| Bimetallic catalysts can often provide enhanced selectivity, stability, and/or activity, and are typically prepared by either co-impregnation or successive impregnation of both metal salts onto the catalyst support. A complex mixture of bimetallic and monometallic nanoparticles often occurs, resulting in poor control over catalyst performance and hindering development of structure-performance relationships. Dendrimer-metal nanocomposites (DMN) provide a novel synthetic route that can been used to produce heterogeneous catalysts. Dendrimers are mono-disperse hyper-branched polymers, whose internal functional groups can coordinate with transition metal ions (e.g., Pt2+, Ru 3+). The resulting complexes can be reduced to form encapsulated metal nanoparticles and/or clusters, which can then be introduced onto high surface area oxide supports. Upon removal of the dendrimer "shell", enhanced control over supported nanoparticles can be achieved.;In this work, both the delivery of the DMNs to the support and the synthesis of catalysts have been investigated. A series of Pt, Ru and Pt-Ru catalysts supported on silica has been synthesized using G4OH PAMAM dendrimer. The dendrimer-derived bimetallic catalysts were prepared by both co- and sequential complexation of the metal salts. They have been characterized by several techniques, including high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), temperature programmed (TP) titration and X-ray diffraction (XRD). Furthermore, liquid phase selective hydrogenation of 3,4-epoxy-1-butene (EpB) was used as probe reaction to evaluate their catalytic performance.;The dendrimer-derived PtRu bimetallic catalyst prepared by the co-complexation method exhibits a superior catalytic activity among all the three bimetallic catalysts, followed by sequential complexation catalyst, and finally the conventional catalyst. With these catalysts, the turnover frequency (TOF) changes by over a factor of five. Comparison of these activities to those predicted by using TOF of monometallic Pt or Ru sites suggests that there must be some type of new bimetallic site created. This hypothesis is supported experimentally by the XRD studies, where the Ru contents in the PtRu alloy phases can be correlated linearly with the corresponding catalytic activities. In addition, the reaction pathway on co-complexation catalyst has been changed toward crotonaldehyde intermediate as opposite to 3-buten-1-ol. This change can also be attributed to the dual PtRu sites, which exhibit different crotonaldehyde adsorption capacities. |
