- Email: [email protected]
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
ooo&(223/81/020071~8$02.00/0 Copyright 4 1981 Pergaman Precs I.td
Carbon. Vol. 19. pp 7542. 1981 Prmkd m Grear Brsain All rights reserved
CATALYTIC GASIFICATIONOF GRAPHITE BY CHROMIUM AND COPPER IN OXYGEN, STEAM AND HYDROGEN R. T. K.
BAKER
and J. J.
CHLUDZINSKI. JR.
Exxon Research and Engineering Company. Linden, NJ 07036. U.S.A (Received
II September
1980)
Abstract-Controlled atmosphere electron microscopy studies have demonstrated that both chromium and copper are extremely active catalysts for the graphite-oxygen reaction over the range 550-800°C. Under these conditions the active catalytic entities are probably Cr20, and CuO, respectively. The behavior of chromium is quite unique. not fitting into the well established patterns, i.e. pitting or channeling. Instead, particles remain motionless throughout the oxidation sequence and the action is seen as an acceleration of edge recession all over the specimen. In contrast, copper exhibited conventional channeling activity, which diminished at higher temperatures ( > 700°C) due to the wetting and spreading of active particles along graphite edges. Such edges underwent rapid recession following this phenomenon. Neither metal showed any tendency to catalyze gasification of graphite in the nresence of steam. Coooer was found to be an active catalyst for graphite hydrogenation at temperatures in excess of 800°C. but chromium remained inactive. L.
York State) by evaporation of wire (Cu) and powder (Cr) from a tungsten filament and boat, respectively, at a residual pressure of lo-” Torr. The metals were added in sufficient amounts to produce a monolayer coverage on the graphite surface. The reactant gases used in the microscopy studies, oxygen, argon and hydrogen were obtained from Scientific Gas Products with stated purities of 99.99% and were used without further purification. Experiments where the effects of steam were being investigated were accomplished by allowing argon to flow through a bubbler containing deionized water maintained at 0°C. prior to entering the gas reaction cell, conditions which produced an argon/water ratio of about 40: I.
I. INTRODUCTION It is surprising
to find that very little information is available on the role of chromium oxide during graphite oxidation. McKee [l] reported that Cr203 was the catalytically active phase, but it was difficult to identify the precise mode by which it operated. He suggested that the oxide might function as a dissociation center for oxygen molecules, rather than interacting directly with the graphite substrate. The same author is also responsible for the most comprehensive published work on the effect of copper on graphite gasification. He used a combination of controlled atmosphere optical microscopy and thermogravimetric analysis to demonstrate that cupric oxide was the active entity in the copper catalyzed graphite-oxygen reaction. He also showed that this action could be inhibited by alloying of the copper with zinc, aluminum or tin [2]. From bulk measurements, Heintz and Parker]31 obtained values of 63.6 and 43.3 kcalmole-‘, respectively, for the activation energies of the chromium and copper-catalyzed decompositions of graphite in air. These values were subsequently amended to 34.1 and 23.5 kcal melee’, respectively, by Harris[4] who reexamined the earlier data. In the present work we have used controlled atmosphere electron microscopy to examine the behavior of copper and chromium on graphite in OZ, H20, and H2 environments. These experiments have enabled us to identify the detailed characteristics and evaluate the kinetics of the catalytic action of these metals towards gasification of graphite under a variety of conditions.
3. RESULTS 3.1 Controlled
3.1.1
description
of
the
CAEM
technique
electron
microscopy
studies
When chromiumigraphite specimens were heated in 5 Torr oxygen particle nucleation occurred at about 400°C. When first formed, particles were in the range 5-10nm dia. and increased in size to around 20nm as the temperature was raised to 550°C. The first signs of graphite gasification were observed at 57s”C, and this took the form of erosion of edges and steps, behavior normally associated with uncatalyzed attack. As the temperature increased so did the speed by which edges receded, often undermining particles which then tended to collect as a residue along the edges. It was apparent that throughout this period particles located on the basal planes of the graphite remained motionless, and there was no evidence of either pitting or channeling action. At 942”C, all particles started to move on the surface, however, this event did not induce any change in the mode of graphite gasification. Figure 1 is a transmission electron micrograph of a specimen which had been heated to 900°C in oxygen. The “saw-toothed” appearance of the graphite edge produced as a result of the gasification reaction is
2. EXPERIMENTAL A detailed
atmosphere
Chromium/graphite-oxygen.
