Kanagawa Accademy of Science and Technology
Photocatalyst Group

Brief summary

TiO2 photocatalysts can decompose organics and bacteria into CO2 and H2O with its strong oxidation ability under UV light1-4. Recently, environmental purification with the photocatalysts has received growing attention. The reactor design for effective photocatalytic environmental purification can be classified into two main strategies: (1) enlargement of reactive surface area and (2) improvement of mass transfer. On the other hands, electrolysis with boron-doped diamond (BDD) electrodes can also decompose organics. The wide potential window of BDD electrodes makes it possible to generate various highly active oxidants which can oxidize aqueous contaminants and waterborne pathogens efficiently5, 6. Therefore, TiO2 photocatalysts and BDD electrodes have a great advantage for environmental applications and disinfections. Based on these backgrounds, now we are studying effective design of the systems for environmental and medical application of TiO2 photocatalysts and BDD electrodes.  
Groop Leader: Ph.D. Akira Fujishima (President of Tokyo University of Science)
Investigation period: 2005-
 KAST Biolab  Photocatalyst Museum
 

Project details

1. Novel photocatalytic filter, TMiPTM (titanium mesh impregnated photocatalyst)
In order to meet the requirements for the effective photocatalytic environmental purification, the titanium mesh impregnated photocatalyst, TMiPTM has been fabricated 7 (Figure 1). We have found many advantages of TMiP and its usefulness for environmental purification. The TiO2 nanoparticles were successfully sintered onto its surface without any binder. This advantage is similar to the usual photocatalytic filter such as ceramic foams, but TMiP does not break as easily as it. Moreover, the high mechanical flexibility of Ti-mesh allows us to manipulate the TMiP. Therefore, any geometry of modules for environmental purification can be designed by combination of UV-sources and the other technologies. Now various environmental purification units by using TMiP have been fabricated for application to various fields7-16 (Figure 2). Among them, we propose a practical air-cleaner using photocatalysis-plasma synergistic reactor in the next section.

 

2. The air-purification ability of the photocatalysis-plasma synergistic air-cleaner in the smoking room17
Figure 3a shows the schematic illustration of the photocatalysis-plasma synergistic reactor. Basic design and fabrication method of the reactor was described previously 18. The atmospheric dielectric barrier discharge between the electrodes of the reactor produces air plasma when AC voltages applied. Air was blown through the gap while maintaining a high level of surface contact with TMiP and plasma region. Figure 3b shows a schematic illustration of a practical air-cleaner. When the fan is turned on, air currents are generated inside the casing from the air inlet toward the air outlet passed through the filters and the reactor. Figure 4 shows the proposed test method. The real-scale smoking room with the air-cleaner was fabricated in the non-smoking office room. In this condition, cigarettes were simultaneously burned in sets of 8. Each set of cigarettes were sequentially burned in the smoking room. After 30 minutes from the burning of a first set of 8 cigarettes, the amounts of gaseous compounds, total volatile organic compounds (TVOC), and total suspended particulates (TSP) were measured at near the inlet (point 1), center of the smoking room (point 2), the outlet of the air-cleaner (point 3), and center of the non-smoking room (point 4). An important point is that contaminated air would be treated by the air-cleaner just once, i.e. this is a “single-pass system”. Figure 4 shows the amounts of the compounds in tobacco smoke at near the air inlet (point 1), center of the smoking room (point 2), air outlet of the air-cleaner (point 3), and center of the non-smoking room (point 4). It seems that the amounts of all compounds increased by simultaneously burning of the cigarettes in the smoking room (point 2). Especially the amount of nicotine dramatically increased. Interestingly, the amounts of the compounds were significantly decreased at air outlet of the air-cleaner (point 3). Finally, the amounts of these compounds were kept at low-level in non-smoking room (point 4). This result indicates that the air-cleaner can decompose and/or remove the compounds efficiently.

3. Pinpoint O3-water production unit by using of BDD microelectrodes for dental treatment19
The application of BDD electrodes is promising for electrolyzing water to produce O3 because of their superior chemical and dimensional stability, as well as their large overpotential for the oxygen evolution reaction.20, 21 Thus, electrolysis with BDD electrodes can disinfect waterborne pathogens efficiently22. On the other hand, while laboratory studies suggest a promising potential of O3 in dentistry, this has not been fully realised in clinical studies to date. Based on these backgrounds, we developed novel pinpoint O3-water production unit for dental treatment by using of BDD microelectrodes19. Figure 5 shows a SEM image of the unit. The BDD thin film was deposited on the prepared tungsten wire (500 μm) using a microwave plasma assisted chemical vapor deposition system. A strip of an Al-foil with an ion-exchange membrane was spirally wound around the BDD microelectrode which was used as an anode. When DC voltage was applied between the anode and the cathode in the water, electrolysis proceeds as follows:

anode (BDD): 3H2O → O3 + 6H+ + 6e-
cathode (Al): 6H+ + 6e- → 3H2

Besides electrolytically yielded O3, OH radicals and oxidative intermediates generate at the BDD microelectrode surface. The unit showed almost the same disinfection ability as 20 ppm of aqueous NaOCl treatment in this in vitro assessment in the root-canal of bovine teeth. This research would be attractive to develop a practical unit for dental treatment.

