Langmuir 1997, 13, 3823-3826
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Specific Feature of Dielectric Behavior of Water Adsorbed on Ag2O Surface Yasushige Kuroda,* Taisuke Watanabe, and Yuzo Yoshikawa Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700, Japan
Ryotaro Kumashiro, Hideaki Hamano, and Mahiko Nagao Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700, Japan Received December 4, 1995. In Final Form: April 30, 1997X The dielectric behavior of Ag2O samples with different amounts of adsorbed H2O was investigated in the temperature range from 308 to 200 K and in the frequency region from 0.1 Hz to 5 MHz. A large relaxation is observed in these ranges, and it is explained by the interfacial polarization due to the heterogeneity of the system. The characteristic frequency (fm) for this relaxation is lowered with decreasing temperature until a complete monolayer is established. However, such a behavior of fm in the region beyond the surface coverage of ca. 1 reveals a new specific feature; the fm value gives a maximal frequency at around 278 K. The cause of this behavior is explored through the measurements of the differential adsorption heat, water content, and Clausius-Clapeyron plots (ln p vs 1/T). As a result, a new phenomenon observed in the Ag2O-H2O system can be explained by considering that the continuous phase change occurs at around 278 K in the adsorbed layer higher than the monolayer; the cluster-like structure formed on the OH groups at a temperature higher than 278 K is transferred to the two- or three-dimensional network structure formed by H-bonding between adsorbed H2O molecules in the lower temperature region.
It is well-known that H2O molecules adsorbed on the solid surfaces have properties that differ considerably from those of bulk water and play significant roles in the various chemical processes taking place on the solid surfaces.1-3 Thus far, their properties have attracted a great deal of attention, because the interaction of H2O with the solid surfaces has an important consequence in their catalytic behavior. On the other hand, carbon dioxide appears as a reactant or a product in a series of reactions catalyzed by metal oxides.4 A large number of studies have been made on the effect of the adsorbed H2O on the adsorption behavior of CO2 on usual metal oxides.5,6 However, little attention has been given to the state of H2O on the Ag2O surface, though the adsorption properties of CO2 on the Ag2O surface, which is basic, are affected by the presence of H2O; the rate for CO2 adsorption on Ag2O is accelerated by the presence of H2O, especially, up to the accomplishment of the first physisorbed layer.7 Dielectric measurement is a useful tool for characterizing the state of adsorbed species on the solid surface, although it needs elaborate measurements in the wide ranges of both temperature and frequency in order to assign the observed relaxation correctly. We have examined the dielectric properties of H2O on the solid surfaces in the wide temperature and frequency regions and have found two relaxations attributable to the interfacial and orientational polarizations at around room temperature and at lower temperatures, respectively. On * To whom all correspondence should be addressed. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, June 15, 1997. (1) Drost-Hansen, W. Ind. Eng. Chem. 1969, 61, 10. (2) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (3) Thiel, P. A. Acc. Chem. Res. 1991, 24, 31. (4) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons, Inc.: New York, 1994. (5) Tanabe, K. Solid Acids and Bases; Kodansha: Tokyo, 1970. (6) Parkyns, N. D. J. Chem. Soc. 1967, 410. (7) Morimoto, T.; Aoki, K. Langmuir 1986, 2, 525.
