Langmuir 1987,3,357-363
357
ported here. The physisorption of CO on a Cu film has been reported4 at low temperature, -20 K. To our knowledge, no other experimental evidence for intrinsic CO precursor states on the other first-row transition metals exists." The transition of a molecule from a precursor state to a chemisorbed state is still a very interesting and open problem for surface scientists. The mechanism will only be understood by further experimental and theoretical studies. We are now beginning more accurate ab initio calculations on the precursor state and chemisorption barrier problems.ls
TI
V
Cr
Mn
Fe
CO
Ni
Cu
Figure 9. CO precursor well, AE2 (eV), and its corresponding distance R2for the parallel orientation. There is recent experimental evidence for a precursor state in the Ni/CO system. Molecular beam scattering experiments1have shown the precursor state in the Ni/CO system, and a model of CO adsorbed parallel to the Ni surface has been suggested. Low-temperature workfunction measurements2 also show an intrinsic precursor state on nickel with a binding energy similar to that re-
Summary The chemisorption barrier and the intrinsic precursor state can be qualitatively understood by simple molecular orbital considerations. The chemisorption barrier results mainly from the interaction of occupied CO orbitals with the metal. The intrinsic precursor state appears when CO is adsorbed with a parallel orientation at about 2.5-3.0 A. From our calculations, nickel appears to be the metal which results in the most stable intrinsic CO precursor state when compared to the other first-row metals. If CO precursor states are observed on metals on the left side of the periodic table, their attractive component is most likely due to dispersion terms, not a molecular orbital attraction. Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, Research Corporation, and the New Jersey Commission on Science and Technology for their support. Registry No. CO, 630-08-0; Ti, 7440-32-6;V, 7440-62-2;Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Co, 7440-48-4; Ni, 7440-02-0; CU, 7440-50-8.
Anomalous Micropore Filling of NO on a-FeOOH-Dispersed Activated Carbon Fiberst K. Kaneko Department of Chemistry, Faculty of Science, Chiba University, 1-33, Chiba 260, Japan Received November 20, 1986 The author tried to disperse ultrafine a-FeOOH particles on activated carbon fibers (ACF) of uniform micropores. The ACF was treated with Fe2(S04)3solution at 303 K and pH 13. X-ray diffraction and X-ray photoelectron spectroscopic data show that ultrafine a-FeOOH particles are formed on the ACF. Adsorption isotherms of NO at 303 K and nitrogen at 77 K on the a-FeOOH-dispersed ACF were compared to each other. Both isotherms for NO and nitrogen are of BDDTI type, indicating micropore filling of both gases on the a-FeOOH-dispersed ACF. The amounts of the NO filled in the micropores reach 1 0 4 5 % of those of nitrogen at 77 K, though the NO adsorption temperature is much higher than the boiling temperature of NO (121 K). Also temperature-programmeddesorption spectra and examination of DR plots for NO adsorption support the conclusion that NO is filled in the micropores even at 303 K.
Introduction Dubinin1-3 has been mainly responsible for developing the concept of micropore filling where adsorption of gases in the narrow pores is affected by overlap of the force fields from the opposite walls of pores; such pore filling is well described by the DR equation. Sing et aL4* have made extensive studies of the mechanism of micropore filling by Presented at the "Kiselev Memorial Symposium", 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W. Sing and R. A. Pierotti, Chairmen.
0743-7463/87/2403-0357$01.50/0
use of a preadsorption technique. The heterogeneity in the micropore structures has been studied by examining the DR Plots.'-'' Typical microporous adsorbents are (1) Dubinin, M. M. Chem. Reu. 1960, 60,235. sei.(2)1966, Bering, 21,B. 378. P.; Dubinin, M. M.; Serpinsky, V. V. J.Colloid Interface (3) Dubinin, M. M. Carbon 1985, 23, 373. (4) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982; Chapter 4. (5) Atkinson, D.; Mcleon, A. I.; Sing, K. S. W. J. Chim. Phys. Phys.-Chem. Bioi. 1984,81, 791. (6) Bohra, J. N.; Sing, K. S. Adsorpt. Sci. Technol. 1985, 2, 89.
