毕业论文-强磁场对透辉石光催化性能影响的实验 - 图文

photocatalytic properties of titania [J], Photochem. Photobiol. A: Chem., 1999,121: 49-53 14. Anpo M, Takeuchi M. The design and development of highly reactive titamium oxide photocatalysts operating under visible light irradiation [J], Catal., 2003, 216:505-516 15. Sarah K and Daniel R. Visible light driven V-doped TiO2 photocatalyst and its photooxidation of ethanol [J], Phys. Chem. B, 2001, 105(14): 2815-2819

16. Yasuo I, Fumitaka K, Hideaki Y, et al. Structure of low concentration of vanadium on TiO2 determined by XANES and ab initio calculations [J], Chem. Commun.2002, 20: 2402-2403

17. Kate H, Akihiko K. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts Co doped with antimony and ahromium [J], Phys. Chem. B,2002, 106(19): 5029-5034

18. Xu X H, Wang M, Hou Y, et al. Preparation and characterization of Bi-doped TiO2 photocatalyst [J], Mater. Sci. Lett., 2002, 21: 1655-1656

19. Shigeru Futamura，Hisahiro Einaga，Hajime Kabashima，etal．Synergistic effect of silent discharge plasma and catalysts on benzene decomposition[J]．Catalysis Today，2004，89：89L95．

20. 晏乃强，吴祖成，施耀等．电晕一催化技术治理甲苯废气的实验研究[J]．环境亿学，1999，20(1)：11-14．

21. 杨建．TiO2(TiN)复合O′-Sialon 基复相陶瓷的制备及功能性质研究[D]，沈阳：东北大学，2003

22. 华北化工学院分析化学教研组，成都科学技术大学分析化学教研组编．分析化学(第三版)[M]，北京：高等教育出版社，1989，330-337，354-356

23.张金龙，陈锋，何斌等．光催化[M]，上海：华东理工大学出版社，2004，10， 67-69

24.杜朝平，杨幼名．磁场助Y2O3/TiO2 粉体的光催化性能研究[J]，工业催化，2005，13(2)：48-50

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附 录

Magnetic Field Effect on the Photocatalytic Reaction with Ultrafine TiO2 Particles

Masanobu Wakasa,*,? Sachiko Suda,? Hisaharu Hayashi,§ Nobuharu Ishii,? and Mitsutoshi Okano?

Department of Chemistry, Faculty of Science, Saitama UniVersity, Shimo-okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan, Department of Chemistry, Faculty of Science, Gakushuin UniVersity, Mejiro, Toshima, Tokyo 171-8588, Japan, and Department of Nanochemistry, Faculty of Engineering, Tokyo Polytechnic UniVersity, Iiyama, Atsugi, Kanagawa 243-0297, Japan ReceiVed: October 25, 2003; In Final Form: June 10, 2004 Magnetically induced acceleration of a photocatalytic reaction was observed for the first time. Upon irradiation of ultrafine TiO2 particles in tert-butyl alcohol at room temperature, the decomposition reaction of tert-butyl alcohol generated acetone and methane as the main products. The yield of acetone was found to increase with increasing magnetic field from 0 to 1.5 T. An increase of about 10% in the yield was observed at 1.5 T. The observed magnetic field effects can be explained by the magnetically induced blocking of the recombination of electrons and holes in the semiconductor. Introduction

Since the discovery of photoelectrochemical decomposition of water into hydrogen and oxygen by Honda and Fujishima in 1972,1,2 the photochemistry and photophysics of semiconductors, especially those of TiO2, have gained much attention. Studies of both fundamental aspects and practical applications have been

carried out extensively.3 It is well-known that the strong oxidation and reduction power of such semiconductors can be used to effect changes in organic molecules as well as to convert light energy into different types of energy. In all such studies, the efficiency has been of utmost significance. To improve the efficiency of light energy and material conversion, (1) the separation of the photogenerated holes and electrons (a physical process) and (2) the back reaction of the products (a chemical process) are both important.4 In electron-transfer

reactions, it is known that magnetic fields can affect the recombination process of cation and anion radicals, and such magnetic field effects have been studied widely.5 Therefore, by considering the similarities between photocatalytic reactions and electrontransfer reactions, the authors wished to utilize magnetic fields to raise the efficiency of photocatalytic reactions on semiconductor

particles. There are, however, few reports for such magnetic field effects on photocatalytic reactions. In 1983, Kiwi reported magnetic field effects on a photosensitized electrontransfer reaction in the presence of TiO2 and CdS loaded particles.6 In this report, a suspension of TiO2 and CdS particles was used and a decrease in H2 evolution was found to occur in the presence of magnetic fields below 0.4 T.

To date, many photocatalytic reactions have been carried out with suspensions of

semiconductors particles. Under such inhomogeneous, opaque conditions, however, the

efficiency of the energy conversion is much smaller than that under homogeneous, transparent

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conditions. In our preliminary results on the photocatalytic decomposition of tert-butyl alcohol with platinized TiO2 particles (100-300 ím), a magnetic field effect on the product yield was observed, but the decomposition reaction was very slow and the experimental errors in the product yields were as large as 5-10%.7

To enhance the reaction efficiency and to avoid poor experimental reproducibility, we synthesized ultrafine colloidal TiO2 particles substituted with a hydrophobic coupling agent and studied the magnetic field effect on the photocatalytic decomposition reaction of tert-butyl alcohol with these particles.

In this paper, we report that an increase in the yield of one of the main products, acetone, was observed with the application of a 1.5 T magnetic field. Therefore, it seems possible that the efficiency of the energy conversion in the process might be enhanced by ordinary magnetic fields.

