Langmuir 1994,10, 807-811
807
Adsorption Sites for Water on Graphite. 6. Effect of Ozone Treatment of Sample Kazuhisa Miura and Tetsuo Morimoto' Department of General Education, Tsuyama National College of Technology, 624-1, Numa, Tsuyama, 708, Japan Received July 12,1993. In Final Form: September 27, 2999 The effect of the 0s treatment of the hydrogenatedgraphite on the adsorbabilityof Ha0 was investigated by measuring the adsorption isotherm of H2O and the amount of gas evolved on heating the sample. After the O3treatment of the graphite, the amount of surface oxidesbecame much larger, the monolayer capacity, Vm, of H2O increased drastically, and the isosteric heat of adsorption, qa, of H2O became much higher than the heat of liquefaction of H20. Namely, the 0 3 treatment made the surface of the graphite very hydrophilic. The further heat treatments of the 03-treated graphite at 500 and lo00 OC made the amount of surface oxides and the V m value of H2O gradually smaller. At the 881118 time, the qa values were depressed far below the heat of liquefaction of H20, and especially the q,t curve on the lo00 OC treated sample revealed a deep minimum. Such a change in the adsorbability of H2O on graphite was interpreted in terms of the species and the amount of surface oxides.
Introduction It has been pointed out1-' that various surface oxides on carbonaceous materials play an important role as the adsorption sites for HzO. The present authors also have selected a natural graphite with a high crystallinity for the sample from among many carbonaceous materials and studied the adsorption of HzO onto the graphites with various amounts of surface oxides.Sl2 The purpose of their investigation has been to know what kind of oxide on carbon is the site for HzO when various oxides are present simultaneously. As a result, the following facts have been brought to light. (1)Surface compounds on the graphite change into six kinds of gases, i.e., HzO, COz, CO, Hz, CH4, and CzHs, oa pyrolysis in vacuo. The desorption of these gases promotes formation of the slit-shaped pore with such a width as a HZO-molecular size. (2) HzO molecules are chemisorbed onto the graphite even at room temperature just after the pyrolysis: they are also filled into such micropores and chemisorbed onto the unsaturated carbon atoms of the pore bottom. Some amounts of surface oxides are reproduced through the chemisorption of HzO, the amount of the reproduced oxides, however, is always smaller than before the pyrolysis. (3) The mechanism of the physisorption of H2O onto the graphite is as follows: HzO molecules are first physisorbed onto the oxide which can evolve HzO and COz by the pyrolysis. In this type of adsorption, the isosteric heat of adsorption, qst,of HzO is larger than the heat of liquefaction of HzO, HL. After this kind of site is completely occupied by HzO ~
~
~~
* To whom correspondencemay be addressed at the Department
of Chemistry, Faculty of Science, Okayama University of Science, 1-1, Ridaichou, Okayama, 700, Japan. 9 Abstract published in Advance ACSAbstracts, January 15,1994. (1) McDermot, H. L.; Amell, J. C. J. Phys. Chem. 1964,58, 492. (2) Healey, F. H.; Yu,Y. F.; Chessick, J. J. J. Phys. Chem. 1955,69, 399.
(3) Puri, B. R.; Murari, K.; Singh, D. D. J. Phys. Chem. 1961,65,37. (4) Walker, P. L., Jr.; Janov, J. J. Colloid Interface Sci. 1968,28,449. (5) Barton, S. S.; Evans, M. J. B.; Harrison,B. H. J. Colloid Interface Sci. 1973, 45,542. (6) Dubinin, M. M; Serpinsky, V. V. Carbon 1981,19,402. (7) Barton, S. S.; Evans, M. J. B.; Holland, J.; Koresh, J. E. Carbon 1984,22, 265. (8)Morimoto, T.; Miura, K. Langmuir 1985,1, 658. (9) Morimoto, T.; Miura, K. Langmuir 1986,2, 43. (10) Miura, K.; Morimoto, T. Langmuir 1986,2,824. (11) Miura, K.; Morimoto, T. Langmuir 1988,4, 1283. (12) Miura, K.; Morimoto, T. Langmuir 1991, 7,374.
