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Kinetics and mechanism of carbon film deposition by acetylene pyrolysis
Carbon
Vol. 30, No. 1, pp. 47-54. 1992 Printedin Great Britain
0008-6223192 $S.OO+.Ml
Copyright0 1991PergamonPressplc
KINETICS AND MECHANISM OF CARBON FILM DEPOSITION BY ACETYLENE PYROLYSIS B. MURPHY and ROBERT W. CARROLL Lehman College (CUNY), Bronx, NY 10468, U.S.A.
DANIEL
(Received 30 July 1990; accepted in revised form 1 February 1991)
Abstract-The pyrolysis of a mixture of 2.5% acetylene in helium in a flow system with and without added hydrogen has been studied between 1100 and 1250 K, under conditions favoring carbon film deposition. The rate of carbon deposition was measured continuously during pyrolysis as a function of the increasing conductivity of the growing film, while the effluent gas was analyzed for ethane, ethylene, and acetylene. An activation energy of 278 kJ/mol was obtained for carbon deposition. While an increase in the partial pressure of hydrogen in the input gas is accompanied by an increase in the concentration of ethylene in the effluent, no corresponding decrease in acetylene concentration is observed; the ratio of output acetylene to input remaining unchanged regardless of the amount of hydrogen added. Acetylene consumption is dependent solely upon residence time in the reaction zone, indicating a zero-order process. The rate of carbon deposition is inversely related to the hydrogen partial pressure. The isolation of carbon and ethylene formation from acetylene consumption is consistent with a process involving surface saturation. A mechanism is proposed involving a slow reaction at active sites, by which acetylene is chemically adsorbed as surface species which then serve as the common source of carbon, ethylene, and other pyrolysis products. Active sites are regenerated in the process, so that the total concentration of active sites remains constant. Key Words-Carbon
films, acetylene, active sites
It has been suggested that such carbon deposits are formed by surface decomposition of the original hydrocarbon[5], although it is also possible that rapid homogeneous pyrolysis to simpler molecules or radicals may occur prior to reaction at the surface[6]. Evidence for the latter is afforded by the observation that, while the crystallinity and graphitizability of the carbon obtained by vapor-phase pyrolysis is independent of the nature of the starting material, the properties of chars produced in the condensed phase are strongly dependent upon the hydrocarbon chosen[7]. In addition, when thiophene or pyridine is pyrolyzed in the vapor state, nearly all of the sulfur or nitrogen is recovered as CS2, H2S, or HCN; little or none of these elements being found in the carbon deposit[8]. Carbon deposition is surface catalyzed[9]. When hydrocarbons are pyrolyzed in a flow apparatus with a high surface-to-volume ratio, and under conditions of low hydrocarbon concentration, high temperature, and low overall conversion of hydrocarbon to carbon, soot formation is almost entirely suppressed and the carbon is deposited as a smooth, uniform, tightly-adhering film on the heated surfaces of the apparatus. The film is electrically conductive, and it is possible to measure the rate of carbon deposition in terms of the rate of change in the resistance of the growing film[lO,ll]. The temperature coefficient of resistivity of the carbon film is sufficiently small between 1000 and 1400 K that it may be assumed to be constant without introducing significant error[l0,12]. This technique has been used to measure the activation energies for carbon film deposition
1. INTRODUCTION
Investigation of the process by which solid carbon is formed in the high-temperature pyrolysis of hydrocarbon vapors is made difficult by the fact that both the nature of the carbon and the mechanism by which it is produced are strongly influenced by the conditions under which pyrolysis is carried out[l]. Whatever the mechanism, the transition from hydrocarbon molecule to solid carbon undoubtedly follows a complex pathway. Very different values for reaction order and activation energy are obtained depending upon the range of temperatures studied, whether a static or flow system is used, the partial pressure of the hydrocarbon, the presence or absence of a carrier gas, whether the reaction rate is measured in terms of loss of hydrocarbon or formation of carbon or other reaction product, and whether one measures total carbon produced or only that deposited on the reactor surface. The various mechanisms that have been proposed for surface deposition of pyrolytic carbon fall essentially into two categories[l-31: (1) vapor-phase nucleation and growth of large molecules, which subsequently diffuse to the surface where they undergo dehydrogenation and incorporation into the growing carbon mass and (2) nucleation and growth on the surface by direct deposition of the original hydrocarbon or a simple decomposition product thereof. While soot is undoubtedly produced by vapor-phase nucleation, it is unlikely that surface deposits of carbon originate by the same process. There is, however, considerable evidence that such deposits are formed by a heterogeneous process at active surface sites[4]. 47
48
D. B.
MURPHY
and R. W. CARROLL
GRAPHITE ELFCTRODFiS
Fig. 1. Schematic
diagram
from a variety of aliphatic and aromatic hydrocarbons[l0,13] and from carbon suboxide[ll]. Porter[l4] first proposed that acetylene, formed in a homogeneous decomposition process, is the ultimate precursor for carbon formation from any aliphatic hydrocarbon. More recently, Carroll and Murphy[l5,16], from analysis of the effluent gases, concluded that ethane rapidly forms a near-equilibrium mixture of ethane, ethylene, and acetylene during pyrolysis in a flow system between 900 and 1000 K, and that the acetylene is the source of the carbon deposit. This has led logically to an examination of the pyrolysis of acetylene itself, and this paper describes the results of that investigation. 2. EXPERIMENTAL
The pyrolysis apparatus is shown schematically in Fig. 1. It consists of a 2.54 cm i.d. refractory porcelain tube ground to a standard taper at each end, and supported within a high-temperature tube furnace. A porcelain thermocouple tube, 0.7 cm in diameter passes concentrically through the outer tube, supported at each end by a threaded bushing in the glass standard-taper end fitting. Gas inlet and outlet tubes are also sealed to each end fitting. The inner tube in turn supports a 1.27 cm o.d. porcelain tube 4.75 cm long upon which the carbon film is deposited, and two graphite tubes of the same diameter that serve as electrodes (Fig. 2). Connection
of pyrolysis
apparatus.
to the resistance-measuring circuit is made by applying a platinum coating to both ends of the inner tube and extending it just far enough into the reactor tube to make good electrical contact with the graphite electrodes. The small annular space between the inner tube and the porcelain tube that it supports is filled with chromatographic-grade alumina powder, effectively preventing deposition of carbon within that region. The system of electrodes and measuring tube is spring-loaded to maintain good electrical contact. During an experiment, the growing carbon film is connected in series with a standard lo-ohm resistor. A constant l-volt potential is maintained in the circuit, and the potential difference across the standard resistor, which is proportional to the conductance of the carbon film, is measured continuously with time using a recording millivolt potentiometer. Shortly after acetylene is introduced into the gas stream, the surface between the electrodes becomes covered with a continuous carbon film and the conductance increases linearly with time; the rate of change remaining constant as the resistance decreases from 6000 to 200 ohms. Assuming an average spacing between carbon layers of 0.35 nm and a specific resistivity of 2.43 x 10m3ohm-cm[lO], this corresponds to a growth in film thickness from about 15 to about 450 carbon layers. In this apparatus, a measured change in conductance of 1 x 10m4ohm-‘, corresponds to the deposition of 4.19 x lo-@ mole/ cm2 of carbon. The rate of carbon deposition in mol x cm-*s-’ is related to the measured rate of change in conductivity in ohm-is-i of the growing film (A) by the expression rate = pdAl12 where: p = 2.43 x lo-’ ohm-cm d = 2.07 g/cm3
Fig. 2. Mode of assembly of graphite electrodes.
