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Effect of platinum salts on mesoporous silica materials synthesized via a non-ionic surfactant templating route
Acta mater. 49 (2001) 1957–1962 www.elsevier.com/locate/actamat
EFFECT OF PLATINUM SALTS ON MESOPOROUS SILICA MATERIALS SYNTHESIZED VIA A NON-IONIC SURFACTANT TEMPLATING ROUTE M. A. ARAMENDI´A, V. BORAU, C. JIME´NEZ, J. M. MARINAS, F. J. ROMERO† and F. J. URBANO Department of Organic Chemistry, University of Co´rdoba, Campus de Rabanales, Edificio C-3, Carretera Nacional IV-A, Km. 396, 14014 Co´rdoba, Spain ( Received 22 November 2000; received in revised form 16 February 2001; accepted 24 February 2001 )
Abstract—Various types of mesoporous silica solids were synthesized using a non-ionic template in a gel medium containing different platinum salts that were found to influence the porosity, morphology and structure of the materials obtained. Thus, our results show that platinum salts promote the hydrolysis of tetraethylorthosilicate (TEOS) and the presence of (NH4)2PtCl4 or H2PtCl6 leads to the formation of materials with larger particles, smaller pore sizes and less condensed than those obtained in the absence of platinum salts or in the presence of (NH3)4PtCl2. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sol-gel; Mesostructure; Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); Nuclear magnetic resonance (NMR)
1. INTRODUCTION
Ever since it was first reported by researchers at Mobil [1, 2], the synthesis of mesoporous solids has aroused vast interest in various fields by virtue of their high potential as catalysts [3], sorbents [4], and optical or conducting materials [5]. Mesoporous solids are synthesized using surfactant assemblies as templates; this ensures the formation of ordered mesostructured materials [6]. As a rule, these materials possess some advantages such as high thermal stability, pore size uniformity and surface area. In this work, we used an electrically neutral polyethylene oxide surfactant and a neutral inorganic precursor to obtain solids of the so-called MSU-X type [7]. This type of surfactant promotes framework assembling through hydrogen bonding between the hydrophilic (EO)n segments and the silanol groups of the neutral inorganic precursor. These materials exhibit a wormhole structure that lacks regular channel packing order; however, they possess uniform channel diameters over a range comparable to M41S materials. The low cost and ready biodegradation of the surfactant are two major advantages of this synthetic procedure.
† To whom all correspondence should be addressed. Fax: +34-957-212-066. E-mail address: [email protected] (F. J. Romero)
The influence of a number of factors including surfactant type [7], temperature [8], hydrogen and Na+ ion concentrations [9], and the presence of fluoride [10] on the synthesis of mesoporous silica has been examined using non-ionic surfactants as templates. Our research group has studied the effect of using platinum salts in the synthesis of mesoporous silica with a view to the preparation of Pt-supported silica solids [11], which possess a high catalytic potential. In addition, this methodology could be extended to the synthesis of other metals supported on alternative materials compatible with the sol-gel route. However, whether or not the metal is incorporated into the support, including these salts in the synthetic gel causes substantial morphological and porosity changes in the resulting solids. This paper reports on the structural and morphological effects of different platinum salts included in the synthetic gel used to synthesize various types of mesoporous silica solids in the presence of a non-ionic surfactant as template.
