Hierarchical MnO₂ nanoflowers blooming on 3D nickel foam: A novel micro-macro catalyst for peroxymonosulfate activation

In this work, birnessite-type δ-MnO₂ nanoflowers were uniformly deposited on 3D nickel foam (NF) by one-step hydrothermal route for high-efficient activation of peroxymonosulfate (PMS) towards degradation of acid orange 7 (AO7). High specific surface area, large pore volume and 3D hierarchical structure promotes the mass and electron transfer for great catalytic activity. Low reaction energy barrier (E_a = 27.5 kJ/mol) and outstanding reusability with extremely low manganese leaching during recycling (<0.06 mg/L) was achieved due to the 3D hierarchical structure which could effectively avoid the agglomeration of nano-sized MnO₂. SO₄ ^[rad]− was confirmed to be the predominant reactive species for AO7 decomposition by electron spin resonance and quenching tests. The synergistic catalytic mechanism of MnO₂/NF and the role of inner-sphere complexation between the active sites of MnO₂ and peroxymonosulfate were thoroughly investigated. Compared with traditional nano/micro-sized catalysts, 3D macroscopic MnO₂/NF with facile recovery and high stability potentially facilitates fascinating applications as green heterogeneous catalysis approach.