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Mass-spectrometric study of vaporization of FeCl3-graphite intercalation compound
1. Php
PII: SOO22-3697~96MO350-9 \
Pergamon
I
Chew
Solids Vol 57, Nos 6-8. pp. 787-790. 1996 Copyright f 1996 Elsewer Science Ltd Printed in Great Britain. All rights reserved W22-3697/96 Si5.00 + 0.00
MASS-SPECTROMETRIC STUDY OF VAPORIZATION FeC13-GRAPHITE INTERCALATION COMPOUND MITSURU
ASANO, TAKESHI
OF
SASAKI, TAKESHI ABE?, YASUO MIZUTANI TOSHIO HARADA
and
Institute of Atomic Energy, Kyoto University, Uji, Kyoto 611, Japan (Received 28 May 1995; accepted 31 May 1995)
Abstract-The vaporization of stage 2 FeCll-graphite intercalation compound (GIC) has been studied by a mass-spectrometric Knudsen effusion method in the temperature range MO-496 K, and compared with that of pure FeC13(s). The main vapor species over pure FeCl, (s) are Fe&&, (g) and Cl* (g), but over stage 2 FeCls-GIC, the vapor species FeCl, (g) is identified in addition to Fe&I, (g) and Cl2 (g). Partial pressures of FeCls (g), Fe2C16(g) and Cl,(g) in equilibrium with stage 2 FeCIs-GIC are much lower than the corresponding pressures over pure FeC13(s). On the basis of the equilibrium partial pressures over FeC13-GIC, the enthalpies of reaction for the transition of stage 2 to stage 3 FeCL-GIC by the vaporization of FeCls (g) and Fe+& (g) are evaluated to be (184.7 f 7.9) and (215.7 k 2.9) kJ mol-‘, respectively, and the enthalpy of reaction for the reduction of FeCls to FeCIZ in FeCls-GIC to be (137.4 + 3.3) kJmol-‘. The chemical activities of the FeCl, and FeCl; components in stage 2 FeCl,-GIC are determined to be 2 x 10e3 and 5 x IO-‘, respectively, at 500 K. Keywords: A. multilayers, B. chemical synthesis, D. thermodynamic properties
1. INTRODUCTION A wide variety of substances (intercalates) such as alkali metals, metal halides and some acids enter the galleries of graphite to form graphite intercalation compounds (GICs). Physical and chemical properties, as well as the preparation techniques of GICs, have been reviewed, e.g. [l]. Among them, thermodynamic properties have been studied mainly on alkali metal GICs. Enthalpies of formation of MCs (M = K, Rb and Cs) have been determined by calorimetry [2,3]. To obtain Gibbs energies of formation for MC*, the electromotive force method has been employed [4, 51. The Knudsen effusion method has also been used for the determination of enthalpies of dissociation for MCs [6, 71. For metal halide-GICs, it has been pointed out that, when enthalpies of oxidation, that is electron affinities, of such metal halides are greater than -460 kJmol_‘, they may intercalate into the galleries of graphite [S]. In the present work, in order to clarify the vaporization behavior of stage 2 FeCl,-GIC, equilibrium partial pressures of the vapor species, monomeric FeCls (g), dimeric FezC16 (g) and Cl2 (g) have been studied by a mass-spectrometric Knudsen effusion method for the first time. From the partial pressures, enthalpies of reaction for the transition of stage 2 to 3 TResearch Fellow of the Japan Society for the Promotion of Science.
FeC13-GIC and the enthalpy of reaction for the reduction of FeCls to FeC12 in FeCls-GIC have been determined by the second law treatment. Furthermore, the chemical activities of the FeCls and FeClz components in FeCls-GIC have been obtained by comparing the partial pressures with those in equilibrium with pure FeCls (s) [9].
