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A thermodesorption study of first stage graphite FeCl3 intercalation compounds
J. Php. Chm
PII: SOO22-3697(%100361-4 .
Pergamon
I
.Solid.r Vol 57. Nos 6-8. pp. 849%854. 1996 Copyright 1, 1996 Ekvier Science Ltd Printed in Great Britain. All rights reserved 0022.3697196 Si5.00 + 0.00
A THERMODESORPTION STUDY OF FIRST STAGE GRAPHITE FeC13 INTERCALATION COMPOUNDS D. BEGIN, Iaboratoire
E. ALAIN,
G.FURDIN
and J. F. MARECHE
de Chimie Mitt&ale Appliqube, URA CNRS 158, Facultt des Sciences, B.P. 239, 54506 Vandoeuvre les Nancy, France (Received 28 May 1995; accepted 3 1 May 1995)
Abstract-With the aim of synthesizing carbonaceous materials with specific adsorbent properties obtained from the pyrolysis of mixtures of coal tar pitch (CTP) and FeCIs graphite intercalation compounds (GIG), we present a study of the thermodesorption of first stage FeCIs GIC. These GICs were synthesized in a two temperature reactor with several kinds of graphites characterized by different granulometry and crystallinity (one a monocrystalline graphite and the others, polycrystalline). During heating under an inert atmosphere, FeCIs is decomposed into FeC$ and partially sublimed. At 5Oo”C, second stage FeClz GIC reflections are observed on X-ray diffractograms, and at 75O”C, all iron and chlorine are desorbed out of the graphene layers in polycrystalline graphite, whereas monocrystalline graphite always contains some amount of iron and chlorine. Thermogravimetric evolution has been followed using a McBain balance. FeCI, desorption mainly occurs between 350 and 550°C and is more quantitative in the case of polycrystalline materials. The graphite granulometry significantly influences the desorption level, and the higher the heating rate, the greater the FeC13 desorption. Keywords: A. inorganic compounds, B. chemical synthesis, C. thermogravimetric analysis (TGA), C. X-ray
diffraction
1. INTRODUCTION
observed a partial removal of FeC13 and its reduction into Fe(O) and FeC12. Later, using Mijssbauer spectroscopy, Hooley et al. [9] showed the reduction of iron chloride into FeC12 after GIC heating at 375°C for 3 h under H2 flow. Morawski and Kalucki [IO] observed FeC13 reduction into FeC12 and cwFedepending on the final temperature of the thermal treatment under a mixture of nitrogen and dihydrogen flow. We describe in this paper the thermal stability and the thermodesorption of first stage FeCl, GIC under nitrogen in a temperature range up to that of CTP pyrolysis. Three kinds of natural graphite (two polycrystalline and one monocrystalline) have been used; the role of the graphite particle size and also that of the heating rate during the thermal treatment have been studied. The final aim is to compare the behaviour of these three graphites and to choose that for which FeCls desorption out of the graphene layers is maximal in the temperature range of the CTP transformation into green-coke (between 350 and 550°C).
In a recent study [1, 21, we proposed a new route to synthesize active charcoals with specific adsorbent properties from the pyrolysis of mixtures of coal tar pitch (CTP) + first stage graphite intercalation compounds (GIC). The GIC serves two purposes: first, it is well known that a Lewis acid such as FeC& or ZnClz mixed with a coal or a parent coal has a dehydrogenating activity during the carbonaceous material pyrolysis [3-51. The consequence is an increase of the green-coke yield and an opening of the microporosity of the green-coke. However, FeCl, decomposition occurs near 3 1SC, a temperature lower than that of coal transformation into green-coke: its higher thermal stability is the reason why first stage FeCls GIC has been used. The second aim of the study is to obtain carbonaceous materials with active sites coming from FeC13 desorption out of the graphite layers during the pyrolysis of the mixture. We hope that these sites will be selective towards different pollutants, but this aspect will not be developed in this work. FeC13 GIC were synthesized for the first time by Thiele [6] and better characterized by Rudorff and Schulz [7]. A detailed study concerning the thermostability of this compound under various atmospheres (H2, N,) was carried out by Gross [8]: depending on the atmosphere and the thermal treatment, he
2. EXPERIMENTAL
Three natural graphites provided by ‘Le Carbone Lorraine PCchiney, France’ were used: two are poly-
crystalline (from Ceylon, and a variety called UF4 from Madagascar) and the other (from China) is 849
