Langmuir 1992,8, 624429
624
Magnetomicropore Filling of Supercritical NO onto Activated Carbon Fibers: Role of Porosity and Surface Functional Groups Hiroyuki Uchiyama, Katsumi Kaneko, and Sumio Ozeki* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Chiba 260, Japan Received July 15, 1991. I n Final Form: November 12, 1991 The micropore filling of a supercritical NO onto activated carbons was enhanced by a static magnetic field, 1-9.6 kG, which is referred to as magnetomicropore filling (MMF). MMF of pitch-based activated carbon fiber occurred just after application of a magnetic field. Cellulose- and polyacrylonitrile-based activated carbon fibers, a coconut shell-based activated carbon, and a molecular sieve carbon showed markedly a transient MMF: rapid NO adsorption due to a magnetic field and a subsequent, exponential decreaseunder the magnetic field. MMF of NO on polyacrylonitrile-based activated carbon fibers increased linearly with an increase in the magnetic field and reached 130 pglg at 9.6 kG. MMF relates closely to slitlike micropores and acid sites. Especially, micropores of less than 1.1-nmwidth seem to be useful for magnetically induced formation of an NO dimer which makes micropore filling of a supercritical NO easy.
Introduction In a previous paper,' we reported that chemisorption of NO on iron oxides is enhanced and depressed by the external magnetic field. The results suggested that the magnetoadsorption and magnetodesorption relate intimately to adsorption states (surface sites) and porosity rather than magnetism of solids. Activated carbons (AC) and activated carbon fibers (ACF) are typical, porous materials. Functional groups on the carbons depend on an origin of the carbons and can be controlled by heating or oxidizing. Therefore, the carbons must be useful for examination of a role of porosity and surface functional groups in the magnetic field-induced adsorption of NO onto solids. NO is a supercritical gas near room temperature. Therefore, usually NO, unlike vapors, cannot be adsorbed by micropore filling which is a physical adsorption enhanced by the pore wall's potential of micropores. However, AC, especially ACF, anomalously adsorbs NO a t 303 K by the micropore filling mechanism, associated with dimerization of NO in the slitlike micropores.2 In addition, NO is a paramagnetic molecule having an unpaired electron, and carbons have relatively high electric conductivity, i.e., free electrons. Thus, it may be expected that the paramagnetic interaction between NO and carbons is perturbed by an external magnetic field. In the preliminary we reported briefly that the micropore fillingof NO onto ACs and ACFs is enhanced by a magnetic field. In this paper, we have examined the relationship between magnetic enhancement of NO adsorption and size of the micropore and/or surface functional groups of various kinds of carbon. Experimental Section Materials. Three kinds of ACF were used: cellulose- (CEL), polyacrylonitrile-(PAN), and pitch-based ACFs (PIT). Three PITSwith different surface areas are denoted P10, P15, and P25.
* To whom correspondences should be addressed. (1) Ozeki, S.; Uchiyama, H. J.Phys. Chem. 1989,92,6485. Ozeki, S.; Uchiyama, H.; Kaneko, K. J.Phys. Chem. 1991,95, 7805. (2) Kaneko, K.; Fukuzaki, N.; Ozeki, S. J. Chem. Phys. 1987,87, 776. Kaneko, K.; Fukuzaki, N.; Kakei, K.; Suzuki, T.; Ozeki, S. Langmuir 1989,5, 960. (3) Uchiyama, H.; Ozeki, S.; Kaneko, K. Chem. Phys. Lett. 1990,166, 531.
P10 was oxidized at different temperatures, T = 373-1173 K, in air for 1h; they are designated P10-T. A coconut shell-basedAC (CAC) and a molecular sieving carbon (MSC)were used. A carbon black (PC) having a slight amount of micropores and a nonporous carbon black (NPC) were also used for comparison. Micropore structures and surfaceareas of the above materials (TableI) were determined by N1 adsorptionat 77 K, as previously rep~rted.~ NO Adsorption under Magnetic Field. The amount of NO adsorbed was determined volumetrically at 303.2 0.1 K, as described in a previous paper.' The sensitivityof the adsorption measurement was about 1pg/(g of adsorbent). Changes of NO pressure were monitored by a pressure sensor upon applying the static magnetic fields of 1.0, 4.0, 7.6, and 9.6 kG to the NOcarbon systems which had been equilibrated for 40 h or more. The homogeneities of the magnetic field at an adsorptioncell are within 2%. The samples were pretreated at 383 K and 1 mPa for 5 h prior to adsorption experiments.
