Simulating chemical reactions assisted by mechanical stresses with ab initio molecular dynamics

The design of new and environmentally friendly materials to reduce friction and wear is very important to save energy and reduce emissions. Among them, carbon films both deposited as coatings or formed in situ through tribochemical reactions have proven to be effective in reducing the friction between metal interfaces. In this thesis we investigate the initial stages of formation of carbon tribofilms with ab initio molecular dynamics simulations. To this aim we consider hydrocarbon molecules confined at metal interfaces and analyse the effect of several parameters, such as the chain length of the molecules, the type of bonds, the pressure applied to the system, the temperature, the presence of shear stress and different substrates. We want to find the most favourable conditions to promote the dissociation of C-H bonds and the formation of new C-C bonds. The hydrocarbons we study mostly are methane, butene, hexene and a molecule belonging to the class of alpha-olefins, while Fe(110) and Ni(111) are the two metallic surfaces considered. Our molecular dynamics simulations confirm that dehydrogenations can be induced by confining hydrocarbon molecules in metal slabs. The results show that the most efficient way to increase the dehydrogenation events is to apply a strong impulsive pressure (7-10 GPa); furthermore, high temperatures help the process. It has been also observed that in longer molecules the fraction of dissociated hydrogens is higher. Regarding the two studied metals, Ni has turned out to be more efficient to induce dehydrogenations than Fe, most likely because in Ni some of the dissociated hydrogen atoms penetrate in the metal slab. In order to have a more complete understanding of the interactions between the hydrocarbons and the metal surfaces, different quantities have been calculated, such as the adsorption energy of hydrogen atoms, the physisorption energy of the molecules, the dissociation energy and the insertion of H atoms in the metal slabs.