Hydrogen Chemisorption on ZnO·Cr2O3, ZnO·MoO3 and Cr2O3

Abstract

Abstract For hydrogen chemisorption on ZnO·Cr2O3, ZnO·MoO3 and Cr2O3, the rates of adsorption and desorption, and adsorption equilibrium were investigated. The activation energy of adsorption ranged from 8 to 12 kcal./mol. for ZnO·Cr2O3, and from 15 to 25 kcal./mol. for ZnO·MoO3. The value for Cr2O3 was about 13 kcal./mol. In the cases of ZnO·MoO3 and Cr2O3, where desorption is negligible during adsorption, the rate of adsorption was proportional to P0.8, while with ZnO·Cr2O3 desorption was found to occur appreciably. The activation energy of desorption increased with a decreasing amount adsorbed from 10 to 35 kcal./mol. for ZnO·Cr2O3, and from 15 to 45 kcal./mol. for Cr2O3, while an approxi-mately constant value of 41 kcal./mol. was obtained with ZnO·MoO3 except in the initial stage of desorption. The maximum heat of adsorption estimated from adsorption isotherms was about 25 kcal./mol. for both ZnO·Cr2O3 and ZnO·MoO3. For all the systems studied, desorption experiments suggested the existence of surface heterogeneity with the activation energy of adsorption and heat of adsorption varying inversely in the cases of ZnO·MoO3 and Cr2O3, while with ZnO·Cr2O3 the variation in the same direction was observed. On the basis of the results hitherto obtained, the general characteristics of hydrogen chemisorption on oxides were discussed.