Journal of Colloid and Interface Science 421 (2014) 22–26
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Investigation of the stability of Platinum nanoparticles incorporated in mesoporous silica with different pore sizes Kazuhisa Yano a,b,⇑, Shuyi Zhang c, Xiaoqing Pan c, Narihito Tatsuda a a
Inorganic Materials Lab., Toyota Central R & D Labs. Inc., Nagakute, Aichi 480-1192, Japan Toyota Research Institute North America, 1555 Woodridge Ave, Ann Arbor, MI 48105, USA c Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA b
a r t i c l e
i n f o
Article history: Received 26 November 2013 Accepted 21 January 2014 Available online 27 January 2014 Keywords: Mesoporous silica Platinum nanoparticles Stability Pore size In situ TEM
a b s t r a c t The effect of the pore size of mesoporous silica on the stability of Pt nanoparticles (NPs) has been investigated. TEM observation and XRD measurement were conducted in situ for Pt loaded mesoporous silica with different mesopore sizes. It turns out that smaller pores are more effective to stabilize Pt NPs below 600 °C. However, aggregation of Pt NPs on the surface of particles is not fully suppressed more than 1000 °C in ambient atmosphere even though smaller mesopore size is applied. The type of precursor does not affect the stability of Pt NPs. Ó 2014 Published by Elsevier Inc.
1. Introduction Precious metal nanoparticles (NPs) have been used as catalysts for various reactions due to their high catalytic activity. Among them, Platinum NPs are the most industrialized and used in a broad range of reactions, such as hydrogenation, oxidation, reforming, NOX abatement, and direct decomposition of alcohols for hydrogen production [1–7]. Although Pt NPs are practically realized in various applications, it is highly common that catalytic performance decreases gradually during operation because of the aggregation [8–12]. In case the aggregation is suppressed, the amount of Pt in an automotive catalytic converter for example could be reduced by a large amount. Thermal resistance of NPs is much lower than that of bulk materials [13,14]. The melting point of Pt is 1768 °C. However, the decrease in the catalytic activity occurs when NPs are used even less than 1000 °C. One of the indices of thermal resistance is Tamman temperature (Tm), briefly expressed by Tm = 0.5 Tmelting point [15]. The Tamman temperature of Pt is calculated to be ca. 750 °C. Mesoporous silica has ordered nano-sized pores with very narrow size distribution [16]. The introduction of various types of nanoparticles into pores of mesoporous silica has been reported
⇑ Corresponding author at: Inorganic Materials Lab., Toyota Central R & D Labs. Inc., Nagakute, Aichi 480-1192, Japan. Fax: +81 561 63 6156. E-mail address: [email protected] (K. Yano). http://dx.doi.org/10.1016/j.jcis.2014.01.027 0021-9797/Ó 2014 Published by Elsevier Inc.
by many researchers. High uniformity in size for NPs is expected since pore size of mesoporous silica is uniform. The size of NPs can be tuned depending on the pore size of mesoporous silica. Since NPs are confined in mesopores, it can be possible to suppress aggregation, leading to high thermal stability. The composites of NPs/mesoporous silica are extended in various applications such as catalysis [17–20], drug delivery [21–24], and optics [25,26]. In this report, we have systematically investigated the effect of the pore size of mesoporous silica on the stability of Pt NPs. Monodispersed mesoporous silica spheres (MMSS) are used as a host to support Pt NPs [27–30]. It is very easy to find aggregated NPs outside MMSS because the particles size of MMSS is uniform and in the range of several hundred nanometers. Nanoparticles in the same area are traced by using in situ TEM observation and XRD measurement.
2. Experimental 2.1. Materials Hexadecyltrimethylammonium chloride (C16TMACl) and tetramethylorthosilicate (TMOS) were purchased from Tokyo Kasei Co. (Japan). Methanol, 1 N NaOH, and 2 N HCl were purchased from Wako Pure Chemical Co. (Japan). Tetraammineplatinum (II) nitrate and hydrogen hexachloroplatinate hexahydrate were purchased from Aldrich. All materials were used as received. The synthesis for monodispersed mesoporous silica spheres (MMSS) and
K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
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Fig. 1. TEM images of Pt/MMSS heated in situ at (a) 400, (b) 600, (c) 800, and (d) 1000 °C.
