Langmuir 1996, 12, 2007-2014
2007
Photoassisted Degradation of n-Butyltin Chlorides in Air-Equilibrated Aqueous TiO2 Suspension J. A. Navio,*,† C. Cerrillos,† F. J. Marchena,† F. Pablos,‡ and M. A. Pradera§ Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla and Dpto. de Quı´mica Inorga´ nica, Facultad de Quı´mica, Universidad de Sevilla, 41012-Sevilla, Spain, Dpto. de Quı´mica Analı´tica, Facultad de Quı´mica, Universidad de Sevilla, 41012-Sevilla, Spain, and Dpto. de Quı´mica Orga´ nica, Facultad de Quı´mica, Universidad de Sevilla, 41012-Sevilla, Spain Received October 27, 1995. In Final Form: January 11, 1996X The photoassisted degradation of n-butyltin chlorides in air-equilibrated aqueous TiO2 suspension has been studied. Several factors affecting the degradation rate, such as the extent of adsorption on the surface of catalyst particles and the effect of pH values, were examined. Particular interest is focused on the TiO2-assisted photodegradation of n-butyltin species. The simultaneous photodeposition of tin oxides/ metallic tin on the TiO2 surfaces is also addressed as a route to form surface-complexed semiconductors. A mechanism consisting of interfacial trapping of a photogenerated electron-hole pair can explain the observed photoassisted degradation of n-butyltin species and the simultaneous photodeposition of tin onto the TiO2 surface. The results are compared with those, previously reported, on the photolytic degradation of these species in water.
1. Introduction Tin has more of its organometallic derivatives in commercial use than any other element.1 Particularly, tributyltin (TBT) compounds have come to be widely used as wood preservatives and in antifouling paints, such as in preventing the attachment of water organisms to immersed structures in sea water, and fishpond factories.2,3 Although inorganic tin compounds are basically harmless, some organotin species are very toxic to both animal and vegetable life. Thus, very low levels of TBT (about 1 µg L-1) are lethal or cause reduced growth and reproduction of commercial shellfish.4 In recent years, concern has grown about the impact of TBT and its degradation products in aquatic environments. This concern has generated several studies5-9 of the photodegradation of TBT in natural sunlight on the surface of waters and sediments. In a previous paper10 we provided data about the mode of UV-photolytic degradation of butyltin chlorides in water, and some of the breakdown products were identified. Our results10 also revealed the important role that the nature of the surrounding environment plays in the rate of photodegradation of these compounds. * Corresponding author. † Instituto de Ciencia de Materiales de Sevilla. ‡ Dpto. de Quı´mica Analı´tica. § Dpto. de Quı´mica Orga ´ nica. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Omae, I. Organotin Chemistry, J. Organomet. Chem. Libr. 1989, 21, Chapter 9. (2) Evans, C. J.; Hill, R. Rev. Silicon, Germanium, Tin Lead Compd. 1983, 7, 55. (3) Blunden, S. J.; Cusack, P. A.; Hill, R. The Industrial Uses of Tin Chemicals; The Royal Society of Chemistry: London, 1985. (4) Alzieu, C.; Heral, M.; Thiband, Y.; Dardignac, M. J.; Fauillet, M. Recl. Trav. Inst. Pech. Marit. 1982, 45, 101. (5) Sherman, L. R.; Yazdi, M.; Hoang, H. Proc. 4th Int. Conf. Organomet. Chem. Ge, Sn, Pb, Montreal, Quebec, 1983. (6) Maguirre, R. J.; Carey, J. H.; Hale, E. J. J. Agric. Food Chem. 1983, 31, 1060. (7) Maguirre, R. J.; Tkacz, R. J. J. Agric. Food Chem. 1985, 33, 947. (8) Seligman, P. F.; Valkirs, A. O.; Lee, R. F. Environ. Sci. Technol. 1986, 20, 1229. (9) Duhamel, K.; Blanchard, G.; Dorange, G.; Martin, G. Appl. Organomet. Chem. 1987, 1, 133. (10) Navio, J. A.; Marchena, F. J.; Cerrillos, C.; Pablos, F. J. Photochem. Photobiol. A: Chem. 1993, 71, 97.
