Langmuir 1986,2, 43-46
43
Adsorption Sites for Water on Graphite. 2. Effect of Autoclave Treatment of Sample Tetsuo Morimoto*+ and Kazuhisa MiuraI Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, Japan, and Department of Metal Engineering, Tsuyama Technical College, Tsuyama 708, Japan Received February 12, 1985. I n Final Form: September 17, 1985 The adsorption sites for H 2 0 on graphite treated with H20 in an autoclave at 300 O C were investigated by measuring the adsorption isotherm of H 2 0 and the amount of surface compounds. As the result, it was found that the autoclave treatment of graphite causes a remarkable decrease in the amount of every kind of surface compound and in the adsorbability for H 2 0 . The succeeding treatment of the sample at higher temperatures also results in a further decrease in the amount of surface compounds as well as the adsorbability for H20. In other words, both the autoclave treatment and the subsequent heat treatment raise the hydrophobicity of the surface of graphite and reduce the isosteric heat of adsorption of HzO, qst, and in an extreme case the qet curve gives a distinct minimum which passes through an energy value much lower than the heat of liquefaction of H20.
Introduction In a previous paper,l the adsorption sites for H 2 0 on carbon have been investigated by using natural graphite as a typical carbonaceous material. As the result, it was found that the H20-,C 0 2 - ,and CO-desorbing oxides on graphite act as the adsorption sites for H20. Among them, the pair site composed of the H20- and C02-desorbing oxides operates as the most intensive adsorption site for HzO, and the CO-desorbing oxides are the next prevailing sites for HzO which give an adsorption energy close to the heat of liquefaction of H20,HL.2 When the surface content of these three kinds of oxides is extremely small, the isosteric heat of adsorption of HzO, qst, decreases sharply and reaches a distinct minimum, at which the qst value is much smaller than the HLvalue. The present study has been undertaken to clarify the adsorption sites for H 2 0 on carbon surfaces and the appearance of the qst minimum in this system. Natural graphite was treated in an autoclave, because the treatment was found to reduce the amount of surface oxides, and H 2 0 adsorption was investigated on this sample. Experimental Section Materials. The original sample of a natural graphite used in
the present investigation was from Sri Lanka and supplied from Nippon Kokuen Co. According to the maker's assay, the sample is 99.5% in purity and contains 0.5% ashes, the particle size distribution being 1-30 pm. First, it was subjected to the extraction by benzene for 6 days and dried in air (G-25). Next, G-25 was treated with HzOin an autoclave at 300 "C under the pressure of 6.44 X lo4 torr (1torr = 133.3 Pa) for 5 h, washed with HzO sufficiently, dried in air, degassed under a vacuum of torr, and heated for 5 h at 25, 400, 700, and 1000 "C, respectively (HG-25, HG-400, HG-700, and HG-1000). The specific surface area of these samples was measured by applying the BET method to the N2adsorption data and found to be 8.63 0.20 m2/g,which is unchanged for every sample, regardless of the various kinds of treatments. Measurement of Adsorption Isotherm of Water. Just after the heat treatment at elevated temperatures, the sample was subjected to the measurement of the adsorption isotherm of HzO at 25 "C. Then the sample was exposed to saturated HzOvapor for 48 h at 25 "C to permit the accomplishment of surface hytorr, dration and degassed at 25 "C for 10 h in a vacuum of and the second adsorption isotherm of HzO on it was measured repeatedly at 10, 18, and 25 "C after every evacuation at 25 "C. Adsorption equilibrium was attained within 30 min after every
*
f
Okayama University.
* Tsuyama Technical College. 0743-7463/S6/2402-0043$01.50/0
dose of HzOvapor for the first adsorption measurement on HG-25 and for the second adsorption measurement on every sample, whereas it took more than 1h for the fiist adsorption measurement on graphite treated at higher temperatures, Le., on HG-400, HG-700, and HG-1000. The adsorption measurement of HzOwas carried out volumetrically by using a conventional adsorption apparatus, equipped with an oil manometer. Since a small amount of H20is absorbed into oil, the pressure dependence of the absorbed amount of HzO was measured preliminarily,and then the data obtained on the HzO adsorption isotherm were corrected on the absorption of HzO into oil. Determination of Surface Compounds. The pyrolysis of graphite produces various kinds of gases through the decomposition of surface compounds. Mass spectrometry and gas chromatography showed that the ignition of graphite at elevated temperatures generates a gas containing HzO,C02,CO, Hz, and CHI, as in the case of G-25.' Therefore, the gas evolved by igniting graphite was analyzed quantitatively by the two-step trapping method.'J On the other hand, the amount of acidic surface oxides was determined by the reaction with four kinds of bases, NaOC,H,, NaOH, Na2C03,and NaHC03.4,5 After equilibrating a sample with a base solution, the supernatant solution was titrated with a standard HC1 solution.'
