Langmuir 1991, 7, 374-379
374
Adsorption Sites for Water on Graphite. 5. Effect of Hydrogen Treatment of Graphite Kazuhisa Miura*v+and Tetsuo Morimoto* Department of General Education, Tsuyama National College of Technology, 624-1, N u m a , Tsuyama 708, Japan, and Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1, Ridaicho, Okayama 700, Japan Received December 19, 1989. I n Final Form: June 8, 1990 The effect of the Hz treatment of graphite on the adsorbability of HzO has been investigated by means of the measurement of the H20 adsorption isotherm, the calculation of the isosteric heat of adsorption q,, therefrom, and the analysis of the surface gas content. The Hz treatment of graphite at 1000 "C reduces remarkably the amount of surface oxides and produces C-H bonds instead. As a result, the amount of adsorbed HzO decreased drastically, and the qst curve of H20 revealed a deep minimum at a lower level than the heat of liquefaction of H20. The effect of further heat treatment of the Hp-treated graphite at 500 and 1000 "C in vacuo on the adsorbability of HzO was also examined. The latter treatment reduced the amount of surface oxides to about one-fifth of that of the Hz-treated graphite but did not change the shape of the qst curve as much; this treatment largely affected the shape of the first adsorption isotherm measured just after the treatment; Le., steps appeared at moderate pressures. This result is interpreted in terms of the pore filling of slit-shaped pores by HzO, formed by the decomposition of surface compounds.
Introduction The adsorption of H20 on Hz-treated carbonaceous materials has been investigated by several Pierce et al. reported H2O adsorption data for Darco G60 charcoal, Spheron 6, and Graphon. The results showed that C-0 complexes were formed by HzO adsorption on the Graphon pretreated with H:! at 1100 O C . l However, this work did not distinguish between chemisorption and physisorption of HzO, nor quantify the C-0 complexes formed. McDermot and Arnell measured the adsorption isotherm of H2O on three kinds of charcoal deoxygenated a t 1000 "C in a H2 stream and also on a sample which was subsequently reoxidized by exposure to H2O. These authors discovered that the reoxidized sample sorbed more H20 over the whole range of humidity than the deoxygenated one,3 but again the surface oxides were not characterized. Kiselev and Kovaleva treated several kinds of carbon black a t various temperatures from 200 to 1700 O C , either in vacuo or in a H2 atmosphere, and measured H2O adsorption isotherm for these samples. Surface functional groups were characterized through elementary analysis and titration of the carbon with NaOH. It was found that the amount of adsorbed HzO decreased markedly with rising temperature of graphitization, which was accompanied by a decrease of surface oxygen^.^ A common result in the literature cited above3s4is that the amount of adsorbed H2O decreased when the sample was treated in a Hz atmosphere at high temperature. In these the energy of adsorption of H2O was not considered, and graphite was not used as an adsorbent. In earlier work, the present authors have examined the adsorbability of HzO on the graphite surfaces modified by treating natural graphite in different ways5-* and have + Tsuyama
National College of Technology.
* Okayama University of Science.
(1) Pierce,C.; Smith, R. N.; Wiley, J. W.;Cordes,H. J . Am. Chem. Sac. 1951, 73, 4551. (2) Smith, R. N.; Pierce, C.; Joel, C. D. J. Phys. Chem. 1954,58, 298. (3) McDermot, H. L.; Arnell, J. C. J. Phys. Chem. 1954,58, 492. (4) Kiselev, A. V.; Kovaleva, N. V. Izu. Akad. Nauk SSSR, Otd. Khim. Nauk 1959, 955. (5) Morimoto, T.; Miura, K. Langmuir 1985, 1, 658. (6) Morimoto, T.; Miura, K. Langmuir 1986,2, 43. (7) Miura, K.; Morimoto, T. Langmuir 1986, 2, 824.