The furnace temperature controller was connected to a Pt/Pt-10% Rh thermocouple inserted into the inner tube with the bead close to the center of the reactor zone. A second, similarly located thermocouple was used to monitor the reaction temperature. The temperature could be maintained within
49
Carbon fiim deposition t2 K from 5 cm upstream to 5 cm downstream of the center of the reaction zone, and within 15 K of the measured temperature throughout the remainder of the heated part of the furnace. Gas flow was regulated by electronically controlled mass flowmeters, from which the gases passed thorugh a mixing manifold into the furnace. Leaving the furnace, the effluent passed through a cold trap and filter and into a lo-cm gas cell mounted in a Miran 980 analytical infrared spectrometer calibrated for ethane, ethylene, and acetylene, and from there through a wet-test meter. This arrangement permitted simultaneous measurement of carbon deposition rate, effluent gas composition, and gas flow rate. The accuracy and reproducibility of gas analyses was ~5% or better.
I’
I 80
3. RESULTS AND DISCUSSION
I a.2 I/T
Carbon deposition rates were measured at intervals of 25 K between 1150 and 1250 K, using a standard mixture of 2.57% acetylene in helium at a flow rate of 800 cm’/min, corresponding to a residence time in the reaction zone of about 1 s (Table 1). An Arrhenius plot of the data is shown in Fig. 3. The activation energy for the process calculated from these data is 278 kJ/mol. Experiments were then run at 1132 and 1179 K and the same flow rate, adding successive molar increments of hydrogen to the input gas and examining the effect on the carbon deposition rates. The rate of carbon deposition was found to vary inversely with the partial pressure of added hydrogen. (Fig. 4). Analyses of the effluent gases for ethane, ethylene, and acetylene, as well as the composition of the input gases are given in Tables 2 and 3. As the hydrogen partial pressure is increased, the concentration of ethylene in the effluent gas increases linearly, as does that of the minor constituent, ethane. However, a corresponding decrease in acetylene concentration, which would be expected if acetylene were reacting in the gas phase to form ethylene, does not take place. In fact, the ratio of output acetylene to input remains essentially unchanged, regardless of the amount of hydrogen added to the input gas. This is shown in Fig. 5, where the ratio of hydrocarbon output to acetylene input is plotted against the ratio of hydrogen input to acetylene input for both hydrocarbons. Ethylene is not formed by reaction of hydrogen
I 6.6
I 0.4
I 8.8
.
X IO4
Fig. 3. Arrhenius plot of carbon deposition rate VS.T-j.
with the already-deposited carbon, since at these temperatures, passing hydrogen at a partial pressure of 0.1 atm over a freshly deposited carbon film was found to cause neither evolution of ethylene nor any change in the conductivity of the film. It would appear, therefore, that of the acetylene introduced into the apparatus, a fixed proportion, depending only on the temperature and not on the amount of hydrogen in the gas mixture, is adsorbed and reacts on the surface, the remainder passing unchanged through the apparatus. Part of this adsorbed acetylene forms carbon and part is hydrogenated; that is, both the carbon and the ethylene are produced on the surface and not in the gas phase. For the
Table 1. Carbon deposition rates at various temperatures T, K 1147 1173 1198 1223 1248
mol C . s-‘cm-2 5.86 1.14 1.88 3.60 6.36
x x x x x
lo-‘* 10-v 10-y IO-9 10-y
0
1
0
,
.
,
,
2
I
3
.
I
4
.
I
5
I’K&WZHZ
Fig. 4. Inverse plot showing effect of partial pressure of input hydrogen on carbon deposition rate. a, 1132 K; 0, 1179 K.
50
D. B. MURPHYand R. W. CARROLL
Table 2. Partial pressures of input and effluent gases at 1132 K Input gas (atm) I-k 8.Z 0:0474 0.0698 0.0888 0.1091
CA 0.0250 0.0243 0.0238 0.0234 0.0229 0.0224
Effluent gas (atm) CA 0.00011 0.00912 0.00026 0.00033 0.00041 0.00043
GH,
GHz
0.00103 0.00234 0.00346 0.00468 0.00565 0.00658
0.0198 0.0193 0.0189 0.0185 0.0184 0.0179
1 8
0.4
-
9 8
NOM./ .A
/O
amount of acetylene consumed to be unaffected by the hydrogen concentration, the initial adsorption step must be decoupled from the process in which products are formed. For example, after adsorption on the surface, the acetylene may undergo a slow reaction at active sites to form a chemisorbed intermediate which then serves as the common source of products. The ratio of output to input acetylene remained unchanged in these experiments, despite the fact that the partial pressure of acetylene was decreased by about 10% by the addition of hydrogen, suggesting that the consumption of acetylene is zero order, again implying a surface reaction. Figure 6 shows the effect of residence time on the consumption of acetylene and production of ethylene, when a mixture of 2.57% acetylene and 4.4% hydrogen in helium at 1 atm was passed through the apparatus at 1130 K. From this is may be seen that, except perhaps at long residence times, acetylene consumption increases linearly with residence time in the system, consistent with a zero-order process. This diagram also provides a crude representation of the conditions that would exist at various points along the reaction tube with the gas flowing at a rate of approximately 1.67 cm/s. Reading from left to right on the diagram corresponds to progressing downstream through the reaction zone. Initially, production of ethylene parallels the consumption of acetylene, but soon lags behind, as other products are produced in increasing quantity. The data used to prepare Fig. 6 are given in Table 4. The difference between the % recovery and 100% is attributed to the relatively small amount of carbon deposited and unanalyzed products, probably benzene, naphthalene, and other polyaromatics which have been detected in the effluent[l7]. The data
d g
-
0.2 -_g&’
D
0.0 0
I ,
.
1 2
,
I 3
,
I 4
~
I 5
PWpCZH2
Fig. 5. Plots of ratios of hydrocarbon output partial pressure to acetylene input partial pressure VS. ratio of hydrogen input partial pressure to acetylene input partial pressure. Upper pair of lines acetylene; lower pair of lines ethylene. 0, 1132 K; 0, 1179 K.
show that, except at the longest residence time, the system is in what has been called the “plateau region”[l3] where the carbon deposition rate is independent of flow rate. In this same region, however, the yield of unanalyzed products varies significantly with residence time. It may be concluded from this that these substances are not intermediates in the production of deposited carbon.
Table 3. Partial pressures of input and effluent gases at 1179 K Effluent gas (atm)
Input gas (atm) I-L
GHz
0.0000 0.0224 0.0438 0.0644 0.0841 0.1028
0.0249 0.0243 0.0239 0.0233 0.0227 0.0224
G& 0.09002 0.09013 0.00013 0.00041 0.00031 0.00043
CJI,
CJIz
0.00101 0.00186 0.00421 0.00553 0.00639 0.00744
0.0176 0.0175 0.0174 0.0171 0.0165 0.0162
Fig. 6. Effect of residence time on effluent composition. 0, acetylene; 0, ethylene; T = 1130 K; input gas: 2.57% acetylene + 4.40% hydrogen in helium at 1 atm.
51
Carbon film deposition Table 4. Effect of residence time on product composition at 1130 K. Input: 2.57% C,H, and 4.40% H2 in He at 1 atm Vatm), effluent gases Residence time s/cm
CzHz
C,H,
% Recovery*
dCldt x 10”’ mol C . cm-?’
