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Cobalt substitution in ETS-10
Microporous and Mesoporous Materials 48 (2001) 65±71
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Cobalt substitution in ETS-10 Abdusallam Eldewik, Russell F. Howe * School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 6 August 2000; received in revised form 20 November 2000; accepted 27 November 2000
Abstract The preparation and characterization of a cobalt substituted ETS-10 titanosilicate are described. X-ray diraction shows that cobalt incorporation causes an increase in unit cell dimensions. UV±VIS, EPR, Raman and Co K-edge XANES spectra all show that Co2 occupies tetrahedral sites, substituting for silicon. The 29 Si NMR spectra do not permit identi®cation of which silicon sites in ETS-10 are substituted, but the Co K-edge EXAFS shows clearly that Co2 substitutes at Si(3Si,1Ti) sites. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: ETS-10 cobalt substitution; EXAFS; EPR
1. Introduction The titanosilicate ETS-10 has a novel structure in which chains of corner sharing TiO6 octahedra are linked to tetrahedral silicate units, generating a three-dimensional 12-ring pore system [1,2]. The presence of tetravalent titanium in octahedral coordination generates a formal negative charge of 2 on the unit cell, which is balanced by exchangeable cations; a typical unit cell composition of the as-synthesized material is Na1:5 K0:5 TiSi5 O13 . ETS-10 ®rst attracted attention as a high capacity ion exchange material [3]. Subsequently it has been shown to have potential as a heterogeneous base catalyst [4], and to stabilize radical cations produced by photo-irradiation of organic molecules
* Corresponding author. Present address: Chemistry Department, University of Aberdeen, Aberdeen AB24 3UE, UK. Tel.: +44-1224-272948; fax: +44-1224-272921. E-mail address: [email protected] (R.F. Howe).
adsorbed in the pores [5]. A particularly intriguing aspect of ETS-10 is the quantum wire concept proposed by Lamberti and coworkers [6±8] to explain the optical properties of ETS-10. These authors have pointed out that the TiO6 chains in ETS-10 behave as one-dimensional semiconductor wires, and band structure calculations of the optical absorption spectrum support this concept [8]. We have noted the potential implications of the nanosized titania chains in ETS-10 for its photoreactivity [9]. The properties of ETS-10 for all of the above applications or potential applications will be modi®ed if other elements are incorporated into the structure. Replacing Si4 or Ti4 with lowervalent elements will increase the negative charge on the framework and hence the ion exchange capacity. Such substitution is expected also to alter the catalytic properties of the material, while incorporation of transition metal ions into or close to the TiO6 chains may modify the electronic properties of the nanowires.
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A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
Anderson et al. have described the incorporation of aluminium [10] and gallium [11] into ETS10. These trivalent ions substitute for silicon. NMR studies showed that the trivalent ions preferentially replace silicon not linked directly through oxygen to titanium; i.e. there is an avoidance (akin to Lowenstein's rule) of Ti±O±Al linkages. On the other hand, we have shown that Fe3 preferentially substitutes for Ti4 in the TiO6 chains, and signi®cantly modi®es the optical absorption spectrum [12]. In the present work we have investigated the incorporation of Co2 into ETS-10. Co2 is known to incorporate readily into tetrahedral sites in aluminophosphate molecular sieves [13]. We show here that in ETS-10 the substitution occurs at Si sites adjacent to the TiO6 chains, again modifying the optical properties. 2. Experimental Synthesis of CoETS-10 was undertaken as follows: 10 g of sodium silicate solution (27% SiO2 , 14% NaOH, 59% H2 O) was stirred with 3.2 g of water. To this solution was added a solution of 1.3 g of NaOH in 9.3 g water. The resulting gel was stirred for a further 30 min, then a solution containing 7.1 g of TiCl3 (as a 15% solution in HCl) and 0.94 g of Co(NO3 )2 6H2 O was added dropwise. 1.5 g of KF 2H2 O and 0.2 g of ETS-10 seed crystals were added, and the mixture stirred until homogeneous. The ®nal gel composition was 3.6 Na2 O:0.94 K2 O:TiO2 :0.07 CoO:5.5 SiO2 :173 H2 O. The gel was autoclaved under autogeneous pressure in a Te¯on-lined autoclave for 10 days at 453 K. Samples of unsubstituted ETS-10 were prepared in a similar manner, without the addition of cobalt. Chemical analysis was undertaken by ICPAES analysis on samples dissolved in hydro¯uoric acid, and gave the unit cell composition Na1:8 K0:33 Co0:06 TiSi4:93 O13 . X-ray powder diraction measurements employed a Siemens D500 diractometer. UV±VIS spectra were measured by diuse re¯ectance (Cary 5 spectrophotometer). EPR spectra were measured at 10 K in a helium cryostat on a Bruker EMX
