Fischer–Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst

Abstract

The effect of noble metal promoters (atomic ratio of promoter to Co=1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493K, 2.2MPa and H2/CO=2.1 using a 1-L continuously stirred tank reactor (CSTR). The results show that all promoted catalysts exhibited markedly higher initial CO conversion rates on a per gram catalyst basis than the unpromoted one, which was ascribed to increased Co site density when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092–0.105s−1 (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was about 40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites.At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was observed for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2–C4 olefin selectivity (7.5–60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addition of Pd and Pt promoters resulted in lower olefin selectivity (4.4–55%) but higher 2-C4 olefin selectivity (14.3 to 27–31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible.Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were observed to slightly enhance oxygenate formation, while Re and Ru slightly decreased it.