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Surface studies on graphite: Immersional energetics
SURFACE STUDIES IMMERSIONAL
ON GRAPHITE: ENERGETICS
S. S. BARTON and B. H. HARRISON Department of Chemistry and Chemical Engineering, Royal Military College Kingston, Ontario, Canada
of Canada,
(Received 28 May 1971) Abstract-The heats of immersion of graphite degassed at temperatures up to 1000°C have been measured in water, methanol and benzene. The removal oloxygen complexes during degassing has been followed quantitatively using a mass-spectrometer system allowing the changes in immersional energetics to be related to the desorbed species. 1. INTRODUCTION
studied. IR studies by normal Lransmission techniques 113,141 and by internal reflectance spectroscopy [ 151 have verified the existence of carbonyl and carboxyl groups, and the existence of adjacent carboxyl groups at the edges of the basal planes, which are desorbed as CO1, leaving a double bond between remaining carbon atoms, has been postulated [15]. It has been shown that the reactivity of graphite is centered predominantly at the prismatic faces (edges of the basal planes) where the layers terminate with unsaturated bonds[l6, 171. It has been speculated that these prismatic or edge planes are responsible for the formation of oxides, and mass spectrometric investigations into the formation of these oxides as intermediates in the oxidation of graphite have helped in the understanding of the oxidation process [18,19]. Desorption studies on graphite, similar to that used in these studies, showed a striking difference between the energetic nature of the CO and CO, desorption [20] leading to the conclusion that the CO and CO, are desorbed independently. Studies on carbon and graphite which attempted to relate the amount of oxide on the surface to immersional energetics have centered around water because of the hydrophilic nature of the oxide surface k1,6,7,21].
Difficulty has been encountered during immersional studies on carbon and graphite surfaces when attempts have been made to relate heats of wetting to surface area [l, 21. Varying amounts of oxide on these surfaces has been used to explain these variations. It has been known for a long time that the surface of graphite contains oxides which can only be removed as CO and CO, on heating to temperatures of up to 1WWC. Surf-aces free frosn any oxides have been shown to be hydrophobic, while those with some portion of the surface covered with oxide have hydrophilic properties pertaining to those regions. It has been shown that the amount of oxide can markedly effect the adsorptive properties of the surface towards gases such as carbon dioxide [3], methanol [4], and water [4-8] as well as adsorption from solution [4,9, 101. The nature of these oxides has been speculated upon by investigators using radical reactions on the carbon surfaces [ll, 121and by IR techniques (1%151. Two recent reviews [I 1, 121 have shown the existence of both acidic and basic oxides on the surfaces of carbons although the latter have received little attention. Tests showed the presence of carboxyl, carbonyl, hydroxyl (phenolic), quinone and lactone functional groups on the surfaces 245
CAR Vol. IO, No. 3-A
S. S. BARTON and B. H. HARRISON
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Attempts to relate immersional energetics to thermal pretreatment 122-241 have been complicated by large changes in surface area [22], and possible changes in surface structure [23]. The study by Wade[23] found a considerable drop in heat of immersion when the carbon they used was degassed between 900 and 1000°C. Being unable to relate this drop to any change in surface area or removal of oxides they correlated it to a change in surface structure, although the marked bulk structural change occurring during graphitisation by heat treatment from 1000°C to 2700°C caused little change in heat of immersion Considerable hydrogen desorption in the temperature range in question was observed. By using a high surface area graphite which undergoes no surface area or structural change during degassing in the temperature range 0-1000°C a comparative study has been made between immersional energetics and the thermally desorbed gases. The results of this study are reported in this paper. f. EXPERIMENTAL Heats of immersion of the graphite in water, benzene and methanol were measured in an isothermal calorimeter described previously [25]. A non-porous graphite was obtained from Acheson’s Colloid (Canada) Ltd., and was purified by the same procedure as used by Thomas et aE. [26] to reduce the impurity level of the graphite to less than 0.075% w/w. Samples to be degassed at temperatures of less than 600°C were degassed in thin Pyrex bulbs on a grease-free vacuum system equipped with mercury diffusion pump and liquid nitrogen cold trap. The vacuum using ionisation pressure was monitored gauges, and the degassing was terminated by sealing the sample in the bulb after at least 24 hr when the pressure had dropped below 10m6Torr. Samples degassed at 600°C and above were treated in a different manner. A horizontal
quartz tube was used to degas the sample. One end was attached to a thin walled Pyrex bulb by a B7 cone and socket which was sealed with Apiezon wax. Part-way down the side of the tube a B14 cone was attached allowing the tube to be attached to the vacuum line,, and also allowing it to be rotated about the joint to a vertical position. The joint was greased with Alpiezon N grease and a small cold trap interposed between the joint and sample to prevent contamination of the sample by the grease. By rotating the tube after degassing, the graphite could be tapped into the bulb, which is subsequently sealed. By weighing the Pyrex bulb with attached socket before sealing, and the sealed bulb, graphite, socket and stem after sealing, the weight of graphite in the bulb could be determined. This procedure maintained a high vacuum (< 10e6 Torr) over the graphite while it was transferred to the Pyrex bulbs used for heats of immersion studies. The benzene and methanol used in this study were spectroscopic grade while the water was de-ionized distilled water. Heats of immersion were measured in duplicate and sometimes triplicate, values being repeatable to 0.05 cals. Samples of approximately 1 g were used and the normal corrections applied to the calculation of heat of immersion. The gases evolved during thermal treatment were analysed using a mass-spectrometer system similar to that described previously [19]. Basically the evolved gases, as the sample is heated in 100°C intervals to IOOO’C, are quantitatively measured by expanding them into a known volume. The pressure of each gaseous component is then measured using a mass spectrometer. The surface area of the sample was measured, by low temperature nitrogen adsorption at 77”K, in a conventional volumetric adsorption apparatus similar to that described by Feath[27]. Surface areas were computed by the BET method with at least 12 points being used for each determination.
