(330b) Initial Pathways to O Vacancy Formation On Pdo(101)

Palladium oxide (PdO) has been identified as the active phase for the oxidation of CH₄ under oxygen-rich conditions. Recent ultrahigh vacuum (UHV) and density functional theory (DFT) studies has shown that the PdO(101) surface provides rows of paired coordinatively unsaturated (cus) Pd and O atoms on the surface that facilitate C-H cleavage in alkanes. To understand methane oxidation on the PdO surface under reaction conditions will require a better understanding the role of O vacancy formation, participation in reaction(s), and healing through dioxygen. Previous scanning tunneling microscopy (STM) studies show that during thermal decomposition of the PdO(101) surface oxygen vacancy clusters initially form before more pronounced reduction of the surface occurs. To understand the early stages of the reduction of the PdO(101) surface, we have investigated oxygen vacancy formation on the PdO(101) surface using density functional theory (DFT).

We initially evaluated the vacancy formation energy (VFE) by removing isolated and clusters of cus O atoms on the PdO(101) surface. The VFE shows negligible differences between clustered versus isolated O vacancies, which conflicts with the STM results that show O vacancy cluster formation. This result suggests that the kinetics of O vacancy formation may influence the observed clustering of O vacancies in the experiments. We will discuss various scenarios for initial and subsequent O vacancy formation and the associated barriers for these mechanisms. We have found that the formation of the O₂ molecule from the O vacancy formation process is more tractable if two adjacent O vacancies form. We will also discuss ongoing work to explore subsequent O vacancy formation around this pair of O vacancies on the PdO(101) surface and correlate DFT results with the observations from STM experiments.