850
D. BEGIN er al.
mainly composed of single crystals. UF4 crystallites present an average size close to 5 pm (it is dry milled), whereas the granulometry of the two others is comprised between 40 and 1000 pm. The coherence lengths L, were measured for the two polycrystalline graphites using the Scherrer formula [I I] for the 004 reflection: it is close to 40 nm for both samples. First stage FeC13 was prepared according to the well known two temperature method [12, 131with freshly twice-sublimed FeC& in a chlorine atmosphere. The tube was introduced in a two temperature furnace for three days at a temperature of 300°C in the graphite compartment and 295°C in the FeC13 compartment. In the following, GIC synthesized from Chinese graphite will be called Ch-GIC and from Ceylon, Ce-GIC. Thermodesorption of FeCls out of the graphene layers was monitored by a McBain balance under nitrogen flow. A thermogravimetric setup allows following the weight changes of a sample inside a nacelle
I 2
I
, 6
suspended from a calibrated silicon spring. The final temperature was 550°C and heating rates varied from 5 to ZO”C/min. X-Ray diffraction was performed with a curved detector, associated with a rotating target X-ray generator (Rigaku-lOkW, with a MoK, source). 3. RESULTS AND DISCUSSION Elemental analyses of the GIC are in agreement with a chemical formula close to C,FeC13. On X-ray diffractograms, 001 reflections of a first stage were indexed: the GIC is characterized by a repeat distance equal to 0.941 nm as previously described [I, 14, 151. The presence of the hkl reflections corresponding to intercalated iron chloride [16, 171 shows the incommensurability of the two macromolecular lattices (graphite and FeCls ) (see Fig. 1A). Figure I represents the evolution of X-ray diffractogram of a first stage FeCls GIC (from Chinese graphite) during its thermal treatment under nitrogen
I
, 10
I
, 14
I
I
theta(“)
Fig. 1. X-Ray diffractograms (MO& radiation) of Chinese graphite-based CTFeC13 at ambient temperature (A), after heating at 300°C (B), 400°C (C), 500°C (D) and 750°C (E) under nitrogen flow (heating rate = ZO”C/min). (1) FeC13 GIC (001) reflections: (2) intercalated FeCi3 (hkl) reflections; (3) FeClz GIC (001) retlections; (4) intercalated FeCIZ (110) reflection; (5) graphite reflections.
1st stage Fe GICs
flow at 300°C (1 B), 400°C (lC), 500°C (ID) and 750°C (1E). The heating rate was ZO”C/min and the final temperature was held for 2 h. We have already seen in the case of a GIC synthesized from UF4 (2) that at 5OO”C,FeC13 was partially removed from the graphite and that the remainder was completely reduced into FeClz. Four 001 reflections are in agreement with the existence of a second stage FeC12 GIC. At 75O”C, all the FeClj has been removed as confirmed by elemental analyses (weight percentage of Fe and Cl = 0.4%) and by the presence of pristine graphite reflections. Similar behaviour is observed in the case of the Ch-GIC up to 500°C: first stage FeC& GIC reflection intensities decrease gradually with increasing
851
temperature and second stage FeC12 GIC reflection intensities are maximal at 500°C. We can also note the presence of the (110) reflection corresponding to the intercalated FeC12 attesting to the incommensurability of the two lattices. However, at 75o”C, in addition to the pristine graphite reflections (Fig. lE), the (110) FeClz reflection is always present. FeC13desorption is not total in the case of the Ch-GIC and it is reduced into FeQ; elemental analyses are in progress to quantify the Fe/Cl ratio. However, the mass percentage of iron + chlorine in the final product is low (around 1 l%), close to that obtained at 900°C under nitrogen by Gross [8]. In Fig. 1, we can see that graphite h/cl reflections are intense by comparison to the 001:indeed, Chinese graphite particles
A I !
I
I !
I
I
1 -5”C/min I --+--lO°C/min
4 .,........... E= 0.9 _ ,......,.
2 2
: ; 0.8 __
,..,.,..,..!
jl ,.,.,,..,,...:
i 0.7
9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
!r ib
i
:
\i
l I
0
t,
L.. . . . . . . ..b . . .. .. . %t.*~+&+.++..
6
0.6
.. ... .. .. ... .. ... ... ... ... ..
1j :-.-._.,*; ; i
b : _-............
i.,..............,............ :? ,,....... i I
..,,, . ,... . . . . . . . . . . . . . \
40
I
I
160
80 120 Time (min)
200
B I !
