H.B. PALMER, J. LAHAYE, AND K. C. Hou
348
If Loand V oare constant, the value of C appearing in eq 2 and 3 is a constant. Slight variations in LOand V O
affect C as shown in eq A2. This variation of accounted for in eq 2 and 3.
C can be
On the Kinetics and Mechanism of the Thermal Decomposition of
Methane in a Flow System’ by H. B. Palmer, J. Lahaye, and K. C. Hou Department of Fuel Science, The Pennsylvania State University, Univereity Parlc, Pennsylvania (Received July $1, 1067)
16802
The rate of thermal decomposition of methane has been studied in a flow system at temperatures from 1323 to 1523’K. The experimental data are generally in close agreement with a recent, similar study by Eisenberg and Bliss. However, a different interpretation is given to the results. I n particular, it is concluded that nucleation of carbon in the gas phase causes the decomposition to accelerate because of heterogeneous decomposition of methane on the nuclei. The conclusion is supported experimentally. A qualitative model of the pyrolysis process is presented and discussed.
Introduction The kinetics and mechanism of the homogeneous thermal decomposition of methane should be completely understood by now, but they are not. Decomposition in more or less conventional static and flow systems has been studied for many years. Kramer and Happe12 have reviewed a number of those studies. Results of three of them are included in Figure 1, an Arrhenius plot of some reported first-order rate constants. The line through the results from flow and static systems is expressed by log Ic(sec-l) = 13.0
- 18.6 X
10*/T
(1)
corresponding to an activation energy of 85 kcal. Shock tube dataJ8v4when combined with the results of measurements of the rate of carbon film formation from pyrolyzing methaneJ6yield a quite different result log k(sec-’) = 14.6 - 22.5 X 10a/T
(2)
corresponding to an activation energy of 103 kcal. This line is also shown in Figure 1, as is the result from rapid compression experiments reported recently by Kondratiev,%which agrees well with eq 2. In the hope of understanding the discrepancy between these expressions, we have carried out some new measurements of the thermal decomposition rate using a conventional flow system. Since doing so, we have become aware of a recent and detailed study of EisenThe Journal of Physical ChemMtry
berg and Bliss7 (hereafter referred to as (EB)) in which a similar flow reactor was used. We shall refer to their work throughout this paper.
Experimental Section The experimental system has been described elsewhere.s-10 It is basically a hot porcelain tube of 5-mm i.d., having a long plateau in its temperature profile. Methane at concentrations in the neighborhood of 10% by volume is carried into the reactor in a stream of helium a t a total pressure of about 740 torr. Products are analyzed by gas chromatography using a 5-ft silica gel column at 50”. (1) Work supported in part by a grant from the J. M. Huber Corp. (2) L. Kramer and J. Happel, “The Chemistry of Petroleum Hydro-
carbons,” Vol. 11, B. T. Brooks, 8. S. Kurtz, C. E. Boord, and L. Sohmerling, Ed., Reinhold Publishing Corp., New York, N. Y., 1955, Chapter 25, p 71. (3) (a) G. B. Skinner and R. A. Ruehrwein, J. Phys. Chem., 63, 1736 (1959); (b) H. 8. Gliok, “Seventh Symposium (International) on Combustion,” Butterworth and Co. Ltd., London, 1959, p 98. (4) V. Kevorkian, C. E. Heath, and M. Boudart, J. Phye. Chem., 64, 964 (1960). (5) H. B. Palmer and T. J. Hirt, ibid., 67,709 (1963). (6) V. N. Kondratiev, “Tenth Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1965, p 319. (7) B. Eisenberg and H. Bliss, Chem. Eng. Progr., Symp. Ser., 63, No. 72, 3 (1967). (8) H. B. Palmer and F. L. Dormish, J. Phys. Chem., 68, 1563 (1964). (9) K. C. Hou and H. B. Palmer, ibid., 69, 858 (1965). (10) K. C. Hou and H. B. Palmer, ibid., 69, 863 (1965).
THERMAL DECOMPOSITION OF METHANE
349
I .o
/1523"K 0.9
0.8
0.7
g
0.6
2 0 0.5
(3
s 0.4
0.3
0.2 0.I
1 0 ~ 1 ~ Figure 1. Arrhenius plot of first-order rate constants for the decomposition of methane: squares, from the work of D. Rudder and H. Biederman, Bull. SOC.Chim. France, 47,710 (1930); crosses, P. S. Shantarovich and B. V. Pavlov, Zh. F i z . Khim., 30, 811 (1956); open circles, L. S. Kassel, J . Am. Chem. SOC.,54, 3949 (1932); solid circles, apparent rate constants derived from the present study (see Discussion). Solid line is eq 2.
