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Deposition of carbon and its hydrogenation catalyzed by nickel
DEPOSITION OF CARBON AND ITS HYDROGENATION CATALYZED BY NICKEL Y. NISHW~MA Chemical Research fnstitute of Non-Aqueous
and Y. TAMA] Solutions. TohokuUniversity,Sendai,Japan
Abstract-Formation of carbon on nickel sheet from benzene vapor carried by hydrogen was studied at a temperature range from 520 to 730°C. A maximum rate was observed at about 63o”C, above which the deposition rate decreased rapidly. The carbon formed was hydrogenated in situ. Methane was the main gaseous product and a m~imum rate was observed at about 67OY. Very high reactivity of deposited carbon toward hydrog~~tion was ascribed to the catalytic action of nickel particles dispersed in the carbon. The hydrogenation rates were divided into three zones and possible interpretations are discussed. A mechanism which is a reverse process to deposition was suggesled. The decrease of the hydrogenation rate at higher temperatures was due to the equilibrium among carbon, hydrogen and methane, where carbon was more reactive than graphite.
hydrogen. The reactions were conducted in a quartz reactor of 26cm Ld., in which nickei specimen, I cm square and 0.01 cm thick, was suspended vertically with a quartz spring balance. From the weight changes of the specimen, the rates of carbon deposition and hydrogenation were obtained. Weight changes were measured with a precision of iO.05 mg. An electric furnace and a controlling device could maintain the temperature of reaction zone within * 3°C. Nickel specimen had a purity of 99.7%. with 0.2% of cobalt as the major impurity. The pretreatment of nickel and the purification of gases were the same as the preceding study [ 121. At the time of carbon deposition, benzene vapor, 60 torr in partial pressure, was mixed into hydrogen carrier,* and total flow rate was kept at 30 + 0.5 cm’ (STP)/min, total pressure being atmospheric. The gasification of deposit was conducted by passing atmospheric hydrogen over the specimen at a rate of 120i- 2 cm’ (STP)/min, except otherwise noted. The deposit adhered well to the vertical wall of the substrate during gasification, so that any holding device was unnecessary. Gas chromaf;ograph analysis of effluent gas was done using activated charcoal column, 2 m, at 100°C and a thermal conductivity detector. X-ray diffraction was conducted with Cu-Kcu ray by usual powder method. For determination of nickel content in the deposit, this was removed from the substrate and burned gently in air. The residue was dissolved in nitric acid and nickel was determined calorimetrically as nickel dimethylglyoxime.
The formation of carbonaceous solid from carboncontaining gases such as hydrocarbons is catalyzed by some metals, especially iron, cobalt and nickel. Carbons formed on these metals at temperatures below 800°C are mostly in a fibrous form and contain metallic particles [ l121.The mechanism of formation has been discussed by several authors. Many of them considered the diffusion of carbon atoms through metal phase as an intermediate step of successive reactions[ IO-141.The whole process is not simple and leaves details to be ascertained. A carbon-Mets composite formed by the deposition process was found to be highly reactive toward gasification[lS]. Gilliland and Harriott noted this earlier[l6], but they did not correlate the enhanced reactivity with metals in carbon. It seems rather obvious that metal particles in the deposit catalyze the reaction. The methanation of carbons draws intensive attention recently in relation to coal gasification, but relativeIy few reports deal with its fundamental aspects of catalytic processes. A research in this laboratory showed that hydrogenation of active carbon catalyzed by transition metals exhibited a few maxima in the rate vs temperature curve[l7]. In a course of carbon deposition study, we noted some resemblance in the kinetic behavior of deposition and hydrogenation processes. It was felt that the investigation of the reactivity of deposited carbon would give some light on the nature of carbon-metal interactions and help one to understand catalytic gasification processes. The present report describes a study initiated from such a point of view. The deposited carbon was revealed to have fairly uniform and quite high reactivity which does not change upon heat treatment.
3.RESULTS
2.MPERIMENTAL
Carbon was deposited from benzene vapor onto cold-rolled sheet of nickel, and was gasified in situ by
Weight changes in three deposition/hydrogenati~ln runs are shown in Fig. 1 to illustrate how the reactions were conducted. Typical features observed were as follows. Below 63O”C, the deposition process normally had an
tin spite of thermodynamic disadvantage, measurable carbon deposition from benzene was observed only in hydrogen atmosphere under the present conditions. In helium, the deposition rate was too small to be measured. The effect of carriers will be discussed in a separate report.
