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.