FTIR and NMR Study of the Adsorbed Water on Nanocrystalline

10590

J. Phys. Chem. C 2007, 111, 10590-10596

FTIR and NMR Study of the Adsorbed Water on Nanocrystalline Anatase Javier Soria,*,† Jesu´ s Sanz,‡ Isabel Sobrados,‡ Juan M. Coronado,§ A. Javier Maira,† Marı´a D. Herna´ ndez-Alonso,† and Fernando Fresno† Instituto de Cata´ lisis y Petroleoquı´mica, CSIC. C/ Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain, Instituto de Ciencia de Materiales de Madrid, CSIC. Cantoblanco, 28049 Madrid, Spain, and Laboratorio de Aplicaciones Medioambientales de la Energı´a Solar, CIEMAT. AV. Complutense 22, 28040 Madrid, Spain ReceiVed: February 20, 2007; In Final Form: May 4, 2007

A hydrated anatase sample with crystal size of 11 nm, prepared by a method involving a calcination treatment, has been studied by FTIR, 1H MAS NMR, and TPD techniques, to determine the characteristics of the hydroxyls and water adsorbed on its surface. The 1H NMR and FTIR spectra of the sample, hydrated and evacuated at increasing temperature, show that most of the hydrated anatase water, weakly adsorbed and highly mobile, is forming the multilayer of water molecular arrangements (MAs). The easy desorption of this water at roomtemperature facilitates the observation of less mobile adsorbed water, whose removal originates two overlapped water desorption peaks, with maxima at 353 and 373 K, observed in the TPD profile. Most of this less weakly adsorbed water is desorbed from the MAs second layer, originating the low-temperature maximum. On the other hand, the removal of some water bound to bridging O2- anions, interacting with terminal hydroxyls, contributes to the maximum at 373 K. The TPD peak with maximum at 430 K is mainly formed by the desorption of water interacting with bridging hydroxyls. The TPD shoulder observed at 540 K is originated by the desorption of water coordinated to Ti4+ cations, forming the first layer of the water MAs. The anatase 1 H MAS NMR spectrum obtained after evacuation at 473 K is mainly formed by the signals of bridging and terminal hydroxyls.

Introduction Heterogeneous photocatalytic oxidation (PCO) is a promising technique for removal of volatile organic pollutants present at low concentration in indoor air and waste gas streams.1,2 Using a semiconductor catalyst and a UV light source, this technology can oxidize contaminant compounds into carbon dioxide and water. During the PCO, the semiconductor absorbs UV light, which excites electrons from the valence band into the conduction band. The resulting electron/hole pairs can then migrate to the surface and initiate redox reactions with adsorbed organic molecules.3 Titanium dioxide is the most widely used photocatalyst, because it provides the best compromise between photocatalytic performance and stability in most chemical environments, being also inexpensive and essentially nontoxic.4 Growing evidence suggests that anatase, thermodynamically less stable than rutile but frequently found in TiO2 particles with sizes near 10 nm,5 is the most active phase for oxidative detoxification reactions. For particles a few nanometers in size, the physical and chemical properties of anatase depend largely on the surface characteristics. This is particularly true for charge carriers trapping, because many defects associated with the surface act as trapping sites. However, in polycrystalline surfaces their exact nature and function have not been well established. Recently, anatase samples with attractive photocatalytic properties have been prepared by treating thermally or hydrothermally TiO2 amorphous precursors obtained by sol-gel methods.6-8 This preparation procedure permits the gradual modification of the anatase crystal size and its photocatalytic * Corresponding author. E-mail: [email protected]. † Instituto de Cata ´ lisis y Petroleoquı´mica. ‡ Instituto de Ciencia de Materiales de Madrid. § Laboratorio de Aplicaciones Medioambientales de la Energı´a Solar.

