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Environmental Photochemistry: Quantitative Adsorption and FTIR Studies during the TiO2-Photocatalyzed Degradation of Orange II L. Lucarelli,† V. Nadtochenko,‡ and J. Kiwi*,§ ThermoQuest Italia S.p A., Strada Rivoltana, 20090 Rodano, Milano, Italy, Institute of Chemical Physics Research RAS, Chernogolovka, Moscow District, Russian Federation 142432, and Institute of Physical Chemistry, Swiss Federal Institute of Technology, EPFL, 1015 Lausanne, Switzerland Received March 8, 1999. In Final Form: September 30, 1999 The degradation rate of Orange II, taken as a model for recalcitrant azo-dyes, was observed to be a function of the type of TiO2 semiconductor used during the process. The surface parameters of the titania material used seem to be a controlling factor in the photosensitized charge-transfer process. The dye abatement was monitored by diffuse reflectance FTIR. When H2O2 was initially added as an oxidant to the photodegradation process, the rate of dye removal from soluton was accelerated significantly. The pore and particle size distribution of the anatase, mixed anatase-rutile (Degussa P-25), and rutile was quantified by gas adsorption and mercury porosimetry. The degradation on rutile and anatase was observed to follow pseudo-first-order kinetics during the initial stages of the reaction. The photodegradation of Orange II was seen to proceed more readily in the case of anatase than in that of rutile. The rate was seen to increase with the surface area and the monolayer volume of the titania variety used.
Introduction Azo-dyes are an abundant class of synthetic, colored, organic compounds that comprise about half of the textile dyestuffs used today.1 It is estimated that the release into the environment without proper treatment represents about 15% of the total world production or 150 tons per day.2 Orange II is a textile azo-dye resistant to light degradation and the action of O2 and common acids and bases. The stability of these dyes useful in textile applications makes them difficult to degrade. Moreover, Orange II does not undergo biological degradation in wastewater treatment plants3,4 and was taken as a model compound for azo-dye degradation Azo-dyes have been treated during the past 30 years by nondestructive technologies such as filtraton, granulated active carbon (GAC), or chemical coagulation using alumina.5 The nondestructive methods transfer the contaminant from the wastewater to the solid phase needing further processing or additional regeneration, as in the case of GAC. The removal of azo-dyes by advanced oxidation technologies (AOTs) has been the subject of several recent studies out of our laboratory.6-9 Photochemical oxidation †
ThermoQuest Italia S.p A. Institute of Chemical Research Physics RAS. § Swiss Federal Institute of Technology. ‡
(1) Zollinger, H. Color Chemistry, Properties and Applications of Organic Dyes and Pigments; VCH Publishers: New York, 1987. (2) Maynard, C. Handbook of Industrial Chemistry; Van Nostrand: New York, 1983. (3) Helz, G.; Zepp, R.; Crosby, D. Aquatic and Surface Chemistry; Lewis Publishers: Boca Raton, FL, 1995. (4) Halmann, M. Photodegradation of Water Pollutants; CRC Press: Boca Raton, FL, 1996. (5) Crowe, T.; O’Melia, C.; Little, L. Am. Dyest. 1978, 52. (6) Morrison, C.; Kiwi, J.; Pulgarin, C. Photochem. Photobiol. A 1996, 99, 57. (7) Morrison, C.; Bandara, J.; Kiwi, J. J. Adv. Oxid. Technol. 1996, 1, 160. (8) Fernandez, J.; Bandara, J.; Lopez, P.; Albers, P.; Kiwi, J. Chem. Commun. 1998, 1493. (9) Fernandez, J.; Bandara, J.; Lopez, P.; Albers, P.; Kiwi, J. Langmuir 1999, 15, 185.