has
been reported previously[S]. Copper and chromium (both 99.99% purity) were introduced onto transmission specimens of single crystal graphite (Ticonderoga, New 75
R. T. K. BAKERand J. J. CHLUDZINSKI, JR.
Fig. 1. Transmission electron micrograph of a chromium/graphite specimen which had been exposed to 5 Torr oxygen at 900%
clearly evident, along with the metal oxide debris which has collected in this region. From quantitative kinetic analysis of edge recession sequences, it has been possible to measure the rate of reaction as a function of temperature. This data is presented in the form of an Arrhenius plot, Fig. 2 and from the slope of the line a value of 23.9 2 3 kcal mole-' has been estimated for the apparent activation energy of chromium catalyzed oxidation of graphite. A correction factor has been applied to the rate measurements obtained at temperatures in excess of 850°C to take into account the contribution of the uncatalyzed reaction, which becomes significant under these conditions. 3.1.2 Chromium/graphite-hydrogen and steam. The behavioural patternof chromium/graphite specimens when treated in either 1 Torr hydrogen or I Torr argon saturated with Hz0 at 0°C was very similar in many respects and will therefore be dealt with collectively. Nucleation of the evaporated metal film into discrete particles (5 nm dia) took place quite readily in steam at 435°C. The same process was much more shtggish in -05 hydrogen, starting at 470°C and not being complete until 08 IO 09 IO3T(K-‘l 675°C. This situation was improved in later experiments by performing the nucleation in oxygen at 400°C and Fig. 2. Arrhenius plot of chromium-catalyzed rate of graphite then switching to hydrogen. This procedure did not apgasificationin 5 Torr oxygen. I
L
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
pear to in&roduce any deviation in the subsequent behavior compared to that where specimens were heated directly in hydrogen. Continuous observations up to 1050°Cshowed no evidence of catalytic attack of the graphite in either systems. Some indication of very slow edge erosion was apparent in steam at approx. 935”C, But this would be expected from uncatalyzed attack. The only other event worthy of note was the observation of occasional particle movement in steam at 965”C, a finding not seen in hydrogen. 3. I .3 Copper/graphite-oxygen. When heated in 5 Torr oxygen, the evaporated copper iilm nucleated to form discrete particles in the size range, 5-25 nm at 325°C. Some particle motion was observed at 4OO”C,a condition where the particles were thin and irregular shaped. This motion ceased as the temperature was gradually raised to 45O”C, and coincided with a change in particle appearance to a more globular morphology. It was noticeable that particles located on edges and steps on the surface were in a “non-wetting” state exhibiting contact angles with the graphite of > 90”. A dramatic transformation in these particular particle characteristics was seen at 52O”C, which is depicted schematically in Fig. 3. The particles tended to become cap-shaped, with contact angles < 90”, typical of a “wetting” condition, and this change preceded the formation of channels. Initially channeling activity was limited to the smallest particles present, 5nm width, causing the channels to be narrow and very shallow. As the temperature was raised, there was an accompanying increase in the average size of particles propagating channels. General particle mobility commenced at 550°C and this increased the number of particles collecting at edges and hence available to produce channels. At 675°C the wetting action of the particles on graphite became so intense that material was left on the sides of the channels as a result of spreading. This effect caused channels to assume a fluted appearance and both the size and rate of forward motion of the particle decreased as the reaction proceeded. Due to this phenomena the average particle size had decreased to 7 nm at 750°C. At this stage in the reaction even those particles which had previously been inactive were beginning to show changes in behavior. Rearrangement from an irregular shape through an intermediate well defined cubic form was followed by prompt disintegration when the particles coniacted edges, resulting in the formation of thin films which covered a considerable edge area. Such edges underwent rapid recession immediately following the spreading action of the particles. Specimens were heated to 820°C when very few parti-
77
cles were visible on the surface and the gasification reaction appeared to be controlled solely by the edge recession process. If the temperature was reduced to 415”C, then after a period of about 10min, particles were observed to reform in the vicinity of edges and on subsequent reheating in oxygen followed the same behavioral pattern as described above Quantitative kinetic analysis showed that, the temperature dependence of channel propagation rate for 25 nm dia. particles cutting channels of similar depth was linear, Fig. 4 and from the slope of the line an apparent activation energy of 21.7 rt 2 kcal mole- ’ was estimated for the copper catalyzed gasification of graphite. 3.1.4 Copper~graphite-hydrogen and steam. The copper film nucleated to form discrete particles 2.5-10.0 nm in size after heating in 1 Torr hydrogen at 415°C for 30min. Inspection of particles located along edges and steps on the graphite showed that they were in a “nonwetting” state with respect to the surface. This situation changed to a “wetting” condition as the temperature was siowly raised to 585°C and during this period the particles increased in size, but showed a reluctance to exhibit any signs of motion. As the temperature was increased to 605°C not only did particles cease to grow, but there was a suggestion that they were actually shrinking. This suspicion was confirmed at slightly higher temperature when the rate of contraction became so appreciable that at 676°C no evidence of any of the original particles remained. Any idea that this process had been the result of metal evaporation from the surface was quickly dispelled, as on raising the temperature to 735°C particles started to reform as fine filaments, often at some distance away from the location of the initial particles. Shortly after their formation, these entities executed a random movement on the surface and tended to “ball-up” into discrete globular particles. 25
20
1
c2 -b
0
Fig. 3. Schematic representation of the change in particle morphology in the copper/graphite-oxygen system observed on increasing the temperature from 450 to 520°C.
L
.._~~
09
!b -
-------i l/T(Kl*
Fig. 4. Arrhenius
i:
‘.\.
___
2.
‘2
IO
plot of copper-catalyzed rate of graphite gasification in 5 Torr oxygen.
