- Email: [email protected]
On the difference between the isoelectric point and the point of zero charge of carbons
Carbon Vol. 33, No. 11, pp. 1655%1659,1995 Copyright 0 1995 Else&x Science Ltd Printed in Great Britain. All rights reserved 0008-6223/95 $9.50 + 0.00
Pergamon
LETTERS TO THE EDITOR On the difference and the point
J.A.
between the isoelectric point of zero charge of carbons
M.J. ILLAN-G~MEZ,C.A. LEON Y LEON* and L.R. RADOVIC Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 *Quantachrome Corporation, 1900 Corporate Drive, Boynton Beach, FL 33426 MENI~NDEZ,
(Received 3 August 1995; accepted in revised form 20 September 1995) Key Words - Point of zero charge; isoelectric point; surface chemistry; activated carbon.
It is well established that the behavior of carbon materials as adsorbents is dependent to a large extent on their physical surface properties (pore size distribution, surface area, pore volume) [ 11. The role of chemical and electrochemical surface properties in the use of carbons as catalyst supports [2-71 and adsorbents of both organic and inorganic species [2,8-111 is increasingly being appreciated. An important (re)current issue is, therefore, the methodology of chemical surface characterization of carbons, having in mind particularly the great affinity of high-surface-area carbons toward oxygen. Electrochemical methods have been widely used in the characterization of carbons [12]. It has been suggested that the isoelectric point (IEP) values are only representative of the external surface charges of carbon particles in solution, whereas the point of zero charge (PZC) varies in response to the net total (external and internal) surface charge of the particles [ 13-151. Hence the difference (PZC - IEP) can be interpreted as a measure of surface charge distribution of porous carbons. Values greater than zero indicate more negatively charged external than internal particle surfaces, and values close to zero correspond to a more homogeneous distribution of the surface charges. These suggestions, if confirmed, can be very useful for the characterization of the chemical nature of activated carbons. In this communication we discuss the interpretation of IEP and PZC data obtained for carbons subjected to a wide variety of conventional pretreatments. The broader issues of the origin of the electric charge on the carbon surface [ 16,171 and the nature of the double layer capacity in porous carbons [ 18-201 are beyond the scope of the present discussion. Twelve samples with very different PZC and IEP values were prepared using a commercial activated carbon, NORIT C (NC), as the precursor. The following treatments have been carried out: (i) oxidation in flowing air at 250°C for three hours (NcA); (ii) heat treatment in flowing N2 at 95O’C for three hours (NcN); (iii) the same as NcN but oxidized in flowing air at 250°C for seven minutes after cooling in N2 (NcNA); (iv) heat treatment in flowing H2 at 950°C for three hours (NcH); (v) room-temperature exposure of sample NcH to ambient air for 1, 7, 20 and 30 days (NcH-lox, -70x.
-200x, and -300x, respectively); (vi) oxidation of sample NcH with HN03 (70 vol.%) for 15 and 30 minutes at boiling point (NcH_HN03cl/4h, NcH-HN03cl/2h) and with HNOs (18 vol.%) during 16 h at 60°C (NcHHN03dl6h). These treatments did not alter drastically the physical surface properties of the resultant carbons. For instance, the BET surface area (N2, 77 K) for carbon NC was 1380 m2/g. After the most severe treatment (NcH), this value decreased to 1240 m2/g, and in the most extreme case (NcA) it descended to 1085 m2/g; the corresponding micropore volumes (obtained from the Dubinin-Radushkevich equation applied to C@ adsorption data at 273 K) were 0.31, 0.34 and 0.30 cm?g, respectively Preparation of samples NcA, NcN, NcNA and NcH for the determination of both PZC and IEP was carried out ten minutes after their treatments in order to avoid atmospheric oxidation as much as possible.The PZC values were determined by mass titra-tion as described elsewhere [ 14,21,22]. The IEP values were determined from electrophoretic mobility measurements using a Zeta-Meter 3.0+ apparatus, using 0.001 M KNOs as the supporting (indifferent) electrolyte. Table 1 lists the PZC and IEP values of all the samples studied. Figures 1 and 2 show representative plots of IEP and PZC determinations.
