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Formation of carbonaceous deposits from the reaction of methane over nickel
Curhon. 1972, Vol
13. pp. 172
Pergamon Presc.
Printed m Great Bntain
FORMATION OF CARBONACEOUS DEPOSITS FROM THE REACTION OF METHANE OVER NICKEL R. T. K. BAKER, P. S. HARRIS, J. HENDERSONand R. B. THOMAS Applied Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Oxfordshire, England (Received 20 May 1974)
Abstract-Controlled atmosphere microscopy techniques have been used to study the formation of several forms of carbon produced when methane was passed over heated nickel surfaces. The predominant forms of carbon were found to be nodular clusters of polycrystalline material and graphite flakes. The latter type of deposit appeared to be favoured under conditions of rapid cooling of specimens which had been held at 1000°Cfor a significant period. Experiments were designed to demonstrate that a solution mechanism could account for a large fraction of the flake deposit. The absence of filamentous carbon growth is accounted for in terms of a previously postulated mechanism.
1. INTRODUCTlON
produced from commercial grade methane though not with high purity methane at temperatures below 900°C. All these studies were limited to post-reaction examination of the carbon products. Since the structures of these products are so complex the interpretation of such results is difficult in the absence of a knowledge of the initial stages of deposition. In the present work we have used controlled atmosphere microscopy techniques to follow the reactions continuously. The use of post-reaction scanning electron microscopy has enabled the observed structures to be related to those reported by other workers, and a definitive experiment has been designed to test the role of solution processes in determining the structure of the deposit.
when carbon-containing gases are decomposed over heated metal surfaces. Some of these deposits may be unique to a particular gas-solid system whilst others consistently appear in widely differing systems. The role of the metal surfaces on the decomposition reactions has, over the past few years, attracted increasing interest and the reaction of methane over nickel is among the most intriguing[l-101 since this is one of the few endothermic reactions where carbon is deposited. Two types of carbon produced from the pyrolysis of 600Torr methane over nickel and other transition metals were identified by Robertson[l, 23, these he termed “flake” and “polycrystalline”. Electron diffraction analysis showed the first to be highly ordered graphite and the polycrystalline form was stated to have an essentially non-graphitic structure, although it occasionally appeared in a fibrous form which, from diffraction evidence, was shown to have the graphite basal planes parallel to the fibre axis. Fryer and co-workers[3,4] have examined similar fibres at high resolution and have resolved the 3.4 .& interlayer lattice period confirming the preferred orientation. They have shown that at the fibre tip the basal plane layers are parallel to the surface of the metal particle which is usually found as an inclusion in the fibre. They also found that all forms of carbon produced from 1 to 600Torr methane over nickel at 7Oo”C,including the flake variety, contained metal in a highly dispersed form. Evans ef a[.[5], investigating the same system, reported that a further filamentous form, similar to that produced in the decomposition of acetylene over nickel [ 111, could be Several forms of carbon deposits
are produced
2. EXPERIMENTAL
2.1 Controlled atmosphere optical microscopy [CAOM] The CAOM studies were carried out using a Leitz 1750 heating stage fitted to a Vickers M41 Photoplan microscope[l2]. Strips of nickel foil, about 5 mm wide, were clamped between steel electrical conductors and the foil heated by passing a current through it. The deposition temperature was measured with an optical pyrometer (Pyrowerke GmbH, optical micropyrometer) which had been calibrated “in situ” against known melting points (Au, Ag, Cu and NaCI). The nickel foils were cleaned in distilled water and acetone by ultrasonic agitation. After the specimen had been clamped into position the cell was flushed with hydrogen several times before the nickel was exposed to 600 Torr hydrogen at 750°C for 2 hr. Finally the specimen was reacted with 600Torr methane at between 750 to 17
R. T. K.
18
1060°Cfor periods of up to 12hr. Micrographs were taken of the metal surface at regular intervals throughout the heating and cooling periods. After the deposition reaction has been terminated some specimens were examined in a Cambridge Stereoscan scanning microscope and others by transmission electron microscopy. The transmission specimens of the carbonaceous deposit were obtained by allowing the specimen to float in approximately 5M HCI for a period of 15hr, when the carbonaceous film was found to separate quite readily from the nickel surface.
