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Langmuir 1993,9, 2523-2529
2623
The Brunauer-Emmett-Teller Equation and the Effects of
Lateral Interactions. A Simulation Study
Alon Seri-Levy and David Avnir’
Department of Organic Chemistry and the F. Haber Research Center for Molecular Dynamics,
The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Received June 18,1992. In Final Form: November 1 0 , 1 9 9 9
Adsorptionldesorption on a smooth surface is studied at the molecular level by new, simple
two-dimensionalMonte Carlo simulation procedures using site-specificadsorptiorddesorption probabilities.
The classical BET conditions are simulated and the resulting adsorption isotherms show a very good fit
with the calculated theoretical parameters of the equation. The BET islands are visualized and analyzed.
Following this test simulation, lateral interactions are added, resulting in type I1 and type 111isotherms.
The ensuing deviations from BET behavior are identified and analyzed. It is shown that accurate surface
area values can only be obtained from an unequivocal B-point. The effects of changes in adsorption1
desorption probabilities on the isotherm shapes are identified and discussed. The changes in the heat
of adsorption with coverage are analyzed. It is found that this type of plot is a very sensitive analytical
tool for locating the monolayer value.
1. Introduction
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roughness factor is 1.0 in order to be able to make a direct
comparison with the BET monolayer value.4
Molecular level computer simulations of adsorption
processes are devoid of this problem. Since here the true
surface area is a known input, it is possible to test the
accuracy of the BET equation under the more realistic
conditions of lateral interactions and an energetically
heterogeneous surface (the effects of the latter to be
addressed in a subsequent report). The usefulness of
computer simulations to the understanding of surface
science is by now well documented.”12 What had been
thought to be a disadvantage, namely the inability of
simulations to provide true-to-life mimicry of all parameters involved, turned out to be an advantage: one can
identify, isolate, and study single important parameters,
free of the fogginess imposed by real conditions. The
approach adopted here is, we believe, novel and concentrates on adsorption/desorption probabilities at the single
molecule level. Its main advantages are as follows: (a) It
allows convenient simulations of adsorptionldesorption
processes on irregular morphologies of any shape and
form13 which are either very difficult or impossible to
perform by classical simulation methods (these morphologies are the topic of a subsequent reportI4). (b) It allows
the visualization of the adsorption process on a molecular
level: in a sense, our approach provides molecular level
microscopy for such processes. (c) We show that the very
basic and elementary rules are capable of retrieving quite
A paper written by Brunauer shortly before his death
provides a rare glimpse into the bitterness with which this
forefather of surface science looked back upon his lifetime
career.’ He opened his paper with “I am not sure that
there is a paper in the last half century that produced as
much adverse criticism as the BET2paper”, and ends his
account with “[Langmuir’sl equation was one of the
reasons for getting his Nobel Prize”, whereas “The BET
paper was recommended for the Nobel Prize but did not
receive it.” Adds Adam~on:~
“However, it is still, after
almost 50 years, among the most frequently quoted papers
in the field. As for practical applications, suffice it to say
that one place alone, the Institute of Catalysis at the USSR
Academy of Sciences, makes some ten thousand BET
surface area measurements per year.” This fascinating
dichotomyof the BET equation reveals an important face
of the modern scientific method: the ability to achieve
tremendous progress in a field, using a theory that far
from reflects reality. In a sense, the BET equation is a
monument to the achievements of imperfection.
The main criticism of the BET model is that it uses two
basic assumptions that are intrinsically inaccurate. The
first is that all adsorbing surface sites are assumed to be
energetically homogeneous, and the second is that only
vertical interactions between adsorbed molecules and the
adsorbent surface take place, thus neglecting lateral
(horizontal) interactions between adjacent adsorbed molecules. In the light of these basic (and problematic)
(4) Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 2nd
assumptions, the key question asked about the BET
ed.; Chapman and Hall: New York, 1987.
equation is whether the surface area measurement it
(5) Nakagawa, T. Kolloid 2.2.Polym. 1967,221,40.
provides is the “true”,correct value. Althoughthe general
(6) Steele, W. A.; Bojan, M. J. Pure Appl. Chem. 1989,62,1927.
