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Catalytic oxidation of graphite
J. Inorg. NucL Chem., 1962, Vol. 24, pp. 1129 to 1137. Pergamon Press Ltd. Printed in England
CATALYTIC
OXIDATION
OF
GRAPHITE*
G. R. H~NNIG Argonne National Laboratory, Argonne, Illinois (Received 15 January1962; in revised form 2 March 1962)
Abstract--Colloidal metal particles placed on graphite single crystals catalyse the oxidation of graphite. The catalysis occurs only at the metal-carbon interface. Spreading of catalytically active fragments from the particles was not observed for colloids of platinum, gold, iron or nickel. The particles produce characteristic etch patterns which are indicative of the crystal perfection. On crystals relatively free of defects, the catalysts attack parallel to cleavage surfaces to produce channels, and occasional pits on prismatic surfaces, but are inactive on the cleavage surfaces. On crystals containing lattice defects or substitutional boron impurities, the catalysts may also attack perpendicular to the cleavage surface and produce pits on it. The channels and pits are often oriented along preferred crystallographic directions. Activation energies for catalysis were estimated from the etch patterns and are about 10 kcal/mole parallel and 56 kcal/mole perpendicular to the cleavage surface. Some details of the mechanism of catalytic action have been deduced. Vanadium is an exceptionally active catalyst for graphite oxidation; it behaves differently from other metals because it forms a liquid pentoxide. THE reactions of graphite with gases, particularly with oxygen or carbon dioxide, are strongly affected by various catalytic agents. The action of such catalysts has been investigated by microscopic studies of the surfaces of graphite single crystals. It was anticipated that catalysts would produce pits on the surfaces of graphite crystals during reaction with oxygen or carbon dioxide. Such pits usually of hexagonal shape are in fact often present on partly reacted crystals; frequently they are found to have penetrated the whole crystal and resulted in hexagonal holes. The presence of such holes has been reported by HOFMANN(1) and has been attributed by GREER and TOPLEY (2) to preferential oxidation of incorporated foreign particle or to catalytic action of impurities. BACH and LEVITIN(3) studied the shape of such holes after reacting impure crystals with carbon dioxide. They observed round holes after reaction with pure CO2 and hexagonal holes after reaction with CO2 containing hydrogen chloride. In preliminary reports it was shownl4) by us that on carefully handled crystals metallic impurities usually do not produce catalytic pits. Special pretreatments such as particle irradiation, quenching or extensive mechanical deformation were required to induce pitting. These pretreatments probably introduced excess vacancies or vacancy clusters into graphite. Pitting was therefore attributed to attack of the catalyst at the vacancies. * Based on work performed under the auspices of the U.S. Atomic Energy Commission. (1) U. HOFMANN, Ber. Dtsch. Chem. Ges. 65 B, 1821 (1932). (2) E. N. GREERand B. TOPLEY, Nature, Lond. 129, 904 (1932). (3) N. BACHand I. LEVITIN,KolloidZ. 68, 152 (1924). (4) G. R. HENNIG, Proceedings of the First and Second Conferences on Carbon, p. 112. Wiley, Buffalo (1956); Proceedings of the Third Conference on Carbon, p. 265. Pergamon Press,
New York 0958). 1129
i 13~)
G.R. I-~r~o
Pretreatment of the crystals by heating was originally carried out to remove residual impurities. However, heat treatment below 2700°C may sometimes convert an inactive impurity to a more active form. It is intended to confine the present report to a description of the details of the catalytic effects. The application of these observations to quantitative studies of quenching, of vacancy motion,~5) and of impurity diffusion, which have been carried out jointly with Dr. M. A. KA~rrER, will be published in a separate report. EXPERIMENTS Materials. Natural graphite crystals were obtained at Ticonderoga, New York. We collected these crystals in their natural matrix of limestone from which they were separated in the laboratory by hydrochloric acid. The crystals were purified by boiling alternately in HCI and HF and by floatation. The crystals were nearly exclusively non-pitting. Their behaviour was indistinguishable from crystals purified further by heating in chlorine to 900°C or heating in helium to 3000°C and cooling slowly. Some experiments were carried out with commercial "Superflake" crystals which had been purified at 3000°C and cooled slowly. Colloidal metal suspensions were prepared by rotating a rod of the desired metal against a wire contact of the same material. The contact was submersed in acetone and caused to arc by applying about 30 V a.c. across the contact. The resulting suspensions were light gray to brown. A few drops placed on the crystal added of the order of one metal atom to 105 carbon atoms. Colloidal gold solutions in water were also preparedt6) by reduction of the chloride with citric acid and dialysis but did not spread well on the crystals and were not sufficiently free of salts to yield unambiguous results. Oxidation. To study the effects of catalysts, particularly on pitting graphite, crystals were cleaved to expose fresh internal surfaces containing only a few irregularities and surface steps. The catalyst was then applied and the crystals were transferred to silica or platinum plates and reacted on these supports in dry or wet oxygen. Wet oxygen had been saturated with water at 20°C. Dry oxygen had been passed through a column, at least 12 inches long, of magnesium perchlorate. To study conditions of very dry oxidation, the crystals were heated to 950°C in helium which had been passed through perchlorate and through a trap cooled with liquid nitrogen. After cooling the crystals to the desired temperature of combustion, oxygen was added to the helium stream. Microscopy. All microscope and electron microscope examinations were confined to the cleavage surfaces of the crystal and to those steps and edges visible by examination parallel to the c-axis. To examine the crystals in the electron microscope, a carbon replica was prepared. The crystal was either pre-shadowed with chromium, or the replica shadowed after separation from the crystal. The separation was accomplished by strengthening the replica with a heavy layer of evaporated gold, reacting the crystal with sulphuric acid contaming 10 per cent of nitric acid which caused the crystal to peel off the replica and floating the replica on aqua regia to dissolve the gold backing. The replicas were examined in an RCA E.M.-3 electron microscope. RESULTS Non-pitting catalysis. Since the crystals were destroyed during replication for electron microscopy, the action of the catalysts had to be inferred from their distribution after a measured period of oxidation. Fig. 1 shows a typical arrangement. Some of the catalyst particles (A) were found at the tip of channels in the crystal surface. The channel sides were nearly perpendicular to the cleavage surface. The channel diameter increased with distance from the particle. All channels started at (5) G. HENNIG and M. KANTER,Proceedings of the Fourth Conference on Carbon, p. 141. Pergamon Press, New York (1960). (6) J. TURKEVITCH,J. HmLmR and P. C. STEVENSON, Discussions Farad. Soc. No. 11, 55 (1951).
