Activating peroxydisulfate by morphology-dependent NiO catalysts: Structural origin of different catalytic properties

Understanding how the morphology of a nanocatalyst tailors its catalytic performance is a crucial issue for the rational design and fabrication of active heterogeneous catalysts at the nanometer level. Herein, we demonstrate novel findings of peroxydisulfate (S2O82-; PDS) activation catalyzed by nickel oxide (NiO) nanocatalysts of different morphologies, in which oxygen vacancies (V-O) were the defective sites that facilitated the chemical bonding with PDS molecules and promoted the reactivity of V-O-connected nickel ions for PDS activation, thus elucidating the structural origin of its catalytic activities. The morphologically tunable NiO with various V-O concentrations exhibited different catalytic performance for the removal of phenol (a model organic pollutant). Based on an electron paramagnetic resonance (EPR) study, radical competition reactions, and quenching tests, the main reactive species was revealed to be the non-radical PDS-NiO complex, which can effectively attack C6H5O- to yield intermediates attached to NiO surface. This work not only improves the fundamental understanding of active sites in morphologically tunable transition metal oxides, but also provides valuable guidelines for the rational design and synthesis of high-performance, morphology-dependent nanocatalysts.