1655
Table 1 Electrochemical Properties of the Carbons Investigated Type of Carbon NC NcA NcN NcNA NcH NcH- 1ox NcH-70x NcH-200x NcH-300x NcH-HN03dl6h NcH-HN03c1/4h NcH-HN03cl/2h
PZC 2.5 2.2 5.2 4.2 9.0 8.9 9.0 8.6 8.4 3.9 3.3 2.8
IEP 2.6 2.2 3.5 2.9 4.9 4.2 4.2 3:; 3.5 1.6 1.4
PZC - IEP -0.1 0.0 1.7 ::: 4.7 ::; 5.1 0.4 1.7 1.4
Letters to the Editor
1656
12,
!
-NC
r~-T-.“-_
,
+NcA
+
NcN
e
NcH
10
a =a
6
I-r-+----
4
0
1
2,
3
_~
0
-3t”‘1’1’1’1’1’1”‘1” 4
5
6
7
8
9
10
0
2
PH
Fig. 1 Determination carbons
of the isoelectric
point of selected
The low and close values of PZC and IEP for the as-received activated carbon (NC) suggest that it has rather homogeneous and acidic external and internal surfaces. The slight decrease in both PZC and IEP observed when NC is subjected to air oxidation at 250 “C (NcA) shows that more acidic oxygen-containing surface groups were added to both the internal and external carbon surfaces. The similar values of PZC and IEP indicate that this treatment affects the entire surface to a similar extent. In the case of the sample heated in a N2 atmosphere (NcN), it is known that most of the functional groups are removed from the surface. However, this treatment creates some very active sites which are oxidized as soon as the sample is exposed to air at room temperature. Hence the values of PZC and IEP are higher than those for NC, albeit not as high as should be expected in oxygen-free environments. In addition, as the diffusion effects for O2 at room temperature are important for microporous materials such as active carbons, this oxidation takes place preferentially on the external surface. The higher value of the PZC compared to the IEP confirms this point. Similar considerations can be made in the case of NcNA. In this case, as a consequence of the higher degree of oxidation of this sample due to its exposure to air at 250°C. the values of PZC and IEP are lower than those of the NcN sample. As with the Nz treatment, heating the as-received carbon in Ha (NcH) removes most of the functional groups from the surface. However, in this case the high value of PZC shows that the internal surface preserves its basic character. This is due to the fact that most of the active sites are stabilized by the hydrogen present during the treatment [ 11,231 and, consequently, the number of acidic oxygen-containing surface groups is much lower than the number of oxygen-free Lewis base sites on the carbon surface [16,24]. In spite of this fact, some oxidation occurs preferentially on the external surface, as indicated by the relatively low IFP value for this carbon. Upon comparing the evolution of PZC and IEP values when sample NcH is exposed to room temperature oxidation for up to 30 days, the following observations are worth pointing out: (a) There was a slight decrease in both PZC and IEP with exposure time. Therefore, even after stabilizing the surface with hydrogen as indicated, some oxidation does take place. (b) The difference between PZC and IEP increased as the
Fig. 2 Determination selected carbons.
4 Carbon
6 fraction
8 (wt%)
10
of the point of zero charge
degree of oxidation increased. This indicates a preferential oxidation of the external surface. (c) The greatest increase in the value of (PZC-IEP) was observed after the first day. This is consistent with the expectation that the degree of oxidation exhibits an exponential decay, indicative of a first-order rate process. When sample NcH was oxidized with HNOa, both the PZC and the IEP values became lower than those observed for the original sample, indicating, as expected [25-281, the increase in the acidic oxygencontaining surface groups. However, the PZC and IEP values and the difference between them were found to depend on temperature, concentration and contact time used in the oxidation treatment. The PZC and IEP values decrease when acid concentration is increased and when longer contact times are used, reflecting the fact that more acidic oxygen-containing surface groups have been introduced [25]. On the other hand, the (PZC-IEP} difference shows that more uniform oxidation takes place when diluted acid is used. In contrast, preferential external surface oxidation is observed if the concentration of acid is increased. The trend of the { PZC-IEP) difference for samples prepared with HNOa using different contact times (NcH-HN03cl/4h and NcH-HN03cl/2h) shows that the decrease in this variable also produces a preferential external oxidation of the carbon. These results confirm that, by carefully selecting the acid oxidation conditions, it is possible to prepare active carbons with different acidity levels and oxygen surface group distributions. This flexibility can be very useful for the preparation of carbon-supported catalysts, for which it is expected that the most uniform surface oxidation treatments lead to higher catalyst dispersions and higher catalytic activities [2,3,6]. It can also be exploited in the design of activated carbon adsorbents for the removal of specific liquid adsorbates [8,29]. Finally, these results also show that the combination of two relatively routine characterization techniques, electrophoresis and mass titration, is a convenient and powerful tool for the analysis and design of surface chemistry of active carbons, especially for determining the spatial (radial) distribution of the acidic oxygen functional groups within carbon particles. For a more thorough analysis, the use of additional techniques (e.g., temperature-programmed desorption, acid and base titrations, surface spectroscopies) is desirable.
