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Langmuir 2009, 25, 1718-1724
Effect of Confinement on the Fluid Properties of Ammonia in Mesopores of MCM-41 and SBA-15 Shigeharu Kittaka,* Mayura Morimura, Shinji Ishimaru, Akiko Morino, and Kyoko Ueda Department of Chemistry, Faculty of Science, Okayama UniVersity of Science, 1-1 Ridaicho, Okayama 700-0005, Japan ReceiVed September 15, 2008. ReVised Manuscript ReceiVed NoVember 13, 2008 The effect of pore size on capillary condensation and solid-liquid phase changes of ammonia in MCM-41 and SBA-15 was studied by adsorption and FTIR measurements of condensed phases at low temperatures. Adsorption isotherms are all typical type IV on the fully hydroxylated surfaces, without hysteresis loops in the smaller pores (d < 2.4 nm). In the larger pores, hysteresis loops appear at lower temperatures and disappear with increasing temperature, i.e., the capillary critical phenomenon was detected (hysteresis critical point). The criticality of the adsorption hysteresis loop is very similar to that for nonpolar nitrogen in mesopores of various shapes, suggesting that this is a universal phenomenon among fluids in mesopores. Freezing and melting of capillary-condensed ammonia were observed by FTIR spectroscopy. The melting temperature of capillary-condensed ammonia decreased with decreasing pore size, which is similar in the behavior of freezing. In the smaller pores (d < 2.4 nm); however, ammonia was not frozen. It is suggested that the capillary-condensed inner part, i.e., inside the ammonia monolayer, is affected too much by the pore wall and/or is too small in volume to crystallize. In the larger pores of SBA-15, crystallization is remarkably segregated from ammonia molecules strongly coordinated to surface hydroxyls.
Introduction Analysis of capillary condensation of fluids in porous materials has long been one of the most important subjects in adsorption science. Questions remain about the mechanisms of capillary condensation and evaporation that are accompanied by the appearance of a hysteresis loop. It has long been understood that capillary condensation with a hysteresis loop does not take place at temperatures above a critical point (Tcc: capillary critical point). This is the traditional view of the disappearance of hysteresis loops in adsorption isotherms.1-6 However, in 1995, Ravikovitch et al. reported that the temperature at which the hysteresis disappears is not the point where capillary condensation vanishes.7 However, it was not recognized experimentally until 1998. Morishige et al. demonstrated that there is a hysteresis critical point, Tch, rather than a capillary critical point.8 They measured adsorption-desorption isotherms of nitrogen in MCM-41 and SBA-15,9 cylindrical mesopores, and MCM-48 and SBA-16,10 which have well-defined three-dimensional networks of cylindrical and cage-like pores. They also recognized true capillary critical points, Tcc, above which first-order phase change (capillary condensation) does not occur.7 By this classification, many historically reported adsorption-desorption isotherms of nonpolar gases in porous materials with less well-defined pore structures, e.g., Vycor, CPG, fumed silica etc., can be reasonably explained, * Corresponding author. (1) Burgess, C. G.; Everett, D. H.; Nuttall, S. Pure Appl. Chem. 1989, 61, 1845. (2) Evans, R.; Marconi, U. M. M. B.; Tarazona, P. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1763. (3) Thommes, M.; Findenegg, G. H. Langmuir 1994, 10, 4270. (4) Morishige, K.; Fujii, H.; Uga, M.; Kinukawa, D. Langmuir 1997, 13, 3494. (5) Denoyel, R.; Pellenq, R. J. M.; Beurroies, I. Proceedings of the 7th Fundamentals of Adsorption; I. K. International, Ltd.: Nagasaki, Japan, 2001, p 967. (6) Machin, W. D. Phys. Chem. Chem. Phys. 2003, 5, 203. (7) Ravikovitch, P. I.; O’Domhnaill, S.; Neimark, C. A.; Schuth, V. F.; Unger, K. K. Langmuir 1995, 11, 4765. (8) Morishige, K.; Shikimi, M. J. Chem. Phys. 1998, 108, 7821. (9) Morishige, K.; Ito, M. J. Chem. Phys. 2002, 117, 8036. (10) Morishige, K.; Tateishi, N.; Fukuma, S. J. Phys. Chem. B 2003, 107, 5177.