processes leading to the abatement of dyes involving •OH and HO2• radicals generated at the surface of TiO2 through photosensitized charge separations on the semiconductor surface show a potential for solar radiation application that is only now beginning to be explored.1-2,6-9 The destruction of azo-dyes through photosensitized degradation on TiO2 has been reported recently by Kamat.10,11 The observed abatement of the dye takes place only in the presence of O2, as reported before for azo-dyes on TiO2.6-11 The objective of the present work is to investigate Orange II photodegradation on TiO2 in relation to the type of TiO2 used. Diffuse reflectance FTIR has been selected as an appropriate surface technique to identify the surface species participating in the degradation process, and HPLC was employed to follow the decrease of the Orange II peak in solution. Gas adsorption and mercury porosimetry were employed to characterize the TiO2-surface active during the photocatalysis. Experimental Section Materials. The Orange II and H2O2 were reagent grade Fluka products and used as received. The titania powders used were as follows: anatase, British Tioxide (105.8 m2/g); anatase British Tioxide (46 m2/g); anatase 80%-rutile 20%, Degussa P-25 (42.7 m2/g); anatase Bayer (64 m2/g); anatase, Tilcom-type British Tioxide (50 m2/g); anatase 70%-rutile 30%, Ultram (15 m2/g); and finally rutile British Tioxide (32.4 m2/g). Photolysis Procedures. Photolysis was carried out on stirred suspensions in 20 cm3 quartz cylindrical cells. Orange II (1 g/L or 2.85 mM) was photolyzed in suspensions containing 1 g/L TiO2. The solutions were air equilibrated. The lamp used was a 400 W Hg lamp. After photolysis titania was separated from the solution by centrifugation and subsequent filtration. The titania was dried with exclusion of light at 40 °C for 24 h under vacuum. The Orange II degradation in the solution was followed by HPLC (Varian 9065 diode array) using a Phenomenex C-18 inverse (10) Vinodgopal, D.; Wynkoop, D.; Kamat, P. V. Environ. Sci. Technol. 1996, 30, 1660. (11) Vinodgopal, D.; Kamat, P. V. Environ. Sci. Technol. 1995, 29, 841.
10.1021/la990272j CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
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Table 1. Monolayer Volumes, Surface Areas, and c Points for Degussa P-25, Anatase, and Rutile Obtained by Physisorption TiO2
monolayer volume, cm3/g
surface area, m2/g
c point
Degussa P-25 anatase rutile
9.8 24.3 7.9
42.7 105.8 32.4
190.0 77.3 37.3
Table 2. Mercury Porosimetry of TiO2 Samples Calculated by the Mayer-Stowe Model21
TiO2
range of particle avg particle surface specific pore size, nm size, nm area, m2/g volume, cm3/g
Degussa P-25 10-103 anatase 10-105 rutile 30-1500
90 9000 500
38.4 88.7 30.6
0.80
phase column. The Orange II peaks were detected at 282 nm (retention time 20.1 min). The solution gradient was regulated with a buffer consisting of ammonium acetate and methanol. Quantification of the HPLC peaks of Orange II was based on the integrated band component. For the analysis of other organic intermediates, an ODS-2 column was employed. The eluant was a mixture of ammonium acetate (0.03 M) and acetonitrile. Analysis of oxalic acid was carried out by means of an H-801 column (Interaction) in isocratic mode at 60 °C. In the latter case, the eluant was H2SO4 (0.01 N). The peaks observed for the reaction intermediates were referenced with appropriate external standards. Diffuse Reflectance FTIR. The adsorbed peaks on titania produced during the photolysis were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy with a Brucker IFS 55 instrument. The samples used were mixed with KBr (1:5) and thoroughly grounded. The Orange II adsorption from the liquid phase was carried out until equilibrium was seen to occur. After adsorption, the oxides were filtered and dried at room temperature under vacuum. IR absorbance was measured in the region 400-4000 cm-1. Due to