0 1987 American Chemical Society
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358 Langmuir, Vol. 3, No. 3, 1987
activated carbons (AC). AC generally show very little selectivity in the adsorption of molecules of varying size, because they have not only micropores but also mesopores,12 but the molecular sieving carbons (MSC) with uniform diameters less than 10 8, have good selectivity for small m~lecules.'~-'~ Activated carbon fibers (ACF) have only micropores of a uniform diameter ranging from 15 to 30 8, and the good characteristic that they can adsorb organic molecules much faster than do the granular activated carbons.17 There is severe damage due to acidic pollutants, such as SO2and NOx.18J9 It is well-known that AC is a good adsorbent for SO2,but it is less effective for NO adsorption, although it has much greater adsorption capacity for NO than silica gels or zeolites.20-22The boiling temperatures of NO2 and SO2are 294 and 263 K, respectively, so they are condensable on microporous solids by pore filling even around room temperature. However, the boiling point of NO is so low (121 K) that one cannot fix NO on microporous solids by pore filling around room temperature. The generally used microporous adsorbents are effective for NO2 and SO2,but they are poor for NO. An effective adsorbent for NO is required to remove NO from atmospheric environments. As the chemical nature of the adsorbent wall is very important in micropore filling, we can expect that surface modification of the ACF by active substances will give rise to a special micropore filling which chemisorption partially assists. Our earlier s t ~ d i e showed s ~ ~ ~that ~ a-FeOOH has 10 times greater chemisorption activity (the adsorption amount per surface area) for NO than AC; a-FeOOH has a very suitable surface structure vhich can accept NO molecules due to the geometrical and electronic nature of the surface. Therefore we modified the ACF by dispersing fine a-FeOOH particles on it. We are submitting the report22that the modified ACF have much greater adsorption capacity for NO than the granular activated carbons and also are effective for SOz at 303 K. In this paper we describe the comparison of micropore filling behaviors of NO at 303 K with that of nitrogen at 77 K in the a-Fe00H-dispersed ACF.
(7) Marsh, H.; Rand, B. J. Colloid Interface Sci. 1970, 33, 101. (8) Masters, K. J.; McEnaney, B. J. Colloid Interface Sci. 1983, 95, 340. (9) Martin-Marinez, J. M.; Rodriguez-Reinoso,F.; Molina-Sabio, M.; McEnaney, B. Carbolt 1986,24, 255. (10) Dubinin, M. M.; Stoekli, H. F. J. Colloid Interface Sci. 1980, 75, 34. (11) Wojsz, R.; Rozwadwski, M. Carbon 1986,24, 225. (12) Juntgen, H. Carbon 1977,15, 273. (13) Adams, L. B.; Boucher, E. A,; Everett, D. H. Carbon 1970,8,761. (14) Kawwoe, K.; Kawai, T.; Eguchi, Y i Itoga, K. J. Chem. Eng. Jpn. 1974, 7, 168. (15) Bird, S. S.; Trimm, D. L. Carbon 1983,21, 177. (16) Barton, S. S.; Koresh, J. E. J.Chem. SOC., Faraday Trans. I 1983, 79,1147. (17) Matauo, T.; Ishizaki, N.; Fukuda, T. Sen'i To Kogyo 1977,33,204. (18) Munger, J. W.; Jacob, D. J.; Waldman, J. M.; Hoffmann, M. R. J . Geophys. Res. 1983,88,5109. (19) Stief-Tauch, H. P. In Pollutants and Their Ecotoxicological Significance;Nurnberg, H. W., Ed.; Wiley: Singapore, 1985; Chapter 3. (20) Ganz, S. N. Zh. Prikl. Khim. 1958, 31, 138. (21) Urano, K.; Tanigawa, N.; Masuda, T.; Kobayashi, Y. Nippon Kagaku Kaishi 1978, 303. (22) Kaneko, K.; Inouye, K., submitted for publication in Colloid
Table I. Binding Energies (eV) Obtained by X-ray Photoelectron Spectroscopy sample CY-CEL CY-CEL(OX) a-FeOOH 0-FeOOH y-FeOOH a-Fe203 y-FezO3 Fe30, ~ ~ 0 2 7
FeZ7
Fe38
018
93.8 94.0 94.0 93.4 93.8 93.6 93.6 93.2 92.5 90.9
530.5 530.8 530.7 530.6 530.2 529.8 530.0 530.2 530.2
Fezp,,* 711.4 711.5 711.6 710.9 711.4 711.0 711.0 708.3 709.5 706.9
Table 11. Adsorption Parameters Determined from Nitrogen Adsomtion surface area SBET, St, m2/9 m2/9 CEL 1400 1470 CY-CEL 1410 1530 CY-CEL(OX) 820 890 PAN 870 975 CY-PAN 810 940 CY-PAN(OX)475 530
pore volume
W&), WJDR), mL/g 0.61 0.62 0.35 0.38 0.35 0.20
mL/a 0.61 0.63 0.37 0.39 0.35 0.20
pore diam,
Eo? kJ/mol 8f1 4.5 9f2 4.5 9f1 4.8 8.5 f 1 5.1 7f1 6.2 9f2 5.8
A
Characteristic adsorption energy.
