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Mechanochemical Dechlorination of Trichlorobenzene on Oxide Surfaces Yasumitsu Tanaka, Qiwu Zhang,*,† and Fumio Saito† Material Solution DeVelopment Department Recording Media Company, Sony Sendai Technology Center, 3-4-1 Sakuragi, Tagajo-shi, Miyagi-ken, Japan ReceiVed: December 10, 2002; In Final Form: July 21, 2003
1,2,3-Trichlorobenzene (TCB) was ground with calcium oxide (CaO) powder in air with a planetary ball mill. A mechanochemical reaction was induced, resulting in the decomposition of TCB through dechlorination from the benzene nucleus. The mechanochemical dechlorination was monitored by a suite of analytical methods including X-ray diffraction (XRD), thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, electron spin resonance (ESR), gas chromatography/mass spectrometry (GC/MS), and ion chromatography. With the increase in grinding time, the remaining amount of TCB decreased and reached 0.02% at 360 min grinding. On the other hand, the water-soluble amount of chlorine increased correspondingly and reached 95%, indicating the successful transformation of chlorine from organics into inorganic chloride. Additionally, the other final products detected after grinding were carbon and some minor methane and ethane, although the formation of some chlorine-containing intermediate phases such as dichlorobenzene and tetrachlorobenzene was observed in the early stage of grinding (for 1 h). A possible decomposition pathway based on dechlorination or dehydrochlorination is compared.
Introduction It is said that more than 10 million different chemicals have already been produced and the number is increasing annually. Concern over chlorinated compounds is increasing due to their toxicity and significant ecological hazards. The most extensively used method to destroy unwanted chlorinated compounds and wastes is incineration; however, there is another concern about the production of toxic byproducts such as chlorine-containing furans and dioxins. Numerous other methods have been studied to decompose the compounds at lower temperatures or by a nonthermal method by means of different types of energy.1-12 For example, it has been reported that the destructive adsorption of chlorinated hydrocarbons on the nanoparticles of metal oxides (SrO, CaO, or MgO) is vastly superior to that by commercial oxides, and that a fast destructive reaction is obtained at relatively low temperature (573-773 K) and the byproduct containing chlorine can be eliminated by using an excess of oxides to give carbon dioxide and metal chlorides as the final decomposition products.13-18 Another interesting phenomenon occurring on the metal surface during the optically and radiation-induced catalytic dechlorination is the charge transfer observed with photocatalysis19-22 and radiolysis23-25 from the excited oxides such as alumina and silica to the absorbed molecules. A novel dechlorinating method by applying mechanical energy26-30 has attracted attention due to the advantage of its simple operation by using a ball mill, especially in the case where a great deal of waste or soil contaminated by toxic chlorinated compounds at quite low concentrations requires disposal. Charge separation occurs during the grinding, and it is expected that the charge transfers from metal oxides to the chlorinated absorbers contribute to the chlorination in a way * Corresponding author. † Institute of Multidisciplinary Research for Advanced Materials (IMRAM),Tohoku University, Katahira 2-1-1, Sendai, Japan.
similar to that of photoinduced and radiation-induced dechlorination. It is also worth noting that nanoparticles of the oxides can be easily obtained in situ by grinding, although the primary nanoparticles are generally agglomerated into micron sizes.31-33 Considering the close contact in nanoscale of chlorinated compounds and metal oxide additives, the grinding operation provides a catalytic effect on the surface of nanoparticles without the need for elaborate preparation of nanosized materials. It is reasonable to believe that the mechanochemical dechlorinating process exhibits some advantages; however, there is still little information about the mechanism. It is easy to imagine that the mechanism would be very complex, possibly involving various pathways. When the problem is simplified, for example, as to whether it is a dechlorinating reaction or a dehydrochlorinating one when chlorinated compounds are decomposed by grinding with metal oxides, the controversial issue remains and needs further work. Clear understanding of the reasonable pathway is helpful for further research to improve the mechanochemical process and to reveal the corresponding reaction relationship between the structures of chlorinated organic compounds and inorganic additives. In this work, a trichlorobenzene sample was chosen as the target and ground with calcium oxide, presuming that it would dehydrochlorinate during grinding as follows, resulting in the most amount of possible product of CaOHCl among chlorinated aromatic compounds.