247
SURFACE STUDIES ON GRAPHITE
The graphite was found to have a surface area of 163 + 2 m2 g-l based on a nitrogen molecule cross-sectional area of 16.2 A2. 3. RESULTS
AND DISCUSSION
The effect of degassing temperature on the heats of immersion of graphite were measured over the temperature range O1000°C. The major gaseous species desorbed as the graphite was heated are shown in Fig. 1. The amount of gas desorbed in each
Water desorption from the surface ceases at 3OO”C, the total quantity being small in relation to the other components. The gaseous species resulting from the decomposition of the surface oxide are CO and CO, which show maximum desorption at 700°C and 400°C respectively. At temperatures above 800°C hydrogen desorption occurs, the quantity desorbed showing a rapid increase between 900 and 1000°C. The extent of surface oxide coverage can be determined from the number of carbon atoms desorbed on the basis of an assigned cross sectional area of 8.3 A2 for a carbon atom on the edge plane of graphite[19]. On this basis the oxide coverage corresponds to 48 m”g-’ or about 30 per cent of the total surface. Figures 2-4 illustrate the effect of degassing temperature on the heats of immersion of the graphite in water, methanol and benzene respectively. The heats are also displayed on a unit area basis (ergs cm-“) for comparative purposes.
r Desorption
temperature.
OC
Fig. 1. Amount of gas desorbed from graphite as it is heated in 100°C increments to 1000°C. Carbon monoxide A; hydrogen Cl; carbon dioxide 0.
t temperature increment as the graphite is heated in 100°C steps to 1000°C is displayed. Table 1 shows the total quantity of each component desorbed. Table
2
i
1. Total quantity of each desorbed species
Gas
Quantity p moles g-r
co co* Hz H,O
765.7 186.3 116.4 5.77
01 Degassing
temperature,
“C
Fig. 2. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in water.
248
S. S. BARTON
and B. H. HARRISON
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I
0
I 200
I 400
I 600
Degassing temperature,
I 800
I low
OC
Fig. 3. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in methanol.
decrease in the heat of immersion. The final heat of 40 ergs crnp2 on removal of all the oxides is close to that value reported by Wade (32 ergs cmM2), whose [23] for “Graphon”, surface is predominantly hydrophobic basal plane. This difference may be expected since the graphite used in this study had some 30 per cent of the surface exposed as edge plane, the remaining surface being basal plane. However Wade’s value was obtained on the basis of a “Graphon” surface area calculated using 20-O & for the area of the nitrogen molecule. If this nitrogen molecule area is used to obtain the surface area of the graphite used in this study, complete agreement between the heats of wetting is achieved. It can be seen from Fig. 5 that the heats of immersion in water vary as a linear function of the total oxygen content of the surface. Oxygen
q0
,
,
200
400
,
600 C&gassing temperature,
,
,
800
1000
desorbed
as
COzs
moles-g-’
400
500
$5
“C
Fig. 4. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in benzene. The behavior with water is similar to that found in other investigations [21,22,24]. The data shows a small increase in heat of immersion on increasing the degassing temperature to 200°C followed by a continuous decrease in heat as the degassing temperature was raised to 1000°C. Up to 2OO”C, as water is removed from the surface exposing extra oxide surface on which it was adsorbed, an increase in heat of immersion is expected. As the degassing temperature was further raised, removing oxide, the continuous fall in heat is in line with the general theory that the hydrophilic surface and sites for hydrogen bonding are being removed, resulting in a
‘-0
I00 Total
200 oxygen
300 desorbed.
