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Chemistry of carbon surface and mechanism of water molecule adsorption
Colloids and Surfaces A: Physicochemical and Engineering Aspects 101 (1995) 227 232
ELSEVIER
COLLOIDS AND SURFACES
A
Chemistry of carbon surface and mechanism of water molecule adsorption Ruben Sh. Vartapetyan a,, Albert M. Voloshchuk a, Armenak A. Isirikyan a, Nikolay S. Polyakov a, Yury I. Tarasevich b a Institute of Physical Chemistry of Russian Academy of Sciences, 31 Lenin Avenue, 117915 Moskow, Russia b Institute of Colloid and Water Chemistry of Ukraine Academy of Sciences, 42 Vernadsky Avenue, 250680 Kiev, Ukraine Received 20 October 1994; accepted 29 March 1995
Abstract The experimental adsorption isotherms of water vapour and nitrogen on nonporous carbon adsorbents with various specific surfaces and concentrations of primary adsorption centres (PACs) have been measured. The adsorption of water molecules ends with the formation of either individual isolated clusters in equilibrium with saturated vapour or a continuous adsorption film depending on the concentration of the PACs. In the former case, the pure differential heats of adsorption are negative over the whole range of adsorption values (differential heat is lower than the heat of condensation), and the threshold adsorption value of water vapour (as) is proportional to the concentration of PACs and corresponds to the formation of fractions of a dense monomolecular layer on the adsorbent surface. In the second case, the value of the pure differential heat of adsorption is positive over the entire range of coverage and decreases continuously to zero at saturation, and the as value depends on the specific surface area of the carbon adsorbents, and corresponds to the formation of a 1.7_+ 0.3 dense monomolecular layer. A method of evaluating the surface of nonporous carbon adsorbents and the mesopore surface of active carbons based on this experimental finding is proposed. The mesopore surfaces determined from the water and nitrogen (benzene) vapour adsorption isotherms are in good agreement. Keywords: Adsorption; Adsorption isotherm; Carbon adsorbents; Clusters of water molecules; Heat of adsorption; Mesopore surface; Specific surface; Water vapour
1. Introduction The adsorption of water molecules onto a c a r b o n surface with hydrophilic functional groups which play the role of primary adsorption centres (PACs) is accompanied by the formation of h y d r o g e n bonds between the adsorbed molecules and PACs. Increasing water v a p o u r pressure leads to the formation of h y d r o g e n b o n d s between the adsorbed molecules, and as a result clusters of * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03180-4
associated water molecules a r o u n d the PACs are formed. It is reasonable to assume that, depending on the P A C concentration, the adsorption of water molecules ends with the formation of either individual isolated clusters in equilibrium with saturated vapours or a continuous adsorption film on the surface. In the former case, the distance between PACs is greater than some critical size of the water clusters, and the threshold adsorption value (as) is proportional to the n u m b e r of PACs on the surface. In the second case, as depends on the specific surface area (Ssp).
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 22~232
228
In this paper, we report the results of a study of the effect of the surface concentration of the PACs on the mechanism of adsorption of water molecules onto a carbon surface, and describe a method of evaluating the surface of nonporous carbon adsorbents and the mesopore surface of active carbons based on the results obtained.