I n the present study, methane has been introduced into the tube a t concentrations between 1 and 20% by volume in the helium. Residence times ranging from 0.1 to 0.9 sec have been employed over the temperature range 1323-1523°K. The available wall surface area of the reactor has been varied twofold by shifting from the single cylindrical tube to a porcelain tube containing four holes of smaller diameter. In the latter stages of the work, the effect of added naphthalene has been studied. Naphthalene of high purity is introduced into the helium stream at a volume fraction of approximately 0.01 by bubbling the carrier gas through molten CloHs a t 80". The purity of the helium is above 99.995%, but nevertheless it is passed through hot copper wool to ensure the complete absence of oxygen. The methane is CP grade (MEbttheson), purity 99% minimum. According to a mass spectrometric analysis, the principal impurity is argon. A small amount of ethane is present. Its upper limit is estimated from the analysis to be 0.3%.
7. / - 1323°K RESIDENCE TIME (MSEC)
Figure 2. Kinetic results a t five temperatures, a t input concentrations close to 10 mole %. The results of Eisenberg and Bliss' are shown for comparison, as follows: crosses, 1371"K, 15-25 mole %; squares, 1409'K, 15 mole %; triangles, 1458'K, 15 mole %.
Results and Discussion The basic data on the fraction of CH, decomposed as a function of time agree remarkably well with the results of EB.' Figure 2 shows our results at five temperatures and the results at the three temperatures employed by EB. The two studies taken together establish the general character of the rate behavior for methane decomposing in a flow system: the reaction accelerates and then decelerates. A steady rate appears only momentarily, as an inflection. At low temperatures, one sees only the acceleration. At high temperatures, the induction period is short and is difficult to observe, though it undoubtedly is present. Our observations on the effect of CHI concentration on the rate are typified by Figure 3, which shows a slightly increasing extent of decomposition as the input concentration rises, except a t the longer times, where there seems to be no effect. However, even a t the shorter times the apparent effect is very slight and may be specious. The concentration effect reported by EB was considerably larger, but it seems to be subject to Volume 7.9, Number 1 January 1968
350
H. B. PALMER, J. LAHAYE, AND K. C. Hou
700 MSEC
500 MSEC
70 -
z
2 8 I 8 w
5w
-
0
L
300 MSEC
60-
50-
-
-
w
200 MSEC
u U
40-
-
-
100 MSEC I
I
l
l
,
1
1
,
5.0
I
10.0
CONCENTRATION (MOLE PERCENT)
Figure 3. The effect of input concentration a t 1473"K, showing the extent of decomposition as a function of the input concentration, at five fixed residence times.
some question, as it relies heavily upon one experimental point (see Figure 5 of their paper). We do not insist that the reaction is simple first order, kinetically. On the contrary, it clearly is complex, exhibiting as it does an induction period and then a deceleration that develops strongly as decomposition proceeds. Nevertheless, the over-all kinetic order of the decomposition, throughout its course, is very close to unity. On the effect of surface-to-volume ratio of the reactor, the present study provides confirmation of EB's tentative conclusion that there is essentially no effect. In Figure 4 are shown the results at two temperatures at which pairs of runs have been made using two reactors: reactor A, a single porcelain cylinder of 5-mm inner diameter, and reactor B, a porcelain rod having four 2.4-mm holes instead of the single large one. The alteration of S/V by about a factor of 2 is not as large a change as one might like, but the data are in such good agreement that the conclusion is clear. EB determined the time-dependent behavior of the three principal gaseous hydrocarbon products, C2H0, C2H4, and C2H2. We have carried out similar determinations and agree with EB that ethane production is always small, and that it goes through a maximum at relatively short contact times. The maximum seems to match up fairly well with the end of the acceleration in the reaction. For c2H4 and C2H2, we obtain the ;ame general shape of the time dependences as EB. The Journal of Physical Chemistry
RESIDENCE TIME (MSEC)
Figure 4. Effect of surface-to-volume ratio: circles, reactor A; triangles, reactor B. (S/V)B = 2 X (S/V)A. Input concentration for 1423°K runs was 10%; for 1373°K runs, 16%.