78
R. T. K. BAKERand J. J. CHLUD~~NS~.JR.
Prior to restoration of particles, there were indications along edges that catalytic attack was occurring. This action could not be classified according to the normal modes of catalyst behavior for graphite gasification, i.e. pitting or channeling, but instead was seen to accelerate the recession of edges leaving a watermark pattern in these regions. It is probable that this edge erosion was caused by the catalytic influence of a thin film of metal spread along the entire length of the step or edge. This claim is supported by the fact that edge recession ceased at the same time that small particles started to grow at the edges, presumably as a result rupture of the film. The onset of catalytic attack by the channeling mode was observed at 845°C. These channels all emanated from edges and were extremely straight, being oriented parallel to (1120) directions and altered course by bending through angles of 60” and 120”. This action was followed up to 105O”C,a condition where the average width of channels was 125nm and even at elevated temperatures the channels maintained a constant width throughout their propagation period, indicating that the rate of the uncatalyzed reaction, which would result in expansion of channels, was negligible. Detailed kinetic analysis of the recorded reaction sequences, a typical example of which is shown in Fig. 5, confirmed the previous finding for catalyzed hydrogenation of graphite (6) that at any given temperature large particles propagated channels at a faster rate than small ones. An Arrhenius plot of the rate data obtained from 25nm dia. particles cutting channels of similar depth yielded an apparent activation energy of 33.32 3 kcal mole-‘, Fig. 6. When the copper/graphite system was reacted in 1Torr argon saturated with water at O”C, there was a marked difference in the activity pattern compared to that seen in either oxygen or hydrogen. In this case particles were not only faceted but also extremely thin, many of the underlying support features being visible through the crystallites. Inspection of some of the larger particles present at 500°C (25 nm dia.) showed that the electron scattering density was uniform across each crystallite indicative of a pillbox morphology. Raising the temperature to 534°C resulted in the onset of particle mobility, but did not induce any changes in structural characteristics of the particles, or any catalytic activity towards gasification of the graphite. The first signs of graphite gasification were apparent at 950°C and occurred by erosion of edges caused by uncatalyzed attack. At this temperature the particles were exhibiting rapid motion on the surface and had become more rounded in outline and dense, characteristics of a globular morphology. However, even at these extreme conditions they showed no tendency to catalyze graphite removal.
4. DISCUSSION The results of this investigation clearly demonstrate that both chromium and copper are extremely active catalysts for the graphite-oxygen reaction over the temperature range, 550-800°C. They do, however, exhibit
completely different qualitative characteristics in their mode of action for this process. The behavior of chromium is quite unique, not fitting into either of the well established patterns, i.e. pitting or channeling. Instead, its action closely resembles that of the uncatalyzed reaction, the rate of which does not become significant until 850°C. It is possible to rationalize the influence of chromium by assuming that edge recession is accelerated by either the formation of a thin film of catalyst material along the graphite edges and steps, in a manner similar to that reported for lead[?], or that the particles act as dissociation centers for oxygen molecules, and that the oxygen atoms produced diffuse to and attack edges and stepsll]. The weight of experimental evidence tends to favor the latter postulate for several reasons. The rate of edge recession was found to be quite uniform over the entire surface at a given temperature. This situation would not be expected from a metal wetting and spreading phenomena since the probability of every edge being coated by metal is not very high. In this case one would expect to observe more erratic behavior with some regions remaining uncontaminated by catalyst and relatively unreactive. Moreover, as the reaction progressed one would predict that a condition would be attained where the metal film became so stretched by the receding edge that eventually it would rupture and formation of discrete particles would occur, and then these entities would continue to attack the graphite, possibly by a channeling mode. Since these effects were not observed, one is left with the conclusion that the situation depicted in Fig. 7 is probably a more accurate description of the influence of chromium on the graphite-oxygen reaction. Thermogravimetric studies by McKee [ I ] indicated that the catalytically active phase for this reaction was Cr203, Results from the present work would support this view as indicated by the finding of particle mobility at 942°C. A temperature which is very close to that of the Tammann temperature of Cr203 (996°C). In contrast to chromium, the catalytic action of copper on the graphite-oxygen reaction was much more conventional. It was, however, the critical observation that the contact angle of particles located on edges changed from obtuse to acute just prior to channel formation that provides further key information on the fundamental aspects of this form of attack. It appears that certainly for the case of copper, the particles have to wet the graphite edges before they proceed to create channels. In order to wet the graphite support the particle surface must be quite viscous; a condition normally prevailing at the Tammann temperature of the material. Such a state could be attained below this characteristic temperature if extra heat was imparted to the particle by exothermic processes occurring at its surfaces. Under such circumstances the particle would probably consist of a solid core su~ounded by a mohen outer layer 181.For a typical particle size range distribution, one might reasonably expect the smaller particles to reach this condition sooner than larger ones and that consequently at a given temperature to become activated first; a situation which is indeed observed experimentally.
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
79
R. T. K. BAKERand J. J. CHLUDZINSKI, JR.
Fig. 5. Sequence showing the development of catalytic channels by copper particles in the graphite-hydrogen reaction.
