Cl^–-Including Rh/IrO_<i>x</i> Cocatalyst Loaded on a Rh,Sb-Codoped SrTiO₃ Photocatalyst for Water Splitting under Visible Light Irradiation

The chemical state of an IrOx cocatalyst and the effect of sequential loading of Rh species on the photocatalytic water splitting activity of a SrTiO3 photocatalyst codoped with Rh and Sb (SrTiO3:Rh,Sb) were investigated. The highest activity was obtained when the Ir species was loaded on SrTiO3:Rh,Sb at 673 K in air using (NH4)2IrCl6. X-ray fluorescence, X-ray absorption near-edge structure, and X-ray photoelectron spectroscopy measurements revealed that Cl– was included in the Ir species. By dispersing the photocatalyst in water, a part of the Ir species was eluted, and some Ir–Cl bonds were hydrolyzed, resulting in decreased amounts of Ir and Cl loaded on the photocatalyst. The elution hardly proceeded anymore during photocatalytic water splitting under visible light irradiation. In contrast, SrTiO3:Rh,Sb loaded with IrOx(OH)y without Cl– using Ir(NO3)4 did not show water splitting activity. Therefore, it was necessary to include Cl– in the Ir species prepared and during water splitting. Moreover, the water splitting activity of SrTiO3:Rh,Sb loaded with Ir species was improved by sequential loading of Rh species by an adsorption method.