0743-7463/96/2412-2007$12.00/0
Although several studies5-10 have proved the positive effect of UV illumination on environmental degradation of butyltin compounds, this process has its limitations. The maximum absorption wavelength of butyltin compounds is within the UV region (190-290 nm). Consequently, TBT species are degraded very slowly by natural sunlight. Moreover, the UV photodegradation of TBT leads to the formation of partially oxidized products from the organic groups attached to the tin atoms. Accordingly, new methods must be developed. Heterogeneous photocatalysis has been shown to be an attractive process for the degradation of many water pollutants.11 Powdered titanium dioxide, TiO2, is one of the most promising photocatalysts for the detoxification of organic or inorganic contaminants in the aquatic environment.11,12 This paper studies the degradation of butyltin compounds by photocatalytic action of powdered TiO2 under UV illumination in aqueous solutions under different experimental conditions. 2. Experimental Details Materials. Titanium dioxide, TiO2, photocatalyst (Degussa, P-25) was subjected to calcination at 500 °C for 24 h before use; a BET surface area of 46.5 m2 g-1, a molar fraction of anatase phase X1 ) 0.52, and a primary particle size of about 1-5 µm were found for the pretreated TiO2 sample.13 The photocatalytic degradation of pollutants over TiO2 in different crystal structures has been reported.14 It was found that, depending on the pollutant, the degradation rate increased with the calcination temperature up to 500 °C or was nearly independent of the calcination temperature over to 500 °C. Tributyltin chloride (TBT) (96%), dibutyltin dichloride (DBT) (97%), and butyltin trichloride (MBT) (95%) were supplied by Aldrich-Chemie GmbH & Co. K.G. The concentration of the compounds studied here (5 µg mL-1) was chosen considering (11) Schiavello, M., Ed. Photocatalysis and Environment, Trends and Applications; NATO ASI Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988; Vol. 237, p 469. (12) Herrmann, J. M.; Guillard, C.; Pichat, P. Catal. Taday 1993, 17, 7. (13) Navio, J. A.; Macias, M.; Gonzalez-Catalan, M.; Justo, A. J. Mater. Sci. 1992, 27, 30. (14) Rivera, A. P.; Tanaka, K.; Hisanaga, T. Appl. Catal. B: Environ. 1993, 3, 37.
© 1996 American Chemical Society
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Figure 1. Lamp’s spectral distribution and diffuse reflectance spectrum for TiO2 photocatalyst. their solubilities in water.15 All solutions were prepared using water from a Millipore Waters Q purification unit. In the experiments in which the effect of pH was investigated, the pH was adjusted with hydrochloric acid, sodium hydroxide, and sodium bicarbonate. All solvents used were HPLC grade, and other chemicals used (such as hydrogen peroxide and methanol) were reagent grade (Aldrich-Chemie). Methods. The photochemical experiments were performed using a Hanovia photochemical reactor (1 L). A medium-pressure mercury lamp (100 W), fitted with a synthetic high-purity quartz envelope, was used as light source. The light passed through a space between two thimbles in which cooling water was circulating. The UV emission from the lamp included energy at 185, 238, 248, 254, 265, 280, 297, 300, and 366 nm. The incident radiant flux reaching the reactor per unit surface was measured using a radiometer (LICOR, Inc. model LI-188B; sensor, LI190SB). From the lamp’s spectral distribution indicated by the manufacturer, and the diffuse reflectance spectrum for the TiO2 photocatalyst (see Figure 1), we calculated the photon flux absorbable by the photocatalyst to be approximately 6.38 × 1020 photons s-1 in the UV region of λ < 366 nm. In all the photoassisted experiments a concentration of 0.25 g of TiO2 L-1 was used, and progressive amounts of butyltins until their respective concentrations in the liquid phase were nearly 5 ppm. In all the experiments the volume of the suspension was 850 mL. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Leybold-Heraeus spectrometer, working at the constant pass energy of 50 eV; Mg KR radiation (hν ) 1253.6 eV) was used as excitation source. A final pressure of 10-9 Torr was attained before XPS recording. Samples for XPS analysis were prepared by recovering illuminated TiO2 powders from the photodegradation reaction mixture of tributyltin compounds as described above. The heterogeneous catalyst was separated from the reaction mixture by filtration and dried at room temperature in a desiccator. IR spectra were recorded on a Perkin-Elmer apparatus, model 883, using KBr disks. Analysis of TBT and the other butyltins was performed on a Perkin-Elmer 8410 gas chromatograph equipped with a flame photometric detector (610 nm filter) and a glass capillary column (12 m × 0.53 mm ID BP-1).16,17 Aliquots of 5 mL of water were analyzed. These samples were freed of photocatalyst by the use of a filter (0.45 µm Millex-Hv, Millipore). The butyltins were extracted into 25 mL of 0.05% tropolone in n-pentane. The tropolone was used to obtain a good recovery of DBT and MBT.16 The butyltins were injected into the gas chromatograph as n-pentyl derivatives. Some of the products formed in the liquid phase during the photochemical process were identified using a Kratos-MS 80 RFA instrument fitted to a Carlo Erba GC. Separations were achieved (15) Ozcan, M.; Good, M. L. Proc. Am. Chem. Soc. Div. Environ. Chem., Houston, TX, 1980. (16) Maguire, R. J.; Huneault, H. J. J. Chromatogr. 1981, 209, 458. (17) Rosales, D.; Pablos, F.; Marr, I. L. Appl. Organomet. Chem. 1992, 6, 27.
Table 1. Stability of TBT in Aqueous Solutions at the Indicated pH Valuesa [TBT] (µg mL-1) time (days)
pH ) 1
pH ≈ 5
pH ≈ 10.5
0 4
4.6 4.8
4.5 4.7
4.4 4.5
a
Solutions stored in the dark at room temperature.
on a CP-SIL 5 CBWCOT (25 m × 0.32 mm) column whose temperature was programmed from 30 °C (5 min) up to 250 °C (15 min) at 10 °C min-1 and S ) 0.