2. EXPERIMENTAL
A reference mesoporous silica, MSU-1, was prepared following a previously reported procedure [7] that involves the use of TEOS as silica source and Tergitol 15-S-12 as template. All other solids were obtained by a similar procedure but including 1 mol% Pt (with respect to Si) in the synthetic medium. The
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 1 ) 0 0 1 0 6 - 9
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ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
platinum salts tested, viz. (NH4)2PtCl4, H2PtCl6 and (NH3)4PtCl2, yielded the solids named MSU-1-1, MSU-1-2 and MSU-1-3, respectively. The Si/Pt/surfactant/H2O mole proportion in the initial mixture was 10:0.1:1:560. By way of example, the synthetic procedure for one of the solids is described. Solid MSU-1-1 was obtained by pouring 10 ml of TEOS over a solution containing 0.171 g of (NH4)2PtCl4 in 46 ml of 0.1 mol l⫺1 Tergitol 15-S12 at room temperature. Following stirring for 24 h, the suspension was allowed to stand for 3 days and the solid was filtered, air-dried and calcined at 873 K for 3 h. All other solids were obtained using a similar procedure and the appropriate platinum salts. X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer using CuKα radiation. Thermogravimetric analyses were conducted on a Cahn 2000 electrobalance by heating at a rate of 10 K min⫺1 in a nitrogen atmosphere. The Pt content was determined by the Service Central d’Analyse, CNRS (Vernaison, France) and the microanalyses (C, H) were performed at Complutense University (Madrid, Spain). N2 isotherms were determined on a Micromeritics ASAP 2000 analyzer. Scanning electron micrographs (SEM) were obtained by using a JEOL 6300 microscope with an Au-sputtered specimen. Transmission electron micrographs (TEM) were recorded on a JEOL 200 CX microscope, using copper grids. 1H MAS NMR and 29Si MAS NMR spectra were recorded at 400.13 and 79.49 MHz, respectively, on a Bruker ACP-400 spectrometer at room temperature. Overall, 1000 free induction decays were accumulated. The excitation pulse and recycle time for 1H MAS NMR spectra were 5 µs and 3 s, respectively, and those for 29Si MAS NMR spectra 6 µs and 60 s. Chemical shifts were measured relative to a tetramethylsilane standard. Prior to measurement, samples were dehydrated in a stove at 423 K for 24 h. 3. RESULTS AND DISCUSSION
The most immediate effect of the addition of a Pt salt to the synthetic medium was an increased yield in the solid obtained, yield that varied in the following sequence: MSU-1-1ⱖ MSU-1-2>>MSU-1-3>MSU-1. Thus, it is arguable that (NH4)2PtCl4 and H2PtCl6, and also, to a lesser extent, (NH3)4PtCl2, might act as catalysts in the process by increasing the extent of hydrolysis and condensation of the inorganic precursor. Several transition metals [e.g. Pd(OAc)2, Ni(acac)2] were found to catalyze the hydrolysis of triethoxysilane for the synthesis of high surface area silica xerogels [12]. Table 1 gives the overall weight losses, C percent weights, and the corresponding C/H ratios for the different solids studied. Tergitol 15-S12 (C39H80O14) possesses a C/H ratio of 5.81, so a lower ratio was to be expected in the solids as they contain occluded residual water and ethanol in addition to surfactant molecules. In any case, the amount of surfactant in the solids MSU-1 and MSU-
Fig. 1. Typical scanning electron micrographs for the solids studied: (a) MSU-1, (b) MSU-1-1, (c) calcined MSU-1-1 and (d) calcined MSU-1-3.
1-3 is higher, which corresponds to their increased void fraction [11]. As expected, the XRD patterns for the solids exhibit a low-angle peak (d100) and second-order peaks typical of mesostructured materials with a sponge-like
ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
Fig. 2. Transmission electron micrographs for the solids studied: (a) MSU-1-1, (b) calcined MSU-1-3 and (c) silica nanotubes on MSU-1-1.
or worm-like pore channel structure (Table 1) [11]. The N2 adsorption–desorption isotherms confirm that, while solids MSU-1 and MSU-1-3 are mesoporous, MSU-1-1 and MSU-1-2 have pore sizes in between those of the micro and meso ranges. The specific surface area exceeds 600 m2 g⫺1 in all cases. The scanning electron micrographs of the solids reveal large morphological differences among them. Thus, while solids MSU-1 and MSU-1-3 (Fig. 1a) are aggregates consisting of small particles of uniform size and glomerular appearance, solids MSU-1-1 and MSU-1-2 (Fig. 1b) consist of smooth, unevenly shaped particles in a wide range of sizes. Following calcination at 873 K for 3 h, all solids exhibit a similar appearance, with large, smooth particles of uneven size and shape. However, one morphological aspect still allows one to discriminate between solids MSU1 and MSU-1-3 on the one hand, and solids MSU-11 and MSU-1-2 on the other. Thus, while the latter
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Fig. 3. 1H MAS NMR spectra for the calcined solids.