spectrometer at 9.5 GHz. Raman spectra were obtained with a Renishaw 2000 imaging microprobe. 29 Si NMR spectra were measured at 59.6 MHz on a Bruker MSL300 spectrometer, using high power proton decoupling and MAS at 5 kHz. X-ray absorption spectra (XANES and EXAFS) were measured at room temperature by ¯uorescence (10-element germanium detector) from pressed disks on BL20B, the Australian National Beam Line Facility at the Photon Factory, Japan. The Si(1 1 0) double crystal monochromator was calibrated at the Co K-edge with a cobalt foil. EXAFS data were analysed with the program XFIT, using the FEFF5.0 code [13]. 3. Results and discussion The chemical analysis of CoETS-10 provides indirect evidence for substitution of Co2 for Si4 in the ETS-10 framework. The Ti: (Co Si) ratio is close to the Ti:Si ratio of unsubstituted ETS-10 (0.20), and the (Na K): Ti ratio in CoETS-10 (2.13) is higher than the corresponding value for ETS-10 (2.00) by an amount consistent with the Co2 content (0.06). The X-ray powder diraction pattern of CoETS-10 showed similar peaks and relative intensities to that of ETS-10. Two additional peaks at 2h 20:8° and 26.55° due to a-quartz were not found in ETS-10, but no additional peaks due to cobalt oxide phases were detected. Fig. 1 shows an expansion of part of the diraction patterns of ETS-10 and CoETS-10. The CoETS-10 shows a marked shift to lower angle in all of the diraction peaks (e.g. by 0.25° for the 24.63° 2h peak). The unit cell parameters calculated from the diraction patterns (tetragonal unit cell) were: ETS-10, c 27:08 A; CoETS-10, a 15:11 A, a 14:85 A, c 27:79 A. Expansion of the unit cell is commonly seen when aluminium substitutes for silicon in aluminosilicate zeolites. Anderson et al. describe similar (although smaller) expansions to that seen with CoETS-10 when Al3 or Ga3 are substituted for silicon in ETS-10 [11]. The respective ionic radii of Si4 (40 pm), Al3 (53 pm), Ga3 (61 pm) and Co2 (72 pm) are consistent with the observed
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Fig. 1. Part of X-ray diraction patterns of ETS-10 (Ð) and CoETS-10 ( ).
magnitudes of the unit cell expansions at dierent loading levels of the substituting ion. This should be contrasted with the case of FeETS-10, where substitution of Fe3 (78.5 ppm) for Ti4 (74.5 ppm) causes negligible change in the unit cell size. The CoETS-10 material is light blue in colour as synthesized, and there is no change in colour on dehydration. The UV±VIS spectrum shown in Fig. 2 con®rms the tetrahedral coordination of Co2 .
Fig. 2. Diuse re¯ectance spectra of (a) ETS-10 and (b) CoETS-10.
In particular, the triplet structure between 14000 and 19 000 cm 1 is closely similar to that observed in as-synthesized CoAPO-5 [14±16], and attributed to d±d transitions of tetrahedral Co2 . The broad band at around 27 000 cm 1 is seen also in the spectra of calcined CoAPO-5, and although this was at ®rst attributed to oxidation of Co2 to Co3 [14], later studies have con®rmed that it is probably associated with distortion of the tetrahedral coordination around Co2 [16,17]. In addition to the presence of bands due to tetrahedral Co2 , the spectrum of ETS-10 shows that cobalt substitution also modi®es the UV absorption bands associated with the TiO6 chains. There is a shift in the apparent band gap to lower energy, and the high energy cm 1 band assigned to Ti±O charge transfer transitions in Ti±O bonds perpendicular to the chain axis (i.e. bridging to Si) [6] is blue shifted by about 5000 cm 1 , suggesting that Co2 is located in proximity to the TiO6 chains. The EPR spectrum of CoETS-10 is shown in Fig. 3. The broad non-axial signal obtained diers in several respects from the corresponding signal obtained from Co2 substituted in ALPO-5 [16,17]. The line shape is typical of an
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A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
Fig. 3. EPR spectrum (X-band) recorded at 10 K of CoETS-10.
orthorhombic powder pattern rather than axial, and there is clearly resolved 59 Co hyperine splitting associated with the low ®eld component. Table 1 summarizes the EPR parameters of this signal for ETS-10 and some related systems. It should be emphasized that the apparent g-values quoted in the table are those deduced from the turning points in the observed spectra. As pointed out by Weckhuysen et al. [17], the apparent g-values of high spin Co2 do not allow unambiguous determination of the coordination symmetry. Comparison with other zeolite systems in which high spin Co2 has been studied (Table 1) nevertheless suggests that the Co2 is tetrahedrally coordinated in ETS-10. The resolution of 57 Co hyper®ne splitting in CoETS-10 does indicate that the Co2 ions are magnetically isolated (dipolar broadening from adjacent 27 Al and 31 P nuclei is presumably responsible for the greater line widths in CoAPO-5).
