Pyridine adsorption reveals high-coordinated cationic centres at the surface of microcrystalline ZnO

The adsorption of pyridine at 300 K on microcrystalline ZnO has been investigated by FTIR spectroscopy. Besides an interaction by H-bonding with surface OH groups, evidence is found for the Lewis coordination of pyridine onto coordinatively unsaturated surface Zn²⁺ centres possessing a regular tetrahedral coordination, as well as onto some other surface Zn²⁺ centres, only slightly uncoordinated. The creation at the surface of the ZnO of cationic centres with an anomalous coordination higher than 4 (quasi-octahedral) is thus postulated. No such high-coordinated Zn²⁺ centres seem to form to an appreciable extent when ZnO is dispersed (at least at up to a 3% weight level) at the surface of other microcrystalline oxides such as, for instance, TiO₂.