| In this study, the preparation of silica-, alumina- and zeolite-based platinum catalysts through noble metal adsorption from aqueous solution is investigated. The Revised Physical Adsorption Model (RPA), derived from first principles without adjustable parameters and solely based on electrostatic interaction between metal complexes and an oxide support surface, was refined through a detailed individual consideration of ionic strength. In conjunction with a proton transfer model, the RPA model was used to simulate platinum (CPA) adsorption over alumina to a very reasonable degree without the use of adjustable parameters. It is shown how the model can in general be used to design a catalyst preparation method and how it was applied to an industrial platinum catalyst preparation problem, where the noble metal was to be selectively placed onto only one support material (zeolite) in the presence of another (silica).; Furthermore, the coordination chemistry of hydrogen hexachloroplatinate (IV) (CPA) in aqueous solution as well as in complexes adsorbed over alumina and its impact on catalyst preparation procedures are investigated using EXAFS. In disagreement with most available speciation models, the chlorine-oxygen ligand exchange process in aqueous solution was found to proceed to a fully oxygen coordinated, chlorine-free complex in dilute solutions at long times and high pH. The chlorine coordination number increases with lowered pH and raised chloride and CPA concentrations. Similarly, the speciation of adsorbed complexes also depends on the chloride concentration in the system, but is mainly a function of the local pH in the adsorbate layer at equilibrium. Given the wide pH shifts observed when large amounts of metal oxide are contacted with aqueous noble metal solutions (as it is commonly practiced in industrial catalyst preparation), the platinum coordination chemistry and thus the chlorine content of the final catalyst are directly influenced by the preparation conditions. |
