Metal nanoparticles (NPs) are attracting interest due to their wide use as heterogeneous catalysts in a large number of chemical processes. Interplay between NPs core sites and undercoordinated surface sites, the quantum confinement effects that modify the electronic properties of metal atoms, and the interactions between metal sites and support and/or adsorbates are responsible for NPs unique catalytic properties. Understanding of NPs atomic structure (based both on experimental evidences and theoretical models) is thus important to understand the catalytic mechanisms, enabled by such NPs. EXAFS spectroscopy is one of a few experimental techniques that are able to probe the structure of small (1 – 2 nm) NPs. Unfortunately, the same reasons that are responsible for the unique catalytic activity of NPs (heterogeneous environments of metal atoms, interactions with support and ligands) make the analysis of EXAFS data for NPs much more challenging than for their bulk counterparts. Large number of structural parameters is required for the description of NPs 3D structure. At the same time, pronounced structural disorder, observed in NPs, result in broad, asymmetric distributions of interatomic distances, which are challenging to account for in conventional analysis. The significant difference in structure and dynamics in nanostructured and bulk materials is a challenge also for theoretical studies. In particular, classical molecular dynamics (MD) simulations that rely on empirical interaction models are a powerful tool to model the structure and atomic dynamics in bulk materials, but to NPs modeling they should be applied with caution, since the interaction models that are able to represent well the properties of bulk material are not necessarily accurate for NPs. In this talk we demonstrate that the problems both in experimental data analysis and in theoretical simulations for nanomaterials can be treated if EXAFS analysis and MD simulations are used together. Information on bond length distributions, extracted from MD, can be used to constrain EXAFS analysis and reduce the ambiguities in the EXAFS data interpretation. The sensitivity of EXAFS method to local structure and dynamics of metallic NPs, in turn, can be used to validate and improve the accuracy of MD models. In this study the possibilities provided by combination of EXAFS analysis with MD simulations are exploited to advance our understanding of the structure of gold and supported platinum NPs.

Probing atomic-scale structure and dynamics in metallic nanocatalysts by x-ray absorption spectroscopy and theoretical simulations

254th National Meeting (2017)

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