SUN Song a, BAO Jun a, GAO Chen a, b, and DING Jianjun a. 1. Introduction

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1 RARE METALS Vol. 30, Spec. Issue, Mar 2011, p. 147 DOI: /s Photocatalytic degradation of gaseous o-xylene over M-TiO 2 (M=Ag, Fe, Cu, Co) in different humidity levels under visible-light irradiation: Activity and kinetic study SUN Song a, BAO Jun a, GAO Chen a, b, and DING Jianjun a a National Synchrotron Radiation Laboratory & School of Nuclear Science and Technology, University of Science and Technology of China, Hefei , China b Department of Materials Science and Engineering, University of Science and Technology of China, Hefei , China Received 29 September 2010; received in revised form 29 December 2010; accepted 10 January 2011 The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2011 Abstract M-TiO 2 (M = Ag, Fe, Cu, Co) photocatalysts were prepared by a sol-gel method with the doping concentration ranging from 0.1 at.% to 1.0 at.% using glacial acetic acid as chelating agent and Ti(OC 4 H 9 ) 4 as precursor. Transition metal ions doping increased the surface area and extended the absorption of TiO 2 to visible light region. The photocatalytic performance and kinetic of M-TiO 2 samples for degradation of gaseous o-xylene in different humidity levels under visible light irradiation were studied in detail. The photocatalytic activity of M-TiO 2 increased with the increasing of humidity level from R.H. 25% to R.H. 60%. The Fe-doped TiO 2 shows the best activity among these M-TiO 2 (M = Ag, Fe, Cu, Co) photocatalysts. The conversion of o-xylene over 0.5 at.% Fe-TiO 2 is 87.3% in R.H. 25% and 95.5% in R.H. 60%, respectively. The photocatalytic process is typical of Langmuir-Hinshelwood model of first-order reaction. The apparent rate constant was calculated. Keywords: photocatalysis; TiO 2 ; metal doping; o-xylene; Langmuir-Hinshelwood 1. Introduction Volatile organic compounds (VOCs), including aromatic hydrocarbons such as o-xylene, are harmful to human health due to their noxiousness and carcinogenesis [1]. Photocatalytic degradation of these compounds on semiconductor catalysts is one of the most attractive methods and efficient destructive technologies among advanced oxidation processes for environmental remediation [2-5]. Photocatalysis uses photons with energy greater than the band gap of a semiconductor to promote valence electrons to the conduction band and thus forming electron-hole pairs to initiate oxidation-reduction reactions and decompose pollutants [5-6]. TiO 2 has been intensively investigated as a semiconductor photocatalyst because of its stability, cheapness and environmental friendliness [3]. An ultraviolet (UV) light illuminated TiO 2 photocatalytic oxidation of Acetone, 1-butanol, butyraldehyde and formaldehyde for air purification was firstly reported [7]. And an overall investigation of 17 VOCs over TiO 2 was obtain using a plug flow ractor in the organic compound concentration range of ppmv by Alberici and Jardim [8]. Many groups also have been devoted to investigate the influence factors, such as VOCs concentration, coexisting gas and temperature, especially humidity (water vapor) level in the degradation of many other VOCs over TiO 2 under UV irradiation [9-15]. Unfortunately, a major obstacle in the popularization of this semiconductor material is the large band gap, 3.2 ev for bulk TiO 2. This severe disadvantage limits the light response to the ultraviolet region, a small fraction (5%) of the solar energy [16]. Therefore, significant efforts have been devoted to developing the efficient photocatalysts capable of using abundant visible light in solar spectrum or artificial light sources. For this purpose, TiO 2 doping with various transition metals have been studied extensively in the last decades due to that doping TiO 2 with transition metals, such as Fe, Cr and La, can not only expand the absorption range of light but also modify its structure or morphology, reduce the electron-hole pairs recombination probability, and consequently enhance its catalytic behavior [17-19]. For instance, Jing et al.[18] found that the probability of charge carrier recombination of Ce 4+ and Cu 2+ doped TiO 2 remarkably decreases and results in higher photocatalytic activity for photocatalytic degradation of phenol. However, to our knowledge, Corresponding author: BAO Jun, GAO Chen baoj@ustc.edu.cn, cgao@ustc.edu.cn

