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

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2013, Vol. 58, No. 7, pp. 94-99 This paper is available online at FABRICATION AND STUDY ON STRUCTURE, PHOTOCATALYSIS OF TiO2:N/Al2O3 MATERIAL FOR CO, NO DEGRADATION Ma Thi Anh Thu1, Nguyen Manh Nghia2 and 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 TiO2 when 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 94 Fabrication and study on structure, photocatalysis of TiO2: N/Al2O3 material... seems to be a better choice to dope with TiO2 due 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 TiO2 was 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-Al2O3 fibers 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 470 0C 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 ofTiO2/Al2O3 dopedN are illustrated in Figure 1. The spectrum peaks at 2θ angles 250, 370, 480, 540 and 630, respectively, correspond to planes , , , and 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. 95 Ma Thi Anh Thu, Nguyen Manh Nghia and Nguyen Thi Hue 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−OR+ HOH/H2NR′ →≡ Ti−OH/HNR′ + ROH (1) ≡ 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) Figure 1. X-ray diffraction patterns of TiO2:N=Al2O3 with sol ratio 1:1:34 (a) and 1:2:34 (b) Figure 2. SEM images of TiO2:N=Al2O3 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 96 Fabrication and study on structure, photocatalysis of TiO2: N/Al2O3 material... 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/Al2O3 anatase 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. 97 Ma Thi Anh Thu, Nguyen Manh Nghia and Nguyen Thi Hue (B1) (B2) Figure 4. Results of the decomposition of CO in B1 and B2 samples under different conditions (B1) (B2) Figure 5. Results of the decomposition ofNO in B1 and B2 samples under different conditions In similar experiments with a carbon dioxide photocatalyst,TiO2:N/Al2O3 samples 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. 98 Fabrication and study on structure, photocatalysis of TiO2: N/Al2O3 material... 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. REFERENCES [1] Hiroshi Taoda, 2008. Development of TiO2 photocatalysts suitable for practical use and their applications in environmental cleanup. Research on Chemical Intermediates, Vol. 34, No. 4, pp. 417-42. [2] H. T. Nguyen, L.Miao, S.Tanemura, S.Toh, M.Kawasaki, 2004. Structural and morphological characterization of anatase TiO2 coating on Alumina scale fiber by sol-gel dip-coating method. J. 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