J. Phys. Chem. C 2007, 111, 773-778
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Local Structure Changes of Microporous Titanosilicate ETS-10 upon Acid Treatment L. Lv, J. K. Zhou, F. Su, and X. S. Zhao* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576 ReceiVed: October 24, 2005; In Final Form: September 11, 2006
In the present work, the local structure changes of microporous titanosilicate ETS-10 upon treatment with citric acid, phosphoric acid, and nitric acid were characterized using XRD, FTIR, Raman, XPS, DR-UV, 29Si MAS NMR, and 1H-29Si CP MAS NMR techniques. The experimental results suggested that regardless of the nature of the acids, the replacement of alkali cations by protons altered the local structure of ETS-10 solid, including partial dissolution of the siliceous matrix and leaching of titanium species from the TiO6 chains. In the meantime, it was observed that the hexacoordinated Ti species of ETS-10 were changed to penta- and/or tetracoordinated ones, generating terminal TiOH groups, accounting for the observed enhancement of photodegradation activity of the acid-treated ETS-10 materials toward phenol compounds.
1. Introduction ETS-10 is a microporous titanosilicate composed of octahedral titanium and tetrahedral silicon atoms linked by cornersharing oxygen atoms, forming a three-dimensional pore system.1-3 A striking feature of ETS-10 is that TiO6 octahedra form linear chains running in two perpendicular directions of the crystal, which are separated from one another by the siliceous matrix,4 giving rise to peculiar optical and electronic properties.5-7 For example, Na,K-ETS-10 has been observed to be photoactive in ethane polymerization.8 Theoretically, a defect-free ETS-10 sample is not supposed to exhibit an oxidative reactivity because of the nature of hexacoordinated titanium.9 However, as the ETS-10 structure is formed by random stacking of polymorphs A and B with chiral and C2/C symmetries, respectively,2 internal point defects are found on the boundary of the two polymorphs,10 resulting in the formation of titanol groups (TiOH). In addition, some TiOH groups locate on the crystal external surface, where O-Ti-O-Ti-O chains end and hydroxyl groups saturate the dangling bonds of titanium ions.11 The isolated titanium wires can capture light and transport it to the external and internal defects, thus leading ETS-10 to display photocatalytic activity.4 It has been reported that the TiOH groups can, on one hand, effectively trap photogenerated holes to minimize recombination with electron-hole pairs, and on the other hand, adsorb O2, which is subsequently oxidized to O2- species to interact with water to form oxygenated radicals ·OH.12 Thus, there have been a couple of recent papers describing how to enhance the number of TiOH groups of ETS10 by posttreatment with acids.12-15 Llabre´s i Xamena and coworkers12 observed that posttreatment using hydrofluoric acid (HF) solution can dramatically enhance the photocatalytic activity of ETS-10 toward organic compounds because of the increase in the amount of TiOH sites created during the acid treatment. However, fluorine is a typical mineralizing reagent that can potentially destroy the crystalline framework of ETS10. In addition, fluorine can poison an oxidation catalyst. Furthermore, HF is a corrosive acid, requiring special handling. Thus, the use of other acids instead of HF to treat ETS-10 while * To whom correspondence should be addressed. E-mail: chezxs@ nus.edu.sg.
