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The accelerating and retarding effects of hydrogen on carbon deposition on metal surfaces
Carbon Vol. 24. No. 6. pp. 687-693, Printed m Great Bntain.
lXM8-6223186 $3.00+ .oO 0 1986 Pergamon Journals Ltd.
19X6
THE ACCELERATING AND RETARDING EFFECTS OF HYDROGEN ON CARBON DEPOSITION ON METAL SURFACES K. L. YANG and R. T. YANG* Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, U.S.A. (Received 21 February 1986; in revised form 7 May 1986)
Abstract--Carbon deposition of benzene on iron was studied at 550-700°C with O-l atm hydrogen in the carrier gas. At least three types of carbon are formed: amorphous, graphitic and carbidic (Fe&Z). The surface of Fe& is essentially inactive for benzene decomposition. In the presence of HZ, a metallic surface is maintained resulting in a high activity and hence an accelerating effect by Hz. In the reaction system five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Based on the results in this study, the retarding effects of H> on carbon deposition reported in the literature can also be explained. The methanation reaction of surface carbon by H, becomes important under conditions when the surface is relatively inactive for hydrocarbon decomposition. and under such conditions, H, has a retarding effect on carbon deposition. Key Words-Carbon
deposition, hydrogen, benzene, iron, retardation of deposition.
1. lNTRODUCTlON
The formation of carbon on metal surfaces during high-temperature hydrocarbon processing has received widespread attention. Carbon formation results in metal corrosion and erosion, deactivation of catalysts, and other undesirable side effects. It may, however, be utilized as a possible source for future production of graphite fibers. Reviews of the general subject area are available[ l-61. It is important to understand the effect of hydrogen on carbon formation because hydrogen is frequently present in the hydrocarbon processing system. The effects of hydrogen can be both accelerating and suppressing, depending on the gas-solid system and temperature range. (Opposite effects on the same system in the same temperature range have also been reported by different groups.) The accelerating effects of hydrogen on carbon formation have been reported for many systems. Walker et al. reported such effects on the coking of iron surfaces with CO/H, at MO-SOo”C[7]. Tamai et al. showed the accelerating effects of hydrogen for coking on iron by CH,, C2H, and CzH,[8], and on nickel by benzene[9]. Similar accelerating effects have also been observed by Lobo and Trimm et al. for several light olefins on Ni[ 10,l l] by LaCava et al. for benzene decomposition on Fe and Ni at 500-65o”C[ 121, and by Tesner et al. for C,Hz on nichrome[l3]. The retarding effects of hydrogen on carbon formation have been reported for methane on (Y-and c-Fe[l4], CH, on Fe-Ni alloys[l5] and on Ni-Cu alloys[ 161. The contradictory accelerating effect of Hz on CH, decomposition on iron reported by Tamai et a/.[81 was apparently due to the coking of impurity hydrocarbons used in their system, viewing the con*Direct all correspondencce
to R. T. Yang. 687
vincing results on the retarding effect reported by all other groups. The retarding effects of hydrogen on coking have also been reported for various hydrocarbons on the relatively inactive catalysts such as Cu (which is much less active than the ferromagnetic metals: Fe, Co and Ni)[17,18], Pt and glass surfaces[19,20]. For the same gas-solid system, the effect of H, can shift from being accelerating to being retarding when the temperature is lowered. This shift can be seen in comparing the results of Renshaw et a[.[211 in the temperature range between 550-800°C and Tottrup[22] in 280-4OO”C, for CO/H, mixture on Ni. The explanation for the retarding effect is a thermodynamic one, i.e. hydrogen drives the reverse reaction for decomposition, the rate of which is significant due to the low equilibrium constant for CH, decomposition[l4-161. The interpretation for the interesting effect of acceleration by hydrogen is not as straight forward, and obviously must resort to invoking side reactions involving hydrogen. Two types of interpretation have been proposed. Trimm and coworkers[lO,ll] and LaCava et a1.[12] suggested that, in the presence of hydrogen, hydrogenation of the adsorbed unsaturated hydrocarbon (olefin) is the ratelimiting step, which forms an intermediate that in turn decomposes rapidly on the surface to form carbon. The other interpretation pertains to the removal, by hydrogen, of the surface carbon and precursors of carbon which otherwise block the active sites[7,21,23]. The decomposition of benzene with and without hydrogen on Fe and Fe,C was studied in this work. The results will be discussed together with the results from the literature in order to obtain a better understanding of the effects of hydrogen on carbon formation.
