210
A. A. ISIRIKYAN AND A. V. KISELEV
Vol. 66
ADSORPTIOX ISOTHEIEtMS OF NITROGEN, BENZENE AND n-HEXANE AND THE HEATS OF ADSORPTIOS OF BENZENE AND n-HEXANE Oh’ GRAPHITIZED CARBON BLACKS. 111. THE THERRIIODYNAMPC CHARACTERISTICS OF ADSORPTION EQUILIBRIA BY A. A. ISIRIKYAN AND A. V. KISELEV Adsorption Luborutoiy, Cheiiwxl Department, il4oscow State Universaty, Moscow, U.S.S.R. liecezved M a y I$. 1901
From cxpcrinicrital quantities for gra;)hitized thermal carbon blacks we have obtained and compared the isotherms of the absolute magnitudes of adsorption, and dirferential heats and entropies of adsorption of n-hexane and benzene on the “bas:d plane of graphite,” and also the corresponding standard differential quantities of free and total energ)’, heat and entropy of :Idsorption and the extrema1 values of the differential heats and entropies of adsorption. All these properties are determined solely by the nature of the adsorbate-ndsorbciit system and practically are free from the influence of surface inhomogeneity of the solid. During filling of a moiioliiyer, the heat of adsorption of n-hexane increases by 2 , and benzene by 0.3 kcal./molc. The latter value is associated with the considerably weaker adsorbate-adsorbate attractions of benzene. Accordingly, the a;dsorpt,ion isothcrm of benzene is ut first concave, and of hexane, convex to the pressure axes. A description of the adsorption isotherms of hcrizerie, hexanc iind nitrogen is discussed in terms of various equations. The BET equation is only :I first approximation. Equations taking into socount adsorbate-adsorbate attractions yield better agreement. A relationship is established between the cotistiints of these equations and the constant of the BET equation. The adsorption of benzene and n-hexane a t 20” arid of nitrogen a t - 1%’ on the “basal plane of graphite” is predominantly localized.
Introduction good agreement between theoretically derived and experimental values. 10113-17 I n our previous studies on the adsorption and There also hope of complete theoretical heats of adsorption of benzene and n-hexane on computation ofis adsorption e q ~ i l i b r i a . ~ 3 ! ~ ~A8 ~ 9 graphitized carbon as well as in a series reduction jn the influence of surface inhomogeneity of researches on the adsorption of the vapors of of thermal blacks on graphitization6 permits various lorn-boiling substaiiccs6-10 including ethyl treating the adsorption properties of such chloride,” alcohols,12 acetone and ether,13 it by semi-empirical mcthods on the basis of systems adsorphas been shown that adsorption on carbon blacks tion isotherm equations on a homogeneous surface, graphitized near 3000°, in particular on thermal the form of which is derived t,heoretically, and the blacks with small specific surface, one is dealing constants found from comparison with experiwith adsorption 011 a fully homogeneous surface, ment.7,13,20-22 Le., on the basal plane of graphite. Investiga4 gives absolute adsorption isotherms tions of the adsorption isotherms of nit,rogen on of Reference nitrogen various samples of graphitized thermal carbon hexane a t a t -195’ and of benzene and nblacks carried on in a number of l a b o r a t ~ r i e s ~ ~ ~ ~ ~ 20°, as well as the differential heats of adsorption of beiizcne and n-hexane on graphit,ized have shown good agrcement of the absolute values thermal hlnck. The present paper comof adsorption (referred to unit s u r f a c ~ ) . ~Thus, pares thecarbon thermodynamic properties of the adthese absolute quantities represent physico-chcmi- sorption systems grnphit8e-benzene graphitmecal constants that characterize the system: ad- hexane arid attempts to describe theand isotherms for sorbate-basal plane of graphite. For some of these systems by approximate semi-empirical these constants, such as the heats of adsorption at; We also shall derive a relationship low surface coverage, it has been possible to find methods. between t,he constants of equilibrium of the BET equationz3and the isotherm equation, taking into (1) N. N. Avgul, G . I. Berezin, A. V. Kiselev a n d I. A. Lygina. Zhur. P i t . Khim., 30, 2106 (1956); Izueut. A k a d . n’auk S.S.S.R. account adsorbate-adsorbate interaction. Dept. Chem. Sei., 1304 (1966). (2) K. N. Avgal, A. V, Kisclev, .I. J. Korolev and I. .i. Lygina, Kolloid. Zhur., 20, 298 (1958). (3) A. A , Isirikyan Itnd it. V. Kiselcv, i6id.. 23, 281 (1961). ( 4 ) A. A . Isirikyaii a n d A. V. Kiuelev, J . Z’hy8. Chem., 66, 001
(1961). ( 5 ) A . 11. lsivikyan and .I. V. Kisclvv, {bid., 66, 203 (1‘302). (6) It. .4. Beebe Itrid I). R1. Yourip, ibid.. 63,93 (19.54); R.A . h e b e , C . 1-1. Arribern and W. 13. Spencer, Can Chem.. 33, 305 (1955). (7) S. Ross and W. LViriklcr, J . Colloid Sct., 10, 319, 330 (1955); S. Ross and M’. W. Pulta, ibid., 13, 307 (1958). (8) R. A . Becbe and R. RI. Dcll, J . l’hys. Chem., 69, 746, 784
(1955). (9) 12. A . Brebe and J . A I . IIolmi:s, i b i d . , 61, 1684 (1957). (10) 15. I,. Pacr and A . It. Sivlwrt, i h i d . , 63, 1398 (1959); 64, 901 (19GO).