properties by systematic changes in the preparation conditions.8 The use of materials with gradually modified crystal size facilitates the study of the influence of this parameter on anatase characteristics,9,10 providing insights on the PCO mechanism, useful for the preparation of improved photocatalysts for specific PCO processes. Previous studies on toluene PCO over calcined anatase samples have shown increasing selectivity to CO2 with decreasing crystal size in the 11-20 nm range, while the steady-state value of the PCO rate remained nearly the same.8 As the toluene PCO selectivity to CO2 is directly related to the characteristics of the toluene adsorption sites at the anatase surface, a detailed study of these sites is needed to explain selectivity modifications. An obvious effect of decreasing the anatase crystal size is the increase of the concentration of surface defects (steps, edges, corners), where low coordinate Ti4+ and O2- ions are located. As these sites are toluene and water adsorption centers, the toluene adsorption can be affected by the water adsorption.11-13 In this way, a modification of the water adsorption originated by decreasing the anatase crystal size can influence toluene adsorption and its PCO carried out at room temperature (RT)8,11 on anatase exposed to humid atmospheres. Many vibrational studies have been carried out to determine the characteristics of water molecules and hydroxyl groups adsorbed on anatase surfaces.14-19 Bands at 3715-3730 and 3675 cm-1 have been assigned to the ν(OH) stretching modes of isolated hydroxyls, while those between 3695-3630 cm-1 and 1640-1625 cm-1 have been assigned to the stretching and bending modes of adsorbed water, respectively.14-20 For hydrated anatase, most of these bands are usually broadened and downward shifted by H-bonding interactions, complicating their analysis. The use of new IR techniques, such as internal

10.1021/jp071440g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007

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TABLE 1: Main Characteristics of the TiO2 Anatase Samples

a

sample

calcination conditions

crystallite size (nm)a

SBET (m2 g-1)

toluene PCO steady rate × 1010 (mol g-1 m-2)

selectivity to benzaldehyde (%)

P11 P16

air @ 723 K for 3 h air @ 773 K for 3 h

11 16

93 34

7.7 6.9

3 13

Anatase primary particle size calculated from the XRD line widths.

reflection or surface enhanced absorption, has allowed the study of the frequency shifts originated by modifications in the H-bonding strength and/or orientation of interfacial water molecules, induced by electric potentials variations and/or by formation of organic compounds/water interfaces.21-25 This information is very useful to understand the modifications of the IR bands produced by water adsorption or by interaction with organic compounds at the catalyst surface. On the other hand, NMR has proven to be an extremely useful technique to study hydroxyls and water. Some NMR studies have provided valuable insights on the characteristics of water adsorbed on anatase.26-31 Thus, quantitative NMR studies of the amount of adsorbed water and of hydroxyls on hydrated anatase, rutile and amorphous TiO2, evacuated at different temperatures, have been reported.26 On the other hand, 1H MAS NMR studies have provided interesting information on the characteristics of hydroxyls and water adsorbed on nanocrystalline anatase.27-31 In the present work, we have studied by FTIR, 1H MAS NMR, and TPD the effect of evacuation at different temperatures on a hydrated anatase, with crystal size of 11 nm, in order to determine the characteristics of adsorbed water and hydroxyls. The analyzed anatase was the calcined sample showing the highest selectivity to CO2 during toluene PCO.8 Taking into account the low crystal size, the amount of adsorbed water and hydroxyls is important. Some results of this study will be compared to those obtained for a calcined anatase with larger crystal size, 16 nm, to get insight into the influence of this parameter on the anatase adsorbed water. Experimental Methods Titanium isopropoxide (TIP) was used as a precursor for the sol-gel synthesis of nanocrystalline TiO2 catalysts. The synthesis was conducted by controlled addition of 1 mL of TIP into a well-mixed isopropanol-water solution under N2 atmosphere. After the addition, the mixture was aged for 1 h at RT. The titania powder was recovered by filtration and dried at 338 K for 24 h, and subsequently calcined in air at 723 K for 3 h (sample P11) or at 773 K for 3 h (sample P16). The main characteristics of these samples and their synthesis conditions are outlined in Table 1. TiO2 powders were analyzed by XRD using a Philips PW 1030 X-ray diffractometer equipped with a CuKR radiation source and a graphite monochromator. The BET surface areas were obtained from the N2 adsorption isotherms measured with a Micromeritics ASAP 2010 equipment. TEM data were obtained with a JEOL 2000 FX II system (3.1 Å point resolution). TPD experiments were performed with a VG 100-D mass spectrometer, monitoring the mass at m/z ) 18. After flowing helium over 50 mg of sample for 1 h at RT, the sample was heated up to 560 K, at a rate of 10 K min-1, in a flow of 50 cm3 min-1 of He. 1H MAS NMR spectra were taken in an AVANCE 400 (Bruker) spectrometer. 1H NMR spectra were recorded after the irradiation of the sample with a π/2 radiofrequency pulse (single pulse technique). The 1H NMR frequency used was 400.13 MHz (B0 ) 9.4 T). In MAS NMR experiments, samples were spun