although sometimes data reported were not precise enough to be assigned to the classifications because of irregular pore shapes, wide pore size distributions, narrow temperature range of measurements etc. The development of well-defined porous materials, MCM-41 and MCM-4811 and SBA-15 and SBA-1612 has enabled marked progress in this area, mainly with nonpolar molecules. As long as the present authors are aware, however, there has been no study of polar molecules involving hydrogen bonding from this point of view, i.e., no detailed analysis of adsorption isotherms in the wide temperature ranges have been done. The present authors have so far been studying the nature of fluids in confinement, e.g., water13-17 and molecules having hydrogen bonding such as methanol,18,19 acetonitrile etc.,20 and have found that confinement has important effects on their properties, structures, dynamics, freezing and melting points, phase transition points, and phase separation of binary mixtures.21 However, because of high reactivity these molecules with siliceous mesoporous materials at temperatures higher than atmospheric conditions, reliable answers for these problems have not been (11) Beck, J.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W. D.; Olson, H.; Sheppard, E. E.; Mc Cullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (12) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (13) Takahara, S.; Nakano, M.; Kittaka, S.; Kuroda, Y.; Mori, T.; Hamano, H.; Yamaguchi, T. J. Phys. Chem. 1999, 103, 5814. (14) Smirnov, P.; Yamaguchi, T.; Kittaka, S.; Takahara, S.; Kuroda, Y. J. Phys. Chem. B 2000, 104, 5498. (15) Takahara, S.; Kittaka, S.; Mori, T.; Kuroda, Y.; Yamaguchi, T.; BellissentFunel, M.-C. Adsorption 2005, 11, 479. (16) Takahara, S.; Sumiyama, N.; Kittaka, S.; Yamaguchi, T.; BellissentFunel, M.-C. J. Phys. Chem. B 2005, 109, 11231. (17) Kittaka, S.; Ishimaru, S.; Kuranishi, M.; Matsuda, T.; Yamaguchi, T. Phys. Chem. Chem. Phys. 2006, 8, 3223. (18) Kittaka, S.; Serizawa, A.; Iwashita, T.; Takahara, S.; Takenaka, T.; Kuroda, Y.; Mori, T. Surf. Sci. Cat. 2001, 132, 653. (19) Takamuku, T.; Maruyama, H.; Kittaka, S.; Takahara, S.; Yamaguchi, T. J. Phys. Chem. B 2005, 109, 892. (20) Kittaka, S.; Iwashita, T.; Serizawa, A.; Kuranishi, M.; Takahara, S.; Kuroda, Y.; Mori, T.; Yamaguchi, T. J. Phys. Chem. B 2005, 109, 23162. (21) Kittaka, S.; Kuranishi, M.; Ishimaru, S.; Umahara, O. J. Chem. Phys. 2007, 126, 091103.
10.1021/la803019h CCC: $40.75 2009 American Chemical Society Published on Web 12/29/2008
Fluid Properties of Ammonia
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Table 1. Characteristic Parameters of the Pore Properties of MCM-41 and SBA-15 sample MCM-41 C10 MCM-41 C12 MCM-41 C14 MCM-41 C16 MCM-41 C18 MCM-41 C22 SBA-15 (80) SBA-15 (140)
pore size (diameter)/nm specific surface area/m2 g-1 2.10 2.38 2.83 3.11 3.60 4.19 7.10 10.4
786 1035 740 870 860 447 677 357
obtained. We found that ammonia presents some clues to the above problems without chemically destroying the matrices of MCM-41 and SBA-15. Ammonia is one of the most common polar molecules displaying hydrogen bonding, because of which it has been utilized as a solvent for various chemical reactions involving both organic and inorganic substances at low temperatures. The reactivity with metals is milder than water but presents electronic character is produced in the liquid by contacting with alkaline metals.22-24 A condensation-evaporation process involving changing the pressure of ammonia has long been used for refrigeration.25 This work is concerned with the criticality of adsorptiondesorption phenomena, and phase changes of ammonia condensed in cylindrical mesopores of MCM-41 and SBA-15. The techniques utilized are adsorption and FTIR measurements over wide temperature ranges.
