677
Langmuir 1990, 6, 677-681
Surface Heterogeneity Studied by a Quasi-Equilibrium Gas Adsorption Procedure L. Michot, M. Francois, and J. M. Cases* Centre de Recherches sur La Valorisation des Minerais, U A 235, BP 40, 54501 Vandoeuvre Cedex, France Received March 30, 1989. I n Final Form: October 26, 1989
It is shown here that for the study of the surface heterogeneity and microporosity of mineral surfaces, the necessary enhancement in the resolution of the adsorption isotherms may quite satisfactorily be obtained by using the quasi-equilibrium procedure. The latter is applied in its volumetric version, where a slow, constant, and continuous flow of adsorbate is introduced into the adsorption cell. From the recording of the quasi-equilibrium pressures (here in the range from to 500 mbars) vs time, the adsorption isotherm is derived. Various tests were carried out for checking the validity of the results. Surface area values obtained for alumina by use of this setup are identical with those obtained by classical volumetric equipments. This quasi-equilibrium volumetric procedure yields the very first stages of the adsorption process and, thus, provides information about surface heterogeneity. In the case of a microporous sample: sepiolite, nitrogen, and argon adsorption isotherms show the high adsorption energy of the pores and the partial filling of the microporosity while the use of CO, at 0 and 20 "C shows the filling of all the structural microporosity, confirming the theoretical value found by Rautureau and Tchoubar.' This apparatus is suitable for samples with surface areas higher than 3 m'/g.
Introduction It is well-known that with most conventional volumet-
t h e interest of t h e continuous volumetric procedure when t h e results are exploited for low relative pressures.
ric equipment for nitrogen a n d argon adsorption, which use point b y point procedures, i t is difficult t o get as many equilibrium points as one would like in t h e initial steep p a r t of t h e adsorption isotherm: actually, a great part of t h e information about t h e surface heterogeneity of t h e adsorbents' a n d about t h e micropore filling steps is lost or o u t of reach. T h i s is why it seemed worthwhile t o draw the best, in this context, from t h e volumetric version of t h e quasi-equilibrium method d e ~ i s e d , ~ - ~ e ~ a l u a t e d ,a~n,d~more recently reviewedg by Rouquerol and Grillet. Until now, very little attention was paid t o t h e s t u d y of t h e influence of surface defects a n d textural properties of t h e adsorbents for low relative pressures. The apparatus was t h e n designed with t h e device proposed by Grillet e t al.7 For calibrating t h e apparatus a n d checking its performance, a well-known adsorbent (Alumina 90a) was studied. T h e n , a microporous adsorbent (~epiolite)'*'~ was studied in order t o demonstrate (1) Rautureau, M.; Tchoubar, C. Clays Clay Miner. 1976, 10,365. (2) Cases, J. M. Bull Mineral. 1979, 102,684. (3) Innes, W. B. Anal. Chem. 1951,23,759-763. (4) Schlosser, E. G. Chem.-Zng.-Tech. 1959,31, 799. (5) Lanee. K. R. J. Colloid Sci. 1963. 18. 65-72. (6) RoGuerol, J. In Thermochimie; Colloques Internationaux du C.N.R.S.: C.N.R.S. Paris, 1972; pp 537-545. (7) Grillet, Y.; Rouquerol, F.; Rouquerol, J. J. Chim. Phys. 1977, 74, ,on 113.