Figure 1. TEM image of a representative area of P11. The inset shows the electron diffraction pattern, which corresponds to polycrystalline anatase.

at 10 kHz around an axis inclined 54°44′ with respect the magnetic field (magic-angle spinning technique). The number of scans, 100, was selected to obtain a signal-to-noise ratio of 40. In order to avoid saturation effects, the interval between successive experiments was chosen 5s. Spectral deconvolution was carried out with the Winfit (Bruker) software package. Intensity, position and line width of components were determined with a nonlinear least-square iterative program. In quantitative analyses, intensities were referred to that of the rotor cap used in MAS experiments. Chemical shift values of NMR components were referred to that of the TMS signal. A Nicolet 5ZDX FTIR-spectrometer, equipped with a MCT detector (4 cm-1 resolution) was used for the IR study. For this work, the TiO2 powder was pressed into thin wafers (25 mg cm-2), and placed in an IR Pyrex cell equipped with NaCl windows. The cell can be evacuated by connecting it to a conventional vacuum line (residual pressure: 1 × 10-4 Torr). Results Transmission Electron Microscopy (TEM) and ThermalProgrammed Desorption (TPD) Studies. A TEM study of calcined anatase samples was carried out to determine their structure and texture. A representative micrograph of P11 illustrates the size homogeneity of the TiO2 particles, Figure 1. XRD and electron diffraction (ED) patterns, the latter in the inset of Figure 1, indicate that the observed crystallites correspond to the anatase TiO2 phase. The mean particle diameter deduced from TEM images matches closely that calculated from the width of the (101) XRD peak, by means of the Scherrer equation, for the two studied samples. These results, together with the measured SBET area values, are included in Table 1. TPD studies, performed to obtain information on the hydrated samples, show that the amount of desorbed water decreases with increasing anatase crystal size, as displayed by the P11 and P16 profiles, Figure 2. The peak observed at the lowest temperature

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Figure 2. Water TPD patterns of anatase samples P11 and P16.

Figure 3. 1H MAS NMR spectra of P11 (a) hydrated and (b) evacuated at RT.

in the P11 profile presents its maximum at 350 K, overlapped by another small and sharp one at 370 K and followed by a broader water desorption with maximum at 430 K, with a shoulder at 540 K. The P16 profile is mainly formed by overlapped peaks with broad maxima at 350, 430, and 500 K. The different intensity and resolution of the TPD peaks observed in the P11 and P16 profiles suggest differences in the amount and characteristics of the adsorbed water. However, calculated ratios between the TPD profile area, measured between 300 and 560 K, and the specific surface areas of the samples gives values near 2.7. This coincidence indicates that the amount of water desorbed from the samples, while heated from 300 to 560 K, depends mainly on the anatase surface area. 1H MAS NMR Study. The 1H MAS NMR spectra of P11, hydrated and evacuated at room temperature, are presented in Figure 3. The sample spinning at 10 kHz improved considerably the experimental resolution, facilitating the differentiation of overlapped signals. The spectrum of hydrated P11 appears mainly formed by a narrow component (line A) at 4 ppm, with a line width of 1.1 ppm, Figure 3a. A small shoulder at -1 ppm indicates the presence of another component (line B). The

Figure 4. (A) 1H MAS NMR spectra of P11 evacuated at (a) 373, (b) 423, and (c) 473 K. The asterisks indicate spinning sidebands. (B) Deconvolution of the spectra. The intensity of the three spectra has been normalized to that of the rotor cap.

