Photocatalytic degradation of gaseous o-xylene over M-TiO2 (M=Ag, Fe, Cu, Co) in different humidity levels under visible-light irradiation: Activity and kinetic study SUN Songa, BAO Juna, GAO Chena, b, and DING Jianjuna a National Synchrotron Radiation Laboratory & School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230029, China b Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
M-TiO2 (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(OC4H9)4 as precursor. Transition metal ions doping increased the surface area and extended the absorption of TiO2 to visible light region. The photocatalytic performance and kinetic of M-TiO2 samples for degradation of gaseous o-xylene in different humidity levels under visible light irradiation were studied in detail. The photocatalytic activity of M-TiO2 in-creased with the increasing of humidity level from R.H. 25% to R.H. 60%. The Fe-doped TiO2 shows the best activity among these M-TiO2 (M = Ag, Fe, Cu, Co) photocatalysts. The conversion of o-xylene over 0.5 at.% Fe-TiO2 is 87.3% in R.H. 25% and 95.5% in R.H. 60%, re-spectively. The photocatalytic process is typical of Langmuir-Hinshelwood model of first-order reaction. The apparent rate constant was cal-culated.
Keywords: photocatalysis; TiO2; 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]. Photocata-lytic degradation of these compounds on semiconductor catalysts is one of the most attractive methods and efficient destructive technologies among advanced oxidation proc-esses for environmental remediation [2-5]. Photocatalysis uses photons with energy greater than the band gap of a semiconductor to promote valence electrons to the conduc-tion band and thus forming electron-hole pairs to initiate oxidation-reduction reactions and decompose pollutants [5-6]. TiO2 has been intensively investigated as a semicon-ductor photocatalyst because of its stability, cheapness and environmental friendliness [3]. An ultraviolet (UV) light il-luminated TiO2 photocatalytic oxidation of Acetone, 1-butanol, butyraldehyde and formaldehyde for air purifica-tion was firstly reported [7]. And an overall investigation of 17 VOCs over TiO2 was obtain using a plug flow ractor in the organic compound concentration range of 400-600 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 TiO2 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 TiO2. 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, TiO2 doping with various transi-tion metals have been studied extensively in the last decades due to that doping TiO2 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 elec-tron-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 recombi-nation of Ce4+ and Cu2+ doped TiO2 remarkably decreases and results in higher photocatalytic activity for photocata-lytic degradation of phenol. However, to our knowledge,
148 RARE METALS, Vol. 30, Spec. Issue, Mar 2011
photocatalytic degradation of gaseous o-xylene over transi-tion metal doped TiO2 under visible light irradiation has been hardly reported.
Reaction kinetics is a primary approach to describe the chemical transformation of heterogeneous catalysis by inte-grating 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 ap-plied 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 de-sorption and reaction kinetics are not understood so far. In this paper, the photocatalytic performance for o-xylene deg-radation over 0.5 at.% M-TiO2 (M = Ag, Fe, Cu, Co) in dif-ferent 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
TiO2 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(OC4H9)4 as precursor. 20 ml of Ti(OC4H9)4 was dissolved in 80 ml of absolute ethanol and the Ti(OC4H9)4 solution was added drop-wise under vigor-ous stirring into 94 ml of a mixture solution containing 80 ml ethanol, 6 ml distilled water and 8 ml of acetic acid. The re-sulting 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 TiO2 sample. Transition metal ions were doped into TiO2 with the doping content of 0.5 at.% by adding a certain amount of M(NO3)x (M = Ag, Fe, Cu, Co )in the distilled water prior to hydroly-sis of Ti(OC4H9)4, respectively.
2.2. Characterization of photocatalysts
X-ray diffraction (XRD) analysis of M-TiO2 series sam-ples was performed using a MacScience MXPAHF diffrac-tometer with Cu Kα radiation (λ = 0.154178 nm). The spe-cific surface areas of the samples were estimated by using the Brunauer-Emmett-Teller (BET) method and determined by N2 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 meas-ured at room temperature with a Shimadzu DUV-3700 UV- visible spectrophotometer using BaSO4 as a reference, and converted from reflection to absorbance by the Kubella- Munk method to estimate the band gap of the samples.
