10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Advanced photocatalytic activity using TiO2/ceramic fiber-based honeycomb Seong Moon Jung*, Ju Hyung Lee, Moon Suk Han, Jong Sik Choi, Sun Joo Kim, Joo Hwan Seo, Ho Yeon Lim. R&D LG Hausys Ltd. 104-1, Moonji-Dong Yuseong-gu Daejeon 305-380, Korea

Abstract A honeycomb-type substrate consisting of a ceramic fiber sheet was used as a practical photocatalyst for water purification. The ceramic substrate has a stable 3-dimensional pore structure, which has a high porosity of over 80% and possesses strong capillary forces due to the highly hydrophilic surface. When the TiO2/ceramic fiber based substrate was partially immersed in water, the photocatalyst, polluted water and UV light could easily be brought in contact within the thin layer of water surrounding the TiO2 . The effect of the substrate was evaluated using experiments with decomposable ink, methylene blue and bisphenol A. Especially, the photocatalytic activity of TiO2 on the ceramic fiber based substrate for the degradation of ink in the presence of water was several times higher than without water. This could be explained by synergism between the hydrophilicity of the ceramic media and the photocatalyst of TiO2. Keywords: photocatalyst, TiO2, ceramic fiber

1. Introduction The photocatalytic activity of TiO2 has been used to convert toxic and non-biodegradable organics into CO2, H2O and inorganics [1]. To apply the photocatalyst for water purification, coating of titanium oxide on supports is most critical. Pozzo et al. proposed several properties for a good support material for titania as a photocatalyst [2]. It is also known that the photocatalytic performance of titanium oxide species loaded onto a substrate depends on the surface properties of the support, especially on the hydrophilic–hydrophobic balance [3]. Yamashita et al. reported that 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 [3]. But for real engineering applications, the slurry process is not practical, because of the difficulty of transmitting UV light. In order to improve the transmission of UV light, we developed TiO2 photocatalysts loaded on a ceramic honeycomb structure having a hydrophilic surface and a three-dimensional pore structure using conventional dip-coating. The uptake of water by capillary force due to the small pore size and the hydrophilic surface created thin water layers surrounding the TiO2. As a result, the ceramic media could promote easy contact between photocatalyst, polluted water and UV light. We have monitored the photocatalytic activity via the degradation of Bisphenol A and Methylene blue under UV-C light.

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2. Materials and experimental

2.1. Materials TiO2 powder (P-25, Degussa) was used as a photocatalyst. The Al-sol contained 20wt% alumina (Nyacol Al-20), Si-Sol containing 15 wt% silica was from Ludox, Aluminum phosphate was used for ceramic fiber honeycomb coating.

Methylene blue (Fluka) and Bisphenol A (Aldrich) were used as the organics for the photocatalytic test.

2.2. Experimental 2.2.1. Preparation of the ceramic fiber-based honeycomb Silica-alumina fiber having an average length of 300 µm was added to water, and dispersed by intensive stirring; pulp of a pine tree was added to the mixture in the amount of 25 wt% to the ceramic fiber, and an acryl binder for providing flexibility to ceramic paper was added to the mixture in the amount of 10 wt% to the ceramic fiber, after which 1% aqueous solution of ammonium aluminum sulfate with pH 3 was added to adjust the pH of the total slurry to about 5.5. Then, a ceramic green paper was prepared from the slurry by using a paper-making machine. The ceramic green paper prepared was shaped by using a wave shaping machine. The wave-shaped ceramic fiber sheets were wound to a diameter of 22 cm. These were dried in the oven at 180oC for 30 min. Then, the first coating was performed with silica sol by dip-coating. The second coating was carried out with aluminium phosphate to give the hydrophilicity on the surface of ceramic media using the same method as for the first coating. Drying was at 180 oC for about 1 h. The coated ceramic structure was calcined at 900, 950 and 1000oC. 2.2.2. Coating of TiO2 A solution was prepared by dispersing 10 wt% TiO2 powder in deionized water after which the mixture was stirred until it became homogeneous. Then 10 wt% ethanol was added. Finally, 10 wt% Al-Sol (Al-20) was dropped in. The TiO2/ceramic fiber based honeycomb was prepared by dip-coating with the TiO2 slurry. The coating thickness was controlled to be around 10µm. Excess coating was removed by air blowing. Thereafter, the ceramic honeycomb structure was dried in an oven at 130oC. It was annealed to remove impure organics and to improve the adherence between coating and ceramic honeycomb structure at around 450oC. 2.2.3. Photocatalytic decomposition & Analyses Irradiation under UV-C light was performed with a 20W mercury lamps (Germicidal lamp, Sankyo Denki Co.) having a wavelength of around 254 nm. Two lamps installed in parallel were fixed in the center of the ceiling of the reactor box made by black acrylic resin. Two identical beakers of 300 ml covered by a transparent vinyl sheet were placed in the reactor box. Each beaker included different organic compounds as Bisphenol A and Methylene blue. The concentration of each solution was different (Methylene blue 10ppm, Bisphenol A 300ppm). During the course of UV radiation, samples of 5 ml were withdrawn at different intervals for measurement of UV-Vis.

