Operando, Multimodal Characterization of Bimetallic Catalysts with Electrons and X-Rays

Alloyed nanoparticles are of increasing interest in many applications, most notably as heterogeneous catalysts. Alloying allows the tuning of composition and structure to increase functionality, most specifically reactivity and selectivity. However, harsh, reactive environments can induce changes in the structure and composition of these materials in unexpected ways, which can inhibit their performance. These materials also present a significant characterization challenge: they are tiny (from single atoms to particles of 10 nm) and can also be heterogeneous in size, composition, and structure.<br/><br/>I will describe how we have developed a new approach to characterize catalysts using so-called 'operando' methods to take measurements. At the same time, the materials are 'in a working condition': i.e., in a reactive environment performing their function. We use a microreactor system compatible with imaging, diffraction, and spectroscopy, using electron, photon, and x-ray probes. The presentation will describe how this multimodal approach can provide unique insights into the dynamic changes in these complex systems as they function.<br/><br/>There will be two specific applications. In the first portion, I will show how we can exploit an innovative colloidal synthesis method to produce highly monodisperse Cu-Pt alloy nanoparticles with a Pt-rich shell and Cu-rich core; the intermetallic CuPt phase formed after annealing shows enhanced and stable catalytic activity for CO oxidation compared to pure Pt nanoparticles or the fresh Cu-Pt particles with a Pt-shell. This synthesis route allows control over Cu-Pt nanostructures, demonstrating Cu-Pt alloys' promising catalytic properties. In the second portion, I will show how Ni-Cu alloy nanoparticles undergo compositional and morphological changes during redox cycles that simulate catalytic reactions; specifically, while Cu segregates in the fresh catalyst, oxidation at 400°C leads to restructuring into hollow particles with heterogenous Ni-Cu composition, explaining the deactivation observed for conversion of biomass-derived 5-hydroxymethylfurfural over NiCu₃/C.