- Email: [email protected]
Physical adsorption of carbon dioxide on Graphon
Cnrbon
1971, Vol. 9. pp, 467-472.
Pergamon Press.
Printed in Great Bntain
PHYSICAL ADSORPTION OF CARBON DIOXIDE ON GRAPHON R. J. TYLER and H. J. WOUTERLOOD Division of Mineral Chemistry, Commonwealth Scientific and Industrial Research Organization, P.O. Box 175, Chatswood, Australia 2067 (Received 8 August 1970)
isotherms of carbon dioxide on Graphon have been determined at temperatures from -80 to +29”C and equilibrium pressures up to 30 atm. Plots of log (fugacity) against l/T at constant amounts adsorbed are linear over the temperature range, showing no change in slope in the region of the triple point of bulk carbon dioxide. With increasing amount adsorbed the isosteric heat of adsorption increases to a maximum of 4.5 kcal mole-’ and then declines. Similar behaviour is shown by carbon dioxide adsorbed on partly combusted specimens of Graphon. Assuming that the maximum heat of adsorption occurs on completion of the first adsorbed layer, the average area of surface occupied by each carbon dioxide molecule at this point is 26A’. The effect of oxygen previously chemisorbed on the carbon surface is to increase the amount of carbon dioxide adsorbed at - 80°C. This effect was studied gravimetrically and it was concluded that the extra amount adsorbed rises with increasing pressure of carbon dioxide to a value corresponding to the adsorption of an extra carbon dioxide molecule for each 3-5 oxygen atoms on the surface.
Abstract-Adsorption
1. INTRODUCTION
‘graphitized’ carbon black, have been described by Schaeffer et al. [5]. Isosteric heats were determined for three materials, i.e. the original Graphon and the same material burnt off to 4.9 per cent and to 19.2 per cent weight loss. Specific areas were X0.5, 92.6 and 95.0 m*g-’ respectively (argon adsorption at - 19VC; 13*8Az per atom; B.E.T. plots linear to Y/P0 = 0.25). Isotherms were measured at - 80.0 * 0.2% by conventional volumetric technique and at -53-l 50.1, 0. and 28.7-+O*l”C using apparatus which operated up to 34 atm pressure. The latter, though, developed independently from the original design by Kini[G], was essentially similar to the apparatus described by Kini and Walker[7] but incorporated a more sensitive transducer read-out system[S]. All samples were degassed at 140°C prior to these measurements. The cryostat for - 53°C consisted of a small dewar flask filled with petroleum ether
The increasing use of carbon dioxide as an adsorbate for measurements of the surface of carbonaceous solids [l-4] has areas prompted the authors to put on record some measurements relating to the properties of carbon dioxide adsorbed on Graphon. Isosteric heats of adsorption have been obtained as a function of coverage from isotherms determined at temperatures from - 80 to + 29% (and pressures up to 30 atm). The results strongly support the view that the first monolayer has liquid-like rather than solid-like properties down to -80°C and that each molecule occupies an area of about 26Ax in the complete layer. In addition, the effects on the isotherms at -80°C of known amounts of oxygen chemisorbed on the carbon surface have been determined. 2. EXPERIMENTAL The nature
and properties
of Graphon,
a 467
R. J. TYLER and H. J. WOUTERLOOD
468
surrounded by a larger one containing liquid Ne. The ether was stirred by bubbling in a slow stream of Nz and its temperature was maintained by a small immersion heater. Power to the heater was controlled by a Matheson ‘Lab. Stat’ using a propane gasthermometer as the sensing device. The effect of chemisorbed oxygen on the by adsorption at -80°C was investigated means of a Cahn R.G. Recording Electrobalance. Chemisorption was effected by exposing the Graphon to oxygen in the balance chamber after the manner described by Tucker and Mulcahy[9]. Briefly, the routine was as follows. The specimen (O-120.16 g), suspended from the balance arm, was first heated in vacw, at 950°C to remove previously chemisorbed oxygen [lo] and then exposed (usually) to 20 torr 0, at 500°C for 1 hr. During the latter period about 0.2% of the carbon was gasified and oxygen was chemisorbed. A CO2 adsorption isotherm was then measured gravimetrically in situ at -80°C. Next the oxygen was desorbed[lO] (mainly as CO) by another
period at 950°C. The measured loss in weight (W,) during this period enabled the weight of chemisorbed oxygen to be calculated (i.e. 16 W,/28). Finally, the CO, isotherm of the oxygen-free carbon was determined (again in situ). In a few experiments adsorption isotherms were also measured at 0°C. A single chemisorptiondesorption cycle produced a negligible change in surface area. However, repetition of the cycle caused a gradual increase in the fraction of the surface covered by oxygen[9] and this procedure was used to obtain a range of coverages for the CO, adsorption measurements. Throughout the work, the adsorption isotherms were highly reproducible and no chemisorption or desorption hysteresis of CO, was observed. 8. RESULTS AND DISCUSSION 3.1 Heats of adsorption Figure 1 shows plots of log,, (fugacity) against l/T for CO2 adsorbed on the original Graphon at several constant degrees of
2.0
BULK
1.0
co2
r t u0
0
LF -J
-1.0
3.5
4.0
4.5 103 r'
OEG
5.0 K-'
Fig. 1. Isosteres for adsorption of CO, on Graphon.
