The state and location of Re in Co–Re/Al2O3 catalysts during Fischer–Tropsch synthesis: Exploring high-energy XAFS for in situ catalysts characterisation

Abstract

A rhenium-promoted Co/Al2O3 catalyst and a sample containing Re/Al2O3 have been studied using XAS measured at both rhenium LIII (10,535eV) and K (71,676eV) edges. Results obtained from both sets of experiments are consistent and demonstrate that the higher penetration power of the X-rays at the Re K edge is a viable option that gives less stringent requirements in the choice of reactor material and X-ray window material. The in situ cell used at the high-energy Re K edge is conveniently of the same dimensions and material as a typical lab-scale fixed-bed reactor. Catalyst performance from such measurements can be directly compared to the results from conventional fixed-bed catalytic experiments. High energy X-rays are thus an attractive option for in situ studies of catalysis at demanding conditions. Both sets of experiments (Re K and LIII edges) indicate that Re is atomically distributed inside the bulk Co particles. The results indicate a local distortion of the Co environment in intimate contact with the Re atoms, which may contribute to the observed increase in catalytic activity for the Re promoted catalysts. A minor fraction of Re (approx. 15%) remains in an oxidic state after a typical reduction procedure for Co-based Fischer–Tropsch synthesis catalysts.