First-Principles Study of Adsorption of CH₄ on a Fluorinated Model NiF₂ Surface

Electrochemical fluorination on nickel anodes, also known as the Simons' process, is an important fluorination method used on an industrial scale. Despite its success, the mechanism is still under debate. One of the proposed mechanisms involves higher valent nickel species formed on an anode acting as effective fluorinating agents. Here we report the first attempt to study fluorination by means of first principles investigation. We have identified a possible surface model from the simplest binary nickel fluoride (NiF₂). A twice oxidized NiF₂(F₂) (001) surface exhibits higher valent nickel centers and a fluorination source that can be best characterized as an [F₂]^- like unit, readily available to aid fluorination. We have studied the adsorption of CH₄ and the co-adsorption of CH₄ and HF on this surface by means of periodic density functional theory. By the adsorption of CH₄, we found two main outcomes on the surface. Unreactive physisorption of CH₄ and dissociative chemisorption resulting in the formation of CH₃F and HF. The co-adsorption with the HF gave rise to four main outcomes, namely the formation of CH₃F, CH₂F₂, CH₃ radical, and also physisorbed CH₄.