Trimethyl-phosphite dissociative adsorption on iron by combined first-principle calculations and XPS experiments

Abstract

The reaction of trimethyl-phosphite, TMPi, with a clean Fe(110) surface has been investigated by ab initio calculations. The most stable configurations and energies are identified for both molecular and dissociative adsorption. The calculated reaction energies indicate that dissociation is energetically more favorable than molecular adsorption and we provide a description of the dissociation path and the associated energy barrier. In situ XPS analysis of adsorbed TMPi on metallic iron confirmed molecular chemisorption and dissociation at high temperature. These results shed light on the mechanism of phosphorus release from organophosphites at the iron surface, which is important for the functionality of these phosphorus-based additives, included in lubricants for automotive applications

First principles study of organophosphorus additives in boundary lubrication conditions: Effects of hydrocarbon chain length

“…The increase of enthalpy is, in fact, ruled by 2 terms, the internal energy and the PV term, and they increase in different ways in TMPi and TBPi, leading to an overall enthalpy increase, which is similar in the two cases. Finally, we observed that the load‐induced dissociation of TMPi proceeds, along the same reaction path as we identified on the open surface in the absence of mechanical stress . The load‐induced dissociation of TBPi proceeds via P─O bond breaking as well.…”

“…The most favorable adsorption site for the TMPi molecule on the Fe(110) surface has been identified in a previous study by our group by comparing the molecule energies at different symmetry locations on the Fe(110) surface. 29 The configuration shown in Figure 1A, where the P atom of the molecule resides OT of a surface Fe atom, turned out to be the most favorable adsorption geometry. 29 The adsorption of TBPi is simulated by positioning the molecule at the same OT site and performing a structural optimization ( Figure 1B).…”

“…29 The configuration shown in Figure 1A, where the P atom of the molecule resides OT of a surface Fe atom, turned out to be the most favorable adsorption geometry. 29 The adsorption of TBPi is simulated by positioning the molecule at the same OT site and performing a structural optimization ( Figure 1B). We assume, in fact, that the chain length does not affect the relative stability of the adsorption sites on the Fe surface since molecular chemisorption occurs through the P atom, which forms the same bonds in the TBPi and the TMPi molecules.…”

“…Here, we apply first principles calculations to study the molecular adsorption, interface confinement, load‐induced enthalpy increase, and subsequent molecular dissociation of TMPi and TBPi additives at the iron interface. These systems have been previously considered also by experiments . The use of first principles calculations has proven to be a relevant tool to provide insight into the tribochemistry of solid and boundary lubricants as real‐time, in situ experimental investigations are still challenging …”

“…These systems have been previously considered also by experiments. [27][28][29][30][31][32] The use of first principles calculations has proven to be a relevant tool to provide insight into the tribochemistry of solid and boundary lubricants 27,33-40 as real-time, in situ experimental investigations are still challenging. [40][41][42][43][44]…”