(8) Northrop, P. S.; Flagan, R. C.; Gavalas, G. R. Langmuir 1987,3, 300. (9) Rouquerol, J.; Rouquerol, F.; Grillet, Y.; Ward, R. J. In Characterization of Porous Solids; Unger, K. K., Rouquerol, J., Sing, K. S. W.; Kral, H., Eds.; Elsevier: Amsterdam, 1988; Vol. 39, p 67. (10) Delon, J. F.; Cases, J. M. J. Chim.Phys. 1970, 4, 662. (11) Fernandez Alvarez, T. In Compte-rendu de la reunion hispanobelga de Minerales de la Arcilla; Serratosa, J. M., Ed.; Consejo Superior de Investigaciones Cientificas: Madrid, 1970 p 202. (12) Fernandex Alvarez, T. Clays Miner. 1978,13,325. (13) Jimenez-Lopez, A.; Lopez-Gonzales, D.; Ramirez-Saenz, A.; Rodriquez-Reinoso, F.; Zurita-Herrera, L. Clays Miner. 1978, 13, 375. (14) Dandy, A. J.; Nadiye-Tabirruka, M. S . Clays Clay Miner. 1975, 23, 428. (15) Grillet, Y.; Cases, J. M.; PranGois, M.; Rouquerol, J.; Poirier, J. E. Clays Clay Miner. 1988, 36, 233.
0743-7463/90/2406-0677$02.50/0
Experimental Section Materials. Aluminum oxide 90a was supplied by Merck. It is made of 30-wm particles and is a poorly crystalline 7alumina. The sepiolite used in this study came from Vallecas (Spain) and was supplied by Tolsa S.A. It is a defibrated sepiolite (trade name Pangel) obtained by wet micronization. Its mineralogical purity was higher than 95%, and the equivalent spherical diameter of the particles was < l o wm (fiber aggregates). Sepiolite is a fibrous magnesium silicate whose formula may be written as Si,,Mg80, ,nH,O. Different studies using X-ray powder diffraction dataf6-'' or selected-area electron diffraction data' have shown that sepiolite is made up of talc-like layers arranged in long ribbons stuck together to form the fibers. A cross section as seen by the "lattice imaging" technique shows numerous (110)faces in which the talc-like layers are arranged in staggered rows separated by channels parallel to the fiber axis. These channels are referred to as structural micropores (size = 13.4 X 6.7 A) developing an intramicroporosity; larger pores having diameters of 20-200 A are also present between the fibers. They are referred to as interfiber rnicrop~res'~'~ and develop an intermicroporosity. Three different adsorbates were used for this study: nitrogen N50 (purity >99.999%), Alphagaz; argon N56 (purity >99.9996%), Alphagaz; and carbon dioxide N48 (purity >99.998%), Alphagaz. Aim of t h e Apparatus. This apparatus was designed for working at pressures from to 500 mbar, Le., relative pressures from to 0.6 for nitrogen and lo4 to 1 for argon. Thus, it is possible to study the first stages of adsorption and to determine as well the specific surface area. Principle. The adsorbate is introduced through a calibrated microleak at a slow enough flow rate to ensure equilibrium all along the adsorption isotherm. The flow rate can be adjusted by changing the pressure upstream of the microleak. Procedure. A scheme of the apparatus is presented in Fig(16) Nagy, B.; Bradley, W. F. Amer. Mineral. 1955,40,885. (17) Brauner, K.; Preisinger, A. Tshmarks Miner. Petr. Mitt. 1956, 6,120. (18) Brindley, G. W. Am. Mineral. 1959, 44, 495. (19) Rautureau, M.; Mifsud, A. Clay Miner. 1977,12, 309.
0 1990 American Chemical Society
Michot et al.
678 Langmuir, Val. 6, No. 3, 1990 Bell-Howell pressure gauge
+
10 LacuLM
2 p l
0
4 bars
-.-.-5 bars
10
- - - 6 h 1 ~
U
-6 h r s I
0
Figure 1. Scheme of the quasi-equilibrium adsorption volumetry apparatus. Table I. Experimental Conditions for Nitrogen Adsorption on Alumina 90a manometer Dressure. bar
flow rate: cm3/min
4 5 6 6, repeat
0.0502 0.0663 0.0940 0.0932
a
dead volumes: cm3 volume V , - V , cell volume 67.13 67.35 67.02 66.97
13.81 13.77 13.85 13.95
STP.
