Theoretical insight into the effect of CO coverage on formic acid formation from dissociated oxygen on χ-Fe₅C₂(510)...

The removal of dissociated oxygen in the form of formic acid over iron-based catalysts can effectively improve atomic utilization efficiency during the Fischer-Tropsch synthesis process. However, surface OH species tend to react with adsorbed hydrogen to generate H₂O, which severely hinders formic acid formation on the χ-Fe₅C₂(510) facet-the key active facet of iron-based catalysts. This work proposes a strategy to promote formic acid formation by increasing CO coverage on this facet, thereby enhancing the reaction probability between CO and OH intermediates. First, the adsorption energies of CO at different coverage degrees are investigated on the hydrogen-covered χ-Fe₅C₂(510) facet. The calculated Gibbs adsorption energies are -1.40 eV, -1.00 eV, and -0.30 eV for 1-3, 4-9, and more than 10 adsorbed CO molecules, respectively. Combined with the effective reaction barriers of 0.94 eV for CO activation and 0.73 eV for formic acid formation, the co-coverage model with 9 adsorbed CO molecules is identified as the most thermodynamically and kinetically favorable configuration for the CO/H co-covered χ-Fe₅C₂(510) facet. Furthermore, microkinetic data reveal that high CO coverage alters the optimal reaction pathway for formic acid synthesis, following the elementary reaction sequence: O + H → OH + CO → COOH + H → HCOOH. Under high CO coverage, the effective reaction barrier of the dominant formic acid formation pathway increases from 0.94 eV to 1.24 eV. In contrast, the energy barriers of competing side reactions for H₂O and CO₂ formation rise more remarkably, increasing from 0.74 eV to 1.26 eV and from 0.85 eV to 1.27 eV, respectively. This kinetic regulation substantially suppresses competitive side reactions and facilitates the elimination of surface dissociated oxygen via formic acid generation. Subsequent kinetic Monte Carlo (kMC) simulations successfully reproduce the formation of formic acid on the CO/H co-covered χ-Fe₅C₂(510) facet, verifying the feasibility and effectiveness of the CO coverage regulation strategy. The theoretical simulation results of this work provide fundamental insights and rational design guidance for the development of advanced Fischer-Tropsch synthesis catalysts with formic acid co-production capability.