Anomalous Structural Disorder in Supported Pt Nanoparticles

Abstract

Supported Pt nanocatalysts generally exhibit anomalous behavior, including negative thermal expansion and large structural disorder. Finite temperature DFT/MD simulations reproduce these properties, showing that they are largely explained by a combination of thermal vibrations and low-frequency disorder. We show here that a full interpretation is more complex and that the DFT/MD mean-square relative displacements (MSRD) can be further separated into vibrational disorder, "dynamic structural disorder" (DSD), and long-time equilibrium fluctuations of the structure dubbed "anomalous structural disorder" (ASD). We find that the vibrational and DSD components behave normally, increasing linearly with temperature while the ASD decreases, reflecting the evolution of mean nanoparticle geometry. As a consequence the usual procedure of fitting the MSRD to normal vibrations plus temperature-independent static disorder results in unphysical bond strengths and Grüneisen parameters.

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.…”

“…To summarize, atomic-scale simulation of nanomaterial structure, guided by XAS data, is a new and rapidly developing field. The importance of and growing interest in such studies are supported by a number of recent publications devoted to this topic (15,17,19,26,(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45). The approaches described in these articles often rely on similar ideas, but their implementation varies because such combined studies are still a relatively new tool for modeling NP structure.…”

Realistic Modelling of Dynamics at Nanostructured Interfaces Relevant to Heterogeneous Catalysis

“…These are key tools to monitor the time-and condition-dependent evolution of the structure of a supported nanocluster. Vila et al [65] utilized DFT modelling and molecular dynamics to characterize the peculiar negative thermal expansion and large structural disorder of Pt nanoparticles supported on a gamma-Al 2 O 3 surface in terms of a combination of thermal vibrations and low-frequency disorder. Vila et al [66] previously fitted a hybrid version of the classical Sutton-Chen force field to predict diffusion coefficients and bond-breaking rates as a function of the nanocluster size.…”

Advanced calculations of X-ray spectroscopies with FEFF10 and Corvus

“…However, for many advanced or high-quality calculations, FEFF must be augmented with calculations of structure and other properties based on DFT, quantum-chemistry, Monte Carlo sampling, etc. Examples include calculations of spectra based on DFT optimized structure, inclusion of vibrational effects through calculations of the dynamical matrix (Vila et al, 2007(Vila et al, , 2018a or molecular dynamics simulations (Vila et al, 2008(Vila et al, , 2012(Vila et al, , 2017, or treatment of multi-electron excitations through realtime time-dependent DFT (TDDFT). For these purposes, an intelligent workflow tool dubbed Corvus has been developed (Story et al, 2019).…”