induction period of 10-20 min, and after this the rate constant within 26% up to 40 mg/cm’ (based on apparant surface area of nickel sheet), which was instrumental limit. At higher temperatures. the
CAR Voiid
13 No. 1-B
was the the
rate
14
Y. NISHIYAMA and Y. TAMAI Table 1. Nature of deposited carbon
“E
9 F20-
610
644
78
81
24
(A,
GrWGlitiZatiO"
0
E
584
@Ia)
do02*
E
s
(“C)
TOtaldeposit Graphitic Str”Ct”re
$
.P
tlepasition temp.
t
A". tryst. sire
IO-
3.40f0.01
3.39
3.38
(X)
47?12
58
74
(8)
60f5
70
70
Nickel crystallite content
(wt %)
A". tryst. size
0
200
100
Cd;)
1.8kO.2
2.0
900~100
480
3.2 _t
300
Time, min
Fig. 1. Weight changes due to carbon deposition and hydrogenation. Broken lines represent the period when benzene vapor was admitted. Numerals attached indicate the reaction temperature (in “C),whichwas changedat the time shownby verticalstrips.
* Graphitic layer spacing. t Ratio Of graptlitization;(3.44-doo2)/o.oB5. * Data scattered Llecween800 and 2500 i.
700
decreased with time as seen in Fig. 1. By turning off the benzene injection, gasification of deposit took place, sometimes with a fairly long induction period, which was partly due to residual benzene vapor. The rate of gasification was usually constant within 2 2% at conversion between 20 and 65%, if conducted at a constant temperature. Thus the main part of deposition and gasification proceed in zero order with respect to the amount of deposit in each run. However, the rate of gasification was seen to be roughly proportional to the total amount of deposit among different runs at temperatures below 680°C. To see the temperature dependence of both processes, temperature jump method was employed, as demonstrated in Fig. 1. Exhaust gas analysis indicated that methane was produced during deposition. The ratio of the rate of methane formation to that of carbon formation was 10 and 6% at 595 and 654°C respectively. At the time of gasification, methane was the only gaseous product detected and its concentration agreed quite well with that calculated from the rate of weight decrease of the specimen. The deposited carbon was a soft bulky mass of black powder and had some degree of crystallinity to diffract X-ray. A few characteristics are tabulated in Table 1. The surface area of deposit was estimated to be around 100m*/g[12]. 3.2 Temperature dependence of deposition rate The Arrhenius type plots of the deposition rate are shown in Fig. 2, which contains three kinds of plot; steady rate, initial rate and “activated” rate. The steady rate, the slope of linear part in Fig. 1, is clearly lambda-shaped. From this, temperature regions above and below 630°C will be referred to as zone A and zone B, respectively, for convenience. It should be noted that the deposition rate tThe experimental data were classified into three sets according to their reliability, i.e. the constancy of the rate itself and the amount of deposition. The ranges given here and in the next section merely indicate the region on which calculated slopes from any combination of sets came, and actually they cover the confidence interval with 90% reliability from most reliable sets.
650
600
550°C
“\ \
1000/T,
K-’
Fig. 2. Arrhenius plot of deposition rate. 0, steady rate; 0, initial rate; I, activated rate. changes reversibly with temperature over zones A and B,
as seen in Fig. 1, though there was a small irreversible deactivation in zone A. It took some time before the rate in zone A became constant. The degree of deactivation was demonstrated by the difference between steady and initial rates. The latter was obtained from extrapolation of the rate against the amount of deposit. The “activated” rate came from experiment in which the deposit was partly hydrogenated and then benzene vapor was introduced. In this case, no induction period was observed and the rate was very high, decreasing rapidly. The plots of steady rate below 630°C gave an apparant activation energy of 36 + 3 kcaUmo1e.t 3.3 Temperature dependence of hydrogenation rate Figure 3 shows the rate of hydrogenation divided by the total amount of deposit in the Arrhenius form. The plots are again lambda-shaped. The temperature regions above 680°C from 680 to 6OO”C,and below 600°C will be referred to as zone A’, B’, and C’, respectively. The reversible nature of hydrogenation rate over these zones is ascertained by the temperature jump method (Fig. 1). In addition, two of the specimens were heat treated in helium
Deposition of carbon and its hydrogenation
O.l,-3%- 800,
/
I-
/
,
09
550°C
600
700 I,
catalyzed by nickel
L
IO
1000/T,
I
1
1
I.1
12
b1
K-’
Fig. 3. Arrhenius plot of hydrogenation rate. The rate is normalized by dividing by the initial amount of deposit. I and A, heat treated at 745°C for 30 min and 865”C, 40 min, respectively. Equilibrium line was calculated for 20 mg of graphite, as the initial amounts for the runs above 700°C were from 19 to 23 mg.