ure 1. The valves V,,,, V, , V, , V,,,, V,, V,, and V, are vacuum bellows valves (type Air Liquide Vic 4/20). The valve V, is a vacuum glass tap. Outgassing of the Samples. Prior to any experiment, the samples must be outgassed; this can be done directly on the device by opening the valve V, to vacuum with the valves V, and V, open and the valve V,,, closed. The adsorption cell is made of Pyrex glass and can be placed in an oven a t the desired temperature. The sample is weighed before and after outgassing. During outgassing, the vacuum is checked on a Pirani gauge (type PRL 10 K low-pressure gauge, head range 10-10-4 mbars) which is also used for recording in the low-pressure range during the adsorption experiment. The values obtained are classicaly around 5 X to mbar. Determination of the flow rate and of the dead volumes of the system was done according to the general procedure described in ref 7. As it is, the volumetric equipment exhibits the following performances: Flow-Rate Stability. The flow rate for an upstream pressure of 4 bar does not vary by more than 2 % for downstream pressures between lo-, and 400 mbar. Kinetics. During adsorption, equilibrium tests have been carried out. When isolating the system, the pressure was found to be stable for the flow rates used (between 3 and 6 bar upstream). Sensitivity. Tests proved that the apparatus gives good results only if a t least 15 m2 is contained in the cell. Due to the size of the adsorption cell, it can be considered that adsorbents with surface areas less than 3 m2/g cannot be treated by using this device. T h e r m a l Equilibrium. For the beginning of the isotherms, the time for thermal equilibrium of the sample in liquid nitrogen is extremely important. If the sample is not left long enough a t 77 K, the pressure increases too quickly a t the very beginning and can even decrease after. Sigmoidal patterns already described" are then obtained for pressures between and lo-' mbar. Some samples have to stay up to a minimum of 6 h in liquid nitrogen for these patterns to be suppressed and to ensure thermal equilibrium. In most experiments, the sample was left all night long in liquid nitrogen. D a t a Treatment. The isotherms are plotted either in the plane (PIP,, adsorbed volume per unit mass of adsorbent) or (20) Hobson, J. P. In The solid-gas interface; Alison Flood, E., Ed.; Marcel Dekker: New York 1967; Vol. 1, p 447.
.05
.
I
.IO
.
l
15
.
I
20
,
repeat 25 30 I
.
Relative pressure Figure 2. Adsorption isotherms of nitrogen a t 77 K on Alumina 90a outgassed a t 120 O C .
in the plane (In (P/Po), with Po being the saturating vapor pressure of the adsorbate and 0 the surface coverage, defined as the ratio of the adsorbed volume to the monolayer capacity obtained generally from the BET treatment. Thus, the saturating vapor pressure of nitrogen is obtained by a mercury manometer used as a nitrogen vapor pressure thermometer. For argon, the value of the saturating vapor pressure is obtained from the temperature of the bath by using the formula In Po = - A / T + B between 66 and 83 K where Po is the saturating vapor pressure of argon in pascals, T is the absolute temperature, A = 953.899, and B = 22.821.''
Results Alumina 90a. Four nitrogen adsorption isotherms have been determined on Alumina 90a after outgassing for 15 h at 120 " C under a vacuum of Torr. The purpose was to check the reproducibility of the results and the influence of the flow rate and to compare the results with those obtained by classical step by step volumetry. The experimental conditions and the results are presented in Table I and Figure 2. Table I shows that the determination of the dead volumes of the system is rather reproducible; Figure 2 exhibits the excellent reproducibility of the results. The flow rate does not seem to have any influence on the isotherm when it varies from 0.05 to 0.095 cm3/min. Table I1 presents the results obtained for the BET surface and the C constant both by a continuous procedure and by classical point by point e q ~ i p m e n t . ' ~ . ~ ~ Except for the C constant, which is slightly different, the reproducibility of the experiment is excellent. The average value for continuous measurements is 118.0 m2/ g; the error can be considered as less than f l % . Two explanations could be given to the differences observed concerning the C constant. (1) The pretreatment on the device presented here could be slightly more efficient for removing superficial impurities and then could increase the energy of the active sites on the surface. (2) The smaller value of C obtained by the point by point procedure could be explained also by the fact that this procedure gives only a few experimental points in the lowpressure region so that the accuracy of the calculation of "C" is lower than with the quasi-equilibrium procedure. Although in the case of alumina upstream pressures up to 6 bar seem to give good results, for safety reasons all (21) Cases, J. M.; Mutafshiev, B. Surf. Sci. 1968, 9, 57. (22) Pollack, J. L. Reu. Mod. Phys. 1964, 36, 748. (23) Thomas, F. These Doct. Etat es Sciences, Universitb Nancy I, 1987, 192 pages. (24) Thomas, F.; Bottero, J. Y.; Cases, J. M.; Grillet, Y. J. Chim. Phys. 1988, 85, 807.