Experimental Section Preparation of Samples. We used cellulose (CEL) and polyacrylonitrile (PAN) based ACF. a-FeOOH was dispersed on the ACF by treating the ACF under the synthetic conditions for a-FeOOH, that is, by hydrolyzing 0.6 M ferric sulfate solution a t 303 K and p H 13.25 We call this sample a-ACF (a-CEL or a-PAN). a-ACF(ox) samples were obtained by treating the ACF preoxidized in 6 M HNO, a t 373 K, under the same conditions as a-ACF. The ACF treated in ferric sulfate solution was washed extensively with 2 L of distilled water per 1g of the sample and dried at 383 K in air. Characterization of Samples. The amount of Fe deposited on the ACF was determined by titrating Fe ions with a 0.01 N K2Crz07solution after elution of Fe ions by HCl solution. The X-ray diffraction patterns were obtained by a recording X-ray diffractometer (Rigaku Denki) a t 30 kV and 10 mA with a Fe target. Photoelectron spectra of the ACF samples and related iron oxides were obtained with the use of a Vacuum Generator ESCA LAB5 instrument with an aluminum X-ray source. The detailed measuring conditions are described elsewhere.26 Photoelectron binding energies were calibrated with respect to the Au 4f7/, binding energy (83.8 eV) of a gold film sublimed onto the sample. Adsorption. We measured adsorption isotherms of nitrogen a t 77 K gravimetrically by means of a quartz spring with a sensitivity of 5 x io3 mg/m. The adsorption isotherms of NO on the ACF samples were determined by the same method a t 288 and 303 K for NO pressures u p to 80 kPa. The amount of NO adsorption was measured 2-5 h after the introduction of NO gas. The samples were preevacuated a t 383 K and P a for 15 h before the nitrogen and NO adsorption experiments. No gas (Takachiho Kagaku) of 99.0% purity was used after purification by vacuum distillation. The nitrogen adsorption isotherms of the samples having preadsorbed NO a t 303 K and 80 kPa of NO were determined a t 77 K. Temperature-Programmed Desorption of NO. We obtained temperature-programmed desorption (TPD) spectra of NO adsorbed on the ACF samples at 303 K and 13 KPa of NO with and temperature the use of a mass filter (ULVAC, MSQ-150 CY) controller (Chino NP161) set a t a constant heating speed (10
Polym Sci.
(23) Hattori, T.; Kaneko, K.; Ishikawa, Tt.; Inouye, K. Nippon Kagaku Kaishi 1979, 423. (24) Kaneko, K.; Inouye, K. Polyhedron 1984,2, 223. (25) Kaneko, K.; Inouye, K. J. Chem. Tech. Biotech., in press.
(26) Seyama,H.; Soma, M. J. Chem. SOC.,Faraday Trans. I 1984,80, 237. (27) McIntyre, N. S.; Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (28) Harvey, D. T.; Linton, R. W. Anal. Chem. 1981,53, 1684.
Langmuir, Vol. 3, No. 3, 1987 359
Micropore Filling of NO on a-FeOOH-ACF 0.75 i
1
I
0
CCFl
IO
5 t
15
CK,
Figure 1. &Plotsfor nitrogen adsorption on a-FeOOH-dispersed
CEL.
K/min). Prior to the TPD experiment, NO gas in the gas phase was removed by evacuation at 303 K for 10 min.
5
IO
1
MICROFORE DIAMETER 2r
Figure 2. Micropore size distributionof a-FeOOH-dispersedCEL from the t-plots.