Experimental Section Samples. MCM-41 and SBA-15 samples with various pore sizes were prepared as previously.17,26 Their pore sizes and surface areas are listed in Table 1. MCM-41 samples were named after the carbon number of the longest alkyl group in the precursor surfactants, e. g., C16 for C16H33(CH3)3N+Br-. SBA-15 samples were prepared by hydrothermal treatment of reagent mixtures at 80 and 140 °C (383. and 413 K respectively) following the method used before.17 These samples were distinguished according to the hydrothermal treatment temperatures, e.g., SBA-15 (140) for the sample treated at 140 °C. Pore parameters for all samples used are presented in Table 1. Adsorption Isotherms of Ammonia. In order to obtain the insight of the phase properties of confined ammonia, adsorption isotherms were measured at temperatures between 193.2 and 263.2 K. All samples were first saturated with adsorbed water vapor, up to the saturated vapor pressure, for standardizing the condition of the surface (extent of hydration of the surface). After evacuation of the sample, ammonia gas was adsorbed on the surface at 298.2 K, followed by evacuation. The pretreated samples were then cooled down to 193.2 K using a methanol bath, whose temperature was regulated by a laboratory-made thermostat using an EYLA cool ECS80 cooling unit and a heating system. Adsorption and desorption measurements were conducted gravimetrically using a Rubotherm balance (BEL Japan, applicable up to 3 bar) equipped with a gas dosing system operated by a PC loaded with laboratory-made control software and Baratron 390HA and 690A capacitance manometers. Instrument limitations on ammonia pressure measurement and cooling unit temperatures restricted measurement temperatures to the range from 193.2 to 263.2 K. FTIR Measurements. Solid-liquid phase transition of confined ammonia was studied by measuring their vibration spectra at varying (22) Wasse, J. C.; Hayama, S.; Skipper, N. J. Chem. Phys. 2000, 112, 7147. (23) Hayama, S.; Skipper, N. T.; Wasse, J. C.; Thompson, H. J. Chem. Phys. 2002, 116, 2991. (24) Wasse, J. C.; Hayama, S.; Masmanidis, S.; Stebbings, S. L.; Skipper, N. J. Chem. Phys. 2003, 118, 7486. (25) Young, K. News Sci. Space 2007, January, 30. (26) Mori, T.; Kuroda, Y.; Yoshikawa, Y.; Nagao, M.; Kittaka, S. Langmuir 2002, 18, 1595.
Figure 1. Adsorption and desorption isotherms of ammonia for (a) MCM41 C14 expressed as a function of P; (b) ln P.
temperatures, because conventional DSC measurement for the system above ambient pressure was not available and this technique had been successfully utilized in the previous work for water in confinement.17 For FTIR measurements, powder samples weighing 2-3 mg were formed into 12 mm diameter thin discs under vacuum for transmission-mode measurements. The pellet was mounted on a copper sample holder in a cryostat manufactured by JEOL that permitted cooling of the sample down to 93 K using liquid nitrogen. The surface properties of the mounted sample were standardized as in the adsorption measurements, i.e., hydroxylated and contacted with ammonia at room temperature. Sequential changes in the spectra due to adsorption and desorption processes were studied at both 298 and 233 K. The phase changes of confined ammonia with temperature were studied as follows. The pretreated samples were cooled to 233.2 K under vacuum, and ammonia was adsorbed to the point of capillary condensation. After filling pores with ammonia, nitrogen gas was introduced into the chamber to keep the ammonia in the pores, as was done for water condensed in mesopores.17 Measurements were conducted during both decreasing and increasing temperatures. The temperature was regulated at each point within (2 K. A JEOL JIR-100 spectrometer was used. For each spectrum, 100 scans were collected at a resolution of 4 cm-1. As a reference, FTIR spectra of bulk ammonia were measured at increasing temperatures after condensing ammonia from the gas phase onto a KCl plate at 213 K.
