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Preparation of high nickel-containing MCM-41-type mesoporous silica via a modified direct synthesis method
Materials Research Bulletin 40 (2005) 1737–1744 www.elsevier.com/locate/matresbu
Preparation of high nickel-containing MCM-41-type mesoporous silica via a modified direct synthesis method Wei Wang, Mo Song * IPTME, Loughborough University, Loughborough LE11 3TU, UK Received 5 January 2005; received in revised form 17 March 2005; accepted 20 May 2005
Abstract A method with modifying tetraethyl orthosilicate (TEOS) with nickel species has been developed for the synthesis of mesoporous silica with high nickel content (11.8 wt.% of Ni or even higher). With the method, MCM41-type materials were obtained with high BET surface area reaching 868 m2/g and pore volume up to 0.73 cm3/g. The materials were characterized by means of X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, N2 adsorption, Fourier transform infrared and X-ray photoelectron spectroscopy. Nickel species were incorporated into the silica frameworks. The mesostructures still remain after activation using H2 at 773 K. # 2005 Elsevier Ltd. All rights reserved. Keywords: B. Chemical synthesis; B. Sol–gel chemistry; C. X-ray diffraction
1. Introduction It is well known that supported nickel catalysts have been widely studied and used due to their very high activity in hydrogenation, hydrotreating and stream-reforming reactions [1,2]. And silica was the most commonly used nickel carrier. Especially, since the discovery of the family of M41S by Mobil scientists in 1992, mesoporous materials have attracted much interest [3,4]. Uniformly hexagonal array of linear channels and mesostructures over long range make them very promising in many areas, such as adsorbents and catalyst supports. Therefore, combining as many catalytically active Ni species as * Corresponding author. E-mail address: [email protected] (M. Song). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.020
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possible with mesoporous silica is of great interest nowadays. Many nickel-containing mesoporous silicas were synthesized and tested in dimerization of ethylene and isomerization of butene [5], desulfurization of thiols and organic sulfides [6,7], deodorization [8] or hydrodechlorination [9], hydrogenation of benzene [10] and so on. The most commonly used method to synthesize nickel-containing mesoporous silica is impregnation one. In this method, mesoporous catalyst supports are impregnated by nickel salt solutions, such as nickel nitrate [7,10–15], nickel bromide [16] and nickel citrate [17]. The disadvantage of this method is that, upon high loadings (higher than 5 wt.% of Ni), it often leads to loss of the BET surface area or even blockage of mesochannels by large Ni (NiO) particles. As reported by Rodrı´guez–Castello´n et al., 48% of BET surface area left after 14.9 wt.% Ni was incorporated and large nickel oxide particles located on the external surface [14]. When 5 wt.% nickel was loaded into MCM-41, only about 73% of total surface area remained [15]. Even only 1.5 wt.% Ni loading into AlMCM-41, about 2 nm NiO particles were formed in the mesochannels [16]. Therefore, an alternative way to introduce nickel species into the mesoporous silica is adding Ni salts, like NiCl2, into the synthesis mixture, followed by the hydrothermal treatment, called direct synthesis method (DS method). However, it is difficult to load high amount of nickel without damaging the mesostructures by using this method [18–20]. For example, with very low nickel loading (<1.5 wt.%), distinct condensation in mesopores was not observed in N2 sorption measurement [20]. So far, to our knowledge, no report has been published on the synthesis of MCM-41-type silica with higher nickel content than 3.6 wt.% using the DS method [19]. Rapid precipitation of nickel ions in basic conditions does not allow full interaction with the template molecules, which results in the low loading into the mesophases and formation of extra-frameworks NiO after calcination. In this communication, we presented a modified sol–gel process for the synthesis of MCM-41-type mesoporous silica with high nickel content. In this method, ‘TEOS-seizing nickel species’ co-hydrolyzes and co-condenses with TEOS during the hydrothermal treatment for building up mesostructures templated by cationic surfactant. This method was proved to be effective to incorporate higher amount of nickel into mesophases.