0743-746319412410-0807$04.50/0
molecules, HzO molecules are adsorbed onto the oxide which can evolve only H2O by the pyrolysis. In this range of coverage, the qBtvalue lies below the HLlevel when the number of CO-desorbing oxides is comparatively small. The completion of occupation of this kind of site is followed by formation of the clusters of Ha0 molecules, during which the qst values increase gradually toward the HLlevel. Here, the graphite whose surface oxides had been almost reduced by the hydrogen treatment at lo00 OC12 was subjected to the 0 3 treatment a t room temperature and was used for the measurement of HzO adsorption and the analysis of the surface gas content. This investigation was undertaken to know the feature of the OStreatment and ita effect on the adsorbability of Hz0 onto the graphite.
Experimental Section Material. The natural graphite was supplied by Nippon Kokuen Co, which was produced in Sri Lanka. Prior to the OS treatment,thisgraphitewas hydrogenated at lo00 O C 1 2 (HYG25). On this OStreatment, the 02 gas containing about 3% OSwas passed through the HYG25 powder at room temperature until the OSconcentration at the outlet of the reaction vessel became constant. It took about 48 h until the OSconcentrationbecame constant. Measurement of Adsorption Isotherm of HzO and Heat Treatment of the Sample. The OS-treated graphite was first degassed under a vacuum of 1V Torr (1Torr = 133.3 Pa) at 25 O C (OZG25). The fist adsorption isotherm of Ha0 onto OZG25 was measured at 25 O C until the relative pressure was attained near 0.4. After that, the sample was exposed to the saturated vapor of HzO for 48 h at 25 "C to ensure complete surface hydration. The sample OZG25 thus hydratedwas degassed under the same condition as before and subjectedto the measurement of the second adsorption isotherm of H2O at 10,18, and 25 O C , respectively. The sample OZG25 was heated in vacuo at 100 O C intervalsfrom room temperatureto 500 "C. During this procew, the sample was maintained for 5 h at each temperature,and the evolved gas waa trapped at liquid nitrogen temperature for gas atqlysis. The gas analysiswas carried out in such a way as written in the previous work.CI2 The graphite sample thus treated at 500 O C , OZG500, was subjected to the measurement of the f i t and second adsorption isotherms of H2O in the same manner as before. After accomplishment of the adsorption measurement, the sample OZGSOO was heated in vacuo at 100 "C intervals from room temperature to lo00OC, and the gas evolved at each temperaturewas analyzed as well. The first and second adsorption isotherme of H2O on 0 1994 American Chemical Society
808 Langmuir, Vol. 10, No. 3, 1994
Miura and Morimoto
Figure 1. Scanning electron micrographs of graphite: (a) OZG25; (b) HYG25. the 1OOO "C treated graphite, OZGlOOO, were then measured in the same way as before. After that, the sample OZGlOOO was again subjected to the gas analysiswith rising temperature from room temperature to lo00 "C successively. The adsorption measurement was carried out volumetrically by using a conventional adsorptionapparatus,equippedwith Baratron capacitance manometers. The adsorption equilibrium was attained within 20 min in both cases of the first and second adsorption for each sample. Moreover, the specific surface area was measured with N2 by means of the BET method just after pyrolysis and evacuationof the hydrated sample, where the cross-sectional area of a N2 molecule at 77 K was assumed to be 0.162 nm2.13 ScanningElectronMicroscope (SEM)Observation. The SEM observationwas carriedout by using an electron micrmcope, Hitachi X-650.
Results SEM Image of 0 2 6 2 5 and Variations in Specific Surface Area Due to Pyrolysis and Hydration. The electron micrographs of the 03-treated graphite, OZG25, and of the starting material, HYG25,12are shown in Figure 1. No difference can be found submicroscopically. The specific surface area is about 4% larger, 8.81 m2/g, than that of HYG25,128.49 m2/g. This is probably due to the fact that graphite particles become finer owing to the oxidation with 0 3 . Besides, there is little difference in the mesopore distribution curves of both samples calculated by the Cranston-Inkley method,16 though the data are not illustrated here. Figure 2 shows the variations in the specific surface area of OZG samples due to the various treatments. The area always increases after the heat treatment and usually decreases just after the exposure to the saturated H20 vapor. Especially, this pattern of the variation in the surface area of graphite is quite similar to those reported in the previous work.11J2 The increment in the area is found to be the largest just after the 500 "C pyrolysis. Adsorption Isothermsof H20and the Isosteric Heat of Adsorption qlt of H2O CalculatedTherefrom. The adsorption isotherms of H20 on the OZG samples are illustrated in Figure 3, where the isotherms of HYG25, the starting material,12 are given for comparison. Here, the amount of adsorbed H20 is expressed as the number of molecules per nanometer squared, and the scales for the adsorbed amount on these samples are different. The (13)Brunauer, S.; Emmett, P.H.J. Am. Chem. SOC.1937,59,1553.