1657
Letters to the Editor Acknowledgments.
This study was made possible by financial support from the Carbon Research Center (Penn State) and postdoctoral grants for JAM (Spanish Scientific Research Council, CSIC) and MJIG (Ministry of Science and Education of Spain).
12. 13. 14. 15.
REFERENCES I. 2. 3. 4. 5. 6. 7.
8.
9.
IO. I I.
J. W. Patrick
Characterization
Porosity and Applications.
(Ed.),
in
Carbons:
Halsted Press, New York, 1995. __ C. A. Leon y Leon and L. R. Radovic, in Chemistry and Physics of Carbon, Vol. 24. (P. A. Thrower, Ed.), Dekker, New York, 1994, p. 213. J. M. Solar, F. J. Derbyshire, V. H. J. de Beer and L. R. Radovic, J. Cutal. 129, 330 (1991). K. T. Kim, J. S. Chung, K. H. Lee, Y. G. Kim and J. Y. Sung, Carbon 30, 467 (1992). C. Moreno-Castilla, M. A. Ferro-Garcia, J. RiveraUtrilla and J. P. Joly, Energy & Fuels 8, 1233
16. 17. 18. 19. 20. 21. 22.
(1994).
M. C. RomBn-Martinez, D. Cazorla-Amor6s, A. Linares-Solano, C. Salinas-Martinez de Lecea, H. Yamashita and M. Anpo. Carbon 33, 3 (1995). L. R. Radovic and C. Sudhakar, “Carbon as a Catalyst Support” in Introduction to Carbon Technologies (H. Marsh and E. A. Heintz, Eds.). Elsevier Science, 1995, in press. L. R. Radovic, J. I. Ume and A. W. Scaroni, “On Tailoring the Surface Chemistry of Activated Carbons for Their Use in Purification of Aqueous Effluents,” in Fundamentals of Adsorption (M. D.
LeVan, Ed.). Elsevier Science, 1995, in press. J. M. Solar, C. A. Leon y Leon, K. Osseo-Asare and L. R. Radovic, Carbon 28, 369 (1990). G. Newcombe, J. Colloid Interf. Sci. 164, 452 (1994).
J. Economy, K. Foster, A. Andreopoulos Jung, CHEMTECH, October 1992, p. 597.
23. 24.
K. Kinoshita, Phvsicochemical
Carbon: Electrochemical and Proverties. Wilev-Interscience. .
19iS. M. 0. Corapcioglu 569 (1987).
and C. P. Huang, Carbon
25,
C. A.‘ Leon y Leon, A. A. Lizzio and L. R. Radovic, Proc. Int. Carbon Con& Paris, France, 1990, p. 24. G. Newcombe, R. Hayes and M. Drikas, Cofloids
Surfaces A 78. 65 (1993).
C. *A.Leon y i-eon; J. G. Solar, V. Calemma and L. R. Radovic, Carbon 30, 797 (1992). B. Kastening, M. Hahn and J. Kremskiitter, J. Electroanal. Chem. 374. 159 (1994). J. Lyklema, J. Electroar&. &em. i8, 341 (1968). B. Kastenlng and S. Spinzig, J. Electroanal. Chem.
214, 295 (1986).
M. Miiller and B. Kastenlng, J. Electroanal.
374,
149 (1994).
Chem.
J. S. Noh and J. A. Schwarz, J. Colloid Inter5 Sci.
130, 157 (1989). S. Zalac and N. Kallav. . .I. Colloid Interf:a Sci. ~~ 149. 233 (1992). S. K. Verma and P. L. Walker, Jr., Carbon 30, 837 (1992). T. J. Fabish and D. E. Schleifer, Carbon 22, 19 (1984).
