Acidity and metal complexation of edge surface of birnessite-type MnO₂: Insight from first principles simulations

Birnessite-type MnO₂ plays key roles in scavenging trace elements in numerous natural environments and has also been regarded as a promising energy storage material. The interfacial properties of birnessite are highly pH-dependent due to the presence of various amphoteric groups on its edges, and, therefore, the acidity constants (pKa) of these groups are vital to the understanding of its electrochemical and environmental performances. However, an accurate acidity dataset for birnessite is absent yet. In this study, we employed first-principles molecular dynamics simulations and the vertical energy gap method to calculate the pKas of groups on the birnessite (010) edge. The interfacial hydration structure was characterized with a focus on the hydrogen bonding network. The obtained pKas suggest that MnOH₂ is active while Mn₂OH remains inert in a common pH range. Based on these results, the incorporation of transition metals on the edge surface was investigated by taking Ni²⁺ and Zn²⁺ as the model cations. The energy changes associated with the incorporation process of Ni²⁺ from the outer-sphere state indicate that incorporation on the edge surface is more feasible than that on the basal surface presumed in previous studies. Overall, the results obtained provide an atomic-scale insight into the acid-base chemistry of birnessite and form a physical basis for understanding the interfacial processes of birnessite.