Langmuir 1988,4, 1283-1288 eicosane/H20 system is above its PIT. Conclusions Disordering of solid eicosane “soil” is a necessity for removal from a surface using an aqueous nonionic surfactant solution as a “detergent”. Neodol23-6.5 provides a greater disordering of solid eicosane, with the assistance of water, than does Neodol25-3. The penetration of water into the eicosane layer occurs after the adsorption of surfactant onto the eicosane surface. Neodol 25-3, although it adsorbs to a much greater extent onto the eicosane surface, is relatively inefficient at disordering the eicosane layer because of a lack of significant participation of water in the process.
1283
FT-IR has been shown to reveal structural and compositional changes in the region of interest in studies of detergency, i.e., the soil-surfactant solution interface. While the spectroscopic data cannot specify the exact detergency pathway due to the complexity of the interfacial phases involved, the key elements of surfactant and water penetration and the disordering of solid eicosane are clearly established. Acknowledgment. We thank Daniel Webster for his operation of the FT-IR and Janice Briones for her determination of the phase diagrams. The support and encouragement by the management of the Clorox Technical Center are also gratefully acknowledged. Registry No. Eicosane, 112-95-8.
Adsorption Sites for Water on Graphite. 4. Chemisorption of Water on Graphite at Room Temperature Kazuhisa Miurat and Tetsuo Morimoto*s$ Department of General Education, Tsuyama National College of Technology, Tsuyama, 708, Japan, Department of Chemistry, Faculty of Science, Okayama University, Okayama, 700, Japan Received December 30, 1987. I n Final Form: April 20, 1988 The chemisorption of H2O on graphite is investigated by repeating the measurement of the adsorption isotherm of H20 at 25 “C and the pyrolytic analysis of surface oxides formed therefrom. It is found that five kinds of gases, H20, C02,CO, H2, and CH,, are evolved by the pyrolysis of the sample after the adsorption of H20. The total amount of the gases evolved increases with increasing final H20vapor pressure at which the adsorption isotherm is measured. Three distinct steps appear in the adsorption isotherm of H20 on the sample treated at 1000 “C in vacuo, i.e., at relative pressures of 0.005, 0.025 and 0.444,respectively, while they disappear when the sample is preexposed to saturated H 2 0 vapor at 25 “C. The first and second steps are clcsely related to the chemisorptionof H20,and the third step is due to physisorption. The second and third steps are associated with pores in graphite. Especially, the second step is mainly due to the pore filling of H20 into the slit-shaped pores that are formed by removal of a sheet of the basal plane of graphite, and it takes a long time for the attainment of the adsorption equilibrium. The mechanism of the chemisorption of H20 on graphite, which takes place at the terminal edge carbon atoms around basal planes, is discussed. Introduction In order to identify the physisorption sites for H 2 0 on graphite, the pyrolysis of surface oxides present on the surface after the adsorption of H 2 0 has been Through these investigations an unexpected phenomenon has been discovered; on the sample treated at 1000 OC in vacuo, two distinct steps appear around relative pressures of 0.1 and 0.5 in the first adsorption isotherm of H20,while they disappear from the sample exposed to saturated H 2 0 vapor. Such steps have never been observed when the sample is pretreated at temperatures lower than 700 “C. The appearance of the steps was accompanied by a long equilibration time for adsorption, which suggests the occurrence of the chemisorption of H 2 0 on the surface of graphite and/or the pore filling of the adsorbate. Some papers report the chemisorption of H20 on carbonaceous materials. Pierce et al.4 contacted a Graphon sample with liquid H 2 0 in a sealed tube at temperatures between 25 and 150 “C and detected C02 and H2 in the gas phase. Smith et aL5 also reacted Spheron 6 and Graphon with H 2 0 in a sealed vessel at temperatures from 25 t Tsuyama
National College of Technology.
