Carbon-supported cobalt–iron catalysts for ammonia synthesis

The cobalt, iron and Co–Fe catalysts deposited on carbon were prepared, characterised (XRD, H₂ TPD) and studied in ammonia synthesis at 90 bar (H₂:N₂ = 3:1). Partly graphitised carbon material obtained via high temperature treatment (1900 °C) of commercial activated carbon was used as a support for the active metals (10 wt.%) and barium or potassium were used as promoters. XRD studies of unpromoted materials have shown that cobalt (5–20% in Co + Fe) dissolves in the iron phase (alloy formation); the average sizes of crystallites (20–30 nm) are roughly independent of the metal kind (Co, Fe, Co–Fe). The effect of Ba and that of K on the catalyst performance proved to be strongly dependent on the choice of an active phase (Co or Fe or Co–Fe). In the case of Co/C, the promotional effect of barium was extremely large. Furthermore, the Ba–Co/C system was found to be less inhibited by the ammonia product than Ba–Fe/C. At low temperature (400 °C) and at high conversion (8% NH₃ in the gas), the surface-based reaction rate (TOF) for Ba–Co/C is about six times higher than that for Ba–Fe/C.