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Specific Adsorption Behavior of Water on a Y2O3 Surface Yasushige Kuroda,*,† Hideaki Hamano,‡ Toshinori Mori,† Yuzo Yoshikawa,† and Mahiko Nagao‡ Department of Chemistry and Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan Received December 31, 1999. In Final Form: May 30, 2000 It was found that in the adsorption isotherms of water on Y2O3 at 298 K a break occurs at around a relative pressure of 0.007, giving rise to a decrease in the vapor pressure and a continuous increase in the adsorbed amount. The water content of the Y2O3 sample was found to be about 100 H2O molecules nm-2, being much larger than that calculated on the basis of crystallographic structure as well as the values obtained experimentally for the typical metal oxides. To clarify this, the adsorbed state of water on the Y2O3 sample was examined by means of near-infrared (NIR) spectroscopy, temperature-programmed desorption (TPD), X-ray absorption fine structure (XAFS), and dielectric measurements. For the hydrated Y2O3 sample three desorption peaks were observed at around 370, 420, and 650 K in the TPD spectra, suggesting the presence of three types of adsorbed water. These are supposed to give the bands at 6750, 7110, and 7250 cm-1 in the NIR spectra; the former two bands are due to the molecular water, i.e., strongly adsorbed water and hydrated bulk species, and the last one to the surface hydroxyl groups (overtone of stretching vibration). Dielectric relaxation of water physisorbed on the Y2O3 surface (coverage of 1.64) was observed at 24.5 kHz and at 158 K. It should be stressed that a new dielectric relaxation is found at around 40 Hz and at 273 K, corresponding to a dipolar relaxation due to the strongly physisorbed water molecule. From these experimental results, it has become apparent that water molecules are strongly adsorbed on Y2O3 and react with bulk oxide layers to form hydroxide layers. Such a process is summarized as follows: Y2O3 + physisorbed water f Y2O3‚H2O (strongly adsorbed on the surface layer) f Y2O3(H2O bulk species) or YOOH f Y(OH)3. This sequential hydration mechanism was also supported by the XAFS data. The reaction with water appears to be limited to such a extent that it proceeds by the heat resulting from a reaction of Y2O3 with water, and hence the intermediate state of the strongly adsorbed water could be recognized in this system. It is the first example detected successfully that the molecular water is strongly adsorbed on the surface layer of Y2O3. The hydration process of Y2O3 can be interpreted by the concept of electronegativity of yttrium ion, Y3+; the Y2O3 sample possesses a basic property, in harmony with the evidence obtained from the point-of-zero charge (PZC) value as well as the O1s XPS data of the sample.
Introduction In the process of a formation of surface by cleaving an oxide crystal, the metal-oxygen bonds have to be broken, and hence the surface ions have fewer numbers of nearest neighbors than the corresponding ions in the bulk. Surface coordinative unsaturation thus formed on the metal oxides contains dangling bonds or defects whose valency requirements are usually satisfied by a chemical reaction with ambient atmosphere. Therefore, water existing in the atmosphere easily reacts with such surface and may influence the surface chemical fields such as electrochemistry, corrosion, and heterogeneous catalysis.1 It is extremely important work to investigate a role of water in the surface chemistry of metal oxides. Rare-earth-metal compounds, especially their oxides, have become of increasing interest in recent years because of their possibility of use as high-energy permanent magnets, high Tc superconductors, solid electrolytes, and photoluminescence materials.2-5 Furthermore, these sub* To whom all correspondences should be addressed. E-mail:
[email protected]. † Department of Chemistry. ‡ Research Laboratory for Surface Science. (1) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: Amsterdam, 1989. (2) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John Wiley & Sons: New York, 1988. West, A. R. Solid State Chemistry and its Applications; John Wiley & Sons: New York, 1984. (3) Adachi, G.; Imanaka, N. Chem. Rev. 1998, 98, 1479, and references therein. (4) Adachi, G. Science of Rare Earths (Japanese); Kagaku-dojinn: Kyoto, 1999.
stances can be utilized for catalysts, such as cracking catalyst, perovskite-type catalyst for complete oxidation and NOx removal reactions, catalyst for the oxidative coupling of methane, and a promoter in the three-way catalyst for auto-exhaust decomposition.4,6,7 In these applications, there are some reports about the segregation of rare-earth substances to surface8 as well as about high reactivity toward oxygen and water.3,9 It is expected that the surface properties of rare-earth metals and their oxides are affected seriously by the presence of water. Since the surfaces of rare-earth-metal compounds are subject to eroding away, the methods for protecting them from oxidation or hydration are requested to be found out for practical use. However, little attention has been given to the adsorption properties of the rare-earth oxides for water.10 The purpose of the present work is to elucidate the adsorption behavior of water on Y2O3 in comparison with the metal-oxide systems studied so far. Experimental Section Materials. The Y2O3 sample was prepared by a decomposition of Y2(C2O4)3 that had been kindly supplied by Mitsui-kinzoku Co. This sample was subjected to electrodialysis and its surface (5) Bednorz, J. G.; Mu¨ller, K. A. Z. Phys. 1986, B64, 189. (6) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (7) Capitan, M. J.; Malet, P.; Centeno, M. A.; Munoz-Paez, A.; Carrizosa, I.; Odriozola, J. A. J. Phys. Chem. 1993, 97, 9233. (8) Langell, M. A.; Ren, Y. G.; Sellmyer, D. J. J. Magn. Magn. Mater. 1989, 82, 213. (9) Bernal, S.; Botana, F. J.; Garcia, R.; Rodriguez-Izquierdo, J. M. React. Solid. 1987, 4, 23. (10) Gammage, R. B.; Fuller, E. L., Jr.; Holmes, H. F. J. Phys. Chem. 1970, 74, 4276. J. Colloid. Interface, Sci. 1970, 34, 428.