* Okayama University.
to 200 “C. As the result, H2, CO, and C 0 2were detected in the gas phase, and two kinds of surface oxygen complexes, which generate C 0 2 and CO on ignition, were postulated. For the investigation of the kinetics and mechanism of the reaction between carbon and steam, Yang and Dum6 examined the growth rate of etch pits and the change in their conformations with the aid of transmission electron microscopy around 700 OC; they concluded that H 2 0 molecules are dissociatively adsorbed on the (1010) surface of the graphite structure. These experiments are concerned with the chemisorption of H 2 0 on the surface oxygen complexes at relatively low temperature8 or on the bare surface of graphite at high temperatures.6 However, the chemisorption of H20 on graphite at room temperature remains to be studied. In the present investigation, we have attempted to clarify the nature of the step in the H20adsorption isotherm and the details of the H20chemisorption on graphite by measuring (1)Morimoto, T.; Miura, K. Langmuir 1985, 1 , 658. (2) Morimoto, T.; Miura, K. Langmuir 1986, 2, 43. (3) Miura, K.; Morimoto, T. Langmuir 1986, 2, 824. (4) Pierce, C.; Smith, R. N.; Wiley, J. W.; Cordes, H. J. Am. Chem. SOC.1951, 73, 4551. (5) Smith, R. N.; Pierce, C.; Joel, C. D. J.Phys. Chem. 1954,58,298. (6) Yang,R. T.; Duan, R. Z. Carbon 1985,23, 325.
0743-7463/88/2404-1283$01.50/0 0 1988 American Chemical Society
1284 Langmuir, Vol. 4, No. 6, 1988
Miura and Morimoto
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Figure 1. Adsorption isotherms of H20 on graphite at 25 "C, measured repeatedly after every evacuation at lo00 "C. Isotherm IV-2 measured after evacuation a t 25 "C. Time required for attainment of adsorption equilibrium is recorded.
the adsorption isotherm of H 2 0 at room temperature on the 1000 "C treated graphite as well as by analyzing the amount of gases evolved by pyrolysis of the sample that has experienced the H 2 0 adsorption. Experimental Section The original graphite (Graphite ACP) is a natural product from Sri Lanka and was supplied by Nippon Kokuen Co. According to the assay, the material is 99.5% in purity and contains 0.5% ash, the particle size being distributed between 1 and 30 wm. In previous works,'-3 it was found that the adsorption isotherm of H20 on graphite reveals steps when the sample is evacuated at 1000 "C and that the steps are distinguished on the sample prepared by 600 "C partial oxidation3 compared with the raw material' and the autoclave-treated graphite.2 Taking these findings into account, the partial oxidation treatment of the sample was carried out in the following way. The original graphite was oxidized at 600 "C in a rotary quartz tube by flowing the mixture of O2 and H20 in the ratio 3 0 1 through the system till 20% of the sample was burnt The sample thus prepared was degassed at 10oO O C for 5 h, and the adsorption isotherm was measured at 25 "C. After the measurement of the adsorption isotherm, the gas expelled by the ignition of the sample in vacuo at every 100 "C interval of rising temperature from room temperature to 1000 "C was analyzed. The gas liberated was first trapped at -196 "C and measured volumetrically after reevaporation at room temperature. Then, each component of the gas, H20, C02, CO, CHI, and H2,was determined separately by combining two techniques: the second trapping at -18 "C7 as well as the gas chromatographic analysis.' After the final ignition at 1000 "C, the next measurement of the HzO adsorption isotherm was carried out on the same sample. The adsorption isotherm of H 2 0 on graphite was measured volumetrically with conventionaladsorption apparatus, equipped (7) Nagao, M.; Morishige, K.; Takeshita, T.; Morimoto, T. Bull. Chem. SOC.Jpn. 1974, 47,2107.
Figure 2. Adsorption isotherms of H20 on graphite at 25 "C, measured repeatedly after every evacuation at 25 "C. Isotherm 111-1measured after evacuation a t 1000 "C. Time required for attainment of adsorption equilibrium is recorded. with an oil manometer. The equilibrium pressure was read after the pressure reduction ceased, and the final value continued for at least 60 min. The specific surface area of the sample was determined by applying the BET theor9 to the N2 adsorption data obtained at liquid N2temperature, where the molecular crow sectional area of N2 was assumed to be 0.162 nm2?
