Chemisorption of Hydrogen on Zinc Oxide. I. Rate of Desorption and Adsorption Isotherm

The rate of desorption of hydrogen chemisorbed on zinc oxide was measured over a wide range of temperatures. The rate at a constant temperature was found to obey approximately the kinetics of the Elovich type. The activation energy of desorption was determined at various coverages, with the result that it increased with decreasing amount from at least about 10∼31 kcal./mol. and then remained constant. The heat of adsorption obtained from adsorption isotherms in the temperature range 300∼370°C was found to be 18 kcal./mol., being roughly constant. Meanwhile, from the isotherms at about 100°C, a heat of adsorption of 6∼8 kcal./mol. was obtained. Comparison between the activation energy of desorption and the heat of adsorption suggests that most part of hydrogen chemisorption on zinc oxide involves an activation energy, its highest value being estimated as 13 kcal./mol.