Results and Discussion Adsorption of Ammonia in Mesopores of MCM-41 and SBA-15. Figure 1a shows an examples of adsorption-desorption isotherms of ammonia for the MCM-41 (C14) determined at temperatures from 193 to 263 K: those plotted as a function of ln P are shown in Figure 1b. The adsorption isotherms are type IV, presenting a steep increase at the lower pressures, slow increase, and steep capillary condensation at higher pressures,
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Figure 2. FTIR spectra of ammonia adsorbed into MCM-41 C14 at 233 K determined at increasing vapor pressures, which are sequentially read in the figures (kPa), 0, 6.92, 13.3, 21.3, 22.61, 23.9, 25.4, 26.6, 31.4, 40.2. A spectrum for the sample evacuated after adsorption of ammonia is also shown and indicates the complete recovery of the surface.
followed by slow adsorption up to condensation. The first increase suggests the occurrence of a strong interaction between ammonia molecules and the solid surface. Figures for other samples’ (MCM15 (C10)-SBA-15 (140)) adsorption isotherms are shown in the Suppporting Information, the shape of which changes dependent upon the pore size. Adsorption was reversible all through the pressure range for smaller pores (C10 and C12), above which hysteresis appeared and became marked with pore size increases (refer to the Supporting Information figures). Figure 2 shows FTIR spectra of ammonia adsorbed on the C14 sample at 233 K as a function of increasing vapor pressures. On the surface of a fresh sample, one can clearly see a sharp band at 3741 cm-1 due to isolated surface hydroxyls and a broadband at ∼3524 cm-1 due to surface hydroxyls interacting with each other through hydrogen bonds. By dosing ammonia at 6.92 kPa, these bands disappeared and new peaks appeared, due to perturbed surface hydroxyls, and various forms of N-H stretching vibrations of ammonia. We can define four different states of ammonia adsorption: (1) less than one monolayer, (2) between monolayer completion and capillary condensation, (3) capillary condensation, and (4) condensation on the outer surface of the particle after capillary condensation. In state (1), adsorption is too fast to control in the temperature range 193-263 K. We could observe a gradual increase in adsorption and spectral changes at room temperature with increasing vapor pressure (not shown), as has been reported by Pichat et al. on amorphous silica27 and by Gianotti et al. on MCM-4128 to occur at room temperature. The 3385 cm-1 band is ascribable to the degenerate stretching N-H vibration of ammonia (ν3).29 A small 3311 cm-1 band is a characteristic N-H stretching vibration caused by interaction between adsorbed ammonia molecules and surface hydroxyls.30 (27) Pichat, P.; Mathieu, M.-V.; Imelik, B. J. Chim. Phys. Phys.-Chim. Biol. 1965, 66, 845. (28) Gianotti, E.; Dellarocca, V.; Marchese, L.; Martra, G.; Coluccia, S.; Maschmeyer, T. Phys. Chem. Chem. Phys. 2002, 4, 6109. (29) Matsumoto, Y.; Honma, K. Symp. Mol. Sci. 2007, Sept., 3B12, Sendai. (30) Corset, J.; Guillermet, J.; Lascombe, J. Bull. Soc. Chim. 1966, 1231.
