Adsorption and removal mechanism of atomic oxygen on different facets of θ-Fe₃C

The Fisher-Tropsch Synthesis (FTS) is one of the most intensively studied reactions in heterogeneous catalysis, which could convert the syngas (CO and H₂) from coal, natural gas, shale gas, and biomass into gasoline, diesel, and a series of important chemical products. The process provides an effective strategy for reducing environmental pollution caused by direct coal combustion, at the meanwhile, alleviating dependence on imported petroleum. During the reaction, iron-based catalysts have attracted the attention of numerous researchers due to their unique advantages, for example, low cost, flexibility in product distribution, suitability for low H₂/CO ratio syngas and large operating space. And the iron-based catalysts in FTS have achieved applications successfully in industry. Iron carbide is recognized as active phases in FTS catalyzed by iron-based catalysts, among which θ-Fe₃C is one of the dominant phases. Additionally, θ-Fe₃C is widely applied in fields such as biomedicine and electrochemistry. However, θ-Fe₃C could be oxidized under realistic conditions, affecting the performance as magnetic materials and catalysts seriously. Considering the case above, the investigation of oxidation of iron carbide θ-Fe₃C is of great significance. And exploring the adsorption, as well as removal mechanism of atomic oxygen over the iron carbide facet is helpful for understanding the oxidation process, and providing a reference for improving the stability of catalysts. In this work, we have explored the adsorption of oxygen atoms from low to high coverage on three θ-Fe₃C surfaces with density functional theory. Ab initio atomistic thermodynamics was utilized to investigate the effect of experimental conditions like temperature and partial pressure of H₂O. According to the calculation, it was found that the adsorption at low coverage on (110) was the strongest, followed by (001), and the adsorption on (011) was the weakest, meaning that (110) can be oxidized easily. As the number of oxygen atoms adsorbed on the surface increases, the stepwise adsorption energy increases, which is a manifestation of the repulsive effect between adsorbed oxygen atoms. The average adsorption energy for each surface increases with the increase of adsorbed oxygens, and the magnitude of increase varies due to the different local structures of each facet. The average adsorption energy on (011) has a relatively small increase, indicating that the repulsion between oxygen atoms adsorbed on this crystal plane is weak; whereas opposite on (001) and (110) facets. Atomistic thermodynamic studies showed that increasing the partial pressure of H₂O or decreasing the temperature will stabilize the O adsorption, leading to surface oxidation. In addition, the highest O coverage on (110) under typical FTS conditions further proved that the facet is easily oxidized, which is consistent with the adsorption results at low coverage. Finally, the removal path of adsorbed oxygen on different facets was calculated, and the results showed that adsorbed O on (011) prefer to react with CO with energy barrier of 0.84 eV. On (001) and (110), removal in the form of H₂O via OH disproportionation is more favored, but the energy barrier to form O−H bond is higher for the latter facet (1.72 vs 1.47 eV).