Results The scanning electron micrographs of graphite have shown that the feature of the graphite crystals remains unchanged by the autoclave treatment as well as by the succeeding heat treatment, though the photographs are not illustrated here. This fact is consistent with the result that the specific surface area of graphite is invariable through the two steps of treatment as described above. Figure 1 shows the change in the surface gas content with the ignition temperature, which is expressed in the number of molecules per manometer squared, on the basis of the N2 adsorption area. This value was obtained by integrating the amount of gas expelled from graphite when heated from the indicated temperature to 1000 "C. Here, the data on the untreated graphite G-25 are also cited.' It is found from Figure 1 that every gas content is reduced by the autoclave treatment, but the decrement depends upon the nature of the evolved gas. The surface content (1) Morimoto, T.; Miura, K. Langmuir 1985, 1, 658. (2) Walker, P. L., Jr.; Janov, J. J. Colloid Interface Sci. 1968,28, 449. (3) Nagao, M.; Morishige, K.; Takeshita,T.; Morimoto, T. Bull. Chem. SOC.Jpn. 1974, 47, 2107. (4) Boehm, H. P.; Diehl, E. Z. Elektrochem. 1962,66, 642. (5) Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem., Int. Ed. Engl. 1964, 3, 669.
0 1986 American Chemical Society
44 Langmuir, Vol. 2, No. I , 1986
Morimoto and Miura
Table I. Monolayer Volume of Water, V,, and Surface Content of Gases surface contenta G-25 HG-25 HG-400 HG-700 HG-1000
V, 1.740 0.496 0.395 0.187 0.101
H,O 1.710 1.406 0.902 (0.769) 0.115 (0.018) 0.054 (0.000)
CQ, 1.644 0.365 0.204 (0.180) 0.024 (0.007) 0.011 (0.000)
co
CHa 1.233 0.939 0.926 (0.913) 0.041 (0.000) 0.011 (0.000)
1.952 1.397 1.344 (1.343) 0.899 (0.856) 0.068 (0.000)
H, 2.294 1.389 1.389 (1.389) 1.518 (1.389) 0.156 (0.000)
"Expressed in molecules/nm2on the basis of N2 area.
L
l
h "
lh
I
0
4
8
I2 16
0
4
8 1 2 1 6 0 4 8 1 2 1 6 0 4 8 1 2 1 6
'0
Pressure /torr
J Temperature/?
Figure 1. Surface gas content on autoclave-treatedgraphite, HzO (A), COz (o),CO (a),CHI (o),and H, (0). Solid and broken lines indicate gas content on G and HG, respectively. a t room temperature increases in the order, H2 > CO > H20, C 0 2 > CH4 for G-25, but in the order Hz, CO, HzO > CHI > COP for HG-25; especially the COPcontent is reduced most drastically to about one-fifth of the original value. Originally, the flat part can be observed in the content curve on Hz and CH4, which implies that the gas is not generated until the indicated temperature is attained. On the autoclave-treated graphite, the flat part appears in every content curve other than that of HzO. Furthermore, the flat part extends to a higher temperature region compared with that in the original sample, G-25, e.g., from 600 to 700 "C for H2, and from 300 to 400 "C for CH,. The flat part is newly developed up to 200 "C for COPand 300 "C for CO. These facts indicate that a certain amount of gas generated by heating the untreated graphite G-25 up to the temperature indicated has been removed by the autoclave treatment a t 300 "C. Oddly, the upper limit of the flat part is not decided by the autoclave treatment temperature, 300 "C. As pointed out above, the content curve for H 2 0 on HG-25 does not reveal the flat part, and in addition, it lies slightly over that on G-25 a t temperatures higher than 200 "C, probably because the H,O-desorbing oxides are partly stabilized through the autoclave treatment. The content curve on CH, also reveals the same feature a t higher temperatures as that on H20. A brief test was carried out to investigate the effect of the autoclave treatment in H 2 0 a t 300 "C: the original sample G-25 was treated in the Nz gas (3.75 X lo4torr) at 300 "C for 5 h, and then the N2-treated sample was subjected to the surface gas content measurement. As the result, it was found that the content curves of CO and COP were almost the same as those on HG-25, though the content of H 2 0 was less than that on HG-25; i.e., it became about two-thirds of the latter. This proves that the autoclave treatment in H 2 0 has only a thermal effect similar
Figure 2. Adsorption isotherms of HzO on autoclave-treated graphite, (a) HG-25, (b) HG-400, (c) HG-700, and (d) HG-1000. Closed circle, first adsorption isotherm; open circle, second adsorption isotherm.