K.L. YANG and R.T. YANG
688 2.EXPERIMENTAL 2.1 Materials
Experiments were performed on coking from benzene vapor on three materials: iron foil, iron powder, and iron carbide powder. All three materials were supplied by Alfa Products with the following purities: iron foil (puratronic grade), iron powder (puratronic grade), and iron carbide (Fe,C = 99%). The iron foil, a polycrystalline material, was 0.025 mm thick. The Fe and Fe& powders were both 325 mesh size. Benzene vapor was generated by saturating a carrier gas in a gas wash bottle (bubbler) containing benzene, the temperature being controlled. Nitrogen and hydrogen, mixed to a desired composition by adjusting their respective flowrates, was the carrier gas. The hydrogen was “ultra high purity” grade from Linde with a minimum purity 99.999%, used without further treatment. The nitrogen was “oxygen free” grade, also from Linde, with a maximum 0, concentration of l/2 ppm. The nitrogen was further purified by passing it through a column of copper turnings at 550°C in order to remove the remaining 0,. 2.2 Reaction products identification The solid coke product was analyzed by X-ray diffraction. This was performed with iron foil, cut into
0
5
1 x 1 cm2, after coking rate measurements. Coke particles were also scraped off the foil for viewing in transmission electron microscope. The gaseous products were analyzed by gas chromatography using a fixed-bed reactor. In each experiment, 13-14 g Fe powder was placed in a quartz tubular reactor that was housed in a clam-shell furnace. Samples of the effluent gas were drawn with syringes (5 ml volume, with locks) at time intervals of 2-10 min and then subjected to GC analyses. 2.3 Rate Measurement All data on rates of coking, Fe& reduction, and coke gasification were measured gravimetrically, using a Mettler Model 2000C thermogravimetric analyzer, which has a sensitivity of 2 x 10e6 g. The coking rate data were measured with 1 x 1 cm2 iron foil specimens. Before each coking experiment, the specimen was cleaned in acetone and then by distilled water, both in an ultrasonic cleaner, and finally dried in oven. The specimen was further treated with hydrogen for 2 hrs at the coking reaction temperature before the admission of benzene. The flowrate of the carrier gas, nitrogen and hydrogen mixture, was 250 cc STPimin. When Fe& powder was used, the procedure was the same with the exception that the sample was not pretreated with hydrogen.
15
10 TIME
,
20
25
MIN
Fig. 1. Coke deposition on iron foil at 650°C with 84 Torr benzene. The carrier gas contained, on a benzene-free basis, 100% H, (0); 60% H2 (A); 30% H2 (0) and 0% H2 (7). Total pressure = 1 attn.
689
The effects of hydrogen on carbon deposition
The surface areas of the iron carbide and iron powder were measured as the N, BET areas by using a Quantasorb analyzer (supplied by Quantachrome Corporation).
3. RESULTS
AND
DISCUSSION
3. I Reaction products for benzene decomposition on Fe with and without hydrogen
Typical rate data on coke deposition are shown in Fig. 1. These were TGA data showing weight gain as a function of time by using 1 x 1 cm? iron foils. After a short initial “incubation” period, a pseudosteady state for coke deposition was reached. The reactions were stopped at 30 min, and the coked iron surfaces were subjected to X-ray diffraction analyses, with diffraction patterns shown in Fig. 2. On the sample coked with benzene and without hydrogen, the amount of coke was 0.12 mg/cm’, yet only a-Fe was shown in the diffraction pattern. Thus the amount of coke was amorphous, which undoubtedly contained a small amount of hydrogen. As the concentration of hydrogen in the carrier gas was increased, some structured carbon species appeared as more of the iron surface was covered. On the sample coked with 60% HZ in the carrier, graphite and iron carbide are seen (Fig. 2). The amounts of graphite and carbidic carbons increased further with 100% HZ carrier
gas. The X-ray diffraction analysis thus revealed three types of carbon: amorphous, graphitic and carbidic (Fe,C). It is difficult to quantitatively assess the relative amounts of these types of carbon. Some remarks may be made on their amounts, however. The sample coked with 30% H, in the carrier contained approximately 0.195 mg/cm2 carbon, which was primarily amorphous. The amorphous carbon is highly reactive to hydrogen. Thus it is uncertain whether the amount of amorphous carbon would increase or decrease with more hydrogen in the carrier gas. It may be concluded, however, that none of the three carbons were in trace amounts, and that the amorphous type was likely the most abundant. The gaseous products of the benzene decomposition reaction were also analyzed, using a fixed-bed reactor with 325 mesh iron. The residence time was short, i.e. approximately 13 s. The effluent compositions were analyzed by GC. and the results are shown in Fig. 3. The gas samples were taken after 30 min, when the pseudo-steady state was reached. The amount of H, in the feed carrier was substracted from that in the effluent. Only H, and CH, were found as the reaction products. Some interesting results are shown in Fig. 3. With no H: in the carrier, only a trace amount of CH, was found, which was probably produced by the reaction between the decomposition products: amorphous carbon + H?. The ratio of H,/
0% II*
6OXW2
FPjC
c
l:‘;“’ 40
60
60
2e
Fig. 2. X-ray diffraction patterns (using CuK,, A = 1.5418 A) of coke deposited on iron foil at 650°C. on samples taken from experiments shown in Fig. 1 with reactions stopped at 30 min.
K. L. YANG and R. T. YANG
690
? 0
I I I I 20 40 60 80 % Hz ( Benzene-free) in Feed
I 100
Fig. 3. Relative amounts of gaseous products for benzene decomposition on iron. Data taken from GC analyses of the effluent from a fixed-bed reactor at quasi-steady state, with 13-14 g iron powder at 600°C. Benzene partial pressure = 84 Torr. HZ% is on benzene-free.basis with a balance of N2. Total feed rate = 100 cc NTP/min.