*
Results Characteristic parameters, specific surface area, ppre volume, and pore width, were analyzed by the t-plot using a standard thickness for Nz film on a graphitized nonporous ~ a r b o n The . ~ parameters are summarized in Table I. The surface areas of the ACFs are in the range 8302150 m2/g, and those of CAC and MSC are 860 and 470 m2/g,respectively. The surface areas of the carbon blacks, 69 m2/g for PC and 81 m2/g for NPC, are much smaller than those of the other carbons, but would be enough to examine their magnetic field effect on NO adsorption, because the surface areas are comparable with or more than those of the metal oxides which first showed large magnetoadsorptivity. Preheating A10 led to increases of specific surface area, pore volume, and pore size of a pristine A10 with increasing preheating temperature. Widths of slitlike micropores of the samples were in the range 0.54-1.7. The widths are not accurate enough because micropore filling of Nz would occur in the carbons, but will be used as a relative value for a pore width.6 NO adsorption onto ACs is generally slow. Therefore, most NO-carbon systems, in which magnetic field effects (4) Kakei, K.; Ozeki, S.; Suzuki, T.; Kaneko, K. J. Chem. Soc., Faraday Trans. 1990,86, 371. (5) Rodroguez-Reinoso, F.; Martin-Marinez, J. M.; Prado-Burguette, C . ; McEnaney, B. J . Phys. Chem. 1987, 91, 515. (6) Ozeki, S. Langmuir 1989, 5 , 186.
0743-7463/92/2408-0624$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 625
Magnetomicropore Filling of Supercritical NO I
Table I. Characterization Parameters of Carbons for NO Adsorption.
'
I
I I
I
~~
samples a,, m2/g V,, mL/g coconut shell-based AC (CAC) 860 molecular sieving carbon (MSC) 468 0.176 cellulose-basedACF (CEL) 1310 0.572 polyacrylonitrile-baedACF (PAN) 830 0.338 pitch-bbed ACF (PIT) P10 1230 0.500 1230 0.498 P10-573 1280 0.558 P10-773 1422 0.618 P10-1173 1450 0.703 P15 2150 1.36 P25 81.0 0.021 carbon black (PC) 0.0 68.5 NPC 0 Some data from refs 3 and 4. a, is the specific surface area obtained by the t-plot. V , is the pore volume obtained by the t-plot. I
I
I
I
I
2oi-A y. -;I -_-
.A
0
~
0
30
~
60
~
90
120
Time lmin
I
I
I
0
30
I
60 Time I min
Figure 2. Magnetoadsorption for NO of various carbons under 7.6 kG at 30 OC. The explanations for symbols are given under Figure 1. Samples: A (0, CAC; A, P10;0,P15; V, P25), B (0, CEL; 0,PAN; A, MSC), C (A, PC; 0,NPC).
I
90
120
Figure 1. Dependence of magnetoadsorption for NO of PAN on adsorption time, 43 h (circle) and 22 days (triangle),which is the time after introduction of NO over PAN at 30 "C. Arrows: ON, applicationof 7.6kG magneticfield; OFF, removal of the magnetic field. Symbols: open, without the magnetic field; solid, under the magnetic field.
on NO adsorption were examined at about 40 h after NO had been introduced over carbons, would not be in a true equilibrium. However, the waiting time of 40 h seems to be enough to evaluate true magnetoadsorptivity for carbons, because Figure 1 shows that there are no differences in magnetoadsorptivitybetween waiting times of 22 days and 43 h. Figure 2 illustrates changes of the amount of NO adsorbed on various carbonsby a 7.6-kG magnetic field, which was applied a t time zero and removed at 60 min. Magnetoadsorption of NO onto the carbons occurred just after application of the magnetic field and reached a maximum within a few minutes. In the case of CAC and PITS,NO adsorption remained almost unchanged while the magnetic field was applied (Figure 2A). In contrast, transient magnetoadsorption was observed in CEL, PAN, and MSC systems (Figure 2B). With application of a magnetic field to adsorption equilibrium systems, NO adsorption is immediately promoted, reaches a maximum within a few minutes, and then exponentially decreases during application of the magnetic field. The characteristic time, 7, for the exponential decrease was estimated from Figure 3, e.g., 34 min for CEL, 37 min for PAN, 40 min for MSC, and 90 min for CAC under 7.6 kG, according to the following equation: Av = Avo exp(-t/7)
(1) where Av is the amount of NO adsorbed by application of the magnetic field. All 7 and AVOvalues are listed in Table 11. AVOis the magnetoadsorption at t = 0: extrapolated
1
0
20
40
60
1
Time lmin
Figure 3. Logarithm of magnetoadsorption (Au) for NO under 7.6 kG at 30 "Cas a function of time. Samples: 0,CEL; 0,PAN, 0, MSC.
