DIFFERENTIAL HEAT OF ADSORPTION AND ENTROPY OF WATER ADSORBED ON ZINC OXIDE SURFACE

The adsorption isotherm and the heat of immersion isotherm of water were measured on two ZnO samples pretreated at 250 and 450°, respectively. By combining these two kinds of data, we obtained the differential heat of adsorption, the free energy of adsorption, the entropy of adsorption, and the site-energy distribution for the system ZnO-H2O. A clear hump appeared at a region of moderate relative pressure of water adsorption isotherm, but we could not find any corresponding anomaly in other properties examined here. Only in the curve of the differential entropy of adsorption could a slight abnormal change be discovered, which seemed to have some connection with the hump of adsorption isotherm. The site-energy distribution curves derived by differentiating the plot of the differential heat of adsorption vs. the amount of adsorption showed two kinds of peaks; one corresponds to a group of physisorption sites with energies less than 5 kcal/mol H2O, and the other to that of chemisorption sites with energies of 20-25 kcal/mol H2O.