Single-atom catalysts (SACs) consisting of late-transition metals on oxide supports have recently gained significant world-wide attention due to their well-defined sites that enable more selective chemistry and the fact that they use precious metals at the ultimate limit of atom efficiency. SACs involving reducible oxide supports are good catalysts for oxidation reactions, but they tend to deactivate by sintering under reducing conditions. On the other hand, single-atom alloys (SAAs) are robust towards sintering and exhibit exceptional catalytic performance for a wide range of reactions under reducing conditions, but some SAAs deactivate under oxidizing environments. This project is aimed at systematically exploring catalytic reactivity between the SAC and SAA oxidation state extremes. Historically, bimetallic RhCu and supported Rh systems have been investigated for several reactions including CO oxidation, NOx reduction, and the hydrogenation of hydrocarbons. Recent studies conducted on RhCu SAAs have shown that while the Rh sites bind CO strongly, CO desorbs reversibly. In contrast, model Rh/CuOx SACs perform CO oxidation just above room temperature. However, there is a lack of understanding of structure-function relationships between these two extremes when the surface is partially oxidized. We report a study of CO oxidation on partially oxidized RhCu alloys using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and low energy electron diffraction (LEED). The TPD results demonstrate that CO oxidation occurs around 456 K on an oxidized RhCu SAA. Increasing the Rh coverage led to gradual decrease of the CO oxidation temperature to 362 K at 25% of a monolayer of Rh. Isotopic TPD experiments provided evidence of a Mars van Krevelen (MvK) CO oxidation mechanism. Furthermore, more fully oxidized RhCu SAAs were inactive for CO oxidation meaning that intermediate oxidation states of the SAA are most efficient for CO oxidation. LEED and IR results provide evidence for how the incorporation of Rh in the system affects its structure and the accessibility of the Rh sites to reactants. Together, these results begin to shed light on the effect of surface oxidation on the structure and reactivity of Rh sites in Cu-based SACs.

Elucidating the effect of oxidation on the structure and reactivity of rhodium copper single-atom alloys

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ACS Fall 2022

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