C6H3Cl3 + 3CaO ) 3CaOHCl + 6C
(1)
Excess addition of CaO (4 times based on eq 1) was used to facilitate the mechanochemical reaction and to assure that the neutralizing capacity of the oxide was enough to absorb the hydrochloric composition. However, the experimental details do not seem to support this pathway and a dechlorination one is proposed.
10.1021/jp0276808 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/12/2003
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Experimental Section All reagents except for specific descriptions were obtained from Wako Pure Chemical Industries, Ltd., Japan, and used as received. The results of the preliminary experiment showed that there is little difference between 1,2,3-trichlorobenzene (1,2,3TCB) and 1,3,5-trichlorobenzene (1,3,5-TCB). 1,2,3-TCB was used and ground with calcium oxide (CaO), which was prepared by heating calcium hydroxide (Ca(OH)2) at 1073 K for 2 h in an electric furnace in an air atmosphere. The TCB was mixed with the CaO powder at a 1:12 molar ratio. A planetary ball mill (Fritsch Pulverisette-7, Germany) was used to grind the starting mixtures. Two grams of the mixture (0.425 g of TCB + 1.575 g of CaO) was put in a zirconia pot with an inner volume of 45 cm3 and seven zirconia balls of 15 mm diameter. The grinding was operated at 700 rpm in air or with argon sweeping before grinding for different periods of time. To prevent excess heating, the milling was stopped 15 min for every 15 min of grinding operation. The ground samples were characterized as follows. X-ray diffraction (XRD) (RAD-B system, Rigaku, Japan) using Cu KR radiation was used to identify phases formed after grinding. Thermogravimetric analysis (TG) (Rigaku TAS200) was conducted at a heating rate of 10 K/min in air. Infrared spectra of the ground samples were measured using a Fourier transform infrared (FTIR) spectrometer (Bio-Rad FTS-40A) with the KBr disk method. Raman spectra were recorded at room temperature using a Labspec Raman spectrograph (Horiba) with a He laser beam at the 632 nm line. Samples for electron spin resonance (ESR) analysis were charged in a quartz tube of 5 mm diameter, and measurements were carried out with an X-band ESR spectrometer (Bruker ESP-380E). Besides the characterization of the ground solid sample, the gas composition was also analyzed by gas chromatography/mass spectrometry (GC/MS). The gas was drawn by a syringe just after the mill was stopped and inserted into the spectrometer for analysis using a Hewlett-Packard gas chromatograph (Model 6890) equipped with a mass-selective detector (Model 5973). The following further treatments of the ground samples were performed for additional analysis. The ground samples were heated at 803 K for 60 min, and the crystalline phases formed in heated samples were determined by X-ray diffraction (XRD) analysis. The ground samples were also washed with water and some organic solvents, respectively. An 0.5 g sample was agitated in 100 mL of distilled water or 50 mL each of organic solvent such as toluene (C6H5CH3), acetone (CH3COCH3), ethyl alcohol (C2H5OH), ethyl acetate (CH3COOC2H5), or hexane (C6H14) with a magnetic stirrer for 30 min to extract chemicals into the solution. After filtration, the filtrates were subjected to an ion chromatograph (IC) (LC10 series, Shimadzu Co. Ltd.) and GC/MS analysis (the same spectrometer as the above with a different column), respectively. The yield of soluble chlorine in water was obtained by measuring the chlorine concentration in the filtrate. The remaining percentage of TCB in the ground sample was obtained by measuring the remaining amount of TCB from toluene washing. The identification of possible products was attempted by changing the solvents from the polar to the nonpolar. The data obtained by washing with acetone were used in this paper unless otherwise specified. Results Decomposition of TCB. In Figure 1, the change in the remaining yield of TCB with grinding time is shown. It is clear that the yield decreases rapidly with the increase in grinding time and only 0.02% of the initial TCB is detected in the sample
Figure 1. Change in yield of the remaining TCB with grinding time.
