Molecular States of O2 Confined in a Carbon Nanospace from the Low

Mar 5, 1997 - Hirofumi Kanoh,Asako Zamma,Norihiko Setoyama,Yoko Hanzawa, ... Takahiro Ueda , Hironori Omi , Takanori Yukioka , Taro Eguchi. Bulletin o...
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Langmuir 1997, 13, 1047-1053

1047

Molecular States of O2 Confined in a Carbon Nanospace from the Low-Temperature Magnetic Susceptibility† Hirofumi Kanoh, Asako Zamma, Norihiko Setoyama, Yoko Hanzawa, and Katsumi Kaneko* Department of Chemistry, Faculty of Science, Chiba University, Yayoi 1-33, Inage, Chiba 263, Japan Received November 16, 1995. In Final Form: April 4, 1996X The magnetic susceptibility χ of O2 adsorbed in micropores of activated carbon fibers (ACFs) having different micropore widths was measured at a wide temperature range of 1.7-100 K. The micropore structure of ACF was determined by the analysis of the N2 adsorption isotherm at 77 K. The adsorption isotherm of O2 on ACF at 77 K was determined and it was shown that O2 is adsorbed in the micropore in a similar way to N2 from the Dubinin-Radushkevich analysis of O2 and N2 adsorption isotherms. The χ-T relation suggested the presence of three kinds of O2 molecular states in the micropore, that is, the isolated, the cluster, and condensed states; their stable region changes with the fractional filling and the temperature. Although the potential energy difference of the O2-surface interaction of different pore systems is only 150 K by the potential calculation, the magnetic susceptibility measurement can sufficiently evidence distinctly the presence of different states of O2 molecules in the micropore.

Introduction Physical adsorption has been classified into adsorption on a flat or macropore surface, adsorption in mesopores, and adsorption in micropores.1-3 Adsorption in micropores, which is an enhanced adsorption due to overlapping of the interaction potentials from the opposite pore walls, has gathered much attention.3,4 Although a lateral interaction between adsorbate molecules should be important, development of a simple theory has been neglected.5 Only adsorption in mesopores, the so-called capillary condensation, is based on the lateral interaction, although recently the molecule-surface interaction is taken into account.7 Capillary condensation theory presumes that the adsorbed layer is the same as the bulk liquid. Also an adsorbed layer in micropores has been believed to be identical to the bulk liquid; the Gurvitch rule has guaranteed that the adsorbed layer in the micropore is the bulk liquid.8 Understanding of the lateral interaction is less advanced compared with the moleculesurface interaction in both the theory and experimental studies regardless of its importance.9 Although recent computer simulation approaches have taken the lateral interaction into account using the Lennard-Jones potential, the physical picture on the intermolecular state in physical adsorption is not sufficiently understood.10-13 † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, September 15, 1996.

(1) Adamson, A. W. Physical Chemistry of Surfaces: John Wiley & Sons; New York, 1990. (2) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity: Academic Press: London, 1984. (3) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Pierotti, R. A.; Rouquerol, J.; Simieniewska. T. Pure Appl. Chem. 1985, 57, 603. (4) Everett, D.; Powl, J. C. J. Chem. Soc., Fraday Trans. 1 1976, 72, 619. (5) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (6) Broekhoff, J. C. P.; Linsen, B. G. Physical and Chemical Aspects of Adsorbents and Catalysts; Linsen, B. G., Ed.; Academic Press: London, 1970; Chapter 1. (7) Keizer, A.; Michalski, T.; Findenegg G. Pure Appl. Chem. 1991, 63, 1495. (8) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic Press: London, 1984; p 113. (9) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Surfaces; Academic Press: London, 1992.