values of the Av - t curves to t = 0. AVOfor PAN, 138 pg/g at 9.6 kG, is the largest of all. PC also exhibits an appreciable magnetoadsorption, but NPC shows no magnetoadsorption (Figure 2C). When the magnetic field was removed, the amounts of NO adsorbed recovered reversibly in all carbon systems (Figure 2). However, a second application of the magnetic field depressed some magnetoadsorption, except for CAC and PITS; i.e., the magnetoadsorptivitydepended strongly on the history (a prior exposure against magnetic field), as shown in Figure 4. The magnetic field effect for NO adsorption onto PITS depended on the preheating temperature (Figure 5). Their magnetoadsorptivity changed with temperature via a maximum at 873 K (Figure 6). This suggests that magnetoadsorptivity relates closely to surface functional
Uchiyama et al.
626 Langmuir, Vol. 8, No. 2,1992 Table 11. Magnetomicropore Filling of NO onto Carbons at 30 O C under a 1-9.6-kG Magnetic Field. samples CAC
H, V , kG mg/g
7.6 7.6 7.6 CEL 1.0 PAN 4.0 PAN 7.6 PAN 7.6 PAN 9.6 PAN P10 7.6 P10-573 7.6 P10-773 7.6 P10-1173 7.6 P15 7.6 7.6 P25 7.6 PC NPC 7.6
MSC
Av,
pg/g 21.2 14 5.1 55 17.8 80 39.0 13 35.4 25 36.5 96 40.2 87 37.2 126 29 21.0 26.4 27 21.7 65 24.2 19 58 20.8 18 14.0 1.9 0.83 0.0 0.40
lOOAV/u, AUO, % mg/g ~,min 0.09 13.2 89.6 1.08 59.0 40.4 0.45 85.6 34.4 0.03 0.07 0.26 107 36.6 (43 h)b 0.22 93.7 60.0 (22 days)b 0.34 138.4 70.6 0.14 0.10 0.30 0.08 0.28 0.13 0.23 0.0
1
0
25 -
a u is the amount of NO adsorbed at a pseudoequilibrium pressure (ca. 15Torr). Au is the increment in NO adsorption due to an applied magnetic field. AUOis the apparent Au at t = 0, which is obtained by eq 1. T is the characteristictime for Au decreaseunder the magnetic field, which is estimhted from eq 1. b Time after introduction of NO over carbons.
0
I
uO;; q >l
-
I
-
-
n
4 I
-3
c
-
I
1
4:: -
>
-0.1
25
-
g-
-0 I
I
1
1
I
H is denoted by
60
h 7
-
c
e
Tinelmin
Figure 4. Reproducibility of magnetoadsorption for NO of carbons under 7.6 kG at 30 O C . Samples: a, CAC;b, MSC; c, CEL; d, PAN; e, P10; f, P15; g, P25; h, PC. The explanations for other symbols are given under Figure 1. groups,because moderate temperatures for preheating may produce acidic functional groups such as -OH and -COOH on their surfaces and higher temperatures (e.g., 1173 K) may promote removal of the functional groups and local graphitization. Figure 7 shows the magnetoadsorption-time curves for PAN under different magnetic fields. The magnetoadsorption increased markedly with an increase in magnetic field (Figure 8). It is very interesting that there is no indication of a saturation of magnetoadsorption up to 10 kG. Figure 9 shows that the relationship between Au and
Av = 8.67 exp(0.29H) (2) Table I1 summarizes amounts of NO adsorbed at an NO pressure, u, and Au. The maximum enhancement was Av = 126 pg/g for PAN at 9.6 kG or 100Aulv = 1.15% for MSC at 7.6 kG.
