Application of NMR to Interfacial Coordination Chemistry: A¹⁹⁵Pt NMR Study of the Interaction of Hexachloroplatinic Acid Aqueous Solutions with Alumina

Abstract

To understand the mechanism of Pt deposition during the synthesis of supported catalysts, liquid-phase 195Pt NMR spectroscopy was applied first to study the speciation of platinum complexes in aqueous solutions of H2PtCl6 as a function of pH and time of hydrolysis and second to follow the adsorption of platinum complexes on γ-alumina from these solutions. Five of the six Pt complexes that can exist in hydrolyzed H2PtCl6 solutions are identified by their 195Pt chemical shift (δPt): [PtCl6]2-, [PtCl5(H2O)]-, [PtCl5(OH)]2-, [PtCl4(H2O)2], and [PtCl4(OH)2]2-. For [PtCl4(OH)(H2O)]-, which cannot be directly detected in NMR spectra due to fast proton exchange, δPt is calculated from the best fit of the δPt = f(pH) dependence to the experimental data. The acid dissociation constants (pKa) for aquaplatinates are also determined and discussed in comparison to literature data. For H2PtCl6 solution−Al2O3 mixtures, NMR signals of [PtCl6]2- and [PtCl5(OH)]2- at the interface were observed for the first time. The [PtCl6]2- signal has a small negative δPt relative to that in H2PtCl6 solutions, which indicates a slight perturbation of the Pt atom coordination sphere by the alumina surface. It is suggested that adsorbed [PtCl5(OH)]2- results from fast deprotonation of the water molecule ligand 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 [PtCl6]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 °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.