Article pubs.acs.org/JPCC
Low-Pressure Hysteresis in Adsorption: An Artifact? Ana M. Silvestre-Albero, Juan Manuel Juárez-Galán, Joaquín Silvestre-Albero,* and Francisco Rodríguez-Reinoso Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica-Instituto Universitario de Materiales, Universidad de Alicante, Ap.99, E-03080 Alicante, Spain ABSTRACT: Hysteresis phenomena are usually observed in the characterization of porous solids using gas adsorption of polar and nonpolar probe molecules. Commonly, hysteresis phenomena take place at high relative pressures due to the presence of metastable states associated with the capillary condensation of the probe molecule on mesopores. However, low-pressure hysteresis phenomena have also been reported for porous solids such as ordered mesoporous silicas, zeolites, and activated carbons. Unfortunately, the reason behind these processes taking place at low pressure is still unclear. Here we prove that the low-pressure hysteresis is rather an artifact associated with the lack of equilibrium in the adsorption isotherm and/or the lack of proper outgassing, mainly found in porous solids where narrow pore constrictions are expected.
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INTRODUCTION Hysteresis phenomena are usually associated with adsorption− desorption processes on porous solids.1 The presence of reproducible hysteresis loops implies well-defined metastable states either in the adsorption or the desorption branch of the isotherm. In any case, a common feature of the hysteresis loop is the presence of a limiting relative pressure where the desorption branch coincides with the adsorption branch, i.e., the presence of a closing point. This closure point occurs at a relative pressure that is almost independent of the nature of the porous adsorbent but highly dependent on the nature of the adsorptive, e.g., for nitrogen at its boiling point, the closure point is at p/p0 ∼ 0.42.2 However, sometimes the desorption branch may persist down to very low pressures giving rise to the well-known, low-pressure hysteresis phenomenon (LPH). The LPH loop is defined by several parameters such as the porosity of the sorbent, the maximum pressure reached in the adsorption analysis, the adsorption temperature, and the adsorptive molecular size. LPH has been widely observed in porous solids, e.g., silicabased micro- and mesoporous materials, when using polar and nonpolar molecules as adsorptives.3−6 This phenomenon has been explained as being due to the presence of different adsorption sites (different strength) on the silica-based material, associated phase transitions (e.g., zeolite MFI), and/or pressure assisted swelling phenomena. Similar LPH processes have been described in the literature for other porous solids such as activated carbons.7−9 When the adsorptive used is a hydrocarbon, the explanation can be based on the presence of adsorption processes in pores of about the same width as the adsorptive molecule and/or the swelling of nonrigid carbon walls.7 However, no clear explanation can be found when the LPH is described for the adsorption of nitrogen at 77 K on activated carbons (it is noteworthy to © 2012 American Chemical Society
mention that in recent literature the presence of LPH is sometimes not even mentioned).8,9 In order to further investigate the nature of this phenomenon and to try to understand why it is more and more frequently found in the literature for microporous carbons without the appropriate explanation, this paper presents a detailed analysis of nitrogen adsorption−desorption isotherms on activated carbons measured under different experimental conditions (equilibration time and outgassing conditions) in order to clarify the real reason behind the hysteresis loop at low relative pressures. The analysis will be extended to other porous materials (e.g., MCM-41, SBA-15, and silicalite) for the sake of comparison.
