Langmuir 1992,8, 1265-1270
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Art i d e s Reactions of Hydrous Titanium Oxide Colloids with Strong Oxidizing Agents Tijana Rajh,' Zoran V. Saponjic, and Olga I. Micic Boris Kidric Institute, Vinca, P.O. Box 522, 11001 Belgrade, Yugoslavia Received October 21,1991. In Final Form: January 27,1992 The absorption spectra of the transients formed in the reactions of titanium hydrous oxide with SO4'-, TP+,or OH' species in alkaline aqueous solution have been studied by using the pulse radiolysis technique. Direct hole capture via interfacial electron transfer from colloidal titanium hydrous oxide to SO4*-radicals leads to the formation of a product which has the broad absorption spectrum with an onset at about 480 nm rising steeply toward the UV region. A completely different absorption spectrum was found for the hydroxyl radical adduct formed by the reaction of OH*with titanium hydrous oxide that has a maximum at 620 nm. This maximum disappears in acidic solution at pH 3 with assistance of H+, which transforms the OH adduct to the product the same as that formed in the reaction with Sod*-. The low-weight aggregates of titanium hydroxide also react with OH' radicals, and in the alkaline solution a similar transient spectrum with a maximum at 570 nm was obtained. In neutral and alkaline solutions the absorption spectra of transients formed in the reactions of OH*,TP+, or SO4'- with Ti02 do not appear to be dependent upon the mode of the colloid preparation. However, decay kinetics of these transients are very sensitive to surface chemistry. Introduction Titanium hydrous oxide colloids are the object of continuous interest because of the photoactivity of Ti02 suspensions in degradation of a wide variety of organic and inorganic Titanium hydrous oxide colloids with very small particle sizes absorb light without light scattering, and because of that powerful optical methods can be used for detection of short-lived intermediates. A number of the transient absorption spectra obtained in irradiated Ti02 colloidshave been rep0rted.6-l~ The trapped holes have been detected by laser spectroscopy in colloidal systems of Ti02-Pt6J and Ti02 with absorbed dyes.g On the other hand, it has been found recently, by using the pulse radiolysis technique, that the OH. radicals react with Ti02 a t a diffusion-controlled rate, which means that the reverse reaction, i.e., the desorption of the OH' radical to the solution, seems unlikely to occur on illuminated TiO2.I3 Extremely small Ti02 colloidal particles show mainly amorphous structure with a small proportion of anatase ( 1 ) Serpone, N.; Pelizzetti, E. Photocatalysis-Fundamentals and Applications; John Wiley & Sons: New York, 1989. ( 2 ) Fox,M. A. Top. Curr. Chem. 1987,142, 72. (3)Ollii, D.F. Enuiron. Sci. Technol. 1985, 19,480. ( 4 ) Anpo, M. Research on Chemical Intermediates-11; Elsevier: Amsterdam, 1989; p 67. ( 5 )Peterson, M. W.; Turner, J. A.; Nozik, A. J. J . Phys. Chem. 1991, 95,221. (6)Bahnemann, D.; Henglein, A.; Lilie, J.;Spanhel, L. J . Phys. Chem. 1984,88,709. (7)Bahnemann, D.;Henglein, A.;Spanhel, L. Faraday Discuss. Chem. SOC.1984, 78, 151. (8) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J . Am. Chem. SOC.1986,107,8054. ( 9 )Arbour, C.; Sharma, D. K.; Langford, C. H. J. Phys. Chem. 1990, 94,331. (10) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982,86, 241. ( 1 1 ) Dimitrijevic, N. M.; Savic, D.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1984,88, 4278. (12)Kolle, U.; Mower, J.; Gratzel, M. Inorg. Chem. 1986, 24, 2253. (13)Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166.
present.14-16 Bahnemann et a1.I6 showed that the Ti02 colloids can be dissolved readily in acidic solution (pH 1) while the crystalline powder (P25,Degussa Corp.) is very resistant. The published values for the solubility of hydrous Ti02 colloids show that even high amounta of the low-weight titanium hydroxide aggregates are present in neutral and alkaline solutions in equilibrium with the solid Ti02.17 Besides, titanium hydrous oxide colloids are water swollen gels and differ from the crystallineTiO2. However, the results obtained in this system may have relevance to Ti02 photocatalysis since large Ti02 particles in contact with the solution also have the water swollen surface with a character very similar to the colloids. In the present study, the pulse radiolysis was applied in order to detect intermediates of the chemical reactions of Sod*-, T12+,or OH' oxidizing species with colloidal titanium hydrous oxide. Experimental Section Materials. Solutions were prepared from analytical grade chemicals (BDH or Merck). Triply distilled water was used. The solution pH was adjusted with HCl, NaOH, or phosphate buffer. The pH was estimated with indicator paper since the colloids block the pores of the glass electrodes. Oxygen was removed by bubbling with argon or N20. The solution of TiO(C104)z (1 X lo-" M) was prepared by addition of the required amount of HC104 to titanium tetraisopropoxide and by consequentevaporation of this solutionunder reduced pressure at 35 O C . This freshly prepared solid was then dissolved in hot HC104solution (1M) and diluted to pH 1.7.The concentration of Ti02+ monomeric ions was determined by addition of HzOz and HClO4 (pH < l),and then the absorption of orange color of the peroxide complex, [(OH),Ti-O-01+, was (14) Maser, J.; Gratzel, M. Helu. Chim. Acta 1982, 65, 1436. (15)Davidson, R. S.;Morrison, C. L.; Abraham,J. J. Photochemistry 1984, 24, 27. (16) Kormann,C.;Bahnemann,D.W.; Hoffmann,M.R. J.Phys. Chem. 1988, 92, 5196. (17)Baes, C . F.,Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976.
0743-7463/9212408-1265$03.00/0 1992 American Chemical Society
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Figure 1. (a) Absorption spectra of titanium hydroxide at pH 10 as a function of concentration: (1) 3 X 10" M, (2) 1 X le6 M, (3) 5.5 x l W 5 M. (b) Absorption spectra of colloidal Ti02 (2.75 X 10-9 M) at pH 10 and after filtration through 20-A pore ultrafilters. Table I. Solubility of Titanium Hydrous Oxide* concentration of Ti(IV), M total
