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

Abstract

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.