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EXPERIMENTAL SECTION A series of porous solids (activated carbons, mesoporous ordered silicas (SBA-15 and MCM-41), and silicalite zeolite) were characterized by gas adsorption at cryogenic temperatures. N2 adsorption/desorption isotherms were performed at 77 K in a homemade fully automated manometric equipment designed and constructed by the Advanced Materials group (LMA), now commercialized as N2Gsorb-6.10 The time of equilibration (the time elapsed between consecutive pressure readings) was modified between 10 s and 300 s, depending on the sample, in order to analyze its effect in the adsorption isotherm. It is noteworthy to comment that the equipment considers that equilibrium is reached when several consecutive pressure readings agree within 1.3 × 10−4 bar. Before any experiment, samples (∼100 mg) were degassed (10−7 bar) at 523 K for 4 h, Received: June 1, 2012 Revised: July 12, 2012 Published: July 13, 2012 16652
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explanations consider the desorption branch as responsible for the observed irreversible uptake. A third hypothesis not explored in the literature concerns the lack of equilibrium in the adsorption branch, i.e., adsorption takes place in narrow micropores where nitrogen accessibility could be restricted at the low temperature of the adsorption measurement. Under these circumstances, nitrogen adsorption at 77 K lacks equilibrium in the adsorption branch, whereas the desorption branch would be under perfect equilibrium conditions, i.e., LPH would not be a real phenomenon but an artifact due to the lack of equilibrium along the adsorption path. To analyze this hypothesis, nitrogen adsorption isotherms in these carbon materials have been determined at increasing equilibration times from 80 s to 300 s. As it can be observed in Figure 1, where both extreme experimental conditions are used, there is an increase in the amount of nitrogen adsorbed at 300 s, thus confirming that the isotherm for 80 s was not obtained under true equilibrium conditions. Furthermore, the high-pressure hysteresis remains constant on both samples, although now the desorption branch perfectly fits the adsorption branch at a relative pressure around p/p0 ∼ 0.45−0.50, i.e., the LPH has completely vanished. In order to extend this analysis to other porous solids, nitrogen adsorption−desorption isotherms on ordered mesoporous silicas (SBA-15 and MCM-41) have been performed under different equilibration times (10−100 s). Figure 2 shows the adsorption isotherms at 77 K for sample SBA-15.
for activated carbons, and 473 K for 12 h, for ordered mesoporous silicas and silicalite zeolite. Briefly, micro/mesoporous activated carbons were prepared from olive stones by physical activation with CO2 and catalyzed by CaCl2.11 Ordered mesoporous silica materials, MCM-41 and SBA-15, were prepared using the recipes described by Zhao12 and Mihai.13 Finally, silicalite-1 zeolite was commercially available from Süd-Chemie AG (Bruckmühl, Germany).
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RESULTS AND DISCUSSION Effect of Equilibration Time. In a first attempt to clarify the real origin of the LPH, the time elapsed between consecutive pressure readings in a common nitrogen adsorption isotherm, i.e., the equilibration time, was modified between 80 s and 300 s for two different micro/mesoporous activated carbons (samples LMA-232 and LMA-233). Figure 1 shows the adsorption−desorption isotherms at 77 K.
Figure 1. N2 adsorption−desorption isotherms at 77 K on activated carbons LMA-232 and LMA-233 measured using different equilibration times (80 s and 300 s).
As can be observed, both samples exhibit a combination of type I and type IV isotherms, according to the IUPAC classification, in close agreement with a micro/mesoporous network. Furthermore, both isotherms exhibit a type H4 hysteresis loop at high relative pressures attributed to capillary condensation of nitrogen in mesocavities present in the synthesized carbons. Interestingly, the adsorption isotherms performed at somewhat low equilibration time (80 s) does not exhibit a closure point for the hysteresis loop even at a relative pressure below the expected value of 0.42 for nitrogen, thus showing the presence of LPH. As described above, the explanation for this phenomenon is very scarce in the literature when using N2 adsorption at 77 K on carbon materials. Traditionally, these observations have been attributed to a real physical phenomenon, i.e., irreversible uptake, due to the presence of pores of the same width of the adsorbate molecule and/or to swelling of the nonrigid porous structure of the activated carbon.8 In any case, both
Figure 2. N2 adsorption−desorption isotherms at 77 K on ordered mesoporous silica SBA-15 using different equilibration times (10 s and 60 s).