3. Results and Discussion Stability of TBT in Aqueous Solution at Various pH Values. One of the problems associated with the use of organotin species is their spontaneous hydrolysis in aqueous solutions. Since in this work we were interested in the TiO2-photocatalytic degradation of TBT at various pH values, a previous study on the stability of TBT in aqueous solution is required. Aqueous stability is defined here in terms of debutylation of TBT. Aqueous TBT solution (5 µg mL-1) in a volumetric flask was incubated in the dark at room temperature for a total period of 4 days. Samples were taken in duplicate, and the experiment was performed at pH ) 1, 5, and 10.5. The constant recoveries over the course of the experiment, shown in Table 1, indicate that there is not a significant cleavage (less than 10%) of butyl groups from tin during 4 days at room temperature in the dark at pH values between 1 and 10.5. However, it is well-known1,18 that organotin halides react with an alkali to give organotin hydroxides that are usually in equilibrium with the oxide and water as follows:
R3SnX F R3SnOH h R3Sn-O-SnR3 + H2O
(i)
On the other hand, triorganotin halides, R3SnX, may be cleaved by their reaction with hydrogen halides,1,19 and the formation of R2SnX2 and SnX4 proceeds stepwise.
R3SnX + HX F R2SnX2 + RH
(ii)
R2SnX2 + 2HX F SnX4 + 2RH
(iii)
(18) Harrison, P. G. In Organotin Compounds: New Chemistry and Applications; Zuckerman, J. J., Ed.; Advances in Chemistry Series No. 157; American Chemical Society: Washington, DC, 1976; p 258. (19) Ingham, R. K.; Rosemberg, S. D.; Gilman, H. Chem. Rev. 1960, 60, 459.
Photoassisted Degradation of n-Butyltin Chlorides
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Figure 3. Variation of the concentration of butyltin chlorides on illuminated aqueous suspension of TiO2 as a function of illumination time. Experimental conditions: air-equilibrated suspension, 0.25 g of TiO2/L, and pH ) 5.5-6.0. Table 2. K and nmax Values for the Room Temperature Adsorption of TBTCl on TiO2 Surfaces substrate
nmax (mol g-1)
K (mM-1 ” 102)
TBTCl
2.1 × 10-6
13.6
Table 3. First-Order Rate Constants in the Degradationa of Butylin Chloride Species at pH ) 5.5 in Air-Equilibrated TiO2 Aqueous Suspensions under UV Illumination kinetics parameters
Figure 2. (A) Adsorption isotherm of TBTCl in aqueous suspensions of TiO2 (0.25 g L-1) at room temperature and pH ) 5.5-6.0. (B) Kinetic plots of the adsorption profile of the results reported in (A).
substates
kobs × 102 (h-1)
〈r2〉 correlation coefficient
TBT DBT MBT
1.4 21.2 129.1
0.998 0.998 0.999
a
0.25 g of TiO2/L.
In practice, these cleavage reactions seem to be difficult to control, and the formation of mixtures of products has been reported as usual even when stoichiometric quantities of reagents are employed, probably by a Korcheskov reaction.20 On the basis of our results (Table 1) and if such reactions (i-iii) occurred during the course of 4 days, its occurrence might have been obscured if the TBT moiety were as readily extractable as it is from other butyltin compounds. However, the nondetection of DBT and MBT and the total agreement between our results and those reported by others21 lead to the suggestion that if the reactions i-iii proceed, under our experimental conditions, their occurrence may be with very low yield. In facts, reactions i-iii take place under very drastic conditions.22 In summary, the tributyltin species dissolved in water seems not to lose butyl groups over a period of at least 4 days at room temperature in the dark at pH values between 1 and 10.5. Adsorption Measurements. The degree of adsorption of butyltin chlorides onto TiO2 was evaluated by monitoring the decreasing concentration of solute (∆C) in aqueous solutions (0.5-5 µg mL-1) containing powdered TiO2 (0.25 g L-1) stirred for 5 h in the dark at room temperature (see Figure 2A). Linear plots were obtained for (∆C)-1 vs Ceq-1, where Ceq is the substrate concentration at equilibrium of adsorption (see Figure 2B). It is concluded that the adsorption of butyltin chlorides can be described by a
Langmuir-type isotherm.23 The adsorption equilibrium constant (K) and the limiting extents of solute adsorption (nmax) that result from competitive adsorption-desorption equilibria between solute and water24 can be deduced and are given in Table 2. Photodegradation of Butyltin Chlorides by UV/ TiO2. Kinetic Study. Tri-, di-, and monobutyltin chlorides were independently subjected to photodegradation on illuminated TiO2 suspended in water (pH ) 5.5). Profiles of these degradations as a function of illumination time are shown in Figure 3, as comparative results. During the course of the TiO2-photoassisted degradation of TBT, an evolution of DBT of about 0.6 ppm as maximum was observed; however no or practically negligible amounts of MBT were detected throughout the photodegradation of TBT or DBT by UV/TiO2 action. The TiO2-assisted photodegradation of each butyltin compound proceeds via reasonably good first-order kinetics. Table 3 lists the values of the first-order rate constants for the TiO2-assisted photodegradation of these species. As can be seen, the degradation of both DBT and MBT is faster than that observed for TBT under the same experimental conditions. Indeed it can be seen that the k values for DBT and MBT are comparatively higher than the k value for TBT under equivalent experimental conditions. In principle, this can be attributed to the stronger adsorption capacity of DBT and MBT with respect
(20) Korcheskov, K. A. Ber. Bunsenges. Phys. Chem. 1929, 62, 996. (21) Maguire, R. J.; Carey, J. M.; Hale, E. J. J. Agric. Food Chem. 1983, 31, 1060. (22) van der Kerk, G. J. M.; Luijten, J. G. A. J. Appl. Chem. 1956, 6, 49.