Experimental Section

The ultrafine colloidal TiO2 particles were synthesized by the coupling of titanium

tetraisopropoxide (TTIP) and isopropyl tris(dioctyl pyrophosphate) titanate (coupling agent) in aqueous ethanol as described in the literature.8 In this work, the molar ratio of the coupling agent and TTIP was 0.1 and the mixed

solution was refluxed for 5 h. The TiO2 particles obtained were dispersed and transparent in methanol, ethanol, tert-butyl alcohol, and acetone. The particle sizes were measured to be 21-43 and 25 nm (mean diameter of the area distribution) by TEM and DLS (dynamic light scattering), respectively. Magnetic fields (B) of up to 1.5 T were provided by a Tokin

SEE-10W electromagnet. The lowest magnetic field, generated by applying a counter-current to cancel the residual field, was less than 0.05 mT. Hereafter, the experiments under the lowest field are denoted as those in the absence of a magnetic field. Photocatalytic Reaction with Ultrafine TiO2 Particles

To measure magnetic field effects on photocatalytic reactions with ultrafine TiO2 particles, we choose the photocatalytic decomposition reaction of neat tert-butyl alcohol, as it has a simple and well-studied reaction mechanism.9 The synthesized ultrafine colloidal TiO2

particles (typically 15 mg) and tertbutyl alcohol (2 mL) were placed in a quartz cell (10 mm) with a PTFE-rubber septum. Each of the solutions was irradiated with a 500-W deep UV lamp at room temperature, in the absence and presence of magnetic fields. The lamp intensity was continually measured by a power meter and its fluctuation was within 1%. UV-vis spectra were recorded with a Shimadzu UV-2550 spectrometer.

At 1 h after irradiation, the products were analyzed by GLC and GC-MS. Acetone and

methane were generated as the main products. Although hydrogen is believed to be generated, its yield is very low under the present nonaqueous conditions. Quantitative analysis of acetone was carried out by GLC as

follows: A Shimadzu GC-14B gas chromatograph coupled with a Shimadzu AOC-20i

auto-injector was used with a RESTEK Stabilwax-DB capillary column (15 m, 0.25 mm ID, 0.25 ím df). The chromatograph was recorded with a Shimadzu chromatopac C-R6A integrator. The yield of acetone (Y) was determined with decane as an internal standard. Results and Discussion

Reaction Scheme. In the present reaction, the electron (e-) and the hole (h+) are generated by irradiation of TiO2 (eq 1). Oxidation of tert-butyl alcohol occurs by reaction with the

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photogenerated hole to generate its cation radical (eq 2). The cation radical decomposes to a 2-propanol cation and a methyl radical (eq 3). There is also the possibility that the cation radical decomposes to a 2-propanol radical and a methyl cation. Since the mass fragment of the 2-propanol cation is much larger than that of the methyl cation, from the GC-MS analysis of tertbutyl alcohol we are able to conclude that the decomposition of a cation radical to a 2-propanol radical and a methyl cation is less dominant than reaction 3 in the present reaction. Next, the 2-propanol cation is reduced by an electron to form a 2-propanol radical (eq 4). Finally, a disproportionation reaction occurs between the 2-propanol and methyl radicals to form acetone and methane (eq 5).

Magnetic Field Effect. Quantitative analysis of acetone was carried out by GLC. The photocatalytic decomposition reaction of tert-butyl alcohol with the ultrafine colloidal TiO2 particles was performed in the absence and presence of an external magnetic field of 1.5 T. The yields of acetone (Y) observed are listed in Table 1. The relative magnetic field effect observed at 1.5 T, R(1.5 T), is represented as follows:

The R(1.5 T) values obtained are also listed in Table 1. This table shows that the yield of acetone increases by about 10% with increasing magnetic field from 0 to 1.5 T. To the best of our knowledge, this is the first report of the magnetically induced acceleration of a photocatalytic reaction. Although Kiwi reported

magnetic field effects on photosensitized electron-transfer reactions in the presence of TiO2 and CdS loaded particles, the magnetic field decelerated the evolution of H2. Furthermore, Kiwi found no magnetic field effect on the direct photolysis with TiO2 and CdS. The effects were only observed in the presence of Ru(bpy)3 2+.6

Mechanism of the Magnetic Field Effects. To clarify the mechanism of the magnetic field effect, we measured the magnetic field dependence of the relative magnetic field effect R(B) for fields between 0 and 1.5 T. The observed R(B) values are plotted against B in Figure 1. It is clear that the R(B) value

gradually increases with increasing B from 0 to 1.5 T. The observed magnetic field effects may occur during either the reencounter of free radicals5 or the recombination of electrons and holes.

First, consider magnetic field effects on the recombination of free radicals. The triplet and singlet radical pairs are generally formed in a three-to-one ratio when the free 2-propanol and methyl radicals reencounter (free radical precursor). The singlet radical pair immediately reacts to form the disproportionation products of acetone and methane (cage products). Since the triplet radical pair cannot react directly, the radicals either escape from the triplet pair or undergo spin-state mixing to produce a singlet pair by the hyperfine coupling and the ￠g mechanisms (HFCM and ￠gM).5 In the case of the HFCM, the yield of cage products from a free radical precursor should decrease with increasing magnetic field.5 Moreover, magnetic field effects caused by the HFCM are usually saturated below 50 mT.5 As shown in Figure 1, however, the yield of acetone increases with increasing magnetic field and no saturation is observed below 1.5 T. From these results, we conclude that the present magnetic field effects are not due to the HFCM and that the HFCM can be safely excluded from consideration. In the case of the ￠gM, the yield of cage products from a free radical precursor should increase with increasing magnetic field. Thus, the yield of acetone should increase with

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