S0743-7463(95)01505-8 CCC: $14.00
the basis of these data, we have obtained useful information on the adsorbed states of H2O on the solid surface.8-11 The purpose of the present study is to clarify the dielectric behavior of H2O adsorbed on the Ag2O surface, especially the features of H2O molecules in the adsorbed layer higher than the monolayer. Experimental Section The Ag2O sample used in this study was prepared by mixing 0.5 mol dm-3 AgNO3 and 0.5 mol dm-3 NaOH solutions at room temperature. The precipitate formed was washed thoroughly with distilled water, and then was dried in a vacuum cell to avoid an easy adsorption of CO2 gas. Prior to the adsorption measurements, the sample was degassed at 623 K for 4 h under a pressure of 1 mPa, followed by exposure to the saturated H2O vapor at 298 K for 12 h in the adsorption apparatus to promote the surface hydration. The sample thus obtained was reevacuated at 298 K before the adsorption measurements. The surface area of this sample was found to be 0.96 m2 g-1 by applying the BET equation to the dinitrogen adsorption isotherm obtained at 77 K. The water content of the sample, which had been re-evacuated at 298 K after evacuation at 623 K and succeeding exposure to H2O vapor, was determined by the successive ignition-loss method. The dielectric measurements were carried out in the frequency range of 0.1 Hz to 5 MHz by using impedance bridges of TR-10C and TR-4 (Ando Electric Co.). The sample cell (dielectric cell) used was composed of two concentric stainless-steel cylinders. The electrodes were coated with a poly(tetrafluoroethylene) film of about 30 µm thickness. The rehydrated sample evacuated at 623 K was packed into the cell with a packing density of 25% in a dry glovebox and then was re-evacuated at 303 K under a reduced pressure of 1 mPa until no further changes in the dielectric permittivity occurred. Prior to every dielectric mea(8) Kuroda, Y.; Yoshikawa, Y.; Morimoto, T.; Nagao, M. Langmuir 1995, 11, 259. (9) Kuroda, Y.; Yoshikawa, Y.; Morimoto, T.; Nagao, M. Langmuir 1995, 11, 2173. (10) Kuroda, Y.; Yoshikawa, Y.; Nagao, M. Langmuir 1995, 11, 4031. (11) Kuroda, Y.; Hamano, H.; Yoshikawa, Y.; Nagao, M. Langmuir 1996, 12, 1399.
© 1997 American Chemical Society
3824 Langmuir, Vol. 13, No. 14, 1997
Kuroda et al.
Figure 1. Adsorption isotherm of H2O on Ag2O surface at 298 K. Table 1. Water Content of the Rehydrated Ag2O Sample 298 K 373 K 423 K 473 K 523 K 573 K 673 K water content / OH groups nm-2
1.92
1.80
1.22
0.48
0.32
0.18
0.0
surement, the amount of water adsorbed on the sample was controlled on the basis of the adsorption data at 298 K. After the adsorption equilibrium was obtained at a certain pressure of water vapor and at 298 K, the sample cell was promptly cooled to the next lower temperature. During the dielectric measurements, the adsorbed amount was little affected by a condensation of residual vapor in the adsorption system due to a lowering of the adsorption temperature, probably because of the sample used in much larger quantities and of the small dead space of the adsorption system including the sample cell. The Clausius-Clapeyron plots for the Ag2O-H2O system were undertaken at various temperatures between 303 and 215 K. The equilibrium vapor pressure was read with a capacitance manometer, MKS Baratron 310-BH. A good deal of sample, as well as a small dead space of the adsorption system, was used in order to avoid a change in the adsorbed amount due to the condensation of H2O vapor when the temperature of the system was lowered. The adsorption heat was measured by the use of an adiabatic adsorption calorimeter connected to the volumetric apparatus. The details of the calorimeter and the procedure for the measurement of heat of adsorption were described elsewhere.12
Results and Discussion Figure 1 shows the adsorption isotherm of H2O measured volumetrically at 298 K on the rehydrated Ag2O sample. This isotherm belongs to the typical type II in the BDDT classification, and the monolayer capacity, Vm, is estimated to be 0.196 cm3 (STP) m-2 by applying the BET equation. The C-value in this equation is 13, being small compared with that for H2O adsorption on usual metal oxides.13 Table 1 represents the water content of the sample. The values indicate the number of hydroxyls existing on the surface treated at respective temperatures. IR spectra (not shown here) obtained for this rehydrated sample showed no deformation band due to molecular water, which suggested that the adsorbed H2O exists in the dissociated hydroxyl form. The water content of Ag2O is suppressed to decrease in the lower temperature range below 373 K and then decreases steeply with increasing temperature. The number of hydroxyls on the rehydrated Ag2O sample is much smaller than that of usual oxides, as well as less than that estimated from the crystallographic consideration by a factor of about 3: 4.5, 6.4, and 5.2 OH groups nm-2 for the (100), (110), and (111) planes, respectively.14 These results are interpreted as (12) Kuroda, Y.; Matsuda, T.; Nagao, M. J. Chem. Soc., Faraday Trans. 1993, 89, 2041. (13) Zettlemoyer, A. C. Water; Franks, F., Ed.; Plenum: New York, 1975.