two (Fig. 1c) consist of very smooth particles, the former two (Fig. 1d) exhibit many surface cracks, consistent with the presence of smaller, strongly bound particles forming agglomerates. The TEM results also reveal large differences between solids MSU-1-1 and MSU-1-2 and the other two. In fact, these two solids exhibit smooth, nonporous particles, both in the freshly calcined materials and the calcined ones (which, as can be seen from Fig. 2a, contain metal particles on their surface). Solid MSU-1 consists of small particles that are either bound to one another or parts of small, entangled filaments. Calcination of this solid produces smooth particles with no appreciable porosity. Solid MSU-13 (Fig. 2b) is morphologically rather different from the others. Thus, it is highly uniform and exhibits filamentous particles that expose the porous structure in both the untreated and calcined solids. The micrographs show a large number of channels cylindrical to hexagonal in shape that are uniform in diameter; however, the solid lacks the typical long-range packing of these materials, where the precursor binds to
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ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS Table 1. Properties of silica samples prepared in the presence of platinum salts
Solid MSU-1 MSU-1-1 MSU-1-2 MSU-1-3 a b
d100 lattice spacinga Percent weight loss ˚) (A (298–873 K) 55 38 37 56
75 52 52 85
C contentb (wt%)
C/H ratiob
Pt content (wt%)
35.20 26.82 27.68 23.80
5.36 5.05 4.82 5.01
– 2.08 0.36 0.23
Calcined in air at 873 K for 3 h. As synthesized.
the flexible worm-like micelles via hydrogen bonds [7]. Some samples—particularly those of MSU-1-1 and MSU-1-2—exhibit long silica nanotubes with diameters from 10 to 20 nm (see Fig. 2c). The 1H MAS NMR spectra for the calcined solids (Fig. 3) exhibit two broad signals centred at 2.4 and 3.5 ppm. The surfactant used exhibits several peaks, namely: two at 3.60 and 3.43 ppm due to CH2 groups in the ethylene oxide units; two others at 1.55 and 1.32 ppm due to CH2 groups in the alkyl chain; and one at 0.93 ppm due to the CH3 end group in the alkyl chain [13]. The absence of such signals rules out the presence of residual surfactant on the solids following the thermal treatment. These results differ from those previously obtained for MCM-41 materials, where carbon chain fragments persist even above 1023 K [14]. Non-acidic (silanol) OH groups in silica exhibit a signal at 1.8–2.3 ppm. Adsorbed water, which interacts with SiOH groups, causes a downfield shift by about 0.4 ppm [15]. Thus, the signal at 2.4 ppm must correspond to the silanol group as perturbed by the presence of water, and that at 3.5 ppm must be due to the water. The solid-state 29Si MAS NMR spectra for the solids (Figs 4 and 5) exhibit several signals centred at about ⫺110, ⫺101 and ⫺91 ppm that can be assigned to the Q4 [(SiO4)Si], Q3 [(SiO3)SiOH] and Q2 [(SiO)2Si(OH)2] sites, respectively, of the framework. The signal at ⫺82 ppm in the spectrum for solid MSU-1 corresponds to a small amount of unreacted TEOS occluded within pores. The spectra for the assynthesized materials (Fig. 4) show that the Q4/Q3 ratios—determined by deconvolution analysis of the spectra—for solids MSU-1-1 and MSU-1-2 are lower than those for MSU-1 and MSU-1-3; this suggests a higher degree of polymerization and cross-linking— and hence a lower content in OH groups per gram of catalyst—in the latter two. Calcination of all these solids yields more condensed materials, as can be inferred from their increased Q4/Q3 ratios (Fig. 5); differences continue to exist among them, however. Solid MSU-1-1 is that exhibiting the highest proportion of Q2 units (10.5%), which, however, decreases upon calcination (to 4.5%). While the Q4/Q3 ratio for solid MSU-1 is consistent with previously reported values [8, 16], our results clearly show that the addition of a platinum salt to the synthetic gel results in less condensed materials—particularly with (NH4)2PtCl4 and H2PtCl6. The effect is
Fig. 4.
