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Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation
Applied Catalysis B: Environmental 85 (2009) 148–154
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation Pengfei Ji a, Jinlong Zhang a,*, Feng Chen a, Masakazu Anpo b a b
Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 May 2008 Received in revised form 3 July 2008 Accepted 3 July 2008 Available online 10 July 2008
Cerium dioxide was prepared by the precipitation method and found to be an efficient photocatalyst to degrade azodyes under visible light irradiation. Nonbiodegradable azodyes acid orange 7 (AO7) was selected as modal target to examine the photocatalytic activity of CeO2. AO7 could be efficiently degraded in aqueous suspension of CeO2 under visible light illumination. The catalyst was characterized by X-ray diffraction (XRD), N2 sorption, transmission electron microscopic image (TEM) and UV/vis absorption spectrum techniques. AO7 solution was quickly decolorized and partly mineralized under visible light irradiation with existing CeO2. The photodegradation rate of this azodye catalyzed by CeO2 is much faster than those occurring on commercial titania (Degussa P25) under otherwise identical conditions of visible light irradiation. Experiments were conducted to examine the adsorption mode of acid orange 7 on CeO2 and adsorption capacity at different pH values. The possible degradation pathway has been proposed for the photocatalytic degradations by using certain radical scavengers and gas chromatography–mass spectrometry (GC–MS) to determine intermediates. The enhanced photoactivity of the lanthanide oxide CeO2 was attributed to the superior adsorption capacity and special 4f electron configuration. ß 2008 Elsevier B.V. All rights reserved.
Keywords: CeO2 photocatalyst Azo dye decomposition Degradation pathway 4f Electron configuration
1. Introduction Azodyes are an abundant class of synthetic, colored, organic compounds, which are characterized by the presence of one or more azo bonds (–N N–). Large quantities of these dyes (about 50–70% of the dyes available on the market today) are manufactured worldwide and used in a variety of applications [1,2]. It is estimated that about 15% of the total world production is lost during synthesis and processing [2,3]. Therefore azodyes are extensively contained in wastewater generated from the textile and dyestuff industries. Such colored dye effluents pose a major threat to the surrounding ecosystems owning to their nonbiodegradability, toxicity and potential carcinogenic nature [2]. Heterogeneous photocatalysis assisted by various semiconductors has been considered as a cost-effective alternative as a pre- or posttreatment of biological treatment processes for the purification of dye-containing wastewater [4,5]. Among various metal oxide semiconductors, TiO2 is the most widely studied photocatalyst for environment remediation owning
* Corresponding author. Fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.07.004
to its unique photocatalytic efficiency, low cost, nontoxicity, and high stability. One obstacle to its effective utilization is the inefficient use of solar energy; less than 5% (UV light) of the sunlight can be absorbed by this photocatalyst. A simple and interesting approach to extend the catalyst absorption toward visible region is the photosensitization by an appropriate dye [6]. The mechanistic details of dye degradation have also been the subject of several studies. In these systems, dyes rather than TiO2 particles are excited by visible light to appropriate singlet and triplet states, processes which were followed by electron injection into the TiO2 conduction band. The TiO2 only acts as an electrontransfer mediator in this reaction. This pathway (Eqs. (1)–(7)) is different from the pathway implicated during UV light irradiation [7,8]. dye þ hn ! dye
(1)
dye þ TiO2 ! dyeþ þ TiO2 ðe Þ
(2)
TiO2 ðe Þ þ O2 ! TiO2 þ O2
(3)
O2 þ TiO2 ðe Þ þ 2Hþ ! H2 O2 þ TiO2
(4)
P. Ji et al. / Applied Catalysis B: Environmental 85 (2009) 148–154
2O2 þ 2Hþ ! O2 þ H2 O2
(5)
H2 O2 þ TiO2 ðe Þ ! OH þ OH þ TiO2
(6)
dyeþ þ ðO2 ; O2 ; or OHÞ ! degraded or mineralized products (7) However, the photodegradation in the dyes/TiO2 sensitization system is relatively inefficient. Kondarides and coworkers have reported that it takes about 50 h to totally decolorize azodyes acid orange 7 (AO7) solution with concentration up to 100 mg/L [9]. To overcome this obstacle, several researches had been carried concerning the effect of transition metal or metal ions on the self-sensitized degradation of dyes under visible light irradiation [10,11]. Zhao et al. have also demonstrated the chemisorbed hexachloroplatinate (PtCl62) anions on the surface of P25 TiO2 particles can accelerate the photodegradation of ethyl orange under visible light illumination [12]. However, the preparations of above mentioned catalysts are economically costly and timeconsuming, which makes the practical application of these novel catalysts hard to achieve. CeO2 has some properties like titania features such as wide band gap, nontoxicity, and high stability. Since its unique 4f electron configuration, CeO2 has been frequently selected as a component to prepare complex oxides or as a dopant to improve titania-based catalysts’ performances [13–15]. As for pure CeO2, it has been investigated under UV irradiation concerning watersplitting for the generation of hydrogen gas [16,17] and photodegradation of toluene in the gas phase [18]. Zhai et al. and Salker et al. have recently reported the photocatalytic behaviors of CeO2 under sunlight irradiation to degrade dyes [19,20]. Herein, we report CeO2 for photocatalytic degradation of sulfo group-containing azodyes in aqueous suspension irradiated by visible light. Acid orange 7 was chosen as model target to examine the adsorption and degradation of azodye on CeO2 irradiated by visible light. The CeO2 showed high photoactivity towards degradation of azodye and has been proven to be a promising alternative for dyecontaining wastewater treatment under visible light irradiation. 2. Materials and methods 2.1. Materials All major chemicals were of reagent grade or higher purity. Ce(NO3)36H2O was supplied by Shanghai Sinopharm Chemical Reagent Co., Ltd., China. Azodye acid orange 7 (AO7) was obtained from Acros and used without further purification (Fig. S1, supporting information). Titania P25 (TiO2; ca. 80% anatase, 20% rutile; BET area, ca. 50 m2/g) was purchased from the Degussa. 1,4Benzoqunone and potassium iodide used as scavengers were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. Sodium azide was purchased from Alfa Aesar. Deionized and double distilled water was used throughout this study. 2.2. Preparation of the catalyst CeO2 was prepared by the precipitation method in our laboratory. Specifically, 4.34 g Ce(NO3)36H2O was dissolved in 100 mL of distilled water to form transparent solution (0.1 M). To this solution 4 mL NH4OH (28 wt% NH3) was quickly added under vigorous stirring. The transparent solution immediately changed to yellowish suspension. This suspension was continued to stir for 2 h followed by 5 h aging at room temperature. Then the purple precipitate was collected by centrifugation and washed 3 times with water and ethanol. Consequently, the precipitate was dried at
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80 8C for 12 h before it was calcined at 550 8C for 4 h (ramp: 2 8C/ min). The powder X-ray diffraction analysis of the prepared catalyst was carried out at room temperature with a Rigaku D/max 2550 VB/PC apparatus using Cu Ka radiation (l = 1.5406 A˚) and a graphite monchromator, operated at 40 kV and 30 mA. The BET of the sample was determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). The sample was degassed at 473 K before the measurement. UV–vis absorbance spectra were obtained for the dry-pressed disk sample using a Scan UV–vis spectrophotometer (Varian, Cary 500) equipped with an intergrating sphere assembly. TEM analysis of the samples was done using a JSM-2100F (JEOL) instrument and the electron beam accelerating voltage was 200 kV. 2.3. Adsorption experiments For the adsorption measurements, 50 mg of CeO2 was added to 50 mL of freshly prepared AO7 solutions of known concentration in the range 104 to 103 M at different pH values which were adjusted by 1 M HNO3 or NH4OH. The suspensions were sonicated for 5 min and stirred for 1 h at room temperature in the dark to reach equilibrium. After filtration through Millipore filter (0.22 mm) membrane, a sample was taken to determine the AO7 concentration in the aqueous phase Ceq by UV/vis spectrophotometer at 485 nm. 2.4. Photocatalytic degradation of azodyes in aqueous CeO2 suspensions In a typical photocatalytic experiment, aqueous suspension of AO7 (50 mL, 70 mg/L) and 0.05 g photocatalyst powder were placed in a quartz tube with vigorous agitation. The used photoreactor was homemade in which a 1000 W halogen lamp was used as illuminated light source and the short-wavelength components (l < 420 nm) of the light were totally cut off using a cut-off glass filter. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The distance between the lamp and the center of quartz tube was 10 cm. The above suspension in the course of irradiation was abstracted from the mixture solution every hour after centrifugation. A Cary 100 UV–vis spectrometer was used to record the change of concentration of AO7 during visible light irradiation. The starting point of the concentration– time plot was collected after the suspension was stirred for 1 h in darkness to reach the adsorption equilibrium and C0 referred to the equilibrated dye concentrations. Total organic carbon of the solution was analyzed with the Elementar TOC analyzer. The SO42 ion was determined by ionic chromatography (Dionex ICS1000). Samples for gas chromatography–mass spectrometry (GC–MS) and FT-IR analysis were prepared by the following procedure. The suspensions at different irradiation times were centrifuged to separate CeO2 particles and dye solutions. The catalyst particles were thoroughly washed, dried and then supported on KBr pellets to record infrared spectrum on a Nicolet 380 Fourier FT-IR spectrophotometer. The dye solutions were extracted with dichloromethane and the extracts were concentrated under reduced pressure to 1 mL at room temperature and then analyzed by a GC–MS system. Agilent 6890GC/5975I MSD equipped with a HP-5MS capillary column (30 m 0.25 mm 0.25 mm film thickness) was employed and GC column was operated in a temperature programmed mode with an initial temperature of 40 8C held for 10 min, ramp first at 100 8C with a 12 8C/min rate, then to 200 8C with 5 8C/min and finally ramp to 270 8C with a 20 8C/min rate, and held at that temperature for 5 min.
