Photocatalytic degradation of Methyl orange Using nanosized TiO2 photocatalyst doped with Cerium

Juncai Lu, Zhimou Wu, Yuying Li School of Chemistry and Environment Engineering,

Wuyi University Jiang Men, Guangdong province 529020, P. R. China

wyuchemljc@126.com

Abstract—Nanosized titanium dioxide particles doped with rare-earth element cerium were prepared by sol-gel method. Particle size and photocatalytic activity was characterized by X-ray Diffraction and Ultraviolet-visible light spectroscopy. Treatment of Methyl orange wastewater using prepared TiO2 photocatalyst was studied and some factors which affected the photocatalytic result were discussed. Ce-doped TiO2 exhibited a remarkable enhancement of the photocatalytic activity compared with the pure TiO2. When Ce/Ti moral ratio was 1.5%, the pH value of simulative methyl orange wastewater was 3, and the photocatalyst concentration in wastewater was 2.0g/L, the removal rate of Methyl orange could reach up 78 % under the 40W power of UV light irradiation for 240 minutes.

Keywords-nanosized TiO2; cerium-doped; photocatalytic activity; degradation; methyl orange

I. INTRODUCTION Titanium dioxide (TiO2) as a fascinating semiconductor

material is increasingly used in photo-assisted degradation and complete mineralization reactions of a large number of toxic organic compounds in the quest to dispose of environmental contaminants in aqueous and atmospheric ecosystems [1–4]. It has a number of advantageous properties such as high dielectric constant, excellent optical transmittance, high chemical stability, low cost and nontoxicity. The photocatalysis mechanism by TiO2 semiconductor is the photo-formed electrons and holes interact on its surfaces when irradiated by UV light. The oxidation-reduction reactions that lead to the organic pollutants translate into CO2, H2O and some mineral acids occurs between electron–hole pairs and surface substance adsorbed by solid particle. TiO2 possesses a wide band gap (3.2 eV) so that its absorption edge occurs below 400 nm and only about 3–5% of sunlight can be utilized to promote the oxide to generate charge carriers which play an important role in the process of reduction and oxidation[5]. Therefore, the enhancement of TiO2 photocatalyst activity is necessary for its more practical applications of this technology. Recently, It has been found that doping TiO2 with ions such as nonmetals N [6-8],C [9], F [10] and S [11],or metals like Fe[12],Ni[13],Cr[14], V [15], rare earth[16], which can be introduced into the TiO2 lattice structure, change its charge density, form lattice defects or change the lattice type to change the band structure of TiO2 to improve the TiO2 photocatalytic efficiency ultimately. Furthermore,nonasized TiO2 possesses high surface

energy,which can produce Quantum size effect and lead to band broadening, and thus TiO2 particles have a stronger redox ability and catalytic activity increases with the increasing degree of quantum size increased with increasing.

Methyl orange as a kind of widely used dye with benzene ring and heteroatoms N and S within its molecular structure, is destructive to the water environment. In this work, we synthesized TiO2 and TiO2 doped with rare earth (Ce) nanoparticles by sol-gel process and subsequently investigated its photocatalytic activity for simulative methyl orange wastewater degradation to validate activity results based on the modification induced by TiO2 doping. In addition, the best degradation conditions were certain.

II. EXPERIMENTAL

A. Preparation of the TiO2 Tetrabutyl titanate mixed with ethanol was slowly added

into the solution combined with ethanol, glacial acetic acid, cerium nitrate and deionized water under stirring. The mixture was stirred until obtaining an even and clear sol. Then it was aging for 56h to form gel. The gel was dried at 80℃ to obtain the precursor. The precursor was crushed and grinded to obtain a fine powder, then the powder was calcined at 500℃ for 3 h. Finally, the TiO2 photocatalyst with various cerium nitrate contents was prepared. The content of cerium nitrate varied according to molar ratio of Ce/Ti as 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%.

B. Photocatalytic degradation of methyl orange A series of different low concentration methyl orange water

solutions with different pH were prepared. Photocatalytic reaction occured after TiO2 particles were dispersed in the solution under magnetic stir and 40w UV lamp irradiation. Samples of liquid were taken every half hour after filter to test absorbency by UV-Vis at the maximum absorption wavelength of methyl orange 465 nm. Calculated the degradation rate of methyl orange according to the standard working curve equation of methyl orange:

A=1.0414*C-0.011 (correlation coefficient R=0.9991). The removal rate of methyl orange was calculated by the following formula:

978-1-4244-5089-3/11/$26.00 ©2011 IEEE

η= (C0-C)/C0×100%

In which C0 is initial concentration of methyl orange solution, while C is concentration of methyl orange solution after photocatalytic degradation. The whole experimental apparatus is as shown in Fig. 1.

Fig.1 Experimental apparatus for photooxidation of organic

compounds in wastewater.

a-apparatus, b-UV lamp(40w), c-beaker, d-methyl orange solution, e-photocatalyst, f-magnetic stirrer

III. RESULTS AND DISCUSSION

A. X-ray diffraction analysis A High Resolution X-ray Diffraction (PANalytical B.V.) are used to test TiO2 crystal type and calculate the crystal size.

a

b

c

d

Fig.2 X-Ray diffraction patterns of: a- 0.5%, b-1.0%, c-1.5%, d-2.0% (mole rate of Ce/Ti )

From XRD spectrogram (Fig.2), we can see that the characteristic peak of anatase TiO2 is seen obviously (2θ = 25.3°). The photocatalytic activity of anatase TiO2 is better than the rutile TiO2 . The particle size can be calculated by the Scherrer formula:

D=k λ/(β*cosθ)

D is the averaged dimension of crystallites (nm); K is the Scherrer constant, somewhat arbitrary value that falls in the range 0.87-1.0 (it is usually assumed to be 1); λ is x-ray diffracted wave length; β is the integral breadth of a reflection located at 2θ. θ is the peak position. In this work θ is 12.65°, so D is 62nm, 55nm, 53nm, 60nm corresponding with molar ratio of Ce/Ti 0.5%, 1.0%, 1.5% and 2.0% respectively by calculation.