BAKER et a[.
possible to discern the grain boundaries. When the specimen was exposed to 600Torr methane and the temperature raised to 850°C deposition of carbonaceous material was apparent after about 5 min and collected at grain boundaries in the form of clusters. Over a period of 5 hr the deposit coverage and size of clusters increased. In Fig. 2 is shown a scanning electron micrograph of the deposit.
2.2 Nickel tube experiments This was a definitive series of experiments designed to allow the examination of graphite produced from solution in the nickel on a surface to which the hydrocarbon did not have access. The experimental arrangement is shown in Fig. 1. The tube was made from spectrographically pure nickel sheet, 0.25 mm thick, bent and welded to shape.
5,um
Fig. I. Experimental arrangement for nickeltube experiments. Stainless steel end plates were welded to the tube so that
gas could be led to the tube through stainless steel pipes. This whole assembly was placed in a silica outer jacket. The tube was cleaned with water and acetone before use and then heated in flowing hydrogen at 750°C at atmospheric pressure for 2 hr. The apparatus was cooled and then methane allowed to flow through the centre tube whilst argon was passed through the jacket and the temperature raised to 1000°C and maintained there for periods of up to 8 hr. When cool the tube was split open and the deposits on both the inner and outer surfaces were examined. Methane (Matheson Research Grade, 9999% pure) was used in all these experiments. The argon and hydrogen were supplied by British Oxygen Ltd. with a stated purity of better than 99.9% and were used without further purification. The nickel, 0.25 mm thick sheet or wire, was Johnson-Matthey spectrographically pure grade (analysis: Fe, 10ppm; Si, 3 ppm; Ag, 2 ppm; Al, Ca, Cu, Mg, < 1ppm, others not stated). 3. RESULTS
3.1 CAOM Experiments After treatment with hydrogen at 750°C the surface of the nickel appeared to be relatively smooth and it was
Fig. 2. Scanning micrograph of a specimen showing the detailed appearance of the deposit.
In experiments where the upper temperature was raised to 900°C the deposit initially took on the same form as at lower temperatures but after 2 hr or so long fibrous growths (2pm width and up to 50pm length) began to form at grain boundaries. On continued reaction these growths expanded in width and increased in length. More dramatic effects were produced by temperature cycling. The temperature was maintained alternately at 1000°C and 500°C to 200°C for periods of 5 min. No deposition was usually observed after the first cycle. After subsequent cycles small nuclei of deposit appeared on cooling, although at the temperatures used these did not appear to increase appreciably in size with increasing time. On reheating, the nuclei grew very rapidly and the whole surface of the metal was covered, though as the temperature increased to 1000°Cthis deposit disappeared to leave a bright surface again. This sequence of events was too fast to record continuously with the facilities available. The nuclei of deposit which appeared at the lower temperature are shown in Fig. 3. Figure 4 is a stereoscan micrograph of a specimen after an extended run in methane at 1000°C and cooled before the appearance of clusters. Depressions in the crystalline
Formation of carbonaceous
deposits from the reaction of methane over nickel
19
surface layer of the type shown were quite common. Figures 5A and 5B are transmissiori micrographs of the stripped deposit from a similar specimen together with its electron diffraction pattern. The deposit is evidently
Fig. 3. Optical micrograph of a specimen after temperature cycling from 1000°C to 200°C in 600 Torr methane, showing the formation of deposit nuclei at the lower temperature.
Fig. 4. Scanning micrograph showing depressions in the laminar deposit produced from cooling a specimen which had been exposed to 600 Torr methane at 1000°C for 35 min.
Fig. 5. (a). Transmission electron micrograph of the stripped laminar deposit. (b). Selected area diffraction pattern indicating the graphitic nature of this material. (c). Diffraction pattern with 12 spots in the innermost ring.