Results Substances Dispersed on the ACF. X-ray diffraction patterns of the treated ACF have only a very broad peak due to the (002) face of graphite; the substances on the ACF must consist of ultrafine particles or amorphous materials. The photoelectron binding energies of a-CEL, a-CEL(ox), CEL, and related iron oxides are shown in Table I. The observed binding energies of a-FeOOH are in good agreement with the values in the l i t e r a t ~ r e . ~ ' * ~ ~ The binding energies of FeOOH are clearly different from those of a-Fe203,y-Fe2O3, Fe304,and FeO. Careful examination of the binding energies for each FeOOH sample shows that there is a difference among three FeOOH forms. I The binding energies of a- and a-CEL(ox) are almost identical with those of a-FeOOH, consequently we con5.0 0 10 20 clude that the deposits on the ACF are not amorphous solids but ultrafine a-FeOOH particles; we regard these In* E / P samples as a-FeOOH-dispersed ACF. Figure 3. DR plots for the nitrogen adsorption on the aThe surface concentration of a-FeOOH on the ACF FeOOH-dispersed CEL. determined from the amount of the deposited iron and specific surface area mentioned below. Table I1 shows the The micropore size distributions obtained from the tamount of Fe deposited and the surface concentration of plots by the MP method30 are shown in Figure 2. The a-FeOOH. There is 0.2-0.7 FeOOH unit per 100 A2 of the micropores of all samples are very uniform, as shown in ACF surface, so the a-FeOOH particles are highly disTable 11, and their sizes are less than 10 A. We also obpersed on the ACF. tained the pore size distribution from the method of Nitrogen Adsorption. The adsorption isotherms of Dollimore and Healtl which gave us about 2 times larger nitrogen at 77 K are of type I in the BDDT classification. pore diameters than the MP method. (The pore diameter The specific surface area SBET of each sample was determentioned in the Introduction is the most probable value mined from the BET plot; Table I1 summarizes the SBET determined from the pore size distribution.) The pore values. The t-plots from nitrogen adsorption on the diameter from the MP method will be used later, because modified CEL samples are shown in Figure 1. In the the pore size distribution arising from the Kelvin equation construction of these plots, we used the value of the is not suitable to micropore systems.32 Table I1 indicates thickness of the adsorbed nitrogen layer, t , as given by that the surface modification does not change the microAll t-plots pass through the origin Broekhoff and pore size by much. in the lower t region and then bend at around 4 A to Figure 3 shows DR plots for the adsorption of nitrogen become nearly parallel to the abscissa. The specific surface at 77 K on the CEL samples. We see good linear relaarea S, was obtained from the slope of each t-plots near tionships which are expressed by the following DR equathe origin. Also the pore volume, Wo(t), was determined tion: from the amount of adsorption corresponding to the W = Woexp(-c2/Eo2) (1) crossing point between the extrapolated linear branch from where W and Wo are the amount of adsorption and the the higher t range and the line passing through the origin. S , and Wo(t)values are also shown in Table 11. (29)Broekhoff, J. C.;Linsen, B. G. In Physical and Chemical Aspects
of Adsorbents and Catalysts; Linsen,
1970;p 33.
B. G., Ed.; Academic: London,
(30)Mikhail, R.S.;Brunauer, S.; Boder, E. E. J. Colloid Interface Sci. 1968, 26, 45. (31)Dollimore, D.;Heal, G. R. J. Appl. Chem. 1964,14, 109. (32) Sing, K.S. W. In Characterization of Powder Surfaces; Parfitt, G. G., Sing, K. S. W., Eds.; Academic: London, 1976;p 44.
Kaneko
360 Langmuir, Vol. 3, No. 3, 1987 I
I
288,303K Y
n ul
- 0
20
0
40
60
TIME (min) - 0
0
40
80
Figure 6. Changes in the NO adsorption on a-ACF(ox) upon introducing 4 kPa of NO at 288 and 303 K.
NO PRESSURE (kPa)
Figure 4. Adsorption isotherms of NO on a-FeOOH-dispersed CEL at 303 K.
,-. 300
-0
r-----l
r 0
-0
40
0
NO PRESSURE ( k P a )
Figure 5. Adsorption isotherms of NO on a-FeOOH-dispersed PAN at 303 K.