0.589 0.286 0.190 0.143 0.111
0.0175 0.0205 0.0218 0.0225 0.0229
0.00385 0.00280 0.00235 0.00190 0.00153
83.1 90.7 94.0 94.9 95.1
3.14 2.73 2.73 2.68 2.64
*Neglecting deposited carbon, which amounted to less than 1% of carbon introduced as acetylene, and negligible amounts of ethane in effluent. The experimental evidence indicates that acetylene undergoes three main processes during pyrolysis under the experimental conditions described here. These are: (1) dehydrogenation and deposition of carbon, (2) hydrogenation to ethylene, and (3) polymerization to higher hydrocarbons, most probably benzene and polyaromatics. The rate of consumption of acetylene at a given temperature has been found to be independent of the partial pressures of both C2H, and H, in the experimental pressure range. This places severe restrictions on any reaction mechanism that may be postulated, and makes it unlikely that any homogeneous reactions are involved in these processes. It is also unlikely that completely independent, parallel surface processes are taking place. In an extensive study of adsorption and pyrolysis of propylene over carbon active sites, Hoffman, et a1.[18.19] observed that the rate of propylene consumption depended upon the initial concentration of active sites, and remained constant throughout pyrolysis, even after the original surface had been covered many times with deposited carbon. They concluded that carbon deposited on an active site is accompanied by regeneration of the active site. The process of carbon deposition is autocatalytic, and the concentration of active surface sites remains unchanged during pyrolysis[20,21]. The structure of the carbon film is turbostratic[22], consisting of small, graphitic crystallites in approximately parallel orientation to the surface. Flat surface regions are relatively inactive in adsorption, while steps, crystallite edges, pits, and other defects afford active sites for adsorption[23]. While this variety of structures will lead to a range of site activities and energies of adsorption, the average, or typical, active site is considered here. Provided that the concentration of active sites remains unchanged, the constant rate of acetylene consumption can be accounted for by a two-stage adsorption process. Fast and reversible non-dissociative adsorption of acetylene at sites, with adsorption near saturation at all times, provides the required constant concentration of reactant. It is necessary to postulate a subsequent, slow, and essentially irreversible stage of chemisorption in order to decouple the initial adsorption step from the pro-
cess leading to products. To maintain the required constant site concentration, this second stage must be accompanied by regeneration of the active site. All of the observed reaction products then result from further reaction of the product of this second stage of chemisorption. As a result, the rate of acetylene consumption is controlled by the rate of the second stage of chemisorption and the concentration of active sites. Since both of these factors are constant under a given set of reaction conditions, the acetylene consumption rate will be independent of the relative rates of the surface processes that follow. Hoffman, et a[.[231 concluded from a study of chemisorption of propylene on a clean Graphon surface at low temperatures and pressures, that the alkene is strongly adsorbed at active sites, and that a surface complex, whose nature was not specified, is formed slowly on different, less active sites. Desorption occurred only at elevated temperatures, and was accompanied by decomposition; hydrogen and methane being the only species desorbed at 873 K. The carbon deposited remained at the active site and became a new active site. Their experimental conditions were very different from those described in the present work, however, so that it is difficult to say how much similarity there is between the twostage chemisorption process that they describe and that postulated herein. Neither acetylene in the gas phase nor the reversibly adsorbed acetylene can undergo hydrogenation to ethylene, for in that case the partial pressure of hydrogen would influence not only the formation of ethylene, but the consumption of acetylene as well, contrary to observation. The linear dependence of the rate of ethylene formation on the partial pressure of hydrogen is, however, consistent with hydrogenation of the strongly chemisorbed species on the surface. Hydrogenation must also change the nature of, and weaken the bonding to the surface in order to permit desorption of ethylene. Carbon already incorporated into the crystal structure of the growing film does not react with hydrogen, but the dehydrogenation process to form carbon would be expected to be inhibited by hydrogen, leading to the observed inverse relationship between the carbon deposition rate and the hydrogen partial pressure. The difference between the amount of carbon in-
52
D. B. MURPHY and R. W. CARROLL traduced as acetylene and that recovered as acetyH,(sur) + sur * C,H2(C2H2)m*C2H2 lene, ethylene, ethane, and carbon is ascribed to e sur . GH,GWn*GH, (4b) higher molecular weight hydrocarbon products. The amount of this unanalyzed material decreases with The product of step (1) does not hydrogenate. Hyincreasing hydrogen partial pressure, and increases drogenation of the chain at the outer end as in eqns with increasing residence time, but is not related to (4) and (4a) leads to ethylene formation. Hydrothe amount of carbon deposited, leading to the congenation at the surface end as in eqn (4b) blocks clusion that these substances are not intermediates carbon deposition. Hydrogenation elsewhere on the in the production of deposited carbon. The constant chain does not affect either process. rate of consumption of acetylene precludes their being formed in the gas phase. The second stage of sur . (C~H*)“C~H~*C~H~ ~hemisorption regenerates an active site to which a = sur - GW,-lGH2* + W%(g) + U%(g) second acetylene molecule can become attached, again regenerating an active site. At lower hydrogen (5) pressures, this process may occur more rapidly than sur - (C,H,),C,H,* hydrogenation to ethylene or dehydrogenation to e sur * (C,H,),_,C,H,* + C,H,(g) (5a) carbon, resulting in the formation of polymeric chains attached to the surface, with an active site at Steps (4a) and (5a) will occur much less frequently the terminal end of each chain. Increasing hydrogen than steps (4) and (5) because of the very low conpressure should lead to shorter chains, with a twocentration of uncovered active sites. carbon chain the limiting case. Unless these chains Deposition of carbon at the surface: are to grow inde~nitely, there must be a process for their removal, perhaps through ring closure and desur . C*H*(C~~~)~*(C~H~) sorption as aromatic species. sur * (C,H,),*C,H, + 2 C + H,(sur) (6) 4. REACTION MECHANISM
Formation
The following sequence of reaction steps is offered to account for the observed results: Reversible nondissociative molecular adsorption of C,H,, covering active surface sites and blocking further adsorption[24]: C,H, + sur’ e
sur*C2H2
(I)
This step is fast in both directions and near equilibrium, with k, > k-,. In the range of acetylene partial pressures used in these experiments, the surface sites at all times are nearly saturated, so that the concentration of adsorbed species is approximately equal to the site density, (a). Second stage of chemisorption with active site regeneration: SW - C,H,*
(2)
Slow, with k_, z=-> k,. The product undergo steps (1) and (2) repeatedly, chain growth on the surface:
may itself leading to
sur*C2H2 -
sur * GH2),(GW3*
+ CzHz e
sur . C2H,*C2H, -
sur * C2H2*C2H2
sur - C,H,C,H,*
H,(g) e H,(sur)
etc.
H,(sur)
(2a)
Equations (1) through more simply as follows:
SW
(C,H,),C,H,*
(5) may be represented
(1) (2)
C2H, + A* $ A A-+A* H,(g) G H,(sur) ii; H,(sur) -t A e B (44 Hz(sur) + A* s B* (4b)H,(sur) + D zz E (5) B* A* + C&(g) + CJMg) (54 B* + A* + C,H,
The following assumptions
are made:
1. All reversible processes, with the exception of (5) and (Sa), are near eq~librium. 2. Steady state conditions exist on the surface. 3. k, >> k_, k-, >> >>
k4 k,(a
Thus: 4. [A] >> [A*]
[Al ‘> PI
(4)
The overall eqn representing ylene may be written:
(4a)
the rate of loss of acet-
~,~C~H~~g)l[A*~ - UAI - k@l + k -5fC,H,(g>ltC,H,fg)l[A*l(8)
-W,H,(gWdt .
