Ni(NH) more efficient than Ni(HO) and Ni(OH) for catalyzing water and phenol oxidation on illuminated BiMoO 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 BiMoO (BMO) by nickel hexammine perchlorate (NiNH). Under visible light, phenol oxidation on BMO was slow. After NiNH, NiOH, and Ni 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 reduction, enhanced water (photo-)oxidation, and facilitated the charge transfer at solid-liquid interface, respectively, the degree of which was always NiNH > NiOH > Ni. Solid emission spectra indicated that all catalysts improved the charge separation of BMO, the degree of which also varied as NiNH > NiOH > Ni. Furthermore, after a phenol-free aqueous suspension of NiNH/BMO was irradiated, there was a considerable Ni(III) species, but a negligible NH 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 reduction. Because NH is a stronger ligand than HO, the Ni(II) oxidation is easier for Ni(NH) than for Ni(HO). This work shows a simple route how to improve BMO photocatalysis through a co-catalyst.