Inducing High-Energy-Barrier Tribochemical Reaction Pathways; Acetic Acid Decomposition on Copper

The surface tribological chemistry of acetic acid on copper is studied using an ultrahigh vacuum tribometer, supplemented by first-principles density functional theory calculations of the surface structure and reaction pathways. Acetic acid forms η2-acetate species on bridge sites at room temperature as identified by reflection–absorption infrared spectroscopy. Rubbing the surface with a tungsten carbide ball reduces the amount of carbon and oxygen in the rubbed region at the same rates to leave some carbon and oxygen on the surface. This is different from the thermal decomposition pathway, where heating to ~ 580 K removes all oxygen, but leave a small amount of carbon on the surface. It is postulated that this arises because sliding along a direction aligned within the plane of the adsorbed acetate species can induce a high-energy-barrier pathway in which the η2-acetate tilts to form an η1-acetate that can react to form a bent CO2δ− species that decomposes to evolve carbon monoxide and deposit atomic oxygen on the surface. Repeated acetic acid dosing and rubbing reduces the total amount of acetic acid that can adsorb on the surface by ~ 50% after ~ 4 cycles, resulting is a stable, low-friction film. At this point, the adsorbed acetic acid is completely tribochemically removed. This suggests that adsorbed acetic acid can form a self-healing film in which any wear of the low-friction film will then allow it to be replenished by shear-induced decomposition of adsorbed acetate species.