Fischer-Tropsch synthesis:: study of the promotion of Re on the reduction property of Co/Al2O3 catalysts by in situ EXAFS/XANES of CoK and Re LIII edges and XPS

A direct relationship between the Fischer-Tropsch synthesis (FTS) rate and the number of surface cobalt atoms available for reaction is usually obtained. For Co/Al2O3 catalysts in particular, the site density depends on two primary factors: (1) the average size of the cobalt clusters on the support; and (2) the fraction of cobalt reduced to the metallic state. The addition of small amounts of noble metal promoters, such as Pt and Ru to cobalt alumina, may catalyze the reduction of cobalt oxide shifting the temperature of reduction of both steps (Co3O4 --> CoO and CoO --> Co-0) to lower temperatures. However, Re affects only the 19 second step. Re is reduced at a higher temperature than Pt or Ru, and at approximately the same temperature as the first step of cobalt reduction (Co3O4 --> CoO). Thus, Re metal is present to catalyze only the second step. In situ extended X-ray absorption fine structure (EXAFS) at the L-III edge of Re has been used to show that there is direct contact of Re with cobalt atoms, while evidence for Re-Re bonds is not observed. Even though direct atom-to-atom contact is found, temperature-programmed reduction (TPR) suggests that hydrogen spillover from the promoter to cobalt oxide clusters is important for the catalysis of cobalt oxide reduction. In situ EXAFS at the K edge of Co shows that the average cobalt cluster size decreases with increasing Re loading. Re promotes reduction of smaller species which interact with the support, and therefore, for a given reduction temperature, the average cobalt metal cluster size decreases as a function of increasing Re content. After reduction at a temperature slightly above the first peak in the TPR (Co3O4 --> CoO), the species remaining on the surface displayed XANES spectra identical to that of Coo. After reduction at a temperature above the second broad TPR peak, XPS showed that a residual oxide species was present, with a binding energy equivalent to cobalt aluminate. (C) 2004 Elsevier B.V. All rights reserved.