Langmuir, Vol. 6, No. 3, 1990 619
Surface Heterogeneity Studied by Gas Adsorption
Table 11. BET Surface Area and C Constant for Nitrogen Adsorption on Alumina 90aa
volumetry range of relative pressure 0.05-0.20 BET surface, m2/g 117 C 59 a QEV = quasi-equilibrium volumetry. 1001
I
4-bar QEV 0.035-0.14 118 125
5-bar QEV 0.04-0.20 118.6
131
1
I
6-bar QEV 0.05-0.22 117.1 105
6-bar-repeat QEV 0.04-0.18 118.9 125
1.5 I
1
.
1
'
a
g
e
1.0
> 0
QI
0.5 c L
3
Ln
0
.05
.10
.15
.20
Relative pressure Figure 3. Adsorption isotherm of argon at 77.23 K on sepiolite outgassed at 25 "C.
-7
-9
-5
-3
-1
In(P/e 1
Figure 6. Adsorption isotherm of nitrogen at 77.21 K on sepiolite outgassed a t 25 "C in the plane (In (P/Po),e).
'
5
M
0
0.5
' 1
-5
0
a5
1
Surface coverage Q Surface coverage 0 Figure 7. Differential enthalpy of adsorption vs coverage for the sepiolite-argon (left) and sepiolite-nitrogen (right) systems at 77 K after outgassing a t 25 "C. 0-l!
-7
-9
-5 In (PIC J
-3
-1
Figure 4. Adsorption isotherm of argon a t 77.23 K on sepiolite outgassed at 25 "C in the plane (In (P/Po),e).
,
1001
.
I
rn
m
E
U
1
E 0
> -0
n L 0
z
Lot
i
2ot
i
0
.05
.10
.IS
.20
Relative pressure Figure 5. Adsorption isotherm of nitrogen a t 77.21 K on sepiolite outgassed a t 25 "C.
the experiments have been carried out with upstream pressures of 4 bar for nitrogen and 3 bar for Argon and CO,. Sepiolite. Argon and Nitrogen Adsorption. Before any experiment, the sepiolite was outgassed a t 25 "C for 24 h under a vacuum of Torr. Under these conditions, only the adsorbed water is outgassed, and the whole of the microporosity is still open. The isotherm obtained
for argon with a flow rate of 0.033 cm3/min is presented in Figure 3. This isotherm is typical of a microporous sample. The BET treatment gives an equivalent specific surface are of 301 m2/g (taking into account a crosssectional area of 13.8 for the argon molecule) and a C constant of 808. The word "equivalent" is of course used here to point out the partial inadequacy of the above calculation for a microporous solid in which the adsorbed molecules do not cover a t all the same surface area as on a flat surface.25 The adsorption isotherm was then plotted in the plane (In P/Po, @,0 being the surface coverage, because this type of representation gives access to the energetic heterogeneity of the surface.2.21 The use of surface coverage as defined here can seem inadequate in the case of microporous adsorbents. However, this parameter was chosen to be able to compare the curves with those obtained by low-temperature adsorption microcalorimetry. This isotherm exhibits three domains before the monolayer: Range 0: a vertical step for In ( P / P o )= -9.7 up to a coverage value of 0.40. All along that step the pressure above the sample as recorded by the Pirani pressure gauge was strictly constant. Range B: for -9.7 < In (PIP,) < -7.2, a second domain on which the isotherm is roughly linear and ends by a plateau for 0 = 0.65.
w2
( 2 5 ) Sing, K. S. W.; Everett, D. A.; Haul, R. A.; Moscou, L.; Pierotte, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1986,57,603.
