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Surface studies of carbon: Acidic oxides on spheron 6
Carh,
1973, Vol. 11, pp. 649-654.
Pergamon
SURFACE
Press.
Prirated
in Great
Britain
STUDIES OF CARBON: ACIDIC OXIDES ON SPHERON 6
S. S. BARTON, D. GILLESPIE and B. H. HARRISON Department of Chemistry & Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada (Received
7 May 1973)
Abstract-Surface
oxygen complexes on Spheron 6, which thermally desorb as CQ, appear to be responsible for the acidity of the carbon. The base uptake of samples degassed at various temperatures has been related to the amounts of oxygen complex remaining on the surface. Two types of acidic oxides, both of which desorbed as C02, were observed. At temperatures around 250°C an oxide which acts as a very weak monobasic acid is decomposed and at about 600°C a second oxide, which is stronger and dibasic is decomposed. The heat of neutralization of this second acid was found to be approximately 12 kcal-mole-r. 1. INTRODUCTION
nearly all the acidic properties could be ascribed to phenol groups. Rivin[7] determined the distribution of phenol, quinone and carboxyl groups and also ascribed most of the acidity to phenol groups. Alternatively Garten and Weiss[S] have proposed that normal lactones and ‘tf-lactones” account for most of this acidity. Extensive studies by Puri [4] with a wide range of carbons, including Spheron 6, have revealed correlation between the base uptake of the carbons and the amount of CO2 evolved on thermal decomposition of the oxygen complexes. However, since phenol groups would evolve CO when they decompose, Pm-i believes that they are not the groups responsible for the acidity of carbon surfaces. A recent study in this laboratory, on the surface of graphite, has also shown a close equivalence between the base uptake and the amount of oxygen complex decomposing to CO, [9]. This result was achieved by degassing graphite samples at different temperatures, determining quantitatively the amount of CO, desorbed during the degassing, and measuring the base uptake of the degassed samples. It was found that for every mole of CO2 desorbed the base uptake was decreased
The study of functional groups on the surfaces of carbons has aroused a great deal of interest, Reports of much of this work can ne found in four recent reviews,[l-4] which describe the probable occurrence of carboxyl, carbonyl, phenol and lactone groups on carbon and graphite surfaces. Depending on the method of preparation, these surfaces can exhibit either acidic or basic properties. However, the acid properties have been more thoroughly investigated. Boehm [l] proposed a scheme for determining the relative amounts of carboxyl, phenol and carbonyl groups by a selective neutralization technique using different strength bases and much work has also been done by using well known organic reagents to both identify and estimate the amounts of different groups present. Unfortunately, the use of such reagents by different workers has not yielded similar results and usually only a small fraction of the oxygen complexes present are accounted for [4,5]. Independent studies using Spheron 6 have also come to quite different conclusions concerning the nature of the acidic oxides on this carbon. Given and Hill161 proposed that 649
S. S. BARTON
650
by one equivalent. Pm-i found, on the other hand, that for the carbons he used the desorption of one mole of CO, resulted in the base uptake being decreased by two equivalents. These results suggest that although the acidic oxides on carbons and graphite are probably both associated with complexes which decompose evolving COZ the type of group may be different on the two surfaces. The present communication describes a study of the acidic oxides on the surface of the carbon black, Spheron 6, using the same techniques which were used earlier to investigate the surface of graphite. 3. EXPERIMENTAL Spheron 6 was supplied by the Cabot Corporation. It was found to have a surface area of 116 & 3 m2g-I, measured by low temperature nitrogen adsorption, and approximately 70% of its surface was covered with oxygen complexes [IO]. The base uptake was measured by breaking sealed thin-walled glass bulbs containing approximately 1 g of degassed carbon under 25.0 ml of 0*050 N base in stoppered bottles which were then shaken mechanically for various times at room temperature. The base uptake was determined by titrating clear aliquots of the basic solution after the carbon had settled. Titration curves, using standardized hydrochloric acid, were determined potentiometrically and the base uptake of the carbon was estimated from the difference between the titration curves of the standard base and the aliquots at pH 8[9]. The gases evolved on thermal decomposition of the surface complexes were quantitatively measured using a mass spectrometer system[9, 10, 111. Heats of immersion were measured on an isothermal change of phase calorimeter. 4. RESULTS AND DISCUSSION The surface complexes on Spheron 6 were found to thermally decompose to CO, COZ,
et al.