As can be observed, there is a high nitrogen uptake at low relative pressure (p/p0 < 0.1), typical of microporous solids, together with the characteristic capillary condensation in the mesopores at p/p0 ∼ 0.68 and the subsequent type H1 hysteresis loop. Apparently, both nitrogen isotherms fit over the whole pressure range, independently of the equilibration time used (either 10 s or 60 s). However, a close inspection of the low pressure region denotes some deviation of the desorption branch compared to the adsorption one. In the analysis measured using a low equilibration time, i.e., 10 s, the high 16653
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pressure loop remains open down to very low pressure (p/p0 < 0.1); this LPH vanishes when the isotherm is performed using an equilibration time of 60 s or longer. Interestingly, a completely different scenario is observed for sample MCM-41 either at low (10 s) or large (100 s) equilibration times (see Figure 3). In both cases, nitrogen
Figure 4. N2 adsorption−desorption isotherms at 77 K on zeolite silicalite-1 measured using different equilibration times (10 s and 60 s).
(low and high pressure) show that for low equilibration times (below 100 s), the adsorption and the desorption branches are not able to match each other, giving rise to an artificial mediumpressure hysteresis. Larger equilibration times (above 100 s) prove that the low and high pressure hystereses are real phenomena on silicalite-1, whereas the artificial mediumpressure hysteresis between these two regions disappears, i.e., it is attributed to the lack of equilibrium in the adsorption branch. Effect of Outgassing Conditions. Another critical parameter in the characterization of the porous structure using gas adsorption is the temperature of the thermal treatment applied under vacuum before the adsorption analysis. The temperature and the final pressure achieved are critical parameters to ensure the cleanness of the porous structure, i.e., the absence of retained moisture and/or volatile species partially blocking the porosity, and to obtain a real image of the porous structure of the synthesized materials. Figure 5 shows the nitrogen adsorption−desorption isotherms for an activated carbon after two different outgassing temperatures, 423 and 523 K, for 4 h. As expected, the amount of nitrogen adsorbed over the whole relative pressure range increases with the temperature of the outgassing treatment, thus suggesting that 423 K for 4 h is not enough to clean the initial carbon material. The larger effect in the low pressure region (above p/p0 ∼ 0.01, both isotherms are parallel) clearly suggests that moisture and/or volatile species are partially blocking the narrow microporosity. In any case, both nitrogen isotherms exhibit a type H4 hysteresis loop, which is mainly unchanged with the outgassing temperature. Interestingly, an amplification of the low pressure region shows that the hysteresis loop does not exhibit a closing pressure, i.e., it remains open, when using a weaker outgassing treatment. Again, this apparent LPH disappears after an increase in the temperature of the thermal treatment applied. Most probably, chemical species (moisture, tars, volatile molecules, and so on) remaining in the inner porous structure after a weak outgassing treatment, partially block narrow micropores and, conse-
Figure 3. N2 adsorption−desorption isotherms at 77 K on ordered mesoporous silica MCM-41 measured using different equilibration times (10 s and 100 s).
