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Controlled atmosphere electron microscopy studies of graphite gasification—The catalytic influence of vanadium and vanadium pentoxide
CONTROLLED ATMOSPHERE ELECTRON MICROSCOPY STUDIES OF GRAPHITE GASIFICATION-THE CATALYTIC INFLUENCE OF VANADIUM AND VANADIUM PEWTOXIDE R. T. K. BAKER, R. B. THOMAS and M. WELLS Applied Chemistry Division, Atomic Energy Research Establishment, Harwell, Didcot, Oxfordshire, England (Receiued 2 September 1974) Abstract-CAEM has been used to examine the behaviour of V and VzOSas catalysts in the graphite-oxygen reaction. The simil~ities in both qualitative and quantitative effects indicates that the same reactive species is operative in both systems. The pronounced activity of these catalysts is attributed to their ability to wet and spread over the graphite surface at moderately low temperatures, It is also shown that V,O, has the ability to penetrate into graphite and that catatvtic attack occurred mainlv _ . _ bv. a channeliing mode, although it is possible that the intercalated material may exert a catalytic influence on the reaction
1.
INTRODUCTION
It was reported by Hennig[l] that vanadium was amongst the most active catalysts for the oxidation of graphite. He attributed this abnormal behaviour to the fact that the catalyst was present as a liquid, probably vanadium pentoxide, which wetted and flowed over the graphite surface. Catalytic channels were formed and often did not carry a visible catalyst particle at their tip; particles which were found at channel tips appeared to wet the graphite surface and conform to the channel shape. In a very limited investigation on this system, Hedleyf21 found that when vanadium-graphite specimens were heated in the presence of oxygen within the electron microscope the vanadium nucleated ta form three different particle ~rangements. Laths, which he identified as vanadium pentoxide, induced very little oxidation; spherical particles, about 100nm dia., attacked the graphite at 845 K to produce channels which originated from crystal steps; and a layer of material, identified as vanadium pentoxide, which was relatively immobile and bad a less vigorous effect on graphite oxidation. From bulk measurements, Heintz and Parker[3] obtained a value of 219K .I. mole-’ for the activation energy for the vanadium catalysed oxidation of graphite in air. McKee[41 studied the oxidation of graphite in the presence of a vanadium pentoxide catalyst using thermogravimetry and optical microscopy techniques. The thermogravimetric analysis showed a sharp transition at 948 K when a 1: 1 mixture of vanadium pentoxide and graphite was heated in nitrogen. An X-ray diffraction examination of the product obtained at 1373K revealed the presence of V6013as well as residual VZO~.The optical microscopy studies of vanadium pentoxide on graphite in 100k . Pa oxygen showed that the oxide particles became molten at 923 K and rapid cbanneiling by these particles on the graphite surface occurred at 973 K. In the present study controlled atmosphere electron microscopy (CAEM) has been used to obtain both qualitative and quantitative information on the catalytic
influence of both the metal and pentokide on the g~phite-oxygen reaction. 2.EXPEZRIMENTAL
The CAEM technique has been described previously [S]. Vanadium pentoxide was introduced on single crystal graphite (Ticonderaga, New York State) transmission specimens as an atomised spray from an ultrasonic dispersion of the oxide powder in alcohol and was present initially on the graphite as a thin film. Vanadium metal was deposited on the graphite as a thin film by vacuum evaporation of a small fragment of spectrographically pure vanadium from a heated tungsten foil, at a residual pressure of 2.6 m ,Pa. Oxygen was obtained from BOC Ltd., with a stated purity df >99% and used without further purificatjon. In some experiments the reaction was terminated at a pre-determined stage, specimens were removed from the cell and surface topo~a~hy subsequently examined after shadowing the specimens with carbon platinum at an angle of 15”. 3. RESULTS 3.1 Behaviour of vanadium and vanndium pentoxide on graphite in fhe presence of oxygen The behavioural pattern of both the metal and
pentoxide in these reactions as identical, differing only in the temperatures at which some events occurred. In order to avoid repetition the results from these two systems will therefore be treated together. When heated in 0% k , Pa oxygen (V at @OK, V20J at 745 K) the evaporated films nucleated to form small particles of about 1Onm dia. As the temperature was raised to 795 K the behaviour of both systems was the same and quite unlike any other catalyst-graphite-oxygen system stydied so far in this laboratory, It was apparent that not only had the particles grown in size, but also several forms of material were prgressively formed classified in three groups, One of these, laths, Fig. I, varied in length (0.1 pm to 2 pm) and width (10 nm to 141
