Development of Highly Active Titanium Oxide Photocatalysts Anchored on Silica Sheets and their Applications for Air Purification Systems
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The purpose of this study is to develop highly active titanium oxide photocatalysts that can be anchored onto a substrate. We have, thus, prepared a titanium oxide photocatalyst using a wet or dry process and the results of this study have led to the successful development of highly active rectangular column-structured titanium oxide photocatalysts, which can be anchored onto silica sheets. These highly active photocatalysts were then applied to develop an effective air purification system.
KeywordsPhotocatalytic Reactivity Silica Fiber TiO2 Photocatalysts TiO2 Crystal Titanium Oxide Photocatalysts
Economic advances in manufacturing and living standards have, unfortunately, led to serious environmental pollution and health hazards caused by chemical substances, bacteria, viruses, and toxic compounds from carbon fuel energy and waste materials. Even in the indoor environment, the volatile organic compounds used in building materials that cause the so-called “sick-house syndrome” have caused serious health problems. Also, contagious diseases such as SARS and bird influenza viruses as well as toxic molds can be easily spread through air. To address these concerns, various air cleaners have been developed and even commercialized. However, most air cleaners use activated carbon and adsorption materials in which the initial adsorptive performance is superior but slowly decreases and finally disappears. Also, unpleasant odors are inevitably caused by the desorption of these adsorptive materials. The used adsorption materials then become industrial waste, leading to greater environmental problems. For this reason, it is necessary to develop air purification systems that do not use such waste-producing adsorption methods as well as find new energy resources that are clean and safe. Photocatalytic reactions using ultraviolet light irradiation that can render toxic or odorous organic compounds harmless are considered a promising field of research in the development of new purification methods to replace conventional adsorption systems.
The development of TiO2 photocatalysts for environmental purification systems is presently being carried out at Andes Electric Co., Ltd., on a commercial scale. Powdered TiO2 photocatalysts such as the commercially available P-25 was first considered for use in these systems. However, powdered TiO2 was not easy to use and various binder materials have to be combined to fix or immobilize a powdered photocatalyst onto substrate materials. The binder materials, however, decrease the photocatalytic reactivity of the TiO2 itself since they act as a physical covering while the mechanical strength of the photocatalyst is also weakened.
We have investigated the development of highly active titanium oxide photocatalysts that can be anchored onto a substrate by using a wet or dry preparation process. These studies have led to the successful development of highly active rectangular column-structured TiO2 photocatalysts anchored onto silica sheets for applications in effective air purification systems. In this work, the preparation method is described along with a characterization of these rectangular column-structured TiO2 photocatalysts. Also, their photocatalytic reactivity and actual performance in air purification systems are introduced.
Preparation of Rectangular Column-Structured TiO2 Photocatalysts
The surface morphology of the samples was observed by scanning electron microscopy (SEM, Model S-4100, S-5000, Hitachi, Ltd) and transmission electron microscopy (TEM, Model H-800). The X-ray diffraction (XRD) patterns of the synthesized samples were recorded with a JEOL JDX-3530 XRD system using CuKα radiation (40 kV and 30 mA) at a scan speed of 1°min−1 in 2θ.
Evaluation of the Photocatalytic Reactivity
To evaluate the photocatalytic reactivity of the synthesized rectangular column-structured TiO2 photocatalysts, the complete oxidation of organic compounds into CO2 in a gas phase reaction system was investigated. The complete oxidation of gaseous acetaldehyde (CH3CHO) was examined by monitoring the gas concentrations of CH3CHO as well as CO2 as a function of the irradiation time under a UV black light (λ: 365 nm; irradiation intensity: 4.0 mW/cm2; irradiation area: 60 × 60 mm).
The reaction was carried out at 25±3°C under humidity of 60 ± 5% in a Pyrex glass reactor with a capacity of 20 L. First, gaseous acetaldehyde (Wako Pure Chemical Industries, Ltd.) was introduced into the reactor at a specified concentration and after reaching an adsorption equilibrium, UV light irradiation was carried out. The decrease in acetaldehyde concentration and its complete oxidation into CO2 were monitored by a photo-acoustic multi-gas monitor (Model 1312-5, INNOVA).