limiting volume of the adsorption space (Wois the micropore volume, Wo(DR), E = RT In Po/P,the adsorption potential, Pois the saturated vapor pressure, and Eo is the characteristic adsorption energy). Wo(DR)was obtained by the extrapolation of the DR plot and Eo was determined from ita slope. Table I1 summarizesadsorption parameters as determined from nitrogen adsorption. According to this table, SBET and S , almost agree with each other, as do Wo(t)and Wo(DR). NO Adsorption. Figure 4 shows adsorption isotherms of NO on the CEL samples at 303 K. The ordinate is expressed in terms of micrograms per meter squared of the sample. The dispersion of a-FeOOH enhances the NO adsorption activity of the CEL; the highest NO adsorption activity of a-CEL(ox) is very clearly shown. It is noteworthy that remarkable hystereses were observed for all isotherms. Figure 5 gives adsorption isotherms of NO on the PAN samples at 303 K. a-PAN(ox) has the greatest adsorption activity and marked hysteresis. The rate of NO adsorption on a-ACF(ox)on introducing 4 kPa of NO at 288 and 303 K are shown in Figure 6. The adsorption rate by a-CEL(ox) is independent of the measuring temperature. On the other hand the adsorption rate on a-PAN(ox) at 288 K is larger than that at 303 K. Figure 7 illustrates NO adsorption isotherms of a-ACF(ox) at 288 and 303 K. The amount of NO adsorption for a-CEL(ox) at 288 K is slightly larger than that at 303 K.
80
40
80
NO PRESSURE ( k P d
Figure 7. Adsorption isotherms of NO on a-ACF(ox)at 288 and 303 K. BEFORE
PRE-ADSOR BE D NO
2 00
\1
..........
2 loo!-
U
"
U
"
-
Q
-
AFTER -0
0
0.2
0.4
0.6
0.8
1
P / Po Figure 8. Adsorption isotherms of nitrogen at 77 K on a-CEL(ox) before and after preadsorption of NO at 303 K.
In the case of a-PAN(ox), NO adsorption at 288 K is much greater than that at 303 K. Nitrogen Adsorption after NO Preadsorption. Figure 8 shows the adsorption isotherms for nitrogen at 77 K on a-CEL(ox) before and after adsorption of NO at 303 K. On preadsorption of NO at 303 K in the amount indicated by the line with arrows in Figure 8, the amount
Micropore Filling of NO on a-FeOOH-ACF
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2001
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Langmuir, Vol. 3, No. 3, 1987 361
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BEFORE
WE-ADSORBED NO
n
0
0
0.2
0.4
0.6
0.8
1.0
0
100
Table 111. Comparison of NO and Nitrogen Adsorption and Density of Adsorbed NO N2 filled NO filled density of at 77 K, at 303 K, [NO]/ adsorbed NO, mg/g mg/g % g/cm3 CEL 510 35 6 0.13 CEL(ox) 200 58 30 wCEL 515 90 16 a-CEL(ox) 295 160 50 0.53 PAN 310 110 32 0.42 a-PAN 290 90 28 a-PAN(ox) 170 120 65 0.39 cod-based AC 395 25 6 coconut-based AC 310 30 10 9"
MSC-5
180
57
32
of nitrogen adsorption significantly decreased to give a much lower lying isotherm. The difference between amounts of nitrogen adsorption before and after the preadsorption of NO does not agree with the amount of preadsorbed NO but is larger than it. Also Figure 9 gives the adsorption isotherms of nitrogen at 77 K on a-PAN(ox) before and after preadsorption of NO at 303 K. The change in nitrogen adsorption due to the NO preadsorption is now less. We can get the density of the NO adsorbed layer from the difference between the amounts of nitrogen adsorption before and after the NO preadsorption and the amount of the preadsorbed NO, as summarized in Table
111. TPD Spectra. TPD spectra of a-CEL samples with and without adsorbed NO are shown in Figure 10. TPD spectra of a-CEL without adsorbed NO are shown as dotted (CO,) and broken (N,) lines. The evolved gases from a-CEL with adsorbed NO consist of NO, C02, and nitrogen. We could detect no NO2 in any of the TPD spectra. There is a single NO peak around 425 K. These spectra indicate that the adsorbed species is not NO2 but NO. Discussion Comparison of NO Adsorption at 303 K and Nitrogen Adsorption at 77 K. Comparing the amount of NO adsorption at 303 K with that of nitrogen adsorption at 77 K in Table Ill leads to an indication of the anomalously high NO adsorption activity of the modified ACF. The observed values for coal- and coconut-shell-based AC and MSC(5A) also are described in Table 111. The percentage of the NO adsorption against nitrogen adsorption in terms of the number of molecules is very important. The percentages for a-CEL(ox) and a-PAN(ox) go over 50%,while the percentages of CEL and coal- and coconut-shell-based AC are less than 10%. This adsorption behavior of aACF(ox) is anomalous, considering that the boiling and
400
TEMPERATURE ( " C )
p / P,
F'igure 9. Adsorption isotherms of nitrogen at 77 K on a-PAN(ox) before and after preadsorption of NO at 303 K.