(23) Langmuir, I. Trans. Faraday Sco. 1921, 17, 621. (24) (a) Cunningham, J.; Al-Sayyed, G. J. Chem. Soc., Faraday Trans. 1 1990, 86, 3935. (b) Cunningham, J.; Al-Sayyed, G.; Srijaranai, S. In Aquatic and Surface Photochemsitry; Helz, G. R., et al., Eds.; Lewis Publishers: London, 1994; Chapter 22, p 317.
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Table 4. Generated Intermediates during the Photoassisted Degradation of TBT in the Presence of TiO2a after 46 h of Illumination retaining time
molecular ion
compound
24:27 16:36 18:13
214 267 227
1,1,2,3,3-pentachloropropane Bu2Sn(OH)2 BuSn(OH)3
a
0.25 g of TiO2/L.
to TBT, probably as a consequence of higher charge and fewer ligands. In fact, the requirement of preadsorption for efficient charge trapping (photoexcited electron or hole, see below) has been repeatedly pointed out,24 indicating that molecules will exhibit reactivity which is ultimately sensitive to preadsorption equilibria.24,25 The turnover numbers considered as indicative of the catalytic regime are calculated as
(moles of butyltin species photodegraded in 30 h)/ (moles of TiO2) The turnover numbers of each ones of the butyltin species, calculated as above, are far of 1 (ca. 10-3) and indicate that the photodegradation processes are not catalytic with respect to TiO2 specimen. It is perhaps stoichiometric because the TiO2 catalyst poisons rapidly due to the deposition of tin oxide (see below). On the other hand, on the basis of our previously reported results,10 the photolytic degradation of butyltin species must be regarded as occurring simultaneously with the TiO2-photoassisted one. In the absence of TiO2 the differences between rate constants of disappearance of TBT, DBT, and MBT were not too large. However, during the TiO2-photoassisted process, under identical experimental conditions, a relatively large difference was found between the rate constant for TBT and the rate constants for DBT and MBT. The first-order rate constants for the degradation of butyltin compounds in the absence of TiO2 (direct photolysis) were higher than the corresponding values when TiO2 was present. These differences could be explained in terms of the adsorption and scattering of light by the catalyst grains and by the adsorption of products by the catalysts surface (see below). Under the conditions of the TiO2-photoassisted experiment reported in Figure 3, the GC-MS analysis after a period of illumination of 46 h showed the appearance of several product peaks, some of which have been identified. With regard to this GC-MS analysis there is a problem mainly related to the TBT species levels used in the experiment (ppm levels). So, the amounts of products after TiO2-photoassisted degradation would be very low, and in spite of working with pure solvent (HPLC grade), any design element might mask some of the product. Nevertheless we have made an effort to identify some of the products during to the photoassisted reaction in the presence of TiO2. Table 4 shows some of these identified products after 46 h of illumination, in which the appearance of some polychlorinated hydrocarbon, probably formed by the reaction between alkyl and chlorine radicals can be observed. The GC-MS analysis did not reveal partial oxidation products in the liquid phase. However the possibility of their formation during the reaction is not excluded. It is possible that their volatile nature and a system of work open to the air would allow their escape from the photoreaction system. Thus, the complete mineralization to CO2 by the photocatalytic oxidation stage would be (25) Matthews, R. W. J. Catal. 1988, 113, 549.
Figure 4. IR spectra from the surface of the following samples: (a) TiO2; (b) commercial TBT (solid); (c) TBT adsorbed at the TiO2 surface, TBT/TiO2; (d) TBT/TiO2 after UV illumination for a period of 46 h.
