Photodegradation of Dichloromethane with Titanium Catalysts - ACS

Chapter 11

Photodegradation of Dichloromethane with Titanium Catalysts 1

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James F. Tanguay, Robert W. Coughlin, and Steven L. Suib 1

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Downloaded by FUDAN UNIV on January 26, 2017 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch011

Department of Chemistry and Department of Chemical Engineering, University of Connecticut, Storrs, CT 06268

Titanium dioxide and titanium incorporated into alumino -silicate photocatalysts have been studied for the photode -gradationof dichloromethane into carbon dioxide and HCl. Different forms of titanium dioxide have been produced such as rutile and anatase as well as an amorphous form. Titanium pillared clays have been found to be more active than titanium clays. Carbon felt was also used as a sup port for titanium species and this material is the most ac tive we have studied. By enriching solutions of dichloromethane with oxygen or by pre-irradiating the ti tanium catalyst, faster rates of reaction and larger conver sions are obtained. Titania (Ti0 ) occurs in nature in three crystal modifications as anatase, rutile and brookite. Rutile is the most common form which has octahedral coordination of titanium ions. Anatase and brookite contain distorted octahedra. Anatase is thermodynamically 8 to 12 kJ/mol more stable than rutile (1). Brookite on the other hand is thermodynamically unstable. Titania is of great interest as a catalyst or photocatalyst. Matsudo and Kato have reviewed the catalytic (thermal) chemistry of Ti0 (2) . For photochemical activity, titania has been used to hydrogenate alkynes and alkenes (3), to oxidize H 0 (4), to oxidize ethylene (5), to oxidize 2-propanol (6), for amine production (Z) and in water splitting reactions (8). Another reaction of great concern is the photodechlorination of chlorinated hydrocarbons. Titania has been used as a photocatalyst in the heterogeneous catalytic decomposition of chloroform and dichloromethane to form carbon dioxide and HCI by Ollis and co-workers (9) These titania catalysts are particularly useful at low levels (parts per million) of chlorinated hydrocarbon. Titania catalysts require near 2

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0097-6156/90/0437-0114$06.00/0 © 1990 American Chemical Society Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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ultraviolet light to degrade such hydrocarbons. One of the common problems of such systems, however, is the suspension of titania and reactant which is not easily separated. This paper deals with a comparison of the activity of various titania catalysts in the photodegradation of dichloromethane. In addition, we will report on the possibility of supporting titania on a carbon felt to ease the separation of catalyst and reactant/product. Finally, the importance of oxygen and pre-irradiation of the catalyst will be reported. EXPERIMENTAL Clays and pillared clay materials were prepared by adding Til or TiCI to aqueous solutions and ion exchanging with the clay or incorporating titanium ions into the aluminochlorohydrate pillar precursor prior to pillaring. Titanium oxide was deposited onto carbon felt by acidifying titanium isopropoxide and forming a sol into which the carbon felt was dipped. Pure titania of the amorphous, anatase, and rutile phases were prepared by treating titania sols at 70°C, 375°C , and 850°C, respectively. Solutions of 1 mL dichloromethane and 100 mL distilled deionized water were irradiated in the presence and absence of titania catalysts with a 1000 watt Xe lamp. About 1 g catalyst was used in these studies. A Corning pH meter was used to monitor pH of the reaction. A 300 nm cutoff filter was used to filter out low wavelength ultraviolet light. The conversion of dichloromethane was monitored with gas chromatography methods.


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RESULTS Results of photocatalytic experiments are given in Table I. The percent conversion was calculated from the observed amount of CI in solution after photolysis. Table I. Conversion of Dichloromethane Catalyst Amorphous Anatase Rutile Anatase on Felt Amorphous Titania on Felt Pre-irradiated Rutile Titania Pillared Clay Pre-irradiated Pillared Clay Rutile, Oxygen Bubbling Rutile, Nitrogen Bubbling

% Conversion 15 37 59 29 6 77 9 10 83 41

Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS

The pre-irradiation lasted for two hours in all cases and all measurements were taken after four hours of photolysis. Oxygen bubbling was done by attaching an oxygen tank, regulator and flow meter to the photolysis apparatus. A plot of the chloride production of pre-irradiated titania pillared clays is given in Figure 1 for samples with no pre-irradiation and for those irradiated for one and two hours.


