UV−Visible Diffuse Reflectance Spectroscopy of Zeolite-Hosted

Mononuclear titanium oxide species in faujasites of different Si/Al ratios are prepared by reaction of dehydrated zeolites with TiCl4 at 100 °C. The ...
0 downloads 0 Views 154KB Size
Download PDF
J. Phys. Chem. B 1997, 101, 1305-1311

1305

UV-Visible Diffuse Reflectance Spectroscopy of Zeolite-Hosted Mononuclear Titanium Oxide Species Joachim Klaas, Gu1 nter Schulz-Ekloff, and Nils I. Jaeger* Institut fu¨ r Angewandte und Physikalische Chemie, UniVersita¨ t Bremen, D-28334 Bremen, Germany ReceiVed: August 30, 1996; In Final Form: December 17, 1996X

Mononuclear titanium oxide species in faujasites of different Si/Al ratios are prepared by reaction of dehydrated zeolites with TiCl4 at 100 °C. The analysis of UV-vis diffuse reflectance spectra in combination with Raman spectroscopy and X-ray diffraction reveals the presence of three different types of titanium oxide species in the faujasite structure exhibiting excitation energies for the charge transfer from the 2p orbitals of oxygen ligands to the 3d orbitals of Ti of around 36 000, 45 000 and 49 000 cm-1. A reliable evaluation of diffuse reflectance spectra requires the calculation of the Kubelka-Munk spectra from the absolute reflectance of the sample. The different titanium oxide types are proposed to result from monofunctional, bifunctional, and multifunctional bonding of TiCl4 to isolated or neighboring OH groups or to hydroxyl nests with coordination numbers ranging from four to six. The blue shifts in the UV-vis spectra of mononuclear titanium oxide species can be discriminated from those of quantum-sized titania particles.

1. Introduction Highly dispersed titanium oxide is widely used in catalysis,1-4 photocatalysis,5-10 and optical filters.11 The latter application focuses on the development of materials with altered electronic properties based on either the size-quantization effect or the different oxidic environments of the titanium. Zeolite molecular sieves are suitable supports for the preparation of highly dispersed semiconductors,12-16 and titanium oxide-loaded zeolites have been studied repeatedly.17-20 Since small titania particles as well as molecular titanium oxide species exhibit blue shifts in the absorption spectra21,22 either due to quantum size effects or to altered excitation energies, an unambiguous identification of the origin of the effect requires a detailed analysis of the type of TiOx species. The characterization of faujasite-supported titanium oxide species by diffuse reflectance spectroscopy (DRS) combined with Raman spectroscopy and X-ray diffraction will be presented and discussed in the following. To obtain reliable results, the influence of absorbing supports on the Kubelka-Munk spectrum must be taken into consideration, since zeolites show some absorption in the same spectral region as the molecular titanium oxide. The method has been used repeatedly to probe the existence of framework and nonframework titanium oxide species, namely, in dependence on the synthesis conditions of TS-1.23 2. Experimental Section 2.1. Preparation. The commonly used ion exchange method for preparing zeolite-hosted clusters is difficult to apply in the case of titanium. TiO2+ ions are only stable in acid solutions (pH < 2), where zeolites with a larger ion exchange capacity are at least partially destroyed. Therefore, a method similar to chemical vapor deposition has been employed, which will be described in the following. Faujasites with Si/Al ratios from 1.2 to 100 were used. NaX (Si/Al ) 1.2) and NaY (Si/Al ) 2.7) were synthezised according to standard procedures. Isomorphously substituted zeolite NaY X

Abstract published in AdVance ACS Abstracts, January 15, 1997.

