Langmuir 1985,1,679-683
679
Adsorption of H 2 0 and Ar on Pulverized CaF, Y asushige Kuroda,? Tohru Takenaka,i Junzo Umemura,f Shigeharu Kittaka,$ Kunimitsu Morishige,! and Tetsuo Morimoto*+ Department of Chemistry, Okayama University, Okayama 700, Japan, Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan, and Department of Chemistry, Okayama College of Science, Ridaicho, Okayama 700, J a p a n Received February 22, 1985. I n Final Form: J u n e 20, 1985 The chemisorption of H 2 0 on pulverized CaF2and its surface homogeneity have been investigated by measuring the electron diffraction, the adsorption isotherms of H 2 0 and Ar, the surface H20 content, and the IR spectra. As the result, it was found that pulverized CaF, has a well-developed (110) plane, and the surface can be hydrated on exposing the pulverized crystal to the atmosphere. Moreover, the homogeneous area for the HzO adsorption, which is estimated from the extent of the sigmoidal jump in the adsorption isotherm, decreases with rising temperature of treatment of the sample, while that for the Ar adsorption estimated from the vertical riser in the adsorption isotherm increases. Rehydration of the surface gives rise to an increase in the homogeneous area for the H 2 0 adsorption, while it causes a decrease in that for the Ar adsorption. The (110) plane exposed by the cleavage of CaF', exhibits an excellent homogeneity for the Ar adsorption either before or after rehydration, but it is heterogeneous for the HzO adsorption.
Introduction
We often encounter the fact that a unique surface phenomenon takes place on a special crystal plane. In our laboratory, it has been discovered that the two-dimensional (2D) condensation of HzO occurs on the surface of limited kinds of metal oxides, Zn0,1-3 Sn02,4-6and Cr,03.71) This phenomenon was considered to occur on a special crstal plane, e.g., on the well-developed and hydroxylated (1010) surface of Zn0.3 Very recently, we have found that the adsorption isotherm of HzO on CaF, reveals a step due to the occurrence of the 2D condensation of HZO,lOJ1 as in the case of metal oxides cited above, where the phenomenon occurs on the (111) plane of the crystal. Moreover, this plane, when hydroxylated, acts as a heterogeneous surface for the Ar adsorption, and the 2D condensation of Ar does not occur on this plane." T h e purpose of the present work is t o investigate the surface homogeneity of another crystal plane (110) of CaF,, which is prepared by pulverizing the single crystal, for the HzO adsorption as well as for the Ar adsorption. Experimental Section Materials. The sample used in the present investigation was prepared by pulverizing the single crystal of CaF,, supplied from Oyo Koken Co., in an agate mortar, and then the powders passing through the no. 150 mesh were collected (CaF,-A). CaF2-A,20 g, was immersed into 1000 cm3of HzO at 25 "C and stirred for 2 h to permit a partial dissolution and at the same time to ensure the surface hydration. The immersing-and-stirring cycle was repeated 20 times by replacing 1000 cm3 of HzO every time (CaF2-B). HzO used as the adsorbate was purified by deionization and redistillation, and the gas dissolved in H20 was removed by the "freeze-evaporate-thaw" cycle. The Ar gas used was 99.99% in purity, being supplied from Teikoku Sanso Co. Measurement of Adsorption Isotherm of Water. First, CaF2-Awas degassed at torr (1torr = 133.3 Pa) for 4 h at various temperatures, 25,150,500, and 600 "C, and CaF2-Bonly at 25 "C, and then the adsorption isotherm of HzO was measured at 10 "C on these samples. After measurement of the first adsorption isotherm, the sample was exposed to saturated H20vapbr at 25 "C for 10 h to accomplish the surface hydration, evacuated at 25 "C and torr for 4 h, and the second adsorption isotherm of H20 was measured on this sample at 0, 10, and 20 "C, repeatedly after every evacuation at 25 "C. Before and after the first H 2 0 Okayama University.