2 148 RARE METALS, Vol. 30, Spec. Issue, Mar 2011 photocatalytic degradation of gaseous o-xylene over transition metal doped TiO 2 under visible light irradiation has been hardly reported. Reaction kinetics is a primary approach to describe the chemical transformation of heterogeneous catalysis by integrating several theories and models such as Langmuir- Hinshelwood (L-H), Eley-Rideal (E-R), Lindemann, etc. [20]. The L-H reaction rate equation was successfully applied to formulate the heterogeneous gas-solid reaction. Some literatures documented the widely use of L-H model for single organic compound degradation by considering different adsorption site types and competitive adsorption with other organics or water vapor [7, 20]. However, in spite of numerous investigations on the gas-solid heterogeneous photocatalysis, the equilibrium between adsorption and desorption and reaction kinetics are not understood so far. In this paper, the photocatalytic performance for o-xylene degradation over 0.5 at.% M-TiO 2 (M = Ag, Fe, Cu, Co) in different humidity levels under visible light irradiation was evaluated. The effect of humidity on the photocatalytic process was investigated. Moreover, the adequacy of L-H model to fit the experimental results is performed and the relevant rate constants are presented. 2. Experimental 2.1. Photocatalysts preparation TiO 2 doped with Ag, Fe, Cu and Co series catalysts were synthesized by the sol-gel method using glacial acetic acid as chelating agent and Ti(OC 4 H 9 ) 4 as precursor. 20 ml of Ti(OC 4 H 9 ) 4 was dissolved in 80 ml of absolute ethanol and the Ti(OC 4 H 9 ) 4 solution was added drop-wise under vigorous stirring into 94 ml of a mixture solution containing 80 ml ethanol, 6 ml distilled water and 8 ml of acetic acid. The resulting transparent colloidal suspension was stirred for 2 h and aged for 2 days till the formation of gel. The gel was dried at 353 K for 48 h and then ground. Then the powder was calcined at 753 K for 3 h to obtain pure TiO 2 sample. Transition metal ions were doped into TiO 2 with the doping content of 0.5 at.% by adding a certain amount of M(NO 3 ) x (M = Ag, Fe, Cu, Co )in the distilled water prior to hydrolysis of Ti(OC 4 H 9 ) 4, respectively Characterization of photocatalysts X-ray diffraction (XRD) analysis of M-TiO 2 series samples was performed using a MacScience MXPAHF diffractometer with Cu Kα radiation (λ = nm). The specific surface areas of the samples were estimated by using the Brunauer-Emmett-Teller (BET) method and determined by N 2 adsorption and desorption at 77 K on a Micromeritics ASAP 2020 M+C equipment after the pretreatment at 473 K for 4 h. UV-visible diffuse reflectance spectrum was measured at room temperature with a Shimadzu DUV-3700 UVvisible spectrophotometer using BaSO 4 as a reference, and converted from reflection to absorbance by the Kubella- Munk method to estimate the band gap of the samples Photocatalytic activity measurements The experimental setup and a photoreactor with volume of 300 ml used for the gas-phase photocatalytic degradation of o-xylene have been designed and operated. The similar equipment was used and discussed in some literatures [8, 21]. For the photocatalysis of o-xylene vapor, 0.3 g M-TiO 2 powder was blended with small amount of distilled water and made a uniform coating on a glass fiber filter as a supporting substrate, then dried at 393 K for 1 h with a temperature gradient of 5.5 K per minute and fixed at the suitable position in the photoreactor at last. The light resource was a 300 W Xe-arc lamp equipped with an IR-cutoff filter and an UV-cutoff filter. The pass-band of the coupled filters was identified as 400 nm λ 900 nm by UV-vis spectrophotometer described above. o-xylene fixed at the initial concentration 370 ppmv was supplied to photoreactor by bubling with O 2 /N 2 (21%O 2 and 79%N 2, equal to air) through the saturator containing o-xylene from a gas cylinder. The humidity level was also controlled by bubling with O 2 /N 2 through the saturator containing water and determined by using an electronic hygrometer fixed in the by-pass line. The temperature of the reaction was controlled at room temperature by a water-cooling system. The concentration of o-xylene and CO 2 in the photoreactor was analyzed by an online gas chromatograph (Shimadzu GC-14C) equipped with a flame ionization detector (FID), a thermal conductivity detector (TCD), and a KX-112 (Lanzhou Institute of Chemical Physics) column. The mineralization rate of o-xylene was calculated from the CO 2 concentrations in the effluent gas according to the principle of carbon balance. 