1.1
I I
i
I
0.4 0
40
I
I I
I ! -
ZO”C/min
I
I
80 120 Time (min)
160
200
Fig. 2. Thermogravimetric analyses of C,FeCl, heated at three heating rates. Vertical lines represent the time at which the final temperature (55PC) is reached. M = sample mass at time I, mO= initial mass. (A) Chinese-GIC (100pm < 8 < 200pm); (B) UFCGIC (f3 = 5pm).
D. BEGIN et al.
852
Table 1. Weight losses (wt%) of first stage FeCI3 GIC from three kinds of graphite after a thermal treatment at 550°C at
different heating rates S”C/min
lO”C/min
ZO”C/min
46
49
52
35
40
42
29
30
36
UF4 8=5pm Ceylon 1OOpm < 8 < 2OOpm Chinese lOOurn < 0 < 2OOfim
are anisotropic and when they are introduced into the capillary, they are preferentially oriented: the packing is parallel to the c axis, which explains the intensity of the hkl reflections. 3.1.
Thermogravimetric
measurements
3.1.1. Influence of the heating rate. Figure 2A and B represents the weight loss as a function of time for two GIC (UF4-5pm and Chinese 100pm < 8 < 200/1m) for three heating rates: 5, 10 and 20°C min. Vertical lines indicate the time at which the final temperature (SSO’C) is reached. Am is equal to m - m. with m = weight of the sample at time t and ma the initial sample weight. Whatever the origin of the GIC, the higher the heating rate, the greater the weight loss. The weight loss (mass%) of the different samples at 5, 10 and 20”C/min is collected in Table 1. We see that most of the desorption occurs between 350 and 480°C and that a plateau is observed in the case of Ch-GIC when the temperature is held at 500°C. In the case of UF4, the weight loss is higher with increasing heating rates and continues during the isothermal treatment at 500°C. The influence of the
1.1
graphite nature will be studied in a later part of this work, and a heating rate of lO”C/min has been selected in all the following results. 3.1.2. Infuence of the granulometry. Figure 3 represents the thermogravimetric curves of Ce-GIC with three different particle diameters: 0 < 40pm, 100pm < 0 < 200pm and 500pm < 0 < 1000pm. The weight losses are, respectively, equal to 45, 40 and 35% after the thermal treatment at 550°C: they increase with decreasing particle size. Desorption is almost terminated for particles of diameter greater than 100 pm at the 550°C isotherm level, whereas it is not terminated when the diameter is lower than 40 pm. This last observation is in line with the behaviour of the UF4 GIC previously described (Fig. 2A): in the case of bigger particles, one can estimate that the main desorption occurs on the edges of the particles and that the residual FeClz is ‘trapped’ between the graphene layers. However, whatever the graphite particle size, it appears that the main desorption occurs in the same temperature range. We have seen the importance of the granulometry on the weight loss on a macroscopic scale, but in the following the microscopic scale will be probed by varying the nature of the graphite and consequently its crystallinity.
!
.-.-0.--- 0<4@“, -
1OQ
--*--500<~<1OOO~m 0.9 0
f
0.8
$ r
0.7 0.6
610
I
8’0
100
Time (mm) Fig. 3. Thermogravimetric analyses of three different granulometry Ceylon graphite-based C,FeCl,. Final temperature = 550°C. heating rate = I O”C/min.
1st stage Fe GlCs
853
-UF4
(Qmdium=5pm1 (lOO
--*--Chinese
EC 0.8 : 0.7 A 0.6
0
Fig. 4. Thermogravimetric
I
I I
i
i
i
20
40
60 Time (mid
80
100
120
analyses of three C7FeC13 synthesized from several graphites. Final temperature = 55O”C, heating rate = 10”Cimin.
