CATALYZED LAMELLAR, R,EACTIONS ON' GRAPHITE*
M¡¡cor-r¡m
L. Dzr¡nus
Argonne N ational
and Gnnse¡,r
Il,. IIrNrrc
Laborato ry, Arg onne, Illi,nois
(1\Ianuscript received September f4, lg6f)
IIosü larnellar compounds of graphite form readily without the need of a catalyst because of a large
favorable free energy of formation.
Ifowever, for some compounds the free energy change is sma"ll
and the formation of such compounds may require a caüalyst. The formabion of thl sodiuá-graphite
lamellar compounds has been found by us to require a catalysü. fü has beon reported that thelaniellar
reactions of graphite with indium chloride or graphite with cadmium chloride-are also catalyzed
reactions'
These reactions are l¡rown to require ühe presence of chlorine which is reported not to enter
into
the compound.
Ilowever, our experiments shov¡ ühaü the compounds of graphite with indium chlorido
or with cadmium chloride contain appreciable amounts of chlorine.
The chlbrine is completely ionized
and in both the indium chloride and cadmium chloride compounds the chlorine is ühe onlylcceptor
present. Tho reacúed chlorine can be recovered as free chlorine by thermal decomposition
of'the
compounds.
Graphite reaets with many substances, such
as bromine, ferric chloride, or pota,ssium, to
form interstitial compounds of graphite
called lamellar compounds. In these lamellar
compounds the planes of carbon atoms
alternate in a definite periodic sequencewith
plenes of the rea,ctant. When lamellar compounds are formed an electron transfer occurs
between the reactant and the graphite.r If
electrons are transferred from the reactant to
the graphite, donor compounds are formed.
If, on the other hand, electrons are transferred
from the graphite to the reactant, acceptor
compounds are formed.
It is believed2that the energy ofthis electron
transfer is the major contribution to the free
energy of reaction. Therefore, substances
which cannot readily accept or donate
electrons should not form lamellar compounds.
Horvever, even these substances can sometimes be incorporatedintolamcllar compound.s
*Based on'work performed under ühe
auspices of
the U.S. Atomic Energy Commission.
1 G. R,. Hennig, Progreso ,in Inorganic Chemi,strg,
Vol. l, fnüerscience (1959) p. 125.
2 G. R. Hennig, Proc, 7st onit Zrul Carbon ConJ.
University of Buffalo (1956) p. f 03.
if an additional reactant capable of exchanging
electrons is prescnt. Under thcse conditions
so-called ternary lamellar compounds a,re
formed. containing two reactants in addition
to the graphite. There is evidence that one
of the components in these compounds seryes
as a "spacer" to separate the chargesrosulting
from the electron transfer of the other component.s As an example,4a ternary compound
is formed when graphite is heated.with iodine
a,nd aluminum chloride. In this case neither
the iodine nor the aluminum chloride will
react, sepa,rately with graphite. Electrical
measurements suggest that only the iodine
acts as the acceptor because the acceptor
concentration equals the ioüne concentration
which is approximately one-third as large as
the concentration of the aluminum chloride.
It has repeatedly been observeds that
lamellar reactions occur when impure chemicals are reacted with graphite whereas the
purified materials do not react. In several
such instances this behavior was traced to tho
3 G. R. Hennig, J. Chen¿. Phys. 20, l44B (tg52l.
a M. L. Dzurus and G. R. Hennig, J. An'¿er.
Chem.
Soc.79,1051 (r957).
s R,. C. Crofü, J . Appl. Chem. 2, 557 (tSlZ).
t39
140
FIFTII
CARBON CONFERENCD
formation of ternary lamellar compounds
incorporating also the impurity. It seemed
possible, however, that the impurities might
act as a catalyst which could. promote the
lamellar reaction without entering into compound formation. At one time such catalysis
by impurities was postulatedo for the reaction
of graphite with impure aluminum chloride
.was believed to bc merely a
where chlorine
catalytic agent. Careful analytical work
revealed, however, that the resulting compound was actually a ternary lamellar compound containing one chloride ion for each
three molecules of aluminum chloride.a
Since the possibility of catalyzed lamellar
reactions seemedplausible, we have examined
several other lamellar reactions which have
been reported to require catalysts. Such
catalysis is most likely to be found for reactions
in which the free energy change is small. Il
was in fact found that the reactions ofgraphite
rvith sodium or lithium are catalyzed by
minute traces ofhydrogen, oxygen, or water.T
In searching for other examples of catalysis,
the reactions of graphite with indium chloride
or cadmium chloride were studied becausethe
free energy of reaction was estimated to be
low and because both of these reactions
proceed only in the presence of chlorine.