S1089-5647(96)02713-7 CCC: $14.00

designated DAY with Si/Al ) 100 was a gift of the DEGUSSA company. For comparison purposes samples with TiOx deposited on silica gel (Merck Kieselgel 40) were prepared as well. The samples were prepared in a flow reactor under shallowbed conditions. Following the dehydration of the zeolite powder (1 g DAY or 1.5 g NaY, NaX) in a stream of dry nitrogen overnight at 400 °C, dry nitrogen (2.5 L/h), loaded with TiCl4 at room temperature, was led through the reactor. The chemisorption of the TiCl4 by the zeolite occurred at temperatures of 100 or 400 °C and at varied times of the zeolite in the TiCl4 stream. Excess TiCl4 was removed by flushing the reactor for at least 1 h with dry nitrogen at the chemisorption temperature. Thereafter, the chemisorbed TiCl4 was hydrolyzed by a watersaturated nitrogen flow again at the same temperature. Finally, remaining HCl was removed by a stream of dry nitrogen at 250 °C. Some samples were calcined in an oven under atmospheric pressure at 600 °C. Preparation conditions, properties, and designation of the samples are listed in Table 1. The samples are labeled by the type of support, the TiCl4 adsorption temperature, and the time of TiCl4 treatment. The titanium content was determined by analyzing the amount of TiCl4 not adsorbed by the zeolite and hydrolyzed in a gaswash bottle. For some samples the results were confirmed within 20% by direct analysis of the loaded zeolite using X-ray fluorescence and chemical analysis. 2.2. Characterization. Diffuse UV-vis spectra of the zeolite powders were recorded on a Varian Cary 4 photospectrometer equipped with a Praying Mantis (light collector). The light spot was about 1-2 mm. The powders were filled in a hole (10 mm in diameter and 3 mm deep) of a sample holder, and the surface was smoothed. The layer can be regarded as infinitely thick, as required by the Kubelka-Munk theory. The selected recording parameters comprised a spectral bandwidth of 4 nm, an integration time of 0.5 s, and a data point distance of 1 nm. The samples were stored under normal conditions, and spectra were recorded directly in air with no further pretreatment. Samples showing an absorbance of F(R) > 3 were diluted with untreated zeolite in the case of TiOx zeolite or with BaSO4 otherwise. The standard from LOT with 75% reflectivity was © 1997 American Chemical Society

1306 J. Phys. Chem. B, Vol. 101, No. 8, 1997

Klaas et al.

TABLE 1: Sample Numbers, Abbreviated Notations, Temperatures (°C), and Times (min) of TiCl4 Treatment, Ti Content (wt %), and Onsets of Absorption Edges (nm) for TiOx Species in Faujasites and on Silicac no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

sample notation TiOxDAY TiOxDAY TiOxDAY TiOxDAY TiOxNaY TiOxNaY TiOxNaY TiOxNaY TiOxNaY TiOxNaY TiOxNaX TiOxNaX TiOxNaX TiOxNaX TiOxSiO TiOxSiO TiOxSiO TS-1a anataseb

temp, °C

time, min

Ti content, wt %

onset, nm

Raman absorption

100 100 400 400 100 100 100 400 400 400 100 100 400 400 100 100 400

10 60 30 60 10 30 60 10 30 60 10 60 30 60 10 60 30

0.6 2.0 0.6 0.5 0.6 1.3 2.7 0.6 1.3 2.2 0.1 0.3 2.1 4.2 0.6 3.6 2.0 1.9

327 342 327 327 340 331 333 283 284 320 298 244 334 334 293 349 346 245 373

+ + + +

a BaSO was used as diluent. b Mixture of 20 wt % anatase and 80 wt % DAY. c Titanium silicalite (TS-1) and anatase are reference materials. 4 The intersection of the tangent through the point of inflection with the X-axis was used as the onset.

used as a reference. The measured intensity of the sample Jsample was related to the measured intensity of the standard reference Jstd and corrected for the absolute reflectivity of the reference σstd ) 75% according to Rsample ) σstdJsample/Jstd. According to Kortu¨m,24 the absolute reflectance R∞ of an infinitely thick layer must be used to calculate Kubelka-Munk spectra