CH, for this reaction was nearly 102. With various amounts of H2 introduced with benzene, a significant amount of methane was produced, and the HJCH, product ratio was approximately 15. The methane was apparently formed by the C + H, reaction. From the gaseous and solid product analyses, the benzene decomposition reaction, with and without H2, may be represented by: C,H, *
Amorphous
C + Graphite
+ Carbide (Fe&)
+ Hz
+ CH, (minor, from secondary rxn.) 3.2 Reaction Kinetics for Coking The pseudo-steady state rates of carbon deposition are shown in Fig. 4. The familiar rate maxima are seen, which are common for coke deposition on metal surfaces. The decline at higher temperatures is not well understood, although several possible reasons have been proposed[lO]. The most reasonable one, as proposed by Lobo et aL[lO], is that of decreased adsorption of the hydrocarbon at higher tempera-
tures. Applied to this case, the heat of adsorption for benzene on iron would be greater than the activation energy of the surface decomposition reaction, resulting in a negative apparent activation energy. The rate dependence on benzene partial pressure is first order at both temperatures investigated, 600 and 650°C. In contrast, the rate dependence on hydrogen partial pressure is strongly temperature dependent: l-1/4 order at 650°C and l/2 order at 600°C. The shifting effect of hydrogen on the coking rate from an accelerating one at high temperatures to a retarding one at low temperatures, as mentioned in the Introduction, can be explained by data shown in Fig. 4. The two rates (with and without H,) apparently will cross over at a certain low temperature, below which a retarding effect by H, should appear. The crossover temperature for the C,H,/H,/Fe system may be too low for the shift to be observed, because the coking rate may not be measurable at the low temperatures. The crossover of the two competing rates, however, can explain the shift of the H, effects for the CO/H,/Ni system[21,22] where the crossover temperature is in the range of 400-500°C.
691
The effects of hydrogen on carbon deposition
1
I
1.0
I
1.2
1.1
i/T
x103,
II”
Fig. 4. Rates of carbon deposition from 84 Torr benzene on iron in 100% HZcarrier (0, AE = 54 kcali mole) and Nz carrier (A, AE = 43 kcal). 3.4 Benzene decomposition on Fe,C with and without HI and on pre-reduced Fe?C-other possible side reactions
The rates of carbon deposition were measured with Fe,C powder (325 mesh, BET N, area = I .426 m’!g) in the TGA reactor. Rates measured with different amounts of Fe,C powder, varied from 33.7 to 98.3 mg, showed no difference in the weight gain rate, indicating no mass transfer limitation on rates. The rates are shown in Fig. 5. These are the pseudosteady state rates. in the experiments with 20% Hr in the carrier gas, pronounced incubation periods were observed, caused by the initial reduction of Fe&X. Comparing Figs. 4 and 5, the rates without Hz addition on the Fe& surface are insignificantly low as compared to those on Fe, i.e. less than lo/O. It is clear that the carbide surface is virtually inactive for benzene decomposition. The activity of Fe,C was greatly increased by using 20% H2 in the carrier gas, to levels nearly equal to the activities of Fe. in a separate experiment, the coking rates were measured with Fe,C powder which was pre-reduced stoichiometrically with Hz (as indicated by weight loss) to Fe. The rates were essentially the same as those on Fe. The rates of the reduction reaction Fe,C I- 2H, -+ 3Fe + CH, were also measured at 550,600 and 650°C and hydrogen partial pressures from 0.2 to 1 atm.
The rates were rapid, e.g. 1 x low2 mglmglmin at 650°C and 0.2 atm hydrogen. The apparent activation energy was 12 kcallmole, and the order of reaction with respect to Hz was 112. The rates of still another reaction, gasification of coke with H,, were also measured with the TG reactor. In these measurements, the benzene was terminated while the Hz flow continued, and the rate was continually measured. The following rates were obtained for the gasification of coke, deposited on Fe, by0.2atmHz: 1.1 x 10-Smglcm~/min(at5500C); 5.9 x 10mJmg/cm2/min (600°C); and 2.5 x lo-’ mgi cm~/min (650°C). 3.5 Reactions involved in the system C,H,tHJFe From the preceeding results, it becomes apparent that the following reactions are involved: (1) Fe + Benzene -+ Carbons including Fe,C (2) Fe& + Hydrogen + Fe and Carbon loss (3) Fe (from 2) + Benzene + Carbons (4) Carbons + Hydrogen -+ Carbon loss (CH,), catalyzed by Fe substrate (5) Fe& + Benzene -+ Carbons Reactions (1)) (3) and (5) result in carbon deposition and hence a weight gain. Reactions (2) and (4) are the gasification reactions (forming mainly CH, at
692
K. L. YANG and R. T. YANG
l/T
x lo3 I I(”
Fig. 5. Rates of carbon deposition from 84 Torr benzene on Fe& in Nz carrier (0) and in 20% HZ/N2 carrier (0).
temperatures below 1800 K) resulting in a weight loss. Reaction (5) is negligible when compared to (1) and (3). The net “coking rates” are the rates of (1) + (3) + (5) - (2) - (4). This is indeed the case within the range of conditions investigated in this study. For example, at 650°C and 20% H, in the carrier gas, the net coking rate was 5.3 x 10-j mg/cm2/min, compared to the rates of reactions (1) + (3) - (2) (4) = 5.4 X 10m3mglcm*/min. 3.6 The role of Hz in carbon deposition As mentioned in the Introduction, two hypotheses have been set forth for the accelerating effects of H2 on carbon deposition. One hypothesis is that the hydrogenation of the adsorbed olefin is the rate limiting step, which forms a surface intermediate that decomposes rapidly to form carbon[lO-121. The accelerating effects have been observed, however, not only for the unsaturated olefins, but also for the saturated hydrocarbons[8] as well as the CO/H, system[7]. The extensive studies on the methanation reaction (CO/ H,/metal) have provided overwhelming evidence which suggests that the reaction mechanism is through the formation of carbidic carbon, followed by hydrogenation of the surface carbon[24]. The aforemen-
tioned hypothesis for the HZ accelerating effects on carbon deposition cannot explain such effects observed for the systems involving saturated hydrocarbons and CO/H,. The results obtained in this study show that the surface carbide is the cause for a low carbon deposition rate. In the presence of hydrogen, the formation of surface carbide is hindered or prevented, resulting a high carbon deposition rate. Our results further show that in the carbon deposition system, C,H,IH,/Fe, five parallel reactions are involved. The net rate of carbon deposition is the sum of the individual rates. More importantly, whether the effects of H2 are accelerating or retarding is dependent on the relative rates of the five competing reactions. The catalytic decomposition of hydrocarbon is highly sensitive to the substrate catalyst, while the hydrogenation of carbon (producing CH,) is relatively less sensitive to the catalyst. Thus for catalysts which are not highly active for decomposition, the hydrogenation reaction is important and the net carbon deposition rate is lowered by H,. Recently Kock et a1.[25] studied the filament carbon formation on Fe from CO and CH, gases, and suggested Fe& and Fe, *C as necessary intermediates for filament growth. The XRD pattern for Fe,C would
The effects of hydrogen on carbon deposition
693
give three strong peaks at equal intensities at 20 = 42.0,42.65 and 43.95”. The lack of these peaks in our results indicate that its amount is very small, perhaps less than 1%. Furthermore, if these carbides are indeed the necessary intermediates for sustained filament growth, based on our results one wouid conclude that Fe,C would be first reduced by H, to Fe and then carburized into these carbides.