Our yields of acetylene agree quite well with theirs, but yields of ethylene do not. EB find acetylene rising to considerably larger concentrations than ethylene at all three of their temperatures (1371, 1409, and 1458°K) and find the relative magnitudes of the two to be essentially independent of the input concentration. However, we find that ethylene tends to exceed acetylene when the temperature is below about 1420°K and the input concentration is less than about 10% CH4. Increasing the input concentration enhances the C2H2 yield relative to C2H4,but at 1373°K the yield of C2H2 is still well below that of C2H4 when the input is 13% CH4. In Figure 5 are shown the yields of C2 hydrocarbon products obtained by us at 1373°K and [CH4]o = 13.1%, and those obtained by EB at 1371°K. In view of the agreement on the extent of decomposition (Figure 2 ) as a function of time, we believe the difference in products is largely a reflection of the difference in cooling rates at the end of the hot zone. EB used a nitrogen-quench method that provided cooling rates several times greater than ours. In both studies the cooling rates were sufficient to freeze reaction steps of high activation energy quite promptly. However, in our reactor, processes of low activation energy, such as perhaps various hydrogenation processes, may continue and affect the product distribution markedly, shifting it toward a distribution characteristic of a lower temperature and perhaps enhancing the yield of hydrocarbons, as opposed to carbon and hydrogen. One
THERMAL
4I V$ 5 . 0 / U
v,
/ ,/
P’
4.0
wV
W
$j
50’ g
35 1
DECOMPOSITION O F hlETHANE
3.0
2.0
&I
x
0
0 1.0
RESIDENCE TIME (MSEC)
Figure 5 . Conversion of original methane to C:! hydrocarbons at 1373°K: initial concentration, 13 mole %; short dashes, ethane; long dashes, ethylene; solid lines, acetylene. Curves labeled EB are constructed by averaging the results from ref 7 a t 1371°K and initial concentrations of 15 and 26 niole %.
notes that among the C2 hydrocarbons, C2H4 tends to be favored over CzH2 at temperatures below about l150°K, and CzHe becomes the more prominent the lower the temperature. This seems to be broadly consistent with the rationalization suggested. Neither EB nor we have determined the carbon formation. Vitreous carbon deposits on the reactor wall, and a small amount of soot escapes from the reactor. Quantitative determination would be extremely inaccurate. Effects of the additives, Hz and CPH6,have not been studied by us (note, however, that our CIf, may have contained as much as 0.3% c2H6). EB found inhibition by H2and acceleration by C2H6. Their study of H2 was systematic and is supported by other, earlier work.ll The observation on C2H6 addition consisted of one experiment. Nevertheless, qualitatively, the effect appears unquestionably real. It is supported also by earlier work in flow r e a c t o r ~ but ~ ~not ~~ by~shock tube studies. The previously discussed agreement of our basic rate data with EB leads us to conclude that we may have overestimated the possible CZHBcontent of our CP grade methane. Later, an explanation is proposed for the difference between the data from shock tubes and from flow reactors. First, let us reconsider the sources of the rate constant expressions given in the Introduction. Equation l is found by fitting data from flow systems and from static systems. In view of the accelerationdeceleration behavior exemplified by the data in Figure 2, how can a first-order rate constant be obtained? The answer is clear: one can draw reasonably good straight lines through the several collections of points, ignoring the decelerations. From the slopes, one ob-
tains the rate constants shown on the Arrhenius plot in Figure 1. They agree very well with eq 1, but not, of course, with eq 2. However, these are not fundamental rate constants for a homogeneous reaction. At best, they represent some combination of rate constants. The shock tube data, which do not show an a c ~ e l e r a t i o nyield , ~ ~ fundamental rate constants. These should be compared with flow system data at very short times, before the reaction has accelerated appreciably. Unfortunately, we cannot obtain these from a system of the present type. All that can be said is that the behavior is fully compatible with a rate that commences at the value given by eq 2 and then accelerates. In their study of methane decomposition, Palmer and Hirt6)l4used a flow reactor, but instead of measuring concentrations in the exiting gas stream, they followed the rate at which carbon deposited on the wall. It is their rate constants that, in combination with shock tube data, specify eq 2. Their residence times were coniparable to those in the present study. One must ask why they did not observe an acceleration and why their rate constants appear to be of the order of those that might apply at the commencement of reaction in the present system. Before presenting a hypothesis, let us summarize the pertinent points above the decomposition. (a) It begins relatively slowly, accelerates, then decelerates. (b) The rate is approximately first order despite its complex behavior. (c) Vitreous carbon deposits on the wall; small amounts of soot and tars collect at the exit. (d) The main gaseous products are CzH2 and C2H4,with lesser quantities of C2Hs. (e) H2 inhibits the decomposition; ethane accelerates it. (f) The rate is not appreciably affected by the S/V ratio. EB do not agree with point b. They believe the decomposition to be of order distinctly greater than unity and to be homogeneous. Accordingly, they construct a mechanistic model that involves a nonsteady-state chain. By suitably choosing rate constants, they are able to fit their observations at one temperature. Their success in doing this is interesting, but we consider some features of their mechanism to be questionable; for example, although the mechanism creates hydrogen atoms, no subsequent reactions of H atoms are included. Perhaps the most obvious difficulty with their treatment is that fitting their results requires that the rate constant for the initial reaction kl
+
CH, CH3 H (1) be approximately 0.1 times the value given by eq 2. (11) J. E. Germain and C . Vaniscotte, Bull. SOC.Chim. France, 319 (1958). (12) J. E.Germain and C. Vaniscotte, ibid., 964 (1958). (13) I. A. Schneider, 2. Physik. Chem., 223, 234 (1963). (14) T.J. Hirt and H. B. Palmer, Carbon, 1, 65 (1963).