spectrum obtained after evacuating P11 at RT shows that line A, with chemical shift of 3.9 ppm and line width of 3.6 ppm, decreases considerably with respect to line B, Figure 3b. Evacuation at 373 K produced a significant decrease of the anatase spectrum intensity and a marked modification of the two observed components, Figure 4a. The band at 3.9 ppm disappeared, leaving the component A formed by two overlapped bands at 5.5 and 3.7 ppm, with line widths of 3.9 and 2.3 ppm. On the other hand, component B, with a marked contribution to the spectrum intensity, was formed by four very narrow lines set on the top of a broad band centered at -1 ppm, with line width of 6.3 ppm. The increasing contribution of line B to the spectrum indicates that this signal is not affected by the evacuation treatments. This line, observed in all spectra of calcined samples, can be ascribed to the protons of the rotor

Adsorbed Water on Nanocrystalline Anatase

Figure 5. FTIR spectra of anatase P11 (a, b, and c) and P16 (d, e, and f): hydrated, (a and d), evacuated at RT (b and d), and evacuated at 373 K (c and f).

cap.29 Line B can be used as an external standard for spectra quantification. Some equally spaced spinning bands at both sides of the NMR spectrum can also be observed in Figure 4a. The spectrum obtained after P11 was evacuated at 423 K showed component A with some new modifications. While the band at 3.7 ppm remained constant, the band at 5.5 ppm decreased markedly, and a new band was detected at 5.1 ppm, Figure 4b. On the other hand, the equally spaced spinning bands present higher intensity after this evacuation The main effect of outgassing P11 at 473 K on its 1H MAS NMR spectrum was the narrowing of the bands at 5.1 and 3.7 ppm forming component A, which now appear with similar intensity and with line width of 1.9 and 1.3 ppm, respectively, Figure 4c. This evacuation decreases the intensity of the equally spaced spinning bands (see asterisks in Figure 4). FTIR Study. The IR spectra of hydrated P11 and P16 are mainly formed by a broad absorption centered at ca. 3400 cm-1 and two narrower bands at ∼3695 and 1645 cm-1, with a broad shoulder at the low wavenumber side of the last band, panels a and d, respectively, of Figure 5. The broad absorption decreased and narrowed with increasing anatase crystal size, disclosing some narrow bands and shoulders in the 3630-3700 cm-1 range of the P16 spectrum. Evacuation for 15 min, at RT, produced a marked decrease of the broad absorption. The new spectra show several overlapped broad bands in the 3600-3000 cm-1 region, with resolved maxima at 3475 and 3120 cm-1, and narrower bands at 3685, 3670, 3660, 3630, and 1625 cm-1, Figure 5, panels b and e. These spectra also present bands at 2970, 2940, and 2870 cm-1, assigned to ν(CH) vibrations of residual organic compounds,22 and some small bands in the 1700-1300 cm-1 range, probably originated by carbonates or hydrogen carbonates.12,18 The spectra difference obtained by subtracting the spectra recorded before and after RT evacuation show that the decrease of the broad absorption corresponded to the decrease of three broad bands centered at 3520, 3400 and 3120 cm-1 for P11, Figure 6a, and mainly of a band at 3300 cm-1, for P16, Figure 6b. These spectra also showed the disappearance of the narrow bands at 3695, 1645, and 1610 cm-1 and the increase of bands at 3685, 3630, and 1625 cm-1. The spectra of the samples evacuated at 373 K for 1 h are mainly formed by a broad band centered at about 3300 cm-1 and small narrow bands at 3730, 3685, 3670, 3650, 3630, 1625,

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10593

Figure 6. FTIR difference spectra obtained for anatase samples P11 (a, c), P16 (b,d) showing the effect of evacuation at 295 K (a, spectrum of Figure 5b minus that of Figure 5a; b, spectrum of Figure 5e minus that of Figure 5d) and at 373 K (c, spectrum of Figure 5c minus that of Figure 5b; d, spectrum of Figure 5f minus that of Figure 5e).