10.1021/la9917031 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/18/2000
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area was found to be 6.4 m2 g-1 by applying the BET equation to the N2-adsorption data obtained at 77 K. Water, H2O and D2O, were purified by repeating the freeze-evaporate-thaw cycles to remove the dissolved gases. Water Adsorption Isotherm and Water Content. The adsorption isotherm of water was determined volumetrically. Prior to the measurement, the Y2O3 sample was evacuated at 298 K under a reduced pressure of 1 mPa to remove the physisorbed water and to leave the chemisorbed or strongly adsorbed water on the Y2O3 sample. The sample was also evacuated at 873 K for 4 h to remove completely the strongly adsorbed water and then the measurement of water adsorption was performed at 298 K (first adsorption). After this the sample was re-evacuated at 298 K, followed by the measurement of second adsorption isotherm at 298 K. The water content was determined by the successive-ignitionloss method for the original Y2O3 sample and the hydrated one. The latter sample was prepared by evacuating the original sample at 873 K, followed by contact with liquid water at 298 K for different periods: 1, 7, and 30 days. Diffuse Reflectance Spectra. The diffuse reflectance spectra in the near-infrared region were recorded at 300 K with a Hitachi 330 spectrophotometer equipped with an integrating sphere. The powder sample was placed in a vacuum reflectance cell made of fused silica. The spectra were evaluated by the SKM theory, which gives a ratio of the absorption to the scattering coefficients.11 TPD Measurements. The temperature-programmed desorption (TPD) method developed by Cvetanovic and Amenomiya12 was applied to the analysis of adsorbed species on the Y2O3 sample. Helium gas was used as a carrier in a rate of 60 ml min-1 and the heating rate was 5-10 K min-1. After the sample was evacuated at room temperature for 4 h, the thermal desorption run was started. Dielectric Properties. The dielectric measurements were carried out in a similar manner described in the previous paper13 in the region 298-130 K by using impedance bridges (TR-4 and TR-10C, Ando Electric Co.) which were able to measure in the frequency region from 0.1 Hz to 5 MHz. XAFS Measurements. The spectra of X-ray absorption spectroscopy (XAS), which includes both X-ray absorption nearedge structure (XANES) and X-ray absorption fine structure (EXAFS), were obtained under a ring operating condition of 2.5 GeV and using a beam line BL-10B in the Photon Factory in the Institute of Materials Structure Science (High Energy Accelerator Organization, KEK, Tsukuba). The photon energy was calibrated by a characteristic pre-edge peak in the spectrum for the copper foil as 8.9788 keV. XAS spectra of Y(OH)3 and the hydrated Y2O3 samples in various levels were recorded at the K-edge of yttrium. All the measurements were carried out in situ with a transmission mode using optimized ion chambers as detectors. The data analysis was performed by using Maeda’s program.14
Results and Discussion Specific Adsorption Behavior of Y2O3 for H2O. Figure 1a shows the adsorption isotherms of water for the Y2O3 samples evacuated at 298 and 873 K. These isotherms exhibit a “knee” after giving a steep rise in the adsorbed amount in the lower pressure region, being classified as type II in the IUPAC classification as found in many metal oxides.15 Moreover, the amount of water adsorbed on the original Y2O3 sample is appreciably larger compared with that on the typical oxides16-18 such as ZnO, R-Fe2O3, etc. (11) Klier, K. Catal. Rev. 1968, 1, 207. (12) Cvetanovic, R. J.; Amenomiya, Y. Adv. Catal. 1967, 17, 103. (13) Kuroda, Y.; Yoshikawa, Y.; Morimoto, T.; Nagao, M. Langmuir 1995, 11, 2173. (14) Maeda, H. J. Phys. Soc. Jpn. 1987, 56, 2777. (15) Zettlemoyer, A. C.; Micale, F. J.; Klier, K. Water, A Comprehensive Treatise; Franks F., Ed.; Plenum: New York, 1975; Vol. 5. (16) Nagao, M.; Morimoto, T. J. Phys. Chem. 1980, 84, 2054. (17) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243. (18) Kno¨zinger, H. The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North-Holland: Amsterdam, 1976; Chap. 27.
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Figure 1. Adsorption isotherms of water on Y2O3 and Y(OH)3 at 298 K. (a) The Y2O3 samples were evacuated at 298 K (O, b) and at 873 K (0, 9: first adsorption; ],[: second adsorption). Open and filled marks represent apparent and equilibrium data, respectively. (b) Adsorption isotherms of water on the samples treated in the different conditions and on the Y(OH)3 sample: (b) Y2O3 evacuated at 298 K; (9) Y2O3 evacuated at 873 K; (2) Y(OH)3 evacuated at 298 K.
