Microkinetic model validation for Fischer-Tropsch synthesis at methanation conditions based on steady state isotopic transient kinetic analysis

A Single-Event MicroKinetic (SEMK) model has been extended towards the simulation of Steady StateIsotopic Transient Kinetic Analysis (SSITKA) data for Co catalyzed Fischer-Tropsch Synthesis (FTS). Theextended 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 experimentsresulted in physicochemically meaningful estimates for the activation energies and atomic chemisorptionenthalpies. The application of the phenomenological UBI-QEP method allows to physically interpret thenature of the two site types considered in the model, i.e., terrace and step sites. A reaction path analysisshows that over 80 percent of the CO reacts on the step sites. Furthermore, chain growth exclusivelyoccurs on these sites. The terrace sites are less reactive for CO dissociation and are identified as responsiblefor methane production. A fraction of the alkenes, produced on the step sites, is hydrogenated toalkanes on the terrace sites. Based on model simulations, the metal particle size effect, i.e., a lowerTOF, 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