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Si MAS NMR spectra for the as-synthesized solids.
of the opposite sign to that of fluoride, which promotes the polymerization of silica, giving rise to mesoporous materials condensed to a higher extent [10]. These results indicate that platinum salts, especially (NH4)2PtCl4 and H2PtCl6, where platinum is forming anionic complexes, induce extensive hydrolysis of TEOS. Several anions (e.g. halides,
ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
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medium, which implies that this salt is strongly interacting with the surfactant–inorganic complexes during the gelling process. The complexation of small cations by the EO groups of polyethylene oxide surfactants [19], in a similar way to crown ether surfactants [20], is well known. In our case, these interactions would lead to the formation of complexes between the surfactant headgroups and the ammonium cations, which would be sorrounded by the counter anions PtCl42⫺. The uncomplexed EO oxygen atoms of the surfactant would form hydrogen bonds with the silanol groups. 4. CONCLUSIONS
In summary, we synthesized various types of silica using a non-ionic surfactant as template in the presence of different platinum salts in the synthetic gel. The salts were found to induce substantial morphological, structural and porosity changes in the materials obtained. In fact, they promote the hydrolysis and condensation of the silicon precursor; the degree of cross-linking achieved, however, depends on the particular salt employed. The procedure can theoretically be extended to the synthesis of other materials obtained by hydrolysis of a precursor; in the presence of suitable salts of platinum or other noble metals, this should allow one to prepare different materials by properly altering other reaction variables (e.g. pH, presence of fluoride).
Fig. 5.
29
Acknowledgements—The authors are grateful to Spain’s DGES, Ministry of Education and Culture, for funding this research within the framework of Project PB97-0446, and to Junta de Andalucı´a for additional financial support.
Si MAS NMR spectra for the calcined solids.
SO42⫺, NO3⫺) have been found to increase the rate of hydrolysis of the TEOS [17]. Monomeric silica species interact with the surfactant headgroups affording surfactant–inorganic complexes. If the condensation between silicic units (equation 2) is slower than the hydrolysis of the silica precursor (equation 1), the rate of formation of nucleus able to grow decreases, giving rise to larger particles, as observed by SEM for solids MSU-1-1 and MSU-1-2 (Fig. 1b). If the hydrolysis rate is low in comparison with condensation rate, the easy condensation between silanol groups on the surface of different micelles brings about a more rapid nucleation and therefore the formation of smaller particles (Fig. 1a) [18]. ⬵Si–OEt + H2O→⬵Si–OH + Et–OH
(1)
⬵Si–OH + HO–Si⬵→⬵Si–O–Si⬵ + H2O
(2)
As can be seen from Table 1, the amount of platinum incorporated into the silica was only significant when (NH4)2PtCl4 was present in the synthetic
REFERENCES 1. Kresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C. and Beck, J. S., Nature, 1992, 359, 710. 2. Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., Chu, C. T. -W., Olson, D. H., Sheppard, E. W., McCullen, S. B., Higgins, J. B. and Schlenker, J. L., J. Am. Chem. Soc., 1992, 114, 10834. 3. Corma, A., Chem. Rev., 1997, 97, 2373. 4. Mercier, L. and Pinnavaia, T. J., Adv. Mater., 1997, 9, 500. 5. Marlow, F., McGehee, M. D., Zhao, D., Chmelka, B. F. and Stucky, G. D., Adv. Mater., 1999, 11, 632. 6. Go¨ ltner, C. and Antonietti, M., Adv. Mater., 1997, 9, 431. 7. Bagshaw, S. A., Prouzet, E. and Pinnavaia, T. J., Science, 1995, 269, 1242. 8. Prouzet, E. and Pinnavaia, T. J., Angew. Chem. Int. Ed. Engl., 1997, 36, 516. 9. Sierra, L., Lopez, B., Gil, H. and Guth, J. -L., Adv. Mater., 1999, 11, 307. 10. Schmidt-Winkel, P., Yang, P., Margolese, D. I., Chmelka, B. F. and Stucky, G. D., Adv. Mater., 1999, 11, 303. 11. Aramendı´a, M. A., Borau, V., Jime´ nez, C., Marinas, J. M. and Romero, F. J., Chem. Commun., 1999, 873. 12. Tour, J. M. and Kafka, C. M., Adv. Mater., 1993, 5, 47. 13. Dag, O., Verma, A., Ozin, G. A. and Kresge, C. T., J. Mater. Chem., 1999, 9, 1475. 14. Keene, M. T. J., Gougeon, R. D. M., Denoyel, R., Harris, R. K., Rouquerol, J. and Llewellyn, P. L., J. Mater. Chem., 1999, 9, 2843.
