Preparation and Photocatalytic Properties of TiO 2 -SiO 2 Mixed Oxides with Different TiO 2 /SiO 2 Ratio and Brownmillerite Type Calcium Ferrite

Transcription

1 Preparation and Photocatalytic Properties of TiO 2 -SiO 2 Mixed Oxides with Different TiO 2 /SiO 2 Ratio and Brownmillerite Type Calcium Ferrite Jiro Ikawa 1, Shin ichi Komai 2, Daisuke Hirabayashi 1, Feng Ya ning 3, Miao Lei 4 and Kenzi Suzuki 1 1 EcoTopia Science Institute, Nagoya University 2 Technical Center of Nagoya University, Nagoya University 3 Department of Environmental Technology & Urban Planning, Nagoya Institute of Technology 4 Materials R&D Laboratory, Japan Fine Ceramics Centre Abstract: TiO 2 -SiO 2 mixed oxides with different TiO 2 /SiO 2 molar ratios in the range, 0.01 ~ 0.2, were prepared by a sol-gel method. Their physicochemical properties were investigated by XRD, UV visible spectroscopy, TEM and N 2 adsorption-desorption measurements. The photocatalytic activity of these samples was also measured by the adsorption of methylene blue (MB). It was found that the photocatalytic activity improved with increasing TiO 2 /SiO 2 molar ratio, the highest at 0.1. It was found that the particle size and the number of TiO 2 particles strongly influenced the photocatalytic activity. The possibility of Brownmillerite type calcium ferrite, Ca 2 Fe 2 O 5, included in SiO 2 as a visible light responding photocatalyst was also examined. The formation of this phase was found to respond to the visible light. Keywords: Photocatalysis, TiO 2 -SiO 2 mixed oxide, CaO-Fe 2 O 3 -SiO 2 mixed oxide, Calcium Ferrite, UV light, Visible Light, Methylene Blue 1. INTRODUCTION It is known that TiO 2 has many functions of deodorization, sterilization, VOCs decomposition, NO x decomposition, antifouling, self cleaning, antifog, antibacterial, etc. Using these functions of TiO 2 a variety of products, air cleaner, refrigerator, road related products, glass coated, mirror coated, medical device, etc. are produced. Our living environment is contaminated with various hazardous chemicals and many health hazards such as respiratory tract disease, allergic disease etc. are arising. Under such situation, the technology for environmental cleanup and preservation is strongly required for the construction of a sustainable society. Photocatalysis is one of the realizing methods to achieve this. The photocatalyst used in the environmental cleanup is mostly TiO 2. Since the concentration of hazardous chemical existed in the environment is very low, the technology of adsorption and concentration of the hazardous chemicals is strongly required. Therefore, the high dispersion technology of TiO 2 to the porous material with high specific surface area is necessary. In this study, the synthesis of TiO 2 -SiO 2 mixed oxide by a sol-gel method was investigated. Though near ultraviolet radiation, less than 380 nm, is necessary for the excitation of TiO 2, the visible light responding photocatalyst is required in order to effectively use the solar energy. SiC (413 nm), CdS (496 nm), Fe 2 O 3 (539 nm), GaP (551 nm), CdSe (730 nm) etc. are already known as a visible light responding photocatalyst by a conventional study [1]. Moreover, RbPb 2 Nb 3 O 10 (475 nm), BiVO 4 (540 nm) and Cu 2+ doped ZnS (520 nm) etc. are also known. We have studied the catalytic properties of Brownmillerite type calcium ferrite, Ca 2 Fe 2 O 5, until now. Our research clearly indicated that, calcium ferrite has high oxidative catalytic activity upon heating [2-4]. Calcium ferrite is an n-type semiconductor, so that, the second content of this study is to confirm the possibility as a visible light responding photocatalyst of Brownmillerite type calcium ferrite, Ca 2 Fe 2 O 5 [5, 6]. In this study, the photocatalytic function of Ca 2 Fe 2 O 5 was evaluated under the coexistence of SiO 2 with high surface area, since the specific