3. Results and discussion As shown in Fig. 1, both M-TiO 2 and pure TiO 2 samples are crystallized in pure anatase phase, and their XRD patterns match well with JCPDS card (No ) and the published literatures [19]. This result indicated that M irons doping with 0.5 at.% did not induce the dopant concentration to attain saturation in the inner surface of the titania framework which is followed by producing the significant isolated metal oxide phase at the surface of the TiO 2 [19]. The crystallite sizes of M-TiO 2 nanoparticles was estimated from the peak half-width by using the Scherrer equation with corrections for instrumental line broadening [22-23] and shown in Table 1. It was found that the crystallite size in

3 Sun S. et al., Photocatalytic degradation of gaseous o-xylene over M-TiO 2 (M=Ag, Fe, Cu, Co) in different 149 Fig. 1. XRD patterns of TiO 2 and 0.5% M-TiO 2 series samples. Table 1. Physics properties of 0.5% M-TiO 2 series samples and TiO 2 Samples Phase Crystallite size / nm SBET / (m 2 /g) TiO 2 anatase %Ag-TiO 2 anatase %Fe-TiO 2 anatase %Cu-TiO 2 Anatase %Co-TiO 2 anatase the present investigation is in the range of nm. The specific surface areas of Ag-TiO 2, Fe-TiO 2, Cu-TiO 2, Co-TiO 2 and TiO 2, also shown in Table 1, are , , , , cm 2 /g respectively. It indicated that M ions doping promoted the increasing of the specific surface areas, probably due to that the doping ions modified the morphology of TiO 2 and promoted the formation of mesopore structure [23]. Fig. 2 shows the UV-vis diffuse reflectance spectrum of M-TiO 2 samples. Compared to that of (~ 390) pure TiO 2, the absorption onset of Ag-TiO 2, Fe-TiO 2, Cu-TiO 2 and Co- TiO 2 is estimated at 411, 404, 401 and 418 nm respectively, Fig. 2. UV-vis diffuse reflectance spectra of TiO 2 and 0.5% M-TiO 2 series samples. with a red shift, according to the following equation: 1/n K (hv -Eg) α = (1) hv Where α is the absorbance, K is a constant, n equals 2 for indirect transition and 1/2 for the indirect transition [24]. Since TiO 2 is considered as an indirect semiconductor, n denotes 1/2 in this study. The absorption of M-TiO 2 in the visible light range is due to the contribution of transition metal ions. It can be attributed to the charge-transfer transition between the d electrons of the transition metal and the TiO 2 conduction or valence band [17]. Noticeably, as a result of the modification of transition metal ion to TiO 2, all of the M-TiO 2 samples show enhanced absorption in the range of nm, in other words, make the response ability to a wide range of solar spectrum, which well agrees with the results reported [17, 19]. Fig. 3 displays the visible light activity of M-TiO 2 for o- xylene degradation under the relative humidity (R.H.) level at R.H. 25% and R.H. 60% in air for 2 h. As the blank tests, shown in Fig. 3(a) (6) and Fig. 3(b) (6), the photolysis of o-xylene itself in the same condition was recorded and could be neglected. Additionally, no CO 2 evolution (data not shown) in the blank test under visible light irradiation, confirmed that the degradation of this typical environmental pollutant occurred through photocatalysis in deed. Compared with the pure TiO 2, all of the doped samples show higher photocatalytic activities for the selective photocatalytic oxidation of o-xylene. The photocatalytic activity increases in the order of 0.5% Fe-TiO 2 (87.3%) > 0.5% Ag- TiO 2 (79.4%) > 0.5% Cu-TiO 2 (65.1%) > 0.5% Co-TiO 2 (45.0%) > pure TiO 2 (9.9%) under R.H.25%, and 0.5% Fe- TiO 2 (95.5%) > 0.5% Ag-TiO 2 (82.7%) > 0.5% Cu-TiO 2 (68.8%) > 0.5% Co-TiO 2 (48.5%) > TiO 2 (11.4%) under R.H. 60%. Noticeably, pure TiO 2 shows poor activity in the test owing to the worse response in the visible light region and the relatively small specific area. On the other hand, although exhibiting the best visible light response, Co-TiO 2 shows the worst activity among M-TiO 2 series catalysts. It may be attributed to that the insignificant emergency of cobalt oxides on the surface of Co-TiO 2, which can be seen from the broad band at nm in the UV-vis DRS spectra shown in Fig. 2. The cobalt oxides may supply the recombination centers of electron-hole pairs and then reduce its catalytic behavior [25]. Since high disperse of the emerged cobalt oxides on the TiO 2 surface, XRD result did not show the impurity peaks except for anatase. In a word, the activity order of series catalysts can contribute by the different adsorption ability of gas, the utilizing of visible light, and the recombination probability of electron-hole pairs [26-27].