142
R. T. K.
Fig. 1. Transmissionmicrographshowing lath formation after heatingV&-graphite in 066 k.Paoxygenat 795K.
BAKERet al.
Fig. 3. Transmission micrograph showing irregular shaped electron dense islands formed after reaction of V-graphite in 066 k.Pa
oxygenat 820K. 2.50nm) with 120”angles discernable across their ends. In general the laths were orientated along the (lOi0) and (11~0) directions of the substrate graphite. This material was immobile and persisted on the surface over a narrow range of temperature, rearranging to spherical particles at 820 K. The other two groups formed after the disappearance of laths were ribbons and islands. The ribbons were straight and interconnected structures giving some areas of the specimen a mosiac-like appearance, Fig. 2. Like the laths they were orientated in (lOi0) and (1120) directions on the graphite surface, but differed from laths in that they appeared to be quite fluid and spread along these directions giving the appearance of growth. In many cases two or more “growing” ribbons would collide and material was seen to be transferred from one to another. From the selected area electron diffraction examination of a ribbon which protruded over the edge of a graphite specimen it was possible to identify this material as VdOu. The electron dense island material, shown in Fig. 3, was seen over most areas of the specimen. Individual islands covered areas of up to 1Ohrn’ and underlay the other forms of material. In some cases areas of three different
Fig. 2. Ribbon formation producedfrom heating V-graphitein O&i k.Pa oxygen at 820K.
islands were seen to overlap each other without any apparent interaction. Particles on both surfaces of the specimen passed over the islands in an uninterrupted path, suggesting that the islands were within the graphite structure as intercalated material rather than on the surface. This suggestion was later confirmed in experiments where the reaction was terminated with islands still present. Subsequent shadowing of both sides of the specimen produced shadows around ribbons and spherical particles, but not around the islands, indicating that they were less than 2nm in height or within the graphite structure. From detailed examination of the formation of ribbons and islands it was clear that they were both produced as a result of the transformation of some of the spherical particles. Those particles which were the precursors of the island material were invariably situated at edges or steps on the graphite surface and appeared to evaporate from the surface suddenly. On repeated viewing of the video record it was clear that this apparent particle loss was always associated with the formation of an island some distance away from the original particle, i.e., the particle had probably diffused into the graphite structure. Electron diffraction examination of a particularly dense area of the intercalated material showed that this was VzOl. Particles responsible for producing ribbons were usually located on the more perfect regions of the graphite, and had no characteristic differences from those which produced islands. It was significant that when either vanadium or vanadium pentoxide were supported on an amorphous carbon film and heated in oxygen these three material structures were not formed. As the temperat~e was raised to 835 K the islands were observed to spread and get perceptibly thinner. It was noticeable that while the material was spreading considerable strain was induced in the graphite and this motion was arrested if the advancing edge reached a twin boundary. At 835 K the first signs of catalytic oxidation of graphite were seen. Holes developed in the graphite around some of the larger spherical particles (about 350 nm. dia.) which then expanded at a much faster rate than would be expected from uncatalysed oxidation to form shallow hexagonal pits. These catalyst particles then changed their
The catalytic influence of vanadium and vanadium pentoxide