Further evidence for the presence of tetrahedral Co2 in CoETS-10 is found in the Raman and Co K-edge XANES spectra (not shown). The Raman spectrum of ETS-10 shows a strong band at 727 cm 1 due to the Ti±O stretching vibration in TiO6 octahedra [19]. In CoETS-10, this band is broadened to higher frequency, consistent with the presence of a band at around 790 cm 1 due to the A1 totally symmetric stretching mode of a CoO4 tetrahedron [20]. In the Co K-edge XANES spectrum, tetrahedral coordination is revealed by the presence of a pre-edge peak associated with dipole forbidden 1s ! 3d transitions which become partially allowed in tetrahedral symmetry. The intensity of this feature relative to the K-edge height for CoETS-10 is comparable to that in CoAPO-5 [16]. Given the spectroscopic evidence for tetrahedral substitution of Co2 for Si4 in the ETS-10 structure, we then consider where the substitution takes place. There are two types of chemical environment for silicon in ETS-10: silicon linked through oxygen to four silicon atoms (4Si, 0Ti), and silicon linked to three silicon atoms and one titanium (3Si, 1Ti). The ratio of Si(4Si, 0Ti) sites to Si(3Si, 1Ti) sites is 1:4. The 29 Si NMR spectrum of ETS-10 shows these two sites clearly resolved (Fig. 4(a)); there is a further resolution of the Si(3Si, 1Ti) sites into several crystallographically inequivalent components [1,10,11]. The 29 Si NMR spectra of aluminium and gallium substituted ETS-10 reported by Anderson et al. show all the resonances found in ETS-10 (broadened to some extent) plus additional down ®eld signals assigned to Si(2Si, 1Ti, 1Al or 1Ga) sites [11]. From the positions and relative intensities of these peaks, the authors concluded that
Table 1 EPR parameters of Co2 in zeolite systems Sample CoETS-10 (59 Co hyper®ne) CoAPO-5 calcined CoAPO-5-as-synthesized CoAPO-5 calcined CoA calcined 400°C (tetrahedral) CoX calcined 600°C (octahedral) a
From turning points in the spectrum.
Eective g-valuesa
Ref.
g1
g2
g3
7.6 (113 Gauss) 5.41 5.80 5.44 5.5 3.6
2.7 5.41 5.80 5.44 5.5 3.6
2.03 2.00 2.00 2.00 2.05 2.0
This work [16] [17] [17] [18] [18]
A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
Fig. 4.
29
69
Si NMR spectra of (a) ETS-10 and (b) CoETS-10.
aluminium and gallium substitution for silicon occurs almost exclusively at the Si(4Si, 0Ti) sites i.e. that Al(Ga)±Ti avoidance is displayed equivalent to the Al±Al avoidance in aluminosilicate zeolites (Lowensteins rule). Semi-empirical quantum chemical modelling calculations support this observation [21]. The 29 Si NMR spectrum of CoETS-10 (Fig. 4(b)) shows the same three signals seen for the unsubstituted material, plus an additional down®eld signal at about 90 ppm. The line widths are substantially increased however. Substitution of paramagnetic Co2 for Si is expected to render the Si atoms adjacent to the substitution sites NMR invisible [22]. van Breukelen et al. suggest that in CoAPO-5 the in¯uence of paramagnetic Co2 extends to both the ®rst and second phosphorus shells, but this is explained by the presence of aggregated clusters of cobalt containing at least ®ve atoms [22]. In the absence of a reliable quantitative description of the eects of an incorporated isolated paramagnetic ion on surrounding nuclei, we are unable to determine from the 29 Si NMR spectrum of CoETS-10 which Si sites are substituted by Co. We can however distinguish between the two types of substitution site, Co(4Si) and Co(3Si, 1Ti) from analysis of the Co K-edge EXAFS. The observed k 3 weighted EXAFS and corresponding Fourier transforms are shown in Figs. 5 and 6, and compared with the best-®t calculated data for re-
Fig. 5. Co K-edge EXAFS (k 3 weighted) and Fourier transform (uncorrected for phase shift) of CoETS-10. Solid curves: observed data (Fourier ®ltered). Dotted curves: best-®t calculated data for the Co(4Si) model.