S0743-7463(95)01042-0 CCC: $14.00

The van der Waals molecule is unstable at room temperature and it is quite difficult to get a concentrated system of van der Waals molecules.14,15 According to these authors supercritical gas molecules such as N2 and NO tend to produce their dimer in micropores.16-20 The NO dimers of a representative van der Waals molecule are highly concentrated in the micropore of ACF even under a subatmospheric pressure. The formation of organized molecular structures, which is associated with the van der Waals molecule, in the micropore was observed.21,22 Hence micropores accelerate the formation of an organized structure of adsorbed molecules. The weak intermolecular interaction in micropores, which provides the van der Waals molecule, should be elucidated in order to understand the micropore filling mechanism. The spin-spin interaction is so weak that it is sensitively affected by the intermolecular interaction. Therefore, the magnetic measurement is a fit method for the study on the molecular association. Furthermore, ACF, which is mainly composed of micrographites, is diamagnetic.23,24 The electronic state of O2 is triplet, and O2 shows paramagnetism in the gas phase. Hence, the intermolecular state of O2 in micropores of ACF can be clarified by the magnetic susceptibility measurement.25,26 In particular, the intermolecular struc(10) Seaton, N. A.; Walton, J. R. P. B.; Quirk, N. Carbon 1989, 27, 853. (11) Bakaev, A. V.; Steele, W. A. Langmuir 1992, 8, 148. (12) Cracknell, R. F.; Nicholson, D.; Gubbins, K. E. J. Chem. Soc., Faraday Trans. 1995, 91, 1377. (13) Cracknell, R. F.; Nicholson, D.; Gubbins, K. E.; Madox, M. Acc. Chem. Res. 1995, 28, 281. (14) Rigby, M.; Smith, E. B.; Wkeham, W. A.; Maitland, G. C. The Forces between Molecules; Oxford Science: Oxford, 1986. (15) Kajimoto, O. Cluster Chemistry (in Japanese); Baihoukan: Tokyo, 1992. (16) Kaneko, K. Lamgmuir 1987, 3, 357. (17) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J. Chem. Phys. 1987, 87, 776. (18) Imai, J.; Souma, M.; Ozeki, S.; Kaneko, K. J. Phys. Chem. 1991, 95, 9955. (19) Kaneko, K.; Shimizu, K.; Suzuki, T. J. Chem. Phys. 1992, 97, 8705. (20) Moreh, R.; Levant, D. Mol. Phys. 1990, 69, 735. (21) Iiyama, T.; Nishikawa, K.; Otowa, T.; Kaneko, K. J. Phys. Chem. 1995, 99, 10075. (22) Fujie, K.; Minagawa, S.; Suzuki, T.; Kaneko, K. Chem. Phys. Lett. 1995, 236, 427. (23) Kaneko, K.; Yamaguchi, K.; Ishii, C.; Ozeki, S.; Hagiwara, S. Chem. Phys. Lett. 1991, 176, 75. (24) Suzuki, T.; Kasuh, T.; Kaneko, K. Chem. Phys. Lett. 1993, 93, 2355.

© 1997 American Chemical Society

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Figure 2. High-resolution R2 plot for N2 adsorption isotherm of ACF-1.4. Table 1. Micropore Structure of ACFs

Figure 1. N2 adsorption isotherms of ACFs at 77 K: O, ACF0.8; 4, ACF-0.9; 0, ACF-1.4.

ture of O2 on the graphite surface has been actively studied27 and we can compare the O2 intermolecular structure in the micropore with that on the graphite surface. In this paper, the adsorption mechanism of O2 in micropores is discussed with a special relevance to the intermolecular state of O2 from the low-temperature magnetic susceptibility measurement. Experimental Section Three kinds of pitch-based ACFs (A5, A10, and A25) were used. Three ACFs are denoted ACF-0.8, ACF-0.9, and ACF-1.45 using the micropore width in this paper. The N2 and O2 adsorption isotherms of ACFs were measured gravimetrically at 77 K. The ACF samples were evacuated at 383 K and 10 mPa for 2 h prior to the adsorption measurement. The samples for the magnetic measurement were prepared as follows. The fiber specimen was weighed, inserted into the quartz tube, evacuated at 383 K and 10 mPa for 2 h, followed by an introduction of oxygen gas at 298 K, and then sealed in a quartz tube (90 mm × 5 mm in diameter). All O2 molecules were assumed to be adsorbed in micropores of ACF at low temperature because of the great surface area of the ACF sample. The amount of O2 adsorption was controlled by introducing O2 of different pressures. The fractional filling φ was obtained from the ratio of the amount of O2 adsorbed in mL (liquid) g-1 to the micropore volume determined by N2 adsorption W0 using the liquid densities of O2 at 87 K (1.15 g mL-1) and of N2 at 77.2 K (0.808 g mL-1). The magnetic susceptibility of ACF itself in vacuo was also measured. The magnetic susceptibility of O2 adsorbed on ACF after sealing was measured with a SQUID (superconducting quantum interference device) magnetometer system MPMS (Quantum Design, CA) over a temperature range of 1.7-100 K and at a magnetic field of 1 T.