2. Experimental All the chemicals were bought from Aldrich Chemicals (UK) and used as received. 0.6 g nickel(II) nitrate hexahydrate was dissolved into 3.5 mL ethylene glycol (EG). The solution was heated and stirred at 353 K for 10 h, and then 4.25 mL TEOS was added dropwise. The mixture was annealed at 363 K for 10 h. This clear solution of TEOS modified with nickel species was dripped into the solution of 0.75 g cetyltrimethylammonium bromide (CTAB) in 32 g de-ionized water. The pH value was adjusted to 11.0. Then the mixture was annealed at 298 K overnight with moderate stirring and then kept at 370 K for 120 h statically. After cooling to room temperature, pH value of the mixture was adjusted to 2.0 using diluted HCl (0.01 M) with magnetic stirring. Stirring was continued for 5 min after acidification. The precipitates were then washed by copious distilled water, and air-dried at 333 K overnight. The calcination was carried out at 823 K for 6 h. Reduction using 5% (v/v) H2 in N2 was conducted at 773 K for 4 h with the heating rate of 10 K/min from the room temperature. Siliceous mesoporous (MNi-00) was also synthesized only using TEOS as the inorganic precursor.
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X-ray powder diffraction (XRD) data were obtained on Bruker D8 diffractometer operated at 40 kV, 40 mA, using Cu Ka radiation (l = 0.1542 nm); transmission electron microscopy (TEM) micrographs were obtained on a JEOL 2000 FX transmission electron microscope equipped with energy dispersive X-ray spectroscopy (EDX) and operated at 200 kV. N2 adsorption–desorption isotherms at 77 K were determined in Micromeritics ASAP surface analyzer. Total pore volume was calculated from the adsorption–desorption isotherm curve at a single point p/p8 = 0.969. The pore size distributions were calculated using the BJH method with corrected Kelvin function, called KJS method [21]. Microporosity was examined by as-plot analysis method [22]. Fourier transform infrared spectroscopy (FTIR) was collected on a Mattson 3000 FTIR spectrometer using the mixtures of the finely ground samples with KBr with a ratio of about 1 mg per 100 mg of KBr. Spectra were acquired at 4 cm1 resolution and averaged over 64 scans. X-ray photoelectron spectroscopy (XPS) studies were performed on an Escalab MKII 200R spectrometer. Si 2p band at 103.4 eV was used as the reference. The spectra were smoothed and nonlinear Shirley-type background was subtracted. De-convolutions were carried out using the theoretical XPS bands—20% Lorenz and 80% Gauss, via a least squares algorithm. The XPS peak fitting software XPSPEAK4.1 was used.
3. Results and discussion Ordered mesostructures can be demonstrated by low angle XRD measurements. The results are shown in Fig. 1. Apparently, the XRD patterns for as-synthesized and calcined mesoporous silica with 11.8 wt.% of Ni (designated as MNi-11) give the expected peaks, analogous to MCM-41 [3,4]. Resolved low-angle peaks indicate long range ordering of the mesopores exists. Additionally, an increase in d100 from 3.38 nm for MNi-00 to 3.98 nm for MNi-11 was observed, which is normally attributed to the incorporation of Ni into the silica frameworks [19,23]. Contraction from as-synthesized MNi-11 (d100 = 4.17 nm) to calcined MNi-11 happened after calcination. From TEM micrographs (Fig. 2, left) for the calcined MNi-11, hexagonally ordered mesopores are clearly visible and no bulk NiO particles are found. EDX (Fig. 2, right) on this area showed 11.8 wt.% nickel was incorporated into the mesophases. The variation of EDX results on different areas was little.
Fig. 1. XRD patterns (1) for as-synthesized MNi-11 and (2) for calcined MNi-11.
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Fig. 2. TEM micrograph of calcined MNi-11 (left) and EDX spectra on this area (right).
Further evidence of MCM-41-type mesostructures is given by N2 adsorption–desorption isotherm of MNi-11 shown in Fig. 3, which indicates a steep rise in the inflection region at about p/p8 = 0.3, characteristic of capillary condensation in the pores and also indicative of the existence of mesopores. Inset is the pore size distribution obtained by KJS method. The average pore size was found to be 3.6 nm. The BET surface area reached 868 m2/g and pore volume was up to 0.73 cm3/g. MCM-41-type mesostructures was confirmed although 11.8 wt.% Ni was incorporated into the silica frameworks. As shown in Fig. 4, the linear extrapolation of the adsorption volume at low as values intersecting the yaxis at origin reveals that MNi-11 is free of micropores. The apparent upward deviation from the linearity on the as-plot starts at about 0.6, at which the capillary condensation takes place, indicating the mesopores are dominantly present in MNi-11.
