EXAFS and XANES Studies on Decomposition of Molybdenum 0, 0-Dialkyl Phosphorgdithioate in Commercial Engine Oils

Abstract

Organo molybdenum compounds are used as an anti-friction additive for fuel-saving engine oils. However, the anti-friction effect of the additive is deteriorated while it is used, to study the mechanism of its deterioration, the local structure around Mo atoms in the compounds have been investigated by an X-ray absorption spectroscopy (EXAFS and XANES)using a laboratory scale equipment.
The EXAFS results s uggested that the local structure of the Mo atoms in the fresh engine oils is similar to that in the molybdenum 0, 0-dialkyl phosphorodithioate (Mo-DTP)whose structure is expressed by the chemical formula as follows;
and the molybdenum compound in the oils has changed to Mo oxide in which an Mo atom is coordinated by six oxygen atoms after 2000 km running. XANES spectra of the compounds show that the coordination geometry around Mo atoms in the Mo oxide is similar to that in MoO₃ rather than MoO₂.
The deterioration of anti-friction effect of the engine oils after use coincides with Mo oxide formation.