* Kyoto University. * Okayama College of Science.
adsorption measurement, the adsorption isotherm of Ar was measured at -196 "C. The adsorption measurement of HzO and Ar was carried out volumetrically by using a conventional adsorption apparatus, equipped with a diaphragm manometer, manufactured by MKS Baratron Co. The specific surface area of the samples was obtained by applying the BET method to the N2 adsorption data. Measurement of Surface Water Content. The surface H20 content was measured by the successive ignition loss method.', Prior to measurement of the surface H20 content, the pretreatment of the sample was carried out in three steps as in the case of the H,O adsorption measurement, i.e., heat treatment at different temperatures, exposure to saturated HzO vapor, and evacuation at room temperature. Since a small amount of C02 was detected in addition to H,O in the gas evolved by ignition, the gas was analyzed by the two-step trapping m e t h ~ d . ~ Measurement of Infrared Spectra. For the measurement of IR spectra of adsorbed HzO on CaF,, 200 mg of CaFz-Awas compressed under the pressure of 200 kg cm-, to form a selfsupporting disk of 20-mm i.d. The disk was pretreated at increasingly elevated temperatures for 2 h at low5torr in an in-situ cell. The IR measurement was carried out at 18 "C, by using a spectrophotometer, Nicolet 6000 FT-IR. The detectors made from InSb and MCT were used and the integration was carried out lo00 times.
Results and Discussion Surface Hydration of Pulverized CaF,. Figure 1 shows a n example of electron micrographs observed on CaF2-A. The shape of particles seems to be very irregular, but the electron diffraction pattern is very distinct. As the result of electron diffraction analysis of more than 100 particles, the pattern demonstrating the well-developed (110) plane of CaF, was most often detected. Though the (1) Morimoto, T.; Nagao, M.; Tokuda,F. Bull. Chem. SOC.Jpn. 1968, 41, 1533. (2) Nagao, M. J . Phys. Chem. 1971, 75, 3822. (3) Mprimoto, T.;Nagao, M. J. Phys. Chem. 1974, 78, 1116. (4) Kittaka, S.;Kanemoto, S.; Morimoto, T. J . Chem. SOC., Faraday Trans. 1, 1978, 74,676. (5) Morimoto, T.;Yokota, Y.; Kittaka, S. J. Phys. Chem. 1978, 82, 1996. (6) Kittaka, S.; Morishige, K.; Fujimoto, T; Morimoto, T. J. Colloid
Interface Sci. 1979, 72, 191. (7) Kittaka, S.; Nishiyama, J.; Morishige, K.; Morimoto, T. Colloids Surf. 1981, 3, 51. (8) Morishige, K.; Kittaka, S.; Morimoto, T. Surf. Sci. 1981,109, 291. (9) Kittaka, S.; Morishige, K.; Nishiyama, J.;Morimoto, T. J. Colloid Interface Sci. 1983, 91, 117. (10) Morimoto, T.; Kadota, T.; Kuroda, Y. J . Colloid Interface Sci. 1985, 106, 104. (11) Kuroda, Y.J . Chem. Soc., Faraday Trans. 1 , 1985,81, 757. (12) Morimoto, T.; Naono, H. Bull. Chem. SOC.J p n . 1973,46, 2000.
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Figure 1. Electron micrograph and electron diffraction pattern of pulverized CaF,. I
3 k $ - z G - ? '
6
18 Wavenumber/
I7
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IO'cm'
Figure 3. IR spectra of pulverized CaF,&r degaPsing at various temperatures.
Temperat ure/"C Figure 2. H,O content of CaFTA, pretreated at 25 (0).150 (G), 500 (a), and 600 OC (4) and of CaF,-B (0). Dotted line indicates H,O content of precipitated CaF,. cleavage plane of the CaF, crystal has been reported to be the (111)plane,13 the present sample reveals another kind of exposed plane, (i10).14J5 The surface H 2 0 content measured on CaF,-A and CaF,-B is illustrated in Figure 2, where the data on the precipitated sample of CaF, are also cited." The H 2 0 content implies the amount of H20 chemisorbed on the surface treated at the indicated temperatures, being expressed in the number of H,O molecules per nanometer squared. Figure 2 manifests that there exists chemisorbed H20 on the surface of pulverized CaF,, the amount of which decreases when the pretreatment temperature is raised. Moreover, the H20 content is slightly enhanced by immersing the sample in liquid H 2 0 for a long time (CaF,B), but nevertheless it is appreciably small compared with the value, 12 H,O nm-, at 25 'C, on the precipitated sample." If we assume that a H 2 0 molecule is removed by the condensation dehydration from two hydroxyls, the amount of hydroxyls on the 25 "C treated CaF,-A can be computed to be 8.54 hydroxyls nm',, which can be compared with the number of the F ions, 9.48 or 7.76 ions nm-,, on the (110)or (111)planes, respectively, of CaF,.I6