(7) Morioka, Y.; Kobayaehi, J.4. Nippon Kagaku Kaishi 1982, 549.
concensus seems to be that the answer is negative, it has
(8) de Keizer, A.; Michalski, T.;Findenegg, G.H. Pure Appl. Chem.
been impossible to evaluate by how much the BET area
1991.63.1495.
- ..-, - -,- ._
-.
is off, since there has been no method that could
(9) Citri, 0.;Kagan, M. L.; Kosloff, R.; Avnir, D. Langmuir 1990, 6,
569 (Erratum: 1991, 7,610).
independently provide that elusive “true” value. Even in
(10) Seri-Levy, A.; Avnir, D. Surf. Sci. 1991,248, 268.
calculations using “perfect” crystals with a known geo(11) Patrykiejew, A.; Binder, K. Surf. Sci. 1992,273, 413.
metric area, the assumption had to be made that the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
(12)Nicholson D.; Parsonage, N. G. Computer Simulation and
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Abstractpublished in Advance ACS Abstracts, August 15,1993.
(1)Brunauer, S. Langmuir 1987,3,3.
(2) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. SOC.1938,
60,309.
(3) Adamson, A. W. note 7 in ref 1.
0
Statistica~hfechanicsof Adsorption; Academic Prese: London, 1982;pp
328-342.
(13) (a) Seri-Levy,A.; Avnir, D. Proceedings of the 4th International
ConferenceFundamentale of Adsorption, Suzuki M., Ed.,in press. (b)
Seri-Levy, A. PbD. Thesis, The Hebrew University of J e d e m ,
Jerusalem, December 1992.
(14) Seri-Levy, A.; Avnir, D. Preprint, 1992.
O743-7463/93/24O9-2523$04.oO/o 0 1993 American Chemical Society
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Seri-Levy and Avnir zyxwvutsrq
2524 Langmuir, Vol. 9, No. 10, 1993
ooo
0
m
o
0
,”,
00
m m
0
O o
00
9
,m
O
00
0 0
o
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8
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00
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Figure 1. Tower-like structure of the BET islands: surface, 0;
adsorptive or neighboring molecules, 0;adsorbed molecules, 0 .
Net adsorption probabilities: A l / D l = 19, Az/Dz = 3. PIP, from
bottom to top: 0.13, 0.27,0.41, 0.55.
accurately realistic shapes of adsorption isotherms and
realistic changes in their shapes, as adsorption parameters
are varied.
The main aim of this study was to construct a Monte
Carlo molecular level simulative procedure of adsorption/
desorption processes, with lateral interactions. This was
achieved by first simulating the classical BET equation,
retrieving all its parameters, and subsequently adding
lateral interactions. Mimicking the BET model of the
adsorption/desorption process before adding the lateral
interactions ensures that the revised algorithm is free of
artifacts. In addition to the estimation of surface area
using the BET equation, the B-point method was also
employed for the resulting isotherms, and guidelines for
increasing the accuracy of the BET monolayer prediction
ability under conditions of lateral interactions are suggested. The simulation of the original BET model also
allows one to observe, perhaps for the first time, the
unnatural arrangement in “islands” of the adsorbed
molecules on the surface. We finally expkorethe potential
use of average adsorption probabilities for the analysis of
surface heterogeneity.
2. Details of the Computer Simulations
2.1. General Properties. The conducted simulations
were two-dimensional and of Monte Carlo type. In all
simulations, the surface was a line 100 pixels long (Figure
1,hollow squares). A reservoir (square lattice), 100 pixels
wide, was placed above the surface and contained an
initially homogeneously-distributed amount of adsorptive
molecules (ideal gas), each molecule occupying an area of
1 pixel. A reservoir pixel can be either empty or occupied
with no more than one adsorptive in it. At any moment,
three types of molecule populations can be found (Figure
1): adsorbed molecules (black circles); neighboring molecules, unadsorbed molecules which are adjacent either
to a surface site or to an adsorbed molecule, and hence
available for adsorption in the next time step (hollow
circ1es);gasphase molecules, unadsorbed molecules which
are not available for adsorption in the next time step (also
hollow circles). Accordingto the BET model, only vertical
interactions are possible and hence an unadsorbed molecule will be considered as a neighboring molecule only if
it is located on the top (position [ x , yl) of either a surface
site or an already adsorbed molecule (located in position
[ x , y - 11). In the case of lateral interactions, the population
of neighboring molecules is extended. Here the “top”
limitation does not exist, and an adsorbed molecule or
surface site in any one of the positions [ x , y + 11, [ x + 1,
y], or [ x - 1,yl will also bring the unadsorbed molecule
at position [ x , y ] into the definition of “neighboring
molecule”.