20
R. T. K. BAKER et al.
graphite of high crystallographic perfection. There is evidence of some rotational mismatch as shown by Moire fringes in the micrograph and satellite spots in the diffraction pattern. Diffraction patterns where there were 12 spots in the innermost ring in a symmetrical arrangement, Fig. X, were frequently encountered.
3.2 Tube experiments In the first experiment methane was passed through the inside of the tube and argon around the outside. After one hour at 1WC the tube was cooled to room temperature and split open. The inner surfaces were black and had a sooty accumulation of carbon. The outer surfaces were matt grey and covered with a thin adherent film. This film was stripped from the outside of the tube and examined by transmission electron microscopy. Electron diffraction analysis showed it to be graphite. The morphology of the deposit reflected the nickel grain structure; deposit was heaviest on grain boundaries and, although the average thickness was the same over an individual grain, it did vary from grain to grain. On some grains the deposit was thick and much overgrowth had occurred, Fig. 6, on other grains rows of well aligned crystallites overlaid a thin substrate of good quality graphite, Fig. 7. These two structures and various forms distinguishable as intermediates between the two accounted for the whole deposit structure. The small crystallites also appeared to possess borders of different contrast to the rest of the crystallite, Fig. 7. The appearance of these crystallites is shown in the scanning electron micrograph, Fig. 8. In a second experiment with a new tube, argon was first passed both through and around the tube to verify that
Fig. 7. Transmission electron micrograph showing the overgrowth of well aligned crystallites on the laminar graphite deposit (Nickel Tube Experiments).
Fig. 8. Scanning micrographof the crystallites shown in Fig. 7.
graphite was not produced in the absence of methane. In the third experiment methane was re-introduced into this tube and graphite was again obtained on the outer surfaces after the tube had been cooled to room temperature.
Fig. 6. Transmission electron micrograph of laminar deposit which had been stripped from the outer wall of a nickel tube after passing 600Torrmethanethroughthe tube at lOUU”C.
4.DISCUSSION The formation of crystalline or “flake” graphite on iron and nickel surfaces from the decomposition of hydrocarbons has been widely reported[l-lo]. Suggested growth mechanisms have involved
Formation of carbonaceous deposits fromthe reactionof methane over nickel a. direct formation of epitaxial layers from decomposition of an adsorbed precursor, b. formation and decomposition of metastable carbide intermediates, and, c. the mobility of carbon atoms generated by the decomposition of the hydrocarbon on the metal surface. These explanations can only be tentative because they lack firm evidence for their validity. This work shows that a further mechanism must also be considered: d. the precipitation of carbon from solution in the metal during the cooling stage of the experiment 1131. For most deposition studies the cooling down of the specimen from the reaction temperature to room temperature prior to examination is an essential part of the experimental technique. Derbyshire et at. [14] showed that such precipitated films could consist of extremely good quality graphite and suggested that these processes could be operative in pyrolytic systems. We have found that graphite flakes grew from the metal”deposit interface. and that crystaliites of graphite were produced on nickel surfaces which had not been in contact with methane. Both these observations are consistent with a mechanism involving the formation of graphite by precipitation of dissolved carbon in the nickel. The possibility of carbide intermediates being involved is discounted on the grounds that the temperature ranges in which carbides are known to be persistent, < 250°C and > 12OO”C, were not entered for a su~~iently long period to enable a carbide phase to establish itself [lS]. It is now necessary to discuss what part of the overall deposit structure we can expect to be due to reorganisat~ons following the cooling down of the specimen. The fraction of the carbon deposited on the nickel surface which enters into solution is determined by both the solubility and the rate of diffusion of carbon in the metal. On cooling the amount of carbon which