Chemistry Reference
In-Depth Information
0
62
n
p
(0.002
＜
＜
0.04)
(3.5)
22
o
n
p
0
0
49
n
p
(0.01
＜
＜
0.16)
(3.6)
15
o
n
p
0
Measurement of the N
2
isotherms at 77.4 K until
p/p
o
=210
-8
made it
possible to determine the three experimental formulas mentioned above.
Equation (3.4) clearly shows that the N
2
adsorption isotherms at 77.4 K by
highly graphitized sample Carbotrap-F-CB obey Henry‟ law quite closely at very
low surface coverages. It is well known that the simplest behavior of adsorbed
phase is expressed as Henry‟s law of equation (3.
7) [33],
n=k
H
p
(3.7)
where
n
is the specific surface excess amount and
k
H
the Henry‟s law constant.
Equation (3.7) is derived by assuming that the dilute adsorbed phase behaves as a
two-dimensional ideal gas on the uniform surface. The experimental fact given by
equation (3.4) and Figure 13 clearly indicates that the N
2
adsorbed phase at 77.4 K
on Carbotrap-F-CB behaves as the ideal two-dimensional gas at the very low
surface coverages of 0.002
＜
＜
0.02 and that the surface of Carbotrap-F-CB is
highly homogeneous or uniform until very low coverage, =0.002.
Rouquerol and co-workers [33] have pointed out that deviation from linearity
(convex curvature with respect to the adsorption axis) may be due to surface
heterogeneity [33]. In our previous paper [8], we have reported that the surface of
#3845-1-CB is partially heterogeneous (
ca.
20%), and the surface of #51-CB is
heterogeneous. From the present results of equations (3.4), (3.5), (3.6) and the
previous
q
st
results (
cf.
Figure 9 of reference 8 and Figure 15), the following
conclusion may be obtained.
If the relation between
and (
p/p
o
) is expressed by equation (3.8),
n
n
k
2
p
(3.8)
n
n
k
0
1
o
p
the constant
k
2
of equation (3.8) is the important parameter of evaluating surface
heterogeneity of CB adsorbents; in other words, the high adsorption energy sites
of heterogeneous CB surfaces (#51, #3845-1-CB) brings about
k
＜
1, and as the
