A molecular-beam study of the tribological chemistry of carbon tetrachloride on oxygen-covered iron

Dc molecular-beam methods are used to examine the reactivity of carbon tetrachloride with oxide films grown on iron in ultrahigh vacuum. The incident CCl4 beam flux is sufficiently low that the nature of the surface oxide is dictated by the annealing temperature allowing the reactivity of Fe2O3, Fe3O4 and FeO films to be examined. Carbon tetrachloride reacts rapidly with Fe2O3 and reaction with Fe3O4 commences at ∼620 K to evolve CO. The activation energy for this process is 20.6± 1.0 kcal/mol. CCl4 reacts with FeO above ∼790 K, also to evolve CO, and the activation energy for this reaction is 5.7 ± 0.4 kcal/mol. X-ray photoelectron spectroscopy shows the formation of a halide after reaction at 900 K. These results are in accord with film-growth kinetics measured using a microbalance at high pressures, where it was found that it was not necessary to remove the oxide layer prior to reaction. This contrasts with the behavior of sulfur-containing molecules, where the oxide layer had to be removed before a film would grow. This effect may contribute to the additive synergies commonly found in extreme-pressure lubricant additives where one of the roles of the chloride may be to reduce the oxide layer.