Results Effect of Repeated Evacuations at 1000 OC on H 2 0 Adsorption. The 1000 O C treatment in vacuo and the subsequent HzO adsorption measurement were repeated 4 times on the same sample. The adsorption isotherms are shown in Figures 1 and 2, where the enlarged illustrations are also inserted. In these figures, the amount of adsorbed HzO is expressed in the number of molecules per nanometer squared on the basis of the N2 area. The first adsorption isotherm I was measured at 25 OC just after the lo00 "C evacuation of the original sample, until the relative pressure, X,reached 0.35 (8.32 Torr).% Two steps appear at X = 0.005 and 0.025, respectively, which are distinct compared to those on the OG sample in previous work.3 The second step reveals a steep rise initially, and later the isotherm continues to rise irregularly until point B is attained around X = 0.072. For the establishment of adsorption equilibrium, it took 2-4 h near the first step and 8-10 h near the second step. After the measurement of the isotherm I, the sample was outgassed in a vacuum of Torr at 25 "C in order to remove physisorbed H20, followed by the analysis of gas evolved by the successive ignition in vacuo until the final heating at 1000 "C was completed. Next, the adsorption isotherm I1 of H 2 0 was measured at 25 "C until X = 0.61 (8) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. SOC.1938, 60, 309. (9) Brunauer, S.; Emmett, P. H. J. Am. Chem. SOC.1937, 59, 1553.
Langmuir, Vol. 4 , No. 6, 1988 1285
Adsorption Sites for Water on Graphite
Table I. Results of Gas Analysis and Adsorption Data" maximum H 2 0 pressure of preexposure, Torr I I1 111-3 IV-2
8.23 14.54 23.76 23.76
surface content, molecules/nm2 HZ0 COZ CO CH, Hz 0.014 0.051 0.128 0.142
0.005 0.010 0.029 0.021
0.103 0.109 0.158 0.164
0.024 0.084 0.113 0.056
0.107 0.107 0.143 0.224
VB,
total 0, atoms/nm2
total H, atoms/nm2
H/O
molecules/ nm2
0.127 0.180 0.344 0.348
0.338 0.652 0.994 0.956
2.66 3.62 2.89 2.75
0.350 0.300 0.215 0.380
"All the data are expressed on the basis of the N2 area given in Figure 4. VB is the amount of H20 adsorbed at the B point in the adsorption isotherm.
(14.54 Torr) was reached. After the same procedure of pyrolysis, the adsorption isotherm 111-1was measured until X = 0.13 (3.09 Torr) was attained. It can be found that the height of the steps in the isotherm decreases with the repetition of the 1000 "C treatment, i.e., in the order I > I1 > 111-1. The first step decays markedly, and finally it is exhausted in the isotherm 111-1. During this process, the time required for the attainment of adsorption equilibrium around the first step is reduced: it took about 40 min on every dose of H 2 0 in the isotherm 111-1. In the vicinity of the second step, it took 8-10 h equally for the equilibration of adsorption in every isotherm, though a considerable decrease in step height was observed. In addition, the third step appeared at X = 0.444 on the isotherm 11, where the equilibration time was 20 min. After the measurement of the isotherm 111-1,the sample was outgassed at 25 "C in a vaccum of loT5Torr, and then the adsorption isotherm 111-2 was measured until X = 0.13 was attained, followed by the measurement of the desorption isotherm. It is interesting to see that a clear hysteresis appears in this adsorption-desorption process. After the completion of the adsorption-desorption cycle 111-2,further measurement of the adsorption isotherm 111-3 was carried out up to a higher relative pressure, i.e., X = 0.70. The height of the second step in the isotherms 111-1, 111-2, and 111-3is almost similar in each case, but the times required for the adsorption equilibrium of the three isotherms are fairly different from one another: it took no more than 3 h for the isotherms 111-2 and 111-3, about one-third of the time for the isotherm 111-1. On the isotherm 111-3,the third step appeared near X = 0.5 and the adsorption equilibrium was established within 20 min. After the measurement of the isotherm 111-3,the sample was exposed to saturated H 2 0 vapor at 25 "C for 48 h to ensure the surface hydration, degassed at the same temperature as before, and subjected to the measurement of gas content through the successive ignition treatment. Then the adsorption isotherm IV-1 was measured on this sample at 25 "C until a pressure near saturation was attained. It can be found from Figure 1 that the height of the first and second steps in the isotherm IV-1 recovers the level in the isotherm I; the height of the first step at X = 0.005 is about twice as much as that of the isotherm I. The third step appears also in the isotherm IV-1 near the same pressure region as those of isotherms I1 and 111-3. After the measurement of isotherm IV-1, the sample was exposed to saturated H 2 0 vapor at 25 "C for 48 h, evacuated at 25 "C in a vacuum of Torr, and then used for the measurement of the isotherm IV-2. It is interesting to see that the shape of the isotherm IV-2 is quite different from those of the other isotherms shown in Figures 1and 2 and belongs to type I1 according to Brunauer's classification;1°the equilibration time is very short, i.e., less than 20 min. Furthermore, it is found that (10) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. SOC.1940,62, 1723.