25.
J. S. Johnson, Jr., C. G. Westmoreland, F. H. Sweeton, K. A. Kraus, E. D. Hagaman, W. P. Eatherly and H. R. Child, J. Chromatography 354,
26. 27. 28.
J. S. Noh and J. A. Schwarz, Carbon 28, 675 (1990). Y. Otake and R. G. Jenkins. Carbon 31. 109 (19931. P. Vinke, M. van der Eijk, M. Vkrbree, A.6. Voskamp and H. van Bekkum, Carbon 32, 675 (1994). H. Tamon and M. Okazaki, in Fundamentals of Adsorption. (M. Suzuki, Ed.), Kodansha. Tokyo, 1992, p. 663.
23 1 (1986).
29.
and H.
Superdense potassium concentration in some graphite intercalation compounds F. GOUTFER-WURMSER, C. HI?ROLD and P. LAGRANGE Laboratoire de Chimie du Solide Mineral (URA CNRS 158), Universitk Henri Poincart? Nancy I, B.P. 239, 54506 Vandoeuvre-l&-Nancy Cedex, France (Received 29 September 2995; Accepted 2 October 1995)
Key Words - Graphite; potassium; oxygen: intercalation, poly-layered sheets
The discovery in 1926 by Fredenhagen and Cadenbach [I] of the first stage KCs graphite intercalation compound was the departure point for much research work concerning the intercalation of the metallic elements into graphite. In these reactions, the intercalated metals behave as electron donors towards graphite, that is consequently reduced by intercalated species. This charge transfer allows precisely the formation of a lamellar intercalation compound, which exhibits, because of this, an iono-metallic bonding. It is well known that only the more electropositive elements (alkali metals, earth alkaline metals and several lanthanides) are able to intercalate by themselves into graphite [2]. These binary
phases show in all cases that the intercalated sheets are mono-layered metallic planes. For a long time, the first stage KCs binary compound was considered as the phase, whose potassium concentration was the highest, even if Fredenhagen and Cadenbach mentioned the probable existence of a compound corresponding to the KCs chemical formula. For more than 60 years, it has been impossible to again synthesize this original graphite intercalation compound. However, numerous ternary compounds, that have been synthesized essentially during the eighties in our laboratory, contain large amounts of potassium. We
Pergamon
LETTERS TO THE EDITOR On the difference and the point
J.A.
between the isoelectric point of zero charge of carbons
M.J. ILLAN-G~MEZ,C.A. LEON Y LEON* and L.R. RADOVIC Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802 *Quantachrome Corporation, 1900 Corporate Drive, Boynton Beach, FL 33426 MENI~NDEZ,
(Received 3 August 1995; accepted in revised form 20 September 1995) Key Words - Point of zero charge; isoelectric point; surface chemistry; activated carbon.
It is well established that the behavior of carbon materials as adsorbents is dependent to a large extent on their physical surface properties (pore size distribution, surface area, pore volume) [ 11. The role of chemical and electrochemical surface properties in the use of carbons as catalyst supports [2-71 and adsorbents of both organic and inorganic species [2,8-111 is increasingly being appreciated. An important (re)current issue is, therefore, the methodology of chemical surface characterization of carbons, having in mind particularly the great affinity of high-surface-area carbons toward oxygen. Electrochemical methods have been widely used in the characterization of carbons [12]. It has been suggested that the isoelectric point (IEP) values are only representative of the external surface charges of carbon particles in solution, whereas the point of zero charge (PZC) varies in response to the net total (external and internal) surface charge of the particles [ 13-151. Hence the difference (PZC - IEP) can be interpreted as a measure of surface charge distribution of porous carbons. Values greater than zero indicate more negatively charged external than internal particle surfaces, and values close to zero correspond to a more homogeneous distribution of the surface charges. These suggestions, if confirmed, can be very useful for the characterization of the chemical nature of activated carbons. In this communication we discuss the interpretation of IEP and PZC data obtained for carbons subjected to a wide variety of conventional pretreatments. The broader issues of the origin of the electric charge on the carbon surface [ 16,171 and the nature of the double layer capacity in porous carbons [ 18-201 are beyond the scope of the present discussion. Twelve samples with very different PZC and IEP values were prepared using a commercial activated carbon, NORIT C (NC), as the precursor. The following treatments have been carried out: (i) oxidation in flowing air at 250°C for three hours (NcA); (ii) heat treatment in flowing N2 at 95O’C for three hours (NcN); (iii) the same as NcN but oxidized in flowing air at 250°C for seven minutes after cooling in N2 (NcNA); (iv) heat treatment in flowing H2 at 950°C for three hours (NcH); (v) room-temperature exposure of sample NcH to ambient air for 1, 7, 20 and 30 days (NcH-lox, -70x.