0743-7463/91/2407-0374$02.50/0
investigated the nature of adsorption sites for HzO on the graphite surface by measuring the surface content of gases. The purpose of the work described here is to examine the effect of Hz treatment of the graphite sample on the amount and character of the adsorption sites for H20 by means of the measurement of the HzO adsorption isotherm, the calculation of the isosteric heat of adsorption therefrom, and the analysis of the surface gas content.
Experimental Section Material. The original graphite, Graphite ACP from Sri Lanka, used for Ha treatment was supplied by Nippon Kokuen Co. The present material is different from the sample used in the previous work;5-8 the purity of the sample is the same as before, 99.5%, but the specific surface area is about twice as large as before, 15.26 m2/g. It was subjected to extraction by benzene for 6 days, dried in air, and then used for H2 treatment. HzTreatment. The graphite extracted by benzene (G25) was first pretreated in a stream of dry NZin a graphite crucible at 2800 "C for 1h. Immediately after the pretreatment, the crucible was held under 1 atm of H2 at 1000 "C for 1 h and then cooled to room temperature. The graphite thus treated in HZwas stored in a desiccator. Measurement of Adsorption Isotherm of HzO and Heat Treatment of the Sample. The H2-treated graphite was outTorr (1Torr = 133.3 Pa) at 25 "C gassed under a vacuum of (HYG25). The first adsorption isotherm of Hz0 was measured at 25 "C on this sample until a relative pressure of 0.6 was reached, followed by exposure to saturated HzO vapor for 48 hat the same temperature to ensure complete surface hydration. The sample thus hydrated was outgassedunder the same conditions as before and subjected to measurementof the second adsorption isotherms of HzOat 25,18,10, and 2 "C, respectively. For comparison, the original graphite G25 was also used for the same measurement. Next, the sample HYG25 was heated at 100 "C intervals from room temperature to 500 "C (HYGSOO). During this process,the sample was maintained for 5 h at each temperature, and the evolved gas was trapped at liquid nitrogen temperature for analysis. Based on the data obtained preliminarily, the trapped gas proved to contain a considerable amount of CzHe in addition to the gases HzO, C02, CO, CHI, and H2. These are the same species as those evolved from the sample used in the previous The amount of each component in the evolved gas was then determined in the following way: first, the amount of each (8) Miura, K.; Morimoto, T. Langmuir 1988, 4, 1283.
0 1991 American Chemical Society
Adsorption Sites for Water on Graphite
2lJm
Langmuir, Vol. 7, No. 2, 1991 375
P
Figure 1. Scanning electron micrographs of graphite: (a) HYG25; (b) G25.
component in the gas uncondensed at liquid nitrogen temperature, i.e., Hz, CO, and CHI, is determined by the same analytical procedure as stated in the previous paper.5 The components of the trapped gas, H20, C02, and C2H6,solidified at liquid nitrogen temperature?JOare measured in the lump after reevaporation at 25 "C. After H20 was removed from the mixture by a dry iceethanol trap, the amount of the remaining uncondensed componentsgJ0is determined volumetrically. One of the components in the remaining gas mixture, C02, is absorbed by KOH. After complete absorption of C02, the amount of the remaining gas,C2H6, is measured volumetrically. As a final step, the amount of H2O is measured volumetrically after reevaporation at 25 "C. Identification of each gas species was carried out by means of both gas chromatography and mass spectroscopy. The graphite sample thus treated at 500 "C, HYG500, was subjected to measurement of the first and second adsorption isotherms of H20 in the sameway as before. Afer accomplishment of the adsorption measurement, the sample HYG500 was heated at 100 "C intervals from room temperature to 1000 "C, and the gas evolved at each temperature was analyzed as well. The first and second adsorption isotherms of H20 on the 1000 "C treated graphite (HYG1000)were measured in the samemanner as before, and after that the sample HYGlOOO was again subjected to gas analysis with rising temperature from room temperature to 1000 "C successively. The adsorption measurement was carried out volumetrically by using a conventional adsorption apparatus, equipped with a Baratron capacitance manometer. The adsorption equilibrium was attained within 120 min in the case of the first adsorption for HYGFjOOand HYGlOOO and within 20 min in the other cases. Moreover, the N2 specific surface area was measured by means of the BET method just after pyrolysis or evacuation of the hydrated sample,where the cross-sectionalarea of the N2molecule at 77 K was assumed to be 0.162 nm2.11 Scanning Electron Microscope(SEM) Observation. The SEM observation was carried out by using the electron microscope, JEOL JSM-35.