Figure 2. Yield of water-soluble Cl as a function of grinding time.
ground for 6 h. For confirmation, the 6 h grinding test was repeated and washed with various solvents. It was found that the remaining yields of TCB vary from 0 to 0.3% at maximum within the experimental range. The water-soluble chlorine was measured, and the results are shown in Figure 2. The yield of the soluble chlorine increased rapidly with the increase in grinding time and reached 95% with the sample ground for 6 h. The results obtained together clearly indicate that TCB was decomposed through a mechanochemical reaction and the chlorines were transformed from the aromatic chlorides into water-soluble inorganic chlorides. Inorganic Products after Decomposition. As the only inorganic compound used, CaO was observed in the ground samples from the XRD patterns (not shown here). However, there were no other peaks observed except those corresponding to the CaO phase. This means that any other reaction products formed after the grinding operation were amorphous ones. The ground samples were heated at 803 K for 1 h in order to obtain some crystalline phases. Figure 3 shows the XRD patterns of the 1 and 6 h samples ground and then heated, respectively. Although the peak intensity of the products in the 1 h ground
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Figure 3. XRD patterns of 1 and 6 h ground mixtures of TCB and CaO, subsequently calcined at 803 K for 1 h.
Figure 5. GC/MS spectra of products extracted by acetone from ground samples. A, 1,2,3-TCB; B, 1,3,5-TCB; C, DCB; D, 1,2,4,5-TeCB.
Figure 4. TG curves of ground samples for different periods of time.
sample is much lower than that of 6 h, the same crystalline productsscalcium hydroxide chloride (CaOHCl) and calcium carbonate (CaCO3)sare identified as having formed in the calcined samples. Since the peak intensity of the products by calcination is found to correspond to the degree of mechanochemical dechlorination, it is reasonable to say that the formation of these products is related to the dechlorination reaction, although not the direct products. Figure 4 shows the TG curves of the ground samples. The weight loss in the region of 373423 K attributed to the vaporization of TCB is observed in the curves of the sample ground within 2 h. The weight loss due to the TCB vaporization has disappeared from the curves of the sample ground over 4 h, indicating the absence of the free TCB after the prolonged grinding. Instead, a weight increase is clearly observed until 873 K, and the longer the grinding time is, the greater this weight increases. The final weight loss at over 923 K can be attributed to the decomposition of the formed calcium carbonate. If the TG analysis is conducted under N2 gas stream, only a weight loss is observed. Combined with the results shown in Figure 3, it is understood that the formation of carbonate contributes to the weight increase because CO2 is not contained
in the reaction system before calcination. The only possible reason for the formation of carbonate, namely CO2 formation and later absorption by CaO, is due to the oxidization of carboncontaining materials in the ground sample calcined in the air. As shown in many related reports, the ground sample typically consists of an agglomerate with the primary particles in nanoscales. The close contact within the nanoscale, the possible chemical absorption between the constituents of the agglomerates, and the excess existence of CaO contribute to the easy absorption of the CO2 gas during calcination to form a carbonate. On the other hand, even if the complete dechlorination was not achieved at 6 h grinding, the subsequent calcination at low temperature will totally decompose the chlorinated compounds because the grinding operation not only produces nanosized oxide particles in situ but also homogenizes the oxide sample and the absorbate within the nanorange at the same time. Organic Products after Decomposition. Both of the gases in the pot and the products extracted from the ground samples were identified by GC/MS analysis. Figure 5 shows the spectra of products extracted by acetone. In the spectrum of the sample ground for 1 h, small peaks corresponding to dichlorobenzene (DCB) and tetrachlorobenzene (TeCB), besides those of TCB and column, have been observed. The peaks due to DCB and TeCB disappear from the spectrum of the sample ground for 6 h, where only a small peak from TCB is detected. The peak area is calculated at almost 0% that of the initial TCB. Considering that some products formed may not dissolve in acetone, different solvents such as toluene, ethanol, ethyl acetate, and hexane were used to wash the ground samples. However, in any case, no other new intermediate phases could be detected from the ground samples excepting the above intermediate phases. Although the chlorinated aromatic compounds have been obtained as the intermediate phases in the early stage of grinding (1 h), they are finally decomposed by the prolonged grinding (6 h). Figure 6 shows the spectra of the gases collected from the pot immediately after grinding. Compared with the results of solid samples, similar ones (not shown here) have been
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Figure 7. FTIR spectra of mixture grinding samples for different periods of time.