Hz and H20. The amount of water desorbed was relatively small and only occurred at temperatures up to 300°C. The total amount of each of the major components desorbed at each temperature up to 1000°C is shown in Fig. 1. The decomposition of the oxygen complexes yielding CO, is confined to the temperature interval lOO-800°C while that yielding CO is confined to the interval 4001OOO“C. Hydrogen desorption occurred at temperatures of 500°C and above but did not become significant until 700-800°C. The total amounts of desorbed material found in this work agree closely with those found in an earlier study on the same batch of carbon but did not agree with the results of other desorption experiments [lo]. ^
PI _
‘.a I500
0
200
400 DEGASSING
600 TEMPERATURE
600
1000
(-3
Fig. 1. Amounts of CO, CO, and Hz desorbed when Spheron 6 is degassed at various temperatures.
Figure 2 shows the differential gas evolution curve, i.e. the amount of gas evolved at each temperature as a sample of Spheron 6 is heated in 50” steps to 1000°C. The curve for CO, shows two maxima corres~nding to two types of surface complexes desorbing as C02, one at about 250°C and the other at 600°C. The CO curve shows a single maximum at 700°C similar to graphite 11211.The evolution of CO:! from graphite only showed one maximum which occurred at 400°C. The complexes desorbing as CO:! on carbon and on graphite must therefore have different
651
SURFACE STUDIES ON CARBON: ACIDIC OXIDES ON SPHERON 6
^
I
‘~‘oo.z 3
-
0.05N
SODIUM
ETHOXIDE
0.05N
SODIUM
HYDROXIDE
3 y
200
-
it! 2
0
:
-
2
100 -
I
I IO
0‘
20
REACTION 200
Fig. 2. Differential
1000
600
TEMPERATURE
(hr)
Fig. 3. Effect of reaction time on the base uptake.
(“C)
gas evolution curve.
if they decompose at these different temperatures. In earlier studies of base uptake by carbons and graphite extremely long reaction times have been required[l, 41. During our work with graphite it was found that the technique of breaking sealed thin walled bulbs of degassed graphite under the base, as one would do for a heat of immersion measurement, markedly shortened the reaction times for base uptake. This technique has been used with Spheron 6 and the base uptake at various reaction times, for samples degassed at 100°C using Oe050N aqueous sodium hydroxide and alcoholic sodium ethoxide, is shown in Fig. 3. As with the graphite, the base uptake was found to be essentially complete after 0.5 hr and showed only a small increase for reaction times to 25 hr. What was apparent though, was that increasing the reaction times made the settling time of the carbon much longer. For convenience the reaction times were therefore limited to 2 hr. The effect of degassing temperature on the base uptake is shown in Fig. 4. The uptake of aqueous sodium hydroxide was found to be about half that observed for alcoholic sodium ethoxide and was insensitive to degassing at temperatures up to 400°C. Sodium ethoxide uptake on the other hand showed a steady structures
TIME
300
IUM
_Z‘w .? s al 200
ETHOXIDE
=t : 2 3 w
100
ii
200
400 DEGASSING
Fig. 4. Effect
600
600
TEMPERATURE
(‘Cl
of degassing temperature base uptake.
on the
fall, on degassing at temperatures up to 4Oo”C, followed by a much faster decrease in the uptake between 400°C and 800°C. The temperature interval over which the drop in acidity occurs appears to be the same as the interval over which COz is desorbed. It is apparent that at 700°C when the evolution of CO is at a maximum the acidity has almost disappeared. The evolution of large amounts of CO would therefore seem to have little effect on the acidity especially above 700°C. Since the temperature intervals associated with the decrease in acidity and evolution of CO, are the same, and the two sections of the base uptake curve occur over the same