repetition of the H 2 0 adsorption measurement at 25 "C and evacuation at 25 "C give the same and the reversible isotherm. Thus, the isotherm IV-2 is certainly the physisorption isotherm of H20. On the other hand, the serial transition from isotherm I to IV-1 was found to be reproducible in trend when the measurement was carried out on the other sampling from the same original sample, although each isotherm was not exactly reproducible. This suggests that these isotherms contain the amount chemisorbed besides the amount physisorbed. In conclusion, the repetition of the exposure of graphite to H20vapor of a lower pressure at 25 "C and the heating in vacuo from room temperature to 1000 "C lower the height of the steps in the H 2 0 adsorption isotherm at X = 0.005 and 0.025, where a very long equilibration time is required. However, the height recovers when the sample is exposed to saturated H 2 0 vapor and then heated in vacuo at 1000 "C. Amount of Gases Evolved by Successive Ignition of Graphite. Just after every measurement of H 2 0 adsorption isotherms I, 11, 111-3, and IV-2, the sample was pyrolyzed and the gas liberated was analyzed at every 100 "C interval of temperature from room temperature to lo00 "C. The data obtained are shown in Figure 3, where the amount of gas is expressed in the number of molecules per nanometer squared on the basis of the N2area. Five kinds of gases were found to be liberated, i.e., H20, C02, CO, CH4,and HP,being the same as in the previous The shapes of the histograms 111-3 and IV-2 in Figure 3 are quite similar to each other and to that obtained on the sample OG-1000 in our previous work.3 The same trend can be observed also on the histograms I and 11, though the amount of gas expelled is small. It can be understood from Figure 3 that the evolution of H 2 0 and C02 is accomplished at a temperature lower than 800 "C, while that of CHI and H2 seems to increase above 700 "C. The liberation of CO initiates at a lower temperature than that of CH4 and H2, and the liberated amount of CO increases with rising temperature similarly to that of CHI and H2. The total amount of each gas evolved after H 2 0 adsorption is integrated from Figure 3 and listed in Table I, together with the total amount of oxygen and hydrogen atoms that are contained in the evolved gas. I t is clear from Figure 3 and Table I that the gas evolution increases with the increasing final H 2 0vapor pressure at which the adsorption isotherm has been measured. The same tendency can be seen also on each of the total amounts of oxygen and hydrogen atoms, respectively. The increasing evolution of three kinds of gases, CO, CHI, and H2, from 800 to 1000 "C in Figure 3 strongly suggests that these gases will be evolved even at higher temperatures or that the sample contains the residual amount of surface species to be decomposed. Therefore, an additional test was carried out; that is, the 5-h pyrolysis of the same original graphite sample was repeated 8 times at 1000 "C without H 2 0 adsorption, and the results obtained are listed in Table 11. The data show that the
1286 Langmuir, Vol. 4, No. 6, 1988
Miura and Morimoto
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1.015 0.064 0.038 0.027 0.020 0.017 0.013 0.012
0.301 0.028 0.017 0.012 0.009 0.007 0.006 0.005
0.357 0.098 0.059 0.040 0.029 0.025 0.022 0.020
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evolution of HzO and C 0 2 is completed through only one pyrolytic treatment, while three kinds of gases, CO, CH4, and H2, continue to be generated decreasingly for a long time. Since every gas in the latter group was found to decrease exponentially, though the amount of the first evolution exceeds the exponential rule, the total amount of gas evolved can be calculated by integrating an appropriate equation, and the calculated data are added to Table 11. Variation of Specific Surface Area. The change in the N2 area is given in Figure 4. The measurement of surface area was made before and after each treatment of both the pyrolysis at 1000 "C and the HzO adsorption at 25 "C. It is found from Figure 4 that the specific surface area always increases immediately after the 1000 "C treatment, strikingly decreases after exposure to saturated HzO vapor, and gradually increases through repetitive exposure to H 2 0 vapor at pressure below X = 0.61. Discussion Chemisorption of H 2 0 on Graphite. As shown in Table I, the amount of every kind of gas expelled by the pyrolysis of graphite increases with increasing final pressure of H 2 0 to which the sample is exposed in the course
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of adsorption measurement. This is the primary evidence for the chemisorption of H20 on graphite, even a t 25 "C. It was reported that carboxyl, phenol, and carbonyl groups would be formed when HzO was chemisorbed on the sample OG-1000 at 25 "C which had been treated at 1000 "C3 These functional groups will be formed on the present sample through chemisorption of H20 even when exposed to a low relative pressure. If we assume that all of the oxygen and hydrogen atoms in the evolved gas are determined by the amount of chemisorbed HzO, the ratio of the total amount of hydrogen to that of oxygen can be expected to be 2. However, the observed values of this ratio, H/O, on 11, 111-3, and IV-2 are considerably larger than 2, as shown in Table I. This may be explained as follows. Even after the first 1000 "C pyrolysis, the graphite sample has a residual amount of surface species, which desorb three kinds of gases, CO, CHI, and H,,on further pyrolysis.