and 1560 cm-1, Figure 5c,f. These bands are stronger for P11 than for P16, particularly those at 3685, 3670, and 3300 cm-1. The spectra difference displaying the effect of evacuation at 373 K, Figure 6c,d, show the decrease of the broad bands in the 2900-3400 cm-1 range and at 3475 cm-1 and of the narrow bands at 3630, 1625, and 1560 cm-1. The P11 spectrum also showed a small decrease of the bands at 3685 and 3670 cm-1. The increase of a new band at 3730 cm-1 was also observed. Discussion The three water desorption peaks observed in the TPD profiles of hydrated anatase, obtained with ultrahigh vacuum TPD systems,32,33 indicate that water was desorbed from, at least, three different types of sites. These differences in desorption strength can be interpreted assuming the formation of threedimensional water molecular arrangements (MAs) on the oxide, induced by the oxide ionic surface field. In this model, when applied to anatase non-defective surfaces, the second layer (SL) water is hydrogen (H) bonded to water coordinated to Ti4+ (WTi) cations.13,32-34 Additional layers, H-bonded to the SL water, are denoted multilayer. Desorption of water H-bonded to bridging O2- anions has also been considered to explain hydrated anatase TPD profiles. The TPD results obtained in the present study, which are relatively similar to those reported by Beck et al. when using an experimental system working at atmospheric pressure,32 can also be interpreted on the basis of the water Mas model. The Weakly Adsorbed Water. The small width of component A, at 4 ppm, in the 1H MAS NMR spectrum of hydrated P11 can be attributed to the cancellation of proton-proton interactions caused by the water motion.29 The marked decrease of the NMR and FTIR spectra intensity observed when P11 was outgassed at RT indicates the desorption of a significant amount of water; however, no water desorption is observed, at RT, in the TPD profiles. This apparent contradiction is due to the desorption of the water most weakly adsorbed on anatase during the treatment with flowing helium at RT, carried out previously to the TPD study. This decrease of the spectrum intensity can, therefore, be attributed to the removal of the most mobile water in the sample. The broadening of component A

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TABLE 2: Characteristics of the Different Types Water Molecules Adsorbed on TiO2 as Identified by FTIRa vibration modes (cm-1) type MAs WW MAs WO B WO A WTi OH a

location

bonding

free OH

multilayer

str. H-bond, bulk str.. H-bond, interfacial weak H-bond str.. H-bond interfacial interm. ads. strong ads. strong ads. isolated isolated

3695 (N.D.) 3685 3630 3630 νasym(OH) (N.D.) 3730 3670

WW-WO SL WO-OHT WO-OHB WTi-Ti4+ terminal bridging

coupled OH

bending H2O

TPD peak (K)

NMR (ppm)

3120 3400-3450 3520 3400-3450 3475 3475 νsym(OH) (N.D.)

1645 1645 1610 1625 1625 1625 1560

RT RT RT 353 373 430 540

4 4 4 3.9 3.9 5.5 3.7 5.1

N.D.: not determined.

in the outgassed sample spectrum indicates a significantly lower mobility29 of the remaining adsorbed water. To determine the IR bands corresponding to the removed water, we have considered that though water monomers present two OH stretching modes, νsym(OH) and νasym(OH) vibrations at about 3616 and 3708 cm-1,25 the distinction between them breaks down for H-bonded water molecules due to intermolecular interactions,22 which force the water vibrational modes to be independent. In interfacial water, one of the oscillators can correspond to “dangling” or “free” OH bonds that straddle the interface, protruding into the gas phase. This OH mode will be energetically uncoupled from its adjoining OH bond, asymmetrically H-bonded to other water molecule22,24,25 of the liquid phase. Both OH should be coupled in bulk water. The decrease of the broad bands at 3120 and 3400 cm-1, wavenumbers close to those of the bands dominating the melting ice35 and liquid water spectra,36 respectively, when the most mobile water has been desorbed indicates the removal of strongly H-bonded water from the multilayer of water MAs. Considering that the multilayer bulk water should be more strongly H-bonded than the multilayer interfacial water, because its two OH are H-bonded, the band at the lowest wavenumber can be attributed to the coupled OH stretching mode of multilayer bulk water. On the other hand, the band at 3400 cm-1 should be due to that mode of multilayer interfacial water. The transformation of the SL water from bulk to interfacial, by the removal of the multilayer, contributes to the decrease of the band at 3130 cm-1. The calculated value of these two bands intensity ratio, a comparative parameter of bond ordering in water MAs,37 indicates a stronger water H-bonding on P11 than on P16, due to a more marked contribution of the multilayer bulk water in P11. This water stronger H-bonding should be originated by a stronger ionic field of the P11 surface, induced by a higher concentration of low coordinated ions where the water MAs can be formed. When H-bonding is not too strong, these two bands can overlap, appearing as a single broad one centered at about 3300 cm-1,37 as observed for P16, Figure 5d. The small band at 3695 cm-1, observed with low intensity in the spectrum of melting ice water,35 probably corresponds to the free OH stretching mode of the multilayer interfacial water, Figure 5. The marked decrease of the band at 1645 cm-1 when P11 and P16 are evacuated at RT indicates that this band corresponds to the bending mode of the strongly H-bonded multilayer water.14,22,38 The relatively broad band at 3520 cm-1, at a higher wavenumber than that of the coupled OH stretching mode of multilayer interfacial water, but lower than those corresponding to the OH stretching modes of water monomers,25 can be assigned to the coupled OH stretching mode of weakly H-bonded water.38 This weakly adsorbed water, denoted WW

