Reaction between Nickel Hydroxide and Cerium(IV) Ammonium Nitrate in Aqueous Solution

Cerium(IV) ammonium nitrate (CAN) has been extensively used as a sacrificial oxidant to study water-oxidation catalysts (WOCs). Although nickel hydroxide has been extensively investigated as WOCs, the water-oxidation reaction (WOR) and mechanistic studies in the presence of CAN and nickel hydroxide were rarely performed. Herein, using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, and in situ electron paramagnetic resonance spectroscopy, WOR in the presence of CAN and β-Ni(OH)₂ was investigated. The proposed WOR mechanism involves the oxidation of β-Ni(OH)₂ by CAN, leading to the formation of γ-NiO(OH). γ-NiO(OH), in the presence of acidic conditions, evolves oxygen and is reduced to Ni(II). In other words, the role of β-Ni(OH)₂ is the storage of four oxidizing equivalents by CAN, and then a four-electron reaction could result in a WOR with low activation energy. β-Ni(OH)₂ in CAN at concentrations of 0.10 M shows WOR with a maximum turnover frequency and a turnover number (for 1000 s) of 5.5 × 10^-5/s and 2.0 × 10^-2 mol (O₂)/mol(Ni), respectively. In contrast to β-Ni(OH)₂, Ni(OH₂)₆²⁺ (aq) could not be oxidized to γ-NiO(OH). Indeed, Ni(OH₂)₆²⁺ (aq) is the decomposition product of β-Ni(OH)₂/CAN.