the strong absorbance of TiO2, the spectral data below 1200 cm-1 could not be analyzed in a quantitative way. Similarly, the IR spectra above 2000 cm-1 were difficult to sort out because of the hydroxyl groups on the titania surface. In most of the measurements the resolution of the IR spectrometer was 1 cm-1. Gas Adsorption Studies. The physisorption was carried out in a Sorptomatic 1990 Micropore configuration (ThermoQuest, Milan, Italy). The samples were degassed at room temperature for 5 h at 10-6 Torr. A precision of 2% was observed in the reproducibility of the piston-operated volumetric apparatus for the observed adsorption isotherms. The physisorption experiments were performed at liquid nitrogen boiling temperature (77 K). Surface area, cumulative pore volume, and pore size distribution were evaluated with the help of a fully computerized unit attached to the Micropore unit system. Mercury Porosimetry. Mercury porosimetry was carried out on Pascal 140 and Pascal 440 instruments in the pressure range 0.01-400 MPa. The data obtained were processed in an IBM PC-2 data processor. The samples were degassed in a vacuum (10 Pa) for 20 min. First and second runs were performed on every sample due to the high degree of aggregation existing in the original samples. This became evident by the meaningful difference between the volumes of mercury used in the first and second pressurization runs. The difference between the first and second Hg pressurization runs is shown in Figure 7. The data reported in this work refer to the second run on disaggregated samples. The first and second pressurization runs are shown in Figure 7. In the first run the pores and the interconnecting channels or transport pores are filled with Hg. After depressurizing, the Hg gets out of the pores in the powder but not out of the interconnecting channels. No meaningful variation of the data reported in Table 2 was found with a third and fourth pressurization-depressurization cycle with the powders used. This is possibly due to the Hg penetrating the pores in titania which are already disaggregated after the first cycle with a constant amount of Hg retained in the interconnecting channels.
Figure 1. Shift in the 1500 cm-1 diffuse reflectance FTIR spectra band observed for different TiO2 (1 g) samples equilibrated for 24 h with Orange II (2.85 mM) and dried.
Figure 2. Orange II disappearance kinetics for three relevant TiO2 powders with different crystallographic compositions and different surface areas followed at λ ) 490 nm. Irradiation of Orange II (2.85 mM), titania 1 g/L at pH 4 by way of a 400 W Hg lamp. The rutile used had the area 32.4 m2/g, DegussaP-25 55 m2/g, and anatase 105.8 m2/g.
Results and Discussion A. Shift of Azo Band Vibration on TiO2 during Orange II-Mediated Decoloration. The FTIR band ∼1500 cm-1 is shown to be inherent to the type of TiO2 material, and this is shown in Figure 1. According to recent work of Kamat,10 this vibration is due to an azo band or an aromatic ring (CdC) vibration sensitive to the dye azo bond. Figure 1 shows in graphical form the position of this band, shifting to higher frequencies from anatase to rutile. The shift is attributed to (a) the variation of the crystallographic phase for the different samples going from anatase to rutile12 and (b) the surface area of the samples. Figure 2 presents the decoloration kinetics monitored at λ ) 490 nm by HPLC for different air-saturated aqueous solutions. During the period immediately preceding the photolysis, the solution was equilibrated for 24 h in the dark. Pseudo-first-order decay kinetics was observed for light irradiation only, since dark control experiments did not show any degradation of Orange II in Figure 2. The lowest decoloration efficiency in Figure 2 was observed in the photocatalysis mediated by rutile (32.4 (12) Parfitt, G. D.; Rochesta, Ch. Adsorption from Solution at the Solid Liquid Interface; Academic Press: New York, 1983.
