(188f) Impact of Pd Single Atoms during CO Oxidation over Ceria-Supported Ni and Pt Single Atom Catalysts: Do Multi-Metal Single Atom Catalysts Demonstrate Synergistic Effects?

Single atom catalysts (SACs) demonstrate unique reactivity due to the ultimate contact between oxidized metal atoms and support. CO oxidation over Pd single atoms supported on ceria nanocubes experimentally measures a CO reaction order greater than 1, which can be captured in a mickrokinetic model if Pd accesses two different redox cycles ultimately oscillating between four oxidation states. Furthermore, experimentally measured turnover frequencies (TOFs, defined as the number of molecules reacting per metal atom per second) increase with Pd loading, suggesting atom ‘synergy’. Other transition metals, such as Ni and Pt, do not demonstrate the same unique kinetic behavior as Pd. Bimetallic catalysts were synthesized containing various ratios of Pd and Ni (or Pt) adsorbed to the same ceria support to determine if multiple, distinct single atoms impact the reactivity of other metals.

Pt is not as active as Pd for CO oxidation and measures higher activation energies (~65 kJ/mol) and lower TOFs. Ni measures a similar activation energy as Pd (~ 40 kJ/mol), but higher TOFs than Pd. For Pd-Pt bimetallic catalysts the activation energy increases with fraction of Pt and TOFs decrease. The kinetic behavior of the Pd-Pt bimetallic catalysts shifts towards Pd monometallic or Pt monometallic kinetics depending on the Pd-Pt ratio, indicating the two metals act independently of one another. Since Ni and Pd measure the same activation energy, no change was observed across the various ratios of Pd-Ni bimetallic catalysts. However, all TOFs fall below the measured value for Ni monometallic catalysts, suggesting Ni is negatively influenced by Pd. Despite Pd single atoms exhibiting unique redox behavior and atom ‘synergy’, Pd does not influence the reactivity of Pt and negatively influences Ni, suggesting Ni cations may impact the ability of Pd to sample the distinct redox cycles.