300
200
Figure 10. TPD spectra of a-CELwith and without adsorbed N O with NO (0,A, 0 ) ;without NO (dotted and broken lines). 5
h
m m
\
E
4
0
z? -
3
d-C EL
\ r CE L
L
2 O
5
10
I"2polP
Figure 11. Pseudo-DR plots for the NO adsorption isotherms of a-FeOOH-dispersed CEL at 303 K.
critical temperatures of NO are 121 and 180 K, respectively.% As the outer surface of modified ACF is less than 20 m2/g from the t-plot, the majority of sites should be in micropores. Almost all NO molecules must be adsorbed in the micropores and occupy most of the space of micropores in the same way as with nitrogen adsorption. However, the apparent density of the adsorbed NO layer is at best 0.5 g/cm3 (see Table 111) and is much smaller than the density value (1.27 g/cm3) of the bulk liquid NO at 120 K.33 Pseudo-DR Plot for the NO Adsorption. NO is assumed to be adsorbed by a kind of micropore-filling mechanism. If the saturated vapor pressure of NO in the micropores is that of bulk liquid at the boiling point, we can obtain the DR plots from the NO adsorption isotherms at 303 K. This plot will hereafter be called a pseudo-DR plot. Figure 11 illustrates the pseudo-DR plots. As the plots are almost linear, the pore volume and the characteristic adsorption energy for NO were determined by use of eq 1. Since the pore volume for NO is almost identical with the saturated amount of adsorption obtained from the Langmuir plot, the pseudo-DR plot seems to be reasonable. Isosteric Heat of Adsorption for NO and Nitrogen. The isosteric heat of adsorntion qst is given by -d -In P - -Qat (2) dT RP ~
~~
~~~
~
(33) Handbook of Chemistry and Physics, 54th ed.; CRC-Express: New York, 1973-1974; B-115, F-76.
Kaneko
362 Langmuir, Vol. 3, No. 3, 1987 Table IV. Pore Volume for NO and Isosteric Heat of Adsorption satd amt of pore vol for NO (9st)e-lle9* NO adsorptn, from DR plot, kJ/mol mL/f mL/g NO nitrogen 0.040 23 10 CEL 0.028 26 10 0.071 0.069 CY-CEL 0.123 25 10 CY-CEL(OX) 0.128 26 11 0.087 0.087 PAN 0.071 25 12 CY-PAN 0.070 0.090 27 11 CX-PAN(OX) 0.095 OThe density of liquid NO in micropores is assumed to be that of bulk liquid at the boiling temperature. *Isosteric heat of adsorption.
The heat of.vaporization H,is related with the saturated vapor pressure Po at T through eq 3. When W /Wo = 6' d In Po H , -=(3) dT RP equals to l / e in eq 1, t = Eo = RT In Po/P. We get, therefore, the relationship (4) (qst)e=l/e = Hv + EO The isosteric heat of adsorption at the coverage of l / e (about 0.4) can be determined from eq 4. Table IV shows qst for NO at 6' = l / e by using the H,value (13.8 kJ/mol)% for bulk liquid NO at 121 K and E,(NO) from the pseudo-DR plot. Also (qst)e=lle for nitrogen adsorption determined by the same equation is written in Table IV for comparison. The (qat)s=l,evalues for nitrogen obtained here are close to the literature values of (qst)e=0.5 for carbon blacks;35 the enhancement of the adsorption energy for these ACF samples is thus not large. The qat values for NO are about 2 times larger than those of nitrogen; the adsorbent-adsorbate and/or interadsorbate interactions in the NO adsorption should be greater than those for the nitrogen adsorption. Model of Micropore Filling of NO. It is generally accepted that micropores in activated carbons are slitlike.% The decrease in the nitrogen adsorption after preadsorbing NO (Figures 8 and 9) indicates that fine a-FeOOH particles are deposited near the entrance of the slitlike pores and most NO molecules are first adsorbed around aFeOOH particles. The maximum thickness of the aFeOOH layer is calculated to be 2-9 FeOOH units with the assumption that all a-FeOOH particles are deposited on only the outer surface. However, in real systems, aFeOOH particles are highly dispersed not only on the outer surface but also on the pore walls, since the pore size is not much changed by the modifications, as shown in Table 11. According to our previous s t ~ d i e s , 2 ~although 9~~ an electron is transferred from the a-FeOOH surface to the adsorbed NO on chemisorption, almost all chemisorbed NO molecules are desorbed in the form of NO. The adsorbed species of NO on the modified ACF is only in the NO form, judging from the TPD spectra; NO is not oxidized to NO2 and does not react with the surface. The literature3'BS indicates that NO molecules form a dimer, (NO),, for which the formation energy is about 11 kJ/mol at 125 K. The difference between the isosteric heats of NO and nitrogen adsorption described in Table (34) Kagaku Binran; Maruzen: Tokyo, 1975; p 933. (35) Sing, K. S. W. In Characterization of Powder Surface; Parfitt, G. G., Sing, K.S. W., Eds.; Academic: London, 1976; p 19. (36) Fryer, J. R. In Characterization of Porous Solids; Gregg, S. J., Sing, K. S. W., Stoekli, H. F., Eds.; Society of Chemistry and Industry: Loidon, 1979, p 41. (37) Dinerman, C. E.; Ewing, G. E. J. Chem. Phys. 1971, 54, 3660. (38) Enault, A.; Larher, Y. Surf. Sci. 1977, 62, 233.
