Initial steps of the alumina-supported platinum catalyst preparation: A molecular study by Pt-195 NMR, UV-visible, EXAFS, and Raman spectroscopy

The compositions of Pt complexes adsorbed on gamma-alumina from acidic H2Pt Cl6 solutions and their transformations upon drying, rewetting, and calcina tion were studied using Pt-195 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 Pt-195 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 Pt-195 ch emical shift (delta(Pt)) varies from -6.5 to 0.1 ppm for [PtCl6](2-) and fr om 638 to 660 ppm for [PtCl5(OH)](2-) (delta(Pt)[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, w hereas a certain part of these species becomes grafted when unmodified alum ina is brought into contact with the H2PtCl6 solution at pH 2.8. After remo val of physisorbed water by drying at 20 degrees 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 degrees C which can be expl ained in terms of a Cl- ligand exchange with neighboring alumina hydroxyls, i.e., grafting. The ligand exchange can be reversed by hydration of the dr ied solids accompanied by the reappearance of the initial NMR signals. Upon drying H2PtCl6/Al2O3 at 90 or 150 degrees 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 t o occur upon drying at temperatures up to 150 degrees C. (C) 1999 Academic Press.