J. Phys. Chem. 1992’96, 10917-10922
free path imposed by the structure, and thereby the rough physical characteristics of the free volume. All the results (both chemical shifts and relaxation rates) indicate there is a discontinuity in the surface properties at around [All, 2. We believe this expresses a nonrandom distribution of aluminum atoms in the lattice, in agreement with other observation of similar materials.
-
Acknowledgment. This work was supported in part by the Sponsors of the Center for Catalytic Science of the University of Delaware, by Grant 19011-ACS of the Petroleum Research Fund of the American Chemical Society, and by the National Science Foundation under grant CHEM 9013926. C.D. gratefully acknowledges a grant-in-aid from Sun Refining and Marketing Co. We acknowledge Ronald Nickle (deceased) and Penrose Hollins for technical assistance. Registry No. Xenon-I 29, 13965-99-6.
References and Notes (1) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350, and references therein. (2) Ito, T.; Fraissard, J. In Proceedings of rhe Fifh International Conference on Zeolltes; Rees, L. V. C., Ed.; Heyden: Frankfurt; 1980; p 510.
10917
(3) Chen, Q.Dissertation, Universitl Pierre et Marie Curie, Paris, 1990. (4) Chen, Q.; Springuel-Huet, M. A.; Fraissard, J. Zeocat 90: Catalysis and Adsorption by Zeolites; Elsevier: Amsterdam; 1991; p 129, and refmnccs therein. ( 5 ) Alexander, S.;Coddington, J. M.; Howe, R. F. Zeolites 1991, 1 1 , 368. (6) Rollman, L. S.;Volyocsik, G. E. Inorg. Synrh. 1983, 22, 61-68. (7) Corbin, D. R.; Burgess, B. F., Jr.; Vega, A. J.; Farlec, R. D. Anal. Chem. 1987, 59, 2722. (8) Jameson, A. K.;Jameson, C. J.; Gutowsky, H. S.J . Chem. Phys. 1970, 53, 2310. (9) Chen, Q.;Fraissard, J. P. J . Phys. Chem. 1992, 96, 1816. (10) Chen, Q.;Fraissard, J. P.J . Phys. Chem. 1992, 96, 1809. (1 1) The vinal coefficients were converted to a common basis using a value for the amagat, the density of an ideal gas at STP, of 2.69 X IOl9 atoms an-’. (12) Hunt, E. R.; Carr, H. Y . Phys. Reo. 1963, 130, 2302. (13) Smith, M. L.; Dybowski, C. J . Phys. Chem. 1991, 95,4942. (14) Torrey, H. C. Phys. Reu. 1963, 130, 2306. (15) Fraissard, J. Unpublished results (16) Derouane, E. G.; Fripiat, J. J. Zeolites 1985, 5, 165. , (17) Debra, G.; Gourgue, A,; B”agy, J.; de Clippeleir, G. Zeolites 1986, 6, 161. (18) Auroux, A.; Gravelle, P. C.; Vedrine, J. C.; Rekas, M. In Proceeding8 of the Fifrh International Conference on Zeolites; R m , L. V. C., Ed.; Heyden: Frankfurt, 1980; p 433. (19) Freude, D. Srud. Surt Sci. Catal. 1989, 52, 169.
Mixed Valence Oxide-Dispersion- Induced Micropore Filling of Supercritical NO Z.-M. Wang: T. Suzuki: N. Uekawa,t K. Asakura,t and K. Kaneko*vt Department of Chemistry, Faculty of Science, Chiba University, Yayoi, Chiba-shi, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, Tokyo, Japan (Received: May 11, 1992; In Final Form: September 23, 1992)
Ultrafine Ti(1V)-doped a-FeOOH particles were dispersed on the high-surface-area carbon fibers of uniform micropores in order to elucidate the role of the surface electronic factor in the micropore filling of supercritical NO. EXAFS and XANES showed that dispersed Ti-doped a-FeOOH particles are ultrafine and have the same local structure as that of bulk a-FeOOH particles. The microporosity of the samples did not change significantly by the doping of Ti(1V) in dispersed a-FeOOH on the basis of Nz adsorption. Also, the surface acidic site concentration measured by the irreversible NH3 adsorption was almost constant regardless of the Ti doping. However, doping of Ti(1V) in the dispersed a-FeOOH enhanced the micropore fdling of supercritical NO by 40%, at the maximum. The enhancement of the NO micropore filling by Ti doping was presumed to come from the increase of quasi-free electrons in the dispersed a-FeOOH fine particles due to mixed valence formation, which is enhanced by the surface segregation of Ti(1V) from the XPS examinations.
