Copper ferrites: A model for investigating the role of copper in the dynamic iron-based Fischer–Tropsch catalyst

Abstract

Copper is a well-known reduction promoter for Fe-based Fischer–Tropsch catalysts. The role of copper is investigated using spinel copper ferrite, CuFe2O4, and delafossite, CuFeO2, as model compounds and compared with hematite. The incorporation of copper within the crystal structure results in a facile, initial reductive decomposition of the structure into magnetite and metallic copper during the catalyst pre-treatment in either hydrogen or carbon monoxide. Copper facilitates the consecutive conversion of magnetite to α-Fe in hydrogen, but not its conversion to predominantly χ-Fe5C2 in CO. The amount of carbide in the catalyst under Fischer–Tropsch conditions is dependent on the presence of copper in close proximity to the iron phases. The observed increase in catalytic activity in the catalysts ex copper ferrites is primarily due to the increased carbide surface area within the catalyst. The copper-containing model compounds show a higher CO2-selectivity, which is ascribed to the presence of a larger amount of small magnetite crystallites present during the Fischer–Tropsch synthesis. Furthermore, copper seems to facilitate secondary olefin hydrogenation.