achieving the same objective of enhancing photocatalytic activity is more desirable. Koermer et al.14 reported the modification of the Si/Ti ratio in ETS-10 using citric acid. The Ti-extracted ETS-10 samples can be used as oxidative catalysts in various reactions. However, the local structural changes of siliceous matrix and TiO6 chains occurred during the acid treatment were not discussed. Recently, Goa et al.13,15 concluded that changes in the coordination environment of ETS-10 samples treated with HCl, NH4Cl, and citric acid may occur due to the extraction of Ti atoms upon acid treatment. Previously, we observed that pH values have a vital impact on the adsorption properties of ETS-10 toward heavy metal ions in both batch and fix-bed reactors.16,17 In addition to the precipitation or the competition from other ions, structural changes of ETS-10 under different pH values, especially acidic conditions, may also account for the variation of adsorption properties. Therefore, a full understanding on the structural changes of ETS-10 upon acid treatment is of importance in exploring the adsorption mechanism of heavy metal ions on ETS-10 solids. In this work, nitric acid, phosphoric acid, and citric acid were used to treat ETS-10 under different pH values. The local structure changes of Ti and Si before and after such treatments were characterized by means of 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR), 1H-29Si cross-polarization (CP) MAS NMR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectanceultraviolet spectroscopy (DR-UV), Fourier transmission infrared spectroscopy (FTIR), Raman spectroscopy, and nitrogen adsorption techniques. In addition, the photodegradation properties of the acid-treated ETS-10 samples toward 2-chlorophenol (CP), 2,4-dichlorophenol (DCP), and 2,4,6-trichlorophenol (TCP) were also examined and compared with that of HF-treated samples.12 The main objectives of this work were to (1) elucidate the local structural changes of ETS-10, (2) understand the mechanism of the formation of TiOH groups, and (3) clarify the key factors responsible for structure changes upon acid treatment. 2. Experimental Section 2.1. Synthesis and Posttreatment of ETS-10. An ETS-10 sample, hereafter named ETS-10-original, was synthesized as
10.1021/jp056107w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006
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TABLE 1: The Elemental Composition and Crystallinity of ETS-10 Samples before and after Acid Treatment bulk atomic ratio acid
pHa
Si/Ti ((0.05)
parent ETS-10 HNO3
7 6 5 4 7 6 5 4 7 6 5 4
4.97 4.97 5.00 5.03 5.09 4.99 5.02 5.07 5.14 5.00 5.02 5.07 5.10
H3PO4
citric acid
surface atomic ratio
(Na + K)/Ti ((0.05)
Si/Ti ((0.1)
(Na + K)/Ti ((0.1)
relative crystallinityb (%)
1.95 1.82 1.63 1.26 1.01 1.76 1.50 1.30 0.99 1.75 1.60 1.41 1.06
4.9 5.0 5.1 5.2 5.3 5.0 5.1 5.2 5.4 5.1 5.1 5.1 5.2
2.0 1.7 1.4 1.0 0.84 1.7 1.3 1.1 0.87 1.7 1.4 1.1 0.91
100 99.1 96.8 95.4 93.6 98.8 97.1 96.3 94.7 99.7 97.4 97.0 95.2
a pH: the equilibrium pH value during acid treatment. b The relative crystallinity was calculated from the intensities of the two main peaks at 2θ ) 20° and 25o.
described previously.18 Posttreatments of sample ETS-10original were performed with 1 M nitric acid (titrisol, Merck), phosphoric acid (99.9%, Aldrich), and citric acid (99.9%, Aldrich), respectively. The liquid to solid ratio used in the treatment was 200 mL:1 g. The equilibrium pH of the treatment system was varied from 7 to 4 using the corresponding acid. During the treatment, ion exchange between protons and the cationic ions in the framework of ETS-10 occurred. After the equilibrium of the ion exchange was attained, the solid ETS10 samples were collected by centrifugation, washed with deionized water, and dried at 373 K. The resultant acid-treated samples are denoted as ETS-10-X, where X stands for the pH value of the acid treatment system. 2.2. Characterization. Bulk elemental analysis was conducted by using an inductive-coupled plasma-atomic emission spectrometer (ICP-AES) on a Perkin-Elmer ICP Optima 3000DV. Physical adsorption of nitrogen was measured on an automatic volumetric sorption analyzer (Quantachrome, NOVA1200). The specific surface areas (SBET) were determined according to the Brunauer-Emmett-Teller (BET) method in the relative pressure range of 0.05-0.2. XPS spectra were obtained on an AXIS HIS (Kratos Analytical Ltd., UK) with an Al KR X-ray source (1486.7 eV) operated at 15 kV and 10 mA. The pressure in the analysis chamber was maintained to be less than 10-8 Torr. The binding energy (BE) of the spectra was referenced to the C 1s electron BE of graphitic carbon (284.6 eV). The peak intensity of the principle spectral feature of each element was used to calculate the surface-atomic concentration.19 FTIR spectra were collected on a Bio-Rad FTIR spectrophotometer using the KBr method. The spectra were recorded with 64 scans and a 4 cm-1 resolution under ambient conditions. Raman spectra were obtained on a FT-Raman spectrometer (Bruker Equinox 55) equipped with a Raman module and an InGaAs detector operated at a power of 200 mW and an excitation wavelength of 1064 nm. The resolution was 4 cm-1. XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer (Cu KR radiation) operated at 40 kV and 30 mA with a scanning speed of 0.02 degree per second. DR-UV spectra were collected on a Shimadzu 3100 with an integrating sphere attachment, ISR 1200, for the diffuse reflectance in the wavelength range of 200-500 nm. Barium sulfate was used as the reference. Both 29Si MAS NMR and 29Si CP-MAS NMR spectra were obtained at 79.49 MHz on a Bruker DRX 400 spectrometer. The CP contact time was 8 ms, which was found to be sufficient to allow full cross polarization for the Si atoms to which hydroxyl groups are directly attached.