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Small bands at 3221 and 3246 cm-1 are due to symmetric stretching (ν1) and overtone bending mode vibrations of ammonia (2ν4), respectively.29 Chemical interactions of ammonia with surface hydroxyls were studied at 233 K by complete exchange reactions between ammonia protons and deuterated surface hydroxyls. It was confirmed that ammonia molecules interacting with deuterated surface hydroxyls form ammonium ions and rotate, followed by departure from the surface hydroxyls as deuterated ammonia molecules leaving behind surface hydroxyls (not shown). This fact indicates that monolayer ammonia is not rigidly anchored to the surface hydroxyls but is quite dynamic and actually has some residence time to react with surface hydroxyls. In the second stage of adsorption up to 23.9 kPa (region 2), one can clearly recognize that all the bands except the 3311 cm-1 stretching band increase and become sharper with increasing vapor pressure: the ν1 (3221 cm-1) and 2ν4 (3246 cm-1) bands of ammonia become more distinct compared with the liquid ammonia bands that are shown later. The 3311 cm-1 band disappears at 25.4 kPa, at which capillary condensation (region 3) commences. This fact suggests that the ammonia molecules coordinated with surface hydroxyls are surrounded by additional adsorbed molecules and become spectroscopically indistinguishable from the latter. Above this vapor pressure, the spectral intensity is somewhat increased but still small, indicating that little adsorption occurs after capillary condensation (region 4). In all of the systems studied, the spectral changes with vapor pressure shown in the FTIR spectra of capillary-condensed ammonia at 233 K are common. Comparison of the adsorption spectra with the spectrum of liquid ammonia at 213 K verifies that the capillary-condensed ammonia is a liquid. A broadband at ∼2800 cm-1 is assigned to the stretching band of surface hydroxyls interacting with ammonia. Determination of Hysteresis Critical Point of Ammonia from Adsorption-Desorption Isotherms. The adsorptiondesorption isotherms of ammonia for MCM-41 C10 and C12 are reversible and do not present a hysteresis loop, while, as shown in Figure 1b, adsorption into C14 does present a hysteresis loop at lower temperatures and becomes reversible above 223 K. With increasing pore size, the hysteresis becomes more marked and the temperature at which reversible adsorption occurs becomes higher. With the largest pores in the SBA-15 (140) sample, hysteresis is marked even at 263 K. In Figure 3, the logarithm of the ratio of middle point pressures between capillary condensation and the saturated vapor pressure, multiplied by temperature, are plotted as a function of temperature. The plots for the adsorption direction give a linear relationship throughout the temperature range while those for the desorption branch cross the former. This is very similar to the results observed by Morishige et al. on various systems.9,10 The crossing-point temperature is denoted as the hysteresis critical temperature, Tch. After the crossing point, both plots run on the same line. This fact signifies that the desorption mechanism above Tch is the reverse of adsorption. In other words, the capillary-condensed phase in this range is in equilibrium with vapor. Below Tch, however, desorption occurs through a mechanism different from that for adsorption. This mechanism is still controversial,31-34 with proposed mechanisms including (1) a receding hemispherical meniscus, and (2) cavitations of gases in the condensed phase. The first mechanism should lead to desorption forming a cylindrical adsorbed layer during the evaporation branch of the (31) Sonwane, C. G.; Bhatia, S. K. Langmuir 1999, 15, 5347. (32) Coasne, B.; Gubbins, K. E.; Pellenq, R. J. M. Adsorption 2005, 11, 289. (33) Puibasset, J. J. Chem. Phys. 2008, 129, 024705. (34) Morishige, K.; Ishino, M. Langmuir 2007, 23, 11021.
Fluid Properties of Ammonia
Figure 3. Relations between T ln(Pm/P0) and T for MCM-41 and SBA15, where Pm and P0 are middle point pressures of adsorption and desorption isotherms in the capillary condensation region of adsorption, and saturated vapor pressure at T.
Figure 4. Relation between Pch/P0 and Tch/Tc, where Pch is a hysteresis critical pressure and P0 a saturated vapor pressure at the hysteresis critical temperature Tch. Tc is the critical temperature of ammonia. Open circle: present. Triangles are referred from the literature,30 which include the nitrogen adsorption in various porous silicas, MCM-41, MCM-48, SBA15, SBA-16, KIT-5, and KIT-6.