surface area of Ca 2 Fe 2 O 5 powder is very small, under 2 m 2 /g. 2. EXPERIMENTAL 2.1 Preparation of sample TiO 2 -SiO 2 mixed oxide Titanium tetraisopropoxide {Ti[OCH(CH 3 ) 2 ] 4, TTIP} and tetramethoxy silane {Si(OCH 3 ) 4, TMOS} were used as starting materials, and TiO 2 -SiO 2 mixed oxides with TiO 2 /SiO 2 molar ratios of 0.01, 0.05, 0.10, 0.15 and 0.20 were prepared by a sol-gel method. Ethanol solutions of TTIP and TMOS were first prepared and they were mixed at a desiring TiO 2 /SiO 2 ratio with stirring. Addition of an aqueous ammonia solution to the TTIP and TMOS solution generated TiO 2 -SiO 2 mixed sol. The obtained sol was kept at room temperature for 12 h in order to progress the polymerization reaction, and then the wet gel was obtained. The xerogel of TiO 2 -SiO 2 was obtained by drying the wet gel at room temperature for 12 h. TiO 2 -SiO 2 mixed oxide was prepared by heating the xerogel at 400 C for 2 h in air. The list of sample prepared is shown in Table 1. Pure TiO 2 was also prepared by the similar method. In addition, a standard TiO 2 sample was also obtained from Catalysis Society of Japan (JRC-TIO-4, crystal structure: anatase, specific surface area: 50±15 m 2 /g) was used without any pretreatment. Corresponding author: K. Suzuki, k-suzuki@esi.nagoya-u.ac.jp 959

2 Where, F(R ): Kubelka-Munk function, k: absorption coefficient, s: scattering coefficient, R : reflectivity of the sufficiently thick sample, r : relative reflectivity. The optical band gap of sample was obtained from equation CaO-Fe 2 O 3 -SiO 2 mixed oxide Ethanol solution of TMOS, and aqueous solutions of Ca(NO 3 ) 2 and Fe(NO 3 ) 3 were mixed at a desiring CaO:Fe 2 O 3 :SiO 2 molar ratio under stirring. The resulting mixture was aged at room temperature for 12 h for crystallization. The wet gel turned into a xerogel of CaO-Fe 2 O 3 -SiO 2 upon drying at room temperature for 12 h. A mixed oxide of CaO-Fe 2 O 3 -SiO 2 was obtained by heating the xerogel at 800 C for 5 h in air. The Ca : Fe : Si mole ratios in CaO-Fe 2 O 3 -SiO 2 mixed oxide were 0.10 : 0.10 : 1.0, 0.20 : 0.20 : 1.0 and 0.30 : 0.30 : 1.0. Binary, Fe 2 O 3 -SiO 2 mixed oxide without CaO was also prepared in the similar manner with the Fe/Si mole ratio of Table 1 shows the list of sample prepared Calcium ferrite, Ca 2 Fe 2 O 5 Powder sample of calcium ferrite, Ca 2 Fe 2 O 5, was prepared by solid state reaction. The starting materials were Ca(OH) 2 and FeO(OH) powders. These hydroxides were calcined individually at 1000 C for 7 h to obtain CaO and Fe 2 O 3, respectively. These oxides were mixed physically, ground in a pestle and mortar and then, calcined at 800 C for 3 h in air. 2.2 Measurement Characterization The crystal structure of samples was measured by X-ray powder diffractmeter by using CuKα radiation (50 kv, 100 ma). The porosity such as specific surface area and pore volume was measured by using N 2 adsorption-desorption measurements at 77 K. The particle size and morphology of photocatalyst particles were observed with a transmission electron microscope (TEM) UV/VIS absorption spectrum The UV/VIS absorption spectrum was measured by diffuse reflectance spectroscopy. MgO was used as a standard sample for calibrating the reflectance spectrum. The absorption spectrum of the sample was obtained by converting the diffuse reflectance spectrum using the Kubelka-Munk function (Equation 1). F(R ) = k/s = (1-R ) 2 /2R = (1-r ) 2 /2r (1) [F(R )hν] 2 = A(hν-E g ) (2) Where, hν: energy of the light, A: constant, E g : energy gap. The optical band gap was obtained by following method, after the graph of [F(R )hν] 2 vs. hν was described. The tangent was described on the graph, and hν at [F(R )hν] 2 =0 was made to be the optical band gap Adsorption isotherm of Methylene Blue (MB) After 10 mg of sample powder and 40 ml of MB (C 16 H 18 ClN 3 S) solution (initial concentration; 1, 5, 