Hydration Process of Rare-Earth Sesquioxides Having Different

oxide (Sm2O3), and yttrium oxide (Y2O3), were different from each other. The difference in .... Yttrium hydroxide, Y(OH)3, thus prepared was evacuated...
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Langmuir 2003, 19, 9201-9209

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Hydration Process of Rare-Earth Sesquioxides Having Different Crystal Structures Mahiko Nagao,*,† Hideaki Hamano,† Koji Hirata,† Ryotaro Kumashiro,† and Yasushige Kuroda‡ Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530, Japan and Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan Received December 9, 2002. In Final Form: June 6, 2003 The reactivity of rare-earth sesquioxides, mainly neodymium oxide, in water vapor and their hydration process have been investigated by measuring the adsorbed amounts, infrared spectra, and water contents. The adsorption of water vapor on neodymium oxides, A-type and C-type Nd2O3, led to the formation of a neodymium hydroxide, Nd(OH)3. The variation of adsorbed amounts (in weight) with exposure time to water vapor suggested that the hydration process is different depending upon the crystal structure of neodymium sesquioxides. Combining with the data of IR spectra and water contents, it has been revealed that the hydration of A-type Nd2O3 takes place in one step [into Nd(OH)3] after an induction period of about 30 min, while that of C-type Nd2O3 proceeds in two steps [into Nd(OH)3 via NdOOH]. The adsorption of water vapor on other rare-earth sesquioxides was also examined; the hydration process of A-type lanthanum oxide (La2O3) was similar to that of A-type Nd2O3, while those of C-type sesquioxides, Nd2O3, samarium oxide (Sm2O3), and yttrium oxide (Y2O3), were different from each other. The difference in the rates of hydration for these oxides can be correlated with a basic nature of oxide. Therefore, the rate of hydration is eventually governed by the ionic radius of rare-earth metal ion because the basicity of oxide depends on the ionic radius of metal ion. To make the Nd2O3 surface resistant to water, the surface modification or coating was also tried by adsorbing a metal-alkoxide vapor. As a result, it was revealed that the surfacecoated neodymium oxide (i.e., titania-coated Nd2O3) has a water-resistant property.

Introduction The compounds involving rare-earth elements are classified into two groups; one group is based on the property of immanent 4f electrons and the other group is based on such factors as ionic radius, charge, chemical nature, and so forth. Magnetic materials such as Sm2Co17, Nd2Fe14B, luminous materials, for example, the Nd3+-YAG laser, and fluorescent materials such as Y2SiO5:Ce3+ belong to the former group. Solid electrolytes such as PSZ utilizing its ionic radius and stable charge (+3), oxide superconductors such as YBa2Cu3O7-x, Nd2-xCexCuO4-y, catalysts such as La2O3, Sm2O3 making use of its chemical property, and hydrogen-absorbing alloys such as LaNi5 are included in the latter group. However, many of them, especially Nd2Fe14B and YBa2Cu3O7-x, have a very high reactivity to water vapor and carbon dioxide, giving rise to serious problems in their practical use. A similar situation is true of the rare-earth oxides that are often employed as catalysts, and hence, it is very useful to evaluate the reactivity of these oxides to water vapor for understanding their catalytic or corrosive behavior. It is generally accepted that the oxides involving light-rareearth elements (from lanthanum to europium) easily react with water vapor to form a hydroxide at room temperature, but for the oxides involving heavy-rare-earth elements (from gadolinium to lutetium) it is not clear whether the reaction with water vapor (i.e., hydration) takes place.1-3 * To whom correspondence should be addressed. E-mail: [email protected]. † Research Laboratory for Surface Science. ‡ Department of Fundamental Material Science. (1) Touret, D.; Queyroux, F. Rev. Chim. Miner. 1972, 9, 883. (2) Alvero, R.; Odriozola, J. A.; Trillo, J. M. J. Mater. Sci. 1985, 20, 1828.

