 K, in order to prevent excessive adsorption of water vapor during sampling. No greased part was submitted to UV irradiation. The dead volume, v d , of the sampling system was 27 cm3. It was evacuated by a rotary pump. The cell itself was evacuated by a water-jet pump which was efficient in outgassing water, provided that the suspension was well stirred and had a temperature slightly higher than that of the water jet. A typical pumping time was 30 min. The sampling procedure was as follows. First the sampling system was evacuated for 6 min, then the cell cock was opened for 5 s to allow the sysfem to be filled. The sampling loop was flushed by the carrier gas for 20 s, a time long enough to desorb water from the loop walls. These time values were not critical. They were chceen in a range that yielded reliable chromatograms. 2.2. Calibration and Accuracy. The evaluation of the chromatograms was based on the heights of the peaks which were sharp. If ni designates the number of moles of oxygen produced during a run until the ith injection, and hj the height of the oxygen peaks in the chromatograms corresponding to the injections one to i, ni (in pmol) is given by
V ni= 0.12028 Jfii TB
(1)
fi, =hi/.
(2)
h, = hi + K
i j=l
hi
(3)
where V, (in cm3) is the gas volume of the photoreactor, Tnis the absolute temperature of the gas, f i i (in Pa) is the pressure immediately before the ith injection corrected for the losses in the dead volume ( v d ) of the gas chromatographic system, hi is the corresponding corrected peak height, u is the 02 sensitivity of the chromatograph assumed to be independent of the pressure, and K is the relative quantity of gas lost as referred to the remaining amount. The value of K was determined from two consecutive injections in the absence of oxygen production: then nl = nz,so that K = (hl/hz) - 1. The value K = 0.13 f 0.01 thus obtained, for pressures below 130 Pa, agreed satisfactorily with K = VdT,/ (VpTd) from Gay Lussac's law, where Td is the temperature in the dead volume v d . Formulas 1 to 3 strictly hold for an ideal gas insoluble in water. We show in the Appendix that, in spite of the rather large volume of solution and the comparatively high solubility of oxygen, the corrections of the 02 rates arising from the solubility can be neglected, since they remain within the relative measurement accuracy of ca. 4 % The calibration was made by use of the electrochemical production of 02 at a Pt electrode in a two-compartment cell containing 1 M HsSO,. A current of 300 pA was imposed by a galvanostat and the cell sampled at intervals of 1 h. After an induction period of about 2 h, a steady state was reached where the corrected peak height hj varied linearly with time, indicating that u did not depend on the 02 pressure (at least below 130 Pa) as presupposed in eq 3. The current efficiency was aseumed to be 100% at steady state. Atypicalvalueforuwas3.7t0.2mm/Pa. Themainadvantage of the electrochemical calibration is to yield u under conditions close to actual measurements, i.e. for low 02 pressures in the presence of water vapor. The calibration was routinely checked. The sensitivity increased slowly, in line with the decreasing retention time because of water accumulation in the molecularsieve column. Accordingly, this column was outgassed periodically. The purpose of a gas-sampling system is obviously to provide the gas analyzer with a sample representative of the atmosphere in the cell. Problems arose from alterations of the composition of the sample, caused by nonlaminar gas flow and by condensation of water vapor. The condition for laminar flow" is K
