Sulfuric Acid Solutions of [Pt(OH)₄(H₂O)₂]: A Platinum Speciation Survey and Hydrated Pt(IV) Oxide Formation for Practical Use

Abstract

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 · x H 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 ] 4–2 n, where n = 0···4) dominate; and (3) 12 M and above, where, along with [Pt(SO 4 ) n (H 2 O) 6– n ] 4–2 n 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 · x H 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 · x H 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.