The interaction of water with iron oxide surfaces is relevant
for several processes of practical interests.
Three different adsorbed water species were distinguished
on Fe-terminated
Fe₃O₄ multilayer films using
thermal desorption spectroscopy (TDS), and
ultraviolet photoelectron spectroscopy (UPS)
measured under adsorption-desorption equilibrium
conditions [1].
By means of density functional theory (DFT)
calculations,
the first species (γ-water) were confirmed to
correspond to dissociative water adsorption with
the resulting hydroxyl (OH) groups of water on the
surface iron (Fe)
sites and the H-atoms adsorbed onto surface
oxygen (O) sites.
The DFT result for the γ-water structure
is consistent with the two OH-stretch lines
observed by infrared-reflection-adsorption-spectroscopy
(IRAS) [2], and the UPS study [1].
The DFT calculations confirm the subsequent formation
of mixed molecular- and dissociated-water structures (β-water) formed by
H-bonded molecular water to the surface OH-groups
on surface-Fe,
and the H on the surface-O sites, respectively,
as suggested by the IRAS [2] and LEED experiments [2].
The DFT results reveal that formation of
the γ-water overlayer structure results
from the diffusion of the mobile H-atoms from
the initially molecular adsorbed water
on iron sites followed by
formation of a transition structrure with
the H-atom adsorbed
the nearest-neighboring oxygen sites,
diffusing
over the surface to adsorb on-top onto the next
O-sites.
[1] W. Weiss and W. Ranke, Prog. Surf. Sci. 70, 1 (2002).
[2] U. Leist, W. Ranke and K. Al-Shamery, Phys. Chem. Chem. Phys. 5,
2435 (2003).

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