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(b)
(c)
(d)
Fig. 2. TEM images of Pt/HT-MMSS heated in situ at (a) 400, (b) 600, (c) 800, and (d) 1000 °C. Pt NPs (bright spots) become larger with increasing temperature and bigger particles are observed at the surface at 1000 °C (d).
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(a)
(b)
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Fig. 3. TEM images of Pt/HT-MMSS heated in situ at 700 (a), 900 (b), and 1000 °C (c). Pt/HT-MMSS was obtained using H2PtCl6 as a precursor. Pt NPs (bright spots) become larger with increasing temperature. Although the precursor of Pt was different, the behavior is similar when (NH3)4Pt(NO3)2 was used as the precursor (Fig. 2).
(200)
(b) (220)
50
(311)
Intensity / cps
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(111)
Intensity / cps
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Fig. 4. XRD patterns for (a) Pt/MMSS, and (b) Pt/HT-MMSS heated in air at 50, 200, 400, 600, 800, and 1000 °C (from the bottom to the top). The numbers in parentheses represent indices of Pt crystalline phase.
hydrothermally treated monodispersed mesoporous silica spheres (HT-MMSS) was based on the literature [28,30]. In a typical MMSS synthesis, 3.52 g of C16TMACl and 2.28 ml of 1 M NaOH solution were dissolved in 800 g of methanol/water (60/40 = w/w) solution. Then 1.32 g of TMOS was added to the solution, with vigorous stirring at 25 °C. Following the addition of TMOS, the clear solution suddenly turned opaque, resulting in a white precipitate. After 8 h of continuous stirring the mixture was aged overnight. The white powder was filtered and washed with distilled water at least three times, and then dried at 45 °C for 72 h. Then the powder was calcined in air at 550 °C for 6 h. HT-MMSS was obtained as follows. A 1 g of MMSS before calcination was dispersed in a 60 mL of 2 N HCl solution. The mixture was heated at 150 °C for 3 days in an autoclave with Teflon container. After rinsing with distilled water and subsequent filtration, the powder was dried at 45 °C and then calcined in air at 550 °C for 6 h. Impregnation of Platinum was done as follows. A 10 mg of tetraammineplatinum (II) nitrate ((NH3)4Pt(NO3)2) was dissolved in a 10 mL of water. A 50 mg of MMSS (or HT-MMSS) was added to the solution. Then, the water was slowly evaporated out at 333 K.
Table 1 Crystalline size of Pt in the MMSS host after heating. Host
MMSS HT-MMSS
Crystalline size (nm) 600 °C
800 °C
1000 °C
3.9 14.7
4.3, 33.4 24.2
5.3, 35.6 30.8
radiation in the microscope. Image of in situ heating experiment was recorded with JEOL JEM-2010F TEM with a 200 kV beam, operated in scanning TEM (STEM) mode with a high-angle annular dark field (HAADF) detector. An in situ heating stage (Gatan, Pleasanton, CA) with ramping rate about 20 °C/min and limit of 1100 °C was used to heat the MMSS in vacuum. Images of ex situ experiment were recorded on Cs-corrected JEOL-2100 in STEM mode. Nitrogen adsorption isotherm was measured using a Quantachrome Autosorb-1 at 196 °C. The sample was evacuated at 150 °C under 0.13 Pa before the measurement. Pore diameter was calculated by the BJH method for the desorption branch.
2.2. Characterization 3. Results and discussion Powder X-ray diffraction measurement was carried out with a Rigaku Rint-2200 X-ray diffractometer using Cu Ka radiation. Sample heating was carried out with a hot stage equipped with the diffractometer. MMSS was dry deposited on Si3N4 high-temperature transmission electron microscope (TEM) support films (Structure Probe, Inc., West Chester, PA), coated with several nanometer carbon film to avoid charging effect under the electron
3.1. Stability of Pt NPs under vacuum The stability of Pt NPs is highly dependent on the atmosphere. First, the stability under vacuum was investigated by in situ TEM observation. Fig. 1 shows TEM images of Pt/MMSS heated in situ in the TEM chamber. The sample holder was heated gradually.
