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 for Co(II)/Ni(II) binding and 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. The results indicate that the average oxidation state (AOS), oxygen vacancies (Ov), and surface reactive oxygen species collectively govern the adsorption performance. Density-functional-theory calculations further confirm the formation of stable inner-sphere complexes, with Co 2p/O 2p orbital hybridization at −1.35 and −1.20 eV for α- and γ-MnO₂, respectively. The large interlayer spacing and electrostatic environment of δ-MnO₂ facilitate co-stabilization of hydrated Co(II)/Ni(II) 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.