 

Conclusions

We surveyed here hopefully underlines the fact that the closely-related TiO2 photocatalysis and BDD electrolysis are extremely versatile and can be used in numerous ways to both environmental purifications and disinfection for dental treatment. Although numerous research and development have proposed, further technological breakthroughs are required for practical applications of TiO2 photocatalysis and BDD electrolysis. We presented several key solutions to meet these requirements. For examples, a novel photocatalytic filter (TMiP) and its environmental applications, a practical air-purifier by using of photocatalysis-plasma synergistic reactor, and a pinpoint ozone-water production unit by using of BDD microelectrodes for dental treatment23. We can expect that the continuous improvements of the material property and the reactor design would create a large number of effective systems. Finally, we welcome joint research with you!

  

References

1.  A. Fujishima, and K. Honda, Nature, 238, 37 (1972).

2.  S. N. Frank, and A. J. Bard, Journal of the American Chemical Society, 99, 303 (1977).

3.  K. Sunada, Y. Kikuchi, K. Hashimoto, and A. Fujishima, Environmental Science & Technology, 32, 726 (1998).

4.  J. Mo, Y. Zhang, Q. Xu, J. J. Lamson, and R. Zhao, Atmospheric Environment, 43, 2229 (2009).

5.  Y. Einaga, in Comprehensive Hard Materials, ed. V. K. Sarin, Elsevier, Oxford, 2014, pp. 493.

6.  C. Comninellis, I. Duo, P.-A. Michaud, B. Marselli, and S.-M. Park, in Diamond Electrochemistry, eds. A. Fujishima, Y. Einaga, T. N. Rao and D. A. Tryk, Elsevier B.V.-BKC, Amsterdam-Tokyo, 2005, pp. 449.

7.  T. Ochiai, T. Hoshi, H. Slimen, K. Nakata, T. Murakami, H. Tatejima, Y. Koide, A. Houas, T. Horie, Y. Morito, and A. Fujishima, Catalysis Science & Technology, 1, 1324 (2011).

8.  T. Ochiai, K. Nakata, T. Murakami, Y. Morito, S. Hosokawa, and A. Fujishima, Electrochemistry, 79, 838 (2011).

9.  T. Ochiai, Y. Niitsu, G. Kobayashi, M. Kurano, I. Serizawa, K. Horio, K. Nakata, T. Murakami, Y. Morito, and A. Fujishima, Catalysis Science & Technology, 1, 1328 (2011).

10.  T. Ochiai, H. Nanba, T. Nakagawa, K. Masuko, K. Nakata, T. Murakami, R. Nakano, M. Hara, Y. Koide, T. Suzuki, M. Ikekita, Y. Morito, and A. Fujishima, Catalysis Science & Technology, 2, 76 (2012).

11.  H. Slimen, T. Ochiai, K. Nakata, T. Murakami, A. Houas, Y. Morito, and A. Fujishima, Industrial & Engineering Chemistry Research, 51, 587 (2012).

12.  A. Fujishima, K. Nakata, T. Ochiai, A. Manivannan, and D. A. Tryk, Interface, 22, 51 (2013).

13.  T. Ochiai, K. Masuko, S. Tago, R. Nakano, K. Nakata, M. Hara, Y. Nojima, T. Suzuki, M. Ikekita, Y. Morito, and A. Fujishima, Water, 5, 1101 (2013).

14.  T. Ochiai, K. Masuko, S. Tago, R. Nakano, Y. Niitsu, G. Kobayashi, K. Horio, K. Nakata, T. Murakami, M. Hara, Y. Nojima, M. Kurano, I. Serizawa, T. Suzuki, M. Ikekita, Y. Morito, and A. Fujishima, Chemical Engineering Journal, 218, 327 (2013).

15.  T. Ochiai, and A. Fujishima, in Encyclopedia of Applied Electrochemistry, eds. G. Kreysa, K.-i. Ota and R. Savinell, Springer New York, 2014, pp. 1542.

16.  T. Ochiai, Y. Hayashi, E. Ichihashi, T. Machida, Y. Uchida, S. Tago, Y. Morito, and A. Fujishima, American Journal of Analytical Chemistry, 5, 467 (2014).

17.  T. Ochiai, Y. Hayashi, M. Ito, K. Nakata, T. Murakami, Y. Morito, and A. Fujishima, Chemical Engineering Journal, 209, 313 (2012).

18.  X. Chen, J. Rozak, J.-C. Lin, S. L. Suib, Y. Hayashi, and H. Matsumoto, Applied Catalysis A: General, 219, 25 (2001).

19.  T. Ochiai, Y. Ishii, S. Tago, M. Hara, T. Sato, K. Hirota, K. Nakata, T. Murakami, Y. Einaga, and A. Fujishima, ChemPhysChem, 14, 2094 (2013).

20.  K. Arihara, C. Terashima, and A. Fujishima, Electrochemical and Solid State Letters, 9, D17 (2006).

21.  S. G. Park, G. S. Kim, J. E. Park, Y. Einaga, and A. Fujishima, Journal of New Materials for Electrochemical Systems, 8, 65 (2005).

22.  Y. Yao, Y. Kubota, T. Murakami, T. Ochiai, H. Ishiguro, K. Nakata, and A. Fujishima, Journal of Water and Health, 9, 534 (2011).

23.  T. Ochiai, Electrochemistry, 82, 720 (2014).