55
PHYSICAL
ADSORPTION
OF CARBON
coverage by the COz. (The data of Houghton et al. [ 1 l] were used to calculate the fugacities at 0 and +29”C from pressures obtained by interpolation between closely adjacent points of the isotherms.) The isostere for bulk CO, in the same temperature range is also shown in Fig. 1. The adsorption isosteres are accurately linear over the whole temperature range, which, as Fig. 1 shows, includes the triple point of the bulk material. There is no indication of a phase change in the adsorbed state and the slopes of the isosteres are close to that of the liquid rather than the solid isostere. Similar behaviour was observed with Graphon burned off to 4.9 per cent and to 19.2 per cent weight loss. This result differs from that obtained by Dubinin et al. [12] from studies of adsorption of CO? on silica gel. When less than a complete monolayer was present on the surface of this material the slope of the isostere was found to change in the vicinity of the triple-point temperature (- 56.6”C) indicating a phase change analogous to the solidliquid transition. Average heats of adsorption determined from the slopes of the isosteres obtained with the three present materials are shown as a function of coverage in Fig. 2. In each case the coverage is expressed as mole rn+ on 5~0
7
u
-
ORIGINAL
-
do., do.,
,-
DIOXIDE
ON GRAPHON
469
the basis of the value of the surface area obtained by argon adsorption. The curves all show a maximum value of 4.5 kcal mole-’ for -AH at a coverage of approximately 6.3 x 10~“mole me2. The rise in -AH with increasing coverage is due to lateral attraction between the CO, molecules and has been observed previously with COz[13] and other adsorbates on homogeneous surfaces. The present results agree with calorimetrically determined heats of adsorption of CO, on other graphitized carbon blacks at -80°C: with Sterling FT(2700”) and Sterling MT (3100”) Spencer et a/.[131 found maxima of 6 kcal mole-’ at coverages of j-7 X 10Ffi and 7.1 X lo-“mole m-’ respectively. Assuming that the maximum value of-AH occurs at completion of the first layer, the present results yield an average value of 26A’ for the area occupied by each molecule in the temperature range -80 to +29”C. The corresponding values [13] at -80°C for the two graphitized blacks just mentioned are 29 and 23A”. In agreement with the results of Walker and Kini [3], these figures indicate that values of the area per molecule based on the densities of the bulk liquid or solid [ 141 (17 or 14&) are too low. Since the high-pressure adsorption apparatus did not allow the specimen to be degassed
GRAPHON
COMEUSTED
To
COMBUSTEO
TO
4’9 ‘I. 19.2
%
6 I
2.0 SURFACE
4.0 COVERAGE
6.0 BY
6.0 CO,x
10:
MOLE
10.0 -2
m
Fig. 2. Variation of heat of adsorption of CO2 with surface coverage.
IL J. TYLER
470
and H. J. WOUTERLOOD
at high temperature in situ, the above measurements refer to surfaces with up to 9 per cent of the area covered with chemisorbed oxygen. Although, as will be seen the presence of the oxygen presently, influences the absolute amount of CO% adsorbed, the data can be corrected, using values for the enhancement of the adsorpassuming the tion obtained at -80°C effect is independent of temperature and a function of surface coverage only. This results in the slopes of the isosteres remaining unaltered but the corresponding values for the coverages are reduced, at the most, by 0.5 X 10V6mole me2 for maximum -AH
A
c
0
ADSORPTION
I
DESORPTION
0
ADSORPTION
.
