| Popis: |
The compositions of Pt complexes adsorbed on γ-alumina from acidic H2PtCl6 solutions and their transformations upon drying, rewetting, and calcination were studied using 195Pt NMR, UV–Vis diffuse reflectance, EXAFS, and Raman spectroscopy. Two acidic H2PtCl6 solutions (at pH 1.15 and 2.8) were used to impregnate unmodified and HCl treated Al2O3. On the basis of 195Pt NMR measurements, platinum is found to be adsorbed on alumina as [PtCl6]2− and [PtCl5(OH)]2−, the latter species being produced by deprotonation of [PtCl5(H2O)]− present in the initial solutions. Depending on the pH of the impregnating H2PtCl6 solutions and alumina pretreatment, the 195Pt chemical shift (δPt) varies from −6.5 to 0.1 ppm for [PtCl6]2− and from 638 to 660 ppm for [PtCl5(OH)]2− (δPt[PtCl6]2−=0 ppm in the H2PtCl6 solution at pH 1.15). The [PtCl6]2−/[PtCl5(OH)]2− ratio depends on the pH of the impregnating solutions, the alumina pretreatment, and the impregnation time. The adsorbed Pt anion complexes are held on the surface by electrostatic interaction with positively charged protonated hydroxyls. No marked grafting of the adsorbed Pt species occurs upon impregnating the unmodified or HCl treated aluminas with the H2PtCl6 solution at pH 1.15, whereas a certain part of these species becomes grafted when unmodified alumina is brought into contact with the H2PtCl6 solution at pH 2.8. After removal of physisorbed water by drying at 20°C, the NMR signals are no longer observed. The UV–Vis, Raman, and EXAFS data evidence a modification of the Pt coordination sphere upon drying at 20°C which can be explained in terms of a Cl− ligand exchange with neighboring alumina hydroxyls, i.e., grafting. The ligand exchange can be reversed by hydration of the dried solids accompanied by the reappearance of the initial NMR signals. Upon drying H2PtCl6/Al2O3 at 90 or 150°C, the number of Cl and O atoms in the Pt coordination sphere does not change markedly, but the adsorbed Pt species become irreversibly grafted to the surface. No ligand exchange and no grafting of [PtCl6]2− adsorbed on the HCl treated alumina was found to occur upon drying at temperatures up to 150°C. |
Full Text from ScienceDirect