Figure 2. Dependence of dielectric permittivity, ′, and dielectric loss, ′′, on frequency for the Ag2O sample at θ ) 0.
that the mutual distance of adsorbed OH groups on the Ag2O surface is rather large, and hence the dehydration by condensation of these OH groups resists the heat treatment at the lower temperatures (≈400 K). By using both data of water content at 298 K and the physisorbed amount (5.27 H2O molecules nm-2) at the monolayer coverage, the ratio of the number of physisorbed water molecules to that of chemisorbed OH groups can be estimated to be 2.8. The dielectric permittivity, ′, and loss, ′′, for the Ag2OH2O system at 298 K and at the surface coverage (θ) of zero are plotted against the frequency in Figure 2. ′ is approximately 80 at 0.1 Hz, and with increasing frequency it decreases steeply and then gradually to give a value of 2 beyond 10 kHz. The ′′ curve reveals a maximum at 39 Hz where ′ undergoes an abrupt decrease in the variation with frequency. The frequency at which ′′ gives a maximum value corresponds to a characteristic frequency (fm) for the observed relaxation.15 From the variation of ′′ with frequency for the samples with different coverages of H2O, it was found that the fm value at 273 K shifted to the higher frequency side with increasing coverage. It was also found from the Cole-Cole plots for the observed relaxation at various coverages that the arc lengths for the relaxations at various coverages are approximately constant, regardless of the coverages, even in the coverage range 0 to 3. On the basis of both these observed data and the reference data of the ′ value for the bulk water16 and of the fm value for the orientational polarization of liquid water (17.7 GHz at 298 K)17 or ice (7.2 kHz at 273 K),18 the relaxation observed can be assigned to the interfacial polarization due to the difference in the conductivities of (14) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1963; Vol. 1. (15) Bo¨ttcher, C. J. F.; Bordewijk, P. Theory of Electric Polarization; Elsevier: Amsterdam, 1973. (16) Hasted, J. B. Water; Franks, F., Ed.; Plenum: New York, 1972. (17) Saxton, J. A. Proc. R. Soc. A 1952, 213, 473. (18) Auty, R. P.; Cole, R. H. J. Chem. Phys. 1952, 20, 1309. (19) Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 282. (20) Morimoto, T. ; Iwaki, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 943. (21) Kuwabara, R.; Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 1059. (22) Fripiat, J. J.; Jelli, A.; Poncelet, G.; Andre, J. J. Phys. Chem. 1965, 69, 2185. (23) Kno¨zinger, The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; Dynamics, Thermodynamics, and Special System; North-Holland: Amsterdam, 1976, Vol. 3.
Water Adsorbed on Ag2O
Langmuir, Vol. 13, No. 14, 1997 3825
a
b Figure 4. Characteristic frequency plotted in Arrhenius form for various coverages. Coverages are 0.6 (9), 1.0 (b), 2.0 (2), and 3.0 ([).