Results and Discussion Micropore Structures. Figure 1 shows the adsorption isotherms of N2 on ACFs at 77 K. The adsorption isotherms of N2 on ACF-0.8 and ACF-0.9 were typical type I, indicating presence of uniform micropores. The adsorption isotherm of ACF-1.45 has a steep uptake at the low P/P0 range and a gradual adsorption until P/P0 ) 0.4. The initial adsorption uptake and the gradual adsorption at the medium P/P0 range are ascribed to the monolayer adsorption on the micropore walls and to the second and/ or third layer adsorption of N2 on the monolayer-covered micropore walls, respectively. This adsorption behavior suggests that the pore width of ACF-1.45 corresponds to (25) Kanoh, K.; Kaneko, K. J. Phys. Chem. 1995, 99, 5746. (26) Kanoh, K.; Kaneko, K. Chem. Phys. Lett. 1995, 237, 329. (27) Steele, W. A. Chem. Rev. 1993, 93, 2355.

ACF-0.8 ACF-0.9 ACF-1.4

surface area, m2‚g-1

pore volume, mL‚g-1

pore width, nm

900 1247 2108

0.34 0.61 1.42

0.75 0.91 1.38

the thickness of more than four adsorbed layers. As the monolayer completion finishes below the relative pressure corresponding to the adsorption on the flat surface, the routine BET evaluation of the surface area leads to an overestimation. The adsorption in micropores is composed of both an enhanced monolayer adsorption on the micropore walls and adsorption in the residual space between the monolayer-covered micropore walls (a kind of condensation). Hence the enhanced adsorption should be subtracted. The high-resolution Rs plot of the N2 adsorption isotherm is quite effective for the evaluation of the specific surface area. The data of the high-resolution N2 adsorption isotherm of nonporous carbon black was used as the standard carbon. The subtracting pore effect (SPE) method for the high resolution Rs plot can remove the enhanced adsorption, leading a correct surface area.28,29 Figure 2 shows the high-resolution Rs plot for ACF-1.45. It has two swings from linearity in the Rs regions of 0-0.3 and 0.7-1. The linear part between both swings passes the origin, and the slope of the line provides the total surface area, that is, the sum of the microporous and external surface areas. The slope of the line of the Rs plot above Rs ) 1 gives the external surface area and its intercept leads to the micropore volume. Hence the highresolution Rs plot provides the total surface area, the microporous surface area, and the micropore volume. The micropores of ACF can be approximated by the slit and thereby the average slit-width (micropore width w) was determined from the microporous surface area and the micropore volume. The Rs plots of ACF-0.8 and ACF-0.9 have only the swing below Rs ) 0.3. Their surface area and micropore volume were determined in a similar way. Table 1 lists the total surface area, the micropore volume, and the average micropore width w. The N2 adsorption isotherm is described by the Dubinin-Radushkevich (DR) equation

W/W0 ) exp(-A2/E2), A ) RT ln P0/P,

(1)

E ) βE0

Here W and W0 are the amount of adsorption at P/P0 and the micropore volume, respectively. β and E0 are the affinity coefficient and characteristic adsorption energy, (28) Kaneko, K.; Ishii, C. Colloid Surf. 1992, 67, 203. (29) Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara. H. Carbon 1992, 30, 1075.

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Figure 5. Dubinin-Radushkevich plots for O2 adsorption isotherms: 4, ACF-0.9; 0, ACF-1.4. Table 2. Micropore Volume and qst,O)1/e for O2 and N2 from DR Plot O2

Figure 3. Adsorption potential distributions of ACFs from N2 adsorption isotherms: dotted line, ACF-0.8; solid bold line, ACF0.9; solid light line, ACF-1.4. ACF-0.9 ACF-1.4