Catalytic oxidation of graphite
I 131
steps (B) running irregularly across the crystal surface. The length of the channels was variable. Many catalyst particles (C) were located on the cleavage surface some distance from the steps, these particles have not produced channels. The observations are interpreted as showing that catalysts remained inactive on the cleavage surface, until they came in contact with steps which moved along the surface due to uncatalysed oxidation. Once undermined by a step, a particle descended partly or completely to the bottom of the step and commenced to catalyse the oxidation along its contact with the "riser" of the step. This contact area became the tip of a channel which resulted from the accelerated oxidation at the contact area. The channels produced by iron particles were usually curved and irregular (Fig. 2); those produced by gold were nearly straight. The orientation of straight channels after oxidation in moist oxygen was preferentially along {2II0} and equivalent directions, whereas it tended to {10i0} in dry oxygen after drying the crystals. It had been observedt 7) that uncatalysed oxidation of graphite in moist oxygen causes preferential orientation of surface steps to {1010} and in dry oxygen to {2Ii0}; therefore the direction of the catalytic channels is perpendicular to and the orientation of the walls of the channel is parallel to the preferred orientation of uncatalysed steps. Reactivity of non-pitting catalysts. The width of the channel at the active tip was equal to the diameter of the catalyst particle. The width increased with distance from the catalyst due to uncatalysed oxidation of the channel sides. Therefore, the difference in half-width between base and tip of a channel divided by the channel length equals the reactivity ratio of uncatalysed to catalysed surface atoms. This ratio was found to increase with temperature as shown by the channels of Fig. I and Fig. 3. An Arrhenius plot of this ratio yielded an apparent activation energy difference of about 36 kcal. Since the activation energy for the uncatalysed oxidation of these crystals was 46 kcal,t 4) the activation energy of the catalysed reaction was 10 kcal. This value constitutes an average for measurements on many different channels. The value is far from exact because at any given temperature the channel dimensions varied between wide limits. Part of this variation in catalyst activity was found to be due to particle size. Very small and very large particles were less active than particles of several hundred Angstrom diameter. The catalytic activity of particles of comparable size depended also upon the channel depth which was determined from the shadow on the replica. The activity was highest for shallow channels. This correlation of channel depth and catalytic activity is apparent from the data of Table 1, which are, however, further complicated by particle size effects. The correlation was also apparent from visual examination of bent channels. Bends in channel direction often occurred when the active particle at the tip of the channel encountered a surface step. The particle was deflected away from ascending steps because they retarded the particle and towards descending steps which accelerated the motion. Channels of depth exceeding the particle radius have never been observed. When a particle interacted with a step higher than its own radius, its catalytic action produced one of three different features, a channel with a ledge, a pit or a serrated step. Usually only one of these features predominated hut the conditions favouring each are not known. A ledge at the base of the channel indicated that the particle had activated the oxidation of only the top of the step. The uncatalysed portion of ~¢IG.R. HENNIG,Proceedingsof the Fifth Conference on Carbon.Pergamon Press. Oxford (1962).
1132
G . R . Hm,rr~G
the step advanced at the uncatalysed rate and remained as a ledge at the base of the channel. Several such ledges are present in Figs. 1-4. The catalyst sometimes produced pits originating on the step and extending parallel to the cleavage plane. They probably would not be detected by electron microscopy but on rare occasions TABLE 1.---CATALYTIC ACTIVITY
d*
d[c*
r*
1450 660 330 300 180 210
5.2 4.4 3.5 3.5 2.1 2.0
70 80 18 26 19
15
*d --- particle diameter in Angstrom c = channel depth r = ratio of catalytic to uncatalysed burning rate were seen in the optical microscope. Pits of this type produced interference colours which are shown by contrast in Fig. 5. Such features were found occasionally on cleaved crystals, principally on steps originating at screw dislocations. In Fig. 5, the dislocation has been etched away leaving a large hole. The pits of Fig. 5 are situated at various depths as shown by the differences in contrast and confirmed by careful cleaving. The most frequently observed mode of action of catalysts at high steps was serration. The particle remained attached to the step and accelerated the oxidation parallel to the step (Fig. 6). The catalyst activity decreased with the age of the catalyst suspension. The activation energy for an aged solution of colloidal gold was measured to be 30 kcal. After extensive oxidation, the catalyst particles tended to coagulate (Fig. 4). Pitting catalysis. Various treatments were found to modify graphite crystals so that they pitted during subsequent catalytic oxidation. These treatments are irradiation with high energy neutrons or electrons, mechanical deformation, or suitable heat treatment. Heating to temperatures above 2700°C followed by quenching usually induced subsequent pitting behaviour probably because excess lattice vacancies were introduced. In crystals which had not previously been oxidised or heat treated, pitting could also be induced by heating for several hours to temperatures between 2400 ° and 2600 °. The rate of subsequent cooling had no influence if it exceeded one degree per minute but very slow cooling rates removed the pitting tendency. Heat treatment below 2700 ° caused a gradual penetration of this tendency into the crystal. As an example, Fig. 7 shows a cleaved and catalytically oxidised section of a crystal heated for one hour at 2600 °. Pitting occurred in a region paralleling the outer surface of the crystal and also along a linear source crossing the crystal which may have been a twin line or low angle boundary present before cleaving. Screw dislocations were also observed to act as sources for the pit forming tendency. This pitting induced below 2700 ° is probably associated, not with excess lattice vacancies, but with a diffusing impurity. The effect could be enhanced by heating the graphite between 2400 and 2600°C with boric acid but was not enhanced by heating with silica or vanadium chloride. The pitting tendency induced in this temperature range
FIG. 1.--Colloidal gold on graphite. One hour oxidation at 550°. A, active catalyst; B, step; C, inactive catalyst; D, crystal bent due to {2TI/} twin.