p moles-
600
g-l
Fig. 5. Heat of immersion of graphite in water in relation to the amount of oxygen desorbed as CO, A, and to the total oxygen desorbed as CO and CO2 0. Puri et aE. [4,21] have reported that the heats of immersion of charcoal in water vary linearly with respect to the evolved COz, but as can be seen from the figure the heat data plotted against evolved CO, does not follow
SURFACE STUDtES
a linear relationship although the deviations from a linear form only occurs during the desorption of the final 20-25% of the CO,. Since the observed heat of immersion/unit area (hi) of the oxide free graphite surface in water agrees with that found for the hydrophobic basal plane of Graphon, it may be assumed that the heat of immersion/unit area on an oxide free edge plane is the same as that ou the hydrophobic basal plane, (h& If it is further assumed that the heat of immersion/unit area on the hydrophilic, oxide covered surface {ho) is independent of the extent of that surface then the equation
249
ON GRAPWTE
Heats of immersion in methanol, as shown in Fig. 3, were found to increase in the Q-200°C interval in a fashion similar to water and to decrease further as the oxide is removed in the 200400°C temperature interval as the opportunity for hydrogen bonding with the oxide is removed. The removal of oxide in this region occurs mainly as C&, Above 6O@‘C, as the remaining oxide is removed as CO, the heat of imm~~rsion increases slightly indicating a higher interaction energy of the methanol with the bare surface than that covered with oxide removable as CU. Figure 6 relates the changes in heats of immersion in methanol with the amount of oxide desorbed. It can be seen that. the
which reduces to
can be written. This &near equation represents the observed heat of immersion as a function of the extent of oxidation of the surface. Here ST represents the total RET surface area of the graphite, S0 represents the total surface which was covered with oxygen and F, represents the fractional coverage of this surface with oxygen. Using the linearity of Fig. 5 to support this view calculations show that h, = 175 ergs trnez. This value is considerably less than the value of 730 ergs cm -2 found by tiealey et al, [7] on oxidised Graphon samples. However, water adsorption on the oxide surface af Graphon@] showed that the isoteric heat of adsorption measured between -3-3X and 20°C closely approached the value for the heat of liquefaction of water. The net heat of adsorption on this surface would therefore be close to Zero and so the heat of immersion on this surface can be approximated to the surface enthalpy of water (I 18 ergs cmF2). A value of 175 ergs cm --2 is therefore not unreasonable. It follows that the energy of hydrogen bonding of water to the surface oxides is similar to that found in bulk water itself.
Oxygen desarbed as CO,,
pmcles-g-’
IOU
200
0
50
I
J
t
150
I
I
1
z f
3-
Fig. 6. Heat of immersion of graphite in methanol in relation t.o the amount of oxygen desorbed as CO2 A, and to the total oxygen desorbed as CO and CC& 0. decrease in heat can be linearly related to the amount of COz desorbed, suggesting hydrogen bonding is occurring between the oxide removed as COz and not with that removed as CO. It is suggested that the heat of immersion in methanol on the hydrophobic surface is slightly higher than that on the hydrophilic oxide surface pertaining to the desorption of CO. The total oxygen curve in the figure is then explained as a combination of two
250
S. S. BARTON and B. H. HARRISON
effects. These are a decrease in heat from hydrogen bonding as the CO, sites are removed occurring simultaneously with an increase in heat as the CO sites are removed. Benzene on the other hand is insensitive to the removal of COP from the surface (cf. Fig. 4). After an ‘initial decrease in the heat of immersion from 0 to 100°C the heat is constant between 100” and 550°C over which temperature CO2 is the main desorption product. The heat then decreased between 550” and 8OO”C, where m~mum desorption of CO occurs, only to increase again between 900” and 1000°C in which temperature interval hydrogen desorption was encountered. Reduction of the heat of immersion between 0” and 100°C can be related to dehydration of the surface and reduced heat of adsorption. Removal of oxides as CO can be related to a reduction in the interaction between the partial charge on the carbonyl carbon atom and the Irr-electron cloud of the benzene molecule. It may be enlightening to review at this point the information obtained from functional group analysis on carbons. Carboxy1 and Carbonyl groups are held mainly responsible for the desorption products CO, and CO respectively. These groups are known to be acidic in nature, although their strengths are quite different. The highly acidic carboxy1 groups have been neutralised with weak bases such as aqueous NaHCO$ solutions while the weakly acidic carbonyl groups require strong bases such as sodium ethoxide in ethanol [12]. Although the exact nature of the COz desorbing surface groups on the present graphite is not known, sufficient hydrogen was desorbed at higher temperatures, after COz removal was complete, for carboxyl (-GOOH) groups to have been originally present. It has been found on a similar graphite[28] that after the graphite surface was initially cleaned to a comparable ratio of CO/CO, as in this study, subsequent oxygen chemisorption and desorption yields pre-
dominantly CO with only a small fraction of CO2 being evolved. It is therefore possible to infer that the CO, and H, could be related as carboxyl groups. The difference in the acidic nature of the two types of oxide surfaces are obviously reflected in their interaction with methanol where the strongly acidic carboxyl or CO2 producing oxides are capable of hydrogen bonding with the oxygen atom of the methanol. The decrease in heat of immersion as the COz is desorbed can be approximated from Fig. 6 to about 7-5 kcalslmofe of CO, desorbed which is representative of physical and not chemical adsorption processes.