It is interesting to notice that the adsorbed amount is much larger for the 298 K-treated sample than for the 873 K-treated sample, though the latter sample is expected to involve the chemisorbed amount. Another characteristic feature for the 298 K-treated sample is the appearance of a break in the adsorption isotherm at around a relative pressure of 0.007, giving rise to a decrease in the equilibrium pressure and a continuous increase in the adsorbed amount, as shown by an arrow in the figure. Beyond this relative pressure, such an adsorption anomaly takes place in a similar manner. The same phenomenon as this is also observed for the sample pretreated at 873 K in vacuo. In this case, in the first adsorption isotherm the initial increase in the adsorbed amount is due to a chemisorption of water and the break is recognized at the lower pressure, compared with the sample evacuated at 298 K. The break is also observed in the second adsorption isotherm for the 873 K-treated sample, though the adsorbed amount is reduced owing to a decrease in the chemisorbed amount. In addition, the longer time was needed to attain the adsorption equilibrium; it took about several tens or hundreds hours for both samples. Similar phenomenon was also observed in the MgO-H2O system,19
Adsorption Behavior of Water on Y2O3 Surface
Figure 2. Water content for the original and hydrated samples: (b) original Y2O3 sample; (4) the sample was evacuated at 873 K and then subjected to hydration in water for 1 day. The ordinate values are obtained by using the surface areas measured for the respective samples evacuated at 298 K.
where the monolayer capacity of water (Vm) is much smaller than that in the present Y2O3-H2O system. The adsorbed amounts of water on the original Y2O3 sample and on the sample evacuated at 873 K were measured at a fixed time of 1 h after every dose of water vapor, and the resulting isotherms are represented in Figure 1b. These isotherms are rather apparent ones. For comparison, the adsorption isotherm of water on the Y(OH)3 sample evacuated at 298 K is also shown; in this case the adsorption equilibrium was attained within 15 min. All the isotherms are well expressed by the BET equation in the lower pressure regions. The Vm value for the Y(OH)3 sample is estimated to be 4.5 H2O molecules nm-2, being well consistent with the value expected from the crystallographic consideration.22 On the other hand, for the original Y2O3 sample and the 873 K-treated sample the monolayer capacities are 25 and 8 H2O molecules nm-2, respectively. It is well understood from these data that the adsorption anomaly of water takes place on the Y2O3 sample, especially on the original Y2O3 sample. Figure 2 shows the water contents obtained for the original and the hydrated (1 day) Y2O3 samples. The water content, which is expressed as the number of water molecules per unit area (nm2), represents the amount of chemisorbed water remaining on the surface after evacuating the sample at the temperature indicated. The water content decreases in a nonsmooth manner with increasing temperature, in contrast to an exponential decrease as seen for many metal oxides. It is found that the maximum value of water content on Y2O3 is much larger compared with that obtained by the crystallographic estimation20 or by the consideration of the cross section of the water molecule23 and that the hydrated sample has much smaller water content than that on the original sample. Analysis of the State of Adsorbed H2O by NIR and TPD Measurements. The NIR spectra provide useful information about the state of water adsorbed on Y2O3. Figure 3 shows the NIR spectra for the samples evacuated at various temperatures. The difference spectra between two adjacent spectra are also depicted in Figure 3c,d. For (19) Kuroda, Y.; Yasugi, E.; Aoi, H.; Miura, K.; Morimoto, T. J. Chem. Soc., Faraday Trans. 1988, 84, 2421. (20) O’Connor, B. H.; Valentine, T. M. Acta Crystallogr. 1969, B25, 2140. (21) Christensen, A. N.; Heidenstam, O. Acta Chem. Scand. 1966, 20, 2658 (22) Christensen, A. N.; Hazell, R. G.; Nilsson, A. Acta Chem. Scand. 1967, 21, 481. (23) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577.
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the sample evacuated at 300 K two major bands are observed in the regions of 7400-6400 and 5300-4750 cm-1, the former being assigned to the overtone band (2νOH) of the stretching vibration of OH group and the latter to the combination band (νOH + δH2O) of the OH stretching vibration and bending vibration of the adsorbed water molecule, respectively.24-26 The appearance of the latter band is indicative of an existence of molecular water on the surface. It should be noted that for the original sample the combination band appears at around 5100 cm-1 close to the wavenumber for the bulk liquid.27 These findings imply that the molecularly adsorbed water strongly interacts with the surface OH groups and/or with each other by forming a hydrogen bonding. The evacuation of the sample at 373 K led to a decrease in intensity of the band in the region 6700-6800 cm-1, being concomitant with an increase in the band intensity at around 7200 cm-1 (Figure 3a, spectra 1 and 2; Figure 3c, spectrum 8). It also reduced the band intensity at 5200 cm-1 (Figure 3d, spectrum 8). By the evacuation at 473 K the band intensities decreased exclusively in the lower wavenumber side of overtone band and in the higher wavenumber side of combination band. After the sample was evacuated at 573 K, a distinct band appears at 7250 cm-1 and the band at 7110 cm-1 weakens, accompanied by a loss of the band at around 5000 cm-1 (Figure 3a,b, spectra 4). The adsorbed species confirmed in the evacuation process at 573 K are strongly adsorbed molecular water and, probably, hydroxyl groups; the former species are situated at the position so as to allow a bending vibration at the relatively lower wavenumber side as in the case of molecular water. This molecular water is expected to get stuck in the bulk through the bonding via lone pair of the oxygen atom, which is simultaneously free from hydrogen bonding with other sites. After the evacuation at 673 K, the band at 7110 cm-1 and the broad band at around 5000 cm-1 become attenuate, while the band at 7250 cm-1 retained its intensity. After the heat treatment of the sample at 873 K, all the bands disappear, which is corresponding to the variation of water content shown in Figure 2. As a result, it is reasonable to assume that there are at least three types of adsorbed species, including two types of molecular water on the Y2O3 surface evacuated at 300 K, i.e., the strongly adsorbed molecular water and bulk water that can be removed in the temperature region from 300 to 673 K, and the hydroxyl groups that may be present on the surface as YOOH and/or in the bulk of Y2O3. To clarify the hydration process of Y2O3, TPD, water content, and NIR spectra were measured for the samples that had been evacuated at 873 K, followed by hydration in the liquid water at 298 K for 1 day (sample A), 7 days (sample B), and 30 days (sample C). The results obtained are shown in Figures 4 and 5. Two distinct bands (at 370 and 420 K) are observed in the TPD spectrum for the sample A in addition to a weak and broad band centered at 650 K, which indicates the existence of at least three types of adsorption sites on the Y2O3 sample (Figure 4a). The water content is larger for the sample subjected to hydration in water for a longer period of time (Figure 4b). By the hydration in water (sample A) the 7250 cm-1 band is recovered in the NIR spectrum, together with an appearance of a new band at around 7000 cm-1 and a distinct shoulder in the region 7100-7200 cm-1 (Figure (24) Klier, K.; Shen, J. H.; Zettlemoyer, A. C. J. Phys. Chem. 1973, 77, 1458. (25) Shen, J. H.; Zettlemoyer, A. C.; Klier, K. J. Phys. Chem. 1980, 84, 1453. Shen, J. H.; Klier, K. J. Colloid Interface Sci. 1980, 75, 56. (26) Burneau, A.; Lepage, J.; Maurice, G. J. Non-Cryst. Solids 1997, 217, 1. (27) Luck, W. A. P. Ber. Bunsen-Ges. Phys. Chem. 1965, 69, 626.