In a few attempts to clarify this, the majority was concerned with a comparison of reactivity to water vapor in terms of difference in the rare-earth elements.1,4,5 It is known that for titanium dioxide and ferric oxide which have a polymorphism the adsorption properties are different depending on their crystal structures.6-9 A similar feature is also expected for rare-earth oxides having different crystal structures. However, only a few attempts have so far been made on this subject.10 Rare-earth sesquioxides, in general, have three different types of crystal structures (i.e., A-type, B-type, and C-type) depending on the ionic radius and calcination temperature.11 The A-type hexagonal sesquioxides are of space group P32/m with one formula per unit cell, and the metal atoms are in a seven-coordination with four oxygen atoms closer than the other three. The B-type sesquioxides, which are monoclinic distortion of the A-form, have a space group C2/m with six formulas per unit cell, and the metal atoms are in a seven- and a six-coordination. The C-type structure is of a cubic bixbyite type with a space group, Ia3, containing 32 metal atoms and 48 oxygen atoms per unit (3) Alvero, R.; Bernal, A.; Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. J. Less-Common. Met. 1985, 110, 425. (4) Bernal, S.; Botana, J. A.; Gadcia, R.; Rodriguez-Izquierdo, J. M. React. Solids 1987, 4, 623. (5) Alvero, R.; Bernal, A.; Carrizosa, I.; Odriozola, J. A.; Trillo, J. M. J. Mater. Sci. 1987, 25, 1517. (6) Morimoto, T.; Nagao, M.; Omori, T. Bull. Chem. Soc. Jpn. 1969, 42, 439. (7) Siriwardane, R. V.; Wightman, J. P. J. Colloid Interface Sci. 1983, 94, 502. (8) Morimoto, T.; Nagao, M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243. (9) Matsuda, T.; Ueno, N.; Nagao, M. Netsu-sokutei 1992, 19, 57. (10) Bernal, S.; Botana, J. A.; Gadcia, R.; Rodriguez-Izquierdo, J. M. J. Chem. Soc., Dalton Trans. 1988, 1765. (11) Warshaw, I.; Roy, R. J. Phys. Chem. 1961, 65, 2048.

10.1021/la020954y CCC: $25.00 © 2003 American Chemical Society Published on Web 09/24/2003

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cell related to double-edge fluorite structure with onefourth of the oxygen sites vacant and regularly ordered. The metal atoms are in a six-coordination. Neodymium oxide, Nd2O3, has two types of crystal structures, A-type and C-type, the former being the most common. In the present work, we have examined the reactivity to water vapor and the hydration process for Nd2O3 having different crystal structures and compared the results with those obtained for other rare-earth oxides. Furthermore, by utilizing the results, we have tried to modify the surface of Nd2O3 bearing a durability against water in mind. Experimental Section Samples. The water-adsorption properties of rare-earth oxides are predicted to vary by their history of thermal treatment. Since the A-type and C-type Nd2O3 can be obtained by the decomposition of Nd(OH)3, all the samples were first prepared in the form of Nd(OH)3 and then subjected to decomposition into oxides. Commercially available Nd2O3 (Kanto Chemicals) was used as a starting material, and it was first evacuated at 1073 K for 3 h to remove surface carbonates that might be formed by the reaction with carbon dioxide in the atmosphere. After cooling to room temperature, the oxide sample was exposed to saturated water vapor at 298 K for 3 days to form Nd(OH)3.12 Final samples of A-type and C-type Nd2O3 were obtained by the thermal decomposition of Nd(OH)3 and subsequent calcination at 1073 and 873 K, respectively. The reference samples for IR measurement, Nd(OH)3 and NdOOH, were prepared by the hydrothermal treatment of Nd2O3 and by the moderate heat treatment of Nd(OH)3, respectively. Lanthanum oxide (La2O3) was obtained by the thermal decomposition of La(NO3)3‚6H2O (from the Nacalai Tesquc) at 1073 K for 5 h in the atmosphere. As in Nd2O3, La2O3 was further evacuated at 1073 K for 3 h to remove carbonates, and then it was exposed to saturated water vapor at 298 K for 1 day to form La(OH)3. Subsequent evacuation of La(OH)3 at 873 K gave a final sample of A-type La2O3. Samarium oxide (Sm2O3) (from the Kanto Chemicals) was treated in the same manner as La2O3 with the exception of exposure period of 20 days. The final sample of Sm2O3 was C-type oxide. Yttrium oxide (Y2O3) was obtained by the thermal decomposition of Y2(C2O4)3‚9H2O (from the Mitsui Metals) at 1073 K in the air. This oxide was then subjected to hydrothermal treatment at 558 K for 16 h in the autoclave. Yttrium hydroxide, Y(OH)3, thus prepared was evacuated at 873 K to produce a final sample of Y2O3 (C-type). XRD Measurement. The powder X-ray diffraction (XRD) patterns were obtained at room temperature in the range 10° e 2θ e 80° by using a X-ray diffractometer (Rigaku RAD-1C) with a carbon monochromator (CuKR). All the samples used in the present study were identified by this XRD measurement. Measurement of Adsorbed Amounts. The oxide sample was exposed to water vapor at 298 K, monitoring the weight of sample. The weight increase due to water adsorption was measured gravimetrically by using a spring balance made of quartz. Infrared Spectra Measurement. Infrared spectra of the samples were measured by using an FTIR spectrophotometer (Mattson 3020) in the transmission mode with a resolution of 4 cm-1. A self-supporting disk of oxide sample was placed in an in-situ cell that can be capable of pretreatment at high temperature (873 K) and of adsorption-desorption (evacuation) operation. Measurement of Water Content. After evacuating the sample at 298 K for 3 h, the water content was determined by the successive-ignition-loss method.13 The water content of the sample evacuated at 873 K was assumed to be zero. Measurements of Adsorption Isotherm and Specific Surface Area. The adsorption isotherm of water vapor was determined volumetrically at 298 K. The specific surface area of (12) (a) Kuroda, Y.; Hamano, H.; Mori, T.; Yoshikawa, Y.; Nagao, M. Langmuir 2000, 16, 6937. (b) Hamano, H.; Kuroda, Y.; Yoshikawa, Y.; Nagao, M. Langmuir 2000, 16, 6961. (13) Morimoto, T.; Nagao, M.; Tokuda, F. Bull. Chem. Soc. Jpn. 1968, 41, 1533.