2.2 Simulations of the BET Conditions. The simulations of the BET test case were carried out as follows.
At each time step, all molecules are treated consecutively
in random order. A selected molecule may belong to one
of the three populations mentioned above and, accordingly,
undergoes one of the following:
If the chosen molecule neighbors a surface site or an
adsorbed molecule, adsorption is attempted with probabilityA1or A2, respectively. If successful,this neighboring
molecule becomes adsorbed. If unsuccessful, it remains
in place.
If the chosen molecule is adsorbed on the surface or
above another adsorbed molecule, with no adsorbed
molecule on top of it, desorption is attempted with
probability D1 or Dz,respectively. If successful,it becomes
a neighboring molecule. If unsuccessful, it remains in
place.
After all the molecules have been treated, all gas-phase
molecules and neighboring molecules (all hollow circles)
are mixed and redistributed randomly and homogeneously,
and the next time step begins. This procedure is repeated
until equilibrium is reached, i.e. until the amount of
adsorbed molecules remains constant for at least lo00time
steps. For a full adsorption isotherm, the whole procedure
is repeated for various initial gas-phase concentrations.
The value of PO,
the “saturation” concentration or pressure,
is calculated from DdA2 as explained below (eq 7).
2.3 Simulations with Added Lateral Interactions.
The addition of lateral interactions diminishes the distinction between adsorption to the first layer and to higher
layers. For each set of simulations, two interaction heats
are defined. The first, Q1, is defined as the heat of
adsorption to the surface, and the second, Qz,is defined
as the heat of interaction between two adjacent adsorbed
molecules (heat of liquefaction). In thermodynamic terms,
the net probability of adsorption to the first layer, A1/D1,
is exp(Q1lRT) and to higher layers, AzlDz, is exp(Q$RT).
Hence, by adding lateral interactions of van der Waals
type, the net adsorption probability, either to the first
layer or to any higher layer, is
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A
- = exp((ZIQl + Z,Q,)/RT)
(1)
D
where 21 is the number of nearest neighboring surface
sites (21 = 1 in the case of smooth line) and 22 is the
number of nearest neighboring adsorbed molecules. The
temperature, T, is also an input parameter of the simulation. For purposes of economizing in computer time, A
was defined as A = 1,and D was determined by the input
values of Q and T, according to eq 1.
zyx
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Effects of Lateral Interactions zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Langmuir, Vol. 9, No. 10,1993 2625
Figure 2. An artist’s (A. W. Adamson) visualization of Figure
1. Reprintedwithpermissionfrom ref 15. Copyright 1990Wiley.
1.0
!
I
0
5
0
I
15
10
20
Island Size (pixels)
Figure 4. Distribution of BET islands obeys eq 2. The results
for 90 lines of 100 pixels each were summed up. AdD1 = 19,
AdDz= 3,PIP, = 0.41 (taken at equal time intervals of 100time
steps at equilibrium).
In addition to the maintenance of constant pressure for
1000 time steps, equilibrium conditions were doubly
ensured by examining the size distribution of the BET
islands. The latter should obey a simple law which is
derived as follows: Taking the fraction of occupied sites
to be 1- Bo, we ask what is the probability of obtaining,
within the randomly distributed occupied sites, an island
of width k. Since 1- 80 is the probability of finding an
occupied site, then the probability of finding a sequence
of k neighboring sites is (1- Bo)c. An island is bound by
two empty sites, and hence the probability of finding an
island of size k is (1- Bo)kBo2. If the totalnumber of surface
sites is Mt, then the total number of islands of size k is
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kfk= ~ , -( eo)ke,2
i
Figure 3. Equilibrium dynamics of BET islands during 30
consecutive time steps (left column from top to bottom then
right column from bottom to top). Al/D*=19,AdDz = 3,PIP0
= 0.41.
