(154c) Chemical Characterization of Pt, Ru, and Sn Multimetallic Catalysts

Magnesia-supported trimetallic Pt-Ru-Sn catalysts prepared through a cluster and a conventional synthetic route have been investigated in terms of their structural properties and their catalytic activity for the hydrogenation of citral and crotonaldehyde.   FTIR results indicate that the majority of the stabilizing ligands remain attached to the PtRu₅(μ-SnPh₂)(C)(CO)₁₅ cluster used following impregnation onto the MgO support. Under H₂ reduction conditions, both partial and full ligand removal are observed at 473 and 573 K, respectively. HRSTEM analysis shows that cluster-derived samples exhibit significantly smaller average metal particle sizes (~1.1 nm), as well as narrower particle size distributions than the corresponding conventionally prepared ones (~4.3 nm). EDS measurements show that in the cluster-derived catalysts, the majority of the metal particles present are trimetallic in nature, with metal compositions similar to those of the original cluster. In contrast, the conventionally prepared materials contain mostly bimetallic and monometallic particles with variable compositions.  For the material prepared using the PtRu₅(C)(CO)₁₆  cluster precursor and SnCl₄^.5H₂O metal salt (PtRu₅-Sn/MgO), EDS measurements show that individual metal particles exhibited Ru and Pt-Ru character and that Sn mostly decorated the MgO support.  XPS was used to determine how the variation in method of Sn addition to bimetallic Pt-Ru affects the electronic state for the trimetallic Pt-Ru-Sn/MgO system prepared by impregnation using multimetallic clusters, metal-salts, and the combination of both precursor types.  Results show that the PtRu₅Sn/MgO material has a significantly higher percentage of Sn⁰ in comparison to Pt-Ru-Sn/MgO and PtRu₅-Sn/MgO, and a corresponding shift in both Pt and Ru peaks can be correlated to this relative change in Sn oxidation state.  The formation of smaller metal particles and electronic modification of Ru and Sn by Pt in the cluster-derived catalysts and the presence of the three metals in these particles in close proximity result in higher activity and selectivity to the unsaturated alcohols for the hydrogenation of both citral and crotonaldehyde.  By characterizing the metals using different techniques, it is clear that the quantity of oxidized Sn, Pt, and Ru varies greatly depending on the type of precursor used, and that these properties can change with the pretreatment temperature.  Therefore, preparation procedures and precursors play a critical role in contributing to the formation of small catalysts of true trimetallic nature which exhibit unique properties.