Thermodynamics and kinetics of carbon deposition from mixtures of hydrogen and carbon tetrachloride

Article abstract of DOI:10.1023/A:1015466920985

The thermodynamics and kinetics of pyrolytic carbon deposition via hydrogen reduction of CCl₄ were studied. Thermodynamic analysis of the C-Cl-H system was used to determine the compositions of the gas and condensed phases at CCl₄:H₂ = 1:4 to 1:90, temperatures from 700 to 1400 K, and a pressure of 10⁵ Pa. The reactions leading to the formation of solid carbon in the H₂-CCl₄ system were considered, and the deposition parameters were optimized in terms of carbon yield.

Full text of DOI:10.1023/A:1015466920985

Inorganic Materials, Vol. 38, No. 5, 2002, pp. 468–470. Translated from Neorganicheskie Materialy, Vol. 38, No. 5, 2002, pp. 573–576.
Original Russian Text Copyright © 2002 by Elyutin, Vorob’eva.
Thermodynamics and Kinetics of Carbon Deposition
from Mixtures of Hydrogen and Carbon Tetrachloride
A. V. Elyutin and M. V. Vorob’eva
State Research Institute for the Rare-Metals Industry, Bol’shoi Tolmachevskii per. 5, Moscow, 109017 Russia
e-mail: vorobjeva@yandex.ru
Received June 19, 2001; in final form, September 28, 2001
Abstract—The thermodynamics and kinetics of pyrolytic carbon deposition via hydrogen reduction of CCl4
were studied. Thermodynamic analysis of the C–Cl–H system was used to determine the compositions of the
gas and condensed phases at CCl : H = 1 : 4 to 1 : 90, temperatures from 700 to 1400 K, and a pressure of
4
2
5
1
0 Pa. The reactions leading to the formation of solid carbon in the H –CCl system were considered, and the
2 4
deposition parameters were optimized in terms of carbon yield.
Heterogeneous processes for hydrogen reduction of using the ASTD system, revealed that carbon forms a
volatile chlorides find practical application in various large number of compounds with chlorine:
fields and open up new possibilities for producing high-
2
CCl (g) + H (g) = 2CCl (g) + 2HCl(g),
4 2 3
purity coatings. The deposition of carbon layers via
hydrogen reduction of carbon tetrachloride at relatively
low temperatures and high rates is a promising tech-
nique for the preparation of new carbon materials.
Understanding the thermodynamics and kinetics of car-
bon deposition from mixtures of hydrogen and carbon
tetrachloride is crucial for the ability to optimize the
deposition conditions and improve the properties of the
coatings. To optimize the deposition temperature and
gas-phase composition in terms of carbon yield, we car-
ried out the thermodynamic evaluation of the C–Cl–H
system. Data on the gas-phase equilibria in the C–Cl–H
system are not available in the literature.
CCl (g) + H (g) = CCl (g) + 2HCl(g),
4
2
2
2
CCl (g) + H (g) = 2CCl(g) + 2HCl(g),
2
2
CCl
(g) + 2H
(g) = CH Cl (g) + 2HCl(g).
2 2
4
2
Consequently, it would be expected that the hydro-
gen reduction of CCl yields a wide variety of vapor
4
species. The possible reactions can be identified using
the principle of maximum entropy. There are several
computer programs for assessing the composition of
the gas or condensed phase, in particular ASTRA
(
B.G. Trusov et al., Bauman State Technical Univer-
Since the presence of chlorine or another halogen in
the gas phase makes it possible to enhance the deposi-
tion rate and improve the structural perfection of the
sity, Moscow) and WORK (V.V. Nechaev et al., State
Research Institute for the Rare-Metals Industry, Mos-
cow). The algorithm of these programs relies on the
principle of maximum entropy in a system at thermody-
deposit, we used carbon tetrachloride, CCl , as the car-
4
bon precursor. Thermal dissociation of CCl , pure or namic equilibrium. Clearly, a real thermodynamic sys-
4
mixed with an inert carrier gas, yields molecular chlo- tem is at least slightly displaced from and tends to equi-
rine (above 700 K), atomic chlorine (above 1600 K), librium. In view of this, we treat thermodynamic
and carbon. In any case, the reaction involves the for- parameters as characterizing separate regions at local
mation of carbon in the gas phase. Therefore, the pro- thermodynamic equilibrium [1].
cess should be run at a reduced pressure or an elevated
To find a constrained extremum in the entropy of a
partial pressure of the carrier gas in order to reduce the
thermodynamic system, we used the Lagrange method
CCl vapor pressure. Since inert carrier gases are inca-
4
of multipliers [2]. In the thermodynamic evaluation of
pable of fully preventing the presence of chlorine, we
the C–Cl–H system, we used the ASTRA program. The
used hydrogen, which reduces CCl , thereby prevent-
4
compositions of the gas and condensed phases were
ing the formation of carbon in the gas phase. The reduc-
tion process can be represented by the following overall
reaction scheme:
determined for CCl : H = 1 : 4 to 1 : 90, temperatures
4
2
5
from 700 to 1400 K, and a pressure of 10 Pa. The con-
densed phase was found to consist of carbon only, inde-
pendent of the CCl : 2 ratio. The presence of hydrogen
CCl (g) + 2H (g) = C + 4HCl(g).
4
4
2
notably reduces the reaction yield in terms of carbon.
5
It seems unlikely that the hydrogen reduction of CCl to At a pressure of 10 Pa, the highest yield of solid carbon
4
carbon proceeds in only one step. A literature search, is expected at temperatures above 1100 K.
0
020-1685/02/3805-0468$27.00 © 2002 MAIK “Nauka/Interperiodica”