Langmuir, Vol. 4, No. 6,1988 1287
Adsorption Sites for Water on Graphite Moreover, the H/O ratio in the remaining surface species is 4.5-5.0, as estimated from the data in Table 11. Thus, the pyrolysis of the sample having such remaining surface species will raise the ratio H/O measured after H20 adsorption. In determining the amount of chemisorbed H20, therefore, neither the total amount of hydrogen atoms nor the total amount of oxygen atoms has much reliability, the former being less reliable. The maximum amount of chemisorbed H20,thus estimated, is found to be more than 0.348 molecules/nm2on the surface after the measurement of the isotherm IV-1. Redmond and Walker,ll Leine et a1.,12 and Barton et al.13J4have estimated an area occupied by a carbon atom on the (1010) and (1150) faces to be 0.083 and 0.071 nm2, respectively, which leads to the surface densities of 12 and 14 carbon atoms/nm2 on the two kinds of prism surfaces. On these terminal edge carbon atoms, H20 will be chemisorbed to form various kinds of functional groups described above. A method to evaluate the prism surface area on graphite was proposed by Barton et al.13J4 They assumed that the number of C02 and CO molecules evolved by ignition of graphitic materials is equal to that of active carbon sites. Then, the prism area was obtained from the total number of evolved C02 and CO molecules multiplied by the area occupied by an active carbon atom, 0.083 nm2.13 By applying this method to the present sample, we can find the prism surface area to be 1.47 m2/g.21 It has been known2 that graphite can chemisorb H 2 0 only when the sample is pretreated in vacuo above 400 "C. This fact suggests that the chemisorption sites for H20 are carbon atoms with unpaired electrons which are left on the surface after the pyrolytic removal of surface oxides. Provided that the density of the H 2 0 chemisorption sites is equal to that of the terminal edge carbon atoms mentioned above, there should be 13 sites/nm2 of the prism surface on the average. After all, the prism surface area of 1.47 m2/g corresponds to 19% of the N2area, 7.84 m2/g (Figure 4), of the sample. Thus, it follows that the maximum amount of chemisorbed H20, 0.348 molecules/nm2 on the basis of the N2 area (Table I), is equal to 1.9 molecules/nm2 on the basis of the prism surface area. If a HzO molecule is chemisorbed dissociatively on two sites, 3.8 sites/nm2 will be occupied primarily by chemisorbed H20 among the total number of 13 sites/nm2 on the prism surface, being equal to 29% occupation. As described above, a small part of surface oxides remains undecomposed even after the first lo00 "C pyrolysis of the sample. Furthermore, it was reported that H2stems from hydrogen atoms bonded directly to edge carbon atcontained in surface oxides13J6and o m or~from ~ those ~ that CO is originated in the thermal decomposition of carbonyl gr0ups.l' On the other hand, there seems to be no report on the evolution of CHI. It may be possible, however, to consider that CHI is formed during the course of pyrolysis of C-H bonds on the terminal edge carbons at high temperature. Taking account of these considerations, we can illustrate schematically a structure of a terminal edge of the basal plane of graphite as shown in Figure 5a. When the dissociative adsorption of H20 takes (11)Redmond, J. P.;Walker, P. L., Jr. J.Phys. Chem. 1960,64,1093. (12)Leine, N.R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67,2030. (13)Barton, S. S.;Harrison, B. H. Carbon 1972,10, 245. (14)Barton, S.S.;Evans, M. J. B.; Harrison, B. H. J. Colloid Interface Sci. 1973,45,542. (15) Rivin, D. Rubber Chem. Technol. 1971,44,307. (16)Barton, S. S.;Boulton, G. L.; Harrison, B. H. Carbon 1972,10, 395. (17)Coltharp, M. T.; Hackerman, N. J.Phys. Chem. 1968,72,1171.