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Fig. 1. (A) X-ray diffraction pattern of the synthesized CeO2 sample. (B) UV–vis absorption spectra of obtained CeO2 sample. (C) Nitrogen adsorption–desorption isotherm and the pore size distribution plot for the synthesized CeO2 sample. The pore size distribution was estimated from the desorption branch of the isotherm. (D) TEM image of the calcined CeO2 and the bar scale is 100 nm.
3. Results and discussion 3.1. Characterizations of photocatalyst The crystal phase and texture of the synthesized CeO2 were characterized by XRD and N2-sorption. Fig. 1A gives the XRD diffraction pattern for the CeO2. Diffraction peaks, corresponding to cubic fluorite structure (JCPDS: 43-1002), are clearly observed. The crystal size estimated by Scherrer equation is 15.8 nm. The photoabsorption property (UV–vis absorption spectrum) for CeO2 (Fig. 1B) shows that it has a strong absorption band in ultraviolet region (200–400 nm) but does not effectively absorb the light beyond wavelength of 420 nm, which is in agreement with the reported value [21]. N2-sorption isotherm (Fig. 1C) for the CeO2 exhibits stepwise adsorption and desorption (Type IV isotherm), indicative of a porous solid [22]. The average pore size for the CeO2 is 7.5 nm with a narrow distribution. The BET specific surface area for the CeO2 is 42 m2/g, whereas that of TiO2 P25 is 50 m2/g. The crystal size and pore size are consistent with the result from TEM measurements. The TEM image (Fig. 1D) also shows the pores resulted from aggregated CeO2 nanocrytalline particles. 3.2. AO7 decolorization by visible light/CeO2 The photodegradation of AO7 was carried out under visible light illumination at wavelengths longer than 420 nm in aqueous CeO2 dispersion. The data displayed in Fig. 2A clearly indicates that under otherwise identical conditions CeO2 exhibits a much higher activity than TiO2. The experiment results showed that AO7 solution was stable under visible light irradiation in the absence of
CeO2 and the CeO2 suspension was unable to initiate the dye degradation in the dark. Both visible light and CeO2 semiconductor particles were indispensable for the degradation of AO7 aqueous solution. The temporal UV/vis spectra showed that the AO7 characteristic band centered at 485 nm decreased promptly upon light irradiation (Fig. 2B), indicating that at least the chromophoric structure of the dye was destroyed. The peaks at 310 and 228 nm corresponding to naphthalene ring and benzene ring in the dye molecule were also reduced partly, showing a variety of organic molecules present in the solution even after the dispersion is totally bleached [9]. According to UV–vis absorption spectrum (Fig. 1B), CeO2 is a semiconducting material which corresponds to the onset of absorption no more than 420 nm. In our experiment, the employed light source wavelength was longer than 420 nm, which is no more absorbed by photocatalyst CeO2 but still able to excite the azodye AO7. Therefore, we proposed the self-sensitization is the governing mechanism in this system. To further confirm this proposal, we repeated this experiment by using light source wavelength above 450 nm and kept other experiment parameters unchanged. The result showed that the photoassisted decolorization rates were almost identical under irradiation of the two light sources (Fig. S2 supporting information). TOC removal of the solution and the formation of sulfate ions with exposure time during the photodegradation of AO7 were also studied to examine the mineralization process (Fig. 3). It is observed that during the early hours of the experiment, when the solution was still colored, the TOC exhibited a substantial decrease with time of irradiation. For instance, after a period of 12 h, when the solution was totally bleached, the TOC was reduced to about 60% of its initial value. Further irradiation did not result in significant decrease of TOC. The
P. Ji et al. / Applied Catalysis B: Environmental 85 (2009) 148–154
Fig. 2. (A) Temporal course of the photodegradation of AO7 in CeO2 aqueous dispersions under visible light irradiation: (a) CeO2 under visible irradiation (l > 420 nm). (b) TiO2 under irradiation (l > 420 nm). (c) No catalyst. (d) CeO2 in the dark. (B) UV/vis spectral changes recorded for (a) as a function of irradiation time. Experimental conditions: [AO7] = 2 104 M (70 mg/L), [CeO2] = 1 g/L, initial pH 6.8 (natural), T = 298 K.
stabilization of TOC values after 12 h irradiation is probably due to (1) generation of refractory organic compounds or (2) the fact that dye transformation can only be proceeded as long as the solution is colored and photosensitization mechanism is operable. The concentration of the sulfate ions continuously increased with time and tended to reach a plateau of ca. 3.7 mg/L after 12 h. This provided additional evidence that the photocatalytic system is active only in the presence of colored compounds. The amount of
Fig. 3. Decolorization and TOC removal of the solution, SO42 ions formation during the photodegradation of AO7 (2 104 M, 50 mL) in CeO2 aqueous dispersions (1 g/ L) under visible irradiation. Other experimental conditions same as Fig. 2.
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Fig. 4. (A) Adsorption isotherm of AO7 over CeO2 catalyst with the different pH value of the suspension being 2.96 (a), 4.15 (b), 6.8 (c) and 10.5 (d). (B) Langmuir analysis of the corresponding adsorption isotherm of AO7 on CeO2 catalyst.
produced SO42 was about five times less than the expected (19.2 mg/L) assuming complete mineralization of the dye. This is because species containing sulfur atoms could be other anionic groups, such as SO3 and the generated SO42 could be strongly bonded on the photocatalyst surface, which was also demonstrated by FT-IR spectra of the catalyst used in the course of photodegradation. 3.3. Adsorption capacity of AO7 on the CeO2 3.3.1. pH effect on adsorption It is well known that the adsorption capacity on photocatalyst of dyes is a key factor for the degradation rate in photocatalytic system. Ranjit et al. reported that the more target molecules adsorbed on catalyst, the faster or more complete these molecules decomposed [23]. Therefore, to explain the enhanced photocatalytic activity of CeO2 catalyst as compared to the TiO2 P25, the adsorption of AO7 on CeO2 nanoparticles were examined. It is well known that the solution pH has great influence on the surface charge of the semiconductor photocatalyst particles, which has been studied extensively in concerns of traditional photocatalyst TiO2 [24]. Fig. 4A shows the adsorption isotherms of AO7 on CeO2 catalyst at pH values of 2.96, 4.5, 6.8 (natural), and 10.5. The amount of AO7 adsorbed onto the photocatalyst increases as the bulk concentration of the substrate is elevated in all the cases. The L-shape isotherms were obtained except at pH 10.5, indicating there is no strong competition between the solvent and the adsorbate for the adsorbent sites in acidic media [25]. This can be explained, since at acidic media, the photocatalyst surface is positively charged (the zero point of charge of CeO2 is pHzpc 6.8)
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Fig. 5. First-order decay curve at pH value of 2.96 (a), 4.5 (b), 6.8 (c), 10.5 (d). Experimental conditions: [AO7] = 2 104 M (70 mg/L), [CeO2] = 1 g/L.