before hydrogenation. Plots of these came on the same line as untreated ones. The rate of hydrogenation was independent of the deposition temperature within the experimental error. The apparant activation energies for zones B’ and C’ are 33 * 4 and 63 rt 3 kcal/mole respectively, the former agreeing well with that reported by Gilliland and Harriott [ 161. 3.4 Eflects of pressure and flow rate of hydrogen on hydrogenation
The effects of flow rate and pressure of hydrogen are shown in Figs. 4 and 5. Increasing of hydrogen flow rate resulted in the increase of the hydrogenation rate over entire temperature range, but there was a tendency that the rate in zones B’ and C’ leveled off at higher flow rate. The pressure dependence was examined by mixing helium into hydrogen flow, keeping total pressure atmospheric. The rate of hydrogenation is roughly second order to partial pressure of hydrogen in zone A’, while it is nearly zero at 1atm and 602°C. Concentration of methane in the effluent gas was 2.7 vol % at 710°C and 120cm’/min, corresponding to about 30%
Vol. fraction of HP Fig. 5. Pressure dependence of hydrogenation rate. 0, 602°C; 0, 788°C; I, 895°C. Broken lines show second order dependence.
of equilibrium value which was calculated on assumptions that whole gas phase was equilibrated with solid phase and that carbon was in the form of graphite. The calculated equilibrium is included in Fig. 3. By reducing the flow rate of hydrogen to 20 cm’lmin, 55% to equilibrium was reached. 3.5 Hydrogenation of carbon powders The deposited carbon was separated from nickel sheet and was rendered to hydrogenation in a quartz pan. This specimen was a little less reactive than those hydrogenated in situ, but quantitative comparison is meaningless because of the difference in geometric arrangement. introduction of benzene to the residue of hydrogenated powder resulted in a weight increase at a rate comparable to the original runs (Fig. 6, a and b). As a control, a carbon black, ISAF, was impregnated with nickel from aqueous solution of nickel chloride at a concentration of 5 wt % of nickel to carbon, and was heated in hydrogen, but no measurable gasification was observed (Fig. 6~). Further consequence of mckelcatalyzed hydrogenation of carbons can be found in a report from this laboratory, in which nickel impregnated active carbon was gasified by hydrogen in two stages [ 171. 4. DISCUSSION
4.1 Mechanism of carbon formation on nickel Recent propositions on the mechanism of carbon formation on catalytic metals mostly include the diffusion
Time, min Flow rate.
cc(STP)/min
Fig. 4. Effect of flow rate of hydrogen. 0, 596°C; 0, 648°C; 0, 796°C; I, 905°C.
Fig. 6. Hydrogenation of carbon powders. A and B, deposited carbon; C, nickel impregnated carbon black. Designations :are the same as in Fig. 1.
16
Y. NISHIYAMA and Y. TAMAI
of carbon atoms through metal phase as an intermediate step[Wl4]. For the motive force which causes diffusion, Baker et al. suggested the temperature gradient due to heat of reaction[lO], but other explanation is possible[ 11,121. Except for the motive force, the proposed mechanism seems reasonable, and the present results are consistent with the mechanism. The apparant activation energy observed in zone B is nearly the same as those reported for the deposition from other hydrocarbon sources[lrl, 181,and is in the same range as that of diffusion of through carbon nickel, 34.838.5 kcal/mole[19,20], leading to the same view as Baker’s[l4] that the rate-determining step in carbon formation in zone B is the diffusion of carbon atoms through nickel. The concurrent formation of methane during deposition seems to suggest the presence of single carbon atom species on nickel surface. The decrease of the rate in zone A is not well understood. Similar changes are reported for deposition from olefins[5,13,18] and from carbon monoxide[2], and Lobo et al. ascribed this to the decrease in the rate of adsorption of olefin[l8]. In Fig. 2, the “activated” rate seems to come on the extension line from zone B, although the plots are somewhat scattered. Then, adsorption of benzene onto clean nickel surface does not decrease at higher temperature. As the rate changed reversibly, the decrease in zone A might be interpreted by the formation of carbon patches or incomplete nickel carbide, which inhibited the dissociative adsorption of benzene and was removed by hydrogen at lower temperature. In other words, overall rate is determined by the competition between the rates of carbon atom formation and diffusion of them into bulk. 4.2 Mechanism of hydrogenation of deposited carbon The high reactivity of carbon-nickel composite presented here is noteworthy. There could be three possible mechanisms for catalytic hydrogenation of carbons: (1) methane formation at the boundaries of three phases, carbon-hydrogen-nickel; (2) methane formation’ at the active sites of carbon surface by dissociated hydrogen atoms which are formed on nickel surface and diffuse on carbon surface to the reaction sites; (3) reaction of adsorbed hydrogen atoms with carbon atoms on nickel