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Langmuir 1998, 14, 388-395
Photoassisted Degradation (in the UV) of Phenyltin(IV) Chlorides in the Presence of Titanium Dioxide J. A. Navio,*,† C. Cerrillos,† M. A. Pradera,‡ E. Morales,§ and J. L. Go´mez-Ariza§ Instituto de Ciencia de Materiales de Sevilla, Centro de Investigaciones Cientı´ficas “Isla de la Cartuja”, c/Ame´ rico Vespucio, s/n, Isla de la cartuja, 41092 Sevilla, Spain, Dpto. De Quı´mica Orga´ nica, Facultad de Quı´mica, Universidad de Sevilla, 41012-Sevilla, Spain, and Dpto. De Quı´mica y Ciencia de los Materiales, Escuela Polite´ cnica Superior, Universidad de Huelva, Campus de la Ra´ bida, Huelva, Spain Received March 26, 1997. In Final Form: October 17, 1997X The photoassisted degradation of phenyltin(IV) chlorides in air-equilibrated aqueous TiO2 suspension has been studied. The stability of the triphenyltin(IV) derivative in aqueous solutions at different pH values and the extent of adsorption of tri- and diphenyltin(IV) chloride species were examined. Particular interest is focused on the TiO2-assisted photodegradation of phenyltin species. Detailed surface analyses of the catalyst particles by IR and of the liquid medium by GC-MS are reported. A mechanism consisting of interfacial trapping of a photogenerated electron-hole pair can explain the TiO2-photoassisted degradation of phenyltin species. The results are compared with those previously reported on the photolytic degradation of theses species in water and with those of n-butyltin species (photolytic and TiO2-photoassisted). Our results allow that this is not a photocatalysis system but a photoassisted one in the presence of TiO2.
1. Introduction Organotin compounds are used commercially in a variety of catalytic applications1-3 as well as for stabilization of polyvinyl chloride (PVC). Other applications include the use of triorganotins for biocidal activity in antifouling paints, wood preservatives, and agriculture.1 Although inorganic tin compunds generally have very low toxicity, some organotin species are very toxic to both animal and vegetable life.4 The highest toxicity is observed in triorganotin compounds while di- and monoorganotin compounds show successively lower toxicity. The toxicity of tetraorganotin compounds is low; however, under environmental conditions they will decompose to toxic triorganotins (either by chemicals or photonic actions). The organic group attached to tin also plays a significant role in the toxicity; triethyltins are the most toxic, followed by butyltins. Also triphenyltins show considerable toxicity. Toxicology data for organotin compounds have been reported.5 In previous papers6,7 we provided data about the mode of UV-photolytic degradation of butyltin chlorides6 and phenyltin chlorides7 in water, and some of the breakdown * Corresponding author. † Instituto de Ciencia de Materiales de Sevilla. ‡ Universidad de Sevilla. § Escuela Polite ´ cnica Superior. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Blunden, S. J.; Cusack, P. A.; Hill, R. The Industrial Uses of TiO2 Chemicals; The Royal Society of Chemistry: London, 1985. (2) Evans, C. J.; Karpel, S. Organotin Compounds in Modern Technology. J. Organomet. Chem. Libr. 1985, 16. (3) Omae, Y. Organotin Chemistry. J. Organomet. Chem. Libr. 1989, 21, Chapter 9. (4) Alzieu, G.; Heral, M.; Thiband, Y.; Dardignac, M. J.; Fauillet, M. Recl. Trav. Inst. Pech. Marit. 1982, 45, 101. (5) Smith, P. J. Toxicological Data of Organotin Compounds. ITRI Publication 538; Internatational Research Institute: Perivale, U.K., 1977. (6) Navio, J. A.; Marchena, F. J.; Cerrillos, C.; Pablos, F. J. Photochem. Photobiol., A: Chem. 1993, 71, 97. (7) Navio, J. A.; Pradera, M. A.; Morales, E.; Go´mez-Ariza, J. L. J. Photochem. Photobiol., A: Chem. 1997, 108, 59.
products were identified. Also the photoassisted degradation of n-butyltin chlorides in air-equilibrated aqueous TiO2 suspension has been studied by us8 as an alernative route to study the photochemical elimination of butyltin species by heterogeneous photocatalysis, since this technology has been shown to be an attractive process for the degradation of many water pollutants in the aquatic environment.9 This paper studies the degradation of phenyltin compounds by the action of powdered TiO2 under UV-illumination in aqueous solutions. 2. Experimental Details Materials. Titanium dioxide, TiO2, photocatalyst (Degussa, P25) was used from the same bath and subjected to the same pretreatment (calcination at 500 °C for 24 h), as previously described;8 a BET surface area of 46.5 m2 g-1, a molar fraction of anatase phase XA ) 0.52, and a primary particle size of about 1-5 µm were found for the pretreated sample. Triphenyltin chloride (TPT) (95%), diphenyltin dichloride (DPT) (96%), and monophenyltin trichloride (MPT) (98%) were supplied by AldrichChemie GmBH & Co. K.G. The concentration of the compounds studied here was chosen considering their solubilities in water. All solutions were prepared using water from a Millipore Waters Q purifications unit. In the experiment 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 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). The lamp’s spectral distribution and the diffuse (8) Navio, J. A.; Cerrillos, C.; Marchena, F. J.; Pablos, F.; Pradera, M. A. Langmuir 1996, 12, 2007. (9) Schiavello, M., Ed. Photocatalysis and Environment, Trends and Applications; NATO ASI Series C; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988; Vol. 237, p 469.