(1 - R∞)2 K F(R∞) ≡ ) ≈c 2R∞ S K and S are the effective absorption and scattering coefficients. The effective absorption coefficient K is proportional but not equal to the absorption coefficient determined by transmission experiments. The Kubelka-Munk theory24 predicts that F(R) is proportional to the concentration c of the absorbing species as long as the assumptions of the theory are fulfilled, namely, that the absorbance of the sample is not too high. F(R) spectra were found to be linear up to F(R)max ) 3.5. A higher absorption causes a change in the spectrum that is observed as a blue shift in the F(R) spectrum. Using a reference with even minor absorbance without correction leads to Kubelka-Munk spectra that differ significantly from those calculated from the absolute reflectance of the sample. Figure 1 shows one example. F(R′) is the Kubelka-Munk spectrum calculated with the reflectance of the sample relative to an absorbing reference NaY. The difference of the F(R) spectrum calculated from the absolute reflectance is significant. The point of interest in this work was the absorbance of the TiOx species but not the absorbance of the support that was used as diluent in most cases as well. A simple subtraction eliminates the absorbance of the diluent:

F(R)TiO ) F(R)sample - F(R)diluent All spectra presented in this work are corrected for the absorbance of the support and diluent. X-ray diffractograms were recorded on a Seifert diffractometer with a Bragg-Brentano setup using Cu KR radiation under ambient conditions. Crystallinity was also checked by physisorption of N2 at 77 K and calculation of the total surface according to the BET method. No significant loss of crystal-

Figure 1. Comparison of diffuse reflectance UV-vis spectra (KubelkaMunk functions) of relative (F(R′), dashed curve) and absolute (F(R), solid curve) reflectivities for TiOx species in NaY (sample 6). For F(R′) the reflectivity of TiOxNaY relative to NaY was used, which has some absorbance in the UV region

linity could be detected in the case of NaY and DAY regardless of the treatment conditions, while a partial framework collapse occurred in the case of NaX treated at 400 °C. Backscattering Raman spectra were measured on a Bruker IFS FT-Raman spectrometer equipped with a Nd:YAG laser operating at a wavelength of 1064 nm. The formation of anatase can be detected by the absorption at 144 cm-1.25 An example is depicted in Figure 2. Anatase is observable only in NaY and SiO2 treated by TiCl4 at 400 °C. The method is very sensitve for anatase as shown by the very strong band at 144 cm-1 in Figure 2. 3. Results 3.1. Low-Temperature Loading with TiCl4. Figure 3 depicts two spectra obtained for TiOx deposited on silica

Titanium Oxide Species

Figure 2. Raman spectra of one TiOxNaY sample loaded at low temperature (sample 6, top) and one sample loaded at high temperature (sample 9, bottom). The position of the strongest anatase line is marked.

Figure 3. Development of the high-frequency and low-frequency domains of the normalized diffuse reflectance UV-vis spectrum F(R) for TiOx species on silica for two treatment times with TiCl4: for 10 min (solid line, sample 15); for 60 min (dashed line, sample 16).

following the exposure to TiCl4 at 100 °C for 10 and for 60 min and subsequent treatments. Distinct absorption can be observed in a low-frequency region (30 000-40 000 cm-1) starting at 28 000 cm-1 and in a high-frequency region above 40 000 cm-1. The relative fraction of the low-frequency domain of the integral intensity increases with the titanium content that rises proportionally to the time of treatment. Similar features with respect to the range of absorption and the temporal development of the spectra are observed for TiOx species in dealuminated faujasite Y (Figure 4) and in NaY (Figure 5). In the case of TiOxNaX samples a similar evolution of the spectra with exposure time could not be observed

J. Phys. Chem. B, Vol. 101, No. 8, 1997 1307

Figure 4. Development of the high-frequency and low-frequency domains of the normalized diffuse reflectance UV-vis spectrum F(R) for TiOx species on dealuminated faujasite Y for two the treatment times with TiCl4: for 10 min (solid line, sample 1); for 60 min (dashed line, sample 2).

Figure 5. Development of the high-frequency and low-frequency domains of the normalized diffuse reflectance UV-vis spectrum F(R) for TiOx species on zeolite NaY for two treatment times with TiCl4: for 10 min (solid line, sample 5); for 60 min (dashed line, sample 7).

probably because of the low loading of the specimen not exceeding 0.3 wt %, which is independent of the reaction time. No clear correlation between the F(R) value of the maximum and the titanium loading was found. Even TiOxNaX samples with a loading of less than 0.3 wt % show F(R) > 1, like spectra of physical mixtures of 20 wt % anatase in zeolite. 3.2. High-Temperature Loading with TiCl4. The spectrum of NaX treated with TiCl4 at 400 °C (Figure 6) shows a strong absorption in the low-frequency region similar to silicasupported TiOx (Figure 3). Freshly prepared TiOxNaY specimens (sample 9 in Table 1) absorb almost exclusivly in the high-frequency regime (Figure