to that of the autoclave treatment in N2. The adsorption isotherm of HzO on HG is shown in Figure 2, the adsorbed amount being expressed in centimeters cubed (STP) per meter squared, on the basis of the N2adsorption area. I t is found from Figure 2 that when the pretreatment temperature of graphite is raised, the knee of the adsorption isotherm becomes lower, and the shape of the isotherm transfers from the type I1 to 111: which implies an increase in hydrophobicity of the surface, as in the case of graphite with no autoclave treatment.* The mutual relationship between the first and second adsorption isotherms changes with the pretreatment temperature of HG; i.e., both isotherms quite agree with each other for HG-25, the first adsorption isotherm lies under the second one for HG-400, both isotherms are close to each other for HG-700, and even on HG-1000 the first adsorption isotherm is similar to the second one. However, the first adsorption isotherm on HG-1000 represents an irregular shape, despite the fact that the second one gives a smooth curve, i.e., the adsorbed amount in the first adsorption isotherm initially increases linearly till the equilibrium pressure of 1torr and enhances sharply near the pressure of 1 2 torr, and the first adsorption isotherm exceeds the second one at this pressure. Similar irregularity was observed on the 1000 "C treated graphite, G1000, with no autoclave treatment.l This discrepancy observed between the first and second adsorption isotherms can be attributed to the chemisorption of HzO on graphite from the two points of view. First, it takes more time, about 1 h, for the equilibration with H 2 0 vapor during the course of the first adsorption measurement on HG-400, HG-700, and HG-1000, compared with the time for the second one. Second, a small amount of surface compounds has been found to be reproduced after the equilibration of a high-temperature-treated graphite with HzO vapor, as will be described later. Since the surface reaction products with H 2 0 can be considered to act as the physisorption sites for HzO, the surface hydration rea(6) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. SOC.1940,62, 1723.
Langmuir, Vol. 2, No. 1, 1986 45
Adsorption Sites for Water on Graphite
Table 11. Amount of Acidic Surface Oxides ~~
'OOt
carboxyl G-25 HG-25 HG-400 HG-700 HG-1000
H,O,CO, 1.12 0.25 0.02 0.00 0.00
Acidic surface oxidea lactone phenol CO, CO, H,O 0.28 0.00 0.00 0.00 0.00
carbonyl CO 1.02 1.13 1.36 0.60 0.44
0.94 0.26 0.16 0.09 0.02
total 3.36 1.64 1.54 0.69 0.46
Expressed in molecules/nm2 on the basis of N2 area. o0
~ 0.I "
'
0.2 " 0.3 " ' 0.4' ' 0.5 ' ~0.6 Amount of adsorbed water/moleCules~nm?
