Support and rhenium effects on the intrinsic site activity and methane selectivity of cobalt Fischer–Tropsch catalysts

Catalyst composition effects on site activity, active carbon coverage, and methane selectivity in the cobalt-catalyzed Fischer–Tropsch synthesis were examined using 12CO→13CO isotope transients at reaction steady state. All catalysts (18 total) were tested at identical conditions: 221 °C, 6.5 atm of H2/CO/Ne=2/1/1. The intrinsic site activity of unsupported Co and Co-supported on SiO2, TiO2, and Al2O3 (Co dispersion < 10%) varies within a relatively narrow band (∼ 2x maximum rate difference). Re present at a Re/Co weight ratio of 0.1 does not affect Fischer–Tropsch activity and selectivity. Since its own activity is 20 times lower than that of Co, Re does not appear to block or modify active Co sites. While Re and the above-noted three common oxide supports do not affect the Fischer–Tropsch activity and selectivity of the relatively large cobalt crystallites, some modifications of SiO2 and Al2O3 supports (with MgO, Y2O3, ZnO, and Ce2O3, by coating and coprecipitation) reduce site activity by up to an order of magnitude. Interestingly, the reduced site activity does not measurably affect methane selectivity. For all catalysts, the observed site-activity differences, whether small or large, do not correlate with bulk composition, and are possibly the result of unidentified impurities introduced during catalyst preparation. These impurities affect the reactivity of both the adsorbed CO and the active carbon present on the catalyst: higher site activity correlates with higher active carbon reactivity. Although this correlation is strong, some variability in the CO/C∗ reactivity ratio does occur. This variability appears to play a key role in methane selectivity, namely, higher values correlate with lower methane selectivity. These observations can be rationalized by our previously published simple kinetic model.