Introduction Pores of less than 2 nm in width are termed micropores according to IUPAC classification,’ although recent statistical mean-field theories predict that the critical width between the micropore and mesopore is 1.3-1.7 nmS2s3In a slit-shaped micropore, the surface potentials from the opposite pore walls are overlapped to enhance the molecular adsorption in low relative pressure; to enhance the molecular adsorption in low relative pressure; such enhanced adsorption is called micropore filling. As micropore filling is an enhanced physical adsorption, it is not a predominant proceap for a supercriticalgas (a gas above the critical temperature) but is for a vapor. Accordingly, great amounts of vapor molecules are sufficiently adsorbed even at low-pressure regions by microporous solids owing to the micropore-filling Zeolites, aluminophosphatcs, and activated carbon fidem (ACFs) are representative microporous solids. Zeolites and aluminophosphatesl2have cylindrical micropores inherent to their crystal structures, while ACFs have uniform slit-shaped micropores whose pore wall is composed of microcrystallites of graphite structure. ACFs have much greater micropore volume than zeolites and can provide great nanospace available for a specific reaction.I3 ‘Chiba University. *TheUniversity of Tokyo.
0022-3654/92/2096- 10917$03.00/0
Adsorption of supercritical gases by microporous solids has been studied by using a high-pressure adsorption techniq~e.’~ Supercritical CH, has especially gathered much attention and interest from both experimental and theoretical researcher~.~~-l’ For example, Gubbins and T a d 5 and Myers and KaraviasI2 have studied, theoretically, CH4adsorption on model carbon microw. Talu et al.” tried to correlate the high-pressure CH4 adsorption with their micropore structure of zeolites. Thus, studies on the adsorption of a supercritical gas are limited to high-pressure adsorption. There has not been sufficient research on the adsorption of a supercritical gas by microprous solids at subambient pressure regions. A N O gas near room temperature is a supercritical gas because its critical temperature is 180 K. NO cannot be adsorbed by microporous solids with the aid of a microporefilling mechanism, although development of a good adsorbent for NO is expected from the field of atmospheric environmental science. N O has an unpaired electron and tends to be dimerized in the micropore to exhibit diamagnetism.’* The dispersion of oxides or oxyhydroxidesof Fe(II1) with great magnetic moments due to their five unpaired electrons on ACF’s induced marked micropore filling of NO through the dimerization on the surface.I9 Application of an external magnetic field enhanced the NO micropore filling over ACF‘S.~O*~~ Not only the magnetic perturbation but also a weak chemisorptive mechanism should be associated with the enhancement of the NO micropore filling.22 0 1992 American Chemical Society
10918 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
Wang et al.
Hence, we can study the supercritical gas adsorption without the high-pressure measurement using the NO/iron oxide-dispersed ACF system. In our preceding study,23we reported that doping of Cu(I1) in the dispersed a-FeOOH on ACFs enhanced the NO micropore filling, which is attributed to the chemisorptive effect of the dispersed oxides. However, dispersion of the Cu-doped a-FeOOH changes the microporosity of ACFs, and the role of the dispersed a-FeOOH in the enhanced NO micropore filling was not elucidated. Doping of metal ions can control the chemisorptive properties due to the nonstoichiometric change. We reported the electrical and the surface chemical properties of the doped aFeOOH particles (not the dispersed one).24 Ti doping enhances the semiconductivity of n-type a-FeOOH and the NO chemisorption rate of a-FeOOH. Consequently, careful examination of Ti doping in the dispersed a-FeOOH on ACFs should elucidate the role of the dispersed a-FeOOH particles in the marked enhancement of the NO micropore filling, which should be helpful in the adsorption of other supercritical gases such as CH4. In this paper, the enhancement of supercritical NO by Ti doping in the dispersed a-FeOOH on ACFs clearly associated with the electrical properties of the bulk Ti-doped a-FeOOH particles, and a general description of supercritical gas adsorption is put forth.