and attractive forces between the Ce–OH2+ surface group and the dye anions are operable [26]. The reverse effect was observed in the basic media where the CeO2 surface is negatively charged which repels the dye. The adsorption isotherm could be analyzed in terms of Langmuir theory (Fig. 4B). A linear relationship between [C]eq/Cads as a function of [C]eq is observed where [C]eq represents the equilibrium bulk concentration of AO7 and Cads is the amount (in moles) of AO7 adsorbed onto the photocatalysts. From Fig. 4B, the derived adsorption constant on CeO2 is Kads = 36,800 M1 at natural pH (6.8). The Langmuir constants calculated from other plots are 96,000, 82,000, 13,000 M1, respectively for the pH of 2.96, 4.50, 10.50. According to Bauer et al. [27], the saturation value of AO7 on TiO2 catalyst is 2.56 106 mol/g and the derived adsorption constant Kads = 18,000 M1 at the pH of 6.0. The adsorption constant of AO7 to
0
Fig. 6. FT-IR spectra obtained from the (a) AO7 powder, and (b–d) photocatalyst samples taken from the suspension after filtration, at 0, 12 and 30 h of irradiation.
form of Eq. (8) is t¼
C0 0 ¼ kr K e t ¼ k t C
ln
dC kr K e C ¼ dt 1 þ KeC
(8)
with C being the dye concentration, kr the apparent reaction rate constant, and Ke the apparent equilibrium constant for the adsorption of the dye on the nanocatalyst surface. The integrated
(10)
The overall rate constants for AO7 decolorization in reciprocal hours are given in the following order:
1
CeO2 is ca. 2-fold higher as compared to the P25 when they are under pHzpc conditions. It is worth notice that the adsorption capacity of CeO2 to acid orange 7 is much greater than TiO2, while the surface area of the CeO2 is even slightly less than that of TiO2. Lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols, etc.) in the interaction of these functional groups with the f-orbitals of the lanthanides. In addition, CeO2 can easily release or store oxygen under different conditions to form large amount of oxygen vacancies on the surface, leading to affluent surface states. These two reasons probably account for the superior adsorption capacity of CeO2 photocatalyst. The kinetic of AO7 decolorization under different initial pH value of solution is presented in Fig. 5 by plotting the logarithm of the normalized dye concentration against irradiation time. Fairly good linear relationships were observed, indicating the all reactions followed first-order kinetics. The decolorization kinetics of the dye can be rationalized by a modified Langmuir–Hinshelwood (L–H) mechanism, where
(9)
When the concentration of the dye is sufficiently low (much less than 103 M), Eq. (9) can be expressed as
kpH 2:96 ð0:65 h
1 C0 1 þ ðC 0 CÞ ln K e kr kr C
0
1
Þ > kpH 4:5 ð0:44 h
0
1
Þ > kpH 6:8 ð0:30 h
0
1
Þ > kpH 10:5 ð0:20 h
Þ;
indicating the degradation rates were pH-dependent and getting down as the pH values were getting up. An increase in pH can lead to a decrease in adsorption and consequently a decrease in degradation rate. Therefore, we can conclude that adsorption capacity played a key role in determining photocatalytic degradation efficiency of azodyes over CeO2. 3.3.2. Adsorption mode of AO7 on CeO2 Ex situ FT-IR was used to investigate the AO7/CeO2 composite in order to get a detailed understanding of the interaction between AO7 and CeO2 surface. Fig. 6 shows the spectra of isolated AO7 (a) and AO7/CeO2 before irradiation (b) and after irradiation for 12 h (c) and 30 h (d), respectively. A detailed analysis of AO7 bands in the region of 2000–1000 cm1 has been reported by Bauer [27]. The intense band at 1514 cm1 has been attributed to the –N N– bond vibrations or to aromatic ring vibrations sensitive to the interaction with the azo bond, or to the bending vibration mode d(N–H) of the hydrazone form of the azo dye. The bands at 1624, 1599, 1572, 1555 and 1452 cm1 are linked to C C aromatic skeletal vibrations. The peaks located at 1402 cm1 can be assigned to O–H bending vibrations, while those located at 1198 cm1 (shoulder) and 1304 cm1 are due to the symmetric and asymmetric vibrations of the sulfonate groups, respectively. The bands at 1256 and 1223 cm1 are assigned to the vibration stretching modes n(C–N) and n(N–N) of the hydrazone tautomer, respectively. Finally, the peaks at 1130
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assumption [27,28]. Furthermore, the adsorption of AO7 occurs through a Lewis acid–base reaction which implies the formation of an inner-sphere complex. The ex situ FT-IR spectra obtained from photocatalyst samples collected at 12 h of irradiation shows the degradation of the adsorbed species. All the bands decrease with time with respect to those at t = 0 (trace b). This is particularly true for the band at 1514 cm1, which reflects the destruction of the chromophore part of the azodye. It is interesting that this azo band still can be observed on FT-IR spectra of the catalyst while the solution has been already totally bleached, indicating the decomposition of dye molecules mainly occurs on the surface of catalyst particles after the adsorption. Almost all the bands on trace b disappear on trace d except the bands at 1130 and 1047 cm1, indicating certain amount of chemicals containing phenyl and sulfo groups still exist on the surface of CeO2 powder even after 30 h irradiation. A new band at 1654 cm1 is generated at 30 h of irradiation, which is attributed to the stretching vibration of C O groups of carboxylic acid. 3.4. Active species to attack dye molecule
Fig. 7. (A) Degradation rate of air equilibrated and N2 purged condition after 9 h irradiation. (B) Variation of normalized concentration of azodye AO7 in solution in the presence of 2 mM of 1,4-benzoquinone (BQ), NaN3 and KI. Other experimental conditions same as Fig. 3.
