Density functional theory study of H2 adsorption on the (100), (001) and (010) surfaces of Fe3C

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.

The Molecular Adsorption of Carbon Monoxide on Cobalt Surfaces: A Dft Study

The theoretical molecular adsorption energies, vibrational frequencies and total density of states of carbon monoxide (CO) on the (100), (110) and (111) surfaces of the face-centred cubic (FCC) crystalline phase of metallic cobalt were investigated using density functional theory calculations. The on-top adsorption state and three surface coverages were used for comparison of the results. The geometries of cobalt FCC surfaces, as well as those with adsorbed CO molecules and the CO binding energies were calculated with the generalised gradient approximation (GGA-D) using the revised revPBE-D3(BJ) functional. The theoretical results for adsorption energies of carbon monoxide were proportional to the electron density of the cobalt surfaces, according to the following order: FCC (100) > FCC (110) > FCC (111). For CO adsorbed on the surface of cobalt metal the C–O distance increases, producing a weakening of the bond and the calculated stretching frequency decreases when compared with the isolated molecule.

Structures and energies of Cu clusters on Fe and Fe3C surfaces from density functional theory computation

Coverage and surface dependent adsorption configurations of Cu_n clusters on the Fe and Fe₃C surfaces.

Insight into CH(4) formation in iron-catalyzed Fischer-Tropsch synthesis

Spin-polarized density functional theory calculations have been performed to investigate the carbon pathways and hydrogenation mechanism for CH(4) formation on Fe(2)C(011), Fe(5)C(2)(010), Fe(3)C(001), and Fe(4)C(100). We find that the surface C atom occupied sites are more active toward CH(4) formation. In Fischer-Tropsch synthesis (FTS), CO direct dissociation is very difficult on perfect Fe(x)C(y) surfaces, while surface C atom hydrogenation could occur easily. With the formation of vacancy sites by C atoms escaping from the Fe(x)C(y) surface, the CO dissociation barrier decreases largely. As a consequence, the active carburized surface is maintained. Based on the calculated reaction energies and effective barriers, CH(4) formation is more favorable on Fe(5)C(2)(010) and Fe(2)C(011), while Fe(4)C(100) and Fe(3)C(001) are inactive toward CH(4) formation. More importantly, it is revealed that the reaction energy and effective barrier of CH(4) formation have a linear relationship with the charge of the surface C atom and the d-band center of the surface, respectively. On the basis of these correlations, one can predict the reactivity of all active surfaces by analyzing their surface properties and further give guides for catalyst design in FTS.