Figure 6. GC/MS spectra of gases collected from mill pot headspace immediately after 1 and 6 h grinding.
observed: chlorinated aromatic compounds are observed to occur as the intermediate phases when the sample is ground for 1 h, and these phases have disappeared when the grinding time is extended to 6 h. Different results show that some light gases such as methane, ethane, and CO2 have been found to occur in the pot, irrespective of the grinding time. A comparative test was performed by injecting Ar gas into the pot before starting the operation (compared to that without Ar gas substitution). Almost the same results were obtained, clearly indicating that these light gases are the products of a mechanochemical reaction. An atmosphere of air or Ar gas does not make a difference for the reaction products. Evidently the dechlorination induces considerable instability in the benzene ring structure. Absorption on oxide to form new bonding, shown later in the following IR spectra, is the most plausible way for stabilization. However, there exists the other possibility for some to go through a complete splitting of the aromatic structure to produce these light gases as well as carbon due to the lack of sufficient hydrogen. FT-IR and Raman Monitorings of the Ground Samples and Subsequently Calcined Ones. Figure 7 shows the IR spectra of ground samples. The spectra are typical of the large peaks from inorganic compounds. The large broad peaks in the region from 1650 to 1350 cm-1 due to the existence of calcium carbonate and hydroxide/hydrate in the samples make it difficult to identify the bands related the aromatic structure lying in the region. However, several small peaks positioned in the range from 1800 to 1650 cm-1 (1800, 1778, 1765, 1711, 1685 cm-1) are observed in the spectrum of 1 h ground sample. These peaks can be assigned as CdO stretches or C-Cl overtone bands,
Figure 8. FTIR spectra of 6 h grinding and subsequently 1 h calcined at 803 K sample.
with the former more reasonable, implying basically that the organic phase still exists in the 1 h ground sample. The CdO stretches strongly suggest that the C atoms in organics (TCB or intermediates) have been bound to O atoms in inorganics (oxide). It is also worth mentioning that these peaks are not observable in the spectrum of 6 h ground sample. The different states of organics after prolonged grinding may be explained by that the dehydration occurs with more formation of C-O bonding after the complete dechlorination.34 Furthermore, the formation of CaCl2 will facilitate the dehydration due to its strong tendency to absorb water to form a hydrated one. The dehydration leads to the cutoff of C-O bonding, and the subsequent disappearance of CdO bonding peaks from the spectrum of 6 h ground sample. The ground sample (6 h) and the calcined one were compared in order to understand whether water-related phases are in a hydrated or hydroxide state. The IR spectra are shown in Figure 8. In the spectrum of the ground sample, the OH stretch, in the shape of a very broadened peak positioned from about 3650 to about 3300 cm-1, is observed, indicating that strong hydrogen bonding occurs in the phases, the typical pattern of a hydrated chloride/bromide such as CaCl2‚nH2O.35 On the other hand, after calcination at 803 K, this peak is reduced to a relatively small
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Figure 10. ESR spectra of ground samples.
