Bottom-up and top-down methods to improve catalytic reactivity for photocatalytic production of hydrogen peroxide using a Ru-complex and water oxidation catalysts

Abstract

Hydrogen peroxide (H₂O₂) was produced from water and dioxygen using [Ru^II(Me₂phen)₃]²⁺ (Me₂phen = 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst and [Ir(Cp*)(H₂O)₃]²⁺ (Cp* = η⁵-pentamethylcyclopentadienyl) as a precursor of a water oxidation catalyst in the presence of Sc³⁺ in water under visible light irradiation. TEM and XPS measurements of residues in the resulting solution after the photocatalytic production of H₂O₂ indicated that [Ir(Cp*)(H₂O)₃]²⁺ was converted to Ir(OH)₃ nanoparticles, which are actual catalytic species. The Ir(OH)₃ nanoparticles produced in situ during the photocatalytic production of H₂O₂ were smaller in size than those prepared independently from hydrogen hexachloroiridiate (H₂IrCl₆), and exhibited higher catalytic reactivity for the photocatalytic production of H₂O₂. The photocatalytic production of H₂O₂ from water and dioxygen was also made possible when Ir(OH)₃ nanoparticles were replaced by nickel ferrite (NiFe₂O₄) nanoparticles, which are composed of more earth abundant metals than iridium. The size of NiFe₂O₄ nanoparticles became smaller during the photocatalytic production of H₂O₂ to exhibit higher catalytic reactivity in the second run as compared with that in the first run. NiFe₂O₄ nanoparticles obtained by the treatment of NiFe₂O₄ in an aqueous solution of Sc³⁺ exhibited 33-times higher catalytic reactivity in H₂O₂-production rates than the as-prepared NiFe₂O₄. Thus, both the bottom-up method starting from a molecular complex [Ir(Cp*)(H₂O)₃]²⁺ and the top-down method starting from as-prepared NiFe₂O₄ to obtain nanoparticles with smaller size resulted in the improvement of the catalytic reactivity for the photocatalytic production of H₂O₂ from water and dioxygen.