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15. Pfeifer, H., Freude, D. and Ka¨ rger, J., Stud. Surf. Sci. Catal., 1991, 65, 89. 16. Richer, R. and Mercier, L., Chem. Commun., 1998, 1775. 17. Bagshaw, S. A., Chem. Commun., 1999, 1785. 18. Di Renzo, F., Testa, F., Chen, J. D., Cambon, H., Galarneau, A., Plee, D. and Fajula, F., Microporous Mesoporous
Mater., 1999, 28, 437. 19. Zhang, W., Glomski, B., Pauly, T. R. and Pinnavaia, T. J., Chem. Commun., 1999, 1803. 20. Danumah, Ch., Zaidi, S. M. J., Xu, G., Voyer, N., Giasson, S. and Kaliaguine, S., Microporous Mesoporous Mater., 2000, 37, 21.
EFFECT OF PLATINUM SALTS ON MESOPOROUS SILICA MATERIALS SYNTHESIZED VIA A NON-IONIC SURFACTANT TEMPLATING ROUTE M. A. ARAMENDI´A, V. BORAU, C. JIME´NEZ, J. M. MARINAS, F. J. ROMERO† and F. J. URBANO Department of Organic Chemistry, University of Co´rdoba, Campus de Rabanales, Edificio C-3, Carretera Nacional IV-A, Km. 396, 14014 Co´rdoba, Spain ( Received 22 November 2000; received in revised form 16 February 2001; accepted 24 February 2001 )
Abstract—Various types of mesoporous silica solids were synthesized using a non-ionic template in a gel medium containing different platinum salts that were found to influence the porosity, morphology and structure of the materials obtained. Thus, our results show that platinum salts promote the hydrolysis of tetraethylorthosilicate (TEOS) and the presence of (NH4)2PtCl4 or H2PtCl6 leads to the formation of materials with larger particles, smaller pore sizes and less condensed than those obtained in the absence of platinum salts or in the presence of (NH3)4PtCl2. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Sol-gel; Mesostructure; Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); Nuclear magnetic resonance (NMR)
1. INTRODUCTION
Ever since it was first reported by researchers at Mobil [1, 2], the synthesis of mesoporous solids has aroused vast interest in various fields by virtue of their high potential as catalysts [3], sorbents [4], and optical or conducting materials [5]. Mesoporous solids are synthesized using surfactant assemblies as templates; this ensures the formation of ordered mesostructured materials [6]. As a rule, these materials possess some advantages such as high thermal stability, pore size uniformity and surface area. In this work, we used an electrically neutral polyethylene oxide surfactant and a neutral inorganic precursor to obtain solids of the so-called MSU-X type [7]. This type of surfactant promotes framework assembling through hydrogen bonding between the hydrophilic (EO)n segments and the silanol groups of the neutral inorganic precursor. These materials exhibit a wormhole structure that lacks regular channel packing order; however, they possess uniform channel diameters over a range comparable to M41S materials. The low cost and ready biodegradation of the surfactant are two major advantages of this synthetic procedure.
† To whom all correspondence should be addressed. Fax: +34-957-212-066. E-mail address: [email protected] (F. J. Romero)
The influence of a number of factors including surfactant type [7], temperature [8], hydrogen and Na+ ion concentrations [9], and the presence of fluoride [10] on the synthesis of mesoporous silica has been examined using non-ionic surfactants as templates. Our research group has studied the effect of using platinum salts in the synthesis of mesoporous silica with a view to the preparation of Pt-supported silica solids [11], which possess a high catalytic potential. In addition, this methodology could be extended to the synthesis of other metals supported on alternative materials compatible with the sol-gel route. However, whether or not the metal is incorporated into the support, including these salts in the synthetic gel causes substantial morphological and porosity changes in the resulting solids. This paper reports on the structural and morphological effects of different platinum salts included in the synthetic gel used to synthesize various types of mesoporous silica solids in the presence of a non-ionic surfactant as template.