and 1047 cm1 are assigned to the coupling between the benzene mode and ns(SO3). The spectrum of AO7 adsorbed on the catalyst (Fig. 6, trace b) is modified with respect to the one of AO7 powder (trace a). In general, a major decrease of the band intensities is observed. No broadening of the bands is observed may suggest a homogeneity in the adsorption sites. The only striking feature is the collapse of the bands linked to the sulfonate stretching vibration mode. The band at 1198 cm1 due to nas(SO3) is shifted to 1168 cm1 with a considerable decrease in intensity after adsorption, while the band associated with vs(SO3) at 1123 is only slightly affected. The ratio of the intensities nas(SO3)/ns(SO3) = 0.5 for adsorbed AO7 is very different from the one of isolated AO7 nas(SO3)/ns(SO3) = 0.9. Surprisingly, the band at 1572 cm1, associated with the C O group of the hydrazone form of the dye, is still noticeable in the AO7–CeO2 spectrum, which is different from AO7–TiO2 system. Therefore we can conclude that the way of AO7 adsorbed on CeO2 is different to that of AO7 adsorbed on TiO2 proposed by Bauer et al. [27]. They showed that two oxygen atoms from the sulfonate group are linked to two TiIV cations in a bidentate binuclear coordination type complex, on the one side, while, the oxygen atom from the carbonyl group of the hydrazone tautomer is linked to TiIV cation in an unidentate coordination type complex. According to FT-IR data in our investigation, we found the adsorption of the azodye onto the CeO2 surface is solely associated with the oxygen atoms of the sulfonate group. It is also observed that Dvas-s (nas(SO3) ns(SO3)) of isolated AO7 is higher than that of absorbed AO7, which is the characteristic of bidentate bridge according to Deacon and Phillips’ theory and Bauer’s
In order to determine the main active species responsible for the degradation of dye molecules, comparison experiments of N2purged condition and scavenger-loaded condition were undertaken. The photo reactor was sealed and purged with N2 for 1 h before irradiation to drive out a part of dissolved oxygen and kept purging on until the reaction was completed. As seen in Fig. 7A, the notable result is that the decolorization rate in the air equilibrium condition is 92%, clearly higher than that in the N2 condition which is only about 10%. In the N2-purged condition, dissolved or adsorbed oxygen may be sufficient to degrade the dye at the early stage but the degradation stopped after oxygen was used up, indicating oxygen is the main source of the active species. The formation of possible oxidative intermediate species, such as singlet oxygen (1O2), superoxide (O2), and hydroperoxy (HO2), or hydroxyl radical (OH) if it exists, under photo-reaction conditions and their role in the dye degradation process has been investigated indirectly, with the use of appropriate quenchers of these species. In this type of experiments, a comparison is made between the original decolorization curves of AO7/CeO2 dispersions with those obtained after addition of millimolar concentrations of quenchers in the initial solution, under otherwise identical conditions. In Fig. 7B, trace a is the decolorization curve without any quenchers and b, c, d are respective for addition of sodium azide (NaN3, a singlet oxygen quencher [29] but may also interact with OH radical [30]), 1,4-benzoquinone (C6H4O2, BQ, a quencher of superoxide radical [31]) and KI (a quencher of positive holes (hVB+) and OH radical on catalyst surface [32,33]). It is observed that NaN3 and KI did not affect the degradation rate of AO7 throughout the experiment. This result indicates that 1O2 and OH are not the active oxidative species involved in this photodegradation system. Herna´ndez-Alonso et al. reported that holes or other type of hole-derived entities are responsible for toluene oxidation [18]. But we concluded that hVB+ was not active species in this system from the performance of CeO2 in KI-loaded experiment. However, addition of 2 mM of BQ, which is a O2 quencher, has a significant suppressing effect on the decolorization of the dye solution, showing the superoxide anion radical is the main oxidative species responsible for the photobleaching of the AO7 dye solution. When AO7/TiO2 system is irradiated by visible light, the AO7 dye is excited and injected an electron into Ti3d conduction band and then this electron is scavenged by the surface oxygen to generate superoxide radical, which can then form hydrogen
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through bidentate coordination mode of oxygen atoms from the sulfonate group. The active species were detected indirectly by adding various radical quenchers and superoxide anion radical was found responsible to attack the dye molecules. We preliminary proposed that the dye self-sensitization is the governing mechanism for the degradation process, in which excited AO7 molecules inject electrons into Ce4f orbits followed by oxygen on the catalyst surface scavenging this electron to produce O2 active species. Therefore, CeO2 could serve as an excellent electron-transfer mediator to transfer the electron coming from photo excited dye molecules, which is also useful for a better understanding of the electronic properties of CeO2. Fig. 8. The photocatalytic mechanism over CeO2 under irradiation of visible light.
peroxide (H2O2), hydroperoxy (HO2) and hydroxyl (OH) radicals (Eqs. (1)–(7)). However, Koelling et al. carried out the selfconsistent-field band calculation for CeO2 and predicted that the band-gap energy between the valence band of O2p character and the Ce5d conduction band is about 6 eV, with a Ce4f narrow band lying within the band gap [34]. Thus the conduction band of cerium dioxide is potentially higher than the excited state of AO7 and the electron transfer could be thermodynamically impossible. Several studies have confirmed the Ce4f plays vital role for the application of CeO2 in photocatalytic process and Li et al. has also demonstrated that the potential of the 4f band of CeO2 is a little bit more positive than that of conduction band of TiO2 [14]. Thus, from a thermodynamical point of view, an electron can be more easily injected from the excited state of the adsorbed dye to the 4f band of CeO2 than to the conduction band of TiO2. Then the electron is scavenged by surface oxygen and initialized a series of redox reactions (Fig. 8). 3.5. Determination of intermediates GC–MS technique was applied to identify the intermediates during the photodegradation of AO7. All the identified compounds were unequivocally identified using the NIST98 library database with fit values higher than 90% (Table S1 supporting information). The main products included substituted benzene, such as benzoic acid, phenol, phthalic anhydride and low molecular weight organic 4-oxo-2-butenoic acid. From the results of GC–MS and scavengerloaded experiments, we can propose a possible degradation pathway of AO7. Firstly, the azodye molecule of AO7 is adsorbed on CeO2 surface. Then the dye molecule is excited by irradiation and injected one electron to Ce4f orbit followed by oxygen on the catalyst surface scavenging this electron to form O2. After that, the superoxide starts to attack dye molecule from –N N– bond to decolorize the solution. Dye molecule is split to generate compounds containing benzene ring and naphthalene ring. These compounds could be attacked further to be deeply oxidized to give out small organic acids. 4. Conclusion The photocatalytic activity of CeO2 to degrade azodye acid orange 7 (AO7) was examined under visible light irradiation (l > 420 nm) and was found to have better performance than commercial reference P25. The superior adsorption capacity of CeO2 accounts for the higher activity. The effect of initial solution pH value on adsorption capacity of CeO2 and consequently its photocatalytic ability was also investigated. The lower the pH is, the more the CeO2 adsorbed and then the higher the degradation rate is. The AO7 molecule is absorbed on the surface of CeO2
Acknowledgements This work has been supported by Science and Technology Commission of Shanghai Municipality (07JC14015); Shanghai Nanotechnology Promotion Centre (0652nm045, 0752nm001), National Nature Science Foundation of China (2057709, 20773039), the National Basic Research Program of China (2004CB719502, 2007CB613306) and the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcatb.2008.07.004. References [1] H.R. Hitz, W. Huber, R.H. Reed, J. Soc. Dyers Colour. 94 (1978) 71. [2] H. Zollinger, Color Chemistry: Synthesis, Properties and Applications of Organic Dyes and Pigments, VCH Publishers, New York, 1991. [3] C. Maynard, Handbook of Industrial Chemistry, Van Nostrand Reinhold, New York, 1983. [4] H.M. Martin, Photodegradation of Water Pollutants, CRC Press, Boca Raton, FL, 1996. [5] C. Hu, Y. Wang, Chemosphere 39 (1999) 2107. [6] J. Zhao, C. Chen, W. Ma, Topics Catal. 35 (2005) 269, and references therein. [7] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. [8] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [9] M. Stylidi, D.I. Kondarides, X.E. Verykios, Appl. Catal. B: Environ. 47 (2004) 189. [10] W. Zhao, C.C. Chen, X.Z. Li, J.C. Zhao, H. Hidaka, N.J. Serpone, Phys. Chem. B 106 (2002) 5022. [11] C.C. Chen, X.Z. Li, W.H. Ma, J.C. Zhao, H. Hidaka, N. Serpone, J. Phys. Chem. B 106 (2002) 318. [12] W. Zhao, C.C. Chen, W.H. Ma, J.C. Zhao, D.X. Wang, H. Hidaka, N. Serpone, Chem. Eur. J. 9 (2003) 3292. [13] S. Song, L. Xu, Z.Q. He, J.M. Chen, Environ. Sci. Technol. 41 (2007) 5849. [14] F.B. Li, X.Z. Li, M.F. Huo, K.W. Cheah, W.C.H. Choy, Appl. Catal. A 285 (2005) 181. [15] A.K. Sinha, K. Suzuki, J. Phys. Chem. B 109 (2005) 1708. [16] K.-H. Chung, D.-C. Park, Catal. Today 30 (1996) 157. [17] G.R. Bamwenda, H.J. Arakawa, Mol. Catal. A: Chem. 161 (2000) 105. [18] M.D. Herna´ndez-Alonso, A.B. Hungrı´a, A. Martı´nez-Arias, M. Ferna´ndez-Garcı´a, J.M. Coronado, J.C. Conesa, J. Soria, Appl. Catal. B: Environ. 50 (2004) 167. [19] Y.Q. Zhai, S.Y. Zhang, H. Pang, Mater. Lett. 61 (2007) 1863. [20] P. Borker, A.V. Salker, Mater. Chem. Phys. 103 (2007) 366. [21] K.B. Sundaram, P. Wahid, Phys. Status Solidi B 161 (1990) K63. [22] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic, London, 1997, p. 111. [23] K.T. Ranjit, I. Willner, S.H. Bossmann, A.M. Braun, Environ. Sci. Technol. 35 (2001) 1544. [24] J. Zhao, H. Hidaka, A. Takamura, E. Pelizzetti, N. Serpone, Langmuir 9 (1993) 1646. [25] A.P. Toor, A. Verma, C.K. Jotshi, P.K. Bajpai, V. Singh, Dyes Pigments 68 (2006) 53. [26] X.L. Song, G.Z. Qiu, P. Qu, Z.H. Yang, Chin. J. Rare Met. 2 (2005) 167. [27] C. Bauer, P. Jacques, A. Kalt, Chem. Phys. Lett. 307 (1999) 397. [28] G. Deacon, R. Phillips, Coord. Chem. Rev. 33 (1980) 227. [29] C. Schweitzer, R. Schmidt, Chem. Rev. 103 (2003) 1685. [30] E. Frati, A.M. Khatib, P. Front, A. Panasyuk, F. Aprile, D.R. Mitrovic, Free Radic. Biol. Med. 22 (1997) 1139. [31] J. Bandara, J. Kiwi, New J. Chem. 23 (1999) 717. [32] G.T. Li, J.H. Qu, X.W. Zhang, H.J. Liu, H.N. Liu, J. Mol. Catal. A 259 (1–2) (2006) 238. [33] S.H. Yoon, J.H. Lee, Environ. Sci. Technol. 39 (2005) 9695. [34] D.D. Koelling, A.M. Boring, J.H. Wood, Solid State Commun. 47 (1983) 227.