Results Figure 1gives the electron micrographsof the Hz-treated graphite, HYG25, and of the original graphite, G25. The prism surface in HYG25 is found to be smoother submicroscopically than that in G25. The specific surface area measured at the same time shows a drastic decrease from 15.26 (G25) to 8.49 m2/g (HYG25). (9) Maass, 0.;Wright, C. H. J. Am. Chem. SOC.1921,43, 1098. (10) Copson, R. L.; Frolich, Per. K. Ind. Eng. Chem. 1929,21, 1116. (11)Brunauer, S.; Emmett, P. H. J. Am. Chem. SOC.1937,59, 1553.
The adsorption isotherms of H20 on the HYG samples are illustrated in Figure 2, where the isotherms on G25 are given for comparison. Here, the amount of adsorbed H20 is expressed as the number of molecules per nanometer squared on the basis of the N2 area, and the scale for the adsorbed amount on the HYG samples is 100 times as large as that on G25. The adsorption isotherms of H2O on HYG25 a t 25 "C are inserted into Figure 2a with the same scale as that on G25. First of all, it is clear from Figure 2a that the amount of H20 adsorbed on the graphite decreases drastically by H2 treatment. The first and second adsorption isotherms of H20 on HYG25 coincide with each other and the same is true for G25, while in the cases of HYG500 and HYGlOOO the first adsorption isotherms surpass the second ones as shown in parts c and d of Figure 2. Furthermore, a prominent step appears in the first adsorption isotherm on HYG500 and three steps on HYG1000. It should be noted that the shape of the second adsorption isotherm of HYGlOOO is peculiar in that the amount of adsorbed H20 begins to increase steeply a t the relative pressure of 0.04. The second adsorption isotherms reveal good reproducibility, which indicates that they are really physisorption isotherms. Figure 3 gives the histograms of the gas evolved by the decomposition of the surface compounds on the HYG samples, where the histograms on G25 are also given for comparison. In this figure, the amount of gas is expressed as the number of molecules per nanometer squared on the basis of the N2 area. The scale for the amount of all the gases evolved from the HYG samples and that for both CH4 and C2H6 from G25 is 30 times as large as that for the other gases from G25. As stated above, the heating of the samples G25 and HYG25 produces C2H6 besides the five gases H20, C02, CO, CH4, and H2. The C2H6 gas is considered to come from C-CH3 terminals as will be shown later. First, it is found from Figure 3 that Ha treatment reduces drastically the amount of the five gases but increases that of C2H6 more than 2 times. Moreover, the C2H6 evolution peak from G25 lies around 100 "C, while that from HYG25 lies a t a higher temperature, 500 "C. This indicates that the C-CH3 bonds formed are strengthened by H2 treatment. Another characteristic feature observed in Figure 3 and not observed in the previous ~ t u d i e s ~is- that ~ the gas evolution is classified roughly into two temperature ranges: one group of gases is evolved up to 500 "C, i.e., H20, derived
Miura and Morimoto
376 Langmuir, Vol. 7, No. 2, 1991
2
4
6
8
1012
Figure 2. Adsorption isotherms of H20 on graphite: (a) G25, (b) HYG25, (c) HYG500, (d) HYG1000; (a)first adsorption isotherm second one. The adsorption isotherm of H20 on HYG25 at 25 "C is inserted in part a for comparison. and (0) 0
co,
H2
co
,02 cH4
n
C2H6
I nil I
WGlwO
+sins
temperature /T
Figure 3. Amount of gas evolved from graphite when heated up to 1000 "C at 100 "C intervals.
