Chemistry Department Seminar

"Dynamic Structural Disorder in Supported Nanocatalysts Under Operando Conditions as probed by X-ray Spectra*"

Presented by John J. Rehr, University of Washington

Monday, September 21, 2015, 2:00 pm — Hamilton Seminar Room, Bldg. 555

Supported nanoparticle catalysts are ubiquitous in heterogeneous catalytic processes, and there is broad interest in their physical and chemical properties. However, global probes such as XAS and XPS generally reveal their ensemble characteristics, obscuring details of their fluctuating internal structure. We have previously shown [1] that a combination of theoretical and experimental techniques is needed to understand the intra-particle heterogeneity of these systems [2], and their changes under operando conditions [3]. Here we briefly review the theoretical calculations of both the dynamic structure and the spectra. Ab initio DFT/MD simulations revealed that the nanoscale structure and charge distribution are inhomogeneous and dynamically fluctuating over several time-scales, ranging from fast (200-400 fs) bond vibrations to slow fluxional bond breaking (>10 ps). The anomalous behavior of the disorder is not static, but rather is driven by stochastic motion over 1-4 ps time-scales. The resulting large scale fluctuations are termed "dynamic structural disorder" (DSD) [2]. Moreover, the nanoparticles tend to exhibit a semi-melted cluster surface, which for alloy clusters can be atomically-segregated. Recent studies of Pt and PtSn nanoclusters on various supports show a variety of spectral and structural trends as a function of temperature. DFT/MD simulations show that adsorption drives local electronic structure changes that are responsible for the energy shifts vs temperature of the absorption edge. In order to examine relaxation dynamics in the much longer, ns-scale time regimes we have recently developed modified Sutton-Chen (SC) potentials supplemented with a Lennard-Jones model potential to account for the interaction with the support. The modified SC parameters are chosen to capture the nanoparticles DFT dynamics. These simulations reveal regimes controlled by internal particle melting and activation of surface mobility. *Supp

Hosted by: Anatoly Frenkel

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