(7)
[A*] >> [B*]
+ sur * (C,H,),C,H,* =
+ GM.4
(3)
f sur * (C,H,),C,H2*CZH2 G== sur - (C,H,),C,H,*C,H,
H,(sur)
(la)
sur + (GHz)~-~GH~*
-
La
sur + C,H,*
of aromatic species, for example:
=
Carbon film deposition If hydrogen adsorption far from saturation,
Assuming steady state conditions: d[A]ldt
= 0 = k,[C,H,(g)][A*] - k&41 -
d[A*]ldt
WWur)I[4
+ bP1
[K(sur)l
-h[GWdl[A*l + k~@l + UAI - ~4aFL(W[A*l + L[B*l + WY - k~,[C,H,(g)l[C,H,(g>l[A*l + ks,[B*l- LaLKk)l[A*l
= 0 = k4[H2(sur)][A]
- ks[Bl +
- k~,[Bl
k~S[C,H,(g)l[C,H,(g)l[A*l(11)
Adding eqns (9) and (11) and subtracting
Since [A] = o, and partial pressures may be substituted for gas concentrations: = a{(k,k,k,/k_j)pH,
d(pC,H,)ldt
- (k-lk-,lk,)pC,H,}l(k-~
(17)
from eqn pH,(input)
= k,[Al = k,(o)
(12)
Substituting
[A] can be replaced by u in this eqn since the concentration of chains on the surface is equal to the site density, and at any moment few of those chains are hydrogenated at the outer end. The mechanism thus yields the experimentally observed zero-order rate of loss of C2H,.
d(pC,H,)ldt
(13)
Applying assumptions 3 and 4, and subtracting eqns (9) and (10) from eqn (13) affords
4GH,(g)lldt = WLW~)I[4 - UBI
(14)
Solving eqns (9) and (11) for [A*], equating the results, solving for [B] and substituting for [B] in eqn (14):
4GHJg)l~~t = WMWIPI k-,[Al{k-s[~H,(g)l(k, + kz + W-Wur)l) + M4~WW~ _ kdk-, + ks) + k-v-K,H&)l
(18) terms:
= ok3k4k5/k~3pH,(,,,,,,/(k-4 + k,) - (o{(k3k4kJk-3) + (k~lk-Slk,)}PC2H4)l(k-~
Upon integration,
(19) + k,)
this expression reduces to
pC,H,
- k-.~[C,H,(g>lIC,H,(g)l[A*l + kaP*l - LLKk)l[A*l
= pH, + pCZH,
in eqn (17) and rearranging
d[C,H,(g)lldt = WI
= [constant]pH,
(20)
in agreement with experiment. The mechanism requires that all of the acetylene consumed appear as ethylene, carbon, polyaromatits (PAH), and negligible amounts of ethane, all arising from a common source, namely chemisorbed surface species: - d[C,H,]/dt = d[C,H,]ldt
+ d[PAH]ldt
+ dC/2dt
(21)
Typically, carbon deposition accounts for less than 2% of the acetylene consumed. Thus, from eqn (12) kZcr = d[C,H,]idt
+ d[PAH]/dt
(22)
c15j
Making the reasonable assumptions that k2, k4, k,, and k-s are small relative to k,, and that k-,/k, is small affords:
-
+ ks)
Since less than 2% of the hydrocarbon introduced is converted to carbon, any hydrogen produced in that process is negligible relative to the hydrogen added to the gas stream, and the increase in the partial pressure of ethylene as the gas proceeds through the apparatus is approximately equal to the decrease in partial pressure of hydrogen. At any point in the system, therefore
(8): -d[C,H,(g)]ldt
Wk~#Mg)l.
=
(9)
(10) d[B]/dt
53 and
- k-,[A]
= 0 =
is close to equilibrium
(k-,k~,lk,)[C,H,(g)l} (16)
Therefore, as hydrogen is added to the system, more ethylene is produced at the expense of PAH, the sum of the two products remaining essentially constant. Carbon deposition by this mechanism occurs where the chain is attached to the carbon surface (eqn 6) and is hindered by hydrogenation of the chain at this point (eqn 4b). dC/2dt
= k,[D]
(23)
D. B. MURPHY and R. W. CARROLL
54
With step (4b) near equilibrium,
~dWP1 = LPI
(24)
Since all chains are either hydrogenated [E] or not hydrogenated [D] at the end attached to the carbon surface, u = [D] + [E] Substituting
(25) Ack~owledge~e~~-The authors wish to thank the City University Research Foundation for support under PSCi CUNY Grant #6-65203.
for [E] in eqn 25:
PI = 41 f (WLb)[H(sur)l~ Wdsur)] = (Wk-dpHZ
(26) (27)
therefore, K/2&
= k,l{l
+ (constant)(pH,)}
(28)
giving a linear relationship between the reciprocal of the carbon deposition rate and the partial pressure of hydrogen: dt/dC
= 1/2k,{(l
+ (constant)(~H~)}
sites remains constant. The product of the second stage of adsorption can then undergo dehydrogenation to carbon, hydrogenation to ethylene, or formation of surface chains by chemisorption at the regenerated active sites. Cyclization and desorption of surface chains can account for the benzene and polyaromatics that have been detected in the effluent gas.
(29)
as observed experimentally. 5. CONCLUSIONS Acetylene undergoes three main processes during pyrolysis in a flow system under the experimental conditions described here: carbon deposition, hydrogenation to ethylene, and polymerization to higher hydrocarbons. The activation energy for carbon deposition is 278 k.I/mol. The rate of carbon deposition is inversely related to the partial pressure of hydrogen in the input gas. Incremental addition of hydrogen to the input gas causes a linear increase in the amount of ethylene in the effluent, but has no effect on acetylene consumption; the ratio of output acetylene to input remained unchanged even when the hydrogen partial pressure was increased to as much as five times that of the acetylene. At a given temperature, acetylene consumption is dependent solely upon residence time in the reaction zone, suggesting a zero-order process at saturated active sites. A mechanism is proposed invoI~ng reversible, non-dissociative molecular adsorption of acetylene, followed by a slow, and essentially irreversible stage of chemisorption, the rate of which controls the rate of acetylene consumption. The second stage of chemisorption is accompanied by regeneration of the active site, so that the total concentration of active
REFERENCES 1. H. B. Palmer and C. F. Cullis, Chemistry and Physics of Carbon 1, 269(1965). 2. J. C. Bokros, Chemisrry and Physics of Carbon 5, 1
(1969). 3. W. V. Kotlensky, Chemistry and Physics of Carbon 9, 176 (1973).
4. A. Wehrer, P. Wehrer, and X. Duval, Carbon 21,247 (1983). 5. P. A. Tesner, In 7th Internation~ Combustion Symposium p. 546. Butte~orths, London, (1959). 6. 8. Fitzer, K. Mueller, and W. Schaeffkr, ~~e~~~ry and Phvsics of Carbon 7. 237 (19711. 7. J. S. Cbnroy,*R. S. Sty&, D. ‘B. Murphy, and C. R. Kinney, In Proceedings, 3rd Conference on Carbon p. 395. Pergamon Press, Elmsford, NY (1959). 8. D. B. Murphy, Carbon 10,323 (1972). 9. S. Jasienko and .I. Machinikowski, Carbon 19, 199 (1981). 10. D. B. Murphy, H. B. Palmer, and C. R. Kinney, In Industrial Carbon and Graphite p. 75. Societv of Chemical Industry, London (19‘58). _ 11. T. J. Hirt and H. B. Palmer. Carbon 1. 65 (19631. 12. D. S. Kupperman, C. K. Chen, and H. Weinstock, Curbon 11, 171(1973). 13. D. B. Murphy and R.’ W. Carroll, High TemperasuresH&h Pressures 13. 229 (1981). 14. G.“Porter, AGARI) ~e~or~~d~rn AC 13iMp (1954). 15. R. W. Carroll and D. B. Murphy, In Extd. Abstr. and Program, 16th Biennial Conference on Carbon, p. 114. (1983). 16. D. B. Murphy and R. W. Carroll, In Extd. Abstr. and Program, 19th Biennial Conference
on Carbon,
p. 446.