'
I
0.7 ~
Figure 3. Isosteric heat of adsorption of HzO, qst, on autoclave-treated graphite ( 0 )HG-25, (0)HG-400, (@)HG-700, and ( 0 ) HG-1000. Arrows indicate V, (solid line), HzO content (broken line), and C02content (dotted line). Horizontal broken line indicates heat of liquefaction of H20 at 25 O C . sonably raises the amount of adsorbed HzO. Therefore, a slow rate of hydration will reduce the adsorbed amount a t low pressures. The BET monolayer volumes of HzO, V,, obtained from the second adsorption isotherm and the gas content on HG, are listed in Table I. For comparison, the data on G-25 are cited here.' The surface content read from Figure 1 is listed in the bracket in Table I. When a high-temperature-treated sample is exposed to HzO vapor a t room temperature, a small amount of surface compounds are reproduced and they are decomposed to generate an additional amount of gas on igniting graphite even a t temperatures lower than the pretreatment temperature. The amount of gas coming from the reproduced surface compounds was measured and added to the value in the bracket, and the total amount was listed outside the bracket in Table I; that is the content that graphite has on the surface in the second adsorption process. The amount of reproduced gas depends on the pretreatment temperature of graphite as well as on the nature of gas evolved. When the pretreatment temperature of graphite is raised, the reproduced amount of HzO, COz, and CH4 decreases, while that of Hz and CO increases. In any case, the surface content of every gas decreases with rising temperature of pretreatment, and in addition it is always smaller than that on the corresponding graphite with no autoclave treatment. Moreover, it should be pointed out that the V , value is much smaller than that on G-25; i.e., the autoclave treatment also leads to an enhanced hydrophobicity of graphite, as the heat treatment at elevated temperatures does. By applying the Clausius-Clapeyron equation to the second adsorption isotherms in Figure 2, we can obtain the qatvalues on HG as plotted against the adsorbed amount of H 2 0 in Figure 3. Here, the certainty of the qatvalue is so important that the data were checked through this investigation a t the stage of the reproducibility of the adsorption isotherm of HzO, by measuring the isotherm more than 3 times a t a temperature. The qat values in Figure 3 were calculated by applying the method of least squares to such data. It is clear from Figure 3 that the qat value on HG-25 decreases monotonously with increasing amount of adsorbed HzO and closes to the HL value around 19 = 0.5. The initial decrease in the qatvalue to the HL value becomes more marked when the pretreatment temperature of graphite is raised. In an extreme case of HG-1000, the qat value traverses the HLlevel a t a small amount of adsorbed HzO, attains a very deep minimum, and then increases toward the HLvalue, as in the case of G-1000.'
2 Q
r;;;;,
0
&::::,...-&loo0
, , ,
,lo'
, ,
.I....-
Calculated amount of gas /mdecuies.nm-2
Figure 4. Relation between amount of gas evolved by igniting graphite and that calculated from decomposition reactions: (0) HzO and (v)COP The amount of the acidic surface oxygen compounds on HG, measured by the reaction with four kinds of bases$5 is listed in Table 11, being expressed in the number of functional groups per nanometer squared on the basis of the Nz adsorption area. The data on G-25 are also cited in this table.' Here, the gases to be formed by the decomposition of these compounds are d e ~ c r i b e d . ~ - 'It~ is found from Table 11that every kind of acidic surface oxides decreases by the autoclave treatment of graphite: especially, the lactone groups disappear, and both carboxyl and phenol groups decrease drastically to the ratio of 1f 4. Moreover, the concentration of every surface functional group decreases with rising temperature of treatment, as in the case of graphite with no autoclave treatment.' The carboxyls cannot be detected on HG-700 and HG-1000.
Discussion Decomposition of Acidic Surface Oxides. In the previous paper,' it has been concluded that the following series of decomposition reactions of acidic surface oxides can account for most satifactorily the amount of gases, HzO and COz, evolved when graphite is ignited. COOH + OH COZ + H2O (1) COOH COZ (2) 4
4
(7) Garten, V. A.; Weiss, D. E.; Willis, J. B.Aust. J . Chem. 1957, 10, 245. (8)Rivin, D. Rubber Chem. Technol. 1963,36,729. (9) Walker, P. L., Jr.; Austin, L. G.; Tietjen, J. J. 'Proceedings of Symposium on Carbon, Tokyo", 1964, VIII-12. (10) Boehm, H. P. Angew. Chem., Znt. Ed. Engl. 1966,5, 533. (11)Lang, F. M.; de Noblet, M.; Donnet, J. B.; Lahaye, J.; Papirer, E. Carbon 1967,5,47. (12) Coltharp,M. T.; Hackerman, N. J . Phys. Chem. 1968, 72,1171. (13) Barton, S. S.; Boulton, G. L.; Harrison, B. H. Carbon 1972, 10,
395. (14) Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978,16, 35.
