Promotional Effects of Cu on Precipitated Fe–K–SiO2 Catalysts: Activation Behavior and Catalytic Performance in the Fischer–Tropsch Synthesis

The promotional effects of Cu on precipitated Fe–K–SiO ₂ catalysts were systematically investigated using a series of Cu-free (0Cu) and Cu-promoted catalysts (1Cu, 3Cu, 5Cu, and 7Cu). Comparative studies of the 0Cu and 5Cu catalysts under both fixed and variable activation times provided direct evidence that Cu promotion shortens the induction period and enhances steady-state CO conversion. This effect is attributed to facilitated formation of the active iron carbide phase (χ-Fe ₅ C ₂ ) via hydrogen spillover on the copper surface. When product selectivities were compared at similar CO conversion levels, only marginal differences were observed between the two catalysts. Extending the analysis to the whole catalyst series under sufficiently activated conditions revealed that CO conversion increased with Cu loading, reaching a maximum of 90.8% at 5Cu before decreasing at higher Cu contents. Increasing Cu loading also slightly reduced the olefin-to-paraffin ratio and heavy hydrocarbon selectivity, though the changes were not substantial. Overall, these findings demonstrate that optimal Cu promotion (5Cu in this study) is crucial for minimizing induction periods without significantly affecting product selectivity.