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Figure 3. NIR spectra (a, b) and their difference spectra (c, d) in the overtone (a, c) and combination (b, d) regions for the original Y2O3 sample. The sample was evacuated at different temperatures: (1) 300; (2) 373; (3) 473, (4) 573; (5) 673; (6) 773; (7) 873 K. Difference spectra were obtained from two adjacent spectra shown above: (8) spectrum 1-spectrum 2; (9) 2-3; (10) 3-4; (11) 4-5; (12) 5-6; (13) 6-7.
5). The intensities of these bands increase with increasing hydration time in water. However, the combination band was not clear, probably owing to a weakness of the transition intensity. For the samples B and C it is clearly seen in the desorption data and NIR spectra (Figure 5) that three different types of water are distinctly recognized when the hydration time is elongated (shown by arrows in Figure 5, spectrum 3). Taking account of the desorption data presented in Figure 3, the species giving a 6900 cm-1 band corresponds to a molecularly adsorbed species. When both sets of data of water content and NIR spectra are referred to, the species responsible for the first and second bands in the TPD spectra seems to increase in amount by the hydration in water. The species giving a desorption peak at around 650 K corresponds to that giving a NIR band at 7250 cm-1 (Figure 5). Although there is ambiguous evidence for the appearance of the combination band, the
species giving a band at 7100 cm-1 can be assigned to the hydrated molecular water (bulk species). The water content for the sample A is 16 H2O molecules nm-2 (Figure 4b), which is twice as large as that expected on the basis of a cross section of a water molecule.23 As shown in Figure 4a, the TPD pattern observed for the hydrated Y2O3 sample is different from that for Y(OH)3, which indicates that the adsorbed states of water are quite different from those on Y(OH)3. The desorption peaks at 370, 420, and 650 K correspond to three distinct desorption processes: the desorption of water molecules adsorbed strongly on the surface layer, the desorption of water incorporated into the bulk in the form of YOOH or Y2O3(H2O), and the condensation-dehydration of surface hydroxyl groups. The NIR spectra in the 2νOH region were also obtained at various stages of water adsorption and are depicted in Figure 6a. The difference spectra, i.e., the difference
Adsorption Behavior of Water on Y2O3 Surface
Figure 4. Desorption of water from the Y2O3 and Y(OH)3 samples. (a) TPD: (1) sample A; (2) Y(OH)3. (b) Water content: (4) sample A; (0) sample B; (O) sample C. The ordinate values for water content data are obtained by using the surface areas measured for the samples treated with liquid water for different periods, followed by re-evacuation at 298 K.
between the spectrum recorded after water adsorption and the background spectrum of the 300 K-treated sample (Figure 6a, spectrum 1), are also presented in Figure 6b. It can be seen from these figures that the band at around 7200 cm-1 decreases in its intensity at the initial adsorption stage, which might be caused by a shift toward the lower wavenumber (6950 cm-1) as the result of interaction with physisorbed water. The band at around 7100 cm-1 is little affected by the addition of water vapor. The band at 6630 cm-1 due to the physisorbed water grows at the initial stage, and the band at 6800 cm-1 increases in intensity as the adsorption proceeds. It should be noted that the wavenumber of the band due to the physisorbed species is lower than that for ice, which indicates that the strong interaction takes place between the physisorbed water and solid surface, resulting in an appearance of the combination band at around 5150 cm-1 (not shown). As the results, the adsorbed species on the Y2O3 surface can be recognized as composing of three types of water: the first species interacting with the surface hydroxyl groups, the second one trapped in the surface layer, and the last one mutually interacting water molecules with the first and second types of physisorbed water. The physisorbed water characterized by the band at 6630 cm-1 may correspond to the water molecules of the first and third types. We interpret that the water molecules of the second type, which give a band at 6800 cm-1, penetrate into the bulk and change to the species that are unable to interact laterally. These species will be discussed later. Classification of the Types of Adsorbed H2O by Dielectric Measurement. (i) State of Physisorbed H2O.