N2

micropore volume, mL/g

qst,φ)1/e, kJ/mol

micropore volume, mL/g

qst,φ)1/e, kJ/mol

0.49 0.79

12.4 9.22

0.45 0.88

11.52 8.68

plot is almost linear, suggesting that O2 molecules are filled homogeneously (Figure 5). The micropore volume from the DR plot for O2 adsorption was the same as that from N2 adsorption, when the bulk liquid density at 87 K was used as the density of the adsorbed O2 in micropores. The isosteric heat of adsorption at φ of 1/e, qst,φ)1/e, was evaluated by eq 3

qst,φ)1/e ) βE0 + ∆Hv Figure 4. O2 adsorption isotherms of ACFs at 77 K: 4, ACF0.9; 0, ACF-1.4.

respectively. A is the adsorption potential. The adsorption potential distribution X(A) is evaluated by eq 230,31

X(A) ) dW/dA

(2)

When the DR plot is composed of several lines, eq 1 must be expressed by the sum of the elementary DR terms.32 X(A) gives the information on the energetic heterogeneity of the micropores. Figure 3 shows the adsorption potential distributions of three ACFs for N2. The peak of ACF having a smaller pore width is situated at a higher energy. Although the Dubinin-Stoeckli relation always holds for N2 adsorption on activated carbons,33 this adsorption potential distribution includes not only pore size distribution but also change in the adsorbate-adsorbate interaction. Hence, we cannot simply correlate the adsorption potential distribution with the pore size distribution; the half-width of the distribution is about 600 K at best, corresponding to the difference in the N2-micropore potential with the change of 0.2 nm at the 1 nm width. Accordingly the adsorption potential distribution suggests that the pore size distribution of ACF is (20% of the average width at best. Micropore Filling of O2. The adsorption isotherms of O2 on ACFs at 77 K are close to those of N2, as shown in Figure 4. Although the adsorption isotherm was measured below the boiling temperature (90.18 K), no condensation occurred during the measurement. The DR (30) McEnaney, B. Carbon 1987, 25, 69. (31) Jaroniec, M.; Madey, R. J. Phys. Chem. 1989, 93, 5225. (32) Dubinin, M. M. Carbon 1985, 23, 373. (33) Dubinin, M. M.; Stoekli, H. F. J. Colloid Interface Sci. 1980, 75, 34.

(3)

Here βE0 is the characteristic adsorption energy and ∆Hv the enthalpy of evaporation at the boiling temperature. The micropore volume and qst,φ)1/e values are shown in Table 2. Although the molecular size and quadrupole moment of O2 are smaller than those of N2, both adsorption behaviors are similar to each other. Magnetic Susceptibility of ACF. In previous papers,17,23 it was shown that ACF exhibits diamagnetism near room temperature except for ACF having a great surface area of more than 2000 m2 g-1. ACF is mainly composed of the micrographites which are the π-conjugated systems showing the diamagnetism. The lowtemperature magnetic susceptibilities of some ACF samples were examined by Nakayama et al.34 They showed the presence of a weak paramagnetic behavior below 10 K. Figure 6 shows the temperature dependences of the magnetic susceptibility of these ACFs in the wide temperature range. ACF-0.8 and ACF-0.9 are diamagnetic above 15 and 35 K, respectively. On the other hand, ACF-1.45 has the positive magnetic susceptibility, being temperature-insensitive above 20 K. The magnetic susceptibility steeply increases below 10 K. ACF-1.45 is highly activated and it has many dangling bonds. Recently it was reported that pure activated carbon having many dangling bonds shows an unusual ferromagnetic behavior.35,36 Consequently, ACF-1.45 has many dangling bonds at edges of small micrographites giving rise to the paramagnetism. Even the low-temperature paramagnetic susceptibilitiy of ACF-1.45 is much smaller than that of adsorbed O2 in micropores, as mentioned below. The (34) Nakayama, A.; Suzuki, K.; Enoki, T.; Vittorio, S. L.; Dresselhaus, M. S.; Koga, K.; Endo, M.; Shindo, N. Synth. Met. 1993, 55-57, 3736. (35) Ishii, C.; Matsumura, Y.; Kaneko, K. J. Phys. Chem. 1995, 99, 5743. (36) Ishii, C.; Shindo, N.; Kaneko, K. Chem. Phys. Lett. 1995, 242, 196.

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Figure 6. χ-T curves of ACFs: O, ACF-0.8; 4, ACF-1; 0, ACF1.45.