is precipitated will depend on the degree of supersaturation achieved and the rate of diffusion back to the surface. These factors cannot be quantified on the basis of present knowledge but the optical microscopy results indicate that precipitation is a rapid process even compared to the rate of cooling of small laboratory specimens and the structure of the carbon deposit could be very much dependent on the detail of the way an experiment is performed. In general the amount of graphite formed will increase with (a) the duration of the ex~riment, probably to some limit, (b) increasing deposition temperature, fc) increased cooling time, and (d) may be influenced by the thickness of the foil-either directly. or indirectly through the increased heat capacity. This crystalline deposit will underlie whatever carbon remains on the surface. it is not unreasonable to suppose that the graphite layer will be formed epitaxially particularly if the flux of carbon atoms
21
appe~ing at the surface is fairly uniform and the sites at which they emerge are determined by the crystal structure of the metal. At temperatures below 4O@‘Cthe thermal expansion coefficients for nickel and graphite have opposite signsflfi]; graphite expands in the basal plane directions with decreasing temperature. The resultant stresses are mostly removed by formation of twin ridges as observed in Figs. 4 and SA. The formation of depressions in the graphite film, Fig. 4, might also arise for the same reasons. Although this work and that of Derbyshire et al.[14] establishes the general characteristics of this underlying layer it certainly cannot be stated that all crystalline graphite is formed in the same manner and it is not yet possible to distinguish graphite formed by a precipitation mode from any formed by other high temperature processes. The failure to find carbon ~laments of the type produced from nickel catalysed decomposition of acetylene[ll] in the present series of experiments is not unexpected. In the suggested mechanism for carbon filament production[l 11 one of the postulated requirements is that the decomposition of the hydrocarbon imparts heat to the exposed face of the metal particle. Some of the carbon produced from this reaction is taken into solution by the metal and filament growth proceeds by a diffusion process of carbon from the hotter leading face of the particle to be precipitated at the cooler regions. In the case of methane, the decomposition reaction is endothermic (A~,~ = +21~4kcal.mole~‘)~l7] and to a greater extent than the solution of carbon in nickel is exothermic, and so the postulated conditions for filament formation are never attained. This seems to suggest that the filaments in the deposits from commercial grade methane[5] are indeed due to the presence of an impurity. It should also be borne in mind, however, that oxygen could in principIe achieve the same effect as a depositing impurity through the release of heat from the exothermic oxidation of methane at the metal surface.
1. Robertson S. D., Carbon 8, 365 (1970). S. D., Carbon 10, 221 (1972). 3. Baird T., Fryer J. R. and Grant B., Nature 233, 329 (1971). 4. Baird T., Fryer J. R. and Grant B.. Int. Carbon Conf., Baden-Baden, June 1972,p. 266. 5. Evans E. L., ThrowerP. A., Thomas.I. M. and Walker P. t. Jr., Int. Carbon Conf., Baden-Badeo. June 1972,D. 284. 6. Presland A. E. B. and Walker P. L. Jr., Carbon 7, I’( 1959). 7. Presland A. E. B., Roscoe C. and Walker P. L. Jr., 3rd Conf. fnd. Carbon and Graphite, London (1970). 8. Presland A. E. B. and Biau 6.. 3rd Conf. Ind. Carbon and Graphite, London (1970). 9. Karu A. E. and Beer M. J., J: Apgl. Phys. 37, 2179 (19%). 2. Robertson
22
R. T. K. BAKERet al.
10. Rostrup-Nielsen J. R., J. Cnfal. 27, 343 (1972). ii. Baker R. T. K., Barber M. A., Feates F. S., Harris P. S. and Waite R. J., f. Catal. 26, 51 (1972). 12. Baker R. T. K. and Thomas R. B., J. Cry~fu~G~o~f~ 12, 185 (19772). 13. Austerman S. B., Chemistry and Physics of Corbon Vol. 4, Marcel Dekker, New York (1968). 14. Derbyshire F. J., Presland A. E. B. and Trimm D. L., Carbon 10, 114(1972).
15. Hansen M., Constitution ofBinary Alloys. McGraw Hill 1958) p. 374; R. P. Elliot, Constitution of Binary Alloys, McGraw Hill I%5 p. 223. 16. Reynolds W. N., physical Properties of graphite. Elsevier Press 1968p. 80. 17. Stuff D. R., Westrum E. F. Jr. and Sinke G. C., The Chemicaf Thermodynamics of Organic Compounds. J. Wiley and Sons 1969p. 243.
