(468g) Dynamic Coordination of Dilute Alloys Under Reactive Environments: Supported Ag-Pt Nanoparticle Catalysts

Single atom alloys (SAAs) are a class of dilute alloys with reactive dopant atoms (e.g., Ni, Pd, Pt) hosted in a coinage metal (e.g., Cu, Ag, Au) matrix at ~1:100 molar ratios. SAA nanoparticle catalysts have gained significant attention due to increased selectivity for chemical conversions, limited coking, and reduced CO poisoning. These unique properties originate from the isolated reactive metal sites that facilitate bond cleavage steps and subsequent spillover of adsorbed species onto the noble metal. Promising results motivate accurate characterization of active site structures in SAA nanoparticle catalysts under reactive environments.

Herein we demonstrate how environmental conditions can significantly impact catalyst structure for supported AgPt dilute alloy catalysts. First we present results from ab-intio thermodynamic models parameterized by density functional theory (DFT) calculations to explore favorable structures under inert and CO rich environments. We then explore the analogous experimental system, where dilute alloy AgPt nanoparticle (NP) catalysts were synthesized colloidally and subsequently characterized. CO-FTIR spectra recorded after H₂ and CO pretreatments at elevated temperature provide experimental evidence that Pt can be stabilized at the surface in the presence of carbonaceous adsorbates, such as CO. Additional in-situ XANES measurements and ab-intio density of states (DOS) calculations suggest that some fraction of surface Pt atoms are coordinated to sub-surface Pt. Consequentially, the electronic structure of the surface Pt atoms can be significantly altered without developing contiguous Pt ensembles that would hamper the selectivity associated with SAA structures.

Finally, we propose how such a methodology can be used to unravel the complex active site structures present in dilute alloy catalysts, how they dynamically evolve in response to operating conditions, and how such changes can influence catalytic processes.