Modélisation atomistqiue des interactions entre les ions chlorures et la surface du nockel passivé

Overview & Synopsis

In order to better understand the mechanisms of breakdown of the passive layer formed on nickel, we have studied by DFT the interactions between chloride ions and the hydroxylated NiO(111) surface using the VASP code. Two reactions have been considered: (i) adsorption of Cl- by substituting the surface hydroxyls, (ii) insertion by exchange between Cl- and O2- from the oxide lattice. The study of the smooth hydroxylated NiO(111) layer suggests that the substitution reaction could take place at Cl- coverages below 70% and would result in Ni(OH)Cl complexes. The insertion reaction becomes favorable for Cl- coverages above 70%. The presence of a solvent (H2O) does not change the nature of the reorganization of the layers observed under vacuum, but leads to a decrease of the substitution energy for all coverages except for 100%. In order to assess the role of surface steps in the reactivity of the hydroxylated NiO(111) layer, we have built a periodical model including a monoatomic step and a terrace with the same orientation (111) as the smooth NiO(111) surface. The presence of steps on the surface induces two important differences during the substitution reaction: the formation, at the step edges, of substructrures which are very similar to the structure of Ni compounds such as Ni(OH)Cl and NiCl2, as well as a clear separation of those substructures from the surface, likely promoting their dissolution. After insertion, the substructures are less similar to the reference compounds and the separation is less pronounced. Finally, the fact that the substitution energy is exothermic for all coverages suggests the preponderance of this reaction in the mechanisms leading to the breakdown of passivity.