Adsorption sites on polymorphic zirconia

Abstract

Surface models of monoclinic and tetragonal zirconia are presented. They are derived from the respective lattice structures of both modifications. In contrast to previous investigations structural deviations of both phases from the ideal CaF₂ structure are taken into account. The existence of trigonally and tetrahedrally coordinated O^2– anions in the monoclinic structure necessarily implies the appearance of two different families of surface sites, which give rise to the two bands of isolated OH groups at 3774 and 3668 cm^–1. On the other hand, exclusively tetrahedrally coordinated O^2– anions are compatible with the lattice of tetragonal zirconia. Consequently, there is only one family of surface sites and only one band of isolated OH groups at 3682 cm^–1, which nearly coincides with the low-frequency band of monoclinic zirconia. Thermal treatment of highly dispersed zirconia has a significant influence on the surface phase composition. Therefore, the intensity ratio of the two ν(OH) bands of isolated surface OH groups reflects the history of a sample. Their contours are determined by the respective distributions of local environments.