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The systematic study of the platinum speciation in sulfuric acid solutions of platinum (IV) hydroxide { [Pt-(OH) 4 (H 2 O) 2 ], HHPA} was performed with the use of a combination of methods. Depending on the prevailing Pt form, the three regions of H 2 SO 4 concentration were marked: (1) up to 3 M H 2 SO 4 forms unstable solutions gradually generating the PtO 2 • xH 2 O particles; (2) 4−12 M H 2 SO 4 , where the series of mononuclear aqua−sulfato complexes ([Pt(SO 4 ) n (H 2 O) 6−n ] 4−2n , where n = 0•••4) dominate; and (3) 12 M and above, where, along with [Pt(SO 4 ) n (H 2 O) 6−n ] 4−2n species, the polynuclear Pt(IV) species and complexes with a bidentate coordination mode of the sulfato ligand are formed. For the first time, the salts of the aqua−hydroxo Pt(IV) cation [Pt(OH) 2 (H 2 O) 4 ]SO 4 (triclinic and monoclinic phases) were isolated and studied with a combination of methods, including the single-crystal X-ray diffraction. The formation of PtO 2 •xH 2 O particles in sulfuric acid solutions (1−3 M) of HHPA and their spectral characteristics and morphology were studied. The deposition of PtO 2 •xH 2 O was highlighted as a convenient method to prepare various Pt-containing heterogeneous catalysts. This possibility was illustrated by the preparation of Pt/g-C 3 N 4 catalysts, which show an excellent performance in catalytic H 2 generation under visible light irradiation with a quantum efficiency up to 5% and a rate of H 2 evolution up to 6.2 mol•h −1 per gram of loaded platinum.

Currently, the synthesis of active photocatalysts for the evolution of hydrogen, including photocatalysts based on graphite-like carbon nitride, is an acute issue. In this review, a comprehensive analysis of the state-of-the-art studies of graphic carbon nitride as a photocatalyst for hydrogen production under visible light is presented. In this review, various approaches to the synthesis of photocatalysts based on g-C3N4 reported in the literature were considered, including various methods for modifying and improving the structural and photocatalytic properties of this material. A thorough analysis of the literature has shown that the most commonly used methods for improving g-C3N4 properties are alterations of textural characteristics by introducing templates, pore formers or pre-treatment method, doping with heteroatoms, modification with metals, and the creation of composite photocatalysts. Next, the authors considered their own detailed study on the synthesis of graphitic carbon nitride with different pre-treatments and respective photocatalysts that demonstrate high efficiency and stability in photocatalytic production of hydrogen. Particular attention was paid to describing the effect of the state of the platinum cocatalyst on the activity of the resulting photocatalyst. The decisive factors leading to the creation of active materials were discussed.

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