-200x, and -300x, respectively); (vi) oxidation of sample NcH with HN03 (70 vol.%) for 15 and 30 minutes at boiling point (NcH_HN03cl/4h, NcH-HN03cl/2h) and with HNOs (18 vol.%) during 16 h at 60°C (NcHHN03dl6h). These treatments did not alter drastically the physical surface properties of the resultant carbons. For instance, the BET surface area (N2, 77 K) for carbon NC was 1380 m2/g. After the most severe treatment (NcH), this value decreased to 1240 m2/g, and in the most extreme case (NcA) it descended to 1085 m2/g; the corresponding micropore volumes (obtained from the Dubinin-Radushkevich equation applied to C@ adsorption data at 273 K) were 0.31, 0.34 and 0.30 cm?g, respectively Preparation of samples NcA, NcN, NcNA and NcH for the determination of both PZC and IEP was carried out ten minutes after their treatments in order to avoid atmospheric oxidation as much as possible.The PZC values were determined by mass titra-tion as described elsewhere [ 14,21,22]. The IEP values were determined from electrophoretic mobility measurements using a Zeta-Meter 3.0+ apparatus, using 0.001 M KNOs as the supporting (indifferent) electrolyte. Table 1 lists the PZC and IEP values of all the samples studied. Figures 1 and 2 show representative plots of IEP and PZC determinations.
1655
Table 1 Electrochemical Properties of the Carbons Investigated Type of Carbon NC NcA NcN NcNA NcH NcH- 1ox NcH-70x NcH-200x NcH-300x NcH-HN03dl6h NcH-HN03c1/4h NcH-HN03cl/2h
PZC 2.5 2.2 5.2 4.2 9.0 8.9 9.0 8.6 8.4 3.9 3.3 2.8
IEP 2.6 2.2 3.5 2.9 4.9 4.2 4.2 3:; 3.5 1.6 1.4
PZC - IEP -0.1 0.0 1.7 ::: 4.7 ::; 5.1 0.4 1.7 1.4
Letters to the Editor
1656
12,
!
-NC
r~-T-.“-_
,
+NcA
+
NcN
e
NcH
10
a =a
6
I-r-+----
4
0
1
2,
3
_~
0
-3t”‘1’1’1’1’1’1”‘1” 4
5
6
7
8
9
10
0
2
PH
Fig. 1 Determination carbons
of the isoelectric
point of selected
The low and close values of PZC and IEP for the as-received activated carbon (NC) suggest that it has rather homogeneous and acidic external and internal surfaces. The slight decrease in both PZC and IEP observed when NC is subjected to air oxidation at 250 “C (NcA) shows that more acidic oxygen-containing surface groups were added to both the internal and external carbon surfaces. The similar values of PZC and IEP indicate that this treatment affects the entire surface to a similar extent. In the case of the sample heated in a N2 atmosphere (NcN), it is known that most of the functional groups are removed from the surface. However, this treatment creates some very active sites which are oxidized as soon as the sample is exposed to air at room temperature. Hence the values of PZC and IEP are higher than those for NC, albeit not as high as should be expected in oxygen-free environments. In addition, as the diffusion effects for O2 at room temperature are important for microporous materials such as active carbons, this oxidation takes place preferentially on the external surface. The higher value of the PZC compared to the IEP confirms this point. Similar considerations can be made in the case of NcNA. In this case, as a consequence of the higher degree of oxidation of this sample due to its exposure to air at 250°C. the values of PZC and IEP are lower than those of the NcN sample. As with the Nz treatment, heating the as-received carbon in Ha (NcH) removes most of the functional groups from the surface. However, in this case the high value of PZC shows that the internal surface preserves its basic character. This is due to the fact that most of the active sites are stabilized by the hydrogen present during the treatment [ 11,231 and, consequently, the number of acidic oxygen-containing surface groups is much lower than the number of oxygen-free Lewis base sites on the carbon surface [16,24]. In spite of this fact, some oxidation occurs preferentially on the external surface, as indicated by the relatively low IFP value for this carbon. Upon comparing the evolution of PZC and IEP values when sample NcH is exposed to room temperature oxidation for up to 30 days, the following observations are worth pointing out: (a) There was a slight decrease in both PZC and IEP with exposure time. Therefore, even after stabilizing the surface with hydrogen as indicated, some oxidation does take place. (b) The difference between PZC and IEP increased as the
Fig. 2 Determination selected carbons.