2.3. Photocatalytic Test. The photocatalytic properties of the samples being acid-treated at pH ) 4, namely the ETS-10-4 samples, were measured. Other catalysts including P25 (Degussa), ETS-10-original, and ETS-10-original after being rinsed with a 2% HF solution (following the procedure described elsewhere)12 were also studied for comparison purposes. A 5 mL suspension containing 5 × 10-4 mmol of 2-chlorophenol (CP), 2,4-dichlorophenol (DCP), or 2,4,6-trichlorophenol (TCP) and 5 mg of solid catalyst was used for the catalytic measurement. The suspension was irradiated with a light of λ ) 365 nm generated using a 125-W high-pressure Hg lamp (Philips, Belgium). The incident light intensity measured using a digital radiometer (Model UVX-36: UVP) was 20.6 mW cm-2. The temperature was maintained at 30 °C. After 30-min irradiation, the suspension was centrifuged at 9000 rpm for 5 min. The concentration of the aromatic compound was assayed using a high-performance liquid chromatography (HPLC, Agilent ZORBAX SB C18 column, 4.6 × 150 mm, 5 µm packing) equipped with a UV-vis detector (Merck Hitachi L-4200). The conversion of the aromatic compound was calculated according to (C0 - Ct)/(C0) × 100%, where C0 and Ct were the concentrations of the phenol compound before and after reaction, respectively. 3. Results and Discussion 3.1. Elemental Analysis. Reported in Table 1 are the bulk and surface elemental compositions of the ETS-10 samples before and after acid treatments determined using the ICP and XPS techniques. It is seen that both the bulk and surface atomic ratios of the original ETS-10 (ETS-10-original) are close to its theoretical stoichiometry, namely Na1.5K0.5TiSi5O13.2,3 When the pH of the acid treatment system was decreased from 7 to 4, the atomic ratio of Si/Ti in the bulk was gradually increased regardless of the acids used, indicating that ETS-10 is reactive to protons. On contrast, the ratios of (Na + K)/Ti were decreased because of replacement of the alkali cations by protons. The data in Table 1 also show that leaching of Ti atoms mainly occurred on the solid surface. The replacement of the alkali cations of ETS-10 by protons can be further understood from the correlation curves between the amount of Na and K ions desorbed and the equilibrium pH shown in Figure 1. It can be seen that the desorbed Na/K ratio was increased from 3 to 6 when the treatment pH was decreased from 7 to 4, suggesting that Na+ ions were easier to be replaced by protons than K+ ions, as was observed by Grillo et al.20 This can be considered to be due to energetic effect instead of the
Microporous Titanosilicate ETS-10
Figure 1. Correlation between desorbed amounts of alkali cations (Na+ or K+) and equilibrium pH. O: desorbed Na+ during treatment with H3PO4; 0: desorbed Na+ during treatment with citric acid; ∆: desorbed Na+ during treatment with HNO3; b: desorbed K+ during treatment with H3PO4; 9: desorbed K+ during treatment with citric acid; 2: desorbed K+ during treatment with HNO3.
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Figure 3. DR-UV spectra of (a) ETS-10-original and HNO3-treated ETS-10 samples at pH ) 7 (b), 6 (c), 5 (d), and 4 (e) after being wetted with 30% H2O2 solution.
Figure 4. FTIR spectra of ETS-10-original (a) and HNO3-treated ETS10 samples at pH ) 7 (b), 6 (c), 5 (d), and 4 (e).