"'i
'=
I
I
,,,.',o----- -d
I
0 '
3
m
1
1
I
(HYG25) 25
I
I
25E
500
1
1
WOE 1000 Treatment
I
1
lOOOE 1OOO
Figure 2. Variations in specific surface area of graphite. The numerical figures on the abscissa are the temperatures for 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 saturated H20 vapor".
starting material, HYG25,shows avery low H2O adsorption capacity, as found in Figure 3. It is because its surface was hydrogenated prior to the H20 adsorption.12 This figure demonstrates how the H20 adsorption capacity of the graphite sample changes after the 03 treatment and the further pyrolysis. First of all, it is clear from a comparison between the adsorption isotherms on HYG25 and those on OZG25 that the 03-treatment widely enhances the amount of H2O adsorbed per unit area. The first adsorption isotherm of H20 on OZG25 surpasses the second one of the same sample, while the first and second adsorption isotherms of H20 on HYG25 coincide with each other.12 Next, it is found that the adsorbed amount of H20 decreases when the sample is heated at higher temperatures, which is the same trend as could be observed in the case of G,8 HG,9 OG,l0and HYG.12 The first adsorption isotherms on both OZG500 and OZGlOOO do not coincide with their second ones, the difference between the first and second adsorption isotherm being very small. All the adsorption isotherms are clearly of type II.14 The second adsorption isotherms reveal good reproducibility,which indicates that they are nothing other than physisorption isotherms. (14) Brunauer, S.;Deming, L.
Chem. SOC.1940,62,1723.
S.;Deming, W.S.;Teller, E. J. Am.
Langmuir, Vol. 10,No. 3,1994 809
Adsorption Sites for Water on Graphite
Figure 3. Adsorption isothermsof HzO on graphite (0)first adsorption isotherm and (0) second one. An arrow indicates monolayer capacity, Vm,of H20. Adsorption isotherms on HYG25 at 25 O C are especially shown together with those on OZG25 in order to afford a better understanding of the effect of 0s treatment. Figure 4. The qatvalue is nearly equal to the HLvalue at the monolayer coverage and coincides with the HLvalue in the region beyond 0.20 molecules/nm2. The shape of the qst curve on OZGlO00 is quite similar to those on the other 1000 "C treated graphites:&l0 also in the case of OZG1000, the C02 and H2O content are found to be closely related with the shape of the qst curve. The curve drops very steeply at the very initial stage of adsorption, and crosses the HLlevel. The amount of H2O adsorbed at the crossing point is approximately equal to the C02 content. After the crossing point, the qat curve reaches a prominent minimum lying far below the HLlevel. The adsorbed amount at the minumum is nearly equal to the H2O content. After passing through the minumum, the qatcurve gradually approaches the HLlevel. Differently from the case of OZG500, the qat value is still less than the HLvalue even at the monolayer coverage. In order to obtain the qatvalue with a high accuracy and a high precision, the present authors f i t drew the enlarged illustration of the second adsorption isotherms, next got the equilibrium pressure (P)indicated by each isotherm at the temperature (2') a t the same amount of H2O adsorbed ( n ) ,and last calculated the qstvalue by using the lnP). The degree method of least squares to the data (P, of fitness of the data for the Clausius-Clapeyron equation can be estimated by evaluating the correlation coefficient ( p ) of the data. In this case, we can understand that the nearer to -1.000 the value of p is, the smaller the error in the data is. Actually, the value of p on OZG25 varies from -0.997 to -1.000 with increasing amount of H2O adsorbed. The coefficient p on OZG500 reaches the worst value, -0.977, at n = 0.0224 molecule/nm2, and it varies between -0.990 and -1.000 above n = 0.0417 molecule/nm2. The data on OZGlOOO give -0.961 at n = 0.0054 molecule/nm2 as the worst value for p and lead to better values between -0.998 and -1.000 above n = 0.0144 molecule/nm2. Amount of Gases Evolved from Each Sample on Successive Ignition and Gas Content Calculated Therefrom. Figure 5 gives the histograms of six kinds of gases evolved through the thermal decomposition of the surface compounds on the OZG samples, where the histograms on HYG2512 are also shown for comparison. The 03 treatment widely enhances the evolved amount of gases other than C2He; i.e., the evolved amount of C02,
