Crystal engineering of MnOx polymorphs for unveiling structure–reactivity relationships in Co(II)/Ni(II) binding

Abstract

Manganese oxides (MnO_x) represent a structurally rich family of transition-metal oxides whose polymorph-dependent lattice topology, redox flexibility, and oxygen-defect chemistry enable highly tunable interactions with metal cations. Harnessing these intrinsic features for selective Co(II)/Ni(II) capture is increasingly important for the recycling, purification, and separation of next-generation energy materials. Here, we establish a cross-polymorph structure-reactivity framework by systematically evaluating six representative MnO_x phases toward Co(II)/Ni(II) retention. δ-MnO₂ exhibits the strongest affinity, removing 99.4% of Co(II) and 90.8% of Ni(II), whereas β-MnO₂ and λ-MnO₂ show removal efficiencies below 40%, underscoring the decisive role of crystallographic topology. Study indicated that the average oxidation state (AOS) and oxygen vacancies (O_v) were pivotal factors influencing the adsorption performance. Density-functional-theory calculations further confirm the formation of stable inner-sphere complexes, with Co-2p/O-2p hybridization at -1.35 and -1.20 eV for αand γ-MnO₂, respectively. The large interlayer spacing and electrostatic environment of δ-MnO₂ facilitate co-stabilization of hydrated Co/Ni ions within its galleries, rationalizing its superior uptake. This work provides fundamental insights into how MnO_x lattice architecture orchestrates transition-metal binding and establishes design principles for Mn-oxide-based materials in selective metal-ion separation and resource recovery.