Figure 1. XRD patterns for the products obtained by decomposition of Nd(OH)3 in vacuo at different temperatures: (1) 873; (2) 973; (3) 1073; (4) 1173; (5) 1273 K. Indices shown are from 1998 JCPDS-International Centre for Diffraction Data. the sample was obtained by the BET method using dinitrogen adsorption data at 77 K and a cross-sectional area of 0.162 nm2. The equilibrium pressure was monitored by an MKS Baratron transducer of type 390B (for H2O vapor) or of type 122A (for N2 gas). The surface areas were 3.0 m2 g-1 for a starting Nd2O3, and 8.1 and 10.4 m2 g-1 for A-type and C-type Nd2O3, respectively. Surface Coating. The oxide sample (A-type Nd2O3) was previously calcined at 1123 K for 3 h in an atmosphere of argon and then cooled to the room temperature. The sample was put into contact with a vapor of tetraethyl orthosilicate [Si(OC2H5)4] or of titanium(IV) ethoxide [Ti(OC2H5)4], and then by decomposing the metal alkoxide, the surface-coated Nd2O3 sample was obtained: silica-coated and titania-coated Nd2O3 samples. The surface area of the sample was decreased to 3.5 m2 g-1 by these treatments. XPS Measurements. X-ray photoelectron spectroscopy (XPS) measurements were performed using an advanced XPS imaging spectrometer AXIS-HS (Shimadzu) with a Mg-KR source, to analyze the coated surface. An electron gun was used to neutralize a surface charging occurring for insulating samples. The peak of C(1s) at 285.0 eV, because of a carbonaceous residual of the sample, was used as internal calibration of the energy scale. The samples were degassed at 300 K under a reduced pressure of 1.3 × 10-7 Pa.

Results and Discussion The Nd2O3 samples of both A and C types can be prepared through the decomposition of neodymium hydroxide. The C-type Nd2O3 is stable in the lower temperature region and the A-type Nd2O3 is of hightemperature type. The transition temperature at which the C-type transforms to the A-type is found differently in the literature.11,14,15 Moreover, the effect of heat treatment of the sample has not been understood satisfactorily yet. First, we elucidated the transition temperature under the reduced pressure by heat treating the Nd(OH)3 samples at different temperatures, and the crystal structures of the resulting oxides were identified by XRD, as shown in Figure 1. The Nd2O3 sample of C-type can be obtained by the heat treatment at 873 K and that of A-type can be formed in the single phase at above 1073 K. The Nd2O3 samples thus prepared were used in the present study. The Nd2O3 sample was exposed to a saturated water vapor of 3.2 kPa at room temperature, during which the weight-increase rate was monitored. Figure 2 shows the (14) Eyring, L. Solid State Chemistry; Marcel Dekker: New York, 1974; p 565. (15) Shafer, M. W.; Roy, R. J. Am. Ceram. Soc. 1959, 42, 563.

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Figure 2. Dependence of weight-increase rate on exposure time to water vapor (saturated vapor pressure at 298 K) for the Nd2O3 samples (A- and C-type): (O) A-type Nd2O3; (b) C-type Nd2O3.

Figure 4. IR spectra of Nd2O3 (A-type) sample. The sample was exposed to saturated water vapor at 298 K for 90 min and successively evacuated at different temperatures: (1) 298; (2) 373; (3) 423; (4) 473; (5) 523 K.

Figure 3. IR spectra of Nd2O3 (A-type), Nd(OH)3, and NdOOH. (a) Nd2O3 under various conditions: (1) evacuated at 873 K; (2) exposed to saturated water vapor for 30 min and successively evacuated at 298 K; exposure time was increased to (3) 60 and (4) 90 min. (b) Nd(OH)3 and NdOOH.