3. Results and discussion Figure 1 shows the Scanning Electron Micrograph (SEM) image of the ceramic fiber honeycomb structure before the TiO2 coating. The ceramic fibers covered with 3-phase binder are well-connected with a three dimensional structure and the pores are fully open without blocking by binders.

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Advanced photocatalytic activity using TiO2/ceramic fiber based honeycomb

Fig. 1. SEM image of ceramic structure calcined at different temperatures: (a) 1173K, (b) 1223K, (c) 1273K.

Upon increasing the temperature of calcination, the phase changed from a dual phase system of Al(PO3)3 and SiP2O7. to a single phase of AlSi2P3O12. This modification of phase improved the affinity to water and the surface stability, which enhanced the stability in water and the capillary force. The ceramic fiber based coupon showed a 5 times higher rate of water absorption than the polymer based one.

The SEM micrograph of the ceramic fiber honeycomb structure after TiO2 powder deposition is shown in Fig 2. The TiO2 powder had aggregated and was dispersed between ceramic fibers. The TiO2 powder adhered strongly to the ceramic media, considering that it was not removed from the ceramic structure by rubbing and by cellophane tape.

T iOT iOT iOT iO2222 coated areacoated areacoated areacoated areaT iOT iOT iOT iO2222 coated areacoated areacoated areacoated area

Fig. 2. SEM image of TiO2 coated ceramic structure.

The photocatalytic performance for the degradation of organics with and without water was compared with the degradation of ink by the naked eye. Fig. 3. is the image of the TiO2 coated ceramic honeycomb-like structure after 1h under treatment of irradiation of UV light in the presence of water. The ink conversion rate in the presence of water was about 10 times higher than in the case of without water.

Fig. 3. The photo-oxidation effect with water.

The results for the TiO2 photocatalyst on the ceramic honeycomb structure are shown in Table 1. Firstly, methylene blue was chosen as the representative of the organic dyes and it was also used to estimate the efficiency of photocatalyst. It was 100% degraded in 24 h. Secondly, Bisphenol A, which is well-known as one of the

(b) (c) (a)

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most difficult chemicals to eliminate in wastewater, was chosen as a target material. 154mg/l of Bisphenol A in the water was eliminated in 48 hours.

Table 1. The degradation of organics.

Materials The degree of decomposition (%)

Initial concentration (mg/L)

Time (h)

Methylene Blue 100 10 24

Bisphenol A 51.33 300 48

The results showed that the photocatalytic ceramic fiber-based honeycomb reactor whose inner space is partially coated with TiO2 was effective for purification of con-taminated water. The significant water uptake of the ceramic honeycomb structure due to the capillary forces could play an important role in photocatalytic reaction by faci-litating contact between photocatalyst, polluted water and UV light. This photocatalytic reactor is now expected to be applied for air and water purification..

References 1. D. F. Ollis and H. Al-Ekabi (eds.), 1993, Photocatalytic Purification and Treatment of Water

and Air, Elsevier, Amsterdam 2. R. L. Pozzo, M. A. Baltanfis and A. E. Cassano, 1997, Supported titanium oxide as

photocatalyst in water, Catal. Today, 39, 219 3. H. Yamashita, H. Nose, Y. Kuwahara, Y. Nishida, S. Yuan and K. Mori , 2008, TiO2

photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water, Appl. Catal. A: 350, 2, 164

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