3.1.3. Znfiuenceof the graphite variety. The thermogravimetric curves of GIC based on UF4 (average 8 = 5 pm), Ceylon and Chinese graphites (100 pm < 8 < 200 ,um) are superimposed on Fig. 4; the heating rate is lO’C/min. Two main differences appear according to the kind of graphite. First, desorption occurs between 350 and 480°C in the case of Chinese and Ceylon graphites, whereas it is not finished at 550°C in the case of UF4. Secondly, the weight loss is much higher with UF4 (49%) compared to 40% with Ceylon and 30% with Chinese graphites. In the case of two graphites of the same granulometry (Ceylon and Chinese), the weight loss differences (10%) must be attributed to the degree of crystallinity degree. The Chinese graphite is made of single crystals and crystallite size is much higher leading to a lower desorption. The difference observed in the behaviour between Ceylon and UF4 is mainly a size effect. Indeed, we saw in Fig. 3 that for a particle size lower than 40 pm the weight loss was 45% and the curve fell off more slowly with UF4. However, no experiments were done with these two kinds of graphite of same size, but in that case, a similar behaviour would be expected because of their close crystallinity. In Figs 2-4, a small weight loss can be seen at the beginning of the heat treatment (from 100 to 150°C): this phenomenon was previously observed by Gross [8]. It can be attributed to the sublimation of a small fraction of FeCls adsorbed onto the graphite particles, and also to water outgassing in spite of precautions taken during the sample transfer into the thermogravimetric apparatus. Our experimental apparatus has not allowed a thermogravimetric study to higher temperatures (75OC), and it is known [8] that a second desorption
occurs at 700°C. However, we saw in Ref. [2] that at 750°C FeCls desorption was complete with UF4, whereas 11% (in weight) of iron and chlorine remain in the Chinese graphite (first part of this work): this illustrates the role of the graphite crystallinity in relation to the desorption of FeCls. Mazieres, et al. [18], and Maire and Mering [19] studied the desorption of a bromine-graphite compound treated under vacuum: they noticed that natural graphites generally release more bromine than pyrocarbons. Hennig [20] observed that small size graphite particles retain less bromine than bigger particles, whereas different graphites of the same size present no difference in the bromine release. These results partially agree with ours in the case of FeCls, and it is generally admitted that the remaining reagents in residue compounds (Brz or FeClz in this study) are trapped near the crystal imperfections such as grain boundaries, closed micropores, etc. Hennig explained that in the case of small particles, crystal imperfections are near the edges of the particles, decreasing their ability to retain the intercalants. The fact that desorption is lower in the case of ChGIC must be related to a diffusional process: indeed, we saw that these particles are anisotropic and consecutively have wider planar domains. That leads to a lower diffusion of the intercalants between the graphene layers in contrast to the two other kinds of graphite for which texture is more or less spheroidal. As regards our heating rates, we can consider that we are not really at equilibrium. The decomposition reaction of FeC13 creates an internal pressure of chlorine, depending on the heating rate (more chlorine formed by unit of time at higher rates) and of the possible gas evolving (diffusion or influence of defects).
854
D. BEGIN et al 4. CONCLUSIONS
This thermogravimetric study shows that many parameters can influence the thermodesorption of first stage FeQ GIG. Increasing the heating rates, and also decreasing the size of the GIC particles lead to an increase of the intercalated species desorption, whatever the nature of the graphite. However, the main parameter is the graphite crystallinity: in the case of a monocrystalline graphite (Chinese), we saw that thermodesorption is less quantitative because of the presence of larger ordered areas, unfavourable to the diffusion of the intercalated species out of the graphene layers. Studies are in progress to analyse and quantify the final products (elemental analyses and Mijssbauer spectroscopy). From these results, it appears that to elaborate the CTP + GIC mixture, the best choice is to use a small particle size polycrystalline graphite which desorbs more Lewis acid. Indeed, most of the desorption occurs during the transformation of the CTP into the green-coke (called ‘plastic phase’), that should generate a microporosity in the green-coke, and secondly, that part desorbed above 550°C will generate additional active sites. Moreover, we have described in a previous paper [I] that the use of small particles leads to their homogeneous distribution in the initial mixture.
discussions. Partial financial support was provided from PIGS 119: Carbochimie et Environnement, CNRS, Ecotech and ADEME, France. REFERENCES I. Furdin G., Begin D., Ma&he J. F., Petitjean D., Alain E. and Lelaurain M., Carbon 32,4, 599 (1994).
2. Begin D., Alain E., Furdin G. and Mareche J. F., Fuel 74,2, 139 (1995). 3. Lessing R. and Banks M. A. L., J. Chem. Sot. 125,2344 (1925).
4. Georgiadis G. and Gaillard G., Chaleur Ind. 374, 247 (1956).
5. Kandioty R., Lazaridis J. I., Dyrrold B. and Weerasinghe C. R., Fuel 63, 1583 (1984). 6. Thiele H., Z. Anorg. Allg. Chem. 207,304 (1932). 7. Riidorff W. and Schulz H., Z. Anorg. Allg. Chem. 245, 121 (1940).