These considerations suggest that the chemisorption of H,O takes place on the newly developed surface of CaF, after the destruction of the crystal in air and that the surface is fully covered with chemisorbed H20. Moreover, it is clear from Figure 2 that a high-temperature treatment of the sample CaF,-A in a vacuum makes the surface rehydration difficult even after the exposure of the sample to saturated H 2 0 vapor; e.g., the H 2 0 content is reduced to less than one-third of original when treated a t 600 "C. Figure 3 shows the IR spectra of H,O adsorbed on the pulverized sample of CaF,. Three absorption bands due to adsorbed H,O can be observed in the spectra: a sharp band at 3675 cm-l and a broad band with the absorption center at 3400 cmP, which can be assigned to the OHstretching vibrations of the isolated and the hydrogenbonded hydroxyls, respectively, and a broad band a t 1650 cm-' due to the H,O-bending vibrations."J7 When the sample is evacuated at 150 "C, the absorption band coming from free hydroxyls disappears. Further evacuation at 200 'C results in the extinction of the band due to the H,Obending mode. Finally, the absorption band originated in the hydrogen-bonded hydroxyls vanishes through the evacuation a t 500 "C. The change in the IR peaks by the heat treatment is similar to that of the precipitated CaF,, though the intensity of the peaks on the present sample is smaller than that on the latter." The only small difference between the two samples is that the extinction temperature for the peak due to the H,O-bending mode is fairly lower on the pulverized CaF, than 350 "C, the extinction temperature on the precipitated CaF,. Water Adsorption on Pulverized CaF,. The adsorption isotherm of H,O on CaF,A and CaF2-B is given in Figure 4. Here, the adsorption isotherm on the precipitated CaF, is also cited (Figure 4a)." It is found from Figure 4 that the second adsorption isotherm of H 2 0 on the pulverized CaF, is quite different from that on the precipitated CaF,; i.e., a t first sight the former appears to have no step in contrast to the fact that there is a distinct step on the latter. However, the isotherm on CaF,-A is not of the type I1 according to the Brunauer's classification18 but of a sigmoidal shape, showing an inflection point near 0.5 torr a t 10 "C. When the pretreatment tempera-
(13) Dana, S.;Ford,W. E."A Textbook of Mineralogy"; Wiley: New
(16) Wells. A. F. "Structural Inorganic Chemism". 4th 4. Clarendon ; Press: Oxford. 1975. (17) Barraclough, P. B.; Hall, P. G. J . Chem. SOL.Foradmy Tmns. I.
(14) Bannon. J.; Cumow, C. E. Noture (London) 1948,161, 136. (15) Bannon. J.; Coogan, C. E. Nature (London) 1949,163.62.
1915, 71,2266. (18) Brunauer. S.;Demimg. L. S.;Deming. W. E.; Teller. E. J . Am. Chem. So?. 1940.62, 1723.
York, 1960.
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Adsorption of H 2 0 and Ar on Pulverized CaF2
Pressure/
"0
1
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2
3
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Figure 4. Adsorption isotherms of H 2 0 on CaF2-A,pretreated at 25 (a), 150 (b), 500 (c),and 600 "C (d) and on CaF2-B(e). ( 0 ) First adsorption at 10 "C; (0) second adsorption. Dotted line indicates the isotherm at 20 "C on precipitated CaF2 (a).
ture of the sample is raised, the sigmoidal appearance becomes faint and almost disappears by the 600 "C treatment. Also in the first adsorption isotherm, the sigmoidal feature appears, it decays when the pretreatment temperature is raised, and disappears by the 500 "C treatment, but it reappears in the second adsorption isotherm after the exposure to saturated HzO vapor. When the sample is treated a t 600 "C, the sigmoidal shape does not reappear even after the succeeding exposure to saturated HzOvapor.