10, 12, 15 and 20 mg/l) were mixed, the mixture was stored in the dark place for 3 h with stirring at room temperature and then the concentration of MB solution was measured. The MB equilibrium uptake on the sample was calculated from the concentration of MB solution Photocatalytic activity The photocatalytic activity of the sample was evaluated by decolorization reaction of MB solution at room temperature by the irradiation of a black light (27 W) or a fluorescent lamp (20 W) with UV cut-off filter under 400 nm. The equipment employed to measure the photocatalytic activity is shown in Fig. 1. The MB concentration was determined by the absorbance of 668 nm by using a spectrophotometer. Before taking measurements, 10 mg of the sample powder was mixed with 40 ml of MB solution (initial concentration; 20 mg/l) and, the mixture was stored in the dark place for 3 h under stirring. The change in the concentration of MB after 3 h was considered caused by MB adsorption on the sample. Afterwards, the mixture was irradiated in UV light or visible light, and the MB concentration was measured periodically 3. RESULTS AND DISCUSSION 3.1 XRD measurement XRD patterns of SG1-SG5, TiO 2 prepared by sol-gel method and JRC-TIO-4 are shown in Fig. 2. From TiO 2 and JRC-TIO-4, it was possible to obtain the diffraction peaks attributed to the anatase TiO 2. On the other hand, the clear diffraction peak was not obtained from SG1-SG5. However, in the diffraction patterns of SG4 and SG5 which contained relatively higher TiO 2 com- 960

3 pared with SG1-SG3, the broad diffraction line corresponded to anatase TiO 2 was identified around 25 2θ. From the above result, it was considered that the content of TiO 2 is too small, the crystallinity of TiO 2 is very low, and/or the particle size of TiO 2 is too small. The XRD results of SGF and SGCF series clearly indicate that, these samples are almost similar to those of the SG series. No diffraction peaks attributed to Fe 2 O 3 or Ca 2 Fe 2 O 5 have been clearly discerned. 3.2 N 2 adsorption / desorption isotherm Fig.3 displays the N 2 adsorption / desorption isotherms of SG1-SG5. Based on the shape of the isotherms, the samples could be classified into two groups: Group-1 samples, which include SG1, SG2 and, SG4 and Group-2 samples that contain SG3, SG5. The isotherm of the former group, (Group-1) samples exhibit a large N 2 uptake at the relative pressure (P/P 0 ) of zero, and the N 2 equilibrium uptake increased with increasing relative pressure. In addition, a hysteresis loop also appear between adsorption and desorption isotherms indicating that these samples posses pores widely distributed from micropore to mesopore, and display high specific surface areas. On the other hand, for the Group-2 samples, although the N 2 equilibrium uptake at the initial p/p 0 is almost similar to those observed for Group-1 samples, the N 2 equilibrium uptake is significantly less compared to Group-1 samples even upon increasing the relative pressure. It is also noted that these samples do not exhibit any hysterisis. These results suggest that the Group-2 samples do not posses mesopore although they exhibit micropores. The specific surface area (S BET ), mean pore diameter and pore volume of both Group-1 and Group-2 samples are shown in Table 2. Except the sample, SG5, which exhibits a BET surface area of about 400 m 2 /g, all other samples show a high BET surface areas over 500 m 2 /g. Though it is not clear still on the above reason, the dispersion state of TiO 2 particles in the SiO 2 seems to influence. Since the molecular size of MB ( nm) is 961

