Photochemical Water Oxidation in a Buffered Tris(2,2'-bipyridyl)ruthenium-Persulfate System Using Iron(III)-Modified Potassium Manganese Oxides as Catalysts

Study of manganese oxides for electrocatalytic and photocatalytic oxidation of water is an active area of research. The starting material in this study is a high-surface-area disordered birnessite-like material with K⁺ in the interlayers (KMnOx). Upon ion-exchange with Fe³⁺, the disordered layer structure collapses (Fe(IE)MnOx), and the surface area is slightly increased. Structural analysis of the Fe(IE)MnOx included examination of its morphology, crystal structure, vibrational spectra, and manganese oxidation states. Using the Ru(bpy)₃ ²⁺-persulfate system, the dissolved and headspace oxygen upon visible light photolysis with highly dispersed Fe(IE)MnOx was measured. The photocatalytic activity for O₂ evolution of the Fe(IE)MnOx was three times better than KMnOx, with the highest rate being 9.3 mmol_O2 mol_Mn ^-1 s^-1. The improvement of the photocatalytic activity was proposed to arise from the increased disorder and interaction of Fe³⁺ with the MnO₆ octahedra. As a benchmark, colloidal IrO₂ was a better photocatalyst by a factor of ∼75 over Fe(IE)MnOx.

Supported Mn3O4 Nanosystems for Hydrogen Production through Ethanol Photoreforming

Photoreforming promoted by metal oxide nanophotocatalysts is an attractive route for clean and sustainable hydrogen generation. In the present work, we propose for the first time the use of supported Mn₃O₄ nanosystems, both pure and functionalized with Au nanoparticles (NPs), for hydrogen generation by photoreforming. The target oxide systems, prepared by chemical vapor deposition (CVD) and decorated with gold NPs by radio frequency (RF) sputtering, were subjected to a thorough chemico-physical characterization and utilized for a proof-of-concept H₂ generation in aqueous ethanolic solutions under simulated solar illumination. Pure Mn₃O₄ nanosystems yielded a constant hydrogen production rate of 10 mmol h^-1 m^-2 even for irradiation times up to 20 h. The introduction of Au NPs yielded a significant enhancement in photocatalytic activity, which decreased as a function of irradiation time. The main phenomena causing the Au-containing photocatalyst deactivation have been investigated by morphological and compositional analysis, providing important insights for the design of Mn₃O₄-based photocatalysts with improved performances.