4. E. E. Wolf and P. Alfani, Catal. Rev.-W and Eng. 24, 329 (1982). 5. J. R. Rostrup-Nielsen, in Catalysis, Science and Technology, J. R. Anderson and M. Boudart, eds., Vol. 5,
4. CONCLUSIONS
7. P. L. Walker, Jr., J. F. Rakszawski and G. R. Imperial, J. Phys. Chem. 63, 133. 140 (1959). 8. Y. Tamai, Y. Nishiyama and M. Takahashi, Carbon 7. 209 (1969). 9. A. Tomita. K. Yoshida. Y. Nishiyama and Y. Tamai, Carbon 10, 601 (1972). 10. L. S. Lobo, D. L. Trimm and J. L. Figueiredo. Pre-
Chap. 1, Springer-Verlag, Berlin (1984). 6. (a) A. Sacco, Jr. and J. C. Caulmare, ACS Symp. Ser. 202, 177 (1982). (b) G. G. Tibbetts, Appl. Phys. Lett. 42, 666 (1983). (c)‘M. Endo, M. Shikata, T. Momose and M. Shiraishi. Ext. Abs. 17th Biennial Conf. on Carbon, Lexington, KY (1985). p. 295. (d) J. R. &adley, Y. L. Chen and H. W. Sturner, Carbon 23, 715 (1985).
The effects of Hz, both accelerating and retarding, on carbon deposition can now be understood from the results of this study, as summarized below. (1) Surface carbide (Fe&) is essentially inactive for benzene decomposition. Carbide is formed, along with amorphous carbon and graphite, which deactivates the Fe activity for carbon deposition. In the presence of Hz, a high activity is obtained probably because a metallic surface is maintained, thus sustaining the activity. (2) Hydrogen can also exert a retarding effect on carbon deposition for (a) tow-activity catalyst surfaces, and (b) the same catalyst at lower temperatures. The reason is that, under these conditions, the gasification rate of the deposited carbon by H, (forming CH,) is relatively high. For the system benzeneihydrogeniiron, five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Ac~nowie~gemenr-The work was supported by the National Science Foundation under Grant CBT-8507525.
REFERENCES R. T. K. Baker and P. S. Harris, in Chemistry and Physics of Carbon, P. L. Walker, Jr. and P. A. Thrower, edi., Vol. 14, 83, Marcel Dekker, New York (1978). D. L. Trimm. Catal. Rev.-Sci. and Enc. 16. 155 (1977). C. H. Bartholomew, Catal. Rev. -Sci.“and Eng. \?4, 67 (1982).
prints of 5th international Congress on Catalysis, 791125 (1972). 11. L. S. Lobo and D. L. Trimm, 1. Catal. 29, 15 (1973).
12. A. I. LaCava, E. D. Fernandez-Raone, L. L. Issacs and M. Caraballo. in Coke Formation on Metal Surfaces. L. F. Albright and R. T. K. Baker, eds., ACS Symp. Series 202. 89 (1982). 13. P. A. Tesner. E. Y. Robinovich, I. S. Rafalkes and E. F. Arefieva. Carbon 8. 435 (1970). 14. H. J. Grabke. Metall. Trans. 1, 2972 (1970). 15. J. I_. Figueiredo and D. L. Ttimm, J. Catal. 40, 154 (1975). 16. C. .4. Bernardo, I. Alstrup and J. R. Rostrup-Nielsen, J. Cutal. 96, 517 (1985). 17. Y. Nishiyama and Y. Tamai, J. Catal. 33, 98 (1974). 18. A.I. LaCava. E. D. Fernandez-Raone and M. Caraballo. in Coke Formation on Metal Surfaces. L. F. Albright and R. T. K. Baker. eds.. ACS S;mn. _ . Series 202, lO~(l982). 19. K. I. Makarov and V. K. Pechik, Carbon 7,279 (1969). 20. G. L. Johnson and R. C. Anderson. Proc. 5th Conference on Carbon, Vol. 1. p. 395. Pergamon Press, New York (1962). 21. G. D. Renshaw, C. Roscoe and P. L. Walker, Jr., J. Catal. 22. 394 (19701. 22 P. B. Tottrup, j. Catal. 42, 29 (1976). 23. Y. Nishiyama and Y. Tami, J. Catai. 45, 1 (1976). 24. P. Biloen and W. M. H. Sachtler, in Advances in Caraiysis, Vol. 311,p. 165. Academic Press, New York (1982). 25 A. J. H. M. Kock, P. K. De Dorx, E. Boeilaard, W. Klop and J. W. Geus. J. Caral. %, 468 (1985).
lXM8-6223186 $3.00+ .oO 0 1986 Pergamon Journals Ltd.
