J Sol-Gel Sci Techn (2006) 37: 207–211 DOI 10.1007/s10971-005-6630-1
ORIGINAL ARTICLE
Photocatalytic degradation of 2,4-dichlorophenoxiacetic acid and 2,4,6-trichlorophenol with ZrO2 and Mn/ZrO2 sol-gel materials T. L´opez · M. Alvarez · F. Tzompantzi · M. Picquart
Published online: 24 February 2006 C Springer Science + Business Media, Inc. 2006
Abstract Because of its semiconductor properties, sol-gel zirconia can be used as a photocatalyst. When zirconia is doped with transition metals, its electronic properties are modified. In this work, sol-gel Mn/ZrO2 and ZrO2 materials were tested for photocatalytic degradation of 2,4dichlorophenoxiacetic acid and 2,4,6-trichlorophenol. The powders were characterized by XRD, UV-Vis and Raman spectroscopy. The apparent rate constants were calculated assuming pseudo-first order kinetics. The results reveal that ZrO2 is effective as a photocatalyst; moreover, its photocatalytic properties improve when it is doped with manganese. Keywords Manganese-zirconia sol-gel . Photocatalysis . Photocatalytic degradation . 2,4-dichlorophenoxiacetic acid . 2,4,6-trichlorophenol
Introduction The Advanced Oxidation Process (AOP’s) has become one of the most widely used methods for degradation of pollutants T. L´opez () · M. Alvarez Department of Chemistry, Universidad Aut´onoma Metropolitana-Iztapalapa, P. O. Box 44-534, M´exico, D. F., 09340 e-mail:
[email protected] F. Tzompantzi Instituto Mexicano del Petr´oleo, Eje Central L´azaro C´ardenas 152, M´exico, 07000 D.F., M´exico M. Picquart Department of Physics, Universidad Aut´onoma Metropolitana-Iztapalapa, P. O. Box 44-534, M´exico, D. F., 09340
[1, 2]. Heterogeneous photocatalysis, which has been recently used for water decontamination, is an effective option because the pollutants are degraded and the photocatalyst can subsequently be recovered by simple filtration. Photocatalysis is a process based on absorption of energy (visible or UV light) by a solid (a semiconductor). The degradation reactions take place in the interphase region between the liquid and photoexcited solid. Heterogeneously dispersed semiconductor surfaces provide a fixed environment to influence the chemical reactivity of a wide range of adsorbates and a means to initiate light-induced redox reactivity in these weakly associated molecules [3]. This technology can provide advantages over other process, but it is limited to low concentrations of pollutants [4]. Because the use of bare TiO2 in photocatalytic reactors has certain limitations (nonporous, low adsorption of the pollutants), several attempts have been made to increase or optimize the performance of TiO2 as a photocatalyst, but in many cases, such modifications did not enhance its photocatalytic activity and were rather detrimental [5]. Since the most important factor in the photocatalytic efficiency is the recombination of the electron-hole pairs, several attempts have been made to improve the efficiency of the process; one way to achieve this is by modifying the semiconductor [6] by means of doping. The sol-gel process is an attractive method for addressing this objective, since it allows the introduction of metals into the oxide network of the support, therefore modifying its electrical properties. Zirconium oxide has properties that make it attractive for many applications, such as catalysis. Since it exhibits semiconducting properties [7], zirconium dioxide is expected to be of value in photochemical applications, particularly when doped with suitable transition metal ions to extend its light absorption to the visible region [8–10].
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J Sol-Gel Sci Techn (2006) 37: 207–211
In a previous study, Cu/ZrO2 and ZrO2 were characterized by UV-Vis spectroscopy, and it was reported that the Eg value diminishes when the dopant metal was added. In this work, we investigated the photocatalytic degradation of 2,4-dichlorophenoxiacetic acid (2,4-D) and 2,4,6trichlorophenol (2,4,6-T) over two sol-gel zirconia based materials: ZrO2 and Mn/ZrO2 .
of 60◦ C. The dopant precursor used for the manganesezirconia system was Mn(NO3 )2 .H2 O (98%) from SigmaAldrich. The amount of manganese doped into the zirconia sol-gel formulation was 1 mol%. Samples were dried for 24 hours at 100◦ C. The resulting powders were annealed at 300◦ C in an atmosphere of air, with a heating rate of 2◦ C per minute.
Experimental
X-Ray diffraction
Sample preparation
XRD measurements were carried out on a Siemens D5000 X-Ray Diffractometer using CuKα radiation with λ = 1.5405 Å and a 0.03◦ step size in a range 2θ = 4 – 70◦ .
The photocatalysts were prepared by the sol-gel method [11] at pH 3, with HNO3 as catalyst and a processing temperature Fig. 1 X-Ray diffraction patterns of ZrO2 and Mn/ZrO2 powders annealed at 300◦ C.