S0743-7463(97)00318-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
Photoassisted Degradation of Phenyltin(IV) Chlorides
Langmuir, Vol. 14, No. 2, 1998 389
Table 1. Stability of TPT in Aqueous Solutions at Room Temperature, in the Dark, after 4 Days and at the pH Values Indicated pH value
TPT (mg/L)
1.5 5.5 10.5
0.9006 0.9462 0.2097
reflectance spectrum for TiO2 are given elsewhere.8 The photon flux absorbable by the photocatalyst was calculated to be approximately 6.38 × 1020 photon s-1 in the UV region of λ < 366 nm. In all the photoassisted experiments a concentration of 0.01 g L-1 was used. In all the experiments the volume of the suspension was 850 mL. IR spectra were recorded on a Perkin-Elmer apparatus, model 883, using a KBr disk. Analysis of TPT and other phenyltins was performed on a Perkin-Elmer 8410 gas chromatograph equipped with a flame photometric detector (610 nm filter) and a glass (SPB-1) semicapillary column (15 m × 0.53 mm i.d.) using helium as a carrier gas. Aliquots of 5 mL of water were analyzed. These samples were freed of photocatalyst by the use of a filter (0.45 nm Millex-Hv, Millipore). Other aspects concerning the analytical procedure can be found elsewhere.8 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 on a CP-SIL 5 CBWCOT (25 m × 0.32 mm) column whose temperature was programmed from 30 °C (20 min) up to 250 °C (15 min) at 5 °C min-1 and S ) 0.
3. Results and Discussion Stability of the TPT Derivative in Aqueous Solutions at Different pH Values. The study has been approached in the same way as for the tributylin derivative reported in a previous work.8 Aqueous solutions of around 1 ppm of TPT were incubated in calibrated flasks in the dark at room temperature for 4 days. Duplicated samples were prepared at pH 1.5, 5.5, and 10.5. The results of their later analysis are shown in Table 1. These data imply that at basic pH values the concentration of triphenyltin chloride decreases by about 78%, while at acidic pH values the concentration of TPT remains practically unaltered. The decrease in the concentration of triphenyltin chloride at basic pH values is not accompanied by a simultaneous increase in the other diand mono-phenyltin species. Triorganotin halides react with alkalis10 to generate the corresponding hydroxylated derivative, which in turn is in equilibrium with the oxide and the water, as described by eq 1: OH-
R3SnX 98 R3SnOH a R3Sn-O-SnR3 + H2O
(1)
The reaction conditions for the particular case of R ) Ph have been described in the literature and do not seem drastic, in contrast to those for the butyltin derivative, in which vigorous heating is necessary for an appreciable reaction to take place.11 Thus it is possible that the species described in eq 1 are formed in the medium. The analysis method used in this work for the speciation of organotins will not distinguish whether the derivative present in the medium is chlorinated, hydroxylated, or forming the corresponding oxide. If this reaction did take place, the analytical method would give a single peak of the (10) 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. (11) Aylett, C. Organometallic Compounds; Chapman and Hall: London; Vol. 1, Part. 2; Group IV and V.
triderivative which could be of any of the three species mentioned. Thus the “apparent” decrease in concentration observed at basic pH values, at room temperature in the dark after 4 days, seems unrelated with the possible formation of the hydroxide and/or oxide of the triphenyltin chloride derivative. What is proposed is the possibility that the TPT species is adsorbed on the glass. There are numerous works proposing organotin sample conservation at acidic pH values12 as a consequence of the fact that at basic pH values the adsorption of such species on the glass wall increases. Triorganotin halides can also react with hydrogen halides3 (in our case, HCl was used to decrease the pH), generating the derivatives R2SnX2 and SnX4 in accord with eqs 2 and 3. These reactions, as already stated in a previous work,8 appear to be difficult to control and could give a mixture.
