Mechanistic aspects of facet-dependent CH4/C2+ selectivity over a χ-Fe5C2 Fischer–Tropsch catalyst

An optimal Fe–C coordination ensemble for hydrocarbon chain growth: a full Fischer–Tropsch synthesis mechanism from machine learning

“…For example, Yin et al 30 found that the coupling barriers of CH x + CH y on Fe 5 C 2 (510) are >2.3 eV using a surface model with C vacancies left, but our results show that the surface with C vacancies healed is much better for C–C coupling (<1.4 eV). Pham et al 31 investigated similar C + CH and CH + CH occurring at two neighboring Fe 4 squares from two A-P5 sites on Fe 5 C 2 (510) and found that the barriers are ∼1.27 eV. Our ML-TS data for the barriers of the direct coupling between two CH x species at two Fe 4 squares are indeed similar ( Fig.…”

“…On the other hand, theoretical studies generally do not support experimental FTS ndings on the hydrocarbon formation rate. [26][27][28][29][30][31][32][33][34] By using density functional theory (DFT) calculations on an Fe-B 5 site from an Fe-terminated Fe 5 C 2 (100) model, Cheng et al 26 showed that CO dissociation can proceed with a barrier of 0.79 eV, 33 but the effective barriers of C hydrogenation to CH 4 and C-C coupling (C + CH 3 ) are very high, ∼1.9 eV. Even on bulk-truncated FeC x surfaces 27,30,31 with a mixed Fe/C termination, the barriers for CH 4 formation or C-C coupling are still unexpectedly high, for example, 2.18 eV for CH 4 formation by Pham et al 31 and >2.34 eV for C-C coupling by Yin et al 30 on Fe 5 C 2 (510).…”

“…[26][27][28][29][30][31][32][33][34] By using density functional theory (DFT) calculations on an Fe-B 5 site from an Fe-terminated Fe 5 C 2 (100) model, Cheng et al 26 showed that CO dissociation can proceed with a barrier of 0.79 eV, 33 but the effective barriers of C hydrogenation to CH 4 and C-C coupling (C + CH 3 ) are very high, ∼1.9 eV. Even on bulk-truncated FeC x surfaces 27,30,31 with a mixed Fe/C termination, the barriers for CH 4 formation or C-C coupling are still unexpectedly high, for example, 2.18 eV for CH 4 formation by Pham et al 31 and >2.34 eV for C-C coupling by Yin et al 30 on Fe 5 C 2 (510). The theoretical turnover frequencies (TOFs) from these calculated barriers are thus at least two orders of magnitude lower for methane and long-chain hydrocarbon formation compared to those from experiments.…”

Theoretical Perspectives on the Modulation of Carbon on Transition-Metal Catalysts for Conversion of Carbon-Containing Resources

“…The last three surfaces together account for only 23.5% of the total exposed surface area of an Fe 5 C 2 model particle, yet they are predicted to be responsible for remarkable kinetic competitiveness in the CH 4 formation (96.4%) . Pham et al , also showed that the CH 4 /C 2+ selectivity depends highly on the exposed surface of Fe 5 C 2 .…”