Discussion Relationshipbetween Magnetoadsorptionand Microporosity. Interactions of NO with graphite are very weak, as shown by no peak shift of adsorbed NO in an XPS spectrum.' This causes a small adsorptivity of nonporous carbon black (NPC) for NO. When a carbon black has micropores, it, like PC, adsorbs more NO. Typical microporous materials such as AC and ACF have large adsorptivity for NO. This dependence of NO adsorptivity on microporosity is parallel to that in magnetoadsorption: no magnetoadsorption of NPC, appreciable one of PC, and notable ones of ACs and ACFs. These suggest that the micropores of solids play an important role in magnetoadsorption of NO, and presumably that a magnetic field assists the enhancement process of NO adsorption due to micropores. (7) Dianis, W.; Lester, J. E. Surf. Sei. 1974, 43, 603.
Langmuir, Vol. 8, No. 2, 1992 627
Magnetomicropore Filling of Supercritical NO 100
I
I
I
\ o
lOO}
I o
P Pore width Inm
Figure 10. Relationship between magnetoadsorption (Au)for NO of carbons under 7.6 kG at 30 "C and width of slitlike micropores. The pore width was estimated from the t - p l ~ t . ~ ~ * I
I
I
I
I
0
30
60
90
120
Time lmin
Figure 7. Magnetoadsorption for NO of PAN under various magnetic fields a t 30 O C . Magnetic field (kG): 0,l.O;A,4.0;0, 7.6; V, 9.6. The explanations for other symbols are given under Figure 1. I5Or---l
Magnetic field I
kG
Figure 8. Dependence of magnetoadsorptivity for NO of PAN on magnetic field.
-5t -
-4
cn cn
3 T >
C
d
-3
1
0
I
5 Magnetic field I kG
I
10
Figure 9. Logarithm of magnetoadsorption (Au)for NO of PAN as a function of intensity of the magnetic field.
Figure 10shows the relationship between the increment of NO adsorption (Au) due to a 7.6-kG magnetic field and the width of slitlike micropores of the carbons. This
demonstrates clearly that the decrease in pore width in the range 0.6-1.7 nm (and m for NPC) promotes magnetoadsorption of NO onto the carbons, except for P10 and CAC which deviate downward. The most preferable pore width for the magnetoadsorption seems to be in the range 0.6-1.1 nm. I t is well known that NO adsorption onto ACs and ACFs occurs by means of micropore filling which is enhanced with a decrease in pore width.8 In the present case, amounts of NO adsorbed are in the range 0.&4% of total pore volume of the carbons so the micropores are vacant enough to adsorb further NO by application of the magnetic field. Therefore, the magnetoadsorption of NO onto the carbons here will be referred to as magnetomicropore filling (MMF). Some iron oxides and NiO also showed reversible magnetoadsorption of N0.I These adsorbents did not clearly show the existence of micropores, except for NiO, because of the limitation of applicability of the t-method. According to the present results, it is plausible that in NiO and some iron oxide systems the MMF mechanism contributes to some extent to the magnetoadsorption. Pore width and other structural parameters (surface areas and pore volumes) of PITS increased in the order P10 < P15 < P25. Therefore, it is expected that NO adsorptivity should change monotonically in the same order. However, the experiment showed that the magnetoadsorptivity is in the order P15 > P10 > P25. Also P10773, which has the same pore size as P10-573, had larger magnetoadsorptivity than P10-573. Additionally,themagnetoadsorption of the preheated P10 showed no relation to the structural parameters which increased monotonically with preheating temperature; on the contrary, the magnetoadsorptivity changed through a maximum. Thus, it should be noted that the porosity dependence of course would include some influence originated from, e.g., functional groups on the carbon surfaces, although micropores must be most responsible for MMF of NO on the carbons. Role of Surface Functional Groups in Magnetomicropore Filling. Activated carbons have generally many acidic functional groups on their surfaces as well as microand mesopores. For example, CEL has 0.96 mmol of -COOH/g, 4.18 mmol of -OH/g, and 0.10 mmol of M / g ? which were determined by a titration method with bases.1° The magnetoadsorptivity of CEL is 5.7 times larger than that of CAC. This tendency reflects the amount of surface functional groups, because the total amount of functional groups of CEL is 5.3 times larger than that of CAC (Le., 0.09 mmol of -COOH/g, 0.29 mmol of -OH/g, and 0.60 (8) Kaneko, K. Langmuir 1987,3, 357.