I
OO
0.1
0.2
Amount
03
Ob
0.5
of adsorbed water/mdecuies.nm-2
Figure 4. Ismteric heat of adsorption q,t of H2O on graphite: (0) OZG25; (0)OZG500, (0)OZG1000; (0)HYG25. Arrows indicate V,,, (solid line), H2O content (broken line), and C02 content (dotted line). The isosteric heat of adsorption, qat,of H20 on the OZG samples is given as Figure 4. The value of qatwas calculated by applying the Clausius-Clapeyron equation to the second adsorption isotherms measured at different temperatures. The qst curve over the initial stage of adsorption on HYG2512 is also inserted as an enlarged illustration into Figure 4. In this figure, the horizontal broken line indicates the heat of liquefaction, HL,of H2O at 25 "C, 43.99 kJ/ mol. The monolayer capacity, V,, of H2O calculated by the BET method as well as both C02 and H2O content, which will be defined later, is also indicated by an arrow on each curve. The qlt curve on OZG25 decreases monotonously with increasing coverage of HzO, settles down at the HLlevel near the adsorbed amount of 0.13 molecules/nm2, and completely coincides with the HLlevel at the monolayer coverage. This fact suggests that H20 molecules get adsorbed successivelyonto such sites as can have stronger interaction with H2O molecules. In the case of OZG500, the qstcurve maybe falls steeply, crossesthe HLleve1, and goes by way of a minimum, though the q.t value could not be calculated. After a minimum, the qat curve approaches the HL level gradually with increasing amount of adsorbed H20, as can be seen in
Miura and Morimoto
810 Langmuir, Vol. 10, No. 3, 1994
OO*bb'
co
co2
H,O
0.03
H2
CH,
i
C2Hs
HYGK
I
@A' 'Sbd A' k&
I1 m o * '' Degassing temperaturePC '
rL
''
Figure 5. Amount of gas evolved from graphite when heated up to 1000 O C at 100 O C intervals.
H20, CO, CH4, H2, and C2H6 is about 45,8,10,3.5,2, and 0.7 times as large as that from HYG25, respectively. The order of amount of the gas evolved from OZG25 is found to be H2O = C02 > CO > H2 = CHI > C2H6, while that from HYG25 was H2 > H2O > CH4 = CO > C2H6 > C02.I2 This fact demonstrates that the oxides have prevailed on the surface treated with 0 3 . The histograms of H20, C02, H2, and C2H6 for OZG25 are little different in their shapes from those for HYG25. The CO histogram for OZG25 is, however, different in ita shapes from that for HYG25. A comparatively large amount of CO comes from OZG25 even when the heat treatment temperature is not very high. The same phenomenon as observed in the previous worka12 is not remarkable here that a large amount of CO is expelled near 700 "C. As in the case of previous work,a12 H2 comes from OZG25 above 700 "C, and almost all the amount of C2H6 is exhausted by 500 "C. The evolution of CH4 will be taken up later. The gases expelled from OZG5OOstemfrom the following two kinds of surface compounds, i.e., the compounds left even after the pyrolysis at 500 "C, and those reproduced through chemisorption of H2O. In Figure 5, the gases evolved from room temperature to 500 "C, i.e., H20, C02, CO, CH4, and C2H6, are from the latter compounds, those evolved from 500 "C to 1000 "C, i.e., H20, C02, CO, CHI, C2H6, and H2, being from the former compounds. When H2O is adsorbed onto OZGlOOO just prepared from OZG500,the reproduction of the same kind of compounds occurs on the surface of OZG1000. All the gases shown in its histograms in Figure 5 are from these reproduced compounds. By the way, the present authors previously ignited HYG1000, Le., the hydrogenated graphite pyrolyzed at 1000 "C,12and found that HYGlOOO evolves the smaller amounts of gas than any other graphite samples.a12 Comparing the amounts of gas expelled from OZGlOOO with those from HYG1000,12 it can be found that the amounts of H2O and C2H6 are a little smaller, and those of C02, CO, CH4, and H2 a little larger. The CH4-desorbingcompound on OZG25 may probably be formed through the reaction between the graphite just after the O3 treatment and the moisture in air, because OZG25 has been stored in air. This speculation will be supported by the fact that the same kind of compound has been reproduced on both surfaces of OZG500 and OZGlOOO through the H20 chemisorption, as found in Figure 5. Now, it should be noted that the second adsorption isotherms of H20 were measured on the surface which had chemisorbed these gases previously.