3. The BET Equation
3.1. The BET Islands. The BET simulationsnot only
serve as a test case but also provide an opportunity to
obtain a visual display of the BET islands. To the best
of our knowledge, this very basic aspect of the BET
equation has not been reported. One does find intuitive
descriptions of these adsorbed patches, such as that
provided by Adamson15 (Figure 2), but not computed
islands. Figure 1shows the picture for the set A1/D1= 19,
AdDz = 3 at four PIP0 (=m/mo,where m is concentration)
values and the increase in the size and height of the islands
as the pressure is increased. Interestingly, on the molecular
islands, Brunauer wrote: “Teller ...felt-very justly-that
the model of columns of different heights of molecules is
not right”.’ The dynamic nature of the BET islands at
equilibrium is demonstrated in Figure 3 for 30 consecutive
time steps, for the same PlPo value. The reader is invited
to follow one of the islands through the 30 time steps in
order to get a first-hand impression of the structural
changes that occur within it. From Figure 1one can see
that Teller’s doubts were justified, especially at high
concentrations. Adding lateral interactions (as will be
demonstrated below) flattens the BET islands into more
realistic shapes.
(15) Adamson, A. W .Physical Chemistry of Surfaces, 6th ed.; Wiley:
New York, 1990;p 611.
(2)
A plot of In Mk vs k should give a straight line. As can be
seen in Figure 4, this equation is nicely obeyed, with Bo =
0.206 from the intercept and 00 = 0.204 from the slope,
compared to the value of 0.189 as obtained by the direct
counting of sites. The cumulative data of 90 equilibrium
steps is shown.
3.2. The BET Isotherm. The comparison between
the simulation results and the BET theory is made as
follows: a,the ratio between 81, the coverage by one layer
of adsorbed molecules, and the uncovered surface Bo, is
given by
B1
k’A,P
a=-=-=00
4
1
kAlm
VlDl
(3)
where k’ is a constant counting the number of collisions
per unit time per unit area normalized to the pressure,
and v1 is the vibration frequency of the adsorbates. Since
we perform our simulations in terms of concentration, m,
rather than P, k replaces k’.
A neighboring molecule collides (attempts an adsorption) only once in each time step. The average rate of
collisions per surface site is, however, less than 1, by a
factor determined by the concentration of the neighboring
molecules. Since this value is normalized to the total
concentration which, in turn, is equal to the concentration
of the neighboring molecules, k = 1. Similarly, VI = 1
because at each time step the adsorbate makes one attempt
to desorb. Equation 3 then simplifies to
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Seri-Levy and Avnir
2526 Langmuir, Vol. 9, No. 10,1993
theoretical and simulation results. The dashed line is the
BET function in the form
where N is the number of adsorbed molecules and Nmis
the monolayer value. N, is known (=loo) and C is
calculated from eq 6 and given in Table I. It is seen that
as C increases, the simulation is able to reproduce the
expected change in the isotherm shape. Figure 1shows
the actual molecular distribution at several P/Povalues
on one of these isotherms. Figure 5b shows the fit of the
three isotherms to the BET equation
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--+-(-)
c-1
(10)
N[(P,,/P) - 13 NmC NmC Po
Notice that since the equation (and consequently its
simulation) is free of lateral interactions and condensation,
the fit is not limited to the 0.05-0.3p/p0range but can be
carried out to higher PIP0 values. Table I shows the
comparison between the calculated parameters (eqs 6 and
8) and the simulation results (eq lo), and the agreement
is good.
Having analyzed these simulations of the classical BET
isotherm equation, and having demonstrated that our
simulation conditions accurately retrieve this equation
and mimic equilibrium states, we proceed to more realistic
adsorption conditions.