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation Pengfei Ji a, Jinlong Zhang a,*, Feng Chen a, Masakazu Anpo b a b
Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 21 May 2008 Received in revised form 3 July 2008 Accepted 3 July 2008 Available online 10 July 2008
Cerium dioxide was prepared by the precipitation method and found to be an efficient photocatalyst to degrade azodyes under visible light irradiation. Nonbiodegradable azodyes acid orange 7 (AO7) was selected as modal target to examine the photocatalytic activity of CeO2. AO7 could be efficiently degraded in aqueous suspension of CeO2 under visible light illumination. The catalyst was characterized by X-ray diffraction (XRD), N2 sorption, transmission electron microscopic image (TEM) and UV/vis absorption spectrum techniques. AO7 solution was quickly decolorized and partly mineralized under visible light irradiation with existing CeO2. The photodegradation rate of this azodye catalyzed by CeO2 is much faster than those occurring on commercial titania (Degussa P25) under otherwise identical conditions of visible light irradiation. Experiments were conducted to examine the adsorption mode of acid orange 7 on CeO2 and adsorption capacity at different pH values. The possible degradation pathway has been proposed for the photocatalytic degradations by using certain radical scavengers and gas chromatography–mass spectrometry (GC–MS) to determine intermediates. The enhanced photoactivity of the lanthanide oxide CeO2 was attributed to the superior adsorption capacity and special 4f electron configuration. ß 2008 Elsevier B.V. All rights reserved.
Keywords: CeO2 photocatalyst Azo dye decomposition Degradation pathway 4f Electron configuration
1. Introduction Azodyes are an abundant class of synthetic, colored, organic compounds, which are characterized by the presence of one or more azo bonds (–N N–). Large quantities of these dyes (about 50–70% of the dyes available on the market today) are manufactured worldwide and used in a variety of applications [1,2]. It is estimated that about 15% of the total world production is lost during synthesis and processing [2,3]. Therefore azodyes are extensively contained in wastewater generated from the textile and dyestuff industries. Such colored dye effluents pose a major threat to the surrounding ecosystems owning to their nonbiodegradability, toxicity and potential carcinogenic nature [2]. Heterogeneous photocatalysis assisted by various semiconductors has been considered as a cost-effective alternative as a pre- or posttreatment of biological treatment processes for the purification of dye-containing wastewater [4,5]. Among various metal oxide semiconductors, TiO2 is the most widely studied photocatalyst for environment remediation owning
* Corresponding author. Fax: +86 21 64252062. E-mail address: [email protected] (J. Zhang). 0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.07.004
to its unique photocatalytic efficiency, low cost, nontoxicity, and high stability. One obstacle to its effective utilization is the inefficient use of solar energy; less than 5% (UV light) of the sunlight can be absorbed by this photocatalyst. A simple and interesting approach to extend the catalyst absorption toward visible region is the photosensitization by an appropriate dye [6]. The mechanistic details of dye degradation have also been the subject of several studies. In these systems, dyes rather than TiO2 particles are excited by visible light to appropriate singlet and triplet states, processes which were followed by electron injection into the TiO2 conduction band. The TiO2 only acts as an electrontransfer mediator in this reaction. This pathway (Eqs. (1)–(7)) is different from the pathway implicated during UV light irradiation [7,8]. dye þ hn ! dye
(1)
dye þ TiO2 ! dyeþ þ TiO2 ðe Þ
(2)
TiO2 ðe Þ þ O2 ! TiO2 þ O2
(3)
O2 þ TiO2 ðe Þ þ 2Hþ ! H2 O2 þ TiO2
(4)
P. Ji et al. / Applied Catalysis B: Environmental 85 (2009) 148–154
2O2 þ 2Hþ ! O2 þ H2 O2
(5)
H2 O2 þ TiO2 ðe Þ ! OH þ OH þ TiO2
(6)
dyeþ þ ðO2 ; O2 ; or OHÞ ! degraded or mineralized products (7) However, the photodegradation in the dyes/TiO2 sensitization system is relatively inefficient. Kondarides and coworkers have reported that it takes about 50 h to totally decolorize azodyes acid orange 7 (AO7) solution with concentration up to 100 mg/L [9]. To overcome this obstacle, several researches had been carried concerning the effect of transition metal or metal ions on the self-sensitized degradation of dyes under visible light irradiation [10,11]. Zhao et al. have also demonstrated the chemisorbed hexachloroplatinate (PtCl62) anions on the surface of P25 TiO2 particles can accelerate the photodegradation of ethyl orange under visible light illumination [12]. However, the preparations of above mentioned catalysts are economically costly and timeconsuming, which makes the practical application of these novel catalysts hard to achieve. CeO2 has some properties like titania features such as wide band gap, nontoxicity, and high stability. Since its unique 4f electron configuration, CeO2 has been frequently selected as a component to prepare complex oxides or as a dopant to improve titania-based catalysts’ performances [13–15]. As for pure CeO2, it has been investigated under UV irradiation concerning watersplitting for the generation of hydrogen gas [16,17] and photodegradation of toluene in the gas phase [18]. Zhai et al. and Salker et al. have recently reported the photocatalytic behaviors of CeO2 under sunlight irradiation to degrade dyes [19,20]. Herein, we report CeO2 for photocatalytic degradation of sulfo group-containing azodyes in aqueous suspension irradiated by visible light. Acid orange 7 was chosen as model target to examine the adsorption and degradation of azodye on CeO2 irradiated by visible light. The CeO2 showed high photoactivity towards degradation of azodye and has been proven to be a promising alternative for dyecontaining wastewater treatment under visible light irradiation. 2. Materials and methods 2.1. Materials All major chemicals were of reagent grade or higher purity. Ce(NO3)36H2O was supplied by Shanghai Sinopharm Chemical Reagent Co., Ltd., China. Azodye acid orange 7 (AO7) was obtained from Acros and used without further purification (Fig. S1, supporting information). Titania P25 (TiO2; ca. 80% anatase, 20% rutile; BET area, ca. 50 m2/g) was purchased from the Degussa. 1,4Benzoqunone and potassium iodide used as scavengers were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. Sodium azide was purchased from Alfa Aesar. Deionized and double distilled water was used throughout this study. 2.2. Preparation of the catalyst CeO2 was prepared by the precipitation method in our laboratory. Specifically, 4.34 g Ce(NO3)36H2O was dissolved in 100 mL of distilled water to form transparent solution (0.1 M). To this solution 4 mL NH4OH (28 wt% NH3) was quickly added under vigorous stirring. The transparent solution immediately changed to yellowish suspension. This suspension was continued to stir for 2 h followed by 5 h aging at room temperature. Then the purple precipitate was collected by centrifugation and washed 3 times with water and ethanol. Consequently, the precipitate was dried at
149