4. CONCLUSION
The different roles played by the CO and CO, evolving oxide surfaces have been clearly evident in the heat of immersion studies. The results on the graphite surface have served as an interesting comparison with those on the surface of Spheron 6 [23] an un~aphitised carbon with an approximately equivalent proportion of its surface covered with oxygen. The magnitude as well as the relative changes in heats of immersion in polar liquids are found to be grossly different on the two surfaces. The large decrease in the heats of immersion on the carbon surface when degassed between 900” and lOOO”C[23] can be probably related to surface ordering as the measured heats were found to drop to values close to those found on graphitic surfaces. It therefore appears that surface structure is as important if not more important than surface oxide in the heats of immersion/unit area values which have been recorded. Acknowledgemxnt- Research supported by Defence Research Board of Canada, Grant No. 9530-72. REFERENCES 1. Marsh IX, Fuel44 253 (1965). 2. Broadbent K. A., Dollimore I)., and Dollimore j., CUYbon4,281 (1966). 3. Deitz V. R., J. Pkys. Ckmz. 71,830 (1967).
SURFACE STUDIES 4. Puri B. R., Carbon4,391 (1966). 5. Puri B. R., Murari K., and Singh D. D., J. Phys. Chem. 65,37 (1961). 6. Young G. J., Chessick J. J., Healey F. H., and Zettlemoyer A. C.,J. Phys. Chem. 58,313 (1954). 7. Healey F. H., Yu Y. F., and Chessick J. J.,J. Phys. Chem. 59,399 (1955). 8. Walker P. L., Jr., and Janov J., J. COB. Inter-f&e Sci. 28,449 (1968). 9. Gasser C. G., and Kipling J. J., Proc. Fourth Carbon Conf , p. 55, Pergamon Press, Oxford (1960). 10. Coughlin R. W., Ezra F. S., and Tan R. N., J. Coil. Interface Sci. 28,386 (1968). 11. DonnettJ. B.,Carbon6,161 (1968). 12. Boehm H. P.,Advan. Catalysis 16,198 (1966). 13. Smith R. N., Young D. A., and Smith R. A., Trans. Faraday Sot. 62,228O (1966). 14. Friedel R. A., and Hofer L. J. E., J. Phys. Chem. 74,292l (1970). 15. Mattson J. S., Lee L., Mark H. B., Jr., and Weber W. J., Jr., J. COB. Interface Sci. 33, 281 (1970). 16. Hennig G. R., Proc. Fifth Carbon Co@, Vol. 1, p. 143 Pergamon Press, Oxford (1963). 17. Thomas J. M., Chem. Phys. Carbon 1,122 (1965).
ON GRAPHITE
251
18. Laine N. R., Vastola F. J. and Walker P. L., Jr., J. Phys. Chem. 67,203O (1963). 19. Dollimore J., Freedman C. M. and Harrison B. H., 17th Annual Conf. on Moss Spectrometry and Allied Topics, Dallas, Texas (1969). 20. Dollimore J., Freedman C. M., and Harrison B. H., Proc. 3rd Conf Ind. Carbon and Graphite, London (1969). 21. Puri B. R., Singh D. D., and Sharma L. R., J. Phys. Chem. 62,756 (1958). 22. Razouk R. L., Nashed S. H., and Mom-ad W. E., Carbon 2,539 (1964). 23. Wade W. H., J. Coil. Znterfuce Sci. 31, 111 (1969). 24. Brusset H., Martin J. J. P. and Mendelbaum H. G., Bull. Sot. Chim. France No. 7. 2346 (1967). 25. Barton S. S., and Boulton G., J. Chem. Eng. Data 15,66 (1970). 26. Hughes E. E. G., Williams B. R., and Thomas J. M., Trans. Faraday Sot. 58,201l (1962). 27. Feath P., Adsorption and Vacuum Techniques, Inst. Sci. Technol, Univ. Michigan (1962). 28. Brown J. G., Dollimore J., Freedman C. M., and Harrison B. H., 8th Conf. on Vacuum Microbalance Techniques, Wakefield, U.S.A. (1969).