R3SnX + HX f R2SnX2 + RH
(2)
R2SnX2 + 2HX f SnX4 + 2RH
(3)
Nevertheless, and given that at acidic pH values the concentration of TPT remains practically unaltered, without appearance of either the DPT or MPT derivative simultaneously in the medium, it can be concluded that reactions 2 and 3 seem not to take place under our experimental conditions, or at least without an appreciable yield. In summary, at acidic pH the triphenyltin species dissolved in water seem not to lose phenyl groups in a period of 4 days, at room temperature and in the dark; that is, there is no sequential dephenylation under these experimental conditions. In contrast, at basic pH values and under the same experimental conditions, a decrease is observed in the concentration of the TPT species, which could be attributed in principle to the adsorption of TPT on the glass walls. Since at basic pH an “apparent” decrease in the concentration of TPT can be observed, it is not possible to study the photoassisted degradation of such derivatives in the presence of TiO2 at basic pH values using this analytical method, as a decrease in concentration cannot be attributed only to the action of TiO2/hν but might include an uncontrolled adsorption of this species on the walls of the container. For this reason, this section of the study does not consider the photoassisted TPT degradation in the presence of TiO2 at basic pH values. Measurement of Adsorption. Prior to studying the photoassited degradations of organotin derivatives in aqueous suspensions of TiO2, the competitive adsorption/ desorption equilibrium between the solute and the water must be known, as mentioned before and in accord with the literature for aqueous suspensions of photodegradable contaminants.13 The degree of adsorption of triphenyltin chloride and diphenyltin dichloride in TiO2 is evaluated similarly to that of the butyltin derivative. The decrease in concentration of the solute (∆C) in aqueous solutions (at pH ) 5.5-6) is measured for different concentrations of the phenyltin derivative (from 0.4 to 4 ppm for the triderivative and from 17 to 40 ppm for the diderivative) which contain a fixed amount of TiO2 in (12) (a) Maguire, R. J.; Tracz, R. J. J. Chromatog. 1983, 99, 268. (b) Boettner, E. A.; Ball, G. L.; Hollingsworth; Aquino. Organic and Organotin Compounds Leached from PVC and CPVC Pipe; 1981. (c) Mu¨ller, M. D.; Z. Anal. Chem. 1984, 32, 317. (13) Cunningham, J.; Al-Sayyed, C. J. Chem. Soc., Faraday Trans. 1990, 86, 3935.
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Figure 1. Adsorption isotherm of TPTCI in aqueous solutions of TiO2 (0.01 g/L) at room temperature and pH ) 5.5-6.
Figure 3. Adsorption isotherm of DPTCl2 in aqueous solutions of TiO 2 (0.01 g/L) at room temperature and pH ) 5.5-6.
Figure 2. Kinetic fit of the results of adsorption of TPTCl/ TiO2 from Figure 1. Table 2. Values of K and nmax for the Adsorption of the TPTCI and TBTCI Species on the TiO2 Surface at Room Temperature substrate
nmax (molecules/m2)
10-2 × K (mM-1)
TPTCl TBTCla
6.8 × 1018 2.5 × 1016
2.39 13.60
a
Taken from ref 8.
suspension (0.01 g/L), homogenized for 5 h in the dark at room temperature. Figure 1 shows the plot ∆C against Ceq (concentration of the substrate at adsorption equilibrium) for triphenyltin chloride. The plot of ∆C-1 against Ceq-1 is linear, and its slope gives the value of the adsorption equilibrium constant, K (Figure 2). It is concluded that the adsorption of triphenyltin chloride can be described by a Langmuirtype isotherm.14 The values of K and nmax, defined as adsorption equilibrium constant and maximum extent of adsorption of solute,13 respectively, are given in Table 2. When the data of nmax for the same chloride derivatives of tributyl- and triphenyl-tin (Table 2) are compared, significant differences can be seen. Thus the value of nmax for the TPT chloride is some 100-fold greater than that of the TBTCI derivative. This could be explained, in principle, by the different natures of the phenyl and butyl ligands. As remarked in the previous work8 concerning (14) Langmuir, L. Trans. Faraday Soc. 1921, 17, 621.
the TiO2-photoassisted degradation of TBT, the degree of adsorption of this species is strongly limited by its hydrophobic nature and steric factors of the butyl ligand. However, in the case of TPTCI the phenyl ligand may interact with the TiO2 surface by π interaction of the phenyl with Ti4+ centers presenting an electron deficiency. This would contribute to the TPT species’ higher adsorption capacity with respect to that of the TBT. In fact, benzene and its derivatives can form a variety of charge transfer complexes depending on their capacity to accept or donate electrons.15 The formation of charge transfer complexes has been reported in certain systems involving metal oxides as substrate and benzene derivatives as adsorbate.16 The interaction of benzene with the TiO2 surface has been studied by Suda.17 Those authors conclude that the benzene ring, a π electron carrier, is adsorbed preferentially in a flat orientation on dehydroxylated centers where Ti4+ ions are present as a result of the formation of a charge transfer complex. The adsorption, weaker than the former, of benzene on the hydroxylated surface of TiO2 has also been found by those authors17 and seems to take place by the interaction OH-‚‚‚π. Similar results have been found for other derivatives such as toluene adsorbed on TiO2 surfaces. In fact if we compare the values of nmax with the OH surface density given by Bohem17c (∼5 × 1018 OH/m2), it could be concluded that one TPT molecule is adsorbed per each surface hydroxyl group. The measurements of the adsorption isotherm for diphenyltin dichloride were made in the same way as for the former derivative. Figure 3 shows the plot of ∆C against Ceq. The fit of this isotherm results in a different type of graph to that observed for the triderivative. Different theoretical models have been proposed in the literature to explain the adsorption. Thus the plots for the adsorption isotherms can be fitted to the distinct types proposed by Giles and co-workers,18 according to which four types of isotherm can be distinguished depending on the initial slope, and a subgroup described for each class based on the upper part of the curve. In accord with such (15) Mulliken, R. S.; Pearson, W. B. Molecular Complexes; WileyInterscience: New York, 1969. (16) (a) Basila, M. R. J. Chem. Phys. 1961, 35, 1151. (b) Cusumano, J. A.; Low, M. J. D. J. Phys. Chem. 1970, 74, 1950. (c) Cusumano, J. A.; Low, M. J. D. J. Colloid Interface Sci. 1972, 38, 245. (17) (a) Suda, Y. Langmuir 1988, 4, 147. (b) Nagao, M.; Suda, Y., Langmuir 1989, 5, 42. (c) Boehm, H. P. Discuss. Faraday Soc. 1971, 52, 264. (18) Giles, G. H.; Macewan, T. H. J. Chem. Soc. 1960, Part IX, 3973.