mode of attack to produce shallow channels which emanated from the edges of the pits. When this change in catalytic behaviour occurred the catalyst particle appeared to gather up material from around the sides of the pit before proceeding to cut the channel. This event was accompanied by a decrease in the rate of expansion of the pit. The particles propagating channels were quite liquid-like, took on hexagonal facets at the graphite-catalyst interface, and were frequently observed to split up into two or more smaller active particles. During this stage of the reaction it was often observed that both the size of the catalyst particles and width of channels they were propagating got progressively smaller as the channel increased in length. This behaviour was accompanied by the gradual formation of intercalated material, at some distance from the leading face of the catalyst particle, leaving behind a channel without a catalyst particle at the tip. This phenomenon is illustrated in the sequence of micrographs, Figs. 4a to 4c, where the dark shadow seen on the right-hand side of Fig. 4c is the intercalated material (Y). There was no indication that material was evaporating from the surface at this stage in the reaction. Although there was no direct evidence to suggest that the intercalcated material was exerting a catalytic influence on the oxidation of graphite it was clear that graphite in close proximity to this material got significantly thinner compared with the rest of the surface. Frequently specimens would collapse if held at 900 K for prolonged periods. As the temperature reached 945 K several different events were observed. In some cases islands of intercalated material reorganised to form initially long needlelike structures, which were quite mobile and did not induce strain in the graphite as they moved, thus indicati~ they were on the surface of the graphite. After a short period of time the needles started to “ball up” and form spherical mobile particles (0.1 to 1*OKm dia.). When first formed these particles were transparent towards the electronlbeam but rapidly contracted in size becoming quite dense. Figures 5a to 5d show the re-structuring behaviour of the intercalated material with the eventual formation of particles. Other intercalation islands were observed to reorganise directly to large diffuse transparent particles over 2 pm dia. At this temperature the majority of particles were creating channels and the rate of channel propagation had increased appreciably. The ribbons were also undergoing a rapid contraction with the eventual formation of spherical particles. By MOOK there was a general reduction in the amount of material present on the surface, intercalation islands and ribbons had completely disappeared and all remaining material was now in the form of mobile particles up to 10 pm dia. Particles did not leave a residue when they disappeared from the surface. i.e. at this temperature evaporation and not intercalation was occurring. In many eases the diameters of active particles were much larger than the widths of channels they were creating, and often such particles would quite suddenly leave the surface. This particle size-channel width relationship can be seen in Pig. 6, which is a micrograph taken after the reaction had been terminated. Clearly some re-crystallisation of
143
Fig. 4a-c. Sequenceshowinggradualdecreasein sizeof a catalyst particle(X)cu~inat~ in the formation of an island (Y) at 850K.
R. T. K. BAKER eta/.
Fig. 5a-d. Sequence showing the restructuring I the island material at 945K in 0.66 k&i oxygen.
,
ture dependence of channel propagation rate for 340 nm dia. particles cutting channels of similar depth, was identical for both vanadium and vanadium pentoxide systems. The relationship is shown as an Arrhenius plot in Fig. 7, and from the slope of the line an activation energy of 221220 k 3. mole-’ was evaluated for the catalysed reaction. 4.DISCUSSION
Fig. 6. Transmission micrograph taken after the reaction of V,O,-graphite with 0.66 k.Pa oxygen at lOOOK,showing the diameter of a catalyst particle to he much larger than the width of the channel it has created. the catalyst particle has occurred during the cooling operation. Quantitative kinetic analysis showed that, the tempera-
Controlled atmosphere electron microscopic study of the catalysis of the graphite-oxygen reaction by either vanadium or vanadium pentoxide has shown the qualitative and quantitative similarity in behaviour of the metal and pentoxide, suggesting that the active species is the same in both systems. This finding is in contrast to the differences found for molybdenum and moly~enum trioxide[(i]. Although other workers have used microscopy techniques to investigate both these systems[f, 2,4] they have not reported the possible formation of intercalated material. The existence of such material is shown by
The catalytic influence of vanadium and vanadium pentoxide
14s
catalytic oxidation obtained here and that found by Heintz and Parker, 219k J . mole-‘, from bulk
2.05 @ c 5 ,u p 1.0-
Fig. 7. Arrehuis plot of vanadium 0, and vanadium pentoxide 0, catalysed rates at 0.66 k.Pa oxygen.