ON GRAPHITE: ENERGETICS
S. S. BARTON and B. H. HARRISON Department of Chemistry and Chemical Engineering, Royal Military College Kingston, Ontario, Canada
of Canada,
(Received 28 May 1971) Abstract-The heats of immersion of graphite degassed at temperatures up to 1000°C have been measured in water, methanol and benzene. The removal oloxygen complexes during degassing has been followed quantitatively using a mass-spectrometer system allowing the changes in immersional energetics to be related to the desorbed species. 1. INTRODUCTION
studied. IR studies by normal Lransmission techniques 113,141 and by internal reflectance spectroscopy [ 151 have verified the existence of carbonyl and carboxyl groups, and the existence of adjacent carboxyl groups at the edges of the basal planes, which are desorbed as CO1, leaving a double bond between remaining carbon atoms, has been postulated [15]. It has been shown that the reactivity of graphite is centered predominantly at the prismatic faces (edges of the basal planes) where the layers terminate with unsaturated bonds[l6, 171. It has been speculated that these prismatic or edge planes are responsible for the formation of oxides, and mass spectrometric investigations into the formation of these oxides as intermediates in the oxidation of graphite have helped in the understanding of the oxidation process [18,19]. Desorption studies on graphite, similar to that used in these studies, showed a striking difference between the energetic nature of the CO and CO, desorption [20] leading to the conclusion that the CO and CO, are desorbed independently. Studies on carbon and graphite which attempted to relate the amount of oxide on the surface to immersional energetics have centered around water because of the hydrophilic nature of the oxide surface k1,6,7,21].
Difficulty has been encountered during immersional studies on carbon and graphite surfaces when attempts have been made to relate heats of wetting to surface area [l, 21. Varying amounts of oxide on these surfaces has been used to explain these variations. It has been known for a long time that the surface of graphite contains oxides which can only be removed as CO and CO, on heating to temperatures of up to 1WWC. Surf-aces free frosn any oxides have been shown to be hydrophobic, while those with some portion of the surface covered with oxide have hydrophilic properties pertaining to those regions. It has been shown that the amount of oxide can markedly effect the adsorptive properties of the surface towards gases such as carbon dioxide [3], methanol [4], and water [4-8] as well as adsorption from solution [4,9, 101. The nature of these oxides has been speculated upon by investigators using radical reactions on the carbon surfaces [ll, 121and by IR techniques (1%151. Two recent reviews [I 1, 121 have shown the existence of both acidic and basic oxides on the surfaces of carbons although the latter have received little attention. Tests showed the presence of carboxyl, carbonyl, hydroxyl (phenolic), quinone and lactone functional groups on the surfaces 245
CAR Vol. IO, No. 3-A
S. S. BARTON and B. H. HARRISON
246
Attempts to relate immersional energetics to thermal pretreatment 122-241 have been complicated by large changes in surface area [22], and possible changes in surface structure [23]. The study by Wade[23] found a considerable drop in heat of immersion when the carbon they used was degassed between 900 and 1000°C. Being unable to relate this drop to any change in surface area or removal of oxides they correlated it to a change in surface structure, although the marked bulk structural change occurring during graphitisation by heat treatment from 1000°C to 2700°C caused little change in heat of immersion Considerable hydrogen desorption in the temperature range in question was observed. By using a high surface area graphite which undergoes no surface area or structural change during degassing in the temperature range 0-1000°C a comparative study has been made between immersional energetics and the thermally desorbed gases. The results of this study are reported in this paper. f. EXPERIMENTAL Heats of immersion of the graphite in water, benzene and methanol were measured in an isothermal calorimeter described previously [25]. A non-porous graphite was obtained from Acheson’s Colloid (Canada) Ltd., and was purified by the same procedure as used by Thomas et aE. [26] to reduce the impurity level of the graphite to less than 0.075% w/w. Samples to be degassed at temperatures of less than 600°C were degassed in thin Pyrex bulbs on a grease-free vacuum system equipped with mercury diffusion pump and liquid nitrogen cold trap. The vacuum using ionisation pressure was monitored gauges, and the degassing was terminated by sealing the sample in the bulb after at least 24 hr when the pressure had dropped below 10m6Torr. Samples degassed at 600°C and above were treated in a different manner. A horizontal
quartz tube was used to degas the sample. One end was attached to a thin walled Pyrex bulb by a B7 cone and socket which was sealed with Apiezon wax. Part-way down the side of the tube a B14 cone was attached allowing the tube to be attached to the vacuum line,, and also allowing it to be rotated about the joint to a vertical position. The joint was greased with Alpiezon N grease and a small cold trap interposed between the joint and sample to prevent contamination of the sample by the grease. By rotating the tube after degassing, the graphite could be tapped into the bulb, which is subsequently sealed. By weighing the Pyrex bulb with attached socket before sealing, and the sealed bulb, graphite, socket and stem after sealing, the weight of graphite in the bulb could be determined. This procedure maintained a high vacuum (< 10e6 Torr) over the graphite while it was transferred to the Pyrex bulbs used for heats of immersion studies. The benzene and methanol used in this study were spectroscopic grade while the water was de-ionized distilled water. Heats of immersion were measured in duplicate and sometimes triplicate, values being repeatable to 0.05 cals. Samples of approximately 1 g were used and the normal corrections applied to the calculation of heat of immersion. The gases evolved during thermal treatment were analysed using a mass-spectrometer system similar to that described previously [19]. Basically the evolved gases, as the sample is heated in 100°C intervals to IOOO’C, are quantitatively measured by expanding them into a known volume. The pressure of each gaseous component is then measured using a mass spectrometer. The surface area of the sample was measured, by low temperature nitrogen adsorption at 77”K, in a conventional volumetric adsorption apparatus similar to that described by Feath[27]. Surface areas were computed by the BET method with at least 12 points being used for each determination.
