Effect of alkalis on iron-based Fischer-Tropsch synthesis catalysts: Alkali-FeOx interaction, reduction, and catalytic performance

Abstract

Alkalis (Li, Na, K, Rb and Cs) promoted Fe/SiO2 catalysts were used to investigate the influences of alkalis on the alkali-FeOx interaction, reduction behavior, and catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts. Laser Raman spectroscopy (LRS), Fourier transform infrared spectroscopy (FTIR), Mössbauer effect spectroscopy (MES), H2 temperature programmed reduction (H2-TPR), in situ X-ray photoelectron spectroscopy (in situ XPS) and in situ X-ray diffraction (in situ XRD) were employed to characterize the catalysts. It was found that alkalis had strong interactions with iron species in catalysts. In as-prepared catalysts, the FeO bonds are strengthened by alkalis, which consequently influenced the reduction of catalysts. Li and Na strongly restrained the reduction of all iron oxides in catalysts. K, Rb and Cs inhibited the initial reduction step (reduction of Fe2O3 to Fe3O4), while improved the reduction of iron oxides to FeO and metallic iron. Besides, alkalis facilitated the carbonization of catalysts and the improvement increased with increasing the atomic number of alkalis. In FTS reactions, Li and Na decreased the FTS activity while K, Rb and Cs largely increased the FTS activity of iron catalysts. The selectivities to methane and alkane were decreased while those to olefin and heavier hydrocarbons were increased by alkalis.