from C-COOH and C-OH C02, derived from C-COOH group and l a ~ t o n e , and ~ - ~C2H6; another group contains gases evolved above 500 "C, i.e., H2, derived from C-H terminals as will be shown later, and CO, derived from C=O and isolated C-OH groups.12J3 The CH4 gas, derived from C-CH3 and C-H terminals, is released between 200 and 700 "C, as in the case of previous There are gases whose desorption peaks are shifted after H2 treatment: the desorption peak of C02 lies around 600 O C on G25, while on HYG25 a t 300 "C, and the CO gas is generated between 200 and 1000 "C from G25, while from HYG25 over 500 "C. The temperatures at which the desorption peaks of H20, H2, and CHI appear are not changed by H2 treatment. In addition, it should be noted that each amount of the evolved gases other than H2O becomes smaller in the order HYG25 > HYG500 > HYG1000. The BET monolayer volume, V,, of H2O calculated from the second adsorption isotherm of H2O and the gas content of the HYG samples are listed in Table I. The gas content in Table I is calculated by the summation of the histogram (12) Coltharp, M. T.; Hackerman, N. J. Phys. Chem. 1968, 72, 1171. (13)Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978,16,
35.
values in Figure 3. In this calculation it was assumed that there is no compound that decomposes to generate gases on the surface just after lo00 "Cpyrolysis. The gas content of HYG25 is the sum of the amount of gas evolved by heating HYG25 up to 500 "C and that by heating HYG500 from 500 to 1000 "C, the latter being the gas content of HYG500, both of which are in parentheses in Table I. These values are considered to be the amount of gases that the samples maintained rigi in ally."^ The gas evolved was also measured by heating HYG500 from room temperature to 500 "C after measurement of the H20 adsorption isotherm; this implies that the amount of surface compounds was reproduced by the chemisorption of H2O during the process of HzO adsorption.8 The same measurement on HY GlOOO from room temperature to 1000 "C also gives the surface compounds reproduced in this way. The amount of gas evolution from the reproduced compounds is noted outside the parentheses. Thus, in the case of HYG25, the adsorption of H20 was measured on the surface with the surface content in parentheses, while in the cases of HYG500 and HYG1000, the surface with the sum of the values in and outside the parentheses are used for measurement of the second adsorption isotherm of H20. The isosteric heat of adsorption qst of H2O on the HYG samples and also on G25 is plotted against the amount of adsorbed H2O as shown in Figure 4. The value of qat is calculated by applying the Clausius-Clapeyron equation to the second adsorption isotherms measured at different temperatures in Figure 2. The horizontal broken line indicates the heat of liquefaction HL of H20 at 25 "C, 43.99 kJ/mol. At the initial stage of adsorption, the qst value of G25 lies at a much higher level than HL,falls gradually with increasing amount of adsorbed HzO, and finally settles down to the HLlevel. The qstcurve of HYG25 is located considerably below the HLlevel over the whole range of coverage, increases with increasing amount of adsorbed H20, and finally approaches the HLlevel. This curve suggests the existence of a minimum of the qst value in a lower coverage region, though the qst values could not be calculated. The qst curves of HYG500 and HYGlOOO are also located below the HL level and have a distinct minimum; they behave in the same way as in the case of
Langmuir, Vol. 7, No. 2, 1991 377
Adsorption Sites for Water on Graphite
Table I. Monolayer Volume of Water, V,, and Surface Contents of Gases on Hydrogen-Treated Graphite surface content! HYG25 HYG500
8.49 8.8gb
0.0065 0.0050
HY G 1000
8.w
0.0050
G 25 a
15.26
0.950
(0.0511) 0.0041 (0.0009) 0.0104 (0.0000) (0.7762)
(0.0090) 0.0008 (0.0010) 0.0008 (0.0000) (1.3609)
(0.0587) 0.0000 (0.0587) 0.0132 (0.0oO0) (0.6702)
(0.0325) 0.0000 (0.0325) 0.0084 (0.0000) (1.2422)
(0.0326) 0.0039 (0.0185) 0.0040
(0.0223) 0.0032 (0.0021) 0.0028
(O.oo00)
(0.0000)
(0.0607)
(0.0097)
Expressed in molecules/nm2 on the basis of the Nz area. * Measured after evacuation at 25 "C of the hydrated sample.
