A promising preparation method for highly active cobalt based Fischer-Tropsch catalysts supported on stabilized Al2O3

Abstract

The effectiveness of the addition of diethylene glycol (DEG) to the impregnating solution in synthesizing highly dispersed Co-based catalysts supported on stabilized alumina (Al2O3(s)) is investigated. Both the properties and catalytic performance in the Fischer-Tropsch synthesis (FTS) of the material obtained using DEG (CoDEG/Al2O3(s)) are compared with those of a catalyst with the same formulation but prepared without the addition of DEG in the impregnating solution (Co/Al2O3(s)). When using Co-nitrate as Co-precursor, the addition of DEG leads to a very fast and exothermic decomposition of Co-nitrate into Co oxides during the calcination step. This prevents the agglomeration of Co3O4 particles, thus generating highly dispersed Co3O4 crystallites on the support. In line with the decrease of the Co3O4 crystallites size, the CoDEG/Al2O3(s) catalyst is more difficult to reduce than the Co/Al2O3(s) catalyst. As a result, the metallic surface of the two catalysts is very similar. Nonetheless, when tested in the FTS, the CoDEG/Al2O3(s) catalyst shows CO conversion higher than the Co/Al2O3(s) sample. This is attributed to the high intrinsic activity (i.e. high Turnover Frequency, TOF) of the small Co0 crystallites of the CoDEG/Al2O3(s) catalyst. We explain this result assuming that the combustion phenomenon occurring during the fast calcination induced by the presence of DEG may generate structural defects on the catalyst surface that are beneficial for the FTS. Notably, the product distribution of CoDEG/Al2O3(s) catalyst is only slightly affected.