Figure 9. Raman spectra of ground samples and calcined samples.
degree, suggesting the hydrogen bond has been weakened. However, the intensity of a sharp peak positioned around 3750 cm-1 increases to some degree. This sharp one is assigned to the stretch band of the free OH, more possibly resulting from a hydroxide. The results suggest that calcining treatment causes the transformation from a hydrated into a hydroxide state. Figure 9 shows the Raman spectrum of the ground sample. The Raman shifts at 1570 and 1312 cm-1 bands are observed, and the peak intensity rises with the increase in grinding time. Although a little lower than others reported, the two peaks are typical of amorphous carbon. The sharp peak (1570 cm-1) is the graphite band and the broadened one can be assigned to the finite in-plane domain size,36,37 with other interpretations of the simple D band.38 XRD analysis does not offer evident information about the carbon sample, suggesting that the small domains with limited quantity are below the limits of X-ray detection. Their presence is detected through the 1312 cm-1 peak of Raman spectroscopy. Also, the widened pattern implies a broad distribution of the formed domain sizes. The results from Raman spectroscopy clearly show the carbonization of the organic phases with the progress of decomposition reaction. In fact, the color of the ground samples becomes black with an increase in grinding time, physically confirming the formation of carbon. The carbonization can be attributed to the dehydration of the organic phases with CdO bonding. On the other hand, these broadened peaks due to carbons disappear from the spectrum of the calcined sample. The spectrum is the same as that of the CaO sample, with one sharp peak corresponding to CO3 in the carbonate. The results indicate that the oxidization of the carbons results in the formation of carbonate during calcination, consistent with the results by XRD and TG analyses shown in Figures 3 and 4, respectively. Figure 10 shows the ESR spectra of the ground samples. Two radical peaks are observed and the intensity of the broad one increases with a longer grinding time, while the sharp one remains unchanged, the same phenomena reported in the case where CaO was ground with chlorobiphenyl.30,39 It is understood that the charge transfer induced by the grinding operation is favorable for the dechlorination occurring in the mechanochemical decomposition of chlorinated compounds. Similar phenom-
ena have been reported in other research involving radiationinduced and optically induced dechlorinating reactions. Discussion The above experimental results have clearly indicated that chlorinated compounds such as TCB can be decomposed by the grinding operation, and that chlorine is ultimately transformed into water-soluble chloride. There is a need to determine the pathway of how the TCB is decomposed based mainly on whether by dehydrochlorination or by dechlorination. First, if dehydrochlorination occurs as the main pathway, CaOHCl will be the ideal product. A previous report has confirmed the formation of CaOHCl during the mechanochemical reaction between poly(vinyl chloride) (PVC, (CH2CHCl)n) and CaO. It is found that this compound is generally stable against grinding and exhibits a clear crystalline state even with prolonged grinding over 12 h.40 However, no such crystalline phase of CaOHCl was observed in the ground sample. Although CaOHCl is observed in the calcined sample shown in Figure 3, it may result from calcination, not directly after grinding, similar to the formation of calcium carbonate. Another example of easy dehydrochlorination is 1,1,1-trichloroethane,15 which undergoes HCl elimination around 403 K. In fact, when PVC sample is heated, it undergoes dehydrochlorination below 573 K to form polyethylene. This may help to understand the dehydrochlorination occurring by grinding with the oxide. Furthermore, if dehydrochlorination occurs to form CaOHCl as the product, there exists no need for Ca-O bonding to disconnect. This implies less chance for the new bonding