Microkinetic Model Validation for Fischer-Tropsch Synthesis at Methanation Conditions based on Steady State Isotopic Transient Kinetic Analysis

Abstract A Single-Event MicroKinetic (SEMK) model has been extended towards the simulation of Steady State Isotopic Transient Kinetic Analysis (SSITKA) data for Co catalyzed Fischer-Tropsch Synthesis (FTS). The extended model considers two types of sites and both direct and H-assisted CO dissociation. Regression of the model parameters to the data obtained from 17 steady state and 11 SSITKA experiments resulted in physicochemically meaningful estimates for the activation energies and atomic chemisorption enthalpies. The application of the phenomenological UBI-QEP method allows to physically interpret the nature of the two site types considered in the model, i.e., terrace and step sites. A reaction path analysis shows that over 80 percent of the CO reacts on the step sites. Furthermore, chain growth exclusively occurs on these sites. The terrace sites are less reactive for CO dissociation and are identified as responsible for methane production. A fraction of the alkenes, produced on the step sites, is hydrogenated to alkanes on the terrace sites. Based on model simulations, the metal particle size effect, i.e., a lower TOF, higher methane selectivity and increasing alkane to alkene ratio with decreasing metal particle size, is attributed to an increasing relative importance of the terrace sites on the reaction kinetics.