FIG. 2.--Colloidal iron on graphite. One hour wet oxidation at 500°. FIG. 3.--Colloidal gold on graphite. One quarter hour wet oxidation at 650 °. FIG. 4.--Coagulation of colloid particles. Gold on graphite. FIG. 5.--Pitting along a-axis. Colloidal gold on graphite. Wet oxygen at
700°C. Optical micrograph.
FIG. 6.--Serrated step. Colloidal gold on graphite. One half hour wet oxygen at 650 ° . FIG. 7.--Pitting tendency partly diffused into graphite. Colloidal gold catalyst. Optical micrograph. FIG. 8.--Initial channelling on pitting graphite. Colloidal gold. One hour low pressure oxygen at 700 ° . FJc. 9.--Pitting by colloidal gold. Two and a half hours low pressure oxygen at 750 °. Twin line showed that pit edges are {10T0}.
FIG. 10.--Irregular hexagonal pit. Colloidal gold. One hour oxygen at 650°. Shadowed after replication. FIG. 11.---Apparent spiral pit. Two hours oxygen 650°. FIG. 12.--Islands of vanadium catalyst on pitting graphite. Extensive oxidation. FIG. 13.--Same material as in Fig. 12, washed and burned ½ hour at 600°.
Catalytic oxidation of graphite
1133
is therefore probably due to boron impurities present on the crystal surface and diffusing, presumably substitutionally. No distinction will be made in the following between pitting due to boron or due to quenched-in vacancies since no qualitative differences in pitting behaviour had been noted and since quantitative measurements are still in progress. Pitting due to irradiation or mechanical deformation will not be described here. When metallic catalysts were not added, the oxidation of crystals which had been pretreated to induce pitting behaviour resulted in the development of only a small number of random pits on the cleavage surfaces. In wet oxygen this spontaneous pit formation was more pronounced than in dry oxygen. Oxidation of pitting crystals which had been contaminated with suspensions of colloidal metal caused very extensive pitting. The progress of this pitting was followed by replicating crystals after progressively increased periods of oxidation. Initially the catalyst particle produced a channel on the surface similar to the channels formed on non-pitting crystals; however, nearly every catalyst particle produced such a channel (Fig. 8), whereas on non-pitting crystals only those particles which were in contact with steps produced channels. Furthermore, on pitting crystals, the channels became progressively deeper; the rate of penetration increased with the defect (boron or vacancy) concentration. When the channel depth was about equal to the radius of the particle, lateral motion became restricted and actual pit formation commenced (Fig. 9). Temperature effects on pitting catalysis. The length of the initial channel decreased as the temperature of oxidation increased. The crystals did not pit at 600 °, unless heavily doped with boron, but at 800 ° the initial channel was very short and steep. The shape of the pits produced after extensive oxidation was also found to be a function of the temperature, being steeper, the higher the temperature of oxidation. The activation energy for catalytic oxidation parallel to the c-axis can be estimated from the pit shape. The burning rate at the rim of a pit is uncatalysed and should therefore show the activation energy of 46 kcal measured for uncatalysed burning of these crystals. The depth of the pit, determined from the shadow length and shadow angle of replicas, is an index of the rate of catalytic penetration. The ratio of width to depth was of the order of ten but was temperature dependent. An Arrhenius plot of this ratio yielded a value of --10 kcal for the difference between the activation energies of uncatalysed and catalysed oxidation. The activation energy of catalysed penetration is therefore 56 kcal/mole. The pits produced by colloidal gold or iron in wet oxygen consisted of rather high (> 500 A) steps oriented 10i0 or equivalently (Fig. 9). At low pressures of oxygen, the pits became perfectly hexagonal in shape. Oxidation at high oxygep. pressures and tow temperatures resulted in less regular pits which were, however, still bounded by steps consisting of straight sections of preferred orientation (Fig. 10). Near the bottom of the pit the steps were often of lower height and closer together than at the top. In dry oxygen the burning behaviour usually did not vary appreciably unless an effort was made to remove residual moisture from the crystals by outgassing in dry helium at 950°C. Such dried crystals usually showed less tendency to pit than moist crystals. An appreciable fraction of catalyst particles was found to remain inactive on the surface, the fraction depending also on the boron or defect concentration of
1134
G.R. HENNIO
the crystal. The pits which were produced under very dry conditions were usually oriented so that their sides were (2iI0}. Some of the steps were not parallel to the c axis but were high index pyramidal surfaces. The proportion of such pyramidal surfaces increased with the temperature of oxidation. Spiral pits. Occasional pits appeared, particularly in the optical microscope, to be spiral shaped. Such apparent spirals were always found to be actually pyramidal, the spiral appearance being due to a local crowding of steps which was probably caused by oscillations of the catalyst particle (Fig. 11). Reactivity of pitting catalysts. The dependence of the catalyst reactivity on moisture and on temperature has already been described. Some differences were noted between various metals. Colloidal gold and iron appeared equally active under moist conditions of oxidation. In dry oxygen, iron was considerably more active than gold in forming pits. Both substances appeared to sinter at 900 ° into more uniformly round particles which initiated more symmetrical pits or straighter channels. Nickel, platinum, and silver appeared to show catalytic activity comparable to gold. Aluminium appeared to be less active. Colloidal suspensions of vanadium produced a very much larger enhancement of oxidation rate than other metals tested. Occasional vanadium catalyst particles were seen on these crystals at the tip of catalytic channels, but, in addition, the reactivity of all steps on non-pitting crystals and of all surfaces on pitting crystals seemed to be considerably enhanced. Irregular channels indicating excess reactivity were often observed which did not carry visible catalyst particles at their tip. Furthermore those particles which could be found at channel tips appeared to wet the graphite surface and conform to the channel shape. The vanadium catalyst was apparently present as a liquid which wetted and flowed over the graphite surface. This suggests that the catalyst was present as the low melting pentoxide. Frequently, residues of less active catalyst remained in isolated positions, possibly as vanadium carbide (Fig. 12). Water at 25°C reactivated such particles; they retained their shape below 600 ° but melted above this temperature (Fig. 13). The melting point of bulk vanadium pentoxide is 680 ° but apparent melting of very small particles due to sintering, surface flow, etc. may occur at lower temperature. DISCUSSION The observations demonstrate several details of the mechanism of catalytic oxidation. All the metals studied except vanadium produced catalytic effects only at the interface of a rather sizeable metal particle and the substrate. No evidence was found for any catalysis by individual metal atoms separating from the colloid, the particles did not become smaller, but actually often increased in size by coalescing. In a few experiments, solutions of metal salts were placed on graphite. After brief oxidation, these metals were again found to have coagulated into active colloidal particles. The metallic particles did not cause catalysis by action at a distance, as for instance by altering the Fermi level of conduction electrons in the graphite. The shape of the channels produced on the graphite surface showed that uncatalysed oxidation occurred up to the immediate vicinity of a catalyst particle. What caused the motion of the colloidal particles which produced the observed channels ? Brownian motion would cause a 200/It particle to be displaced 0.05 mm/ second at 500 ° in air unless retarded by friction against the crystal surface. However, this friction or bonding of the particle to the cleavage surface appears to prevent
Catalytic oxidation of graphite
1135
Brownian motion nearly completely since the colloid particles remain randomly distributed instead of moving to surface steps and becoming catalytically active. The forces between the particle and the cleavage surface are probably van der Waals forces. The motion of a particle along its catalytic channel is probably due to additional attractive forces to the carbon atoms along the edge of the channel tip. Since these carbon atoms are constantly being oxidised, only the leading surface of the catalyst particle can be in contact with edge atoms. Thus the attractive forces are unbalanced and cause motion of the particle along the channel direction. Motion by this mechanism rather than by Brownian motion is also deduced from the observation that active particles are always located in the tip of the channels. Particles located elsewhere in the channel are inactive. The straightness and preferential orientation of the channels can be caused by anisotropic reactivity. If the catalysed oxidation at the interface is faster in certain crystallographic directions, the resulting motion which maintains maximum contact area between particle and graphite, and thus the channel direction, will be perpendicular to one of the directions of fastest oxidation (Fig. 14). It should be noted
///
//~.