2. EXPERIMENTAL
A reference mesoporous silica, MSU-1, was prepared following a previously reported procedure [7] that involves the use of TEOS as silica source and Tergitol 15-S-12 as template. All other solids were obtained by a similar procedure but including 1 mol% Pt (with respect to Si) in the synthetic medium. The
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 1 ) 0 0 1 0 6 - 9
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ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
platinum salts tested, viz. (NH4)2PtCl4, H2PtCl6 and (NH3)4PtCl2, yielded the solids named MSU-1-1, MSU-1-2 and MSU-1-3, respectively. The Si/Pt/surfactant/H2O mole proportion in the initial mixture was 10:0.1:1:560. By way of example, the synthetic procedure for one of the solids is described. Solid MSU-1-1 was obtained by pouring 10 ml of TEOS over a solution containing 0.171 g of (NH4)2PtCl4 in 46 ml of 0.1 mol l⫺1 Tergitol 15-S12 at room temperature. Following stirring for 24 h, the suspension was allowed to stand for 3 days and the solid was filtered, air-dried and calcined at 873 K for 3 h. All other solids were obtained using a similar procedure and the appropriate platinum salts. X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer using CuKα radiation. Thermogravimetric analyses were conducted on a Cahn 2000 electrobalance by heating at a rate of 10 K min⫺1 in a nitrogen atmosphere. The Pt content was determined by the Service Central d’Analyse, CNRS (Vernaison, France) and the microanalyses (C, H) were performed at Complutense University (Madrid, Spain). N2 isotherms were determined on a Micromeritics ASAP 2000 analyzer. Scanning electron micrographs (SEM) were obtained by using a JEOL 6300 microscope with an Au-sputtered specimen. Transmission electron micrographs (TEM) were recorded on a JEOL 200 CX microscope, using copper grids. 1H MAS NMR and 29Si MAS NMR spectra were recorded at 400.13 and 79.49 MHz, respectively, on a Bruker ACP-400 spectrometer at room temperature. Overall, 1000 free induction decays were accumulated. The excitation pulse and recycle time for 1H MAS NMR spectra were 5 µs and 3 s, respectively, and those for 29Si MAS NMR spectra 6 µs and 60 s. Chemical shifts were measured relative to a tetramethylsilane standard. Prior to measurement, samples were dehydrated in a stove at 423 K for 24 h. 3. RESULTS AND DISCUSSION
The most immediate effect of the addition of a Pt salt to the synthetic medium was an increased yield in the solid obtained, yield that varied in the following sequence: MSU-1-1ⱖ MSU-1-2>>MSU-1-3>MSU-1. Thus, it is arguable that (NH4)2PtCl4 and H2PtCl6, and also, to a lesser extent, (NH3)4PtCl2, might act as catalysts in the process by increasing the extent of hydrolysis and condensation of the inorganic precursor. Several transition metals [e.g. Pd(OAc)2, Ni(acac)2] were found to catalyze the hydrolysis of triethoxysilane for the synthesis of high surface area silica xerogels [12]. Table 1 gives the overall weight losses, C percent weights, and the corresponding C/H ratios for the different solids studied. Tergitol 15-S12 (C39H80O14) possesses a C/H ratio of 5.81, so a lower ratio was to be expected in the solids as they contain occluded residual water and ethanol in addition to surfactant molecules. In any case, the amount of surfactant in the solids MSU-1 and MSU-
Fig. 1. Typical scanning electron micrographs for the solids studied: (a) MSU-1, (b) MSU-1-1, (c) calcined MSU-1-1 and (d) calcined MSU-1-3.
