Re-Co/NaY and Re-Co/Al₂O₃ Bimetallic Catalysts: In Situ EXAFS Study and Catalytic Activity

Abstract

To reveal possible relations between the structure and catalytic activity, in situ EXAFS and catalytic studies complemented with XRD, XPS, and TPR measurements have been performed on the promotion of cobalt catalysts by rhenium prepared by the incipient-wetness technique on Al₂O₃ and NaY zeolite.

In situ EXAFS data collected at the Co K-edge and at the Re L_III-edge provided direct evidence of the rhenium-cobalt bond formation. The degree of reducibility depends on the support. There are two structural features, that is, on Re-Co/NaY nearly all rhenium atoms are in contact with Co atoms, whereas the cobalt atoms are surrounded by cobalt atoms in the first coordination sphere. In the case of Re-Co/Al₂O₃ samples the rhenium in oxide form may prevent the development of the “cobalt surface phase” (CSP), which is hardly reducible.

The rate, α value and olefin/paraffin ratio showing special features in the CO hydrogenation and CH₄ conversion to higher hydrocarbons are in line with the structural architecture of the catalysts. Despite the difference in the degree of reducibility, the various activity of Re-Co/NaY and Re-Co/Al₂O₃ may be interpreted by the formation of mixed oxide on alumina preventing the deactivation and agglomeration of small metal particles. Furthermore, the rhenium promotes cobalt activity and the selective formation of higher hydrocarbon. In the mechanism the rhenium also prevents fast deactivation of cobalt.