Adsorption of Water on Nd2O3: Protecting a

process is proposed; first Nd2O3 changes to NdOOH and then to Nd(OH)3. ... water at a coverage of 2.5 was observed at 50 kHz and at 169 K, indicating ...
11 downloads 0 Views 177KB Size
Download PDF
Langmuir 2000, 16, 6961-6967

6961

Adsorption of Water on Nd2O3: Protecting a Nd2O3 Sample from Hydration through Surface Fluoridation Hideaki Hamano,† Yasushige Kuroda,*,‡ Yuzo Yoshikawa,‡ and Mahiko Nagao† Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan, and Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan Received January 31, 2000. In Final Form: June 1, 2000 The adsorption of water on Nd2O3 has been investigated by measuring the adsorption isotherm of water, water content, temperature-programmed desorption (TPD), and near-infrared (NIR) spectra and also by dielectric measurement. The water content of Nd2O3 was found to be appreciably larger than that of typical metal oxides. Two distinct desorption peaks were observed in the TPD spectrum of Nd2O3, as in the case of Nd(OH)3. The most characteristic feature of the adsorption of water on Nd2O3 was the appearance of a break in the adsorption isotherm at a relative pressure of approximately 0.028. The NIR spectra of this sample gave a broad band at around 7050 cm-1 and a sharp band at 7140 cm-1 due to the overtone of the stretching vibration of the OH group, though the combination mode was not observed. These facts are interpreted in terms of the occurrence of bulk hydration at room temperature. The following hydration process is proposed; first Nd2O3 changes to NdOOH and then to Nd(OH)3. Dielectric relaxation of physisorbed water at a coverage of 2.5 was observed at 50 kHz and at 169 K, indicating the restricted motion of water owing to a strong interaction with the surface layer. On the basis of the O1s binding energy observed at 528.9 eV for Nd2O3, these phenomena are explained by the concept of basicity due to the large ionic radius of the rare-earth metal ion. To protect the surface of Nd2O3 from hydration, surface fluoridation was tried. As a result, it was clearly shown that surface fluoridation is very effective for protecting against hydration of Nd2O3. The depression of surface conduction observed in the dielectric behavior of the fluoridated sample also supports the depression of bulk hydration.

Introduction In recent years, rare-earth compounds have attracted a lot of interest because of their potential for applications as hydrogen-absorbing alloys, high-energy permanent magnets, high-Tc superconductors, solid electrolytes, and photoluminescence materials.1 Their additional use can be found in a wide variety of catalysts, such as cracking catalysts, catalysts for hydrogenation and isomerization reactions, catalysts for the oxidative coupling of methane, and also promoters in the three-way catalyst in autoexhaust decomposition.2-4 Few attempts have so far been made to elucidate the adsorption properties of rare-earth oxides, especially their reactivity to water, though water is present invariably at ambient conditions and is expected to exert a great influence on the surface properties of these materials.5-12 Recently, we have elucidated the adsorption characteristics of Y2O3 as a typical and simple rare-earth * To whom all correspondences should be addressed. E-mail: [email protected]. † Research Laboratory for Surface Science. ‡ Department of Chemistry. (1) Adachi, G.; Imanaka, N. Chem. Rev. 1998, 98, 1479 and references therein. (2) (a) Breysse, M.; Claudel, B.; Faure, L.; Guenin, M. J. Colloid Interface. Sci. 1979, 70, 201. (b) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (3) Minachev, K. M.; Khodakov, Y. S.; Nakhshounov, V. S. J. Catal. 1977, 49, 207. (4) Rosynek, M. P.; Fox, J. S.; Jensen, J. L. J. Catal. 1981, 71, 64. (5) Warshaw, I.; Roy, R. J. Phys. Chem. 1961, 65, 2048. (6) Gammage, R. B.; Fuller, E. L., Jr.; Holmes, H. F. J. Phys. Chem. 1970, 74, 4276; J. Colloid Interface. Sci. 1970, 34, 428. (7) Touret, D.; Queyroux, F. Rev. Chim. Miner. 1972, 9, 883. (8) Alvero, R., Odriozola, J. A.; Trillo, J. M. J. Mater. Sci. 1985, 20, 1828. (9) Alvero, R.; Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. J. LessCommon Met. 1985, 109, 197. (10) Alvero, R.; Bernal, A.; Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. J. Less-Common Met. 1985, 110, 425.