8. Gross R., Thesis, Nancy, France (1962). 9. Hooley J. G., Bartlett M. W., Liengme B. V. and Sams J. R., Carbon 6,681 (1968). 10. Morawski A. W. and Kalucki K.. Carbon 27.951(1989L 11. Guinier A., Theorie et technique de la Radio&ailographie. 2nd edn. Dunod, Paris (1956). 12. Fredenhagen K. and Cadenbach G., Z. Anorg. Chem. 158,249 (1926). 13. Herold A., Bull. Sot. Chim. Fr. 999 (1955). 14. Cowley J. M. and Ibers J. A., Acta Cryst. 9,421 (1956). 15. Herold C., Ma&he J. F., Gerardin R., Mabchour A. and Furdin G., Mater. Res. Bull. 27,851 (1992). 16. Rousseaux F., Vangelisti R., Plancon A. and Tchoubar D. Rev. Chim. Mint?. 19,572 (1982). 17. Metz W. and Schulz E., Z. Krysf. 142,409 (1975). 18. Mazieres C., Cohn G., Jegoudez J. and Setton R.. Carbon 13,289 (1975).
Acknowledgements-The authors would like to acknowledge G. Medjadi for X-Ray diffraction and E. McRae for helpful
19. Maire J. and Mering J., Proc. 3th Carbon Con/., Buffalo, 337 (1959). 20. Hennig G., J. Chem. Phys. 20, 1438 (1952).
PII: SOO22-3697(%100361-4 .
Pergamon
I
.Solid.r Vol 57. Nos 6-8. pp. 849%854. 1996 Copyright 1, 1996 Ekvier Science Ltd Printed in Great Britain. All rights reserved 0022.3697196 Si5.00 + 0.00
A THERMODESORPTION STUDY OF FIRST STAGE GRAPHITE FeC13 INTERCALATION COMPOUNDS D. BEGIN, Iaboratoire
E. ALAIN,
G.FURDIN
and J. F. MARECHE
de Chimie Mitt&ale Appliqube, URA CNRS 158, Facultt des Sciences, B.P. 239, 54506 Vandoeuvre les Nancy, France (Received 28 May 1995; accepted 3 1 May 1995)
Abstract-With the aim of synthesizing carbonaceous materials with specific adsorbent properties obtained from the pyrolysis of mixtures of coal tar pitch (CTP) and FeCIs graphite intercalation compounds (GIG), we present a study of the thermodesorption of first stage FeCIs GIC. These GICs were synthesized in a two temperature reactor with several kinds of graphites characterized by different granulometry and crystallinity (one a monocrystalline graphite and the others, polycrystalline). During heating under an inert atmosphere, FeCIs is decomposed into FeC$ and partially sublimed. At 5Oo”C, second stage FeClz GIC reflections are observed on X-ray diffractograms, and at 75O”C, all iron and chlorine are desorbed out of the graphene layers in polycrystalline graphite, whereas monocrystalline graphite always contains some amount of iron and chlorine. Thermogravimetric evolution has been followed using a McBain balance. FeCI, desorption mainly occurs between 350 and 550°C and is more quantitative in the case of polycrystalline materials. The graphite granulometry significantly influences the desorption level, and the higher the heating rate, the greater the FeC13 desorption. Keywords: A. inorganic compounds, B. chemical synthesis, C. thermogravimetric analysis (TGA), C. X-ray
diffraction
1. INTRODUCTION
observed a partial removal of FeC13 and its reduction into Fe(O) and FeC12. Later, using Mijssbauer spectroscopy, Hooley et al. [9] showed the reduction of iron chloride into FeC12 after GIC heating at 375°C for 3 h under H2 flow. Morawski and Kalucki [IO] observed FeC13 reduction into FeC12 and cwFedepending on the final temperature of the thermal treatment under a mixture of nitrogen and dihydrogen flow. We describe in this paper the thermal stability and the thermodesorption of first stage FeCl, GIC under nitrogen in a temperature range up to that of CTP pyrolysis. Three kinds of natural graphite (two polycrystalline and one monocrystalline) have been used; the role of the graphite particle size and also that of the heating rate during the thermal treatment have been studied. The final aim is to compare the behaviour of these three graphites and to choose that for which FeCls desorption out of the graphene layers is maximal in the temperature range of the CTP transformation into green-coke (between 350 and 550°C).
