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TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water
Applied Catalysis A: General 350 (2008) 164–168
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water Hiromi Yamashita *, Hiroyuki Nose, Yasutaka Kuwahara, Yoshikatsu Nishida, Shuai Yuan, Kohsuke Mori Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2008 Received in revised form 6 August 2008 Accepted 7 August 2008 Available online 26 August 2008
TiO2 photocatalyst loaded on Si3N4 (TiO2/Si3N4) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO2 photocatalysts (TiO2/SiO2, TiO2/Al2O3, and TiO2/SiC) loaded on various types of supports (SiO2, Al2O3, and SiC). The formation of the well-crystallized anatase phase of TiO2 was observed on the calcined TiO2/Si3N4 photocatalyst, while a small anatase phase of TiO2 was observed on the TiO2/SiC photocatalyst and amorphous TiO2 species was the main component on the TiO2/SiO2 and TiO2/Al2O3 photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO2/Si3N4 photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO2/Si3N4 photocatalyst decomposed 2-propanol diluted in water into acetone, CO2, and H2O, and finally, acetone was also decomposed into CO2 and H2O. The TiO2/Si3N4 photocatalyst showed higher photocatalytic activity than TiO2 photocatalyst loaded on other supports. The well-crystallized TiO2 phase deposited on Si3N4 and the hydrophobic surface of Si3N4 support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Photocatalyst TiO2 Si3N4 Photocatalytic degradation
1. Introduction The design of highly efficient photocatalytic systems is of vital interest and one of the most desirable goals in the research on environmentally friendly catalysts. The utilization of extremely small TiO2 particles as photocatalysts has attracted a great deal of attention, especially for the degradation of organics pollutants diluted in air and water. TiO2 semiconductor material is known as one of the most stable and highly reactive photocatalysts [1–7]. Because TiO2 photocatalyst can completely mineralize toxic and nonbiodegradable organics to CO2, H2O, and inorganic constituents, many researches have been undertaken to design highly efficient TiO2 photocatalytic systems which can be applied especially to purify polluted water. For practical applications of photocatalytic systems, TiO2 having higher photocatalytic activity is desirable and the photocatalyst will have has to be separated from the treated water after the process of purification of polluted water. To separate the photocatalyst from water, one must support the photocatalyst on
* Corresponding author. Tel.: +81 6 6879 7457; fax: +81 6 6879 7457. E-mail addresses: [email protected], [email protected] (H. Yamashita). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.08.015
bulk materials. It has been found that the highly dispersed titanium oxide photocatalysts anchored on the porous silica-based materials exhibit high and characteristic photocatalytic activity better than those of bulk TiO2 powder [4–7]. Especially the titanium oxide species prepared within the pores and frameworks of zeolite and mesoporous silica have been revealed to have unique local structures as well as high activity in the various photocatalytic reactions [8,9]. The photocatalytic performance of titanium oxide species loaded on silica matrixes depended on the surface properties of supports, especially on the surface hydrophilic–hydrophobic properties. Because the surface modification of silica support by the addition of fluorine is effective to create the hydrophobic surfaces and can enhance the photocatalytic activity of loaded TiO2 photocatalysts significantly, the hydrophobic surface of support is found to be suitable for the efficient photocatalytic degradation of organics diluted in water [10–12]. Furthermore, TiO2 photocatalyst loaded on SiC has also been successfully utilized as the catalyst for the photocatalytic degradation of pollutants diluted in water, because SiC has a more hydrophobic surface in comparison to the hydrophilic surface of silica support [13,14]. On the other hand, silicon nitride (Si3N4) has various attractive properties such as high thermostabilty and high mechanical strength. In addition, Si3N4 can easily be molded into a filter and