Fig. 3. Nitrogen adsorption–desorption isotherm curves for template-free MNi-11. Inset is the pore size distribution calculated by KJS method.
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Fig. 4. as-Plot analysis for MNi-11.
XPS is a sensitive technique for surface analysis. XPS, here, was used to examine the chemical state of Ni in the mesophases. Surface atom ratio (Ni/Si) for calcined MNi-11 decreased from 0.097 to 0.078. The bulk Ni/Si ratio calculated from the EDX results was found to be 0.14, indicating much of Ni incorporated into the frameworks is beyond this detection. In addition, as shown in Fig. 5, predominant peak centered at 857.8 eV in Ni 2p3/2 can be observed, indicating interactions exist between the nickel and the silica base, or even formation of nickel silicates. A small peak at 855.0 assigned to Ni2+ in NiO was found. However, compared with the dominant peak at 857.8 eV, the formation of pure NiO phase should be scarce. In the FTIR spectra as shown in Fig. 6, a weak peak (due to the low crystallinity) centered at 665 cm1 appeared. This corroborates the presence of ill-crystallized nickel phyllosilicates [24,25]. It was reported that cyclic nickel glycoxide ((CH2O)2Ni) can be generated by dissolution of nickel hydroxide into ethylene glycol at 353 K [26]. After adding the ethyl silicate, the product of ethylene glycol diethyl ether and formation of Ni–O–Si bonds were detected by C13 NMR spectrum and IR,
Fig. 5. XPS spectra for MNi-11. Inset is the de-convoluted Ni 2p3/2 spectrum.
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Fig. 6. FTIR spectra (1) for mesoporous silica and (2) for MNi-11.
respectively [26,27]. So the proposed reaction between the nickel glycoxide and TEOS was as follows [27]: 2ðC2 H5 OÞ4 Si þ ðCH2 OÞ2 Ni ! ðC2 H5 OÞ3 SiONiOSiðC2 H5 0Þ3 þ ðC2 H5 OCH2 Þ2 It is believed that the connection of nickel ions with TEOS makes the co-hydrolysis and co-condensation possibly occur at molecular level so that the homogeneous dispersion of the Ni in the frameworks of silica can be realized. This allows the incorporation of high amount of nickel into the silica frameworks. In order to verify this conclusion, one sample was also synthesized by conventional DS method (MNi-11D). Aqueous solution of nickel nitrate was added into preformed mixture of TEOS, ethylene glycol and CTAB in de-ionized water with the same composition as for MNi-11. The incorporation of nickel species into the mesophases was not as well realized as MNi-11. Only 4.1 wt.% was achieved according to EDX.
Fig. 7. XRD patterns (1) for calcined MNi-11-D synthesized by conventional DS method and (2) for the calcined MNi-11 (*Al sample holder, #bulk NiO reflections and +amorphous silica).
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Fig. 8. TEM micrograph of reduced MNi-11. The bar is 50 nm.
As shown in Fig. 7, significant increase in bulk NiO phase in MNi-11-D (well-resolved peaks at about 37.38, 43.38 and 62.98, characteristic of bulk NiO particles can be observed) was demonstrated by XRD compared with MNi-11. Formation of extra-frameworks bulk metal oxide was also reported upon high loading metal ions using conventional DS method [19]. Therefore, the modification of TEOS with nickel species is a key step to incorporate higher amount of nickel species into the silica frameworks in the modified direct synthesis method. Through this method, mesoporous silica with varying Ni content from 0 to 15 wt.% were synthesized. It is worthy to note that 15 wt.% of Ni led to the formation of disordered mesostructures. After activation at 773 K with H2, the mesostructures are intact as clearly seen in Fig. 8. Small black spots can be seen and very small nickel clusters (1–2 nm) embedded in the silica walls can be identified. The low-angle XRD pattern (not shown here) also shows the integrity of mesostructures, similar to that shown in Fig. 1.