Figure 8. χ-1-T curves of O2/ACF-0.9. φ values are (O) 0.029, (4) 0.069, (0) 0.15, (3) 0.29, (]) 0.47, (b) 0.69, and (2) 0.82. Table 3. Curie and Weiss Constants of O2 in Various States states free spin adsorbed in ACF-0.8 ACF-0.9 ACF-1.4

Figure 7. χ-T curves of O2/ACF-0.9. φ values are (O) 0.029, (4) 0.069, (0) 0.15, (3) 0.29, (]) 0.47, (b) 0.69, and (2) 0.82.

magnetic susceptibility of ACF was subtracted in order to show clearly the magnetic susceptibility of O2 adsorbed in micropores using the Pascal law.37 Magnetic Susceptibility of O2 Confined in a Micropore. Figure 7 shows the temperature dependence of the magnetic susceptibility χ of O2 adsorbed on ACF-0.9 as a function of φ. The χ decreases steeply with the increase of temperature at a low fractional filling below 0.1 which is characteristic of paramagnetism. In fact, the χ-T relation can be well described by the Curie-Weiss law38

χ ) C/(T - Θ)

(4)

Here C and Θ are the Curie and Weiss constants. The Curie-Weiss plots are shown in Figure 8. The data for φ < 0.1 give good linear relations. Those constants of O2 adsorbates are quite close to those of the bulk O2 gas in the literature (Table 3). This good coincidence guarantees that the spin of O2 confined in the micropore can behave freely without any interaction with the neighbor spin. In the case of a two-dimensional O2 on graphite surface the gaseous state must coexist with solid δ-phase below 27 K and with the liquid phase above 27 K at the initial stage of adsorption.39,40 Accordingly, the intermolecular state of O2 adsorbed in micropores should be different from that on the graphite surface. The gaseous free spin state of O2 was observed in ACF-0.8 and ACF-1.45, too. The gas region of the fractional filling of ACF-0.8 is wider than (37) Weiss, A.; Witte, H.: Magnetochemie (Translated into Japanese by Sorai, M.); Misuzu: Tokyo, 1980; Chapter 4. (38) Weiss, A.; Witte, H. Magnetochemie (Translated into Japanese by Sorai, M.); Misuzu: Tokyo, 1980; Chapter 1. (39) Ko¨bler, U.; Marx, Y. Phys. Rev. B 1987, 35, 9809. (40) Suematsu, H.; Murakami, Y. J. Magnet. Magnet. Mater. 1990, 90/91, 749.

φ

0.044 0.029 0.012

C/emu‚g-1‚K

Θ/K

3.1 × 10-2

0

2.3 × 10-2 2.8 × 10-2 2.6 × 10-2

-1.8 -7.4 -2.3

that of ACF-0.9, whereas the gas phase was observed only below φ ) 0.03 in the case of ACF-1.45. The Curie-Weiss constants for these ACFs are summarized in Table 3 together with those of the bulk O2 gas. The constants are close to those of the bulk O2 gas. The basic state of O2 in the free spin state should be identical for all systems. There is the temperature-independence region in the χ-T relationship (see Figure 7) above φ > 0.3. The bulk solid phases of R, β, and γ exhibit similar temperatureindependent magnetic behavior,41 which is believed to be associated with the antiferromagnetism. However, the temperature-independent magnetism of O2 adsorbed in micropores has a completely different nature from the bulk solid phases. In the coverage of this temperature independence we observed a thermal hysteresis of the χ-T relation, as shown in Figure 9. The χ-T curve of O2 at φ ) 0.47 in Figure 9a has a hysteresis between heating and cooling processes. The sample was cooled down to 1.7 K without the magnetic fields, and the χ-T relation was measured on heating. This χ-T relation on heating has a maximum at 2.1 K, while the χ-T relation upon cooling from 4 K under the magnetic field has no maximum and it can be described by the Curie-Weiss law regardless of the narrow temperature range. This thermal hysteresis should be associated with the random magnetism such as spin glass or mictomagnetism. The maximum of the χ-T relation is often called the cusp in spin glass.42 Therefore, the temperature-independent magnetism at the medium fractional filling region must be attributed to the random magnetism which is associated with the spin cluster formation. In this case, the presence of the random magnetism indicates the cluster formation of O2 molecules in micropores. The temperature-independent magnetism was observed in the medium fractional filling range for ACF-0.8 and ACF-1.45. In the case of ACF-1.45, the fractional filling range showing the temperature independent magnetism extends significantly compared with ACF-0.9. It is noteworthy that the adsorbed layer of φ ) 0.63 having the bulk phase transition exhibited the random magnetism. The thermal hysteresis in the χ-T relation was shown for the O2 adsorbed layer of φ ) 0.63. (41) DeFotis, G. C. Phys. Rev. B 1981, 23, 4714. (42) Mydosh, J. A. Spin Glass; an Experimental Introduction; Taylor and Francs: London, 1993.