247
SURFACE STUDIES ON GRAPHITE
The graphite was found to have a surface area of 163 + 2 m2 g-l based on a nitrogen molecule cross-sectional area of 16.2 A2. 3. RESULTS
AND DISCUSSION
The effect of degassing temperature on the heats of immersion of graphite were measured over the temperature range O1000°C. The major gaseous species desorbed as the graphite was heated are shown in Fig. 1. The amount of gas desorbed in each
Water desorption from the surface ceases at 3OO”C, the total quantity being small in relation to the other components. The gaseous species resulting from the decomposition of the surface oxide are CO and CO, which show maximum desorption at 700°C and 400°C respectively. At temperatures above 800°C hydrogen desorption occurs, the quantity desorbed showing a rapid increase between 900 and 1000°C. The extent of surface oxide coverage can be determined from the number of carbon atoms desorbed on the basis of an assigned cross sectional area of 8.3 A2 for a carbon atom on the edge plane of graphite[19]. On this basis the oxide coverage corresponds to 48 m”g-’ or about 30 per cent of the total surface. Figures 2-4 illustrate the effect of degassing temperature on the heats of immersion of the graphite in water, methanol and benzene respectively. The heats are also displayed on a unit area basis (ergs cm-“) for comparative purposes.
r Desorption
temperature.
OC
Fig. 1. Amount of gas desorbed from graphite as it is heated in 100°C increments to 1000°C. Carbon monoxide A; hydrogen Cl; carbon dioxide 0.
t temperature increment as the graphite is heated in 100°C steps to 1000°C is displayed. Table 1 shows the total quantity of each component desorbed. Table
2
i
1. Total quantity of each desorbed species
Gas
Quantity p moles g-r
co co* Hz H,O
765.7 186.3 116.4 5.77
01 Degassing
temperature,
“C
Fig. 2. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in water.
248
S. S. BARTON
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I
0
I 200
I 400
I 600
Degassing temperature,
I 800
I low
OC
Fig. 3. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in methanol.
decrease in the heat of immersion. The final heat of 40 ergs crnp2 on removal of all the oxides is close to that value reported by Wade (32 ergs cmM2), whose [23] for “Graphon”, surface is predominantly hydrophobic basal plane. This difference may be expected since the graphite used in this study had some 30 per cent of the surface exposed as edge plane, the remaining surface being basal plane. However Wade’s value was obtained on the basis of a “Graphon” surface area calculated using 20-O & for the area of the nitrogen molecule. If this nitrogen molecule area is used to obtain the surface area of the graphite used in this study, complete agreement between the heats of wetting is achieved. It can be seen from Fig. 5 that the heats of immersion in water vary as a linear function of the total oxygen content of the surface. Oxygen
q0
,
,
200
400
,
600 C&gassing temperature,
,
,
800
1000
desorbed
as
COzs
moles-g-’
400
500
$5
“C
Fig. 4. Heat of immersion of graphite, degassed at temperatures up to lOOO”C,in benzene. The behavior with water is similar to that found in other investigations [21,22,24]. The data shows a small increase in heat of immersion on increasing the degassing temperature to 200°C followed by a continuous decrease in heat as the degassing temperature was raised to 1000°C. Up to 2OO”C, as water is removed from the surface exposing extra oxide surface on which it was adsorbed, an increase in heat of immersion is expected. As the degassing temperature was further raised, removing oxide, the continuous fall in heat is in line with the general theory that the hydrophilic surface and sites for hydrogen bonding are being removed, resulting in a
‘-0
I00 Total
200 oxygen
300 desorbed.