4. Conclusions Mesoporous silica with high nickel content was prepared by modifying TEOS with Ni species followed by templating processes. This modification was proved to play an important role for effective incorporation of high amount of Ni (11.8 wt.%) into the silica frameworks. High surface area and wellordered mesostructures analogous to MCM-41 were confirmed by XRD, TEM and N2 sorption measurements. After activation with H2 at 773 K, small nickel clusters were obtained and embedded in the silica walls.
References [1] [2] [3] [4]
C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980. G.C. Bond, Heterogeneous Catalysis, Oxford University Press, Oxford, 1988. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. J.S. Beck, J.C. VartUli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenkert, J. Am. Chem. Soc. 114 (1992) 10834. [5] M. Hartmann, A. Po˝ppl, L. Kevan, J. Phys. Chem. 100 (1996) 9906.
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W. Wang, M. Song / Materials Research Bulletin 40 (2005) 1737–1744 M. Ziolek, I. Nowak, I. Sobczak, A. Lewandowska, P. Decyk, J. Kujawa, Stud. Surf. Sci. Catal. 129 (2000) 813. M. Ziolek, I. Nowak, I. Sobczak, H. Poltorak, Stud. Surf. Sci. Catal. 130 (2000) 3047. M. Ziolek, I. Nowak, H. Poltorak, A. Lewandowska, I. Sobczak, Stud. Surf. Sci. Catal. 125 (1999) 691. Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, G.L. Haller, Stud. Surf. Sci. Catal. 135 (2001) 233. A. Lewandowska, S. Monteverdi, M. Bettahar, M. Ziolek, J. Mol. Catal. A: Chem. 188 (2002) 85. A. Corma, A. Martı´nez, V. Martı´nez-Soria, J.B. Monto´n, J. Catal. 153 (1995) 25. T. Halachev, R. Nava, L. Dimitrov, Appl. Catal. A: Gen. 169 (1998) 111. Y. Yue, Y. Sun, Z. Gao, Catal. Lett. 47 (1997) 167. E. Rodrı´guez-Castello´n, L. Dı´az, P. Braos-Garcı´a, J. Me´rida-Robles, P. Maireles-Torres, A. Jime´nez-Lo´pez, A. Vaccari, Appl. Catal. A: Gen. 240 (2003) 83. R. Wojcieszak, S. Monteverdi, M. Mercy, I. Nowak, M. Ziolek, M.M. Bettahar, Appl. Catal. A: Gen. 268 (2004) 241. J.-S. Jung, W.-S. Chae, R.A. McIntyre, C.T. Seip, J.B. Wiley, C.J. O’Connor, Mater. Res. Bull. 34 (1999) 1353. D.J. Lensveld, J.G. Mesu, A. Jos van Dillen, K.P. De Jong, Microporous Mesoporous Mater. 44–45 (2001) 401. M. Hartmann, A. Po˝ppl, L. Kevan, J. Phys. Chem. 100 (1996) 9906. V. Parvulescu, B.-L. Su, Catal. Today 69 (2001) 315. U. Junges, S. Disser, G. Schu¨th, Stud. Surf. Sci. Catal. 117 (1998) 391. M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267. M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410. A. Tuel, Microporous Mesoporous Mater. 27 (1999) 151. M. Kermarec, J.Y. Carriat, P. Burattin, M. Che, A. Decarreau, J. Phys. Chem. 98 (1994) 12008. P. Burattin, M. Che, C. Louis, J. Phys. Chem. B 101 (1997) 7060. A. Ueno, H. Suzuki, Y. Kotera, J. Chem. Soc., Faraday Trans. 1 79 (1983) 127. K. Tohji, Y. Udagawa, S. Tanabe, A. Ueno, J. Am. Chem. Soc. 106 (1984) 612.