2. EXPERIMENTAL
Reagent grade anhydrous FeCls was supplied from Merck. The sample of stage 2 FeCls-GIC was prepared from natural graphite powder from China and anhydrous FeCls by the ordinary two-bulb method. The reaction time was chosen to be 20 h. The temperature of graphite was set at 673 K and that of FeC13 at 573 K in the two-zone system [IO]. The sharp (OOZ) lines of the X-ray diffraction pattern indicated that the sample was well crystallized. A 0.2m radius, 90” sector single focusing Hitachi RM-6K mass spectrometer was used in combination with a quartz Knudsen cell. The cell has an inside diameter of 7 mm and an inside height of 9 mm. The diameter of the effusion orifice is 0.6 mm. The vapors effusing from the orifice were ionized by electron impact and the measured partial pressure pmeas was converted from the ion intensity I by the relation [ 111, where k is the pressure calibration P meas = kIT/ugn, 787
M ASANO et al.
788
500 I
450 I
2.0
2.2
T(K)
400
2.4
I
2.6
2.8
IO3x l/T(K“)
Fig. 1. log IT vs. 1/T over pure FeCl, (s) (right) and stage 2
FeCIJ-GIC (left). constant,
Tis the temperature,
tion cross-section, plier and
0 is the relative ioniza-
g is the gain of the electron
n is the isotopic
abundance
equilibrium partial pressure was determined ing the apparent 3.
condensation
multi-
ratio.
The
by correct-
coefficient.
RESULTS AND DISCUSSION
3.1. Vapor species and their partial pressures The vaporization reference
of pure FeC& (s) was studied as a
in the temperature
range
3633404K.
Ion
species identified over pure FeC13 (s) are Fe+, FeCl+, Fe&$, Fe$l:, FeCll , FeClz, Fe&l+, Fe&l:, Fe$ZI:, Fe&, Cl+, Cl: and HCl+. Over stage 2 FeC&-GIC the following ion species are found: Fe+, Fe@, FeCll, FeCl:, Fe&l:, Fe2Cl:, Fe&l:, Fe*@, Cl+, Cl; and HCl+. The amount of ion species over FeCls-GIC, is small as can be seen from the weak ion intensities (I). Trimeric ion species such as Fe3Cl: and Fe&l: [12] could not be observed. In Fig. 1, logarithms of the products of I and T for some of the above ion species are shown as a function of reciprocal temperature over pure FeC& (s) and stage 2 FeCljGIC. Here, the product of I and T is a quantity proportional to the corresponding partial pressure. As can be seen in Fig. 1, the relative ion intensities over FeClj-GIC are different from those over FeQ (s). For example, the relative intensity of FeCli ion is average over FeC13-GIC, but very low over FeCls (s). Furthermore, the appearance energies Table 1. Appearance energies (eV) of ions containing
of the ions containing one Fe atom over FeC13-GIC are always small in comparison with those over FeC& (s) as shown in Table 1. The main vapor species over pure FeC13 (s) have been reported to be Fe&l6 (g) and Cl2 (g) [9], and thereby it is known that all of the ions containing one Fe atom in addition to the ions containing two Fe atoms are formed only from Fe2C16(g). However, over FeCls-GIC the ions containing one Fe atom are partly produced from FeC& (g), because the ions from FeC& (g) are formed by lower electron energies; in other words, they have lower appearance energies than the ions formed from Fe&l6 (g). As mentioned above, all of the ions over FeCl, (s) containing Fe originate only from Fe2C16(g). Thus, the fractions of respective ions containing one Fe atom originating from FeC& (g) over stage 2 FeC13-GIG may be obtained by comparing the intensity ratios between FeC13(s) and FeClj-GIC. By considering the fractions, the partial pressures of FeC13 (g) and FeQ (g) over stage 2 FeC13-GIC can be determined from the intensities. Since the partial pressure of FeCls (g) is eight orders of magnitude larger than that of FeCl*(g) [9], the formation of ions from FeC12 (g) is not observed. The origin of the Cl: ion is found to be Cl* (g) and those of the Cl’ and HCl+ ions to be HCl(g) from the time dependence of their intensities. Figure 2 shows the partial pressures of Fe&l6 (g) and Cl2 (g) determined from the ion intensities over pure FeCl, (s) together with the corresponding equilibrium ones [9]. The experimental values, particularly for Cl2 (g), are small in comparison with the equilibrium ones [9]. Hammer and Gregory [13] have reported that the partial pressures over FeCls (s) measured by an effusion method are also smaller than the equilibrium ones [9]. They pointed out that the reason is due to the small condensation coefficients for the r(K) -O.JFII
reference Fe+&,(g)
one
Fe atom Ion species
Fe+ FeCl+ FeCI+ FeCli
FeC& (s)
md
Stage 2 FeCll-GIC
23.1 18.4 13.6 12.9
Uncertainty is within ho.5 eV.