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Figure 5. NIR spectra in the overtone region of the stretching vibration of the OH group: (1) sample A; (2) sample B; (3) sample C.
Dielectric behavior of adsorbed water depends greatly upon the surface state of Y2O3. It is well-known that the curves of ′ and ” change remarkably in the region where the relaxation is observed. The shapes of curves of ′ and ′′ with frequency are correlated with each other; ′′ has such a characteristic-frequency dependence that it passes through a maximum value at the frequency where ′ undergoes its maximum rate of change with frequency. Figure 7a,b shows the typical examples of relaxation observed in the Y2O3-H2O system at a coverage of 1.64. The frequency giving a maximum ′′ corresponds to a characteristic frequency for the observed dispersion. For other coverages of 0 (Figure 7c), 1.12 (Figure 7d), and 2.24 (Figure 7e), only the dielectric losses, ′′, which were obtained in the temperature range 298-130 K, are plotted against the frequency. The ′′ value obtained at 298 K and at a coverage of 1.64 is approximately 200 at 0.1 Hz and it decreases gradually with increasing frequency. Moreover, the ′′ value becomes smaller as the temperature is lowered. On the other hand, below 193 K another relaxation appears in the higher frequency region, though the ′′ value for this relaxation is much smaller than that observed at 298 K. As described above, the frequency where ′′ gives a maximum value corresponds to a characteristic frequency (fm) for the observed dispersion. Further lowering of temperature causes a shift of the characteristic frequency of this relaxation to the lower frequency side. It has become apparent at this coverage that at least two kinds of relaxations are observed in both frequency and temperature regions examined. The relaxation that has a large ′′ value and is observed at the lower frequency seems to shift to the higher frequency side with increasing coverage, while the characteristic frequency of other relaxations, which are observed at higher frequencies and at lower temperatures, does not have an appreciable coverage dependence. When our dielectric study on the
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Figure 6. NIR spectra (a) and their difference spectra (b) in the overtone region of the stretching vibration of the OH group on Y2O3. The sample was equilibrated with water vapor for 1 h at different relative pressures: (1) nearly zero; (2) 0.03; (3) 0.19; (4) 0.29. Difference spectra were obtained by subtracting respective spectrum 2 (5), spectrum 3 (6), and spectrum 4 (7) from spectrum 1 shown above.
SrF2-H2O system is referred to,13 a large relaxation observed in the lower frequency region and at 298 K can be assigned to the interfacial polarization arising from a heterogeneity of the system. In accordance with this assignment, the arc length of the Cole-Cole plot of the relaxation observed at 298 K in the present Y2O3-H2O system remained constant, regardless of the coverage. Therefore, another relaxation observed at the lower temperature and in the higher frequency region can be ascribed to the orientational polarization due to the adsorbed water. The basis for this assignment can also be confirmed by the Cole-Cole plots shown in Figure 8. The arc length of Cole-Cole plots is found to increase with increasing coverage, obeying Onsager’s equation,28 and hence, this relaxation can be assigned to the orientational polarization of physisorbed water. Furthermore, there is an interesting point not to be ignored; when we look more closely the data at a coverage zero, some additional relaxation is also observable. In the present stage, however, it is undefined if such a relaxation is really present. The plots of τ against T-1, where τ [)1/(2πfm)] means a relaxation time, for the relaxation due to the adsorbed water give a linear relationship, as shown in Figure 9. The values for liquid water and ice are also shown in this figure.29,30 From the slope of these straight lines the activation energy for the relaxation is evaluated to be ca. 38.6 kJ mol-1 for the adsorbed water on Y2O3 (at coverage of 1.64). Moreover, it is seen from this figure that the property of adsorbed layer on Y2O3 is intermediate between liquid water and ice. From the slope and intercept of these lines, the relaxation frequency of orientational polarization at 298 K is estimated to be 4.4 GHz, which is compared with those on SiO2,31 Al2O3,32 the matrix of poly(2(28) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486. (29) Auty, R. P.; Cole, R. H. J. Chem. Phys. 1952, 20, 1309. (30) Hasted, J. B. Water, A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1972; Vol. 1. (31) Kamiyoshi, M. K.-I.; Ripoche, M. J. J. Phys. Radium 1958, 19, 943.
hydroxyethyl methacrylate),33 etc. Since the strong interaction of water molecules with the surface prevails over their mutual interaction, the interaction between water molecules on the Y2O3 surface is restricted. Thus, the water molecules adsorbed on the Y2O3 surface take an amorphous state rather than the ice-like rigid structure.34 Furthermore, it is interesting to compare the present fm values with those for the SrF2 system in which the twodimensional condensation of water occurs;13 the fm values for Y2O3 system at 158 K are 52.7 and 8.9 kHz at coverages of 1.44 and 1.88, respectively, and that for SrF2 is 840 kHz at 159 K and at a coverage of 1.26.13 This fact indicates that water on Y2O3 is rather immobile in the alternating field, compared with that on SrF2, and also suggests a strong interaction with the underlying surface, being consistent with the conclusion derived in the previous work.13,35 On the other hand, the fm values of 0.09 kHz at 162 K for SiO2 and of 112 kHz at 183 K for the Al2O3 systems were also reported,31,32 which indicates a rather restricted motion of water molecules adsorbed on these samples compared with those on Y2O3. These facts can be interpreted by considering that the interaction in the Y2O3-H2O system is not so strong as that water molecules are trapped in the pores by feeling a pore potential. Here, a little description is devoted to examine the change of the state of adsorbed water on Y2O3 with coverage. The data shown in Figure 7b,d,e clearly indicates that the motion of the water molecule tends to become smaller gradually as the coverage is increased. The characteristic frequency at 158 K varies from 96 to 24.5 or 9.3 kHz, corresponding to the coverage from 1.12 to 1.64 or 2.24. This tendency can be explained quite naturally as due to the mutual interaction of adsorbed (32) Dransfeld, K.; Frisch, H. L.; Wood, E. A. J. Chem. Phys. 1962, 36, 1574. (33) Johare, G. P. J. Chem. Phys. 1996, 105, 7079, and references therein. (34) Bertoline, B.; Cassettari, M.; Salvetti, G. J. Chem. Phys. 1982, 76, 3285. (35) Kuroda, Y.; Yoshikawa, Y.; Hamano, H.; Nagao, M. Langmuir, 1996, 12, 1399.