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has been used as a catalyst support. Because it may be a useful support for photocatalysts used in liquid-phase, the application of Si3N4 as the support of TiO2 photocatalysts is very interesting to design more useful photocatalytic systems. In the present study, we present the preparation and characterization of TiO2 photocatalysts loaded on Si3N4 using the conventional impregnation. We successfully used this material for the photocatalytic degradation of 2-propanol diluted in water. 2. Experimental 2.1. Preparation of catalysts The powders of a-Si3N4 (SN-E03, Ube Co.), b-SiC (Wako Chemical Co.), SiO2 (G-3, Fuji Silysia Co.), and Al2O3 (AKP-G015, Sumitomo Chemical Co.) were used as commercially obtained. TiO2/Si3N4 (mainly 10 wt.% as TiO2) photocatalyst was prepared by an impregnation method as follows: Si3N4 was impregnated with an aqueous solution of (NH4)2[TiO(C2O4)2]2H2O at 323 K and then evaporated at 343 K. The obtained sample was dried at 373 K for 12 h and then calcined in air at 873–1273 K (mainly at 1073 K) for 5 h. In order to compare photocatalytic activity of TiO2/SiO2, TiO2/ Al2O3, and TiO2/SiC (10 wt.% as TiO2) photocatalysts were also prepared by the same method. 2.2. Characterization of catalysts X-ray diffraction (XRD) patterns of the samples were measured with a Rigaku RINT2500 diffractometer with Cu Ka1 radiation. UV– vis spectra of the samples were recorded at 295 K with a Shimadzu UV-2400A spectrophotometer. BET surface area measurements were performed using an ASAP 2010 system (Shimadzu) at 77 K. The sample was degassed under vacuum at 473 K prior to data collection. Water adsorption isotherms of the catalysts were recorded at 295 K using a vacuum line system. 2.3. Photocatalytic degradation The photocatalyst (50 mg) was transferred to a quartz cell with an aqueous solution of 2-propanol (2.6 10 3 mol dm 3, 25 ml). Prior to UV-light irradiation, the suspension was stirred in a flow of O2 for 1 h under dark conditions. The sample was then irradiated (1300 mW cm 2 at 340–370 nm) at 295 K using UV light (l > 250 nm) from a 100 W high-pressure Hg lamp with continuous stirring under O2 atmosphere in the system [13,14]. The products were analyzed by gas chromatography. The photocatalytic activity was estimated from the initial decrease in the concentration of 2-propanol after preadsorption of 2-propanol on the catalyst under dark condition.
Fig. 1. The XRD patterns of TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K.
Fig. 2 shows the intensity of XRD peak assigned to anatase TiO2 crystalline observed with the photocatalysts calcined at various temperatures. All the photocatalysts exhibited the peaks assigned to anatase TiO2 crystalline and only TiO2/Al2O3 photocatalyst calcined at 1273 K exhibited small peaks assigned to rutile TiO2 crystalline phase together with the peaks of anatase crystalline. Although the peak intensity due to anatase TiO2 crystalline increased with the calcination temperatures for all samples, the intensity of TiO2/Si3N4 photocatalyst was always stronger than those of other photocatalysts, indicating that the well-crystallized anatase TiO2 crystalline was easily formed on TiO2/Si3N4 photocatalyst. Fig. 3 shows the XRD patterns of TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The formation of the well-crystallized anatase TiO2 crystalline was observed on TiO2/Si3N4 photocatalyst with the amount of TiO2 less than 10 wt.% TiO2, while the mixtures of anatase and rutile TiO2 crystalline were observed on photocatalysts with a larger amount of 20 wt.% TiO2.
3. Results and discussion Fig. 1 shows the XRD patterns of TiO2 photocatalysts loaded on various types of supports (TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3) and calcined at 1073 K. The formation of the wellcrystallized anatase TiO2 crystalline was observed on TiO2/Si3N4 photocatalyst. Also the formation of small anatase TiO2 crystalline particles was observed with TiO2/SiC photocatalyst. We suppose that the formation of well-crystallized TiO2 is enhanced on the relatively hydrophobic Si3N4 surface with the smaller amounts of surface OH groups. On the other hand, TiO2/SiO2 and TiO2/Al2O3 photocatalysts exhibited very weak XRD peaks due to the crystallized anatase phases, indicating that the TiO2 species mainly exist in an amorphous phase.
Fig. 2. The calcination temperature dependence of the intensity of XRD peak assigned to anatase TiO2 crystalline with TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/ Al2O3 photocatalysts prepared by the calcination at various temperatures.
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Fig. 4. The UV–vis absorption spectra of TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/ Al2O3 photocatalysts prepared by the calcination at 1073 K.