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
I
i
P
025;
OL-.___-_ 072
075
OL8
_.-A
085
09
103/7YK.‘,
Fig. 6. Arrhenius plot of copper-catalyzedhydrogenation of graphite. In order to establish that the above phenomenon is a general criterion for channel propagation, detailed examination of many more systems will be necessary. The observation of two distinct mobility temperatures coupled with a variation in wetting characteristics of particles with temperature indicates that we are dealing with a complex reaction system. It is possible that the motion observed at 400°C is associated with particles in the metallic state (Tammann temperature for Cu= 4OYC),whereas that seen at 550°C is due to the presence of cupric oxide (Tamman tempera~re for CuO = 558’C). Thermo~avime~ic results also point to cupric oxide being the active entity in the copper catalyzed graphiteoxygen reaction[2]. This being the case then one can rationalize the observed changes in particle behavior according to the following arguments: It is known that copper does not wet graphite[9], however, addition of oxygen to form CuO results in a drastic lowering of the surface tension of the metal[lO] and an increased tendency for wetting. It is therefore not
Fig. 7. Model of the influence of chromium oxide particles on the graphite-oxygen reaction. CAR Vol
IT’. No. l--H
81
surprising to find that at high reaction temperatures virtually all the catalyst material has spread in the form of a thin film along the edges and steps on the graphite surface. This result would seem to imply that the maximum catalytic rate will be obtained when all the graphite edges are coated with a film of copper and that further additions would have no influence. This rationale might explain McKees’ finding that the optimum concentration of copper was 50 ppm (21.On subsequent lowering of the temperature to about 400°C reformation of copper might be promoted, now in a non-wetting condition, so that particle nucleation will once again be favored, and catalyst be available in a form for further reaction. Although this explanation is attractive, its final verification must await the results of in-situ Electron Paramagnetic Resonance (EPR) experiments from which it should be possible to identify the state of copper at various stages during the reaction. Al~ough the precise significance of the values obtained here for the apparent activation energies for these two systems, chromium catalyzed reaction. 23.9 kcal melee’ and copper catalyzed reaction, 21.7 kcal mole-‘, is not clearly understood at this time, it is interesting to find that they show close agreement with those derived from bulk measurements, 24.1 kcal mole-’ and 23.5 kcal mole-’ for chromium and copper, respectively [3,4]. This suggests that the rate controlling step is the same for micro- and macro-systems and any other factors affecting the overall values are also operative in both cases. In order to put the activity of these materials into some perspective a collective plot, Fig. 8, has been compiled of the catalytic influence of a number of oxides for graphite oxidation, together with that for the uncatalyzed reaction. Data for the other oxides was also obtained from CAEM studies; V20s[12], ~00~~131, ‘Zn0[14], WO,[lS] and the uncatalyzed reaction[l4]. It is
Fig. 8. Comparison of the effect of various oxides on the graphite-oxygen reaction.
82
R. T. K. BAKER and J. J. CHLUDZINSKL JR.
clear that whilst both chromium and copper exhibit appreciable activity over the low temperature region, 600800°C they fall far short of that displayed by vanadium. The failure to find any trace of catalytic activity for the graphite-steam reaction leaves one with the conclusion that neither of these oxides is an effective agent for dissociation of water moiecules, the proposed rate determiningstep in this process[ 1I]. The apparent lack of activity of chromium for the graphite-hydrogen reaction may be associated with the inability to reduce the oxide to the metallic state under
the prevailingconditionsof 1Ton hydrogen. The events preceding the formation of channels in the copper/graphite-hydrogenreaction are intriguingand completely in line with previous observationson the behavior of other metals for this reaction[6]. Rather than speculate as to possiblereasons for this behavior,we prefer to await the results of EPR experiments, which will compIimentthose conducted under oxidizingconditions.
1. D. W. McKee,Carbon 8,623 (1970). 2. D. W. McKee. Carbon 8. 131(1970). 3. E. A. Heintz and W. E. Parker, Caibon 4,473 (1966). 4. P. S. Harris, Carbon 10,643 (1972). 5. R. T. K. Baker, Catal. Rev. Sci. Engng 19(2),161(1979). 6. R. T. K. Baker. R. D. Sherwood and J. A. Dumesic. J. C&L. 6656 (1980). 7. P. S. Harris, F. S. Feates and B. G. Reuben, Carbon 11,565 (1973). 8. E. G. Derouane, R. T. K. Baker, J. A. Dumesic and R. D. Sherwood, J. Catal., in press. 9. W. Weisweiler and V. Madadevan, High Temp.-H/gh PresI~
_
sure 4,21(1972).
10. M. F. Felsen and P. Regnier, Surface Sci. 68,410 (1977). 11.F. J. Long and K. W. Sykes, Froc. Roy. Sot. Land A215,lOO
(1952). 12. R. T. K. Baker,R. B. Thomasand M. Wells, Carbon 13, 141 (1975). 13. R. T. K. Baker, P. S. Harris, D. J. Kemper and R. J. Waite, Carbon 12, 179(1974). 14. R. T. K. Baker and P. S. Harris, C&on ii,25 (1973). 15. R. T. K. Baker and J. J. Chludzinski, Jr., to he published.