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Figure 9. Magnetic moment M-T curves of (a) O2/ACF-0.9 at φ ) 0.47 and (b) O2/ACF-1.4 at φ ) 0.63: solid symbol, heating process after cooled without the magnetic field; open symbol, cooling process under the magnetic field.

Hence, the O2 molecular aggregate formed in the micropore of ACF-1.45 should have some disorder due to a geometrical restriction. At a high fractional filling region, the χ-T curve becomes close to that of the bulk solid O2. We can observe the magnetic anomalies at 24 and 44 K, which are the phase transition temperatures from R-β and β-γ, respectively (Figure 10). However, the jump upon the β-γ transition is not sharp. Possibly the structure of O2 molecular assembly in micropores is slightly disordered. Figure 10 compares the χ-T curves of O2 filled on ACF samples and bulk O2. All three χ-T curves are similar to each other; all show the magnetic anomalies due to R-β and β-γ transitions. The magnetic transitions on ACF-0.9 and ACF-1.45 are not sharp, compared with those on that of bulk O2. The O2 molecular assembly in narrow micropores cannot form a perfect solid-like ordered structure due to a serious restriction of their packing. The defective structure of O2 adsorbed in narrow micropore should give rise to an ambiguous transition. The strikingly important thing is that the bulk phase formation occurs on the flat graphite surface under more accumulated adsorbed conditions at least after completion of the forth layer. In the case of ACF-1.45, the bilayer adsorption can be possible on each micropore wall at least. Above all in the micropores of ACF-0.9 three layers of O2 are adsorbed between two graphitic micropore walls at maximum. Thus, the O2 molecular system in micropores of ACF is much smaller than the adsorbed layer on the graphite surface exhibiting bulk like behavior. As O2 molecules in micropores are confined in deep potential wells (see Figure 11), molecules tend to be packed in the potential well to enhance the intermolecular interaction. Accordingly, the

Figure 10. χ-T curves of (a) O2 and (b) O2/ACF-0.9 at φ ) 0.82 and (c) O2/ACF-1.4 at φ ) 0.63.

Figure 11. Interaction potential profiles of O2 with the graphitic slit pores of different widths. Here z denotes the vertical distance from the midplane of two graphitic surfaces.

strong molecule-surface interaction makes O2 molecules in micropores crystallize irrespective of the thin adsorbed layer. The micropores of ACF-1.45 are fit for formation of the well-crystallized thin film in the micropore, whereas smaller micropores of ACF-0.9 induce the slight hindrance of the lattice formation. The χ-T curve of the bulk O2 shows the magnetic anomaly of γ-liquid transition at 54 K, whereas those of O2 adsorbed in micropores do not show any transition around 54 K, but do above 67 K (Figure 10). The magnetic anomaly around 67 K should be attributed to the transition to a fluid-like gas rather than the bulk liquid. The O2

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Figure 13. Model of physisorbed molecular states of O2 in micropores of ACF-0.8 and ACF-0.9.