(1989). 17. P. Lucas and A. Marchand, Carbon 28, 207 (1990). 18. W. P. Hoffman, F. J. Vastola, and P. L. Walker, Jr., Carbon 23, 151 (198.5). 19. W. P. Hoffman, F. J. Vastola, and P. L. Walker, Jr., Carbon 26, 485 (1988).
20. S. B. Tong, P. Pareja, and M. H. Back, Carbon 20, 191 (1982). 21. T. Jacklewski and R. J. Diefendorf, In Exrd. Abstr. and Program, 15th Biennial Conference on Carbon, p. 284. (1981). 22. R. Iley and H. L. Riley, J. Chem. Sot. 1362 (1948). 23. W. P. Hoffman, F. J. Vastola, and P. L. Walker, Jr., Carbon 22, 585 (1984).
24. L. L. Kesmodel, P. C. Stair, R. C. Baetzold, and G. A. Samoraj, Phys. Rev. Letters 36, 1316 (1976).
Vol. 30, No. 1, pp. 47-54. 1992 Printedin Great Britain
0008-6223192 $S.OO+.Ml
Copyright0 1991PergamonPressplc
KINETICS AND MECHANISM OF CARBON FILM DEPOSITION BY ACETYLENE PYROLYSIS B. MURPHY and ROBERT W. CARROLL Lehman College (CUNY), Bronx, NY 10468, U.S.A.
DANIEL
(Received 30 July 1990; accepted in revised form 1 February 1991)
Abstract-The pyrolysis of a mixture of 2.5% acetylene in helium in a flow system with and without added hydrogen has been studied between 1100 and 1250 K, under conditions favoring carbon film deposition. The rate of carbon deposition was measured continuously during pyrolysis as a function of the increasing conductivity of the growing film, while the effluent gas was analyzed for ethane, ethylene, and acetylene. An activation energy of 278 kJ/mol was obtained for carbon deposition. While an increase in the partial pressure of hydrogen in the input gas is accompanied by an increase in the concentration of ethylene in the effluent, no corresponding decrease in acetylene concentration is observed; the ratio of output acetylene to input remaining unchanged regardless of the amount of hydrogen added. Acetylene consumption is dependent solely upon residence time in the reaction zone, indicating a zero-order process. The rate of carbon deposition is inversely related to the hydrogen partial pressure. The isolation of carbon and ethylene formation from acetylene consumption is consistent with a process involving surface saturation. A mechanism is proposed involving a slow reaction at active sites, by which acetylene is chemically adsorbed as surface species which then serve as the common source of carbon, ethylene, and other pyrolysis products. Active sites are regenerated in the process, so that the total concentration of active sites remains constant. Key Words-Carbon
films, acetylene, active sites
It has been suggested that such carbon deposits are formed by surface decomposition of the original hydrocarbon[5], although it is also possible that rapid homogeneous pyrolysis to simpler molecules or radicals may occur prior to reaction at the surface[6]. Evidence for the latter is afforded by the observation that, while the crystallinity and graphitizability of the carbon obtained by vapor-phase pyrolysis is independent of the nature of the starting material, the properties of chars produced in the condensed phase are strongly dependent upon the hydrocarbon chosen[7]. In addition, when thiophene or pyridine is pyrolyzed in the vapor state, nearly all of the sulfur or nitrogen is recovered as CS2, H2S, or HCN; little or none of these elements being found in the carbon deposit[8]. Carbon deposition is surface catalyzed[9]. When hydrocarbons are pyrolyzed in a flow apparatus with a high surface-to-volume ratio, and under conditions of low hydrocarbon concentration, high temperature, and low overall conversion of hydrocarbon to carbon, soot formation is almost entirely suppressed and the carbon is deposited as a smooth, uniform, tightly-adhering film on the heated surfaces of the apparatus. The film is electrically conductive, and it is possible to measure the rate of carbon deposition in terms of the rate of change in the resistance of the growing film[lO,ll]. The temperature coefficient of resistivity of the carbon film is sufficiently small between 1000 and 1400 K that it may be assumed to be constant without introducing significant error[l0,12]. This technique has been used to measure the activation energies for carbon film deposition
1. INTRODUCTION
Investigation of the process by which solid carbon is formed in the high-temperature pyrolysis of hydrocarbon vapors is made difficult by the fact that both the nature of the carbon and the mechanism by which it is produced are strongly influenced by the conditions under which pyrolysis is carried out[l]. Whatever the mechanism, the transition from hydrocarbon molecule to solid carbon undoubtedly follows a complex pathway. Very different values for reaction order and activation energy are obtained depending upon the range of temperatures studied, whether a static or flow system is used, the partial pressure of the hydrocarbon, the presence or absence of a carrier gas, whether the reaction rate is measured in terms of loss of hydrocarbon or formation of carbon or other reaction product, and whether one measures total carbon produced or only that deposited on the reactor surface. The various mechanisms that have been proposed for surface deposition of pyrolytic carbon fall essentially into two categories[l-31: (1) vapor-phase nucleation and growth of large molecules, which subsequently diffuse to the surface where they undergo dehydrogenation and incorporation into the growing carbon mass and (2) nucleation and growth on the surface by direct deposition of the original hydrocarbon or a simple decomposition product thereof. While soot is undoubtedly produced by vapor-phase nucleation, it is unlikely that surface deposits of carbon originate by the same process. There is, however, considerable evidence that such deposits are formed by a heterogeneous process at active surface sites[4]. 47
48
D. B.
MURPHY
and R. W. CARROLL
GRAPHITE ELFCTRODFiS
Fig. 1. Schematic
diagram
from a variety of aliphatic and aromatic hydrocarbons[l0,13] and from carbon suboxide[ll]. Porter[l4] first proposed that acetylene, formed in a homogeneous decomposition process, is the ultimate precursor for carbon formation from any aliphatic hydrocarbon. More recently, Carroll and Murphy[l5,16], from analysis of the effluent gases, concluded that ethane rapidly forms a near-equilibrium mixture of ethane, ethylene, and acetylene during pyrolysis in a flow system between 900 and 1000 K, and that the acetylene is the source of the carbon deposit. This has led logically to an examination of the pyrolysis of acetylene itself, and this paper describes the results of that investigation. 2. EXPERIMENTAL
The pyrolysis apparatus is shown schematically in Fig. 1. It consists of a 2.54 cm i.d. refractory porcelain tube ground to a standard taper at each end, and supported within a high-temperature tube furnace. A porcelain thermocouple tube, 0.7 cm in diameter passes concentrically through the outer tube, supported at each end by a threaded bushing in the glass standard-taper end fitting. Gas inlet and outlet tubes are also sealed to each end fitting. The inner tube in turn supports a 1.27 cm o.d. porcelain tube 4.75 cm long upon which the carbon film is deposited, and two graphite tubes of the same diameter that serve as electrodes (Fig. 2). Connection
of pyrolysis
apparatus.