Photoassisted Degradation of Phenyltin(IV) Chlorides
Langmuir, Vol. 14, No. 2, 1998 391 Table 3. First-Order Rate Constants for the Photodegradation of the TPT Species (in Water at pH ) 5.5-6) in the Absence and Presence of TiO2a kinetic parameters
a
substrate
kobs (h-1)
〈r2〉
TPT/TiO2 TPTb
0.97 1.06
0.997 0.995
0.01 g L-1. b Taken from ref 7.
Figure 4. Kinetic profiles for the variations in concentration of species detected during the photodegradation of TPTCl in aqueous suspensions (pH ) 5.5.5-6) of TiO2 under UV illumination.
Figure 6. Linear plot of the reaction rate of TPTCl against the square root of the intensity of illumination (data taken from Figure 4).
Figure 5. Variations in concentration of species detected during the photodegradation of DPTCl2 in aqueous suspensions (pH ) 5.5-6) of TiO2 under UV illumination.
models, the L- or Langmuir-type isotherm means the existence of affinity between adsorbent and adsorbate in the initial part of the isotherm, but as the number of adsorbed molecules increases, it becomes more difficult for the solute molecules to find vacant sites. In accord with the results shown in Figure 1, the adsorption of TPT on TiO2 seems to fit this model. However, when there is cooperative adsorption, the adsorption takes place with increasing ease in the final part of the isotherm as the concentration of solute increases, suggesting that the molecules adsorbed increase the retention of additional similar molecules by interaction between them. This type is defined by an S-type isotherm, the type that seems to take place in our studies of the adsorption of the DPT species on TiO2 (Figure 3). Photodegradation of Phenyltin Chlorides by UV/ TiO2. Kinetic Studies. Photodegradation under conditions of UV illumination of suspensions of TiO2 in water (at pH ) 5.5-6) containing species of TPT or DPT has been studied. Figures 4 and 5 show the kinetic profiles for the degradation of these species with illumination time. For triphenyltin chloride, the results of the photoassisted degradation in the presence of TiO2 fitted firstorder kinetics. The rate constant of this photodegradation is shown in Table 3, together with the value of the same rate constant but in the absence of the catalyst (taken from ref 7). As can be seen, the rate constant in the presence of TiO2 is slightly lower than that in the absence of the
semiconductor. This could be attributed in principle to the phenomena of reflection and loss of the light by the catalyst grains, to the adsorption of other productssalso susceptible to photocatalytic degradationson the surface, to the parallel photolytic degradation of the phenyltin compounds present in the aqueous medium,7 or to other derivatives also photocatalytically degradable by the action of UV light, and lastly to the effect of the intensity of light used. These results suggest that the working system is not photocatalytic at all but photochemical, with similar rate constants obtained with and without titania. Figure 6 shows the linear plot of the reaction rate against the square root of the light intensity, under experimental conditions such as C0 ) 4 ppm, pH ) 5.5-6, and flux intensity absorbed by the TiO2 below 360 nm equal to 4.9 × 1020 photons/s. The data are taken from Figure 4 for the TPT derivative. The literature provides many examples on the effect of light intensity. Thus, Egerton and co-workers19a and Pichat and co-workers19b suggest that this type of linear dependence indicates that electron-hole recombination predominates over the reaction at the surface of the semiconductor particle. Thus light arriving at the catalyst surface produces the electron-hole pairs, but their recombination means that the amount of light used in the process is not used potentially for the TiO2-photoassited reaction of the organotin derivative adsorbed on the surface. In other words, even, for relative small radiant fluxes, the second-order recombination of charges19a predominates over the degradation of TPT; this could be a disadvantage for implementation of the method. In the curve for photoassisted degradation of diphenyltin dichloride in the presence of TiO2, the points showing the concentration of this species are greatly dispersed in the first 8 h of illumination (Figure 5). This may be because the action of the light modifies the adsorption equilibrium of the DPT species on the TiO2 surface and possibly that of the DPT species adsorbed in the monolayers following (19) (a) Egerton, T. A.; King, G. G. J. Oil Colour Chem. Assoc. 1979, 62, 386. (b) Al-Sayyed, C.; D’Oliveira, J.; Pichat, P. J. Photochem. Photobiol., A: Chem. 1991, 158, 99.