jl ~
~
~
" a005 " " " "0.010 " ' ~ " 0.015" ' Amount of adsorbed wter /-dleculecnm-'
0.020
Figure 4. Isosteric heat of adsorption qst of HzO on graphite: ( 0 ) HYG25; ( 0 ) HYG5OO; (0)HYG1000; (0)G25. Arrows indicate V,,, (solid line), HzO content (broken line), and COz content (dotted line).
I
A I 25
25e
500
500e 1000 lOOOe 1000
Treatment
Figure 5. Variation of specific surface area of graphite. The numerical figures on the abscissa are the temperatures of pyrolysis. Numbers alone mean "just after pyrolysis at the temperature", and the attached "e" means "just after evacuation at 25 "C of the sample exposed to H20 vapor". HYG25 when the coverage of H20 increases. The qstcurve of HYG500 is situated above that of HYG25, and the curve of HYGlOOO above that of HYG500. Figure 5 shows the variation in the specific surface area which is measured when a series of treatments are carried out after H2 treatment. It is found from Figure 5 that the surface area increases after pyrolysis a t 500 "C and further increases even after evacuation a t 25 "C of the sample exposed to HzO. When the 1000 "C treated sample is exposed to H20, the surface area measured just after evacuation a t 25 "C once decreases, but it again increases after the second pyrolysis a t 1000 "C. Discussion E f f e c t of Hz T r e a t m e n t of G r a p h i t e o n HzO Adsorption. As can be seen in Figure 2a, H2 treatment
reduces the amount of adsorbed H2O drastically, and apparently changes the shape of the adsorption isotherm from type I1 to type I11 according to Brunauer's classifi~iation.1~ From the enlarged scale of the ordinate (Figure 2b), however, the isotherm on HYG25 is certainly perceived to be of type 11. This fact makes it possible to infer that H2 treatment drastically reduces the number of H2O adsorption sites on graphite, but not the energy of the adsorption sites. This is an extreme enhancement of the apparent surface hydrophobicity. As shown in Table I, the V , value of H20 on G25 is 0.950 molecules/nm2, while that on HYG25 only amounts to 0.0065 molecules/nm2. In other words, the number of the HzO adsorption sites on graphite falls off to 1/150 by H2 treatment. As has been often d e s ~ r i b e d the , ~ ~number of H2O adsorption sites originates from surface oxides on the substrate, especially from the H20- and CO2-desorbing oxides. In the case of the H2-treated sample, the most drastic decrease in the amount of surface oxides (Figure 3) is observed among the graphite samples treated in different ways in the previous work;" this is the reason for an extreme falling off of the HzO adsorption sites on the present graphite samples. Next, as seen in Figure 1, the prism surface of graphite is found to become smoother submicroscopically after Hz treatment, accompanied by a decrease in the specific surface area to about half of the original graphite. This fact suggests that surface recrystallization took place and, therefore, that the surface roughness of the prism surface of graphite decreased during Hz treatment. Hitherto, the reason for the drastic reduction both in the number of the HzO adsorption sites and in the amount of surface oxides on graphite has been described as if it depends only on the H2 treatment of graphite a t 1000 "C. But the fact is that the sample G25 was severely treated in N2 a t 2800 "C prior to Hz treatment. Therefore, the two treatments must change cooperativelythe properties of the present graphite samples. The single effect of the severe treatment in N2 has not been examined. Thus, the term, HZ treatment, stated in the above and the succeeding discussions must be considered to involve the effects of both treatments. Anomaly i n Isosteric Heat of Adsorption Curve of H20 on He-Treated Graphite. It is surprising to see in Figure 4 that the samples HYG500 and HYGlOOO reveal the qst curve with a very deep minimum lying a t a much lower level than HL, and the sample HYG25 suggests the existence of a minimum. Such a minimum appeared only in the cases of 1000 "C treated samples used in the previous work."' In a number of inve~tigations,"~ it has been found that the appearance of such a qstminimum arises from the presence of a very small amount of surface oxides, i.e., of the HzO-, COz-, and CO-desorbing oxides. Among the graphite samples used in the previous the sample HG1000, which was autoclaved in H2O a t 300 "C and then treated in vacuo at 1000 "C, had the smallest amount of (14)Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. SOC.1940,62, 1723.