4 Carbon
6 fraction
8 (wt%)
10
of the point of zero charge
degree of oxidation increased. This indicates a preferential oxidation of the external surface. (c) The greatest increase in the value of (PZC-IEP) was observed after the first day. This is consistent with the expectation that the degree of oxidation exhibits an exponential decay, indicative of a first-order rate process. When sample NcH was oxidized with HNOa, both the PZC and the IEP values became lower than those observed for the original sample, indicating, as expected [25-281, the increase in the acidic oxygencontaining surface groups. However, the PZC and IEP values and the difference between them were found to depend on temperature, concentration and contact time used in the oxidation treatment. The PZC and IEP values decrease when acid concentration is increased and when longer contact times are used, reflecting the fact that more acidic oxygen-containing surface groups have been introduced [25]. On the other hand, the (PZC-IEP} difference shows that more uniform oxidation takes place when diluted acid is used. In contrast, preferential external surface oxidation is observed if the concentration of acid is increased. The trend of the { PZC-IEP) difference for samples prepared with HNOa using different contact times (NcH-HN03cl/4h and NcH-HN03cl/2h) shows that the decrease in this variable also produces a preferential external oxidation of the carbon. These results confirm that, by carefully selecting the acid oxidation conditions, it is possible to prepare active carbons with different acidity levels and oxygen surface group distributions. This flexibility can be very useful for the preparation of carbon-supported catalysts, for which it is expected that the most uniform surface oxidation treatments lead to higher catalyst dispersions and higher catalytic activities [2,3,6]. It can also be exploited in the design of activated carbon adsorbents for the removal of specific liquid adsorbates [8,29]. Finally, these results also show that the combination of two relatively routine characterization techniques, electrophoresis and mass titration, is a convenient and powerful tool for the analysis and design of surface chemistry of active carbons, especially for determining the spatial (radial) distribution of the acidic oxygen functional groups within carbon particles. For a more thorough analysis, the use of additional techniques (e.g., temperature-programmed desorption, acid and base titrations, surface spectroscopies) is desirable.
1657
Letters to the Editor Acknowledgments.
This study was made possible by financial support from the Carbon Research Center (Penn State) and postdoctoral grants for JAM (Spanish Scientific Research Council, CSIC) and MJIG (Ministry of Science and Education of Spain).
12. 13. 14. 15.
REFERENCES I. 2. 3. 4. 5. 6. 7.
8.
9.
IO. I I.
J. W. Patrick
Characterization
Porosity and Applications.
(Ed.),
in
Carbons:
Halsted Press, New York, 1995. __ C. A. Leon y Leon and L. R. Radovic, in Chemistry and Physics of Carbon, Vol. 24. (P. A. Thrower, Ed.), Dekker, New York, 1994, p. 213. J. M. Solar, F. J. Derbyshire, V. H. J. de Beer and L. R. Radovic, J. Cutal. 129, 330 (1991). K. T. Kim, J. S. Chung, K. H. Lee, Y. G. Kim and J. Y. Sung, Carbon 30, 467 (1992). C. Moreno-Castilla, M. A. Ferro-Garcia, J. RiveraUtrilla and J. P. Joly, Energy & Fuels 8, 1233
16. 17. 18. 19. 20. 21. 22.
(1994).
M. C. RomBn-Martinez, D. Cazorla-Amor6s, A. Linares-Solano, C. Salinas-Martinez de Lecea, H. Yamashita and M. Anpo. Carbon 33, 3 (1995). L. R. Radovic and C. Sudhakar, “Carbon as a Catalyst Support” in Introduction to Carbon Technologies (H. Marsh and E. A. Heintz, Eds.). Elsevier Science, 1995, in press. L. R. Radovic, J. I. Ume and A. W. Scaroni, “On Tailoring the Surface Chemistry of Activated Carbons for Their Use in Purification of Aqueous Effluents,” in Fundamentals of Adsorption (M. D.