Figure 2. (A) XRD patterns of as-synthesized ETS-10 before (a) and after treatment with HNO3 at pH ) 7 (b), 6 (c), 5 (d), and 4 (e). (B) XRD patterns of calcined ETS-10 before (a) and after HNO3 treatment at pH ) 7 (b), 6 (c), 5 (d), and 4 (e).
kinetic effect because the mobility of hydrated K+ is about 50% higher than that of hydrated Na+ ions.21 3.2. XRD Analysis. Figure 2A shows the XRD patterns of ETS-10 before and after HNO3 treatments under different pH values. It is seen that the XRD patterns of all samples match well with that of pure ETS-10.1 The relative crystallinities of the acid-treated ETS-10 samples were calculated using the sum of the intensities of the diffraction peaks at 2θ ) 20 and 25° over that of the original sample, of which the crystallinity was assumed to be 100%, and are reported in Table 1. As can be seen, the relative crystallinities of the acid-treated samples are all larger than 93%, indicating that the acid treatments had little effect on the crystalline structure of ETS-10. However, it can be seen from Figure 2B that calcination of the acid-treated samples at 550 °C for 6 h led to a significant decrease in the diffraction intensities, suggesting that the acid treatment lowered the thermal stability of ETS-10. This may be due to the formation of a substantial amount of terminal TiOH and or SiOH groups upon the acid treatment, which will condense at 550 °C, leading to structural dislocations and/or disorder. 3.3. Diffuse-Reflectance UV Spectra. It is known that the tetravalent Ti sites of Ti-substituted silicalite-1 (TS-1) can react
with hydrogen peroxide to display a yellow color and to give a UV absorption band at about 360-385 nm, which is attributed to the ligand-to-metal charge-transfer transition (LMCT) of peroxo-type Ti species.22,23 It has also been reported that the amount of accessible TiOH groups is proportionally correlated to the intensity of the UV absorbance of the peroxo-titanate.12 In this work, the original and HNO3-treated ETS-10 samples were wetted with a 30 wt % H2O2 solution. It was found that the yellow color of the wetted samples virtually became more and more dark with decreasing the equilibrium pH, implying that more Ti(IV) centers were generated when ETS-10 was acidified. This speculation was confirmed by the UV-vis spectra shown in Figure 3. The intensity of the absorption band centered at about 360 nm due to the LMCT of peroxo-titanate formed via interaction with H2O212 was increased as the acidifying pH was decreased. The above observations demonstrate that acid treatment can indeed create the formation of more accessible Ti(IV) centers. 3.4. FTIR Spectra. Shown in Figure 4 are the FTIR spectra of the original and HNO3-treated ETS-10 samples in the vibration range of 400-1400 cm-1. The absorption band at about 1038 cm-1 is assigned to the asymmetric Si-O stretching vibration mode while the peak at 749 cm-1 is attributed to the Ti-O-Ti stretching mode.24 The bands at 548 and 447 cm-1 are associated with the Ti-O stretching, Si-O rocking, O-Ti-O bending, Ti-O rocking, and O-Si-O bending vibrations of ETS-10.25,26 After acid treatment, some changes can be seen from the FTIR spectra. With decreasing the treatment pH, the intensity of the peak at 749 cm-1 was gradually decreased and finally disappeared when the treatment pH was lowered to 5 (see Figure 4d), showing that the TiO-Ti bonds were gradually broken up. For sample ETS-10-4, the absorption band assigned to the Si-O stretching mode was shifted to a wavenumber of 1061 cm-1 with two shoulders at 1140 and 985 cm-1, respectively, which were also observed on ETS-10 samples treated with a 0.5 M H2SO4.25 The blue shift of the Si-O stretching band is likely due to the weakened strength of the Si-O bonds caused by the electrostatic interac-
776 J. Phys. Chem. C, Vol. 111, No. 2, 2007
Figure 5. Raman spectra in the wavelength range of (A) 2001200 cm-1 and (B) 200-700 cm-1 of ETS-10-original (a) and HNO3treated ETS-10 samples at pH ) 7 (b), 6 (c), 5 (d), and 4 (e).