4 150 RARE METALS, Vol. 30, Spec. Issue, Mar 2011 Fig. 4. Stability test of 0.5% Fe-TiO 2 for 10 consecutive runs of photocatalytic degradation of gaseous o-xylene under visible light irradiation in R.H. 25% and R.H. 60%. Fig. 3. (a) The mineralization rate of o-xylene over (1) 0.5% Fe-TiO 2 (87.3%), (2) 0.5% Ag-TiO 2 (79.4%), (3) 0.5% Cu-TiO 2 (65.1%), (4) 0.5% Co-TiO 2 (45.0%), (5) TiO 2 (9.9%), (6) no catalyst (1.9%) in R.H. 25%, and (b) the mineralization rate of o-xylene over (1) 0.5% Fe-TiO 2 (95.5%), (2) 0.5% Ag-TiO 2 (82.7%), (3) 0.5% Cu-TiO 2 (68.8%), (4) 0.5% Co-TiO 2 (48.5%), (5) TiO 2 (11.4%), (6) no catalyst (1.9%) in R.H. 60% under visible light irradiation. Obviously, in our results, increasing of humidity level promoted the catalytic behavior significantly. It proved water vapor play positive effect on the photocatalytic oxidation due to that the primary attack species OH to the o-xylene molecules were formed via photo-generated holes and the adsorbed hydroxyls, which supplied by water vapor. Similarly, some literatures documented the positive effect of humidity on the other VOCs, such as toluene, in the given humidity range [28-29]. The experiment was carried out for 10 consecutive runs to examine the stability of the 0.5% Fe-TiO 2, which shows best reactivity in R.H. 25% and R.H. 60%, under visible light irradiation as shown in Fig. 4. The o-xylene conversion over 0.5% Fe-TiO 2 decreased slightly from 95.5% in the first run to 88.0% in the tenth run, and the amount of CO 2 produced in the reactions almost maintain stable with the value difference of ± 90 ppmv in R.H. 60%. However, the o-xylene conversion decreased significantly in R.H. 25%. These results indicated that 0.5% Fe-TiO 2 showed extremely stability in R.H.60%, while deactivated in some degree in R.H. 25%. The positive effect of water vapor for the o-xylene degradation was proved again. The deactivation results may due to that during the photocatalytic degradation of o-xylene, some intermediates such as benzaldehyde and benzoic acid are formed and adsorbed on the surface. These intermediates occupy the active sites and are stable and hardly to be oxidized [11]. For further describe the chemical transformation of heterogeneous photocatalytic degradation of o-xylene over M-TiO 2, L-H model was used to fit the experimental results [7]. The L-H reaction rate equation shown in equation (2) is applied to describe the adsorption and degradation of o-xylene since water vapor play positive effect on this photocatalytic process. _ d C k K C r = = (2) d t 1 + K C Where r denotes the photocatalytic degradation rate, K is Langmuir adsorption equilibrium constants, k represents the rate constant, and C is gaseous reactant concentration. Assuming that the concentration of reactant is very low (KC << 1), integration of equation (2), will gives equation (3). ln (C 0 /C) = kkt + α = k 1 t + α (3) If the assumed L-H model is valid, then a plot of ln(c 0 /C) versus t should be linear, and the reaction rate is in direct proportion to the reactant concentration. That is, this type of photocatalytic process is L-H model of first-order reaction [7]. Fig. 5 indicates that the experimental data are in good agreement with this integral rate-law analysis, suggesting that the photocatalytic degradation of gaseous o-xylene over M-TiO 2 is typically L-H model. And k 1, the apparent rate constant of first-order are estimated according to the slope of fitting line in Fig. 5 and shown in Table 2.

