3768
S. A. PURSLEY, R. A. MATULA,AND 0. W. WITZELL
The Kinetics of Carbon Dioxide and Carbon Formation from Carbon Monoxidela
by Stephen A. Pursley, Department of Mechanicel Engineering, Purdue Univeraity, Lafagette, Indiana
Richard A. Matula,lb and Otto W. Witzell" School of Engineering, Mechanicul Engineering Department, University of California, Santa Barbara, California (Received August 8,1066)
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The initial kinetics of the reaction 2CO GO2 C have been investigated in Vycor vessels in the temperature range from 740 to 860" and in the pressure range from 25 to 945 mm. The rate has been shown to be extremely slow and the reaction is essentially zero order and heterogeneous with an activation energy of 35 kcal/mole. --f
Introduction
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The reaction 2CO --t COZ C was first encountered by Be112 in his work involving the reactions occurring in blast furnaces. B o u d o ~ a r d and ~ ~ Clemiiison and Briscoeab catalyzed the same reaction in an effort to determine the equilibrium composition of the COCO& system. It has been known for some time4 that the commercially significant COZ C + 2CO reaction is strongly retarded by carbon monoxide. StricklandConstable,s Reif,g Kawana,' Erguqs and Blackwood and Ingemeg have attributed this retardation to a firstorder CO (CO) +COz C reaction. Reiflo undertook a kinetic study of the 2CO + COz C reaction in an attempt to characterize the CO retardation. The reaction, when carried out on degassed high-temperature coke, was shown to follow first-order kinetics with CO adsorption as the rate-determining step. Brandner and Urey" and Hayakawa,12 in their work on the isotopic exchange between CO and COz, briefly investigated the 2CO -+ CO? C reaction in quartz vessels to determine if its rate influenced the exchange rate. The present investigation was conducted in order to determine the initial kinetics in CO pyrolysis.
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Experimental Section A static reaction vessel having 394 cc volume was constructed from Corning Code 7900 glass. The vessel had a surface to volume ratio of 0.90 cm-l. The temperature of the reactor vessel during a given experiment was maintained at +lo. Prior to installation in the The Journal of Physical Chemistry
furnace, the vessel was cleaned with reagent grade nitric acid and distilled water. The reactant gas (CP CO) was passed through a gas purification train before being admitted to the reactor vessel. Gas chromatography techniques were used for the quantitative analysis. Preliminary experiments conducted without cleaning the reactor vessel between successive points indicated that a slow continual carbon buildup on the xalls of the reactor was retarding the reaction. I n order to eliminate this problem, a cleaning technique m-as developed which allowed the carbon to be removed from the vessel by introducing oxygen between subsequent runs. This procedure allowed reproducible and consistent kinetic data to be obtained. (1) (a) This research is based on the Ph.D. dissertation of S. A. Pursley, Purdue University, 1965. (b) Department of hIechanica1 Engineering, University of Michigan, Ann Arbor, Xich. (e) Dean of the Graduate School, Drexel Institute of Technology, Philndelphia, Pa. (2) I. L. Bell, J . Chem. Soc., 22, 203 (1869). (3) (a) 0. Boudouard, A n n . Chim. Phys., 2 4 , 5 (1901); (b) J. Cleminson and H. V. A. Briscoe, J . Chem. Soc., 1926, 2148 (1926). (4) J. Gadsby, F. J. Long, P. Sleightholm, and K. W. Sykes, Proc. Roy. SOC.(London), A193, 357 (1948). (5) R. F. Strickland-Constable, J . Chim. Phgs., 47, 356 (1950). (6) A. E. Reif, J . Phys. Chem., 56, 785 (1952). (7) Y. Kawana, Bull. Chem. SOC. Japan, 2 6 , 507 (1953). (8) S. Ergun, J . Phys. Chem., 60,480 (1956). (9) J. D. Blackwood and A. J. Ingeme, Australian J . Chem., 13, 194 (1960). (10) A. E. Reif, J . Phys. Chem., 56, 778 (1952).
(11) J. D.Brandner and H. C. Urey, J . Chem. Phys., 13, 351 (1945). (12) T. Hayakawa, Bull. Chem. Soc. Japan, 2 6 , 165 (1953).
KINETICSOF
THE
REACTION 2CO
-,COz
+C
3769
Table I : Initial Rates of Carbon Dioxide Formation
TIME
,
HOURS
Figure 1. Plot 01 COz concentration us. time (T = 819", P = 761 mm).
PRESSURE
T, OC
Po mm
RCOZ, moles/cmz hr X 1010
742 779 781 819 820 858
762 760 382 761 382 945
1.03 2.03 2.00 3.72 3.74 5.99
9
l l l l l l l l ~
- 20.6 '21.0
t
TORR
Figure 2. Variation in the rate as a function of pressure (2' = 858).