hereafter, should be interacting with weakly H-bonded water more strongly adsorbed on the anatase surface, hampering water WW incorporation to the multilayer of water MAs. The observation of the single narrow line of component A in the 1H MAS NMR spectrum of hydrated P11 indicates the exchange between the multilayer and WW water. The band at about 1610 cm-1, close to that of the bending mode of water monomers, should correspond to the bending mode of WW water, Figure 5, panels b and e. The assignment of the water bands, as well as their correspondence with TPD peaks, is summarized in Table 2. Water Adsorbed with Intermediate Strength. Water adsorbed with intermediate strength can be associated with the overlapped peaks with maxima at 350 and 370 K in the P11 TPD profile, not well resolved in the P16 profile. Significant information on the characteristics of the water originating these TPD peaks is provided by the FTIR and NMR spectra showing the effect of evacuation at 373 K. The assignment of the NMR bands is not straightforward, because proton exchange processes hamper the differentiation of water and hydroxyl protons. However, some useful information can be derived from the analysis of the intensity and line width of the bands contributing to the component A. The elimination of the band at 3.9 ppm by evacuating P11 at 373 K is consistent with the complete desorption of a less mobile water than that of the multilayer. On the other hand, the spectrum evolution from a single band at 3.9 ppm to two bands at 5.5 and 3.7 ppm, broader and narrower respectively than the band originated by the removed water, indicates that the broad band corresponds to a more rigid water than that desorbed and that the narrow band could be originated by hydroxyl groups. The constant intensity of the band at 3.7 ppm and its narrowing with increasing evacuation temperature support the assignment of this band to OH groups. The observation of the bands at 3.7 ppm and at 3730 cm-1 in the NMR and FTIR spectra, respectively, after evacuating P11 at 373 K, indicates that both bands correspond to terminal hydroxyls.20 The chemical shift of the broad fixed water band at 5.5 ppm suggests that it is interacting with more acidic hydroxyls than those originating the band at 3.7 ppm, probably, bridging hydroxyls. The decreasing broad band centered at about 3400 cm-1 in the IR difference spectra, showing the effect of evacuating P11 and P16 at 373 K, can be assigned to the coupled OH stretching mode of water desorbed from the SL of water MAs, Figure 6, panels c and d. The H-bonding strength of SL and multilayer water is not significantly different.39 On the other hand, the higher wavenumber of the relatively broad band at 3475 cm-1 corresponds to the coupled OH stretching mode of water, denoted WO hereafter, more weakly H-bonded than SL water. In a similar way, Finnie et al. have ascribed the bands at 3630 and 3475 cm-1 to the free and coupled OH stretching modes

