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Langmuir 2003, 19, 218-219
Encapsulation of He and Ne in Carbon Molecular Sieves Y. Finkelstein, A. Saig, A. Danon, and J. E. Koresh* Chemistry Division, Nuclear Research Center of the Negev (NRCN), Beer-Sheva 84190, Israel Received October 10, 2002. In Final Form: November 24, 2002 The adsorption-desorption pattern of He and Ne from amorphous carbon molecular sieve fibers (CMSF) was found to be governed by an encapsulation mechanism. He and Ne undergo reversible and efficient entrapment by the micropores of CMSF. Selective adjustment of pore openings to meet the critical dimensions of He and Ne which allow their admission is achieved via two principal steps: (i) the well-known pore widening by means of an irreversible removal of surface oxide groups upon evacuation at elevated temperatures and (ii) regulation of the effective pore opening via reversible thermal dilation/contraction. The occurrence of a markedly high activation energy, which previously could not be solely attributed to pure adsorption, is now understood as that needed for overcoming the energetic barrier imposed by the thermally constricted pores. The present results imply that the encapsulation phenomenon can considerably affect dead volume measurements in porous materials.
I. Introduction The phenomenon of encapsulation of pressurized noble gases is well established in solids of regular cage structure such as that occurring in zeolites.1,2 The occurrence of such a mechanism in amorphous solids, however, is not known. In previous publications,3,4 it was shown that carbon molecular sieve fibers (CMSF) exhibit a highly selective atomic sieving between He and Ne. In that study, markedly large effective activation energies for desorption were deduced, whose origin could not be attributed solely to the occurrence of strong adsorption interactions. An alternative plausible mechanism was suggested,3 in which CMSF constrictions undergo dilations/contractions, eventually leading to an enhanced traplike confinement effect on the sorbed species. Indications for the occurrence of dilations in amorphous carbon were first proposed by Koresh,5,6 and those were attributed to the flexibility of the carbon skeleton. In the present study, it is shown that reversible thermal dilations/ contractions of the CMSF pore opening lead to He and Ne encapsulation at low temperature and ambient pressure. II. Experimental Section Temperature-Programmed Desorption (TPD)-Mass Spectrometry (MS)-Supersonic Molecular Beam (SMB). A detailed description of the experimental setup is given in ref 7. A single as-received CMSF sample (∼100 mg fibrous cloth TCM-128, Carbonne-Lorraine Ltd., France) was used for all sets of Ne and He measurements. Initially, the as-received CMSF was loaded into a stainless steel U-shaped 1/4 in. tube fitted with a heating element and a thermocouple in direct contact with the CMSF sample, by means of a feedthrough connection. For each gas, sorption procedures were carried out on the sample preevacuated at various temperatures, T0 ) 100, 200, 300, and 400 °C. For each T0 and gas type, two sorption procedures were (1) Barrer, R. M.; Vaughan, D. E. W. Trans. Faraday Soc. 1971, 67, 2129. Breck, D. W. In Zeolite Molecular Sieves; Wiley: New York, 1974. Fraenkel, D. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2029. Jameson, C. J.; Jameson, A. K.; Gerald, R., II; de Dios, A. C. J. Chem. Phys. 1992, 96, 1676. (2) Saig, A.; Danon, A.; Finkelstein, Y.; Koresh, J. E. J. Chem. Phys., accepted for publication. (3) Danon, A.; Finkelstein, Y.; Koresh, J. E. Langmuir 2002, 18, 638. (4) Danon, A.; Koresh, J. E. Langmuir 2001, 17, 2739. (5) Koresh, J. E.; Kim, T. H.; Walker, D. R. B.; Koros, W. J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1557. (6) Koresh, J. E. J. Chem. Soc., Faraday Trans. 1993, 89, 935. (7) Danon, A.; Avraham, I.; Koresh, J. E. Rev. Sci. Instrum. 1997, 68, 4359.
performed by introducing the gas (∼1 bar, 50 sccm ultrahigh purity (UHP) He) to the pre-evacuated sample (i) after stabilizing its temperature to that of liquid nitrogen (LN2) and (ii) during sample cooling from room temperature (RT) down to LN2. To set off uncertainties due to possible kinetic effects, both sorption procedures were taken along comparable time scales. Sorption periods along the RT to LN2 temperature range lasted for about several minutes. For sorption procedures taken at LN2, periods of up to 3 h were used. A total of four measurements were performed for each T0 (two sorption procedures for each of the two gases used). All sorption procedures were followed by removal of residual atmosphere by evacuation at LN2 until the background signal (m/z.).4 in the case of He sorption) practically vanished. In all cases, care was taken to evacuate for periods exceeding by far those of sorption (ca. 20 min and 3.5 h for the [RT, LN2] and LN2 sorption temperatures, respectively). On completing evacuation, a Ne carrier gas flow (UHP, ∼1 bar, 20 sccm) was initiated, additionally removing residual impurities from the gas phase and from low binding energy sites, while leaving any pretrapped gas practically intact. Desorption acquisition was initiated as soon as all sampled signals (He, H2O, N2, O2, Ar, CO2) dropped to their background levels. TPD was performed by linearly (20.°C/ min) ramping the sample temperature from LN2 to T0. On completing all four measurements for a given T0, the preevacuation temperature was raised to T0′ ) T0 + 100 °C, which was then used as the new T0 for the next successive set. The desorbed products were probed with a Balzers QMG-422 quadrupole mass spectrometer (QMS). In cases where Ne (UHP, ∼1 bar, 20 sccm) was used as the sorbent, masses 20 and 22 were probed instead of mass 4 and He (∼1 bar, 50 sccm) was used as the carrier gas. A final remark concerns the precision of the recorded TPD profiles. Numerous repetitions were carried out for each complete set of measurements (including He and Ne for all T0 values). In each set, a new piece of as-received CMSF was used. In that case, apart from testing data reproducibility, also any possible local effects that could possibly be introduced by chance, for example, due to defects in a certain selected CMSF sample, were eliminated. In this respect, it should be mentioned that the observed results are also exact reproductions of those previously observed in refs 3 and 4 on the same raw CMSF.