Michot et al.
680 Langmuir, Vol. 6, No. 3, 1990 Table 111. Quasi-Equilibrium Adsorption Volumetry Results for Sepiolite Outgassed at 25 "C' nitrogen adsorption at 77.21 K argon adsorption at 77.23 K C 1300
range
Vi, cm3/g
m2/g 366
Stotal1
0.0582 0.0298 0.0880
a
d a+$
C
m2/g 117
sex,,
%tal*
809
Vi, cm3/g
m2/g
301
sex,,
m2/g
105
0.0406 0.0254 0.0660
a Stow total specific surface area; Vi micropore volume per unit mass of adsorbent as calculated with density of the liquid adsorptive; S,,,, external specific surface area.
Table IV. Low-Temperature Adsorption Microcalorimetry Results for Sepiolite Outgassed at 25 "C' nitrogen adsorption at 77 K argon adsorption at 77 K range
Vi, cm3/g
m2/g
%€,tal>
351
Sext,
0.0499 0.0312 0.0811
a
P a+B
S & t,
m2/g
123
m2/g
vi,
332
cm3/g
Sextt
m2/g
108
0.0560 0.0197 0.0757
StOm1 specific surface area; Vi, micropore volume per unit mass of adsorbent as calculated with density of the liquid adsorptive; Sext, external specific surface area. a
I
25r
I
I
1
1
t
I
I
10 log (P/Po)l
15
t cn
0.5
005
0
010
-'"O
020
015
Relative pressure
[
Figure 8. Adsorption isotherms of carbon dioxide at 273 and 293 K on sepiolite outgassed at 25 O C .
2 2.0
-5:cn
1.
I
1
05 00
\
-05 I
i
I
20
Figure 10. Dubinin plot for carbon dioxide adsorption at 20 "C on sepiolite.
Range a: a vertical step for In (P/Po)= -10.2 up to 0 = 0.45.
Range j3: a nearly horizontal part for -10.2 < In (P/ Po) < -8.4 followed by a roughly linear part for -8.4 < In (P/Po)< -7.2, ending up by a change of slope for 0 =
\;
10-
5
0.68. Range y: a roughly linear part for -7.2 < In (P/Po)< -3.6 up to the monolayer capacity. It is very interesting to compare the two curves obtained in the plane (In ( P / Po),0) with the curves obtained by low-temperature adsorption microcalorimetry on the same sample outgassed under the same condition^'^ (Figure 7). All the breaks described on the low-temperature adsorption microcalorimetry can be noticed on the adsorption isotherms. The domains are even better defined on the adsorption isotherm. It is possible to give the same interpretation to the existence of these different domains. The first part of the isotherm (range a ) describes the filling of the structural or intramicroporosity. Taking 0.808 and 1.427 for the values of the density of nitrogen and argon, respectively, this part corresponds to a liquid volume of 0.0582 and 0.0406 cm3/g for nitrogen and argon. The second part of the isotherm (range 6 ) describes the filling of the interfiber microporosity. It corresponds to liquid volumes of 0.0298 and 0.0254 cm3/g, respectively, for nitrogen and argon. The third part of the isotherm up to the completion of a monolayer (range y) corresponds to the adsorption on the external surfaces of fibers. The external surfaces as calculated by this part are 117 and 105 m2/g, respectively, for nitrogen and argon. This value
Surface Heterogeneity Studied by Gas Adsorption is very close to the value calculated from the size of the fibers, i.e., 120 m2/g.15 All these results are summarized in Tables I11 and IV. The fact that nitrogen fills more than argon could be explained by the difference in the relative pressures of the two gases at the beginning of the isotherm (Figures 4 and 6); thus, it is likely that more nitrogen has access to the pores. An other explanation could lie in the differences in molecular sizes and shapes (the small diameter of the nitrogen molecule is smaller than the diameter of the argon molecule). However, the first explanation is supported by the following fact. A nitrogen adsorption isotherm was carried out on sepiolite by using a flow rate of 0.09 cm3/min, i.e., starting the isotherm at a higher relative pressure