Several investigatorss,ehave reported that the
resulting compounds do not contain excess
chlorine. These observations intimate that
the chlorine merely acts as a catalyst.
This apparent catalysis by chlorine was
confirmed by us in preliminary experiments
rvhich utilized a sensitive quantitative test for
very dilute lamellar compounds. The reaction
was detected by an increase in electrical
conductiviúy which is a nearly quantitative
index of acceptor or donor concentration.
6 W. Rüdorff and R. ZeIler, Z. Anorg. Allgem'.
Chern., 279, r82 (1955).
? M. L. Dzurus, G. R. Ilennig and G. L. Montel,,
Proc. Fourüh Carbon ConJ, Pergamon Press (1960)
p. 165.
eW. Rüdorff and A. Landel, Z. Anorg. Allgern.
Chem.293, 327 (1958).
e R,. C. Croft, Awt. J. Che¡n. 9, 184 (f 956).
When graphite was outgassed ín aacuo at
1000'C and rvas heated with triply distilled
indium chloride at 450'C or cadmium chloride
at 550'C, no detectablereaction occurred. To
deternine whether the adsorption of chlorine
on the surface of graphite is sufficient to
promote subsequent reactions, a sample of
graphite was heated for 15 min at 900"C in a
stre¿m of chlorine and cooledto room temperature in a stream of helium. This sample was
then heated with triply sublimed indium
chloride for 17 hr at 450"C and showed no
conductivity or weight changes. More than
chemisorption of chlorine is needed,therefore,
to promote the reaction.
Before examining this apparent catalytic
mechanism of chlorine in further detail, it
seemednecessaryto confirm that chlorine is in
fact only a catalyst and that no ternary
lamellar compound incorporating both chlorine and metal halide had been formed. Such
experiments showed that ternary lamellar
compounds had actually been formed and
that chlorine is, therefore, not a catalyst but
a reactant. The reactions were, however,
complicated and analyses made difficult by a
side reaction ofchlorine and carbon w-hichrvas
catalyzed by the metal halide. This side
reaction often consumed' a considerable
fraction of the chlorine and converted it to an
orgarric chlorine compound which escaped
conventional analytical methods for chlorine.
Because of this side reaction, the simple
analysis, used by other investigatorss,s,e of
decomposing a lamellar compound by complete combustion will lead to inaccurate
results due to loss of chlorine.
The consumption of chlorine in the side
reaction during formation of the indium
chloride compound. is shown in Table I' The
amount of chlorine consumedduring the formation of the lamellar compound at 450"C is
listed in the first column. The lamellar
compound can be decomposed nearly completely at 1000'C; during this decomposition
an amount of chlorine is released which is
listed in the second column. The difference
CATAIYZED LAMELLAR RDACTTONS Of,'GRAPIIITE
TABLE 1
Consumptdon oJ Chlori,ne d,uri,ng the Reocti,on oJ
Inil'iunt, Chlorid,eanil Graphiüe
104 x Cl/C
Consumed ¿t 450'C.
neleased at 1000"C
Artiñcial
graphite
2.r
2r.5
t4.7
4.4
4.9
24.7
Natural graphite
103.0
t4.5
cu.c
148.0
104 x
Acceptors/C
2.5
5.4
o.D
23.4
between the two values is unrecovered
chlorine which is presumably consumed by
the side reaction. The table also lists, in the
third column, values for the acceptor concentration which was determined by electrical
measurements. This acceptor concentration
agreed reasonably well rvith the concentration
ofchlorine releasedat I000"C. It is, therefore,
concluded that the side reaction occurred only
during the formation and not during the
decomposition of the lamellar compound.
This can readily be explained because during
the formation of the compound, chlorine and
carbon lyere heated together for many hours
at nearly atmospheric pressure whereas the
decomposition was carried out in high
vacuum. The data also show that a smaller
fraction of the chlorine was consumed in the
side reaction when natural rather than artificial graphite was used.
Table II lists similar observations for the
reaction of cadmium chloride and chlorine
with graphite. Again a cosiderable fraction of
TABLE II
the Reactíon
Consumpüion oJ Chlorine During
Cailmium Chlordde and, Graph'ite
104 x Cl/C
Consumetl at 550"C
Released at 1000'C
Artificial
25.7
r 7.3
29.5
ro.2
t2.3
Natural
37.8
graphite
ó.o
graphite
9.0
of
104 x
Acceptors/C
8.4
9.1
10.3
I4I
the chlorine was consunred in a side reaction.