In Table 1, the gas content of each OZG sample as well as the monolayer capacity, Vm, of H2O is listed together with the data on HYG25 for comparison. The gas content is the sum of the values shown in the histograms (Figure 5). The gas content in parentheses is what the sample had before the first adsorption of H2O. The gas content outside the parentheses is from only the compounds reproduced during the first adsorption of H2O. The surface content of both OZG500 and OZGlOOO after the first adsorption is given as the sum of values in and outside the parentheses. As stated above, the second adsorption of H2O on these samples occurs on the surface including both gas contents.
Discussion Enhancement of Surface Hydrophilicity Due to O3 Treatment. As can be seen in Figure 3, the 0 3 treatment enhances the amount of adsorbed H2O drastically and apparently changes the shape of the adsorption isotherm from type I11 (for HYG25) to type I1 (for OZG25) according to Brunauer's ~1assification.l~ As shown in Table 1,the Vm value of H20 on OZG25 is 0.460 moleculehm2, while that on HYG25 was 0.0065 molecule/nm2,which indicates that the adsorption sites for H2O have increased in number by about 71 times by virtue of the 0 3 treatment. As shown in Figure 4, the 03treatment raises the qstvalue very much. These facts are all the evidence for extreme enhancement of the surface hydrophilicity. By the way, the number of adsorption sites for H2O is known to be in proportion to that of the surface oxides.a10J2 As shown in Figure 5 and Table 1, the amount of the surface oxides on OZG25 is remarkably large as compared with that on HYG25, which is the reason for surface hydrophilicity to be prompted. Furthermore, the amounts of H2O adsorbed per unit area of OZG500 and OZGlO00are small;when the graphite is heated at higher temperatures, the Vm value of H2O decreases. The qst value of the two at the initial stage of adsorption becomes depressed far below the HL level (Figure 4). This is due to the fact that the amount of oxides is smaller when the heat treatment temperature is higher (Table 1). The surface is made more hydrophobic than before the heat treatment, which is quite similar to the trend observed in the previous work.a10J2 Effect of Collapse of Surface Oxides on Specific Surface Area and on First Adsorption of HzO. As is seen in Figure 2, the specificsurface area showsthe greatest increase before and after the pyrolysis at 500 "C. Figure 5 demonstrates that all the gases except H2 and CO are almost completely extinguished by 500 "C. Therefore, the cause of such a remarkable increase in the surface area is reasonably considered to be the desorption of HzO, C02, and CO, because the total amount of CH4 and C2& is very small by nature. The disintegration of the surface oxides probably made the prism surface rougher than before the pyrolysis.ll However,the first adsorption isotherm of H2O onto OZG500 is of typical type II,l6 different from the case of HYG500 which has a prominent step in the isotherm just after the same heat treatment.12 The treatment at 1000 "C also increases the specific surface area, as shown in Figure 2. This increase is probably ascribed to the desorption of CO from OZG500, because the desorption of H2 results from collapse of C-H bonds on the outer surface and then likely makes no contribution to an increase in the specific surface areaS12 By the way, we can consider that the CO-desorbing oxideslsJ6 exist even on the terminal edge carbon atoms (15) Coltharp,
M.T.;Hackerman, N. J. Phya. Chem. 1968, 72, 1171.
Adsorption Sites for Water on Graphite
Langmuir, Vol. 10, No. 3, 1994 811
Table 1. Monolayer Volume of Water, V,, and Surface Contents of Gas on Ozonized Graphite surface contenta
V,.
a
OZG25 OZG500
0.460 0.147
OZGlOOO
0.031
HYG25
0.0065
COZ (0.4019) 0.0150 (0.0761) 0.0045
(0.3312) 0.0127 (0.2205) 0.0108
Hz (0.1241) O.oo00 (0.1241) 0.0242
(O.oo00)
(O.oo00)
(O.oo00)
(O.oo00)
(0.0511)
(0.0090)
(0.0325)
(0.0587)
Expressed in molecule/nmz on the basis of the NBarea.