1-3 is higher, which corresponds to their increased void fraction [11]. As expected, the XRD patterns for the solids exhibit a low-angle peak (d100) and second-order peaks typical of mesostructured materials with a sponge-like
ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
Fig. 2. Transmission electron micrographs for the solids studied: (a) MSU-1-1, (b) calcined MSU-1-3 and (c) silica nanotubes on MSU-1-1.
or worm-like pore channel structure (Table 1) [11]. The N2 adsorption–desorption isotherms confirm that, while solids MSU-1 and MSU-1-3 are mesoporous, MSU-1-1 and MSU-1-2 have pore sizes in between those of the micro and meso ranges. The specific surface area exceeds 600 m2 g⫺1 in all cases. The scanning electron micrographs of the solids reveal large morphological differences among them. Thus, while solids MSU-1 and MSU-1-3 (Fig. 1a) are aggregates consisting of small particles of uniform size and glomerular appearance, solids MSU-1-1 and MSU-1-2 (Fig. 1b) consist of smooth, unevenly shaped particles in a wide range of sizes. Following calcination at 873 K for 3 h, all solids exhibit a similar appearance, with large, smooth particles of uneven size and shape. However, one morphological aspect still allows one to discriminate between solids MSU1 and MSU-1-3 on the one hand, and solids MSU-11 and MSU-1-2 on the other. Thus, while the latter
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Fig. 3. 1H MAS NMR spectra for the calcined solids.
two (Fig. 1c) consist of very smooth particles, the former two (Fig. 1d) exhibit many surface cracks, consistent with the presence of smaller, strongly bound particles forming agglomerates. The TEM results also reveal large differences between solids MSU-1-1 and MSU-1-2 and the other two. In fact, these two solids exhibit smooth, nonporous particles, both in the freshly calcined materials and the calcined ones (which, as can be seen from Fig. 2a, contain metal particles on their surface). Solid MSU-1 consists of small particles that are either bound to one another or parts of small, entangled filaments. Calcination of this solid produces smooth particles with no appreciable porosity. Solid MSU-13 (Fig. 2b) is morphologically rather different from the others. Thus, it is highly uniform and exhibits filamentous particles that expose the porous structure in both the untreated and calcined solids. The micrographs show a large number of channels cylindrical to hexagonal in shape that are uniform in diameter; however, the solid lacks the typical long-range packing of these materials, where the precursor binds to
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ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS Table 1. Properties of silica samples prepared in the presence of platinum salts
Solid MSU-1 MSU-1-1 MSU-1-2 MSU-1-3 a b
d100 lattice spacinga Percent weight loss ˚) (A (298–873 K) 55 38 37 56
75 52 52 85
C contentb (wt%)
C/H ratiob
Pt content (wt%)
35.20 26.82 27.68 23.80
5.36 5.05 4.82 5.01
– 2.08 0.36 0.23
Calcined in air at 873 K for 3 h. As synthesized.
the flexible worm-like micelles via hydrogen bonds [7]. Some samples—particularly those of MSU-1-1 and MSU-1-2—exhibit long silica nanotubes with diameters from 10 to 20 nm (see Fig. 2c). The 1H MAS NMR spectra for the calcined solids (Fig. 3) exhibit two broad signals centred at 2.4 and 3.5 ppm. The surfactant used exhibits several peaks, namely: two at 3.60 and 3.43 ppm due to CH2 groups in the ethylene oxide units; two others at 1.55 and 1.32 ppm due to CH2 groups in the alkyl chain; and one at 0.93 ppm due to the CH3 end group in the alkyl chain [13]. The absence of such signals rules out the presence of residual surfactant on the solids following the thermal treatment. These results differ from those previously obtained for MCM-41 materials, where carbon chain fragments persist even above 1023 K [14]. Non-acidic (silanol) OH groups in silica exhibit a signal at 1.8–2.3 ppm. Adsorbed water, which interacts with SiOH groups, causes a downfield shift by about 0.4 ppm [15]. Thus, the signal at 2.4 ppm must correspond to the silanol group as perturbed by the presence of water, and that at 3.5 ppm must be due to the water. The solid-state 29Si MAS NMR spectra for the solids (Figs 4 and 5) exhibit several signals centred