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samples absorb significantly in the visible range, differComments on the Paper "Spectroscopic, ently from a corresponding pure Ti02 sample. In ref 2 we Photoconductivity, and Photocatalytic Studies of did not report definite attributions for the bands recorded Ti02 Colloids: Naked and with the Lattice Doped in the diffise reflectance spectra, although plausible with C 9 + , Fe3+,and V+Cations" assignments were reported in ref 3 on the basis of While the work reported in ref 1is of great value, there previously published papers. The spectra were not well are a few points which need clarification. This paper structured. At the time of publication of refs 2 and 3 we reports the hypothesis that CIA+ ions may be present in did not believe that sufficient experimental evidence the Ti02 matrix when it is doped with chromium(II1). This existed for hypothesizing the presence of such an unusual statement, i.e., the occurrence ofthe unusual d2oxidation CIA+ species in our chromium-doped Ti02 samples, alstate of chromium in a T i 0 2 matrix, is based on the though they were fired in an oxidizing atmosphere (ca. interpretation by the authors of some diffise reflectance 773 K for 24 h). spectra reported in the paper by Palmisano and coIn this Comment we wish to provide additional inforworkers.2 Serpone et al.' report in Figure l a the spectra mation deriving from very recent results obtained on the of their own chromium-doped titania samples and in samples examined in ref 2. A n X-ray photoelectron Figure l b some spectra, adapted from refs 2 and 3, relative spectroscopy (XPS) study' carried out on the same to other chromium-doped titania samples. The two types chromium-doped T i 0 2 samples used in ref 2 unequivocally of solids are completely different and therefore the shows that only Cr(II1) and Cr(VI) are present in those expected chemical properties also ought to be different. samples and that Cr(VI)reducedto Cr(II1)after irradiation The solid samples, whose spectra are reported in Figure by X-rays. An additional indication of the presence of la, were indeed obtained by rotoevaporating under Cr(II1) rather that CdIW arises from a structural study vacuum at ambient temperature colloidal solutions of T i 0 2 described in ref 8. Thus, to the best of our knowledge, the prepared via the controlled hydrolysis of Tic14 at 273 K pertinent literature reports no definite evidence on the in the presence of appropriate quantity of C P ions. By existence of CIA+ in titania (anatase or rutile) catalysts. contrast, the samples whose spectra are reported in Figure Two recent publications9J0report the presence of C r W ) l b are polycrystalline solids prepared by a coprecipitation in the form of CrOz segregated on the T i 0 2 particle surface method, i.e., by reacting an aqueous solution of Tic13 for samples containing more than -5 wt % chromium. containing the required quantity of Cr3+ ions with an However, we wish to note that the authors of refs 9 and aqueous solution of ammonia. After repeated operations 10 only hypothesize the presence of CrO2 as a separated of filtering and washing, the solids were dried at 393 K phase and that chromium content in the samples invesfor 24 h and finally calcined in air for 24 h at 773 K. From tigated by us in ref 2 was always less than 5 wt %. the observation of spectra reported in Figure lb, the red Another consideration reported in ref 1on Fe3+-doped absorption bands to ca. 700-800 nm were attributed by Ti02 samples also deserves some clarification. In ref 1, Serpone et a1.l to the possible presence of Cr4+ions in the Serpone and co-workers noted that some results reported lattice of T i 0 2 . The above statement was supported by by us in a recent EPR work1' contrasted with