Initial Steps of the Alumina-Supported platinum Catalyst Preparation: A Molecular Study by ~(195)Pt NMR, UV-Visible, EXAFS, and Raman Spectroscopy

摘要

The compositions of Pt complexes adsorbed on #gamma#-alumina from acidic H_2PtCl_6 solutions and their transformations upon drying, rewetting, and calcination were studied using ~(195)Pt NMR, UV-Vis diffuse reflectance, EXAFS, and Raman spectroscopy. Two acidic H_2PtCl_6 solutions (at pH 1.15 and 2.8) were used to impregnate unmodified and HCl treated Al_2O_3. On the basis of ~(195)Pt NMR measurements, platinum is found to be adsorbed on alumina as [PtCl_6]~(2-) and [PtCl_5(OH)]~(2-), the latter species being produced by deprotonation of [PtCl_5(H_2O)]~- present in the initial solutions. Depending on the pH of the impregnating H_2PtCl_6 solutions and alumina pretreatment, the ~(195)Pt chemical shift (#delta#_(pt)) varies from -6.5 to 0.1 [PtCl_6]~(2-) and from 638 to 660 ppm for [PtCl_5(OH)]~(2-) (#delta#_(Pt)[PtCl_6]~(2-) = 0 ppm in the H_2PtCl_6 solutions at pH 1.15). The [PtCl_6]~(2-)/[PtCl_5OH]~(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 H_2PtCl_6 solution at pH 1.15, whereas a certain part of these species becomes grafted when unmodified alumina is brought into contact with the H_2PtCl_6 solution at pH 2.8. After removal of physisorbed water by drying at 20 deg 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 deg 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 H_2PtCl_6/Al_2O_3 at 90 or 150 deg 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 [PtCl_6]~(2-) adsorbed on the HCl treated alumina was found to occur upon drying at temperatures up to 150 deg C.