The Relation between the Amounts of Chemisorbed and Physisorbed Water on Zinc Oxide

Abstract

In the system ZnO-H2O, in which a jump appears in the adsorption isotherm, the relation between the amounts of chemisorbed and physisorbed water has been investigated, on the basis of the measurements of the water adsorption isotherms and the water content. The first adsorption isotherm of water was measured at 18°C on a sample degassed at 450°C. After degassing the sample again at 30°C, the second adsorption isotherm was obtained at the same temperature as before. The water content (Vh) on the surface of the sample was determined by the successive ignition-loss method. In the present system, the amount of water physisorbed in the first layer is composed of two parts: one (Vp1) is the amount adsorbed at lower vapor pressures, up to the beginning of the jump in the isotherm, and the other (Vp2), the amount adsorbed during the jump. The amount of water chemisorbed (Vc) during the adsorption process could be estimated as the difference between the monolayer capacities of the first and second isotherms, the total amount of chemisorbed water being the sum of Vh and Vc. The ratio of the total amount of physisorbed water (Vp1+Vp2=Vp), expressed in the number of water molecules per unit of area, to that of chemisorbed water (Vc+Vh), expressed in the number of hydroxyl groups per unit of area, was found to be about 1 : 1 in the first physisorption layer for all the samples tested, irrespective of their origin, while the ratio of Vp1 to Vp2 varied extensively with the origin of the samples.