S. S. BARTON et al.
652
temperature intervals as the desorption of the two CO2 complexes it seems most proable that the acidity is associated with the COZ complexes. By plotting the milliequivalents of base uptake vs the amount of CO2 desorbing oxide (expressed as mmoles of CQ) remaining on the surface, it is possible to establish the basic&y of the surface oxide[9]. The plot for Spheron-6 is shown in Fig. 5. The sodium ethoxide data fall on two intersecting straight lines. The initial line corresponding to the desorption of CO, at temperatures around 600°C has a slope of two, indicating that the surface complex being destroyed is dibasic. The second linear portion has a slope of one, indicating that the less thermally stable surface oxide which decomposes around 250°C is monobasic in character. From the aqueous sodium hydroxide uptake data it is apparent that the complex desorbing at 250°C is not neutralized by this base but it can be seen that a 1: 1 relationship exists between the CO*, desorbing with a maximum at 600X, and the base uptake, indicating that the sodium hydroxide is neutralizing a monobasic acid. If it is assumed that the ethanolic sodium ethoxide neutralizes all the acidic oxides, and this appears reasonable because the uptake
SODIUM
ETHO
DIUM
CO2
Fig.
ON
HYDROXIDE
SURFACE
Q~moles.q-~)
5. Relation between base uptake oxides desorbing as CO%.
and the
is close to values reported by other authors [4, 61, then there are two possible explanations for the sodium hydroxide values. Either the 1: 1 relationship found with this base is fortuitous and the carbon actually contains a continuum of acidic oxides of varying strength, only part of which are neutralized by the sodium hydroxide or the acidic oxide is a lactone or some similar group which is monobasic towards aqueous sodium hydroxide but is hydrolysed by the action of a stronger base into a dibasic structure. The COZ complex which desorbs at 250°C is obviously a very weak acid which does not react with aqueous sodium hydroxide. The complex is monobasic and obviously not a carboxyl group which react readily with aqueous sodium hydroxide. It is possibly a structure which acts as a monobasic acid on hydrolysis in the presence of a strong base. The second CO, complex desorbing at 600°C is dibasic with a strong base such as alcoholic sodium ethoxide but only monobasic when neutralized by the weaker base aqueous sodium hydroxide. The study on graphite191 showed a CO, complex which was monobasic with sodium hydroxide, but in that instance there was no difference betieen the alcoholic sodium ethoxide and aqueous sodium hydroxide uptakes. In the earlier study on graphite an estimate of the heat of neutralization was also made. This estimate was made by preadsorbing enough water on the graphite to effectively eliminate the heat effects which one would measure in an immersion process (i.e. heat of adsorption, etc.) and then breaking a bulb of the graphite, with preadsorbed water, in excess base in a calorimeter. Using the same technique with Spheron 6 degassed at 25”C, bulbs of pre-wetted carbon were broken in 0.050 N sodium hydroxide. On relating the observed heat to the uptake of the base and enthalpy change for the reaction acidic oxide (aq) 4 OH-(aq) = acid oxide anion (aq) + H20(1) was found to be - 12.5 I+ O-4 kcal . mole-‘.
653
SURFACE STUDIES ON CARBON: ACIDIC OXIDES ON SPHERON 6
As before the calorimetric measurement took approximately 0.75 hr and showed no slow evolution of heat. Since it was also observed, Fig. 3, that the base uptake was essentially complete after O-5 hr the reaction should have reached equilibrium during the heat measurement. This enthalphy value refers to the monobasic moiety of the most thermally stable CO, desorbing surface oxide. A variation of this technique has also been used in the present case. The heat of immersion of carbon samples degassed at different temperatures have been measured in water and O-050 N sodium hydroxide. Since the base uptake is limited to the portion of the surface containing acidic oxides, the heat of immersion in water and the base should be the same on the rest of the surface. This was found to be correct. When the carbon was degassed at temperatures of 800% and above, when all the acidic oxides had been removed, the heats were found to be identical. At temperatures below 800°C the heat of immersion in base was found to be higher than that in water when the carbon was degassed at the same temperature. The difference in the heats is shown in Fig. 6a for the different degassing temperatures.
I
I
Oxide surface
I
I
H,O,
OH-
Is)
+ Ha+0
I
i
Neutralized surface I
+ HZ0
I
_-iOH-
(2)
Considering only the portion of the surface covered by acidic oxides the above diagram can be used to interpret the difference in the heat measurements. Step (1) represents the enthalpy change when the acidic oxides are ionized by the immersion in water. Similarly step (3) respresents the immersion in aqueous base. It is thus apparent that the enthalpy difference between the two processes, i.e.