Figure 3. Dependence of dielectric loss, ′′, on frequency for various temperatures. Coverages are (a) 0.6 and (b) 2.0.
two layers which are composed of the interface of electrode-particle and particle-particle in the Ag2O-H2O system.8,19-23 The ′- and ′′-f curves can be correlated to each other, and therefore, only the dielectric losses (′′) obtained at various temperatures are plotted against the frequency in Figure 3 (for the coverages of 0.6 and 2.0). The fm value for the coverage of 0.6 is about 60 Hz at 298 K, and it becomes small as the temperature of measurement is lowered; 20 Hz at 278 K, 10 Hz at 248 K, and 0.6 Hz at 211 K. As a result of the interfacial polarization due to the electric conduction, there is such a clear tendency that fm shifts toward the lower frequencies with decreasing temperature; the proton conduction falls in its value with decreasing temperature, because of a requirement of activation energy. Such behavior is general one for the interfacial polarization, as found for the metal oxides19-21 and ionic crystal-H2O systems.8 However, it is worth noting here that there is a strange tendency in the relation between fm and temperature at the coverage of 2 (Figure 3b); the characteristic frequency observed at 278 K exhibits the highest frequency among those examined at the temperatures between 308 and 224 K. Such tendency appears only in the sample with more than monolayer coverage. It seems quite strange to think that the relaxation observed here is caused by the interfacial polarization, because this polarization is characterized by the ease of proton conduction in the system. The above behavior can be understood easily by plotting the characteristic frequency for the observed relaxation in the Arrhenius form, as shown in Figure 4. If a linear relationship is established, then the activation energy for the observed relaxation can be estimated from the slope of the straight line.8,24-26 In the present case, it is (24) Kamiyoshi, K.; Ripoche, J. J. Phys. Radium 1958, 19, 943. (25) Dransfeld, K.; Frisch, J. L.; Wood, E. A. J. Chem. Phys. 1962, 36, 1574.
Figure 5. Clausius-Clapeyron plots of equilibrium vapor pressures at θ ) 0.35 (+), 0.6 (9), 1.0 (b), and 2.0 (2).
important to note that the Arrhenius plots give a nonlinear relationship. In the lower coverage, e.g., θ ) 0.6, the fm value decreases with decreasing temperature. However, in the monolayer coverage (θ ) 1.0), the fm value slightly changes in the temperature range above 278 K, and it decreases steeply below that temperature. An approximately linear relationship is established in the fm-1/T curve in the temperature range below 268 K. Furthermore, it is clearly seen that the fm value for the coverage of 2 increases with increasing temperature in the lower temperature region, giving rise to a maximum at around 0.0036 (278 K), and then decreases with increasing temperature. Such variation of fm with temperature is more marked for the sample with higher coverage than monolayer. Figure 5 shows the Clausius-Clapeyron plots, ln p vs 1/T, for the coverages of 0.35, 0.6, 1.0, and 2.0. A straight line was obtained for the coverage of 0.35 in the whole (26) Sakamoto, T.; Nakamura, H.; Uedaira, H.; Wada, A. J. Phys. Chem. 1989, 93, 357.
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Figure 6. Heats of adsorption of H2O on Ag2O at 301 K.
range of temperatures. However, for the coverage of 0.6, a straight line seems to be obtained only in the temperature region lower than 273 K and a slight bending nature is seen in the higher temperature region. This finding suggests an occurrence of phase transition in the H2O layer adsorbed on the Ag2O surface at around 273 K. A more noteworthy feature appears on the adsorbed H2O at the coverage exceeding monolayer; the increasing tendency in ln p is depressed with increasing temperature beyond 278 K. In the temperature region lower than 278 K, a linear relationship establishes. On the whole, this curvature becomes large and pronounced as the coverage increases. The appearance of a bend in these straight lines is characteristic of the present system, and the temperature at which the bending point is found can be read off as 278 K. On the basis of these results, it may be summarized in the following manner. In the temperature region below 278 K, H2O molecules are in the solid or liquid state, i.e., the adsorbed state of H2O changes successively without exhibiting a distinct phase change in this temperature region. The appearance of a bend in these plots at the coverage exceeding monolayer can be interpreted as that the phase change of adsorbed H2O proceeds partly and continuously or that the adsorbed layer is composed from some phases. The temperature at which the break of straight line occurs is in fairly good agreement with that where the fm-1/T curve gives a maximum. These results are useful for our understanding of the dielectric behavior mentioned above. A calorimetric study of adsorption also provides useful information on the state of adsorbed H2O. The differential heat of adsorption of H2O on the surface of Ag2O is shown in Figure 6 as a function of surface coverage. At the initial stage the adsorption heat gives a value of about 80 kJ mol-1, and then it falls with increasing coverage, reflecting an energetic heterogeneity of the adsorption sites. The initial heat indicates a strong interaction between the OH groups existing on the active sites and the adsorbed H2O, and subsequent decrease in the heat corresponds to a decrease in the number of such active sites. It should be noted that there is a serious difference in the heat curves beyond the monolayer coverage between the present Ag2O system and the usual metal oxide systems; the heat of adsorption of H2O on Ag2O is smaller than the heat of liquefaction of water vapor (HL) near the monolayer coverage. In general, H2O molecules interact strongly with polar sites on the solid surface through the formation of hydrogen bonding, and therefore, the value of heat of
Kuroda et al.
