Nanometer-sized bi-metallic clusters are known to exhibit unique reactivities for hydrocarbon activation (Ni-Au, Pt-Ni), alcohol oxidation (Pd-Au), hydrogenation (Pt-Pd), and water-gas-shift (Pt-Au) reactions. The bulk and surface compositions of these clusters may deviate from their large structure, because of the much higher surface area-to-volume ratios, and thus larger effects of the surface free energies on their thermodynamically stable structures and compositions. These clusters may undergo active surface reconstruction as they are exposed to reaction mixtures, and even experience bulk phase transition and segregation with changing oxygen chemical potentials. We investigate the structural dynamics and the transition in Pt based bimetallic clusters (Pt-Ru, Pt-Pd) during methane and methanol oxidation reactions with combined kinetic assessments, isotopic methods, and in-situ and ex-situ electron microscopic studies. We explore the combination of Pt with a more oxophilic metal (M=Ru, Pd) in nanosized clusters and explain their promotional effects on methane and methanol activation. Pt-Pd bimetallic clusters catalyze the C-H bond activation of methane at much higher rates than Pt and Pd monometallic clusters in the presence of H2O and SO2 whereas Pt-Ru clusters catalyze the activation of methanol with rate dependencies unlike those of their monometallic counterparts. In-situ and ex-situ high-resolution scanning and transmission electron microscopic studies unravel the dynamics of these nanometer sized clusters and especially, the oxygen solvation and oxide shell formation on the Pt-M clusters in response to an increase in oxygen chemical potentials. During oxidation reactions, the oxophilic metal preferentially migrates to the shell of the Pt-M clusters and forms an oxide layer, but a small portion of Pt remains as the active site during catalysis. As the oxygen dissolution step occurs, the clusters change their overall shape and size. Such migration of the oxophilic metal to the surface and the formation of the oxide layers lead to the generation of a reactive metal-oxygen site-pair, previously not found on monometallic clusters, that is highly effectively for the activation of methane during methane oxidation and of methanol during oxidative dehydrogenation reactions.

Unraveling the dynamics of surface- and bulk-phase transition of Pt-based, bi-metallic clusters and their kinetic consequences during oxidation catalysis

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.