The Role of the Stage of Metal Complex Deposition on Support in the Properties Formation of Supported Platinum Catalysts

The subject of this review is related to the development of research on the mechanism of interaction of platinum complex compounds, primarily, H₂[PtCl₆], with alumina supports and the role of this interaction in properties formation of the corresponding platinum catalysts. Based on the study of the composition of Pt(IV) chloride complexes adsorbed on the γ-Al₂O₃ surface, it was proposed that the complexes should be quantitatively discriminated as inner- and outer-sphere complexes in accordance with the nature of their binding with the support. The chloride ligands were found to be dominant in the bound outer-sphere Pt(IV) complexes, and adsorption of the complexes on the surface occurs due to the protonation of the hydroxyl groups of the support without any significant changes in the composition and geometry of the [PtCl₆]^2– complex anion. The binding of Pt(IV) in the outer-sphere complexes leads (after their reduction with hydrogen) to the formation of Pt particles with a relatively low dispersion containing platinum atoms mainly in a nearly Pt⁰ state, which are characterized by hydrogenating and dehydrogenating activity. The inner-sphere platinum(IV) complexes are the hydrolyzed forms of the chloride complexes formed by the replacement of chloride ligands by the hydroxyl groups of the support. Their reduction forms finely dispersed platinum particles containing a significant fraction of atoms in the ionic form, which increases the activity of the Pt/Al₂O₃ catalyst in dehydrocyclizations of n-alkanes. Several approaches were proposed that make it possible to vary the ratio between the outer- and inner-sphere platinum(IV) complexes (increasing the degree of hydrolysis of the complexes, or varying the nature of the surface OH groups of the alumina support) and hence to modify the properties of the catalyst. The possibility of selective synthesis of outer- and inner-sphere platinum(IV) complexes bound with other supports was shown for the Pt/MgAlO_x system. Using the structural features of layered aluminum-magnesium hydroxides makes it possible to bind platinum(IV) in the complexes of the given type at high selectivity and to change the electronic state of Pt atoms in Pt particles and their activity in alkane dehydrogenations without changing the chemical composition of the catalyst.