Carbon. Vol. 19. pp 7542. 1981 Prmkd m Grear Brsain All rights reserved
CATALYTIC GASIFICATIONOF GRAPHITE BY CHROMIUM AND COPPER IN OXYGEN, STEAM AND HYDROGEN R. T. K.
BAKER
and J. J.
CHLUDZINSKI. JR.
Exxon Research and Engineering Company. Linden, NJ 07036. U.S.A (Received
II September
1980)
Abstract-Controlled atmosphere electron microscopy studies have demonstrated that both chromium and copper are extremely active catalysts for the graphite-oxygen reaction over the range 550-800°C. Under these conditions the active catalytic entities are probably Cr20, and CuO, respectively. The behavior of chromium is quite unique. not fitting into the well established patterns, i.e. pitting or channeling. Instead, particles remain motionless throughout the oxidation sequence and the action is seen as an acceleration of edge recession all over the specimen. In contrast, copper exhibited conventional channeling activity, which diminished at higher temperatures ( > 700°C) due to the wetting and spreading of active particles along graphite edges. Such edges underwent rapid recession following this phenomenon. Neither metal showed any tendency to catalyze gasification of graphite in the nresence of steam. Coooer was found to be an active catalyst for graphite hydrogenation at temperatures in excess of 800°C. but chromium remained inactive. L.
York State) by evaporation of wire (Cu) and powder (Cr) from a tungsten filament and boat, respectively, at a residual pressure of lo-” Torr. The metals were added in sufficient amounts to produce a monolayer coverage on the graphite surface. The reactant gases used in the microscopy studies, oxygen, argon and hydrogen were obtained from Scientific Gas Products with stated purities of 99.99% and were used without further purification. Experiments where the effects of steam were being investigated were accomplished by allowing argon to flow through a bubbler containing deionized water maintained at 0°C. prior to entering the gas reaction cell, conditions which produced an argon/water ratio of about 40: I.
I. INTRODUCTION It is surprising
to find that very little information is available on the role of chromium oxide during graphite oxidation. McKee [l] reported that Cr203 was the catalytically active phase, but it was difficult to identify the precise mode by which it operated. He suggested that the oxide might function as a dissociation center for oxygen molecules, rather than interacting directly with the graphite substrate. The same author is also responsible for the most comprehensive published work on the effect of copper on graphite gasification. He used a combination of controlled atmosphere optical microscopy and thermogravimetric analysis to demonstrate that cupric oxide was the active entity in the copper catalyzed graphite-oxygen reaction. He also showed that this action could be inhibited by alloying of the copper with zinc, aluminum or tin [2]. From bulk measurements, Heintz and Parker]31 obtained values of 63.6 and 43.3 kcalmole-‘, respectively, for the activation energies of the chromium and copper-catalyzed decompositions of graphite in air. These values were subsequently amended to 34.1 and 23.5 kcal melee’, respectively, by Harris[4] who reexamined the earlier data. In the present work we have used controlled atmosphere electron microscopy to examine the behavior of copper and chromium on graphite in OZ, H20, and H2 environments. These experiments have enabled us to identify the detailed characteristics and evaluate the kinetics of the catalytic action of these metals towards gasification of graphite under a variety of conditions.
3. RESULTS 3.1 Controlled
3.1.1
description
of
the
CAEM
technique
electron
microscopy
studies
When chromiumigraphite specimens were heated in 5 Torr oxygen particle nucleation occurred at about 400°C. When first formed, particles were in the range 5-10nm dia. and increased in size to around 20nm as the temperature was raised to 550°C. The first signs of graphite gasification were observed at 57s”C, and this took the form of erosion of edges and steps, behavior normally associated with uncatalyzed attack. As the temperature increased so did the speed by which edges receded, often undermining particles which then tended to collect as a residue along the edges. It was apparent that throughout this period particles located on the basal planes of the graphite remained motionless, and there was no evidence of either pitting or channeling action. At 942”C, all particles started to move on the surface, however, this event did not induce any change in the mode of graphite gasification. Figure 1 is a transmission electron micrograph of a specimen which had been heated to 900°C in oxygen. The “saw-toothed” appearance of the graphite edge produced as a result of the gasification reaction is
2. EXPERIMENTAL A detailed
atmosphere
Chromium/graphite-oxygen.