Volume 78, Number 1
January 106’8
H. B. PALMER, J. LAHAYE,
352 The only ways to reconcile this with shock tube data are to suggest either that the activation energy for reaction I is much larger than 103 kcal or that there was a chain length of about 10 under the shock tube reaction conditions. The chain length argument is the only one that seems conceivable, but it would require that the reaction
+ CH4 +C2HG + H b2
CH3
(11)
be very fast. Skinner has considered this possibility (the rate has not been measured) and has rejected it by estimating the probable value of the rate constant. It can also be ruled out on another basis. If reaction I1 is important, a chain results. If it is not in a steady state, then an acceleration of the rate should have been observed in the shock tube experiments, but it was not. If there is a steady-state chain, it is very probably represented by
+
CH4 -3 CH3 H (initiation) CH3 CH4 +CZH6 H (propagation)
+ H + CH4
4
+ CHI + H2
(propagation)
2CH3 ---f C2H6 (termination) H
+ CH3+CH4
(termination)
The Journal of Physical Chemistry
+H
(1%)
CH, +CH,
+ Hz
(Ib)
and/or
((Ia) seems the more probable); (b) deposition of carbon on the wall wall
CHI ---t C(s)
(IV)
Whether one chooses (IV) or (V) as the ending reaction (IV is more probable), the reaction order will be close to or equal to 1.5, but in the shock tube experiments it was found to be accurately equal to unity. Thus, it seems necessary to look for an explanation of the acceleration of the rate in a flow system that is based upon a process other than a homogeneous chain reaction. The obvious possibility that comes to mind is acceleration by a heterogeneous process, namely nucleation of soot. Soots are known to contain a large concentration of spin centers and should present active surfaces for heterogeneous decomposition of CHI in contrast to the vitreous carbon on the wall. Tesner’5 has in fact studied the heterogeneous decomposition of methane on carbon black, using an electron microscopic technique. Acceleration by added Cz& might then simply be a consequence of its more rapid decomposition, with accompanying formation of soot. The inhibition by added Hz is also qualitatively understood on this basis. If nuclei are significant, then one should be able to accelerate the reaction very strongly by adding a hydrocarbon that nucleates with great rapidity. Accordingly, we added naphthalene in one experiment. The reaction temperature was not known accurately, but it was steady. At a concentration of 10% CH4 and a residence time of about 0.8 sec, the extent of decomposition without added naphthalene was 1.2%. Addition of 1% naphthalene (by running the helium through a vaporizer) increased the decomposition to 37.2%. Sooting
.
CH, +CH3
(111)
(V)
c. Hou
was heavy. When the naphthalene addition was discontinued, the decomposition reverted to 3.8%. The inability to return completely to the original low figure was probably due to the large amount of carbon black that clung to the reactor wall. Thus addition of a small quantity of an active sootformer increased the rate by roughly a factor of 30. This very striking effect convinces us that nucleation in the gas phase is indeed responsible for the acceleration of the reaction as observed in flow systems. Since the kinetics of nucleation in sooting systems are not understood, we cannot propose a detailed mechanism that includes it. A general model can be suggested, however. It includes: (a) homogeneous decomposition
(1) (11)
AND E(.