tion of protons in the vicinity of the framework anionic oxygen.27 The two shoulders can be assigned to the asymmetric Si-O stretch of the dispersed Si-O-T (where T ) Si or Ti) bonds coupled with the formation of hydroxyl groups.27,28 3.5. FT-Raman Spectra. Figure 5A shows the Raman spectra of the ETS-10-original and HNO3-treated ETS-10 samples. A prominent band at ca. 724 cm-1 with a tail in the higher frequency region, together with other small excitation peaks at 636, 535, 457, and 320 cm-1, can be seen in sample ETS-10-original. The intensive band at 724 cm-1 is due to symmetric Ti-O stretching vibrations in octahedral TiO6 units along the -TiO-Ti- chains29 while the tail at the higher frequency is due to the asymmetric Ti-O stretching modes caused by the stretching of the Si-O-Si bonds in the three-membered rings.4,30 However, a significant reduction in the intensities of the Raman peaks can be seen on the HNO3-treated ETS-10 samples. It should be particularly noted that the characteristic band at 724 cm-1 entirely disappeared for sample ETS-10-4. For other acid-treated samples, this peak was gradually broadened and shifted to a higher frequency. Southon and Howe10 concluded that both the position and the broadness of a Raman active band at about 725 cm-1 are an indicative of the average -Ti-O-Ti- chain length and the concentration of defects. In a highly defective ETS-10 material, this band can be considerably broadened and positioned at about 775 cm-1.10 Given that ETS-10 material is quite stable under the excitation of laser beams of various wavelengths,11 the decrease in the intensity and disappearance of the peak at 724 cm-1 can be ascribed to the partial or entire disruption of the Ti-O-Ti chains, leading to the decrease in the symmetric Ti-O stretching in the framework of ETS-10. Figure 5B shows the Raman spectra in the range of 200700 cm-1. As can be seen, the relative intensities of the peaks at 320, 457, 535, and 636 cm-1 of sample ETS-10-original, having been ascribed to the various T-O-T, O-T-O (T ) Ti or Si) bending and skeletal deformation modes,10,29 were significantly decreased as the treatment pH was decreased. The distortion of the skeletal bonds, as a consequence of acid treatment, probably accounts for this observation. 3.6. XPS Spectra. Figure 6 shows the XPS spectra of Ti 2p and O 1s. As can be seen from Figure 6A, the BE of the Ti 2p3/2 core level electrons of sample ETS-10-original is at about
Lv et al.
Figure 6. (A) Ti 2p and (B) O 1s XPS core level spectra of ETS-10original (a) and HNO3-treated ETS-10 samples at pH ) 7 (b), 6 (c), 5 (d), and 4 (e).
458.4 eV, very close to that of P25.31 Thus, this peak can be attributed to octahedral Ti atoms in the -O-Ti-O-Ti-Ochains.31,32 With decreasing the treatment pH value, the peak was broadened and shifted to a higher BE. The observed broadening of the Ti 2p3/2 peak in comparison with that of P2531 is probably due to the siliceous matrix in ETS-10 and the existence of titanium species of coordination states other than the octahedral state in the TiO6 chains.32,33 The O 1s core-level electron spectra of the ETS-10 samples, as shown in Figure 6B, display a very broad peak at about 531.9 eV with a shoulder at 530.1 eV. The main peak is attributed to the O atoms in Si-O-Si and Si-O-Ti bonds, and the shoulder peak is due to Ti-O-Ti.34 It is observed that the relative intensity of the shoulder peak was significantly lowered as the treatment pH was decreased, suggesting the occurrence of partial leaching of Ti atoms or the interruption of -O-Ti-O-Ti-O- chains under the acidic conditions, which had led to the change in the coordination state of Ti atoms as discussed earlier. 3.7. 29Si MAS NMR and 1H-29Si CP-MAS NMR Spectra. Figure 7A shows the 29Si MAS NMR spectra of the ETS-10 samples. For sample ETS-10-original, the 29Si NMR spectrum exhibits three resonances at -94.7, -96.3, and -103.8 ppm, respectively, with relative intensities of about 2:2:1. The peaks at -94.7 and -96.3 ppm correspond to four crystallographically inequivalent Si sites with three of them having the same NMR signal, namely Si(3Si, 1Ti),2 while the resonance at -103.8 ppm is assigned to Si(4Si, 0Ti) site.2 It can be seen that, as the treatment pH was decreased, the three peaks were weakened and broadened, especially when the pH was decreased to be lower than 6, indicating that the Si environments were increasingly changed with decreasing the treatment pH value. Replacement of alkali cations by protons during the acid treatment may also account for the observed broadening of the MAS NMR spectra because protons can lead to the increase in net electron density for all Si sites.35 When the pH was 4, the peaks at -94.7 and -96.3 ppm essentially merged into one peak at about -97.6 ppm while the peak at -103.8 ppm totally disappeared, suggesting the breaking of the Si-O-Si bonds, leading to the partial loss of both Si(4Si, 0Ti) and Si(3Si, 1Ti) sites. A new
Microporous Titanosilicate ETS-10
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Figure 8. Four possible locations of extra framework cations in the framework of ETS-10.12 Ti is shown in black, silicon in dark gray, and oxygen in light gray. Figure 7. (A) 29Si MAS NMR and (B) 29Si CP/MAS NMR spectra of ETS-10-original (a) and HNO3-treated ETS-10 samples at pH ) 7 (b), 6 (c), 5 (d), and 4 (e).