K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
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25
(b)
100 nm
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100 nm
100 nm
Fig. 5. TEM images of Pt/MMSS (a), (b) and Pt/HT-MMSS (c), (d) heated in air at 600 °C (a), (c) and at 1000 °C (b), (d), respectively. Most of the nanoparticles are observed at the surface when Pt/HT-MMSS was heated at 1000 °C.
Crystalline size / nm
(a)
(b)
(c)
Temperature / ºC Fig. 6. Effect of temperature on Pt crystalline size in Pt/HTMMSS under (a) air, (b) Ar and (c) vacuum. The dashed line represents particle sizes of Pt NPs heated under vacuum. The sizes are estimated from TEM images.
On the contrary, when mesoporous silica with larger pore size was used, the aggregation of Pt NPs was observed. TEM images at different temperatures are shown in Fig. 2. The pore size of the mesoporous silica host, HT-MMSS, was estimated to be 17 nm. Pt NPs are stabilized and no aggregation is observed through 800 °C. However, several particles bigger than 7 nm are observed when Pt/HT-MMSS was heated at 1000 °C (Fig. 2(d)). Those bigger particles seem to be located on the surface of HT-MMSS. It is assumed that Pt NPs migrated to the surface and aggregated upon heating. It is expected that types of precursor affect the stability of NPs. The precursor was changed from tetraammineplatinum (II) nitrate to hydrogen hexachloroplatinate hexahydrate and heated in situ. TEM images are shown in Fig. 3. No significant difference is seen between Figs. 3 and 2. Although the most of the NPs are inside HT-MMSS, a few bigger particles are seen on the surface. These results explain that Pt NPs are highly stabilized by the confinement effect if NPs are introduced in mesopores with the size equal to the particle size. The interaction between Pt and silica might contribute to the confinement effect.
3.2. Stability of Pt NPs under ambient atmosphere The image of the same particle was captured at the temperature of 400, 600, 800, and 1000 °C, respectively. The pore size of MMSS was estimated to be 2.3 nm from the BJH method. Pt NPs of ca. 2 nm in size are dispersed homogeneously in the mesoporous silica host. During the heating process, no change in size as well as in the position is observed, mentioning that Pt NPs are stabilized inside mesopore channels. It should be noted that the shrinkage of MMSS occurred upon heating due to the progress of silanol condensation.
It was reported that the existence of oxygen makes Pt NPs aggregation much easier [31,32]. To understand the effect of atmosphere on the stability of Pt NPs, the samples were heated in air and XRD patterns were recorded. The XRD specimen holder was inserted in a heat stage and patterns were obtained with the same area at different temperatures. Fig. 4 shows XRD patterns for Pt/MMSS (a) and Pt/HT-MMSS (b) heated at 50, 200, 400, 600, 800, and 1000 °C, respectively. Crystalline phase of Platinum was not detected 6400 °C for both Pt/MMSS and Pt/HT-MMSS samples.
