Effect of hydrothermal treatment on the composition and structure of Pt(IV) hydroxo complexes as model precursors of the active component in Pt/Al2O3 catalysts

The influence of hydrothermal treatment conditions (temperature, duration, acidity of the medium) on the structural and chemical transformations of the complex H2[Pt(OH)6] was studied. The composition and structure of the resulting compounds were determined by several physicochemical methods. Thermal analysis coupled with mass spectrometry showed that, as the hydrothermal treatment temperature is raised from 25 to 120 and 150°C, the product composition in terms of empirical formulas changes as follows: PtO2 · 4H2O → PtO2 · 3.5H2O → PtO2 · 1.5H2O. X-ray diffraction and UV and IR spectroscopy demonstrated that the changes in the chemical composition are accompanied by the amorphization of the structure and Pt-O bond strengthening. X-ray structure determination using the radial electron density distribution method showed that polynuclear species ∼10–15 Å in size with a structure similar to that of orthorhombic PtO2 form in the complexes subjected to “hard” hydrothermal treatment (T ≥ 150°C).