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J Sol-Gel Sci Techn (2006) 37: 207–211
UV-Vis spectroscopy UV-Vis (diffuse reflectance) spectra were obtained in a Varian model Cary-1 UV-Vis spectrophotometer, with an integrating sphere. To obtain the spectra, self supporting pellets were prepared. The band gap values were calculated [13] by linearization of the slope of the spectrum for each sample with the equation: 1239∗ b (1) −a where, Eg is the band gap in eV of the material, and b and a are obtained by a linear fit. Eg =
Raman scattering Raman spectroscopy measurements were performed at room temperature on a computerized Spex 1403 double monochro-
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mator in combination with the 514.5-nm line of an argon ion laser (Lexel lasers) at a power level of 100 mW at the laser head. Raman spectra were taken directly from zirconia powder aggregates in back scattering geometry. Photocatalysis The photocatalytic degradation of the pollutants was carried out in a closed and sealed box, with the agitated powder suspension irradiated with a high pressure mercury lamp emitting 254 nm radiation. In order to assure the adsorption of the molecule on the photocatalyst, the suspension was stirred for 30 min without light, with an air flow of 1 ml/sec. It was then irradiated with the UV lamp. The degradation of the pollutants was followed by the depletion of the main absorption bands for each molecule using UV-Vis spectroscopy. Both reactions follow pseudo-first order kinetics, and hence the
Fig. 2 UV-Vis spectra of ZrO2 and Mn/ZrO2 powders annealed at 300◦ C.
Fig. 3 Raman spectra of ZrO2 and Mn/ZrO2 powders annealed at 300◦ C.
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apparent rate constant was calculated by plotting ln (C0 /C) vs time. The slope of the plot after applying a linear fit represents the apparent rate constant [12]. Results and discussion
with the key features of the tetragonal phase indicated by the peaks at 30, 51 and 61◦ (2θ ). This crystalline phase exhibits better textural and catalytic properties [14], so it is expected to have an acceptable photocatalytic behavior. The crystallite size can be estimated using the Scherer equation:
X-Ray diffraction
D=
XRD patterns of ZrO2 and Mn/ZrO2 are shown in Fig. 1. We observed both amorphous (minor) and tetragonal phases,
where λ is the X-ray wavelength, θ is the Bragg angle and β is the pure full width of the diffraction line at half of the
Fig. 4 Variation in the relative concentrations of a) 2,4,6-T and b) 2,4-D vs irradiation time for ZrO2 and Mn/ZrO2 powders annealed at 300◦ C.
0.9λ , β cos θ
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maximum intensity using the peak at 30.3◦ . In the case of pure ZrO2 , the crystallite size is 90 ◦ , whilst it is reduced at 69 ◦ in the case of the sample containing manganese. UV-Vis The Eg values were calculated from the UV-Vis absorption spectra (Fig. 2) for both photocatalysts. The observed band gaps were 4.3 eV and 4.8 for the undoped and doped samples, respectively. The greatest value is obtained for Mn/ZrO2 , although both values exceeded 4 eV. However, it must also be noted that photocatalytic activity is not only dependent on Eg, but is also influenced by other parameters such as the specific area and interactions between the metal and the support. Raman spectroscopy The Raman spectra (Fig. 3) exhibit characteristic bands of the tetragonal phase at 164, 240 and 625 cm−1 , together with an amorphous phase that is more abundant in the case of the sample containing manganese. The poor quality of the spectra (in particular the Mn/ZrO2 sample) is due to strong luminescence that masks the Raman signal. Nevertheless we confirm the results observed by X-Ray Diffraction.
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Conclusions We have compared the characteristics of pure sol-gel zirconia with the corresponding manganese-doped material after annealing at 300◦ C. Under these conditions, the band gaps of ZrO2 and Mn/ZrO2 are nearly the same, although better degradation of selected pollutants is obtained with the doped sample. The adsorption capacity of the doped catalyst is higher due to its higher extent of hydroxylation, and hence it is concluded that the OH groups behave as adsorption centers. XRD confirmed that crystalline materials are obtained at 300◦ C, and the observed Eg values were influenced by the thermal treatment parameters. The specific surface area did not influence the catalytic activity. However, dehydroxylation of the thermally treated samples had a significant effect on the photocatalytic properties. Therefore, it is concluded that the crystal structure is an important factor which has to be considered in the design of photocatalytic processes. Aknowledgments We would like to acknowledge CONACyT for the grant given to M. Alvarez (No.171497).
References 1. 2. 3. 4.
Photocatalysis 5.
The natural logarithm of the variation in the relative concentrations of 2,4-D and 2,4,6-T as a function of irradiation time are shown in Figs. 4(a) and 5(b), respectively. In both cases, the lowest irradiation time required for the degradation was for the Mn/ZrO2 photocatalyst. This is attributed to the interaction between the metal and the support. In fact, the introduction of low levels of manganese modifies the structure of the support, giving rise to defects via the incorporation of the dopant within the oxide framework. Moreover at 300◦ C, we suspect that the presence of terminal hydroxyls on the surface of the material favors the formation of OH radicals, thus increasing the photoactivity of the oxide. The results obtained in this work show the viability of ZrO2 for further applications as a photocatalyst and photocatalyst support.
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