80 8C for 12 h before it was calcined at 550 8C for 4 h (ramp: 2 8C/ min). The powder X-ray diffraction analysis of the prepared catalyst was carried out at room temperature with a Rigaku D/max 2550 VB/PC apparatus using Cu Ka radiation (l = 1.5406 A˚) and a graphite monchromator, operated at 40 kV and 30 mA. The BET of the sample was determined through nitrogen adsorption at 77 K (Micromeritics ASAP 2010). The sample was degassed at 473 K before the measurement. UV–vis absorbance spectra were obtained for the dry-pressed disk sample using a Scan UV–vis spectrophotometer (Varian, Cary 500) equipped with an intergrating sphere assembly. TEM analysis of the samples was done using a JSM-2100F (JEOL) instrument and the electron beam accelerating voltage was 200 kV. 2.3. Adsorption experiments For the adsorption measurements, 50 mg of CeO2 was added to 50 mL of freshly prepared AO7 solutions of known concentration in the range 104 to 103 M at different pH values which were adjusted by 1 M HNO3 or NH4OH. The suspensions were sonicated for 5 min and stirred for 1 h at room temperature in the dark to reach equilibrium. After filtration through Millipore filter (0.22 mm) membrane, a sample was taken to determine the AO7 concentration in the aqueous phase Ceq by UV/vis spectrophotometer at 485 nm. 2.4. Photocatalytic degradation of azodyes in aqueous CeO2 suspensions In a typical photocatalytic experiment, aqueous suspension of AO7 (50 mL, 70 mg/L) and 0.05 g photocatalyst powder were placed in a quartz tube with vigorous agitation. The used photoreactor was homemade in which a 1000 W halogen lamp was used as illuminated light source and the short-wavelength components (l < 420 nm) of the light were totally cut off using a cut-off glass filter. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The distance between the lamp and the center of quartz tube was 10 cm. The above suspension in the course of irradiation was abstracted from the mixture solution every hour after centrifugation. A Cary 100 UV–vis spectrometer was used to record the change of concentration of AO7 during visible light irradiation. The starting point of the concentration– time plot was collected after the suspension was stirred for 1 h in darkness to reach the adsorption equilibrium and C0 referred to the equilibrated dye concentrations. Total organic carbon of the solution was analyzed with the Elementar TOC analyzer. The SO42 ion was determined by ionic chromatography (Dionex ICS1000). Samples for gas chromatography–mass spectrometry (GC–MS) and FT-IR analysis were prepared by the following procedure. The suspensions at different irradiation times were centrifuged to separate CeO2 particles and dye solutions. The catalyst particles were thoroughly washed, dried and then supported on KBr pellets to record infrared spectrum on a Nicolet 380 Fourier FT-IR spectrophotometer. The dye solutions were extracted with dichloromethane and the extracts were concentrated under reduced pressure to 1 mL at room temperature and then analyzed by a GC–MS system. Agilent 6890GC/5975I MSD equipped with a HP-5MS capillary column (30 m 0.25 mm 0.25 mm film thickness) was employed and GC column was operated in a temperature programmed mode with an initial temperature of 40 8C held for 10 min, ramp first at 100 8C with a 12 8C/min rate, then to 200 8C with 5 8C/min and finally ramp to 270 8C with a 20 8C/min rate, and held at that temperature for 5 min.
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Fig. 1. (A) X-ray diffraction pattern of the synthesized CeO2 sample. (B) UV–vis absorption spectra of obtained CeO2 sample. (C) Nitrogen adsorption–desorption isotherm and the pore size distribution plot for the synthesized CeO2 sample. The pore size distribution was estimated from the desorption branch of the isotherm. (D) TEM image of the calcined CeO2 and the bar scale is 100 nm.
3. Results and discussion 3.1. Characterizations of photocatalyst The crystal phase and texture of the synthesized CeO2 were characterized by XRD and N2-sorption. Fig. 1A gives the XRD diffraction pattern for the CeO2. Diffraction peaks, corresponding to cubic fluorite structure (JCPDS: 43-1002), are clearly observed. The crystal size estimated by Scherrer equation is 15.8 nm. The photoabsorption property (UV–vis absorption spectrum) for CeO2 (Fig. 1B) shows that it has a strong absorption band in ultraviolet region (200–400 nm) but does not effectively absorb the light beyond wavelength of 420 nm, which is in agreement with the reported value [21]. N2-sorption isotherm (Fig. 1C) for the CeO2 exhibits stepwise adsorption and desorption (Type IV isotherm), indicative of a porous solid [22]. The average pore size for the CeO2 is 7.5 nm with a narrow distribution. The BET specific surface area for the CeO2 is 42 m2/g, whereas that of TiO2 P25 is 50 m2/g. The crystal size and pore size are consistent with the result from TEM measurements. The TEM image (Fig. 1D) also shows the pores resulted from aggregated CeO2 nanocrytalline particles. 3.2. AO7 decolorization by visible light/CeO2 The photodegradation of AO7 was carried out under visible light illumination at wavelengths longer than 420 nm in aqueous CeO2 dispersion. The data displayed in Fig. 2A clearly indicates that under otherwise identical conditions CeO2 exhibits a much higher activity than TiO2. The experiment results showed that AO7 solution was stable under visible light irradiation in the absence of
CeO2 and the CeO2 suspension was unable to initiate the dye degradation in the dark. Both visible light and CeO2 semiconductor particles were indispensable for the degradation of AO7 aqueous solution. The temporal UV/vis spectra showed that the AO7 characteristic band centered at 485 nm decreased promptly upon light irradiation (Fig. 2B), indicating that at least the chromophoric structure of the dye was destroyed. The peaks at 310 and 228 nm corresponding to naphthalene ring and benzene ring in the dye molecule were also reduced partly, showing a variety of organic molecules present in the solution even after the dispersion is totally bleached [9]. According to UV–vis absorption spectrum (Fig. 1B), CeO2 is a semiconducting material which corresponds to the onset of absorption no more than 420 nm. In our experiment, the employed light source wavelength was longer than 420 nm, which is no more absorbed by photocatalyst CeO2 but still able to excite the azodye AO7. Therefore, we proposed the self-sensitization is the governing mechanism in this system. To further confirm this proposal, we repeated this experiment by using light source wavelength above 450 nm and kept other experiment parameters unchanged. The result showed that the photoassisted decolorization rates were almost identical under irradiation of the two light sources (Fig. S2 supporting information). TOC removal of the solution and the formation of sulfate ions with exposure time during the photodegradation of AO7 were also studied to examine the mineralization process (Fig. 3). It is observed that during the early hours of the experiment, when the solution was still colored, the TOC exhibited a substantial decrease with time of irradiation. For instance, after a period of 12 h, when the solution was totally bleached, the TOC was reduced to about 60% of its initial value. Further irradiation did not result in significant decrease of TOC. The
P. Ji et al. / Applied Catalysis B: Environmental 85 (2009) 148–154
Fig. 2. (A) Temporal course of the photodegradation of AO7 in CeO2 aqueous dispersions under visible light irradiation: (a) CeO2 under visible irradiation (l > 420 nm). (b) TiO2 under irradiation (l > 420 nm). (c) No catalyst. (d) CeO2 in the dark. (B) UV/vis spectral changes recorded for (a) as a function of irradiation time. Experimental conditions: [AO7] = 2 104 M (70 mg/L), [CeO2] = 1 g/L, initial pH 6.8 (natural), T = 298 K.
stabilization of TOC values after 12 h irradiation is probably due to (1) generation of refractory organic compounds or (2) the fact that dye transformation can only be proceeded as long as the solution is colored and photosensitization mechanism is operable. The concentration of the sulfate ions continuously increased with time and tended to reach a plateau of ca. 3.7 mg/L after 12 h. This provided additional evidence that the photocatalytic system is active only in the presence of colored compounds. The amount of
Fig. 3. Decolorization and TOC removal of the solution, SO42 ions formation during the photodegradation of AO7 (2 104 M, 50 mL) in CeO2 aqueous dispersions (1 g/ L) under visible irradiation. Other experimental conditions same as Fig. 2.
