Specific adsorption behavior of water on a Y₂O₃ surface

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 wit bulkd oxide layers to form hydroxide layers. Such a process is summarized as follows: Y₂O₃ + physisorbed water → Y₂O₃·H₂O (strongly adsorbed on the surface layer) → Y₂O₃(H₂O bulk species) or YOOH → 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.