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Table 4. Intermediates Generated in the Photoassisted Degradation of TPT in the Presence of TiO2 at 5 h of UV Illumination retention time
molecular ion
name of compound
7:10 12:51 29:50 35:28 57:01
106 94 122 170 309
benzaldehyde phenol benzoic acid hydroxybiphenyl Ph2SnCl
Table 5. Intermediates Generated in the Photoassisted Degradation of DPT in the Presence of TiO2 at 40 h and 30 min of UV Illumination retention time
molecular ion
name of compound
6:46 12:51 13:27 30:50 34:47 35:30 41:08 43:03 43:19
106 94 122 122 154 170 182 182 198
benzaldehyde phenol hydroxybenzaldehyde benzoic acid biphenyl hydroxybiphenyl benzophenone formylbiphenyl formylhydroxybiphenyl
the first. After illumination of 8.5 h, the sequence of points on the curve shows a “pseudo-first-order” kinectics (data not shown), with a correlation coefficient of 〈r2〉 ) 0.996. The rate constant from that illumination time on is kobs ) 6.005 × 10-2 (h-1). Under the conditions of the TiO2-photoassited experiments shown in Figures 4 and 5 for the TPT and DPT derivatives, the GC-MS analysis at illumination periods of 5 and 40 h, respectively, showed the presence of peaks attributed to intermediates formed during the degradation process. As in the case of the TBT species,8 the possibility arises of masking by trace elements accompanying both the starting product and the solvents used, as a result of working at such low concentration levels (levels of ppm). Tables 4 and 5 list some of the possible products formed during the photoassisted reaction in the presence of TiO2. These photogenerated intermediates are products of partial oxidation detected in the aqueous phase and whose formation can be explained by the mechanism proposed below. In general, their concentration levels would be so low as to consider them trace elements in the solution (the initial concentrations of the reagent are very low). It is important to point out that, in the case of the photodegradation of both TPT and DPT assisted by TiO2, the photogenerated intermediate found at a retention time of 12:51 (phenol) appears by far the major compound. The possibility of total mineralization to CO2 of these intermediates via photocatalytic oxidation steps cannot be concluded with certainty since our system was open to the air and we did not monitor the amount of CO2 that may have been photogenerated. However, the literature reports total mineralization to CO2 of organic derivatives.20 In particular, Palmisano and co-workers have studied the total mineralization of phenol by photocatalytic oxidation in the presence of TiO2.21 Surface Analysis by IR. Figure 7 shows the IR spectra in the same frequency range of the following samples: (a) TiO2; (b) TPT; (c) TPT/TiO2 equilibrated in the saturation step; and (d) TPT/TIO2 after a UV illumination time of 5 h. (20) Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991, 25 (9), 1523. (21) Palmisano, L.; Schiavello, M.; Sclafani, A.; Marta, G.; Borello, E.; Coluccia, S. Appl. Catal. B: Environ. 1994, 3, 117.
Figure 7. IR spectra of sample surface: (a) TiO2; (b) TPT (commercial); (c) TPT/TiO2; (d) TPT/TiO2 + 5 h of illumination.
Comparison of spectra 7a, 7b, and 7c reveals qualitative evidence of the adsorption of triphenyltin chloride on the TiO2 surface, sincesas can be seen in the figuresmost of the bands assigned to TPT22 (Figure 7b) are present in the spectrum of the TPT/TiO2 sample (Figure 7c). Some examples of such bands are the following: a band at 1075 cm-1 assigned to breathing of the benzene ring linked to the tin,23 a band at 3030 cm-1 assigned to the CsH bond frequency in aromatics, and bands at approximately 750 and 700 cm-1 corresponding to the out-of-plane twisting of the CsH bond in aromatic rings.24 The IR bands associated with dissociatively adsorbed water, present on the original TiO2 surface (region 36003200 cm-1) (Figure 7a), are practically unchanged after adsorption of the TPT species (Figure 7c). This condition of the hydroxyl zone could be related with the mode of adsorption of the TPT species via OH- groups present on the surface, with consequent weakening of the Sn-Cl bond and the parallel formation of Ph3Sn+‚‚‚OH- and Cl-‚‚‚H+ interactions, in the same way as described for the butyltin derivatives.8 In the case of the phenyltin derivatives, adsorption is aided by the possible π interaction of the phenyl ligand with the Ti4+ center or OH- group, as mentioned before. Comparison between the profiles of spectra 7c and 7d and spectra 7a and 7d shows that they are quite different. In the case of spectra 7c and 7d, we go from a spectrum with narrow, pronounced bands (spectrum 7c) to another with wide bands (spectrum 7d). In particular, band widening is seen in the region between 3700 and 2800 cm-1 and two wide bands are present, centered at 1400 and 1100 cm-1, respectively. From the results obtained in the GC-MS analysis of the effect of illumination on aqueous suspensions containing TPT/TiO2 (Table 4), various organic products were detected (22) Kriegsmann, H.; Geissler, H. Z. Anorg. Chem. 1963, 323, 170. (23) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley and Sons: New York, 1978. (24) Pretsch, E.; Clerc, T.; Seibl, J.; Simon, W. In Tablas para la Ebullicio´ n de Compuestos Orga´ nicos por Me´ todos Espectrosco´ picos; Castells, J., Camps, F., Eds.; Alhambra: 1988.
