How does activation affect the cobalt crystallographic structure? An in situ XRD and magnetic study
Résumé
Fischer-Tropsch synthesis uses cobalt catalysts to convert syngas into linear hydrocarbons. Activation is the key step generating and impacting the active phase and more precisely the crystallographic structure of the metallic cobalt particles. This study investigates the selective crystallographic orientation of cobalt-supported catalysts and its impact on the degree of reduction. Three catalysts supported on alumina and silica with various particle sizes were prepared and characterized by ex situ techniques. They underwent two kinds of reduction processes carried out under different atmospheres. A carbidisation-decarbidisation cycle was also performed on reduced catalysts. The evolution of the active phase under reactive conditions was followed by in situ X-Ray Diffraction and magnetic measurements. The different reduction processes gave rise to the formation of metallic cobalt with mainly a face-centered cubic structure. The degree of reduction was also affected by the type of reduction process. Carbidisation-decarbidisation had no global impact on the reduction extent and allowed the formation of mainly hexagonal closed packed metallic cobalt. (C) 2013 Elsevier B.V. All rights reserved.