In a recent study [1, 21, we proposed a new route to synthesize active charcoals with specific adsorbent properties from the pyrolysis of mixtures of coal tar pitch (CTP) + first stage graphite intercalation compounds (GIC). The GIC serves two purposes: first, it is well known that a Lewis acid such as FeC& or ZnClz mixed with a coal or a parent coal has a dehydrogenating activity during the carbonaceous material pyrolysis [3-51. The consequence is an increase of the green-coke yield and an opening of the microporosity of the green-coke. However, FeCl, decomposition occurs near 3 1SC, a temperature lower than that of coal transformation into green-coke: its higher thermal stability is the reason why first stage FeCls GIC has been used. The second aim of the study is to obtain carbonaceous materials with active sites coming from FeC13 desorption out of the graphite layers during the pyrolysis of the mixture. We hope that these sites will be selective towards different pollutants, but this aspect will not be developed in this work. FeC13 GIC were synthesized for the first time by Thiele [6] and better characterized by Rudorff and Schulz [7]. A detailed study concerning the thermostability of this compound under various atmospheres (H2, N,) was carried out by Gross [8]: depending on the atmosphere and the thermal treatment, he
2. EXPERIMENTAL
Three natural graphites provided by ‘Le Carbone Lorraine PCchiney, France’ were used: two are poly-
crystalline (from Ceylon, and a variety called UF4 from Madagascar) and the other (from China) is 849
850
D. BEGIN er al.
mainly composed of single crystals. UF4 crystallites present an average size close to 5 pm (it is dry milled), whereas the granulometry of the two others is comprised between 40 and 1000 pm. The coherence lengths L, were measured for the two polycrystalline graphites using the Scherrer formula [I I] for the 004 reflection: it is close to 40 nm for both samples. First stage FeC13 was prepared according to the well known two temperature method [12, 131with freshly twice-sublimed FeC& in a chlorine atmosphere. The tube was introduced in a two temperature furnace for three days at a temperature of 300°C in the graphite compartment and 295°C in the FeC13 compartment. In the following, GIC synthesized from Chinese graphite will be called Ch-GIC and from Ceylon, Ce-GIC. Thermodesorption of FeCls out of the graphene layers was monitored by a McBain balance under nitrogen flow. A thermogravimetric setup allows following the weight changes of a sample inside a nacelle
I 2
I
, 6
suspended from a calibrated silicon spring. The final temperature was 550°C and heating rates varied from 5 to ZO”C/min. X-Ray diffraction was performed with a curved detector, associated with a rotating target X-ray generator (Rigaku-lOkW, with a MoK, source). 3. RESULTS AND DISCUSSION Elemental analyses of the GIC are in agreement with a chemical formula close to C,FeC13. On X-ray diffractograms, 001 reflections of a first stage were indexed: the GIC is characterized by a repeat distance equal to 0.941 nm as previously described [I, 14, 151. The presence of the hkl reflections corresponding to intercalated iron chloride [16, 171 shows the incommensurability of the two macromolecular lattices (graphite and FeCls ) (see Fig. 1A). Figure I represents the evolution of X-ray diffractogram of a first stage FeCls GIC (from Chinese graphite) during its thermal treatment under nitrogen
I
, 10
I
, 14
I
I
theta(“)
Fig. 1. X-Ray diffractograms (MO& radiation) of Chinese graphite-based CTFeC13 at ambient temperature (A), after heating at 300°C (B), 400°C (C), 500°C (D) and 750°C (E) under nitrogen flow (heating rate = ZO”C/min). (1) FeC13 GIC (001) reflections: (2) intercalated FeCi3 (hkl) reflections; (3) FeClz GIC (001) retlections; (4) intercalated FeCIZ (110) reflection; (5) graphite reflections.
1st stage Fe GICs
flow at 300°C (1 B), 400°C (lC), 500°C (ID) and 750°C (1E). The heating rate was ZO”C/min and the final temperature was held for 2 h. We have already seen in the case of a GIC synthesized from UF4 (2) that at 5OO”C,FeC13 was partially removed from the graphite and that the remainder was completely reduced into FeClz. Four 001 reflections are in agreement with the existence of a second stage FeC12 GIC. At 75O”C, all the FeClj has been removed as confirmed by elemental analyses (weight percentage of Fe and Cl = 0.4%) and by the presence of pristine graphite reflections. Similar behaviour is observed in the case of the Ch-GIC up to 500°C: first stage FeC& GIC reflection intensities decrease gradually with increasing
851
temperature and second stage FeC12 GIC reflection intensities are maximal at 500°C. We can also note the presence of the (110) reflection corresponding to the intercalated FeC12 attesting to the incommensurability of the two lattices. However, at 75o”C, in addition to the pristine graphite reflections (Fig. lE), the (110) FeClz reflection is always present. FeC13desorption is not total in the case of the Ch-GIC and it is reduced into FeQ; elemental analyses are in progress to quantify the Fe/Cl ratio. However, the mass percentage of iron + chlorine in the final product is low (around 1 l%), close to that obtained at 900°C under nitrogen by Gross [8]. In Fig. 1, we can see that graphite h/cl reflections are intense by comparison to the 001:indeed, Chinese graphite particles
A I !