¢;I~-s.~S;;~S2SS ~"
0° b° FI~. 14.iMotion of catalyst. Dashed lines: position near beginning of catalysis. Solid fines: position when channel has formed. Arrows denote directions of highest reactivity. Burning along these directions is highly exaggerated in drawing. that these directions of fastest catalysed oxidation coincide with the directions of fastest uncatalysed oxidation since the orientation of the channel sides is usually parallel to the orientation of surface steps in uncatalysed burning, even when this preferred orientation is altered by the action of water. The catalytic activity of colloidal particles of a given size is lower in deep than in shallow channels. Since the depth of a channel in non-pitting crystals is determined by the height of the step at which catalysis commenced, this correlation cannot be due to inherent differences between particles but may be due to differences in the
1136
G.R. I-ImqraG
area of the particle-graphite interface. The diffusion length of oxygen or reaction products in this interface increases with the channel depth, thus slowing the reaction. When the step height exceeds the particle radius, the particle usually does not form a catalytic channel but moves parallel to the surface step. In this case, the accumulation of reaction products probably prevents the formation of three lines of attachment required for channel formation (Fig. 14). When attached to the step as in Fig. 6, the particle probably changes its area of attachment constantly thereby facilitating the release of reaction products from the contact area. Mechanism of catalysis. Three sets of observations on non-pitting crystals yield information on the mechanism of catalysis. These are the preferred direction of catalytic attack, the motion of the catalyst particles, and tho measured activation energies. The uncatalysed oxidation of graphite proceeds in several steps which probably include adsorption of oxygen, activation and dissociation of the oxygen, formation of mobile or immobile surface oxide complexes, activation and rupture of carbon bonds between complexes and interior carbon atoms, and removal of adsorbed reaction products. Of these partial reactions, the step which determines the crystallographic orientation of the oxidised surface is probably the rupture of carbon-carbon bonds. It is likely that this particular step in the reaction is not affected by metal catalysis because the preferred orientation is insensitive to catalysis as described earlier. The observed motion of the catalyst particles demonstrates that bonds are formed between the catalyst and a large number of reactive carbon atoms at the metal-c arbon interface. Since this bonding may be incidental to the catalytic action, it does not reveal which of the reaction steps is affected or by-passed by the catalytic action. If the bonding were concomitant with the catalytic action, activation of adsorbed oxygen to form metal-oxygen-carbon bridges would be a likely mechanism of catalytic action. The measured activation energies for non-pitting catalysis demonstrate that the catalysts cause a pronounced change in the pre-exponential term of the Arrhenius rate equation. The observed reactivity ratio of catalysed to uncatalysed oxidation of carbon atoms was measured to be about 50 at 550°C (Table 1). The activation energy was reduced by catalysis from 46 to about 10 kcal/mole. Therefore the preexponential term was reduced by a factor of 108 due to catalysis. Such a large change suggests that the catalyst does not merely accelerate one of the reaction steps of oxidation but affects several steps, or bypasses the normal reaction sequence possibly by the formation of metal-oxygen-carbon bridges. Pit formation appears to be initiated by the accelerated oxidation at those vacancies and/or boron atoms which are present in the initial contact region between catalyst and graphite. This attack is followed by catalytic oxidation of edge atoms until the hole in the surface layer of the crystal is large enough to expose additional vacancies or boron atoms in the next layer. As these are oxidized in turn, the particle gradually penetrates into the crystal while producing a surface channel. When the channel depth has increased to the particle radius, lateral motion is slowed but penetration continues resulting in a pit. The stepped geometry of the pits is not due to periodic bursts of catalytic activity but to excessive reactivity of pyramidal faces initially produced. Even in the absence of metallic catalysts, such pyramidal faces
Catalytic oxidation of graphite
1137
are nearly always converted to prismatic faces. The reason for this preferential burning to prismatic (i.e. "vertical") steps on graphite is not understood but is believed to be due to catalysis by traces of water. The temperature dependence of the catalytic penetration yielded an apparent activation energy of 56 kcal. It is not surprising that this value is higher than the activation energy of uncatalysed oxidation because the penetration reaction involves oxidation of carbon atoms adjacent to vacancies or boron atoms which may be more tightly bonded than carbon atoms at edges of layer planes. The conclusions do not apply to vanadium catalysts which melt and wet the surface.
Acknowledgements--The author is indebted to J. BALDWIN,P. ROACHand C. KAZMIEROWICZ for help in sample preparations. The author gratefully acknowledges the co-operation of the electron microscope group of the Biology Division and particularly O. T. MI~CK in making the instrument available for these studies.
