Reducibility of alumina-supported cobalt Fischer–Tropsch catalysts: Effects of noble metal type, distribution, retention, chemical state, bonding, and influence on cobalt crystallite size

Abstract

The distribution, retention, chemical state, and bonding of noble metal (NM) promoters (Pt, Re, or Ru), their influence on Co crystallite size, and the resulting Co reducibility and dispersion were investigated in commercially-representative Co Fischer–Tropsch catalysts (nominally 25wt% Co and 0 to 0.6wt% NM on a La stabilized alumina support). NM retention during preparation ranged from complete for Pt and Re to<50% for Ru. Co concentrations were spatially uniform; however, NM concentration varied dramatically from a sharp concentration gradient at the pellet edge in the case of Ru to a homogeneous distribution for Re and Pt. After 360°C reduction for 1h, Pt0 bonds directly to Co0, Ru forms separate metal crystallites and Re remains as Re2O7. Despite these differences, all three NMs substantially improved Co reducibility. Pt is the most effective promoter for increasing cobalt reduction rate and gives the highest extent of reduction (EOR). Preliminary activity tests showed no statistical differences in the CO depletion rates of the four catalysts of this study.