19X6
THE ACCELERATING AND RETARDING EFFECTS OF HYDROGEN ON CARBON DEPOSITION ON METAL SURFACES K. L. YANG and R. T. YANG* Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, U.S.A. (Received 21 February 1986; in revised form 7 May 1986)
Abstract--Carbon deposition of benzene on iron was studied at 550-700°C with O-l atm hydrogen in the carrier gas. At least three types of carbon are formed: amorphous, graphitic and carbidic (Fe&Z). The surface of Fe& is essentially inactive for benzene decomposition. In the presence of HZ, a metallic surface is maintained resulting in a high activity and hence an accelerating effect by Hz. In the reaction system five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Based on the results in this study, the retarding effects of H> on carbon deposition reported in the literature can also be explained. The methanation reaction of surface carbon by H, becomes important under conditions when the surface is relatively inactive for hydrocarbon decomposition. and under such conditions, H, has a retarding effect on carbon deposition. Key Words-Carbon
deposition, hydrogen, benzene, iron, retardation of deposition.
1. lNTRODUCTlON
The formation of carbon on metal surfaces during high-temperature hydrocarbon processing has received widespread attention. Carbon formation results in metal corrosion and erosion, deactivation of catalysts, and other undesirable side effects. It may, however, be utilized as a possible source for future production of graphite fibers. Reviews of the general subject area are available[ l-61. It is important to understand the effect of hydrogen on carbon formation because hydrogen is frequently present in the hydrocarbon processing system. The effects of hydrogen can be both accelerating and suppressing, depending on the gas-solid system and temperature range. (Opposite effects on the same system in the same temperature range have also been reported by different groups.) The accelerating effects of hydrogen on carbon formation have been reported for many systems. Walker et al. reported such effects on the coking of iron surfaces with CO/H, at MO-SOo”C[7]. Tamai et al. showed the accelerating effects of hydrogen for coking on iron by CH,, C2H, and CzH,[8], and on nickel by benzene[9]. Similar accelerating effects have also been observed by Lobo and Trimm et al. for several light olefins on Ni[ 10,l l] by LaCava et al. for benzene decomposition on Fe and Ni at 500-65o”C[ 121, and by Tesner et al. for C,Hz on nichrome[l3]. The retarding effects of hydrogen on carbon formation have been reported for methane on (Y-and c-Fe[l4], CH, on Fe-Ni alloys[l5] and on Ni-Cu alloys[ 161. The contradictory accelerating effect of Hz on CH, decomposition on iron reported by Tamai et a/.[81 was apparently due to the coking of impurity hydrocarbons used in their system, viewing the con*Direct all correspondencce
to R. T. Yang. 687
vincing results on the retarding effect reported by all other groups. The retarding effects of hydrogen on coking have also been reported for various hydrocarbons on the relatively inactive catalysts such as Cu (which is much less active than the ferromagnetic metals: Fe, Co and Ni)[17,18], Pt and glass surfaces[19,20]. For the same gas-solid system, the effect of H, can shift from being accelerating to being retarding when the temperature is lowered. This shift can be seen in comparing the results of Renshaw et a[.[211 in the temperature range between 550-800°C and Tottrup[22] in 280-4OO”C, for CO/H, mixture on Ni. The explanation for the retarding effect is a thermodynamic one, i.e. hydrogen drives the reverse reaction for decomposition, the rate of which is significant due to the low equilibrium constant for CH, decomposition[l4-161. The interpretation for the interesting effect of acceleration by hydrogen is not as straight forward, and obviously must resort to invoking side reactions involving hydrogen. Two types of interpretation have been proposed. Trimm and coworkers[lO,ll] and LaCava et a1.[12] suggested that, in the presence of hydrogen, hydrogenation of the adsorbed unsaturated hydrocarbon (olefin) is the ratelimiting step, which forms an intermediate that in turn decomposes rapidly on the surface to form carbon. The other interpretation pertains to the removal, by hydrogen, of the surface carbon and precursors of carbon which otherwise block the active sites[7,21,23]. The decomposition of benzene with and without hydrogen on Fe and Fe,C was studied in this work. The results will be discussed together with the results from the literature in order to obtain a better understanding of the effects of hydrogen on carbon formation.
K.L. YANG and R.T. YANG
688 2.EXPERIMENTAL 2.1 Materials
Experiments were performed on coking from benzene vapor on three materials: iron foil, iron powder, and iron carbide powder. All three materials were supplied by Alfa Products with the following purities: iron foil (puratronic grade), iron powder (puratronic grade), and iron carbide (Fe,C = 99%). The iron foil, a polycrystalline material, was 0.025 mm thick. The Fe and Fe& powders were both 325 mesh size. Benzene vapor was generated by saturating a carrier gas in a gas wash bottle (bubbler) containing benzene, the temperature being controlled. Nitrogen and hydrogen, mixed to a desired composition by adjusting their respective flowrates, was the carrier gas. The hydrogen was “ultra high purity” grade from Linde with a minimum purity 99.999%, used without further treatment. The nitrogen was “oxygen free” grade, also from Linde, with a maximum 0, concentration of l/2 ppm. The nitrogen was further purified by passing it through a column of copper turnings at 550°C in order to remove the remaining 0,. 2.2 Reaction products identification The solid coke product was analyzed by X-ray diffraction. This was performed with iron foil, cut into
0
5
1 x 1 cm2, after coking rate measurements. Coke particles were also scraped off the foil for viewing in transmission electron microscope. The gaseous products were analyzed by gas chromatography using a fixed-bed reactor. In each experiment, 13-14 g Fe powder was placed in a quartz tubular reactor that was housed in a clam-shell furnace. Samples of the effluent gas were drawn with syringes (5 ml volume, with locks) at time intervals of 2-10 min and then subjected to GC analyses. 2.3 Rate Measurement All data on rates of coking, Fe& reduction, and coke gasification were measured gravimetrically, using a Mettler Model 2000C thermogravimetric analyzer, which has a sensitivity of 2 x 10e6 g. The coking rate data were measured with 1 x 1 cm2 iron foil specimens. Before each coking experiment, the specimen was cleaned in acetone and then by distilled water, both in an ultrasonic cleaner, and finally dried in oven. The specimen was further treated with hydrogen for 2 hrs at the coking reaction temperature before the admission of benzene. The flowrate of the carrier gas, nitrogen and hydrogen mixture, was 250 cc STPimin. When Fe& powder was used, the procedure was the same with the exception that the sample was not pretreated with hydrogen.
