Different Functions of the Noble Metals Added to Cobalt Catalysts for Fischer–Tropsch Synthesis

The influence of the addition of very small amounts of Ru, Pt, and Pd to supported cobalt catalysts, which were prepared by mixing cobalt nitrate and cobalt acetate, has been investigated for the Fischer–Tropsch synthesis. Catalysts containing 10 wt% Co (5 wt% from cobalt nitrate and 5 wt% from cobalt acetate) and a very small amount of Ru, Pt, or Pd were prepared by coimpregnation. The weight ratio of M to total Co was 1/50 (M = Ru, Pt, Pd). At T=513 K, P=1.0 MPa, H2/CO=2, and W/F=5 g-cat. h mol−1, CO hydrogenation rates followed the order RuCo>PdCo>PtCo>Co. The addition of a small amount of Ru to Co/SiO2 catalyst increased the catalytic activity and the reduction degree remarkably. The turnover frequency (TOF) increased but the CH4 selectivity was unchanged. However, added Pt or Pd catalyst exhibited higher CH4 selectivity. Although Pt and Pd exerted hardly any effect on the degree of cobalt reduction, they promoted cobalt dispersion and decreased the TOF. Characterization of these bimetallic catalysts suggested that different contacts between Co and Ru, Pt, or Pd existed. Ru was enriched on cobalt but Pt, or Pd dispersed well in the form of Pt–Co or Pd–Co alloy.