1308 J. Phys. Chem. B, Vol. 101, No. 8, 1997

Klaas et al.

excess TiCl4, i.e., the formation of multinuclear (tSisOs)mTixCly clusters, is excluded.27 This means that the subsequent hydrolysis

(tSisOs)mTiCl4-m + (4 - m)H2O f (tSisOs)mTi(OH)4-m + (4 - m)HCl (3)

Figure 6. Normalized diffuse reflectance UV-vis spectra F(R) of NaX TiCl4 treated at 400 °C (dotted line, sample 13), of NaY freshly prepared (short dashed line, sample 9), and after 3 months (long dashed line, sample 9a). TS-1 (solid line, sample 18) is presented for comparison purposes. The curves are labeled with the sample number (Table 1).

6, sample 9). The spectrum is very similar to that obtained for a TS-1 reference sample (Figure 6, sample 18) with titanium isomorphously substituted in the zeolite framework. Aging of the TiOxNaY specimen led to a red shift of the spectrum (Figure 6, sample 9a) with rising absorption below 40 000 cm-1. The treatment of zeolite NaY with TiCl4 at 400 °C results in spectra dominated by TiOx species absorbing in the highfrequency region (Figure 6, sample 9). This feature is largely maintained even for the aged sample. Zeolite NaX behaves quite differently if loaded at high temperature, i.e., the spectrum is dominated by TiOx species absorbing in the low-frequency domain (Figure 6, sample 13). The high-temperature treatment of dealuminated faujasite Y does not change the pattern of the UV-vis spectra. The temperature of treatment with TiCl4 influences the resulting loading with Ti, depending on the support used, i.e., the Ti content for the high-temperature loading is (1) much higher for NaX, (2) unchanged for NaY and SiO2, and (3) lower for DAY (cf. Table 1). In the case of NaX the high-temperature treatment leads to a much higher titanium content accompanied by increasing framework collapse with time-on-stream. However, no anatase particles could be observed in the Raman spectrum in contrast to the NaY and SiO2 supports where a strong band at 144 cm-1 appeared. 4. Discussion 4.1. Low-Temperature Loading. 4.1.1. UV-Visible Band Patterns. The following interpretation of the UV-vis bands of TiCl4-treated and hydrolyzed zeolites will be based on arguments developed for the analysis of TiCl4-loaded and hydrolyzed silica.21,26,27 For the latter system it was concluded27 that TiCl4 reacts with one OH group at the silica surface under evolution of HCl (1) or in a multifunctional manner with vicinal

