Fabrication and study on structure, photocatalysis of TiO2:N Al2O3 material for CO, NO degradation

An Al2O3 overcoated TiO2:N layer was synthesized using the solgel method. XRD, SEM and UV-vis measurements were used to study the structure and optical properties of the material. The XRD results indicated that the TiO2:N layer was identified only as an anatase phase.

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FABRICATION AND STUDY ON STRUCTURE, PHOTOCATALYSIS

OF TiO2:N/Al2O3 MATERIAL FOR CO, NO DEGRADATION

Ma Thi Anh Thu1, Nguyen Manh Nghia2and Nguyen Thi Hue3

1Faculty of Natural Sciences, Cao Bang Teacher Training College

2Faculty of Physics, Hanoi National University of Education

3Institute of Environmental Technology, Vietnam Academy of Science and Technology

Abstract.An Al2O3 overcoated TiO2:N layer was synthesized using the solgel

method XRD, SEM and UV-vis measurements were used to study the structure and

optical properties of the material The XRD results indicated that the TiO2:N layer

was identified only as an anatase phase SEM images showed that the diameter of

particles was around 15-30 nm depending on the NH(C2H4OH)2 concentration

in the sol solution UV-vis spectrums suggested that a TiO2:N/Al2O3 sample

can absorb visible light These results indicate that fabricated material degrades

CO, NO better than Degussa P25 commercial TiO2when exposed to natural light

Keywords: TiO2, Al2O3, photocatalysis

1 Introduction

Titanium dioxide is a catalytic material that is used to treat contaminants at room temperature and normal atmospheric pressure [1, 5-7] This material is a typical photocatalyst which is capable of degrading contaminants with the aid of sunlight or artificial illumination without requiring special conditions such as high temperature or high pressure [2, 3] However, the large bandgap of about 3.2 eV of pure TiO2 in the anatase phase causes the photocatalytic property of TiO2 to be limited in the ultraviolet radiation range This is a major limitation because no more than 5% of solar radiation can be used to stimulate photocatalysis of TiO2 Therefore, improving the efficiency of TiO2 photocatalysis by expanding its absorption region to the visible radiation range has potential and is therefore attracting the attention of scientists Recent studies have shown that TiO2 can absorb visible light by doping metallic elements such as Fe, Co, La, Zr and

Pt [1, 5, 6], or nonmetal elements such as N, S and C [1, 5, 6] Among these elements, N

Received September 26, 2013 Accepted October 30, 2013.

Contact Nguyen Manh Nghia, e-mail address: nghianm@hnue.edu.vn

seems to be a better choice to dope with TiO2due to its potential to increase the strength and uniformity of materials, and to show stronger catalytic properties under natural light conditions

In this paper we present the synthesis and structural studies, as well as the optical properties, of nano TiO2:N/Al2O3 materials The study also focuses on testing the photochemical catalytic ability of materials to decompose the pollutants CO and NO in natural light conditions

2 Content

2.1 Experiment

TiO2was synthesized using a sol-gel process The sol solution was prepared using

hydrolysis and a condensation Ti(O-iC3H7)4 alkoxide A uniform TiO2 coating layer on

the surface of the χ-Al2O3 layer was obtained using two different mixtures we call sol B1 and sol B2 Sols B1 and B2 consisted of TTIP, DEA and EtOH with a molar ratio

of 1:1:34 and 1:2:34, respectively Titanium tetraisopropoxide (TTIP) and ethyl alcohol (EtOH) with a purity of 99.8% were provided by Wako Pure Chemical Industries Co., Ltd Diethylamine (DEA) purchased from the Kishida Chemical Company (Japan) was

used in the preparation of the sol solution χ-Al2O3fibers having a diameter of about 0.1

mm and a surface area of 0.015 m2/g were provided by Alus Co., Ltd (Japan) Al2O3 fibers were soaked for 60 minutes in the sol solution The best samples were annealed at

4700C for 3 hours to obtain crystalline TiO2 in the anatase form [4]

The crystalline structure of the TiO2 layer was determined by X-ray diffraction (XRD, Siemens D5000) Surface morphology and crystal size was measured by scanning using electron microscopy SEM (Hitachi S-4800) The band gap was calculated from the results of absorption spectra measured by a Jasco V670 system Photocatalytic experiments were conducted in a 1 m×1 m×1 m sealed test chamber The light sources

were 20 W UV lamps with wavelengths of 254 nm, 365 nm and a 10 W visible light lamp Photocatalysis materials placed 30cm from the light source were synthesized TiO2:N/Al2O3, 35 cm×35 cm in size A pollution gas in the chamber was moved through

the material several times by convection fan The concentration of CO and NO were determined using a colorimetric method (Shimadzu UV-Vis 2450)

2.2 Results and Discussion

The XRD patterns of TiO2/Al2O3doped N are illustrated in Figure 1 The spectrum

peaks at 2θ angles 250, 370, 480, 540 and 630, respectively, correspond to planes <101>,

<004>, <200>, <105> and <204> of the anatase phase Two diffraction peaks at positions

380 and 450 belong to χ-Al2O3 This result shows that the sample heated at 470 0C for 3 hours exhibits only an anatase phase and no diffraction peak relating to a rutile or brookite phase was observed