Experimental Section Matealnls. Cellulose-based ACF's were used. The ACF surface was treated under synthetic conditions for the preparation of Ti-doped a-FeOOH with the same procedures reported in our previous work.23 The surface-modified ACF samples are designated as Ti(x)/Fe-ACF (x = 0-3). Here x is the atomic % of Ti/Fe of the starting Ti-Fe mixed ionic solution. The chemical composition of the Ti-doped a-FeOOH on ACFs was determined after dissolution in HCl solution using atomic absorption spectroscopy. A NO gas of 99.0% purity was used after successive vacuum distillations. An NH3 gas with a purity of 99.9% was used after drying with a mixture of ice and NaCI. Characterization. The adsorption isotherms of N2 on the surface-modified ACF samples were gravimetrically measured at 77 K after preevacuation at 383 K and 10 mPa for 2 h. The Fe K edge EXAFS and XANES spectra of the Ti(x)/Fe-ACF samples were measured by the use of the EXAFS facilities at BL7C of the Photon Factory in the National Laboratory for High Energy Physics (Tsukuba, Japan). The surface concentration of doped Ti ions in the dispersed iron hydroxide on ACF's was examined by the use of X-ray photoelectron spectroscopy (Shimazu ESCA-850) with a magnesium X-ray source. The X-ray diffraction of powdered Fe-ACF was measured with an automatic X-ray diffractometer. The adsorption rate for NH3 during 50 min was determined upon introducing 1 kPa of NH3 at 303 K. The concentration of acidic sites of these samples was obtained from the irreversible NH3 adsorption amount after evacuation at 10 mPa for 1 h. NO Admption. The adsorption isotherms of NO up to 80 kPa were measured gravimetrically at 303 K. The samples were preevacuated at 383 K below 10 mPa for 2 h. The irreversible amount of NO was measured after evacuation at 10 mPa and 303 K for about 12 h. Resdts and Discussion
Mixed Valence State of Ti-Doped a-FeOOH Dispersed on ACF's. Iron hydroxides of 7-12 wt % to ACF's were dispersed on ACF's. If synthetic a-FeOOH crystals of this amount are mixed with ACF's, we can determine the presence of a-FeOOH crystals by use of X-ray diffraction. However, there was no diffraction peak in a-FeOOH-dispersed ACFs due to a-FeOOH crystals other than the broad peaks of an ACF itself; the dispersed iron hydroxides on ACF's should be very fine. Figure 1 shows the phaseuncorrected Fe K edge EXAFS of Ti-doped a-FeOOH particles on the surface of ACF's. There are two main peaks: A and B. The A and B peaks are ascribed to the coordination of oxygen to the central Fe ion and the inter-Fe ion coordination, respectively. Basically, all spectra are similar to each other ir-
A
Ti/Fe: Ool0
Il
Ti/Fe : lolo
D i st ance/nm
Figure 1. Fourier transforms of the EXAFS oscillation for Ti(x)/FeACF samples.
I
7.1
7.3
7.2
Photon energy/ Kev
Figure 2. XANES of the Fe K edge of Ti(x)/Fe-ACF samples.
