(205c) Synthesis and Characterization of Au-Pt/SiO2 Catalysts Prepared by Electroless Deposition Methods

Electroless deposition (ED) has been used for the preparation of continuous, thin film metal coatings, but also has the potential for the preparation of novel, bimetallic catalysts by adjustment of the deposition parameters. Unlike traditional methods for the preparation of bimetallic catalysts, ED permits the selective deposition of a secondary metal (e.g., Au) on the surface of a catalytically active, primary metal surface (e.g., Pt). In order to prepare catalysts using this technique it is necessary to design a reactive, yet stable ED solution by the careful selection of the secondary metal salt and reducing agent(s), as well as the pH and temperature of the ED bath. In order to determine the stability and specificity of the bath, the ED solution is exposed to the support (e.g., SiO₂) to ensure that neither metal salt adsorption nor spontaneous reduction and precipitation of the secondary metal from solution occurs. For the case of Pt/SiO₂, a stable, room temperature solution was obtained at basic conditions (pH 8-10) for two different electroless baths. One bath used AuCl₄¯ as the metal source and HCHO as the reducing agent, while the second employed Au(CN)₂¯ and hydrazine, respectively. For both systems, predictive, sub-monolayer coverages of Au on Pt were achieved. Thus, a series of Au-Pt/SiO₂ catalysts was prepared and characterized using H2 chemisorption and transmission Fourier-transform infrared (FT-IR) spectroscopy of CO adsorption. Chemisorption results show a decrease in H₂ uptake on Pt as Au is added, indicating Au deposition on Pt has occurred. Changes in the population of adsorbed CO species with increasing Au content also corroborate this finding, as do XPS results. These catalysts have been initially evaluated for propylene hydrogenation to determine any obvious bimetallic ensemble effects. As expected, this hydrogenation reaction was determined to be structure insensitive as shown by the Pt-based, constant TOF values achieved for the different Au loadings. The efficacy of this ED approach for the synthesis of Au-Pt (and other Au-containing) bimetallic catalysts will also be discussed.