DEGASSING
TEMPERATURE (a)
(“Cl
BASE
UPTAKE
(pquiv.g“)
(bf
Fig. 6a. Effect of degassing temperature upon the difference between the heat of immersion in water and 0.050 N sodium hydroxide (AH& Fig. 6b. Relation between AH, and the base uptake.
step (2), is the heat of neutralization of the acidic oxides. If the heat difference between the two immersion processes is related to the base uptake, (Fig. 6b), the heat of neutralization calculated from the slope of the graph, by least squares analysis, is 12t 1 kcal . mole-‘. This value is chasacteristic of a weak acid. The error in the determination in the present case is higher than with the determination using pre-wetted carbon because the heat measurement in this case arises from the difference between two measurements each of which has an associated error. The above scheme accounts for only the chemical processes taking place. It has been assumed that heat effects arising from the immersion processes are very similar in steps (I) and (3) so that the enthalpy difference is only due to the neutralization of acidic oxides. This assumption seems justified when one considers the close agreement between the heat of neutralization obtained by the two different procedures. It thus seems that Spheron 6 contains two types of acidic oxide each of which will decompose during thermal treatment evolving CO%. Several other workers have also concluded that two groups are responsible for the acidity with this type of carbon. Using non-aqueous titrations, Studebaker [13], identified two types of acidic groups on a similar type of carbon and found that only one
S. S. BARTON
654
of these groups contained active hydrogen that could be measured by the Zerewitinoff procedure. Rivin [7] also concluded from his aqueous titration curves that basically two types of acidic oxide were present. He found that his base uptake was relatively independent of the hydroxide ion concentration between 10m3N and 0.02 N and at concentrations above O-6 N. Garten and Weiss [8] also interpreted most of their acidity to two groups, an n-lactone and an f-lactone. Al&ough no conclusive proof has been advanced for the existence of lactone groups they appear to at least fit in with the present facts. They decompose evolving COz and are weakly acidid. That the groups found on carbon are unlike the ones found on graphite [9] seems quite possible when one considers the difference in the structure of carbon and graphite and the fact that the ease of hydrolysis of lactone groups is quite dependent of the structure associated with them. 5. CONCLUSION The existence of two types of acidic oxide on Spheron 6 seems to have been established. Both oxides thermally decompose evolving CO*. One decomposes at 25o”C, the other at 600°C. These structures are unlike the one found on graphite which decomposes at 400°C and have different acidic properties. The acidic oxide on Spheron 6 which decomposes at 250°C was not neutralized by 0.050 N aqueous sodium hydroxide but was neutralized by 0.050 N alcoholic sodium ethoxide and was monobasic. The oxide
et al.
decomposing at 600°C appears to be dibasic neutralized by alcoholic sodium when ethoxide but only monobasic when neutralized with aqueous sodium hydroxide. The heat of neutralization of this oxide with sodium hydroxide was 12 kcal . mole-‘. It appeared that the stronger base was capable of hydrolysing the structure into a dibasic acid. Acknowledgement- Research supported by Defence Research Board of Canada, Grant No. 9530-72. REFERENCES :: 3. 4. 5. 6. 7. 8.
9. 10. 11.
12. 13.
Boehm H. P.,Advan. Catulysb16,198 (1966). Donnett J. B.,CurbonG, 161 (1968). Deviney M. L. Jr., Advan. Colloid Interface Se-i.4, 237 (1969). Puri B. R., In Chemistry and Physics of Carbon, (Edited by P. L. Walker, Jr.), Vol. 6, p. 191, (1970). Van Der Plas Th., In Physical and Chemical Aspects of Adsorbents and Catalysts, (Edited by B. G. Linsen). Academic Press (1970). Given P. H. and Hill L. W., Carbon 7, 649 (1969). Rivin D., Rubber Chem. Technol. 36,729 (1963). Garten V. A. and Weiss D. E., Proc. Third Carbon Co@ p. 295. Pergamon Press, New York (1957). Barton S. S., Boulton G. and Harrison B. H., Carbon 10,395 (1972). Barton S. S. and Harrison B. H., to be published. Brown J. G., Dollimore J., Freedman C. M. and Harrison B. H., Thermochimica Actu 1,499 (1970). Barton S. S. and Harrison B. H., Carbon 10, 245 (1972). Studebaker M. L., Proc. F$th Carbon Conj, p. 189, Vol. 2. Pergamon Press, New York (1963).
