Guest Speaker, Sharani Roy, Ph.D., University of Tennessee

Digging deeper into the surface – Investigating subsurface oxygen and its reactivity in silver using lattice-gas models, DFT, and Monte Carlo simulations

First-row atoms, such as hydrogen, carbon, and oxygen, not only adsorb on the surface of a solid but are small enough to diffuse into the near-surface or subsurface region. The percolation of adsorbates through the surface raises many fundamental questions, such as, what conditions promote subsurface adsorption? Does the same adsorbate have different chemical properties in the subsurface compared to the surface? How do subsurface adsorbates influence chemical reactions on surfaces? To address these questions, we extended the theoretical framework of lattice-gas models to describe both coverage-dependent surface and subsurface adsorption in crystalline solids. Using this framework, we developed an all-site DFT-parameterized lattice-gas model for O/Ag(111) and integrated it with Monte Carlo simulations to calculate the thermodynamic distributions of atomic oxygen on the surface and in the subsurface of Ag(111). The results show that subsurface adsorption becomes thermodynamically favorable for oxygen coverages greater than 0.375 ML. Furthermore, we applied the simulations to construct the first ab initio phase diagram of O/Ag(111) that shows the pressure and temperature ranges within which subsurface oxygen coexists with surface oxygen on Ag(111). Our results indicate that subsurface oxygen is present under the industrial conditions used for the catalytic partial oxidation of olefins on silver nanoparticles. Finally, we computed the reaction pathway for the conversion of ethylene to ethylene oxide on Ag(111) using DFT and found significant changes to the reaction barriers with increased oxygen coverage and the presence of subsurface oxygen.