weight-increase rate as a function of exposure time, where the ratio of the increased weight by the exposure to the sample weight before exposure is expressed in percentage. Such a weight-increase rate may be regarded as a measure of progress of hydration. When the C-type Nd2O3 sample was exposed to the saturated water vapor, the sample weight increased immediately and then gradually and almost linearly with time. This finding leads to the assumption that the hydration of the C-type Nd2O3 sample takes place in two stages, that is, rapid stage and slow stage. On the other hand, for the A-type Nd2O3 sample the weight increase is very slow in the initial stage (for about 30 min) and then shows an abrupt rise. This implies that the hydration occurs in one step after an induction period of about 30 min. Such a different manner of hydration between type-A and type-C suggests that the hydration process is different depending upon the crystal structure of oxide. The hydration process of Nd2O3 samples was examined in further detail by the measurement of IR spectra. Figure 3 represents the IR spectra of Nd2O3 (A-type) sample, together with those for reference samples of Nd(OH)3 and NdOOH. After evacuation at 873 K, the Nd2O3 sample was exposed to water vapor of 2.7 kPa at 298 K for 30 min

and then evacuated again at the same temperature; this exposure time was increased to 60 and 90 min. The resulting IR spectra exhibit a broad absorption band in the range 3300-3600 cm-1, without giving any absorption bands in the bending-vibration region for water molecules (inserted in Figure 3a). Therefore, this broad band is obviously assigned to the OH stretching vibration in the surface hydroxyl groups that were produced by the dissociative adsorption of water molecules. This result is different from the case of a similar rare-earth oxide, Y2O3, where the water molecules are taken up in the form of a molecule.12 With increasing exposure time, a distinctive absorption band appears at 3608 cm-1, which can be assigned to the OH stretching vibration in Nd(OH)316 rather than in NdOOH (Figure 3b). This fact indicates a production of Nd(OH)3 by the hydration of Nd2O3. To confirm the validity of such assignment, the IR spectra were examined for the A-type Nd2O3 sample evacuated at different temperatures after exposure to the saturated water vapor (Figure 4). The absorption band at 3608 cm-1 showed practically no change after evacuation at the temperatures up to 373 K. However, this band reduced its intensity markedly by the evacuation at 423 K and finally disappeared after treatment at 473 K. Instead, another band appeared at 3595 cm-1 after evacuating the sample at 423 K, and it disappeared eventually at 523 K. The latter absorption band is well correspondent to the OH stretching vibration in NdOOH (Figure 3b). From these observations, the decomposition of Nd(OH)3 is assumed to occur in the following two stages: -2H2O

-H2O

2Nd(OH)3 98 2NdOOH 98 Nd2O3

(I)

Figure 5 shows the water content of Nd(OH)3 at the different evacuation temperatures. The water content decreases markedly in the temperature ranges 373-473 K and 473-623 K and the decreasing amount in the former (16) Glushkova, V. B.; Suglobov, D. N. Zh. Strukt. Khim. 1965, 6, 837.

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Figure 5. Water contents of (0) Nd(OH)3 and (b) Nd2O3 (Ctype).

range is twice as large as that in the latter range. Thus, it is reasonable to assume that Nd(OH)3 decomposes into NdOOH in the temperature range 373-473 K and subsequently into Nd2O3 in the temperature range 473623 K. The correspondence of the former temperature range, where the intensity of IR band at 3608 cm-1 is reduced, to the decomposition temperature of Nd(OH)3 indicates that the absorption band at 3608 cm-1 is obviously due to Nd(OH)3. Furthermore, the appearance of a band at 3595 cm-1, accompanying a decrease in the band intensity at 3608 cm-1, can be ascribed to the OH stretching vibration in NdOOH (Figure 4). The 3595 cm-1 band disappeared completely by the evacuation at 523 K, being close to the decomposition temperature of NdOOH. On the other hand, taking account of the weight change for C-type Nd2O3 (Figure 2), the hydration of this sample is assumed to take place in two stages. To clarify each hydration stage, the IR spectra were taken for the samples with different hydration levels, as shown in Figure 6. By exposing the sample to water vapor for 5 min and subsequent evacuation at 298 K, a new absorption band appeared at 3593 cm-1, corresponding to a termination of hydration in the first stage. This band is very close to the OH band for NdOOH (3594 cm-1), which suggests that NdOOH is formed in the first hydration stage for Nd2O3 (C-type). The weight-increase rate is about 6% at the end of the first hydration stage (Figure 2). A similar weight increase (5.4%) was estimated by assuming that Nd2O3 is perfectly converted into NdOOH through the hydration. These results also support the interpretation that NdOOH is formed in the initial hydration stage for C-type Nd2O3. Furthermore, it is found from the difference spectrum (Figure 6, spectrum 4) that the intensity of 3606 cm-1 band increases with increasing exposure time up to 3.5 h, corresponding to the second stage of hydration. Since this band is ascribed to the OH stretching vibration in Nd(OH)3, the second hydration process can be regarded as a process of Nd(OH)3 formation. To verify the production of NdOOH in the initial hydration stage, the water content was determined for the C-type Nd2O3 sample that had been exposed to the saturated water vapor for 1 min at 298 K. The result shown in Figure 5, in comparison with the data for Nd(OH)3, indicates that such a sample decomposes by one step in the temperature range 423-523 K. Furthermore, the fact that this temperature range is higher compared with the temperature range (373-473 K) in which Nd(OH)3

Figure 6. IR spectra of Nd2O3 (C-type) sample: (1) evacuated at 873 K; (2) exposed to water vapor for 5 min and successively evacuated at 298 K; (3) exposed to water vapor for 3.5 h and successively evacuated at 298 K; (4) difference spectrum between spectra 2 and 3.

