Ni(NH₃)6²⁺ more efficient than Ni(H₂O)6²⁺ and Ni(OH)₂ for catalyzing water and phenol oxidation on illuminated Bi₂MoO₆ with visible light

Nickel (hydr)oxide (NiOH) is known to be good co-catalyst for the photoelectrochemical oxidation of water, and for the photocatalytic oxidation of organics on different semiconductors. Herein we report a greatly improved activity of Bi 2 MoO 6 (BMO) by nickel hexammine perchlorate (NiNH). Under visible light, phenol oxidation on BMO was slow. After NiNH, NiOH, and Ni 2 + loading, a maximum rate of phenol oxidation increased by factors of approximately 16, 8.8, and 4.7, respectively. With a BMO electrode, all catalysts inhibited O 2 reduction, enhanced water (photo-)oxidation, and facilitated the charge transfer at solidliquid interface, respectively, the degree of which was always NiNH > NiOH > Ni 2 + . Solid emission spectra indicated that all catalysts improved the charge separation of BMO, the degree of which also varied as NiNH > NiOH > Ni 2 + . Furthermore, after a phenol-free aqueous suspension of NiNH/BMO was irradiated, there was a considerable Ni(III) species, but a negligible NH 2 radical. Accordingly, a plausible mechanism is proposed, involving the hole oxidation of Ni(II) into Ni(IV), which is reactive to phenol oxidation, and hence promotes O 2 reduction. Because NH 3 is a stronger ligand than H 2 O, the Ni(II) oxidation is easier for Ni(NH 3 ) 6 + than for Ni(H 2 O) 6 + . This work shows a simple route how to improve BMO photocatalysis through a co-catalyst. ?? 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.