15
10 TIME
,
20
25
MIN
Fig. 1. Coke deposition on iron foil at 650°C with 84 Torr benzene. The carrier gas contained, on a benzene-free basis, 100% H, (0); 60% H2 (A); 30% H2 (0) and 0% H2 (7). Total pressure = 1 attn.
689
The effects of hydrogen on carbon deposition
The surface areas of the iron carbide and iron powder were measured as the N, BET areas by using a Quantasorb analyzer (supplied by Quantachrome Corporation).
3. RESULTS
AND
DISCUSSION
3. I Reaction products for benzene decomposition on Fe with and without hydrogen
Typical rate data on coke deposition are shown in Fig. 1. These were TGA data showing weight gain as a function of time by using 1 x 1 cm? iron foils. After a short initial “incubation” period, a pseudosteady state for coke deposition was reached. The reactions were stopped at 30 min, and the coked iron surfaces were subjected to X-ray diffraction analyses, with diffraction patterns shown in Fig. 2. On the sample coked with benzene and without hydrogen, the amount of coke was 0.12 mg/cm’, yet only a-Fe was shown in the diffraction pattern. Thus the amount of coke was amorphous, which undoubtedly contained a small amount of hydrogen. As the concentration of hydrogen in the carrier gas was increased, some structured carbon species appeared as more of the iron surface was covered. On the sample coked with 60% HZ in the carrier, graphite and iron carbide are seen (Fig. 2). The amounts of graphite and carbidic carbons increased further with 100% HZ carrier
gas. The X-ray diffraction analysis thus revealed three types of carbon: amorphous, graphitic and carbidic (Fe,C). It is difficult to quantitatively assess the relative amounts of these types of carbon. Some remarks may be made on their amounts, however. The sample coked with 30% H, in the carrier contained approximately 0.195 mg/cm2 carbon, which was primarily amorphous. The amorphous carbon is highly reactive to hydrogen. Thus it is uncertain whether the amount of amorphous carbon would increase or decrease with more hydrogen in the carrier gas. It may be concluded, however, that none of the three carbons were in trace amounts, and that the amorphous type was likely the most abundant. The gaseous products of the benzene decomposition reaction were also analyzed, using a fixed-bed reactor with 325 mesh iron. The residence time was short, i.e. approximately 13 s. The effluent compositions were analyzed by GC. and the results are shown in Fig. 3. The gas samples were taken after 30 min, when the pseudo-steady state was reached. The amount of H, in the feed carrier was substracted from that in the effluent. Only H, and CH, were found as the reaction products. Some interesting results are shown in Fig. 3. With no H: in the carrier, only a trace amount of CH, was found, which was probably produced by the reaction between the decomposition products: amorphous carbon + H?. The ratio of H,/
0% II*
6OXW2
FPjC
c
l:‘;“’ 40
60
60
2e
Fig. 2. X-ray diffraction patterns (using CuK,, A = 1.5418 A) of coke deposited on iron foil at 650°C. on samples taken from experiments shown in Fig. 1 with reactions stopped at 30 min.
K. L. YANG and R. T. YANG
690
? 0
I I I I 20 40 60 80 % Hz ( Benzene-free) in Feed
I 100
Fig. 3. Relative amounts of gaseous products for benzene decomposition on iron. Data taken from GC analyses of the effluent from a fixed-bed reactor at quasi-steady state, with 13-14 g iron powder at 600°C. Benzene partial pressure = 84 Torr. HZ% is on benzene-free.basis with a balance of N2. Total feed rate = 100 cc NTP/min.
CH, for this reaction was nearly 102. With various amounts of H2 introduced with benzene, a significant amount of methane was produced, and the HJCH, product ratio was approximately 15. The methane was apparently formed by the C + H, reaction. From the gaseous and solid product analyses, the benzene decomposition reaction, with and without H2, may be represented by: C,H, *
Amorphous
C + Graphite
+ Carbide (Fe&)
+ Hz
+ CH, (minor, from secondary rxn.) 3.2 Reaction Kinetics for Coking The pseudo-steady state rates of carbon deposition are shown in Fig. 4. The familiar rate maxima are seen, which are common for coke deposition on metal surfaces. The decline at higher temperatures is not well understood, although several possible reasons have been proposed[lO]. The most reasonable one, as proposed by Lobo et aL[lO], is that of decreased adsorption of the hydrocarbon at higher tempera-
tures. Applied to this case, the heat of adsorption for benzene on iron would be greater than the activation energy of the surface decomposition reaction, resulting in a negative apparent activation energy. The rate dependence on benzene partial pressure is first order at both temperatures investigated, 600 and 650°C. In contrast, the rate dependence on hydrogen partial pressure is strongly temperature dependent: l-1/4 order at 650°C and l/2 order at 600°C. The shifting effect of hydrogen on the coking rate from an accelerating one at high temperatures to a retarding one at low temperatures, as mentioned in the Introduction, can be explained by data shown in Fig. 4. The two rates (with and without H,) apparently will cross over at a certain low temperature, below which a retarding effect by H, should appear. The crossover temperature for the C,H,/H,/Fe system may be too low for the shift to be observed, because the coking rate may not be measurable at the low temperatures. The crossover of the two competing rates, however, can explain the shift of the H, effects for the CO/H,/Ni system[21,22] where the crossover temperature is in the range of 400-500°C.