Morimoto and Miura
46 Langmuir, Vol. 2, No. 1 , 1986
2 OH
-
+
lactone
HZO C02
(3)
(4)
By assuming the same series of reactions also for iG,'-I4 we can compute the amount of HzO and COz to be formed by the decomposition of surface oxides, and it is plotted against the amount of actually determined gases in Figure 4. Since the lactones are absent on the surface of HG, reaction 4 can be omitted. The plot shown in Figure 4 reveals a large deviation from the 1:1 relationship: the amount of actually evolved gases is much greater than that calculated by the decomposition reaction, the deviation being remarkable compared with that on graphite with no autoclave treatment.' This suggests that the evolved gas comes in large quantities from nonacidic surface oxide^.'^'' Adsorption Sites for Water on Autoclave-Treated Graphite. The V , value of H 2 0 and the HzO and COz contents are indicated by arrows on the qst curve of H 2 0 in Figure 3, all the values being expressed in the number of molecules per nanometer squared on the basis of the Nzadsorption area. It appears from Figure 3 that the shape of the qst curve is closely related with these values. On HG-1000, in which the qst curve reveals a deep minimum, the H 2 0 content is very close to the amount of adsorbed H,O at which the qst minimum appears, and the COz content approximates the adsorbed amount at which the qst curve crosses the H L level. The same feature was observed on G-1000, which led to the conclusion that the initial adsorption sites for HzO on G-1000 are the pair sites, which expel HzO and COSsimultaneously according to eq 1. After the accomplishment of this type of adsorption, HzO molecules are adsorbed on the sites that desorb HzO on ignition according to eq 3, where the qst value is reduced to a deep minimum, and a further increase in the collision frequency of HzO molecules onto the adsorption sites leads to the cluster formation2*lswhich raises the qst value to the HLlevel.' This can be considered to be valid also in the case of HG-1000. As stated above, the gas content is smaller on HG than on G. This is reflected on the shape of the qst curve as follows. Both the cross point and the minimum point in the qst curve are shifted to lower values of the adsorbed amount of HzO on HG-1000 compared to those on G-1000, and at the same time the minimum value is much depressed. Additionally, the CO content also becomes smaller on HG-1000 than on G-1000.l Thus, it can be concluded from the present investigation that the mini(15) Smith, W. R.; Schaeffer, W. D. Rubber Chem. Technol. 1950,23, 625. (16) Puri, B. R.; Sharma, G. K.; Sharma, S. K. J . Indian Chem. SOC. 1967, 44, 64. (17) Voll. M.: Boehm. H. P. Carbon 1970. 8. 741. (18)Young, G.J ; Chessick, J J.; Healey, F H.; Zettlemoyer, A C J Ph%s Chem 1954, 58,313
mum in the qst curve, of a smaller value than the HLvalue, appears only when the gas content is very small, as expected in the previous paper.' The initial declining slope of the qst curve becomes steeper when the pretreatment temperature of graphite is raised, but it does not traverse the HLvalue through the pretreatment till 700 "C. Thus, a fairly sharp knickpoint appears in the qst curve on HG-700, and after that the qst value close to the HL value is kept, in spite of a small content of COz and H20. For this reason, it may be reasonable to consider that a large amount of the CO content, 0.899 molecule/nm2, can be found on HG-700, compared with the value, 0.068 molecule/nm2, on HG-1000, which plays an important role., Thus, HzO molecules can be adsorbed on the CO-desorbing sites prevailing on the surface on HG-700, after the pair sites of the H20- and Cop-desorbingoxides have been covered; this keeps the qst value at least higher than the HLvalue. On HG-400 and HG-25, the quantity of HzO-, COz-, and CO-desorbing oxides is much more than that on HG-700 (Table I). This is reasonably favorable for the adsorption of HzO, which will bring about higher qst values. Here, it should be added that the qst curve on HG-25 lies under that on G-25,' and it reaches the HL value at 0 = 0.5, because the autoclave treatment of graphite reduces the amount of every gas content. Effect of Autoclave Treatment on Surface Hydrophobicity of Graphite. The heat treatment of the autoclave-treated graphite at increasingly elevated temperatures causes an increase in hydrophobicity of the surface in some points: the amount of adsorbed HzO, e.g., the V , value, decreases, the shape of the H,O adsorption isotherm changes from the type I1 to 111: the declining slope of the qst curve becomes sharp, and in an extreme case a deep minimum appears in the qst curve. These phenomena can be accounted for quantitatively by a successive decrease in surface gas content, especially in the content of HzO, COz, and CO. Moreover, it has been found that the autoclave treatment of graphite with H 2 0at 300 OC also leads to a decrease in surface gas content and accordingly results in an enhancement of surface hydrophobicity. When the autoclave-treated graphite is succeedingly treated at an elevated temperature, the surface gas content is further reduced, which makes the surface more hydrophobic. Thus, on HG-1000, which is autoclave-treated and then heated at 1000 "C, the qst minimum is shifted to a lower amount of adsorbed H20, accompanied by a depression of the minimum value from that on G-1000 heated only at 1000 OC.l
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan. Registry No. H20, 7732-18-5; graphite, 7782-42-5.