2. Experimental The adsorption isotherms of nitrogen vapour at 77 K and water vapour at 293 K on carbon adsorbents prepared from ultradispersed gas channel carbon blacks and graphitized at 2800°C (Table 1), and on nonporous carbon adsorbents with various specific surfaces and comparatively high concentrations of PACs (Table 2) have been measured. The heats of immersion in water (Qi) of the samples with various quantities of pre-adsorbed water were measured using a calorimetric method. Sample 1 (Table 1) has a narrow pore-size distribution with a mean pore radius of about 25 nm. The sample was treated with gaseous C12 and then hydrated (samples 2 and 3), and oxidized in HNO3 at 298 K (sample 4) and 373 K (sample 5) to form surface acidic OH groups. In addition, the surface OH groups were deposited by the pre-adsorption of phenol molecules from aqueous solution (sample 6). Sample 7 has a smaller specific surface area and higher concentration of PACs in comparison with
Table 1 Specific surfaces (S,p), numbers of primary adsorption centres (am), distances between them (1), threshold adsorption values (as) and numbers of statistic monomolecular layers (N) of water vapour adsorbed onto samples of graphitized carbon blacks Sample
Ssp (m z g - l )
am (gmol g 1)
1 (nm)
a~ (mmol g 1)
N
1 2 3 4 5 6 7 8
47 50 51 47 63 50 33 100
0.5 8.0 12.0 5.0 15.0 2.5 1.6 4.0
12.50 3.22 2.66 3.96 2.65 5.76 5.85 6.44
0.24 1.20 1.79 0.61 1.79 0.39 0.25 0.59
0.32 1.50 2.20 0.82 1.80 0.49 0.48 0.37
Table 2 Values of S~p, am, 1, as and N for nongraphitized carbon blacks Sample
Sap (m z g - 1 )
am (gmolg-1)
1 (nm)
as ( m m o l g 1)
N
1 2 3 4 5 6 7 8 9 10 11 12
33 87 97 73 126 91 90 90 85 68 130 120
13.0 23.2 22.0 27.2 68.0 60.1 220.3 132.2 283.0 61.1 337.0 311.0
2.05 2.50 2.71 2.11 1.75 1.59 0.82 1.06 0.71 1.36 0.80 0.80
0.85 2.25 2.75 1.90 2.95 2.50 2.20 2.50 2.85 1.85 3.25 2.90
1.63 1.63 1.79 1.64 1.48 1.74 1.54 1.76 2.11 1.72 1.58 1.53
sample 1. Carbon black Vulkan-7H graphitized at 2800°C (sample 8) was used as a reference sample. Nonporous carbon adsorbents with different specific surfaces and sufficiently high PAC concentrations, and active carbons containing mesopores were also studied. The following carbon blacks (CBs) (Table2) were studied: commercial CB P-154 used in the tyre industry (sample 1), gas channel CB K-354 with 20% precipitated pyrolytic carbon (sample 2), acetylene CB with 5.5% precipitated pyrolytic carbon (sample 3), Czechoslovakian CB HAF (sample 4), CB Vulkan-7H (5), CB Spheron-6 (sample 6) and CB Vulkan (sample 7), furnace CB P-234 (samples 8 and 9, varying in the value of the specific surface), CB PM-75 (sample 10), gas channel CB K-354 (sample 11), and Ukhta channel CB (sample 12). The mesoporous active carbons (ACs) (Table 3) were represented by samples of coal-based AC AG type (samples 1-8), peat-based AC SKT and ART type (samples 9-11), furfurol-based AC FAS type (samples 12-17), AC KAU type prepared from fruit stones (samples 18 and 19), and laboratory AC of progressive activation (samples 20-23). The specific surfaces of the carbon black samples and the mesopore surface areas (S. . . . ) of ACs were determined from the nitrogen adsorption isotherms at 77 K or benzene adsorption isotherms at 298 K, using a comparative method [1]. The concentrations of PACs (am) were determined from the slopes of the comparative plots of adsorption on
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 227 232 Table 3 Water vapour adsorption in mesopores of active carbons (a. . . . ) and surface area of mesopores (S . . . . isotherms of water and nitrogen (benzene) vapours Sample
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
229
) determined from adsorption
a. . . . ( H 2 0 )
S. . . . (N2)
S. . . . ( H 2 0 )
AS . . . . /S . . . . ( N 2 )
(mmol g - l )
(m 2 g - l )
(m 2 g - l )
(%)
0.5 1.4 2.8 4.4 1.7 5.0 2.0 2.9 1.4 0.9 3.5 0.6 1.2 1.5 2.7 2.9 6.8 2.2 2.5 1.0 1.6 3.1 4.5
25 60 137 208 85 230 90 130 60 37 140 30 60 74 120 120 300 105 100 45 80 130 200
22 62 124 194 75 221 88 128 62 39 155 27 53 66 119 129 301 98 111 44 71 137 199
12.0 3.3 9.4 6.7 11.8 3.9 2.2 1.5 3.3 5.4 10.7 10.0 11.7 10.8 0.8 7.5 0.3 6.7 11.0 2.2 11.3 5.4 0.5
the studied and reference carbon blacks [2]. The distances between PACs (1) were calculated, assuming that they were uniformly distributed on the carbon black surface. The values of Ssp, l, am, as and the number of statistical monomolecular layers at saturation (N) for the samples of CB are given in Tables 1 and 2.