THERMODYNAMICS AND KINETICS OF CARBON DEPOSITION
469
logp [Pa]
logp [Pa]
0
0
–
–
–
–
2
4
6
8
–
–
–
–
2
4
6
*
*
*
*
*
*
*
8
*
–
–
10
12
–
–
10
12
6
00
800
1000
1200
1400 T, K
6
00
800
1000
1200
1400 T, K
1
2
3
4
5
6
7
8
9
13
14
15
16
17
18
19
20
10
11
12
1
2
3
4
5
6
7
8
9
13
14
15
16
*
10
11
12
Fig. 2. Pseudoequilibrium composition in the C–Cl–H sys-
tem as a function of temperature at an H : CCl molar ratio
of 9 : 1; (1) H , (2) C H , (3) C H Cl, (4) C HCl, (5) HCl,
Fig. 1. Equilibrium composition in the C–Cl–H system as a
function of temperature at an H : CCl molar ratio of 9 : 1;
2
4
2
4
2
2
4
2
3
2
(
(
(
1) H , (2) CH Cl, (3) CH , (4) Cl , (5) HCl, (6) H,
(6) Cl, (7) C H , (8) CH Cl , (9) CH , (10) CH , (11) C H ,
(12) C H, (13) C H , (14) C H , (15) CH Cl, (16) CH Cl,
2 2 6 2 2 2 3
2
3
3
2
2 5 2 2 4 3 2 3
7) C H , (8) C H , (9) CH , (10) C H , (11) C(solid),
2
2
2
3
4
2
4
12) CH Cl, (13) C H , (14) Cl, (15) C H Cl, (16) C HCl.
(17) H, (18) C(solid), (19) C HCl, (20) C H Cl .
2
2
6
2
3
2
2 2 2 2
The calculation results for CCl : H = 1 : 9 are dis- the entire temperature range of interest. The results for
4
2
CCl : H = 1 : 9 are shown in Fig. 2.
played in Fig. 1. The reaction is likely to proceed in sev-
eral steps, via consecutive substitution of hydrogen for
4
2
The data displayed in Figs. 1 and 2 for the C–Cl–H
system and earlier data for similar systems suggest that
the process under consideration has a homogeneous–
heterogeneous character. There is no predominant reac-
chlorine, as evidenced by the presence of CH Cl through-
3
out the temperature range studied. Above 1200 K, we
observe the formation of C H Cl. With increasing tem-
2
2
perature, the CH concentration decreases slightly owing tion, and carbon deposition is due to a number of paral-
4
to CH dehydrogenation. The presence of excess hydro- lel and consecutive reactions. According to our calcula-
4
tions, thermal decomposition of CCl in argon carrier
gen in the reaction mixture inhibits CH decomposition.
4
4
gas yields appreciable amounts of atomic and molecu-
lar chlorine and lower carbon chlorides. On the surface,
carbon is probably transported in the form of lower car-
bon chlorides. In the presence of hydrogen, some of the
chlorine atoms may be replaced by hydrogen, but the
mechanism of carbon transport (in the form of lower
carbon chlorides) remains unchanged.
The CH concentration is higher by 4 to 5 orders of mag-
4
nitude than the concentrations of intermediate hydrocar-
bons (C H , C H , and C H ) and radicals.
2
4
2
6
2
2
These results suggest that condensed carbon is ther-
modynamically stable only above 1300 K; at lower
temperatures, only hydrocarbons are stable. However,
in real systems, carbon exits in a stable state at low tem-
Thus, there are two possible mechanisms of carbon
peratures, down to room temperature. In view of this, to deposition in the H –CCl system. One of them
2
4
take into account the nonequilibrium state of the system involves steplike decomposition of carbon tetrachloride
due to the presence of condensed carbon, we calculated and formation of intermediate hydrocarbons, consecu-
a pseudoequilibrium composition of the gas phase over tive dehydrogenation of the intermediate hydrocarbons,
INORGANIC MATERIALS Vol. 38 No. 5 2002