between C and O to form. The results from IR analysis, where CdO bonding has been observed, do not support the dehydrochlorination pathway, consistent with the XRD analysis results. More important are the intermediate phases observed in the 1 h ground sample. Although in very tiny amounts, the formation of dichlorobenzene and tetrachlorobenzene provides important evidence that dehydrochlorination seems to be impossible. Such a substitution of H by Cl seems to be possible with dechlorination as the pathway. Generally, the radical Cl formed by dechlorination from TCB is trapped by CaO oxide to form chloride. It is still possible for some Cl radicals to have a chance to contact other TCB molecules and to replace H to form tetrachlorobenzene, accompanied by the formation of dichlorobenzene through the uptake of the substituted H. If HCl is
11096 J. Phys. Chem. B, Vol. 107, No. 40, 2003 formed through dehydrochlorination, it seems more difficult to expect that a stable HCl can replace H from TCB to form tetrachlorobenzene and H2 as a byproduct. The formation of CH4 and C2H6 does not support the dehydrochlorination pathway, either, because TCB has the same number of H and Cl atoms so that only carbon, not hydrocarbon, can be finally formed if dehydrochlorination occurs. The existence of radicals also offers more evidence that dechlorination is more reasonable than dehydrochlorination. The dechlorination pathway means the separation of C-Cl bonding, leaving free radicals in the ground sample. The dehydrochlorination pathway implies that both C-Cl and C-H bondings are cut off simultaneously to form HCl absorbed by CaO as CaOHCl and an intermediate phase with a structure similar to benzyne. This will leave less chance for the formation of free radicals. Other reports have indicated that poly(tetrafluoroethylene) ((CF2)n) and hexabromobenzene (C6Br6) can be decomposed by grinding with the oxides to form alkaline earth metal fluoride and bromide, respectively.35,41 Successful debromination and defluorination occur as extreme examples where dehydrohalogenation is impossible without the existence of hydrogen. In general, when TCB and CaO are ground together, the TCB is absorbed on the surfaces of the oxide. The grinding operation induces excitations in metal oxide particles and a subsequent charge transfer to TCB molecules absorbed at the oxide surfaces. The dechlorination occurs through the charge transfer induced by grinding from the inorganic oxide to the organic phases. After dechlorination and the formation of Ca-Cl bonding, C-O bonding arises instead. Part of the dechlorinated TCB molecules may go through a complete decomposition of their structure to form CH4, C2H6, carbon, etc. The recombination of some radicals with TCB and dechlorinated TCB molecules to form some CB, DCB, and TeCB as the intermediate phases is also apparent. As the grinding progresses and dechlorination proceeds, the oxygen tends to bind carbon strongly, therefore allowing Ca more easily to take chlorine to form chloride. The dehydration will occur with the progress of mechanochemical dechlorination and the subsequent formation of C-O bonding. This again results in the formation of carbon and the hydration of the chloride. The main components as the final reaction products are carbon and calcium chloride hydrate. As to the products formed in the calcined sample, the calcination leads to the crystallization of CaOHCl from the chloride hydrate and CaO. The formation of calcium carbonate comes from the absorption of CO2 by CaO, after carbon is oxidized. Based on the discussion with dechlorination as a more reasonable pathway than dehydrochlorination, further research on the mechanochemical decompositions of chlorinated aromatic compounds, such as the search for more efficient inorganic additives, can be performed toward assessing those oxides from which chlorides and oxychlorides are easily formed and stable against grinding, without focusing on those oxides from which formations of metal hydroxide chlorides (MOHCl) through the so-called dehydrochlorination occur. Conclusions Trichlorobenzene, as an example of chlorinated compound, can be decomposed by dry grinding with CaO, with calcium chloride hydrate and carbon as the main final products besides the excess of CaO, achieving the goal of transforming toxic organics into inorganics that are safe and can be stored without problem or can be used in appropriate fields. The prolonged grinding (6 h) results in calcium chloride hydrate and carbon