(11) .J. Mooi, C. I’irrcc and R. N. Siiiith, ibid., 67, 657 (1953). (121 N. N. A v ~ t i l ,G . I . Uovezin, A . V. Kiselev a n d I. :I Lygina, . I m e s f . A k a d . JYauk S.S.S.IL’., Dupt. Cheni. Sci., 203 (1961); N. N. .\vaul, A , V. l 1.1 is associated
-
( 2 5 ) A. V. Xiselov a n d D. P. Poshkus, Izvcat. Akad. Nauk D e p t . Chem. Soc., 5.90 (1958).
S.S.S.R.,
(26) A. A. Iairikyan and . I . V. Kiselev, Kolloid. Zhur., 23, 281
(1901).
Ii’cb., 1902
TIIERMODPN.~MIC CHARACTERISTICS OF ADSORPTION EQUILIBRI.\
with the approximate nature of assumptions in the BET associated with the equality of the constants of equilibrium during formation of multiple poly molecular complexcs. Deviations downward of experimental points in the field of small 6 are due to the influence of residual inhomogeneity, which increases the initial heats of adsorption. This lcads to enhanced values of adsorption a t small h (as compared with adsorption on a smooth surface of a basal plane of graphite for the same values of h). The reason for upward deviations of expcrimental isotherms from the straight BET line for diminishing 8 from 0.7-0.1 and below is adsorbate-adsorbate interactions m the first layer which are not taken into account in the BET theory. These deviations increase as we pass from an isotherm initially concave to the pressure axes, L e . , bcnzcne, to isotherms initially convex, as with n-hexane and nitrogen, and are due to an increase in adsorbate-adsorbate attraction. For this reason, in the case of adsorption on graphitized carbon blacks of less homogeneous surface, such as graphitized channel blacks (Graphon), an increase in 6 (and z ) in a broader range of small h leads to a better coincidence of experimental points on the straight B E T line. The isotherm for nitrogen on Graphon blacks of more heterogeneous surface is at first concavez1*5 and for this reason is better described by the B E T equation (from h-0 to hc0.15). This equation gives an even better description of the adsorption isotherm of nitrogen on the even more heterogeneous surface of non-graphitized blacks and active ~ s r b o n s . ~However, ~ * ~ ~ in these cases, the mechanism of formation of the adsorption layer still is different from the assumptions of the BET theory, because a t the most active sites (in cracks, fine pores, and at points of particle contact) a denser layer is formed, whereas a t less active sites this layer is more tenuous. Therefore, to quantities a, obtained by the B E T method for inhomogeneous surfaces we cannot, strictly speaking, attribute values of monolayer capacities, and the approach should be cautious to the quantities s, a and 6 determined in this manner. The significance of this conclusion21v28increases with increasing heterogeneity of the adsorbent surface. It follows from the foregoing that the various equations for adsorption isotherms derived for homogeneous surfaces that still are meaningful in the case of carbon blacks are applicable only for graphitized thermal blacks with low speciJic surface. We already have pointed out that the deviation, which is evident from the graphs in the upper part of Fig. 2, of cxperimental isotherms from the B E T plot is associated with manifestation of adsorbate-adsorbate attraction and is in accord with increase in differential heat of adsorption with increasing 6 (see Fig. 1). For this reason, the BET equation should be regarded only as a first approximation. The next approximation is given by taking account of adsorbate-adsorbate interactions (27) A. V. Kiaelevand E. V. Khrapova, Imest. Akad. NaukS.S.S.R., D e p t . Chem. Sci., 389 (1958). (28) A. A. Isirikyan, A. V. Kiselev and V. V. Kulichenko, “Proc. of the Second Intern. Congr. on Surface Activity,” Vol. 2, London, 1957, p. 199.
110
1 I
I
100
3
213
.