Such behavior of the sigmoidal isotherm by heat treatment of the sample and succeeding exposure to saturated H,O vapor is quite similar to that of the step in the H 2 0 isotherm on the precipitated CaF, reported previously.'l Next, it should be pointed out that the heat treatment of the sample reduces the pressure at which the inflection point appears, and finally it reaches the pressure at which the step appears on the precipitated CaFP Also on CaF2-B, the H 2 0 adsorption isotherm reveals a sigmoidal shape (Figure 4e), where the pressure a t which the inflection point appears is close to the pressure a t which it appears on the 500 "C treated CaF,-A. In other words, the equilibration of the pulverized CaF, with liquid HzO has the same effect as that of the high-temperature treatment of the sample. Another significant result which can be perceived from Figure 4 is the difference between the first and second adsorption isotherms. When the sample is pretreated a t increasingly elevated temperatures, the first adsorption isotherm of HzO deviates from the second one; the former lies over the latter on the 150 "C treated CaF2-A (Figure 4b), while on the 500 or 600 "C treated CaF2-A the situation becomes complicated; Le., the former is below the latter in the low-pressure range and crosses the latter at an intermediate pressure; e.g., a t about 2 torr for the 500 "C treated CaFP The same phenomenon was observed also on the precpitated CaF, and explained in terms of a slow rehydration of the surface.llJg Since the chemisorbed H20 can be considered to form new physisorption sites for HzO, the chemisorption of H20 w ill reasonably raise the amount of physisorbed HzO. Thus, a slow rehydration will reduce the amount of adsorbed HzO in the low-pressure range in the first adsorption isotherm, also in the case of the pulverized CaF,. In the previous paper," it was reported that the 2D condensation of H 2 0 occurs on the hydroxylated (111) surface of CaF,, which results in the appearance of a distinct step in the H20 adsorption isotherm. However, the hydroxylated (110) surface of the pulverized CaF2-A does not give the same step as that on the (111)surface, as shown in Figure 4. Only a sigmoidal jump appears on CaF2-A. Chung and Dash20have reported that when the extent of the patches of the uniform surface is reduced, the step of the isotherm which indicates the occurrence of the 2D condensation of an adsorbate becomes slow. Furthermore, as stated above, the inflection point of the sigmoidal isotherm on CaF,-A moves to the pressure a t which the step in the isotherm on the (111)plane of CaF, appears, through severe heat treatment or equilibration with liquid HzO. Thus, it is reasonable to infer that a sigmoidal jump is originated in the ill-developed (111) plane on CaF2-A which has a well-developed (110) plane and that severe heat treatment or equilibration with liquid H 2 0 brings about an enhancement in the extent of the (111)plane on CaF2-A. By applying the Clausius-Clapeyron equation to the adsorption data in Figure 4, we can calculate the isosteric heat of adsorption of H20,qst,as shown in Figure 5. For comparison, the qst curve on the precipitated CaF, is cited in this figure." It can be seen from Figure 5 that the qst value on the 25 "C treated CaF,-A first decreases to a minimum, increases, passes a weak maximum near 0 = 0.6, and finally approaches the heat of liquefaction of H20HL a t 0 = 1. When the pretreatment temperature is raised, the weak maximum becomes weaker and at the same time (19) Kuroda, Y.; Sato, H.; Morimoto, T. J. Colloid Interface Sci., in press. (20) Chung, T. T.; Dash, J. G. Surf. Sci. 1977, 66, 559.
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682 Langmuir, Vol. I, No. 6, 1985
1:: 0.1
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Figure 5. Isosteric heat of adsorption of H20, qst, on CaF,-A, pretreated at 25 (o),150 (@), 500 (e),and 600 "C ( 0 )and on CaF,-B (0). Dotted line indicates qst on precipitated CaF,.
"0
IO
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30
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Figure 7. Adsorption isotherms of Ar on CaF,-A, pretreated at 25 (a),500 (b),600 "C (c), before ( 0 )and after (0) H20adsorption
measurement.
Tempera!ure /OC
Figure 6. H20evolved from CaF2-A,measured at every 25 "C interval of temperature. Dotted line indicates HzO evolved from
precipitated CaF,.
shifts to the lower coverage side and finally becomes close to a smooth decreasing curve, corresponding to the fact that the inflection point of the sigmoidal isotherm shifts to lower pressures. The qst curve on CaF2-B resembles to that on the 500 "C treated CaFz-A, and especially the coverage a t which the qst maximum appears is the same on both samples. These maxima shown in the qst curve of the pulverized CaF2 are very much smaller than that on the precipitated CaF,. This must be caused by a weaker lateral interaction of adsorbed H20 molecules, because of an inferior surface homogeneity of the pulverized CaF,. Figure 6 shows the histogram of the amount of desorbed H20 from the 25 "C treated CaF2-A,which was measured a t every 25 "C interval of rising temperature. This gives the original data for the calculation of the surface HzO content as shown in Figure 2. For comparison, the data on the precipitated CaF, are also cited in this figure." In Figure 6, a large difference can be observed between the histograms of the two samples. In the previous paper,*l it was reported that among three desorption peaks, the peaks A and B are associated with the chemisorbed water, on which the 2D condensation of H 2 0 occurs. It is clear