1
1
P
P I Po zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
-
"
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b
011
012
013
014
015
016
017
018
PIP,
Figure 5. (a) BET adsorption isotherms. The parameters are
collectedin Table I. Isotherms a, b, and c refer to bottom, middle,
0 , and 0 are the simulation
and upper lines, respectively. .,
results. The dashed lines are the fit of these results to the BET
equation (eq 9). Figure 1 shows four equilibrium points on
isotherm a. (b) BET analysis (eq 10)of the isotherms in a. The
resulting parameters are shown in Table I.
(4)
Similarly
4. Adsorption with Lateral Interactions
4.1. The Adsorption Isotherms. The BET equation,
as will be recalled, does not take lateral interactions into
account, although these cannot be neglected in most, if
not all, adsorption processes. Isotherm equations including lateral interactions have been suggestedl6J7but never
gained wide use. Instead, the BET equation is universally
used, at least in the sense that all manufacturers of
adsorption instruments install the equation as the standard
automated way to evaluate surface area. Perhaps the main
reason for this situation is the fact that quite often, analysis
of experimental data according to eq 10, over the traditionally recommended PIP0 range of 0.05-0.3,does provide
a good straight line. With the popular single-point BET
determinations, there is clearly no fitting whatsoever.
It is the aim of this section to estimate the error involved
in BET surface area determinations, to find under what
conditions eq 10 appears to work in the presence of lateral
interactions, and to see how one can minimize BET errors
in such cases.
Figure 6a shows a set of five adsorption/desorption
isotherms for which QdRT = 0.5195 and Q1/RT decreases
gradually from 12.987 to 0.649 (curves a-g, respectively).
The following observations and interpretations are made:
No hysteresis of the adsorption/desorption loop is
observed. This is an important test since we show in a
subsequent report that introduction of geometric irregularity is sufficient for the full development of such
hystereses. This test also corroborates the fact that in
our simulations a good equilibrium state is obtained at
each point.
For the four highest QlIRT values, i.e. strongest adsorption, the classical type I1isotherm is obtained (Figure
6a, curves a-d). We recall that this shape of isotherm is
typical for adsorptions on nonporous or macroporous
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where B1,, B,, are the fractions of surface covered by
columns of n - 1and n adsorbed molecules, respectively.
The BET constant, C, can be calculated from eqs 4 and
5
This equation allows comparison between the C obtained
from the simulation and the calculated one.
Finally, since under BET conditions4
(7)
where mo is the saturation concentration, mo can be
calculated from eqs 5 and 7
mo = D2/A2
(8)
The BET simulations were carried out for three pairs
of A1/D1, Ad02 values. The three resulting isotherms are
shown in Figure 5a, with an excellent agreement between
(16)Hill, T. L. J. Chem. Phys. 1947,16, 767.
(17)Gregg, S.J.;Sing,K. S. W. Adsorption, Surface Area andPorosity;
Academic Press: London, 1982.
Effects of Lateral Interactions
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zyx
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Langmuir, Vol. 9, No. 10,1993 2627 zyxwvutsrqp
Table I. Simulation of the BET Conditions
experimental results zyxwvutsrqponmlkjihgfedcbaZYXWV
BET prediction
input values
BET PIP0
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Po
isotherm
(Figures 5a,5b)
a, 0
N,cP from
AdD1
AdDz
(~oP
Cb
simulation
101
19
3
113
6.3
100
213
12.7
19
1.5
b, 0
100
213
132.7
C, 8
199
1.5
F’rom eq 8. * From eq 6. Using eq 10. Input value: N , = 100 pixels.
250
i:
c
f
200
a
a
150
”c
100
4
i2
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
PIPo
-
0.002
FE
5
0.001
0
PIPo
Figure 6. (a) Adsorption isotherms with QdRT = 0.5195 and
lateral interactions: 0,adsorption; 0 , desorption. From top to
bottom (a-g): QdRT = 12.987,7.792,5.299,3.896, 1.948,1.208,
0.649. (b)BET analysis of the type I1 isotherms of Figure 7.0,
Q1IRT = 12.987;A, Q1IRT = 7.792;W, QIIRT = 5.229;0 , Q1IRT
= 3.896. PIP0 range: 0.05-0.30.