surface, in which carbon atoms are supplied from inside of metals. The last one implies a reverse process to that of carbon formation discussed in the preceding section. Though the present results are not sufficient to discriminate three mechanisms, the authors would suggest the third one to be most likely, assuming that the whole body of the deposit consists of fibrous carbons containing nickel particles. The following are a few reasons for this view: (1) uniform rate of hydrogenation from 20 to 65% burnoff for fibrous carbon containing catalytic particles at some locations is most easily visualized by the third mechanism, in which reaction sites could be preserved at the same conformation; (2) the hydrogenation of graphite is reported to proceed through drilling of channels [21,22], which can be interpreted by the same mechanism, otherwise the pathway of gaseous reactant and product is a problem; (3) the temperature dependence in zone B’ is
very close to that of deposition, suggesting a similar rate controlling step. It should be pointed out that diffusion of carbon through nickel need not occur across the diameter of several hundred angstroms, rather short paths near periphery would be sufficient for gasification of carbon filament which have hollows [8,9, 111. There are some ambiguities in the above discussion. For example, the observed activation energy can be either that of diffusion of hydrogen atoms on carbon surface[23], or a half of activation energy of surface reaction when diffusion of reactant gas through pores is a predominating factor[24], as both would give values of similar magnitude. Also there can be catalytic contribution of finely dispersed nickel atoms or small aggregates [6,9], working in the second mechanism. These points will be studied further. The rate in zone C’ would be explained by one of surface reactions as rate-determining step. The activation energy of 63 kcal/mole could be compared with 55? 3 kcahmole for platinum catalyzed hydrogenation of carbon blacks reported by Rewick et a1.[25], who supposed the dissociation of hydrogen on platinum to be rate controlling. The decrease of hydrogenation rate in zone A’ should be ascribed to the equilibrium concentration of methane near reaction sites, since the rate is second order to hydrogen pressure. As the rate is not exactly proportional to the flow rate (Fig. 4), there is some contribution of diffusion of gaseous product to the overall rate. One difficulty associated with this interpretation was that the slope of the plots in zone A’ in Fig. 3 is larger than expected. It yields about -34 kcal/mole for heat of reaction, while that of graphite is - 21.6 kcal/mole at 800°C. The difference could be attributed to the structural incompleteness of deposited carbon, but it is somewhat larger than reported[26], so that there might be another effect that caused hydrogenation rate to decrease at higher temperature. The hydrogenation of nickel-catalyzed active carbon shows an enhanced activity at 540°C which vanishes by heat treatment [ 171.The structural unit involved there is different from deposited carbon, since this was unaffected by heat treatment. 4.3 Enhanced reactivity of deposited carbon The high reactivity of deposited carbon was reported by Gilliland[16]. So far as hydrogenation is concerned, it is obvious that the nickel particles or dispersed atoms incorporated in the deposit catalyze the reaction, in addition to the structural irregularity which enhances the reactivity. But a simple impregnation of nickel to carbon black, which is less graphitic to diffract X-ray, cannot be as effective as the present cases. Both the state of dispersion of nickel and the nature of contact between carbon and nickel may play important roles. On the basis of “diffusion of carbon through nickel” mechanism, the nature of contact, i.e. degree of adhesion and relative crystallographic orientation, might be such that transfer of carbon atoms from graphitic lattice occurs easily. Apart from the explanation, the observation suggests a possibility that catalytic hydrogenation of graphitic carbon
Deposition of carbon and its hydrogenation can proceed method
in high rate at around
of impregnation
of catalytic
65o”C,if an adequate metals
is achieved.
REFERENCES I.
1. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Davis W. R., Slawson R. J. and Rigby G. R., Nature 171,756 (1953). Walker P. L., Jr., Rakszawski J. F. and Imperial G. R., J. Phys. Chem. 63, 133. 140 (1959). Renshaw G. D., Roscoe C. and Walker P. L., Jr., .I Catal. 18, 164 (1970). Robertson S. I)., Carbon 8, 365 (1970). Tesner P. A., Robinovich E. Y., Rafalkes I. S. and Arefieva E. F., Carbon 8, 435 (1970). Baird T., Fryer J. R. and Grant B., Nature 233, 329 (1971). Harris P. S., Baker R. T. K. and Birch R. A., Carbon 11,531 (1973). Evans E. L., Thomas J. M., Thrower P. A. and Walker P. L., Jr., Curbon 11, 441 (1973). Baird T., Fryer .I. R. and Grant B., Carbon 12, 591 (1974). Baker R. T., Barber M. A., Harris P. S., Feates F. S. and Waite R. J.. J. Catal. 26, 51 (1972). Boehm H. P.. Carbon 11. 583 (1973).