151
Fig. 4. (A) Adsorption isotherm of AO7 over CeO2 catalyst with the different pH value of the suspension being 2.96 (a), 4.15 (b), 6.8 (c) and 10.5 (d). (B) Langmuir analysis of the corresponding adsorption isotherm of AO7 on CeO2 catalyst.
produced SO42 was about five times less than the expected (19.2 mg/L) assuming complete mineralization of the dye. This is because species containing sulfur atoms could be other anionic groups, such as SO3 and the generated SO42 could be strongly bonded on the photocatalyst surface, which was also demonstrated by FT-IR spectra of the catalyst used in the course of photodegradation. 3.3. Adsorption capacity of AO7 on the CeO2 3.3.1. pH effect on adsorption It is well known that the adsorption capacity on photocatalyst of dyes is a key factor for the degradation rate in photocatalytic system. Ranjit et al. reported that the more target molecules adsorbed on catalyst, the faster or more complete these molecules decomposed [23]. Therefore, to explain the enhanced photocatalytic activity of CeO2 catalyst as compared to the TiO2 P25, the adsorption of AO7 on CeO2 nanoparticles were examined. It is well known that the solution pH has great influence on the surface charge of the semiconductor photocatalyst particles, which has been studied extensively in concerns of traditional photocatalyst TiO2 [24]. Fig. 4A shows the adsorption isotherms of AO7 on CeO2 catalyst at pH values of 2.96, 4.5, 6.8 (natural), and 10.5. The amount of AO7 adsorbed onto the photocatalyst increases as the bulk concentration of the substrate is elevated in all the cases. The L-shape isotherms were obtained except at pH 10.5, indicating there is no strong competition between the solvent and the adsorbate for the adsorbent sites in acidic media [25]. This can be explained, since at acidic media, the photocatalyst surface is positively charged (the zero point of charge of CeO2 is pHzpc 6.8)
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Fig. 5. First-order decay curve at pH value of 2.96 (a), 4.5 (b), 6.8 (c), 10.5 (d). Experimental conditions: [AO7] = 2 104 M (70 mg/L), [CeO2] = 1 g/L.
and attractive forces between the Ce–OH2+ surface group and the dye anions are operable [26]. The reverse effect was observed in the basic media where the CeO2 surface is negatively charged which repels the dye. The adsorption isotherm could be analyzed in terms of Langmuir theory (Fig. 4B). A linear relationship between [C]eq/Cads as a function of [C]eq is observed where [C]eq represents the equilibrium bulk concentration of AO7 and Cads is the amount (in moles) of AO7 adsorbed onto the photocatalysts. From Fig. 4B, the derived adsorption constant on CeO2 is Kads = 36,800 M1 at natural pH (6.8). The Langmuir constants calculated from other plots are 96,000, 82,000, 13,000 M1, respectively for the pH of 2.96, 4.50, 10.50. According to Bauer et al. [27], the saturation value of AO7 on TiO2 catalyst is 2.56 106 mol/g and the derived adsorption constant Kads = 18,000 M1 at the pH of 6.0. The adsorption constant of AO7 to
0
Fig. 6. FT-IR spectra obtained from the (a) AO7 powder, and (b–d) photocatalyst samples taken from the suspension after filtration, at 0, 12 and 30 h of irradiation.
form of Eq. (8) is t¼
C0 0 ¼ kr K e t ¼ k t C
ln
dC kr K e C ¼ dt 1 þ KeC
(8)
with C being the dye concentration, kr the apparent reaction rate constant, and Ke the apparent equilibrium constant for the adsorption of the dye on the nanocatalyst surface. The integrated
(10)
The overall rate constants for AO7 decolorization in reciprocal hours are given in the following order:
1
CeO2 is ca. 2-fold higher as compared to the P25 when they are under pHzpc conditions. It is worth notice that the adsorption capacity of CeO2 to acid orange 7 is much greater than TiO2, while the surface area of the CeO2 is even slightly less than that of TiO2. Lanthanide ions are known for their ability to form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols, etc.) in the interaction of these functional groups with the f-orbitals of the lanthanides. In addition, CeO2 can easily release or store oxygen under different conditions to form large amount of oxygen vacancies on the surface, leading to affluent surface states. These two reasons probably account for the superior adsorption capacity of CeO2 photocatalyst. The kinetic of AO7 decolorization under different initial pH value of solution is presented in Fig. 5 by plotting the logarithm of the normalized dye concentration against irradiation time. Fairly good linear relationships were observed, indicating the all reactions followed first-order kinetics. The decolorization kinetics of the dye can be rationalized by a modified Langmuir–Hinshelwood (L–H) mechanism, where
(9)
When the concentration of the dye is sufficiently low (much less than 103 M), Eq. (9) can be expressed as
kpH 2:96 ð0:65 h
1 C0 1 þ ðC 0 CÞ ln K e kr kr C
0
1
Þ > kpH 4:5 ð0:44 h
0
1
Þ > kpH 6:8 ð0:30 h
0
1
Þ > kpH 10:5 ð0:20 h
Þ;
indicating the degradation rates were pH-dependent and getting down as the pH values were getting up. An increase in pH can lead to a decrease in adsorption and consequently a decrease in degradation rate. Therefore, we can conclude that adsorption capacity played a key role in determining photocatalytic degradation efficiency of azodyes over CeO2. 3.3.2. Adsorption mode of AO7 on CeO2 Ex situ FT-IR was used to investigate the AO7/CeO2 composite in order to get a detailed understanding of the interaction between AO7 and CeO2 surface. Fig. 6 shows the spectra of isolated AO7 (a) and AO7/CeO2 before irradiation (b) and after irradiation for 12 h (c) and 30 h (d), respectively. A detailed analysis of AO7 bands in the region of 2000–1000 cm1 has been reported by Bauer [27]. The intense band at 1514 cm1 has been attributed to the –N N– bond vibrations or to aromatic ring vibrations sensitive to the interaction with the azo bond, or to the bending vibration mode d(N–H) of the hydrazone form of the azo dye. The bands at 1624, 1599, 1572, 1555 and 1452 cm1 are linked to C C aromatic skeletal vibrations. The peaks located at 1402 cm1 can be assigned to O–H bending vibrations, while those located at 1198 cm1 (shoulder) and 1304 cm1 are due to the symmetric and asymmetric vibrations of the sulfonate groups, respectively. The bands at 1256 and 1223 cm1 are assigned to the vibration stretching modes n(C–N) and n(N–N) of the hydrazone tautomer, respectively. Finally, the peaks at 1130
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assumption [27,28]. Furthermore, the adsorption of AO7 occurs through a Lewis acid–base reaction which implies the formation of an inner-sphere complex. The ex situ FT-IR spectra obtained from photocatalyst samples collected at 12 h of irradiation shows the degradation of the adsorbed species. All the bands decrease with time with respect to those at t = 0 (trace b). This is particularly true for the band at 1514 cm1, which reflects the destruction of the chromophore part of the azodye. It is interesting that this azo band still can be observed on FT-IR spectra of the catalyst while the solution has been already totally bleached, indicating the decomposition of dye molecules mainly occurs on the surface of catalyst particles after the adsorption. Almost all the bands on trace b disappear on trace d except the bands at 1130 and 1047 cm1, indicating certain amount of chemicals containing phenyl and sulfo groups still exist on the surface of CeO2 powder even after 30 h irradiation. A new band at 1654 cm1 is generated at 30 h of irradiation, which is attributed to the stretching vibration of C O groups of carboxylic acid. 3.4. Active species to attack dye molecule
Fig. 7. (A) Degradation rate of air equilibrated and N2 purged condition after 9 h irradiation. (B) Variation of normalized concentration of azodye AO7 in solution in the presence of 2 mM of 1,4-benzoquinone (BQ), NaN3 and KI. Other experimental conditions same as Fig. 3.
