A density functional theory based elementary reaction mechanism for early steps of Fischer-Tropsch synthesis over cobalt catalyst. 2. Microkinetic modeling of liquid-phase vs. gaseous-phase process

The process of Fischer-Tropsch synthesis (FTS) is conducted commercially under both gaseous- and liquid-phase conditions. For decades, numerous studies have been performed to better understand this heterogeneous catalytic process, where some of them have tried to compare the reaction rates and product distributions of FTS under different phase conditions. In spite of several provided models, the actual reaction mechanisms and kinetics of important elementary steps at each phase condition are still under investigation, especially in the liquid-phase. In this work, we have utilized the generalized gradient approximation method of density functional theory (DFT-GGA) to estimate the rate parameters of elementary reactions in liquid-phase FTS on a flat Co(0001) catalyst surface. In this regard, using the continuum-based conductor-like screening model (COSMO) for the liquid solvent environment, the activation energies and rate constants of individual reactions, suggested in an existing microkinetic model, have been calculated in the presence of n-hexane solvent at T = 493 K and P = 25 bar. The preferred reaction pathway has been discussed and the quantitative results have been compared to that of the gaseous-phase in order to find the impact of liquid solvent, as well as differences in the probable surface reactions under each phase condition. It has been found that while the carbide mechanism is the dominant mechanism of chain growth under both phase conditions, the presence of the liquid solvent enhanced the CO insertion sub-mechanism of chain growth, which can describe some of the experimental observations in FTS studies. (C) 2017 Elsevier B.V. All rights reserved.