13. pp. 172
Pergamon Presc.
Printed m Great Bntain
FORMATION OF CARBONACEOUS DEPOSITS FROM THE REACTION OF METHANE OVER NICKEL R. T. K. BAKER, P. S. HARRIS, J. HENDERSONand R. B. THOMAS Applied Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Oxfordshire, England (Received 20 May 1974)
Abstract-Controlled atmosphere microscopy techniques have been used to study the formation of several forms of carbon produced when methane was passed over heated nickel surfaces. The predominant forms of carbon were found to be nodular clusters of polycrystalline material and graphite flakes. The latter type of deposit appeared to be favoured under conditions of rapid cooling of specimens which had been held at 1000°Cfor a significant period. Experiments were designed to demonstrate that a solution mechanism could account for a large fraction of the flake deposit. The absence of filamentous carbon growth is accounted for in terms of a previously postulated mechanism.
1. INTRODUCTlON
produced from commercial grade methane though not with high purity methane at temperatures below 900°C. All these studies were limited to post-reaction examination of the carbon products. Since the structures of these products are so complex the interpretation of such results is difficult in the absence of a knowledge of the initial stages of deposition. In the present work we have used controlled atmosphere microscopy techniques to follow the reactions continuously. The use of post-reaction scanning electron microscopy has enabled the observed structures to be related to those reported by other workers, and a definitive experiment has been designed to test the role of solution processes in determining the structure of the deposit.
when carbon-containing gases are decomposed over heated metal surfaces. Some of these deposits may be unique to a particular gas-solid system whilst others consistently appear in widely differing systems. The role of the metal surfaces on the decomposition reactions has, over the past few years, attracted increasing interest and the reaction of methane over nickel is among the most intriguing[l-101 since this is one of the few endothermic reactions where carbon is deposited. Two types of carbon produced from the pyrolysis of 600Torr methane over nickel and other transition metals were identified by Robertson[l, 23, these he termed “flake” and “polycrystalline”. Electron diffraction analysis showed the first to be highly ordered graphite and the polycrystalline form was stated to have an essentially non-graphitic structure, although it occasionally appeared in a fibrous form which, from diffraction evidence, was shown to have the graphite basal planes parallel to the fibre axis. Fryer and co-workers[3,4] have examined similar fibres at high resolution and have resolved the 3.4 .& interlayer lattice period confirming the preferred orientation. They have shown that at the fibre tip the basal plane layers are parallel to the surface of the metal particle which is usually found as an inclusion in the fibre. They also found that all forms of carbon produced from 1 to 600Torr methane over nickel at 7Oo”C,including the flake variety, contained metal in a highly dispersed form. Evans ef a[.[5], investigating the same system, reported that a further filamentous form, similar to that produced in the decomposition of acetylene over nickel [ 111, could be Several forms of carbon deposits
are produced
2. EXPERIMENTAL
2.1 Controlled atmosphere optical microscopy [CAOM] The CAOM studies were carried out using a Leitz 1750 heating stage fitted to a Vickers M41 Photoplan microscope[l2]. Strips of nickel foil, about 5 mm wide, were clamped between steel electrical conductors and the foil heated by passing a current through it. The deposition temperature was measured with an optical pyrometer (Pyrowerke GmbH, optical micropyrometer) which had been calibrated “in situ” against known melting points (Au, Ag, Cu and NaCI). The nickel foils were cleaned in distilled water and acetone by ultrasonic agitation. After the specimen had been clamped into position the cell was flushed with hydrogen several times before the nickel was exposed to 600 Torr hydrogen at 750°C for 2 hr. Finally the specimen was reacted with 600Torr methane at between 750 to 17
R. T. K.
18
1060°Cfor periods of up to 12hr. Micrographs were taken of the metal surface at regular intervals throughout the heating and cooling periods. After the deposition reaction has been terminated some specimens were examined in a Cambridge Stereoscan scanning microscope and others by transmission electron microscopy. The transmission specimens of the carbonaceous deposit were obtained by allowing the specimen to float in approximately 5M HCI for a period of 15hr, when the carbonaceous film was found to separate quite readily from the nickel surface.