Photoassisted Degradation of Phenyltin(IV) Chlorides
Langmuir, Vol. 14, No. 2, 1998 393 Scheme 1. Sequence Proposed for the Photocatalytic Degradation of Phenyltin Compounds in the Presence of TiO2 (Substrate TPTCI)
Figure 8. IR spectra of sample surface: (a) TiO2; (b) DPT (commercial); (c) DPT/TiO2; (d) DPT/TiO2 + 40 h and 30 min of UV illumination.
in the medium. Consequently, these may also be adsorbed on the TiO2 surface. In fact, the IR spectra of these products (benzaldehyde, phenol, and benzoic acid, for example) have IR bands25 that could be included in the bands centered at 1400 and 1100 cm-1. In addition, new bands appear at 2750, 2925, and 3200 cm-1 (spectrum 7d) which are within the range of vibration frequency (35502500 cm-1) of a carboxylic acid OH bond.24 It is therefore proposed in principle that these organic species are adsorbed, giving components that generate the wide bands in the IR. The contributions of each group would cause the band widening observed at both 1400 and 1100 cm-1. As mentioned previously, it is possible that CO2 is generated by successive steps of total oxidative photodegradation of the intermediates. If this were so, this species could also be adsorbed on the semiconductor surface, giving rise to carbonate/bicarbonate species, which would present IR bands23 around 1100 and 1300 cm-1. A similar discussion could be applied to the DPT species, observing Figure 8: (a) TiO2; (b) DPT; (c) DPT TiO2 equilibrated in the saturation step; and (d) DPT/TiO2 after 40 h and 30 min of UV illumination. Mechanistic Characteristics and Photoactivity. The mechanism of photoassisted degradation for phenyltin chlorides (Scheme 1) is defined in the same terms as that proposed for butyltin derivatives.8 For this reason the detailed discussion of the mechanism is not repeated here. The main difference between these proposed mechanisms for the two organotin series studied (butyl- and phenyltin) lies in the nature of the ligand accompanying the tin atom, so that the partial oxidation products or photogenerated intermediates would have different natures. Thus, the phenyltin derivatives will be aromatic, as can be seen in Tables 4 and 5 from the GC-MS analysis. From Tables 4 and 5 and the mechanism of Scheme 1, the following can be deduced: the TiO2-photoassisted degradation of phenyltin derivatives takes place via certain nonvolatile intermediates of partial oxidation of the phenyl ligand, such as phenol (in accord with eqs 2123 of Scheme 1), which remain in the photoreaction system. There they may be susceptible to adsorption onto the catalyst surface (as remarked in the section on IR analysis (25) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 2nd Ed.
of the surface) and to photocatalytic mineralization to CO2 and H2O once adsorbed. Data are known of the total mineralization of phenols in TiO2 at initial concentrations of approximately 370 ppm after UV illumination of 3-4 h.21 The CO2 formed could generate CO radicals by light absorption26a of low wavenumber, which in turn could attack Ph radical to generate the oxygenated derivatives that contain carbonyl CdO as a functional group, as is the case of benzaldehyde, benzoic acid, etc. Another plausible explanation to account for the formation of aromatic carbonyl group-containing molecules could be that CO2 is reduced as follows
CO2 + e- f CO2-
(4)
CO2- + H2O f HCOO• + OH-
(5)
and that these radicals attack the phenyl groups or react with C6H5• radicals. In fact Inoue et al.26b first reported that a suspension of TiO2 photocatalyzes the reduction of CO2 with water to produce formic acid, formaldehyde, methanol, and a trace amount of methane. In an alternative route, surface carbonate reacts with hydroxyl radicals
OH• + HCO3- f H2O + CO3- f CO• + O2-
(6)
and CO• can subsequently react with C6H5• radicals. These two latter mechanisms do not necessarily imply the formation of CO• radicals from CO2 photoactivation, since CO2 is known as a stable final product. In any case, this proposal is still speculative. Scheme 1 omits the step oxidation of the Cl- species by holes and consequent formation of the Cl• radical, since if the method of adsorption (see ref 8) is accepted, the chloride would remain in the second coordination sphere without the possibility of oxidation, in accord with the (26) (a) Wayne, R. P. J. Photochem. Photobiol., A: Chem. 1992, 62, 379. (b) Inoue, I.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637.
