First principles investigation of b-MnO2 surfaces: Response to oxygen pressure
Gloria A. E. Oxford and Anne M. Chaka
Ubiquitous in geological settings, manganese oxides form environmentally relevant surfaces that adsorb and oxidize heavy metal ions, thus affecting their toxicity and bioavailability. Of particular interest is the ability of manganese oxides to oxidize trivalent chromium to the toxic hexavalent form. Because the reactivity of mineral surfaces is related to their surface structures, compositions, and chemical properties, a fundamental understanding of manganese oxide surface reconstructions and their redox behavior is essential to develop accurate geochemical models to determine the fate of hexavalent chromium in the environment.
The rutile-type b-MnO2, also known as pyrolusite, is the most stable polymorph of MnO2, the surface oxidation product of weathered manganese minerals, and found in natural deposits worldwide. It is therefore likely to play a role in chromium oxidation. To identify stable surface reconstructions of the rutile-type structure b-MnO2 and to probe the redox behavior of the surfaces under varying oxygen chemical potential, periodic density functional theory calculations have been combined with ab initio thermodynamics. The (110), (100), and (101) surfaces were investigated, and it was determined that for each of these surfaces, the stoichiometric surface is most stable under a wide range of oxygen chemical potentials. The (110) and (101) surfaces may be oxidized at low temperatures and ambient oxygen partial pressure ( = 0.20 bar). At low oxygen chemical potentials achieved at high temperatures and UHV-like oxygen partial pressure ( = 10-10 bar), all b-MnO2 surfaces studied can be reduced.
Reduction of b-MnO2 surfaces leads to surface reconstructions not observed for rutile-type SnO2 or for rutile TiO2. These surface reconstructions demonstrate the importance of symmetry-breaking at the surface to allow reduced MnIII, a d4 system, to undergo Jahn-Teller distortion. Another important driver for these interesting surface reconstructions is the interplay between d-orbital occupation and manganese coordination geometry. Reduced manganese at b-MnO2 surfaces will adopt coordination geometries that optimize the d-orbital occupations within the steric and electrostatic constraints of the lattice. Because SnO2 is a d10 system and TiO2 has no d-electrons, rearrangement of the surface atoms does not lead to energetic gains by lowering d-orbital energies, explaining why the new surface reconstructions predicted in this work have not been observed for other common rutile-type metal oxide surfaces.