18.4
14.1 12.7 11.6
360
380
400
t
-3.51,
Fe$&(g)
\I&) I 2.5
I 2.6
I, 2.7
1,
1
IO3x I/T(K-')
Fig. 2. logpvs. l/T over pure FeCl, (s). Solid line: measured partial pressure, dottrd line: equilibrium partial pressure [9].
FeClj-GIC
789
Table 2. Enthalpies (kJ mol-‘) of reaction for transition of stage 2 to 3 and for reduction of FeC& to FeCl* in FeCIJ-GIC, and enthalpies (kJ mol-‘) of vaporization of FeC&(g), Fe&(g) and Cl2(g) over pure FeCIJ (s) Reaction
No.
=
(1) GFeCb W
(2) 6Ci2FeC13 (s) (3) (4) (5) (6)
= = = = =
2ClzFeC13 (s) FeCh (s) 2FeC13(s) 2FeC1, (s)
Present work
2ClsFeC13 (s)+FeC13 (g) 4CIsFeC13 (s)+Fe2C16 (g) 2C12FeClz (s)+C12 (g) FeC13(g) Fe&h (g) 2FeQ (s)+C12 (g)
vapors. The values of the apparent condensation coefficients can be evaluated from the ratios between the measured and the equilibrium pressures in Fig. 2 to be 0.4 for Fe&l6 (g) and 0.01 for Cl* (g). Actual partial pressures in equilibrium with stage 2 FeCl,-GIC are determined from the fractions of the corresponding ion intensities by using the above given apparent condensation coefficients under the assumption that the coefficients are applicable to the vaporization of FeC13-GIC, and further, the value for FeC& (g) is equal to that for Fe2C16(g). The results are shown in Fig. 3. The least-squares treatment of data is given by the following equations: log(peq,FecIIPa) = (16.85 f 0.86) - (9.65 *0.41)103/T log@e4,Fel,-inPa) = (19.88 f 0.32) - (11.27 f 0.15)103/T log(p,,,c,zPa) = (13.38 -f 0.36) - (7.18 f0.17)103/T From the equilibrium partial pressures given above the enthalpy of the dimerization reaction for FeClj (g) is calculated by the third law treatment to be A,H” (2FeC13 = Fe&&, g, 298) = -(145.3 f 11.3) kJ mol-’ 500
450
T(K)
400
/
-Fe,CI,, (g)
~
(
over FeCI, [9] over FeCI,-GIG
184.7 rt 7.9 215.7 * 2.9 137.4 f 3.3 133.2 f 2.7 127.5 i 11.9
JANAF tables [9] 142.6 139.8 112.8
and by the second law treatment to be -(153.7 f16.1) kJmol-‘, which are in good agreement with the value of -( 148.1 f 9.8) kJ mol-’ previously determined [9]. 3.2. Enthalpies of reaction for stage transition Whenthevaporizationofstage2FeC13-GICproceeds for a long time, very weak lines corresponding to stage 3 FeC13-GIC are observed in the XRD pattern. Thus, the reactions for the stage transition caused by the vaporization of FeC13 (g) and Fe& (g) correspond to eqns (1) and (2) in Table 2. The enthalpies of reaction for eqns (1) and (2) are equivalent to the partial molar enthalpies of vaporization of FeC13 (g) and FeQ (g), respectively. These values are obtained from their equilibrium partial pressures by the second law treatment. In FeC13-GIC, the existence of FeCl* has been verified by MGssbauer spectroscopy [14]. Furthermore, the presence of Cl2 (g) is very important for the preparation ofmetal chloride-GICs by a two-bulb method. The reaction for the reduction of FeCl, to FeC12 in stage 2 FeC13-GIC by vaporization of Cl2 (g) is given by eqn (3) in Table 