Figure 12. Phase diagrams of O2/ACF-0.9 (top) and O2/ACF1.4 (bottom): I, gas phase; II, gas + cluster; III, gas + cluster + bulk phases (R, β, and γ phases); IV, a fluid.

adsorption isotherm at 77 K supports the Gurvitch rule. Consequently, the density of the fluid state must be close to the bulk liquid density. As the transition of the solid γ phase of O2 confined in the micropore is different from that of bulk O2, as mentioned above, the fluid should have some organized structure. The fluid phase discussed here will be expressed by IV in Figure 12. Different Confined States of O2 in a Micropore. The chemisorbed species on the solid surface has been widely studied and almost established methods are available for determination of the different chemisorbed species. For example, representative five species of chemisorbed O2, O2-, O22-, O-, and O2- are evident with the aid of IR and ESR for O2 chemisorbed on the surface of transition metal oxides.43,44 However, interactions in physical adsorption, even micropore filling, are so weak that we have no established method for assignment of the different physisorbed states. The interaction potential profiles of an O2 molecule with the slit graphite was obtained using Steele’s 10-4-3 potential funciton45 and the Lennard-Jones parameters,46 as shown in Figure 11. Although the potential minimum difference of three ACFs shown in Figure 11 is 150 K, it still is great enough for the magnetic interaction. There is a possibility that a slight difference in the weak interaction can be measured by the magnetic method. The magnetic method is quite (43) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: Amsterdam, 1989; Chapter 7. (44) Kase, K.; Yamaguchi, M.; Kaneko, K. J. Phys. Chem. 1995, 36, 13307. (45) Steele, W. A. Surf. Sci. 1973, 36, 317. (46) Maurice, R.; Smith, E. B.; Wakeham, W. A.; Maitland, G. C. The Forces Between Molecules; Oxford Science Publishers: Oxford, 1986; p 216.

sensitive and we identify different O2 states in micropores with the aid of magnetic susceptibility measurements at low temperature, as described above. The magnetic behaviors of O2 in micropores can provide an adsorbed mechanism. Figure 12 shows phase diagrams of the O2/ACF systems.26 Briefly speaking, each system has four regions of gas phase [I], coexistent phase of gas and cluster [II], coexistent phase of gas, cluster, and bulk solid [III], and a fluid [IV]. The smaller the pore width, the wider the I region. On the contrary the wider the pore width, the wider the III region. Thus the phase diagram sensitively depends on the pore width, indicating that the magnetic susceptibility reflects the slight difference in the molecular state. The average intermolecular distance di is calculated from the fractional filling using the assumption of the isotropic and homogeneous distribution of O2 molecules in the pore. di is shown on the other abscissa of Figure 12. Generally speaking, di of 1 nm is necessary for the spin dipole-spin dipole interaction. It has recently been elucidated that a new spin relaxation of bulk gaseous O2 occurs through a quadrupole-mediated mechanism at the contact distance of Lennard-Jones molecules.47 Consequently, the critical distances are 0.34 and 1 nm in the magnetic interactions of O2. Figure 13 shows the model of O2 confined in the micropores of ACF-0.8 and ACF-0.9. O2 molecules are not magnetically interacting with each other and they are isolated at a distance of more than 1 nm for O2 molecules below φ ) 0.2. In the middle fractional filling range of 0.2 < φ < 0.6, a part of the isolated molecules are associated to form the cluster which causes random magnetism. Near complete filling clusters are aggregated to produce the condensed phase. The condensed phase in the micropore exhibits the same magnetic phase transitions as the bulk phase. However, the χ-T jump upon the magnetic phase transition is not sharp, and thereby the condensed molecular layer should contain many defects due to geometrical restrictions. The recent grand canonical Monte Carlo simulation of O2 adsorption by a graphite slit at 100 K showed that O2 molecules tend to be orientationally ordered on the micropore walls.48 If O2 molecules have an orientationally ordered structure in the micropore as suggested by the simulation work, such an ordered structure coincides with the cluster and condensed phase formations. (47) Zamma, A.; Kaneko, K.; Sugihara, K. J. Phys. Chem., to be submitted. (48) Sokotowski, S. Mol. Phys. 1992, 75, 999.

O2 Confined in a Carbon Nanospace

Thus the magnetic measurements are quite effective to identify the weakly interacted molecular states in micropores. Acknowledgment. We are grateful to Professor Isao Yamada (Department of Physics, Chiba University) for his help concerning the measurement with the SQUID magnetometer of the Analytical Center of Chiba Univer-

Langmuir, Vol. 13, No. 5, 1997 1053

sity. We acknowledge support from the Science and Technology Agency, Japanese Government for H.K. to study at Chiba University. This work was supported by the Grant-in-Aid for scientific research from the Ministry of Education and Science, Japanese Government. LA951042H