decomposes into NdOOH suggests that the present decomposition is from NdOOH into Nd2O3. Thus, it is confirmed that NdOOH is produced preferentially by exposing Nd2O3 (C-type) to the saturated water vapor for only 1 min at 298 K. Moreover, the XRD analysis revealed that NdOOH produced is amorphous. This seems to be a reason why the decomposition temperature of such a sample is slightly lower than that of NdOOH prepared by the decomposition of Nd(OH)3. These results lead us to conclude that for the samples of A-type and C-type Nd2O3 placed in the saturated water vapor at 298 K, Nd(OH)3 is formed via different hydration steps depending on the crystal structure of Nd2O3: 3H2O

Nd2O3 (A-type) 98 2Nd(OH)3 H2O

2H2O

Nd2O3 (C-type) 98 2NdOOH 98 2Nd(OH)3

(II) (III)

For C-type Nd2O3, two stages of weight increase in the water-vapor adsorption (Figure 2) correspond to each hydration step in reaction III. The induction period observed in the hydration process of A-type Nd2O3 is particularly interesting in the elucidation of its hydration mechanism. The analysis of hydration process for A-type Nd2O3 will be given later in some detail. Use of heavy-water (D2O) is helpful in examining the IR spectra in more detail. Figure 7 shows the feature of H-D or D-H exchange in the hydration process for Nd2O3 (A-type). The sample was first evacuated at 873 K (spectrum 1), and then was exposed to D2O vapor at 2.7 kPa for 1 h and successively evacuated at 298 K (spectrum 2). A broad band observed in the range 2400-2700 cm-1 is obviously due to the OD stretching vibration in the surface species formed in the induction period (Figure 3). Further increase of exposure time to D2O vapor brought about a sharp band at 2661 cm-1 (spectrum 4), being assignable to the OD stretching vibration in Nd(OD)3.16 After that, the D2O vapor was replaced by water vapor,

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Figure 8. Comparison of the hydration process between Nd2O3 (A-type) sample (O) and Avrami-Erofeev’s model for solid-phase reaction (solid line).

in Figure 8. The result can be expressed successfully by the Avrami-Erofeev equation:

[- ln(1 - R)]1/n ) kt Figure 7. IR spectra of Nd2O3 (A-type) sample: (1) evacuated at 873 K; (2) exposed to saturated D2O vapor for 1 h and successively evacuated at 298 K; exposure time was increased to (3) 2 h and (4) 4 h; (5) exposed to saturated H2O vapor for 5 min and successively evacuated at 298 K.

and the sample was kept for 5 min, followed by evacuation at 298 K. By such treatments a new band appeared at 3606 cm-1, accompanying a shift of broad band from the range 2400-2700 cm-1 to 3300-3600 cm-1 (spectrum 5). This band shift is obviously due to the D-H exchange in some precursor formed during the induction period. Furthermore, from the consideration that in the D-H exchange the hydrogen (H) donor is water vapor and that the exchange time is only 5 min, this exchange reaction seems to take place easily. Both hydrogen (H) and deuterium (D) in the precursor may be situated in the exchangeable state to facilitate an easy supply of H (and D) to the oxide and thereby an easy hydration. Thus, it can be concluded that the precursor formed in the induction period promotes a hydration reaction. Taking into consideration that the shape of curve representing the weight change of Nd2O3 (A-type) sample during the hydration is similar to that of reaction curve for the solid-phase reaction, we tried to elucidate the hydration mechanism for the present Nd2O3-H2O system by applying some typical model equations for the solidphase reaction: Jander equation (diffusion-controlled contracting reaction type),17 Prout-Tompkins equation (autocatalytic reaction type),18 and Avrami-Erofeev equation (nucleus-growth type).19,20 As a result, the experimental data could not be expressed satisfactorily by any equations. The reason for this is considered as follows; a large quantity of the heat generated by the hydration of Nd2O3 promoted a further hydration reaction, giving rise to an obstruction in analyzing the hydration phenomenon. Therefore, the vapor pressure of water was lowered to 610 Pa so as to reduce such heat effects, and the weight change in the hydration process was monitored, as shown (17) Jander, W. Z. Anorg. Chem. 1927, 163, 1. (18) Prout, E. G.; Tompkins, F. C. Trans. Faraday Soc. 1944, 40, 488. (19) (a) Avrami, M. J. Chem. Phys. 1939, 7, 1103. (b) Avrami, M. J. Chem. Phys. 1940, 8, 212. (c) Avrami, M. J. Chem. Phys. 1941, 9, 177. (20) Erofeev, B. V. Dokl. Akad. Nauk. SSSR 1946, 52, 511.