compound.13 As a result, it has become apparent that water molecules are strongly adsorbed on Y2O3 and successively react with bulk oxide layers to form hydroxide layers. Such a process is summarized as follows: Y2O3 + physisorbed water f Y2O3‚H2O (strongly adsorbed in the surface layer) f Y2O3 (H2O bulk species) or YOOH f Y(OH)3. These hydration processes can be explained using the electronegativity of the yttrium ion, Y3+; the tendency for hydration is well explained by assuming that the high electron density on the oxygen ion in metal oxides is responsible for the reactivity of solids, as in the case of alkaline and alkaline-earth oxides. On the basis of such a background, the present work was done to provide data relevant to understanding the surface properties of rare-earth metals, especially neodymium metal, which is used as a magnetic material.1 At the first stage, we have attempted to elucidate the reactivity of Nd2O3 with water, since at ambient conditions the oxide layers are produced naturally and easily on the surface of neodymium metal. An efficient procedure for protecting the surface from hydration was also sought in order to control the reactivity of the Nd2O3 surface with water. Experimental Section Materials. The original Nd2O3 powder sample purchased from Kanto Chemicals was reagent grade. To prepare the carbonatefree sample, the Nd2O3 sample was evacuated at 873 K under a reduced pressure of 1 mPa. The sample thus treated was exposed to saturated H2O vapor for 2 days to achieve surface hydration, (11) Bernal, S.; Botana, F. J.; Garcia, R.; Rodriguez-Izquierdo, J. M. React. Solids 1987, 4, 23. (12) Alvero, R., Bernal, A.; Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. J. Mater. Sci. 1987, 22, 1517; J. Therm. Anal. 1987, 32, 637. (13) Kuroda, Y.; Hamano, H.; Mori, T.; Yoshikawa, Y.; Nagao, M. Langmuir, in press.

10.1021/la0001169 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000

6962

Langmuir, Vol. 16, No. 17, 2000

followed by keeping it in vacuo until the measurements are performed. The specific surface area was found to be 2.37 m2 g-1 by applying the BET equation to the N2-adsorption isotherm obtained at 77 K. Redistilled water was further purified by repeating the freeze-evaporate-thaw cycles to remove the dissolved gases. Surface Fluoridation. Surface fluoridation was carried out by stirring the Nd2O3 sample in an aqueous solution of NH4F. The extent of surface fluoridation was controlled by changing the reaction time or the concentration of the NH4F solution. The number of F- incorporated was estimated by lanthanum-alizarin complexone absorptiometry. Fluoridated samples were abbreviated as F-Nd2O3(x), where x means the number of fluoride ions nm-2. Water Adsorption Isotherm and Water Content. The measurement of adsorption isotherm of water was performed volumetrically. Prior to this measurement the 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 sample. The water content of the original Nd2O3 sample was determined by the successive-ignition-loss method.14 TPD Measurements. For Nd2O3 and Nd(OH)3, the temperature-programmed desorption (TPD) of water was performed in order to analyze the adsorbed species. The sample was first evacuated at 300 K for 4 h under a pressure of 1 mPa to remove physisorbed water and then kept in a helium flow (the rate of 60 ml min-1) at this temperature until a stable baseline was monitored. The TPD experiment was performed at a heating rate of 5 K min-1 in the temperature region 300-900 K. Diffuse Reflectance Spectra. 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. Powdered tetrafluoroethylene vacuum-sealed in an optical cell was employed as a reference material. The spectra were evaluated by SKM theory which gives a ratio of the absorption coefficient to the scattering one.15 Dielectric Properties. The dielectric measurements were carried out in the same manner as described in the previous paper16 in the temperature region 298-145 K by using the impedance bridges (TR-4 and TR-10C, Ando Electrics) which were able to measure in the frequency region from 0.1 Hz to 5 MHz. XPS Measurements. XPS spectra were taken on a Fisons Instruments S-Probe ESCA using a monochromatized Al KR1,2 (1486.6 eV) X-ray incident radiation and a low-energy electron gun in a vacuum. Electron binding energies, Eb, were given relative to Eb(C1s) ) 285.0 eV of the C1s electrons of hydrocarbons at the surface of the tested samples.

