Chemisorption of H2and H2—O2on polymorphic zirconia

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Abstract
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 H2 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 (O2, O). Their reaction with physisorbed O2 leads to irreversible formation of multiple-coordinated OH groups and O2 .– radical anions [ν(O—O).–; 1118 cm–1] evidenced by 18O2 isotopic experiments. The appearance of these OH groups necessarily implies surface mobility of intermediately existing H atoms on both monoclinic and tetragonal surface domains.