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Two series of bimetallic photocatalysts (0.5% Pt/0.01–0.5% IrOx/g-C3N4 and 0.1% Pt/0.01–0.1% IrOx/g-C3N4) were synthesized by the thermolysis of melamine cyanurate and a successive deposition of platinum and iridium labile complexes (Me4N)2[Pt2(μ-OH)2(NO3)8] and fac-[Ir(H2O)3(NO2)3. The synthesized photocatalysts were studied by a set of physicochemical analysis techniques. Platinum exists in two states, with up to 60% in metallic form and the rest in the Pt2+ state, while iridium is primarily oxidized to the Ir3+ state, which was determined by X-ray photoelectron spectroscopy (XPS). The specific surface area (SBET), which is determined by low-temperature nitrogen adsorption, ranges from 80 to 100 m2 g−1 and the band gap energy (Eg) value is in the range of 2.75–2.80 eV as found by diffuse reflectance spectroscopy (DRS). The activity of the photocatalysts was tested in the photocatalytic production of hydrogen from ultrapure water under visible light (λ = 400 nm). It was found that the splitting of water occurs with the formation of the stochiometric amount of H2O2 as an oxidation product. Two photocatalysts 0.5% Pt/0.01% IrOx/g-C3N4 and 0.1% Pt/0.01% IrOx/g-C3N4 showed the highest activity at 100 μmol h−1 gcat−1, which is among the highest in H2 production published for such systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sharp color changes under solid-state photolysis of hexanitratoplatinate(IV) salt. • The Pt(III) nitratocomplexes are generated during photolysis. • The trapped Pt species are disclosed by spectroscopy methods and DFT. The solid state photolysis of (Bu 4 N) 2 [Pt(NO 3) 6 ] salt, accompanied by sharp changing of the colour from light-yellow to brown and green is investigated by a combination of techniques such as electronic paramagnetic resonance, infrared spectroscopy, Raman spectroscopy, and diffuse reflectance spectroscopy. A paramagnetic Pt(III) complex [Pt(NO 3) 5 ] 2− is shown to be a product of photolysis at 455 nm and shorter wavelength sources generated due to reduction of Pt(IV) centres with the elimination of the reactive nitrate-radical. Prolonged irradiation or aging in the dark results in the formation of mixed-valence Pt(III)-Pt(IV) species from trapped [Pt(NO 3) 5 ] 2− and [Pt(NO 3) 6 ]2− anions of the host structure. The obtained experimental results are supplemented and compared with the data of density functional theory (DFT) calculations, which allows suggesting a possible mechanism of the studied process. [ABSTRACT FROM AUTHOR]