Stability, Structural, and Electronic Properties of Hausmannite (Mn3O4) Surfaces and Their Interaction with Water

Hausmannite (Mn3O4) is the stable phase of manganese oxides and has attracted interest due to its technological applications such as in molecular adsorption, ion exchange, catalysis, and water treatment. Hausmannite is a normal spinel structure that presents the formula A(2+)B(2)(3+)O(4), where the tetrahedral sites are occupied by Mn2+ and the octahedral sites by Mn3+ cations. The present work intends to fulfill the lack of information about the electronic and structural properties of hausmannite surfaces and their interaction with water. This is an important aspect for understanding the reaction mechanism occurring at the hausmannite-water interface. Density functional calculations have been employed to investigate the structural and electronic properties of hausmannite. Energy surfaces have been estimated for the low Miller indexes, and the water adsorption on the most favored surface was investigated in detail. The results indicate that the (001) cleavage surface is the most favored, in good agreement with the experimental results with an estimated surface energy of 1.40 J m(-2). The water prefers to adsorb molecularly at the Mn2+ adsorption sites with an estimated adsorption energy of -16.5 kcal mol(-1). The dissociative mechanism is estimated to be at least 5.2 kcal mol(-1) higher in energy. For the monolayer limit for water adsorption, the energy is estimated to be -10.9 kcal mo1(-1) and the dissociative mechanism is only 4.7 kcal mol(-1) higher in energy. This is an important step to understand the chemical reactivity of the hausmannite surfaces in the solid-water interface.