SEM images of two samples showed that the particle size of B1 and B2 samples were 30 nm, and 15 nm, respectively In both samples, the TiO2 particles were quite uniform and no cloud phenomenon was seen The influence of amine concentration

in different samples on particle size was considered Chemical reactions related to the formation of TiO2 nanocrystals can be described as a two-step process: hydrolysis and condensation [6] The reaction mechanism is written as follows:

≡ Ti − OH/HNR ′+ R′ NH/HO − Ti ≡→ Ti − O(N) − Ti ≡ + HOH/H2NR′ (2a)

≡ Ti − OH/HNR ′ + RO− Ti ≡→≡ Ti − O(N) − Ti ≡ + ROH/R ′OR (2b)

with sol ratio 1:1:34 (a) and 1:2:34 (b)

Hydrolysis reaction (Equation 1) occurs when the hydroxyl group in diethanolamine (H2NR′) undergoes nucleophilic substitution on the metallic center leading to the

exchange of alkyl groups in the titanium alkoxides (Ti− OR) Following a condensation

reaction, which involves the formation of≡ Ti − O(N) − Ti ≡, the byproduct is water

or a condensed 2a amino, alcohol or ether 2b The rate of hydrolysis and condensation

is inversely proportional to the amount of ligand added This means that the hydrolysis rate will determine the particle properties The rate of hydrolysis in solution with a molar ratio of Ti(O− iC3H7)4 : NH(C2H4OH)2 (1:1) is faster than that of the solution using a molar ratio of Ti(O− iC3H7)4 : NH(C2H4OH)2 (1: 2) This is the main reason for the difference in TiO2 particle size in two samples

Figure 3 Absorption spectra of Degussa P25 (a), sample B1 (b) and B2 (c)

To examine the changes of the electronic band structure of TiO2:N/Al2O3 materials, we studied the absorption spectra of the synthesized samples Figure 3

is the absorption spectrum of Degussa P25, TiO2:N/Al2O3 with the molar ratio of

Ti(O-iC3H7)4 : NH(C2H4OH)2 of 1:1 and 1:2, respectively From the results of the absorption spectra, we determined the band gap of Degussa P25 powder to be 3.3 eV and that of the samples B1 and B2 to 3.0 eV Thus, the doped nitrogen atom causes the band gap of TiO2/Al2O3anatase to become smaller than that of P25 Degussa commercial TiO2 and it increases the ability of the photocatalyst under visible radiation

We assessed the ability of the catalyst by carrying out a decomposition experiment

of CO and NO Figure 4 is the result of the decay of CO by materials under different lighting conditions Under UV light with the wavelength of 254 nm, both B1 and B2 samples showed good catalytic activity, and the carbon dioxide with 5 ppm initial concentration was almost completely decomposed after 30 minutes Potential degradation

of the two samples differed when light wavelength was increased to 365 nm Particularly, the B1 sample could convert CO almost completely in the test box after 100 minutes irradiation while the B2 sample needed just 60 minutes to do that A special feature of this catalytic result is that both samples can decompose CO when exposed to visible radiation The time needed to convert the total amount of CO in the test was 180 minutes and 150 minutes when using B1 and B2 samples, respectively

(B1) (B2)

Figure 4 Results of the decomposition of CO

in B1 and B2 samples under different conditions

Figure 5 Results of the decomposition of NO

in B1 and B2 samples under different conditions

In similar experiments with a carbon dioxide photocatalyst, TiO2:N/Al2O3samples also showed a photocatalytic decomposition of NO in the test box Using ultraviolet radiation with a wavelength of 254 nm, an initial 5 ppm concentration of NO was significantly reduced after 30 minutes when treated by samples B1 and B2 as shown in Figure 5 Under natural light conditions, the reaction times for the decay of NO were 210 and 180 minutes when using samples B1 and sample B2, respectively In both experiments involving the decomposition of CO and NO, sample B2 (particle size 15 nm) displayed a higher catalytic efficiency than that of sample B1 (particle size 30 nm) This is mainly due

to particle size changes The increase in surface area as well as the decrease in particle size lead to the phenomenon that the gas molecules can be exposed more efficiently to

a catalyst material, a capillary in a medium capillary, and thus increase photocatalyst efficiency

3 Conclusion

We succeeded in synthesizing a nanomaterial covering Al2O3 From the XRD results, TiO2:N/Al2O3 particles exhibited an anatase phase structure and there was

no evidence of a rutile or brooket phase The molar ratio between Ti(O-iC3H7)4 and NH(C2H4OH)2 in the initial solution was the key factor in the reaction rate and thus

it would decide the particle size of TiO2 crystalline Compared to the Degussa P25 commercial TiO2, TiO2:N/Al2O3 extended the radiation absorption to the visible range This made the TiO2: N/Al2O3 capable of decomposing CO and NO even in natural light conditions The difference in performance degradation between sample B1 and sample B2 was due to the difference in particle size: particle size reduction increased the surface area, which increased the catalytic performance

Acknowledgements This work was financially supported by a grant from the

KC.08.26/06-10 program of the Ministry of Sciences and Technologies

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