possible. As our experiments were performed in an open air-equilibrated system, CO2 was not monitored. Others products identified are the hydroxylated di- and monobutyltin species. This indicates the debutylation sequence during the the TiO2-photoassisted degradation of the TBT species. Also, the appearance of hydroxylated butyltin species would show the important role of the hydroxyl radical in the reaction mechanism, as will be discussed below. Surface Analysis by IR and XPS. Figure 4 shows the IR spectra, in the same wavelength range, of the following samples: (a) TiO2, (b) solid TBT, and (c) TBT/ TiO2 equilibrated at the saturation step, as well as the TBT/TiO2 after having been subjected to UV illumination for a period of 46 h (spectrum d), under the same experimental conditions as reported in Figure 2. The comparison between spectra a-c recorded in Figure 4 supports qualitative evidence of the adsorption of TBT on the TiO2 surface. Most of the bands assigned to tributyltin chloride26 (Figure 4b) were also found in TBT/TiO2 surface IR spectrum (Figure 4c), although they are slightly displaced in Figure 4c with respect to Figure 4b, possibly by an adsorption effect. It is interesting that the IR bands associated with dissociatively adsorbed water which are present in the original surface of TiO2 (region 3600-3200 cm-1) (Figure 4a) are practically unmodified after the adsorption of TBT species (Figure 4c). The practically unalterated hydroxyl region profile could be related with the adsorption of TBTCl species through the OH- sites present at the surface, with the subsequent weakness of the Cl-Sn bond and the parallel formation of Bu3Sn+‚‚‚OH- and Cl-‚‚‚H+ interactions. In such a case, OH (or O) species can acts as bridges between Sn(IV) and Ti(IV). In fact, it is well-known27 that the adsorption of TBTCl on laminar clays and oxides takes place at the surface hydroxyl group level, and these results support the hypothesis previously mentioned. The extent of the adsorption might be limited by the steric (26) Geissler, H.; Kriegsmann, H. J. Organomet. Chem. 1968, 11, 85. (27) (a) Serratosa, J. M. Proceedings of the International Clay Conference, 1978. (b) Serratosa, J. M. Development in Sedimentology 27; Mortland, M. M., Farmer, V. C., Eds.; Elseiver Science Publishing Co.: Amsterdam, 1979.
Photoassisted Degradation of n-Butyltin Chlorides
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Scheme 1. Proposal for the Adsorption of Organostannic Species on the TiO2 Surface
Figure 5. Results of the XPS analysis: Sn 3d5/2 peaks (spectra a, c, and e); C 1s peaks (spectra b, d, and f); TBT/TiO2 surfaces (spectra a and b); TBT/TiO2 + 5 h of UV illumination (spectra c and d); TBT/TiO2 + 46 h of UV illumination (spectra e and f).
effect of the butyl ligand and also by its hydrophobic character. A proposed mechanism for the adsorption of TBT on the TiO2 surface is summarized in Scheme 1. On the other hand the profiles of spectra a and d of Figure 4 are more or less identical, indicating that prolonged UV illumination of TBT in aqueous suspensions of TiO2 leads to a TiO2 surface free of adsorbed butyltin or other organic compounds, probably because practically total mineralization has occurred. XPS characterization of the TiO2 at the saturation step shows the appearance of a band centered at 487.0 eV, suggestive of an Sn 3d5/2 transition (Figure 5a). This Sn peak is shifted from the binding energy of elemental tin and correlates reasonably with those expected for SnRx.28 The spectrum also shows a single C(1s) peak at 284.3 eV (Figure 5b) attributed to the carbon in the CH2 functional group.29 In contranst, several distinct Sn bands can be observed in the XPS analysis of TiO2 recovered after UV photodegradation of TBT for a short period of illumination (Figure 5c). As can be seen, the initial observed peak at 487.0 seemed to be still present in the spectrum but overlapping with two other lower binding energy peaks centered at 486.4 and 484.6 eV, respectively. These two peaks correlate reasonably with those expected for SnOx (SnO or SnO2) and elemental tin, respectively,28,30 and seem to be present also after prolonged illumination time (28) Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Published by Physical Electronics Division, USA, 1979. (29) Kurtvatov, G.; Darque Ceretti, E.; Aucouturier, M. Surf. Interface Anal. 1992, 18, 811. (30) Meenan, B. J.; Brown, N. M. D.; Wilson, J. W. Appl. Surf. Sci. 1994, 74, 221.
(see broad band in Figure 5e). Although the high binding energy component observed around 487.0 in spectrum c of Figure 5 could be associated with residual SnRx species, we do not exclude the possibility that this component could also result from a Sn(IV) material, namely stannic acid, SnO2‚2H2O, i.e., an “Sn(OH)4” species;30 the observed binding energy at 486.4 eV could be attributed, in terms of the known chemistry, to an Sn(II) compound, i.e., SnO or Sn(OH)2.30 Since there is no evidence for a Cl- peak in the XPS data, it is unlikely that the Sn(II) species is caused by SnCl2 species adsorbed on the substrate surface. Similarly the spectrum (Figure 5d) shows a C(1s) region in which peaks at 284.3, 286.2, and 288.4 eV can be distinguished, and could be attributed to the carbon in the CH2, CH2OCH2, and CH2OH functional groups, respectively, according to data from the literature.29 This result clearly indicates that during the TiO2-photoassisted degradation of TBT, several oxidation products from the butyl ligand can be generated as intermediates, which initially are partially adsorbed at the TiO2 surface where they can then be totally oxidized to CO2 after a prolonged illumination time. In fact, the C(1s) region of the TBT/ TiO2 samples after 46 h of illumination showed a practical absence of carbon atoms (Figure 5f). As mentioned, no evidence for surface-bound chloride could be seen in these XPS analysis. Mechanistic Features and Photocatalytic Activity. Illumination of titanium dioxide powder with photons of energy equal to or greater than its band gap energy (3.0 eV) results in the formation of an electron (e-) and hole (h+) pair (eq 1). Recombination of e--h+ (eq 2) can be hindered by electron trapping by adsorbed oxygen (eqs 4 and 5), which is renewed from the air saturation as it is consumed at the photocatalyst surface. The oxidative degradation of organometallic compounds adsorbed on the semiconductor can then be initiated by direct trapping of the photogenerated hole (eq 7). This results in the formation of a singly oxidized species (the organometallic radical cation) as has been pointed out by other authors using various groups IVA organometallic compounds.31 Such organometallic cation radicals could fragment to give either hydroxyl or butyl radicals or butyl cations (eqs 8 and 10). These intermediates initate thermal reactions of oxidative degradation, leading finally to carbon dioxide and water as shown in the remaining steps (eqs 11-16). Since XPS studies indicate the formation of SnOx and Sn(0) on TiO2, these formations must been taken into account in the mechanistic reaction series by including an additional inorganic process. Scheme 2 summarizes the proposed sequence for the TiO2-photoassisted degradation of butyltin chlorides. The tin-containing fragment becomes associated with the surface, probably more strongly adsorbed, upon subsequent repetition of the Sn-C cleavage by parallel routes: adsorption of DBT and MBT and deposition of SnO2 or Sn(0). (31) Dulay, M. T.; Washington-Dedeaux, D.; Fox, M. A. J. Photochem. Photobiol. A: Chem. 1991, 61, 153.