DESORPTIDN
A
REPEAT
DIFFERENCE
for Graphon combusted to 19.2 per cent weight loss. Further, correcting only the -80°C data for the enhancement and assuming the effect does not occur at + 29°C results in a change in slope of the isosteres giving a decrease of approximately 0.1 kcal mole-’ in the maximum -AH value for the 19.2 per cent combusted sample. Corrections at lower degrees of coverage are correspondingly smaller. Hence the higher values of -AH observed with the partly combusted materials at low coverage must be due, at least partly, to changes in the nature of the carbon surface (cf. the results of Griffiths et aZ.[15]). It is interesting to OEGASSEO
OXIDIZED
SAMPLE
SAMPLE
ADSORPTION BETWEEN
A AN0 8
a
c 0
50 PRESSURE,
100
ooc 150
200
TORR
Fig. 3. Adsorption of CO, on Graphon combusted to 52% weight loss; oxygen content 349 X IoN6atom g-l.
471
PHYSICAL ADSORPTION OF CARBON DIOXIDE ON GRAPHON
with increasing pressure but increases becomes independent of pressure beyond about 150 torr, i.e., just before completion of the first monolayer over the whole surface. Contrary to this behaviour, argon isotherms determined at - 196°C were found to be completely unaffected by the presence or otherwise of oxygen on the surface (as also therefore were the B.E.T. areas). Hence the existence of polar attraction between CO, and chemisorbed oxygen seems to be established. Figure 4 shows the enhancement of CO2 adsorption at -8O”C, measured at 200 torr pressure, plotted against the amount of chemisorbed oxygen on the surface. The latter was varied in several ways as indicated in the legend to the figure. Three main facts emerge from the results: (i) there is a genera1 increase of CO2 adsorption with increasing amounts of oxygen on the surface; (ii) since the scatter of the results is well outside the range of experimental error, the effect of the oxygen on the adsorption depends to some extent on the history of the specimen; and (iii) under the conditions
note, however, that the surface of the specimen combusted to 19.2 per cent weight loss is still sufficiently homogeneous to show an increase in - AN with coverage. 3.2 E$ect of chemisorbed oxygen on adsorption of co2 Several investigators have found that the adsorption of CO, by carbons increases with increasing oxygen content of the carbon [2, 13, 161, but it does not appear that the amount of CO:! adsorbed has ever been correlated quantitatively with direct measurements of the concentration of oxygen atoms or groups on the surface. The present data provide such a correlation for the particular case of oxygen chemisorbed on Graphon. Figure 3 shows representative examples of graviobtained adsorption isotherms metrically at -80 and 0°C with the same specimen in the absence (A) and presence (B) of a precisely determined amount of oxygen on the surface. The amount of extra COz adsorbed by the oxygenated surface, obtained by subtracting isotherm A from isotherm B, is given by curve C. At low pressures this 0
OXIOIZEO
0
do.,
5OOQC/ZO
A
OXIOIZEO
PARTIALLY
TORR
O2
DEGASSEO
600aC/170
AT 600-
TORR
650%
02
A
0 0
I
I
I
0
100 SURFACE
CONCENTRATION
Fig. 4. Enhancement
zoo OF
CHEMISOREED
I
I
300
LOO
OXYGEN
of CO* adsorption at -80°C oxygen.