2. The enthalpy of reaction (3) is equivalent to the partial molar enthalpy of vaporization of Cl2 (g). These values are listed in Table 2 together with the corresponding enthalpies of vaporization of FeC13 (g), Fe+&(g) and Cl2 (g) over pure FeC13 (s) [9]. The values over FeC13-GIC are greater than those over FeC13 (s), which probably results from the interaction due to the charge transfer between FeC& intercalate and graphene layer in FeC13-GIC. 3.3. Chemical activities of FeC13 and FeClz
2.0
2.1
2.2
2.3
2.4
2.5
IO3 x l!T(K-‘)
Fig. 3. logp,,vs. l/T over stage 2 FeCl,-GIC and pure FeCI, (s). Solid line: equilibrium partial pressure over FeCI,-GIC, dotted line: equilibrium partial pressure over pure FeCI, (s) [9].
Information on the chemical activities of the FeC13 and FeCl* components in FeC13-GIC is very useful for the evaluation of the degree of the interaction between the FeC13 intercalate and the graphene layer. The chemical activity of FeCl, can be calculated from the equilibrium partial pressures of FeC13 (g) and Fe&l6 (g) over stage 2 FeC13-GIC and pure FeC13 (s) [9] using the following equations: aFeCl,
= Peq,FeCl,
/PO.eq.FeClj
t
(7)
M. ASANO et (11 0.5 aFeCl,
=
(P~~,F~,cI,/P~,~~.F~~cI~) )
(8)
where a is the chemical activity, peq is the equilibrium partial pressure over FeC13-GIC and pO.eqis that over pure FeClj (s) [9]. From eqns (7) and (8) the chemical activities are calculated to be aF&Jl = 3 x 10m3 and = 2 x 10m3.respectively, at 500 K. Similarly, the equation:
is used for the determination of the chemical activity of FeC12, aF@&= 5 X 10m2. Although the concentration of FeC13 is actually higher than that of FeC12 in FeC13-GIC, the activity of FeC13 is small in comparison with that of FeCl?. These results show that the interaction of the FeC13 intercalate with the graphene layer is stronger than that of FeC12.
4. CONCLUSIONS
Mass
spectrometry
effusion method
coupled
with
the
Knudsen
was used for the study of the vapor-
ization of stage 2 FeC13-GIC. This technique is suited for the study of physical and chemical properties of GICs, particularly thermodynamic properties such as equilibrium partial pressures of vapor species,
enthalpies of stage transitions, enthalpies of reductions of intercalates and chemical activities of intercalates. Acknowledgements~This
work was financially supported by
Grants-in-Aid for Scientific Research and Installations from the Ministry of Education, Science and Culture, Japan.
REFERENCES
3. 4. 5. 6. 7.
Zabel H. and Solin S. A., Graphite Intercalation Compounds I and II. Springer, Berlin (1990) and (1992). H&old A. and Saehr D., Bull. Sot. Chim. Fr. 1287 (1964). Boehm H. P. and Coughlin R. W., Carbon 2, I (1964). Diebold R. and H&old A., Bull. Sot. Chim. Fr. 578 (1963). Aronson S., Salzano F. J. and Bellaflore D., J. Chem. Phys. 49,434 ( 1968). Salzano F. J. and Aronson S., J. Chem. Phys. 45,4551 (1966). Salzano F. J. and Aronson S., J. Chem. Phys. 47, 2978 (1967).
