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Direct observation of bromine penetration into a pyrocarbon sample
LETTERS
666
TO THE
Our observations reveal that catalytic reactions occurring under “mild” conditions may be accompanied by the formation of not only amorphous carbon layers but of graphitic whiskers aiso. These must have been formed in processes which are not at present elucidated but it is without doubt that the formation of such whiskers requires very strong interactions between their precursors and the metal, as shown by the small metal particles embedded into the fibres torn off from the bulk metal as reported previouslyf3,4] and seen also in some of our micrographs. It is of particular interest therefore that such reactions may take place at much lower temperatures than believed so far on the surface of platinum which has not been regarded as a catalyst for the promotion of fibrous carbon formation below 600°C.
*Permanent address: Institute of Isotopes of the Hungarian Academy of Sciences P-0. Box 77, Budapest, Hungary
Carh,
1973, Vol. 11, pp. 666-668.
Pergamon Press.
EDITOR
REFERENCES 1. Robertson S. D., Carbon 8,365 (1970). 2. Ruston W. R., Warzee M., Hennaut J. and Waty J., Cur6on 7,47 (1969). 3. Baird T., Fryer J. R. and Grant B., Nature 233, 329 (1971). 4. Baker R. T. K., Barber M. A., Harris P. S., Feates F. S. and Waite R. J., J. Catalysis 26, 51 (1972). 5. Tomita A., Yoshida K., Mishiyama Y. and Tamai Y., Cc&on 10,601(1972). 6. Koyama T., Carbon 10,757 (1972). 7. Baird T., Fryer J. R. and Grant B., “Carbon 72” Znt. Carbon Co@ Baden-Baden, p. 266 (1972). 8. Presland A. and Walker P., Carbon 7, 1 (1969). 9. Lobo L. F. G., Ph.D. Thesis, (1971). 10. Taylor G. F., Thomson S. J, and Webb G., J.Catalysis 12,150(1968). 11.Paal Z. and Tetenyi P., Acta Chim. Acad. Sci. Hung.,53,193 (1967). 12. Paal Z., Thomson S. J. Webb G. and McCorkindale N. R., Acta Khim. Sci. Hung. (in Press). 13. Baird T., Paal Z. and Thomson S. J., J.C.S. Faraday Trans. I., 69,50 (1973). 14. Paal Z. and Tetenyi P., J. Catalysis, 29, 176 (1973).
Printed in Great Britain
Direct Observation of Bromine Penetration into a Pyrocarbon Sample (Received 7 June 1973) Bromine lamellar compounds of carbon being stable only under bromine pressure, a detailed investigation of the halogen dist~bution throughout a sample is difficult. However, when bromine is driven out, at room temperature, from a pyrocarbon (deposited at ZlOO%), a residue compound is obtained with a bromine content approximately 25% that of the parent lamellar compound. So that, when a carbon sample has been incompletely brominated, a study of the bromine distribution in the resulting residue compound gives a good
picture of the penetration of bromine into the parent sample. In order to clarify the kinetics of the first bromine insertion into a carbon, we submitted pyrocarbon samples (deposited at 2100%) to a partial bromine intercalation, and studied the resulting residue compounds by means of an electron microprobe@]. The results lead us to ~ncl~ions somewhat different from those previously published by other authors[2] and ourselves [3], The samples Si and S; (10X5X 1 mm) had their larger dimensions parallel to the deposition plane. They were submitted to a bromine pressure of 176 Torr at 25°C during210min (8,) and400min (&) respectively, up to bromine weight uptakes WI
LETTERS
TO THE
and W, equal to 26% and 43% of the maximum uptake (corresponding to the CR Br formula). After desorption the residual bromine contents were 0.245 WI and O-265 W, respectively. Figures 1 and 2 show the distribution of bromine in various planes through samples S1 and Se There are irregularities due to microcracks in the sample, but the overall picture is quite clear. It is noteworthy that: l-Penetration of bromine along a direction perpendicular to the plane of deposition is extremely slow: approximately 25 pm after 210 min and 30 pm after 400 min which is at least 10 times slower than along the other direction. This finding confirms the results of a previous study by Hooley and coworkers[4]. It may be assumed that bromine diffusion occurs only along the graphite layers, since in these samples the average value of sin2 @ is 0.1 (@ is the angle between the c-axis of a crys-
IOmm D
5mm
43 %
lmmg
of
total
bromine
m
%
Bromine
= 20
%
of carbon
weight
a
%
Bromine
-C 20
%
of carbon
weight
0
%
Bromine
= 0%
of
weight
carbon
Fig. 1.
Ezl
%
Bromine
=
20
cl
46
Bromine
=
0 %
%
Fig. 2.