3 Results and Discussions
However, without the titanium oxide seeds, even by applying the sputtering or spray methods, such rectangular column-structured TiO2 crystals could not be formed. The preparation method for the TiO2 crystal nuclei was, thus, seen to be the most important factor for the silica fibers, TiO2 crystal nuclei, and rectangular column-structured TiO2 crystals to be chemically combined in order to synthesize a stable photofunctional material.
Applications for Rectangular Column-Structured TiO2 Photocatalysts
The air purifying systems incorporating the rectangular column-structured titanium oxide photocatalysts are shown in Fig. 8. Figure 8a shows the air purifier which addresses the noxious fumes which cause “Sick House Syndrome” (Model BF-H102A) and Fig. 8b shows the air purifier for “Industrial Use” (Model BF-S103A). The inner structures of these purifiers are rather simple, consisting of anchored rectangular column-structured TiO2 photocatalyst sheets, a UV light source, and a fan for air circulation.
Study institution: Research Center for Medical Environment, The Kitasato Institute
Study No.: 00228 March 19, 2003
Test method (virus): Cytopathic effect method (CPE)
Test method (bacteria): Nutrient agar plate cultivation
Tested model: Air purifier (BF-H201A)
Elimination capacity of air purifier using TiO2 photocatalysts (BF-H201A)
Elimination effects (%)
MRSA (methicillin resistant Staphylococcus aureus)
Investigations in the synthesis of highly active TiO2 photocatalysts that can be directly anchored onto silica sheets were carried out in order to develop an effective and stable air purification system. The results obtained from the present study are as follows.
Highly active “rectangular column-structured TiO2 crystals” which could be anchored onto silica sheets were developed. The rectangular column-structured TiO2 crystals could be anchored perpendicularly onto a silica fiber substrate in a very dense state with stable chemical bonds. The TiO2 crystals had a width of 100–500 nm and length of 1,000–5,000 nm, with anatase TiO2 nanoparticles of 10–30 nm. Moreover, the rectangular columnar crystals were observed to have a hollow structure. Investigations on the complete oxidation reaction of acetaldehyde into CO2 with these rectangular column-structured TiO2 photocatalysts showed a high performance equivalent to or even higher than the most efficient standard P-25 powdered photocatalysts. Thus, effective and stable air purifying systems could be successfully developed with the incorporation of these TiO2 photocatalyst sheets for the complete oxidation of organic compounds and bacteria in the gas phase.
This work was supported by NEDO (New Energy and Industrial Technology Development Organization of Japan). We would like to express our gratitude for their kind support.
- Anpo M (ed) (2000) Photofunctional zeolites. Nova Sci, New YorkGoogle Scholar
- Anpo M (2004) Preparation, characterization, and reactivities of highly functional titanium oxide-based photocatalysts able to operate under UV-visible light irradiation: Approaches in realizing high efficiency in the use of visible light Bull Chem Soc Jpn 77:1427Google Scholar
- Anpo M, Dohshi S, Kitano M, Hu Y, Takeuchi M, Matsuoka M (2005) The Preparation and Characterization of Highly Efficient Titanium Oxide-Based Photofunctional Materials Annu Rev Mater Res 35:1Google Scholar
- Hwang JS, Chang JS, Park SE, Ikeue K, Anpo M (2005) Photoreduction of carbondioxide on surface functionalized nanoporous catalysts Top Catal 35:311Google Scholar
- Kudo T, Kudo Y, Ruike A, Hasegawa A, Kitano M, Anpo M (2007) The design of highly active rectangular column-structured titanium oxide photocatalysts and their application in purification systems Catal Today 122:14Google Scholar
- Kudo T, Kudo Y, Ruike A, Hasegawa A (2005) Design and development of highly efficient rectangular column structured titanium oxide photocatalysts anchored onto silica sheets Top Catal 35:225Google Scholar
- Kudo T, Nakamura Y, Ruike A (2003) Development of rectangular column structured titanium oxide photocatalysts anchored on silica sheets by a wet process Res Chem Intermed 29:631Google Scholar
- Yamashita H, Takeuchi M, Anpo M (2004) American Scientific Publishers, Loa Angels Visible-Light-Sensitive Photocatalysts In: Nalwa HS (ed) Encyclopedia of nanoscience and nanotechnology 10:639Google Scholar