broad peak at about -110.9 ppm, which is due to the presence of Q4 sites [(SiO)4*Si],36 appeared, confirming the breaking of the Si-O-Si bonds and further condensation of the leached Si species. Figure 7B shows the 1H-29Si CP MAS NMR spectra of the ETS-10 samples. The featureless 1H-29Si CP MAS NMR signal of sample ETS-10-original indicates that no protons are present in ETS-10-original, indicating that Si in the as-synthesized ETS10 sample has a single environment. When the sample was acidtreated at pH ) 7, a peak at about -97.4 ppm appeared (spectrum b). Further decreasing the treatment pH led to the appearance of two more resonances at -99.5 and -103 ppm, respectively. In combination with the 29Si MAS NMR data shown in Figure 7A, the peak at -97.4, -99.5, and -103 ppm can be attributed to the Si sites (*Si) in (SiO)2OH*Si-O-Ti, (SiO)2*Si-(OH)2, and (SiO)3*Si-OH, respectively. This further indicates the breaking of the Si-O-Si bonds because of acid treatment. The above MAS NMR data strongly support that the acid treatments under different pH values resulted in local structure changes and distortion, in good agreement with the wide-angle XRD results shown in Figure 2A. 3.8. Summary of Local Structural Changes of ETS-10 upon Acid Treatment. From the above characterization data it can be concluded that acid treatment can lead to partial dissolution (breaking up and removal) of the siliceous matrix and the disruption of -O-Ti-O-Ti-O- chains without obviously affecting the crystalline structure of ETS-10. The FTIR and Raman spectra (Figures 4 and 5) clearly suggest that Ti leaching and/or breaking of [TiO6] chains occurred during the acid treatment. It is known that ETS-10 exhibits a totally symmetric combination of asymmetric Ti-O-Ti stretches along the -O-Ti-O-Ti-O- chains, in which four equivalent TiO-Si bonds perpendicular to the chain have a unique Ti-O distance of 2.02 Å and two unequivalent Ti-O bonds parallel to the chain have significantly different first-shell Ti-O distances of 1.73 and 2.11 Å, respectively, alternately existing in the -O-Ti-O-Ti-O- chains.37 Since the apical oxygen in the short Ti-O bond is strongly bonded to titanium, its binding strength is similar to that of a TidO double bond,32 thus more stable. As a result, the long-distanced Ti-O bonds are liable to be attacked by protons, accounting for the shortening of the average chain length of -O-Ti-O-Ti-Orods and/or partial Ti leaching from the -O-Ti-O-Ti-O-
TABLE 2: Comparison of the Photocatalytic Activities and Specific Surface Areas of P25 (TiO2), ETS-10-original, and Acid-Treated ETS-10 Samples acid P25 ETS-10 Original ETS-10 (HF) ETS-10-4 ETS-10-4 ETS-10-4
HF HNO3 H3PO4 citric acid
SBET (m2/g) 290 279 350 317 323
conversio n (%)a CP
DCP
71.2(1) 72.9(1.02) 24.0(1) 33.0(1.26) 25.7(1) 58.5(2.28) 25.2(1) 56(2.22) 26.0(1) 53.6(2.06) 26.5(1) 56.6(2.14)
TCP 72.2(1.01) 36.4(1.36) 90.5(3.52) 87.8(3.48) 85.1(3.27) 81.1(3.06)
a The values in parentheses are the relative conversion of DCP and TCP compared to that of CP.
