The impact of pre-reduction thermal history on the metal surface topology and site-catalytic activity of Co=SiO2 Fischer Tropsch catalysts

Abstract

[EN] For ex-nitrate Co/SiO2 Fischer Tropsch synthesis (FTS) catalysts, the influence of the early thermal treatments during catalyst synthesis on the ultimate catalytic properties is studied without overlapping of particle size effects (d(Co) > 15 nm). The bulk and surface properties of the Co3O4 crystallites in the oxidic catalyst precursors depend on the previous thermal history and are decisive for the oxide reduction kinetics. Even after a homogeneous reduction treatment (673 K, H2 flow), catalysts with different prereduction thermal histories showed dissimilar TOF (per surface Co0 atom) under FTS conditions (493 K, 2.0 MPa). As revealed by CO-TPSR and CO-FTIR, the different surface-specific activitites relate to differences in the surface topology of the Co0 nanoparticles which likely result from the varying reduction kinetics. By tracking the effects of the earlier thermal treatments on the properties of the Co species, this work evidences that the pre-reduction thermal history not only determines metal dispersion and reducibility, but also the ultimate TOF. Such memory effect underscores the significance of a rational design of the entire synthesis and activation procedures to achieve Co-based Fischer Tropsch catalysts with improved activity.