First principle calculations including ab initio molecular dynamics studies for the activation of hydrogen fluoride on Ni(111)

Abstract

The formation of an insulating nickel fluoride film on a nickel surface in contact with hydrogen fluoride (HF) is predicted to cause the low-voltage passivation of Ni anodes used in the Simons process. In this work, we examine the energetics of the activation of a single HF molecule adsorbing onto a Ni(111) surface using density functional theory with periodic boundary conditions. First, we calculate structures and energies of the minimum energy path towards dissociative chemisorption: physisorption intermediates, transition states and chemisorbed products. The calculated energies of the transition states are in the order of 380 meV, suggesting a surmountable activation barrier for chemisorption. To learn more in detail about the mechanism of the dissociative chemisorption, we additionally consider the adsorption of HF as a simulation of a molecular beam experiment by using ab initio molecular dynamics. Through analysis of the trajectories, the position of the center of mass of the HF molecule, upon collision with the Ni surface, is the most important factor for the probability of chemisorption. By comparison of the chemisorpion probability of an incoming HF molecule with the minimum energy path, we find that favorable HF impact angles for dissociative chemisorption do not correspond to the minimum energy path, due to the difference in mass between hydrogen and fluoride. The data points from the molecular dynamics simulations are fed into a neural network in order to fit a six-dimensional potential energy surface of HF on Ni(111), which is discussed.