Effects of CO2 on the deactivation behaviors of Co/Al2O3 and Co/SiO2 in CO hydrogenation to hydrocarbons

Abstract

Different deactivation behaviors of the prototype Co/γ-Al₂O₃ (CoAl) and Co/SiO₂ (CoSi) catalysts under an excess CO₂ environment were investigated in terms of the surface oxidation and aggregation of cobalt crystallites for the Fischer–Tropsch synthesis (FTS) reaction. The presence of excess CO₂ in the syngas feed largely altered the catalytic activity and product distribution, especially on the CoAl catalyst. A relatively faster deactivation and lower C₅₊ selectivity under an excess CO₂ environment were observed on CoAl compared with the CoSi, which also were fairly stable under the same reaction conditions. From measurements of the reduction–oxidation behaviors of the cobalt crystallites, it could be seen that CO₂ molecules acted as a mild oxidant by partially oxidizing the exposed metallic cobalt surfaces. The dramatic decrease in CO conversion and an increase in CH₄ selectivity under the CO₂ environment over CoAl were mainly attributed to the irreducible oxidation of the metallic cobalt surfaces through strong interactions with the Al₂O₃ support. Meanwhile, the marginal deactivation rate and lower changes of selectivity on CoSi were mainly attributed to the reversible oxidation–reduction property of the metallic cobalt crystallites by forming larger cobalt crystallites with relatively weak interactions with the SiO₂ support. An excess exposure to the mild oxidant of CO₂ on the FTS catalysts generally decreased the catalytic activity irreversibly by forming strongly interacted large cobalt crystallites, which was more predominantly seen on the acidic Co/Al₂O₃ than on the Co/SiO₂ catalyst. The easy reversibility of the oxidation–reduction of the surface metallic cobalt crystallites on SiO₂ even in the presence of excess CO₂ could prevent catalyst deactivation during the FTS reaction.