Results and Discussion Analysis of the Adsorption of Water on Nd2O3. Figure 1 shows the adsorption isotherm (298 K) of water on the Nd2O3 sample evacuated at 298 K. The adsorption isotherm gives a “knee” after a steep increase in the adsorbed amount in the lower pressure range, and it can be regarded as type II in BDDT classification, similarly to many cases of metal oxides.17 The amount of water adsorbed on Nd2O3 is similar to that on typical oxides,17-21 (14) Morimoto. T.; Naono, H. Bull. Chem. Soc. Jpn. 1973, 46, 2000. (15) Klier, K. Catal. Rev. 1968, 1, 207. (16) Kuroda, Y.; Yoshikawa, Y.; Morimoto, T.; Nagao, M. Langmuir 1995, 11, 259, 2173, and 4031. (17) Nagao, M.; Morimoto, T. J. Phys. Chem. 1980, 84, 2054. (18) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243. (19) Zettlemoyer, A. C.; Micale, F. J.; Klier, K. In Water, A Comprehensive Treatise; Franks F., Ed.; Plenum: New York, 1975; Vol. 5. (20) Kno¨zinger, H. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C. Eds.; North-Holland: Amsterdam, 1976; Chapter 27. (21) Kung, H. H. In Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: Amsterdam, 1989.

Hamano et al.

Figure 1. Adsorption isotherm (b, 9) of water on the original Nd2O3 sample evacuated at 298 K.

Figure 2. Water content of the original Nd2O3 sample.

such as ZnO, R-Fe2O3, etc, and is much smaller than on the similar rare-earth oxide, Y2O3.13 Another characteristic feature of the adsorption isotherm is the appearance of a break. It took several tens of hours to attain an apparent adsorption equilibrium, and the break appeared abruptly when the relative pressure reached approximately 0.028, which resulted in a decrease in the vapor pressure and a continuous increase in the adsorbed amount (shown by an arrow in Figure 1). Such a phenomenon was observed beyond this pressure. By reference to similar phenomena observed in the Y2O3H2O and MgO-H2O systems,13,22 this phenomenon can be interpreted in terms of hydration onto the bulk. The rate of hydration of Nd2O3 seems to be faster than that of Y2O3.13 The water content obtained for the original Nd2O3 sample is shown in Figure 2. Here, the water content, which is expressed as the number of water molecules per unit area (nm2), indicates the amount of chemisorbed water remaining on the surface after evacuating the sample at the temperature indicated. The water content decreases slowly up to a temperature of 423 K and then it rapidly decreases to a value of zero. It is also found that the amount of water adsorbed on Nd2O3 definitely exceeds the value expected from crystallographic considerations23 and even the value deduced from the cross section of a water molecule.24 Figure 3 shows the TPD curve for water adsorbed on the Nd2O3 sample. For comparison, the TPD curve for (22) Kuroda, Y.; Yasugi, E.; Aoi, H.; Miura, K.; Morimoto, T. J. Chem. Soc., Faraday Trans. 1988, 84, 2421. (23) Wolf, L.; Schwab, H.; Sieler, S. J. Prakt. Chem. 1966, 32, 113. (24) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577.

Adsorption of Water on Nd2O3

Langmuir, Vol. 16, No. 17, 2000 6963

Figure 3. TPD curve of Nd2O3 (1) and of Nd(OH)3 (2).

Nd(OH)3 is also depicted in this figure. For Nd(OH)3, two distinct desorption peaks are observed at 507 and 609 K. On the other hand, combined with XRD analysis, the removal of water from Nd(OH)3 is found to occur via the following two steps:

2Nd(OH)3 f 2NdOOH + 2H2O 2NdOOH f Nd2O3 + H2O The similarity in the TPD curves between Nd2O3 and Nd(OH)3 suggests that the surface layers on Nd2O3 are at least composed of Nd(OH)3-like species. Consequently, it may be reasonable to consider that the surface hydration on Nd2O3 proceeds through a process forming NdOOH or Nd(OH)3 layers. The TPD data for Nd2O3 are consistent with the fact that the decrease in water content is not appreciable in the temperature range up to 423 K (Figure 2), which implies that the strongly adsorbed molecular water is hardly discernible in the surface layer of Nd2O3, in contrast to the case of Y2O3.13 Figure 4 shows the NIR spectra in the evacuation process for the original sample. For the sample evacuated at 300 K, a strong 2νOH band is observed at 7050 cm-1 and no band was found below 6000 cm-1, indicating the absence of molecular water. The present NIR data clearly verify the foregoing statement that there is no molecular water on Nd2O3. Heat treatment at 373 K brings about a slight decrease in intensity of the band at 7050 cm-1, together with the appearance of a distinct shoulder at around 7140 cm-1. By heat treatment at 473 K, a further decrease in intensity of the 7050 cm-1 band is observed and the 7140 cm-1 band weakens. Corresponding to the desorption peak at 507 K in the TPD spectra, the band intensity at 7050 cm-1 is reduced greatly after heat treatment at 573 K. In this stage, an important point to note is that the broad band at around 7050 cm-1 has a distinctly lower wavenumber (6980 cm-1) component. Both bands are completely removed when the sample is treated at 673 K, keeping a weak band at 7140 cm-1. Treatment at 773 K brings about the loss of the band at 7140 cm-1. On the basis of the above observations, the species responsible for the 7140 cm-1 band is assumed to be an OH group strongly held on the surface that can interact with another OH group. The species giving desorption peaks at 507

Figure 4. NIR spectra of the original Nd2O3 sample evacuated at different temperatures (K): (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773.

and 609 K in the TPD curve correspond to the species giving the NIR bands at 7050 and 6980 cm-1, and they may be identified as the bulk hydroxide (Nd(OH)3) and NdOOH, respectively. On the other hand, the species giving the 7140 cm-1 band is assigned to the surface hydroxyl group. To get information on the hydration process, the NIR spectra were obtained after evacuation at 300 K for the samples that had been exposed to water vapor at 300 K and at different pressures or for different times, and the resulting spectra are shown in Figure 5. The 7140 cm-1 band, which is ascribable to the overtone of the stretching vibration of the surface OH group, is recovered at the lower pressure. The appearance of this band is strong evidence for the existence of surface hydroxyl groups. The band at 7050 cm-1 appears at a vapor pressure exceeding the threshold value at which the continuous increase of the adsorbed amount is observed (spectra 3-5). This band increases in intensity with increasing vapor pressure of water and exposure time, and simultaneously the lower wavenumber component (7000-6900 cm-1) increases in intensity (spectra 5-7). These findings also support the hydration process forming bulk hydroxide layers. Dielectric measurement is a useful technique for evaluating the molecular motion of adsorbed water. As described above, the rate of hydration toward bulk Nd2O3 is too fast to analyze a dielectric relaxation meaningfully, that is, as a function of coverage or temperature, especially near room temperature. Analysis of the relaxation measured at low temperatures is expected to provide significant information about the orientational polarization, such as the characteristic frequency and the activation energy, though the exact coverage is hard to obtain. The dielectric loss factor ′′(f) is plotted against the frequency, f, in Figure 6. Since ′′(f) has a peak in the neighborhood of the frequencies where dielectric permittivity ′(f) changes,25 (25) Bo¨ttcher, C. J. F. In Theory of Electric Polarization; Elsevier: Amsterdam, 1973.

6964

Langmuir, Vol. 16, No. 17, 2000

Hamano et al.

Figure 5. NIR spectra obtained in the processes of water adsorption on Nd2O3 pretreated at 873 K. The sample was equilibrated with water vapor at different relative pressures and for different times: (1) nearly zero pressure; (2) 0.01 for 0.5 h; (3) 0.1 for 0.5 h; (4) 0.1 for 1 h; (5) 0.1 for 8 h; (6) 1.0 for 4 h; (7) 1.0 for 12 h; (8) 1.0 for 24 h; (9) 1.0 for 36 h.

Figure 7. (a) Adsorption isotherm of water on Nd2O3 (b, 9), F-Nd2O3(5) (2, [), and F-Nd2O3(9) (O). Prior to the measurement the samples were evacuated at 298 K. (b) Adsorption isotherm of water on F-Nd2O3(9) up to the higher pressure. Table 1

samples F-Nd2O3(5) F-Nd2O3(9) F-Nd2O3(18) F-Nd2O3(61)

Figure 6. ′′-f curves of the Nd2O3-H2O system at a coverage of 2.5 and at different temperatures: (b) 298; (]) 273; (O) 226; (!) 193; (2) 169; (4) 158; ([) 145 K.