1973, Vol. 11, pp. 649-654.
Pergamon
SURFACE
Press.
Prirated
in Great
Britain
STUDIES OF CARBON: ACIDIC OXIDES ON SPHERON 6
S. S. BARTON, D. GILLESPIE and B. H. HARRISON Department of Chemistry & Chemical Engineering, Royal Military College of Canada, Kingston, Ontario, Canada (Received
7 May 1973)
Abstract-Surface
oxygen complexes on Spheron 6, which thermally desorb as CQ, appear to be responsible for the acidity of the carbon. The base uptake of samples degassed at various temperatures has been related to the amounts of oxygen complex remaining on the surface. Two types of acidic oxides, both of which desorbed as C02, were observed. At temperatures around 250°C an oxide which acts as a very weak monobasic acid is decomposed and at about 600°C a second oxide, which is stronger and dibasic is decomposed. The heat of neutralization of this second acid was found to be approximately 12 kcal-mole-r. 1. INTRODUCTION
nearly all the acidic properties could be ascribed to phenol groups. Rivin[7] determined the distribution of phenol, quinone and carboxyl groups and also ascribed most of the acidity to phenol groups. Alternatively Garten and Weiss[S] have proposed that normal lactones and ‘tf-lactones” account for most of this acidity. Extensive studies by Puri [4] with a wide range of carbons, including Spheron 6, have revealed correlation between the base uptake of the carbons and the amount of CO2 evolved on thermal decomposition of the oxygen complexes. However, since phenol groups would evolve CO when they decompose, Pm-i believes that they are not the groups responsible for the acidity of carbon surfaces. A recent study in this laboratory, on the surface of graphite, has also shown a close equivalence between the base uptake and the amount of oxygen complex decomposing to CO, [9]. This result was achieved by degassing graphite samples at different temperatures, determining quantitatively the amount of CO, desorbed during the degassing, and measuring the base uptake of the degassed samples. It was found that for every mole of CO2 desorbed the base uptake was decreased
The study of functional groups on the surfaces of carbons has aroused a great deal of interest, Reports of much of this work can ne found in four recent reviews,[l-4] which describe the probable occurrence of carboxyl, carbonyl, phenol and lactone groups on carbon and graphite surfaces. Depending on the method of preparation, these surfaces can exhibit either acidic or basic properties. However, the acid properties have been more thoroughly investigated. Boehm [l] proposed a scheme for determining the relative amounts of carboxyl, phenol and carbonyl groups by a selective neutralization technique using different strength bases and much work has also been done by using well known organic reagents to both identify and estimate the amounts of different groups present. Unfortunately, the use of such reagents by different workers has not yielded similar results and usually only a small fraction of the oxygen complexes present are accounted for [4,5]. Independent studies using Spheron 6 have also come to quite different conclusions concerning the nature of the acidic oxides on this carbon. Given and Hill161 proposed that 649
S. S. BARTON
650
by one equivalent. Pm-i found, on the other hand, that for the carbons he used the desorption of one mole of CO, resulted in the base uptake being decreased by two equivalents. These results suggest that although the acidic oxides on carbons and graphite are probably both associated with complexes which decompose evolving COZ the type of group may be different on the two surfaces. The present communication describes a study of the acidic oxides on the surface of the carbon black, Spheron 6, using the same techniques which were used earlier to investigate the surface of graphite. 3. EXPERIMENTAL Spheron 6 was supplied by the Cabot Corporation. It was found to have a surface area of 116 & 3 m2g-I, measured by low temperature nitrogen adsorption, and approximately 70% of its surface was covered with oxygen complexes [IO]. The base uptake was measured by breaking sealed thin-walled glass bulbs containing approximately 1 g of degassed carbon under 25.0 ml of 0*050 N base in stoppered bottles which were then shaken mechanically for various times at room temperature. The base uptake was determined by titrating clear aliquots of the basic solution after the carbon had settled. Titration curves, using standardized hydrochloric acid, were determined potentiometrically and the base uptake of the carbon was estimated from the difference between the titration curves of the standard base and the aliquots at pH 8[9]. The gases evolved on thermal decomposition of the surface complexes were quantitatively measured using a mass spectrometer system[9, 10, 111. Heats of immersion were measured on an isothermal change of phase calorimeter. 4. RESULTS AND DISCUSSION The surface complexes on Spheron 6 were found to thermally decompose to CO, COZ,
et al.
