Density functional theory study of the γ-MnOOH (010) surface: Response to oxygen and water partial pressures and temperature

Ab initio thermodynamics was combined with density functional theory calculations to identify stable gamma-MnOOH (010) surface terminations in response to varying oxygen and water partial pressures. Within the range of accessible oxygen chemical potentials, reduced manganese atoms are not thermodynamically stable at the surface. Oxidation of the surface by addition of oxygen is favorable at oxygen chemical potentials typically found in experiments. Entropy drives the removal of H(2) from the stoichiometric surface above 603 K under ambient conditions, in close agreement with the experimental decomposition temperature of 573 K. Molecular adsorption of water at half-monolayer and monolayer coverages is highly exothermic and significantly lowers the surface free energy of the clean surface. Dissociative adsorption of water is only possible at monolayer coverage, where it is stabilized by the formation of a hydrogen-bonding network on the surface. The most thermodynamically stable surfaces are oxidized surfaces, but the stoichiometric and fully hydrated surfaces may be accessible in experiments due to slow oxidation kinetics of the surface.