(9) Kaneko, K.; Ozeki, S.; Inouye, K. Colloid Polym. Sci. 1987,265, 1018. (10)Boehm, H.P. Adu. Catal. 1966,16,179.
Uchiyama et al.
628 Langmuir,Vol. 8, No. 2, 1992
75
t L
OO
50
I00
NH, adsorbed at 20 k h l m g l g
Figure 11. Relationship between magnetoadsorption (Au)for NO of PITSunder 7.6 kG at 30 O C and NHs uptake at 20 kPa. Samples: 0, P10-573, P10-773, P10-1173; 0, P10, P15, P25. mmol of -C=O/g);ll in contrast, e.g., Brunauer-EmmettTeller (BET) surface areas of CEL are only 1.6 times. Thus, the surface functional groups should be taken into account in considering magnetoadsorption. PITShave much fewer surface functional groups than CEL and PAN.3 Therefore, using PIT, the amount of functional groups may be easily controlled through oxidation at various temperatures, without serious modification of structural parameters (Table I): P10 preheated in air had a maximum adsorptivity for NH3 (gas) around 773 K because of the formation of functional groups on the carbon below 773 K and their removal above 773 K.12 Figure 11shows the relationship between the increment of NO adsorption due to a 7.6-kG magnetic field (Av) and the amount of NH3 adsorbed on the preheated PITS,where the amount of NH3 adsorbed on ACF is used as a measure of the amount of acidic functional groups on ACF surfaces. This figure, which also includes the data for three P1Ts,l3 shows obviously that an increase in the amount of acidic functional groups leads to an enhancement of magnetoadsorption. NO is only weakly adsorbed on graphite7 Therefore, functional groups on activated carbons should play an important role in adsorption of NO as an adsorption site or an origin of the crystal field, e.g., a local electric field.14 This is one possibility of a role of surface functional groups in MMF. The magnetoadsorption on metal oxides relates intimately to the kind of adsorption site or the form of adsorption species.' A heterogeneous reaction of NO, 3 N 0 NzO + NO2, N20 + C N2 + CO, may occur, but only much less than 1% of the total NO reacts at 30 OC.15 Nevertheless, such reactions might relate to no MMF (Figure 4) during the second application of the magnetic field. MagnetomicroporeFilling of NO. There seem to be a few types in the time course of the magnetic field-induced adsorption of NO, such as magnetoadsorption, magnetodesorption, and MMF, which are classified furthermore by the response of the adsorptivity against application and removal of the magnetic field. The former two were observed in metal oxide systems, depending on the kind of chemisorbed species on surfaces.' On the other hand, MMF here is a physisorption process. We found in previous papers2 that a supercritical NO is reversibly adsorbed on ACFs and ACs by the micropore filling mechanism, and that some portion of NO in the
-
-
(11) Ozeki, S. Unpublished data. (12) Kaneko, K.; Kakei, K. J. Colloid Interface Sci. 1989, 129, 583. (13) Kaneko, K.; Nakahigashi, Y.; Nagata, K. Carbon 1988,26, 327. (14) Hoffman, B. M.; Nelson, N. J. J. Chem. Phys. 1969, 50, 2598. Gardner, C. L.; Weinberger, M. A. Can. J. Chem. 1970,48, 1317. (15) Imai, J.; Kaneko, K. Manuscript in preparation. StricklandConstable, R. F. Trans. Faraday SOC.1938, 34,1374.