4 smaller than the pore diameter of the samples, it can expect that MB could be introduced in the pore of all samples. In addition, because all the samples exhibit a high BET surface areas, a high adsorption capacity for MB could also be expected. In contrast to SG samples, the BET surface areas of Ca 2 Fe 2 O 5 and TiO 2 are significantly less, 1.8 and 5.0 m 2 /g, respectively. It should be noted that the BET surface area of TiO 2 is drastically decreased from 150 to 5.0 m 2 /g after heating at 400 C. It is considered that the heat-resistance is very low for TiO 2 xerogel. The BET surface area of SGCF10 and SGCF20, and that of CaO-Fe 2 O 3 -SiO 2 mixed oxide are 385 and 403 m 2 /g, respectively. However, the BET surface area of SGCF30 sample that incorporates both CaO and Fe 2 O 3 is also less, 273 m 2 /g. 3.3 TEM images TEM images of SG1, SG2, SG3 and SG4 are shown in Fig. 4. The black particles in the photograph are the TiO 2 particles. The TiO 2 particle sizes in SG2, SG3 and SG4 are almost ~5, 5~20 and 5~50 nm, respectively, and they increased with increasing TiO 2 content. The TiO 2 particles could be formed in the following way: at the initial stage of the TiO 2 sol generation, the crystalline nucleus of TiO 2 is formed first. Then the growth of the crystalline nucleus is continuously generated in proportion to the TTIP concentration. On the other hand, the formation of TiO 2 particle could not be observed in SG1, because the TiO 2 content in this sample is very low with a TiO 2 /SiO 2 molar ratio of UV/VIS absorption spectrum The UV/VIS absorption spectra of samples are shown in Fig. 5. The validity of this measurement method could be confirmed, because the absorption edge for JRC-TIO-4 which posses primarily anatase TiO 2 phase, exhibit absorption at 371nm (3.34eV). Though the absorption edges for SG3 - SG5 are also about 380nm, very close to anatase TiO 2, the absorption edges for SG1 and SG2 are shifted to low-wavelength side by blue shift and this is caused by smaller TiO 2 particle sizes, especially below 10 nm [7]. It is clear from TEM results that the particle sizes of TiO 2 in SG1 and SG2 are smaller than those in SG3 - SG5. In addition, the absorption spectra of SG3-SG5 are recognized even in longer-wavelength side over 400nm. From this fact, the possibility of the excitation by the visible light could be considered. The samples, SG3 - SG5 exhibited a faint brown coloration, while the samples SG1 and SG2 962

5 showed white color. The observed difference would have caused by the use of aqueous ammonia in the synthesis. On the other hand, the absorption spectra of SGF10, SGCF10, Fe 2 O 3 and Ca 2 Fe 2 O 5 of which exhibited brown body color absorb in the visible light region. This fact shows the possibility as a visible light responding photocatalyst in these samples. The bandgap obtained from equation 2 and wavelength of SG1 - SG5, JRC-TIO-4 and Ca 2 Fe 2 O 5 are summarized in Table Adsorption isotherm of MB The MB adsorption isotherm for SG4 is shown in Fig. 6 and that is typical Langmuir type adsorption isotherm. The adsorption phenomenon of MB on the sample surface therefore corresponds to monolayer adsorption. The saturation adsorption amount of MB for SG4 could be calculated using Langmuir equation with g/g. This value is about 1/4 in comparison with the MB quantity (0.283 g/g) of monolayer adsorbed on the surface of SG4 (682 m 2 /g). 3.6 Photocatalytic activity MB solution of 20 mg/l was stored with SG samples, JRC-TIO-4 and TiO 2 for 3 h under stirring before UV irradiation. While the MB concentration lowered significantly after the adsorption equilibrium over SG samples, it remained almost unchanged over both JRC-TIO-4 and TiO 2 samples (Table 4). A plot of the amount of MB adsorbed versus BET surface areas of samples shown in Fig. 7 indicates that the adsorption capacity increases and shows a maximum around 600 m 2 /g. However, we believe that there are other parameters such as pore diameter, pore shape, pore volume and other surface properties of TiO 2 particle that influences the adsorption properties rather than mere BET surface areas. Further experiments are in progress