Tanaka et al. as the main products through dechlorination and subsequent dehydration. The successful decomposition of TCB in 6 h ground sample with a weight ratio as high as 1:3.7 to the additive oxide indicates that the grinding offers an effective method for treating wastes and soils contaminated with chlorinated compounds at low concentrations. Acknowledgment. The authors thank Dr. Ikoma, Tohoku University, for his assistance in the ESR measurements. References and Notes (1) Brunelle, D. J.; Singleton, D. A. Chemosphere 1983, 12, 183. (2) Erikson, M. D.; Swanson, S. E.; Flora, J. D.; Hinshaw, G. D. EnViron. Sci. Technol. 1989, 23, 462. (3) Woodyard, J. P. EnViron. Prog. 1990, 9, 131. (4) Roth, J. A.; Dakoji, S. R.; Hughes, R. C.; Carmody, R. E. EnViron. Sci. Technol. 1994, 28, 80-87. (5) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. EnViron. Sci. Technol. 1995, 29, 439. (6) Liu, Y.; Schwartz, J.; Cavallaro, C. L. EnViron. Sci. Technol. 1995, 29, 836. (7) Zhang, S.; Rusling, J. F. EnViron. Sci. Technol. 1995, 29, 11951199. (8) Ukisu, Y.; Iimura, S.; Uchida, R. Chemosphere 1996, 33, 15231530. (9) Matsunaga, M.; Miyamoto, K.; Iida, M.; Miyake, A.; Ogawa, T. J. Resources EnViron. (Japanese) 1998, 34, 751-756. (10) De Filippis, P.; Scarsella, M.; Pochetti, F. Ind. Eng. Chem. Res. 1999, 38, 380-384. (11) Zhang, G. M.; Hua, I. EnViron. Sci. Technol. 2000, 34, 15291534. (12) El-morsi, T. M.; Budakowski, W. R.; Abd-el-aziz, A. S.; Friesen, K. J. EnViron. Sci. Technol. 2000, 34, 1018-1022. (13) Li, Y. X.; Li, H.; Klabunde, K. J. EnViron. Sci. Technol. 1994, 28, 1248-1253. (14) Koper, O. B.; Lagadic, I.; Volodin, A.; Klabunde, K. J. Chem. Mater. 1997, 9, 2468-2480. (15) Koper, O.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1997, 9, 838848. (16) Koper, O.; Klabunde, K. J. Chem. Mater. 1997, 9, 2481-2485. (17) Decker, S.; Lagadic, I.; Klabunde, K. J. Chem. Mater. 1997, 10, 674-678. (18) Mishakov, I. V.; Bedilo, A. F.; Richards, R. M.; Chesnokov, V.; Vodolin, A. M.; Zaikovskii, V. I.; Buyanov, R. A.; Klabunde, K. J. J. Catal. 2002, 206, 40-48. (19) Mao, Y.; Thomas, J. K. J. Chem. Soc., Faraday Trans. 1992, 88, 3079-3086. (20) Thomas, J. K. Chem. ReV. 1993, 93, 301-320. (21) Mao, Y.; Zhang, G. H.; Thomas, J. K. Langmuir 1993, 9, 12991305. (22) Balko, B. A.; Tratnyek, P. G. J. Phys. Chem. B 1998, 102, 14591465. (23) Zacheis, G. A.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2142-2150. (24) Zaheis, G. A.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 4715-4720. (25) Raffi, J.; Gelly, S.; Barral. L.; Burger, F.; Piccerelle, P.; Prinderre, P.; Baron, M.; Chamayou, A. Spectrochim. Acta, Part A 2002, 58, 13131320. (26) Hall, A. K.; Rowlands, S. A.; Hart, R. J.; Ebell, G.; Donecker, P.; Street, R.; McCormick, P. G. Nature 1994, 367, 223. (27) Loiselle, S.; Branca, M.; Mulas, G.; Cocco, G. EnViron. Sci. Technol. 1997, 31, 261-265. (28) Hall, A. K.; Harrowfield, J. M.; Hart, R. J.; McCormick, P. G. EnViron. Sci. Technol. 1996, 30, 3401-3407. (29) Cao, G.; Doppiu, S.; Monagheddu, M.; Orru, R.; Sannia, M.; Cocco, G. Ind. Eng. Chem. Res. 1999, 38, 3218. (30) Zhang, Q.; Saito, F.; Ikoma, T.; Tero-Kubota, S.; Hatakeda, K. EnViron. Sci. Technol. 2001, 35, 4933-4935. (31) Zhang, Q.; Saito, F. J. Alloys Compd. 2000, 297, 99-103. (32) Zhang, Q.; Saito. F. J. Mater. Sci. 2001, 36, 2287-2290. (33) Lee, J.; Zhang, Q. Saito, F. J. Am. Ceram. Soc. 2001, 84, 863865. (34) Zhang, Q.; Lu, J.; Saito, F.; Baron, M. J. Appl. Polym. Sci. 2001, 81, 2249-2252. (35) Zhang, Q.; Matsumoto, H.; Saito, F.; Baron, M. Chemosphere 2002, 48, 787-793.
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