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K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
Crystallization of Pt NPs started at 600 °C for both samples. The crystalline sizes of Pt obtained by the Scherrer equation for both samples at different temperatures are summarized in Table 1. The crystalline size of Pt in Pt/MMSS is much smaller than that in Pt/HT-MMSS at 600 °C. Since Pt nanoparticles can migrate easily in HT-MMSS (pore size: 17 nm), aggregation is supposed to occur readily upon heating. On the contrary, Pt NPs are confined in mesopores of MMSS and the migration could be largely suppressed. Crystalline sizes increase with increasing heating temperature for both samples. However, two different sizes of crystalline formed in MMSS. The size of bigger particles is similar to that for Pt NPs in HT-MMSS, indicating aggregation occurred on the surface of particles. The smaller particles are assumed to form inside the particle due to the confinement effect of mesopores. Fig. 5 shows TEM images of Pt/MMSS and Pt/HT-MMSS heated in air at 600 °C and 1000 °C. Pt NPs are confined in radially aligned mesopores in Pt/MMSS after heated at 600 °C. No aggregation is shown in Fig. 5(a), agreeing well with the result from the XRD measurement. Dense center part also suggests that Pt NPs are dispersed well in mesopores of MMSS. After heating at 1000 °C, large aggregated particles as well as small particles are seen (Fig. 5(b)). Since less particles are seen at the center part of MMSS by comparing with Pt/MMSS heated at 600 °C, the most of the Pt NPs were assumed to migrate to the surface of MMSS and then aggregated. Similar image was obtained for Pt/HT-MMSS heated in air at 600 °C (Fig. 5(C)), mentioning that the migration of Pt NPs occurred at lower temperature in HT-MMSS. Large particles are seen on the surface of HT-MMSS after heated at 1000 °C (Fig. 5(d)) and the numbers of Pt NPs confined in mesopores are very small. These results explain well the result by the XRD measurements. To understand the effect of atmospheric condition on the stability of Pt NPs, crystalline sizes of Pt NPs heated under air, Ar, and vacuum are plotted in Fig. 6. To ease the migration of NPs, large pore-sized HT-MMSS was used as a host. The sizes are the biggest when Pt NPs are heated in air. The sizes decrease in a large amount when heated in Ar, indicating that oxygen plays an important role in migration of Pt NPs [31,32]. It is surprising that the particle size unchanged up to 1000 °C under vacuum. Although Tamman temperature of Pt is calculated to be ca. 750 °C, Pt nanoparticles are assumed to be stabilized by the interaction with silica. 4. Conclusion We have investigated the effect of the pore size of mesoporous silica on the stability of Pt NPs by using in situ TEM and XRD measurement. Pt NPs are more stabilized in smaller mesopores under vacuum or in air. Atmospheric condition plays an important role on the stability of Pt NPs. Aggregation occurs very easily in air
regardless of the pore size of mesoporous silica. However, the aggregation is highly suppressed in Ar or vacuum. The type of precursor does not affect the stability of Pt NPs at all. It is important to understand the stability of NPs. The results obtained here could be useful to design Pt NPs loaded mesoporous silica toward various applications. References [1] S.E. Golunski, Platinum Met. Rev. 51 (2007) 162. [2] J.R. Croy, S. Mostafa, H. Heinrich, B. Roldan Cuenya, Catal. Lett. 131 (2009) 21. [3] Y. Wei, Z. Zhao, T. Li, J. Liu, A. Duan, G. Jiang, Appl. Catal. B-Environ. 146 (2014) 57. [4] G.J. Leong, M.C. Schulze, M.B. Strand, D. Maloney, S.L. Frisco, H.N. Dinh, B. Pivovar, R.M. Richards, Appl. Organomet. Chem. 28 (2014) 1. [5] S.O. Blavo, E. Qayyum, L.M. Baldyga, V.A. Castillo, M.D. Sanchez, K. Warrington, M.A. Barakat, J.N. Kuhn, Top. Catal. 