value corresponds to the formation of a fraction of the dense monolayer for samples 1, 4, 6 and 7, as well as for the reference graphitized carbon black Vulkan-7H. Fig. 1 shows a plot of threshold value of adsorption as a function of PAC concentration. This plot
a, (mmol g-l)
3. Results and discussion Table 1 presents the values of am, Ssp, 1 and the threshold adsorption of water vapour at saturation pressure p/ps = 1.0 for the studied samples as well the number of statistical monolayers at saturation, calculated for a cross-sectional area of water molecule equal to 0.105 nm 2. It can be noted that the number of PACs increases dramatically after hydroxylation of the sample surface. S~p is practically constant for samples 1-4 and increases slightly after boiling in HNO3 (sample 5). The as
-
3
5
10
5
15
Fig. 1. Threshold values of water vapour adsorption (as) as a function of number of primary adsorption centres (am) on graphitized carbon samples.
230
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicoehem. Eng. Aspects 101 (1995) 227-232
yields a straight line through the origin, the slope of which leads to a value of the number of water molecules in the cluster formed around PAC of about 150 molecules per centre. According to the numerical modelling data [-3 ], the diameter of this hemispherical cluster is about 3 nm. Hence, if the distance between PACs exceeds 3 nm, no uniform adsorption layer will be formed on the surface of graphitized carbon blacks. It is necessary to point out that the correct evaluation of the number of PACs for water molecules plays an important role in the suggested approach. The comparative technique is based on the Langmuir equation for localized adsorption which was used to determine the number of PACs on the reference carbon black [2]. This value is equal to the number of acid surface groups which may be determined by use of an independent method of chemical neutralization of the acidic groups with NazCO 3. The heats of immersion of sample 7 were measured as a function of the amount of pre-adsorbed water. Fig. 2 shows that the heat of immersion increases continuously with an increase in the amount of pre-adsorbed water up to maximum adsorption. This means that the differential heat of adsorption is lower than the heats of condensation over the entire range of extent of filling. Assuming the surface energy of normal water to be 118 m J m -2, the disappearance of 12.7m 2 g of aqueous surface area corresponds to a heat of immersion of 1.5 J g 1. As a consequence, only part of the surface (about 40%) of the carbon black is occupied by water clusters at saturation pressure, and the distance between them is greater than their diameter. For carbon blacks with higher concentrations of PACs, the plots of heat of immersion as a function of the amount of pre-adsorbed water pass through a maximum owing to formation of a continuous adsorption film [4]. One can see from Table 2 that the values of 1 for all of the samples are smaller than the critical size of water clusters on the carbon surface (about 3 nm), which indicates the formation of a continuous adsorption film. In this case, the threshold adsorption value should be proportional to the specific surfaces (Ssp) of the CB samples.
•
50
tO0
I
I
1.5
193
a (#tool g-t) 2~,
l
I
0
~-
0
_i
,.3
iI
! _opf
,
,i:
4
8
12 a (,amolg-~)
Fig. 2. (1) Heat of immersion of graphitized sample 7 in water at 25°C as a function of the amount of pre-adsorbed water. (2) differential heat of adsorption calculated from heat of immersion. (3) isosteric heat of water vapour adsorption on the same sample in region of low coverage, calculated from the adsorption isotherms at 0, 20 and 25°C.
Fig. 3 shows a plot of as as a function of Ssp for the carbon blacks studied. This plot yields a straight line through the origin. As follows from a,(mmol g-l)
/-,-117
1 ~ 0
Ssp (m2 g-*) I
I
1
30
60
90
I
120
Fig. 3. Threshold value of water vapour adsorption (as) as a function of specific surface area (Ssp) of nongraphitized carbon black samples.