I
I !
I
I
1 -5”C/min I --+--lO°C/min
4 .,........... E= 0.9 _ ,......,.
2 2
: ; 0.8 __
,..,.,..,..!
jl ,.,.,,..,,...:
i 0.7
9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
!r ib
i
:
\i
l I
0
t,
L.. . . . . . . ..b . . .. .. . %t.*~+&+.++..
6
0.6
.. ... .. .. ... .. ... ... ... ... ..
1j :-.-._.,*; ; i
b : _-............
i.,..............,............ :? ,,....... i I
..,,, . ,... . . . . . . . . . . . . . \
40
I
I
160
80 120 Time (min)
200
B I !
1.1
I I
i
I
0.4 0
40
I
I I
I ! -
ZO”C/min
I
I
80 120 Time (min)
160
200
Fig. 2. Thermogravimetric analyses of C,FeCl, heated at three heating rates. Vertical lines represent the time at which the final temperature (55PC) is reached. M = sample mass at time I, mO= initial mass. (A) Chinese-GIC (100pm < 8 < 200pm); (B) UFCGIC (f3 = 5pm).
D. BEGIN et al.
852
Table 1. Weight losses (wt%) of first stage FeCI3 GIC from three kinds of graphite after a thermal treatment at 550°C at
different heating rates S”C/min
lO”C/min
ZO”C/min
46
49
52
35
40
42
29
30
36
UF4 8=5pm Ceylon 1OOpm < 8 < 2OOpm Chinese lOOurn < 0 < 2OOfim
are anisotropic and when they are introduced into the capillary, they are preferentially oriented: the packing is parallel to the c axis, which explains the intensity of the hkl reflections. 3.1.
Thermogravimetric
measurements
3.1.1. Influence of the heating rate. Figure 2A and B represents the weight loss as a function of time for two GIC (UF4-5pm and Chinese 100pm < 8 < 200/1m) for three heating rates: 5, 10 and 20°C min. Vertical lines indicate the time at which the final temperature (SSO’C) is reached. Am is equal to m - m. with m = weight of the sample at time t and ma the initial sample weight. Whatever the origin of the GIC, the higher the heating rate, the greater the weight loss. The weight loss (mass%) of the different samples at 5, 10 and 20”C/min is collected in Table 1. We see that most of the desorption occurs between 350 and 480°C and that a plateau is observed in the case of Ch-GIC when the temperature is held at 500°C. In the case of UF4, the weight loss is higher with increasing heating rates and continues during the isothermal treatment at 500°C. The influence of the
1.1
graphite nature will be studied in a later part of this work, and a heating rate of lO”C/min has been selected in all the following results. 3.1.2. Infuence of the granulometry. Figure 3 represents the thermogravimetric curves of Ce-GIC with three different particle diameters: 0 < 40pm, 100pm < 0 < 200pm and 500pm < 0 < 1000pm. The weight losses are, respectively, equal to 45, 40 and 35% after the thermal treatment at 550°C: they increase with decreasing particle size. Desorption is almost terminated for particles of diameter greater than 100 pm at the 550°C isotherm level, whereas it is not terminated when the diameter is lower than 40 pm. This last observation is in line with the behaviour of the UF4 GIC previously described (Fig. 2A): in the case of bigger particles, one can estimate that the main desorption occurs on the edges of the particles and that the residual FeClz is ‘trapped’ between the graphene layers. However, whatever the graphite particle size, it appears that the main desorption occurs in the same temperature range. We have seen the importance of the granulometry on the weight loss on a macroscopic scale, but in the following the microscopic scale will be probed by varying the nature of the graphite and consequently its crystallinity.
!
.-.-0.--- 0<4@“, -
1OQ
--*--500<~<1OOO~m 0.9 0
f
0.8
$ r
0.7 0.6
610
I
8’0
100
Time (mm) Fig. 3. Thermogravimetric analyses of three different granulometry Ceylon graphite-based C,FeCl,. Final temperature = 550°C. heating rate = I O”C/min.
1st stage Fe GlCs
853
-UF4
(Qmdium=5pm1 (lOO
--*--Chinese
EC 0.8 : 0.7 A 0.6
0
Fig. 4. Thermogravimetric
I
I I
i
i
i
20
40
60 Time (mid
80
100
120
analyses of three C7FeC13 synthesized from several graphites. Final temperature = 55O”C, heating rate = 10”Cimin.