p moles-
600
g-l
Fig. 5. Heat of immersion of graphite in water in relation to the amount of oxygen desorbed as CO, A, and to the total oxygen desorbed as CO and CO2 0. Puri et aE. [4,21] have reported that the heats of immersion of charcoal in water vary linearly with respect to the evolved COz, but as can be seen from the figure the heat data plotted against evolved CO, does not follow
SURFACE STUDtES
a linear relationship although the deviations from a linear form only occurs during the desorption of the final 20-25% of the CO,. Since the observed heat of immersion/unit area (hi) of the oxide free graphite surface in water agrees with that found for the hydrophobic basal plane of Graphon, it may be assumed that the heat of immersion/unit area on an oxide free edge plane is the same as that ou the hydrophobic basal plane, (h& If it is further assumed that the heat of immersion/unit area on the hydrophilic, oxide covered surface {ho) is independent of the extent of that surface then the equation
249
ON GRAPWTE
Heats of immersion in methanol, as shown in Fig. 3, were found to increase in the Q-200°C interval in a fashion similar to water and to decrease further as the oxide is removed in the 200400°C temperature interval as the opportunity for hydrogen bonding with the oxide is removed. The removal of oxide in this region occurs mainly as C&, Above 6O@‘C, as the remaining oxide is removed as CO, the heat of imm~~rsion increases slightly indicating a higher interaction energy of the methanol with the bare surface than that covered with oxide removable as CU. Figure 6 relates the changes in heats of immersion in methanol with the amount of oxide desorbed. It can be seen that. the
which reduces to
can be written. This &near equation represents the observed heat of immersion as a function of the extent of oxidation of the surface. Here ST represents the total RET surface area of the graphite, S0 represents the total surface which was covered with oxygen and F, represents the fractional coverage of this surface with oxygen. Using the linearity of Fig. 5 to support this view calculations show that h, = 175 ergs trnez. This value is considerably less than the value of 730 ergs cm -2 found by tiealey et al, [7] on oxidised Graphon samples. However, water adsorption on the oxide surface af Graphon@] showed that the isoteric heat of adsorption measured between -3-3X and 20°C closely approached the value for the heat of liquefaction of water. The net heat of adsorption on this surface would therefore be close to Zero and so the heat of immersion on this surface can be approximated to the surface enthalpy of water (I 18 ergs cmF2). A value of 175 ergs cm --2 is therefore not unreasonable. It follows that the energy of hydrogen bonding of water to the surface oxides is similar to that found in bulk water itself.
Oxygen desarbed as CO,,
pmcles-g-’
IOU
200
0
50
I
J
t
150
I
I
1
z f
3-
Fig. 6. Heat of immersion of graphite in methanol in relation t.o the amount of oxygen desorbed as CO2 A, and to the total oxygen desorbed as CO and CC& 0. decrease in heat can be linearly related to the amount of COz desorbed, suggesting hydrogen bonding is occurring between the oxide removed as COz and not with that removed as CO. It is suggested that the heat of immersion in methanol on the hydrophobic surface is slightly higher than that on the hydrophilic oxide surface pertaining to the desorption of CO. The total oxygen curve in the figure is then explained as a combination of two
250
S. S. BARTON and B. H. HARRISON
effects. These are a decrease in heat from hydrogen bonding as the CO, sites are removed occurring simultaneously with an increase in heat as the CO sites are removed. Benzene on the other hand is insensitive to the removal of COP from the surface (cf. Fig. 4). After an ‘initial decrease in the heat of immersion from 0 to 100°C the heat is constant between 100” and 550°C over which temperature CO2 is the main desorption product. The heat then decreased between 550” and 8OO”C, where m~mum desorption of CO occurs, only to increase again between 900” and 1000°C in which temperature interval hydrogen desorption was encountered. Reduction of the heat of immersion between 0” and 100°C can be related to dehydration of the surface and reduced heat of adsorption. Removal of oxides as CO can be related to a reduction in the interaction between the partial charge on the carbonyl carbon atom and the Irr-electron cloud of the benzene molecule. It may be enlightening to review at this point the information obtained from functional group analysis on carbons. Carboxy1 and Carbonyl groups are held mainly responsible for the desorption products CO, and CO respectively. These groups are known to be acidic in nature, although their strengths are quite different. The highly acidic carboxy1 groups have been neutralised with weak bases such as aqueous NaHCO$ solutions while the weakly acidic carbonyl groups require strong bases such as sodium ethoxide in ethanol [12]. Although the exact nature of the COz desorbing surface groups on the present graphite is not known, sufficient hydrogen was desorbed at higher temperatures, after COz removal was complete, for carboxyl (-GOOH) groups to have been originally present. It has been found on a similar graphite[28] that after the graphite surface was initially cleaned to a comparable ratio of CO/CO, as in this study, subsequent oxygen chemisorption and desorption yields pre-
dominantly CO with only a small fraction of CO2 being evolved. It is therefore possible to infer that the CO, and H, could be related as carboxyl groups. The difference in the acidic nature of the two types of oxide surfaces are obviously reflected in their interaction with methanol where the strongly acidic carboxyl or CO2 producing oxides are capable of hydrogen bonding with the oxygen atom of the methanol. The decrease in heat of immersion as the COz is desorbed can be approximated from Fig. 6 to about 7-5 kcalslmofe of CO, desorbed which is representative of physical and not chemical adsorption processes.