continuous observations of its formation. This study probably represents the first time anyone has seen the process of forming intercalated groups in graphite. Several workers have used electron microscopy to examine such compounds with sodium, potassium[7] and ferric chloride181but there are no reports of the formation of an intercalation compound of vanadium pentoxide with graphite. The present study merely shows that the pentoxide has the ability to enter between the layers of the graphite crystal. The existence of the type of material, shown in Figs. 4a to 4c could explain Hennigs’ observation[l] that catalytic channels were formed with vanadium at 873 K with no visible particle at their tip. One cannot be certain whether the intercalated material exerts a catalytic influence on the oxidation of graphite. The agreement between the activation energy for
experiments[3] is excellent. There is little doubt that the high activity found for both vanadium and vanadium pentoxide catalysts is associated with the ability of these materials to wet and spread over the graphite surface. Although the bulk melting point of vanadium pentoxide is 963 K, it is well-known that small metal and metal oxide particles show liquid-like behaviour at temperatures well below that of their bulk melting points and therefore it is not unexpected to find such phenomena as wetting and particle coalescence at about 800 K. At 1000K there are indications that evaporation of material from the surface is taking place. Under these conditions particles would tend to have a relatively small area of contact with the surface, which would account for the observation at this temperature of active particle diameters being larger than the width of channels they were creating. One would also expect to find a number of channels without particles at their head for the same reasons. 5. CONCLUSIONS
Vanadium and vanadium pentoxide have been shown to be extremely active catalysts for the oxidation of graphite. It has been demonstrated that V20s has the ability to enter between the layers of graphite crystals. The similarity in behaviour of the two systems suggests that the same reactive species is involved. At higher temperatures the catalyst tends to evaporate from the graphite surface. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
Hennig G. R., J. Inorg. Nucl. Chem. 24, II29 (1962). Hedley J. A., M.Sc. Thesis, Newcastle University (1962). Heintz E. A. and Parker W. E.. Carbon 4. 473 (1966). McKee D. W., Carbon 8, 623 (1970). Baker R. T. K. and Harris P. S., J. Phys. E. 5 (1972). Baker R. T. K., Harris P. S. and Kemner D. J.. Carbon 12.179 (1974). Halin M. K. and Jenkins G. M., 3rd Conf Industrial Carbons and Graphite, p. 53. London, (I 970). Heerchap M. and Delavignette P., Carbon 5, 383 (1967).
1.
INTRODUCTION
It was reported by Hennig[l] that vanadium was amongst the most active catalysts for the oxidation of graphite. He attributed this abnormal behaviour to the fact that the catalyst was present as a liquid, probably vanadium pentoxide, which wetted and flowed over the graphite surface. Catalytic channels were formed and often did not carry a visible catalyst particle at their tip; particles which were found at channel tips appeared to wet the graphite surface and conform to the channel shape. In a very limited investigation on this system, Hedleyf21 found that when vanadium-graphite specimens were heated in the presence of oxygen within the electron microscope the vanadium nucleated ta form three different particle ~rangements. Laths, which he identified as vanadium pentoxide, induced very little oxidation; spherical particles, about 100nm dia., attacked the graphite at 845 K to produce channels which originated from crystal steps; and a layer of material, identified as vanadium pentoxide, which was relatively immobile and bad a less vigorous effect on graphite oxidation. From bulk measurements, Heintz and Parker[3] obtained a value of 219K .I. mole-’ for the activation energy for the vanadium catalysed oxidation of graphite in air. McKee[41 studied the oxidation of graphite in the presence of a vanadium