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.

Automated summary

This study reports an improved activity of Bi2MoO6 (BMO) through the addition of nickel hexammine perchlorate (NiNH). NiNH greatly increased the rate of phenol oxidation on BMO and facilitated the charge separation of BMO.