TABLE I: (a) h
I Structure Panmeters and (b) Electroaic Stat- of
Iron Oxides Ti/Fe
0 0.2 1 (a) Local Structures
3
a-FeOOH
position
peak A, nm peak B, nm intensity ratio of B to A
0.15 0.26 0.6
0.14
0.15
0.15
0.15
0.27
0.26 0.5
0.26 0.5
0.27
0.4
0.9
(b) Electronic States of Iron Oxides energy diff between A and B 18.4 18.3 18.5 18.3 absorption peaks, eV absorbance ratio of A to B 0.3 0.3 0.3 0.3
respective of the doping of the Ti ions. Table Ia shows the position of the A and B peaks and the intensity ratio of the B peak to the A peak. The position of both A and B peaks does not change by dispersion and Ti doping. However, the intensity ratio of the dispersed samples is much smaller than that of bulk a-FeOOH particles, indicating that the dispersed a-FeOOH is ultrafine. Thus, the Ti doping slightly lowered the intensity ratio. The Ti-doped iron oxides on the surfacc of ACFs have the same local structure as a-FeOOH bulk particlw, but they should be ultrafine. Figure 2 shows the XANES spectra of Ti-doped a-FeOOH dispersed on ACF's. All spectra have a strong peak [B] and a weak preedge peak [A], which come from the allowed l s 4 p transition and the dipole-forbidden 1s-3d transition under the Oh
The Journal of Physical Chemistry, Vol. 96, No. 26, I992 10919
Micropore Filling of Supercritical NO
A
01s
v) Y
C 0 3
0
0
1 2 a t o m3 i c % (Ti/FeIbulk,
4
Figure 5. Relationship between the bulk and surface Ti/Fe ratio. TABLE II: MicroPorositv of Ti(x)/FeACF Samples ~~
42
532 Bi ndi n g energy/ev
537
527
sample ACF Ti(O)/FeOH-ACF Ti( 0.2)/ FeOH-ACF Ti( l)/FeOH-ACF Ti(3) / FeOH-ACF
Figure 3. XPS spectra of ACF's and Ti(x)/Fe-ACF (0 1s).
T i/Fe ,Q l ~
Ti/Fe ,%
--
U ln
C 3
0
\ I "
0
J
31
I
721
Ti/Fe, Fe/ACF, a,, aScrt. W,, w, atom % wt B m2/g mi/g mL/g nm 0 0 1340 10 0.60 0.90 0 12.6 1575 14 0.65 0.83 0.3 7.1 1415 9 0.61 0.87 0.8 12.3 1376 5 0.60 0.88 3.6 7.8 1290 6 0.54 0.84
I
711
I 1 472
Binding energy
I
L 62
/
I
152
ev
Figure 4. XPS spectra of Ti(x)/FeACF. (a) Fezp, (b) Tizp
symmetry around a central Fe atom, respectively. The intensity of the preedge A peak is important, because it sensitively depends on the structural deviation of the Fe-(0,0H)6 complex from Oh symmetry. As d and p orbitals can be mixed under Td symmetry, the intensity of the prcedge A peak increases with deviation to Tdsymmetry. Table Ib shows the energy difference At?, between A and B peaks and the absorbance ratio of the A peak to the B peak. No significant changes of DAB and the absorbance ratio were observed. In the case of Ti(1V) doping in a-FeOOH dispersed on ACF's, the crystal structure of a-FeOOH does not severely change due to doping in accordance with the preceding work on the Tidoping effect of bulk a-FeOOH parti~les.~ Hence, a-FeOOH particles dispersed on ACF's should be ultrafine and have a F t ( 0 , 0 H ) 6 structure of slightly distorted symmetry on the ACFs, giving rise to the weak preedge absorption. Accordingly, Ti doping can control only the electronic property of the dispersed a-FeOOH fine particle without causing severe atomic structural changes. Although X-ray absorption spectroscopy cannot mention the state of doped Ti ion, XPS can provide important information. Figure 3 shows the 0 1s spectra of ACF's and Ti(x)/Fe-ACF. The 0 Is band of the iron oxide-dispersed ACF has the doublet structure (531.3 and 530.2 eV) whose peaks correspond to those of 02-and OH-, respectively. The band does not change with Ti doping. Only an ACF has a broad peak at 532.5 eV, which arises from the surface functional groups such as C 4 , COOH, etc.25 Thus, dispersed iron oxides should have the a-FeOOH structure irrespective of Ti doping. Figure 4, parts a and b, show Fe 2p and Ti 2p bands, rcspectively. The Fe 2p bands which have the same peaks as bulk a-FeOOH coincide with each other,