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m2/g). The highest decoloration activity was observed in the case of anatase (105.8 m2/g). In the case of rutile, a plateau was attained after ∼8 h, suggesting the formation of intermediates under light which preclude further Orange II degradation. The results shown in Figure 2 show the importance of the TiO2 crystallographic phase as a controlling factor during dye degradation. After 4 h of light irradiation, 4-hxdroxybenzenesulfonic acid, 1,2naphthol, and 1,2-naphthoquinone were detected as photoproducts in solution. After 24 h, the compounds found in solution were oxalic acid, 4-hydroxybenzenesulfonic acid, acetic acid, 1,3-isobenzofurandione, formic acid, and 4-hydroxyglioxalic acid. For Degussa P-25, the competition between Orange II and the degradation intermediates generated in solution can be readily seen when comparing with the anatase photodegradation, since the amount of dye degraded in ∼20 h is about the same. TiO2 Degussa P-25 (42.7 m2/g) presents intermediate kinetics between those of rutile and anatase during the initial stages of Orange II decoloration (Figure 2). If the degradation of Orange II reported in Figure 2 is corrected for the surface area of the TiO2, there is little difference between the initial activities for the titania samples used. But there are clearly substantial differences between rutile and anatase associated with blocking of the surface sites in the former case. This point will be taken up again and discussed in section D below. Sensitized degradation is known to be dependent on the interaction between the dye and the specific TiO2 used. Different kinetics for the charge injection8,9 from photoexcited Orange II into the different TiO2 samples is invoked to explain the difference observed in the degradation rates. In Figure 2, a concentration of Orange II of 2.85 mM produces multilayer coverage9 on TiO2. These results were obtained after a series of preliminary optimization photodegradation experiments. These preliminary experiments varied systematically the amount of catalyst added in solution, the light flux reaching the solution, and the O2 dissolved in solution to attain the most efficient photodegradation. B. Diffuse Reflectance FTIR during Decoloration of Orange II on Degussa P-25 Powder. Figure 3a-1 shows the diffuse reflectance FTIR spectrum in the region 1100-2000 cm-1 of Degussa P-25 titania powder. The samples were equilibrated in water for 24 h at room temperature. The supernatant containing the dye in solution was then removed and the precipitate subsequently dried. The four FTIR bands in Figure 3a-1 are due to OH group vibrations in the TiO2 crystal lattice.12 Figure 3a-2, refers to a sample of Orange II on Degussa P-25. Five peaks are observed in the 1350-1700 cm-1 region for a nonirradiated sample. These vibrations with νmax ) 1454.2, 1502.4, 1553.5, 1596.1, and 1619.1 cm-1 originate from the aromatic skeletal vibrations of Orange II.13,14 In addition to these peaks, Kamat10 observed additional peaks at 1568 and 1405 cm-1 which were not observed during the present experiments. This latter effect is probably due to the noise-to-signal ratio in the present experiments. Figure 3b shows the vibrations at 1420.4 cm-1 due to the OH bending vibration.13 The band at 1255.5 cm-1 is due to the C-O-H stretching vibration.15 Figure 3b shows the diffuse reflectance FTIR spectra of Orange II/TiO2 (13) Vien Lin, D.; Norman, C.; Fateley, J.; Grasselli, R. Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (14) Varsany, G.; Lang, L. In Assignment for Vibrational Spectra of Seven Hundred Benzene Derivatives; Adam Holger, Ed.; Manchester, U.K., 1974.
Lucarelli et al.
Figure 3. (a-1) Diffuse reflectance FTIR peaks of fully hydroxylated Degussa P-25 in the region 1100-2000 cm-1. (a2) Diffuse reflectance FTIR peaks for a nonirradiated sample of Orange II (2.85 mM) loaded on P-25 Degussa (42.7 m2/g). (b) Diffuse reflectance FTIR peaks of Orange II (2.85 mM) loaded on Degussa P-25 under light irradiation (400 W Hg lamp). Airsaturated solutions at pH 4.
under light irradiation in air-saturated solutions plotted in Kubelka-Munk units. Between 0 and 16 h the ∼1500 cm-1 band was observed to gradually disappear. The degradation of Orange II on Degussa P-25 in Figure 3 relies on the assumption that the surface dye concentration on titania is directly proportional to the normalized Kubelka-Munk (K-M) function in eq 1
K-M function )
(1 - R)2 2R
(1)
rather than to the dye absorbance. The Kubelka-Munk (15) Silverstein, R.; Clayton, G.; Morril, T. Spectrophotometric Identification of Organic Compounds; Wiley: New York, 1991.
