Influence of operating conditions in a continuously stirred tank reactor on the formation of carbon species on alumina supported cobalt Fischer–Tropsch catalysts

The paper focuses on the identification of carbon species using a wide range of ex situ characterization techniques (TPH/MS, TGA-DTA, XPS, TOF-SIMS) in spent 15 wt% Co/Al2O3 supported catalysts exposed to different conditions of Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR) at 20 bar of total pressure, different gas-space velocities and H2/CO ratios. Three types of carbonaceous species were uncovered in the used catalysts: (i) hydrcarbobns (probably wax), (ii) strongly adsorbed hydrocarbons and (iii) amorphous polymeric carbon. The amounts of deposed carbon species strongly depend on the operating conditions. In agreement with the observed slow catalyst deactivation, only very small amounts of strongly adsorbed hydrocarbons and polymeric carbon were observed when the catalyst was exposed to a H2/CO = 2 syngas ratio and moderate carbon monoxide conversions. Lower gas space velocity and lower H2/CO ratio in syngas lead to larger amounts of deposed carbon species. Strongly adsorbed hydrocarbons and amorphous polymeric carbon seem to contribute to catalyst deactivation. Cobalt carbide and graphitic carbon were not detected in the used catalysts by ex situ methods.