hysteresis, accompanied by a concave hemispherical surface having the same vapor pressure as the former. The second mechanism indicates that fluid is drained by evaporation from the inside of the liquid in the pores, which has been used to reasonably explain desorption from a system with cage-type pores, i.e., the opening of the pore structure is smaller than the radius of the interior cages. Morishige et al. claimed that similar desorption processes occur between cylindrical, interconnecting and independent cylindrical porous materials.34 Neimark et al. proposed that the desorption branch is for the equilibrium condition.7 In order to gain insight, characteristics of the hysteresis critical point were investigated by plotting the relative pressure against the reduced hysteresis critical temperature (Figure 4). Here, similar
Langmuir, Vol. 25, No. 3, 2009 1721
Figure 5. The relation between (Tch - Tc)/Tc and the inverse of d/R, where d is the diameter of fluid molecule. Here, this was estimated by fitting of melting points to Gibbs-Thomson relation together with R which is shown in Figure 9. d is assumed to be equal to t (interfacial ammonia thickness). Least square fitting of the plots with linear relation presents the slope 1.10.
plots for nitrogen adsorption into various types of silica mesopores, cylindrical, interconnected cylindrical, and cage-like, were plotted.34 It is to note to find that plots for both ammonia and nitrogen are allocated on the common line. This indicates that appearance and extinction derive from the intrinsic fluidindependent properties of the materials and pore shapes and, furthermore, mechanisms for both are reversible. If the phenomena are independent of the nature of the solid, one can say that the hysteresis criticality is controlled only by the curvature of the fluid surface, which is determined by the pore size. In order to define these mechanisms, more examples are necessary. Above the capillary critical point Tcc, the adsorption isotherm should increase slowly around the pressures corresponding to capillary condensation. Experimentally, in the present work, no system displayed such change in the adsorption isotherm at temperatures above the hysteresis critical point Tch, even in samples C10 and C12. Evans et al. derived the theoretical relation between Tcc and pore size in equation 1.2
(Tc - Tcc) d ) Tc R
(1)
This equation should work for the case of d/R < 1, where d is the molecular diameter. In Figure 5, this relation was tested by using Tch instead of Tcc. Apparently, the experimental data obey this relation, giving a slope close to 1 but not passing through the origin. There have been several reports in which similar plots were done and extrapolated to origin. Simple leastsquares analysis of the plots, however, makes us believe that they can be extrapolated to the ordinate above the origin as found here. This fact suggests that we are plotting exactly hysteresis critical points rather than the capillary critical points Tcc which is higher than Tch. Puibaset claimed that Tch is not related with pore size inverse through straight line.33 The temperature and pressure range to be tested for the Tcc is beyond the ability of the present experimental apparatus. Freezing and Melting of Ammonia Confined within Mesoporous Silica. Radhakrishnan et al. predicted reduction of the melting point of ammonia confined within MCM-41 by chemical
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Figure 6. Temperature changes of FTIR spectra of ammonia frozen on the KCl plate determined at increasing temperatures across the melting point 195.3 K. Inset is the peak edges of degenerate stretching vibration band ν3 at ∼3375 cm-1, which were determined by defining the inflection point (wavenumber) for both sides of the peak: H, higher wavenumber edge; L, lower wavenumber edge.
simulation studies.35,36 However, there has been no experimental work on this system. Here, the phase properties of ammonia condensed in mesopores were studied by FTIR measurements. The melting point of ammonia has been reported to be 195.3 K, which was reconfirmed by FTIR spectral change in ammonia condensed on a KCl plate in Figure 6. The sharp band ν3 (3377 cm-1) changed to a less sharp one when the system was warmed from freezing temperature. A broad 3282 cm-1 band for the solid phase ammonia (P213)37,38 was displaced to 2ν4 (3253 cm-1) and the ν1 band (3215 cm-1) became clearer between 193 and 203 K, corresponding to the melting of frozen ammonia.30 The present authors employed this change to characterize capillarycondensed ammonia within mesopores. Apparently, changes in the bands other than ν3 are marked with temperature change, but it is more profitable to trace the phase change of ammonia in the mesopores using the change in the broadening of the ν3 band shown in the experimental data. Note that, unlike water, no hysteresis has been observed in the freezing and melting of bulk ammonia. In the inset of Figure 6 the shifts in the ν3 band peak position and band edges (H: higher wavenumber edge; L: lower wavenumber edge) are shown with temperature change: band edges were defined as inflection points in the bands. The band edge H clearly displaces at the temperature corresponding to the melting point of the solid ammonia phase. This also occurred with the peak position, but was less clear in the confined system. Thus, we have traced the band edge H. Parts a-c of Figure 7 show typical examples of FTIR spectral changes upon cooling and warming of capillary-condensed frozen ammonia in MCM-41 (C10, C18) and SBA-15 (140); spectra for (35) Radhakrishnan, R.; Gubbins, K. E.; Sliwinska-Bartkowiak, M. J. Chem. Phys. 2000, 112, 11048. (36) Alba-Simionesco, C.; Coasne, B.; Dosseh, G.; Dudziak, G.; Gubbins, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. J. Phys., Condens. Matter 2006, 18, 15–68. (37) Von Dreele, R. B.; Hanson, R. C. Acta Crystallogr. 1984, C40, 1635. (38) Reding, F. P.; Hornig, D. F. J. Chem. Phys. 1951, 19, 594.