Preparation of high nickel-containing MCM-41-type mesoporous silica via a modified direct synthesis method Wei Wang, Mo Song * IPTME, Loughborough University, Loughborough LE11 3TU, UK Received 5 January 2005; received in revised form 17 March 2005; accepted 20 May 2005
Abstract A method with modifying tetraethyl orthosilicate (TEOS) with nickel species has been developed for the synthesis of mesoporous silica with high nickel content (11.8 wt.% of Ni or even higher). With the method, MCM41-type materials were obtained with high BET surface area reaching 868 m2/g and pore volume up to 0.73 cm3/g. The materials were characterized by means of X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, N2 adsorption, Fourier transform infrared and X-ray photoelectron spectroscopy. Nickel species were incorporated into the silica frameworks. The mesostructures still remain after activation using H2 at 773 K. # 2005 Elsevier Ltd. All rights reserved. Keywords: B. Chemical synthesis; B. Sol–gel chemistry; C. X-ray diffraction
1. Introduction It is well known that supported nickel catalysts have been widely studied and used due to their very high activity in hydrogenation, hydrotreating and stream-reforming reactions [1,2]. And silica was the most commonly used nickel carrier. Especially, since the discovery of the family of M41S by Mobil scientists in 1992, mesoporous materials have attracted much interest [3,4]. Uniformly hexagonal array of linear channels and mesostructures over long range make them very promising in many areas, such as adsorbents and catalyst supports. Therefore, combining as many catalytically active Ni species as * Corresponding author. E-mail address: [email protected] (M. Song). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.020
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possible with mesoporous silica is of great interest nowadays. Many nickel-containing mesoporous silicas were synthesized and tested in dimerization of ethylene and isomerization of butene [5], desulfurization of thiols and organic sulfides [6,7], deodorization [8] or hydrodechlorination [9], hydrogenation of benzene [10] and so on. The most commonly used method to synthesize nickel-containing mesoporous silica is impregnation one. In this method, mesoporous catalyst supports are impregnated by nickel salt solutions, such as nickel nitrate [7,10–15], nickel bromide [16] and nickel citrate [17]. The disadvantage of this method is that, upon high loadings (higher than 5 wt.% of Ni), it often leads to loss of the BET surface area or even blockage of mesochannels by large Ni (NiO) particles. As reported by Rodrı´guez–Castello´n et al., 48% of BET surface area left after 14.9 wt.% Ni was incorporated and large nickel oxide particles located on the external surface [14]. When 5 wt.% nickel was loaded into MCM-41, only about 73% of total surface area remained [15]. Even only 1.5 wt.% Ni loading into AlMCM-41, about 2 nm NiO particles were formed in the mesochannels [16]. Therefore, an alternative way to introduce nickel species into the mesoporous silica is adding Ni salts, like NiCl2, into the synthesis mixture, followed by the hydrothermal treatment, called direct synthesis method (DS method). However, it is difficult to load high amount of nickel without damaging the mesostructures by using this method [18–20]. For example, with very low nickel loading (<1.5 wt.%), distinct condensation in mesopores was not observed in N2 sorption measurement [20]. So far, to our knowledge, no report has been published on the synthesis of MCM-41-type silica with higher nickel content than 3.6 wt.% using the DS method [19]. Rapid precipitation of nickel ions in basic conditions does not allow full interaction with the template molecules, which results in the low loading into the mesophases and formation of extra-frameworks NiO after calcination. In this communication, we presented a modified sol–gel process for the synthesis of MCM-41-type mesoporous silica with high nickel content. In this method, ‘TEOS-seizing nickel species’ co-hydrolyzes and co-condenses with TEOS during the hydrothermal treatment for building up mesostructures templated by cationic surfactant. This method was proved to be effective to incorporate higher amount of nickel into mesophases.
2. Experimental All the chemicals were bought from Aldrich Chemicals (UK) and used as received. 0.6 g nickel(II) nitrate hexahydrate was dissolved into 3.5 mL ethylene glycol (EG). The solution was heated and stirred at 353 K for 10 h, and then 4.25 mL TEOS was added dropwise. The mixture was annealed at 363 K for 10 h. This clear solution of TEOS modified with nickel species was dripped into the solution of 0.75 g cetyltrimethylammonium bromide (CTAB) in 32 g de-ionized water. The pH value was adjusted to 11.0. Then the mixture was annealed at 298 K overnight with moderate stirring and then kept at 370 K for 120 h statically. After cooling to room temperature, pH value of the mixture was adjusted to 2.0 using diluted HCl (0.01 M) with magnetic stirring. Stirring was continued for 5 min after acidification. The precipitates were then washed by copious distilled water, and air-dried at 333 K overnight. The calcination was carried out at 823 K for 6 h. Reduction using 5% (v/v) H2 in N2 was conducted at 773 K for 4 h with the heating rate of 10 K/min from the room temperature. Siliceous mesoporous (MNi-00) was also synthesized only using TEOS as the inorganic precursor.