2012 Langmuir, Vol. 12, No. 8, 1996 Scheme 2. Proposed Sequence for the TiO2-Photocatalytic Degradation of Butyltin Chlorides
Navio et al. Table 5. Quatum Yields of Butylin Species during Photodegradation in the Presence or Absence of TiO2 quantum yield values (×107)
a
It should be noted that in the proposed mechanism the formation of the hydroxylated organometallic cation radical by eq 7 is equivalent to considering that it is formed by the direct attack of the adsorbed cation R3Sn+ species by the adsorbed hydroxyl radical OH. The degradation of TBT on hydroxyl radicals to yield this cation might be considered an important step in the pathway mechanism. Although some authors31 have proposed that the oxidative photodegradation of various IVA organometallic compounds (including organostannes) adsorbed on TiO2 is initiated by direct trapping of photogenerated holes, at present we cannot discriminate between the two possibilities. In fact, direct oxidation of organic substrates by photogenerated holes has been a matter of some debate.32 At the same time, the direct attack of the adsorbed butyltin cations by the adsorbed molecular dioxygen (eq 16) must be considered as another possible step in the TiO2 photodegradation of butyltin compounds. If the adsorption model for TBT at the TiO2 surfaces is assumed, then the oxidation of Cl- species by holes and the subsequent formation of chlorine radicals or hypochlorite anions might be omitted from our proposed mechanism because it is well-known that chlorine ions are not photooxidized either in oxygenated aqueous suspensions of TiO233 or upon UV illumination when they are adsorbed at the hydroxylated TiO2 surfaces.34 However, the formation of polychlorinated hydrocarbons (see GC-MS analysis) clearly indicated the formation of chlorine radicals. This species could be formed by the homolytic breaking of the Sn-Cl bond during the photolytic process of TBT in aqueous medium. Although the (32) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (33) Herrmann, J. M.; Pichat, P. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1138. (34) Munuera, G.; Navio, J. A.; Soria, J.; Gonzalez-Elipe, A. R. Proceedings of the 7th International Congress on Catalysis; Seiyama, T., Tanabe, K., Eds.; Kodansha LTD: Tokyo, 1981; p 1185.
substrate
without TiO2a
with TiO2
TBT DBT MBT
54.8 50.4 32.1
0.56 4.30 7.60
Data obtained from ref 10.
