Fischer−Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts

Abstract

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hägg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Hägg carbide. A model containing Hägg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.