Adsorption of a water molecule on Fe(100): Density-functional calculations

Molecular and dissociative adsorption of a single water molecule on the Fe(100) surface has been studied by using density-functional theory calculations. We found that there exists a locally stable molecular adsorption state with an adsorption energy of 0.39 eV, where the H₂O molecule adsorbs on top of a surface Fe atom in a flat-lying molecular configuration. This molecular configuration is found to well reproduce the water-induced vibrational frequencies measured in a low-temperature electron-energy-loss spectroscopy (EELS) study. The H₂O molecular state is subject to a dissociation into H+OH species with an activation barrier of 0.35 eV. A further dissociation of the OH group into H+O species requires a higher activation energy of 0.79 eV. The prediction of the H₂O molecular precursor and the energy diagram for its dissociation is in good accordance with the adsorption picture which was suggested in a previous EELS study but has been incompatible with a previous density-functional study predicting a barrierless H+OH dissociation of water molecule.