and 1047 cm1 are assigned to the coupling between the benzene mode and ns(SO3). The spectrum of AO7 adsorbed on the catalyst (Fig. 6, trace b) is modified with respect to the one of AO7 powder (trace a). In general, a major decrease of the band intensities is observed. No broadening of the bands is observed may suggest a homogeneity in the adsorption sites. The only striking feature is the collapse of the bands linked to the sulfonate stretching vibration mode. The band at 1198 cm1 due to nas(SO3) is shifted to 1168 cm1 with a considerable decrease in intensity after adsorption, while the band associated with vs(SO3) at 1123 is only slightly affected. The ratio of the intensities nas(SO3)/ns(SO3) = 0.5 for adsorbed AO7 is very different from the one of isolated AO7 nas(SO3)/ns(SO3) = 0.9. Surprisingly, the band at 1572 cm1, associated with the C O group of the hydrazone form of the dye, is still noticeable in the AO7–CeO2 spectrum, which is different from AO7–TiO2 system. Therefore we can conclude that the way of AO7 adsorbed on CeO2 is different to that of AO7 adsorbed on TiO2 proposed by Bauer et al. [27]. They showed that two oxygen atoms from the sulfonate group are linked to two TiIV cations in a bidentate binuclear coordination type complex, on the one side, while, the oxygen atom from the carbonyl group of the hydrazone tautomer is linked to TiIV cation in an unidentate coordination type complex. According to FT-IR data in our investigation, we found the adsorption of the azodye onto the CeO2 surface is solely associated with the oxygen atoms of the sulfonate group. It is also observed that Dvas-s (nas(SO3) ns(SO3)) of isolated AO7 is higher than that of absorbed AO7, which is the characteristic of bidentate bridge according to Deacon and Phillips’ theory and Bauer’s
In order to determine the main active species responsible for the degradation of dye molecules, comparison experiments of N2purged condition and scavenger-loaded condition were undertaken. The photo reactor was sealed and purged with N2 for 1 h before irradiation to drive out a part of dissolved oxygen and kept purging on until the reaction was completed. As seen in Fig. 7A, the notable result is that the decolorization rate in the air equilibrium condition is 92%, clearly higher than that in the N2 condition which is only about 10%. In the N2-purged condition, dissolved or adsorbed oxygen may be sufficient to degrade the dye at the early stage but the degradation stopped after oxygen was used up, indicating oxygen is the main source of the active species. The formation of possible oxidative intermediate species, such as singlet oxygen (1O2), superoxide (O2), and hydroperoxy (HO2), or hydroxyl radical (OH) if it exists, under photo-reaction conditions and their role in the dye degradation process has been investigated indirectly, with the use of appropriate quenchers of these species. In this type of experiments, a comparison is made between the original decolorization curves of AO7/CeO2 dispersions with those obtained after addition of millimolar concentrations of quenchers in the initial solution, under otherwise identical conditions. In Fig. 7B, trace a is the decolorization curve without any quenchers and b, c, d are respective for addition of sodium azide (NaN3, a singlet oxygen quencher [29] but may also interact with OH radical [30]), 1,4-benzoquinone (C6H4O2, BQ, a quencher of superoxide radical [31]) and KI (a quencher of positive holes (hVB+) and OH radical on catalyst surface [32,33]). It is observed that NaN3 and KI did not affect the degradation rate of AO7 throughout the experiment. This result indicates that 1O2 and OH are not the active oxidative species involved in this photodegradation system. Herna´ndez-Alonso et al. reported that holes or other type of hole-derived entities are responsible for toluene oxidation [18]. But we concluded that hVB+ was not active species in this system from the performance of CeO2 in KI-loaded experiment. However, addition of 2 mM of BQ, which is a O2 quencher, has a significant suppressing effect on the decolorization of the dye solution, showing the superoxide anion radical is the main oxidative species responsible for the photobleaching of the AO7 dye solution. When AO7/TiO2 system is irradiated by visible light, the AO7 dye is excited and injected an electron into Ti3d conduction band and then this electron is scavenged by the surface oxygen to generate superoxide radical, which can then form hydrogen
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through bidentate coordination mode of oxygen atoms from the sulfonate group. The active species were detected indirectly by adding various radical quenchers and superoxide anion radical was found responsible to attack the dye molecules. We preliminary proposed that the dye self-sensitization is the governing mechanism for the degradation process, in which excited AO7 molecules inject electrons into Ce4f orbits followed by oxygen on the catalyst surface scavenging this electron to produce O2 active species. Therefore, CeO2 could serve as an excellent electron-transfer mediator to transfer the electron coming from photo excited dye molecules, which is also useful for a better understanding of the electronic properties of CeO2. Fig. 8. The photocatalytic mechanism over CeO2 under irradiation of visible light.
peroxide (H2O2), hydroperoxy (HO2) and hydroxyl (OH) radicals (Eqs. (1)–(7)). However, Koelling et al. carried out the selfconsistent-field band calculation for CeO2 and predicted that the band-gap energy between the valence band of O2p character and the Ce5d conduction band is about 6 eV, with a Ce4f narrow band lying within the band gap [34]. Thus the conduction band of cerium dioxide is potentially higher than the excited state of AO7 and the electron transfer could be thermodynamically impossible. Several studies have confirmed the Ce4f plays vital role for the application of CeO2 in photocatalytic process and Li et al. has also demonstrated that the potential of the 4f band of CeO2 is a little bit more positive than that of conduction band of TiO2 [14]. Thus, from a thermodynamical point of view, an electron can be more easily injected from the excited state of the adsorbed dye to the 4f band of CeO2 than to the conduction band of TiO2. Then the electron is scavenged by surface oxygen and initialized a series of redox reactions (Fig. 8). 3.5. Determination of intermediates GC–MS technique was applied to identify the intermediates during the photodegradation of AO7. All the identified compounds were unequivocally identified using the NIST98 library database with fit values higher than 90% (Table S1 supporting information). The main products included substituted benzene, such as benzoic acid, phenol, phthalic anhydride and low molecular weight organic 4-oxo-2-butenoic acid. From the results of GC–MS and scavengerloaded experiments, we can propose a possible degradation pathway of AO7. Firstly, the azodye molecule of AO7 is adsorbed on CeO2 surface. Then the dye molecule is excited by irradiation and injected one electron to Ce4f orbit followed by oxygen on the catalyst surface scavenging this electron to form O2. After that, the superoxide starts to attack dye molecule from –N N– bond to decolorize the solution. Dye molecule is split to generate compounds containing benzene ring and naphthalene ring. These compounds could be attacked further to be deeply oxidized to give out small organic acids. 4. Conclusion The photocatalytic activity of CeO2 to degrade azodye acid orange 7 (AO7) was examined under visible light irradiation (l > 420 nm) and was found to have better performance than commercial reference P25. The superior adsorption capacity of CeO2 accounts for the higher activity. The effect of initial solution pH value on adsorption capacity of CeO2 and consequently its photocatalytic ability was also investigated. The lower the pH is, the more the CeO2 adsorbed and then the higher the degradation rate is. The AO7 molecule is absorbed on the surface of CeO2
Acknowledgements This work has been supported by Science and Technology Commission of Shanghai Municipality (07JC14015); Shanghai Nanotechnology Promotion Centre (0652nm045, 0752nm001), National Nature Science Foundation of China (2057709, 20773039), the National Basic Research Program of China (2004CB719502, 2007CB613306) and the Ministry of Science and Technology of China (2006AA06Z379, 2006DFA52710).
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