CATALYTIC
OXIDATION
OF
GRAPHITE*
G. R. H~NNIG Argonne National Laboratory, Argonne, Illinois (Received 15 January1962; in revised form 2 March 1962)
Abstract--Colloidal metal particles placed on graphite single crystals catalyse the oxidation of graphite. The catalysis occurs only at the metal-carbon interface. Spreading of catalytically active fragments from the particles was not observed for colloids of platinum, gold, iron or nickel. The particles produce characteristic etch patterns which are indicative of the crystal perfection. On crystals relatively free of defects, the catalysts attack parallel to cleavage surfaces to produce channels, and occasional pits on prismatic surfaces, but are inactive on the cleavage surfaces. On crystals containing lattice defects or substitutional boron impurities, the catalysts may also attack perpendicular to the cleavage surface and produce pits on it. The channels and pits are often oriented along preferred crystallographic directions. Activation energies for catalysis were estimated from the etch patterns and are about 10 kcal/mole parallel and 56 kcal/mole perpendicular to the cleavage surface. Some details of the mechanism of catalytic action have been deduced. Vanadium is an exceptionally active catalyst for graphite oxidation; it behaves differently from other metals because it forms a liquid pentoxide. THE reactions of graphite with gases, particularly with oxygen or carbon dioxide, are strongly affected by various catalytic agents. The action of such catalysts has been investigated by microscopic studies of the surfaces of graphite single crystals. It was anticipated that catalysts would produce pits on the surfaces of graphite crystals during reaction with oxygen or carbon dioxide. Such pits usually of hexagonal shape are in fact often present on partly reacted crystals; frequently they are found to have penetrated the whole crystal and resulted in hexagonal holes. The presence of such holes has been reported by HOFMANN(1) and has been attributed by GREER and TOPLEY (2) to preferential oxidation of incorporated foreign particle or to catalytic action of impurities. BACH and LEVITIN(3) studied the shape of such holes after reacting impure crystals with carbon dioxide. They observed round holes after reaction with pure CO2 and hexagonal holes after reaction with CO2 containing hydrogen chloride. In preliminary reports it was shownl4) by us that on carefully handled crystals metallic impurities usually do not produce catalytic pits. Special pretreatments such as particle irradiation, quenching or extensive mechanical deformation were required to induce pitting. These pretreatments probably introduced excess vacancies or vacancy clusters into graphite. Pitting was therefore attributed to attack of the catalyst at the vacancies. * Based on work performed under the auspices of the U.S. Atomic Energy Commission. (1) U. HOFMANN, Ber. Dtsch. Chem. Ges. 65 B, 1821 (1932). (2) E. N. GREERand B. TOPLEY, Nature, Lond. 129, 904 (1932). (3) N. BACHand I. LEVITIN,KolloidZ. 68, 152 (1924). (4) G. R. HENNIG, Proceedings of the First and Second Conferences on Carbon, p. 112. Wiley, Buffalo (1956); Proceedings of the Third Conference on Carbon, p. 265. Pergamon Press,
New York 0958). 1129
i 13~)
G.R. I-~r~o
Pretreatment of the crystals by heating was originally carried out to remove residual impurities. However, heat treatment below 2700°C may sometimes convert an inactive impurity to a more active form. It is intended to confine the present report to a description of the details of the catalytic effects. The application of these observations to quantitative studies of quenching, of vacancy motion,~5) and of impurity diffusion, which have been carried out jointly with Dr. M. A. KA~rrER, will be published in a separate report. EXPERIMENTS Materials. Natural graphite crystals were obtained at Ticonderoga, New York. We collected these crystals in their natural matrix of limestone from which they were separated in the laboratory by hydrochloric acid. The crystals were purified by boiling alternately in HCI and HF and by floatation. The crystals were nearly exclusively non-pitting. Their behaviour was indistinguishable from crystals purified further by heating in chlorine to 900°C or heating in helium to 3000°C and cooling slowly. Some experiments were carried out with commercial "Superflake" crystals which had been purified at 3000°C and cooled slowly. Colloidal metal suspensions were prepared by rotating a rod of the desired metal against a wire contact of the same material. The contact was submersed in acetone and caused to arc by applying about 30 V a.c. across the contact. The resulting suspensions were light gray to brown. A few drops placed on the crystal added of the order of one metal atom to 105 carbon atoms. Colloidal gold solutions in water were also preparedt6) by reduction of the chloride with citric acid and dialysis but did not spread well on the crystals and were not sufficiently free of salts to yield unambiguous results. Oxidation. To study the effects of catalysts, particularly on pitting graphite, crystals were cleaved to expose fresh internal surfaces containing only a few irregularities and surface steps. The catalyst was then applied and the crystals were transferred to silica or platinum plates and reacted on these supports in dry or wet oxygen. Wet oxygen had been saturated with water at 20°C. Dry oxygen had been passed through a column, at least 12 inches long, of magnesium perchlorate. To study conditions of very dry oxidation, the crystals were heated to 950°C in helium which had been passed through perchlorate and through a trap cooled with liquid nitrogen. After cooling the crystals to the desired temperature of combustion, oxygen was added to the helium stream. Microscopy. All microscope and electron microscope examinations were confined to the cleavage surfaces of the crystal and to those steps and edges visible by examination parallel to the c-axis. To examine the crystals in the electron microscope, a carbon replica was prepared. The crystal was either pre-shadowed with chromium, or the replica shadowed after separation from the crystal. The separation was accomplished by strengthening the replica with a heavy layer of evaporated gold, reacting the crystal with sulphuric acid contaming 10 per cent of nitric acid which caused the crystal to peel off the replica and floating the replica on aqua regia to dissolve the gold backing. The replicas were examined in an RCA E.M.-3 electron microscope. RESULTS Non-pitting catalysis. Since the crystals were destroyed during replication for electron microscopy, the action of the catalysts had to be inferred from their distribution after a measured period of oxidation. Fig. 1 shows a typical arrangement. Some of the catalyst particles (A) were found at the tip of channels in the crystal surface. The channel sides were nearly perpendicular to the cleavage surface. The channel diameter increased with distance from the particle. All channels started at (5) G. HENNIG and M. KANTER,Proceedings of the Fourth Conference on Carbon, p. 141. Pergamon Press, New York (1960). (6) J. TURKEVITCH,J. HmLmR and P. C. STEVENSON, Discussions Farad. Soc. No. 11, 55 (1951).