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the cooling direction were not shown since they are very similar to the increasing direction. Note that the spectral changes for ammonia in C10 were gradual with temperature change. The degenerate stretching band was broadened by warming. The symmetric stretching band ν1 (3213 cm-1) remained at lower temperatures, suggesting that the structure of liquid ammonia condensed in mesopores was maintained. When the pore size was increased, spectral changes became discontinuous at definite temperatures for both cooling (not shown) and warming procedures. Referring to the spectral change of bulk ammonia in Figure 6, the changes are related to the melting of confined ammonia. However, when confined, definition of melting points is not easy. Peak shift is not large, nor is the change in bandwidth. The edge of the ν3 band (3375 cm-1) band on the higher wavenumber side (H), however, changed rather clearly with temperature change and can be reasonably utilized to trace the phase changes. Spectra of the SBA-15 (140) system show different features at low temperatures from the samples with smaller pores. The ν3 band acquired a new shoulder on the higher wavenumber side with decreasing temperature. In addition, a small peak at 3311 cm-1 appeared and increased in intensity. The appearance of these additional peaks indicates that the ammonia molecules coordinated to hydroxyls on the silica surface are separate from those in the inner part of the pores (see FTIR spectra at low pressure regions in Figure 2). In other words, the freezing of ammonia in the inner part of the pores proceeds free from the effects of pore walls and/or interfacial ammonia. Similar behavior was found with the SBA-15 (80) sample, but less sharp than the former. In Figure 8, the inflection point shifts of this side of the bands with temperature are shown for the systems with larger pores. With smaller pore samples, the changes are gradual and do not clearly identify the phase-change temperature. With increasing pore size, stepwise change appears and its position displaces to higher temperatures. In the case of SBA-15 (140), as predicted above, the inflection point changes differently from those for smaller pores and decreases with increasing temperature, followed by flattening. That is, interfacial ammonia segregated from the freezing of inner ammonia represents the band at higher wavenumbers. Figure 9 shows the phase-change temperatures where discontinuous change appeared (shown by arrows in Figure 8) as a function of the inverse of the pore size, 1/R. Here, pore size was defined by subtracting the interfacial ammonia thickness so that a linear relationship was maintained between the two. As predicted in the chemical simulation studies by Radhakrishnan et al., phase-change temperature decreases as the pore size decreases.31,32 Apparently, the classical Gibbs-Thomson relation [eq 2] describes this relation.
T0 - TR ) ∆T ) -
2V∆γII-IT0 1 j II-I R ∆H
(2)
Here TR is the melting point, ∆γII-I is the interfacial free energy change from solid I to liquid II, and ∆HII-I is the enthalpy change for melting. However, as reported previously, eq 2 should be applied in a limited range of temperature changes (up to 10%). Thus, we should understand that eq 2 presents a rough measure of the pore size effect on the melting of frozen ammonia. Effect of Confinement on the Ammonia Cluster Sizes. When comparing the FTIR spectral changes of bulk ammonia and those in confinement (Figures 6 and 7, parts a and b), one finds noncorresponding changes with phase changes for the latter. The ∼3210 cm-1 symmetric N-H stretching band (ν1) for the ammonia in the pores (C18 and SBA-15 (140)) that is frozen at
Fluid Properties of Ammonia
Langmuir, Vol. 25, No. 3, 2009 1723
Figure 7. Effect of temperature changes (warming) on the FTIR spectra of ammonia capillary condensed and frozen in (a) MCM-41 (C10), (b) MCM-41 (C18), and (c) SBA-15 (140) where each spectrum was subtracted by that of gas phase ammonia measured at 233 K. Temperatures were changed stepwise by 10 K and some of them are shown. Because of very strong absorption coefficient of gas phase ammonia, thorough reduction was not possible in some parts.