to clarify this. The UV irradiation on the MB solution started after 3 h. The relationship between MB concentration and irradiation time is shown in Fig. 8. The MB concentration decreased with the irradiation time in all samples. Especially, the decreasing rate of the MB concentration is large, when JRC-TIO-4 and TiO 2 are used as photocatalysts. These samples, in fact, exhibit higher photocatalytic activities. In case of SG samples, on the other hand, the decreasing rate of MB concentration is small at SG1 and SG5, and it increases in the order SG4<, SG2<SG3. The graph of ln (C/C 0 ) vs. irradiation is shown in Fig. 9. The C o is the initial concentration of MB in UV irradiation start, and C is the MB concentration after the irradiation time elapsed. The decomposition reaction of MB was first-order reaction, because the graph of ln (C/C 0 ) vs. irradiation time fitted first order. Therefore, the rate constant of the reaction for each sample has been calculated from the slope of the graph, and the results are summarized in Table 5. In addition, the rate constant of reaction 963

6 suggest that the SG3 sample could be employed as a photocatalyst in both UV and visible range. The catalytic activities of Fe 2 O 3 and SGF10 which are considered to be promising materials as a visible light responding photocatalyst show very low photocatalytic activity compared to SG3 sample. The catalytic activity of SGCF10 in which CaO coexisted with Fe 2 O 3 showed the high photocatalytic activity in either irradiation of UV and visible light. The rate constants were and s -1 in each irradiation. The absorption wave length of SGCF10 exists in the visible light region, indicating that SGCF10 also is a promising material as a visible light responding photocatalyst. for the MB decomposition in visible-light irradiation is also summarized in Table 5. It can be seen that both JRC-TIO-4 and TiO 2 show high photocatalytic activity by the UV irradiation. The SG3 sample also exhibits a high photocatalytic activity by the UV excitation. It is interesting to note that although the JRC-TIO-4 sample exhibit a high photocatalytic activity of rate of s -1 under UV irradiation, the sample exhibit a very low rate s -1 under visible light irradiation. This is about 30 times loss in activity when the switching from UV to Visible light irradiation. On the other hand, the photocatalytic activity decreases only by about 50 % in the case of SG3. The rate constant for SG3 ( s -1 ) is about 5 times higher than that of JRC-TIO-4 sample ( s -1 ) highlighting that the SG3 developed in this work is much more active under visible light than the commercial JRC-TIO-4 photocatalyst. The results also 4. CONCLUSIONS The photocatalyst with high dispersion of TiO 2 particles and high specific surface area could be obtained from sol-gel synthesized TiO 2 -SiO 2 mixed oxides. The TiO 2 /SiO 2 ratio was found to greatly influence the photocatalytic activity. In this study, mixed oxides with TiO 2 /SiO 2 = 0.10 in which TiO 2 particles of about 10nm diameter highly dispersed in the silica showed the highest photocatalytic activity. On the other hand, the catalytic activity of Fe 2 O 3 -SiO 2 mixed oxide under visible-light irradiation was found to be very low. Its catalytic activity could be improved when CaO is doped. Although the information of Ca 2 Fe 2 O 5 is unclear it is highly probable that this phase is present in these samples that are contributing to high catalytic activity. REFERENCES 1. The Chemical Society of Japan, Jikken Kagaku Koza, Maruzen, p.558 (in Japanese). 2. D. Hirabayashi et al., Adv. Sci. Tech., 45, (2006) pp D. Hirabayashi et al., Catal. Lett., 110, (2006) pp D. Hirabayashi et al., Hyperfine Interact., 167, (2006) pp Y. Yang et al., Mater. Chem. Phys., 96, (2006) pp Y. Yang et al., Mater. Sci. Eng. B, 132, (2006) pp M. Anpo et al., J. Phys. Chem., 91, (1987) pp