56 (2013) 18. [6] D. Varade, H. Abe, Y. Yamauchi, K. Haraguchi, ACS Appl. Mater. Interfaces 5 (2013) 22. [7] C. Chaudhari, S.M.A. Siddiki, K. Shimizu, Tetrahedron Lett. 54 (2013) 6490. [8] E.M. Larsson, J. Millet, S. Gustafsson, M. Skoglundh, V.P. Zhdanov, C. Langhammer, ACS Catal. 2 (2012) 238. [9] S. Porsgaard, L.R. Merte, L.K. Ono, F. Behafarid, J. Matos, S. Helveg, M. Salmeron, Beatriz R. Cuenya, F. Besenbacher, ACS Nano 6 (2012) 10743. [10] S.E.F. Kleijn, B. Serrano-Bou, A.I. Yanson, M.T.M. Koper, Langmuir 29 (2013) 2054. [11] L.R. Baker, G. Kennedy, J.M. Krier, M. Van Spronsen, R.M. Onorato, G.A. Somorjai, Catal. Lett. 142 (2012) 1286. [12] J. Dupont, J.D. Scholten, Chem. Soc. Rev. 39 (2010) 1780. [13] Y.-H. Wena, H. Fang, Z.-Z. Zhu, S.-G. Sun, Chem. Phys. Lett. 471 (2009) 295. [14] M. Rauber, F. Muench, M.E. Toimil-Molares, W. Ensinger, Nanotechnology 23 (2012) 475710. [15] A. Cao, R. Luc, G. Veser, Phys. Chem. Chem. Phys. 12 (2010) 13499. [16] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [17] A.K. Prashar, S. Mayadevi, P.R. Rajamohanan, R.N. Devi, Appl. Catal. A: Gen. 403 (2011) 91. [18] Z. Konya, E. Molnar, G. Tasi, K. Niesz, G.A. Somorjai, I. Kiricsi, Catal. Lett. 113 (2007) 19. [19] M.-Y. Kim, S.M. Park, J.-H. Park, C.-H. Shin, W.-J. Moon, N.-E. Sung, G. Seo, React. Kinet. Mech. Cat. 103 (2011) 463. [20] A. Chen, W. Zhang, X. Li, D. Tan, X. Han, X. Bao, Catal. Lett. 119 (2007) 159. [21] Y. Cui, H. Dong, X. Cai, D. Wang, Y. Li, ACS Appl. Mater. Interfaces 4 (2012) 3177. [22] Z.-Y. Li, Y. Liu, X.-Q. Wang, L.-H. Liu, J.-J. Hu, G.-F. Luo, W.-H. Chen, L. Rong, X.-Z. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 7995. [23] P. Yang, P. Yang, X. Teng, J. Linb, L. Huang, J. Mater. Chem. 21 (2011) 5505. [24] G.F. Andrade, D.C.F. Soares, R.G. dos Santos, E.M.B. Sousa, Microporous Mesoporous Mater. 168 (2013) 102. [25] T. Nakamura, Y. Yamada, H. Yamada, K. Yano, J. Mater. Chem. 19 (2009) 6699. [26] E. Bovero, K. Yano, T. Nakamura, Y. Yamada, F.C.J.M. van Veggel, ChemPhysChem 11 (2010) 2550. [27] K. Yano, F. Fukushima, J. Mater. Chem. 13 (2003) 2577. [28] K. Yano, F. Fukushima, J. Mater. Chem. 14 (2004) 1579. [29] T. Nakamura, M. Mizutani, H. Nozaki, N. Suzuki, K. Yano, J. Phys. Chem. C 111 (2007) 1093. [30] K. Yano, T. Nishi, Microporous Mesoporous Mater. 158 (2012) 257. [31] P.J.F. Harris, J. Catal. 97 (1986) 527. [32] R.M.J. Fiedorow, B.S. Chahar, S.E. Wanke, J. Catal. 51 (1978) 193.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Investigation of the stability of Platinum nanoparticles incorporated in mesoporous silica with different pore sizes Kazuhisa Yano a,b,⇑, Shuyi Zhang c, Xiaoqing Pan c, Narihito Tatsuda a a
Inorganic Materials Lab., Toyota Central R & D Labs. Inc., Nagakute, Aichi 480-1192, Japan Toyota Research Institute North America, 1555 Woodridge Ave, Ann Arbor, MI 48105, USA c Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA b
a r t i c l e
i n f o
Article history: Received 26 November 2013 Accepted 21 January 2014 Available online 27 January 2014 Keywords: Mesoporous silica Platinum nanoparticles Stability Pore size In situ TEM
a b s t r a c t The effect of the pore size of mesoporous silica on the stability of Pt nanoparticles (NPs) has been investigated. TEM observation and XRD measurement were conducted in situ for Pt loaded mesoporous silica with different mesopore sizes. It turns out that smaller pores are more effective to stabilize Pt NPs below 600 °C. However, aggregation of Pt NPs on the surface of particles is not fully suppressed more than 1000 °C in ambient atmosphere even though smaller mesopore size is applied. The type of precursor does not affect the stability of Pt NPs. Ó 2014 Published by Elsevier Inc.