spectively Co(4Si) and Co(3Si, 1Ti). The structural parameters for both models are given in Table 2. The two peaks in the Fourier transforms correspond to the nearest neighbour Co±O shells and the next nearest neighbour Co±Si and Co±Ti shells. The ®tting procedure involved Fourier ®l ®xing tering the raw data over the r range 1±4 A, the coordination numbers at the values speci®ed in the models, then allowing the edge shift E0 , the distances and the Debye±Waller factors to ¯oat. There is clearly an improved ®t obtained to the experimental data when the second shell is assumed to contain contributions from both Co±Ti and Co±Si next nearest neighbours i.e. the model
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tain that cobalt substitution is exclusively at this site. is signi®cantly The Co±O bond length (2.04 A) longer than the average Si±O bond length in ETS [2]). The observed Co±Ti distance 10 (1.61±1.70 A (3.13 A) is however shorter than the Si±Ti distance according to the in unsubstituted ETS-10 (3.31 A Ti K-edge EXAFS measurements of Sankar et al. [23]), suggesting that the larger ionic size of Co2 is accommodated by a distortion which involves narrowing of the Ti±O±Co bond angle. The Co±Si distance, on the other hand, is close to the Ti±Si [23]), so distance in unsubstituted ETS-10 (3.31 A that the distortion of geometry around the three silicons linked to cobalt is similar to that around the silicons linked to titanium in the unsubstituted ETS-10. It should be recalled that the level of cobalt substitution corresponds to only 1.5% of the Si(3Si, 1Ti) sites in the ETS-10 lattice, so that any lattice strain induced by the local distortions is readily accommodated.
4. Conclusions
Fig. 6. Co K-edge EXAFS (k 3 weighted) and Fourier transform (uncorrected for phase shift) of CoETS-10. Solid curves: observed data (Fourier ®ltered). Dotted curves: best-®t calculated data for the Co(3Si, 1Ti) model.
Co(3Si, 1Ti) is preferred. The scattering amplitudes and phase shifts of Ti (Z 22) and Si (Z 14) are suciently dierent that this distinction is unambiguous. We cannot however be cer-
We have shown through the application of dierent spectroscopic techniques that Co2 can be substituted for silicon at tetrahedral sites in ETS10, and that substitution occurs preferentially at silicon sites adjacent to the TiO6 chains. The resulting modi®cations to the UV±VIS absorption spectrum of ETS-10 suggest that the photoelectronic properties of CoETS-10 may dier signi®cantly from those of the unsubstituted material. We are currently investigating the consequences of these dierences, particularly for the photocatalytic properties of ETS-10.
Table 2 Best-®t structural parameters from Co K-edge EXAFS Model
Shell
Na
rb
2 r2 A
Co(4Si)
Co±O Co±Si Co±O Co±Ti Co±Si
4.0 4.0 4.0 1.0 3.0
2.02 3.28 2.04 3.12 3.33
0.007 0.007 0.007 0.003 0.013
Co(3Si, 1Ti)
a b
Fixed values. 0.05 A.
Residual R 24% 9.8%
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Acknowledgements This work was supported by a grant from the Australian Research Council. AE acknowledges scholarship support from the Libyan government. Access to the Photon Factory was funded by the Australian Synchrotron Research Program. References [1] M.W. Anderson, O. Terasaki, T. Ohsana, P.J. O'Malley, A. Phillipou, S.P. Mackay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [2] X. Wang, A. Jacobsen, Chem. Commun. (1999) 973. [3] S.M. Kuznicki, US patent 48 53 202 (1989). [4] A. Phillipou, J. Rocha, M.W. Anderson, Catal. Lett. 57 (1999) 151. [5] R.M. Krishna, A.M. Prakash, V. Kurshev, L. Kevan, Phys. Chem. Chem. Phys. 1 (1999) 4119. [6] E. Borello, C. Lamberti, S. Bordiga, A. Zecchina, C.O. Arean, Appl. Phys. Lett. 71 (1997) 2319. [7] C. Lamberti, Micropor. Mesopor. Mater. 30 (1999) 155. [8] S. Bordiga, G.T. Palomino, A. Zecchina, G. Ranghino, E. Giamello, C. Lamberti, J. Chem. Phys. 112 (2000) 3895. [9] R.F. Howe, Y. Krisnandi, Chem. Commun. (2001) in press.
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[10] M.W. Anderson, A. Phillipou, Z. Lin, A. Ferreira, J. Rocha, Angew. Chem. Int. Ed. 34 (1995) 1003. [11] M.W. Anderson, J. Rocha, Z. Lin, A. Phillipou, I. Orion, A. Ferreira, Micropor. Mater. 6 (1996) 195. [12] A. Eldewik, V. Luca, N.K. Singh, R.F. Howe, In: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proc. 12th International Zeolite Conference 1999, Materials Research Society, Warrendale, PA, p. 1507. [13] P. Ellis, H. Freeman, J. Synchrotron Radiat. 2 (1995) 190. [14] A.A. Verbeckmoes, M.G. Uytterhoeven, R.A. Schoonheydt, Zeolites 19 (1997) 180. [15] A.A. Verbeckmoes, B.M. Weckhuysen, R.A. Schoonheydt, Micropor. Mesopor. Mater. 22 (1998) 165. [16] S. Thomson, V. Luca, R.F. Howe, Phys. Chem. Chem. Phys. 1 (1999) 615. [17] B.M. Weckhuysen, A.A. Verbeckmoes, M.G. Uytterhoeven, F.E. Mabbs, D. Collison, E. de Boer, R.A. Schoonheydt, J. Phys. Chem. B 104 (2000) 37. [18] M.A. Heilbron, J.C. Vickerman, J. Catal. 33 (1974) 434. [19] S. Asteakar, A.M. Prakash, L. Kevan, L.F. Gladden, Chem. Commun. (1998) 91. [20] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., Wiley, New York, 1986. [21] M. Grillo, J. Carrazza, J. Phys. Chem. B 101 (1997) 6749. [22] H.F.W. van Breukelen, G.J.C. Kraaijveld, L.J.M. van de Ven, J.W. de Haan, J.H.C. van Hoof, Micropor. Mater. 12 (1997) 313. [23] G. Sankar, R.G. Bell, J.M. Thomas, M.W. Anderson, P.A. Wright, J. Rocha, J. Phys. Chem. 100 (1996) 449.