at the bottom of the slit, i.e., interlayer space, which was formed by the removal of one sheet of basal plane of graphite, because the size of the oxides is comparatively smal1.l' The desorption of these oxides, therefore, will make the slits still deeper, i.e., it will leave "slit-shaped pores" on the surface behind. In the previous work,8-12 a very large amount of CO was expelled at temperatures above 700 "C,slit-shaped pores were actually formed on the surface of graphite just after the pyrolysis at 1000 "C, and the prominent steps appeared in the first adsorption isotherm of HzO onto the 1000 "C treated graphite. Especially, in the case of OG1000, i.e., the graphite treated with wet 02and further pyrolyzed a t 1000"C)OJ1 the much greater steps appeared. This fact suggests that O2 molecules could intrude into such an interlayer space to react with C atoms at the bottom and form many COdesorbing oxides and that deep slit-shaped pores were formed through collapse of these oxides by the pyrolysis at 1000 "C. The present authors have thus considered that the steps are caused by the pores. Now, 0 3moleculeswill not be able to enter the interlayer space so easily as 0 2 molecules because of its greater size. 0 3 molecules can erode some projected basal planes, so that the oxidation must occur only on the outer surface. It is likely, then, that comparatively shorter is the walls of the slit-shaped pores formed when the Os-treated graphite is pyrolyzed. Actually, steps do not appear in the first adsorption isotherm of HzO on OZG1000. Mechanism of Physisorption of H2O. As stated above, both contents of H2O and COa on OZGlO00 are all ascribed to the reproduced surface oxides, and then the H2O molecules approaching the surface of OZGlOOO inevitably interact with these hydrophilic oxides. As shown in Figure 4, the correspondence between both contents on OZGlOOO and the shape of its qet curve is quite similar to that of the 1000 "C treated graphite of G,8 HG? or OGlO sample: the C02 content is very close to the amount of adsorbed H2O at which the qst curve crosses the HLlevel, and the H2O content approximates the adsorbed amount at which the q,t minimum appears. And moreover, the C02 content is smaller than the H2O content. The amount of HzO adsorbed in this range of coverage is so small that the influence of errors is not small. Nevertheless, the feature stated just above can be recognized clearly. The present authors concluded in the previous workglo that C02 and HzO come from the following oxides: (16) Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978,16, 35.
co
HzO (0.4220) 0.0415 (0.0240) 0.0053
COOH + OH
-
-
CHI (0.1146) 0.0014 (0.0519) 0.0128 (O.oo00) (0.0326)
C02 + H 2 0
CZH6 (0.0155) '0.0018 (0.0004) 0.0017
(O.oo00) (0.0223)
(1)
2 0 H H,O (2) According to these reactions, the results shown in Figure 4 can be elucidated in the following way. H20 molecules first are physisorbed onto the oxide site which expels H2O and C02 simultaneously on pyrolysis. This kind of site is probably a pair of carboxyl and phenol groups as shown in eq 1. The qst value is larger than the HLvalue during this process. After these kinds of oxides are covered, H2O molecules are next physisorbed onto the oxide site which expels only HzO on thermal decomposition. Such a site must be a pair of neighboring phenol groups as shown in eq 2. In this stage of adsorption, the q B t curve lies far below the HL1evel.l' When the neighboring phenol groups are completely occupied, HzO molecules will subsequently be physisorbed onto the preadsorbed HzO molecules to form clusters, which results in a gradual increase in the qst value toward the HLlevel. The qstvalue on OZG500 is found to be less than the HL value, though the contents of C02 and HzO on this sample are much larger than those on OZG1000. Such a surprising occurrence of negative net heat of adsorption is definitely due to the fact that the CO content on OZG500 is very small.4JO Incidentally, the CO content on OZG500 is smaller than that on OG1000, which gave the qlt values below the HLvalue almost all the coverage.1° The COdesorbing oxides are known to be C=O and isolated C-OH groups.16J6 Unless these kinds of groups exist near a pair of oxides stated above, the HzO molecule adsorbed on the pair can neither form more hydrogen bonds nor behave like a molecule in the liquid phase. This situation will give rise to the negative net heat of adsorption. Also in the case of OZG500,the physisorption of H 2 0 in the final stage will certainly proceed through clustering. The shape of the q a t curve on OZG25 is very similar to that of the qst curve on the 25 "C treated graphites in the previous work,a10 where many hydrophilic surface oxides were present.
Acknowledgment. We thank Dr. Shigeji Hagiwara of the Institute of Industrial Science a t the University of Tokyo for his kind help in the OStreatment of graphite. We also thank Professor Shigeharu Kittaka of Okayama University of Science for much help in the electron microscopic experiments. (17) Young, G.J.; Chessick, J. J.; Healey, F. H.; Zettlemoyer, A. C. J. Phys. Chem. 1954,58,313.
