Adsorption of water, ammonia, and carbon dioxide on zinc oxide at elevated temperatures

The adsorption isotherms of H/sub 2/O, NH/sub 3/, and CO/sub 2/ were measured at a variety of temperatures on the surface of ZnO which was pretreated at 723 K in vacuo. The adsorption isotherm of H/sub 2/O at room temperature showed multilayer adsorption, involving two-dimensional condensation of H/sub 2/O, but when the adsorption temperature exceeded 373 K, it changed to the Langmuir type which represents the chemisorption of H/sub 2/O. The desorption of H/sub 2/O was so retarded in a lower pressure region that a small number of strongly adsorbed H/sub 2/O molecules were left on the surface even after their evacuation into a trap cooled by liquid nitrogen, the number was found to decrease with the elevation of adsorption temperatures. The isosteric heat of adsorption, q/sub st/, of H/sub 2/O in the chemisorption region showed a monotonic decrease with increase of surface coverage, while the q/sub st/ value in the physisorption region showed a maximum before the completion of the monolayer. It was found that there is a wide temperature range, 337-623 K, at which the coverage of about 3 molecules/nm/sup 2/ is thermally stable for chemisorbed H/sub 2/O, whereas for chemisorbed NH/sub 3/ and CO/sub 2/ it shiftsmore » to a lower temperature range, corresponding to the q/sub st/ values of 110, 80, and 60 kJ/mol for H/sub 2/O for H/sub 2/O, NH/sub 3/, and CO/sub 2/, respectively. 23 references, 7 figures.« less