O<sub>2</sub> Dissociation\non Cu(111) Dynamics on\na Novel Screened Hybrid van der Waals DFT Potential Energy Surface

The dissociative chemisorption (DC) of O<sub>2</sub> on\nCu(111)\nhas been extensively studied by both theory and experimentation. Different\nexperiments disagree on the underlying mechanisms (direct or indirect)\nfor the sticking of O<sub>2</sub>. Thus far, studies based on density\nfunctional theory (DFT) favor the indirect mechanism. However, DFT\nhas not fully resolved the discussion as DFT based on the generalized\ngradient approximation (GGA) has always substantially overestimated\nthe reactivity and sticking probabilities of O<sub>2</sub> on Cu(111)\nand other Cu surfaces. Recent work indicated that this overestimation\nis due to the failure of GGA DFT to describe molecule–metal\nsystems where the charge transfer energy (<i>E</i><sub>CT</sub>), i.e., the work function of the metal surface minus the electron\naffinity of the molecule, is below 7 eV. O<sub>2</sub> + Cu(111) is\none such system. This work presents computed sticking probabilities\nfor O<sub>2</sub> + Cu(111) based on the HSE06-1/2x-VdWDF2 screened\nhybrid van der Waals density functional (DF), which is applied self-consistently.\nA six-dimensional static potential energy surface (PES) was constructed\nusing the corrugation-reducing procedure, keeping the surface atoms\nfixed. This PES was used to perform quasi-classical trajectory calculations\nto compute the sticking probabilities of O<sub>2</sub> + Cu(111).\nFor the first time, we present DFT-based sticking probabilities that\nunderestimate the experimental sticking probabilities. While reproducing\nthe experimental results would have been even more desirable, the\nfact that we found a DF which underestimates the measured sticking\nprobabilities means that a DF using a lower fraction of exact exchange\nwill most likely describe the O<sub>2</sub> + Cu(111) system with\nhigh accuracy. Furthermore, our work shows evidence for the presence\nof both indirect and direct dissociative chemisorptions. The indirect\nprecursor-mediated mechanism occurs for low-incidence energy O<sub>2</sub>. The mechanism is supplanted by a direct dissociative mechanism\nat higher incidence energies. Lastly, our work suggests that the Cu\nsurface temperature may also affect the dissociation mechanism, but\nthis still needs further verification with a different theoretical\nframework that allows for the simulation of surface temperature.