I +3t) ! II
90
I v M
r=
3
80
I rl v
70
0.2
0.4
0.6
0.8
1
2.
Fig. 3.-Adsorption isotherm of benzene on graphitized thermal carbon black, biT-1 (3100’) in the coordinates of equation 2 for various cross-section areas of benzene.
in the first layer, Le., equation 2 for localized adsorption. I n turn, this equation is approximate, for it does not take into account the differences in the mutual coardination of adsorbate molecules.20 However, in the region of not too large 8, this assumption is sufficiently justified. Figure 3 gives the results of construction of an experimental isotherm of benzene in coordinates of linear form of equation 2 . This construction requires the choice of a value of WmC& and a determination of the degree of coverage 6 = a/a, from the values of adsorption In Fig. 3, this construction is done for several values of q,,c,I16. Equation 2 embraces the largest interval of expcrimental isotherm (from 8.30.2 to eZ1.1) a t WmCsH, = 39.5 i e . , for the probable value of this quantity. Deviations of the experimental isotherm from linearity for large z (large h) are connected with inaccuracies of assumptions, the atlsorbate-adsorbate interactions being independent of the multiplicity of complexes both perpendicular to the surface (the 13ET assumption) and along it (assumption of the present theory).20 lcor this reason, the linear region of the isotherm is of special interest. The values of the constants of equation 2, K I = 57 and K n = 0.88 (K,/KI = 0.0155) arc obtained from Fig. 3 at wmCaIIs = 39.5 A.2. In this case, Kn < 1, which is due to the concave beginning of the isotherm as ZL result of relatively weak adsorbate-adsorbate attraction2O (I?ig. 1). Upward deviations of the experimental isotherm
-
(29) The quantities a a/8 are computed in terms of the values of 8 determined by the RET method from the isotherm of benzene vapor st 20’ and W ~ C ~ H , 40 A . 2 . which nre close to the values of a determined b y this method from the portion of the nitrogen isotherm a t -195’ in the interval h, 0.0003 to 0.09 for W ~ N ,= 16.2 A.a. We consider more justified the determination of the magnitude s required for computation of B b y the BET method from the adsorption iaotherrn of benzene vapor at W ~ C = ~ H 40 ~A.a.
A. A. ISIRIKYAN AND A. V. KISELEV
2 14
(?) Z
2
=
h-+O
h_0
=
Vol. 66 0.0175, whence (;)h-.-O
=
kl
=
57
the adsorbate-adsorbenb constant of equilibrium in accord with the linear graph of equation 2 in Fig. 3 (at W,C~H~ = 39.5 A,*>. From this, as me have seen, it also follows that
1.5
1 =
(;)h4
0.2 0.4 0.6 0.8 I 0.2 0.4 0.6 0.8 1 2. 2. Fig. 4.-The adsorption isotherm of n-hexane on graphitized thcnnal carbon black, ;MT-1 (3100") in coordinates of equation 2 (left) and equation (6') (right) for various crosssection areas of n-hexane.
K1
=
57
Since in the region of small B equation 2 gives a bettcr description of the experimental isotherm than the BET equation, so K1 = 57, as compared with CBET = 108, is a better approximation to the true adsorbate-adsorbent equilibrium constant (the "Henry constant") in the system benzenebasal plane of graphite. The relationship between the constant CBET and the constants of equation 2, K1 and K,. may be established by taking advantage of the fact that a t large and h the differences in the adsorption mechanism in the first layer are no longer of importance. Since in the derivation of our equation 2 the same mechanism of polymolecular adsorption was taken as in the BET theory, both the BET equation (1) and our equation ( 2 ) approach each other in the region of large 0 and h. Indeed, from (2) it follows that h_ = 1-
+ (Kr - 1)h - KiK,(I -
Z)
(2) Ki approaches a linear dependence on h, since at h + 1, x + 1 and h/x 4 1 just as in the case of the BET equation 1. For this reason, we can equate expressions for h from (1) and (2) at the upper limit h = 1 and z = 1,whence it follows that CBET= Ki(1 ZCJ (4) In the case of adsorption of benzene on graphite K 1 = 57, K , = 0.88, whence from (4) CBET = 107 in accordance with the limiting value of the quantity Z
0.2 0.4 0.6 0.8
0.2 0.4 0.6 0.8 1
1 8.
Fig. 5.-The adsorption isotherm of nitrogen on graphitized thermal carbon black, SIT-I (3100") in coordinates of equation 3 (left) and equation 5 (right) for various crossscctional areas of nitrogen.
in Fig. 3 for small h (small x ) are due to the influence of residual surface inhomogeneity that increases the quantities ~ (-l h ) e +[@(l-;j1h40.
0 ' 2
h
+
~ - =(-l h )
h(l
- 2)
Z
