Adsorption of water on Nd2O3:: Protecting a Nd2O3 sample from hydration through surface fluoridation

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 Ols 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.