x10:
ATOM
g-’
by chemisorbed
R. J. TYLER and H. J. WOUTERLOOD
472
examined molecule
a maximum is adsorbed
of one extra COs 3 to 5 oxygen
for every
atoms on the surface. A similar conclusion to (iii) can be drawn from the measurements of adsorption of CO* at -79% on Spheron 6 (i.e. Graphon in ungraphitized form) made by Spencer et al. [13]. These authors found the adsorption to be notably reduced by prior heating of the carbon to lOOO”C,a treatment which totally removes its content of combined oxygen [ 13,171. The CO, adsorption isotherms of the original and heat-treated materials become parallel above about 100 torr, indicating saturation of the enhancing effect of oxygen above that pressure. At this point the extra COz adsorbed by the original material amounts to 5.8 X lob6 mole me2 (see Fig. 1 of Ref. 13). The oxygen content of Spheron 6 has been given[17] as 2.3 X 10e3 mole g-l and its specific surface area[5] as 115 m2g-‘. If the oxygen is assumed to be present entirely on the surface[17] these data indicate the adsorption of one extra molecule of CO, for every 3.5 oxygen atoms on the surface, a value in remarkable agreement with that obtained in the present work with Graphon. Acknowledgments-The
Godfrey
authors are L. Cabot Inc. for providing
grateful to the samples
of Graphon
and to Dr. M. F. R. Mulcahy for his
interest and advice. REFERENCES 1. Marsh H. and Wynne-Jones
W. F. K., Carbon
1,269 (1964).
2. Anderson
R. B., Bayer J. and Hofer L. J. E.,
Fuel 44,443 (1965).
3. Walker P. L. and Kini K. A., Fuel 44,453 (1965). 4. Walker P. L. and Pate1 R. L., Fuel49,91 (1970). 5. Schaeffer W. D., Smith W. R. and Polley M. H.,Znd. Eng. Chem. 45,1721(1953). Kini K. A., Fuel43, 173 (1964). ;: Kini K. A. and Walker P. L., J. Sci. Ins&urn.
42,821 (1965). A. B., J. Sci. In&urn. Series 2, 1, 86 (1968). 9. Tucker B. G. and Mulcahy M. F. R., Trans. 8. Ayling
Faraday SOL.65,274 (1969). 10. Laine N. R., Vastola, F. J. and Walker P. L., J. Phys. Chem. 67,203O (1963).
G., McLean A. M. and Ritchie 11. Houghton P. D., Chem. Eng. Sci. 6, 132 (1957). 12. Dubinin M. M., Bering B. P., Serpinsky V. V. and Vasil’ev B. N., Surface Phenomena in Chemistry and Biology, pp. 172-188. Pergamon Press, London (1958). 13. Spencer W. B., Amberg C. H. and Beebe R. A., J. Phys. Chem. 62,719 (1958). 14. Emmett P. H. and Brunauer S., J. Amer. Chem. sot. 59, 1553 (1937). 15. Griffiths D. W. L., Thomas W. J. and Walker P. L., Carbon 1,515 (1964). F. G. and Arnold 16. Deitz V. R., Carpenter R. G., Carbon 1,245 (1964). 17. Coltharp M. T. and Hackerman N., J. Phys. Chem. 72,1171 (1968).
1971, Vol. 9. pp, 467-472.
Pergamon Press.
Printed in Great Bntain
PHYSICAL ADSORPTION OF CARBON DIOXIDE ON GRAPHON R. J. TYLER and H. J. WOUTERLOOD Division of Mineral Chemistry, Commonwealth Scientific and Industrial Research Organization, P.O. Box 175, Chatswood, Australia 2067 (Received 8 August 1970)
isotherms of carbon dioxide on Graphon have been determined at temperatures from -80 to +29”C and equilibrium pressures up to 30 atm. Plots of log (fugacity) against l/T at constant amounts adsorbed are linear over the temperature range, showing no change in slope in the region of the triple point of bulk carbon dioxide. With increasing amount adsorbed the isosteric heat of adsorption increases to a maximum of 4.5 kcal mole-’ and then declines. Similar behaviour is shown by carbon dioxide adsorbed on partly combusted specimens of Graphon. Assuming that the maximum heat of adsorption occurs on completion of the first adsorbed layer, the average area of surface occupied by each carbon dioxide molecule at this point is 26A’. The effect of oxygen previously chemisorbed on the carbon surface is to increase the amount of carbon dioxide adsorbed at - 80°C. This effect was studied gravimetrically and it was concluded that the extra amount adsorbed rises with increasing pressure of carbon dioxide to a value corresponding to the adsorption of an extra carbon dioxide molecule for each 3-5 oxygen atoms on the surface.