has
been reported previously[S]. Copper and chromium (both 99.99% purity) were introduced onto transmission specimens of single crystal graphite (Ticonderoga, New 75
R. T. K. BAKERand J. J. CHLUDZINSKI, JR.
Fig. 1. Transmission electron micrograph of a chromium/graphite specimen which had been exposed to 5 Torr oxygen at 900%
clearly evident, along with the metal oxide debris which has collected in this region. From quantitative kinetic analysis of edge recession sequences, it has been possible to measure the rate of reaction as a function of temperature. This data is presented in the form of an Arrhenius plot, Fig. 2 and from the slope of the line a value of 23.9 2 3 kcal mole-' has been estimated for the apparent activation energy of chromium catalyzed oxidation of graphite. A correction factor has been applied to the rate measurements obtained at temperatures in excess of 850°C to take into account the contribution of the uncatalyzed reaction, which becomes significant under these conditions. 3.1.2 Chromium/graphite-hydrogen and steam. The behavioural patternof chromium/graphite specimens when treated in either 1 Torr hydrogen or I Torr argon saturated with Hz0 at 0°C was very similar in many respects and will therefore be dealt with collectively. Nucleation of the evaporated metal film into discrete particles (5 nm dia) took place quite readily in steam at 435°C. The same process was much more shtggish in -05 hydrogen, starting at 470°C and not being complete until 08 IO 09 IO3T(K-‘l 675°C. This situation was improved in later experiments by performing the nucleation in oxygen at 400°C and Fig. 2. Arrhenius plot of chromium-catalyzed rate of graphite then switching to hydrogen. This procedure did not apgasificationin 5 Torr oxygen. I
L
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
pear to in&roduce any deviation in the subsequent behavior compared to that where specimens were heated directly in hydrogen. Continuous observations up to 1050°Cshowed no evidence of catalytic attack of the graphite in either systems. Some indication of very slow edge erosion was apparent in steam at approx. 935”C, But this would be expected from uncatalyzed attack. The only other event worthy of note was the observation of occasional particle movement in steam at 965”C, a finding not seen in hydrogen. 3. I .3 Copper/graphite-oxygen. When heated in 5 Torr oxygen, the evaporated copper iilm nucleated to form discrete particles in the size range, 5-25 nm at 325°C. Some particle motion was observed at 4OO”C,a condition where the particles were thin and irregular shaped. This motion ceased as the temperature was gradually raised to 45O”C, and coincided with a change in particle appearance to a more globular morphology. It was noticeable that particles located on edges and steps on the surface were in a “non-wetting” state exhibiting contact angles with the graphite of > 90”. A dramatic transformation in these particular particle characteristics was seen at 52O”C, which is depicted schematically in Fig. 3. The particles tended to become cap-shaped, with contact angles < 90”, typical of a “wetting” condition, and this change preceded the formation of channels. Initially channeling activity was limited to the smallest particles present, 5nm width, causing the channels to be narrow and very shallow. As the temperature was raised, there was an accompanying increase in the average size of particles propagating channels. General particle mobility commenced at 550°C and this increased the number of particles collecting at edges and hence available to produce channels. At 675°C the wetting action of the particles on graphite became so intense that material was left on the sides of the channels as a result of spreading. This effect caused channels to assume a fluted appearance and both the size and rate of forward motion of the particle decreased as the reaction proceeded. Due to this phenomena the average particle size had decreased to 7 nm at 750°C. At this stage in the reaction even those particles which had previously been inactive were beginning to show changes in behavior. Rearrangement from an irregular shape through an intermediate well defined cubic form was followed by prompt disintegration when the particles coniacted edges, resulting in the formation of thin films which covered a considerable edge area. Such edges underwent rapid recession immediately following the spreading action of the particles. Specimens were heated to 820°C when very few parti-
77
cles were visible on the surface and the gasification reaction appeared to be controlled solely by the edge recession process. If the temperature was reduced to 415”C, then after a period of about 10min, particles were observed to reform in the vicinity of edges and on subsequent reheating in oxygen followed the same behavioral pattern as described above Quantitative kinetic analysis showed that, the temperature dependence of channel propagation rate for 25 nm dia. particles cutting channels of similar depth was linear, Fig. 4 and from the slope of the line an apparent activation energy of 21.7 rt 2 kcal mole- ’ was estimated for the copper catalyzed gasification of graphite. 3.1.4 Copper~graphite-hydrogen and steam. The copper film nucleated to form discrete particles 2.5-10.0 nm in size after heating in 1 Torr hydrogen at 415°C for 30min. Inspection of particles located along edges and steps on the graphite showed that they were in a “nonwetting” state with respect to the surface. This situation changed to a “wetting” condition as the temperature was siowly raised to 585°C and during this period the particles increased in size, but showed a reluctance to exhibit any signs of motion. As the temperature was increased to 605°C not only did particles cease to grow, but there was a suggestion that they were actually shrinking. This suspicion was confirmed at slightly higher temperature when the rate of contraction became so appreciable that at 676°C no evidence of any of the original particles remained. Any idea that this process had been the result of metal evaporation from the surface was quickly dispelled, as on raising the temperature to 735°C particles started to reform as fine filaments, often at some distance away from the location of the initial particles. Shortly after their formation, these entities executed a random movement on the surface and tended to “ball-up” into discrete globular particles. 25
20
1
c2 -b
0
Fig. 3. Schematic representation of the change in particle morphology in the copper/graphite-oxygen system observed on increasing the temperature from 450 to 520°C.
L
.._~~
09
!b -
-------i l/T(Kl*
Fig. 4. Arrhenius
i:
‘.\.
___
2.
‘2
IO
plot of copper-catalyzed rate of graphite gasification in 5 Torr oxygen.