Figure 12. Schematic diagram of the NO micropore filling on a-Fe00H-dispersed ACF.
IV is almost equal to the formation energy of (NO),. Therefore NO molecules filling the micropores probably exist in the form of dimers even at 303 K. The formation of NO dimers in the micropores should inhibit desorption of NO from pores at around room temperature and bring about the marked hystereses observed in all the NO isotherms. The author proposes a model of chemisorption-assisted NO micropore filling, as illustrated in Figure 12. The diameter of the pore is about 9 A, 3 times larger than the short axis of an NO molecule. a-FeOOH deposited near the entrance can rapidly chemisorb NO molecules and the adsorbed NO moves into the pore to form the NO dimer; this may be a special example of the enhanced adsorbate-adsorbate interactions proposed by Sing et al.5 Almost all NO molecules are adsorbed in the pore near the entrance. The NO molecules chemisorbed on a-FeOOH at the opposite sides of the wall hinder desorption of NO molecules in the pore as a steric effect; not only the dimer formation but also this steric effect should give rise to hystereses. Thus, fine a-FeOOH particles assist the NO micropore filling at around room temperature through their chemisorptive action. Motchida et al.39 reported that PAN-ACF is a good catalyst for the reaction of NO with NH,. The high NO adsorption activity of the original PAN should be related to the above catalytic activity; this may be due to the surface polarity owing to the presence of nitrogen atoms in carbon chains. In the modified PAN samples not only the a-FeOOH particles present but also the surface polarity must accelerate the dimer formation to cause extensive NO micropore filling. We found a clear difference between the a-FeOOH-dispersed CEL and PAN samples in the temperature dependence of NO adsorption (Figures 6 and 7). These results suggest that the chemisorptive action in the CEL systems is more dominant than in the PAN ones, and the effect of the presence of a-FeOOH particles is greater than that of the polar C-N bonds on the NO adsorption activity.
Conclusions We have prepared an a-FeOOH highly dispersed ACF which has a high adsorption capacity for NO molecules around room temperature. The adsorption of NO at 303 K is similar to that of nitrogen at 77 K, being of the BDDTI type. The adsorbed species of NO is not NO2but (39) Komatsubara, Y.; Ida, S.; Fujitsu, H.; Mochida, I. Fuel 1984,63, 1739.
Langmuir 1987, 3, 363-368
NO from the TPD experiment. The DR plot is applicable to the NO adsorption isotherms. These observations lead to the conclusion that NO is adsorbed by a kind of micropore-filling mechanism; the fine a-FeOOH particles on the ACF assist the micropore filling of NO through their chemisorptive action. Acknowledgment. This work was partly supported by
363
the Grant in Aid for Fundamental Scientific Research from the Ministry of Education. Special thanks are given to Professor emeritus Katsuya Inouye for his comments and Dr. Sumio Ozeki for discussion, Dr. Mitsuyuki Soma for measuring photoelectron spectroscopic data, and Professor Yasuhiro Iwasawa for help in the TPD experiments. Registry No. FeO(OH),20344-49-4; NO, 10102-43-9.