Fig. 3. The XRD patterns of TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
With increase in the amount of TiO2 in TiO2/Si3N4, the ratio of rutile phase against anatase phase increased. As shown in Fig. 4, the UV–vis spectra of the TiO2 loading photocatalysts exhibited the absorption band at around 260– 400 nm, indicating the presence of the fine particles of anatase TiO2 [15]. The absorption edge of the catalysts can be observed in the longer wavelength regions with TiO2/Si3N4 and TiO2/SiC photocatalysts. Because the smaller particles of TiO2 crystalline usually exhibit the absorption band at the shorter wavelength regions, the presence of absorption band in the longer wavelength regions suggests that the crystallinity of fine anatase TiO2 is higher with TiO2/Si3N4 and TiO2/SiC photocatalysts, than with TiO2/SiO2 and TiO2/Al2O3 photocatalysts. Fig. 5 shows the UV–vis spectra of the TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The absorption band of catalysts shifted to the longer wavelength region with the content of TiO2, indicating that the particle size of TiO2 crystalline had increased. Fig. 6 shows the water adsorption isotherm on various photocatalysts. Amounts of water adsorption on TiO2/Si3N4 and TiO2/SiC photocatalysts calcined at 1073 K are much smaller than those on TiO2/SiO2 and TiO2/Al2O3 photocatalysts. This result suggests that Si3N4 and SiC supports have the hydrophobic surface properties. As reported previously [10–12,16], the photocatalytic activity of TiO2 loaded on supports depend significantly on the hydrophilic–hydrophobic surface properties of supports; also, the hydrophobic surface property is suitable for the photocatalytic degradation of organics diluted in water. In the present reaction system, the hydrophobic surface properties of Si3N4 and SiC supports are also very important factors to control the photocatalytic activity of TiO2/Si3N4 and TiO2/SiC photocatalysts for the liquid-phase reaction.
Figs. 7 and 8 show the reaction time profiles of the liquid-phase photocatalytic degradation of 2-propanol diluted in water on the TiO2 loading photocatalysts calcined at 1073 K. Under UV-light irradiation of photocatalysts 2-propanol is decomposed into acetone, CO2 and H2O, and finally, acetone is also decomposed into CO2 and H2O, similar to the reaction scheme observed on TiO2/ HMS mesoporous silica [10–12]. Among the TiO2 loading photocatalysts, the TiO2/Si3N4 photocatalyst decomposed 2-propanol faster than TiO2/SiC, TiO2/SiO2 and TiO2/Al2O3 photocatalysts, as
Fig. 5. The UV–vis absorption spectra of Si3N4 support and TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
H. Yamashita et al. / Applied Catalysis A: General 350 (2008) 164–168
Fig. 6. The water adsorption isotherm on TiO2/Si3N4, TiO2/SiC, and TiO2/SiO2 photocatalysts prepared by the calcination at 1073 K.
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Fig. 9. Photocatalytic reactivity normalized per weight of photocatalyst including supports for degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K and commercial TiO2 powder (P25).
shown in Fig. 9. The TiO2/Si3N4 photocatalyst can perform the high activity similar to that of TiO2 (P-25) power photocatalyst, which is well known as the reactive TiO2 standard photocatalyst, even though the TiO2/Si3N4 photocatalyst contains only 10 wt.% of TiO2. This result indicates that hydrophobic Si3N4 is a useful support for TiO2 photocatalyst for degradation of organic compounds diluted in water. The surface areas of the TiO2/Si3N4 calcined at 1073 K and P-25 were determined to be 6 and 65 m2/g, respectively. The catalytic activity of P-25 proportionally increased with increasing the amount employed. These results suggest that the surface area of the photocatalyst have little influence on the catalytic efficiency. Fig. 10 shows the photocatalytic reaction rate estimated from the initial decrease in the concentration of 2-propanol on various photocatalysts calcined at different temperatures. The TiO2/Si3N4
Fig. 7. The time profile for photocatalytic degradation of 2-propanol diluted in water on TiO2/Si3N4 photocatalysts prepared by the calcination at 1073 K.
Fig. 8. The time profile for photocatalytic degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K and commercial TiO2 powder (P25).
Fig. 10. The calcination temperature dependence of photocatalytic reactivity for degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at various temperatures.
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catalytic activity for the degradation of 2-propanol diluted in water. TiO2/Si3N4 photocatalyst showed the higher photocatalytic activity than TiO2 photocatalysts loaded on various types of supports (TiO2/SiO2, TiO2/Al2O3, and TiO2/SiC). The formation of well-crystallized anatase TiO2 on Si3N4 and the hydrophobic surface of Si3N4 were found to be related to the efficient photocatalytic activity of TiO2/Si3N4 photocatalyst. Since Si3N4 is mechanically strong enough to be used as a filter for water purification, Si3N4 is a good support for TiO2 photocatalysts used in liquid-phase reactions.