4. CONCLUSION
The different roles played by the CO and CO, evolving oxide surfaces have been clearly evident in the heat of immersion studies. The results on the graphite surface have served as an interesting comparison with those on the surface of Spheron 6 [23] an un~aphitised carbon with an approximately equivalent proportion of its surface covered with oxygen. The magnitude as well as the relative changes in heats of immersion in polar liquids are found to be grossly different on the two surfaces. The large decrease in the heats of immersion on the carbon surface when degassed between 900” and lOOO”C[23] can be probably related to surface ordering as the measured heats were found to drop to values close to those found on graphitic surfaces. It therefore appears that surface structure is as important if not more important than surface oxide in the heats of immersion/unit area values which have been recorded. Acknowledgemxnt- Research supported by Defence Research Board of Canada, Grant No. 9530-72. REFERENCES 1. Marsh IX, Fuel44 253 (1965). 2. Broadbent K. A., Dollimore I)., and Dollimore j., CUYbon4,281 (1966). 3. Deitz V. R., J. Pkys. Ckmz. 71,830 (1967).
SURFACE STUDIES 4. Puri B. R., Carbon4,391 (1966). 5. Puri B. R., Murari K., and Singh D. D., J. Phys. Chem. 65,37 (1961). 6. Young G. J., Chessick J. J., Healey F. H., and Zettlemoyer A. C.,J. Phys. Chem. 58,313 (1954). 7. Healey F. H., Yu Y. F., and Chessick J. J.,J. Phys. Chem. 59,399 (1955). 8. Walker P. L., Jr., and Janov J., J. COB. Inter-f&e Sci. 28,449 (1968). 9. Gasser C. G., and Kipling J. J., Proc. Fourth Carbon Conf , p. 55, Pergamon Press, Oxford (1960). 10. Coughlin R. W., Ezra F. S., and Tan R. N., J. Coil. Interface Sci. 28,386 (1968). 11. DonnettJ. B.,Carbon6,161 (1968). 12. Boehm H. P.,Advan. Catalysis 16,198 (1966). 13. Smith R. N., Young D. A., and Smith R. A., Trans. Faraday Sot. 62,228O (1966). 14. Friedel R. A., and Hofer L. J. E., J. Phys. Chem. 74,292l (1970). 15. Mattson J. S., Lee L., Mark H. B., Jr., and Weber W. J., Jr., J. COB. Interface Sci. 33, 281 (1970). 16. Hennig G. R., Proc. Fifth Carbon Co@, Vol. 1, p. 143 Pergamon Press, Oxford (1963). 17. Thomas J. M., Chem. Phys. Carbon 1,122 (1965).
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18. Laine N. R., Vastola F. J. and Walker P. L., Jr., J. Phys. Chem. 67,203O (1963). 19. Dollimore J., Freedman C. M. and Harrison B. H., 17th Annual Conf. on Moss Spectrometry and Allied Topics, Dallas, Texas (1969). 20. Dollimore J., Freedman C. M., and Harrison B. H., Proc. 3rd Conf Ind. Carbon and Graphite, London (1969). 21. Puri B. R., Singh D. D., and Sharma L. R., J. Phys. Chem. 62,756 (1958). 22. Razouk R. L., Nashed S. H., and Mom-ad W. E., Carbon 2,539 (1964). 23. Wade W. H., J. Coil. Znterfuce Sci. 31, 111 (1969). 24. Brusset H., Martin J. J. P. and Mendelbaum H. G., Bull. Sot. Chim. France No. 7. 2346 (1967). 25. Barton S. S., and Boulton G., J. Chem. Eng. Data 15,66 (1970). 26. Hughes E. E. G., Williams B. R., and Thomas J. M., Trans. Faraday Sot. 58,201l (1962). 27. Feath P., Adsorption and Vacuum Techniques, Inst. Sci. Technol, Univ. Michigan (1962). 28. Brown J. G., Dollimore J., Freedman C. M., and Harrison B. H., 8th Conf. on Vacuum Microbalance Techniques, Wakefield, U.S.A. (1969).