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(K-M)16 function is related to the absorbance A by
R ) 10-A
(2)
C. Diffuse Reflectance FTIR Spectra of Orange II during Decoloration Mediated by Anatase. Figure 4 shows the decrease of the azo bands of Orange II during TiO2 photocatalytic degradation photolyzed in the presence of H2O2. The photolysis of Orange II abatement was seen to proceed in the presence of the oxidant at a much faster rate. In both cases the peak at 1500 cm-1 was observed to be the most prominent peak in the FTIR spectrogram. The upper trace in Figure 4b-1 shows in Kubelka-Monk units the decrease of the vibrational band at ν ) 1500 cm-1 during the photolysis of the Orange II-anatase sample. The degradation of the dye does not follow pseudofirst-order kinetics as a function of time, as commonly observed during the photocatalytic degradation of organic compounds.3-7 This could be due to several reasons, and two of them could be outlined as follows: (a) the tails of the peaks due to the intermediates produced during the photodegradation extending into the 1500 cm-1 region (Figure 4) overlap with the Orange II vibration in this region and make the latter peak no longer directly proportional to the concentration of the dye, and (b) the intermediates formed during the photocatalysis are colored, competing with the light absorption by Orange II in solution. Most likely the observed degradation kinetics of Orange II on TiO2 is due to a combination of these two factors. After 2 h a decrease in surface coverage by the dye is expected, since the dye undergoes photolysis on the anatase surface. As shown in Figure 4b-1 and b-2, the degradation kinetics slowdown was observed either in the absence or in the presence of H2O2. The slower degradation in the absence of H2O2 suggests photolysis intermediates precluding the full degradation of the dye up to 16 h (Figure 4b-1). Upon addition of H2O2 (10 mM), complete removal of the dye is observed, since the intermediates precluding dye degradation could be eliminated. Figure 4b-2 presents the growth curve for the peaks observed at ν ) 1700 cm-1 which are characteristic for carboxylic groups.13,15 The plot relates to Orange II irradiated on anatase in the absence and presence of added H2O2. No lag period was observed for the peak growth at 1700 cm-1 in Figure 4b-2, indicating that the intermediates formed during Orange II photocatalysis undergo oxidation fairly rapidly into carboxylic compounds. D. Diffuse Reflectance FTIR Spectra of Orange II during Decoloration Mediated by Rutile Powders. Progressive decrease of the azo dye ∼1500 cm-1 peak and other minor peaks up to 16 h during Orange II photocatalysis was seen to occur on rutile (32.4 m2/g). This decrease is slower than previously observed on anatase (Figure 4a) under identical experimental conditions. Moreover, additional peaks are observed in the rutile IR spectogram between 1100 and 1800 cm-1. This suggests a larger number of intermediates during the photolysis on rutile than in the case of anatase. The deactivation of Orange II on rutile is therefore more probably associated with the blocking of the surface species by adsorbed molecules. The reaction mechanism seems to be the same when samples of anatase, rutile, and Degussa P-25 were used. Qualitatively the intermediates in solution and the peaks on the titania showed common components for the three different titania species. (16) Kortu¨m, G. Reflectance Spectrometry; Springer-Verlag: Berlin, 1969.
Figure 4. (a) Diffuse reflectance FTIR of a Orange II (2.85 mM)-loaded anatase (105.8 m2/g) sample as a function of irradiation times: (A) 6 h; (B) 4 h; (C) 2 h; (D) 0 h (anatase without Orange II). Irradiation was by means of a 400 W Hg lamp. Air-saturated solutions at pH 4 in the presence of H2O2 (10 mM) for the irradiation times (A) 6 h, (B) 4 h, (C) 2 h, (D) 0 h. (b-1) Decrease of the ν ) 1500 cm-1 peak in normalized Kubelka-Munk units for anatase (105.8 m2/g) under light irradiation in air-saturated solutions with Orange II (2.85 mM) or anatase (1 g/L) in the absence and presence of H2O2 (10 mM). (b-2) Growth of the ν ) 1700 cm-1 peak in normalized KubelkaMunk units for anatase (105.8 m2/g) under light irradiation in air-saturated solutions with Orange II (2.85 mM) or anatase (1 g/L) in the absence and presence of H2O2 (10 mM).
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Figure 5. Diffuse reflectance FTIR of Orange II (2.85 mM) loaded on TiO2 rutile (32.7 m2/g) under light irradiation and of H2O2 (10 mM) at the following times: (A) 12 h; (B) 6 h; (C) 4 h; (D) 1 h; (E) 0 h. Solution pH 4. Air-saturated solutions.
Figure 5 shows that when H2O2 (10 mM) is added in solution, the abatement of the 1500 cm-1 peak is complete in