OH groups (2) A reaction between (tSisOs)mTiCl4-m and

yields exclusively mononuclear, isolated TiOx species.21,26 x is used to account for the stoichiometry of the mononuclear species deviating from bulk TiO2. The existence of mononuclear, isolated TiOx was concluded by Lassaletta et al.28 for TiO2 prepared on quartz from evaporated titanium metal in an oxygen atmosphere. The features of the UV-vis spectrum of Ti-loaded silica (Figure 3, samples 15 and 16) suggest the presence of at least two types of TiOx species. These two species can be related to a monofunctional bonding of TiOx to isolated OH groups and a bifunctional attachment to vicinal silanol groups.29 However, spectra synthesis for all samples under survey in this work can be carried out satisfactorily only if the presence of three different bands is assumed with absorption maxima at about 36 000, 45 000, and 49 000 cm-1. This is shown in Figure 7 for NaX (sample 11 in Table 1) and in Figure 8 for NaY (sample 5 in Table 1). Both samples were prepared at 100 °C with low titanium content. The third band has a maximum at about 49 000 cm-1 as shown in Figures 7 and 8. This is about the same energy as the absorption in the spectrum of TS-1 shown in Figure 6. 4.1.2. Excitation Energies of Different TiOx Species. The following assignment of the absorption maxima at 36 000, 45 000, and 49 000 cm-1, gleaned from the spectra synthesis, to the different TiOx species bonded in a monofunctional, bifunctional, or multifunctional manner to the zeolite framework is mainly based on (i) theoretical considerations30 and (ii) interpretations of the UV-vis spectra of TS-1.31 The theoretical calculations of the energies required for the excitation of the charge transfer (CT) from the 2p orbitals of oxygen ligands to the 3d orbitals of Ti(VI)30 consider published electronegativities of Ti and OH ligands.32 For perfect symmetries an increase of the excitation energy from 42 000 cm-1 for octahedral coordination, i.e., Ti(OH)6, to 48 000 cm-1 for the tetrahedral one, i.e., Ti(OH)4, is calculated.32 For TiOx species bonded to supports such as zeolites, a strong deviation from perfect symmetries can be expected so that exact calculations of excitation energies will not be possible. However, the basic conclusion that the energies will increase with decreasing coordination number should be applicable for an assignment. The UV-vis spectra of samples treated with TiCl4 at 100 °C are dominated by the band around 45 000 cm-1. Presumably, the reaction of TiCl4 with neighboring hydroxyl groups is preferred as with SiO2.27 Thus, the 45 000 cm-1 band relates to titanium with two bonds to the support. This band may correspond to the UV absorption of hydrated TS-1. Since EXAFS data of TS-1 were interpreted in the way that a fraction of titanium is 5-fold coordinated,33,34 the 45 000 cm-1 band may correspond to such titanium species. The growth of the band at 36 000 cm-1 with rising titanium loading corresponds to titanium with one bond to the zeolithe framework originating from the less favorable reaction with isolated OH groups.

Titanium Oxide Species

J. Phys. Chem. B, Vol. 101, No. 8, 1997 1309 TABLE 2: Correlation between Preparation Conditions As Summarized in Table 1 and Predominant Mononuclear Ti Oxide Species According to the Diffuse Reflectance UV-Visible Spectra no.

Figure 7. Normalized diffuse reflectance UV-vis spectrum F(R) of TiOx species in zeolite NaX (solid line, sample 11) and synthesized spectrum (dashed line) based on three species absorbing at 36 000, 45 000, and 49 000 cm-1 (dotted lines).

sample notation

1 2

TiOxDAY TiOxDAY

5

TiOxNaY

7

TiOxNaY

9 11 13

TiOxNaY TiOxNaX TiOxNaX

18

TS-1

predominant bands cm-1

coordination to the support

45 000 45 000 36 000 45 000 36 000 45 000 36 000 49 000 45 000 45 000 36 000 49 000

bifunctional bifunctional monofunctional bifunctional monofunctional bifunctional monofunctional tetrahedral bifunctional bifunctional monofunctional tetrahedral

anatase + -

will be possible in the case of silica gel in pores of the rough surface, explaining the presence of a band at 49 000 cm-1 in all samples. The relative fraction of the integrated intensity is low for TiOxSiO2 and high for TiOxNaY prepared by TiCl4 treatment at 400 °C, which will be discussed below. 4.1.3. Origin of Different TiOx Species in Zeolites. Since the Raman spectra, which sensitively indicate multinuclear TixOy species with octahedrally coordinated titanium if present,35 exhibit no absorption at the position of the strongest anatase band (144 cm-1), the existence of any kind of titania clusters in the zeolite framework can be ruled out. The three different types of mononuclear TiOx species required for the spectra deconvolution should be related to three different types of OH groups existing in the zeolite structure. Silanol groups analogous to those in silica exist also in zeolites and are characterized by infrared spectroscopy (3740 cm-1)36 and H NMR spectroscopy.37 Isolated OH groups are assumed to exist at silica impurities or as terminal silanol groups at defects. Vicinal OH groups can be generated by hydrolysis of siloxane bonds according to

tSisOsSit + H2O f 2(tSisOH)

(4)

Thus, monofunctional as well as bifunctional bonding of TiCl4 to a zeolite support is possible, such as on silica surfaces. The third species, i.e., the one absorbing at ca. 49 000 cm-1, is assumed to result from multifunctional bonding of TiCl4 to more than two OH groups. A multiple interaction is possible at a peculiar defect in zeolites denoted as hydroxyl nest,38 which originates from a removal of aluminum atoms from framework positions according to

(tSisOs)4Al- + H+ + 3H2O f 4(tSisOH) + Al(OH)3 Figure 8. Normalized diffuse reflectance UV-vis spectrum F(R) of TiOx species in zeolite NaY (solid line, sample 5) and synthesized spectrum F(R) (dashed line) based on three species absorbing at 36 000, 45 000, and 49 000 cm-1 (dotted lines).