indicating that Ti doping does not change the a-FeOOH structure. On the other hand, the Ti 2p band depends on the doping amount. The peak position is close to that of a Ti(1V) ion surrounded by six oxygen ions.26 Consequently, the Ti(IV) ion occupies the lattice point of an Fe(II1) ion of the deviated Oh symmetry. The peak height is proportional to the Ti concentration of the synthetic solution. The surface concentrationof Ti(1V) to Fe(II1) is compared with the bulk concentration in Figure 5. The surface Ti(1V) concentration is 5 times greater than the bulk concentration. The Ti ions are segregated near the surface of the ultrafine a-FeOOH. Thus, the surface of the dispersed a-FeOOH should have greater quasi-free electrons due to efficient valence control. Microprosity. The N2 adsorption isotherms were of type I, which is indicative of the presence of uniform micropores. We observed slight changes in the saturated amount of N2 adsorption and in the rising points at low relative pressures. The decrease in the saturated adsorption with Ti doping suggests the partial blocking of micropores; gradual uptake in the low pressure indicates the blocking of micropores of smaller pore width due to Ti doping. The micropore structures were quantitatively evaluated by the a,plot of the N2 adsorption isotherms. Here the standard adsorption isotherm of N2 on nonporous carbon black was used for construction of the a,plots2' All a,plots were seen to bend in the a,range of 0.6-0.8. The total specific surface area a,, the external surface area as,ut,and the micropore volume W,were determined by the a,plots, which are collected in Table 11. The width w of the slit-shaped micropore was evaluated from the relationship of 2W,/(a, - a,,ex,),which is also shown in Table 11. Dispersion of a-FeOOH slightly narrowed the micropore width. Surface Acidity. Addition of Ti(1V) ions to Fe(II1) ionic oxides produces not only the mixed valence state but also the surface acidity. The excess positive charge due to the Ti ion possibly leads to Lewis-type acidic sites.28 The NH, adsorption examination provides important information on the surface acidity. The adsorption of NH3 molecules was almost finished within 10 min. We used the amount of NH3 adsorption at 1 min (the first measuring point) as the measure of the NH3 adsorption rate. The irreversible amount of NH3 adsorption was used to determine the concentration of the surface acidic sites. Figure 6 shows the changes in the amount of NH, adsorption at 1 min after NH, introduction and the surface acidic site concentration in mmol/m* with Ti doping. The concentration of the acidic sites is almost constant irrespective of Ti doping. On the other hand, the NH, adsorption rate increases by about 2 times with Ti doping. The Ti doping does not increase the acidic site but strengthens the acidity. But, possibly, a new Lewis acidic site by Ti doping is
10920 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
Wang et al.
0
1 2 3 Ti/Fe a t o m i c % FIgm 6. Changes in the NH3 adsorption rate and the numbers of acidic sites with Ti doping.
20
'
0
20
40
60
80
100
P(N0) / k P a Figure 8. Adsorption isotherms of NO on Ti(x)/Fc-ACF samples.
50 1601 7- 40 E
-. bo
E
7-
30
2 -
h
0
z
g
1LO-
E
\
20
$20-
10
0
0
100
200
300
400
Time / m i n Figure 7. NO adsorption rate. TABLE UI: Ratio of Irreversible Adsorption Amount of NO to W L and Initial hngmuir Rate Constants ( k , and k,) sample WJW,, % k,,P a d k,, s-I 61 5 6.33 X lo-* 6.81 X Ti(O)/FeOH-ACF 5.4 X 62 5 5.63 X Ti(l)/FeOH-ACF 4.2 X 58 5 6.70 X Ti(3)/FeOH-ACF
**
formed. As a NO molecule has an unpaired electron, it can be chemisorbed on the Lewis acidic site. However, the direct chemisorption of NO to the acidic sites cannot be a predominant factor of the adsorption of NO on the Ti(x)/FtACF samples, as will be described later. NO Adsorption Rate Cbmge. The initial adsorption rate of introducing 1.3 kPa of NO at 303 K as a function of Ti doping is shown in Figure 7. Ti doping of 1% enhances the initial adsorption rate by about 60%, although further doping does not greatly enhance the adsorption rate. There are several empirical or theoretical equations for adsorption kinetics.29 Equation 1 is the Langmuir adsorption rate equation. Here u and D, are the u = o,(l - e-(Pkl+W) (1) amounts of adsorption at a certaintime t and the saturated amount
of adsorption at the equilibrium pressure P,respectively. k, and k2are the adsorption and desorption rate constants, respectively. The adsorption rate process up until 100 min was well described by the Langmuir rate equation. Thus, the NO adsorption obeys the Langmuir equation as if NO molecules were chemisorbed. The value of k, and k2of the various samples are shown in Table 111. klis not seriously changed, but k2 decreases by Ti doping. Decreasing of the desorption rate means that adsorption of NO
0
1
2
3
Doped Ti/Fe,"h Figure 9. Saturated amount of NO adsorption WL(NO) in mg/mL as a function of the amount of Ti added.