pentoxide catalyst using thermogravimetry and optical microscopy techniques. The thermogravimetric analysis showed a sharp transition at 948 K when a 1: 1 mixture of vanadium pentoxide and graphite was heated in nitrogen. An X-ray diffraction examination of the product obtained at 1373K revealed the presence of V6013as well as residual VZO~.The optical microscopy studies of vanadium pentoxide on graphite in 100k . Pa oxygen showed that the oxide particles became molten at 923 K and rapid cbanneiling by these particles on the graphite surface occurred at 973 K. In the present study controlled atmosphere electron microscopy (CAEM) has been used to obtain both qualitative and quantitative information on the catalytic
influence of both the metal and pentokide on the g~phite-oxygen reaction. 2.EXPEZRIMENTAL
The CAEM technique has been described previously [S]. Vanadium pentoxide was introduced on single crystal graphite (Ticonderaga, New York State) transmission specimens as an atomised spray from an ultrasonic dispersion of the oxide powder in alcohol and was present initially on the graphite as a thin film. Vanadium metal was deposited on the graphite as a thin film by vacuum evaporation of a small fragment of spectrographically pure vanadium from a heated tungsten foil, at a residual pressure of 2.6 m ,Pa. Oxygen was obtained from BOC Ltd., with a stated purity df >99% and used without further purificatjon. In some experiments the reaction was terminated at a pre-determined stage, specimens were removed from the cell and surface topo~a~hy subsequently examined after shadowing the specimens with carbon platinum at an angle of 15”. 3. RESULTS 3.1 Behaviour of vanadium and vanndium pentoxide on graphite in fhe presence of oxygen The behavioural pattern of both the metal and
pentoxide in these reactions as identical, differing only in the temperatures at which some events occurred. In order to avoid repetition the results from these two systems will therefore be treated together. When heated in 0% k , Pa oxygen (V at @OK, V20J at 745 K) the evaporated films nucleated to form small particles of about 1Onm dia. As the temperature was raised to 795 K the behaviour of both systems was the same and quite unlike any other catalyst-graphite-oxygen system stydied so far in this laboratory, It was apparent that not only had the particles grown in size, but also several forms of material were prgressively formed classified in three groups, One of these, laths, Fig. I, varied in length (0.1 pm to 2 pm) and width (10 nm to 141
142
R. T. K.
Fig. 1. Transmissionmicrographshowing lath formation after heatingV&-graphite in 066 k.Paoxygenat 795K.
BAKERet al.
Fig. 3. Transmission micrograph showing irregular shaped electron dense islands formed after reaction of V-graphite in 066 k.Pa
oxygenat 820K. 2.50nm) with 120”angles discernable across their ends. In general the laths were orientated along the (lOi0) and (11~0) directions of the substrate graphite. This material was immobile and persisted on the surface over a narrow range of temperature, rearranging to spherical particles at 820 K. The other two groups formed after the disappearance of laths were ribbons and islands. The ribbons were straight and interconnected structures giving some areas of the specimen a mosiac-like appearance, Fig. 2. Like the laths they were orientated in (lOi0) and (1120) directions on the graphite surface, but differed from laths in that they appeared to be quite fluid and spread along these directions giving the appearance of growth. In many cases two or more “growing” ribbons would collide and material was seen to be transferred from one to another. From the selected area electron diffraction examination of a ribbon which protruded over the edge of a graphite specimen it was possible to identify this material as VdOu. The electron dense island material, shown in Fig. 3, was seen over most areas of the specimen. Individual islands covered areas of up to 1Ohrn’ and underlay the other forms of material. In some cases areas of three different
Fig. 2. Ribbon formation producedfrom heating V-graphitein O&i k.Pa oxygen at 820K.