Catalytic oxidation of graphite
I 131
steps (B) running irregularly across the crystal surface. The length of the channels was variable. Many catalyst particles (C) were located on the cleavage surface some distance from the steps, these particles have not produced channels. The observations are interpreted as showing that catalysts remained inactive on the cleavage surface, until they came in contact with steps which moved along the surface due to uncatalysed oxidation. Once undermined by a step, a particle descended partly or completely to the bottom of the step and commenced to catalyse the oxidation along its contact with the "riser" of the step. This contact area became the tip of a channel which resulted from the accelerated oxidation at the contact area. The channels produced by iron particles were usually curved and irregular (Fig. 2); those produced by gold were nearly straight. The orientation of straight channels after oxidation in moist oxygen was preferentially along {2II0} and equivalent directions, whereas it tended to {10i0} in dry oxygen after drying the crystals. It had been observedt 7) that uncatalysed oxidation of graphite in moist oxygen causes preferential orientation of surface steps to {1010} and in dry oxygen to {2Ii0}; therefore the direction of the catalytic channels is perpendicular to and the orientation of the walls of the channel is parallel to the preferred orientation of uncatalysed steps. Reactivity of non-pitting catalysts. The width of the channel at the active tip was equal to the diameter of the catalyst particle. The width increased with distance from the catalyst due to uncatalysed oxidation of the channel sides. Therefore, the difference in half-width between base and tip of a channel divided by the channel length equals the reactivity ratio of uncatalysed to catalysed surface atoms. This ratio was found to increase with temperature as shown by the channels of Fig. I and Fig. 3. An Arrhenius plot of this ratio yielded an apparent activation energy difference of about 36 kcal. Since the activation energy for the uncatalysed oxidation of these crystals was 46 kcal,t 4) the activation energy of the catalysed reaction was 10 kcal. This value constitutes an average for measurements on many different channels. The value is far from exact because at any given temperature the channel dimensions varied between wide limits. Part of this variation in catalyst activity was found to be due to particle size. Very small and very large particles were less active than particles of several hundred Angstrom diameter. The catalytic activity of particles of comparable size depended also upon the channel depth which was determined from the shadow on the replica. The activity was highest for shallow channels. This correlation of channel depth and catalytic activity is apparent from the data of Table 1, which are, however, further complicated by particle size effects. The correlation was also apparent from visual examination of bent channels. Bends in channel direction often occurred when the active particle at the tip of the channel encountered a surface step. The particle was deflected away from ascending steps because they retarded the particle and towards descending steps which accelerated the motion. Channels of depth exceeding the particle radius have never been observed. When a particle interacted with a step higher than its own radius, its catalytic action produced one of three different features, a channel with a ledge, a pit or a serrated step. Usually only one of these features predominated hut the conditions favouring each are not known. A ledge at the base of the channel indicated that the particle had activated the oxidation of only the top of the step. The uncatalysed portion of ~¢IG.R. HENNIG,Proceedingsof the Fifth Conference on Carbon.Pergamon Press. Oxford (1962).
1132
G . R . Hm,rr~G
the step advanced at the uncatalysed rate and remained as a ledge at the base of the channel. Several such ledges are present in Figs. 1-4. The catalyst sometimes produced pits originating on the step and extending parallel to the cleavage plane. They probably would not be detected by electron microscopy but on rare occasions TABLE 1.---CATALYTIC ACTIVITY
d*
d[c*
r*
1450 660 330 300 180 210
5.2 4.4 3.5 3.5 2.1 2.0
70 80 18 26 19
15
*d --- particle diameter in Angstrom c = channel depth r = ratio of catalytic to uncatalysed burning rate were seen in the optical microscope. Pits of this type produced interference colours which are shown by contrast in Fig. 5. Such features were found occasionally on cleaved crystals, principally on steps originating at screw dislocations. In Fig. 5, the dislocation has been etched away leaving a large hole. The pits of Fig. 5 are situated at various depths as shown by the differences in contrast and confirmed by careful cleaving. The most frequently observed mode of action of catalysts at high steps was serration. The particle remained attached to the step and accelerated the oxidation parallel to the step (Fig. 6). The catalyst activity decreased with the age of the catalyst suspension. The activation energy for an aged solution of colloidal gold was measured to be 30 kcal. After extensive oxidation, the catalyst particles tended to coagulate (Fig. 4). Pitting catalysis. Various treatments were found to modify graphite crystals so that they pitted during subsequent catalytic oxidation. These treatments are irradiation with high energy neutrons or electrons, mechanical deformation, or suitable heat treatment. Heating to temperatures above 2700°C followed by quenching usually induced subsequent pitting behaviour probably because excess lattice vacancies were introduced. In crystals which had not previously been oxidised or heat treated, pitting could also be induced by heating for several hours to temperatures between 2400 ° and 2600 °. The rate of subsequent cooling had no influence if it exceeded one degree per minute but very slow cooling rates removed the pitting tendency. Heat treatment below 2700 ° caused a gradual penetration of this tendency into the crystal. As an example, Fig. 7 shows a cleaved and catalytically oxidised section of a crystal heated for one hour at 2600 °. Pitting occurred in a region paralleling the outer surface of the crystal and also along a linear source crossing the crystal which may have been a twin line or low angle boundary present before cleaving. Screw dislocations were also observed to act as sources for the pit forming tendency. This pitting induced below 2700 ° is probably associated, not with excess lattice vacancies, but with a diffusing impurity. The effect could be enhanced by heating the graphite between 2400 and 2600°C with boric acid but was not enhanced by heating with silica or vanadium chloride. The pitting tendency induced in this temperature range
FIG. 1.--Colloidal gold on graphite. One hour oxidation at 550°. A, active catalyst; B, step; C, inactive catalyst; D, crystal bent due to {2TI/} twin.
FIG. 2.--Colloidal iron on graphite. One hour wet oxidation at 500°. FIG. 3.--Colloidal gold on graphite. One quarter hour wet oxidation at 650 °. FIG. 4.--Coagulation of colloid particles. Gold on graphite. FIG. 5.--Pitting along a-axis. Colloidal gold on graphite. Wet oxygen at
700°C. Optical micrograph.