01 CDibo”
Of
Carbon
weight
weight
insertion
EDITOR
667
tallite and the perpendicular to the plane of deposition) [5]. 2 -The central areas of the sample are entirely devoid of bromine. There is no doubt that the bromine did not reach this zone during insertion. This also confirms previous conclusions of Hooley obtained by a different method [4]. 3 -The bromine front is very sharp: as an example Fig. 1 shows the zone where the bromine content decreases from 20% to 0% of the carbon weight; in order to show it clearly, it was necessary to enlarge it; actually it extends usually over less than 091 mm. 4 - Measurement of the bromine content behind the front is performed with an accuracy of & 7% by the microprobe: the regions reached by bromine, behind the front, have a q~~-~~~~ bromine content (20-23% of the carbon weight). This suggests that a fully intercalated lamellar compound was formed behind the bromine diffusion front. From these findings, and assuming also that the bromine front moves at a constant rate inside the sample, a most simple two-dimensional model of the first intercalation process may be suggested, where the weight uptake would be a parabolic function of time. Comparison with experimental data shows a satisfactory agreement up to approximately 50% of the maximum bromine insertion. Later stages however are much slower than predicted by this model. This is probably an indication that, contrary to what was just assumed, the bromine concentration is not, during intercalation, quite uniform behind the front, and that there is a gradient between the inner and outer zones of the sample. Such a gradient, provided it does not exceed 10% of the total bromine content, remains compatible with the microprobe results. It would play an important part in supplying the driving force during the end of the first intercalation. The first intercalation kinetics would then be interpreted as a two-step process (which implies the non-validity of Pacault’s rule). During the first step, the bromine front would move through the sample along the graphitic layers (with a constant velocity because of the intercalation threshold[3]). Behind the front a nearly (but not quite) complete lamellar compound would be formed. The second step would then be a simple diffusion process, rather analogous to the second and subsequent brominations (where the starting material already contains bromine, and there is no intercalation threshoid). It would transform the not quite complete lameflar compound into a fully intercalated one.
668
LETTERS
TO THE
Acknodedgement- We wish to express our thanks to Pr. Naslain and Dr. Bourdeau from the Electron Micropro~ Laboratory of the University of Bordeaux for their very helpful assistance in this work. A. MARCHAND J. C. ROUXLLON Centre a2 Recherche Paul Pascal Domaine Uniuersitaire-33 Talewe France
EDITOR
REl?ERENCES 1. Electron Probe for microanalysis, Cameca M. S. 46 (licence ONERA). 2. Saunders G. A., UbbeIohde A. R. and Young D. A., Proc. Roy. Sot. A271,499 (1963). 3. Marchand A., Rouillon J. C. and De Macuzo M. H., 11 th Carbon Conf. (1973) - paper IC3. 4. Hooley J. G., Garby W. P. and Valentin J., Carbon $7 (1965). 5. Poquet E. ,J. Chim. Phy. 60,566 (1963).
666
TO THE
Our observations reveal that catalytic reactions occurring under “mild” conditions may be accompanied by the formation of not only amorphous carbon layers but of graphitic whiskers aiso. These must have been formed in processes which are not at present elucidated but it is without doubt that the formation of such whiskers requires very strong interactions between their precursors and the metal, as shown by the small metal particles embedded into the fibres torn off from the bulk metal as reported previouslyf3,4] and seen also in some of our micrographs. It is of particular interest therefore that such reactions may take place at much lower temperatures than believed so far on the surface of platinum which has not been regarded as a catalyst for the promotion of fibrous carbon formation below 600°C.
*Permanent address: Institute of Isotopes of the Hungarian Academy of Sciences P-0. Box 77, Budapest, Hungary
Carh,
1973, Vol. 11, pp. 666-668.
Pergamon Press.
EDITOR
REFERENCES 1. Robertson S. D., Carbon 8,365 (1970). 2. Ruston W. R., Warzee M., Hennaut J. and Waty J., Cur6on 7,47 (1969). 3. Baird T., Fryer J. R. and Grant B., Nature 233, 329 (1971). 4. Baker R. T. K., Barber M. A., Harris P. S., Feates F. S. and Waite R. J., J. Catalysis 26, 51 (1972). 5. Tomita A., Yoshida K., Mishiyama Y. and Tamai Y., Cc&on 10,601(1972). 6. Koyama T., Carbon 10,757 (1972). 7. Baird T., Fryer J. R. and Grant B., “Carbon 72” Znt. Carbon Co@ Baden-Baden, p. 266 (1972). 8. Presland A. and Walker P., Carbon 7, 1 (1969). 9. Lobo L. F. G., Ph.D. Thesis, (1971). 10. Taylor G. F., Thomson S. J, and Webb G., J.Catalysis 12,150(1968). 11.Paal Z. and Tetenyi P., Acta Chim. Acad. Sci. Hung.,53,193 (1967). 12. Paal Z., Thomson S. J. Webb G. and McCorkindale N. R., Acta Khim. Sci. Hung. (in Press). 13. Baird T., Paal Z. and Thomson S. J., J.C.S. Faraday Trans. I., 69,50 (1973). 14. Paal Z. and Tetenyi P., J. Catalysis, 29, 176 (1973).