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Cobalt substitution in ETS-10 Abdusallam Eldewik, Russell F. Howe * School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 6 August 2000; received in revised form 20 November 2000; accepted 27 November 2000
Abstract The preparation and characterization of a cobalt substituted ETS-10 titanosilicate are described. X-ray diraction shows that cobalt incorporation causes an increase in unit cell dimensions. UV±VIS, EPR, Raman and Co K-edge XANES spectra all show that Co2 occupies tetrahedral sites, substituting for silicon. The 29 Si NMR spectra do not permit identi®cation of which silicon sites in ETS-10 are substituted, but the Co K-edge EXAFS shows clearly that Co2 substitutes at Si(3Si,1Ti) sites. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: ETS-10 cobalt substitution; EXAFS; EPR
1. Introduction The titanosilicate ETS-10 has a novel structure in which chains of corner sharing TiO6 octahedra are linked to tetrahedral silicate units, generating a three-dimensional 12-ring pore system [1,2]. The presence of tetravalent titanium in octahedral coordination generates a formal negative charge of 2 on the unit cell, which is balanced by exchangeable cations; a typical unit cell composition of the as-synthesized material is Na1:5 K0:5 TiSi5 O13 . ETS-10 ®rst attracted attention as a high capacity ion exchange material [3]. Subsequently it has been shown to have potential as a heterogeneous base catalyst [4], and to stabilize radical cations produced by photo-irradiation of organic molecules
* Corresponding author. Present address: Chemistry Department, University of Aberdeen, Aberdeen AB24 3UE, UK. Tel.: +44-1224-272948; fax: +44-1224-272921. E-mail address: [email protected] (R.F. Howe).
adsorbed in the pores [5]. A particularly intriguing aspect of ETS-10 is the quantum wire concept proposed by Lamberti and coworkers [6±8] to explain the optical properties of ETS-10. These authors have pointed out that the TiO6 chains in ETS-10 behave as one-dimensional semiconductor wires, and band structure calculations of the optical absorption spectrum support this concept [8]. We have noted the potential implications of the nanosized titania chains in ETS-10 for its photoreactivity [9]. The properties of ETS-10 for all of the above applications or potential applications will be modi®ed if other elements are incorporated into the structure. Replacing Si4 or Ti4 with lowervalent elements will increase the negative charge on the framework and hence the ion exchange capacity. Such substitution is expected also to alter the catalytic properties of the material, while incorporation of transition metal ions into or close to the TiO6 chains may modify the electronic properties of the nanowires.