Acknowledgements
Fig. 11. Photocatalytic reactivity normalized per weight of photocatalyst including supports and photocatalytic reactivity normalized per weight of TiO2 for degradation of 2-propanol diluted in water on TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
photocatalyst exhibited the higher activity than other photocatalysts even after calcination at various temperatures between 873 and 1273 K. The maximum reactivity was observed with the TiO2/Si3N4 photocatalyst calcined at 1073 K. Fig. 11 shows the photocatalytic reaction rate estimated from the initial decrease in the concentration of 2-propanol on various TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The activity increased with TiO2 contents and becomes maximum with photocatalyst having 10 wt.% TiO2, in which anatase was the main crystalline phase. On the other hand, the photocatalysts with amounts of TiO2 larger than 10 wt.% and with rutile phase as main component exhibited the lower activity. Fig. 11 also shows the photocatalytic activity normalized with the unit weight of TiO2. The values decrease remarkably for the samples having TiO2 larger than 10 wt.% in which the formation of rutile phase was observed. This result indicates that well-crystallized anatase TiO2 deposited on Si3N4 can demonstrate efficient photocatalytic activity. 4. Conclusions TiO2 photocatalysts loaded on Si3N4 (TiO2/Si3N4) which was prepared by the conventional impregnation showed high photo-
The present work is supported by some Grants-in-Aid for Scientific Research (KAKENHI) in Priority Area ‘‘Molecular Nano Dynamics’’ from Ministry of Education, Culture, Sports, Science and Technology (No. 17034036, No. 17360388, No. 18656238). This work is partly performed under the project of collaborative research at the Joining and Welding Research Institute (JWRI) of Osaka University.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water Hiromi Yamashita *, Hiroyuki Nose, Yasutaka Kuwahara, Yoshikatsu Nishida, Shuai Yuan, Kohsuke Mori Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2008 Received in revised form 6 August 2008 Accepted 7 August 2008 Available online 26 August 2008
TiO2 photocatalyst loaded on Si3N4 (TiO2/Si3N4) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO2 photocatalysts (TiO2/SiO2, TiO2/Al2O3, and TiO2/SiC) loaded on various types of supports (SiO2, Al2O3, and SiC). The formation of the well-crystallized anatase phase of TiO2 was observed on the calcined TiO2/Si3N4 photocatalyst, while a small anatase phase of TiO2 was observed on the TiO2/SiC photocatalyst and amorphous TiO2 species was the main component on the TiO2/SiO2 and TiO2/Al2O3 photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO2/Si3N4 photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO2/Si3N4 photocatalyst decomposed 2-propanol diluted in water into acetone, CO2, and H2O, and finally, acetone was also decomposed into CO2 and H2O. The TiO2/Si3N4 photocatalyst showed higher photocatalytic activity than TiO2 photocatalyst loaded on other supports. The well-crystallized TiO2 phase deposited on Si3N4 and the hydrophobic surface of Si3N4 support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Photocatalyst TiO2 Si3N4 Photocatalytic degradation
1. Introduction The design of highly efficient photocatalytic systems is of vital interest and one of the most desirable goals in the research on environmentally friendly catalysts. The utilization of extremely small TiO2 particles as photocatalysts has attracted a great deal of attention, especially for the degradation of organics pollutants diluted in air and water. TiO2 semiconductor material is known as one of the most stable and highly reactive photocatalysts [1–7]. Because TiO2 photocatalyst can completely mineralize toxic and nonbiodegradable organics to CO2, H2O, and inorganic constituents, many researches have been undertaken to design highly efficient TiO2 photocatalytic systems which can be applied especially to purify polluted water. For practical applications of photocatalytic systems, TiO2 having higher photocatalytic activity is desirable and the photocatalyst will have has to be separated from the treated water after the process of purification of polluted water. To separate the photocatalyst from water, one must support the photocatalyst on