1. Introduction Precious metal nanoparticles (NPs) have been used as catalysts for various reactions due to their high catalytic activity. Among them, Platinum NPs are the most industrialized and used in a broad range of reactions, such as hydrogenation, oxidation, reforming, NOX abatement, and direct decomposition of alcohols for hydrogen production [1–7]. Although Pt NPs are practically realized in various applications, it is highly common that catalytic performance decreases gradually during operation because of the aggregation [8–12]. In case the aggregation is suppressed, the amount of Pt in an automotive catalytic converter for example could be reduced by a large amount. Thermal resistance of NPs is much lower than that of bulk materials [13,14]. The melting point of Pt is 1768 °C. However, the decrease in the catalytic activity occurs when NPs are used even less than 1000 °C. One of the indices of thermal resistance is Tamman temperature (Tm), briefly expressed by Tm = 0.5 Tmelting point [15]. The Tamman temperature of Pt is calculated to be ca. 750 °C. Mesoporous silica has ordered nano-sized pores with very narrow size distribution [16]. The introduction of various types of nanoparticles into pores of mesoporous silica has been reported
⇑ Corresponding author at: Inorganic Materials Lab., Toyota Central R & D Labs. Inc., Nagakute, Aichi 480-1192, Japan. Fax: +81 561 63 6156. E-mail address: [email protected] (K. Yano). http://dx.doi.org/10.1016/j.jcis.2014.01.027 0021-9797/Ó 2014 Published by Elsevier Inc.
by many researchers. High uniformity in size for NPs is expected since pore size of mesoporous silica is uniform. The size of NPs can be tuned depending on the pore size of mesoporous silica. Since NPs are confined in mesopores, it can be possible to suppress aggregation, leading to high thermal stability. The composites of NPs/mesoporous silica are extended in various applications such as catalysis [17–20], drug delivery [21–24], and optics [25,26]. In this report, we have systematically investigated the effect of the pore size of mesoporous silica on the stability of Pt NPs. Monodispersed mesoporous silica spheres (MMSS) are used as a host to support Pt NPs [27–30]. It is very easy to find aggregated NPs outside MMSS because the particles size of MMSS is uniform and in the range of several hundred nanometers. Nanoparticles in the same area are traced by using in situ TEM observation and XRD measurement.
2. Experimental 2.1. Materials Hexadecyltrimethylammonium chloride (C16TMACl) and tetramethylorthosilicate (TMOS) were purchased from Tokyo Kasei Co. (Japan). Methanol, 1 N NaOH, and 2 N HCl were purchased from Wako Pure Chemical Co. (Japan). Tetraammineplatinum (II) nitrate and hydrogen hexachloroplatinate hexahydrate were purchased from Aldrich. All materials were used as received. The synthesis for monodispersed mesoporous silica spheres (MMSS) and
K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
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Fig. 1. TEM images of Pt/MMSS heated in situ at (a) 400, (b) 600, (c) 800, and (d) 1000 °C.
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(b)
(c)
(d)
Fig. 2. TEM images of Pt/HT-MMSS heated in situ at (a) 400, (b) 600, (c) 800, and (d) 1000 °C. Pt NPs (bright spots) become larger with increasing temperature and bigger particles are observed at the surface at 1000 °C (d).
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K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
(a)
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Fig. 3. TEM images of Pt/HT-MMSS heated in situ at 700 (a), 900 (b), and 1000 °C (c). Pt/HT-MMSS was obtained using H2PtCl6 as a precursor. Pt NPs (bright spots) become larger with increasing temperature. Although the precursor of Pt was different, the behavior is similar when (NH3)4Pt(NO3)2 was used as the precursor (Fig. 2).
(200)
(b) (220)
50
(311)
Intensity / cps
50
(111)
Intensity / cps
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Fig. 4. XRD patterns for (a) Pt/MMSS, and (b) Pt/HT-MMSS heated in air at 50, 200, 400, 600, 800, and 1000 °C (from the bottom to the top). The numbers in parentheses represent indices of Pt crystalline phase.