(1)

where R is the fraction of reaction, t is the reaction time, and n and k are constants. Taking account of this equation expressing a nucleus-growth-type reaction, it is assumed that the nucleation of Nd(OH)3 takes place in the initial induction period and then the nucleus growth becomes more prominent with increasing time. Moreover, n in eq 1 is named Avrami exponent that is related to the shape and state of nucleus; the values of 4-3, 3-2, and 2-1 correspond to the three-dimensional (spherical) nucleus, two-dimensional (circular) nucleus, and one-dimensional (rodlike) nucleus, respectively.19,20 The value of n obtained by analyzing the present weight-increase-rate curve is 2.57 (k ) 2.51 × 10-3), which implies that the nucleus of Nd(OH)3 formed in the induction period grows twodimensionally. Here, from the crystallographic point of view, it is meaningful to examine the structural change during the hydration process of Nd2O3 to Nd(OH)3. Figure 9 represents the crystal structures of Nd2O3 (A-type) and Nd(OH)3 that are projected on the (001) plane. These crystal structures are both hexagonal ones, and there exist two Nd3+ ions in the unit cell. The Nd3+ ions are situated at almost the same positions: (1/3, 2/3, 0.235; 2/3, 1/3, 0.765) in Nd2O3 (A-type) and (1/3, 2/3, 1/4; 2/ 3, 1/3, 3/4) in Nd(OH)3,21 which implies a resemblance of the crystal structures for both compounds. Assuming that the crystal structure develops two-dimensionally throughout the transformation from Nd2O3 to Nd(OH)3, the hydration process of Nd2O3 (A-type) can be shown schematically in Figure 10; the Nd(OH)3 crystal grows two-dimensionally. Here, the oxide ions and hydroxide ions are assumed to be supplied from the array on the (001) plane. Two types of oxide ions, that is, six-coordinated and four-coordinated ions, exist on the (001) plane of A-type Nd2O3 and the former oxide ions are relatively mobile.22,23 Although this figure is drawn on the basis of some assumption, the hydration process from Nd2O3 (A-type) to Nd(OH)3 can be depicted satisfactorily, and thus the following two descriptions are possible: (a) crystallographically, the (21) Galasso, F. S. Structure and Properties of Inorganic Solids; Pergamon Press: Oxford, 1970. (22) Louis, C.; Chang, T. L.; Kermarec, M.; Van, T. L.; Tatibouet, J. M.; Che, M. Colloids Surf., A 1993, 72, 217. (23) Vanbaelinghem, F.; Pelloux, A.; Deportes, C. J. Appl. Electrochem. 1976, 6, 67.

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Figure 9. Crystal structures projected on the (001) plane of (a) Nd2O3 (A-type) and (b) Nd(OH)3. A large grayish circle and a black circle represent Nd3+ and O2- ions, respectively, and a small circle represents a H atom.

structure of Nd2O3 (A-type) is very similar to that of Nd(OH)3, and (b) the hydration process of Nd2O3 (A-type) can be expressed by the Avrami-Erofeev’s equation, as described above. In conclusion, the presented results suggest that the hydration of Nd2O3 progresses through the two-dimensional crystal growth of hydroxide. On the other hand, the crystal structure of C-type Nd2O3 is very complicated, and its resemblance to the crystal structure of NdOOH or Nd(OH)3 could not be recognized, which is different from the A-type Nd2O3. Considering the fact that Nd2O3 (C-type) can be produced by the thermal decomposition of NdOOH, it may be inferred that the structure of NdOOH is close to that of Nd2O3 (C-type). This seems to be one of the reasons for the production of Nd(OH)3 from Nd2O3 (C-type) via NdOOH. The effect of crystal structure on the hydration properties was examined from the standpoint of the rate of hydration. An easiness of hydration for oxides, in general, has a correspondence to the basicity of oxides.12 This is due to the supposition that if the oxide has a basic nature the OH-species produced by the chemisorption of water can exist in a stable manner on the oxide surface. Indeed, comparing the desorption temperatures for surface carbonates, the carbonate species on Nd2O3 (C-type) exhibits a higher desorption temperature than that on Nd2O3 (Atype), which predicts that the basicity of Nd2O3 (C-type) is stronger than that of Nd2O3 (A-type). Therefore, it is interesting to investigate the relationship between the crystal structure and the basicity for some kinds of rareearth sesquioxides including Nd2O3. There are several methods for obtaining a numerical value of the acidity or basicity of oxide surface, for example,

Figure 10. Schematic representation of hydration process from Nd2O3 (A-type) to Nd(OH)3. Open and filled circles represent Nd3+ and O2- ions, respectively, and H atoms are omitted here.

the method based on the model assuming a uniform crystal plane or the method using an electronegativity. Since the surfaces of rare-earth oxides are heterogeneous, and hence the crystal planes cannot be specified, in addition to a similarity in the electronegativity for rare-earth elements, it is impractical to estimate the acidity (or basicity) on the basis of such models. For these reasons, we evaluated the basicity by using the Parks model26 that is relatively simple and capable of calculation of acidity (or basicity) on the basis of structural data. It is well-known that the isoelectric point of surface (IEPS) is a measure of numerical expression of acidity or basicity of oxide surface. The IEPS is defined as a pH of solution contacting with oxide surface when the net charge of surface is zero; the larger IEPS means the stronger basicity and vice versa as to the acidity. The IEPS value can be estimated using the equation:

