Co 0 from partial reduction of La(Co,Fe)O 3 perovskites for Fischer–Tropsch synthesis

Abstract

Small Co clusters ( d<10 nm) supported over mixed La–Co–Fe perovskites were successfully synthesized. These catalysts were active for Fischer–Tropsch (FT). Depending on the Co to Fe ratios the mixed perovskite exhibited two different forms: the rhombohedral phase of LaCoO 3 is maintained for the mixed perovskite when x>0.5, the orthorhombic phase of LaFeO 3 is found for x<0.5. Interestingly only one of these structures is active for the FT reaction: the orthorhombic structure. This is most likely due to the capacity of this material to maintain its structure even with a high number of cation vacancies. These cations (mostly Co) were on purpose extracted and reduced. Magnetic measurements clearly showed their metallic nature. Rhombohedral Co–Fe mixed perovskites ( x≥0.5) cannot be used as precursors for Fischer–Tropsch catalysts: their partial reduction only consists in a complete reduction of Co 3+ into Co 2+. The partial reduction of orthorhombic perovskites ( x<0.5) leads to active Fischer–Tropsch (FT) catalysts by formation of a metal phase well dispersed on a cation-deficient perovskite. The FT activity is related to the stability of the precursor perovskite. When initially calcined at 600 °C, a maximum of 8.6 wt.% of Co 0 can be extracted from LaCo 0.40Fe 0.60O 3 (compared to only 2 wt.% after calcination at 750 °C). The catalyst is then composed of Co 0 particles of 10 nm on a stable deficient perovskite LaCo 0.05 3+Fe 0.60 3+O 2.40. Catalytic tests showed that up to 70% in the molar selectivity for hydrocarbons was obtained at 250 °C, 40% of which was composed of the C 2–C 4 fraction.