Adsorption Behavior of Water on Y2O3 Surface
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Figure 7. Dielectric curves of the Y2O3-H2O system obtained at various temperatures. (a) ′-f and (b) ′′-f curves at a coverage of 1.64; ′′-f curves at a coverages of (c) 0, (d) 1.12, and (e) 2.24. The measurement temperatures are 298 (b), 273 (]), 226 (O), 193 (2), 170 (0), 158 ([), 145 (4), and 130 (9) K.
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Figure 8. Coverage dependence of the arc length of ColeCole plots for the Y2O3-H2O system measured at 158 K. Coverages are 0.56 ([), 1.12 (b), 1.44 (~), 1.64 (O), and 2.48 (2).
Figure 9. Plots of the relaxation time (τ) obtained from the dielectric studies against 1/T for various systems: ([), Y2O3H2O at a coverage of 1.64; (4), polymer-H2O;33 (9), SiO2-H2O;31 (b), Al2O3-H2O;32 (2), SrF2-H2O at a coverage of 0.99;13 (O), ice;30 (0), bulk water;29 (]), supercooled water.34
Figure 10. Cole-Cole plots representing the bulk hydration: (O), obtained at 158 K immediately after exposure to water vapor at a coverage of 1.64; (b), obtained at 158 K after keeping the sample in water vapor for 7 days at 298 K.
molecules and also to the interaction between water molecules and the underlying surface hydroxyl layers. (ii) State of Strongly Adsorbed H2O. The dielectric measurement was also carried out in the adsorption range where a break appears in the isotherm in order to reveal the difference of adsorbed species. The arc length of the Cole-Cole plots decreases with a lapse of hydration time (Figure 10). According to Onsager’s equation, the amount of physisorbed water that responds to the applied alternating-current field decreases, which suggests that some water molecules change their states; these water molecules, which bring about an adsorption anomaly (i.e., steep increase in the adsorbed amount), do not respond in these frequencies and temperature region. These data, together with the data of NIR and TPD, reveal that water molecules are physisorbed on Y2O3 and that some of them are get stuck strongly in the surface layer and then
Kuroda et al.
penetrate slowly into the bulk to form a hydrated or hydroxide layer. It may be hard for such water molecules to respond to the applied field. (iii) Existence of Another Kind of Physisorbed H2O. In the next stage, we shall examine carefully the coverage dependence of the ′′-f curves obtained at 273 K (Figure 11). These curves exhibit a discernible shoulder at the higher frequency side of large relaxation (shown by arrows in Figure 11a). According to Cole’s method,36,37 we tried to deconvolute the observed spectrum into two components by assuming the Debye-type relaxation that has a distribution of relaxation times, and the result obtained is shown in Figure 11b. The existence of two types of relaxations is evidently proved from this figure. The large relaxation situated at the lower frequency side is ascribed to the interfacial polarization arising from a heterogeneity of the system, as mentioned above. Taking account of the fact that the strongly adsorbed water exists on the Y2O3 surface, it is reasonable to suppose that the other relaxation appearing at 40 Hz and at a coverage of 1.64 can be assigned to the orientational polarization of strongly adsorbed water with a restricted motion. Such a relaxation was not observed in the SrF2 system.13 From the observed facts, it may be concluded that there are at least two types of physisorbed water, including the relaxation observed in the lower temperature region. This conclusion is not in conflict with the NIR result, indicating the existence of two types of adsorbed water molecules. On the basis of the results described above, the surface model of Y2O3 can be constructed, as shown in Scheme 1. Roughly speaking, there are three types of water on the Y2O3 surface: the strongly adsorbed water (species I), the hydrated molecular water or hydroxide-like species (species II), and the hydroxyl species (species III). The physisorbed water on the outermost surface penetrates slowly into the inner layer to form species I. The species I thus formed reacts with the Y2O3 surface and penetrates into the bulk to form Y2O3(H2O) or YOOH-like species as a transitional state to the bulk species of YOOH or Y(OH)3. On the other hand, species III, which is the surface OH group, may be formed preferentially on the 873 K-treated Y2O3 sample at the initial adsorption stage of water. In regard to the formation of the strongly adsorbed water on the 873 K-treated Y2O3 sample, the surface rearrangement would occur during the evacuation of Y2O3 at a higher temperature as 873 K, and hence much time would be required for the hydration in which the available sites for adsorption of water molecule as a species I are necessary. H-D Exchange Reaction of the Respective Adsorption Sites. It is also interesting to evaluate the H-D exchange property of the Y2O3 sample. Figure 12 shows the NIR spectra for the sample at different stages of H-D exchange using a deuterium oxide (D2O). At first, the H-D exchange was performed on the 300 K-treated sample (spectrum 1) by keeping an apparent equilibrium vapor pressure of nearly zero for 5.5 h and then the sample was degassed for 1 h at a pressure of ca. 1 Pa, followed by the measurement of NIR spectra (spectrum 2). The same procedure was repeated by varying equilibrium time and pressure of D2O (spectra 3-6). It seems that the band intensity at around 6800 cm-1 decreases at an equilibrium pressure of nearly zero. At a relative pressure of 0.04, the bands at 6800 and 7250 cm-1 reduce their intensities. The 6800 cm-1 band is hardly discernible and the 7110 cm-1 band loses significantly its intensity at a relative (36) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341. (37) Kuroda, Y.; Kittaka, S.; Takahara, S.; Yamaguchi, T.; BellissentFunel, M.-C. J. Phys. Chem. B. 1999, 103, 11064.