adsorption is primarily a criterion of hydrophilicity or hydrophobicity of the surface; the adsorption heat higher than HL suggests the presence of hydrophilic sites and that lower than HL suggests the presence of hydrophobic sites.27,28 Therefore, it may be said that the surface of Ag2O has a slight hydrophobic nature and interacts weakly with coming H2O molecules in the region where HL is smaller than the heat of liquefaction of water. This interpretation is supported by the data described above, i.e., the small quantity of surface OH groups and the small C-value. On the basis of the consideration mentioned above, the following model can be constructed. A structural change of the adsorbed H2O layer appears to set in as soon as the surface coverage attains the monolayer at the temperatures above 278 K. Taking into account the value of water content, it is assumed that the OH groups exist on the Ag2O surface keeping their distance at about 7 Å. H2O molecules are adsorbed onto these OH groups, in the ratio of H2O molecules to OH groups being ca. 3 within monolayer coverage. It can be said that H2O molecules coming to the surface interact with each other, i.e., lateral interaction, as well as with OH groups in the temperature range lower than 278 K, resulting in the formation of a two- or three-dimensional network depending on the surface coverage. On the other hand, at temperatures higher than 278 K, the adsorbed H2O molecules may have a cluster-like structure on the surface hydroxyl groups as in a similar case on the hydrophobic surface. The proton mobility is responsible for the surface electrical conductance when H2O molecules are adsorbed on the solid surface. Therefore, it is expected that the electric current flows more easily on the surface forming a network structure of H2O molecules through the hydrogen bonding than on the surface having a cluster-like structure, owing to a lacking of the mutual interaction between the clusters. On the other hand, other factors, for example, the temperature, affect the mobility of the proton in such manner that the mobility of proton is larger in the higher temperature region than in the lower temperature region. As a result, the easier proton transfer at the lower temperatures is in competition with the harder proton transfer at lower temperatures to promote the proton transfer at around 278 K. It should be emphasized that the hydroxylated oxides having a small number of OH groups exhibit a specific dielectric behavior described here. In conclusion, the new phenomenon observed in the Ag2OH2O system can be explained by considering that the continuous phase change occurs at around 278 K in the adsorbed layer higher than the monolayer; the clusterlike structure formed on the OH groups at a temperature higher than 278 K is transformed to the two- or threedimensional network structure formed by H-bonding between adsorbed H2O molecules in the lower temperature region. Acknowledgment. Thanks are due to Mr. Shin-ichi Konno for his assistance in preparing Ag2O sample and to Mr. Daisuke Kataoka who carried out a part of the dielectric measurements. LA951505J (27) Bolis, V.; Fubini, B.; Marchese, L.; Martra, G.; Costa, D. J. Chem. Soc., Faraday Trans. 1991, 87, 497. (28) Fubini, B.; Bolis, V.; Cavenago, A.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1992, 88, 277.