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 227-232
the slope of the straight line, the threshold adsorption value per 1 m 2 of the carbon black surface (a*) is equal to about 26 ~tmol. The as values for the studied carbon blacks correspond to the formation of a (1.7 + 0.3) statistical dense monomolecular layer (see Table 2). The linear dependence of the threshold adsorption value on the specific surface area of carbon adsorbents may be used as the basis of the method of determining the surface areas of nonporous (and mesoporous) carbon adsorbents, and of the method of determining the mesopore surface of active carbons as well. Assuming that the mechanism of generation of a continuous adsorption film on the surface of nonporous CBs and mesopores of ACs are similar, one can estimate the mesopore surface of an AC from the value of the water vapour adsorption onto the surface of the AC mesopores and from a*. As a first approximation, the adsorption onto the mesopore surfaces can be estimated from the "tail" of an adsorption isotherm on an AC containing mesopores in the UPs range from 0.93-0.95 to 1.0. The adsorption isotherms of water vapour on the FAS type AC (curve 1) and on CB 2 (curve 2) are shown in Fig. 4 as an example. At p/ps=0.95,
a (mmol g t)
t6
t~
0.2
0.4
~.6
-
-0.8
1.0
P/P,
Fig. 4. Water vapour adsorption isotherms at 20°C on FAS type active carbon (curve 1) and on gas channel carbon black 2 (curve 2).
231
the AC micropore volume is virtually filled with water. At higher PIPs, the adsorption continues on the mesopore surface, and the adsorption isotherms on the AC and the CB are closely corresponding. In this case, the adsorption onto the mesopore surfaces (a.... ) may be calculated as the difference between the as value and the adsorption value at pips=0.95 (ao.95). The mesopore surface of active carbons may be calculated from the water vapour adsorption onto the mesopore surface (as - a0.95), with an allowance made for the threshold water adsorption per unit of surface area of nonporous carbon blacks (26~tmolm -2) and the average adsorbed film thickness (1.7 monolayers). Table 3 presents the (a s - - ao.95 ) values and the mesopore surfaces of the ACs, determined from the water vapour adsorption isotherms. The Smeso values determined from the nitrogen (benzene) vapour adsorption isotherms are given for comparison. As follows from Table 3, the mesopore surfaces calculated from the water and nitrogen (benzene) adsorption isotherms are in satisfactory agreement. The maximum deviation in the S. . . . values obtained from adsorption isotherms of water and nitrogen vapours is 12%. The threshold adsorption on the mesopore surface of ACs corresponds to the formation of a (1.5 + 0.2) dense monomolecular layer, which is consistent with the data obtained for the carbon blacks. Note that all the active carbons studied are characterized by an almost complete absence of supermicropores, and the isotherms of water vapour adsorption onto them resemble that shown in Fig. 4. The isotherms have a well-defined S-shape with an additional rise at UPs= 0.93-0.95. For ACs which, in addition to micro- and mesopores, contain supermicropores, and particularly for ACs with a broad pore size distribution, the water vapour adsorption isotherm does not have a noticeable bend at PIPs--0.95. This fact complicates and even makes impossible the determination of the mesopore surface area by the proposed method. Thus, the specific surface area of nonporous (and mesoporous) carbon adsorbents with a comparatively high PAC concentration and the mesopore surface of ACs with a homogeneous microporous
232
R. Sh. Vartapetyan et al./Colloids Surfaces A." Physicochem. Eng. Aspects 101 (1995) 227~32
structure m a y be d e t e r m i n e d from w a t e r v a p o u r a d s o r p t i o n isotherms. This m e t h o d m a y be further e x t e n d e d to A C s with m i c r o p o r o u s structures of v a r i o u s types.
Acknowledgement This study was s u p p o r t e d by the Russian F o u n d a t i o n of F u n d a m e n t a l Researches ( G r a n t No. 94-03-09950).
References [1] A.M. Voloshchuk, M.M. Dubinin, T.A. Moskovskaya, Y.K. Ivakhnyuk and N.F. Fedorov, Izv. Akad. Nauk SSSR Set. Khim., 277 (1988). [2] R.Sh. Vartapetyan, A.M. Voloshchuk, M.M. Dubinin and T.A. Moskovskaya, Izv. Akad. Nauk SSSR Ser. Khim., 1961 (1988). E3] M.M Dubinin and G.G. Malenkov, Izv. Akad. Nauk SSSR Ser. Khim., 1217 (1984). [4] M.M. Dubinin, A.A. Isirikyan, K.M. Nikolaev, N.S. Polyakov and L.I. Tatarinova, Izv. Akad. Nauk SSSR Ser. Khim., 7 (1987).