to the resistance-measuring circuit is made by applying a platinum coating to both ends of the inner tube and extending it just far enough into the reactor tube to make good electrical contact with the graphite electrodes. The small annular space between the inner tube and the porcelain tube that it supports is filled with chromatographic-grade alumina powder, effectively preventing deposition of carbon within that region. The system of electrodes and measuring tube is spring-loaded to maintain good electrical contact. During an experiment, the growing carbon film is connected in series with a standard lo-ohm resistor. A constant l-volt potential is maintained in the circuit, and the potential difference across the standard resistor, which is proportional to the conductance of the carbon film, is measured continuously with time using a recording millivolt potentiometer. Shortly after acetylene is introduced into the gas stream, the surface between the electrodes becomes covered with a continuous carbon film and the conductance increases linearly with time; the rate of change remaining constant as the resistance decreases from 6000 to 200 ohms. Assuming an average spacing between carbon layers of 0.35 nm and a specific resistivity of 2.43 x 10m3ohm-cm[lO], this corresponds to a growth in film thickness from about 15 to about 450 carbon layers. In this apparatus, a measured change in conductance of 1 x 10m4ohm-‘, corresponds to the deposition of 4.19 x lo-@ mole/ cm2 of carbon. The rate of carbon deposition in mol x cm-*s-’ is related to the measured rate of change in conductivity in ohm-is-i of the growing film (A) by the expression rate = pdAl12 where: p = 2.43 x lo-’ ohm-cm d = 2.07 g/cm3
Fig. 2. Mode of assembly of graphite electrodes.
The furnace temperature controller was connected to a Pt/Pt-10% Rh thermocouple inserted into the inner tube with the bead close to the center of the reactor zone. A second, similarly located thermocouple was used to monitor the reaction temperature. The temperature could be maintained within
49
Carbon fiim deposition t2 K from 5 cm upstream to 5 cm downstream of the center of the reaction zone, and within 15 K of the measured temperature throughout the remainder of the heated part of the furnace. Gas flow was regulated by electronically controlled mass flowmeters, from which the gases passed thorugh a mixing manifold into the furnace. Leaving the furnace, the effluent passed through a cold trap and filter and into a lo-cm gas cell mounted in a Miran 980 analytical infrared spectrometer calibrated for ethane, ethylene, and acetylene, and from there through a wet-test meter. This arrangement permitted simultaneous measurement of carbon deposition rate, effluent gas composition, and gas flow rate. The accuracy and reproducibility of gas analyses was ~5% or better.
I’
I 80
3. RESULTS AND DISCUSSION
I a.2 I/T
Carbon deposition rates were measured at intervals of 25 K between 1150 and 1250 K, using a standard mixture of 2.57% acetylene in helium at a flow rate of 800 cm’/min, corresponding to a residence time in the reaction zone of about 1 s (Table 1). An Arrhenius plot of the data is shown in Fig. 3. The activation energy for the process calculated from these data is 278 kJ/mol. Experiments were then run at 1132 and 1179 K and the same flow rate, adding successive molar increments of hydrogen to the input gas and examining the effect on the carbon deposition rates. The rate of carbon deposition was found to vary inversely with the partial pressure of added hydrogen. (Fig. 4). Analyses of the effluent gases for ethane, ethylene, and acetylene, as well as the composition of the input gases are given in Tables 2 and 3. As the hydrogen partial pressure is increased, the concentration of ethylene in the effluent gas increases linearly, as does that of the minor constituent, ethane. However, a corresponding decrease in acetylene concentration, which would be expected if acetylene were reacting in the gas phase to form ethylene, does not take place. In fact, the ratio of output acetylene to input remains essentially unchanged, regardless of the amount of hydrogen added to the input gas. This is shown in Fig. 5, where the ratio of hydrocarbon output to acetylene input is plotted against the ratio of hydrogen input to acetylene input for both hydrocarbons. Ethylene is not formed by reaction of hydrogen
I 6.6
I 0.4
I 8.8
.
X IO4
Fig. 3. Arrhenius plot of carbon deposition rate VS.T-j.
with the already-deposited carbon, since at these temperatures, passing hydrogen at a partial pressure of 0.1 atm over a freshly deposited carbon film was found to cause neither evolution of ethylene nor any change in the conductivity of the film. It would appear, therefore, that of the acetylene introduced into the apparatus, a fixed proportion, depending only on the temperature and not on the amount of hydrogen in the gas mixture, is adsorbed and reacts on the surface, the remainder passing unchanged through the apparatus. Part of this adsorbed acetylene forms carbon and part is hydrogenated; that is, both the carbon and the ethylene are produced on the surface and not in the gas phase. For the
Table 1. Carbon deposition rates at various temperatures T, K 1147 1173 1198 1223 1248
mol C . s-‘cm-2 5.86 1.14 1.88 3.60 6.36
x x x x x
lo-‘* 10-v 10-y IO-9 10-y
0
1
0
,
.
,
,
2
I
3
.
I
4
.
I
5
I’K&WZHZ
Fig. 4. Inverse plot showing effect of partial pressure of input hydrogen on carbon deposition rate. a, 1132 K; 0, 1179 K.
50
D. B. MURPHYand R. W. CARROLL
Table 2. Partial pressures of input and effluent gases at 1132 K Input gas (atm) I-k 8.Z 0:0474 0.0698 0.0888 0.1091
CA 0.0250 0.0243 0.0238 0.0234 0.0229 0.0224
Effluent gas (atm) CA 0.00011 0.00912 0.00026 0.00033 0.00041 0.00043
GH,
GHz
0.00103 0.00234 0.00346 0.00468 0.00565 0.00658
0.0198 0.0193 0.0189 0.0185 0.0184 0.0179
1 8
0.4
-
9 8
NOM./ .A
/O
amount of acetylene consumed to be unaffected by the hydrogen concentration, the initial adsorption step must be decoupled from the process in which products are formed. For example, after adsorption on the surface, the acetylene may undergo a slow reaction at active sites to form a chemisorbed intermediate which then serves as the common source of products. The ratio of output to input acetylene remained unchanged in these experiments, despite the fact that the partial pressure of acetylene was decreased by about 10% by the addition of hydrogen, suggesting that the consumption of acetylene is zero order, again implying a surface reaction. Figure 6 shows the effect of residence time on the consumption of acetylene and production of ethylene, when a mixture of 2.57% acetylene and 4.4% hydrogen in helium at 1 atm was passed through the apparatus at 1130 K. From this is may be seen that, except perhaps at long residence times, acetylene consumption increases linearly with residence time in the system, consistent with a zero-order process. This diagram also provides a crude representation of the conditions that would exist at various points along the reaction tube with the gas flowing at a rate of approximately 1.67 cm/s. Reading from left to right on the diagram corresponds to progressing downstream through the reaction zone. Initially, production of ethylene parallels the consumption of acetylene, but soon lags behind, as other products are produced in increasing quantity. The data used to prepare Fig. 6 are given in Table 4. The difference between the % recovery and 100% is attributed to the relatively small amount of carbon deposited and unanalyzed products, probably benzene, naphthalene, and other polyaromatics which have been detected in the effluent[l7]. The data
d g
-
0.2 -_g&’
D
0.0 0
I ,
.
1 2
,
I 3
,
I 4
~
I 5
PWpCZH2
Fig. 5. Plots of ratios of hydrocarbon output partial pressure to acetylene input partial pressure VS. ratio of hydrogen input partial pressure to acetylene input partial pressure. Upper pair of lines acetylene; lower pair of lines ethylene. 0, 1132 K; 0, 1179 K.
show that, except at the longest residence time, the system is in what has been called the “plateau region”[l3] where the carbon deposition rate is independent of flow rate. In this same region, however, the yield of unanalyzed products varies significantly with residence time. It may be concluded from this that these substances are not intermediates in the production of deposited carbon.