catalyzed by nickel
17
12. Nishiyama Y. and Tamai Y., J. Catal. 33, 98 (1974). 13. Lobo L. S., Trimm D. L. and Figueiredo .I. L., Prepr. 5th Int. Congr. Catal. (1972). 14. Baker R. T., Harris P. S., Thomas R. B. and Waite R. J.. J. Catal. 30, 86 (1973). 15. Tomita A., Yoshida K., Nishiyama Y. and Tamai Y.. Carbon 10, 601 (1972). 16. Gilliland E. R. and Harriott P., Ind. Engng Chm 46. 1195 (1954). 17. Tomita A., Sato N. and Tamai Y., Curbon 12, 143 (19’4). 18. Lobo L. S. and Trimm D. L., Nature, Phys. Sci. 234, 15 (1971). 19. Diamond S. and Wert C., Trans. Met. Sot. ,1IME 239’. 705 (1967). 20. Lafitau H., Gendrel P. and Jacque L., C. K. Acad. Sci., Ik’uria, Ser. C 263, 1033 (1966). 21. McKee D. W., Carbon 12, 453 (1974). 22. Tomita A. and Tamai Y., .I. Phys. Chem. 78, 2254 (1971). 23. Robe11 A. J., Ballou E. V. and Boudart M.. J. Ph!\. Chew 68. 2748 (1964). 24. Walker P. L., Jr., Rusinko F., Jr. and Austin L. G., -tdl-. (‘atal. 11, 133 (1959). 215. Rewick R. T., Wentrcek P. R. and Wise H., Far/ 52’. 274 (1974). ;!6. Rostrup-Nielsen J. R., J. Cutal. 27, 343 (1972)
and Y. TAMA] Solutions. TohokuUniversity,Sendai,Japan
Abstract-Formation of carbon on nickel sheet from benzene vapor carried by hydrogen was studied at a temperature range from 520 to 730°C. A maximum rate was observed at about 63o”C, above which the deposition rate decreased rapidly. The carbon formed was hydrogenated in situ. Methane was the main gaseous product and a m~imum rate was observed at about 67OY. Very high reactivity of deposited carbon toward hydrog~~tion was ascribed to the catalytic action of nickel particles dispersed in the carbon. The hydrogenation rates were divided into three zones and possible interpretations are discussed. A mechanism which is a reverse process to deposition was suggesled. The decrease of the hydrogenation rate at higher temperatures was due to the equilibrium among carbon, hydrogen and methane, where carbon was more reactive than graphite.
hydrogen. The reactions were conducted in a quartz reactor of 26cm Ld., in which nickei specimen, I cm square and 0.01 cm thick, was suspended vertically with a quartz spring balance. From the weight changes of the specimen, the rates of carbon deposition and hydrogenation were obtained. Weight changes were measured with a precision of iO.05 mg. An electric furnace and a controlling device could maintain the temperature of reaction zone within * 3°C. Nickel specimen had a purity of 99.7%. with 0.2% of cobalt as the major impurity. The pretreatment of nickel and the purification of gases were the same as the preceding study [ 121. At the time of carbon deposition, benzene vapor, 60 torr in partial pressure, was mixed into hydrogen carrier,* and total flow rate was kept at 30 + 0.5 cm’ (STP)/min, total pressure being atmospheric. The gasification of deposit was conducted by passing atmospheric hydrogen over the specimen at a rate of 120i- 2 cm’ (STP)/min, except otherwise noted. The deposit adhered well to the vertical wall of the substrate during gasification, so that any holding device was unnecessary. Gas chromaf;ograph analysis of effluent gas was done using activated charcoal column, 2 m, at 100°C and a thermal conductivity detector. X-ray diffraction was conducted with Cu-Kcu ray by usual powder method. For determination of nickel content in the deposit, this was removed from the substrate and burned gently in air. The residue was dissolved in nitric acid and nickel was determined calorimetrically as nickel dimethylglyoxime.
The formation of carbonaceous solid from carboncontaining gases such as hydrocarbons is catalyzed by some metals, especially iron, cobalt and nickel. Carbons formed on these metals at temperatures below 800°C are mostly in a fibrous form and contain metallic particles [ l121.The mechanism of formation has been discussed by several authors. Many of them considered the diffusion of carbon atoms through metal phase as an intermediate step of successive reactions[ IO-141.The whole process is not simple and leaves details to be ascertained. A carbon-Mets composite formed by the deposition process was found to be highly reactive toward gasification[lS]. Gilliland and Harriott noted this earlier[l6], but they did not correlate the enhanced reactivity with metals in carbon. It seems rather obvious that metal particles in the deposit catalyze the reaction. The methanation of carbons draws intensive attention recently in relation to coal gasification, but relativeIy few reports deal with its fundamental aspects of catalytic processes. A research in this laboratory showed that hydrogenation of active carbon catalyzed by transition metals exhibited a few maxima in the rate vs temperature curve[l7]. In a course of carbon deposition study, we noted some resemblance in the kinetic behavior of deposition and hydrogenation processes. It was felt that the investigation of the reactivity of deposited carbon would give some light on the nature of carbon-metal interactions and help one to understand catalytic gasification processes. The present report describes a study initiated from such a point of view. The deposited carbon was revealed to have fairly uniform and quite high reactivity which does not change upon heat treatment.