III. Results Shown in Figure 1a are typical TPD spectra measured for Ne sorbed on CMSF. The sample was pre-evacuated at T0 ) 100, 300, and 400 °C and exposed to Ne (i) at LN2 (lower dashed curves) and (ii) while cooling the sample from RT down to LN2 (solid curves). Similarly, Figure 1b depicts the results obtained for the He/CMSF case with pre-evacuation temperatures of T0 ) 100, 200, and 300 °C. The occurrence of encapsulation becomes clearly
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Letters
Figure 1. Decapsulation spectra of: (a) Ne (upper) and (b) He (lower) from CMSF pre-evacuated at various temperatures (indicated in °C). Solid and dashed curves denote TPD spectra measured following dynamic adsorption during cooling between RT and LN2 and at LN2, respectively.
evident by noting the following observations: (1) Figure 1 shows the practical flatness of the dashed TPD curves which correspond to TPD spectra acquired following sorption procedures performed at LN2 (i.e., gas was introduced only after stabilizing the sample temperature at -196 °C) on samples pretreated at various T0’s. The overall flatness (relative to the solid TPD curves) of the dashed desorption curves clearly indicates that at LN2, the pores of the CMSF are inaccessible for He and Ne. (2) For all T0, prolonged evacuation at LN2 of the sample preloaded with either He and Ne during cooling from RT down to LN2 showed no detectable consumption of the desorption peaks, that is, the solid curves of Figure 1 remained practically intact. When the above observations are combined, the occurrence of an efficient entrapment of these gases within the pores of the CMSF is clearly underlined. Hence, the detected TPD curves should be referred to as decapsulation curves rather than pure desorption. Careful examination of the lower dashed curves of Figure 1 reveals the systematic occurrence of minor “desorption” bumps near LN2 temperature, irrespective of T0. These may be attributed to pure physical adsorption on the external surface, occurring in the vicinity
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of LN2. Much more interesting are those prominent peaks occurring in the temperature range where decapsulation occurs. Interestingly, these peaks emerge with growing magnitudes in a systematic fashion with increasing T0, and much more significantly for the smaller He atom. This may be understood in the following way: low preevacuation temperatures mildly remove surface oxides thus resulting in a slight widening of the pore opening. In such a case, cooling to LN2 contracts the pore aperture to the extent of practical pore enclosure. Higher evacuation temperatures remove surface oxides more efficiently to such an extent that closure of the pore opening following cooling to LN2 evidently becomes less effective. In accordance with the critical diameter of He and Ne, pore enclosure is always less effective for the smaller He. In refs 3 and 4, He and Ne adsorption procedures were taken at LN2; however, this was done following sample cooling from T0 down to LN2 under He and Ne atmospheres, respectively. Yet, lacking the strict adsorption procedure at LN2, the occurrence of gas encapsulation could not be revealed in these earlier works. The calculated huge activation energies for desorption,3,4 in the order of tens of kJ mol-1, could not be explained since such high values resemble chemical adsorption which seems unrealistic, in particular when Ne and He are considered. Accounting for the present new finding of He and Ne encapsulation by the CMSF, the high activation energies are now understood as those for decapsulation, that is, overcoming the energetic barrier for pore penetrating, and not the pure adsorption phenomenon. From both fundamental and applied perspectives, an interesting consequence concerning the practice of dead volume measurements has emerged from the present study. Determining the dead volume of a solid is conventionally achieved by using He as the reference probe gas, under the assumption that it is a noninteracting nonadsorbable species. The above results clearly imply that the conventional concept of dead volume in solids is expected to introduce errors in cases of porous substances whose effective pore openings undergo reversible temperature-dependent variations. Dead volumes should thus be determined at the sorption temperature and individually for each of the sample’s pre-evacuation temperatures. In summary, it was shown that at ambient pressure, the amorphous CMSF undergoes thermal dilations/ contractions, which in turn lead to variations in the effective pore openings, the degree of which can be thermally controlled to meet the critical dimension of different adsorbates. Thus, for substrates of a sufficiently flexible skeleton the accessibility for certain gases can be strongly affected by the specific (T, P) sorption conditions. In extreme cases, gas was shown to undergo efficient (partially or fully) encapsulation within the solid. The possible occurrence of such a phenomenon should be carefully accounted for when performing adsorption experiments. Dead volumes of given species could be affected by the sorption under a set of specific physical conditions. LA026671N