at about ⫺110, ⫺101 and ⫺91 ppm that can be assigned to the Q4 [(SiO4)Si], Q3 [(SiO3)SiOH] and Q2 [(SiO)2Si(OH)2] sites, respectively, of the framework. The signal at ⫺82 ppm in the spectrum for solid MSU-1 corresponds to a small amount of unreacted TEOS occluded within pores. The spectra for the assynthesized materials (Fig. 4) show that the Q4/Q3 ratios—determined by deconvolution analysis of the spectra—for solids MSU-1-1 and MSU-1-2 are lower than those for MSU-1 and MSU-1-3; this suggests a higher degree of polymerization and cross-linking— and hence a lower content in OH groups per gram of catalyst—in the latter two. Calcination of all these solids yields more condensed materials, as can be inferred from their increased Q4/Q3 ratios (Fig. 5); differences continue to exist among them, however. Solid MSU-1-1 is that exhibiting the highest proportion of Q2 units (10.5%), which, however, decreases upon calcination (to 4.5%). While the Q4/Q3 ratio for solid MSU-1 is consistent with previously reported values [8, 16], our results clearly show that the addition of a platinum salt to the synthetic gel results in less condensed materials—particularly with (NH4)2PtCl4 and H2PtCl6. The effect is
Fig. 4.
29
Si MAS NMR spectra for the as-synthesized solids.
of the opposite sign to that of fluoride, which promotes the polymerization of silica, giving rise to mesoporous materials condensed to a higher extent [10]. These results indicate that platinum salts, especially (NH4)2PtCl4 and H2PtCl6, where platinum is forming anionic complexes, induce extensive hydrolysis of TEOS. Several anions (e.g. halides,
ARAMENDı´A et al.: EFFECT OF PLATINUM SALTS ON SILICA MATERIALS
1961
medium, which implies that this salt is strongly interacting with the surfactant–inorganic complexes during the gelling process. The complexation of small cations by the EO groups of polyethylene oxide surfactants [19], in a similar way to crown ether surfactants [20], is well known. In our case, these interactions would lead to the formation of complexes between the surfactant headgroups and the ammonium cations, which would be sorrounded by the counter anions PtCl42⫺. The uncomplexed EO oxygen atoms of the surfactant would form hydrogen bonds with the silanol groups. 4. CONCLUSIONS
In summary, we synthesized various types of silica using a non-ionic surfactant as template in the presence of different platinum salts in the synthetic gel. The salts were found to induce substantial morphological, structural and porosity changes in the materials obtained. In fact, they promote the hydrolysis and condensation of the silicon precursor; the degree of cross-linking achieved, however, depends on the particular salt employed. The procedure can theoretically be extended to the synthesis of other materials obtained by hydrolysis of a precursor; in the presence of suitable salts of platinum or other noble metals, this should allow one to prepare different materials by properly altering other reaction variables (e.g. pH, presence of fluoride).
Fig. 5.
29
Acknowledgements—The authors are grateful to Spain’s DGES, Ministry of Education and Culture, for funding this research within the framework of Project PB97-0446, and to Junta de Andalucı´a for additional financial support.
Si MAS NMR spectra for the calcined solids.
SO42⫺, NO3⫺) have been found to increase the rate of hydrolysis of the TEOS [17]. Monomeric silica species interact with the surfactant headgroups affording surfactant–inorganic complexes. If the condensation between silicic units (equation 2) is slower than the hydrolysis of the silica precursor (equation 1), the rate of formation of nucleus able to grow decreases, giving rise to larger particles, as observed by SEM for solids MSU-1-1 and MSU-1-2 (Fig. 1b). If the hydrolysis rate is low in comparison with condensation rate, the easy condensation between silanol groups on the surface of different micelles brings about a more rapid nucleation and therefore the formation of smaller particles (Fig. 1a) [18]. ⬵Si–OEt + H2O→⬵Si–OH + Et–OH
(1)
⬵Si–OH + HO–Si⬵→⬵Si–O–Si⬵ + H2O
(2)
As can be seen from Table 1, the amount of platinum incorporated into the silica was only significant when (NH4)2PtCl4 was present in the synthetic
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