Furthermore, the amount, of chlorine liberated
from the lamellar compound at 1000'C was
nearly equal to the acceptor cóncentration.
A complete analysis of the lamellar compounds formed can now be given sirrce it is
clear that the chlorine concentration in these
compounds can be obtained from the amount
of chlorine which is released at 1000"C.
Table III shows these analyses expressed as
ratios ofcarbon to indium chloride to chlorine.
As in all lamellar compounds the concentration ratio of reagent to carbon is highly
TABLE
III
An<tl,gsis of the Inili,nmt Chloride-Chlorine-Gra,phi,te
Compound,
c
4363
2281
2045
406
97.9
InCl3
Artificial Graphite
o.t
6.2
6.7
5.3
Natural graphite
6.7
l"t
I
I
I
I
I
Cn+ CI- . 6(InCfu) or Cn+ . (InoClrg)-
variable depending on whether every second,
every third, or eyery n-th interlayer space is
occupied. The ratio of chlorine to iudium
'a
chloride remains constant at
value of
essentially 6. Since the chlorine concentration
is also equal to the acceptor concentratiorr as
pointed out earlier the ideal formula for the
compound is probably C"+Cl-.6(InCl3) with n
variable from approximately 78 to infinity.
The value of n equal to 78 corresponds to a
ratio ofcarbon to indium chloride of 13. This
value has been reported8 to be the secondstage
lamellar compound in which two carbon
layers alternate with one layer of indium
chloride. In our experiments this concentrated
compound was never formed from artificial
graphite. Large flakes of natural graphite
ga,vea yalue of n eqaal to 98 which is somewhat lessthan the reported concentration for a
second stage compound.
i42
I.ITTH
CARBON
CONFEIiENCE
stabilized by the acceptor action ofappreciable
amounts ofexcesschlorine inthese compounds.
Our experiments also show that this excess
chlorine may easily be lost in the ordinary
anal¡rtical proced.ures because the metal
halides catalyze a side reaction of chlorine
with carbon. It should be emphasized that
the side reaction does not occur in the absence
of metal halides as evidenced by our obseryation that no chlorine is consumed when
heated for many hours with graphite at 450'C.
The exact composition of the product from
this side reaction was not determined, but
from the fact that it is volatile at room
temperature and contains little carbon, it
appears to be carbon tetrachloride. It escapes
before the lamellar compound can be weighed
TABLE IV
at room temperature becausethe weight ofthe
Analysis oÍ the C adftLiunx Chlorid'e-Chlorine-Graphi'te
Compound,
lamellar compound is in fact equal to the
original weight of carbon plus the amount of
CdCIe
c
metal halide consumed plus the amount of
Artificial Graphite
I
I175
6.3
chlorine which can be driven off asain at
I
5,1
984
1000'c.
5.6
I
8t5
In conclusion the compounds of indium
Natural Graphite
I
8.2
tLt2
chlorid.e or cadmium chloride and graphite
'werefound to be ternary lamellar compounds.
'
Cr+ Cl-. 6(CdCl2) or Cu+ (CdoCIrs)Theyresemble the compounds of graphite with
aluminum chloride with the principal differThe analyses reported in the tables contain ence that the ratio of metal halide to excess
one unavoidable error. These analyses have chlorine is 6 in these compounds and only 3
been obtained by analyzing the decomposition in the aluminum chloride compound. Thereproducts of the compound. Decomposition fore, at any given stage the aluminum
at 1000"C leaves, however, a small residue of chloride compound contains twice as many
reactant trapped in the graphite. The acceptors as these other compounds. Our
composition of this residue cannot be deter- present, knowledge of the bonding in these
mined precisely because its chemical com- compounds does not permit us to explain this
position will be altered. during more vigorous difference.
The experiments again confirm our postuanalytical methods such as complete combustion of the graphite.
late1,2 that lamellar reactions can only occur
The principal result of this investigation is when electrons are transferred between graphthat both indium chloride and cadmium ite and reactant.
chloride form lamellar comnounds which a're
One experiment was carried out to determine whether additional ind.ium chloride could
be forced into a compound without changing
the acceptor concentration by heating d
rather dilute compound with additional
indium chloride in the absence of chlorine.
The composition did not change appreciably
suggestingthat the ratio of indium chloride to
chlorine must remain fixed at a value of 6.
Table IV shows that nearly the same results
were obtained for cadmium chloride as for
indium chloride compounds, excepting that
the scatter in results is somewhat greater and
the compounds obtained were not as concentrated even for natural graphite.
1",