Fischer-Tropsch synthesis: Direct cobalt nitrate reduction of promoted Co/Al2O3 catalysts

Direct reduction of cobalt nitrate versus conventional calcination/reduction treatment was conducted using alumina with identical methodology as previously applied to SiO2 and TiO2. Similar BET surface areas, pore volumes and pore size distributions were obtained for the activated calcined and uncalcined catalysts indicating no significant difference in morphological properties. However, the reducibility slightly increases and Co crystallite size is smaller for activated uncalcined samples. Reduction phenomena were analyzed by TPR-MS and TPR-EXAFS/XANES. Combining these techniques allows an explanation of the complex phenomena occurring during the direct reduction of cobalt nitrate, as both nitrate decomposition and cobalt oxide reduction are involved. Cobalt nitrate species are converted to CoOx intermediates. These species are oxidized by NOX (from nitrate decomposition) to Co3O4 spinel, which is converted to CoO prior to Co0 formation. Noble metals (Pt, Re, Ru and Ag) improve cobalt oxide reducibility, especially for the final reduction step (i.e., CoO to Co0). The effect of direct nitrate reduction on FT activity was investigated using a 1 L CSTR. Activated unpromoted and Ptpromoted uncalcined catalysts achieved higher initial and steady-state CO conversions in comparison to the corresponding calcined catalysts. The best performance was achieved with direct reduction of uncalcined 0.5 % Pt-25 %Co/Al2O3.

Automated summary

Direct reduction of cobalt nitrate using alumina showed differences in reducibility and Co crystallite size compared to traditional calcination/reduction treatment. Noble metals improved cobalt oxide reducibility, especially in the final reduction step. The best performance was achieved with direct reduction of uncalcined 0.5% Pt-25%Co/Al2O3.