Revisiting the role of NiOOH in catalytic chlorination degradation of 4-chlorophenol

To clarify the role of Ni-based compounds in the catalytic chlorination process, NiOOH, Ni(OH)₂ and NiO catalysts were prepared and their catalytic performances were investigated by employing 4-chlorophenol (4-CP) as a model pollutant in this study. Compared with the nearly pure phases of Ni(OH)₂ and NiO, the NiOOH catalyst is dominated by β-NiOOH phase, and exhibits a mixed state of Ni^II and Ni^III with an actual formula of NiO_0.91(OH)_1.09. The average valence state of Ni in the NiOOH is +2.90. A cycle of Ni species for catalytic oxidation process of free chlorine (FC) is proposed for NiOOH. β-NiOOH phase is oxidized by FC to γ-NiOOH phase containing Ni^IV species, and then the γ-NiOOH phase are reduced by surface water to β-NiOOH phase or Ni(OH)₂ phase, accompanied by the generation of ¹O₂ species. Also, the reduction of the high-valence NiOOH phases such as β-NiOOH and γ-NiOOH can be reduced by organics since their Fermi levels are lower than the level of highest occupied molecular orbital of 4-CP. The NiOOH catalyst exhibits the TOC removal of around 50% in 100 mg/L 4-CP + 100 mg/L FC at pH 7, higher than the Ni^II catalysts (less than 10%). ¹O₂ species dominate the mineralization of 4-CP via the successive oxidation and open-ring reactions, and reactive chlorine species mainly contributes to the conversion of 4-CP to the polychlorinated products. In addition, the high-valence Ni species (Ni^III/Ni^IV) significantly contribute to the removal of 4-CP (∼26% contribution ratio) via the direct surface oxidation.