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This work is devoted to the study of the process of hydrolysis of chloride Pt(II) complexes and Pt(IV) nitrate complexes existing in aqueous solutions of different nature by EXAFS spectroscopy.
The deposited platinum catalysts are inevitable in various industrial oxidation and reduction processes due to their unique properties,. The variety of catalytic properties is determined by the possibility of varying the dispersion and lability of the active component. It is quite obvious that the development of methods for fine control of Pt dispersion is necessary to study the fundamental relationships between the size, state of the active component nanoparticles, their detailed electronic structure, and the actual properties of a catalyst in a chemical reaction. As a rule, various precursor solutions based on Pt(IV) and Pt(II) salts are used for the synthesis of platinum catalysts; after precipitation to a carrier the system undergoes various heat treatments. The main factor affecting the dispersion of Pt forms existing in salt solutions is the ongoing hydrolysis process. To implement the research goals, ~0.01-0.05 M aqueous solutions of K2 PtCl4 and nitrate solutions of Pt (IV) with different ligand environments were prepared and EXAFS spectra (Pt-L3 edge) of studied samples were recorded at SSTRC, Novosibirsk. The composition and structures of the hydrolysis products being polynuclear platinum (II) hydroxocomplexes (PHC-Pt) were studied. It was found that during the hydrolysis reaction of [PtCl4]2-complexes, oligomeric chains containing Pt-O-Pt fragments are formed, in which neighboring Pt atoms are connected by a single O–bridge atom. Aging of solutions containing PHC-Pt for a long time, by formation of 3D-oxide structures. does not take place, in contrast to solutions of PHC-Pd (II) and PHC-Pt (IV). Increase in size of PHC-Pt particles is shown to take place while the process of oligomerization of Pt(II) mononuclear complexes proceeds. It was shown that during hydrolysis of Pt(IV) nitrate solutions, Pt(IV) polynuclear hydroxocomplexes were formed. It was established that the nearest environment of Pt in these solution is always octahedral, with the distortion caused by replacing the O part with the N part. However, formation and stabilization of various agglomerated Pt-containing forms of different nuclea are possible. It was found out that the nuclearity of various forms and their resistance to hydrolysis depend on the preliminary history of the sample and the nature of O, N-ligands in the platinum coordination sphere platinum in the solutions. The Pt-O, N, and Pt-Pt distances and corresponding coordination numbers are determined. Variants of possible structural models are considered. Additionally, all prepared samples of solutions were examined by NMR, TEM methods. Data from all these methods agree with each other.
The work was carried out in the framework of the RFBR projects no. 19-05-50046, 18-03-01251. The work was done at the shared research center SSTRC on the basis of the Novosibirsk FEL/VEPP-4-VEPP-2000 complex at BINP SB RAS, using equipment supported by project RFMEFI62119X0022.