Hz and H20. The amount of water desorbed was relatively small and only occurred at temperatures up to 300°C. The total amount of each of the major components desorbed at each temperature up to 1000°C is shown in Fig. 1. The decomposition of the oxygen complexes yielding CO, is confined to the temperature interval lOO-800°C while that yielding CO is confined to the interval 4001OOO“C. Hydrogen desorption occurred at temperatures of 500°C and above but did not become significant until 700-800°C. The total amounts of desorbed material found in this work agree closely with those found in an earlier study on the same batch of carbon but did not agree with the results of other desorption experiments [lo]. ^
PI _
‘.a I500
0
200
400 DEGASSING
600 TEMPERATURE
600
1000
(-3
Fig. 1. Amounts of CO, CO, and Hz desorbed when Spheron 6 is degassed at various temperatures.
Figure 2 shows the differential gas evolution curve, i.e. the amount of gas evolved at each temperature as a sample of Spheron 6 is heated in 50” steps to 1000°C. The curve for CO, shows two maxima corres~nding to two types of surface complexes desorbing as C02, one at about 250°C and the other at 600°C. The CO curve shows a single maximum at 700°C similar to graphite 11211.The evolution of CO:! from graphite only showed one maximum which occurred at 400°C. The complexes desorbing as CO:! on carbon and on graphite must therefore have different
651
SURFACE STUDIES ON CARBON: ACIDIC OXIDES ON SPHERON 6
^
I
‘~‘oo.z 3
-
0.05N
SODIUM
ETHOXIDE
0.05N
SODIUM
HYDROXIDE
3 y
200
-
it! 2
0
:
-
2
100 -
I
I IO
0‘
20
REACTION 200
Fig. 2. Differential
1000
600
TEMPERATURE
(hr)
Fig. 3. Effect of reaction time on the base uptake.
(“C)
gas evolution curve.
if they decompose at these different temperatures. In earlier studies of base uptake by carbons and graphite extremely long reaction times have been required[l, 41. During our work with graphite it was found that the technique of breaking sealed thin walled bulbs of degassed graphite under the base, as one would do for a heat of immersion measurement, markedly shortened the reaction times for base uptake. This technique has been used with Spheron 6 and the base uptake at various reaction times, for samples degassed at 100°C using Oe050N aqueous sodium hydroxide and alcoholic sodium ethoxide, is shown in Fig. 3. As with the graphite, the base uptake was found to be essentially complete after 0.5 hr and showed only a small increase for reaction times to 25 hr. What was apparent though, was that increasing the reaction times made the settling time of the carbon much longer. For convenience the reaction times were therefore limited to 2 hr. The effect of degassing temperature on the base uptake is shown in Fig. 4. The uptake of aqueous sodium hydroxide was found to be about half that observed for alcoholic sodium ethoxide and was insensitive to degassing at temperatures up to 400°C. Sodium ethoxide uptake on the other hand showed a steady structures
TIME
300
IUM
_Z‘w .? s al 200
ETHOXIDE
=t : 2 3 w
100
ii
200
400 DEGASSING
Fig. 4. Effect
600
600
TEMPERATURE
(‘Cl
of degassing temperature base uptake.
on the
fall, on degassing at temperatures up to 4Oo”C, followed by a much faster decrease in the uptake between 400°C and 800°C. The temperature interval over which the drop in acidity occurs appears to be the same as the interval over which COz is desorbed. It is apparent that at 700°C when the evolution of CO is at a maximum the acidity has almost disappeared. The evolution of large amounts of CO would therefore seem to have little effect on the acidity especially above 700°C. Since the temperature intervals associated with the decrease in acidity and evolution of CO, are the same, and the two sections of the base uptake curve occur over the same
