Nanomaterials of platinum (Pt) alloyed with transition metals such as gold (Au) are effective catalysts for a wide range of chemical reactions, particularly the oxidation of small organic molecules (SOMs) such as methanol and glucose. These reactions are integral to the function of fuel cells and glucose sensors, respectively. However, the oxidation of SOMs on Pt-based catalysts results in the formation of carbon monoxide (CO) and other partially-oxidized intermediates, which poison the catalyst. Previous findings have shown that alloying Pt with other transition metals may facilitate SOM oxidation via a CO-free pathway. This project employs computational methods to study the surface distribution and properties of PtAu nanomaterials. These findings can then be used to inform our understanding of the different possible mechanisms of SOM oxidation. A random PtAu nanoslab containing 900-2016 total atoms, of varying atomic ratios, is generated, and parallel-tempering Metropolis Monte Carlo simulations are performed to determine equilibrium configurations, using a cohesive energy model for metal alloys. Slabs are studied in vacuum and with oxygen adsorbates to simulate the oxidizing conditions of an electrochemical experiment. Results suggest that at levels of adsorbate coverage that mirror experimental conditions, a significant fraction of surface Pt atoms are paired to other Pt atoms regardless of the amount of Au present. Increasing the Au content promotes the formation of Au-Pt pair sites up to 50% added Au, after which the Au atoms begin to pool and the percentage of Pt-Au sites decreases. The correlation between the AuPt pair sites in our calculations and related experimental measurements suggests that the AuPt pair sites determine the mechanistic pathway for SOM oxidation, and tuning the amount of Au added can therefore regulate the tendency for catalytic poisoning.

Monte Carlo simulations of surface distribution in PtAu nanostructures as a function of composition

ACS Spring 2022

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