′′-f curves are shown in Figure 6. From this figure the following conclusions are drawn. First, two dominant types of relaxation are clearly seen. The first one, observed at near room temperature, can be ascribed to the Maxwell-Wagner type relaxation.13,16 Its discussion is difficult because bulk hydration takes place. Nevertheless, a brief discussion will be given later. The second type of relaxation observed at lower temperatures (e.g. at 169 K) and at 50 kHz is obviously ascribed to the orientational polarization of physisorbed water. The relaxation frequency observed is far lower than that observed in other systems examined so far. Second, another striking feature is the appearance of a shoulder at around 1 kHz and at 226 K. Such a shoulder has not been observed in other systems.13,16,26-28 Taking account of the data shown above, we tentatively assigned it to the (26) Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 282. (27) Morimoto, T.; Iwaki, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 943. (28) Kuwabara, R.; Iwaki, T.; Morimoto, T. Langmuir 1987, 3, 1059.

reacn no. of specific concn of aq time/ fluoride surf soln of -1 -1 -2 min ion/F nm area/m2 g-1 NH4F/mol L 0.0125 0.025 0.050 0.10

10 10 15 180

4.7 8.6 18.0 61.0

3.1 2.9 2.8 4.8

relaxation of the OH group that might be expected to exist on the surface; the surface OH group is situated such that the interaction among OH groups is moderate so as to follow the alternating electric field. It is obvious from the dielectric data that the OH groups and physisorbed water exist on the Nd2O3 surface. The fact that relaxation due to the physisorbed water molecules is observed at 50 kHz and at 169 K implies that they appreciably resist the applied field, indicating their strong interaction with the Nd2O3 surface. Surface Fluoridation for Protecting the Nd2O3 Sample from Surface Hydration. Figure 7a shows the adsorption isotherms of water at 298 K on the fluoridated Nd2O3 samples with different extents of fluoridation, together with the adsorption isotherm on the original Nd2O3 sample. The extent of fluoridation was changed from less than monolayer to more than seven layers, as shown in Table 1, where the fluoridated sample is expressed as F-Nd2O3(x), x being the number of fluoride ions nm-2. The number of O2- on the Nd2O3 surface is estimated to be 7.8 ions nm-2 by taking account of the crystallographic data. Surface fluoridation of Nd2O3 within monolayer coverage, e.g. the sample F-Nd2O3(5), retards surface hydration so that it proceeds quite slowly into the

Adsorption of Water on Nd2O3

Figure 8. Dielectric data (′′-f curves) of the F-Nd2O3(9)H2O system at a coverage of 2.1 and at various temperatures (K): (b) 298; (]) 273; (O) 226; (!) 193; (2) 169; (4) 158; ([) 145 K.

Langmuir, Vol. 16, No. 17, 2000 6965

Figure 10. Variation of ′′ with frequency (f) for the F-Nd2O3(9)-H2O system at a coverage of 1.5 and at different temperatures: (!) 193; (2) 169; (4) 158; ([) 145 K.

Figure 9. Coverage dependence of the arc length of ColeCole plots for the F-Nd2O3(9)-H2O system measured at 158 K. Coverages are 0.5 ((), 1.0 (2), 2.1 (9), and 2.8 (b).