8. Bartlett N., McCarron E. M. and McQuillan B. W., Synth. Metab 1,221 (1979/1980). 9. Chase M. W. et al., J. Phys. Chem. Ref: Data 14, (1985). Suppl. No. 1, JANAF Thermochemical Tables, 3rd Ed. 10. Mitzutani Y., Abe T., Asano M. and Harada T., J. Mater. Res. 8, 1586 (1993). Il. Kato Y _,Asano M., Harada T. and Mizutani Y., J. Nucl. Mater. 203,27 (1993). 12. Fowler R. M. and Melford S. S., Inorg. Chem. 15, 473 (1976). 13. Hammer R. R. and Gregory N. W., J. Phys. Chem. 66, 1705 (1962).
14. Schlagl R., Bowen P., Millward G. R. and Jones W., J. Chem. Sot.. Faraday Trans. 79, 1793 (1983).
PII: SOO22-3697~96MO350-9 \
Pergamon
I
Chew
Solids Vol 57, Nos 6-8. pp. 787-790. 1996 Copyright f 1996 Elsewer Science Ltd Printed in Great Britain. All rights reserved W22-3697/96 Si5.00 + 0.00
MASS-SPECTROMETRIC STUDY OF VAPORIZATION FeC13-GRAPHITE INTERCALATION COMPOUND MITSURU
ASANO, TAKESHI
OF
SASAKI, TAKESHI ABE?, YASUO MIZUTANI TOSHIO HARADA
and
Institute of Atomic Energy, Kyoto University, Uji, Kyoto 611, Japan (Received 28 May 1995; accepted 31 May 1995)
Abstract-The vaporization of stage 2 FeCll-graphite intercalation compound (GIC) has been studied by a mass-spectrometric Knudsen effusion method in the temperature range MO-496 K, and compared with that of pure FeC13(s). The main vapor species over pure FeCl, (s) are Fe&&, (g) and Cl* (g), but over stage 2 FeCls-GIC, the vapor species FeCl, (g) is identified in addition to Fe&I, (g) and Cl2 (g). Partial pressures of FeCls (g), Fe2C16(g) and Cl,(g) in equilibrium with stage 2 FeCIs-GIC are much lower than the corresponding pressures over pure FeC13(s). On the basis of the equilibrium partial pressures over FeC13-GIC, the enthalpies of reaction for the transition of stage 2 to stage 3 FeCL-GIC by the vaporization of FeCls (g) and Fe+& (g) are evaluated to be (184.7 f 7.9) and (215.7 k 2.9) kJ mol-‘, respectively, and the enthalpy of reaction for the reduction of FeCls to FeCIZ in FeCls-GIC to be (137.4 + 3.3) kJmol-‘. The chemical activities of the FeCl, and FeCl; components in stage 2 FeCl,-GIC are determined to be 2 x 10e3 and 5 x IO-‘, respectively, at 500 K. Keywords: A. multilayers, B. chemical synthesis, D. thermodynamic properties
1. INTRODUCTION A wide variety of substances (intercalates) such as alkali metals, metal halides and some acids enter the galleries of graphite to form graphite intercalation compounds (GICs). Physical and chemical properties, as well as the preparation techniques of GICs, have been reviewed, e.g. [l]. Among them, thermodynamic properties have been studied mainly on alkali metal GICs. Enthalpies of formation of MCs (M = K, Rb and Cs) have been determined by calorimetry [2,3]. To obtain Gibbs energies of formation for MC*, the electromotive force method has been employed [4, 51. The Knudsen effusion method has also been used for the determination of enthalpies of dissociation for MCs [6, 71. For metal halide-GICs, it has been pointed out that, when enthalpies of oxidation, that is electron affinities, of such metal halides are greater than -460 kJmol_‘, they may intercalate into the galleries of graphite [S]. In the present work, in order to clarify the vaporization behavior of stage 2 FeCl,-GIC, equilibrium partial pressures of the vapor species, monomeric FeCls (g), dimeric FezC16 (g) and Cl2 (g) have been studied by a mass-spectrometric Knudsen effusion method for the first time. From the partial pressures, enthalpies of reaction for the transition of stage 2 to 3 TResearch Fellow of the Japan Society for the Promotion of Science.