from Figure 6 that the desorbed HzO from the pulverized CaFz decreases monotonously, where the peaks B and C are lacking in the histogram. This indicates that the absence of chemisorbed H 2 0 corresponding to peak B brings about the unique HzO adsorption isotherm which differs from that on the precipitated CaF, (Figure 4). Surface Homogeneity of Pulverized CaF,. The adsorption isotherm of Ar, measured at 196 "C, on CaF2-A treated at various emperatures is given in Figure 7. A t first sight, a very similar isotherm can be observed on every sample; i.e., a distinct step appears a t 12 torr. The same phenomenon has been found on the precipitated CaFz and ascribed to the 2D condensation of Ar on the uniform surface of CaF2."rZ1 Therefore, the present data indicate that the pulverized CaF, haing a well-developed (110) plane is also homogeneous for the Ar adsorption, similar to that of the precipitated CaF, having a well-developed (111)plane, though the surface homogeneity of both samples for H20 adsorption is quite different as stated above. It is interesting to see in Figure 7 that the 600 "C treated CaF2-A,which has no sigmoidal jump in the HzO adsorption isotherm before and after exposing to saturated H20 vapor, gives rise to a clear step in the Ar adsorption isotherm in either case. Also, it can be seen that exposure of a high-temperature-treated sample to H20 vapor results in an enhancement in the adsorbed amount of Ar in the initial part of the isotherm. Ross and his co-workers22,23 tried to evaluate the extent of the homogeneity on CdBr, by analyzing the Ar adsorption isotherm having a step. For this purpose, the monolayer capacity for Ar is first obtained from the ad(21) Edelhoch, H.; Taylor, H. s. J. Phys. Chem. 1954, 58, 344. (22) Ross, S.; Olivier, J. P.; Hinchen, J. J. A&. Chem. Ser. 1961, 33, 317. (23) Ross, S.; Olivier, J. P. "On Physical Adsorption"; Interscience: New York, 1964.
Adsorption of HzOand Ar on Pulverized CaFz
VmC A r- I 1 9 Vm(Ar-ll) .......e a
n
e
Langmuir, Vol. I , No. 6, 1985 683
e ........... 0................................. '""v .... ......8....................................
m (H20-1)
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. 0....... 200 ":
, 100
Vu,i(H20
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-1)
\
h . . . l . . . . . . . . A
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500
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Figure 8. Change in V , and homogeneous area Vh on CaF2-A for H20 and Ar adsorption with pretreatment temperature.
sorption isotherm. Second, the initial part of the isotherm is extended by assuming the validity of the BET equation, the extended isotherm being considered to be the adsorption isotherm on the heterogeneous part of the surface. Then, by subtracting the adsorbed amount Vhetero on the extended isotherm a t the same pressure as that at which the V , value apears from the V , value, we can obtain the adsorbed amount Vuni on the homogeneous part of the surface. The vertical riser in the adsorption isotherm containing the 2D condensation depends on the temperature of measurement and becomes shorter when the 2D critical point is approached. However, the height of the step Vmi measured a t a constant temperature may be considered to be a relative measure of the homogeneous area for samples treated under various conditions. The V , and V~
values thus calculated for Ar adsorption are plotted against the pretreatment temperature of CaF,-A in Figure 8. Here, the values estimated by applying the same method to the sigmoidal HzO adsorption isotherms in Figure 4 are also added, though the surface homogeneity of the present samples for the HzO adsorption is extremely deteriorated. From Figure 8, we can deduce several characteristic features on the change in the surface homogeneity of the pulverized CaF, which appears when the sample is treated at increasingly elevated temperatures. V , for the Ar adsorption is unchanged by the heat treatment and regardless of whether the surface is hydrated or not, whereas that for the HzO adsorption changes slightly. Second, VWifor the HzOadsorption is diminished drastically by treatment over 200 "C, and it recovers when the sample is exposed to saturated HzO vapor, but the recovery ceases after heat treatment over 500 "C. On the other hand, the change in Vunifor the Ar adsorption is different from that for the HzO adsorption; it increases with rising temperature of pretreatment, while it is reduced when the sample is exposed to saturated HzO vapor. Third, Vu,, for Ar adsorption is very much larger than that for H20 adsorption, which suggests that the homogeneous surface for the Ar adsorption is different from that for the H 2 0 adsorption. These trends have been found on the precipitated CaF, as well," though Vmi for HzO adsorption is very large, i.e., Vu,;/ V , = 60%, compared with 3070,the value on the pulverized CaF,. In the previous paper, the adsorption studies on the precipitated CaF, having the well-developed (111)plane have led to the conclusion that the homogeneous surface of CaF, for the Ar adsorption acts as the heterogeneous one for the HzO adsorption. The same conclusion is found to be valid also on the pulverized CaF, having the welldeveloped (110) plane. Furthermore, it has been clarified from the present investigation that the well-developed (110) surface of CaF, is homogeneous for the Ar adsorption, but it is heterogeneous for the HzO adsorption.
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. CaF'2,7789-75-5; H20, 7732-18-5;Ar, 7440-37-1.