3.RESULTS
2.MPERIMENTAL
Carbon was deposited from benzene vapor onto cold-rolled sheet of nickel, and was gasified in situ by
Weight changes in three deposition/hydrogenati~ln runs are shown in Fig. 1 to illustrate how the reactions were conducted. Typical features observed were as follows. Below 63O”C, the deposition process normally had an
tin spite of thermodynamic disadvantage, measurable carbon deposition from benzene was observed only in hydrogen atmosphere under the present conditions. In helium, the deposition rate was too small to be measured. The effect of carriers will be discussed in a separate report.
induction period of 10-20 min, and after this the rate constant within 26% up to 40 mg/cm’ (based on apparant surface area of nickel sheet), which was instrumental limit. At higher temperatures. the
CAR Voiid
13 No. 1-B
was the the
rate
14
Y. NISHIYAMA and Y. TAMAI Table 1. Nature of deposited carbon
“E
9 F20-
610
644
78
81
24
(A,
GrWGlitiZatiO"
0
E
584
@Ia)
do02*
E
s
(“C)
TOtaldeposit Graphitic Str”Ct”re
$
.P
tlepasition temp.
t
A". tryst. sire
IO-
3.40f0.01
3.39
3.38
(X)
47?12
58
74
(8)
60f5
70
70
Nickel crystallite content
(wt %)
A". tryst. size
0
200
100
Cd;)
1.8kO.2
2.0
900~100
480
3.2 _t
300
Time, min
Fig. 1. Weight changes due to carbon deposition and hydrogenation. Broken lines represent the period when benzene vapor was admitted. Numerals attached indicate the reaction temperature (in “C),whichwas changedat the time shownby verticalstrips.
* Graphitic layer spacing. t Ratio Of graptlitization;(3.44-doo2)/o.oB5. * Data scattered Llecween800 and 2500 i.
700
decreased with time as seen in Fig. 1. By turning off the benzene injection, gasification of deposit took place, sometimes with a fairly long induction period, which was partly due to residual benzene vapor. The rate of gasification was usually constant within 2 2% at conversion between 20 and 65%, if conducted at a constant temperature. Thus the main part of deposition and gasification proceed in zero order with respect to the amount of deposit in each run. However, the rate of gasification was seen to be roughly proportional to the total amount of deposit among different runs at temperatures below 680°C. To see the temperature dependence of both processes, temperature jump method was employed, as demonstrated in Fig. 1. Exhaust gas analysis indicated that methane was produced during deposition. The ratio of the rate of methane formation to that of carbon formation was 10 and 6% at 595 and 654°C respectively. At the time of gasification, methane was the only gaseous product detected and its concentration agreed quite well with that calculated from the rate of weight decrease of the specimen. The deposited carbon was a soft bulky mass of black powder and had some degree of crystallinity to diffract X-ray. A few characteristics are tabulated in Table 1. The surface area of deposit was estimated to be around 100m*/g[12]. 3.2 Temperature dependence of deposition rate The Arrhenius type plots of the deposition rate are shown in Fig. 2, which contains three kinds of plot; steady rate, initial rate and “activated” rate. The steady rate, the slope of linear part in Fig. 1, is clearly lambda-shaped. From this, temperature regions above and below 630°C will be referred to as zone A and zone B, respectively, for convenience. It should be noted that the deposition rate tThe experimental data were classified into three sets according to their reliability, i.e. the constancy of the rate itself and the amount of deposition. The ranges given here and in the next section merely indicate the region on which calculated slopes from any combination of sets came, and actually they cover the confidence interval with 90% reliability from most reliable sets.