ELSEVIER
COLLOIDS AND SURFACES
A
Chemistry of carbon surface and mechanism of water molecule adsorption Ruben Sh. Vartapetyan a,, Albert M. Voloshchuk a, Armenak A. Isirikyan a, Nikolay S. Polyakov a, Yury I. Tarasevich b a Institute of Physical Chemistry of Russian Academy of Sciences, 31 Lenin Avenue, 117915 Moskow, Russia b Institute of Colloid and Water Chemistry of Ukraine Academy of Sciences, 42 Vernadsky Avenue, 250680 Kiev, Ukraine Received 20 October 1994; accepted 29 March 1995
Abstract The experimental adsorption isotherms of water vapour and nitrogen on nonporous carbon adsorbents with various specific surfaces and concentrations of primary adsorption centres (PACs) have been measured. The adsorption of water molecules ends with the formation of either individual isolated clusters in equilibrium with saturated vapour or a continuous adsorption film depending on the concentration of the PACs. In the former case, the pure differential heats of adsorption are negative over the whole range of adsorption values (differential heat is lower than the heat of condensation), and the threshold adsorption value of water vapour (as) is proportional to the concentration of PACs and corresponds to the formation of fractions of a dense monomolecular layer on the adsorbent surface. In the second case, the value of the pure differential heat of adsorption is positive over the entire range of coverage and decreases continuously to zero at saturation, and the as value depends on the specific surface area of the carbon adsorbents, and corresponds to the formation of a 1.7_+ 0.3 dense monomolecular layer. A method of evaluating the surface of nonporous carbon adsorbents and the mesopore surface of active carbons based on this experimental finding is proposed. The mesopore surfaces determined from the water and nitrogen (benzene) vapour adsorption isotherms are in good agreement. Keywords: Adsorption; Adsorption isotherm; Carbon adsorbents; Clusters of water molecules; Heat of adsorption; Mesopore surface; Specific surface; Water vapour
1. Introduction The adsorption of water molecules onto a c a r b o n surface with hydrophilic functional groups which play the role of primary adsorption centres (PACs) is accompanied by the formation of h y d r o g e n bonds between the adsorbed molecules and PACs. Increasing water v a p o u r pressure leads to the formation of h y d r o g e n b o n d s between the adsorbed molecules, and as a result clusters of * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03180-4
associated water molecules a r o u n d the PACs are formed. It is reasonable to assume that, depending on the P A C concentration, the adsorption of water molecules ends with the formation of either individual isolated clusters in equilibrium with saturated vapours or a continuous adsorption film on the surface. In the former case, the distance between PACs is greater than some critical size of the water clusters, and the threshold adsorption value (as) is proportional to the n u m b e r of PACs on the surface. In the second case, as depends on the specific surface area (Ssp).
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 22~232
228
In this paper, we report the results of a study of the effect of the surface concentration of the PACs on the mechanism of adsorption of water molecules onto a carbon surface, and describe a method of evaluating the surface of nonporous carbon adsorbents and the mesopore surface of active carbons based on the results obtained.
