O₂ Dissociation on Cu(111) Dynamics on a Novel Screened Hybrid van der Waals DFT Potential Energy Surface

Abstract

The dissociative chemisorption (DC) of O 2 on Cu(111) has been extensively studied by both theory and experimentation. Different experiments disagree on the underlying mechanisms (direct or indirect) for the sticking of O 2 . Thus far, studies based on density functional theory (DFT) favor the indirect mechanism. However, DFT has not fully resolved the discussion as DFT based on the generalized gradient approximation (GGA) has always substantially overestimated the reactivity and sticking probabilities of O 2 on Cu(111) and other Cu surfaces. Recent work indicated that this overestimation is due to the failure of GGA DFT to describe molecule−metal systems where the charge transfer energy (E CT ), i.e., the work function of the metal surface minus the electron affinity of the molecule, is below 7 eV. O 2 + Cu( 111) is one such system. This work presents computed sticking probabilities for O 2 + Cu(111) based on the HSE06-1/2x-VdWDF2 screened hybrid van der Waals density functional (DF), which is applied self-consistently. A six-dimensional static potential energy surface (PES) was constructed using the corrugation-reducing procedure, keeping the surface atoms fixed. This PES was used to perform quasi-classical trajectory calculations to compute the sticking probabilities of O 2 + Cu(111). For the first time, we present DFT-based sticking probabilities that underestimate the experimental sticking probabilities. While reproducing the experimental results would have been even more desirable, the fact that we found a DF which underestimates the measured sticking probabilities means that a DF using a lower fraction of exact exchange will most likely describe the O 2 + Cu(111) system with high accuracy. Furthermore, our work shows evidence for the presence of both indirect and direct dissociative chemisorptions. The indirect precursor-mediated mechanism occurs for low-incidence energy O 2 . The mechanism is supplanted by a direct dissociative mechanism at higher incidence energies. Lastly, our work suggests that the Cu surface temperature may also affect the dissociation mechanism, but this still needs further verification with a different theoretical framework that allows for the simulation of surface temperature.