micropores exists as an NO dimer by stability energy (7 kJ/mol) for a dimer due to the enhanced potential between micropore walls. MMF is apt to occur notably on the samples which have large adsorptivity for NO: the larger v, the larger Av or Av/v, suggesting that the magnetic field should act on micropore filling of NO. The size of an NO dimer (-0.53 X 0.41 X 0.30 nm3 for both a trans and cis form) f i b the most preferable pore size for MMF, 0.6-1.1 nm (Figure 10) which corresponds to a one to two layer thickness of (N0)z. Thus, it is inferred that the magnetic field may enhance the adsorption of NO through both stabilization of a paramagnetic NO and promotion of an NO dimer formation in micropores. In some cases, Av decreased exponentially even under a magnetic field: Le., the transient enhancement of NO adsorption occurred. This is suggestive for the mechanism of MMF. There seem to be at least two possibilities for the exponential decay of Av: (i) temperature increment due to heat of adsorption or decrease of NO pressure due to a rapid adsorption and (ii) escape of an adsorbed species of NO in micropores from the magnetic field with, e.g., a gradual disappearance of induction current produced by application of the magnetic field. Since the initial rate of MMF is not so different from carbon to carbon, e.g., between MSC and PIT, (i) if any seems not to be a major cause. It is possible that the induction current in carbons is responsible for MMF and ita exponential decrease, because the carbons (e.g., 25 Seem-' for P10 at 30 OC16) have relatively high electric conductivity and also electrons may be produced during NO adsorption on carbons, as in the case on ze01ites.l~ The induction current might enhance the lattice vibration for some distorted modes in the carbons17or temporarily weaken the effective magnetic field in the carbons. The enhanced energy may be transferred to NO molecules to form a species, probably NO dimer, which is quasistable under the local current, and the reduced magnetic field assists the NO adsorption via stabilization of a diamagnetic (NO)2. Then, a subsequent deduction of the induction current should lead to the decrease in Av. Since the Zeeman splitting of a u orbital of an NO molecule due to the external magnetic field would be about 1cm-' for 10 kG, the population of electrons at the lower energy level would be only less than 1%larger than that at the higher level. Also, the magnetic contribution, 0.01 J/mol, to the free energy of reaction, 2N0 (NO)2,cannot modify the equilibrium constant, which is estimated by assuming the change, emu/mol, of magnetic susceptibility during the reaction (magnetic susceptibility/lO+ emu/g, 49 for NO18 and -0.6 for (NOI2l9). Consequently, another possibility for the MMF mechanism comes from a cooperative effect on NO via the magnetic field. The NO dimer has a weak chemical bond which arises from electron pairing between two N0(211)molecules, but the coupling between two interacting NO molecules will still be weak and the unpaired electron will mainly be localized on each NO molecule.20 Then, one may regard an (NO)? as a two-spin system, whose ground state is a singlet
-
(16) Imai, J.; Suzuki, T.; Ozeki, S.; Kaneko, K. Manuscript in preparation. (17) Uchiyama, H.; Ozeki, S.; Kaneko, K.; Natsume, Y.; Suzuki, T. In Dynamics and Patterns in Complex Fluids (Springer Proceeding8 in Physics);Onoki, A., Kawasaki, K., Ede.; Srpinger-Verlag: Berlin, 1990; Vol. 52, p 221. (18) Bauer, E.; Piccard, A. J. Phys. 1920, I , 97. (19) Smith, A. L.; Jonson, H. L. J. Am. Chem. SOC.1952, 74,4696. (20) Ng, C. Y.; Tiedemann. P.W.; Mahan, B. H.; Lee, Y.T. J. Chem. Phys. 1977,66, 3985.
Magnetomicropore Filling of Supercritical NO
When ( N o h is under the magnetic field, the triplet (T) state separates into three energy levels. Considering that Au increased proportionally with magnetic field (Figure 8), a so-called Ag mechanismz2seems to assist the dimer formation via the triplet-singlet transition. From the aspecta above, a possible mechanism for MMF is the following. Some portion of NO in the micropores is dimerized by the enhanced potential in the micropores.z When a magnetic field is applied to the carbon-NO systems, the diamagnetic (NO12 in the micropores is (21) Brechignac,Ph.;De Benedictis, S.;Halberstadt,N.;Whitaker, B. J.; Avrillier, S. J. Chem. Phys. 1985, 83, 2064. (22) Steiner, U. E.; Ulrich, T. Chem. Reu. 1989,89,51.
Langmuir, Vol. 8, No. 2,1992 629
produced through the singlet-triplet conversion under the magnetic field and stabilized via the lattice-vibration enhancement due to the induction current. The fact that (NO)z has a boiling point in contrast with the supercritical NO can lead to further NO micropore filliig, as in the vapor adsorption. Thus, the application of a magnetic field brings about an increase in the amount of NO adsorbed (Au), i.e., MMF. Surface functional groups and defects around microporesmay assistNO micropore filling, e.g., through enhancement in polarization and crystal field on the surfaces. Registry No. NO,10102-43-9.