S. S. BARTON et al.
652
temperature intervals as the desorption of the two CO2 complexes it seems most proable that the acidity is associated with the COZ complexes. By plotting the milliequivalents of base uptake vs the amount of CO2 desorbing oxide (expressed as mmoles of CQ) remaining on the surface, it is possible to establish the basic&y of the surface oxide[9]. The plot for Spheron-6 is shown in Fig. 5. The sodium ethoxide data fall on two intersecting straight lines. The initial line corresponding to the desorption of CO, at temperatures around 600°C has a slope of two, indicating that the surface complex being destroyed is dibasic. The second linear portion has a slope of one, indicating that the less thermally stable surface oxide which decomposes around 250°C is monobasic in character. From the aqueous sodium hydroxide uptake data it is apparent that the complex desorbing at 250°C is not neutralized by this base but it can be seen that a 1: 1 relationship exists between the CO*, desorbing with a maximum at 600X, and the base uptake, indicating that the sodium hydroxide is neutralizing a monobasic acid. If it is assumed that the ethanolic sodium ethoxide neutralizes all the acidic oxides, and this appears reasonable because the uptake
SODIUM
ETHO
DIUM
CO2
Fig.
ON
HYDROXIDE
SURFACE
Q~moles.q-~)
5. Relation between base uptake oxides desorbing as CO%.
and the
is close to values reported by other authors [4, 61, then there are two possible explanations for the sodium hydroxide values. Either the 1: 1 relationship found with this base is fortuitous and the carbon actually contains a continuum of acidic oxides of varying strength, only part of which are neutralized by the sodium hydroxide or the acidic oxide is a lactone or some similar group which is monobasic towards aqueous sodium hydroxide but is hydrolysed by the action of a stronger base into a dibasic structure. The COZ complex which desorbs at 250°C is obviously a very weak acid which does not react with aqueous sodium hydroxide. The complex is monobasic and obviously not a carboxyl group which react readily with aqueous sodium hydroxide. It is possibly a structure which acts as a monobasic acid on hydrolysis in the presence of a strong base. The second CO, complex desorbing at 600°C is dibasic with a strong base such as alcoholic sodium ethoxide but only monobasic when neutralized by the weaker base aqueous sodium hydroxide. The study on graphite191 showed a CO, complex which was monobasic with sodium hydroxide, but in that instance there was no difference betieen the alcoholic sodium ethoxide and aqueous sodium hydroxide uptakes. In the earlier study on graphite an estimate of the heat of neutralization was also made. This estimate was made by preadsorbing enough water on the graphite to effectively eliminate the heat effects which one would measure in an immersion process (i.e. heat of adsorption, etc.) and then breaking a bulb of the graphite, with preadsorbed water, in excess base in a calorimeter. Using the same technique with Spheron 6 degassed at 25”C, bulbs of pre-wetted carbon were broken in 0.050 N sodium hydroxide. On relating the observed heat to the uptake of the base and enthalpy change for the reaction acidic oxide (aq) 4 OH-(aq) = acid oxide anion (aq) + H20(1) was found to be - 12.5 I+ O-4 kcal . mole-‘.
653
SURFACE STUDIES ON CARBON: ACIDIC OXIDES ON SPHERON 6
As before the calorimetric measurement took approximately 0.75 hr and showed no slow evolution of heat. Since it was also observed, Fig. 3, that the base uptake was essentially complete after O-5 hr the reaction should have reached equilibrium during the heat measurement. This enthalphy value refers to the monobasic moiety of the most thermally stable CO, desorbing surface oxide. A variation of this technique has also been used in the present case. The heat of immersion of carbon samples degassed at different temperatures have been measured in water and O-050 N sodium hydroxide. Since the base uptake is limited to the portion of the surface containing acidic oxides, the heat of immersion in water and the base should be the same on the rest of the surface. This was found to be correct. When the carbon was degassed at temperatures of 800% and above, when all the acidic oxides had been removed, the heats were found to be identical. At temperatures below 800°C the heat of immersion in base was found to be higher than that in water when the carbon was degassed at the same temperature. The difference in the heats is shown in Fig. 6a for the different degassing temperatures.
I
I
Oxide surface
I
I
H,O,
OH-
Is)
+ Ha+0
I
i
Neutralized surface I
+ HZ0
I
_-iOH-
(2)
Considering only the portion of the surface covered by acidic oxides the above diagram can be used to interpret the difference in the heat measurements. Step (1) represents the enthalpy change when the acidic oxides are ionized by the immersion in water. Similarly step (3) respresents the immersion in aqueous base. It is thus apparent that the enthalpy difference between the two processes, i.e.