C from
BETC
6.2
12.6
130.1
C from
(alB)
5.9
12.8
127.7
range analysis and
(no. of winta)
0.13-0.80 (6)
0.07-0.43 (6)
0.06-0.43(6)
corr coeff
(Figure 5b)
0.9994
0.9999
1.oooO
exp(Ql/RT), to exp((QllQz)lRT),to exp((Q1+ 2Q2)IRT)
etc., and so the loop continues.
Figure 6a re-emphasizesthe strength and the weakness
of the B-point in monolayer evaluations. It is seen that
while the sharp knee in curves a-c appears at the correct
value of Nm= 100 pixels, the flatter knee in curve d leads
to a false evaluation of the monolayer capacity. This
finding supports Gregg et al.17 and Halsey,20who claimed
that accurate surface area values can only be obtained
from an unequivocal B-point. The sharper the knee, the
lower the PIP0 at which the B-point is attained, and the
greater the accuracy of the calculated monolayer capacity.
4.2 Apparent BET Behavior in the Presence of
Lateral Interactions. Type I1 isotherms, i.e. isotherms
with a well-defined B-point, tend to obey the BET equation
in the sense that a straight line is obtained by applying
eq l O l g for PIP0 between 0.05 and 0.3. This is shown in
Figure 6b for isotherms a-d in Figure 6a. For all four
cases the line is slightly concave, but one can readily
appreciate that under experimental conditions and with
the 3-point BET practice, this concavity is not detected.
The apparent surface areas are somewhat below the true
monolayer value of 100 pixels; i.e. the apparent area
obtained by the inappropriate use of the BET equation
’ . Both underestimations
is underestimated here by -7 %
and overestimations of the nitrogen BET monolayer
capacity, relative to the B-point value, have been reported
by many authors. For instance, Young and Crowell
collected 68different solidsmand claimed that ‘there seems
to be no way of telling whether a low C value causes the
point B or the BET methods (or both) to be in error”. As
mentioned in the Introduction, the strength of a simulation
is that it allows comparison to the true surface area which
in most experimenta is unknown.
For high C values, improvement of the monolayer
estimation is achieved by shifting the analysis from the
standard PIP0 range of 0.05-0.3, to PIP0 values around
the B-point. Indeed, as shown in Table 11, for isotherms
a-c in Figure 6a,Nmimproves with shifting the PIP0 range,
reaching very good values of N m = 100. Although isotherm
d in Figure 6b is of type 11, shifting the PIP0 BET range
does not improve the monolayer result. It should be
recalled that it is not possible to estimate surface area
from the B-point of curve d. As noted by Halsey,2l the
BET method is in effect a graphical representation of the
B-point. Indeed, from these simulations it can be seen
that the BET monolayer value accuracy is equivalent to
the sharpness of the B-point.
Also seen in Table I1 is the effect of this procedure on
the BET constant C, which changes gradually from
negative values (as indeed observed in experiments=) to
“legitimate” positive high C values, for which the BET
equation operates well. The first two type I11 isotherms
zyxwvutsrqp
objecta,4J’ where’ adsorbate-adsorbate interactions are
much weaker than adsorbate-adsorbent interactions, and
the latter, in turn, are relatively strong. Under these
conditions, monolayer coverage already occurs at relatively
small PIP0 values.
As Q1IRT is lowered, a type I11 isotherm is obtained
(Figure 6a, curves e-g). Type 111 is observed on flat
surfaces18Jgwhere adsorbate-adsorbate interactions are
comparable to adsorbateadsorbent interactions, and
hence multilayer adsorption starts before monolayer
coverage is completed. Ita concavity has been explained17
in terms of a positive feedback loop: Molecules which are
adsorbed at the first layer convert the surface into a
stronger adsorbent, since the net probability of adsorption
near an adsorbed molecule increases significantly from
(20) Young, D. M.; Crowell A. D. Physical Adsorption of Gores;
(18) Sing,K.S.W.;Everett,D.H.;Haul,R.A.W.;Moscou,L.;Pierotti, Butterwortha. London, 1962.
(21) Loeser, E. H.; Harkim, W. D.; Twiss, S. B. J. Phys. Chem. 1953,
R. A.; Rouquerol, J.; Siemienieweka, T. Pure Appl. Chem. 1985,57,603.
67, 591.
