ЖУРНАЛ НЕОРГАНИЧЕСКОМ ХИМИИ, 2009, том 54, № 8, с. 1275-1281
СИНТЕЗ И СВОЙСТВА НЕОРГАНИЧЕСКИХ СОЕДИНЕНИЙ
УДК 546.824-31:541.128
A Study on the Degradation of Methamidophos in the Presence of Nano-TiO2 Catalyst Doped with Re
© 2009 Lei Zhang*, Fei Yan, Mingming Su, Guangxi Han, Pingli Kang
College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China
Поступила в редакцию 05.07.2007 г.
A new Re-doped nano-TiO2 photocatalyst was synthesized by immersion method. The novel doped nano-TiO2 photocatalyst utilizing visible light was firstly prepared. The doped nano-TiO2 powder was charactered by XRD, FTIR, UV/Vis, and its photocatalytic activity was tested through the photocatalytic degradation of methamidophos as a model compound under ultraviolet irradiating and in sunlight, respectively. In order to compare the photocatalytic activities, the same experiment was carried out for undoped nano-TiO2. The degradation ratio of methamidophos in the presence of doped nano-TiO2 reached 64.40% under sunlight for 12 h, which was 2.64% in the presence of undoped nano-TiO2. The degradation ratio of methamidophos in the presence of doped nanoTiO2 reached 90.39% under UV irradiationat 2.5 h, which was 51.29% in the presence of undoped nano-TiO2. All the results show that the doped TiO2 is a promising photocatalyst using sunlight for treating the organophosphorous pesticide wastewater.
Keywords: doped; nano-TiO2; photocatalyst; degradation; methamidophos.
1. INTRODUCTION
Methamidophos pesticide is a kind of high-effective pesticide that is well applied in agriculture. However, it also leaves a negative effect on the environment, even on the human health because of its toxicity and stability. So it has great value to study about treating metha-midophos waste water and transforming organophos-phorous into inorganophosphate. Many traditional treatment methods, including activated sludge [1, 2], activated carbon adsorption, wet air oxidation and basic hydrolysis, have been used in waste water treatment and recycling but the results are not satisfied because the concentration of organophosphorous remained still keep at a high level.
Therefore, the innovative technologies for effectively and completely decomposing all organic pollutants and turning these organic pollutants into environmental compatible compounds are agog required. In recent years, the technology of ultraviolet photocatalyst degradation has been studied and extensively used to treat some organic pollutants. The research showed that nano-TiO2 photocatalyst could achieve a better effect in oxidizing the waste water which containing dyes, phenol, pesticides and so on. However, TiO2 only absorbs near-UV light (Eg = 3.2 eV for anatase) and does not act with the solar light effectively. Moreover, electron (e-) and hole (h+) can easily recombine, so the catalytic rate became lower. Therefore, current research has sought to improve the photocatalystic properties of TiO2 by doping with metals and oxides. Many attempts
* Corresponding author: Tel.: +86-24-62207816, Fax: +86-2462202380, E-mail address: zhanglei63@126.com
have been made to extend the light absorption of the catalyst to the visible light region and to enhance its photonic efficiency [3-5]. Doping TiO2 with various transition metal ions has been proved to be one of the useful methods, which have been shown to reduce band gap energy and improve charge separation between photogenerated electrons and holes [6, 7].
Unfortunately, there has been no report available for the degradation of methamidophos using Re-doped TiO2. In this paper, we report for the first time on the photocatalytic degradation of methamidophos, a model pollutant. It has been found that the photocatalytic activity of Re-doped TiO2 is much higher as compared to that of undoped TiO2. Moreover, Re-doped TiO2 pho-tocatalysts require shorter irradiation time for complete mineralization than undoped TiO2. The relative photonic efficiency of TiO2, Re-doped TiO2 has also been compared and the results have been discussed. Interestingly, the relative photonic efficiency of Re-doped TiO2 is much higher as compared to that of undoped TiO2.
The band gap energy of the doped nano-TiO2 will be less than that of titanium dioxide, which induces the red shift of the absorption edge to respond to visible light. Therefore, the absorption apex of doped nano-TiO2 and the sunlight spectra are a well match; it is importance for the practical application of the photocatalytic [8, 9]. It enhanced the utilizing efficiency of the visible light and broke through the limitation that nano-TiO2 can only utilize ultraviolet (k < 387 nm) that is only about 3-8% of irritation totally in the sunlight.
П, % 100
80 -
60 40
20 -
120 180 Time, min
240
CH3S-CH3O'
O II
;P-NH2
TiO2 was irradiated in natural light. The degradation throughout experiments, aliquot of samples were withdrawn regularly from the reactor and centrifuged to remove TiO2. The concentration of PO3-, degradation final product, could be determined with phosphomolyb-denum blue colorimetry and the degree of phosphate evolution of organophosphorous pesticides (n) could be determined.