691
The effects of hydrogen on carbon deposition
1
I
1.0
I
1.2
1.1
i/T
x103,
II”
Fig. 4. Rates of carbon deposition from 84 Torr benzene on iron in 100% HZcarrier (0, AE = 54 kcali mole) and Nz carrier (A, AE = 43 kcal). 3.4 Benzene decomposition on Fe,C with and without HI and on pre-reduced Fe?C-other possible side reactions
The rates of carbon deposition were measured with Fe,C powder (325 mesh, BET N, area = I .426 m’!g) in the TGA reactor. Rates measured with different amounts of Fe,C powder, varied from 33.7 to 98.3 mg, showed no difference in the weight gain rate, indicating no mass transfer limitation on rates. The rates are shown in Fig. 5. These are the pseudosteady state rates. in the experiments with 20% Hr in the carrier gas, pronounced incubation periods were observed, caused by the initial reduction of Fe&X. Comparing Figs. 4 and 5, the rates without Hz addition on the Fe& surface are insignificantly low as compared to those on Fe, i.e. less than lo/O. It is clear that the carbide surface is virtually inactive for benzene decomposition. The activity of Fe,C was greatly increased by using 20% H2 in the carrier gas, to levels nearly equal to the activities of Fe. in a separate experiment, the coking rates were measured with Fe,C powder which was pre-reduced stoichiometrically with Hz (as indicated by weight loss) to Fe. The rates were essentially the same as those on Fe. The rates of the reduction reaction Fe,C I- 2H, -+ 3Fe + CH, were also measured at 550,600 and 650°C and hydrogen partial pressures from 0.2 to 1 atm.
The rates were rapid, e.g. 1 x low2 mglmglmin at 650°C and 0.2 atm hydrogen. The apparent activation energy was 12 kcallmole, and the order of reaction with respect to Hz was 112. The rates of still another reaction, gasification of coke with H,, were also measured with the TG reactor. In these measurements, the benzene was terminated while the Hz flow continued, and the rate was continually measured. The following rates were obtained for the gasification of coke, deposited on Fe, by0.2atmHz: 1.1 x 10-Smglcm~/min(at5500C); 5.9 x 10mJmg/cm2/min (600°C); and 2.5 x lo-’ mgi cm~/min (650°C). 3.5 Reactions involved in the system C,H,tHJFe From the preceeding results, it becomes apparent that the following reactions are involved: (1) Fe + Benzene -+ Carbons including Fe,C (2) Fe& + Hydrogen + Fe and Carbon loss (3) Fe (from 2) + Benzene + Carbons (4) Carbons + Hydrogen -+ Carbon loss (CH,), catalyzed by Fe substrate (5) Fe& + Benzene -+ Carbons Reactions (1)) (3) and (5) result in carbon deposition and hence a weight gain. Reactions (2) and (4) are the gasification reactions (forming mainly CH, at
692
K. L. YANG and R. T. YANG
l/T
x lo3 I I(”
Fig. 5. Rates of carbon deposition from 84 Torr benzene on Fe& in Nz carrier (0) and in 20% HZ/N2 carrier (0).
temperatures below 1800 K) resulting in a weight loss. Reaction (5) is negligible when compared to (1) and (3). The net “coking rates” are the rates of (1) + (3) + (5) - (2) - (4). This is indeed the case within the range of conditions investigated in this study. For example, at 650°C and 20% H, in the carrier gas, the net coking rate was 5.3 x 10-j mg/cm2/min, compared to the rates of reactions (1) + (3) - (2) (4) = 5.4 X 10m3mglcm*/min. 3.6 The role of Hz in carbon deposition As mentioned in the Introduction, two hypotheses have been set forth for the accelerating effects of H2 on carbon deposition. One hypothesis is that the hydrogenation of the adsorbed olefin is the rate limiting step, which forms a surface intermediate that decomposes rapidly to form carbon[lO-121. The accelerating effects have been observed, however, not only for the unsaturated olefins, but also for the saturated hydrocarbons[8] as well as the CO/H, system[7]. The extensive studies on the methanation reaction (CO/ H,/metal) have provided overwhelming evidence which suggests that the reaction mechanism is through the formation of carbidic carbon, followed by hydrogenation of the surface carbon[24]. The aforemen-
tioned hypothesis for the HZ accelerating effects on carbon deposition cannot explain such effects observed for the systems involving saturated hydrocarbons and CO/H,. The results obtained in this study show that the surface carbide is the cause for a low carbon deposition rate. In the presence of hydrogen, the formation of surface carbide is hindered or prevented, resulting a high carbon deposition rate. Our results further show that in the carbon deposition system, C,H,IH,/Fe, five parallel reactions are involved. The net rate of carbon deposition is the sum of the individual rates. More importantly, whether the effects of H2 are accelerating or retarding is dependent on the relative rates of the five competing reactions. The catalytic decomposition of hydrocarbon is highly sensitive to the substrate catalyst, while the hydrogenation of carbon (producing CH,) is relatively less sensitive to the catalyst. Thus for catalysts which are not highly active for decomposition, the hydrogenation reaction is important and the net carbon deposition rate is lowered by H,. Recently Kock et a1.[25] studied the filament carbon formation on Fe from CO and CH, gases, and suggested Fe& and Fe, *C as necessary intermediates for filament growth. The XRD pattern for Fe,C would
The effects of hydrogen on carbon deposition
693
give three strong peaks at equal intensities at 20 = 42.0,42.65 and 43.95”. The lack of these peaks in our results indicate that its amount is very small, perhaps less than 1%. Furthermore, if these carbides are indeed the necessary intermediates for sustained filament growth, based on our results one wouid conclude that Fe,C would be first reduced by H, to Fe and then carburized into these carbides.