(19) Brennan, D.; Graham, M. J.; Hayes, F. H. Nature 1963,199,1152.
(22) Halsey, G. D. Diacrces. Faraday SOC.1950,8,54.
Sing, K.5.W. Chem. Ind. 1964, 321.
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2528 Langmuir, Vol. 9, No. 10,1993
Seri-Levy and Avnir
Table 11. Effect of Narrowing the PIP0 Interval on the BET-Determined Monolayer Value in Adsorption
isotherm'
curve a
QiIRT
QdRT
12.987
0.5195
curve b
7.792
0.5195
curve c
5.299
0.5195
curve d
3.896
0.5195
curve e
curve f
curve g
1.948
1.208
0.649
7.792
0.5195
0.5195
0.5195
0.7792
5.299
0.7792
with Lateral Interactions
isotherm
BET analysis
type
PIP0 min
PIP0 max
NI2
0.27
92.6
I1
0.06
0.0004
0.04
99.0
I1
0.06
0.27
92.6
0.0003
0.08
98.3
I1
0.06
0.27
92.8
0.0033
0.11
99.1
0.27
93.1
I1
0.07
0.05
0.17
91.7
0.28
110.2
I11
0.06
I11
0.06
0.28
106.8
I11
0.06
0.29
78.8
I1
0.08
0.26
87.3
0.004
0.19
96.6
I1
0.08
0.27
87.2
0.03
0.15
92.7
I11
0.07
0.28
135.9
1.948
0.7792
In Figure 6a. Input value: N,,, = 100.e Does not obey the BET equation. d Negative C value.
C
d
+OD
d
5344
d
181
77
113
3.6
1.5
1.1
d
10447
d
424
1.6
corr coeff
0.9998
1.oooO
0.9998
1.oooO
0.9997
0.9998
0.9997
1.oooO
0.984of
0.9489
0.6781'
0.9996
1.oooO
0.9998
0.9999
0.8898C
300
0
&
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
PIP,
Figure 7. Adsorption isotherms of two type I1 isotherms with
QdRT= 5.229and different QJRTvalues. The dotted isotherm
is for QJRT = 0.5195 and the solid is for QJRT = 0.7792.
(Figure 6a, curves e and f) only superficially resemble the
BET isotherm, while the third one (Figure 6a curve g)
does not obey the BET equation at all. This finding also
agrees with experimental observations.17
Additional simulations carried out with QdRT = 1.208
instead of QdRT = 0.7792 as previously, led to similar
qualitative results both for type I1 and type I11isotherms
(see Table 11). With the increase in lateral interactions,
the error in the BET surface area increased to 13 7%. In
Figure 7 a comparison between two isotherms with the
same Ql/RT = 5.229 but different Q2/RT values is made.
The increasing influence of the lateral interactions on the
shape of the isotherm is observed immediately after the
B-point. Hence, calculating the BET surface area from
PIP0 values beyond the B-point increases the error in the
calculated surface area.
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N
5. Enthalpy of Adsorption
Adsorption is usually visualized in a simplified way as
the interaction between a reservoir of adsorptives and a
surface of fixed properties, acting as an equilibrium sink
for the molecules. In reality, however, the picture is much
more complicated, because the surface itself is a dynamically changing entity, either when equilibrium is being
established at a given PIP0 value, or when one sweeps the
system along the isotherm. The reason is simple: The
original bare homogeneous surface is quickly replaced by
a new surface, which is the outer blanket of sites composed
Figure 8. An effectively heterogeneous surface: the outer
contour of the adsorbed (black) molecules. QI/RT= 5.229,Qd
RT = 0.5196. PIP, from bottom to top: 0.40, 0.52, 0.64, 0.80.
both of empty surface sites and of adsorbed molecules.
This new, ever-changing surface is quite heterogeneous,
and is composed of sites with a variety of adsorption/
desorption probabilities.