2.3. 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-TiO2 powder was blended with small amount of distilled water and made a uniform coating on a glass fiber filter as a sup-porting substrate, then dried at 393 K for 1 h with a tem-perature gradient of 5.5 K per minute and fixed at the suit-able 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 spectro-photometer described above. o-xylene fixed at the initial concentration 370 ppmv was supplied to photoreactor by bubling with O2/N2 (21%O2 and 79%N2, equal to air) through the saturator containing o-xylene from a gas cylin-der. The humidity level was also controlled by bubling with O2/N2 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 CO2 in the photoreactor was analyzed by an online gas chromatograph (Shimadzu GC-14C) equipped with a flame ionization detector (FID), a thermal conductiv-ity detector (TCD), and a KX-112 (Lanzhou Institute of Chemical Physics) column. The mineralization rate of o-xylene was calculated from the CO2 concentrations in the effluent gas according to the principle of carbon balance.
3. Results and discussion
As shown in Fig. 1, both M-TiO2 and pure TiO2 samples are crystallized in pure anatase phase, and their XRD pat-terns match well with JCPDS card (No. 73-1764) and the published literatures [19]. This result indicated that M irons doping with 0.5 at.% did not induce the dopant concentra-tion 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 TiO2 [19]. The crystallite sizes of M-TiO2 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
Sun S. et al., Photocatalytic degradation of gaseous o-xylene over M-TiO2 (M=Ag, Fe, Cu, Co) in different … 149
Fig. 1. XRD patterns of TiO2 and 0.5% M-TiO2 series samples.
Table 1. Physics properties of 0.5% M-TiO2 series samples and TiO2
the present investigation is in the range of 12.29 – 17.02 nm. The specific surface areas of Ag-TiO2, Fe-TiO2, Cu-TiO2, Co-TiO2 and TiO2, also shown in Table 1, are 34.9858, 38.1758, 30.6665, 30.0825, 16.7840 cm2/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 TiO2 and promoted the forma-tion of mesopore structure [23].
Fig. 2 shows the UV-vis diffuse reflectance spectrum of M-TiO2 samples. Compared to that of (~ 390) pure TiO2, the absorption onset of Ag-TiO2, Fe-TiO2, Cu-TiO2 and Co- TiO2 is estimated at 411, 404, 401 and 418 nm respectively,
Fig. 2. UV-vis diffuse reflectance spectra of TiO2 and 0.5% M-TiO2 series samples.
with a red shift, according to the following equation: α =
K (hv -Eg)hv
1/n
(1)
Where α is the absorbance, K is a constant, n equals 2 for indirect transition and 1/2 for the indirect transition [24]. Since TiO2 is considered as an indirect semiconductor, n denotes 1/2 in this study. The absorption of M-TiO2 in the visible light range is due to the contribution of transition metal ions. It can be attributed to the charge-transfer transi-tion between the d electrons of the transition metal and the TiO2 conduction or valence band [17]. Noticeably, as a re-sult of the modification of transition metal ion to TiO2, all of the M-TiO2 samples show enhanced absorption in the range of 450 – 600 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-TiO2 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 CO2 evolution (data not shown) in the blank test under visible light irradiation, con-firmed that the degradation of this typical environmental pollutant occurred through photocatalysis in deed. Com-pared with the pure TiO2, all of the doped samples show higher photocatalytic activities for the selective photocata-lytic oxidation of o-xylene. The photocatalytic activity in-creases in the order of 0.5% Fe-TiO2(87.3%) > 0.5% Ag- TiO2 (79.4%) > 0.5% Cu-TiO2 (65.1%) > 0.5% Co-TiO2
(45.0%) > pure TiO2 (9.9%) under R.H.25%, and 0.5% Fe- TiO2 (95.5%) > 0.5% Ag-TiO2(82.7%) > 0.5% Cu-TiO2
(68.8%) > 0.5% Co-TiO2 (48.5%) > TiO2 (11.4%) under R.H. 60%. Noticeably, pure TiO2 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, al-though exhibiting the best visible light response, Co-TiO2 shows the worst activity among M-TiO2 series catalysts. It may be attributed to that the insignificant emergency of co-balt oxides on the surface of Co-TiO2, which can be seen from the broad band at 600 – 700 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 TiO2 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].
