Water Interaction with Iron Oxides

We present a mechanistic study on the interaction of water with a well‐defined model Fe3O4(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol−1) is considerably higher than all previously reported values obtained by indirect desorption‐based methods. By employing 18O‐labeled water molecules and an Fe3O4 substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non‐dissociated water molecule creating a thermodynamically very stable dimer‐like complex. The adsorption of water on Fe3O4 surfaces is experimentally and theoretically investigated. Calorimetric and IR spectroscopy measurements under ultrahigh vacuum conditions combined with state‐of‐the‐art density functional calculations show that adsorbed water molecules on the Fe3O4(111) surface tend to form dimer‐like complexes consisting of one intact and one dissociated water molecule (see picture).