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Anderson et al. have described the incorporation of aluminium [10] and gallium [11] into ETS10. These trivalent ions substitute for silicon. NMR studies showed that the trivalent ions preferentially replace silicon not linked directly through oxygen to titanium; i.e. there is an avoidance (akin to Lowenstein's rule) of Ti±O±Al linkages. On the other hand, we have shown that Fe3 preferentially substitutes for Ti4 in the TiO6 chains, and signi®cantly modi®es the optical absorption spectrum [12]. In the present work we have investigated the incorporation of Co2 into ETS-10. Co2 is known to incorporate readily into tetrahedral sites in aluminophosphate molecular sieves [13]. We show here that in ETS-10 the substitution occurs at Si sites adjacent to the TiO6 chains, again modifying the optical properties. 2. Experimental Synthesis of CoETS-10 was undertaken as follows: 10 g of sodium silicate solution (27% SiO2 , 14% NaOH, 59% H2 O) was stirred with 3.2 g of water. To this solution was added a solution of 1.3 g of NaOH in 9.3 g water. The resulting gel was stirred for a further 30 min, then a solution containing 7.1 g of TiCl3 (as a 15% solution in HCl) and 0.94 g of Co(NO3 )2 6H2 O was added dropwise. 1.5 g of KF 2H2 O and 0.2 g of ETS-10 seed crystals were added, and the mixture stirred until homogeneous. The ®nal gel composition was 3.6 Na2 O:0.94 K2 O:TiO2 :0.07 CoO:5.5 SiO2 :173 H2 O. The gel was autoclaved under autogeneous pressure in a Te¯on-lined autoclave for 10 days at 453 K. Samples of unsubstituted ETS-10 were prepared in a similar manner, without the addition of cobalt. Chemical analysis was undertaken by ICPAES analysis on samples dissolved in hydro¯uoric acid, and gave the unit cell composition Na1:8 K0:33 Co0:06 TiSi4:93 O13 . X-ray powder diraction measurements employed a Siemens D500 diractometer. UV±VIS spectra were measured by diuse re¯ectance (Cary 5 spectrophotometer). EPR spectra were measured at 10 K in a helium cryostat on a Bruker EMX
spectrometer at 9.5 GHz. Raman spectra were obtained with a Renishaw 2000 imaging microprobe. 29 Si NMR spectra were measured at 59.6 MHz on a Bruker MSL300 spectrometer, using high power proton decoupling and MAS at 5 kHz. X-ray absorption spectra (XANES and EXAFS) were measured at room temperature by ¯uorescence (10-element germanium detector) from pressed disks on BL20B, the Australian National Beam Line Facility at the Photon Factory, Japan. The Si(1 1 0) double crystal monochromator was calibrated at the Co K-edge with a cobalt foil. EXAFS data were analysed with the program XFIT, using the FEFF5.0 code [13]. 3. Results and discussion The chemical analysis of CoETS-10 provides indirect evidence for substitution of Co2 for Si4 in the ETS-10 framework. The Ti: (Co Si) ratio is close to the Ti:Si ratio of unsubstituted ETS-10 (0.20), and the (Na K): Ti ratio in CoETS-10 (2.13) is higher than the corresponding value for ETS-10 (2.00) by an amount consistent with the Co2 content (0.06). The X-ray powder diraction pattern of CoETS-10 showed similar peaks and relative intensities to that of ETS-10. Two additional peaks at 2h 20:8° and 26.55° due to a-quartz were not found in ETS-10, but no additional peaks due to cobalt oxide phases were detected. Fig. 1 shows an expansion of part of the diraction patterns of ETS-10 and CoETS-10. The CoETS-10 shows a marked shift to lower angle in all of the diraction peaks (e.g. by 0.25° for the 24.63° 2h peak). The unit cell parameters calculated from the diraction patterns (tetragonal unit cell) were: ETS-10, c 27:08 A; CoETS-10, a 15:11 A, a 14:85 A, c 27:79 A. Expansion of the unit cell is commonly seen when aluminium substitutes for silicon in aluminosilicate zeolites. Anderson et al. describe similar (although smaller) expansions to that seen with CoETS-10 when Al3 or Ga3 are substituted for silicon in ETS-10 [11]. The respective ionic radii of Si4 (40 pm), Al3 (53 pm), Ga3 (61 pm) and Co2 (72 pm) are consistent with the observed
A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
67
Fig. 1. Part of X-ray diraction patterns of ETS-10 (Ð) and CoETS-10 ( ).
magnitudes of the unit cell expansions at dierent loading levels of the substituting ion. This should be contrasted with the case of FeETS-10, where substitution of Fe3 (78.5 ppm) for Ti4 (74.5 ppm) causes negligible change in the unit cell size. The CoETS-10 material is light blue in colour as synthesized, and there is no change in colour on dehydration. The UV±VIS spectrum shown in Fig. 2 con®rms the tetrahedral coordination of Co2 .
Fig. 2. Diuse re¯ectance spectra of (a) ETS-10 and (b) CoETS-10.
In particular, the triplet structure between 14000 and 19 000 cm 1 is closely similar to that observed in as-synthesized CoAPO-5 [14±16], and attributed to d±d transitions of tetrahedral Co2 . The broad band at around 27 000 cm 1 is seen also in the spectra of calcined CoAPO-5, and although this was at ®rst attributed to oxidation of Co2 to Co3 [14], later studies have con®rmed that it is probably associated with distortion of the tetrahedral coordination around Co2 [16,17]. In addition to the presence of bands due to tetrahedral Co2 , the spectrum of ETS-10 shows that cobalt substitution also modi®es the UV absorption bands associated with the TiO6 chains. There is a shift in the apparent band gap to lower energy, and the high energy cm 1 band assigned to Ti±O charge transfer transitions in Ti±O bonds perpendicular to the chain axis (i.e. bridging to Si) [6] is blue shifted by about 5000 cm 1 , suggesting that Co2 is located in proximity to the TiO6 chains. The EPR spectrum of CoETS-10 is shown in Fig. 3. The broad non-axial signal obtained diers in several respects from the corresponding signal obtained from Co2 substituted in ALPO-5 [16,17]. The line shape is typical of an
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A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
Fig. 3. EPR spectrum (X-band) recorded at 10 K of CoETS-10.