+
(c) formation of light hydrocarbons
(CHI, CH2, CH4) --t C2Hs, CzK, CzHz (d) nucleation light hydrocarbons +nuclei (e) and heterogeneous decomposition nuclei
CH, -+ growth of nuclei (+H2) According to this model, shock tube measurements yield the rate of the homogeneous first step because nuclei do not form in time to influence the rate appreciably. Carbon deposition measurements also yield this rate because the principal fate of CHa is to disappear at the wall, when the system is of small diameter. This process is not perfectly efficient, however, and some CH3 survives to form light hydrocarbons and, eventually, soot nuclei. The reaction then accelerates. The rate of wall deposition is not appreciably affected by the nucleation because the total surface area presented by the nuclei is very small relative to the wall area, and most of the CH3 continues to deposit carbon on the wall. However, CHI is much more stable than CH3; it does not decompose efficiently on the vitreous carbon deposit on the wall, but does decompose well on the active surfaces presented by the nuclei. Deceleration occurs as hydrogen accumulates and as the nuclei age and become less active. A decrease in activity of (15) P. A. Tesner, “Seventh Symposium (International) on Combustion,” Butterworth and Co. Ltd., London, 1969, p 646.
SURFACE TENSION OF LIQUIDCADMIUM CHLORIDE-ALKALI CHLORIDE SYSTEMS this type has been observed in the flame studies of Bonne, et ~ 1 . ~ 6
Conclusions Studies of hydrocarbon pyrolysis in flow systems have much technological utility, but they will yield fundamental information on homogeneous reaction rates only with great difficulty. Other types of reactions, including reactions involving hydrocarbons, can be effectively studied in flow systems if the enormous complication presented by sooting phenomena can be avoided. The present study of methane pyrolysis shows it to be an example of a system that is very sensitive to the
353
effect of nucleation. The accumulation of evidence from this and other studies, particularly the valuable contribution of Eisenberg and Bliss, had led us to the conclusion that the controversy over the rate constant for the first-order homogeneous decomposition of methane is a consequence of the kinetic complications in flow- and static-system studies. On this basis, we believe that eq 2 must be chosen as the best available representation of the rate constant for the homogeneous first step in the decomposition mechanism. (16) U. Bonne, K.H. Homann, and H. Gg. Wagner, “Tenth Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1965,p 503.
Surface Tension of Liquid Cadmium Chloride-Alkali Chloride Systems by G. Bertozzil and G. Soldani’ Materials Department, Electrochemistry Group, Euratom CCR, Petten, Holland Accepted and Transmitted by The Faraday Society
(March 13, 1967)
Surface tension measurements have been carried out on four cadmium-alkali metal chloride systems, from the melting point up to 650°, and the results are given in the form of linear equations for each mixture. All the surface tension isotherms are S shaped. The behavior of the systems is discussed in terms of associated entities in pure CdC12 and complex anions in the mixtures. The effect of temperature is also taken into account.
Introduction The study of molten cadmium halide-alkali halide liquid mixtures is not only interesting because of the association phenomena which occur in them, but also because it could provide indications on the structure of pure liquid CdC12. The structure of pure liquid cadmium chloride is not yet well understood and is still open to discussion. In Bues’ opinion,2 the Raman spectrum of liquid CdClz maintains the peak of the solid polymer up to 100” above the melting point, thus indicating the presence in the liquid of an appreciable degree of polymerization. However, its low viscosity and high vapor pressure do not agree with this point of view; they seem rather to suggest a molecular structure similar to that of the mercuric halides, but with a much higher degree of ionization. In fact, the electrical conductivity of molten CdCl2 is similar to that of the alkaline earth halides, while that of HgClz is lo4 times lower. The dominant ionic species present must be the monatomic iona, since the frequencies corresponding to
the two possible complex ions (CdCl)+ and (CdCl8)do not appear in the vibrational spectrum of molten CdC12.a The principal equilibria which follow could be (CdC12),
JrnCdClz
CdCl2 _r Cd2+
+ 2C1-
the associated entities (CdCL), being so small and their concentration so low that no effect on the viscosity results. A structure of this type was proposed by Bockris and Angell.4 On addition of C1- ions, a different ioniaation process takes place (1) Address correspondence to the authors at Euratom COR, Ispra, Italy. (2) W. Bues, 2. Anorg. Allgem. Chem., 279, 104 (1955). (3) D. W. James in “Molten Salt Chemistry,” M. Blander, Ed., 1964,p 515.
(4) J. 0.M . Bockris and C . A. Angel], Electrochim. Acta, 1, 308 (1969).
Volume 72, Numbw 1 January 1068