Data for a fresh anatase catalyst and a used anatase catalyst are given in Figure 2. The amount of chloride produced from dichloromethane as a function of time for both materials is given there. At long periods of time, the used material shows less chloride ion production than the fresh catalyst. DISCUSSION The data of Figure 1 clearly show that irradiation of the titania pillared clay systems leads to an enhancement in the moles of CI produced during photodegradation of dichloromethane. Prolonged pre-irradiation leads to lower conversion than shorter periods of pre-irradiation. This is a general result and was found for all supported systems studied here. The significance of pre-irradiation is that defect sites must be created at the titania surface during irradiation that serve as adsorption sites for the dichloromethane. These may be sites for dissociation of CI since we have found (10) that poisoning of various substrates by NaCI causes a general decrease in overall conversion. Prolonged irradiation may cause degradation of these activated sites. Data for several titania systems are given in Table I for the photodegradation of dichloromethane. First of all, data for unsupported titania should be compared. These data suggest that the rutile form of titania is more active than the anatase form which in turn is more active than the amorphous titania. In general this is also true for the supported analogs. Reasons why an ordered high temperature form is more active than a low temperature or amorphous form may be due to the propensity of the former to form defect adsorption sites (See below). For the amorphous and anatase forms supported on carbon felt, the percent conversion is markedly higher for the anatase form than the amorphous form. It was not possible to produce the rutile form on the carbon felt due to degradation of the felt during thermal activation at the high temperature ( > 600°C) needed to produce rutile. Data for pre-irradiated rutile and titania pillared clay clearly show that pre-irradiation causes the % conversion to increase with respect to non-irradiated materials as discussed earlier. Data in Table I for oxygen bubbling of rutile versus nitrogen bubbling show that oxygen can enhance the overall % conversion of dichloromethane into HCI and C0 . These data may indicate that the rutile surface provides oxygen for the oxidation of the carbon part of the chlorinated hydrocarbon and that lattice oxygen is then replaced with gas phase oxygen that absorbs at the surface. Data of Figure 2 clearly show that the used anatase catalyst produces less CI than the fresh catalyst. Poisoning studies with titania 2



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TIME (mm)

Figure 1. Chloride production of Pre-irradiated Titania Pillared Clays, 750 Watts Power, Wavelengths of 300 nm to 800 nm. (a) 1 Hr Pre-irradiation, (b) 2 Hr Pre-irradiation, (c) No Pre-irradiation.

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Figure 2. Chloride production from Used Anatase: (A) Fresh catalyst, (B) Catalyst in 2A, washed with distilled deionized H 0. 2


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catalysts treated with NaCI suggest that CI ions adhere to the surface of titania and react with sites that are active in the photodechlorination process. In our systems it appears that CI ions are produced during the photodegradation and then poison active sites. Ollis and co-workers have suggested that dichloromethane absorbs as an ion by reacting with a hole on the titania surface (9). Other studies have shown, however, that both CI" and H inhibit the degradation of simple hydrocarbons (H). These inhibitors could influence the Bronsted acidity of the titania surface. This could lead to a different mode of binding of dichloromethane and other hydrocarbons to the surface such as by abstraction of H from the hydrocarbon as well as binding of a hydrocarbon anion to the surface of titania (i.e. as CHCI ). In this case, H could be bound to lattice oxygen and the anionic hydrocarbon fragment could be bound to Ti in the lattice. This mech anism is supported by the data of Ollis and co-workers (9). Our pre-ir radiation data can be explained by the fact that light consists of an electron-hole pair that can form on the titania surface during irradiation. Pre-irradiation of the surface of titania leads to a build-up of such defect sites that are not initially destroyed before reaction with dichlorometh ane since there is no chloride ion around. +

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CONCLUSION We have shown here that different crystalline forms of titania as well as different supports for the titania influence the overall reactivity of Ti0 in the photodechlorination of dichloromethane. The importance of pre-ir radiation to produce defect centers on the surface has also been ob served. Oxygen plays a role in these photodegradations since saturation of the solution with oxygen enhances the rate of photode gradation. Chloride ions poison Ti0 during irradiation. Further studies should focus on removing CI ions from the surface. 2

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Literature Cited 1. Cotton, F. Α.; Wilkinson, G. Advanced Inorganic Chemistry, Fourth Edition, John Wiley and Sons, NY, 1980. 2. Matsudo, S.; Kato, A. Appl. Catal. 1983, 8, 149-165. 3. Anpo, M.; Aikawa, N.; Kadama, S.; Kubukawa, Y. J. Phys. Chem. 1984, 88, 2569-2572. 4. Rivers-Arnau, V. J. Electroanal. Chem. 1985, 190, 279-281. 5. Gonzalez-Elire, A. R.; Che, M. J. Chim. Phys. 1982, 79, 355-359. 6. Henglein, A. Ber. Buns. Phys. Chem. 1982, 86, 241-246. 7. Miyama, H.; Nosaka, Y.; Fukushima, T.; Toi, H. J. Photochem. 1987, 36, 121-123. 8. Grätzel, M.; Borgarello, Ε.; Kiwi, J., Pelizetti, E.; Visca, M. J. Am. Chem. Soc. 1981, 103, 6324-6329. 9. Ollis, D. F.; Hsiao, C-Y.; Lee, C-L. J. Catal. 1983, 82, 418-423. 10. Tanguay, J. F.; Coughlin, R. W.; Suib, S. L. J. Catal. 1989, 117, 335-347. 11. West, A. R. Solid State Chemistry, John Wiley and Sons, NY, 1987. RECEIVED May 9, 1990 Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Photodegradation of Dichloromethane with Titanium Catalysts - ACS

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