molecules by the ACF surface is strengthened by Ti doping, which coincides with the concentration change of the strong surface acidic sites. Quasi-Free Electroa Induced NO Micropore Filllng. Figure 8 shows the adsorption isotherms of NO at 303 K. Ti doping raises the adsorption isotherm. As the saturated vapor pressure of NO at 303 K cannot be defined, the isotherms are described approximately by the Langmuir equation. The saturated amount of NO adsorption W,was determined by the Langmuir plot. The W Lvalues of Ti-doped a-FeOOH-dispersed ACF samples were greater than that of a-FeOOH-dispersed ACFs. The relationship between WLand the amount of Ti doped is shown in Figure 9; Ti doping increases markedly in WL.Ti doping of 3% enhances WLby about 40%. Table 111 shows the irreversible amount of NO adsorption W,,,. The ratio of W,,to WLis almost constant regardless of Ti doping. NO molecules of about 60?6 are irreversibly adsorbed. Thus, the adsorption of NO exhibits chemisorptive characteristics. However, evacuation at 1 pPa using an ultrahigh-vacuumsystem almost completely moved the adsorbed NO. Also, the preceding magnetic susceptibility'* and temperature-programmed desorption studid showed that the irreversible adsorption arises mainly from dimerization of NO molecules. Therefore, NO molecules are not chemisorbed but are strongly physisorbed by the micropore field. That is, NO molecules are adsorbed in the micropom by the micropore filling. The constant Wh/WLvalue suggests that the stability of the NO dimers formed in microporesis not affectedby Ti doping. The dispersed FeOOH particles should accelerate the dimerization to give rise to the marked micropore filling; Ti doping must be associated with the
n
Micropore Filling of Supercritical NO
g
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10921 TABLE IV: P (NO) and qs,,,=,/eof NO Adsorption for Ti(x)/FeACF%mples
TilFe, %
1.0-
P,. Wa
sample Ti(0)/ FeOH-AC F Ti(O.Z)/FeOH-ACF Ti( 1 )/FeOH-ACF Ti(3)/FeOH-ACF
z
I
5- 0.52.0
LO
a,
I-I
480 305 290 215
kJ.mo1-l 28 26 26 23
6.0
In P( NO/ k Pa)
Figure 10. Extended DR plots for Ti(x)/Fe-ACF samples.
acceleration of the dimerization. Electrical conductivity and FT-IR examinations showed that NO molecules are weakly chemisorbed on the a-FeOOH surface.30 If similar weak chemisorption of NO occurs on the dispersed ultrafine a-FeOOH at the entrance of the micropores, NO molecules weakly chemisorbed at the FeOOH dispersed on ACF’s should be changed into the dimer, which is stored in the micropore of the strong micropore field. Also in the case of NO chemisorption on bulk a-FeOOH particles, Ti doping in the a-FeOOH lattice creates a quasi-free electron according to the defect reaction published before;24a Ti4+ ion on the Fe lattice can create an Fez+ion, that is, the quasi-free electron, which causes the weak chemisorption of NO on the surface of bulk a-FeOOH particles. The same effect of Ti doping should be applied to the dispersed fine a-Fe00H. Consequently, dispersion of Ti-doped a-FeOOH on the ACF surface gives rise to weak chemisorption of NO, leading to the dimerization of NO; the NO dimers are stored in the micropores. Also, there is another possibility that the strong surface acidic sites are associated with the acceleration of the NO dimer formation. This is because the strong acidic site increases with Ti doping (in Figure 6), which is close to the WLchange with doped Ti. Thus, it should be noted that both the electronic conductivity increase and the surface acidic site increase due to Ti doping in the dispersed a-FeOOH cause the enhancement of NO micropore filling. C e n d Descriptionof Micropore Filling of Supercritical NO. When NO molecules near the mixed valence FeOOH on the ACF surface are dimerized to become a vapor, the adsorption process can be described by the ordinary micropore-filling mechanism. The micropore filling of vapor molecules can be well described by the Dubinin-Radushkevich (DR) e q u a t i ~ n : ~ ~ . ~ ~
