Evaluation of Reoxidation Thresholds for γ-AI_2O_3-Supported Cobalt Catalysts under Fischer-Tropsch Synthesis Conditions

摘要

Size-dependent phenomena at the nanoscale influence many applications, notably in the science of heterogeneous catalysis. In cobalt-based Fischer-Tropsch synthesis (FTS), the size of Co nanoparticles (NPs) dictates to a high degree catalyst's performance in terms of activity, selectivity, and stability. Here, a highly dispersed Re/Co/χ-Al_2O_3 catalyst with high Co surface area per gram of catalyst was exposed to industrially relevant FTS conditions and monitored in situ by synchrotron X-ray radiation. X-ray absorption near-edge structure spectra were obtained on the cobalt K edge and Re L_3 edge of the working catalyst. The experimental results demonstrate development of tetrahedrally coordinated Co~(2+) forming at the expense of metallic Co~(0). The structure of the oxide resembles CoA1_2O_4 and appears at the onset (first 5-10 h) of the reaction. Reoxidation of Co~(0) is more pronounced close to the outlet of the reactor, where higherpH_2O is anticipated. The state of the Re promoter does not change during the FT process. We propose that reoxidation of small Co NPs is followed by spreading of Co oxide that leads to the formation of Co_xAi_yO_z phases. Hence, in order to avoid an irreversible loss of the active phase during process start-up, catalyst design should be restricted to Co NPs larger than 5.3 nm.