b
Figure 5. Model of terminal edge of basal plane of graphite (a) after pyrolysis at 1000 "C and (b) after adsorption of HzO.
place on such terminal carbon atoms, the COP-,H20-,and CO-desorbing oxides, i.e., the carboxyl, phenol, and carbonyl groups, will be produced as given in a model in Figure 5b. As represented in Table I, the adsorbed amount of H20 at the end of the second step of the adsorption isotherm, i.e., the value at the B point, VB,decreases in the order I > I1 > 111-3, while the chemisorbed amount of H 2 0 measured by pyrolysis increases in the order I < I1 < 111-3 and reaches a maximum just after exposure to saturated H20vapor at 25 "C. These facts strongly suggest that the chemisorption of H 2 0 occurs on graphite not only in the region of adsorption steps but also over the whole pressure range of the adsorption measurement. Role of Pores in Chemisorption of H20. As described above, the adsorption isotherm 111-2 reveals a distinct hysteresis around the second step. From the fact that this step appears at a small pressure, it is reasonable to infer that the step is caused by the filling of H20 molecules into small pores. In order to evaluate the pore diameter, the t plot of the isotherm 111-1was examined as shown in Figure 6. For this calculation, the C value was first determined by applying the BET equation to the low-pressure region of the adsorption isotherm 111-1. Second, a group o f t values most appropriate for the C value calculated was selected from the list given by Hagymassy et al.'* It is found from Figure 6 that the amount of adsorbed H20 increases suddenly at t = 0.184 nm in the adsorption isotherm 111-1, corresponding to one-half of the distance between the neighboring two layers in graphite, 0.34 nm.19 This fact convinces us that the second step of the isotherm at X = 0.025 in Figure 1 is caused by the pore filling of H 2 0 molecules into the slit-shaped pores formed by the removal of one sheet of basal plane of graphite. It is seen from Table I that the amount of chemisorbed HzO, 0.127 molecules/nm2, in the isotherm I is only 36% of the step height, i.e., of the amount of adsorbed H 2 0 at the B point, VB = 0.350 molecules/nm2. This fact indicates that when the slit-shaped pores are filled with H20 at the second step the majority of H20molecules are physisorbed. Moreover, the equilibration time for H20 adsorption at the second step becomes shorter when the adsorption and the succeeding evacuation are repeated at 25 "C, as shown in the isotherms 111-1to 111-3. Therefore, it follows that the chemisorbed species of H 2 0 on graphite accelerate the (18)Hagymassy, J., Jr.; Brunauer, S.; Mikhail, R. Sh. J. Colloid Znterface Sci. 1969,29,485. (19)International committee for characterization and terminology of carbon (Carbon 1982,20,485). (20)1 Torr = 133.3 Pa. (21)Each amount of evolved C02 and CO is given in Table 11.
1288 Langmuir, Val. 4, No. 6, 1988 2.5 1
Miura and Morimoto 1
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Figure 7. Model of pore f i i n g of H20 molecules into slit-shaped pores: shaded circles, chemisorbed H20;open circles, physisorbed HzO.
t /nm
Figure 6. t plot of H20 adsorption isotherm 111-1.