4
70
ELYUTIN, VOROB’EVA
2
The deposition rate was measured between 1100
and 1400 K at CCl : H = 1 : 9 to 1 : 13, a pressure of
lnv [g/(cm s)]
4
2
5
3
1
0 Pa, and a total flow rate of 240 m /h. The Arrhenius
plots of the deposition rate (Fig. 3) show a maximum.
Below 1300 K, the deposition rate varies exponentially
with inverse temperature, suggesting that the process is
kinetically limited (by the reaction rate). Since the pro-
cess involves a number of parallel reactions, it cannot be
described by a single rate equation. The activation
energy determined from the slope of the Arrhenius plots
is 220 kJ/mol. According to earlier results, this value is
typical of kinetically limited heterogeneous reactions in
chemical vapor deposition processes. In the range 1300–
–
9
–
–
10
11
1
3
2
1573 K, the deposition rate decreases with increasing
temperature, which cannot be accounted for by diffu-
sional limitations in the gas phase. The likely reason is
homogeneous carbon formation in the gas phase, as sup-
ported by the observed formation of carbon black. This
effect can be eliminated by cooling the reactor wall.
0
.71
0.77
0.83
0.89
03/T, K–1
1
Fig. 3. Arrhenius plots of the deposition rate at CCl : H =
4
2
(1) 1 : 9, (2) 1 : 12, and (3) 1 : 13.
The deposition rate of carbon depends strongly on
the CCl : H ratio. The best results were obtained at
4
2
CCl : H = 1 : 9. The plots of the deposition rate versus
1
1
1
1
1
80
60
40
20
00
4
2
total gas flow rate are displayed in Fig. 4. These data,
3
obtained at constant temperature and CCl concentra-
4
tion, lead us to conclude that the process is kinetically
limited: the flow rate has little or no effect on the deposi-
tion rate, which decreases slightly only at very high flow
rates. This can be accounted for by the substantial
decrease in the CCl consumption in homogeneous reac-
4
tions, the increased CCl concentration at the substrate
4
surface, and the accelerated rate of the reaction with the
2
1
8
6
4
2
0
0
0
0
deposited carbon, which reduces the deposition rate.
Thus, the present thermodynamic calculations and
kinetic studies of the hydrogen reduction of carbon tet-
rachloride allowed us to optimize the process parame-
ters: T = 1523–1573 K, CCl : H = 1 : 9, and total gas
4
2
3
flow rate of 400–450 m /h. Under these conditions, we
obtained high-quality pyrolytic carbon layers (10 to
0 wt % impurities, density of 2.19 g/cm , interlayer
–
3
–
4
3
1
0
100
200
300
400
500
600
3
spacing of 3.45 Å, average crystallite diameter of 51 Å)
on graphite substrates at a deposition rate of up to
Total flow rate, m /h
2
00 µm/h, which is about 30 times higher than the rate
Fig. 4. Deposition rate as a function of total gas flow rate at
1) 1423, (2) 1533, and (3) 1623 K.
of carbon deposition via conventional vacuum pyroly-
sis of hydrocarbons [3].
(
and direct decomposition of the hydrocarbons. The
other mechanism involves consecutive detachment of
REFERENCES
chlorine atoms from CCl to give compounds such as
4
1. Haase, R., Thermodynamik der irreversiblen Prozesse,
Darmstadt, 1963. Translated under the title Termodi-
namika neobratimykh protsessov, Moscow: Mir, 1967.
CCl and CCl , and the transport of carbon to the sub-
3
2
strate in the form of lower carbon chlorides. According
to the thermodynamic calculation results, the optimal
2
. Vatolin, N.A., Moiseev, G.K., and Trusov, B.G., Termo-
dinamicheskoe modelirovanie v vysokotemperaturnykh
neorganicheskikh sistemakh (Thermodynamic Modeling
in High-Temperature Inorganic Systems), Moscow:
Metallurgiya, 1994.
conditions for carbon deposition from H + CCl mix-
2
4
tures are T = 1100–1400 K and CCl : H = 1 : 4 to
4
2
1
: 50. To identify the rate-limiting process and exam-
ine its effect on the morphology and structure of the
deposits, we investigated the kinetics of carbon depo-
sition.
3. Ostrovskii, A.A., Iskusstvennyi grafit (Artificial Graph-
ite), Moscow: Metallurgiya, 1987.
INORGANIC MATERIALS Vol. 38 No. 5 2002