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

Abstract

The reaction of & beta;-Ni(OH)(2) in the presenceof cerium(IV) ammonium nitrate during oxygen-evolution reaction wasinvestigated. Cerium(IV) ammonium nitrate (CAN) has been extensivelyused asa sacrificial oxidant to study water-oxidation catalysts (WOCs). Althoughnickel hydroxide has been extensively investigated as WOCs, the water-oxidationreaction (WOR) and mechanistic studies in the presence of CAN andnickel hydroxide were rarely performed. Herein, using in situ Ramanspectroscopy, in situ X-ray absorption spectroscopy, and in situ electronparamagnetic resonance spectroscopy, WOR in the presence of CAN and & beta;-Ni(OH)(2) was investigated. The proposed WOR mechanisminvolves the oxidation of & beta;-Ni(OH)(2) by CAN, leadingto the formation of & gamma;-NiO(OH). & gamma;-NiO(OH), in the presenceof acidic conditions, evolves oxygen and is reduced to Ni(II). Inother words, the role of & beta;-Ni(OH)(2) is the storageof four oxidizing equivalents by CAN, and then a four-electron reactioncould result in a WOR with low activation energy. & beta;-Ni(OH)(2) in CAN at concentrations of 0.10 M shows WOR with a maximumturnover frequency and a turnover number (for 1000 s) of 5.5 x10(-5)/s and 2.0 x 10(-2) mol(O-2)/mol(Ni), respectively. In contrast to & beta;-Ni(OH)(2), Ni(OH2)(6) (2+) (aq) could notbe oxidized to & gamma;-NiO(OH). Indeed, Ni(OH2)(6) (2+) (aq) is the decomposition product of & beta;-Ni(OH)(2)/CAN.