hydrothermally treated monodispersed mesoporous silica spheres (HT-MMSS) was based on the literature [28,30]. In a typical MMSS synthesis, 3.52 g of C16TMACl and 2.28 ml of 1 M NaOH solution were dissolved in 800 g of methanol/water (60/40 = w/w) solution. Then 1.32 g of TMOS was added to the solution, with vigorous stirring at 25 °C. Following the addition of TMOS, the clear solution suddenly turned opaque, resulting in a white precipitate. After 8 h of continuous stirring the mixture was aged overnight. The white powder was filtered and washed with distilled water at least three times, and then dried at 45 °C for 72 h. Then the powder was calcined in air at 550 °C for 6 h. HT-MMSS was obtained as follows. A 1 g of MMSS before calcination was dispersed in a 60 mL of 2 N HCl solution. The mixture was heated at 150 °C for 3 days in an autoclave with Teflon container. After rinsing with distilled water and subsequent filtration, the powder was dried at 45 °C and then calcined in air at 550 °C for 6 h. Impregnation of Platinum was done as follows. A 10 mg of tetraammineplatinum (II) nitrate ((NH3)4Pt(NO3)2) was dissolved in a 10 mL of water. A 50 mg of MMSS (or HT-MMSS) was added to the solution. Then, the water was slowly evaporated out at 333 K.
Table 1 Crystalline size of Pt in the MMSS host after heating. Host
MMSS HT-MMSS
Crystalline size (nm) 600 °C
800 °C
1000 °C
3.9 14.7
4.3, 33.4 24.2
5.3, 35.6 30.8
radiation in the microscope. Image of in situ heating experiment was recorded with JEOL JEM-2010F TEM with a 200 kV beam, operated in scanning TEM (STEM) mode with a high-angle annular dark field (HAADF) detector. An in situ heating stage (Gatan, Pleasanton, CA) with ramping rate about 20 °C/min and limit of 1100 °C was used to heat the MMSS in vacuum. Images of ex situ experiment were recorded on Cs-corrected JEOL-2100 in STEM mode. Nitrogen adsorption isotherm was measured using a Quantachrome Autosorb-1 at 196 °C. The sample was evacuated at 150 °C under 0.13 Pa before the measurement. Pore diameter was calculated by the BJH method for the desorption branch.
2.2. Characterization 3. Results and discussion Powder X-ray diffraction measurement was carried out with a Rigaku Rint-2200 X-ray diffractometer using Cu Ka radiation. Sample heating was carried out with a hot stage equipped with the diffractometer. MMSS was dry deposited on Si3N4 high-temperature transmission electron microscope (TEM) support films (Structure Probe, Inc., West Chester, PA), coated with several nanometer carbon film to avoid charging effect under the electron
3.1. Stability of Pt NPs under vacuum The stability of Pt NPs is highly dependent on the atmosphere. First, the stability under vacuum was investigated by in situ TEM observation. Fig. 1 shows TEM images of Pt/MMSS heated in situ in the TEM chamber. The sample holder was heated gradually.
K. Yano et al. / Journal of Colloid and Interface Science 421 (2014) 22–26
(a)
25
(b)
100 nm
(c)
(d)
100 nm
100 nm
Fig. 5. TEM images of Pt/MMSS (a), (b) and Pt/HT-MMSS (c), (d) heated in air at 600 °C (a), (c) and at 1000 °C (b), (d), respectively. Most of the nanoparticles are observed at the surface when Pt/HT-MMSS was heated at 1000 °C.
Crystalline size / nm
(a)
(b)
(c)
Temperature / ºC Fig. 6. Effect of temperature on Pt crystalline size in Pt/HTMMSS under (a) air, (b) Ar and (c) vacuum. The dashed line represents particle sizes of Pt NPs heated under vacuum. The sizes are estimated from TEM images.
On the contrary, when mesoporous silica with larger pore size was used, the aggregation of Pt NPs was observed. TEM images at different temperatures are shown in Fig. 2. The pore size of the mesoporous silica host, HT-MMSS, was estimated to be 17 nm. Pt NPs are stabilized and no aggregation is observed through 800 °C. However, several particles bigger than 7 nm are observed when Pt/HT-MMSS was heated at 1000 °C (Fig. 2(d)). Those bigger particles seem to be located on the surface of HT-MMSS. It is assumed that Pt NPs migrated to the surface and aggregated upon heating. It is expected that types of precursor affect the stability of NPs. The precursor was changed from tetraammineplatinum (II) nitrate to hydrogen hexachloroplatinate hexahydrate and heated in situ. TEM images are shown in Fig. 3. No significant difference is seen between Figs. 3 and 2. Although the most of the NPs are inside HT-MMSS, a few bigger particles are seen on the surface. These results explain that Pt NPs are highly stabilized by the confinement effect if NPs are introduced in mesopores with the size equal to the particle size. The interaction between Pt and silica might contribute to the confinement effect.