LeVan, Ed.). Elsevier Science, 1995, in press. J. M. Solar, C. A. Leon y Leon, K. Osseo-Asare and L. R. Radovic, Carbon 28, 369 (1990). G. Newcombe, J. Colloid Interf. Sci. 164, 452 (1994).
J. Economy, K. Foster, A. Andreopoulos Jung, CHEMTECH, October 1992, p. 597.
23. 24.
K. Kinoshita, Phvsicochemical
Carbon: Electrochemical and Proverties. Wilev-Interscience. .
19iS. M. 0. Corapcioglu 569 (1987).
and C. P. Huang, Carbon
25,
C. A.‘ Leon y Leon, A. A. Lizzio and L. R. Radovic, Proc. Int. Carbon Con& Paris, France, 1990, p. 24. G. Newcombe, R. Hayes and M. Drikas, Cofloids
Surfaces A 78. 65 (1993).
C. *A.Leon y i-eon; J. G. Solar, V. Calemma and L. R. Radovic, Carbon 30, 797 (1992). B. Kastening, M. Hahn and J. Kremskiitter, J. Electroanal. Chem. 374. 159 (1994). J. Lyklema, J. Electroar&. &em. i8, 341 (1968). B. Kastenlng and S. Spinzig, J. Electroanal. Chem.
214, 295 (1986).
M. Miiller and B. Kastenlng, J. Electroanal.
374,
149 (1994).
Chem.
J. S. Noh and J. A. Schwarz, J. Colloid Inter5 Sci.
130, 157 (1989). S. Zalac and N. Kallav. . .I. Colloid Interf:a Sci. ~~ 149. 233 (1992). S. K. Verma and P. L. Walker, Jr., Carbon 30, 837 (1992). T. J. Fabish and D. E. Schleifer, Carbon 22, 19 (1984).
25.
J. S. Johnson, Jr., C. G. Westmoreland, F. H. Sweeton, K. A. Kraus, E. D. Hagaman, W. P. Eatherly and H. R. Child, J. Chromatography 354,
26. 27. 28.
J. S. Noh and J. A. Schwarz, Carbon 28, 675 (1990). Y. Otake and R. G. Jenkins. Carbon 31. 109 (19931. P. Vinke, M. van der Eijk, M. Vkrbree, A.6. Voskamp and H. van Bekkum, Carbon 32, 675 (1994). H. Tamon and M. Okazaki, in Fundamentals of Adsorption. (M. Suzuki, Ed.), Kodansha. Tokyo, 1992, p. 663.
23 1 (1986).
29.
and H.
Superdense potassium concentration in some graphite intercalation compounds F. GOUTFER-WURMSER, C. HI?ROLD and P. LAGRANGE Laboratoire de Chimie du Solide Mineral (URA CNRS 158), Universitk Henri Poincart? Nancy I, B.P. 239, 54506 Vandoeuvre-l&-Nancy Cedex, France (Received 29 September 2995; Accepted 2 October 1995)
Key Words - Graphite; potassium; oxygen: intercalation, poly-layered sheets
The discovery in 1926 by Fredenhagen and Cadenbach [I] of the first stage KCs graphite intercalation compound was the departure point for much research work concerning the intercalation of the metallic elements into graphite. In these reactions, the intercalated metals behave as electron donors towards graphite, that is consequently reduced by intercalated species. This charge transfer allows precisely the formation of a lamellar intercalation compound, which exhibits, because of this, an iono-metallic bonding. It is well known that only the more electropositive elements (alkali metals, earth alkaline metals and several lanthanides) are able to intercalate by themselves into graphite [2]. These binary
phases show in all cases that the intercalated sheets are mono-layered metallic planes. For a long time, the first stage KCs binary compound was considered as the phase, whose potassium concentration was the highest, even if Fredenhagen and Cadenbach mentioned the probable existence of a compound corresponding to the KCs chemical formula. For more than 60 years, it has been impossible to again synthesize this original graphite intercalation compound. However, numerous ternary compounds, that have been synthesized essentially during the eighties in our laboratory, contain large amounts of potassium. We