bulk. The most striking feature observed for the fluoridated samples, e.g. F-Nd2O3(9), is that the adsorption isotherm of water is typical of type II and that adsorption equilibrium is established within 15 min in the whole pressure region examined. This is different from the case of the original Nd2O3 sample in which bulk hydration takes place. Figure 7b shows the adsorption isotherm of water on the F-Nd2O3(9) sample up to the higher equilibrium pressure. From these figures, it is evident that bulk hydration proceeds with difficulty after surface fluoridation of Nd2O3. Surface protection, of course, is effective for the samples covered with F- ions in more than a monolayer. These facts indicate that bulk hydration can be hindered by covering the surface of Nd2O3 with F- and that monolayer coverage suffices for adequate protection from bulk hydration. These findings were also verified by NIR spectra of the F-Nd2O3(9) sample; this sample gave no absorption due to 2νOH even after exposing the sample to H2O vapor. Dielectric studies allow one to determine the state of adsorbed water. Curves showing the temperature variation of ′′-f at a coverage of 2.1 are shown in Figure 8 for the F-Nd2O3(9)-H2O system. Two relaxations can be observed in the frequency region examined: a large relaxation at around 298 K and in the lower frequency side and a small relaxation in the lower temperature and higher frequency regions. The coverage dependence of Cole-Cole plots for the relaxation observed in the lower temperature region is shown in Figure 9. The arc length for the latter relaxation changes with coverage, though the shape of the Cole-Cole plots for the former relaxation scarcely changes (not shown here). According to the analysis by Maxwell-Wagner29 and by Onsager,30 the former relaxation can be ascribed to the interfacial polarization caused by a heterogeneity of the system and the latter to the orientational polarization of adsorbed (29) Van Beek, L. K. H. Prog. Dielectr. 1967, 7, 69. (30) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486.

Figure 11. Variation of ′′ with frequency (f) for various systems measured at the coverage of ca. 1.5 and at 158 K: (b) Nd2O3; (O) F-Nd2O3(9); (9) Y2O3; (2) SrF2; (0) SiO2.

water molecules. This consideration is consistent with our previous results concerning the Y2O3 system.13 The variation of the relaxation of the latter type with temperature (Figure 10) gives an activation energy, which is estimated to be ca. 45 kJ mol-1. This value is larger than that for other systems having no pores, which indicates a strong interaction of physisorbed water with the surface. Such an interpretation is also supported by the following data. The relaxation of the latter type at 158 K for various systems is depicted in Figure 11. The water molecules in the present system are less mobile than those adsorbed on SrF2 or Y2O3, though they are more mobile than those confined in the small pores. It seems that water on Nd2O3 is more immobile, compared with that on F-Nd2O3. These tendencies are well explained in terms of the strong interaction of water with the Nd2O3 surface. It should be noted that relaxation due to the surface hydroxyls could not be found for the F-Nd2O3 system (Figure 8), as distinct from the result shown in Figure 6 for the Nd2O3 system. Figure 12 shows the ′′-f and G (conductivity)-f curves obtained at 298 K and at an apparent coverage of 1.5 for the Nd2O3 and F-Nd2O3 systems. In this figure, the data of the Y2O3 system are also included for comparison. The frequency that gives a maximum ′′ corresponds to the frequency giving a maximum rate in the variation of G with f. This fact clearly indicates that the observed relaxation is caused by a surface conduction of MaxwellWagner type as described above.29 It is interesting to note that the values of conductivity are larger for the Nd2O3H2O system, compared with the F-Nd2O3-H2O and Y2O3H2O systems. Taking into account the fact that the surface conductivity for all systems is too small to obtain a

6966

Langmuir, Vol. 16, No. 17, 2000

Figure 12. ′′-f (a) and G-f curves (b) for various systems: (b) Nd2O3; (O) F-Nd2O3(9); (9) Y2O3.

meaningful value at zero coverage, it is apparent that the surface conductivity in the Nd2O3 system changes drastically with water adsorption. By assuming that the hydroxide layer is formed during water adsorption on the Nd2O3 surface, such a phenomenon can be explained satisfactorily. The surface hydroxide layer may accelerate proton conduction, in combination with the conduction due to the adsorbed water. It is reasonable to suppose that the surface conduction of the system including the hydroxide layer is large compared with the system consisting of the adsorbed water alone, because of the smaller portion of current path for the system containing only the physisorbed layer. Therefore, the surface hydration process accompanying formation of a surface hydroxide layer is supported from the concept of the increase in surface conductivity. The data obtained so far are interpreted by considering that surface fluoridation protects the surface against hydration. X-ray photoelectron spectroscopy (XPS) is one of the most widely used techniques for the characterization of the electronic structure of inorganic substances, and it was applied to the present samples of Nd2O3, F-Nd2O3, and NdF3. For the Nd2O3 sample, the XPS spectrum of O1s showed a broad band at around 530 eV. The band observed at 528.9 eV is assigned to the O1s band for the O2- species of Nd2O3. This value can be compared with those of MgO and Y2O3. The O1s binding energies of these compounds increase in the order Nd2O3 (528.9 eV), Y2O3 (529.3 eV),31 and MgO (530.9 eV).32 The observed tendency for hydration is well explained by assuming that the electron density on the oxygen ion in metal oxides is responsible for the reactivity of the solids; Nd2O3 has the most strongly basic nature. Nd2O3 first interacts strongly with water molecules in the vapor phase even at room temperature and then the adsorbed water takes part in the formation of surface and bulk hydroxide layers (31) Barr, T. L. J. Phys. Chem. 1978, 82, 1801. (32) Barr, T. L.; Brundle, C. R. Phys. Rev. 1992, B46, 9199.