* Corresponding author. Tel.: +81 6 6879 7457; fax: +81 6 6879 7457. E-mail addresses: [email protected], [email protected] (H. Yamashita). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.08.015
bulk materials. It has been found that the highly dispersed titanium oxide photocatalysts anchored on the porous silica-based materials exhibit high and characteristic photocatalytic activity better than those of bulk TiO2 powder [4–7]. Especially the titanium oxide species prepared within the pores and frameworks of zeolite and mesoporous silica have been revealed to have unique local structures as well as high activity in the various photocatalytic reactions [8,9]. The photocatalytic performance of titanium oxide species loaded on silica matrixes depended on the surface properties of supports, especially on the surface hydrophilic–hydrophobic properties. Because the surface modification of silica support by the addition of fluorine is effective to create the hydrophobic surfaces and can enhance the photocatalytic activity of loaded TiO2 photocatalysts significantly, the hydrophobic surface of support is found to be suitable for the efficient photocatalytic degradation of organics diluted in water [10–12]. Furthermore, TiO2 photocatalyst loaded on SiC has also been successfully utilized as the catalyst for the photocatalytic degradation of pollutants diluted in water, because SiC has a more hydrophobic surface in comparison to the hydrophilic surface of silica support [13,14]. On the other hand, silicon nitride (Si3N4) has various attractive properties such as high thermostabilty and high mechanical strength. In addition, Si3N4 can easily be molded into a filter and
H. Yamashita et al. / Applied Catalysis A: General 350 (2008) 164–168
165
has been used as a catalyst support. Because it may be a useful support for photocatalysts used in liquid-phase, the application of Si3N4 as the support of TiO2 photocatalysts is very interesting to design more useful photocatalytic systems. In the present study, we present the preparation and characterization of TiO2 photocatalysts loaded on Si3N4 using the conventional impregnation. We successfully used this material for the photocatalytic degradation of 2-propanol diluted in water. 2. Experimental 2.1. Preparation of catalysts The powders of a-Si3N4 (SN-E03, Ube Co.), b-SiC (Wako Chemical Co.), SiO2 (G-3, Fuji Silysia Co.), and Al2O3 (AKP-G015, Sumitomo Chemical Co.) were used as commercially obtained. TiO2/Si3N4 (mainly 10 wt.% as TiO2) photocatalyst was prepared by an impregnation method as follows: Si3N4 was impregnated with an aqueous solution of (NH4)2[TiO(C2O4)2]2H2O at 323 K and then evaporated at 343 K. The obtained sample was dried at 373 K for 12 h and then calcined in air at 873–1273 K (mainly at 1073 K) for 5 h. In order to compare photocatalytic activity of TiO2/SiO2, TiO2/ Al2O3, and TiO2/SiC (10 wt.% as TiO2) photocatalysts were also prepared by the same method. 2.2. Characterization of catalysts X-ray diffraction (XRD) patterns of the samples were measured with a Rigaku RINT2500 diffractometer with Cu Ka1 radiation. UV– vis spectra of the samples were recorded at 295 K with a Shimadzu UV-2400A spectrophotometer. BET surface area measurements were performed using an ASAP 2010 system (Shimadzu) at 77 K. The sample was degassed under vacuum at 473 K prior to data collection. Water adsorption isotherms of the catalysts were recorded at 295 K using a vacuum line system. 2.3. Photocatalytic degradation The photocatalyst (50 mg) was transferred to a quartz cell with an aqueous solution of 2-propanol (2.6 10 3 mol dm 3, 25 ml). Prior to UV-light irradiation, the suspension was stirred in a flow of O2 for 1 h under dark conditions. The sample was then irradiated (1300 mW cm 2 at 340–370 nm) at 295 K using UV light (l > 250 nm) from a 100 W high-pressure Hg lamp with continuous stirring under O2 atmosphere in the system [13,14]. The products were analyzed by gas chromatography. The photocatalytic activity was estimated from the initial decrease in the concentration of 2-propanol after preadsorption of 2-propanol on the catalyst under dark condition.
Fig. 1. The XRD patterns of TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K.