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X-ray powder diffraction (XRD) data were obtained on Bruker D8 diffractometer operated at 40 kV, 40 mA, using Cu Ka radiation (l = 0.1542 nm); transmission electron microscopy (TEM) micrographs were obtained on a JEOL 2000 FX transmission electron microscope equipped with energy dispersive X-ray spectroscopy (EDX) and operated at 200 kV. N2 adsorption–desorption isotherms at 77 K were determined in Micromeritics ASAP surface analyzer. Total pore volume was calculated from the adsorption–desorption isotherm curve at a single point p/p8 = 0.969. The pore size distributions were calculated using the BJH method with corrected Kelvin function, called KJS method [21]. Microporosity was examined by as-plot analysis method [22]. Fourier transform infrared spectroscopy (FTIR) was collected on a Mattson 3000 FTIR spectrometer using the mixtures of the finely ground samples with KBr with a ratio of about 1 mg per 100 mg of KBr. Spectra were acquired at 4 cm1 resolution and averaged over 64 scans. X-ray photoelectron spectroscopy (XPS) studies were performed on an Escalab MKII 200R spectrometer. Si 2p band at 103.4 eV was used as the reference. The spectra were smoothed and nonlinear Shirley-type background was subtracted. De-convolutions were carried out using the theoretical XPS bands—20% Lorenz and 80% Gauss, via a least squares algorithm. The XPS peak fitting software XPSPEAK4.1 was used.
3. Results and discussion Ordered mesostructures can be demonstrated by low angle XRD measurements. The results are shown in Fig. 1. Apparently, the XRD patterns for as-synthesized and calcined mesoporous silica with 11.8 wt.% of Ni (designated as MNi-11) give the expected peaks, analogous to MCM-41 [3,4]. Resolved low-angle peaks indicate long range ordering of the mesopores exists. Additionally, an increase in d100 from 3.38 nm for MNi-00 to 3.98 nm for MNi-11 was observed, which is normally attributed to the incorporation of Ni into the silica frameworks [19,23]. Contraction from as-synthesized MNi-11 (d100 = 4.17 nm) to calcined MNi-11 happened after calcination. From TEM micrographs (Fig. 2, left) for the calcined MNi-11, hexagonally ordered mesopores are clearly visible and no bulk NiO particles are found. EDX (Fig. 2, right) on this area showed 11.8 wt.% nickel was incorporated into the mesophases. The variation of EDX results on different areas was little.
Fig. 1. XRD patterns (1) for as-synthesized MNi-11 and (2) for calcined MNi-11.
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Fig. 2. TEM micrograph of calcined MNi-11 (left) and EDX spectra on this area (right).
Further evidence of MCM-41-type mesostructures is given by N2 adsorption–desorption isotherm of MNi-11 shown in Fig. 3, which indicates a steep rise in the inflection region at about p/p8 = 0.3, characteristic of capillary condensation in the pores and also indicative of the existence of mesopores. Inset is the pore size distribution obtained by KJS method. The average pore size was found to be 3.6 nm. The BET surface area reached 868 m2/g and pore volume was up to 0.73 cm3/g. MCM-41-type mesostructures was confirmed although 11.8 wt.% Ni was incorporated into the silica frameworks. As shown in Fig. 4, the linear extrapolation of the adsorption volume at low as values intersecting the yaxis at origin reveals that MNi-11 is free of micropores. The apparent upward deviation from the linearity on the as-plot starts at about 0.6, at which the capillary condensation takes place, indicating the mesopores are dominantly present in MNi-11.
Fig. 3. Nitrogen adsorption–desorption isotherm curves for template-free MNi-11. Inset is the pore size distribution calculated by KJS method.