FeC13-GIC and the enthalpy of reaction for the reduction of FeCls to FeC12 in FeCls-GIC have been determined by the second law treatment. Furthermore, the chemical activities of the FeCls and FeClz components in FeCls-GIC have been obtained by comparing the partial pressures with those in equilibrium with pure FeCls (s) [9].
2. EXPERIMENTAL
Reagent grade anhydrous FeCls was supplied from Merck. The sample of stage 2 FeCls-GIC was prepared from natural graphite powder from China and anhydrous FeCls by the ordinary two-bulb method. The reaction time was chosen to be 20 h. The temperature of graphite was set at 673 K and that of FeC13 at 573 K in the two-zone system [IO]. The sharp (OOZ) lines of the X-ray diffraction pattern indicated that the sample was well crystallized. A 0.2m radius, 90” sector single focusing Hitachi RM-6K mass spectrometer was used in combination with a quartz Knudsen cell. The cell has an inside diameter of 7 mm and an inside height of 9 mm. The diameter of the effusion orifice is 0.6 mm. The vapors effusing from the orifice were ionized by electron impact and the measured partial pressure pmeas was converted from the ion intensity I by the relation [ 111, where k is the pressure calibration P meas = kIT/ugn, 787
M ASANO et al.
788
500 I
450 I
2.0
2.2
T(K)
400
2.4
I
2.6
2.8
IO3x l/T(K“)
Fig. 1. log IT vs. 1/T over pure FeCl, (s) (right) and stage 2
FeCIJ-GIC (left). constant,
Tis the temperature,
tion cross-section, plier and
0 is the relative ioniza-
g is the gain of the electron
n is the isotopic
abundance
equilibrium partial pressure was determined ing the apparent 3.
condensation
multi-
ratio.
The
by correct-
coefficient.
RESULTS AND DISCUSSION
3.1. Vapor species and their partial pressures The vaporization reference
of pure FeC& (s) was studied as a
in the temperature
range
3633404K.
Ion
species identified over pure FeC13 (s) are Fe+, FeCl+, Fe&$, Fe$l:, FeCll , FeClz, Fe&l+, Fe&l:, Fe$ZI:, Fe&, Cl+, Cl: and HCl+. Over stage 2 FeC&-GIC the following ion species are found: Fe+, Fe@, FeCll, FeCl:, Fe&l:, Fe2Cl:, Fe&l:, Fe*@, Cl+, Cl; and HCl+. The amount of ion species over FeCls-GIC, is small as can be seen from the weak ion intensities (I). Trimeric ion species such as Fe3Cl: and Fe&l: [12] could not be observed. In Fig. 1, logarithms of the products of I and T for some of the above ion species are shown as a function of reciprocal temperature over pure FeC& (s) and stage 2 FeCljGIC. Here, the product of I and T is a quantity proportional to the corresponding partial pressure. As can be seen in Fig. 1, the relative ion intensities over FeClj-GIC are different from those over FeQ (s). For example, the relative intensity of FeCli ion is average over FeC13-GIC, but very low over FeCls (s). Furthermore, the appearance energies Table 1. Appearance energies (eV) of ions containing
of the ions containing one Fe atom over FeC13-GIC are always small in comparison with those over FeC& (s) as shown in Table 1. The main vapor species over pure FeC13 (s) have been reported to be Fe&l6 (g) and Cl2 (g) [9], and thereby it is known that all of the ions containing one Fe atom in addition to the ions containing two Fe atoms are formed only from Fe2C16(g). However, over FeCls-GIC the ions containing one Fe atom are partly produced from FeC& (g), because the ions from FeC& (g) are formed by lower electron energies; in other words, they have lower appearance energies than the ions formed from Fe&l6 (g). As