- 24.4 Results Figure 1 is a typical curve showing the experimentally determined COZ concentration as a function of time. Initial normalized rates, Reo,, obtained by multiplying the experimental slopes by the reciprocal of the vessel X/V ratio are listed in Table I. The variation of the rate of formation of COz, (Rco,) (XITI),a t 858" in a vessel with S/V = 0.90 em-l as a function of initial CO pressure is given in Figure 2. At 858' it is seen that the rate increases slightly as the initial CO pressure is increased from 43 to 760 mm. However, at initial pressures greater than 760 mm, the rate is essentially independent of pressure. The data in Table I indicate that at temperatures lower than 858' the rate is essentially independent of initial CO pressures exceeding 0.5 atm. Further experiments with a vessel having an S / V ratio of 8.5 cm-' indicated that within 10% the rate was linearly dependent on the S/V ratio. The reaction was, therefore, assumed to be heterogeneous. It may he assumed that the rate is essentially independent of the initial CO concentration. Hence, Rco, can be equated to the zero-order rate constant K . Figure 3 is an Arrhenius plot of the temperature variation of the zero-order rate constant, as determined
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t 0.88
0.90
0.92
0.94 0.96 lo3 TEMPERATURE, K
0.98
1.00
Figure 3. Arrhenius plot for the zero-order rate constant.
from the dat,a in Table I. A least-mean-squares line resulted in an activation energy of 34.9 kcal/mole. The experimental zero-order rate expression for the formation of COn is
RCO,= K
= 9.9 X lo-' exp(-34.9/RUT)
moles/cm2 see (1)
Discussion The following equat,ions are proposed to account for the initial kinetics of the over-all reaction 2CO -, C 0 2 S C
co (CO)
ki
(CO)
k2
+ (CO) -% (C02) + c (cod
(2)
(3)
kr ka
c02
(4)
Volume 70, Wumber 12 December 1966
S. A. PURSLEY, R. A. MATULA, AND 0. W. WITZELL
3770
where (CO) and (COz) refer to chemisorbed carbon monoxide and carbon dioxide, respectively. It is well known that CO, as well as COZ, is chemisorbed on glass at the temperatures of interest in this investigation. Hayakatva12 has reported isotherms for CO and COz on quartz at 900’. These were low-pressure isotherms, up to 16 mm, but these results showed that CO was much more strongly adsorbed than COz in separate is0therms. Although adsorption as well as desorption are found to be rate controlling in many surface reactions, rates for adsorption and desorption processes are usually orders of magnitude higher than those encountered in this work. It appears reasonable, therefore, that the over-all rate is controlled by reaction 3 involving a Langmuir-Hinshelwood mechanism. Under this assumption, the rate is conveniently expressed as
RCO,= k&02
(5)
where 6 ~ is0 the fractional surface coverage of adsorbed CO. Equation 5 expresses the results consistent with those found experimentally. The reaction is zero order at high pressures, while a t lower pressures, the rate decreases with decreasing pressure, which is characteristic of chemisorption isotherm behavior. The retardation found in the preliminary experiments was apparently due to carbon deposition on the surface resulting in a decrease in the number of active reaction sites. It is not reasonable to attribute this retardation to COz adsorption. The quantity of COZ adsorbed was very small compared to the quantity of CO. Furthermore, the introduction of oxygen into the system should not have an appreciable effect on adsorbed COZ. A theoretical estimate of the rate constant can be calculated by invoking the absolute rate theory which has been detailed by Glasstone, Laidler, and Eyring.I3 If the experimental activation energy is used in the formulation, the value of the theoretical rate constant is de-
The Journal of Physical Chemistry
pendent on the choice of the number of active sites per square centimeter and the transmission coefficient. In order for the experimental and theoretical rate constants to agree, both the transmission coefficient and the number of active sites per square Centimeter must be small. The choice of the number of sites per square centimeter is dependent on the total number of sites per square centimeter and an estimate of the maximum distance between nearest neighbors which would allow a reaction to proceed. One class of reactions which have small transmission coefficients is nonadiabatic reactions. Although the form of the condensed carbon is not known, it is reasonable to assume that the carbon exists initially as condensed atomic carbon. Both CO and CO, have singlet ground states; however, atomic carbon has a triplet ground state. Hence, the reaction would be aceompanied by a change of multiplicity. Under these circumstances, a transmission coefficient of the order of to would not be unreasonable. Another possible cause for the low experimental rate is that the rate of the reaction is controlled by a surface diffusion process which may be very slow. The experimental results may be summarized as follows. The reaction 2CO +.COZ C has been shown to be heterogeneous when the reaction is allowed to proceed in Corning Code ’7900 glass reactors. The initial rate of production of COZ is linearily dependent on the reactor S/V ratio. The zero-order rate constant has an activation energy of 34.9 kcal/mole and a frequency factor of 9.9 X lo-’ mole/cm2 scc. The reaction has also been shown to be essentially zero order at high pressures, while a t lower pressures the rate decreases with decreasing pressure. The above result is characteristic of chemisorption isotherm behavior.
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(13) S. Glasstone, K. J. Laidler, and H. Eyring, “The Theory of Rate Processes,” McGraw-Hill Book Co., Inc., New York, N. Y . , 1941, pp 377-382.
