(617dd) Use of Stability Diagrams to Predict Catalyst Speciation during Fischer Tropsch Reduction and Reaction

An understanding of the speciation of iron catalyst precursor during reduction is an important piece of knowledge in Fischer-Tropsch (FT) reaction. The thermodynamics of the precursor speciation is governed by the reducing gases used, and this tends to influence the activity, selectivity and the catalyst life span. Economic benefits can potentially be obtained if the same syngas used for the normal FT reaction can also be used to activate the catalyst at the normal FT reaction temperature.

During catalyst activation or FT synthesis, the interaction of reducing agents or syngas with Fe-based catalyst results in the formation of several gaseous components such as CO, H₂, CO₂, H₂O, and CH₄. The partial pressure of each gaseous component determines the predominant state of the catalyst. Catalyst speciation happens due to different partial pressures of the gaseous components, and as a result, the stable iron phases formed during FT synthesis are those that are in equilibrium with the gas composition. Although the ratio of P_H2/P_H2O and P_CO/P_CO2 or the partial pressure of water and carbon dioxide does not have any appreciable effect on the deactivation by oxidation on the FT reaction rate in Cobalt catalysts, in the iron based catalyst deactivation by oxidation is predominant. Based on thermodynamic calculations of the FT system, the maximum allowable oxygen partial pressure during catalyst activation is 10^-45 bar. The oxygen partial pressure in therefore dependent on the PH₂/PH₂O and PCO₂/P_CO  ratios, and the equilibrium constants. Plots of O₂ partial pressure against PH₂/PH₂O , PCO₂/P_CO  ratios have been used to predict the stability of the different species.