adsorption−desorption isotherms perfectly mach each other, low and high pressure hysteresis loops being absent. Previous studies described in the literature have shown that the MCM41 used in this work is exclusively mesoporous, whereas SBA15 exhibits both narrow micropores (below 0.56 nm) and mesopores.14 The presence of low pressure hysteresis in sample SBA-15 and its absence in sample MCM-41 is in close agreement with the aforementioned observations, thus suggesting that the LPH loop observed on sample SBA-15 when using a short equilibration time is merely an artifact due to the lack of equilibrium in the adsorption branch, i.e., nitrogen accessibility to the narrow micropores is kinetically restricted at the low temperature of the measurement, these kinetic restrictions being absent in a nonmicroporous solid such as MCM-41. Finally, these analyses have been extended to a traditional zeolite such as silicalite-1, which exhibits two well-defined hysteresis loops at low and high pressure in the nitrogen adsorption isotherms at 77 K. Figure 4 shows the nitrogen isotherms for sample silicalite-1 using different equilibration times. As it can be observed, both nitrogen isotherms exhibit the LPH loop at p/p0 ∼ 0.25 and the high-pressure hysteresis loop above p/p0 ∼ 0.5. Previous studies described in the literature have attributed these hysteresis phenomena on nitrogen adsorption to specific adsorbate−adsorbent interactions, phase transitions, and/or pressure induced swelling processes.6−9 In order to ascertain the real effect of the equilibration time in the final adsorption isotherm, sample silicalite-1 has been analyzed using two different equilibration times: 10 s and 100 s (see Figure 4). Although both measurements are coincident over the whole pressure range, an amplification of the region connecting both hysteresis loops 16654
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(2) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603−619. (3) Llewellyn, P. L.; Grillet, Y.; Patarin, J.; Faust, A. C. Microporous Mater. 1993, 1, 247−256. (4) Kyriakou, G.; Theocharis, C. R. Stud. Surf. Sci. Catal. 2002, 144, 709−716. (5) Chu, Y.-H.; Yamagishi, M.; Wang, Z.-M.; Kanoh, H.; Hirotsu, T. J. Colloid Interface Sci. 2007, 312, 186−192. (6) Aguilar-Armenta, G.; Díaz-Jiménez, L. Colloids Surf., A: Phys. Eng. Aspects 2001, 176, 245−252. (7) Rodríguez-Reinoso, F.; Martín-Martínez, J. M.; Linares-Solano, A.; Torregrosa, R. Stud. Surf. Sci. Catal. 1991, 62, 419−427. (8) Puziy, A. M.; Poddubnaya, O. I.; Martínez-Alonso, A.; SuárezGarcía, F.; Tascón, J. M. D. Carbon 2002, 40, 1507−1519. (9) Tseng, R.-L.; Tseng, S.-K. J. Colloid Interface Sci. 2005, 287, 428− 437. (10) Gas to Materials Technologies; www.g2mtech.com (accessed July 22, 2012). (11) Juárez-Galán, J. M.; Silvestre-Albero, A.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. Microporous Mesoporous Mater. 2009, 117, 519−521. (12) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548−552. (13) Mihai, G. D.; Meynen, V.; Beyers, E.; Mertens, M.; Bilba, N.; Cool, P.; Vasanth, E. F. J. Porous Mater. 2009, 16, 109−118. (14) Silvestre-Albero, A.; Jardim, E. O.; Bruijn, E.; Meynen, V.; Cool, P.; Sepúlveda-Escribano, A.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. Langmuir 2009, 25, 939−943.
Figure 5. N2 adsorption−desorption isotherms at 77 K on an activated carbon measured using different outgassing conditions (423 and 523 K for 4 h).
quently, give rise to kinetic restrictions for nitrogen adsorption at cryogenic temperatures.
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CONCLUSIONS In summary, nitrogen adsorption−desorption isotherms on porous solids performed under different experimental conditions show that both the equilibration time and/or the outgassing conditions are critical parameters to take into account in the understanding of the textural and structural properties. A deficient degassing treatment and/or a low equilibration time, i.e., lack of real equilibrium, can give rise to unreal hysteresis phenomena at low−medium relative pressures. Consequently, care must be taken when performing the adsorption isotherms in porous solids to ensure the cleanness of the sample and to avoid the lack of equilibrium. Either an increase in the equilibration time or an increase in the temperature of the thermal treatment is mandatory to ensure a true isotherm and/or a true hysteresis phenomenon. On the other hand, the presence of an LPH loop should be taken as a hint of the probable nonreal adsorption isotherm and the corresponding mistake in the characterization of the porous solid.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors acknowledge financial support from the EU (Contract FRESP ECGA No. 218138), MICINN (project PLE20090052) and Generalitat Valenciana (PROMETEO/2009/002).
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REFERENCES
(1) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powder and Porous Solids; Academic Press: New York, 1999; pp 204−212. 16655
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