2. Experimental The adsorption isotherms of nitrogen vapour at 77 K and water vapour at 293 K on carbon adsorbents prepared from ultradispersed gas channel carbon blacks and graphitized at 2800°C (Table 1), and on nonporous carbon adsorbents with various specific surfaces and comparatively high concentrations of PACs (Table 2) have been measured. The heats of immersion in water (Qi) of the samples with various quantities of pre-adsorbed water were measured using a calorimetric method. Sample 1 (Table 1) has a narrow pore-size distribution with a mean pore radius of about 25 nm. The sample was treated with gaseous C12 and then hydrated (samples 2 and 3), and oxidized in HNO3 at 298 K (sample 4) and 373 K (sample 5) to form surface acidic OH groups. In addition, the surface OH groups were deposited by the pre-adsorption of phenol molecules from aqueous solution (sample 6). Sample 7 has a smaller specific surface area and higher concentration of PACs in comparison with
Table 1 Specific surfaces (S,p), numbers of primary adsorption centres (am), distances between them (1), threshold adsorption values (as) and numbers of statistic monomolecular layers (N) of water vapour adsorbed onto samples of graphitized carbon blacks Sample
Ssp (m z g - l )
am (gmol g 1)
1 (nm)
a~ (mmol g 1)
N
1 2 3 4 5 6 7 8
47 50 51 47 63 50 33 100
0.5 8.0 12.0 5.0 15.0 2.5 1.6 4.0
12.50 3.22 2.66 3.96 2.65 5.76 5.85 6.44
0.24 1.20 1.79 0.61 1.79 0.39 0.25 0.59
0.32 1.50 2.20 0.82 1.80 0.49 0.48 0.37
Table 2 Values of S~p, am, 1, as and N for nongraphitized carbon blacks Sample
Sap (m z g - 1 )
am (gmolg-1)
1 (nm)
as ( m m o l g 1)
N
1 2 3 4 5 6 7 8 9 10 11 12
33 87 97 73 126 91 90 90 85 68 130 120
13.0 23.2 22.0 27.2 68.0 60.1 220.3 132.2 283.0 61.1 337.0 311.0
2.05 2.50 2.71 2.11 1.75 1.59 0.82 1.06 0.71 1.36 0.80 0.80
0.85 2.25 2.75 1.90 2.95 2.50 2.20 2.50 2.85 1.85 3.25 2.90
1.63 1.63 1.79 1.64 1.48 1.74 1.54 1.76 2.11 1.72 1.58 1.53
sample 1. Carbon black Vulkan-7H graphitized at 2800°C (sample 8) was used as a reference sample. Nonporous carbon adsorbents with different specific surfaces and sufficiently high PAC concentrations, and active carbons containing mesopores were also studied. The following carbon blacks (CBs) (Table2) were studied: commercial CB P-154 used in the tyre industry (sample 1), gas channel CB K-354 with 20% precipitated pyrolytic carbon (sample 2), acetylene CB with 5.5% precipitated pyrolytic carbon (sample 3), Czechoslovakian CB HAF (sample 4), CB Vulkan-7H (5), CB Spheron-6 (sample 6) and CB Vulkan (sample 7), furnace CB P-234 (samples 8 and 9, varying in the value of the specific surface), CB PM-75 (sample 10), gas channel CB K-354 (sample 11), and Ukhta channel CB (sample 12). The mesoporous active carbons (ACs) (Table 3) were represented by samples of coal-based AC AG type (samples 1-8), peat-based AC SKT and ART type (samples 9-11), furfurol-based AC FAS type (samples 12-17), AC KAU type prepared from fruit stones (samples 18 and 19), and laboratory AC of progressive activation (samples 20-23). The specific surfaces of the carbon black samples and the mesopore surface areas (S. . . . ) of ACs were determined from the nitrogen adsorption isotherms at 77 K or benzene adsorption isotherms at 298 K, using a comparative method [1]. The concentrations of PACs (am) were determined from the slopes of the comparative plots of adsorption on
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 227 232 Table 3 Water vapour adsorption in mesopores of active carbons (a. . . . ) and surface area of mesopores (S . . . . isotherms of water and nitrogen (benzene) vapours Sample
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
229
) determined from adsorption
a. . . . ( H 2 0 )
S. . . . (N2)
S. . . . ( H 2 0 )
AS . . . . /S . . . . ( N 2 )
(mmol g - l )
(m 2 g - l )
(m 2 g - l )
(%)
0.5 1.4 2.8 4.4 1.7 5.0 2.0 2.9 1.4 0.9 3.5 0.6 1.2 1.5 2.7 2.9 6.8 2.2 2.5 1.0 1.6 3.1 4.5
25 60 137 208 85 230 90 130 60 37 140 30 60 74 120 120 300 105 100 45 80 130 200
22 62 124 194 75 221 88 128 62 39 155 27 53 66 119 129 301 98 111 44 71 137 199
12.0 3.3 9.4 6.7 11.8 3.9 2.2 1.5 3.3 5.4 10.7 10.0 11.7 10.8 0.8 7.5 0.3 6.7 11.0 2.2 11.3 5.4 0.5
the studied and reference carbon blacks [2]. The distances between PACs (1) were calculated, assuming that they were uniformly distributed on the carbon black surface. The values of Ssp, l, am, as and the number of statistical monomolecular layers at saturation (N) for the samples of CB are given in Tables 1 and 2.