Table 3. Partial pressures of input and effluent gases at 1179 K Effluent gas (atm)
Input gas (atm) I-L
GHz
0.0000 0.0224 0.0438 0.0644 0.0841 0.1028
0.0249 0.0243 0.0239 0.0233 0.0227 0.0224
G& 0.09002 0.09013 0.00013 0.00041 0.00031 0.00043
CJI,
CJIz
0.00101 0.00186 0.00421 0.00553 0.00639 0.00744
0.0176 0.0175 0.0174 0.0171 0.0165 0.0162
Fig. 6. Effect of residence time on effluent composition. 0, acetylene; 0, ethylene; T = 1130 K; input gas: 2.57% acetylene + 4.40% hydrogen in helium at 1 atm.
51
Carbon film deposition Table 4. Effect of residence time on product composition at 1130 K. Input: 2.57% C,H, and 4.40% H2 in He at 1 atm Vatm), effluent gases Residence time s/cm
CzHz
C,H,
% Recovery*
dCldt x 10”’ mol C . cm-?’
0.589 0.286 0.190 0.143 0.111
0.0175 0.0205 0.0218 0.0225 0.0229
0.00385 0.00280 0.00235 0.00190 0.00153
83.1 90.7 94.0 94.9 95.1
3.14 2.73 2.73 2.68 2.64
*Neglecting deposited carbon, which amounted to less than 1% of carbon introduced as acetylene, and negligible amounts of ethane in effluent. The experimental evidence indicates that acetylene undergoes three main processes during pyrolysis under the experimental conditions described here. These are: (1) dehydrogenation and deposition of carbon, (2) hydrogenation to ethylene, and (3) polymerization to higher hydrocarbons, most probably benzene and polyaromatics. The rate of consumption of acetylene at a given temperature has been found to be independent of the partial pressures of both C2H, and H, in the experimental pressure range. This places severe restrictions on any reaction mechanism that may be postulated, and makes it unlikely that any homogeneous reactions are involved in these processes. It is also unlikely that completely independent, parallel surface processes are taking place. In an extensive study of adsorption and pyrolysis of propylene over carbon active sites, Hoffman, et a1.[18.19] observed that the rate of propylene consumption depended upon the initial concentration of active sites, and remained constant throughout pyrolysis, even after the original surface had been covered many times with deposited carbon. They concluded that carbon deposited on an active site is accompanied by regeneration of the active site. The process of carbon deposition is autocatalytic, and the concentration of active surface sites remains unchanged during pyrolysis[20,21]. The structure of the carbon film is turbostratic[22], consisting of small, graphitic crystallites in approximately parallel orientation to the surface. Flat surface regions are relatively inactive in adsorption, while steps, crystallite edges, pits, and other defects afford active sites for adsorption[23]. While this variety of structures will lead to a range of site activities and energies of adsorption, the average, or typical, active site is considered here. Provided that the concentration of active sites remains unchanged, the constant rate of acetylene consumption can be accounted for by a two-stage adsorption process. Fast and reversible non-dissociative adsorption of acetylene at sites, with adsorption near saturation at all times, provides the required constant concentration of reactant. It is necessary to postulate a subsequent, slow, and essentially irreversible stage of chemisorption in order to decouple the initial adsorption step from the pro-
cess leading to products. To maintain the required constant site concentration, this second stage must be accompanied by regeneration of the active site. All of the observed reaction products then result from further reaction of the product of this second stage of chemisorption. As a result, the rate of acetylene consumption is controlled by the rate of the second stage of chemisorption and the concentration of active sites. Since both of these factors are constant under a given set of reaction conditions, the acetylene consumption rate will be independent of the relative rates of the surface processes that follow. Hoffman, et a[.[231 concluded from a study of chemisorption of propylene on a clean Graphon surface at low temperatures and pressures, that the alkene is strongly adsorbed at active sites, and that a surface complex, whose nature was not specified, is formed slowly on different, less active sites. Desorption occurred only at elevated temperatures, and was accompanied by decomposition; hydrogen and methane being the only species desorbed at 873 K. The carbon deposited remained at the active site and became a new active site. Their experimental conditions were very different from those described in the present work, however, so that it is difficult to say how much similarity there is between the twostage chemisorption process that they describe and that postulated herein. Neither acetylene in the gas phase nor the reversibly adsorbed acetylene can undergo hydrogenation to ethylene, for in that case the partial pressure of hydrogen would influence not only the formation of ethylene, but the consumption of acetylene as well, contrary to observation. The linear dependence of the rate of ethylene formation on the partial pressure of hydrogen is, however, consistent with hydrogenation of the strongly chemisorbed species on the surface. Hydrogenation must also change the nature of, and weaken the bonding to the surface in order to permit desorption of ethylene. Carbon already incorporated into the crystal structure of the growing film does not react with hydrogen, but the dehydrogenation process to form carbon would be expected to be inhibited by hydrogen, leading to the observed inverse relationship between the carbon deposition rate and the hydrogen partial pressure. The difference between the amount of carbon in-
52
D. B. MURPHY and R. W. CARROLL traduced as acetylene and that recovered as acetyH,(sur) + sur * C,H2(C2H2)m*C2H2 lene, ethylene, ethane, and carbon is ascribed to e sur . GH,GWn*GH, (4b) higher molecular weight hydrocarbon products. The amount of this unanalyzed material decreases with The product of step (1) does not hydrogenate. Hyincreasing hydrogen partial pressure, and increases drogenation of the chain at the outer end as in eqns with increasing residence time, but is not related to (4) and (4a) leads to ethylene formation. Hydrothe amount of carbon deposited, leading to the congenation at the surface end as in eqn (4b) blocks clusion that these substances are not intermediates carbon deposition. Hydrogenation elsewhere on the in the production of deposited carbon. The constant chain does not affect either process. rate of consumption of acetylene precludes their being formed in the gas phase. The second stage of sur . (C~H*)“C~H~*C~H~ ~hemisorption regenerates an active site to which a = sur - GW,-lGH2* + W%(g) + U%(g) second acetylene molecule can become attached, again regenerating an active site. At lower hydrogen (5) pressures, this process may occur more rapidly than sur - (C,H,),C,H,* hydrogenation to ethylene or dehydrogenation to e sur * (C,H,),_,C,H,* + C,H,(g) (5a) carbon, resulting in the formation of polymeric chains attached to the surface, with an active site at Steps (4a) and (5a) will occur much less frequently the terminal end of each chain. Increasing hydrogen than steps (4) and (5) because of the very low conpressure should lead to shorter chains, with a twocentration of uncovered active sites. carbon chain the limiting case. Unless these chains Deposition of carbon at the surface: are to grow inde~nitely, there must be a process for their removal, perhaps through ring closure and desur . C*H*(C~~~)~*(C~H~) sorption as aromatic species. sur * (C,H,),*C,H, + 2 C + H,(sur) (6) 4. REACTION MECHANISM
Formation
The following sequence of reaction steps is offered to account for the observed results: Reversible nondissociative molecular adsorption of C,H,, covering active surface sites and blocking further adsorption[24]: C,H, + sur’ e
sur*C2H2
(I)
This step is fast in both directions and near equilibrium, with k, > k-,. In the range of acetylene partial pressures used in these experiments, the surface sites at all times are nearly saturated, so that the concentration of adsorbed species is approximately equal to the site density, (a). Second stage of chemisorption with active site regeneration: SW - C,H,*
(2)
Slow, with k_, z=-> k,. The product undergo steps (1) and (2) repeatedly, chain growth on the surface:
may itself leading to
sur*C2H2 -
sur * GH2),(GW3*
+ CzHz e
sur . C2H,*C2H, -
sur * C2H2*C2H2
sur - C,H,C,H,*
H,(g) e H,(sur)
etc.