proposed mechanism is speculative, there is some evidence for several steps. Although there is not evidence for most of the steps shown in Scheme 2, the proposed mechanism is consistent with the reported results. Quantum Yields. Table 5 gives the values of the apparent quantum yiled (qy) for the photodegradation of butyltin chloride compounds in the presence of TiO2 (from results reported in Figure 2) or in the absence of TiO2 (from results reported in ref 10), under equivalent experimental conditions. However, it should be noted, when comparing the qy values reported by both procedures (absence or presence of TiO2), that in the absence of TiO2 qy has been evaluated, taking into account the different absorptions of the three butyltins (TBT and DBT absorb at λ < 230 nm and MBT at λ < 270 nm). Therefore the amounts of light absorbed by each tin compound have been evaluated by considering the photon flow corresponding to each emission line of the lamp below the respective absorption values for each substrate. However, in the presence of TiO2 the qy has been estimated assuming that all the photons supplied by the lamp below 360 nm are absorbed by the catalyst grains. This latter assumption is not strictly accurate because for heterogeneous systems as is the case for the present study, the photons below 360 nm entering the reactor during a given time are in part scattered; in part adsorbed by the solid particles, producing or not a photocatalytic reaction; in part transmitted through the dispersion; and in part adsorbed by the substrate, producing or not a photolytic reaction. On the other hand, as mentioned above, the parallel photolytic process of butyltin species and the subsequent formation of partial oxidation products10 cannot be ruled out during the TiO2-photoassisted process. Consequently, if the intermediates are adsorbed at the TiO2 surface, they will be further degraded by the direct photocatalytic mode. This latter effect must be considered when the efficiency of the two procedures (absence and presence of TiO2) is evaluated. Effect of Solution pH. A common feature of photolytic reactions occurring on metal oxide semiconductor powders suspended in aqueous solution is the weak dependence of the reaction rate on solution pH.35 Several parameters such as particle size, surface charge, and band edge positions of TiO2 are strongly influenced by pH.36 Therefore it was of interest to study the influence of pH on the TiO2-photoassisted degradation of butyltins. Tributyltin chloride was chosen for this investigation since its photodegradation rate in the presence of TiO2 at natural pH (5.5) was found to be far lower than those for DBT and MBT (Table 3). Likewise, TBT is much more toxic than DBT or MBT, and these latter species can be formed from the former. Air-equilibrated aqueous TiO2 suspensions of TBT were examined under UV illumination at pH ) 2, ) 5.5, ) 10.5, (35) (a) Sabin, F.; Tu¨rk, T.; Vogler, A. J. Photochem. Photobiol. A: Chem. 1992, 63, 99. (b) Harada, H.; Ueda, T.; Sakata, T. J. Phys. Chem. 1989, 93, 1542. (c) Rose, T. L.; Nanjundiah, C. J. Phys. Chem. 1985, 89, 3766. (36) Siffert, B.; Metzger, J. M. Colloids Surf. 1991, 53, 79.
Photoassisted Degradation of n-Butyltin Chlorides
Figure 6. Kinetics profiles of the photoassisted degradation of TBT in air-equilibrated aqueous suspensions of TiO2 at the indicated pH values. Table 6. First-Order Rate Constants in the Degradation of Tributyltin Chloride at the Indicated pH Values in Air-Equilibrated TiO2 Aqueous Suspensions under UV Illumination kinetics parameter pH values
kobs × 102 (h-1)
correlation coefficient
2.0 5.5 10.5 13.5
0.9 1.4 3.4 6.5
0.967 0.998 0.995 0.998
and 13.5. Figure 6 illustrates the time course of the TiO2photoassisted degradation of TBT at the four mentioned pHs. A progressive increase of the degradation rate with the increase of pH is observed for TBT (Figure 5). Each of the TiO2-photoassisted degradation processes reported in Figure 6 follows reasonably good first-order kinetics; the corresponding apparent first-order rate constant for each is reported in Table 6. A linear relationship was found with ln(kobs) vs pH, ln(kobs) ) 0.160 (pH) - 5.038 (k in h-1 units), with a reasonably good correlation coefficient (0.988). As mentioned above, several factors can be invoked to explain the pH dependence. One of these is the surface characteristics of the catalyst. The isoelectric point for TiO2 in water is about pH ) 5.6-6.0,37 close to the pH value of studies at natural pH (approximately 5.5). At more acidic pH the semiconductor particle surface is positively charged, while at pH > 5.6 the surface is negatively charged, as illustrated by the following surface reactions:
Ti4+-OH h T4 -OH2+ +
at pH < 5.6
Ti4+-OH + OH- h Ti4+-O- + H2O
at pH > 5.6
This has important consequences on the adsorption/ desorption properties of the catalyst particles’ surface for a given substrate, as well on the photoadsorption/ photodesorption features (e.g. O2) of such surface. Thus, according to the considerations mentioned in the adsorption measurements section, a negatively charged surface must favor the adsorption of positively charged species as R3Sn+. Also, because of the role of dioxygen such both for electron capture and as a reactant, directly or indirectly as superoxide ion (see Scheme 2), good adsorbing properties for dioxygen are a prerequisite for the oxidizing properties of the photocatalyst.38,39 Redox processes are aided by the presence of adsorbed molecular oxygen, which (37) Augustinski, J. Struct. Bonding 1988, 69, 1.