islands were seen to overlap each other without any apparent interaction. Particles on both surfaces of the specimen passed over the islands in an uninterrupted path, suggesting that the islands were within the graphite structure as intercalated material rather than on the surface. This suggestion was later confirmed in experiments where the reaction was terminated with islands still present. Subsequent shadowing of both sides of the specimen produced shadows around ribbons and spherical particles, but not around the islands, indicating that they were less than 2nm in height or within the graphite structure. From detailed examination of the formation of ribbons and islands it was clear that they were both produced as a result of the transformation of some of the spherical particles. Those particles which were the precursors of the island material were invariably situated at edges or steps on the graphite surface and appeared to evaporate from the surface suddenly. On repeated viewing of the video record it was clear that this apparent particle loss was always associated with the formation of an island some distance away from the original particle, i.e., the particle had probably diffused into the graphite structure. Electron diffraction examination of a particularly dense area of the intercalated material showed that this was VzOl. Particles responsible for producing ribbons were usually located on the more perfect regions of the graphite, and had no characteristic differences from those which produced islands. It was significant that when either vanadium or vanadium pentoxide were supported on an amorphous carbon film and heated in oxygen these three material structures were not formed. As the temperat~e was raised to 835 K the islands were observed to spread and get perceptibly thinner. It was noticeable that while the material was spreading considerable strain was induced in the graphite and this motion was arrested if the advancing edge reached a twin boundary. At 835 K the first signs of catalytic oxidation of graphite were seen. Holes developed in the graphite around some of the larger spherical particles (about 350 nm. dia.) which then expanded at a much faster rate than would be expected from uncatalysed oxidation to form shallow hexagonal pits. These catalyst particles then changed their
The catalytic influence of vanadium and vanadium pentoxide
mode of attack to produce shallow channels which emanated from the edges of the pits. When this change in catalytic behaviour occurred the catalyst particle appeared to gather up material from around the sides of the pit before proceeding to cut the channel. This event was accompanied by a decrease in the rate of expansion of the pit. The particles propagating channels were quite liquid-like, took on hexagonal facets at the graphite-catalyst interface, and were frequently observed to split up into two or more smaller active particles. During this stage of the reaction it was often observed that both the size of the catalyst particles and width of channels they were propagating got progressively smaller as the channel increased in length. This behaviour was accompanied by the gradual formation of intercalated material, at some distance from the leading face of the catalyst particle, leaving behind a channel without a catalyst particle at the tip. This phenomenon is illustrated in the sequence of micrographs, Figs. 4a to 4c, where the dark shadow seen on the right-hand side of Fig. 4c is the intercalated material (Y). There was no indication that material was evaporating from the surface at this stage in the reaction. Although there was no direct evidence to suggest that the intercalcated material was exerting a catalytic influence on the oxidation of graphite it was clear that graphite in close proximity to this material got significantly thinner compared with the rest of the surface. Frequently specimens would collapse if held at 900 K for prolonged periods. As the temperature reached 945 K several different events were observed. In some cases islands of intercalated material reorganised to form initially long needlelike structures, which were quite mobile and did not induce strain in the graphite as they moved, thus indicati~ they were on the surface of the graphite. After a short period of time the needles started to “ball up” and form spherical mobile particles (0.1 to 1*OKm dia.). When first formed these particles were transparent towards the electronlbeam but rapidly contracted in size becoming quite dense. Figures 5a to 5d show the re-structuring behaviour of the intercalated material with the eventual formation of particles. Other intercalation islands were observed to reorganise directly to large diffuse transparent particles over 2 pm dia. At this temperature the majority of particles were creating channels and the rate of channel propagation had increased appreciably. The ribbons were also undergoing a rapid contraction with the eventual formation of spherical particles. By MOOK there was a general reduction in the amount of material present on the surface, intercalation islands and ribbons had completely disappeared and all remaining material was now in the form of mobile particles up to 10 pm dia. Particles did not leave a residue when they disappeared from the surface. i.e. at this temperature evaporation and not intercalation was occurring. In many eases the diameters of active particles were much larger than the widths of channels they were creating, and often such particles would quite suddenly leave the surface. This particle size-channel width relationship can be seen in Pig. 6, which is a micrograph taken after the reaction had been terminated. Clearly some re-crystallisation of
143
Fig. 4a-c. Sequenceshowinggradualdecreasein sizeof a catalyst particle(X)cu~inat~ in the formation of an island (Y) at 850K.
R. T. K. BAKER eta/.
Fig. 5a-d. Sequence showing the restructuring I the island material at 945K in 0.66 k&i oxygen.
