Surface Structures of Mn₃O₄and the Partition of Oxidation States of Mn

The Mn(III) ions at Mn3O4 surface are hypothesized to contribute to catalytic activity in oxygen reduction reaction. However, the surface structure and stability of Mn3O4 are far less understood. Here, the atomic structures of the widespread (101) and (001) surfaces of Mn3O4 are determined by combining aberration-corrected transmission electron microscopy and DFT calculations. The surface stabilization mechanisms and the oxidation states of Mn are revealed and correlated to the catalytic activity of the surfaces. The results show that the (101) surface undergoes a subsurface reconstruction, forming a rock-salt-type surface layer. The Mn(III) ions are in the outermost layer of the (001) surface but in the subsurface of the (101) surface. The surface partition of the Mn(III) ions provides a microscopic understanding to the observed higher catalytic activity of the (001) surface relative to the (101) surface and would contribute to further development of novel catalysts based on Mn3O4.