4. E. E. Wolf and P. Alfani, Catal. Rev.-W and Eng. 24, 329 (1982). 5. J. R. Rostrup-Nielsen, in Catalysis, Science and Technology, J. R. Anderson and M. Boudart, eds., Vol. 5,
4. CONCLUSIONS
7. P. L. Walker, Jr., J. F. Rakszawski and G. R. Imperial, J. Phys. Chem. 63, 133. 140 (1959). 8. Y. Tamai, Y. Nishiyama and M. Takahashi, Carbon 7. 209 (1969). 9. A. Tomita. K. Yoshida. Y. Nishiyama and Y. Tamai, Carbon 10, 601 (1972). 10. L. S. Lobo, D. L. Trimm and J. L. Figueiredo. Pre-
Chap. 1, Springer-Verlag, Berlin (1984). 6. (a) A. Sacco, Jr. and J. C. Caulmare, ACS Symp. Ser. 202, 177 (1982). (b) G. G. Tibbetts, Appl. Phys. Lett. 42, 666 (1983). (c)‘M. Endo, M. Shikata, T. Momose and M. Shiraishi. Ext. Abs. 17th Biennial Conf. on Carbon, Lexington, KY (1985). p. 295. (d) J. R. &adley, Y. L. Chen and H. W. Sturner, Carbon 23, 715 (1985).
The effects of Hz, both accelerating and retarding, on carbon deposition can now be understood from the results of this study, as summarized below. (1) Surface carbide (Fe&) is essentially inactive for benzene decomposition. Carbide is formed, along with amorphous carbon and graphite, which deactivates the Fe activity for carbon deposition. In the presence of Hz, a high activity is obtained probably because a metallic surface is maintained, thus sustaining the activity. (2) Hydrogen can also exert a retarding effect on carbon deposition for (a) tow-activity catalyst surfaces, and (b) the same catalyst at lower temperatures. The reason is that, under these conditions, the gasification rate of the deposited carbon by H, (forming CH,) is relatively high. For the system benzeneihydrogeniiron, five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Ac~nowie~gemenr-The work was supported by the National Science Foundation under Grant CBT-8507525.
REFERENCES R. T. K. Baker and P. S. Harris, in Chemistry and Physics of Carbon, P. L. Walker, Jr. and P. A. Thrower, edi., Vol. 14, 83, Marcel Dekker, New York (1978). D. L. Trimm. Catal. Rev.-Sci. and Enc. 16. 155 (1977). C. H. Bartholomew, Catal. Rev. -Sci.“and Eng. \?4, 67 (1982).
prints of 5th international Congress on Catalysis, 791125 (1972). 11. L. S. Lobo and D. L. Trimm, 1. Catal. 29, 15 (1973).
12. A. I. LaCava, E. D. Fernandez-Raone, L. L. Issacs and M. Caraballo. in Coke Formation on Metal Surfaces. L. F. Albright and R. T. K. Baker, eds., ACS Symp. Series 202. 89 (1982). 13. P. A. Tesner. E. Y. Robinovich, I. S. Rafalkes and E. F. Arefieva. Carbon 8. 435 (1970). 14. H. J. Grabke. Metall. Trans. 1, 2972 (1970). 15. J. I_. Figueiredo and D. L. Ttimm, J. Catal. 40, 154 (1975). 16. C. .4. Bernardo, I. Alstrup and J. R. Rostrup-Nielsen, J. Cutal. 96, 517 (1985). 17. Y. Nishiyama and Y. Tamai, J. Catal. 33, 98 (1974). 18. A.I. LaCava. E. D. Fernandez-Raone and M. Caraballo. in Coke Formation on Metal Surfaces. L. F. Albright and R. T. K. Baker. eds.. ACS S;mn. _ . Series 202, lO~(l982). 19. K. I. Makarov and V. K. Pechik, Carbon 7,279 (1969). 20. G. L. Johnson and R. C. Anderson. Proc. 5th Conference on Carbon, Vol. 1. p. 395. Pergamon Press, New York (1962). 21. G. D. Renshaw, C. Roscoe and P. L. Walker, Jr., J. Catal. 22. 394 (19701. 22 P. B. Tottrup, j. Catal. 42, 29 (1976). 23. Y. Nishiyama and Y. Tami, J. Catai. 45, 1 (1976). 24. P. Biloen and W. M. H. Sachtler, in Advances in Caraiysis, Vol. 311,p. 165. Academic Press, New York (1982). 25 A. J. H. M. Kock, P. K. De Dorx, E. Boeilaard, W. Klop and J. W. Geus. J. Caral. %, 468 (1985).