(451h) Manganese Oxide Surface Chemistry and Adsorption of Cr(III)

            Manganese oxides are
prevalent in geological settings and play a significant role in the geochemical
cycling of heavy metals due to their high adsorption capacity.  Of particular interest is their ability to
oxidize Cr(III) to toxic Cr(IV).  While
much work has focused on the mechanism of oxidation, several questions remain
about the specific manganese-chromium interactions prior to and during
oxidation.  An understanding of the
structure, chemical properties, and redox reactivity of manganese oxides
surfaces is an essential foundation for determining possible Cr(III) oxidation
mechanisms.

            Periodic density
functional theory calculations have been combined with ab initio thermodynamics to identify stable beta‑MnO₂
(110) and gamma‑MnOOH (010) surface
terminations and to probe their redox behavior in response to varying oxygen
and water chemical potentials.  Under ambient
conditions, oxidation of the beta‑MnO₂ (110) surface is not
likely, while oxidation of the gamma‑MnOOH
(010) surface is predicted to be favorable. 
Reduction of the clean surfaces leads to significant surface
reconstructions that result from the competition between optimal d‑orbital
occupation and manganese coordination geometry and lattice constraints of the
bulk.  Jahn‑Teller
effects allow multiple oxidation states to exist at the surface.  While these surfaces are not predicted to be
stable at ambient conditions, they may be important in the oxidation
mechanism.  Hydration of the clean beta‑MnO₂
(110) and gamma‑MnOOH (010) surfaces generally
lowers the surface free energies significantly with hydrogen bonding networks
formed at monolayer coverage.  Adsorption
of Cr(III) has been investigated on the hydrated surfaces.  The effect of manganese oxidation state on
adsorption geometries and electron transfer will be discussed.