Printed in Great Britain
Direct Observation of Bromine Penetration into a Pyrocarbon Sample (Received 7 June 1973) Bromine lamellar compounds of carbon being stable only under bromine pressure, a detailed investigation of the halogen dist~bution throughout a sample is difficult. However, when bromine is driven out, at room temperature, from a pyrocarbon (deposited at ZlOO%), a residue compound is obtained with a bromine content approximately 25% that of the parent lamellar compound. So that, when a carbon sample has been incompletely brominated, a study of the bromine distribution in the resulting residue compound gives a good
picture of the penetration of bromine into the parent sample. In order to clarify the kinetics of the first bromine insertion into a carbon, we submitted pyrocarbon samples (deposited at 2100%) to a partial bromine intercalation, and studied the resulting residue compounds by means of an electron microprobe@]. The results lead us to ~ncl~ions somewhat different from those previously published by other authors[2] and ourselves [3], The samples Si and S; (10X5X 1 mm) had their larger dimensions parallel to the deposition plane. They were submitted to a bromine pressure of 176 Torr at 25°C during210min (8,) and400min (&) respectively, up to bromine weight uptakes WI
LETTERS
TO THE
and W, equal to 26% and 43% of the maximum uptake (corresponding to the CR Br formula). After desorption the residual bromine contents were 0.245 WI and O-265 W, respectively. Figures 1 and 2 show the distribution of bromine in various planes through samples S1 and Se There are irregularities due to microcracks in the sample, but the overall picture is quite clear. It is noteworthy that: l-Penetration of bromine along a direction perpendicular to the plane of deposition is extremely slow: approximately 25 pm after 210 min and 30 pm after 400 min which is at least 10 times slower than along the other direction. This finding confirms the results of a previous study by Hooley and coworkers[4]. It may be assumed that bromine diffusion occurs only along the graphite layers, since in these samples the average value of sin2 @ is 0.1 (@ is the angle between the c-axis of a crys-
IOmm D
5mm
43 %
lmmg
of
total
bromine
m
%
Bromine
= 20
%
of carbon
weight
a
%
Bromine
-C 20
%
of carbon
weight
0
%
Bromine
= 0%
of
weight
carbon
Fig. 1.
Ezl
%
Bromine
=
20
cl
46
Bromine
=
0 %
%
Fig. 2.
01 CDibo”
Of
Carbon
weight
weight
insertion
EDITOR
667
tallite and the perpendicular to the plane of deposition) [5]. 2 -The central areas of the sample are entirely devoid of bromine. There is no doubt that the bromine did not reach this zone during insertion. This also confirms previous conclusions of Hooley obtained by a different method [4]. 3 -The bromine front is very sharp: as an example Fig. 1 shows the zone where the bromine content decreases from 20% to 0% of the carbon weight; in order to show it clearly, it was necessary to enlarge it; actually it extends usually over less than 091 mm. 4 - Measurement of the bromine content behind the front is performed with an accuracy of & 7% by the microprobe: the regions reached by bromine, behind the front, have a q~~-~~~~ bromine content (20-23% of the carbon weight). This suggests that a fully intercalated lamellar compound was formed behind the bromine diffusion front. From these findings, and assuming also that the bromine front moves at a constant rate inside the sample, a most simple two-dimensional model of the first intercalation process may be suggested, where the weight uptake would be a parabolic function of time. Comparison with experimental data shows a satisfactory agreement up to approximately 50% of the maximum bromine insertion. Later stages however are much slower than predicted by this model. This is probably an indication that, contrary to what was just assumed, the bromine concentration is not, during intercalation, quite uniform behind the front, and that there is a gradient between the inner and outer zones of the sample. Such a gradient, provided it does not exceed 10% of the total bromine content, remains compatible with the microprobe results. It would play an important part in supplying the driving force during the end of the first intercalation. The first intercalation kinetics would then be interpreted as a two-step process (which implies the non-validity of Pacault’s rule). During the first step, the bromine front would move through the sample along the graphitic layers (with a constant velocity because of the intercalation threshold[3]). Behind the front a nearly (but not quite) complete lamellar compound would be formed. The second step would then be a simple diffusion process, rather analogous to the second and subsequent brominations (where the starting material already contains bromine, and there is no intercalation threshoid). It would transform the not quite complete lameflar compound into a fully intercalated one.
668
LETTERS
TO THE
Acknodedgement- We wish to express our thanks to Pr. Naslain and Dr. Bourdeau from the Electron Micropro~ Laboratory of the University of Bordeaux for their very helpful assistance in this work. A. MARCHAND J. C. ROUXLLON Centre a2 Recherche Paul Pascal Domaine Uniuersitaire-33 Talewe France
EDITOR
REl?ERENCES 1. Electron Probe for microanalysis, Cameca M. S. 46 (licence ONERA). 2. Saunders G. A., UbbeIohde A. R. and Young D. A., Proc. Roy. Sot. A271,499 (1963). 3. Marchand A., Rouillon J. C. and De Macuzo M. H., 11 th Carbon Conf. (1973) - paper IC3. 4. Hooley J. G., Garby W. P. and Valentin J., Carbon $7 (1965). 5. Poquet E. ,J. Chim. Phy. 60,566 (1963).