Dynamic structural disorder in supported nanoscale catalysts

Abstract

We investigate the origin and physical effects of "dynamic structural disorder" (DSD) in supported nano-scale catalysts. DSD refers to the intrinsic fluctuating, inhomogeneous structure of such nano-scale systems. In contrast to bulk materials, nano-scale systems exhibit substantial fluctuations in structure, charge, temperature, and other quantities, as well as large surface effects. The DSD is driven largely by the stochastic librational motion of the center of mass and fluxional bonding at the nanoparticle surface due to thermal coupling with the substrate. Our approach for calculating and understanding DSD is based on a combination of real-time density functional theory/molecular dynamics simulations, transient coupled-oscillator models, and statistical mechanics. This approach treats thermal and dynamic effects over multiple time-scales, and includes bond-stretching and -bending vibrations, and transient tethering to the substrate at longer ps time-scales. Potential effects on the catalytic properties of these clusters are briefly explored. Model calculations of molecule-cluster interactions and molecular dissociation reaction paths are presented in which the reactant molecules are adsorbed on the surface of dynamically sampled clusters. This model suggests that DSD can affect both the prefactors and distribution of energy barriers in reaction rates, and thus can significantly affect catalytic activity at the nano-scale.

Anomalous Structural Disorder in Supported Pt Nanoparticles

“…Thus to obtain bond-strength-associated Grüneisen parameters from EXAFS, one has to subtract both the DSD and ASD components of the disorder. Finally, given that these anomalous properties likely arise from the synergistic effects of limited dimensionality of these systems, intraparticle heterogeneity, and the effects of oxide support, our results suggest the existence of other particle–support systems with similar anomalous properties.…”

“…In contrast ∼2.9 nm NPs have positive thermal expansion on average and mean bond lengths closer to those in the bulk. Using DFT/MD simulations, 8,9 this behavior was attributed to "dynamic structural disorder" (DSD), i.e. the inhomogeneous, fluctuating nature of structure at the nanoscale.…”

“…In contrast, ∼2.9 nm NPs have positive thermal expansion on average and mean bond lengths closer to those in the bulk. Using DFT/MD simulations, , this behavior was attributed to “dynamic structural disorder” (DSD), that is, the inhomogeneous, fluctuating nature of structure on the nanoscale. In particular, the DSD exhibits multiple time scales, being driven by the chaotic motion of the center of mass (CM) of the NPs, their librational motion, and transient tethering to the support on the picosecond time scale and their internal fluxional bonding on substantially slower time scales.…”

“…The partitioned σ Vib 2 and σ DSD 2 are obtained similarly to σ P 2 in eq . The ASD component is defined as where the brackets indicate an average over all selected trajectories and initial conditions and where the mean trajectory distance r̅ L is A simpler version of this partitioning that did not separate the ASD from DSD was previously used , to investigate dynamic effects on Pt and PtSn NPs.…”

Solving the Structure and Dynamics of Metal Nanoparticles by Combining X-Ray Absorption Fine Structure Spectroscopy and Atomistic Structure Simulations

“…Similar analysis and comparison of CNs and interatomic distances obtained in MD simulations with EXAFS data were carried out for a cobalt-phosphate catalyst (109). Finally, AIMD-based investigations of the influence of dynamic effects on disorder factors and interatomic distances for supported Pt and PtSn catalysts have been carried out by Rehr, Vila, and colleagues (42,110,111). Direct EXAFS simulations using MD coordinates for interpretation of experimental data were introduced in the 1990s (112), and they allow one to avoid many deficiencies of conventional EXAFS analysis.…”