건조상 산화 티탄늄에 의한 광촉매 반응 :

4-클로로페놀의 In Situ 표면 분해 반응에 대한 연구

김호, 김용수, 공성호

한양대학교 화학공학과

Dry phase Titanium Dioxide-Mediated Photocatalysis : A basic study for the In Situ Surface degradation of 4-chlorophenol

. Ho Kim, Yong Soo Kim, and Sung Ho Kong

Department of Chemical Engineering, Hanyang University

1. Introduction

The contamination of surfaces by hazardous organic chemical spills is a significant problem. Organic compounds are spilled onto concrete or asphalt from leaking containers in hazardous waste storage areas, or by the leakage of petroleum products in refineries, fuel depots, and gasoline stations. In addition, nonpoint highway runoff often poses a threat to sensitive surface and ground waters.1 To mitigate the effect of this contamination on the surrounding environment, control measures such as grassy swales, berms, or retention basins are used to retard the migration of hazardous pollutants. These controls use considerable land space and have high costs associated with their maintenance to ensure effective pollution control. However, if the contaminated surface can be modified to destroy organic pollutants in situ, an effective and economical means of protecting adjacent soils and water may be achieved. The promotion of surface photocatalysis to degrade the spilled contaminants on the pavement surface may be an effective mechanism of such in situ control. Furthermore, the rates of natural photodegradtion of the spilled chemicals can be enhanced by photocatalyst like TiO2.

The purpose of this study is to investigate the surface degradation of 4-chlorophenol(4-CP) on dry TiO2 and to explore the mechanisms responsible for the dry phase photodegradation of 4-CP as a basis for the development of solid-phase surface catalysis systems.

2. Theory

Semiconductors(e.g. TiO2, ZnO) can act as sensitizers for light-reduced redox processes due to their electronic structures, which is characterized by a filled valence band and an empty conduction band. When a photon with an energy of hv matches or exceeds the bandgap energy, Eg, of the semiconductor, an electron, ecb-, is promoted from the valence band, VB, into the conduction band, CB, leaving a hole, hvb+ behind (see Figure 2). Excited state conduction-band electrons and valence-band holes can recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles. If a suitable scavenger of surface defect state is available to trap the electron or hole, recombination is prevented and subsequent redox reactions may occur. The valence-band holes are powerful oxidants, while the conduction-band electrons are good reductants. Most organic photodegradation reactions utilize the oxidizing power of the holes either directly or indirectly; however, to prevent a buildup of charge, one must also provide a reducible species to react with the electrons.2

The application of illuminated semiconductors for the remediation of contaminants has been used successfully for a wide variety of compounds at the liquid2,3 and gas phase.2,4 To apply this system to in situ surface destruction, the reaction and degradation of contaminants should be investigated. To do this, the state of 4-CP on the TiO2 surface through FT-IR, photocatalytic degradation trend under different oxygen and humidity conditions, and reaction intermediate through  GC and FT-IR  were investigated.

3. Experimental Section

Three gram of the TiO2 was mixed with 100ml of distilled water and then agitated for about 30min to form a well dispersed slurry. The mixture was loaded on pyrex glass dish (7.07cm2), which has been dried for 12 h at 60 ℃ and then for 1.5 h at 140℃. The sample was spiked with 4-CP dissolved in acetone. The spiked TiO2 was placed at the dark hood to minimize photodegradation  and to evaporate acetone. The cell was then placed inside the reaction chamber as shown in figure 1. Controls were established by blocking the UV ray and compared with the residual concentrations to that of the illuminated samples. To elucidate the role of oxygen and water on oxidation rate, the reaction chamber was controlled under tight closure. and the experiment was performed when the reaction chamber reached at steady-state. Titanium-dioxide samples were collected over time and 4-CP was extracted with acetone using microwave extraction system(MES 1000, CEM). The 4-CP residual was analyzed using a Hewlett Packard 6890 gas chromatograph with flame ionization detector. And the surface state of TiO2 samples was analyzed using FT-IR.

4. Results and Conclusion

Titanium Dioxide adsorbs significant quantities of 4-CP. This solid state adsorption results in a color change of the TiO2 from white to yellow. FT-IR spectra shows that 4-CP exists on the TiO2 surface as three states, i.e., condensed phase, physisorbed phase, and chemisorbed phase. As shown in figure 3. the concentration of dark control reduced about 30% after 12 hours, which means that condensed and physisorbed 4-CP was volatilized.

The data of figure 4 show the effect of the addition of water vapor  to the dry TiO2 photocatalytic system in air environment. 4-CP oxidation rate is higher at low level of humidity. Water vapor inhibits the 4-CP oxidation rate at high humidity level.

Oxygen also plays a significant role on 4-CP oxidation rate as indicated in figure 5. In the absence of oxygen, the difference of residual concentration between dark controls and illuminated samples is negligible. However, dry O2 and UV irradiation produce 4-CP degradation in the absence of water, although the rate and extent of these reactions were much less than that under hydrated conditions. The fact that degradation occurs under dry conditions suggests that the reactions can be mediated by reaction with oxygen in either its molecular or reduced form.

The results indicate that degradation of 4-CP on the surface systems is not mediated solely by hydroxyl radicals at the positive hole and that oxygen has specific role in the disappearance of 4-CP besides trapping of electrons.

Because of significant amount of volatilized 4-CP and limitation of experimental system, complete photocatalytic mineralization of 4-CP was not observed. However, research carried out by the other researchers and FT-IR spectra reveal evidence for the formation of intermediates (mainly hydroquinone) before complete mineralization.

Although the types of compounds that may be degraded via this system are limited to those that relatively nonvolatile, this in situ surface destruction can be effective method for treating spilled chemicals.

5. References

1. Mikkelsen, P.S., Weyer, G., Berry, C., Walden, Y., Colandini, V., Plulsen, S., Grotehusmann, D., and Rohlfing R. Wat. Sci. & Technol., 29, 293, (1990).

2. Hoffmann, M.R., Martin, S.T., Choi, W., and Bahnemann, D.W. Chem. Rev. 95, 69-96, (1995).

3. Pearl, J., and Ollis, D.F. J. Catal. 136, 553-565, (1992)

4. Mills, A., Morris, S., and Davies, R. J. Photochem. Photobiol. A: Chem. 70, 183-191 (1993)

Figure 1. The experimental system used for photocatalytic degradation experiments.

Figure 3. Photocatalytic degradation of 4-CP on the surface of dry TiO2

Figure 2. Primary steps in the photochemical mechanism

Figure 5. Oxidation rate dependence on oxygen concentration

Figure 4. Oxidation rate depedence on humidity in air environment