П = P 100%,
(1)
Fig. 1. The ultraviolet degradation ratio of the doped nano-TiO2 and nano-TiO2.
2. EXPERIMENTAL 2.1. Reagents and Instruments
Methamidophos (50%) - o,s-dimethylphosramidot-hooate was Shenyang Academy of Agricultural Sciences Products and it is well applied in agriculture now. Metal Re sample was purchased from Shanghai Chemical Company. Nano-TiO2 (anatase, particle-size, 2050 nm) was used as semiconductor photocatalyst in this study, they were provided by Zhoushanmingri Nanometer Material Company. Molecular structure of methamidophos is as follow:
The light source was a high-pressure mercury vapour UV lamp (Shanghai Yaming Lighting Co. LTD.) that emited its maximum radiation at 365 nm. LAMB-DA-17 UV-vis spectrophotometer (Perkin-Elmer Company, USA) was used to inspect the degradation process of methamidophos. FTIR spectra of nano-TiO2 were measured using FTIR 5700 (Nicolet company, USA). The variation temperature of the nano-TiO2 powders X-ray diffraction (XRD) patterns were recorded on Siemens D5000 Diffractometer.
2.2. Experimental Methods
It was composed of a suspended system, which methamidophos aqueous solution (1.0 x 10-4 mol/L) with amount of doped photocatalyst was put into the reactor. The high-pressure mercury vapour lamp should be hung above the reactor, degradation should be carried out below 40°C, air was bubbled through the reactor from the bottom of it to ensure a constant dissolved O2 concentration and stirred the aqueous suspensions. Or methamidophos aqueous solution with Re-doped
where Pt - the concentration of inorganic phosphorous after irritating time (t), P0 - the content of organic phosphorous before irritating.
2.3. Prepare Catalyst
Stock solution of Re (4 mg/mL) was prepared by heat dissolving 0.0040 g pure rhenium (99.999%) with 20 mL strong nitric acid. After it got cool, it was moved to 100 mL flask. After that, it was diluted with quadratic distilled water to maintain a constant volume.
Nano-TiO2 powders were put into solution of difference content Re (2.0, 3.0, 4.0, 5.0, 6.0%). The mixed solution was dispersed in the ultrasound for 15 min, and stirred for 5.0 h, and then dried below 100°C. After that, it was milled to powder by ceramic mortar. Then, the powder samples were calcined at experimental temperature (i.e. 400, 500, 600, 700, and 800°C, respectively) for 2 h. At the same time, un-doped TiO2 powder samples were calcined at the same experimental conditions. The active photocatalysts were obtained and grinded before using.
3. RESULTS AND DISCUSSION 3.1. Photocatalytic Activity
A series of solution of 1.0 x 10-4 mol/L methamidophos adding different photocatalysts (Re-doped TiO2 and TiO2) were investigated efficacies of degradation
at UV irradiation. Fig. 1 shows changes of PO^- concentrations during photocatalytic processes. The comparative experiment showed that the degradation efficiency in the presence of Re doped TiO2 is remarkably superior to the nano-TiO2. The degree of phosphate evolution had reached 90.39% by using doped nano-TiO2 when the methamidophos solution was ultraviolet light irradiated for 2.5 h. However, 51.29% of methamidophos was degraded in the presence of nano-TiO2 powder at same time. And the degradation ratio of methamidophos in the presence of doped nano-TiO2 reached 64.40% under sunlight for 12 h, which was 2.64% in the presence of undoped nano-TiO2.
It embodies the optimum effect of preventing recombination of photoinduced electron and hole by dop-
0
0
П, % 80
60
40
20
2 3 4 5 6
W, %
Fig. 2. The effect of Re adding amount on the nano-TiO2 photocatalytic degradation of methamidophos.
n, % 80
70
60
50
40
30
20
0.8
nano-TiO2, g/L
Fig. 3. Effect of catalyst amount on degradation.
ing, and much more •OH and • O2 on the surface of nano-TiO2 is produced to enhance the photocatalytic activity.
3.2. Effect of Re Doped Amount on the Degradation
The oxidation-reduction potential of Re7+/Re4+ is 0.5 eV and that of Ti4+/Ti3+ is 0.1 eV [10], so Re7+ can capture photogenerated electrons more easily, thus can avail separate of carriers and enhance photocatalytic activity. The suitable amount dopants can capture photogenerated electrons and decrease the rate of recombination of electron-hole and accelerate photocatalytic reaction. As the concentration of dopant increases, electron-hole pairs captured overcome barrier and recombine. However, at a certain level of dopant, the rate of recombination starts dominating the reaction [11]:
^recomb ^
exp
-2R
(2)
where A"recomb is the rate of recombination, R the distance between the trap site of e- and h+ and a0 is the hy-drogenic radius of the wave function for the charge carrier. It can be seen from equation (2), as above optimal concentration, with redu
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