Fig. 2 shows the intensity of XRD peak assigned to anatase TiO2 crystalline observed with the photocatalysts calcined at various temperatures. All the photocatalysts exhibited the peaks assigned to anatase TiO2 crystalline and only TiO2/Al2O3 photocatalyst calcined at 1273 K exhibited small peaks assigned to rutile TiO2 crystalline phase together with the peaks of anatase crystalline. Although the peak intensity due to anatase TiO2 crystalline increased with the calcination temperatures for all samples, the intensity of TiO2/Si3N4 photocatalyst was always stronger than those of other photocatalysts, indicating that the well-crystallized anatase TiO2 crystalline was easily formed on TiO2/Si3N4 photocatalyst. Fig. 3 shows the XRD patterns of TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The formation of the well-crystallized anatase TiO2 crystalline was observed on TiO2/Si3N4 photocatalyst with the amount of TiO2 less than 10 wt.% TiO2, while the mixtures of anatase and rutile TiO2 crystalline were observed on photocatalysts with a larger amount of 20 wt.% TiO2.
3. Results and discussion Fig. 1 shows the XRD patterns of TiO2 photocatalysts loaded on various types of supports (TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3) and calcined at 1073 K. The formation of the wellcrystallized anatase TiO2 crystalline was observed on TiO2/Si3N4 photocatalyst. Also the formation of small anatase TiO2 crystalline particles was observed with TiO2/SiC photocatalyst. We suppose that the formation of well-crystallized TiO2 is enhanced on the relatively hydrophobic Si3N4 surface with the smaller amounts of surface OH groups. On the other hand, TiO2/SiO2 and TiO2/Al2O3 photocatalysts exhibited very weak XRD peaks due to the crystallized anatase phases, indicating that the TiO2 species mainly exist in an amorphous phase.
Fig. 2. The calcination temperature dependence of the intensity of XRD peak assigned to anatase TiO2 crystalline with TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/ Al2O3 photocatalysts prepared by the calcination at various temperatures.
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Fig. 4. The UV–vis absorption spectra of TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/ Al2O3 photocatalysts prepared by the calcination at 1073 K.
Fig. 3. The XRD patterns of TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
With increase in the amount of TiO2 in TiO2/Si3N4, the ratio of rutile phase against anatase phase increased. As shown in Fig. 4, the UV–vis spectra of the TiO2 loading photocatalysts exhibited the absorption band at around 260– 400 nm, indicating the presence of the fine particles of anatase TiO2 [15]. The absorption edge of the catalysts can be observed in the longer wavelength regions with TiO2/Si3N4 and TiO2/SiC photocatalysts. Because the smaller particles of TiO2 crystalline usually exhibit the absorption band at the shorter wavelength regions, the presence of absorption band in the longer wavelength regions suggests that the crystallinity of fine anatase TiO2 is higher with TiO2/Si3N4 and TiO2/SiC photocatalysts, than with TiO2/SiO2 and TiO2/Al2O3 photocatalysts. Fig. 5 shows the UV–vis spectra of the TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The absorption band of catalysts shifted to the longer wavelength region with the content of TiO2, indicating that the particle size of TiO2 crystalline had increased. Fig. 6 shows the water adsorption isotherm on various photocatalysts. Amounts of water adsorption on TiO2/Si3N4 and TiO2/SiC photocatalysts calcined at 1073 K are much smaller than those on TiO2/SiO2 and TiO2/Al2O3 photocatalysts. This result suggests that Si3N4 and SiC supports have the hydrophobic surface properties. As reported previously [10–12,16], the photocatalytic activity of TiO2 loaded on supports depend significantly on the hydrophilic–hydrophobic surface properties of supports; also, the hydrophobic surface property is suitable for the photocatalytic degradation of organics diluted in water. In the present reaction system, the hydrophobic surface properties of Si3N4 and SiC supports are also very important factors to control the photocatalytic activity of TiO2/Si3N4 and TiO2/SiC photocatalysts for the liquid-phase reaction.
Figs. 7 and 8 show the reaction time profiles of the liquid-phase photocatalytic degradation of 2-propanol diluted in water on the TiO2 loading photocatalysts calcined at 1073 K. Under UV-light irradiation of photocatalysts 2-propanol is decomposed into acetone, CO2 and H2O, and finally, acetone is also decomposed into CO2 and H2O, similar to the reaction scheme observed on TiO2/ HMS mesoporous silica [10–12]. Among the TiO2 loading photocatalysts, the TiO2/Si3N4 photocatalyst decomposed 2-propanol faster than TiO2/SiC, TiO2/SiO2 and TiO2/Al2O3 photocatalysts, as
Fig. 5. The UV–vis absorption spectra of Si3N4 support and TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
H. Yamashita et al. / Applied Catalysis A: General 350 (2008) 164–168
Fig. 6. The water adsorption isotherm on TiO2/Si3N4, TiO2/SiC, and TiO2/SiO2 photocatalysts prepared by the calcination at 1073 K.
