CHEMISORPTION OF H-2 AND H-2-O-2 ON POLYMORPHIC ZIRCONIA

Powders of polymorphic zirconia contain crystallites incorporating monoclinic and tetragonal domains. Thermal activation in high vacuum results in a significant amount of highly reactive radical surface centres (O-, Zr3+) mainly located in tetragonal surface sites. Their exposure to molecular H-2 leads uniquely to homolytic dissociation processes clearly evidenced by IR spectroscopy. These processes give rise to an irreversible formation of multiple-coordinated surface OH groups on tetragonal surface sites (3668 cm(-1)) and to a reversible formation of simple (ZrH; 1565 cm(-1)) and bridged hydrides (Zr2H; 1360 cm(-1)). Simple hydrides are very reactive towards any kind of oxygen species (O-2, O). Their reaction with physisorbed O-2, leads to irreversible formation of multiple-coordinated OH groups and O-2(.-), radical anions [nu(O-O)(.-); 1118 cm(-1)] evidenced by O-18(2), isotopic experiments. The appearance of these OH groups necessarily implies surface mobility of intermediately existing H atoms on both monoclinic and tetragonal surface domains.