The results show that the TiO2 photocatalyst size is on a nanoscale, which means that the photocatalytic reaction will occur more thoroughly because of its more specific surface area.

B. The absorbency photocatalytic result of TiO2,Ce-doped TiO2 In photocatalytic reaction apparatus, 0.2g TiO2（ 0.5%

Ce/Ti ）and undoped TiO2 are respectively dispersed in 100

ml methyl orange solution with its concentration 5mg/L respectively as well as a same sample without TiO2 photocatalyst, adjust pH to 3.0,and determine the absorbency and calculate the removal rate of methyl orange. The results are as shown in Fig. 3.

0

20

40

60

80

100

0 50 100 150 200 250 300

Photocatalytic degradation time(min)

Removal rate(%)

without TiO2

pure TiO2

doped TiO2 (Ce/Ti 0.5%)

Fig.3 Contrast of different photocatalytic degradation

results It can be observed that the presence of TiO2 photocatalyst

could improve the reaction efficiency evidently. Besides, the TiO2 nanoparticle doped with Ce has the better photocatalytic activity than the pure TiO2. At the same time, we could discover that the removal rate of methyl orange gradually increased during the irradiation time 240 minutes, after 4hours, the degradation rate keep changed hardly. It is not economically to control the treatment time more than 4 hours. The removal rate of methyl orange using the doped TiO2 photocatalyst could be improved nearly three times compared with undoped TiO2.

C. Determination of the optimum photocatalytic reaction conditions

1) The influence of different doped rate of Ce When we keep the irradiation time is 240mins, and other

reaction conditions are same as above experimental conditions, compare the degradation results of nanoparticles with Ce/Ti 0.5％,1.0％,1.5％,2.0％,2.5％.

Fig.4 The removal rate changes with different mole rate of

doped Ce during photocalytic oxidation process

Fig.4 indicates the removal rate of methyl orange changes with the amount of doped Ce. And the TiO2 with Ce/Ti 1.5% has the optimal photocatalytic ability on treating methyl orange. The reason may be only a bit of Ce4+ ions are doped into TiO2 lattice, while most Ce4+ ions disperse in TiO2 nanoparticles or cover on its surface in the form of CeO2 clusters. When UV light irradiate, Ce4+ can easily capture electron to convert into Ce3+, therefore the electron could complex with the cavity hardly, the photocatalytic efficiency would be improved consequently. But when the doping amount of Ce is larger than 1.5%, redundant Ce can not disperse on its surface, on the contrary, it would prevent the primary clusters from capturing electron.

2) The influence of TiO2 photocatalyst usage amount By changing the usage amount of photocatalyst, and

keeping other conditions unaltered, we can find out that the corresponding amount of TiO2 doped with 1.5% Ce to the best treating effect is 2.0g/L.When its usage is more than 2.0g/L, TiO2 as a kind of insoluble substance, its particle interaction would influence UV light irradiation depth.

40

50

60

70

80

90

0 0.05 0.1 0.15 0.2 0.25 0.3

Usage amount (g/100ml)

Removal rate(%)

Fig.5 The removal rate changes with different usage

amount of TiO2 during photocalytic oxidation process

3) The influence of pH

40

50

60

70

80

90

0 2 4 6 8 10

pH

Removal rate(%)

Fig.6 The removal rate changes with different pH during

photocalytic oxidation process

From Fig.6 we discover that the best treatment efficiency could reach up to 78% when pH is 3 and the amount of TiO2 doped with 1.5%Ce is 2.0g/L.With the increasing of pH, degradation rate significantly reduces. Possible reasons are:

one is the change of pollutants existence form and external adsorption on the surface of photocatalysts, and the other is pH would affect the electric band position of TiO2, as a result, the photocatalysis mechanism would be changed.

IV. CONCLUSIONS According to obtained results of photocatalytic oxidation

of methyl orange in water, it may be concluded that: • The most convenient method for preparation of TiO2

doped with Ce is sol-gel method. The anatase TiO2 photocatalyst with various cerium nitrate contents is prepared. The content of cerium nitrate varied according to molar ratio of Ce/Ti as 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%.

• It is significant that the photocatalytic activity of TiO2 can be improved by modification. The removal rate of methyl orange could be improved nearly three times compared the doped TiO2 with undoped TiO2. TiO2 doped with Ce/Ti 1.5% has the optimal photocatalytic ability on treating methyl orange.

• When Ce/Ti moral ratio of TiO2 is 1.5%, the pH value of methyl orange solution is 3, and the photocatalyst usage is 2.0g/L, the removal rate of methyl orange could reach to 78 % under the 40W power of UV light irradiation for 240 minutes. The results can be regarded as a reference to photocatalytic degradation of low concentration organic pollutants.

ACKNOWLEDGMENT The financial support of this work is provided by Youth

Foundation of Wuyi University. The authors also thank our colleagues and other students participating in this work.

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