mentioned above, all of the ions over FeCl, (s) containing Fe originate only from Fe2C16(g). Thus, the fractions of respective ions containing one Fe atom originating from FeC& (g) over stage 2 FeC13-GIG may be obtained by comparing the intensity ratios between FeC13(s) and FeClj-GIC. By considering the fractions, the partial pressures of FeC13 (g) and FeQ (g) over stage 2 FeC13-GIC can be determined from the intensities. Since the partial pressure of FeCls (g) is eight orders of magnitude larger than that of FeCl*(g) [9], the formation of ions from FeC12 (g) is not observed. The origin of the Cl: ion is found to be Cl* (g) and those of the Cl’ and HCl+ ions to be HCl(g) from the time dependence of their intensities. Figure 2 shows the partial pressures of Fe&l6 (g) and Cl2 (g) determined from the ion intensities over pure FeCl, (s) together with the corresponding equilibrium ones [9]. The experimental values, particularly for Cl2 (g), are small in comparison with the equilibrium ones [9]. Hammer and Gregory [13] have reported that the partial pressures over FeCls (s) measured by an effusion method are also smaller than the equilibrium ones [9]. They pointed out that the reason is due to the small condensation coefficients for the r(K) -O.JFII
reference Fe+&,(g)
one
Fe atom Ion species
Fe+ FeCl+ FeCI+ FeCli
FeC& (s)
md
Stage 2 FeCll-GIC
23.1 18.4 13.6 12.9
Uncertainty is within ho.5 eV.
18.4
14.1 12.7 11.6
360
380
400
t
-3.51,
Fe$&(g)
\I&) I 2.5
I 2.6
I, 2.7
1,
1
IO3x I/T(K-')
Fig. 2. logpvs. l/T over pure FeCl, (s). Solid line: measured partial pressure, dottrd line: equilibrium partial pressure [9].
FeClj-GIC
789
Table 2. Enthalpies (kJ mol-‘) of reaction for transition of stage 2 to 3 and for reduction of FeC& to FeCl* in FeCIJ-GIC, and enthalpies (kJ mol-‘) of vaporization of FeC&(g), Fe&(g) and Cl2(g) over pure FeCIJ (s) Reaction
No.
=
(1) GFeCb W
(2) 6Ci2FeC13 (s) (3) (4) (5) (6)
= = = = =
2ClzFeC13 (s) FeCh (s) 2FeC13(s) 2FeC1, (s)
Present work
2ClsFeC13 (s)+FeC13 (g) 4CIsFeC13 (s)+Fe2C16 (g) 2C12FeClz (s)+C12 (g) FeC13(g) Fe&h (g) 2FeQ (s)+C12 (g)
vapors. The values of the apparent condensation coefficients can be evaluated from the ratios between the measured and the equilibrium pressures in Fig. 2 to be 0.4 for Fe&l6 (g) and 0.01 for Cl* (g). Actual partial pressures in equilibrium with stage 2 FeCl,-GIC are determined from the fractions of the corresponding ion intensities by using the above given apparent condensation coefficients under the assumption that the coefficients are applicable to the vaporization of FeC13-GIC, and further, the value for FeC& (g) is equal to that for Fe2C16(g). The results are shown in Fig. 3. The least-squares treatment of data is given by the following equations: log(peq,FecIIPa) = (16.85 f 0.86) - (9.65 *0.41)103/T log@e4,Fel,-inPa) = (19.88 f 0.32) - (11.27 f 0.15)103/T log(p,,,c,zPa) = (13.38 -f 0.36) - (7.18 f0.17)103/T From the equilibrium partial pressures given above the enthalpy of the dimerization reaction for FeClj (g) is calculated by the third law treatment to be A,H” (2FeC13 = Fe&&, g, 298) = -(145.3 f 11.3) kJ mol-’ 500
450
T(K)
400
/
-Fe,CI,, (g)
~
(
over FeCI, [9] over FeCI,-GIG
184.7 rt 7.9 215.7 * 2.9 137.4 f 3.3 133.2 f 2.7 127.5 i 11.9
JANAF tables [9] 142.6 139.8 112.8