IEPS )

1 -∆G° 2 2.3 RT

(

)

(2)

(24) (a) Parks, G. A. Chem. Rev. 1965, 65, 177. (b) Yoon, R.-H.; Salman, T.; Donnay, G. J. J. Colloid Interface Sci. 1979, 70, 483. (25) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. (26) Bacquet, G.; Bouysset, C. J. Solid State Chem. 1976, 18, 247.

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Figure 11. Variation of weight-increase rate with exposure time to saturated water vapor at 298 K for rare-earth sesquioxides: (O) Nd2O3 (A-type); (b) Nd2O3 (C-type); ([) La2O3 (Atype); (2) Sm2O3 (C-type); (9) Y2O3 (C-type). Initial hydration process for La2O3 (A-type) is shown in the inserted figure. Table 1. Distance of Ln3+-O2- and IEPS of Rare-Earth Sesquioxides La2O3 Nd2O3 Sm2O3 Nd2O3 Y2O3 (A-type) (C-type) (C-type) (A-type) (C-type) Ln3+-O2-/pm IEPS

242 9.17

240 9.12

237 9.04

235 8.99

230 8.83

Here, ∆G° is a free-energy change in the following reaction:

MO- + 2H+ f MOH2+

(IV)

This value can be obtained by using the Parks' electrostatic model:24

∆G° )

2QMQH 2QOQH + + ∆G′ rO (2rO + rM)

(3)

where QM, QO, and QH are formal charges of M (metal), O (oxygen), and H (hydrogen), respectively, (QO ) -2, QH ) +1), rM and rO are ionic radii of M and O, respectively,  the permittivity of oxide, and ∆G′ the nonelectrostatic contribution of free-energy change (constant value for all the oxides). Since  and ∆G′ are assumed to be constant, the above equation can be further simplified:

IEPS ) 18.6-11.5 ×

QM (2rO + rM)

(4)

For the rare-earth sesquioxides, QM ) +3, and if the ionic radius of four-coordinated oxide ion is assumed to be 0.124 nm (Shannon’s value25), the IEPS depends on only rM. The ionic radius of seven-coordinated metal ion can be obtained from the Ln3+-O2- distance in the crystal. The IEPS values were obtained by using eq 4 for Nd2O3 (Atype), Nd2O3 (C-type), La2O3 (A-type), Sm2O3 (C-type), and Y2O3 (C-type), as shown in Table 1. The oxide having a larger Ln3+-O2- distance (i.e., a larger ionic radius of metal ion) is more basic. To clarify the relationship between the basicity of rareearth oxide and the reactivity to water vapor, the weight changes were also examined for the La2O3 (A-type), Sm2O3 (C-type), and Y2O3 (C-type) samples that had been exposed to saturated water vapor at 298 K. These results are shown in Figure 11, in comparison with the results for A-type

Figure 12. Differentiation of weight-increase rate plotted against exposure time for the system of rare-earth sesquioxide and water vapor.

and C-type Nd2O3. When La2O3 (A-type) was in contact with water vapor, the hydration took place in a moment. Comparing the weight increase in time interval 0-1 min with that in 1-2 min after exposure to water vapor, the latter case was larger than the former case, which suggests that there is an induction period, though very short, in the hydration of La2O3 (A-type). Such an induction period has been observed in the hydration process for a single crystal of La2O3 (A-type).26 From the fact that for La2O3 (A-type) and Nd2O3 (A-type) as well as for La(OH)3 and Nd(OH)3 their crystal structures resemble each other, it can be reasonably presumed that the hydration process for La2O3 (A-type) is similar to that for Nd2O3 (A-type). For Sm2O3 (C-type), differently from Nd2O3 (C-type), the hydration appears to proceed in one step without an abrupt hydration after the introduction of water vapor. The weight increase of Y2O3 (C-type) continues slowly up to about 7% in increasing rate, and after that it practically stops. For the Y2O3 (C-type) sample exposed to water vapor for 1500 min, the weight-increase rate was zero after the evacuation at 298 K. It can be interpreted that the weight increase is only due to a physisorption and that the hydration can hardly proceed into the bulk phase. For the comparison of reactivity of rare-earth oxides to water vapor, it is worthwhile to direct our attention to the rate of hydration immediately after exposing the oxide sample to water vapor. The result of differentiating the weight-increase rate by time can be regarded as a rate of hydration, as shown in Figure 12. The rate of hydration for Y2O3 (C-type) seems to be the slowest among these rare-earth oxides, because the bulk hydration was not observed. Thus, the initial rate of hydration is assumed to increase in the following order:

Y2O3 (C-type) < Nd2O3 (A-type) < Sm2O3 (C-type) < Nd2O3 (C-type) < La2O3 (A-type) This order is consistent with the order of increasing IEPS (Table 1); the order of reactivity of rare-earth oxide to water vapor is compatible with the order of increasing basicity, that is, the order of ionic radius of rare-earth metal ion. According to such conception, it is expected to improve a water-resistant property of rare-earth oxides by making their basicity lowered. One possible method is a neutral-

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Nagao et al.