low temperatures does not change markedly its intensity, although crystallization of bulk ammonia leads to a marked decrease or even disappearance below the melting point. According to Matsumoto and Honma,29 who made IR measurements of ammonia clusters produced in a He-ammonia (10%) jet using cavity ring-down spectroscopy, the intensity of this band decreases with increasing particle size, while the ν3 band is mainly structure dependent. Matsumoto and Honma performed experiments on the effect of the number of component ammonia molecules, in the range of 1000-10000 molecules, in the 3-5 nm cluster size range, on the spectra. The pore size range is exactly the range studied here. This signifies that ammonia is frozen at low temperatures but its crystallite is inhibited to grow, at most, to the pore size.
Finally, considering the fact that capillary-condensed ammonia in C10 and C12 does not freeze. The idea is given that the cluster size of ammonia has not grown enough to reach the size critical size of crystalline ammonia. The 3211 cm-1 bands (ν1) are similar in shape and size at high temperatures for all systems but are distinguished in the samples C10 and C12 at low temperatures (Figure 10), i.e., cluster sizes are smaller than those in larger pores.
Conclusions Interactions of ammonia molecules with hydroxylated MCM41 and SBA-15 pore surfaces were examined and were confirmed to be similar to those observed with silica gel surfaces. Ammonia is adsorbed sharply at low pressures and presents a typical type
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Figure 8. Temperature shift of FTIR spectral edge H of degenerate ν3 band of ammonia confined in mesopores of MCM-41 with different pore sizes, which have been determined in the warming direction of the system after cooling.
Figure 9. Melting points of ammonia confined in MCM-41 as a function of the inverse of effective pore, where effective pore size R ) r - t was determined by adjusting the interfacial molecular thickness t so that linear relation is held; r is an intrinsic pore radius.
IV isotherm, which is ascribable to the acidic character of silica surface hydroxyls. Ammonia adsorbed on the surface hydroxyls presents a characteristic N-H stretching band (3311 cm-1) together with absorption bands typical of ammonia: 3385 (ν3), 3246 (2ν4), and 3221 cm-1 (ν1). With increasing adsorption over a monolayer, the 3319 cm-1 band decreases in intensity because of additional coordinations with multilayer molecules.
Kittaka et al.
Figure 10. Effect of pore size on the FTIR spectra of ammonia condensed in the MCM-41 determined at 93 K.
Hysteresis critical points of ammonia were determined in the pores (d ) 2.8-10.4 nm) of MCM-41 and SBA-5 using adsorption measurements in the temperature range 193-263 K. Plots of relative hysteresis critical pressure (Pch/P0) against reduced critical temperature are very similar to the relation for nonpolar gas (nitrogen) in various siliceous mesopores. The capillary condensation phenomenon is dependent on neither the fluid nor the pore structure. The effect of confinement on freezing and melting of confined ammonia was studied by FTIR measurements. Melting point decreases linearly with the inverse of pore size; apparently, the Gibbs-Thomson relation is observed. The symmetric stretching band (ν1) clearly remains in the FTIR spectra of frozen ammonia in mesopores, signifying the limited crystal growth in confinement. With the larger pores of SBA-15, on the other hand, a frozen phase is segregated from monolayer ammonia because of the predominated crystal growth. Acknowledgment. This work was partly supported by Grants in Aid for Science Research No. 17550223622, 17550023, and 15076211 from the Ministry of Education Science and Culture of Japan and a Special Grant for Cooperative Research administered by the Japan Private School Promotion Foundation. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. LA803019H