and by the second law treatment to be -(153.7 f16.1) kJmol-‘, which are in good agreement with the value of -( 148.1 f 9.8) kJ mol-’ previously determined [9]. 3.2. Enthalpies of reaction for stage transition Whenthevaporizationofstage2FeC13-GICproceeds for a long time, very weak lines corresponding to stage 3 FeC13-GIC are observed in the XRD pattern. Thus, the reactions for the stage transition caused by the vaporization of FeC13 (g) and Fe& (g) correspond to eqns (1) and (2) in Table 2. The enthalpies of reaction for eqns (1) and (2) are equivalent to the partial molar enthalpies of vaporization of FeC13 (g) and FeQ (g), respectively. These values are obtained from their equilibrium partial pressures by the second law treatment. In FeC13-GIC, the existence of FeCl* has been verified by MGssbauer spectroscopy [14]. Furthermore, the presence of Cl2 (g) is very important for the preparation ofmetal chloride-GICs by a two-bulb method. The reaction for the reduction of FeCl, to FeC12 in stage 2 FeC13-GIC by vaporization of Cl2 (g) is given by eqn (3) in Table 2. The enthalpy of reaction (3) is equivalent to the partial molar enthalpy of vaporization of Cl2 (g). These values are listed in Table 2 together with the corresponding enthalpies of vaporization of FeC13 (g), Fe+&(g) and Cl2 (g) over pure FeC13 (s) [9]. The values over FeC13-GIC are greater than those over FeC13 (s), which probably results from the interaction due to the charge transfer between FeC& intercalate and graphene layer in FeC13-GIC. 3.3. Chemical activities of FeC13 and FeClz
2.0
2.1
2.2
2.3
2.4
2.5
IO3 x l!T(K-‘)
Fig. 3. logp,,vs. l/T over stage 2 FeCl,-GIC and pure FeCI, (s). Solid line: equilibrium partial pressure over FeCI,-GIC, dotted line: equilibrium partial pressure over pure FeCI, (s) [9].
Information on the chemical activities of the FeC13 and FeCl* components in FeC13-GIC is very useful for the evaluation of the degree of the interaction between the FeC13 intercalate and the graphene layer. The chemical activity of FeCl, can be calculated from the equilibrium partial pressures of FeC13 (g) and Fe&l6 (g) over stage 2 FeC13-GIC and pure FeC13 (s) [9] using the following equations: aFeCl,
= Peq,FeCl,
/PO.eq.FeClj
t
(7)
M. ASANO et (11 0.5 aFeCl,
=
(P~~,F~,cI,/P~,~~.F~~cI~) )
(8)
where a is the chemical activity, peq is the equilibrium partial pressure over FeC13-GIC and pO.eqis that over pure FeClj (s) [9]. From eqns (7) and (8) the chemical activities are calculated to be aF&Jl = 3 x 10m3 and = 2 x 10m3.respectively, at 500 K. Similarly, the equation:
is used for the determination of the chemical activity of FeC12, aF@&= 5 X 10m2. Although the concentration of FeC13 is actually higher than that of FeC12 in FeC13-GIC, the activity of FeC13 is small in comparison with that of FeCl?. These results show that the interaction of the FeC13 intercalate with the graphene layer is stronger than that of FeC12.
4. CONCLUSIONS
Mass
spectrometry
effusion method
coupled
with
the
Knudsen
was used for the study of the vapor-
ization of stage 2 FeC13-GIC. This technique is suited for the study of physical and chemical properties of GICs, particularly thermodynamic properties such as equilibrium partial pressures of vapor species,
enthalpies of stage transitions, enthalpies of reductions of intercalates and chemical activities of intercalates. Acknowledgements~This
work was financially supported by
Grants-in-Aid for Scientific Research and Installations from the Ministry of Education, Science and Culture, Japan.
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3. 4. 5. 6. 7.
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