Density Functional Theory Study of CO Adsorption on the CaFe₂O₄ (001) Surface

The density functional theory (DFT) has been employed to investigate the relative stability of CaFe2O4 (001) surface at seven terminations and the adsorption of CO onto the six possible topmost sites of the CaFe2O4 (001) surface. The results showed the surface energies ranged from 8.16 to 23.71 eV·nm−2 at the seven terminations and the surface with the O1 atom termination was the most stable structure. For the O1-terminated CaFe2O4 (001) surface, compared to the Ca, Fe1 and Fe2 atoms, CO molecule tended to adsorb to the topmost three O atoms. The adsorption energies were −3.19, −0.82 and −0.80 eV when CO molecule adsorbed on the O1, O2 and O3 atoms, respectively. More than 0.5 electrons were transferred to CO molecule from surface on the O sites, whereas CO molecule obtained few electrons with CO molecule adsorption on others atoms. Furthermore, two new C–O singe bonds, a new C–O double bonds, a new C–O and C–Fe single bond were generated with CO adsorption on O1, O2 and O3 sites, respectively, which implied the CO2 species formation. However, the CO2 dissociation energies were 1.80, −0.33 and 0.32 eV from the O1, O2 and O3 sites, respectively. Although CO preferentially adsorbed on O1 site to form a CaCO3-like stable sturcture, the CO2 dissociation was difficult from O1 site. Moreover, since the CO adsorption and CO2 dissociation energies were negative values, CO molecule spontaneously adsorbed on O2 site and deprived oxygen bound to iron to form the CO2 molecule.