succeeding physisorption of H 2 0 into pores. In contrast to the fact that the second step lies in the pore filling of H20, the first step at a lower relative pressure, X = 0.005, can be considered to be due to the chemisorption of H 2 0 on the outer surfaces of graphite, because it is impossible to expect pores on graphite with diameters much less than 0.34 nm. This is also supported by the fact that the time required for adsorption equilibrium in the first step is much shorter than that in the second step. The third step appears near X = 0.4-0.5 on isotherms 11, 111-3, and IV-1, in which it takes about 20 min for equilibration. This step is certainly due to the physisorption of H20. The t value corresponding to X = 0.444 at the third step on the isotherm I1 was plotted in the same way18 as in the case of the second step (Figure 6), though the t plot is not given here, and it was found that a steep increase in the adsorbed amount of HzO is observed at t = 0.50 nm, corresponding to one-half of the thickness of three sheets of graphite layers, 1.01 nm.19 This fact convinces us that the third step is caused by capillary condensation of HzOin the slit-shaped pores with the distance of 1.01 nm. Chemisorption of H 2 0 and Pore Model. On the prism plane of graphite, it is possible to consider three kinds of active carbon atoms lying on the terminal edge of the basal plane as shown in Figure 5. The a-type carbon atoms form projecting edges, on which HzO molecules can be chemisorbed most readily even at low pressures. The carbon atoms of @-typemay be less active compared with those of a-type because of a lower degree of unsaturation, and therefore they will react with H20 at higher pressures. The reactivity of the third type of carbon atoms, Le., ytype, will be smallest, because the Cy-€, distance is smaller than the H 2 0 molecule in addition to having a lower degree of unsaturation. If a partially hydrated surface having the hydrated a-type carbon atom is pyrolyzed at lo00 "C, this type of carbon atom will be removed to leave the @-type of carbon atom, which is less active. As shown in Figure 1, the first step in the adsorption isotherm decays by the repetition of pyrolysis after low-pressure exposure, e.g., below X = 0.61. This will be explained by the process described above. The active sites for H20chemisorption should be located also at the sequestered edges in the pores of graphite. Models for the slit-shaped pores formed on the prism surface are postulated in Figure 7 , where the solid line represents the continuum of carbon atoms lying on a basal
plane. As described above, two kinds of slit-shaped pores can be considered on the present sample: one is 0.34 nm in width for the second step and the other 1.01 nm in width for the third step (Figures 1 and 2), both of which are shown schematically in parts a and d of Figure 7 , respectively. When HzO molecules are initially chemisorbed on terminal edge carbon atoms of the outermost layers, A and C, hydrophilic surface oxides are formed which promote the physisorption of HzO (Figure 7a). H20 chemisorption of this kind can produce surface species with larger sizes, i.e., the HzO- and COz-desorbing oxides, as well as those with smaller sizes, i.e., the CO-, CH4-, and H2-desorbing groups. Next, H 2 0 molecules will be chemisorbed successively on the terminal edge carbon atoms at the bottom of the slit-shaped pores, the B layer (Figure 7b). Then, HzO is physisorbed around the surface species thus formed (Figure 7c). Thus, the adsorption of HzO at the second step at X = 0.025 will give rise to this type of pore filling. Figure 7d shows the filling of H 2 0 molecules into pores with a larger width of 1.01 nm, which corresponds to the capillary condensation of HzO at X = 0.444. In conclusion, it has been found that the chemisorption of H20 occurs on various kinds of sites of graphite, which makes the phenomenon complicated. The chemisorption of H20 at the bottom in the narrow and deep pores will be slowest and will therefore require long equilibration times. Once the chemisorption of H20 occurs on sites in deep pores, the successive physisorption of H20 will proceed to form clusters until the pore filling is accomplished. Additionally, chemisorption of this kind will form surface species with smaller size such as the CO-, CHI-, and H,-desorbing groups. The 1000 "C pyrolysis of the graphite, which has been exposed to saturated HzO vapor, will release most of the chemisorbed H20 together with surface carbon atoms, which results in the reproduction of the surface with a morphology similar to that before HzO chemisorption, as understood by the step height recovery. In other words, the opening and the closing of every kind of slit-shaped pores are repeated alternately by the pyrolysis at 1000 "C and the succeeding exposure to saturated HzO vapor. This will give rise to the H20 adsorption isotherm with the same shape as before, as shown in the isotherms I and IV-1 in Figure 1;this will also reveal the repetition of an increase and a decrease in surface area as shown in Figure 4. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research, No. 5747007, from the Ministry of Education, Science, and Culture of the Japanese Government. Registry No. HzO, 7732-18-5; graphite, 7782-42-5.