Langmuir, Vol. 12, No. 8, 1996 2013
acts as an electron scavenger to form the superoxide radical anion O2-; this prevents the recombination of electrons and holes (whether free or trapped).40 Thus it can be reasonably proposed that the extent of photoadsorbed dioxygen could be correlated with the amount of holes free or trapped by adsorbed species. In fact, measurements of the amount of photoadsorbed dioxygen have been used as a means of showing the effects of modifications upon the density of electrons available at the surface of a semiconductor oxide.41 Boonstra and Mutsaers42 have quantitatively related the amount of photoadsorbed oxygen with the surface hydroxyl content on TiO2. On the other hand Munuera et al. have shown43 that when the protonic species H+ are in the majority at the TiO2 surface (sample H/TiO2), the electron-hole recombination process strongly competes with the electron trapping by H+ species to give the H• radical. This latter process accounts for the weak photoadsorption of dioxygen observed on H/TiO2 surfaces, in contrast with the strong dioxygen photoadsorption observed on the very hydroxylated TiO2 surfaces. In view of the data in the literature and since the oxidative degradation of butyltin compounds adsorbed on the TiO2 surface could be initiated by direct trapping of the photogenerated holes (eqs 6 and 7) and/or by direct attack of adsorbed butyltin cations molecular adsorbed dioxygen (eq 16), the surface factors governing the photoadsorption of dioxygen (eqs 4 and 5) must account for the extent of the TBT photodegradation processes when TiO2 is present. Given that the surface characteristics of TiO2 are dictated by the pH values, which indirectly govern the capacity for adsorption-desorption of substrate and products, as well as the electron-hole recombination and dioxygen photoadsorption, we can explain the effect of pH in the TiO2 photodegradation of TBT (Figure 6) in terms of the considerations discussed above. Hydroxyl OH and perhydroxyl HO2 radicals have been considered as primary oxidants in the heterogeneous photocatalytic oxidation of organic molecules, and photoadsorption of oxygen seems to be more efficient at high pH values. Consequently, in accord with the proposed mechanism, the photooxidation of the butyl radicals (eqs 12-14) might be more efficient at high pH values. In such a case, the fast elimination of the butyl radicals should prevent their coupling39 with the partially degraded butyltin fragments. Such effects of recombination lead to an increased yield of the TiO2 photodegradation of TBT by raising the pH of solution, as observed. Finally the consideration of pH-dependent change in energy levels of the band edges of TiO244-46 and the redox potentials of the species in solution should also be considered. Unfortunately, due to the lack of data on the redox potentials of butyltin chlorides in water, we cannot (38) Lewis, N. S.; Rosenbluth, M. L. In Photocatalysis; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: New York, 1989; p 99. (39) Gerischer, H. Electrochim. Acta 1993, 38, 3. (40) Meyer, G. J.; Leubker, E. R. M.; Lisensky, G. C.; Ellis, A. B. In Photochemistry on Solid Surfaces; Anpo, M.; Matsuura, T., Eds.; Elseiver: Amsterdam, 1989; p 388. (41) Courbon, H.; Herrmann, J. M.; Pichat, P. J. Phys. Chem. 1984, 88, 5210. (42) (a) Boonstra, A. H.; Mutsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1694. (b) Boonstra, A. H.; Mutsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1940. (43) Munuera, G.; Gonzalez-Elipe, A. R.; Rives-Arnau, V.; Navio, J. A.; Malet, P.; Soria, J.; Conesa, J. C.; Sanz, J. In Adsorption and Catalysis on Oxides Surfaces; Che, M., Bond, G. C., Eds.; Elsevier Science Publishers B. V.: Amsterdam, The Netherlands, 1985; p 113. (44) Bolts, J. M.; Wrighton, M. S. J. Phys. Chem. 1976, 80, 2614. (45) Dutoit, E. C.; Cardon, F.; Gomes, W. P. Ber. Bunsenges. Phys. Chem. 1976, 80, 475. (46) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. Soc. 1983, 27, 105.
2014 Langmuir, Vol. 12, No. 8, 1996
rationalize this aspect at the present stage. A detailed study of these redox potentials is currently in progress. 4. Conclusions Our results allows several conclusions: For the tri-n-butyltin(IV) chloride species (TBT) in aqueous medium, there is no significant breaking (less than 10%) of the butyl-tin bonds, at least during 4 days at room temperature in the dark and within the range of pH values between 1 and 13.5. The adsorption of TBT at the TiO2 surface can be described by a Langmuir type isotherm with an equilibrium constant K ) 13.6 × 102 (mM-1) and a limit extent for the solute adsorption nmax ) 2.1 × 10-6 mol g-1. The IR results infer that the adsorption of TBT at the TiO2 surface seems to proceed through the surface hydroxyl groups acting as bridges between Ti4+ and Sn4+. The photoassisted degradation, in the presence of TiO2, of n-butyl(IV) compounds follows pseudo-first-order kinetics with the following sequence for the rates of photodegradation: kTBT < kDBT , kMBT, under our experimental conditions. Increasing the pH of the solution leads to a progressive increase in the rate of photodegradation. From the apparent observed quantum yields of the TiO2-
Navio et al.
photoassisted processes (as low as 10-7 and lower than that observed for the photolytic process) it may be concluded that butyltins offer high resistance to photodegradation by either photolytic or TiO2-photoassisted processes. During the photoassisted degradations of n-butyltin species in air-equilibrated aqueous TiO2 suspensions, several products were identified as polychlorinated hydrocarbons and hydroxylated species of di- and monobutyltin. The prolonged UV illumination (46 h) of TBT species in air-equilibrated aqueous TiO2 specimens leads to a TiO2 surface free of carbonaceous species; tin, however, remains at the TiO2 surface as metallic tin and/or as SnOx. Finally, our results appears rather as a negation of heterogeneous photoassisted reactions. Perhaps, titania’s surface covered by organotin intermediates or SnOx species can occult the photosensitive part of TiO2. Acknowledgment. J.A.N. wishes to thank the “Direccio´n General de Investigacio´n Cientı´fica y Te´cnica” (DGICYT, Projects PB90-0911 and PB93-0917) for supporting part of this work. The help of the XPS service of Sevilla University is also gratefully acknowledged. LA950947S