FIG. 6.--Serrated step. Colloidal gold on graphite. One half hour wet oxygen at 650 ° . FIG. 7.--Pitting tendency partly diffused into graphite. Colloidal gold catalyst. Optical micrograph. FIG. 8.--Initial channelling on pitting graphite. Colloidal gold. One hour low pressure oxygen at 700 ° . FJc. 9.--Pitting by colloidal gold. Two and a half hours low pressure oxygen at 750 °. Twin line showed that pit edges are {10T0}.
FIG. 10.--Irregular hexagonal pit. Colloidal gold. One hour oxygen at 650°. Shadowed after replication. FIG. 11.---Apparent spiral pit. Two hours oxygen 650°. FIG. 12.--Islands of vanadium catalyst on pitting graphite. Extensive oxidation. FIG. 13.--Same material as in Fig. 12, washed and burned ½ hour at 600°.
Catalytic oxidation of graphite
1133
is therefore probably due to boron impurities present on the crystal surface and diffusing, presumably substitutionally. No distinction will be made in the following between pitting due to boron or due to quenched-in vacancies since no qualitative differences in pitting behaviour had been noted and since quantitative measurements are still in progress. Pitting due to irradiation or mechanical deformation will not be described here. When metallic catalysts were not added, the oxidation of crystals which had been pretreated to induce pitting behaviour resulted in the development of only a small number of random pits on the cleavage surfaces. In wet oxygen this spontaneous pit formation was more pronounced than in dry oxygen. Oxidation of pitting crystals which had been contaminated with suspensions of colloidal metal caused very extensive pitting. The progress of this pitting was followed by replicating crystals after progressively increased periods of oxidation. Initially the catalyst particle produced a channel on the surface similar to the channels formed on non-pitting crystals; however, nearly every catalyst particle produced such a channel (Fig. 8), whereas on non-pitting crystals only those particles which were in contact with steps produced channels. Furthermore, on pitting crystals, the channels became progressively deeper; the rate of penetration increased with the defect (boron or vacancy) concentration. When the channel depth was about equal to the radius of the particle, lateral motion became restricted and actual pit formation commenced (Fig. 9). Temperature effects on pitting catalysis. The length of the initial channel decreased as the temperature of oxidation increased. The crystals did not pit at 600 °, unless heavily doped with boron, but at 800 ° the initial channel was very short and steep. The shape of the pits produced after extensive oxidation was also found to be a function of the temperature, being steeper, the higher the temperature of oxidation. The activation energy for catalytic oxidation parallel to the c-axis can be estimated from the pit shape. The burning rate at the rim of a pit is uncatalysed and should therefore show the activation energy of 46 kcal measured for uncatalysed burning of these crystals. The depth of the pit, determined from the shadow length and shadow angle of replicas, is an index of the rate of catalytic penetration. The ratio of width to depth was of the order of ten but was temperature dependent. An Arrhenius plot of this ratio yielded a value of --10 kcal for the difference between the activation energies of uncatalysed and catalysed oxidation. The activation energy of catalysed penetration is therefore 56 kcal/mole. The pits produced by colloidal gold or iron in wet oxygen consisted of rather high (> 500 A) steps oriented 10i0 or equivalently (Fig. 9). At low pressures of oxygen, the pits became perfectly hexagonal in shape. Oxidation at high oxygep. pressures and tow temperatures resulted in less regular pits which were, however, still bounded by steps consisting of straight sections of preferred orientation (Fig. 10). Near the bottom of the pit the steps were often of lower height and closer together than at the top. In dry oxygen the burning behaviour usually did not vary appreciably unless an effort was made to remove residual moisture from the crystals by outgassing in dry helium at 950°C. Such dried crystals usually showed less tendency to pit than moist crystals. An appreciable fraction of catalyst particles was found to remain inactive on the surface, the fraction depending also on the boron or defect concentration of
1134
G.R. HENNIO
the crystal. The pits which were produced under very dry conditions were usually oriented so that their sides were (2iI0}. Some of the steps were not parallel to the c axis but were high index pyramidal surfaces. The proportion of such pyramidal surfaces increased with the temperature of oxidation. Spiral pits. Occasional pits appeared, particularly in the optical microscope, to be spiral shaped. Such apparent spirals were always found to be actually pyramidal, the spiral appearance being due to a local crowding of steps which was probably caused by oscillations of the catalyst particle (Fig. 11). Reactivity of pitting catalysts. The dependence of the catalyst reactivity on moisture and on temperature has already been described. Some differences were noted between various metals. Colloidal gold and iron appeared equally active under moist conditions of oxidation. In dry oxygen, iron was considerably more active than gold in forming pits. Both substances appeared to sinter at 900 ° into more uniformly round particles which initiated more symmetrical pits or straighter channels. Nickel, platinum, and silver appeared to show catalytic activity comparable to gold. Aluminium appeared to be less active. Colloidal suspensions of vanadium produced a very much larger enhancement of oxidation rate than other metals tested. Occasional vanadium catalyst particles were seen on these crystals at the tip of catalytic channels, but, in addition, the reactivity of all steps on non-pitting crystals and of all surfaces on pitting crystals seemed to be considerably enhanced. Irregular channels indicating excess reactivity were often observed which did not carry visible catalyst particles at their tip. Furthermore those particles which could be found at channel tips appeared to wet the graphite surface and conform to the channel shape. The vanadium catalyst was apparently present as a liquid which wetted and flowed over the graphite surface. This suggests that the catalyst was present as the low melting pentoxide. Frequently, residues of less active catalyst remained in isolated positions, possibly as vanadium carbide (Fig. 12). Water at 25°C reactivated such particles; they retained their shape below 600 ° but melted above this temperature (Fig. 13). The melting point of bulk vanadium pentoxide is 680 ° but apparent melting of very small particles due to sintering, surface flow, etc. may occur at lower temperature. DISCUSSION The observations demonstrate several details of the mechanism of catalytic oxidation. All the metals studied except vanadium produced catalytic effects only at the interface of a rather sizeable metal particle and the substrate. No evidence was found for any catalysis by individual metal atoms separating from the colloid, the particles did not become smaller, but actually often increased in size by coalescing. In a few experiments, solutions of metal salts were placed on graphite. After brief oxidation, these metals were again found to have coagulated into active colloidal particles. The metallic particles did not cause catalysis by action at a distance, as for instance by altering the Fermi level of conduction electrons in the graphite. The shape of the channels produced on the graphite surface showed that uncatalysed oxidation occurred up to the immediate vicinity of a catalyst particle. What caused the motion of the colloidal particles which produced the observed channels ? Brownian motion would cause a 200/It particle to be displaced 0.05 mm/ second at 500 ° in air unless retarded by friction against the crystal surface. However, this friction or bonding of the particle to the cleavage surface appears to prevent
Catalytic oxidation of graphite
1135
Brownian motion nearly completely since the colloid particles remain randomly distributed instead of moving to surface steps and becoming catalytically active. The forces between the particle and the cleavage surface are probably van der Waals forces. The motion of a particle along its catalytic channel is probably due to additional attractive forces to the carbon atoms along the edge of the channel tip. Since these carbon atoms are constantly being oxidised, only the leading surface of the catalyst particle can be in contact with edge atoms. Thus the attractive forces are unbalanced and cause motion of the particle along the channel direction. Motion by this mechanism rather than by Brownian motion is also deduced from the observation that active particles are always located in the tip of the channels. Particles located elsewhere in the channel are inactive. The straightness and preferential orientation of the channels can be caused by anisotropic reactivity. If the catalysed oxidation at the interface is faster in certain crystallographic directions, the resulting motion which maintains maximum contact area between particle and graphite, and thus the channel direction, will be perpendicular to one of the directions of fastest oxidation (Fig. 14). It should be noted
///
//~.
