DIFFERENTIAL HEAT OF CHEMISORPTION .1. CHEMISORPTION OF WATER ON ZINC-OXIDE AND TITANIUM-DIOXIDE

The heat of immersion in water was measured at 28°C on samples of ZnO and TiO2 (rutile) having different amounts of surface hydroxyls, from which the integral heat of water chemisorption was calculated. The differential heat of water chemisorption, i.e., that of formation of hydroxyl groups, was obtained by differentiating the curve of the integral heat of chemisorption vs. the amount of prechemisorbed water. The differential heat on TiO2 decreases with increasing amount of chemisorbed water, suggesting an ordinary type of surface heterogeneity. However, ZnO gives an unusual differential heat curve which exhibits a maximum in the vicinity of monolayer coverage. This phenomenon is best understood in terms of the additional contribution of mutual hydrogen bondings between surface hydroxyls formed on the well-developed (1010) plane of ZnO, on the basis of the model postulated previously for the elucidation of an adsorption anomaly in the system ZnO-H2O. © 1978 American Chemical Society.