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

摘要

The influence of hydrothermal treatment conditions (temperature, duration, acidity of the medium) on the structural and chemical transformations of the complex H-2[Pt(OH)(6)] was studied. The com position 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 degrees C, the product composition in terms of empirical formulas changes as fol lows: PtO2 center dot 4H(2)O -> PtO2 center dot 3.5H(2)O -> PtO2 center dot 1.5H(2)O. 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 dis tribution method showed that polynuclear species similar to 10-15 angstrom in size with a structure similar to that of orthor hombic PtO2 form in the complexes subjected to "hard" hydrothermal treatment (T >= 150 degrees C).