5 Sun S. et al., Photocatalytic degradation of gaseous o-xylene over M-TiO 2 (M=Ag, Fe, Cu, Co) in different 151 Hinshelwood model of first-order reaction. Acknowledgement This work was supported by National Nature Science Foundation of China ( , ) and Anhui Provincial Natural Science Foundation ( ). References: Fig. 5. Plots of ln(c 0 /C) versus t for photocatalytic degradation of gaseous o-xylene in (a) R.H.25% and (b) R.H. 60%. Table 2. Apparent rate constant k 1 of photocatalytic degradation of o-xylene over M-TiO 2 Photocatalyst k 1 (o-xylene) R.H. 25% R.H. 60% 0.5%Ag-TiO %Fe-TiO %Cu-TiO %Co-TiO Conclusion The nanoscale M-TiO 2 (M = Ag, Fe, Cu, Co) series photocatalysts were prepared by using so-gel method and characterized by means of XRD, BET and UV-vis DRS. The activity evaluation of o-xylene degradation in gas phase over the transition metal ions doped TiO 2 under visible light irradiation was firstly studied. Compared to the pure TiO 2, M-TiO 2 series photocatalysts exhibited good activity of toluene degradation under visible light irradiation due to the enhanced surface area and visible light utilizing with the small quantities (0.5 at.%) of transition metal. Water vapor plays positive effect for the photocatalytic degradation of o-xylene. 0.5% Fe-TiO 2, with the best photocatalytic activity, showed extremely stable under R.H. 60%, while deactivated in some degree in R.H. 25%. The photocatalytic degradation of gaseous o-xylene over M-TiO 2 is typically Langmuir- [1] Wilkinson C.F., Environ. Sci. Technol., 1987, 21: 843. [2] Hoffmann M.R., Martin S.T., and Choi W., Chem. Rev., 1995, 95: 69. [3] Wang S.B., Ang H.M., and Tade M.O., Environ, Int., 2007, 33: 694. [4] Maira A.J., Coronado J.M., Augugliaro V., Yeung K.L., and Conesa J.C., J. Soria, J. Catal., 2001, 202: 413. [5] Dibble L.A., and Raupp G.B., Environ. Sci. Technol., 1992, 26: 492. [6] Alberici R.M., and Jardim W.F., Appl. Catal. B: Environ., 1997, 14: 55. [7] Kisch H., Zhang L., Lange C., Maier W.F., Antonius C., and Meissner D., Modified, Angew. Chem. Int. Ed. Engl., 1998, 37: [8] Sidheswaran M., and Tavlarides L., Ind. Eng. Chem. Res., 2008, 47: [9] Choi J., Park H., and Hoffmann M.R., J. Phys. Chem. C, 2010, 114: 783. [10] Cao Y., Yang W., Zhang W., Liu G., and Yue P., New J. Chem., 2004, 28: 218. [11] Wang E., Yang W., and Cao Y., J. Phys. Chem. C, 2009, 113: [12] Belver C., Bellod R., Stewart S.J., Requejo F.G., and Fernández-García M., Appl. Catal. B: Environ., 2006, 65: 309. [13] Yang X., Cao C., Erickson L., Hohn K., Maghirang R., and Klabunde K., Appl. Catal. B: Environ., 2009, 91: 657. [14] Miyauchi M., Nakajima A., Hashimoto K., and Watanabe T., Adv. Mater., 2000, 12: [15] Li F.B., Li X.Z., Ao C.H., Lee S.C., and Hou M.F., Chemosphere, 2005, 59: 787. [16] Choi W., Termin A., and Hoffmann M.R., J. Phys. Chem., 1994, 98: [17] Jing L., Qu Y., Wang B., Li S., Jiang B., Yang L., Fu W., Fu H., and Sun J., Sol. Energy Mater. Sol. Cells, 2006, 90: [18] Zhu J., Chen F., Zhang J., Chen H., and Anpo M., J. Photochem. Photobiol. A: Chem., 2006, 180: 196. [19] Li X., Yue P.L., and Kutal C., New J. Chem., 2003, 27: [20] Murakami N., Chiyoya T., Tsubota T., and Ohno T., Appl. Catal. A: Gen., 2008, 348: 148. [21] Sun S., Ding J.J., Bao J., Gao C., Qi Z., and Li C., Catal. Lett., 2010, 137: 239. [22] Rouquerol F., Rouquerol J., and Sing K.S.W., Academic Press, San Diego, 1999.

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