Adsorption Behavior of Water on Y2O3 Surface
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Figure 11. (a) ′′-f curves obtained at 273 K for various coverages: (b), 0.56; (4), 1.12; (O), 1.44; (]), 1.64; (2), 1.88. (b) Curveanalyzed ′′-f for a coverage of 1.64 and at 273 K: (]), measured data; (b), relaxation due to the interfacial polarization; (O), relaxation due to the orientational polarization. Scheme 1
pressure of 0.14 (spectrum 4). It is found that the spectrum is little affected by the treatment with D2O for 0.5 h, even if the relative pressure is so high as 0.72 (spectrum 5). However, the prolonged contact with D2O vapor (30 h) at this pressure brings about a drastic change of spectrum (spectrum 6). By this operation the H-D exchange appears to be almost complete and the band due to the OH overtone remains slightly. These properties clearly bear out an occurrence of diffusional penetration of water into the bulk. Taking account of the model presented above, it is presumed that the H-D exchange takes place between the physisorbed water and water molecules of type I and then it proceeds on the hydroxyl species of type III, followed by the molecular or oxo-hydroxide species of type II. Electronic and Structural Features of Y2O3. From the electronic and structural points of view, the results described above can be confirmed by the examination of XAFS including EXAFS and XANES. Figure 13 represents the EXAFS spectra. These spectra were analyzed by using a curve-fitting technique, and the resultant coordination number and the first nearest distance between yttrium and oxygen ions are summarized in Table 1. For the Y(OH)3 sample, the coordination number, N, is found to be the largest and the distance, R, is the longest among the samples examined. These are consistent with the crystallographic data.22,38 For the hydrated Y2O3 samples, N (38) Malet, P.; Capitan, M. J.; Centeno, M. A.; Odriozola, J. A.; Carrizosa, I. J. Chem. Soc. Faraday Trans. 1994, 90, 2783.
Figure 12. NIR spectra in the overtone region during the H-D exchange reaction: (1) hydrated in water (H2O) for 30 days, followed by evacuation at 300 K; (2) exposed to D2O vapor at 300 K for 5.5 h at an equilibrium relative pressure of nearly zero; the sample was exposed to D2O vapor at 300 K (3) for 30 h at an equilibrium relative pressure of 0.04; (4) for 0.5 h at an equilibrium relative pressure of 0.14; (5) for 0.5 h at an equilibrium relative pressure of 0.72; (6) for 30 h at an equilibrium relative pressure of 0.72.
decreases from 7.4 to 6.9 and R from 2.34 to 2.33 Å with an increase in the treatment temperature from 300 to 873 K, which is very consistent with the decrease in water content shown in Figure 2. On the basis of water-content data, the chemical formula of the original sample is presumed to be Y2O3‚0.31H2O. The decrease in the values
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Figure 13. (a) EXAFS spectra of Y2O3 evacuated at (1) 300 and (2) 873 K. (b) EXAFS spectra of Y(OH)3 evacuated at (1) 300, (2) 558, (3) 623, (4) 703, and (5) 873 K. Table 1. Coordination Number, N, and Average Distance, R, Obtained from EXAFS Data samples
N
R (Å)
Y(OH)3 evacuated at 300 K Y2O3 evacuated at 300 K Y2O3 evacuated at 873 K
10 7.4 6.9
2.41 2.34 2.33
of N and R may come from a loss of water and a structural change in the vicinity of surface. The values of N and R for the Y2O3 sample re-evacuated at 873 K indicate that this sample is dominantly composed of the bulk oxide, which has the values of 6 and 2.284 Å.20 Figure 14 shows the XANES spectra of the Y2O3 samples evacuated at 300 and 873 K and of the Y(OH)3 sample evacuated at 300 K. For the latter sample, a distinct band at 17.045 keV and a blurry band at 17.056 keV are observed, which are due to the yttrium K-edge. The ratio of the former band intensity to the latter one is 1.7. On the other hand, in the spectrum of the Y2O3 sample evacuated at 873 K, two bands are clearly observed; both bands slightly shift toward the lower energy side compared to the case of Y(OH)3 sample. The ratio of the band intensity at 17.043 keV to that at 17.053 keV is nearly equal to 1. Such a ratio was larger for the hydrated sample, and it became smaller when the sample was evacuated at increasing temperature. This result also supports the interpretation that the hydrated sample takes an intermediate electronic state between the hydroxide and oxide of yttrium. As shown above, the Y2O3 sample evacuated at 873 K corresponds to a pure Y2O3 and the Y(OH)3 sample can be represented by a chemical formula of Y2O3‚3H2O. Consequently, we can simulate a corresponding XANES spectrum of the original sample by combining the spectrum of Y(OH)3 and that of pure Y2O3 sample in an appropriate ratio. Here, it should be remembered that the original Y2O3 sample used in the present study can be expressed as Y2O3‚0.31H2O. Figure 14b shows the simulated spectrum of Y2O3‚0.31H2O, together with the spectrum of the original Y2O3 sample; the former spectrum is well superposed upon the latter one. This fact also gives a strong evidence that the original sample is composed of Y2O3‚ 0.31H2O. Taking into consideration that the chemical formula of Y2O3‚H2O is also expressed as 2YOOH, the
Figure 14. (a) XANES spectra of Y2O3 evacuated at (1) 873 and (2) 300 K and of Y(OH)3 evacuated at 300 K (3). (b) Simulated XANES spectrum of Y2O3‚0.31H2O (1) and experimentally obtained XANES spectrum of Y2O3 evacuated at 300 K (O).