chains.15 From the XPS spectra (Figure 6), it is concluded that the disruption of the TiO6 chains is due to the conversion of the hexacoordinated Ti species to tetra- or pentacoordinated Ti species, thus resulting in the generation of TiOH groups. It can be inferred from the results of the MAS NMR spectra and Figure 1 that the siliceous framework surrounding the TiO-Ti chains was partially dissolved by the acid. As illustrated in Figure 8,12 in the framework of ETS-10, K+ ions preferentially locate on sites III and IV.20 On the basis of the MAS NMR data, it can be concluded that most of the Si-O bonds of the Si (4Si, 0Ti) sites and partial of the Si-O bonds of the Si (3Si, 1Ti) sites were broken upon acid treatment, resulting in the partial removal of the siliceous matrix and the formation of Si-OH groups. Consequently, the local structure or local electron density of most of the cation sites I and II and a small fraction of the cation sites III and IV were affected. This may account for the preferable replacement of Na+ ions by protons than K+ ions. As a result of the breaking of Ti-O-Si and Ti-O-Ti bonds under acidic conditions, the density of accessible TiOH groups was increased as reflected by the DR-UV spectrum of sample ETS-10-4 wetted with H2O2. 3.9. Photodegradation of Chlorinated Aromatic Phenol Compounds. Table 2 presents the photocatalytic conversions of three (poly)chlorinated aromatic compounds over catalysts P25, ETS-10-original, ETS-10-original rinsed with HF, and ETS-10 treated with nitric acid, phosphoric acid, and citric acid at pH ) 4. It is seen that P25 exhibited a high activity toward the three compounds. About 72% of the aromatic molecules were decomposed by P25 in 30 min. In contrast, ETS-10-original showed a very low catalytic activity toward the three compounds, indicating that Ti species in defect-free ETS-10 were photocatalytically inactive.9 However, the ETS-10 samples
778 J. Phys. Chem. C, Vol. 111, No. 2, 2007 treated with the acids were all photocatalytically active but with different activities toward different organic compounds. They exhibited a 2-3 times higher activity toward DCP and TCP than toward CP. The enhancement of active TiOH sites on the external surface of the acid-treated ETS-10 samples contributed to the observed improvement in the photocatalytic activity toward DCP and TCP. On the other hand, the partial dissolution of the siliceous matrix because of acid treatment facilitated the smallest chlorinated phenol compound, CP, to diffuse into the cavities of ETS-10 against photodegradation. It can also be seen that the ETS-10 samples treated with HNO3, H3PO4, and citric acid all displayed an activity comparable to that of the the HFtreated ETS-10 sample, showing that treatment of ETS-10 for enhancing the density of TiOH groups can be implemented with common acids like HNO3. Table 2 lists the surface areas of the samples. The treatments with HNO3, H3PO4, and citric acid largely increased the specific surface area. However, the surface area of ETS-10 treated with HF was slightly lowered because of the partial collapse of the pore structure of ETS-10. Nevertheless, it can be concluded that the surface area of ETS-10 materials is not a significant factor affecting their photocatalytic properties. 4. Conclusions The Ti-O-Ti chains surrounded by the siliceous matrix in the framework of ETS-10 are unstable toward acid treatment regardless of the nature of the acids. With decreasing the treatment pH, the leaching of Ti species and/or the disruption of the Ti-O-Ti chains become more and more remarkable, resulting in the conversion of hexacoordinated Ti species to tetra- or pentacoordinated Ti species. Such acid treatment also leads to the dissolution of the siliceous moiety. Both the interruption of Ti-O-Ti chains and the dissolution of the siliceous matrix collectively bring about the increase in the density of TiOH groups, which are responsible for the observed enhancement of photodegradation activity toward aromatic compounds. This work further confirmed that friendly acids such as HNO3, H3PO4, and citric acid can be used to modify ETS10 materials. Acknowledgment. This work was financially supported by NUS (RP279-000-171-112). L.L. thanks NUS for offering a scholarship. References and Notes (1) Kuznicki, S. M. U.S. Patent, 4,853,202, 1989. (2) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (3) Anderson, M. W.; Terasaki, O.; Oshuna, T.; O’Malley, P. J.; Philippou, A.; Mackay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Philos. Mag. B 1995, 71, 813.
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