Unique Chain-Growth and oxygen removal mechanisms in Fischer-Tropsch synthesis on the ε-Fe2C (1-21) Surface: Insights from DFT calculations and microkinetic modeling

Iron catalyzed Fischer-Tropsch Synthesis (FTS) process has been successfully applied in indirect coal liquefaction industry. The working industrial catalyst is typically a mixture of various iron carbide phases. While mechanistic studies have predominantly focused on the primary active phase chi-Fe5C2, the limited investigation into reaction processes on other phases has constrained a comprehensive understanding of the overall reaction mechanism. Among the iron carbide phases, epsilon-iron carbide (epsilon-Fe2C/Fe2.2C) is commonly detected in industrial catalyst and has been identified one of important active phase with high performance for FTS. In this work, a comprehensive theoretical study including density functional theory (DFT) calculation and microkinetic modeling has been performed on the epsilon-Fe2C (12 1) surface to provide valuable new insights into FTS. A chain-growth mechanism involving both CO insertion and CH* coupling has been identified. The formation of C2 hydrocarbons is more feasible through CO/CHO insertion followed by hydrogenation and deoxygenation. These C2 intermediates can decompose into CHx species at iron sites, which serve as monomers for further chain propagation and methanation. For the removal of oxygen, we identified CH species at carbon vacancies can significantly reduce the energy barriers for OH* and H2O formation, making water the main oxygen removal product instead of CO2. These findings reveal significant differences between the reaction network on C-rich epsilon-Fe2C (12 1) surface and those on other known iron-terminated surface, highlighting the need for FTS mechanism studies on various types of carbide iron surfaces.