BAKER et a[.
possible to discern the grain boundaries. When the specimen was exposed to 600Torr methane and the temperature raised to 850°C deposition of carbonaceous material was apparent after about 5 min and collected at grain boundaries in the form of clusters. Over a period of 5 hr the deposit coverage and size of clusters increased. In Fig. 2 is shown a scanning electron micrograph of the deposit.
2.2 Nickel tube experiments This was a definitive series of experiments designed to allow the examination of graphite produced from solution in the nickel on a surface to which the hydrocarbon did not have access. The experimental arrangement is shown in Fig. 1. The tube was made from spectrographically pure nickel sheet, 0.25 mm thick, bent and welded to shape.
5,um
Fig. I. Experimental arrangement for nickeltube experiments. Stainless steel end plates were welded to the tube so that
gas could be led to the tube through stainless steel pipes. This whole assembly was placed in a silica outer jacket. The tube was cleaned with water and acetone before use and then heated in flowing hydrogen at 750°C at atmospheric pressure for 2 hr. The apparatus was cooled and then methane allowed to flow through the centre tube whilst argon was passed through the jacket and the temperature raised to 1000°C and maintained there for periods of up to 8 hr. When cool the tube was split open and the deposits on both the inner and outer surfaces were examined. Methane (Matheson Research Grade, 9999% pure) was used in all these experiments. The argon and hydrogen were supplied by British Oxygen Ltd. with a stated purity of better than 99.9% and were used without further purification. The nickel, 0.25 mm thick sheet or wire, was Johnson-Matthey spectrographically pure grade (analysis: Fe, 10ppm; Si, 3 ppm; Ag, 2 ppm; Al, Ca, Cu, Mg, < 1ppm, others not stated). 3. RESULTS
3.1 CAOM Experiments After treatment with hydrogen at 750°C the surface of the nickel appeared to be relatively smooth and it was
Fig. 2. Scanning micrograph of a specimen showing the detailed appearance of the deposit.
In experiments where the upper temperature was raised to 900°C the deposit initially took on the same form as at lower temperatures but after 2 hr or so long fibrous growths (2pm width and up to 50pm length) began to form at grain boundaries. On continued reaction these growths expanded in width and increased in length. More dramatic effects were produced by temperature cycling. The temperature was maintained alternately at 1000°C and 500°C to 200°C for periods of 5 min. No deposition was usually observed after the first cycle. After subsequent cycles small nuclei of deposit appeared on cooling, although at the temperatures used these did not appear to increase appreciably in size with increasing time. On reheating, the nuclei grew very rapidly and the whole surface of the metal was covered, though as the temperature increased to 1000°Cthis deposit disappeared to leave a bright surface again. This sequence of events was too fast to record continuously with the facilities available. The nuclei of deposit which appeared at the lower temperature are shown in Fig. 3. Figure 4 is a stereoscan micrograph of a specimen after an extended run in methane at 1000°C and cooled before the appearance of clusters. Depressions in the crystalline
Formation of carbonaceous
deposits from the reaction of methane over nickel
19
surface layer of the type shown were quite common. Figures 5A and 5B are transmissiori micrographs of the stripped deposit from a similar specimen together with its electron diffraction pattern. The deposit is evidently
Fig. 3. Optical micrograph of a specimen after temperature cycling from 1000°C to 200°C in 600 Torr methane, showing the formation of deposit nuclei at the lower temperature.
Fig. 4. Scanning micrograph showing depressions in the laminar deposit produced from cooling a specimen which had been exposed to 600 Torr methane at 1000°C for 35 min.
Fig. 5. (a). Transmission electron micrograph of the stripped laminar deposit. (b). Selected area diffraction pattern indicating the graphitic nature of this material. (c). Diffraction pattern with 12 spots in the innermost ring.
