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

Abstract

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 ${\text{H}}_{2}\text{O}$ 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 ${\text{H}}_{2}\text{O}$ molecular state is subject to a dissociation into $\text{H}+\text{OH}$ species with an activation barrier of 0.35 eV. A further dissociation of the OH group into $\text{H}+\text{O}$ species requires a higher activation energy of 0.79 eV. The prediction of the ${\text{H}}_{2}\text{O}$ 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 $\text{H}+\text{OH}$ dissociation of water molecule.