H,(sur)
(2a)
Equations (1) through more simply as follows:
SW
(C,H,),C,H,*
(5) may be represented
(1) (2)
C2H, + A* $ A A-+A* H,(g) G H,(sur) ii; H,(sur) -t A e B (44 Hz(sur) + A* s B* (4b)H,(sur) + D zz E (5) B* A* + C&(g) + CJMg) (54 B* + A* + C,H,
The following assumptions
are made:
1. All reversible processes, with the exception of (5) and (Sa), are near eq~librium. 2. Steady state conditions exist on the surface. 3. k, >> k_, k-, >> >>
k4 k,(a
Thus: 4. [A] >> [A*]
[Al ‘> PI
(4)
The overall eqn representing ylene may be written:
(4a)
the rate of loss of acet-
~,~C~H~~g)l[A*~ - UAI - k@l + k -5fC,H,(g>ltC,H,fg)l[A*l(8)
-W,H,(gWdt .
(7)
[A*] >> [B*]
+ sur * (C,H,),C,H,* =
+ GM.4
(3)
f sur * (C,H,),C,H2*CZH2 G== sur - (C,H,),C,H,*C,H,
H,(sur)
(la)
sur + (GHz)~-~GH~*
-
La
sur + C,H,*
of aromatic species, for example:
=
Carbon film deposition If hydrogen adsorption far from saturation,
Assuming steady state conditions: d[A]ldt
= 0 = k,[C,H,(g)][A*] - k&41 -
d[A*]ldt
WWur)I[4
+ bP1
[K(sur)l
-h[GWdl[A*l + k~@l + UAI - ~4aFL(W[A*l + L[B*l + WY - k~,[C,H,(g)l[C,H,(g>l[A*l + ks,[B*l- LaLKk)l[A*l
= 0 = k4[H2(sur)][A]
- ks[Bl +
- k~,[Bl
k~S[C,H,(g)l[C,H,(g)l[A*l(11)
Adding eqns (9) and (11) and subtracting
Since [A] = o, and partial pressures may be substituted for gas concentrations: = a{(k,k,k,/k_j)pH,
d(pC,H,)ldt
- (k-lk-,lk,)pC,H,}l(k-~
(17)
from eqn pH,(input)
= k,[Al = k,(o)
(12)
Substituting
[A] can be replaced by u in this eqn since the concentration of chains on the surface is equal to the site density, and at any moment few of those chains are hydrogenated at the outer end. The mechanism thus yields the experimentally observed zero-order rate of loss of C2H,.
d(pC,H,)ldt
(13)
Applying assumptions 3 and 4, and subtracting eqns (9) and (10) from eqn (13) affords
4GH,(g)lldt = WLW~)I[4 - UBI
(14)
Solving eqns (9) and (11) for [A*], equating the results, solving for [B] and substituting for [B] in eqn (14):
4GHJg)l~~t = WMWIPI k-,[Al{k-s[~H,(g)l(k, + kz + W-Wur)l) + M4~WW~ _ kdk-, + ks) + k-v-K,H&)l
(18) terms:
= ok3k4k5/k~3pH,(,,,,,,/(k-4 + k,) - (o{(k3k4kJk-3) + (k~lk-Slk,)}PC2H4)l(k-~
Upon integration,
(19) + k,)
this expression reduces to
pC,H,
- k-.~[C,H,(g>lIC,H,(g)l[A*l + kaP*l - LLKk)l[A*l
= pH, + pCZH,
in eqn (17) and rearranging
d[C,H,(g)lldt = WI
= [constant]pH,
(20)
in agreement with experiment. The mechanism requires that all of the acetylene consumed appear as ethylene, carbon, polyaromatits (PAH), and negligible amounts of ethane, all arising from a common source, namely chemisorbed surface species: - d[C,H,]/dt = d[C,H,]ldt
+ d[PAH]ldt
+ dC/2dt
(21)
Typically, carbon deposition accounts for less than 2% of the acetylene consumed. Thus, from eqn (12) kZcr = d[C,H,]idt
+ d[PAH]/dt
(22)
c15j
Making the reasonable assumptions that k2, k4, k,, and k-s are small relative to k,, and that k-,/k, is small affords:
-
+ ks)
Since less than 2% of the hydrocarbon introduced is converted to carbon, any hydrogen produced in that process is negligible relative to the hydrogen added to the gas stream, and the increase in the partial pressure of ethylene as the gas proceeds through the apparatus is approximately equal to the decrease in partial pressure of hydrogen. At any point in the system, therefore
(8): -d[C,H,(g)]ldt
Wk~#Mg)l.
=
(9)
(10) d[B]/dt
53 and
- k-,[A]
= 0 =
is close to equilibrium
(k-,k~,lk,)[C,H,(g)l} (16)
Therefore, as hydrogen is added to the system, more ethylene is produced at the expense of PAH, the sum of the two products remaining essentially constant. Carbon deposition by this mechanism occurs where the chain is attached to the carbon surface (eqn 6) and is hindered by hydrogenation of the chain at this point (eqn 4b). dC/2dt
= k,[D]
(23)
D. B. MURPHY and R. W. CARROLL
54
With step (4b) near equilibrium,
~dWP1 = LPI
(24)
Since all chains are either hydrogenated [E] or not hydrogenated [D] at the end attached to the carbon surface, u = [D] + [E] Substituting
(25) Ack~owledge~e~~-The authors wish to thank the City University Research Foundation for support under PSCi CUNY Grant #6-65203.
for [E] in eqn 25:
PI = 41 f (WLb)[H(sur)l~ Wdsur)] = (Wk-dpHZ
(26) (27)
therefore, K/2&
= k,l{l
+ (constant)(pH,)}
(28)
giving a linear relationship between the reciprocal of the carbon deposition rate and the partial pressure of hydrogen: dt/dC
= 1/2k,{(l
+ (constant)(~H~)}
sites remains constant. The product of the second stage of adsorption can then undergo dehydrogenation to carbon, hydrogenation to ethylene, or formation of surface chains by chemisorption at the regenerated active sites. Cyclization and desorption of surface chains can account for the benzene and polyaromatics that have been detected in the effluent gas.
(29)
as observed experimentally. 5. CONCLUSIONS Acetylene undergoes three main processes during pyrolysis in a flow system under the experimental conditions described here: carbon deposition, hydrogenation to ethylene, and polymerization to higher hydrocarbons. The activation energy for carbon deposition is 278 k.I/mol. The rate of carbon deposition is inversely related to the partial pressure of hydrogen in the input gas. Incremental addition of hydrogen to the input gas causes a linear increase in the amount of ethylene in the effluent, but has no effect on acetylene consumption; the ratio of output acetylene to input remained unchanged even when the hydrogen partial pressure was increased to as much as five times that of the acetylene. At a given temperature, acetylene consumption is dependent solely upon residence time in the reaction zone, suggesting a zero-order process at saturated active sites. A mechanism is proposed invoI~ng reversible, non-dissociative molecular adsorption of acetylene, followed by a slow, and essentially irreversible stage of chemisorption, the rate of which controls the rate of acetylene consumption. The second stage of chemisorption is accompanied by regeneration of the active site, so that the total concentration of active
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