Adsorption of atmospheric gases on cementite $010$ surfaces

We study the adsorption of a series of small molecules on the nonstoichiometric {010} surface of cementite (θ-Fe₃C) by means of first-principles calculations. We find that CO, N₂, H₂O, and CH₄ prefer to adsorb over iron atoms in an atop configuration. O₂, CO₂, and the OH radical prefer a configuration bridging two iron atoms and CH₂O adsorbs in a configuration bridging a surface iron atom and a surface carbon atom. Adsorption energies are small for H₂, CO₂, and CH₄, indicating a physisorption process, while those for CO, CH₂O and especially for O₂ and the OH radical are large, indicating a strong chemisorption process. H₂O and N₂ display adsorption energies between these two extremes, indicating moderate chemisorption. The dissociation of H₂, CH₂O, the OH radical, and O₂ is favoured on this surface. Comparison with adsorption on Fe{100} surfaces indicates that most of these gases have similar adsorption energies on both surfaces, with the exception of CO and the OH radical. In addition, we find similarities between the reactivities of cementite and Mo₂C surfaces, due to the similar covalent character of both carbides.