Kinetic origins of the metastable zone width in the manganese oxide Pourbaix diagram

Abstract

Pourbaix diagrams show solid–aqueous phase stability as a function of pH and redox potential and can be a valuable tool to guide the hydrothermal synthesis of transition metal oxides. However, the Pourbaix diagram is based on thermodynamics, and nucleation kinetics are not readily apparent in this framework. Here, we conduct a combined experimental and theoretical study to measure the onset of MnO₂ precipitation from an MnO₄⁻(aq) solution at various pH conditions, which we then compare against Pourbaix diagram phase boundaries. Using a combination of in situ X-ray absorption spectroscopy and X-ray wide-angle scattering, we directly observe the transformation kinetics from the tetrahedral MnO₄⁻(aq) ion to octahedrally coordinated Mn pre-nuclei, as well as its initiation into crystalline δ′-MnO₂ at various pH. The kinetics of octahedral-Mn precursor availability is observed to govern induction times of crystalline MnO₂ nucleation, which results in a metastable zone width in the Mn–H₂O Pourbaix diagram. These results suggest that synthesis conditions often need to be prepared far beyond the phase boundaries in a Pourbaix diagram to initiate crystallization within a reasonable timescale.