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Fig. 9. Photocatalytic reactivity normalized per weight of photocatalyst including supports for degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K and commercial TiO2 powder (P25).
shown in Fig. 9. The TiO2/Si3N4 photocatalyst can perform the high activity similar to that of TiO2 (P-25) power photocatalyst, which is well known as the reactive TiO2 standard photocatalyst, even though the TiO2/Si3N4 photocatalyst contains only 10 wt.% of TiO2. This result indicates that hydrophobic Si3N4 is a useful support for TiO2 photocatalyst for degradation of organic compounds diluted in water. The surface areas of the TiO2/Si3N4 calcined at 1073 K and P-25 were determined to be 6 and 65 m2/g, respectively. The catalytic activity of P-25 proportionally increased with increasing the amount employed. These results suggest that the surface area of the photocatalyst have little influence on the catalytic efficiency. Fig. 10 shows the photocatalytic reaction rate estimated from the initial decrease in the concentration of 2-propanol on various photocatalysts calcined at different temperatures. The TiO2/Si3N4
Fig. 7. The time profile for photocatalytic degradation of 2-propanol diluted in water on TiO2/Si3N4 photocatalysts prepared by the calcination at 1073 K.
Fig. 8. The time profile for photocatalytic degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at 1073 K and commercial TiO2 powder (P25).
Fig. 10. The calcination temperature dependence of photocatalytic reactivity for degradation of 2-propanol diluted in water on TiO2/Si3N4, TiO2/SiC, TiO2/SiO2, and TiO2/Al2O3 photocatalysts prepared by the calcination at various temperatures.
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H. Yamashita et al. / Applied Catalysis A: General 350 (2008) 164–168
catalytic activity for the degradation of 2-propanol diluted in water. TiO2/Si3N4 photocatalyst showed the higher photocatalytic activity than TiO2 photocatalysts loaded on various types of supports (TiO2/SiO2, TiO2/Al2O3, and TiO2/SiC). The formation of well-crystallized anatase TiO2 on Si3N4 and the hydrophobic surface of Si3N4 were found to be related to the efficient photocatalytic activity of TiO2/Si3N4 photocatalyst. Since Si3N4 is mechanically strong enough to be used as a filter for water purification, Si3N4 is a good support for TiO2 photocatalysts used in liquid-phase reactions.
Acknowledgements
Fig. 11. Photocatalytic reactivity normalized per weight of photocatalyst including supports and photocatalytic reactivity normalized per weight of TiO2 for degradation of 2-propanol diluted in water on TiO2/Si3N4 photocatalysts with different amounts of TiO2 prepared by the calcination at 1073 K.
photocatalyst exhibited the higher activity than other photocatalysts even after calcination at various temperatures between 873 and 1273 K. The maximum reactivity was observed with the TiO2/Si3N4 photocatalyst calcined at 1073 K. Fig. 11 shows the photocatalytic reaction rate estimated from the initial decrease in the concentration of 2-propanol on various TiO2/Si3N4 photocatalysts with the different amounts of TiO2 calcined at 1073 K. The activity increased with TiO2 contents and becomes maximum with photocatalyst having 10 wt.% TiO2, in which anatase was the main crystalline phase. On the other hand, the photocatalysts with amounts of TiO2 larger than 10 wt.% and with rutile phase as main component exhibited the lower activity. Fig. 11 also shows the photocatalytic activity normalized with the unit weight of TiO2. The values decrease remarkably for the samples having TiO2 larger than 10 wt.% in which the formation of rutile phase was observed. This result indicates that well-crystallized anatase TiO2 deposited on Si3N4 can demonstrate efficient photocatalytic activity. 4. Conclusions TiO2 photocatalysts loaded on Si3N4 (TiO2/Si3N4) which was prepared by the conventional impregnation showed high photo-
The present work is supported by some Grants-in-Aid for Scientific Research (KAKENHI) in Priority Area ‘‘Molecular Nano Dynamics’’ from Ministry of Education, Culture, Sports, Science and Technology (No. 17034036, No. 17360388, No. 18656238). This work is partly performed under the project of collaborative research at the Joining and Welding Research Institute (JWRI) of Osaka University.
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