Effect of ruthenium addition on the performance of Co/SiO₂ catalyst for Fischer-Tropsch synthesis

Three Co/SiO₂ catalysts doped with different amounts of Ru were prepared by incipient wetness impregnation. These catalysts were characterized by N₂ physisorption, XRD, H₂-TPD, DRIFTS, etc.; their catalytic performance in Fischer-Tropsch (F-T) synthesis was investigated in a micro fixed-bed reactor. The F-T reaction results showed that the Co/SiO₂ catalysts doped with Ru exhibit higher CO conversion, higher turnover frequency (TOF), lower selectivity to CO₂ and CH₄, as well as lower ratio of olefin to paraffin, in comparison with undoped Co/SiO₂. FT-IR spectra indicated that the Co-O bond in the as-prepared catalyst is weakened by the addition of Ru, which facilitates the reduction of the Co/SiO₂ catalysts; such results are also supported by the H₂-TPR profiles and XRD patterns of the reduced catalysts. The main cobalt phase in the reduced catalyst with 0.5% (by weight) of Ru is in a hexagonal close packing (hcp) structure. CO-DRIFTS results revealed that the peak of linearly adsorbed CO is red-shifted by the addition of Ru, suggesting an improvement on the dissociation of adsorbed CO. CO-TPD results showed that the ratio of CO_ads/Co_s and CO^*/Co_s on catalysts surface is increased by the addition of Ru, which may contribute to the decrease of the selectivity to CH₄ in F-T synthesis.