RELATION BETWEEN AMOUNTS OF CHEMISORBED AND PHYSISORBED WATER ON METAL OXIDES

The first and second water adsorption isotherms and the water content on the metal oxide surfaces have been measured for samples of TiO2 (rutile) and α-Fe2O3 (hematite). After the first adsorption isotherm had been obtained, the sample was degassed under a reduced pressure of 10-5 torr at room temperature for 4 hr, and then the second adsorption isotherm was measured at the same temperature as before. The determination of water content was carried out by the successive ignition-loss method. The amounts of physisorbed water (Vp) and chemisorbed water (Vc) were estimated from the two isotherms. The total amount of chemisorbed water was obtained by adding the water content (Vh) to the value of Vc. The sum of Vc and Vh decreases slightly with an increase in the temperature of pretreatment of the sample, which indicates that the extent of reversible rehydroxylation of the sites dehydrated at higher temperatures is somewhat diminished. The ratio of Vp, expressed in the number of water molecules per unit area, to the sum of Vc and Vh, both of which are expressed in the number of hydroxyl groups per unit area, was found to be about 1:2 for all samples used, which indicates that a water molecule is adsorbed on two hydroxyl groups through hydrogen bonding.