earlier two earlier papers which reported the finding of the observations.12 We hasten to note, however, that the existence of Cr4+ions in a forsterite (Mg2SiO4)~ r y s t a l . ~ ~authors ~ of ref 12 examined colloidal suspensions; conseAccording to Serpone et al.,l the reflectance bands reported quently, the comparison with the polycrystalline samples in ref 2 are similar to those attributed to CIA+in forsterite used in ref 11 is tenuous. On the other hand, the and, consequently, they hypothesized on the existence of experimental evidence that Fe(II1)present in the samples Cr4+in samples examined in ref 2. is reduced to Fe(I1)before any reduction of Ti(1V)to Ti(II1) In our opinion the finding that CIA+ ions present in seems to us not questionable on the basis of recent EPR" other materials give rise to some absorption bands similar evidence and also of temperature programmed reduction to those observed in our samples2 is not sufficient to experiment^.'^ In these latter experiments, no reduction support the hypothesis of the existence of such an unusual of Ti(IV) to Ti(II1) was observed before the reduction of oxidation state of chromium in a Ti02 matrix. It must be the entire quantity of Fe(II1) to Fe(I1). Hence, it seems noted that reflectance spectra may be strongly affected likely, as reported earlier,l' that Fe(II1)is a better electron by the morphological and textural features of the particles.6 trap than Ti(IV),at least in the polycrystalline samples The principal focus of the presentation of the reflectance studied by us. Finally, it is worth noting that results spectra in ref 2 was only to indicate that chromium-doped reported in ref 2 showed that the photoactivity of the irondoped titania specimen can be completely recovered by (1)Serpone, N.; Lawless D.; Disdier, J.; Herrmann, J.-M. Langmuir subjecting the spent catalyst to a strong thermal treatment 1994,10, 643. at -823 K in the presence of oxygen. The deactivation/ (2)Palmisano, L.;Augugliaro,V.; Sclafani, A.; Schiavello, M. J.Phys. activation of the powders was always accompanied by the Chem. 1988,92,6710. (3)Martin, C.;Martin, I.; Rives, V.; Palmisano, L.; Schiavello, M. J. disappearancdappearance of Fe(II1) detected by EPR Catal. 1992,134,434. measurements. This finding suggests that Fe(II1)reduces (4)Moncorge, R.; Cormier, G.; Simkin, D. J.; Capobianco, J . A. ZEEE to Fe(I1) during the photocatalytic process. J. Quantum Electron 1991,27, 114. (5)Whitmore, M.H.; Sacra, A.; Singel, D. J. J. Chem. Phys. 1993, 96, 3656. M. Schiavello,*J V. AugugliaroJ L. PalmisanoJ (6)Stone, F.S. In Surface Properties and Catalysis by Non-Metals; A. Sclafani? and A. M. Venezia' Bonnelle, J. P.; Delmon, B.; Derouane, E., Eds.; D. Reidel: Dordrecht,
1983;p 237. (7)Venezia, A. M.;Palmisano, L.; Schiavello, M.; Martin, C.; Martin, I.; Rives, V. J. Catal. 1994,147, 115. (8) Venezia, A.M.; Palmisano, L.; Schiavello, M. J.Solid State Chem. 1995,114,364. (9)Kahler, K.; Schlapfer,C. W.;vonZelewsky, A.; Nickl, J.; Engweiler, J.; Baiker, A. J. Catal. 1993,143, 201. (10)Engweiler, J.;Nickl, J.; Baiker, A.; Kohler, K.; Schlapfer, C. W.; von Zelewsky, A. J. Catal. 1994,145, 141. (11)Soria, J.;Conesa, J. C.;Augugliaro,V.;Palmisano,L.;Schiavello, M.; Sclafani, A. J. Phys. Chem. 1991,95,274. (12)Moser, J.; Gratzel, J.; Gallay, R. Helu. Chi&. Acta 1987, 70, 1596.
(13)Bickley, R. I.; Gonzllez-Carreiio,T.; Palmisano, L. Mater. Chem. Phys. 1991,29, 47.
Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy, and Dipartimento di Chimica Inorganica, University of Palermo, Via Archirafi, 26-28, 90123 Palermo, Italy Received February 27, 1995 LA950151T Dipartimento cli Ingegneria Chimica dei Processi e dei Materiali.
* Dipartimento di Chimica Inorganica.
0743-746319512411-3278$09.00/00 1995 American Chemical Society