W = Woexp(-c2/E2)
E = @Eo
(2)
where W is the amount of adsorption at a pressure P; Wois the micropore volume; e = R T In (Po/P), the adsorption potential; Po is the saturated vapor pressure; Eo is the characteristic adsorption energy; and @ is an affinity coefficient. However, we cannot apply the DR equation to the NO micropore filling because the saturated vapor pressure of the NO dimer in the micropore is not known. We can determine the saturated amount of NO adsorption, WL,which expresses the volume of the micropores whose surface field is strong enough to induce the micropore filling of NO. As supercritical gas molecules in the micropore of the sufficient surface field can be regarded as vapor molecules, we can extend the ordinary DR equation to supercritical NO, as expressed by eq 3. Here P,(NO) is the saturated vapor pressure [In (Wo(NO)/WJ]112 = (RT/@Eo)[lnP,(NO)
- In P(NO)] (3)
of quasi-NO vapor at temperature T and W is the adsorption amount of NO at pressure P. Also, the Eo value can be connected to the isosteric heat of adsorption according to eq 4. Here qsLBllle Qst.e=lle
@EO+ A H v
(4)
is the isosteric heat of adsorption at the fractional filling of l/e and A?Iv is the heat of vaporization of NO (1 3.8 kJ/mol) at 303 K. Hence, the extended DR plot, that is, the [In 1 WL/ W)] vs
“L Ti /Fe,%
11I
I
I. I
I. 1
I. 1
1.
’
P/P, (NO) Figure 11. Reduced adsorption isotherm of supercritical NO at 303 K.
In P plot, provides the P,(NO) and qwl e values. These extended DR plots are linear in the wide range (Figure 10); P,(NO) at 303 K and the qSt,#=[ values were determined from these linear plots, as shown in dable IV. The P, value is in the range of 215-480 kPa. Ti doping lowered the P, value so that the NO micropore filling was enhanced. The qst,B=lle values for all the samples are higher than the dissociation enthalpy (12-16 kJ/mol) of the NO dimer in the condensed bulk phase at a low temperature.[* Adsorbed NO dimers are stabilized by the enhanced intrapore potential field. If the extended DR equation is correctly applicable to the description of the micropore filling for supercritical NO, all adsorption data of different samples must be expressed by only one reduced adsorption isotherm of the abscissa of the relative pressure determined by eq 3. Figure 1 1 shows the reduced adsorption isotherm of NO vaporized in the micropore. Almost all the observed points form one curve. Thus, the extended DR equation can describe well the micropore filling of supercritical NO. Here, both the WLand the TWvalues which are inherent to the system are governed by the mcropore field strength. Good applicability of the extended DR equation to the micropore filling of supercritical NO is indicative that supercritical NO molecules are adsorbed accompanying a transition from supercritical gas to vapor by the micropore field. In principle, the extended DR equation can be generally applied to any other supercritical gases even in the case of high-pressure adsorption; the analysis with the extended DR equation leads to valuable information of qst,ell/r and P, having clear physical meaning. Acknowledgment. We thank Dr.A. Matsumoto for cooperative experiments of EXAFS and XANES. The financial support by the Science Research Grant and an overseas student scholarship from the Ministry of Education of the Japanese Government are greatly appreciated. R e t r y No. Ti, 7440-32-6; FeO(OH), 20344-49-4; NO, 10102-43-9.
References and Notes ( 1 ) 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. (2) Seaton, N. A.; Walton, J. P. R. B.; Quike, N. Carbon 1989,27,853. (3) Balbuena, P. B.; Gubbins, K. E. Fluid Phase Equilib. 1992. 76, 21. (4) Gregg, S.J.; Sing, K. S.W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982; Chapter 4.
10922
J. Phys. Chem. 1992,96, 10922-10928
( 5 ) Kaneko, K.; Katori, K.;Shimizu, K.; Shindo, N.; Maeda, T. J . Chem. Soc., Faraday Trans. I 1992,88,1305.
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1 ILL.