orthorhombic powder pattern rather than axial, and there is clearly resolved 59 Co hyperine splitting associated with the low ®eld component. Table 1 summarizes the EPR parameters of this signal for ETS-10 and some related systems. It should be emphasized that the apparent g-values quoted in the table are those deduced from the turning points in the observed spectra. As pointed out by Weckhuysen et al. [17], the apparent g-values of high spin Co2 do not allow unambiguous determination of the coordination symmetry. Comparison with other zeolite systems in which high spin Co2 has been studied (Table 1) nevertheless suggests that the Co2 is tetrahedrally coordinated in ETS-10. The resolution of 57 Co hyper®ne splitting in CoETS-10 does indicate that the Co2 ions are magnetically isolated (dipolar broadening from adjacent 27 Al and 31 P nuclei is presumably responsible for the greater line widths in CoAPO-5).
Further evidence for the presence of tetrahedral Co2 in CoETS-10 is found in the Raman and Co K-edge XANES spectra (not shown). The Raman spectrum of ETS-10 shows a strong band at 727 cm 1 due to the Ti±O stretching vibration in TiO6 octahedra [19]. In CoETS-10, this band is broadened to higher frequency, consistent with the presence of a band at around 790 cm 1 due to the A1 totally symmetric stretching mode of a CoO4 tetrahedron [20]. In the Co K-edge XANES spectrum, tetrahedral coordination is revealed by the presence of a pre-edge peak associated with dipole forbidden 1s ! 3d transitions which become partially allowed in tetrahedral symmetry. The intensity of this feature relative to the K-edge height for CoETS-10 is comparable to that in CoAPO-5 [16]. Given the spectroscopic evidence for tetrahedral substitution of Co2 for Si4 in the ETS-10 structure, we then consider where the substitution takes place. There are two types of chemical environment for silicon in ETS-10: silicon linked through oxygen to four silicon atoms (4Si, 0Ti), and silicon linked to three silicon atoms and one titanium (3Si, 1Ti). The ratio of Si(4Si, 0Ti) sites to Si(3Si, 1Ti) sites is 1:4. The 29 Si NMR spectrum of ETS-10 shows these two sites clearly resolved (Fig. 4(a)); there is a further resolution of the Si(3Si, 1Ti) sites into several crystallographically inequivalent components [1,10,11]. The 29 Si NMR spectra of aluminium and gallium substituted ETS-10 reported by Anderson et al. show all the resonances found in ETS-10 (broadened to some extent) plus additional down ®eld signals assigned to Si(2Si, 1Ti, 1Al or 1Ga) sites [11]. From the positions and relative intensities of these peaks, the authors concluded that
Table 1 EPR parameters of Co2 in zeolite systems Sample CoETS-10 (59 Co hyper®ne) CoAPO-5 calcined CoAPO-5-as-synthesized CoAPO-5 calcined CoA calcined 400°C (tetrahedral) CoX calcined 600°C (octahedral) a
From turning points in the spectrum.
Eective g-valuesa
Ref.
g1
g2
g3
7.6 (113 Gauss) 5.41 5.80 5.44 5.5 3.6
2.7 5.41 5.80 5.44 5.5 3.6
2.03 2.00 2.00 2.00 2.05 2.0
This work [16] [17] [17] [18] [18]
A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
Fig. 4.
29
69
Si NMR spectra of (a) ETS-10 and (b) CoETS-10.
aluminium and gallium substitution for silicon occurs almost exclusively at the Si(4Si, 0Ti) sites i.e. that Al(Ga)±Ti avoidance is displayed equivalent to the Al±Al avoidance in aluminosilicate zeolites (Lowensteins rule). Semi-empirical quantum chemical modelling calculations support this observation [21]. The 29 Si NMR spectrum of CoETS-10 (Fig. 4(b)) shows the same three signals seen for the unsubstituted material, plus an additional down®eld signal at about 90 ppm. The line widths are substantially increased however. Substitution of paramagnetic Co2 for Si is expected to render the Si atoms adjacent to the substitution sites NMR invisible [22]. van Breukelen et al. suggest that in CoAPO-5 the in¯uence of paramagnetic Co2 extends to both the ®rst and second phosphorus shells, but this is explained by the presence of aggregated clusters of cobalt containing at least ®ve atoms [22]. In the absence of a reliable quantitative description of the eects of an incorporated isolated paramagnetic ion on surrounding nuclei, we are unable to determine from the 29 Si NMR spectrum of CoETS-10 which Si sites are substituted by Co. We can however distinguish between the two types of substitution site, Co(4Si) and Co(3Si, 1Ti) from analysis of the Co K-edge EXAFS. The observed k 3 weighted EXAFS and corresponding Fourier transforms are shown in Figs. 5 and 6, and compared with the best-®t calculated data for re-
Fig. 5. Co K-edge EXAFS (k 3 weighted) and Fourier transform (uncorrected for phase shift) of CoETS-10. Solid curves: observed data (Fourier ®ltered). Dotted curves: best-®t calculated data for the Co(4Si) model.