20
R. T. K. BAKER et al.
graphite of high crystallographic perfection. There is evidence of some rotational mismatch as shown by Moire fringes in the micrograph and satellite spots in the diffraction pattern. Diffraction patterns where there were 12 spots in the innermost ring in a symmetrical arrangement, Fig. X, were frequently encountered.
3.2 Tube experiments In the first experiment methane was passed through the inside of the tube and argon around the outside. After one hour at 1WC the tube was cooled to room temperature and split open. The inner surfaces were black and had a sooty accumulation of carbon. The outer surfaces were matt grey and covered with a thin adherent film. This film was stripped from the outside of the tube and examined by transmission electron microscopy. Electron diffraction analysis showed it to be graphite. The morphology of the deposit reflected the nickel grain structure; deposit was heaviest on grain boundaries and, although the average thickness was the same over an individual grain, it did vary from grain to grain. On some grains the deposit was thick and much overgrowth had occurred, Fig. 6, on other grains rows of well aligned crystallites overlaid a thin substrate of good quality graphite, Fig. 7. These two structures and various forms distinguishable as intermediates between the two accounted for the whole deposit structure. The small crystallites also appeared to possess borders of different contrast to the rest of the crystallite, Fig. 7. The appearance of these crystallites is shown in the scanning electron micrograph, Fig. 8. In a second experiment with a new tube, argon was first passed both through and around the tube to verify that
Fig. 7. Transmission electron micrograph showing the overgrowth of well aligned crystallites on the laminar graphite deposit (Nickel Tube Experiments).
Fig. 8. Scanning micrographof the crystallites shown in Fig. 7.
graphite was not produced in the absence of methane. In the third experiment methane was re-introduced into this tube and graphite was again obtained on the outer surfaces after the tube had been cooled to room temperature.
Fig. 6. Transmission electron micrograph of laminar deposit which had been stripped from the outer wall of a nickel tube after passing 600Torrmethanethroughthe tube at lOUU”C.
4.DISCUSSION The formation of crystalline or “flake” graphite on iron and nickel surfaces from the decomposition of hydrocarbons has been widely reported[l-lo]. Suggested growth mechanisms have involved
Formation of carbonaceous deposits fromthe reactionof methane over nickel a. direct formation of epitaxial layers from decomposition of an adsorbed precursor, b. formation and decomposition of metastable carbide intermediates, and, c. the mobility of carbon atoms generated by the decomposition of the hydrocarbon on the metal surface. These explanations can only be tentative because they lack firm evidence for their validity. This work shows that a further mechanism must also be considered: d. the precipitation of carbon from solution in the metal during the cooling stage of the experiment 1131. For most deposition studies the cooling down of the specimen from the reaction temperature to room temperature prior to examination is an essential part of the experimental technique. Derbyshire et at. [14] showed that such precipitated films could consist of extremely good quality graphite and suggested that these processes could be operative in pyrolytic systems. We have found that graphite flakes grew from the metal”deposit interface. and that crystaliites of graphite were produced on nickel surfaces which had not been in contact with methane. Both these observations are consistent with a mechanism involving the formation of graphite by precipitation of dissolved carbon in the nickel. The possibility of carbide intermediates being involved is discounted on the grounds that the temperature ranges in which carbides are known to be persistent, < 250°C and > 12OO”C, were not entered for a su~~iently long period to enable a carbide phase to establish itself [lS]. It is now necessary to discuss what part of the overall deposit structure we can expect to be due to reorganisat~ons following the cooling down of the specimen. The fraction of the carbon deposited on the nickel surface which enters into solution is determined by both the solubility and the rate of diffusion of carbon in the metal. On cooling the amount of carbon which