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NMR Study of Kaolinite. 1. *'SI, 27AI,and 'H Spectra Shigenobu Hayashi,* Takahiro Ueda, Kikuko Hayamizu, and Etsuo Akiba National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305, Japan (Received: May 26, 1992; In Final Form: September 1 , 1992)
We have measured z9Si,27Al,and IH NMR spectra of kaolinites with natural and synthetic origins, using high-resolution solid-state techniques such as the magic angle spinning and the multipulse sequences. The z9Sispectra demonstrate clearly that two inequivalent Si sites are present, and the origin of the line width has been discussed quantitatively. The line shape of the z7Alspectra is governed by the second-order quadrupole interaction and suggests the presence of two inequivalent A1 sites with different quadrupole coupling constants and asymmetry factors. The IH static spectra are broadened by the dipoldipole interactions between 'Hspins and between 'Hand 27Alspins, indicating that the hydrogen atoms in the hydroxyl groups are in a rigid lattice state at room temperature. The technique of combined rotation and multiple-pulse spectroscopy has been successfully applied to kaolinites. The 'Hchemical shift of the hydroxyl groups is 2.8 ppm from tetramethylsilane.
Introduction
TABLE I: Kaolinite Samplea Studied
Kaolinite, A14Si40,,(OH),, is a layered aluminosilicate with a dioctahedral 1:l layer structure consisting of an octahedral aluminum hydroxide sheet and a tetrahedral silica sheet. Numerous structure studies have been made using diffraction meth0ds.l Figure 1 shows the structure of kaolinite. Since a large single crystal cannot be obtained, powder techniques have been used inevitably in diffraction studies. The positions of non-hydmgm atoms have been well established, while the crystal structure including the hydrogen sites has not been well understood until mxntly.'J However, Akiba et aL3have recently studied a detailed crystal structure including the sites of hydrogen atoms by Rietveld analysis using neutron powder diffraction data of synthetic deuterated kaolinites. The structure they obtained is as follows: the space group is C1, a = 5.169 (1) A, b = 8.953 (2) A, c = 7.399 (1) A, a = 91.36 (1)". 9, = 104.72 (2)", and y = 89.96 (2)". In contrast to the diffraction techniques, NMR is suitable to study local structures, especially when high-resolution solid-state techniques are used. Many NMR studies on kaolinite have been reported so far.e1s They measured 29Siand z7Alhigh-resolution solid-state NMR spectra, and most of them dealt with thermal transformation from kaolinite to metakaolinite. Correlations between the NMR data and the local structures have rarely been discussed quantitatively. Furthermore, a 'H high-resolution solid-state NMR study has not been subjected for kablinite up to now, although the broad line shapes for static samples have been studied in In the present work, we have studied 29Si,27Al,and 'H NMR spectra using high-resolution solid-state techniques such as the magic angle spinning (MAS) and the multipulse sequences.
origin
sample natural kaolinite
N1 N2
N3 N4
N5 N6 synthesized kaolinite
s1
s2 a
Hinckley index.
Kanpaku, Japan API No. 9,U.S.A. Georgia, U.S.A. Hokone, Japan Kibushi, Japan Gairome, Japan H20, 290 O C H20,220 O C
content of HI" Fe20? (wt W ) 1.4 1.4
co.01
0.7
0.46
0.4 0.2 0.2
1.18 1.25
0.9
0.8
0.01
From ref 19.
Analyzing the spectra theoretically, correlations between the NMR data and the local structures are discussed quantitatively. Experimental Section Mnterials. Most kaolinite samples were kindly supplied by Dr. R. Miyawaki at the Government Industrial Research Institute, Nagoya, Japan. An API No. 9 standard kaolinite specimen was supplied by Dr. K. Kuroda at Waseda University, Japan. A total of eight samples were used in this work, as listed in Table 1. Six samples were natural, which were Kanpaku kaolin (called N l ) , API No. 9 standard kaolinite specimen (NZ), Georgia kaolin (N3), Hakone Oowakudani kaolin (N4), Kibushi clay (NS), and Gairome clay (N6). Two synthetic samples were used, which were synthesized according to the literaturez0at 290 "C (Sl) and 220 "C (S2).
0022-3654/92/2096-10922S03.00/00 1992 American Chemical Society
