Microcalorimetric Studies on Co–Re/γ-Al₂O₃ Catalysts with Na Impurities for Fischer–Tropsch Synthesis

Abstract

In Fischer–Tropsch synthesis (FTS), sodium impurities originating from biomass feedstock can be introduced into the catalyst as traces via the synthesis gas. The addition of small amounts of Na to a Co-based Fischer–Tropsch catalyst has been studied in terms of catalytic performance and key characteristics. The Na addition had a severe effect on the catalytic performance, causing significant suppression of the catalytic activity. Small amounts of Na (up to 1000 ppm) had no effect on chemisorption amounts or heats of adsorption for H 2 or CO. Higher amounts of Na added (10 000 or 20 000 ppm) had a dramatic effect on the activity and selectivity in CO hydrogenation. Minor changes in heats of adsorption and chemisorption uptake suggest that the effect of large amounts of Na can be assigned both to geometric and electronic effects. Stronger adsorption of the reactants in parallel to the diffusion barrier caused by the site blocking from Na atoms could explain the dramatic drop in the catalytic activity. For the samples containing low amounts of Na (<1000 ppm), the results cannot support the previous hypothesis; in this case, the negative effect on the activity is suggested to be ascribed to the mobility of the Na, triggered by the presence of water under FTS conditions.