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literature.27 The formation of Cl• radicals in the medium could occur by homolytic cleavage of the Sn-Cl bond of the TPT derivative during the photolytic process, in accord with the description in the section on the photolytic degradation of phenyltin derivatives. As mentioned before, there is a photolytic process of the phenyltin species in the medium at the same time as the TiO2-photoassisted one. However, observing Tables 4 and 5, chlorinated derivatives do not appear among the intermediate products of partial oxidation. This does not necessarily indicate that they have not been formedsthey could be masked by other intermediates, due to their low concentration in the medium. It is noteworthy that, under TiO2-photoassisted conditions similar to those of the butyltin derivatives in the presence of TiO2, these chlorinated derivatives were readily detected.8 This is difficult to explain, and further study, using techniques such as laser flash photolysis, is necessary. Nevertheless, there are certain factors that may help us to understand this aspect: the possibility that the photogenerated Cl• radical could be deactivated by O2 dissolved in the medium;7 the much lower concentration of organotin species in the medium for the phenyltin series; the greater stability of the Bu• radical than the Ph• radical; and lastly, the factor related with the dissociation equilibrium of the organotin halides in the presence of an electron donor,28 as illustrated in eq 7:
R3SnX + :Y a R3SnY+ + X-
(7)
The species R3SnY+ can undergo later reactionsfor instance the transfer of a proton in the case that Y ) H2O, eq 8:
R3SnOH2+ + H2O a R3SnOH + H3O+
(8)
The extent of this reaction depends on the nature of both R and Y. In the concrete case of aqueous solutions of phenyl- and butyltin chlorides, the donor would be Y ) H2O, while R varies in the two organotin series, phenyl or butyl. The effect of the variation of R has been studied previously29 by measuring conductivity, which gives an approximate measurement of the degree of dissoaciation. Organotin halides of the general formula R3SnCl, where R varies from an alkane such as Et or Pri to an aromatic ring such as phenyl, have been studied. It was observed that conductivity increased from the ethyl radical to the phenyl one, implying that the dissociation constant would increase in this same order. Thus we can conclude in principle that in the phenyltin derivatives the equilibrium of hydrolysis is displaced further toward the formation of Cl- and Ph3SnOH2+ ions than in the butyltin series. If this is so, a lower concentration of Ph3SnCl species as such would remain in the medium. Thefore the homolytic cleavage of Sn-Cl to generate the Cl• radical (photolytically) would be less probable in the case of the phenyltin series. Quantum Yields (QYs), Turnover Number, and Relative Photonic Efficiency. Table 6 summarizes the values of the “apparent” quantum yields of the phenyland butyltin species for the photoassisted degradation in the absence or in the presence of TiO2. (27) (a) Herrmann, J. M.; Pichat, P. J. Chem. Soc., Faraday Trans. 1980, 7b, 1138. (b) Munuera, G.; Navı´o, J. A.; Soria, J.; Gonza´lez-Elipe, A. R.Proc. 7th International Congress on Catalysis; Seiyama, T., Tanabe, K., Eds.; Kodancha LTD: Tokyo, 1981, 1185. (28) Ingham, R. K.; Rosenberg, S. D.; Gilman, H. Chem. Rev. 1960, 60, 459. (29) Prince, R. H. J. Chem. Soc. 1959, 1783.
Table 6. “Apparent” Quantum Yields of the Phenyl- and Butyltin Species for the Photoassisted Degradation in the Absence or in the Presence of TiO2 106 × (QY) compound
absence of TiO2
presence of TiO2
TPT DPT TBT DBT
1.12a
1.580 1.110 0.056c 0.430c
3.72a 5.48b 5.04b
a Data obtained from ref 7. b Data obtained from ref 6. c Data obtained from ref 8.