650
600
550°C
“\ \
1000/T,
K-’
Fig. 2. Arrhenius plot of deposition rate. 0, steady rate; 0, initial rate; I, activated rate. changes reversibly with temperature over zones A and B,
as seen in Fig. 1, though there was a small irreversible deactivation in zone A. It took some time before the rate in zone A became constant. The degree of deactivation was demonstrated by the difference between steady and initial rates. The latter was obtained from extrapolation of the rate against the amount of deposit. The “activated” rate came from experiment in which the deposit was partly hydrogenated and then benzene vapor was introduced. In this case, no induction period was observed and the rate was very high, decreasing rapidly. The plots of steady rate below 630°C gave an apparant activation energy of 36 + 3 kcaUmo1e.t 3.3 Temperature dependence of hydrogenation rate Figure 3 shows the rate of hydrogenation divided by the total amount of deposit in the Arrhenius form. The plots are again lambda-shaped. The temperature regions above 680°C from 680 to 6OO”C,and below 600°C will be referred to as zone A’, B’, and C’, respectively. The reversible nature of hydrogenation rate over these zones is ascertained by the temperature jump method (Fig. 1). In addition, two of the specimens were heat treated in helium
Deposition of carbon and its hydrogenation
O.l,-3%- 800,
/
I-
/
,
09
550°C
600
700 I,
catalyzed by nickel
L
IO
1000/T,
I
1
1
I.1
12
b1
K-’
Fig. 3. Arrhenius plot of hydrogenation rate. The rate is normalized by dividing by the initial amount of deposit. I and A, heat treated at 745°C for 30 min and 865”C, 40 min, respectively. Equilibrium line was calculated for 20 mg of graphite, as the initial amounts for the runs above 700°C were from 19 to 23 mg.
before hydrogenation. Plots of these came on the same line as untreated ones. The rate of hydrogenation was independent of the deposition temperature within the experimental error. The apparant activation energies for zones B’ and C’ are 33 * 4 and 63 rt 3 kcal/mole respectively, the former agreeing well with that reported by Gilliland and Harriott [ 161. 3.4 Eflects of pressure and flow rate of hydrogen on hydrogenation
The effects of flow rate and pressure of hydrogen are shown in Figs. 4 and 5. Increasing of hydrogen flow rate resulted in the increase of the hydrogenation rate over entire temperature range, but there was a tendency that the rate in zones B’ and C’ leveled off at higher flow rate. The pressure dependence was examined by mixing helium into hydrogen flow, keeping total pressure atmospheric. The rate of hydrogenation is roughly second order to partial pressure of hydrogen in zone A’, while it is nearly zero at 1atm and 602°C. Concentration of methane in the effluent gas was 2.7 vol % at 710°C and 120cm’/min, corresponding to about 30%
Vol. fraction of HP Fig. 5. Pressure dependence of hydrogenation rate. 0, 602°C; 0, 788°C; I, 895°C. Broken lines show second order dependence.
of equilibrium value which was calculated on assumptions that whole gas phase was equilibrated with solid phase and that carbon was in the form of graphite. The calculated equilibrium is included in Fig. 3. By reducing the flow rate of hydrogen to 20 cm’lmin, 55% to equilibrium was reached. 3.5 Hydrogenation of carbon powders The deposited carbon was separated from nickel sheet and was rendered to hydrogenation in a quartz pan. This specimen was a little less reactive than those hydrogenated in situ, but quantitative comparison is meaningless because of the difference in geometric arrangement. introduction of benzene to the residue of hydrogenated powder resulted in a weight increase at a rate comparable to the original runs (Fig. 6, a and b). As a control, a carbon black, ISAF, was impregnated with nickel from aqueous solution of nickel chloride at a concentration of 5 wt % of nickel to carbon, and was heated in hydrogen, but no measurable gasification was observed (Fig. 6~). Further consequence of mckelcatalyzed hydrogenation of carbons can be found in a report from this laboratory, in which nickel impregnated active carbon was gasified by hydrogen in two stages [ 171. 4. DISCUSSION
4.1 Mechanism of carbon formation on nickel Recent propositions on the mechanism of carbon formation on catalytic metals mostly include the diffusion
Time, min Flow rate.
cc(STP)/min
Fig. 4. Effect of flow rate of hydrogen. 0, 596°C; 0, 648°C; 0, 796°C; I, 905°C.
Fig. 6. Hydrogenation of carbon powders. A and B, deposited carbon; C, nickel impregnated carbon black. Designations :are the same as in Fig. 1.