DEGASSING
TEMPERATURE (a)
(“Cl
BASE
UPTAKE
(pquiv.g“)
(bf
Fig. 6a. Effect of degassing temperature upon the difference between the heat of immersion in water and 0.050 N sodium hydroxide (AH& Fig. 6b. Relation between AH, and the base uptake.
step (2), is the heat of neutralization of the acidic oxides. If the heat difference between the two immersion processes is related to the base uptake, (Fig. 6b), the heat of neutralization calculated from the slope of the graph, by least squares analysis, is 12t 1 kcal . mole-‘. This value is chasacteristic of a weak acid. The error in the determination in the present case is higher than with the determination using pre-wetted carbon because the heat measurement in this case arises from the difference between two measurements each of which has an associated error. The above scheme accounts for only the chemical processes taking place. It has been assumed that heat effects arising from the immersion processes are very similar in steps (I) and (3) so that the enthalpy difference is only due to the neutralization of acidic oxides. This assumption seems justified when one considers the close agreement between the heat of neutralization obtained by the two different procedures. It thus seems that Spheron 6 contains two types of acidic oxide each of which will decompose during thermal treatment evolving CO%. Several other workers have also concluded that two groups are responsible for the acidity with this type of carbon. Using non-aqueous titrations, Studebaker [13], identified two types of acidic groups on a similar type of carbon and found that only one
S. S. BARTON
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of these groups contained active hydrogen that could be measured by the Zerewitinoff procedure. Rivin [7] also concluded from his aqueous titration curves that basically two types of acidic oxide were present. He found that his base uptake was relatively independent of the hydroxide ion concentration between 10m3N and 0.02 N and at concentrations above O-6 N. Garten and Weiss [8] also interpreted most of their acidity to two groups, an n-lactone and an f-lactone. Al&ough no conclusive proof has been advanced for the existence of lactone groups they appear to at least fit in with the present facts. They decompose evolving COz and are weakly acidid. That the groups found on carbon are unlike the ones found on graphite [9] seems quite possible when one considers the difference in the structure of carbon and graphite and the fact that the ease of hydrolysis of lactone groups is quite dependent of the structure associated with them. 5. CONCLUSION The existence of two types of acidic oxide on Spheron 6 seems to have been established. Both oxides thermally decompose evolving CO*. One decomposes at 25o”C, the other at 600°C. These structures are unlike the one found on graphite which decomposes at 400°C and have different acidic properties. The acidic oxide on Spheron 6 which decomposes at 250°C was not neutralized by 0.050 N aqueous sodium hydroxide but was neutralized by 0.050 N alcoholic sodium ethoxide and was monobasic. The oxide
et al.
decomposing at 600°C appears to be dibasic neutralized by alcoholic sodium when ethoxide but only monobasic when neutralized with aqueous sodium hydroxide. The heat of neutralization of this oxide with sodium hydroxide was 12 kcal . mole-‘. It appeared that the stronger base was capable of hydrolysing the structure into a dibasic acid. Acknowledgement- Research supported by Defence Research Board of Canada, Grant No. 9530-72. REFERENCES :: 3. 4. 5. 6. 7. 8.
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Boehm H. P.,Advan. Catulysb16,198 (1966). Donnett J. B.,CurbonG, 161 (1968). Deviney M. L. Jr., Advan. Colloid Interface Se-i.4, 237 (1969). Puri B. R., In Chemistry and Physics of Carbon, (Edited by P. L. Walker, Jr.), Vol. 6, p. 191, (1970). Van Der Plas Th., In Physical and Chemical Aspects of Adsorbents and Catalysts, (Edited by B. G. Linsen). Academic Press (1970). Given P. H. and Hill L. W., Carbon 7, 649 (1969). Rivin D., Rubber Chem. Technol. 36,729 (1963). Garten V. A. and Weiss D. E., Proc. Third Carbon Co@ p. 295. Pergamon Press, New York (1957). Barton S. S., Boulton G. and Harrison B. H., Carbon 10,395 (1972). Barton S. S. and Harrison B. H., to be published. Brown J. G., Dollimore J., Freedman C. M. and Harrison B. H., Thermochimica Actu 1,499 (1970). Barton S. S. and Harrison B. H., Carbon 10, 245 (1972). Studebaker M. L., Proc. F$th Carbon Conj, p. 189, Vol. 2. Pergamon Press, New York (1963).