Oxyanion-Sensitive Catalytic Activity of Ni(II)/Oxyanion Systems for Heterogeneous Organic Degradation: Differential Oxidizing Capacity of Ni(III) and Ni(IV) as High-Valent Intermediates.

This study demonstrated that NiO and Ni(OH) as Ni(II) catalysts exhibited significant activity for organic oxidation in the presence of various oxyanions, such as hypochlorous acid (HOCl), peroxymonosulfate (PMS), and peroxydisulfate (PDS), which markedly contrasted with Co-based counterparts exclusively activating PMS to yield sulfate radicals. The oxidizing capacity of the Ni catalyst/oxyanion varied depending on the oxyanion type. Ni catalyst/PMS (or HOCl) degraded a broad spectrum of organics, whereas PDS enabled selective phenol oxidation. This stemmed from the differential reactivity of two high-valent Ni intermediates, Ni(III) and Ni(IV). A high similarity with Ni(III)OOH in a substrate-specific reactivity indicated the role of Ni(III) as the primary oxidant of Ni-activated PDS. With the minor progress of redox reactions with radical probes and multiple spectroscopic evidence on moderate Ni(III) accumulation, the significant elimination of non-phenolic contaminants by NiOOH/PMS (or HOCl) suggested the involvement of Ni(IV) in the substrate-insensitive treatment capability of Ni catalyst/PMS (or HOCl). Since the electron-transfer oxidation of organics by high-valent Ni species involved Ni(II) regeneration, the loss of the treatment efficiency of Ni/oxyanion was marginal over multiple catalytic cycles.