In the presence of TiO2 the value of the quantum yield is indicative, as its formulation assumes that all the photons emerging from the lamp below 360 nm are totally adsorbed by the grains of the catalyst. However, this is not strictly true, as they are partly dispersed by the solid particles of the catalyst and partly absorbed by sustances in the medium (for instance, reaction intermediates), producing or not producing photolytic reaction. Photolytic processes may also affect the phenyltin species dissolved in the medium. Another important process that may take place during the TiO2-photoassisted reaction is the adsorption of the intermediates formed on the catalyst surface. It seems that this does happen, according to the surface analysis data. The species studied may also undergo parallel photocatalytic reactions. These must all be taken into account when comparing QY data in photolytic experiments (without catalyst) and TiO2-photoassisted experiments. In the absence of TiO2 the QY has been evaluated considering the photon flux for each emission line of the lamp below the respective values of adsorption for each species: for the TPT derivative, below 265 nm, and for the DPT, below 250 nm. If the TiO2-photoassisted processes of the two organotin series compared in this work are compared overall, differences can be seen in their quantum yields (Table 6). Thus, for the phenyltin series the quantum yields are greater than those for the butyltin series. This may be attributable in principle to the higher adsorption capacity of the phenyltin derivatives, in accord with the foregoing. The turnover number (TN), defined as the number of the reacting molecules or product molecules formed per surface active site in heterogeneous catalysis (photocatalysis), will be established whether this process is truly catalytic (TN . 1) or is stoichiometric (TN ) 1).30 In order to estimate the number of surface active sites we propose the assumption of Somorjai31 that about 10% or less of the surface sites are active in any given reaction. Assuming the above considerations and a surface density of OHgroups of 1013 per cm2 for TiO2, the specific TN for the TiO2-photoassisted degradation of TPT species was determined as 0.81, which is close to 1. This value, considered as a conservative estimate of the real turnover, clearly indicates that TiO2-photoassisted degradation of TPT is stoichimetric, as observed for TBT species.8 At the same time, because term quantum yields seem to be ambiguous and inapplicable in heterogeneous photocatalysis, as has been emphasized by Serpone et al.,30 we have estimated the relative photonic efficiency by the (30) Serpone, N.; Pelizzetti, E.; Hidaka, H. In Photochemical and Photoelectrochemical Conversion and Storage of Solar Energy; Tian, Z. W., Cao, Y., Eds.; International Academic Publishers: Beijing, 1993; p. 33. (31) Somorjai, G. A. in PhotocatalysissFundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: New York, 1989; Chapter 9.
Photoassisted Degradation of Phenyltin(IV) Chlorides
methods protocol suggested earlier32 which relates the initial rate of substrate disappearance with the rate of incident photons reaching inside the front window of the reactor. According to this method a value of ca. 2 × 10-6 is given for the photonic efficency of the TiO2-photoassisted degradation of TPT. 4. Conclusions For triphenyltin(IV) chloride in water, the initial concentration (1 ppm) falls approximately 78% at basic pH, in the dark at room temperature for an overall period of 4 days; while under the same conditions but at acidic pH, the initial concentration remains practically unaltered. The apparent fall observed for the concentration of TPT under the former conditions seems to be associated with the capacity of adsorption on the glass walls of the container at basic pH. The adsorption of triphenyltin(IV) chloride in aqueous suspensions (pH ) 5.5) of TiO2 can be described by a Langmuir-type isotherm with an equilibrium constant (K) of 2.39 × 102 (mM-1) and a maximum adsorption extent (nmax) of 5.71 × 10-4 (mol g-1) under our experimental conditions. In contrast, the adsorption isotherm for the diphenyltin(IV) dichloride in aqueous suspensions of TiO2 seems to be type S. The method of adsorption of triphenyltin chloride on the TiO2 surfacesfrom the data obtained from IR analysissseems to be via surface hydroxyl groups, which act as bridges between Sn(IV) and Ti(IV). The adsorption of the TPT species is favored over that of tributyltin chloride by the possible π interaction of the phenyl ligand with the Ti4+ center or OH- groups. The rate constant for the photodegradation of triphenyltin(IV) chloride in the presence of TiO2 is slightly lower than that in the absence of the catalyst. GC-MS analysis of the photoassisted degradation of triphenyltin(IV) chloride and diphenyltin(IV) dichloride in the presence of TiO2 during illumination periods of up (32) Serpone, N.; Sauve`, G.; Koch, R.; Tahiri, H.; Pichat, P.; Piccinini, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol., A: Chem. 1996, 94, 191.
Langmuir, Vol. 14, No. 2, 1998 395
to 5 and 40 h, respectively, shows the appearance of peaks attributed to some of the intermediates photogenerated and present in the liquid phase. The products of partial oxidation detected were the following: in the case of the photoassisted degradation of triphenyltin(IV) chloride in the presence of TiO2 after 5 h of illumination, benzaldehyde, phenol, benzoic acid, hydroxybiphenyl, and diphenyltin chloride; in the case of the photoassisted degradation of diphenyltin(IV) chloride in the presence of TiO2 after 40 h of illumination, benzaldehyde, phenol, hydroxybenzaldehyde, benzoic acid, biphenyl, hydroxybiphenyl and benzophenone. The “apparent” quantum yield of the photoassisted degradation of diphenyltin dichloride in the presence of TiO2 is slightly lower than that of the photoassisted degradation of triphenyltin(IV) chloride in the absence of TiO2. The quantum yields for the photoassisted degradation of phenyltin(IV) species in the presence of TiO2 are higher than those for the photoassisted degradation of the butyltin(IV) species in the presence of TiO2. In general, the values of the quantum yields of the phenyltin(IV) species in the presence of TiO2 are plausible, given that the process of photoassisted degradation implies the partial or total photooxidation of species generated from the phenyl ligands. This parallel process of photooxidation may affect the overall quantum yields. Parallel photoassisted processes take place in the species dissolved in the medium, also affecting the quantum yield value. In any case the relative photonic efficiency of the TiO2photoassisted degradation of triphenyltin(IV) chloride is as low as 10-6. Finally, our results allow the conclusion that this is not a photocatalysis system but a photoassisted one in the presence 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 PB93-917) for supporting part of this work. LA970318D