¢;I~-s.~S;;~S2SS ~"
0° b° FI~. 14.iMotion of catalyst. Dashed lines: position near beginning of catalysis. Solid fines: position when channel has formed. Arrows denote directions of highest reactivity. Burning along these directions is highly exaggerated in drawing. that these directions of fastest catalysed oxidation coincide with the directions of fastest uncatalysed oxidation since the orientation of the channel sides is usually parallel to the orientation of surface steps in uncatalysed burning, even when this preferred orientation is altered by the action of water. The catalytic activity of colloidal particles of a given size is lower in deep than in shallow channels. Since the depth of a channel in non-pitting crystals is determined by the height of the step at which catalysis commenced, this correlation cannot be due to inherent differences between particles but may be due to differences in the
1136
G.R. I-ImqraG
area of the particle-graphite interface. The diffusion length of oxygen or reaction products in this interface increases with the channel depth, thus slowing the reaction. When the step height exceeds the particle radius, the particle usually does not form a catalytic channel but moves parallel to the surface step. In this case, the accumulation of reaction products probably prevents the formation of three lines of attachment required for channel formation (Fig. 14). When attached to the step as in Fig. 6, the particle probably changes its area of attachment constantly thereby facilitating the release of reaction products from the contact area. Mechanism of catalysis. Three sets of observations on non-pitting crystals yield information on the mechanism of catalysis. These are the preferred direction of catalytic attack, the motion of the catalyst particles, and tho measured activation energies. The uncatalysed oxidation of graphite proceeds in several steps which probably include adsorption of oxygen, activation and dissociation of the oxygen, formation of mobile or immobile surface oxide complexes, activation and rupture of carbon bonds between complexes and interior carbon atoms, and removal of adsorbed reaction products. Of these partial reactions, the step which determines the crystallographic orientation of the oxidised surface is probably the rupture of carbon-carbon bonds. It is likely that this particular step in the reaction is not affected by metal catalysis because the preferred orientation is insensitive to catalysis as described earlier. The observed motion of the catalyst particles demonstrates that bonds are formed between the catalyst and a large number of reactive carbon atoms at the metal-c arbon interface. Since this bonding may be incidental to the catalytic action, it does not reveal which of the reaction steps is affected or by-passed by the catalytic action. If the bonding were concomitant with the catalytic action, activation of adsorbed oxygen to form metal-oxygen-carbon bridges would be a likely mechanism of catalytic action. The measured activation energies for non-pitting catalysis demonstrate that the catalysts cause a pronounced change in the pre-exponential term of the Arrhenius rate equation. The observed reactivity ratio of catalysed to uncatalysed oxidation of carbon atoms was measured to be about 50 at 550°C (Table 1). The activation energy was reduced by catalysis from 46 to about 10 kcal/mole. Therefore the preexponential term was reduced by a factor of 108 due to catalysis. Such a large change suggests that the catalyst does not merely accelerate one of the reaction steps of oxidation but affects several steps, or bypasses the normal reaction sequence possibly by the formation of metal-oxygen-carbon bridges. Pit formation appears to be initiated by the accelerated oxidation at those vacancies and/or boron atoms which are present in the initial contact region between catalyst and graphite. This attack is followed by catalytic oxidation of edge atoms until the hole in the surface layer of the crystal is large enough to expose additional vacancies or boron atoms in the next layer. As these are oxidized in turn, the particle gradually penetrates into the crystal while producing a surface channel. When the channel depth has increased to the particle radius, lateral motion is slowed but penetration continues resulting in a pit. The stepped geometry of the pits is not due to periodic bursts of catalytic activity but to excessive reactivity of pyramidal faces initially produced. Even in the absence of metallic catalysts, such pyramidal faces
Catalytic oxidation of graphite
1137
are nearly always converted to prismatic faces. The reason for this preferential burning to prismatic (i.e. "vertical") steps on graphite is not understood but is believed to be due to catalysis by traces of water. The temperature dependence of the catalytic penetration yielded an apparent activation energy of 56 kcal. It is not surprising that this value is higher than the activation energy of uncatalysed oxidation because the penetration reaction involves oxidation of carbon atoms adjacent to vacancies or boron atoms which may be more tightly bonded than carbon atoms at edges of layer planes. The conclusions do not apply to vanadium catalysts which melt and wet the surface.
Acknowledgements--The author is indebted to J. BALDWIN,P. ROACHand C. KAZMIEROWICZ for help in sample preparations. The author gratefully acknowledges the co-operation of the electron microscope group of the Biology Division and particularly O. T. MI~CK in making the instrument available for these studies.