Application of NMR to interfacial coordination chemistry: A Pt-195 NMR study of the interaction of hexachloroplatinic acid aqueous solutions with alumina

To understand the mechanism of Pt deposition during the synthesis of suppor ted catalysts, liquid phase Pt-195 NMR spectroscopy was applied first to st udy the speciation of platinum complexes in aqueous solutions of H2PtCl6 as a function of pH and time of hydrolysis and second to follow the adsorptio n of platinum complexes on gamma-alumina from these solutions. Five of the six Pt complexes that can exist in hydrolyzed Hz PtCl6 solutions are identi fied by their Pt-195 chemical shift(delta(Pt)): [PtCl6](2-), [PtCl5(H2O)](- ), [PtCl5(OH)](2-), [PtCl4(H2O)(2)], and [PtC4(OH)(2)](2-). For [PtCl4(OH)( H2O)](-), which cannot be directly detected in NMR spectra due to fast prot on exchange. delta(Pt), is calculated from the best fit of the delta(Pt) = f(pH) dependence to the experimental data. The acid dissociation constants (pK(a),) for aquaplatinates are also determined and discussed in comparison to literature data. For H2PtCl6 solution-Al2O3 mixtures, NMR signals of [P tCl6](2-) and [PtCl5(OH)](2-) at the interface were observed for the first time. The [PtCl6](2-) signal has a small negative delta(Pt) relative to tha t in H2PtCl6 solutions, which indicates a slight perturbation of the Pt ato m coordination sphere by the alumina surface. It is suggested that adsorbed [PtCl5(OH)](2-) results from fast deprotonation of the water molecule liga nd in [PtCl5(H2O)](-) by alumina basic hydroxyls. Most likely, the platinum anions are held on the positively charged alumina surface by electrostatic interaction. The fraction of adsorbed [PtCl5(OH)(2-) rises with increasing initial pH of H2PtCl6 solutions and becomes dominant at higher pH. The [Pt Cl6](2-) and [PtCl5(OH)(2-) signals disappear after removal of physisorbed water from alumina powder at room temperature and reappear almost unchanged upon subsequent rewetting of the solid. In contrast, after drying at 90 de grees C for 1.5 h, no NMR signal can be detected for the dried samples, and much weaker signals than those in the original solid are observed for the rewetted sample. The disappearance of the NMR signals is supposed to be due to formation of lower symmetry grafted Pt complexes with a higher chemical shift anisotropy. The likely mechanisms of platinum deposition from acidic H2PtCl6 solutions on positively charged alumina surface are discussed.