Adsorbed Water on Nanocrystalline Anatase of water desorbed from nanocrystalline anatase22,40,41 by heating above 350 K.20 The observation of the band at 3730 cm-1, assigned to terminal hydroxyls,20 when water WO is partially removed, indicates that a part of water WO removed by evacuation at 373 K, denoted WOB, was interacting with those hydroxyls. The relatively high stability of this water suggests that it is also interacting with low coordinate bridging O2anions. Considering the reported downward shift of the water free OH stretching mode with decreasing water H-bonding,25 the band at 3685 and 3630 cm-1 can be ascribed to the free OH stretching mode of water with the strongest and weakest H-bonding, SL and WO water respectively. The presence of only the band at 1625 cm-1 in the 1645-1600 cm-1 range of the spectra difference indicates that it corresponds to the bending mode of both SL and WO water, Figure 6, panels c and d. The removal of water WW and the resolution of the IR bands corresponding to WO, both weakly H-bonded, by evacuation at RT suggests that water WW was interacting by H-bonding with water WO. The desorption of water SL and WOB and the disappearance of the NMR line at 3.9 ppm, observed after evacuating P11 at 373 K, indicate that the NMR signal was originate by water exchange between the sites where SL and WOB water was adsorbed. Selloni et al. have indicated that the interaction with water SL can favor the stability of water H-bonded to bridging O2- anions.34 The decrease of the IR bands corresponding to water WO, by heating the samples at temperatures above 353 K,20 suggests that the TPD peak with maximum at 373 K is originated by the desorption of water WOB. On the other hand, the broad peak with maximum at 353 K should correspond to the removal of water SL. Strongly Adsorbed Water. The marked decrease of the broad NMR band at 5.5 ppm and the formation of a new narrow one at 5.1 ppm by evacuation at 423 K, while the hydroxyl band at 3.7 ppm remains nearly constant, indicate that the broad band corresponds to water interacting with the hydroxyls originating the new narrow band. The higher chemical shift of this hydroxyls band than that of the terminal hydroxyls could indicate that they are bridging hydroxyls. Peaks at 6.4 and 2.3 ppm observed in the 1H MAS NMR spectra of anatase samples have been attributed to the positively charged acidic protons located on bridging O atoms and to the basic H atoms bound to terminal oxygen.28 The similar mean value of the chemical shift of these hydroxyls and of those reported at 3.7 and 5.1 ppm in this work, and those at 2.3 and 6.4 ppm indicates that the difference between the chemical shift of both types of hydroxyls is, probably, due to differences of the studied anatase samples surface.29 Though the band at 3475 cm-1 is not well resolved, the observation of the bands at 3630 and 1625 cm-1 in the FTIR spectra of the samples evacuated at 373 K indicates that some WO water, denoted WOA hereafter, remained adsorbed after such treatment. This water should correspond to the band observed at 5.5 ppm in the NMR spectrum of P11 evacuated at 373 K. The stretching mode of acidic bridging hydroxyls in anatase, usually appearing at about 3670 cm-1,20,23 should be downward shifted by the H-bonding interactions with WOA water in the spectra obtained after evacuation at 373 K.15,17 The observation of two types of water molecules, WOB and WOA, desorbed at different temperature and with their protons originating NMR lines with different chemical shifts, but with their vibration modes appearing at similar frequencies in their FTIR spectra, indicates that both types of water are adsorbed on similar