spectively Co(4Si) and Co(3Si, 1Ti). The structural parameters for both models are given in Table 2. The two peaks in the Fourier transforms correspond to the nearest neighbour Co±O shells and the next nearest neighbour Co±Si and Co±Ti shells. The ®tting procedure involved Fourier ®l ®xing tering the raw data over the r range 1±4 A, the coordination numbers at the values speci®ed in the models, then allowing the edge shift E0 , the distances and the Debye±Waller factors to ¯oat. There is clearly an improved ®t obtained to the experimental data when the second shell is assumed to contain contributions from both Co±Ti and Co±Si next nearest neighbours i.e. the model
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A. Eldewik, R.F. Howe / Microporous and Mesoporous Materials 48 (2001) 65±71
tain that cobalt substitution is exclusively at this site. is signi®cantly The Co±O bond length (2.04 A) longer than the average Si±O bond length in ETS [2]). The observed Co±Ti distance 10 (1.61±1.70 A (3.13 A) is however shorter than the Si±Ti distance according to the in unsubstituted ETS-10 (3.31 A Ti K-edge EXAFS measurements of Sankar et al. [23]), suggesting that the larger ionic size of Co2 is accommodated by a distortion which involves narrowing of the Ti±O±Co bond angle. The Co±Si distance, on the other hand, is close to the Ti±Si [23]), so distance in unsubstituted ETS-10 (3.31 A that the distortion of geometry around the three silicons linked to cobalt is similar to that around the silicons linked to titanium in the unsubstituted ETS-10. It should be recalled that the level of cobalt substitution corresponds to only 1.5% of the Si(3Si, 1Ti) sites in the ETS-10 lattice, so that any lattice strain induced by the local distortions is readily accommodated.
4. Conclusions
Fig. 6. Co K-edge EXAFS (k 3 weighted) and Fourier transform (uncorrected for phase shift) of CoETS-10. Solid curves: observed data (Fourier ®ltered). Dotted curves: best-®t calculated data for the Co(3Si, 1Ti) model.
Co(3Si, 1Ti) is preferred. The scattering amplitudes and phase shifts of Ti (Z 22) and Si (Z 14) are suciently dierent that this distinction is unambiguous. We cannot however be cer-
We have shown through the application of dierent spectroscopic techniques that Co2 can be substituted for silicon at tetrahedral sites in ETS10, and that substitution occurs preferentially at silicon sites adjacent to the TiO6 chains. The resulting modi®cations to the UV±VIS absorption spectrum of ETS-10 suggest that the photoelectronic properties of CoETS-10 may dier signi®cantly from those of the unsubstituted material. We are currently investigating the consequences of these dierences, particularly for the photocatalytic properties of ETS-10.
Table 2 Best-®t structural parameters from Co K-edge EXAFS Model
Shell
Na
rb
2 r2 A
Co(4Si)
Co±O Co±Si Co±O Co±Ti Co±Si
4.0 4.0 4.0 1.0 3.0
2.02 3.28 2.04 3.12 3.33
0.007 0.007 0.007 0.003 0.013
Co(3Si, 1Ti)
a b
Fixed values. 0.05 A.
Residual R 24% 9.8%
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Acknowledgements This work was supported by a grant from the Australian Research Council. AE acknowledges scholarship support from the Libyan government. Access to the Photon Factory was funded by the Australian Synchrotron Research Program. References [1] M.W. Anderson, O. Terasaki, T. Ohsana, P.J. O'Malley, A. Phillipou, S.P. Mackay, A. Ferreira, J. Rocha, S. Lidin, Philos. Mag. B 71 (1995) 813. [2] X. Wang, A. Jacobsen, Chem. Commun. (1999) 973. [3] S.M. Kuznicki, US patent 48 53 202 (1989). [4] A. Phillipou, J. Rocha, M.W. Anderson, Catal. Lett. 57 (1999) 151. [5] R.M. Krishna, A.M. Prakash, V. Kurshev, L. Kevan, Phys. Chem. Chem. Phys. 1 (1999) 4119. [6] E. Borello, C. Lamberti, S. Bordiga, A. Zecchina, C.O. Arean, Appl. Phys. Lett. 71 (1997) 2319. [7] C. Lamberti, Micropor. Mesopor. Mater. 30 (1999) 155. [8] S. Bordiga, G.T. Palomino, A. Zecchina, G. Ranghino, E. Giamello, C. Lamberti, J. Chem. Phys. 112 (2000) 3895. [9] R.F. Howe, Y. Krisnandi, Chem. Commun. (2001) in press.
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