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378 Langmuir, Vol. 7, No. 2, 1991
surface oxides and showed the most pronounced minimum of the qstcurve of H z O . ~The present authors have deduced that the HzO molecules are initially adsorbed on the sites, the neighboring CO2-desorbing and HzO-desorbing oxides, when each amount of H20-, COz-,and CO-desorbing oxides is extremely small, and that the subsequent adsorption of H20 proceeds on the preadsorbed HzO molecules to form cl~sters.5-~The oxide content of the present sample of HYG25 (Figure 3, Table I) is less than one-fourth of that of HGlOOO in total, and each amount of HzO-, COZ-,and CO-desorbing oxides of HYG25 is also less compared with those of HG1000.6 The appearance of the distinct qst minimum for HYG500 or HYGlOOO is, therefore, due to an extremely small amount of surface oxides. Thus, the adsorption of HzO onto the HYG samples will proceed on only a small number of oxide sites following the same adsorption model as discussed p r e v i ~ u s l y . ~Furthermore, -~ it is interesting to see that the qst minimum is very deep, and the bottom of the qst curve becomes slightly higher in the order HYG25 < HYG5OO < HYG1000, despite the fact that the surface content of HzO, COZ,and CO decreases in this order. There is another characteristic feature for the present sample: the evolution of HzO and COSalmost ceases by the treatment up to 500 "C, but that of CHI, CO, and HZbecomes predominant above 500 "C. A majority of the HzO-desorbing and COz-desorbing oxides will be reduced during H2: treatment, and at the same time C-H bonds will be formed on the prism surface instead of surface oxides, which results in the evolution of CHI, C2H6, and Ha. The present authors have assumed in the previous work7 that the surface of graphite treated in an 0 2 atmosphere is covered with C-0 bonds, and as a result the qstcurve of H20 gives high values initially but settles down to the HLlevel with increasing coverage of Hz0. The qst curve of the sample G25 in the present study is in the same situation. On the contrary, the fundamental surface of the Hz-treated samples will be covered extensively with C-H bonds just like the surface of paraffins; such a hydrogenated part of the surface will rather repel H2:O molecules more than a surface covered with C-0 bonds. Certainly, the surface of HYG25 will be the most extensively hydrogenated of the three HYG samples. When HYG25 is evacuated at elevated temperatures, i.e., a t 500 "C and succeedingly 1000 " C ,the H-containinggasessuch as CZH6, CH4, and Hz, are removed progressively, so that these surfaces will act less as a water-repelling surface. This results in elevation of the qst curve as a whole. As stated above, CZH6 can be removed at relatively low temperatures, Le., below 500 "C. This will come from a simultaneous breaking of the adjacent 0 H C-C-H H '
bonds.15 Irregularity of First Adsorption Isotherm of HzO. As shown in Figure 2, prominent steps appear in the first adsorption isotherms of HzO on HYG500 and HYGlOOO and they disappear in the second adsorption isotherms. The specific surface area of HYGlOOO increases after pyrolysis at 1000 "C and decreases after hydration (Figure 5). This is quite similar to the properties observed in the case of OG1000, which was partially oxidized and then evacuated at 1000 oC.8 In the previous paper,8 this phenomenon was considered to be due to both the pore opening by the removal of surface oxides on 1000 "C py(15) Cao, J.-R.; Back, M. H. Carbon 1982, 20, 505.