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10595 sites, but with different acidity. Water WOA interaction with acidic hydroxyls should favor proton exchange between hydroxyl and water, facilitating the formation of H3O+ cations. The formation of these cations should strengthen the water WOA adsorption on anatase, evidenced by the higher temperature (about 50 K more) required for its desorption than for that of water WOB. On the other hand, WOB water should be interacting with basic bridging O2- anions. The presence of equally spaced spinning side bands at both sides of the NMR spectra indicates the presence of water WTi with very restricted mobility. In this case, the presence of intense proton-proton dipolar interactions is responsible for detection of several spinning bands at both sides of the central region (see asterisks in the Figure 4). This interaction produces a considerable broadening of the NMR line, hampering the precise determination of chemical shift values. The intensity of this signal reaches a maximum after evacuation at 423 K. Above this temperature, elimination of coordinated water increases the amount of oxygen vacancies at the anatase surface. The strong bond of this water to Ti4+ cations should hinder its bending vibrations, inducing a downward shift of this mode, which can appear below1600 cm-1.14,22 On this basis, the shoulder at 1560 cm-1 observed in the spectra of the samples evacuated at 373 K could be ascribed to the bending mode of WTi water, Figure 5, panels c and f and in the difference spectra c and d in Figure 6. Considering that water WTi should not be H-bonded34 in P11 evacuated at 423 K, its two OH stretching modes, νsym(OH) and νasym(OH) vibrations, should appear in between those of water monomers, observed at about 3616 and 3708 cm-1.25 The NMR spectrum obtained after evacuating P11 at 473 K shows that component A is only formed by the narrow bands at 3.7 and 5.1 ppm ascribed to terminal and bridging hydroxyls. The absence of broad bands in Figure 4c indicates that very little water remains adsorbed on the sample. The equally spaced spinning side bands originated by WTi water also contribute to this spectrum. Though a progressive samples dehydroxylation should favor the narrowing of the NMR component A, by reducing the H-H interactions, the relative hydroxyls stability at 473 K indicates that the water desorption is the main responsible for the line narrowing of OH signals. Based on the NMR and TPD results, the TPD peak with maximum at 430 K can be ascribed mainly to the desorption of WOA water, while the TPD shoulder at 540 K can be attributed to the desorption of WTi water.33,34 The increasing H-bonding strength of the water MAs with decreasing anatase crystal size, due to the increasing anatase surface ionic field, can hamper the adsorption of aromatic organic compounds25 and oxygen41 on low coordinated Ti4+ and O2- ions, the anatase photocatalytic active sites. The hampered strong adsorption of organic compounds on Ti4+ cations can hinder the photocatalyst deactivation,8 while the more hydrophilic anatase surface can offer new adsorption sites for these organic molecules. However, by hampering O2 adsorption, its electron trapping effect should be hindered, favoring electronhole recombination and decreasing the anatase photoactivity. According to these arguments, the nearly constant value of the toluene PCO steady rate, observed for calcined anatase samples with decreasing anatase crystal size, should indicate the compensation of the positive and negative effects of a stronger water H-bonding on these samples. However, a more hydrophilic anatase surface could provide new adsorption sites for organic molecules, favoring the oxidation of intermediates of the toluene PCO, improving this reaction selectivity to CO2. Photocatalytic and spectroscopic studies are currently being carried out to

10596 J. Phys. Chem. C, Vol. 111, No. 28, 2007 determine the influence of water adsorbed on these samples with intermediate and strong strength on the toluene PCO selectivity to CO2. Conclusions The 1H-MAS NMR and FTIR spectra of P11 show that the water adsorbed on the anatase surface can be differentiated in several types, according to their adsorption site. The most weakly adsorbed water on hydrated anatase consists of strongly H-bonded water, forming the multilayer of water molecular arrangements, and of water weakly H-bonded to other interacting with low coordinated bridging O2- anions. The multilayer water H-bonding strength increases with decreasing anatase crystal size. Both types of water are removed during the flowing inert gas treatment at RT, previously to the TPD study. The first water desorption peak observed in the TPD profile, with maxima at 353 and 373 K, corresponds to desorption of less mobile water. Part of this desorbed water is removed from the second layer of water molecular arrangements, while other part had been bound to bridging O2- anions. The TPD peak with maximum at 430 K is mainly originated by desorption of water H-bonded to bridging hydroxyls. The desorption of the most strongly adsorbed water, coordinated to Ti4+ cations and forming the first layer of water molecular arrangements, originates the TPD shoulder observed at 540 K. The anatase NMR spectrum obtained after evacuation at 473 K is mainly formed by protons signals of terminal and bridging hydroxyls. Acknowledgment. This study has received financial support from the project MAT2001-2112-C01-01 funded by the Spanish Ministerio de Educacio´n y Ciencia. References and Notes (1) Peral, J.; Ollis, D. F. J. Catal. 1992, 136, 554. (2) Alberici, R. M.; Jardim, W. F. Appl. Catal., B: EnViron. 1995, 14, 55. (3) In Photocatalysis: Fundamentals and Applications; Serpone, N., Pelezzetti, E., Eds.; Wiley-Interscience: New York, 1989. (4) In Heterogeneous Photocatalysis; Schiavello, M., Ed.; Wiley Series in Photoscience and Photoengineering; J. Wiley & Sons: Chichester, U.K., 1997; Vol. 3. (5) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (6) Cao, L.; Huang, A.; Spiess, F. J.; Suib, S. L. J. Catal. 1999, 188, 48. (7) Maira, A. J.; Yeung, K. L.; Lee, C. Y.; Yue, P. L.; Chan, C. K. J. Catal. 2000, 192, 185.

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