3.2. Stability of Pt NPs under ambient atmosphere The image of the same particle was captured at the temperature of 400, 600, 800, and 1000 °C, respectively. The pore size of MMSS was estimated to be 2.3 nm from the BJH method. Pt NPs of ca. 2 nm in size are dispersed homogeneously in the mesoporous silica host. During the heating process, no change in size as well as in the position is observed, mentioning that Pt NPs are stabilized inside mesopore channels. It should be noted that the shrinkage of MMSS occurred upon heating due to the progress of silanol condensation.
It was reported that the existence of oxygen makes Pt NPs aggregation much easier [31,32]. To understand the effect of atmosphere on the stability of Pt NPs, the samples were heated in air and XRD patterns were recorded. The XRD specimen holder was inserted in a heat stage and patterns were obtained with the same area at different temperatures. Fig. 4 shows XRD patterns for Pt/MMSS (a) and Pt/HT-MMSS (b) heated at 50, 200, 400, 600, 800, and 1000 °C, respectively. Crystalline phase of Platinum was not detected 6400 °C for both Pt/MMSS and Pt/HT-MMSS samples.
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Crystallization of Pt NPs started at 600 °C for both samples. The crystalline sizes of Pt obtained by the Scherrer equation for both samples at different temperatures are summarized in Table 1. The crystalline size of Pt in Pt/MMSS is much smaller than that in Pt/HT-MMSS at 600 °C. Since Pt nanoparticles can migrate easily in HT-MMSS (pore size: 17 nm), aggregation is supposed to occur readily upon heating. On the contrary, Pt NPs are confined in mesopores of MMSS and the migration could be largely suppressed. Crystalline sizes increase with increasing heating temperature for both samples. However, two different sizes of crystalline formed in MMSS. The size of bigger particles is similar to that for Pt NPs in HT-MMSS, indicating aggregation occurred on the surface of particles. The smaller particles are assumed to form inside the particle due to the confinement effect of mesopores. Fig. 5 shows TEM images of Pt/MMSS and Pt/HT-MMSS heated in air at 600 °C and 1000 °C. Pt NPs are confined in radially aligned mesopores in Pt/MMSS after heated at 600 °C. No aggregation is shown in Fig. 5(a), agreeing well with the result from the XRD measurement. Dense center part also suggests that Pt NPs are dispersed well in mesopores of MMSS. After heating at 1000 °C, large aggregated particles as well as small particles are seen (Fig. 5(b)). Since less particles are seen at the center part of MMSS by comparing with Pt/MMSS heated at 600 °C, the most of the Pt NPs were assumed to migrate to the surface of MMSS and then aggregated. Similar image was obtained for Pt/HT-MMSS heated in air at 600 °C (Fig. 5(C)), mentioning that the migration of Pt NPs occurred at lower temperature in HT-MMSS. Large particles are seen on the surface of HT-MMSS after heated at 1000 °C (Fig. 5(d)) and the numbers of Pt NPs confined in mesopores are very small. These results explain well the result by the XRD measurements. To understand the effect of atmospheric condition on the stability of Pt NPs, crystalline sizes of Pt NPs heated under air, Ar, and vacuum are plotted in Fig. 6. To ease the migration of NPs, large pore-sized HT-MMSS was used as a host. The sizes are the biggest when Pt NPs are heated in air. The sizes decrease in a large amount when heated in Ar, indicating that oxygen plays an important role in migration of Pt NPs [31,32]. It is surprising that the particle size unchanged up to 1000 °C under vacuum. Although Tamman temperature of Pt is calculated to be ca. 750 °C, Pt nanoparticles are assumed to be stabilized by the interaction with silica. 4. Conclusion We have investigated the effect of the pore size of mesoporous silica on the stability of Pt NPs by using in situ TEM and XRD measurement. Pt NPs are more stabilized in smaller mesopores under vacuum or in air. Atmospheric condition plays an important role on the stability of Pt NPs. Aggregation occurs very easily in air
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