CO adsorption, oxidation and carbonate formation mechanisms on Fe₃O₄ surfaces

By means of density functional theory calculations that account for the on-site Coulomb interaction via a Hubbard term (DFT+U), we systematically investigated CO adsorption on Fe₃O₄ surfaces at different coverages. It has been found that more than one CO can coadsorb on one surface iron atom on both Fe_tet1 and Fe_oct2 terminations of Fe₃O₄(111). The uncapped oxygen atom is the active site for CO oxidation on both Fe_tet1 and Fe_oct2 terminations of Fe₃O₄(111). For Fe₃O₄(110), two CO molecules prefer to coadsorb on one surface iron atom on the A layer; CO prefers to adsorb at the bridge site of the surface octahedral iron atoms at low coverage, while CO prefers to adsorb at the surface tetrahedral iron atom at high coverage on the B layer. It has been found that the surface oxygen atom which is not coordinated to the tetrahedral iron atom is the active site for CO oxidation on the B termination of Fe₃O₄(001). On the Fe₃O₄ surfaces, the formation of carbonate has been found to be very stable thermodynamically, which agrees well with experiments. The adsorption mechanism has been analyzed on the basis of projected density of states (PDOS).