Spectra synthesis, carried out with a commercial program (Spectra Calc) using Gauss profiles exclusively, yields 49 000 cm-1 as the center for the absorption maximum of the third band. This is about the same position as for TS-1 with tetrahedrally coordinated titanium. It must be concluded that the band at about 49 000 cm-1 relates to titanium with multiple (more than two) bonds to the support. Since the absorption coefficient rises with deviation from octatedral symmetry and since tetrahedral coordination represents the largest deviation from octahedral symmetry of the possible TiOx species, a rather small number of multiple bonded TiOx species generate a detectable absorption in the UV-vis spectrum. This small number of species with multiple bonding

(5) The formation of such hydroxyl nests should be favored by the HCl formed from the interaction of TiCl4 with silanol groups (cf. eq 1). A generated hydroxyl nest is characterized by four neighboring OH groups enabling the multiple bonding of TiCl4 and the formation of TiOx species strongly attached to the zeolite framework. The absorption maxima and their assignment to different TiOx species in correlation with the preparation conditions are summarized in Table 2. 4.2. High-Temperature Loading with TiCl4. The correlation between time-on-stream and the amount of titanium found in TiOxNaX by analysis of the product (Table 1, column 4, samples 13 and 14) and the high absolute values can be readily understood, considering the zeolite framework collapse gleaned from strong background increase in the X-ray diffractograms and the corresponding decrease in the BET values. The framework collapse by the formed HCl (cf. eqs 1-3) is

1310 J. Phys. Chem. B, Vol. 101, No. 8, 1997

Klaas et al.

presumably accompanied by an increase of the number of silanol groups according to

(tSisOsAlt)-Na+ + HCl f NaCl + tSisOH + Alt (6) This self-accelerating process, i.e., reaction with TiCl4 in combination with the high temperature and framework collapse, favors the attachement of large amounts of TiCl4 in a monofunctional way. This conclusion is confirmed by the strong absorption in the low-energy region at about 36 000 cm-1 (Figure 6, sample 13) and the absence of anatase bands in the Raman spectrum. The framework collapse impedes the migration and aggregation of TiOx to titania species. However, Si/ Ti mixed oxide species were described by several authors39-41 for silica with a low titanium content where titanium is molecularly distributed with different coordination numbers. Since the UV-vis spectra of these Si/Ti mixed oxides42 show similar features as the spectra of TiOxNaX prepared by TiCl4 treatment at 400 °C, the presence of mononuclear TiOx species can be assumed in the latter system, too. The framework of zeolite NaY stays intact during the TiCl4 treatment at 400 °C. Two different titanium oxide species are generated. Raman spectra and, at higher titanium content, XRD patterns show the formation of bulk anatase, as found previously on TiCl4-treated silicalite.43 The UV-vis spectra of TiOxNaY (400) exhibit an absorption rising above 40 000 cm-1 (Figure 6), which is well-known for TS-1. At least a larger fraction of the titanium in TiOxNaY (400) has become part of the zeolite framework and is tetrahedrally coordinated. This is not really surprising, since Kraushaar et al.38 showed that a reaction of TiCl4 with ZSM-5 at 450 °C resulted in an incorporation of titanium into the zeolite framework on tetrahedral positions. They assumed that defects in the zeolite structure are necessary for an incorporation of titanium into the framework. But unlike in Ti-ZSM-5 samples tetrahedral titanium in TiOxNaY (400) is not stable. After several months under ambient conditions, the UV-vis spectra show a red shift, indicating an increace in the coordination number of TiOx (Figure 6, sample 9a). High-temperature treatment of DAY with TiCl4 is also accompanied by a maintenance of the framework but results in lower loadings with TiOx. This can be related to the low stability of silanol groups in DAY, the density of which will readily decrease by OH group recombination and water liberation at high temperatures. The decreased OH group density decreases the absolute amount of bonded TiCl4. 4.3. Blue Shifts of Absorption Edges. Size quantization (q-size) effects are reported for colloidal titania.44,45 Anpo et al.44 observed blue shifts of some nanometers in the absorption spectrum for colloidal TiO2 particles with sizes ranging from 5.5 to 3.8 nm. A blue shift of 15 nm was reported for 2.4 nm particles by Kormann et al.45 More pronounced effects with blue shifts in the absorption spectrum on the order of 30 nm were observed by Yoneyama and Haga46 for TiO2 incorporated in sheet silicates. Blue shifts of absorption edges for small titania particles have been reported repeatedly,44,45 and some data are given exemplarily in Table 3. The blue shifts are referred to the bandgap broadening of the semiconductor titania and are expected to be detectable for particle sizes below 10 nm.44 Therefore, various publications dealing with titanium oxide-loaded zeolites assign the observed blue shifts to quantum-sized titania particles.18-20 In most cases these assignments are based largely on the analysis of UV-vis spectra. The data presented here, however, indicate that blue shifts can also be expected if mononuclear TiOx species are formed via various kinds of attachment to different types