value corresponds to the formation of a fraction of the dense monolayer for samples 1, 4, 6 and 7, as well as for the reference graphitized carbon black Vulkan-7H. Fig. 1 shows a plot of threshold value of adsorption as a function of PAC concentration. This plot
a, (mmol g-l)
3. Results and discussion Table 1 presents the values of am, Ssp, 1 and the threshold adsorption of water vapour at saturation pressure p/ps = 1.0 for the studied samples as well the number of statistical monolayers at saturation, calculated for a cross-sectional area of water molecule equal to 0.105 nm 2. It can be noted that the number of PACs increases dramatically after hydroxylation of the sample surface. S~p is practically constant for samples 1-4 and increases slightly after boiling in HNO3 (sample 5). The as
-
3
5
10
5
15
Fig. 1. Threshold values of water vapour adsorption (as) as a function of number of primary adsorption centres (am) on graphitized carbon samples.
230
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicoehem. Eng. Aspects 101 (1995) 227-232
yields a straight line through the origin, the slope of which leads to a value of the number of water molecules in the cluster formed around PAC of about 150 molecules per centre. According to the numerical modelling data [-3 ], the diameter of this hemispherical cluster is about 3 nm. Hence, if the distance between PACs exceeds 3 nm, no uniform adsorption layer will be formed on the surface of graphitized carbon blacks. It is necessary to point out that the correct evaluation of the number of PACs for water molecules plays an important role in the suggested approach. The comparative technique is based on the Langmuir equation for localized adsorption which was used to determine the number of PACs on the reference carbon black [2]. This value is equal to the number of acid surface groups which may be determined by use of an independent method of chemical neutralization of the acidic groups with NazCO 3. The heats of immersion of sample 7 were measured as a function of the amount of pre-adsorbed water. Fig. 2 shows that the heat of immersion increases continuously with an increase in the amount of pre-adsorbed water up to maximum adsorption. This means that the differential heat of adsorption is lower than the heats of condensation over the entire range of extent of filling. Assuming the surface energy of normal water to be 118 m J m -2, the disappearance of 12.7m 2 g of aqueous surface area corresponds to a heat of immersion of 1.5 J g 1. As a consequence, only part of the surface (about 40%) of the carbon black is occupied by water clusters at saturation pressure, and the distance between them is greater than their diameter. For carbon blacks with higher concentrations of PACs, the plots of heat of immersion as a function of the amount of pre-adsorbed water pass through a maximum owing to formation of a continuous adsorption film [4]. One can see from Table 2 that the values of 1 for all of the samples are smaller than the critical size of water clusters on the carbon surface (about 3 nm), which indicates the formation of a continuous adsorption film. In this case, the threshold adsorption value should be proportional to the specific surfaces (Ssp) of the CB samples.
•
50
tO0
I
I
1.5
193
a (#tool g-t) 2~,
l
I
0
~-
0
_i
,.3
iI
! _opf
,
,i:
4
8
12 a (,amolg-~)
Fig. 2. (1) Heat of immersion of graphitized sample 7 in water at 25°C as a function of the amount of pre-adsorbed water. (2) differential heat of adsorption calculated from heat of immersion. (3) isosteric heat of water vapour adsorption on the same sample in region of low coverage, calculated from the adsorption isotherms at 0, 20 and 25°C.
Fig. 3 shows a plot of as as a function of Ssp for the carbon blacks studied. This plot yields a straight line through the origin. As follows from a,(mmol g-l)
/-,-117
1 ~ 0
Ssp (m2 g-*) I
I
1
30
60
90
I
120
Fig. 3. Threshold value of water vapour adsorption (as) as a function of specific surface area (Ssp) of nongraphitized carbon black samples.