3.1.3. Znfiuenceof the graphite variety. The thermogravimetric curves of GIC based on UF4 (average 8 = 5 pm), Ceylon and Chinese graphites (100 pm < 8 < 200 ,um) are superimposed on Fig. 4; the heating rate is lO’C/min. Two main differences appear according to the kind of graphite. First, desorption occurs between 350 and 480°C in the case of Chinese and Ceylon graphites, whereas it is not finished at 550°C in the case of UF4. Secondly, the weight loss is much higher with UF4 (49%) compared to 40% with Ceylon and 30% with Chinese graphites. In the case of two graphites of the same granulometry (Ceylon and Chinese), the weight loss differences (10%) must be attributed to the degree of crystallinity degree. The Chinese graphite is made of single crystals and crystallite size is much higher leading to a lower desorption. The difference observed in the behaviour between Ceylon and UF4 is mainly a size effect. Indeed, we saw in Fig. 3 that for a particle size lower than 40 pm the weight loss was 45% and the curve fell off more slowly with UF4. However, no experiments were done with these two kinds of graphite of same size, but in that case, a similar behaviour would be expected because of their close crystallinity. In Figs 2-4, a small weight loss can be seen at the beginning of the heat treatment (from 100 to 150°C): this phenomenon was previously observed by Gross [8]. It can be attributed to the sublimation of a small fraction of FeCls adsorbed onto the graphite particles, and also to water outgassing in spite of precautions taken during the sample transfer into the thermogravimetric apparatus. Our experimental apparatus has not allowed a thermogravimetric study to higher temperatures (75OC), and it is known [8] that a second desorption
occurs at 700°C. However, we saw in Ref. [2] that at 750°C FeCls desorption was complete with UF4, whereas 11% (in weight) of iron and chlorine remain in the Chinese graphite (first part of this work): this illustrates the role of the graphite crystallinity in relation to the desorption of FeCls. Mazieres, et al. [18], and Maire and Mering [19] studied the desorption of a bromine-graphite compound treated under vacuum: they noticed that natural graphites generally release more bromine than pyrocarbons. Hennig [20] observed that small size graphite particles retain less bromine than bigger particles, whereas different graphites of the same size present no difference in the bromine release. These results partially agree with ours in the case of FeCls, and it is generally admitted that the remaining reagents in residue compounds (Brz or FeClz in this study) are trapped near the crystal imperfections such as grain boundaries, closed micropores, etc. Hennig explained that in the case of small particles, crystal imperfections are near the edges of the particles, decreasing their ability to retain the intercalants. The fact that desorption is lower in the case of ChGIC must be related to a diffusional process: indeed, we saw that these particles are anisotropic and consecutively have wider planar domains. That leads to a lower diffusion of the intercalants between the graphene layers in contrast to the two other kinds of graphite for which texture is more or less spheroidal. As regards our heating rates, we can consider that we are not really at equilibrium. The decomposition reaction of FeC13 creates an internal pressure of chlorine, depending on the heating rate (more chlorine formed by unit of time at higher rates) and of the possible gas evolving (diffusion or influence of defects).
854
D. BEGIN et al 4. CONCLUSIONS
This thermogravimetric study shows that many parameters can influence the thermodesorption of first stage FeQ GIG. Increasing the heating rates, and also decreasing the size of the GIC particles lead to an increase of the intercalated species desorption, whatever the nature of the graphite. However, the main parameter is the graphite crystallinity: in the case of a monocrystalline graphite (Chinese), we saw that thermodesorption is less quantitative because of the presence of larger ordered areas, unfavourable to the diffusion of the intercalated species out of the graphene layers. Studies are in progress to analyse and quantify the final products (elemental analyses and Mijssbauer spectroscopy). From these results, it appears that to elaborate the CTP + GIC mixture, the best choice is to use a small particle size polycrystalline graphite which desorbs more Lewis acid. Indeed, most of the desorption occurs during the transformation of the CTP into the green-coke (called ‘plastic phase’), that should generate a microporosity in the green-coke, and secondly, that part desorbed above 550°C will generate additional active sites. Moreover, we have described in a previous paper [I] that the use of small particles leads to their homogeneous distribution in the initial mixture.
discussions. Partial financial support was provided from PIGS 119: Carbochimie et Environnement, CNRS, Ecotech and ADEME, France. REFERENCES I. Furdin G., Begin D., Ma&he J. F., Petitjean D., Alain E. and Lelaurain M., Carbon 32,4, 599 (1994).
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Acknowledgements-The authors would like to acknowledge G. Medjadi for X-Ray diffraction and E. McRae for helpful
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