actual state of Y2O3‚0.31H2O may be the intermediate between Y2O3 and YOOH. Reactivity of Y2O3 with H2O: Acid-Base Properties. It is well-known that the surface acidity and basicity have a great influence on the reactivity of solids. Sanderson has pointed out that the electronegativity of the metal ion, Xi, would be expected to change linearly with its charge: Xi = (1 + 2z)X0, where X0 is the electronegativity of the neutral atom and z is the charge of the metal ion.39
Adsorption Behavior of Water on Y2O3 Surface
In the first approximation, Tanaka and Ozaki pointed out that this parameter can be regarded as a semiempirical acidity parameter of the metal ion,40 though there are some interpretations based on the different points of view.41 According to their arguments, the Y2O3 sample can be classified as a weak basic oxide. Actually, the acidic-basic tendency obtained from the XPS data is completely consistent with the above consideration,42 as seen from the XPS O1s binding energy for various oxides, such as 529.3 eV for Y2O3, 530.9 eV for MgO, and 532.65 eV for SiO2.43-45 The reactivity is related to a high electrondonating strength of oxygen (low 1s binding energy). The result based on the PZC values for Y2O3 and SiO2 is also in accordance with this interpretation.46,47 As a result, the trend for water adsorption on Y2O3 is well explained by assuming that the electron density on oxygen ion in the metal oxides is responsible for the reactivity of solids; Y2O3 reacts easily with water vapor even at room temperature to form a strongly hydrated species and then to form progressively the surface and bulk hydroxide layers in such a way that alkaline and alkaline-earth oxides do. The basic oxides such as Y2O3 and MgO easily react with water, liberating a large amount of heats. The heats thus evolved may assist the reaction of Y2O3 with water, and hence, water penetrates into the bulk. Summary Appearance of a break in the adsorption isotherms was found in the Y2O3-H2O system. This phenomenon is interpreted as follows: water molecules interact strongly with the surface of Y2O3 and the adsorbed water becomes incorporated into the surface layer progressively, and finally the bulk hydration or the formation of bulk (39) Sanderson, R. T. Chemical Periodicity; Reinhold Publishing: New York, 1960. (40) Tanaka K.-I.; Ozaki, A. J. Catal. 1967, 6, 1. (41) Noguera, C. Physics and Chemistry at Oxide Surfaces; Cambridge University Press: Cambridge, 1996. (42) Vinek, H.; Noller, H.; Ebel, M.; Schwarz, K. J. Chem. Soc., Faraday Trans. 1 1977, 73, 734. (43) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (44) Barr, T. L.; Brundle, C. R. Phys. Rev. 1992, B46, 9199. (45) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: Chichester, 1994; p 412. (46) Parks, G. A. Chem. Rev. 1965, 65, 177. (47) Yoon, R. H.; Salman, T.; Donnay, G. J. Colloid Interface Sci. 1979, 70, 483.
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hydroxide layers proceeds step by step at room temperature. Such interpretation is supported from the data of water content, which is appreciably larger compared with those on typical metal oxides, NIR, XAFS, and dielectric measurements of adsorbed water on Y2O3. The following reaction mechanisms are supposed for the Y2O3-H2O system (1) Water molecules are adsorbed on the Y2O3 surface to take the amorphous state. The formation of a network structure among the adsorbed water molecules is restricted because of a strong interaction with the Y2O3 surface. (2) Some amounts of physisorbed water are transformed into the strongly adsorbed species, i.e., strongly incorporated into the surface layer. The existence of such water molecules is the first example detected in the water adsorption on metal oxides. (3) This strongly adsorbed water reacts with the Y2O3 surface and penetrates into the bulk, resulting in the formation of Y2O3(H2O) or YOOH-like species. (4) A progressive reaction proceeds slowly into the bulk to form Y(OH)3. These mechanisms are interpreted on the concept of electronegativity of yttrium ion, as suggested by the binding energy of O1s and PZC value of this oxide. The Y2O3 sample has somewhat basic properties, and it reacts easily with water to form hydroxide. Such behavior is analogous to that of alkaline-earth oxides. Acknowledgment. This research was supported in part by grants from Nippon Sheet Glass Foundation for Materials Science and Engineering and from Hitachi Metal Ltd. One (Y.K.) of the authors would like to acknowledge the generosity of these organizations. A part of this work has been performed under the proposal of the Photon Factory Program Advisory Committee. We thank Drs. M. Nomura, A. Koyama, and N. Usami of the Photon Factory (KEK) in Tsukuba for their kind assistance in measuring the XAFS spectra. Thanks are also due to Mr. T. Ozaki of the glassblowing workshop of Hiroshima University for the technical assistance in making an in situ glass cell. LA9917031