Abstract-Adsorption
1. INTRODUCTION
‘graphitized’ carbon black, have been described by Schaeffer et al. [5]. Isosteric heats were determined for three materials, i.e. the original Graphon and the same material burnt off to 4.9 per cent and to 19.2 per cent weight loss. Specific areas were X0.5, 92.6 and 95.0 m*g-’ respectively (argon adsorption at - 19VC; 13*8Az per atom; B.E.T. plots linear to Y/P0 = 0.25). Isotherms were measured at - 80.0 * 0.2% by conventional volumetric technique and at -53-l 50.1, 0. and 28.7-+O*l”C using apparatus which operated up to 34 atm pressure. The latter, though, developed independently from the original design by Kini[G], was essentially similar to the apparatus described by Kini and Walker[7] but incorporated a more sensitive transducer read-out system[S]. All samples were degassed at 140°C prior to these measurements. The cryostat for - 53°C consisted of a small dewar flask filled with petroleum ether
The increasing use of carbon dioxide as an adsorbate for measurements of the surface of carbonaceous solids [l-4] has areas prompted the authors to put on record some measurements relating to the properties of carbon dioxide adsorbed on Graphon. Isosteric heats of adsorption have been obtained as a function of coverage from isotherms determined at temperatures from - 80 to + 29% (and pressures up to 30 atm). The results strongly support the view that the first monolayer has liquid-like rather than solid-like properties down to -80°C and that each molecule occupies an area of about 26Ax in the complete layer. In addition, the effects on the isotherms at -80°C of known amounts of oxygen chemisorbed on the carbon surface have been determined. 2. EXPERIMENTAL The nature
and properties
of Graphon,
a 467
R. J. TYLER and H. J. WOUTERLOOD
468
surrounded by a larger one containing liquid Ne. The ether was stirred by bubbling in a slow stream of Nz and its temperature was maintained by a small immersion heater. Power to the heater was controlled by a Matheson ‘Lab. Stat’ using a propane gasthermometer as the sensing device. The effect of chemisorbed oxygen on the by adsorption at -80°C was investigated means of a Cahn R.G. Recording Electrobalance. Chemisorption was effected by exposing the Graphon to oxygen in the balance chamber after the manner described by Tucker and Mulcahy[9]. Briefly, the routine was as follows. The specimen (O-120.16 g), suspended from the balance arm, was first heated in vacw, at 950°C to remove previously chemisorbed oxygen [lo] and then exposed (usually) to 20 torr 0, at 500°C for 1 hr. During the latter period about 0.2% of the carbon was gasified and oxygen was chemisorbed. A CO2 adsorption isotherm was then measured gravimetrically in situ at -80°C. Next the oxygen was desorbed[lO] (mainly as CO) by another
period at 950°C. The measured loss in weight (W,) during this period enabled the weight of chemisorbed oxygen to be calculated (i.e. 16 W,/28). Finally, the CO, isotherm of the oxygen-free carbon was determined (again in situ). In a few experiments adsorption isotherms were also measured at 0°C. A single chemisorptiondesorption cycle produced a negligible change in surface area. However, repetition of the cycle caused a gradual increase in the fraction of the surface covered by oxygen[9] and this procedure was used to obtain a range of coverages for the CO, adsorption measurements. Throughout the work, the adsorption isotherms were highly reproducible and no chemisorption or desorption hysteresis of CO, was observed. 8. RESULTS AND DISCUSSION 3.1 Heats of adsorption Figure 1 shows plots of log,, (fugacity) against l/T for CO2 adsorbed on the original Graphon at several constant degrees of
2.0
BULK
1.0
co2
r t u0
0
LF -J
-1.0
3.5
4.0
4.5 103 r'
OEG
5.0 K-'
Fig. 1. Isosteres for adsorption of CO, on Graphon.