Hamano et al.

progressively, as in the cases of alkaline and alkalineearth oxides. The electronic state of O1s may account for the reason the reactivity of Nd2O3 toward water is large. It can also be considered that the core-level binding energies of fluoride ion are correlated with the amount of charge transfer from the fluoride ion to the cation. The electronic charge around the fluoride ion is reduced by the bonding overlap with the neighboring cations, which would result in a more attractive potential at the fluorine nucleus and in a corresponding increase of the core-level binding energy of fluorine (F1s). In the present case, a CaF2 sample, whose F1s value is 684.8 eV,33 was taken as the reference sample. For the fluoride ion on F-Nd2O3 and NdF3, the binding energy of F1s is shifted to the higher energies of 685.9 and 687.0 eV, respectively. These shifts can be interpreted in terms of increasing covalency of the fluorides, which is due to the reduction of the electron density around the fluoride ion, as is suspected from the same discussion performed on the alkaline-earth oxides.34 This is one of the reasons it resists hydration when the surface is fluoridated. Another possibility for the reduction of reactivity of F-Nd2O3 can be found in the viewpoint of the structural feature, that is, the lowering of the surface electric field due to the presence of monovalent charge on the surface, instead of divalent ion, O2-. Both effects may additionally contribute to protecting the surface from hydration. A detailed study using a quantum chemical method will be performed soon. Summary The surface characteristics of Nd2O3 were investigated and the results are summarized below. (1) The characteristic features of the original Nd2O3 sample are as follows. In the adsorption isotherm of water on Nd2O3, a break appears at a relative pressure of approximately 0.028 and the water content of this sample is appreciably larger than that of typical metal oxides. Two major desorption peaks due to the adsorbed water were observed at 507 and 609 K. It can be concluded that the Nd2O3 surface readily reacts with water to form hydroxide layers at room temperature, as is supported by the NIR and dielectric data. For the Nd2O3 sample, the increase of adsorbed amount was faster than the increase in the case of Y2O3, which suggests that penetration of water into the bulk, i.e., bulk hydration, is easier in the Nd2O3 sample than in the Y2O3 sample. If one takes account of the binding energy of O1s, the reactivity of the Nd2O3 sample is explained by the concept of basicity due to the large ionic radius of the rare-earth metal ion. As a general trend, the rare-earth oxides can adsorb water strongly and react with it to produce surface and bulk hydroxide layers. (2) Surface fluoridation of the Nd2O3 sample was performed. It is clearly observed that surface fluoridation prevents hydration of the Nd2O3 sample. The dielectric measurement showed that fluoridation of the sample depresses the surface conduction and bulk hydration. The dielectric relaxation due to the orientational polarization of physisorbed water at 158 K on Nd2O3, as well as on F-Nd2O3, were observed at several kilohertz, which was a lower frequency region than that observed for other nonporous solid surfaces. This fact was interpreted in terms of the restricted motion of the adsorbed water molecules owing to a strong interaction with the surface. (33) Wu, Y.; Mayer, J. T.; Garfunkel, E.; Madey, T. E. Langmuir 1994, 10, 1482. (34) Pacchioni, G.; Bagus, P. Phys. Rev. 1994, B50, 2576.

Adsorption of Water on Nd2O3

Acknowledgment. This research was supported in part by grants from Nippon Sheet Glass Foundation for Materials Science and Engineering and from Hitachi Metal Ltd. Y. K. acknowledges the generosity of these organiza-

Langmuir, Vol. 16, No. 17, 2000 6967

tions. The authors thank Professor M. Ohshima for helpful suggestions related to the technique for the determination of fluoride ion. LA0001169