This heterogeneity of the equilibrium state, which is
generated by the presence of lateral interactions, is
demonstrated in Figure 8. It can be noticed that the
adsorbed molecules are not organized in high columns (a
structure Teller has already questioned). One can actually
distinguish between the inherent heterogeneity of the
surface, i.e. the heterogeneity that is due to static
parameters such as geometric irregularities and chemical
impurities, and heterogeneity that is due to adsorption
Effects of Lateral Interactions
-6
I
I
0
0.5
I
1.o
zyx
zy
z
zyxwv
Langmuir, Vol. 9, No. 10, 1993 2629 zyxwvutsrqpo
1.5
I
2.0
2.5
NINm
Figure 9. Enthalpy of adsorption profiles for Figure 6a. Solid
lines are for type 11 isotherms, dotted l i e s are for type I11
isotherms, and the dashed line is for the BET model (no lateral
interactions). The deepest type I1 profie is for isotherm a and
the shalloweet is for isotherm.d. The type I11 isotherm profiles
(e (top), f (middle), g (bottom)), as well as the BET profile, do
not give any indication about the monolayer coverage.
itself. The latter is a changing,N/N,-dependent property.
The former, of course, dictates the latter, at least at low
coverages.
How then is it possible to characterize this complex,
PI&-dependent situation? Traditionally, a global thermodynamic function, such as enthalpy of adsorption, is
used.2s The simulation allows us to calcuIate the averaged
net adsorption probability at equilibrium on the irregular
hull contour line that comprises the effective surface
available for adsorption and, consequently, to calculate
the averaged enthalpy of adsorption.
Figure 9 shows the results of this analysis for the seven
isotherms of Figure 6a. It is observed that this analysis
is a sensitive probe for the detection of the monolayer
value and provides a vivid presentation of the B-point.
The drop toward the monolayer is due to a decrease in the
average net adsorption probability following an increase
in the number of adsorption sites with low net adsorption
probability. This is most acute just beyond N/Nm = 1,
while later it asymptoticallyreaches a constant value. Since
Q2 is the same for the seven isotherms, the lines merge at
the domain where 81 does not contribute, as already
observed above. Perhaps unexpectedly, the highest heat
~~
(23)Joyner, L.G.;Emmett, P.H.J. Am. Chem. SOC.1948,70,2353.
of adsorption to the surfaceleads to the deepest minimum.
This, however, is understood by noticing that the highest
Q1 also means a sharp distinction between the Q1 and Q2
domains; i.e. a Langmuirian monolayer is obtained. The
enthalpy of adsorption analysis also sharpens the distinction between BET conditions (with AI/D1= exp(5.299)
and AdD2 = exp(0.5195)and lateral interaction conditions.
While isotherms may look similar, the enthalpy of adsorption probability curve is different, as shown in Figure
9; the BET line provides no indication of where the
monolayer might be. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP
6. Conclusions
A simulation tool has been constructed and applied to
the analysis of adsorption on a flat homogeneous surface,
with and without lateral interactions. In this study:
We have displayed for the first time the BET islands
of adsorbates and analyzed their size distribution.
We have demonstrated and analyzed correlations between the adsorption/desorption probabilities (the C
constant in the case of BET) and the shape of the
adsorption isotherm (isotherms of type I1 and I11 in our
case).
By comparing known input data on the monolayer value
to simulation results, we showed that the experimentallyderived B-point, when such is apparent, is a reliable
monolayer indicator. This conclusion is indeed widely
practiced by experimentalists.
We showed that applying the BET equation to the
realistic conditions of adsorption with lateral interactions
on smooth surfaces leads to underestimated monolayer
values.
We showed that for those cases of a well-defined B-point
(high C value), this error can be significantly reduced by
applying the BET equation to a PIP0 range below the
B-point. Since high C value and low PIP0 range convert
the BET equation into the Langmuir equation, this
suggeststhe preferable use of the latter whenever possible.
We have demonstrated that true homogeneous surfaces
in principle never exist by virtue of the heterogenizing
action of the very adsorption process. This dynamic
property was analyzed in terms of plots of enthalpy of
adsorption as a function of coverage. These plots turned
out to be sensitive indicators of the monolayer value.
Acknowledgment. Supported by the US.-Israel Binational Foundation. D.A. is a member of the Farkas
Center for Light Energy Conversion.