Figure 14. Adsorption isotherms of water vapor on Nd2O3 (A-type) at 298 K: (]), noncoated Nd2O3; (b) silica-coated Nd2O3; (O) titania-coated Nd2O3. Table 2. Distance of Mn+-O2- and IEPS of Silica and Titania Mn+-O2-/pm IEPS

Figure 13. XPS results for silica-coated Nd2O3 and titaniacoated Nd2O3.

ization of rare-earth oxide surface by acidic substances. Thus, we tried to neutralize the Nd2O3 sample by coating its surface with acidic oxides (e.g., silica and titania). The vapor of Si(OC2H5)4 or Ti(OC2H5)4 evolved by argon-gas bubbling was adsorbed onto the surface of Nd2O3 (A-type) that had been pretreated at 1123 K, and then the adsorbed species were subjected to decomposition into the acidic oxide. Figure 13 shows the results of XPS for these surfacecoated samples. The presence of acidic oxides, silica and titania (rutile), on the Nd2O3 surface is confirmed by these spectra, though their quantitative determination was not given at present. The water-vapor adsorption isotherms obtained for the noncoated and surface-coated Nd2O3 samples are shown in Figure 14. For the noncoated Nd2O3 sample, the adsorbed amount increases abruptly at a relative pressure of around 0.2, accompanying a drop in equilibrium pressure, which is indicative of a commencement of surface hydration. Such backward going of equilibrium pressure, however, was not observed in the adsorption isotherm for the silica-coated or titania-coated Nd2O3 sample. These isotherms in the lower pressure region were well expressed by the BET equation, and the monolayer capacities obtained by applying this equation were 10.5 and 11.0 H2O-molecules nm-2 for the silica-coated and titaniacoated Nd2O3 samples, respectively. These values are in fair agreement with the monolayer coverage estimated on the basis of cross-sectional area of a water molecule (0.105 nm2). Therefore, it is reasonable to consider that the water-resistant property of the Nd2O3 surface could be improved by the surface coating, and this property is more superior on the titania-coated Nd2O3 sample than

SiO2 (β-cristobalite)

TiO2 (rutile)

155 1.93

195 4.07

on the silica-coated Nd2O3 sample; for the latter sample the larger amount of adsorption in the relatively higher pressure region may arise from a hydration of incompletely or heterogeneously coated surface (Figure 13). Taking account of the acid strength of solid, the materials having more acidic nature should react easily with the basic metal oxides such as Nd2O3 and La2O3. However, the present results are entirely unexpected; the Nd2O3 sample coated with a less acidic titania exhibits more effective surface modification than the sample coated with a more acidic silica does (Table 2). The reason for this might be found in the property of metal alkoxide itself. The rate of hydrolysis of metal alkoxide is much faster for Ti(OC2H5)4 than for Si(OC2H5)4,27 and hence, it is assumed to have a correspondence of the reactivity of metal alkoxide. Thus, the highly reactive Ti(OC2H5)4 reacts easily with the Nd2O3 surface to give oxide layers that can protect the surface from the penetration of water molecules into bulk (i.e., bulk hydration). Conclusions The hydration processes of Nd2O3 samples were different depending on the crystal structure; A-type Nd2O3 was transformed directly to Nd(OH)3, while C-type Nd2O3 was transformed to Nd(OH)3 via NdOOH. Moreover, the hydration reaction of Nd2O3 (A-type) could be interpreted in terms of the nucleus-growth model, namely, the twodimensional nucleus growth of Nd(OH)3. For Nd2O3 and La2O3 that have the same crystal structure (A-type) the hydration processes were also identical, while for Nd2O3, Sm2O3, and Y2O3 that have C-type structure the hydration processes were different from one another. This implies that the hydration process is not always the same for the oxides having the same crystal structure. The rare-earth sesquioxide having a stronger basicity (i.e., larger ionic radius of rare-earth element) exhibits a higher reactivity to water vapor. The water-resistant property of Nd2O3 could be improved by the surface-coating with an acidic oxide derived from the metal alkoxide and it was more (27) Sakka, S. Science of Sol-Gel Method; Agune-shofusha: Tokyo, 1988.

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superior for the sample surface-coated with a reactive Ti(OC2H5)4 than for that with a less reactive Si(OC2H5)4.

Foundation, and the author (M. N.) would like to acknowledge the generosity of this foundation.

Acknowledgment. This research was supported in part by a grant from the Hosokawa Powder Technology

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