,
ture dependence of channel propagation rate for 340 nm dia. particles cutting channels of similar depth, was identical for both vanadium and vanadium pentoxide systems. The relationship is shown as an Arrhenius plot in Fig. 7, and from the slope of the line an activation energy of 221220 k 3. mole-’ was evaluated for the catalysed reaction. 4.DISCUSSION
Fig. 6. Transmission micrograph taken after the reaction of V,O,-graphite with 0.66 k.Pa oxygen at lOOOK,showing the diameter of a catalyst particle to he much larger than the width of the channel it has created. the catalyst particle has occurred during the cooling operation. Quantitative kinetic analysis showed that, the tempera-
Controlled atmosphere electron microscopic study of the catalysis of the graphite-oxygen reaction by either vanadium or vanadium pentoxide has shown the qualitative and quantitative similarity in behaviour of the metal and pentoxide, suggesting that the active species is the same in both systems. This finding is in contrast to the differences found for molybdenum and moly~enum trioxide[(i]. Although other workers have used microscopy techniques to investigate both these systems[f, 2,4] they have not reported the possible formation of intercalated material. The existence of such material is shown by
The catalytic influence of vanadium and vanadium pentoxide
14s
catalytic oxidation obtained here and that found by Heintz and Parker, 219k J . mole-‘, from bulk
2.05 @ c 5 ,u p 1.0-
Fig. 7. Arrehuis plot of vanadium 0, and vanadium pentoxide 0, catalysed rates at 0.66 k.Pa oxygen.
continuous observations of its formation. This study probably represents the first time anyone has seen the process of forming intercalated groups in graphite. Several workers have used electron microscopy to examine such compounds with sodium, potassium[7] and ferric chloride181but there are no reports of the formation of an intercalation compound of vanadium pentoxide with graphite. The present study merely shows that the pentoxide has the ability to enter between the layers of the graphite crystal. The existence of the type of material, shown in Figs. 4a to 4c could explain Hennigs’ observation[l] that catalytic channels were formed with vanadium at 873 K with no visible particle at their tip. One cannot be certain whether the intercalated material exerts a catalytic influence on the oxidation of graphite. The agreement between the activation energy for
experiments[3] is excellent. There is little doubt that the high activity found for both vanadium and vanadium pentoxide catalysts is associated with the ability of these materials to wet and spread over the graphite surface. Although the bulk melting point of vanadium pentoxide is 963 K, it is well-known that small metal and metal oxide particles show liquid-like behaviour at temperatures well below that of their bulk melting points and therefore it is not unexpected to find such phenomena as wetting and particle coalescence at about 800 K. At 1000K there are indications that evaporation of material from the surface is taking place. Under these conditions particles would tend to have a relatively small area of contact with the surface, which would account for the observation at this temperature of active particle diameters being larger than the width of channels they were creating. One would also expect to find a number of channels without particles at their head for the same reasons. 5. CONCLUSIONS
Vanadium and vanadium pentoxide have been shown to be extremely active catalysts for the oxidation of graphite. It has been demonstrated that V20s has the ability to enter between the layers of graphite crystals. The similarity in behaviour of the two systems suggests that the same reactive species is involved. At higher temperatures the catalyst tends to evaporate from the graphite surface. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8.
Hennig G. R., J. Inorg. Nucl. Chem. 24, II29 (1962). Hedley J. A., M.Sc. Thesis, Newcastle University (1962). Heintz E. A. and Parker W. E.. Carbon 4. 473 (1966). McKee D. W., Carbon 8, 623 (1970). Baker R. T. K. and Harris P. S., J. Phys. E. 5 (1972). Baker R. T. K., Harris P. S. and Kemner D. J.. Carbon 12.179 (1974). Halin M. K. and Jenkins G. M., 3rd Conf Industrial Carbons and Graphite, p. 53. London, (I 970). Heerchap M. and Delavignette P., Carbon 5, 383 (1967).