R. Sh. Vartapetyan et al./Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 227-232
the slope of the straight line, the threshold adsorption value per 1 m 2 of the carbon black surface (a*) is equal to about 26 ~tmol. The as values for the studied carbon blacks correspond to the formation of a (1.7 + 0.3) statistical dense monomolecular layer (see Table 2). The linear dependence of the threshold adsorption value on the specific surface area of carbon adsorbents may be used as the basis of the method of determining the surface areas of nonporous (and mesoporous) carbon adsorbents, and of the method of determining the mesopore surface of active carbons as well. Assuming that the mechanism of generation of a continuous adsorption film on the surface of nonporous CBs and mesopores of ACs are similar, one can estimate the mesopore surface of an AC from the value of the water vapour adsorption onto the surface of the AC mesopores and from a*. As a first approximation, the adsorption onto the mesopore surfaces can be estimated from the "tail" of an adsorption isotherm on an AC containing mesopores in the UPs range from 0.93-0.95 to 1.0. The adsorption isotherms of water vapour on the FAS type AC (curve 1) and on CB 2 (curve 2) are shown in Fig. 4 as an example. At p/ps=0.95,
a (mmol g t)
t6
t~
0.2
0.4
~.6
-
-0.8
1.0
P/P,
Fig. 4. Water vapour adsorption isotherms at 20°C on FAS type active carbon (curve 1) and on gas channel carbon black 2 (curve 2).
231
the AC micropore volume is virtually filled with water. At higher PIPs, the adsorption continues on the mesopore surface, and the adsorption isotherms on the AC and the CB are closely corresponding. In this case, the adsorption onto the mesopore surfaces (a.... ) may be calculated as the difference between the as value and the adsorption value at pips=0.95 (ao.95). The mesopore surface of active carbons may be calculated from the water vapour adsorption onto the mesopore surface (as - a0.95), with an allowance made for the threshold water adsorption per unit of surface area of nonporous carbon blacks (26~tmolm -2) and the average adsorbed film thickness (1.7 monolayers). Table 3 presents the (a s - - ao.95 ) values and the mesopore surfaces of the ACs, determined from the water vapour adsorption isotherms. The Smeso values determined from the nitrogen (benzene) vapour adsorption isotherms are given for comparison. As follows from Table 3, the mesopore surfaces calculated from the water and nitrogen (benzene) adsorption isotherms are in satisfactory agreement. The maximum deviation in the S. . . . values obtained from adsorption isotherms of water and nitrogen vapours is 12%. The threshold adsorption on the mesopore surface of ACs corresponds to the formation of a (1.5 + 0.2) dense monomolecular layer, which is consistent with the data obtained for the carbon blacks. Note that all the active carbons studied are characterized by an almost complete absence of supermicropores, and the isotherms of water vapour adsorption onto them resemble that shown in Fig. 4. The isotherms have a well-defined S-shape with an additional rise at UPs= 0.93-0.95. For ACs which, in addition to micro- and mesopores, contain supermicropores, and particularly for ACs with a broad pore size distribution, the water vapour adsorption isotherm does not have a noticeable bend at PIPs--0.95. This fact complicates and even makes impossible the determination of the mesopore surface area by the proposed method. Thus, the specific surface area of nonporous (and mesoporous) carbon adsorbents with a comparatively high PAC concentration and the mesopore surface of ACs with a homogeneous microporous
232
R. Sh. Vartapetyan et al./Colloids Surfaces A." Physicochem. Eng. Aspects 101 (1995) 227~32
structure m a y be d e t e r m i n e d from w a t e r v a p o u r a d s o r p t i o n isotherms. This m e t h o d m a y be further e x t e n d e d to A C s with m i c r o p o r o u s structures of v a r i o u s types.
Acknowledgement This study was s u p p o r t e d by the Russian F o u n d a t i o n of F u n d a m e n t a l Researches ( G r a n t No. 94-03-09950).
References [1] A.M. Voloshchuk, M.M. Dubinin, T.A. Moskovskaya, Y.K. Ivakhnyuk and N.F. Fedorov, Izv. Akad. Nauk SSSR Set. Khim., 277 (1988). [2] R.Sh. Vartapetyan, A.M. Voloshchuk, M.M. Dubinin and T.A. Moskovskaya, Izv. Akad. Nauk SSSR Ser. Khim., 1961 (1988). E3] M.M Dubinin and G.G. Malenkov, Izv. Akad. Nauk SSSR Ser. Khim., 1217 (1984). [4] M.M. Dubinin, A.A. Isirikyan, K.M. Nikolaev, N.S. Polyakov and L.I. Tatarinova, Izv. Akad. Nauk SSSR Ser. Khim., 7 (1987).