is precipitated will depend on the degree of supersaturation achieved and the rate of diffusion back to the surface. These factors cannot be quantified on the basis of present knowledge but the optical microscopy results indicate that precipitation is a rapid process even compared to the rate of cooling of small laboratory specimens and the structure of the carbon deposit could be very much dependent on the detail of the way an experiment is performed. In general the amount of graphite formed will increase with (a) the duration of the ex~riment, probably to some limit, (b) increasing deposition temperature, fc) increased cooling time, and (d) may be influenced by the thickness of the foil-either directly. or indirectly through the increased heat capacity. This crystalline deposit will underlie whatever carbon remains on the surface. it is not unreasonable to suppose that the graphite layer will be formed epitaxially particularly if the flux of carbon atoms
21
appe~ing at the surface is fairly uniform and the sites at which they emerge are determined by the crystal structure of the metal. At temperatures below 4O@‘Cthe thermal expansion coefficients for nickel and graphite have opposite signsflfi]; graphite expands in the basal plane directions with decreasing temperature. The resultant stresses are mostly removed by formation of twin ridges as observed in Figs. 4 and SA. The formation of depressions in the graphite film, Fig. 4, might also arise for the same reasons. Although this work and that of Derbyshire et al.[14] establishes the general characteristics of this underlying layer it certainly cannot be stated that all crystalline graphite is formed in the same manner and it is not yet possible to distinguish graphite formed by a precipitation mode from any formed by other high temperature processes. The failure to find carbon ~laments of the type produced from nickel catalysed decomposition of acetylene[ll] in the present series of experiments is not unexpected. In the suggested mechanism for carbon filament production[l 11 one of the postulated requirements is that the decomposition of the hydrocarbon imparts heat to the exposed face of the metal particle. Some of the carbon produced from this reaction is taken into solution by the metal and filament growth proceeds by a diffusion process of carbon from the hotter leading face of the particle to be precipitated at the cooler regions. In the case of methane, the decomposition reaction is endothermic (A~,~ = +21~4kcal.mole~‘)~l7] and to a greater extent than the solution of carbon in nickel is exothermic, and so the postulated conditions for filament formation are never attained. This seems to suggest that the filaments in the deposits from commercial grade methane[5] are indeed due to the presence of an impurity. It should also be borne in mind, however, that oxygen could in principIe achieve the same effect as a depositing impurity through the release of heat from the exothermic oxidation of methane at the metal surface.
1. Robertson S. D., Carbon 8, 365 (1970). S. D., Carbon 10, 221 (1972). 3. Baird T., Fryer J. R. and Grant B., Nature 233, 329 (1971). 4. Baird T., Fryer J. R. and Grant B.. Int. Carbon Conf., Baden-Baden, June 1972,p. 266. 5. Evans E. L., ThrowerP. A., Thomas.I. M. and Walker P. t. Jr., Int. Carbon Conf., Baden-Badeo. June 1972,D. 284. 6. Presland A. E. B. and Walker P. L. Jr., Carbon 7, I’( 1959). 7. Presland A. E. B., Roscoe C. and Walker P. L. Jr., 3rd Conf. fnd. Carbon and Graphite, London (1970). 8. Presland A. E. B. and Biau 6.. 3rd Conf. Ind. Carbon and Graphite, London (1970). 9. Karu A. E. and Beer M. J., J: Apgl. Phys. 37, 2179 (19%). 2. Robertson
22
R. T. K. BAKERet al.
10. Rostrup-Nielsen J. R., J. Cnfal. 27, 343 (1972). ii. Baker R. T. K., Barber M. A., Feates F. S., Harris P. S. and Waite R. J., f. Catal. 26, 51 (1972). 12. Baker R. T. K. and Thomas R. B., J. Cry~fu~G~o~f~ 12, 185 (19772). 13. Austerman S. B., Chemistry and Physics of Corbon Vol. 4, Marcel Dekker, New York (1968). 14. Derbyshire F. J., Presland A. E. B. and Trimm D. L., Carbon 10, 114(1972).
15. Hansen M., Constitution ofBinary Alloys. McGraw Hill 1958) p. 374; R. P. Elliot, Constitution of Binary Alloys, McGraw Hill I%5 p. 223. 16. Reynolds W. N., physical Properties of graphite. Elsevier Press 1968p. 80. 17. Stuff D. R., Westrum E. F. Jr. and Sinke G. C., The Chemicaf Thermodynamics of Organic Compounds. J. Wiley and Sons 1969p. 243.