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Fig. 4. as-Plot analysis for MNi-11.
XPS is a sensitive technique for surface analysis. XPS, here, was used to examine the chemical state of Ni in the mesophases. Surface atom ratio (Ni/Si) for calcined MNi-11 decreased from 0.097 to 0.078. The bulk Ni/Si ratio calculated from the EDX results was found to be 0.14, indicating much of Ni incorporated into the frameworks is beyond this detection. In addition, as shown in Fig. 5, predominant peak centered at 857.8 eV in Ni 2p3/2 can be observed, indicating interactions exist between the nickel and the silica base, or even formation of nickel silicates. A small peak at 855.0 assigned to Ni2+ in NiO was found. However, compared with the dominant peak at 857.8 eV, the formation of pure NiO phase should be scarce. In the FTIR spectra as shown in Fig. 6, a weak peak (due to the low crystallinity) centered at 665 cm1 appeared. This corroborates the presence of ill-crystallized nickel phyllosilicates [24,25]. It was reported that cyclic nickel glycoxide ((CH2O)2Ni) can be generated by dissolution of nickel hydroxide into ethylene glycol at 353 K [26]. After adding the ethyl silicate, the product of ethylene glycol diethyl ether and formation of Ni–O–Si bonds were detected by C13 NMR spectrum and IR,
Fig. 5. XPS spectra for MNi-11. Inset is the de-convoluted Ni 2p3/2 spectrum.
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Fig. 6. FTIR spectra (1) for mesoporous silica and (2) for MNi-11.
respectively [26,27]. So the proposed reaction between the nickel glycoxide and TEOS was as follows [27]: 2ðC2 H5 OÞ4 Si þ ðCH2 OÞ2 Ni ! ðC2 H5 OÞ3 SiONiOSiðC2 H5 0Þ3 þ ðC2 H5 OCH2 Þ2 It is believed that the connection of nickel ions with TEOS makes the co-hydrolysis and co-condensation possibly occur at molecular level so that the homogeneous dispersion of the Ni in the frameworks of silica can be realized. This allows the incorporation of high amount of nickel into the silica frameworks. In order to verify this conclusion, one sample was also synthesized by conventional DS method (MNi-11D). Aqueous solution of nickel nitrate was added into preformed mixture of TEOS, ethylene glycol and CTAB in de-ionized water with the same composition as for MNi-11. The incorporation of nickel species into the mesophases was not as well realized as MNi-11. Only 4.1 wt.% was achieved according to EDX.
Fig. 7. XRD patterns (1) for calcined MNi-11-D synthesized by conventional DS method and (2) for the calcined MNi-11 (*Al sample holder, #bulk NiO reflections and +amorphous silica).
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Fig. 8. TEM micrograph of reduced MNi-11. The bar is 50 nm.
As shown in Fig. 7, significant increase in bulk NiO phase in MNi-11-D (well-resolved peaks at about 37.38, 43.38 and 62.98, characteristic of bulk NiO particles can be observed) was demonstrated by XRD compared with MNi-11. Formation of extra-frameworks bulk metal oxide was also reported upon high loading metal ions using conventional DS method [19]. Therefore, the modification of TEOS with nickel species is a key step to incorporate higher amount of nickel species into the silica frameworks in the modified direct synthesis method. Through this method, mesoporous silica with varying Ni content from 0 to 15 wt.% were synthesized. It is worthy to note that 15 wt.% of Ni led to the formation of disordered mesostructures. After activation at 773 K with H2, the mesostructures are intact as clearly seen in Fig. 8. Small black spots can be seen and very small nickel clusters (1–2 nm) embedded in the silica walls can be identified. The low-angle XRD pattern (not shown here) also shows the integrity of mesostructures, similar to that shown in Fig. 1.
4. Conclusions Mesoporous silica with high nickel content was prepared by modifying TEOS with Ni species followed by templating processes. This modification was proved to play an important role for effective incorporation of high amount of Ni (11.8 wt.%) into the silica frameworks. High surface area and wellordered mesostructures analogous to MCM-41 were confirmed by XRD, TEM and N2 sorption measurements. After activation with H2 at 773 K, small nickel clusters were obtained and embedded in the silica walls.
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