The kinetics of H 2 adsorption on and desorption from cobalt and the effects of support thereon

Abstract

The technique of temperature-programmed desorption (TPD) was applied to the investigation of hydrogen adsorption/desorption from a series of cobalt catalysts. Experiments in which the adsorption temperature was varied revealed that hydrogen adsorption on cobalt is activated and that the activation energy for adsorption is a function of catalyst support and metal loading. The activation energy for adsorption ranged from 5.8 kJ/mol for unsupported cobalt to 43 kj/mol for 3% cobalt on silica. Desorption runs on selected catalysts showed hydrogen desorption to be second order, the heat of adsorption ranging from 145 kJ/mol on 10% Co SiO 2 to 105 kj/mol on 10% Co Al 2O 3 . Desorption from unsupported, silica-, and alumina-supported catalysts involved one major peak suggesting the presence of only one type of site while the titania- and carbon-supported catalysts displayed multiple desorption peaks providing evidence of a more complex distribution of surface sites possibly as a result of metal-support interactions. No H 2 adsorption was detected for magnesia-, ZSM-5, and lower loading (1 and 3%) alumina-supported cobalt catalysts at any temperature using flow adsorption/desorption techniques, even though these catalyst adsorbed hydrogen at temperatures ranging from 25 to 150 °C using the static, equilibrium technique. These results suggest that strong metal-support interactions in these catalysts may lead to a strongly activated adsorption of hydrogen on cobalt.