Irreversible Adsorption from Solution. 2. Barium Dinonylnaphthalenesulfonate on Anatase? Mary E. Zawadzki, Y. Harel, and Arthur W. Adamson* Department of Chemistry, University of Southern California, Los Angeles, California 90089-1062 Received October 20, 1986 Barium dinonylnaphthalenesulfonate, BaDNNS, adsorbs on Ti02 anatase from n-heptane to give Langmuirian adsorption isotherms; adsorption is slow, and three or more hours are needed for final equilibration. Isotherms for intermediate times of adsorption are also Langmuirian and similar except for the case of adsorption time limited to 3 min-this isotherm, while Langmuirian, is of different shape and cuts the other isotherms. The temperature dependence of the “equilibrium”adsorption gives a van’t Hoff adsorption enthalpy of 7-12 kcal mol-’, while direct calorimetric measurement gives AH = -15 5 kcal mol-’, or of opposite sign. The adsorption is essentially reversible up to about a 2-h equilibration time: dilution with solvent gives equilibrium isotherm points. If the age of the adsorbed state is 12 h, desorption is now irreversible on washing with n-heptane but is readily reversed with more polar solvents. The adsorption vs. time data show a maximum in the amount adsorbed at about 10 min; also, desorption rates depend on the age of the adsorbed state, being faster the smaller the age. It appears that two types of adsorbed states are present: the initially formed state which adsorbs reversibly and a second state which forms slowly from the first and which is irreversibly bound to the Ti02. The discrepancy between isosteric and calorimetric heats of adsorption is noted as caution against using the former unless the adsorption is known to be in dynamic equilibrium.
*
Introduction We continue here a study of the phenomenon of irreversible adsorption from solution.’ The typical situation to which we refer is the following. If an adsorbent is equilibrated with solutions of increasing concentration, the analytical amount of adsorption is conventionally determined by the drop in solution concentration. Workup of the data typically produces a well-behaved analytical adsorption isotherm, often of the Langmuir type in that a smooth rise to a limiting amount adsorbed occurs as the solution concentration is increased. If, now, the “equilibrium” solution is removed and replaced by the pure solvent, little or, in many cases, essentially no desorption occurs. There is no experimentally obvious reason for such irreversibility-adsorption energies are usually not large for such systems, either as calculated from the temperature dependence of the adsorption isotherms or from direct calorimetric data. Moreover, the adsorbed material is easily displaced with the use of a better solvent or by the addition to the solution of a more strongly adsorbing solute. Polymer adsorption is well-known to show the above type of irreversibility, and there has been some study of the effect.2 Stuart and co-workers3 have suggested that Presented at the “Kiselev Memorial Symposium”, 60th Colloid and Surface Science Symposium, Atlanta, GA,June 15-18,1986; K. S. W.Sing and R. A. Pierotti, Chairmen.
0743-7463/87/2403-0363$01.50/0
irreversibility in polymer adsorption is essentially artifactual, being due to polymer polydispersity. This type of explanation does not seem important, however, in various recent studies involving both natural and synthetic In the case of the adsorption of plasma proteins on polymer surfaces, for example, desorption was relatively fast if the adsorbate was freshly adsorbed but became very slow if the period of adsorption was long. Moreover, what then could be desorbed was now denatured.’ In the case of polymer adsorption, the attractive explanation for irreversibility is that multiple points of attachment develop, the complete severing of which is rel(1) Zawadzki, M.; Adamson, A. W., Engineering Foundation, Fundamentals of Adsorption, in press. (2) (a) Adamson, A. W. T h e Physical Chemistry of Surfaces, 4th ed.; Wiley: New York, 1982. (b) Lipatove, Yu. S.; Sergeeva,L. M. Adsorption of Polymers; translated by Kondor, R., Wiley: New York, 1974 (a review
more of adsorption rather than desorption behavior). (c) Eirich, F. R. In Interface Conuersion for Polymer Coatings; Weiss, P., Chewer, G. D., Eds.; American Elsevier: New York, 1968. Id) Stromberg, R.R.; Grant, W. H.; Passaglia, E. J. Res. Natl. Bur. Stand. (V. S.) 1984,68A,391. (3) Stuart, M. A. C.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Polym. Sci. 1980,18,559. Stuart, M. A. C.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982,90,310. (4)Penners, G.;Priel, Z.; Silberberg, A. J.Colloid Interface Sci. 1981, 80,437. (5) de Bruin, H.G.; Van Om,C. J.; Absolom, D. R. J . Colloid Interface Sci. 1980, 76, 254. (6) Gramain, Ph.; Myard, Ph. J. Colloid Interface Sci. 1981,84,114. (7) Soderquist, M. E.; Walton, A. G. J.Colloid Interface Sci. 1980,75, 386.
0 1987 American Chemical Society