value. In this case, the liquid volume corresponding to range a is equal to 0.0462. It is more difficult to explain the small discrepancies shown by the two methods (Tables I11 and IV) especially in the case of argon adsorption in range a. This point is still under discussion at the moment. In all cases, the theoretical value of the structural microporosity is never reached. The amount of range a corresponds to 23.5% and 16% of the theoretical microporosity for nitrogen and argon, respectively. Thus it seemed interesting to use carbon dioxide for studying more accurately the microporosity of sepiolite. Carbon Dioxide Adsorption. Carbon dioxide is used for microporous adsorbents because of its linear shape and because of its adsorption temperature, which corresponds to very low relative pressures. The use of the continuous device for CO, adsorption could yield more information. The curves obtained at 0 and 20 "C are presented in Figure 8. For studying the microporosity, Dubinin's equation26 was applied log v = log - D[log (P/Po)12 where V is the volume of gas adsorbed, V , is the volume of gas which once adsorbed is able to fill completely the micropores, and Po is the saturating vapor pressure (here, 3485.565 Pa a t 0 "C and 5727.809 Pa at 20 "C). V, may be converted into a liquid volume by using a density of 1.08 and 1.05 g/cm3 for liquid CO, a t 0 and 20 "C, respec-
vo
(26) Dubinin, M.M.Pure Appl. Chem. 1966, 10, 309.
Langrnuir, Vol. 6, No. 3, 1990 681 tively. The Dubinin plots for 0 and 20 "C are presented respectively in Figures 9 and 10. In a recent study already quoted,15the authors found, using a conventional point by point procedure, that CO, was filling 44% and 52% of the structural microporosity of sepiolite as calculated by Rautureau and Tchoubarl at 0 and 20 "C, respectively. The continuous volumetric equipment gives values of V , of 0.256 and 0.230 cm3/g, respectively. The value of the calculated structural microporosity is 0.2478 cm3/g. It was computed from the dimensions of the unit cell obtained by the lattice imaging technique, knowing the structural formula of the sepiolite used in this study. Thus this result means that with quasi-equilibrium volumetric equipment with a low flow rate it is possible to reach, using CO,, the total amount of the microporosity because the filling of microporosity requires very low relative pressures. Conclusions (1)Quasi-equilibrium devices equipped with pressure sensors that work a t low pressures enable one to study in satisfactory conditions the surface heterogeneity and the microporosity of adsorbents. (2) In the case of a microporous adsorbent, sepiolite, the method yields, similar to low-temperature adsorption calorimetry, three different domains before the monolayer: for very low relative pressures, an adsorption domain corresponding to the structural microporosity (intramicroporosity); for higher pressures, a domain corresponding to the interfiber microporosity; and then a domain corresponding to the adsorption on external surfaces until the completion of a monolayer. (3) In the case of ultramicroporous adsorbents, using gases like carbon dioxide allows one to work a t very low relative pressures, and the continuous method yields better information about the microporosity of adsorbents than the case of classical step by step procedures. Particularly in the case of sepiolite, the continuous device shows the filling of all the structural microporosity, i.e., 0.25 cm3/g, while step by step procedures showed the filling of only 52% of the structural microporosity. Acknowledgment. We thank Dr. J. E. Poirier and Dr. J. Rouquerol for helpful discussions and corrections of the manuscript.