78
R. T. K. BAKERand J. J. CHLUD~~NS~.JR.
Prior to restoration of particles, there were indications along edges that catalytic attack was occurring. This action could not be classified according to the normal modes of catalyst behavior for graphite gasification, i.e. pitting or channeling, but instead was seen to accelerate the recession of edges leaving a watermark pattern in these regions. It is probable that this edge erosion was caused by the catalytic influence of a thin film of metal spread along the entire length of the step or edge. This claim is supported by the fact that edge recession ceased at the same time that small particles started to grow at the edges, presumably as a result rupture of the film. The onset of catalytic attack by the channeling mode was observed at 845°C. These channels all emanated from edges and were extremely straight, being oriented parallel to (1120) directions and altered course by bending through angles of 60” and 120”. This action was followed up to 105O”C,a condition where the average width of channels was 125nm and even at elevated temperatures the channels maintained a constant width throughout their propagation period, indicating that the rate of the uncatalyzed reaction, which would result in expansion of channels, was negligible. Detailed kinetic analysis of the recorded reaction sequences, a typical example of which is shown in Fig. 5, confirmed the previous finding for catalyzed hydrogenation of graphite (6) that at any given temperature large particles propagated channels at a faster rate than small ones. An Arrhenius plot of the rate data obtained from 25nm dia. particles cutting channels of similar depth yielded an apparent activation energy of 33.32 3 kcal mole-‘, Fig. 6. When the copper/graphite system was reacted in 1Torr argon saturated with water at O”C, there was a marked difference in the activity pattern compared to that seen in either oxygen or hydrogen. In this case particles were not only faceted but also extremely thin, many of the underlying support features being visible through the crystallites. Inspection of some of the larger particles present at 500°C (25 nm dia.) showed that the electron scattering density was uniform across each crystallite indicative of a pillbox morphology. Raising the temperature to 534°C resulted in the onset of particle mobility, but did not induce any changes in structural characteristics of the particles, or any catalytic activity towards gasification of the graphite. The first signs of graphite gasification were apparent at 950°C and occurred by erosion of edges caused by uncatalyzed attack. At this temperature the particles were exhibiting rapid motion on the surface and had become more rounded in outline and dense, characteristics of a globular morphology. However, even at these extreme conditions they showed no tendency to catalyze graphite removal.
4. DISCUSSION The results of this investigation clearly demonstrate that both chromium and copper are extremely active catalysts for the graphite-oxygen reaction over the temperature range, 550-800°C. They do, however, exhibit
completely different qualitative characteristics in their mode of action for this process. The behavior of chromium is quite unique, not fitting into either of the well established patterns, i.e. pitting or channeling. Instead, its action closely resembles that of the uncatalyzed reaction, the rate of which does not become significant until 850°C. It is possible to rationalize the influence of chromium by assuming that edge recession is accelerated by either the formation of a thin film of catalyst material along the graphite edges and steps, in a manner similar to that reported for lead[?], or that the particles act as dissociation centers for oxygen molecules, and that the oxygen atoms produced diffuse to and attack edges and stepsll]. The weight of experimental evidence tends to favor the latter postulate for several reasons. The rate of edge recession was found to be quite uniform over the entire surface at a given temperature. This situation would not be expected from a metal wetting and spreading phenomena since the probability of every edge being coated by metal is not very high. In this case one would expect to observe more erratic behavior with some regions remaining uncontaminated by catalyst and relatively unreactive. Moreover, as the reaction progressed one would predict that a condition would be attained where the metal film became so stretched by the receding edge that eventually it would rupture and formation of discrete particles would occur, and then these entities would continue to attack the graphite, possibly by a channeling mode. Since these effects were not observed, one is left with the conclusion that the situation depicted in Fig. 7 is probably a more accurate description of the influence of chromium on the graphite-oxygen reaction. Thermogravimetric studies by McKee [ I ] indicated that the catalytically active phase for this reaction was Cr203, Results from the present work would support this view as indicated by the finding of particle mobility at 942°C. A temperature which is very close to that of the Tammann temperature of Cr203 (996°C). In contrast to chromium, the catalytic action of copper on the graphite-oxygen reaction was much more conventional. It was, however, the critical observation that the contact angle of particles located on edges changed from obtuse to acute just prior to channel formation that provides further key information on the fundamental aspects of this form of attack. It appears that certainly for the case of copper, the particles have to wet the graphite edges before they proceed to create channels. In order to wet the graphite support the particle surface must be quite viscous; a condition normally prevailing at the Tammann temperature of the material. Such a state could be attained below this characteristic temperature if extra heat was imparted to the particle by exothermic processes occurring at its surfaces. Under such circumstances the particle would probably consist of a solid core su~ounded by a mohen outer layer 181.For a typical particle size range distribution, one might reasonably expect the smaller particles to reach this condition sooner than larger ones and that consequently at a given temperature to become activated first; a situation which is indeed observed experimentally.
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
79
R. T. K. BAKERand J. J. CHLUDZINSKI, JR.
Fig. 5. Sequence showing the development of catalytic channels by copper particles in the graphite-hydrogen reaction.
Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen
I
i
P
025;
OL-.___-_ 072
075
OL8
_.-A
085
09
103/7YK.‘,
Fig. 6. Arrhenius plot of copper-catalyzedhydrogenation of graphite. In order to establish that the above phenomenon is a general criterion for channel propagation, detailed examination of many more systems will be necessary. The observation of two distinct mobility temperatures coupled with a variation in wetting characteristics of particles with temperature indicates that we are dealing with a complex reaction system. It is possible that the motion observed at 400°C is associated with particles in the metallic state (Tammann temperature for Cu= 4OYC),whereas that seen at 550°C is due to the presence of cupric oxide (Tamman tempera~re for CuO = 558’C). Thermo~avime~ic results also point to cupric oxide being the active entity in the copper catalyzed graphiteoxygen reaction[2]. This being the case then one can rationalize the observed changes in particle behavior according to the following arguments: It is known that copper does not wet graphite[9], however, addition of oxygen to form CuO results in a drastic lowering of the surface tension of the metal[lO] and an increased tendency for wetting. It is therefore not
Fig. 7. Model of the influence of chromium oxide particles on the graphite-oxygen reaction. CAR Vol
IT’. No. l--H
81
surprising to find that at high reaction temperatures virtually all the catalyst material has spread in the form of a thin film along the edges and steps on the graphite surface. This result would seem to imply that the maximum catalytic rate will be obtained when all the graphite edges are coated with a film of copper and that further additions would have no influence. This rationale might explain McKees’ finding that the optimum concentration of copper was 50 ppm (21.On subsequent lowering of the temperature to about 400°C reformation of copper might be promoted, now in a non-wetting condition, so that particle nucleation will once again be favored, and catalyst be available in a form for further reaction. Although this explanation is attractive, its final verification must await the results of in-situ Electron Paramagnetic Resonance (EPR) experiments from which it should be possible to identify the state of copper at various stages during the reaction. Al~ough the precise significance of the values obtained here for the apparent activation energies for these two systems, chromium catalyzed reaction. 23.9 kcal melee’ and copper catalyzed reaction, 21.7 kcal mole-‘, is not clearly understood at this time, it is interesting to find that they show close agreement with those derived from bulk measurements, 24.1 kcal mole-’ and 23.5 kcal mole-’ for chromium and copper, respectively [3,4]. This suggests that the rate controlling step is the same for micro- and macro-systems and any other factors affecting the overall values are also operative in both cases. In order to put the activity of these materials into some perspective a collective plot, Fig. 8, has been compiled of the catalytic influence of a number of oxides for graphite oxidation, together with that for the uncatalyzed reaction. Data for the other oxides was also obtained from CAEM studies; V20s[12], ~00~~131, ‘Zn0[14], WO,[lS] and the uncatalyzed reaction[l4]. It is
Fig. 8. Comparison of the effect of various oxides on the graphite-oxygen reaction.
82
R. T. K. BAKER and J. J. CHLUDZINSKL JR.
clear that whilst both chromium and copper exhibit appreciable activity over the low temperature region, 600800°C they fall far short of that displayed by vanadium. The failure to find any trace of catalytic activity for the graphite-steam reaction leaves one with the conclusion that neither of these oxides is an effective agent for dissociation of water moiecules, the proposed rate determiningstep in this process[ 1I]. The apparent lack of activity of chromium for the graphite-hydrogen reaction may be associated with the inability to reduce the oxide to the metallic state under
the prevailingconditionsof 1Ton hydrogen. The events preceding the formation of channels in the copper/graphite-hydrogenreaction are intriguingand completely in line with previous observationson the behavior of other metals for this reaction[6]. Rather than speculate as to possiblereasons for this behavior,we prefer to await the results of EPR experiments, which will compIimentthose conducted under oxidizingconditions.
1. D. W. McKee,Carbon 8,623 (1970). 2. D. W. McKee. Carbon 8. 131(1970). 3. E. A. Heintz and W. E. Parker, Caibon 4,473 (1966). 4. P. S. Harris, Carbon 10,643 (1972). 5. R. T. K. Baker, Catal. Rev. Sci. Engng 19(2),161(1979). 6. R. T. K. Baker. R. D. Sherwood and J. A. Dumesic. J. C&L. 6656 (1980). 7. P. S. Harris, F. S. Feates and B. G. Reuben, Carbon 11,565 (1973). 8. E. G. Derouane, R. T. K. Baker, J. A. Dumesic and R. D. Sherwood, J. Catal., in press. 9. W. Weisweiler and V. Madadevan, High Temp.-H/gh PresI~
_
sure 4,21(1972).
10. M. F. Felsen and P. Regnier, Surface Sci. 68,410 (1977). 11.F. J. Long and K. W. Sykes, Froc. Roy. Sot. Land A215,lOO
(1952). 12. R. T. K. Baker,R. B. Thomasand M. Wells, Carbon 13, 141 (1975). 13. R. T. K. Baker, P. S. Harris, D. J. Kemper and R. J. Waite, Carbon 12, 179(1974). 14. R. T. K. Baker and P. S. Harris, C&on ii,25 (1973). 15. R. T. K. Baker and J. J. Chludzinski, Jr., to he published.