Is Subsurface Oxygen Necessary for the Electrochemical Reduction of CO₂ on Copper?

It has recently been proposed that subsurface oxygen is crucial for the adsorption and subsequent electroreduction of CO₂ on copper. Using density functional theory, we have studied the stability and diffusion of subsurface oxygen in single crystals of copper exposing (111) and (100) facets. Oxygen is at least 1.5 eV more stable on the surface than beneath it for both crystal orientations; interstitial sites are too small to accommodate oxygen. Here, the rate of atomic oxygen diffusion from one layer below a Cu(111) surface to the surface is 5 × 10³ s^–1. Oxygen can survive longer in deeper layers, but it does not promote CO₂ adsorption there. Diffusion of subsurface oxygen is easier to the less-dense Cu(100) surface, even from lower layers (rate ≈ 1 × 107 s–1). Finally, once the applied voltage and dispersion forces are properly modeled, we find that subsurface oxygen is unnecessary for CO₂ adsorption on copper.