X-ray Chemical Imaging of Mn-doped Co/TiO2 Pellets reveals the significance of Cobalt Carbide Formation and Distribution on CO conversion and Alcohol Selectivity during Fischer Tropsch Synthesis

Abstract

Abstract X-ray diffraction/scattering computed tomography (XRS-CT) were used to create 2D images, with 20 µm resolution, of passivated Co/TiO2/Mn Fischer Tropsch catalyst extrudates post-reaction after 300 h on stream under industrially relevant conditions. This combination of scattering techniques provided unprecedented insights into both the spatial variation of the various cobalt phases and the influence of increasing Mn loading has on this. Also observed is the presence of a wax coating throughout the extrudate and its capacity to preserve the Co/Mn species in the same state as in the reactor. Correlating these findings with catalytic performance, highlights the crucial phases and active sites within Fischer Tropsch catalysts required for understanding the tunability of the product distribution between saturated hydrocarbons or oxygenate and olefin products. In particular a Mn loading of 3 wt. % led to an optimum equilibrium between the amount of hexagonal close-packed Co and Co2C phases resulting in maximum oxygenate selectivity. XRS-CT revealed Co2C to be located on the extrudates' periphery, while metallic Co phases were more prevalent towards the centre, possibly due to a lower [CO] ratio there. Reduction at 450°C of a 10 wt. % Mn sample resulted in MnTiO3 formation which inhibited carbide formation and alcohol selectivity. It is suggested that small MnO particles promote Co carburisation by decreasing the CO dissociation barrier and the Co2C phase promotes CO non-dissociative adsorption leading to increased oxygenate selectivity. This study highlights the influence of Mn on catalyst structure and function and the importance of studying catalysts under industrially relevant reaction times.