Site-activity, active carbon coverage and methane selectivity of cobalt Fischer-Tropsch catalysts: Effects of catalyst composition and reaction conditions

Catalyst composition and reaction conditions effects on site-activity, active carbon coverage and methane selectivity in cobalt-catalyzed Fischer-Tropsch synthesis are examined using 12CO→13CO isotope transients, applied for the first time under commercially important high-pressure conditions. To address catalyst composition effects, a large suite of catalysts was tested under a single, constant set of reaction conditions: 221°C, 6.5 atm, H₂/CO/Ne = 2/1/1, CO conversion = 12–16%. Intrinsic site-activities of adsorbed CO (k_CO) and active carbon (k_C*), as well as methane selectivity, are unaffected by the support identity (TiO₂, Al₂O₃ and SiO), Re addition, and secondary reactions. Modified support catalysts (-Y₂O, -MgO, -ZnO modification of Al₂O₃, SiO₂, SiO _2·Al₂O₃ and Ce₂O₃) show reduced site-activities, but normal methane selectivities. For all catalysts, the variability in site-activity indicates either mild structure sensitivity for cobalt FT or susceptibility to chemical inhibition by impurities. Although the cause of the site-activity variability is unknown, lower methane selectivity correlates with higher active carbon coverage, resulting from variable active carbon reactivity. To examine reaction conditions effects, an unsupported cobalt catalyst was tested over a wide range of conditions: 190–228°C, 3–15 atm syngas, H₂/CO = 1/1–4/1, $PH2O$ = 0–8 atm, CO conversion = 12%. The amount of reversibly adsorbed CO is constant under all conditions, except for a small decrease (10–20%) during water co-feeding. By contrast, the active carbon inventory, not affected by temperature or $PH2$, displays saturation behaviour with increasing P_CO. Active carbon does not compete for CO adsorption sites. Water co-feeding causes substantial catalyst deactivation, resulting from both a decrease in the number of active sites, due to particle sintering, and a decrease in site-activity. Water also invokes reversible catalyst changes. Water decreases methane selectivity, and increases alpha and the reactivity of adsorbed CO without affecting the active carbon reactivity, leading to higher active carbon coverage. A simple model of hydrocarbon formation correlates the majority of selectivity changes observed in the catalyst composition and reaction conditions studies with the active carbon coverage on the catalyst. In addition, the model reveals that the rate coefficient ratio of chain initiation to CO turnover (k _i/k_CO) may be the dominant factor determining FT selectivity.