55
PHYSICAL
ADSORPTION
OF CARBON
coverage by the COz. (The data of Houghton et al. [ 1 l] were used to calculate the fugacities at 0 and +29”C from pressures obtained by interpolation between closely adjacent points of the isotherms.) The isostere for bulk CO, in the same temperature range is also shown in Fig. 1. The adsorption isosteres are accurately linear over the whole temperature range, which, as Fig. 1 shows, includes the triple point of the bulk material. There is no indication of a phase change in the adsorbed state and the slopes of the isosteres are close to that of the liquid rather than the solid isostere. Similar behaviour was observed with Graphon burned off to 4.9 per cent and to 19.2 per cent weight loss. This result differs from that obtained by Dubinin et al. [12] from studies of adsorption of CO? on silica gel. When less than a complete monolayer was present on the surface of this material the slope of the isostere was found to change in the vicinity of the triple-point temperature (- 56.6”C) indicating a phase change analogous to the solidliquid transition. Average heats of adsorption determined from the slopes of the isosteres obtained with the three present materials are shown as a function of coverage in Fig. 2. In each case the coverage is expressed as mole rn+ on 5~0
7
u
-
ORIGINAL
-
do., do.,
,-
DIOXIDE
ON GRAPHON
469
the basis of the value of the surface area obtained by argon adsorption. The curves all show a maximum value of 4.5 kcal mole-’ for -AH at a coverage of approximately 6.3 x 10~“mole me2. The rise in -AH with increasing coverage is due to lateral attraction between the CO, molecules and has been observed previously with COz[13] and other adsorbates on homogeneous surfaces. The present results agree with calorimetrically determined heats of adsorption of CO, on other graphitized carbon blacks at -80°C: with Sterling FT(2700”) and Sterling MT (3100”) Spencer et a/.[131 found maxima of 6 kcal mole-’ at coverages of j-7 X 10Ffi and 7.1 X lo-“mole m-’ respectively. Assuming that the maximum value of-AH occurs at completion of the first layer, the present results yield an average value of 26A’ for the area occupied by each molecule in the temperature range -80 to +29”C. The corresponding values [13] at -80°C for the two graphitized blacks just mentioned are 29 and 23A”. In agreement with the results of Walker and Kini [3], these figures indicate that values of the area per molecule based on the densities of the bulk liquid or solid [ 141 (17 or 14&) are too low. Since the high-pressure adsorption apparatus did not allow the specimen to be degassed
GRAPHON
COMEUSTED
To
COMBUSTEO
TO
4’9 ‘I. 19.2
%
6 I
2.0 SURFACE
4.0 COVERAGE
6.0 BY
6.0 CO,x
10:
MOLE
10.0 -2
m
Fig. 2. Variation of heat of adsorption of CO2 with surface coverage.
IL J. TYLER
470
and H. J. WOUTERLOOD
at high temperature in situ, the above measurements refer to surfaces with up to 9 per cent of the area covered with chemisorbed oxygen. Although, as will be seen the presence of the oxygen presently, influences the absolute amount of CO% adsorbed, the data can be corrected, using values for the enhancement of the adsorpassuming the tion obtained at -80°C effect is independent of temperature and a function of surface coverage only. This results in the slopes of the isosteres remaining unaltered but the corresponding values for the coverages are reduced, at the most, by 0.5 X 10V6mole me2 for maximum -AH
A
c
0
ADSORPTION
I
DESORPTION
0
ADSORPTION
.
DESORPTIDN
A
REPEAT
DIFFERENCE
for Graphon combusted to 19.2 per cent weight loss. Further, correcting only the -80°C data for the enhancement and assuming the effect does not occur at + 29°C results in a change in slope of the isosteres giving a decrease of approximately 0.1 kcal mole-’ in the maximum -AH value for the 19.2 per cent combusted sample. Corrections at lower degrees of coverage are correspondingly smaller. Hence the higher values of -AH observed with the partly combusted materials at low coverage must be due, at least partly, to changes in the nature of the carbon surface (cf. the results of Griffiths et aZ.[15]). It is interesting to OEGASSEO
OXIDIZED
SAMPLE
SAMPLE
ADSORPTION BETWEEN
A AN0 8
a
c 0
50 PRESSURE,
100
ooc 150
200
TORR
Fig. 3. Adsorption of CO, on Graphon combusted to 52% weight loss; oxygen content 349 X IoN6atom g-l.