150 RARE METALS, Vol. 30, Spec. Issue, Mar 2011
Fig. 3. (a) The mineralization rate of o-xylene over (1) 0.5% Fe-TiO2 (87.3%), (2) 0.5% Ag-TiO2 (79.4%), (3) 0.5% Cu-TiO2 (65.1%), (4) 0.5% Co-TiO2 (45.0%), (5) TiO2 (9.9%), (6) no catalyst (1.9%) in R.H. 25%, and (b) the mineralization rate of o-xylene over (1) 0.5% Fe-TiO2 (95.5%), (2) 0.5% Ag-TiO2 (82.7%), (3) 0.5% Cu-TiO2 (68.8%), (4) 0.5% Co-TiO2 (48.5%), (5) TiO2 (11.4%), (6) no catalyst (1.9%) in R.H. 60% under visible light irradiation.
Obviously, in our results, increasing of humidity level pro-moted 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. Simi-larly, some literatures documented the positive effect of hu-midity on the other VOCs, such as toluene, in the given hu-midity range [28-29].
The experiment was carried out for 10 consecutive runs to examine the stability of the 0.5% Fe-TiO2, which shows best reactivity in R.H. 25% and R.H. 60%, under visible light ir-radiation as shown in Fig. 4. The o-xylene conversion over 0.5% Fe-TiO2 decreased slightly from 95.5% in the first run to 88.0% in the tenth run, and the amount of CO2 produced in the reactions almost maintain stable with the value dif-ference of ± 90 ppmv in R.H. 60%. However, the o-xylene conversion decreased significantly in R.H. 25%. These re-sults indicated that 0.5% Fe-TiO2 showed extremely stability in R.H.60%, while deactivated in some degree in R.H. 25%.
Fig. 4. Stability test of 0.5% Fe-TiO2 for 10 consecutive runs of photocatalytic degradation of gaseous o-xylene under visible light irradiation in R.H. 25% and R.H. 60%.
The positive effect of water vapor for the o-xylene degrada-tion 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 oxi-dized [11].
For further describe the chemical transformation of het-erogeneous photocatalytic degradation of o-xylene over M-TiO2, 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-xy-lene since water vapor play positive effect on this photo-catalytic process. r =
k K C
1 + K C
d C
d t=_
(2)
Where r denotes the photocatalytic degradation rate, K is Langmuir adsorption equilibrium constants, k represents the rate constant, and C is gaseous reactant concentration. As-suming that the concentration of reactant is very low (KC << 1), integration of equation (2), will gives equation (3). ln (C0/C) = kKt + α = k1t + α (3)
If the assumed L-H model is valid, then a plot of ln(C0/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-TiO2 is typically L-H model. And k1, the apparent rate constant of first-order are estimated according to the slope of fitting line in Fig. 5 and shown in Table 2.
Sun S. et al., Photocatalytic degradation of gaseous o-xylene over M-TiO2 (M=Ag, Fe, Cu, Co) in different … 151
Fig. 5. Plots of ln(C0/C) versus t for photocatalytic degrada-tion of gaseous o-xylene in (a) R.H.25% and (b) R.H. 60%.
Table 2. Apparent rate constant k1 of photocatalytic degrada-tion of o-xylene over M-TiO2
The nanoscale M-TiO2 (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 TiO2 under visible light irradiation was firstly studied. Compared to the pure TiO2, M-TiO2 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-TiO2, 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-TiO2 is typically Langmuir-
Hinshelwood model of first-order reaction.
Acknowledgement
This work was supported by National Nature Science Foundation of China (50721061, 20903084) and Anhui Pro-vincial Natural Science Foundation (090416226).
References:
[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,