16
Y. NISHIYAMA and Y. TAMAI
of carbon atoms through metal phase as an intermediate step[Wl4]. For the motive force which causes diffusion, Baker et al. suggested the temperature gradient due to heat of reaction[lO], but other explanation is possible[ 11,121. Except for the motive force, the proposed mechanism seems reasonable, and the present results are consistent with the mechanism. The apparant activation energy observed in zone B is nearly the same as those reported for the deposition from other hydrocarbon sources[lrl, 181,and is in the same range as that of diffusion of through carbon nickel, 34.838.5 kcal/mole[19,20], leading to the same view as Baker’s[l4] that the rate-determining step in carbon formation in zone B is the diffusion of carbon atoms through nickel. The concurrent formation of methane during deposition seems to suggest the presence of single carbon atom species on nickel surface. The decrease of the rate in zone A is not well understood. Similar changes are reported for deposition from olefins[5,13,18] and from carbon monoxide[2], and Lobo et al. ascribed this to the decrease in the rate of adsorption of olefin[l8]. In Fig. 2, the “activated” rate seems to come on the extension line from zone B, although the plots are somewhat scattered. Then, adsorption of benzene onto clean nickel surface does not decrease at higher temperature. As the rate changed reversibly, the decrease in zone A might be interpreted by the formation of carbon patches or incomplete nickel carbide, which inhibited the dissociative adsorption of benzene and was removed by hydrogen at lower temperature. In other words, overall rate is determined by the competition between the rates of carbon atom formation and diffusion of them into bulk. 4.2 Mechanism of hydrogenation of deposited carbon The high reactivity of carbon-nickel composite presented here is noteworthy. There could be three possible mechanisms for catalytic hydrogenation of carbons: (1) methane formation at the boundaries of three phases, carbon-hydrogen-nickel; (2) methane formation’ at the active sites of carbon surface by dissociated hydrogen atoms which are formed on nickel surface and diffuse on carbon surface to the reaction sites; (3) reaction of adsorbed hydrogen atoms with carbon atoms on nickel surface, in which carbon atoms are supplied from inside of metals. The last one implies a reverse process to that of carbon formation discussed in the preceding section. Though the present results are not sufficient to discriminate three mechanisms, the authors would suggest the third one to be most likely, assuming that the whole body of the deposit consists of fibrous carbons containing nickel particles. The following are a few reasons for this view: (1) uniform rate of hydrogenation from 20 to 65% burnoff for fibrous carbon containing catalytic particles at some locations is most easily visualized by the third mechanism, in which reaction sites could be preserved at the same conformation; (2) the hydrogenation of graphite is reported to proceed through drilling of channels [21,22], which can be interpreted by the same mechanism, otherwise the pathway of gaseous reactant and product is a problem; (3) the temperature dependence in zone B’ is
very close to that of deposition, suggesting a similar rate controlling step. It should be pointed out that diffusion of carbon through nickel need not occur across the diameter of several hundred angstroms, rather short paths near periphery would be sufficient for gasification of carbon filament which have hollows [8,9, 111. There are some ambiguities in the above discussion. For example, the observed activation energy can be either that of diffusion of hydrogen atoms on carbon surface[23], or a half of activation energy of surface reaction when diffusion of reactant gas through pores is a predominating factor[24], as both would give values of similar magnitude. Also there can be catalytic contribution of finely dispersed nickel atoms or small aggregates [6,9], working in the second mechanism. These points will be studied further. The rate in zone C’ would be explained by one of surface reactions as rate-determining step. The activation energy of 63 kcal/mole could be compared with 55? 3 kcahmole for platinum catalyzed hydrogenation of carbon blacks reported by Rewick et a1.[25], who supposed the dissociation of hydrogen on platinum to be rate controlling. The decrease of hydrogenation rate in zone A’ should be ascribed to the equilibrium concentration of methane near reaction sites, since the rate is second order to hydrogen pressure. As the rate is not exactly proportional to the flow rate (Fig. 4), there is some contribution of diffusion of gaseous product to the overall rate. One difficulty associated with this interpretation was that the slope of the plots in zone A’ in Fig. 3 is larger than expected. It yields about -34 kcal/mole for heat of reaction, while that of graphite is - 21.6 kcal/mole at 800°C. The difference could be attributed to the structural incompleteness of deposited carbon, but it is somewhat larger than reported[26], so that there might be another effect that caused hydrogenation rate to decrease at higher temperature. The hydrogenation of nickel-catalyzed active carbon shows an enhanced activity at 540°C which vanishes by heat treatment [ 171.The structural unit involved there is different from deposited carbon, since this was unaffected by heat treatment. 4.3 Enhanced reactivity of deposited carbon The high reactivity of deposited carbon was reported by Gilliland[16]. So far as hydrogenation is concerned, it is obvious that the nickel particles or dispersed atoms incorporated in the deposit catalyze the reaction, in addition to the structural irregularity which enhances the reactivity. But a simple impregnation of nickel to carbon black, which is less graphitic to diffract X-ray, cannot be as effective as the present cases. Both the state of dispersion of nickel and the nature of contact between carbon and nickel may play important roles. On the basis of “diffusion of carbon through nickel” mechanism, the nature of contact, i.e. degree of adhesion and relative crystallographic orientation, might be such that transfer of carbon atoms from graphitic lattice occurs easily. Apart from the explanation, the observation suggests a possibility that catalytic hydrogenation of graphitic carbon
Deposition of carbon and its hydrogenation can proceed method
in high rate at around
of impregnation
of catalytic
65o”C,if an adequate metals
is achieved.
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