Figure 6. t plot of the first adsorption isotherm of H@: (0)
HYG500; ( 0 )HYG1000.
rolysis and the succeeding pore closure by the chemisorption of HzO. Also, in the present case, the fact mentioned above can be considered to originate in the same reason as before.8 In order to obtain information on the pore structure of HYG500 and HYG1000, the t plots of the lower pressure region of the first adsorption isotherm of HzO on both samples are drawn in Figure 6 in a similar way as before,8 because the t plot method is known to be effective in micropore analysis. The t values used here were cited from the data of Hagymassy et a1.16 The step observed a t the initial stage of the first adsorption of H20 on HYG500 corresponds to the line AB in Figure 6. A t point A, the t value is equal to 0.167 nm, this being equal to half of the distance between the adjacent basal planes in graphite, 0.335 nm.17 On the other hand, - the first step on HYGlOOO corresponds to the line CD in Figure 6. The t value at point C is also about half of the distance between the basal planes. These facts indicate that the first step is the process of micropore filling of the slit-shaped pores by H20, formed by the removal of one sheet of the basal plane. The volume of this kind of pore will be given as the difference between the points A and B for HYG500 or between the points C and D for HYG1000. The pore volume of HYG500 is then estimated to be about 3 times as large as that of HYG1000. As stated above, a large part of the surface oxides, such as the HzO-, COZ-,and CO-desorbing oxides, might be reduced by the Hz treatment at 1000 "C to produce C-H bonds instead. The bond strength of C-H bonds is known to be stronger than those of both C-0 bonds of C-OH groups and C-C bonds.'* Therefore, the surface oxides, which must remain on the sequestered edges of the basal planes even after Hz treatment, will be decomposed by the pyrolysis of HYG25 a t relatively lower temperature, i.e., a t 500 "C, to give COz and HzO, which will leave the pores. Because of stronger bonds, the C-H bonds located on the outer surfaces of graphite will be removed a t higher temperatures, i.e., over 500 "C, to evolve Hz and CH4. In conclusion, the pores are formed by the removal of HzOand COz-desorbing oxides at lower temperature, 500 " C , and then the removal of Hz from the C-H bonds remaining on the outer surfaces succeeds at higher temperature, 1000 (16) Hagymassy, J., Jr.; Brunauer, S.; Mikhail, R. Sn. J. Colloid Interface Sci. 1969, 29, 485. (17) International Committee for Characterization and Terminology of Carbon (Carbon 1982,20, 485). (18) Pauling, L. In The Nature of the Chemical Bond, 3rd ed.; Cornel1 University Press: Ithaca, NY, 1960.
Adsorption Sites for Water on Graphite
"C. This results in a decrease in height of the pore wall preformed. Thus, the pore volume of HYGlOOO will be smaller than that of HYG500. Furthermore, the second or third step in the first adsorption isotherm of HzO on HYGlOOO is probably due to the capillary condensation of HzO in the wider pores.
Langmuir, Vol. 7, No. 2, 1991 319
Acknowledgment. We thank Dr. Shigeji Hagiwara of the Institute of Industrial Science a t the University of Tokyo for his kind help in the Hz treatment of graphite. We also thank Professor Shigeharu Kittaka of Okayama University of Science for much help in the electron microscopic experiments.