Figure 9. Normalized diffuse reflectance UV-vis spectra F(R) of TiOx species in zeolites NaX (sample 11), NaY (sample 5), and dealuminated Y (sample 1). The spectra of anatase (sample 19) and TS-1 (sample 18) are presented for comparison purposes. The curves are labeled with the sample number (Table 1).

TABLE 3: Data Representing Bandgap Energies Eg for Nanometer-Sized TiO2 Particles Taken from Absorbance or Luminescence Spectra sample (particle size) anatase TiO2 clay (1.5 nm) TiO2 clay (1.5 nm) TiO2 colloid (2.4 nm) TiO2 colloid (3.8 nm) TiO2 colloid (5.0 nm)

source spectra

Eg [nm]

refs

absorbance luminescence absorbance luminescence luminescence

384 348 332 369 371 375

47 45 45 44 43 43

of OH groups (Figure 9). Very helpful for the identification of the type of titanium oxide species that cause the observed blue shift is Raman spectroscopy, which sensitively indicates the presence of multinuclear TixOy clusters by the characteristic band at 144 cm-1. Another indicator of the existence of either mononuclear TiOx species or quantum-sized anatase particles is the absorbance that is more than 1 order of magnitude larger for the former species at comparable Ti content. The strongly enhanced absorption of the TiOx species can be referred to their distorted symmetry, thus canceling the inversion symmetry, which is valid for the octahedrally coordinated TiO6 units in anatase or rutile. The absorption coefficient of cations usually rises by 1 or 2 orders of magnitude if the coordination changes from centrosymmetry to noncentrosymmetry,47 i.e., from ideal to strongly distorted octahedral or to lower coordination. The absence of centrosymmetry is also the reason for the Raman inactivity of TiOx species attached to the zeolite framework. The Raman-active vibrations of TiO2 are vibrations of centrosymmetric units.25 5. Conclusions The interaction of thoroughly dehydrated zeolites of the faujasite type with TiCl4 at 100 °C followed by hydrolysis results in UV-vis spectra, which can be synthesized on the basis of three different bands with maxima around 36 000, 45 000, and 40 900 cm-1. The three bands are related to three different kinds of mononuclear TiOx species because of the absence of any indication for polynuclear species in the Raman spectra.

Titanium Oxide Species Increasing excitation energies for mononuclear TiOx species are expected for decreasing coordination numbers. The three TiOx types are proposed (i) to result from monofunctional, bifunctional, and multifunctional bonding of TiCl4 to the zeolite framework and (ii) to exhibit decreasing coordination numbers ranging from six for the 1-fold attachment to isolated OH groups to four in the hydroxyl nests. Blue shifts of small titanium oxide species prepared by interaction of TiCl4 with carriers exhibiting OH groups can be due to either mononuclear TiOx species with different coordination numbers or titania species of quantum size, i.e.,