471
PHYSICAL ADSORPTION OF CARBON DIOXIDE ON GRAPHON
with increasing pressure but increases becomes independent of pressure beyond about 150 torr, i.e., just before completion of the first monolayer over the whole surface. Contrary to this behaviour, argon isotherms determined at - 196°C were found to be completely unaffected by the presence or otherwise of oxygen on the surface (as also therefore were the B.E.T. areas). Hence the existence of polar attraction between CO, and chemisorbed oxygen seems to be established. Figure 4 shows the enhancement of CO2 adsorption at -8O”C, measured at 200 torr pressure, plotted against the amount of chemisorbed oxygen on the surface. The latter was varied in several ways as indicated in the legend to the figure. Three main facts emerge from the results: (i) there is a genera1 increase of CO2 adsorption with increasing amounts of oxygen on the surface; (ii) since the scatter of the results is well outside the range of experimental error, the effect of the oxygen on the adsorption depends to some extent on the history of the specimen; and (iii) under the conditions
note, however, that the surface of the specimen combusted to 19.2 per cent weight loss is still sufficiently homogeneous to show an increase in - AN with coverage. 3.2 E$ect of chemisorbed oxygen on adsorption of co2 Several investigators have found that the adsorption of CO, by carbons increases with increasing oxygen content of the carbon [2, 13, 161, but it does not appear that the amount of CO:! adsorbed has ever been correlated quantitatively with direct measurements of the concentration of oxygen atoms or groups on the surface. The present data provide such a correlation for the particular case of oxygen chemisorbed on Graphon. Figure 3 shows representative examples of graviobtained adsorption isotherms metrically at -80 and 0°C with the same specimen in the absence (A) and presence (B) of a precisely determined amount of oxygen on the surface. The amount of extra COz adsorbed by the oxygenated surface, obtained by subtracting isotherm A from isotherm B, is given by curve C. At low pressures this 0
OXIOIZEO
0
do.,
5OOQC/ZO
A
OXIOIZEO
PARTIALLY
TORR
O2
DEGASSEO
600aC/170
AT 600-
TORR
650%
02
A
0 0
I
I
I
0
100 SURFACE
CONCENTRATION
Fig. 4. Enhancement
zoo OF
CHEMISOREED
I
I
300
LOO
OXYGEN
of CO* adsorption at -80°C oxygen.
x10:
ATOM
g-’
by chemisorbed
R. J. TYLER and H. J. WOUTERLOOD
472
examined molecule
a maximum is adsorbed
of one extra COs 3 to 5 oxygen
for every
atoms on the surface. A similar conclusion to (iii) can be drawn from the measurements of adsorption of CO* at -79% on Spheron 6 (i.e. Graphon in ungraphitized form) made by Spencer et al. [13]. These authors found the adsorption to be notably reduced by prior heating of the carbon to lOOO”C,a treatment which totally removes its content of combined oxygen [ 13,171. The CO, adsorption isotherms of the original and heat-treated materials become parallel above about 100 torr, indicating saturation of the enhancing effect of oxygen above that pressure. At this point the extra COz adsorbed by the original material amounts to 5.8 X lob6 mole me2 (see Fig. 1 of Ref. 13). The oxygen content of Spheron 6 has been given[17] as 2.3 X 10e3 mole g-l and its specific surface area[5] as 115 m2g-‘. If the oxygen is assumed to be present entirely on the surface[17] these data indicate the adsorption of one extra molecule of CO, for every 3.5 oxygen atoms on the surface, a value in remarkable agreement with that obtained in the present work with Graphon. Acknowledgments-The
Godfrey
authors are L. Cabot Inc. for providing
grateful to the samples
of Graphon
and to Dr. M. F. R. Mulcahy for his
interest and advice. REFERENCES 1. Marsh H. and Wynne-Jones
W. F. K., Carbon
1,269 (1964).
2. Anderson
R. B., Bayer J. and Hofer L. J. E.,
Fuel 44,443 (1965).
3. Walker P. L. and Kini K. A., Fuel 44,453 (1965). 4. Walker P. L. and Pate1 R. L., Fuel49,91 (1970). 5. Schaeffer W. D., Smith W. R. and Polley M. H.,Znd. Eng. Chem. 45,1721(1953). Kini K. A., Fuel43, 173 (1964). ;: Kini K. A. and Walker P. L., J. Sci. Ins&urn.
42,821 (1965). A. B., J. Sci. In&urn. Series 2, 1, 86 (1968). 9. Tucker B. G. and Mulcahy M. F. R., Trans. 8. Ayling
Faraday SOL.65,274 (1969). 10. Laine N. R., Vastola, F. J. and Walker P. L., J. Phys. Chem. 67,203O (1963).
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