The surface chemistry of alkyl and arylphosphate vapor phase lubricants on Fe foil

Abstract

The surface chemistry of tributylphosphate (TBP) and tricresylphosphate (TCP) on a polycrystalline Fe surface was studied using temperature programmed reaction spectroscopy and Auger electron spectroscopy to illustrate some of the initial steps in the reaction mechanisms of alkyl and arylphosphate vapor phase lubricants. During heating, TBP [(C 4H 9O) 3P=O] adsorbed on the Fe surface decomposes via C–O bond scission to give butyl surface intermediates [C 4H 9–] that react via β-hydride elimination to desorb as 1-butene [CH 3CH 2CH=CH 2] and H 2 without appreciable carbon deposition onto the surface. The thermal decomposition of 1-iodobutane [I-C 4H 9] on Fe was observed to proceed via the same β-hydride elimination mechanism. In contrast to tributylphosphate, meta-tricresylphosphate ( m-TCP) [(CH 3–C 6H 4O) 3P=O] decomposes on Fe via P–O bond scission to produce methylphenoxy intermediates [CH 3–C 6H 4O–]. During heating to 800 K, methylphenoxy intermediates either desorb as m-cresol [CH 3–C 6H 4–OH] via hydrogenation or decompose further to generate tolyl intermediates [CH 3–C 6H 4–]. Some of the tolyl intermediates desorb as toluene [CH 3–C 6H 5] via hydrogenation but the majority decompose resulting in H 2 and CO desorption and carbon deposition onto the Fe surface. The P–O bond scission mechanism of m-TCP was verified by showing that the temperature programmed reaction spectra of m-cresol yield products that are almost identical to those of m-TCP. These results provide insight into the origin of the differences in the performance of alkyl and arylphosphates as vapor phase lubricants. The alkylphosphates decompose via alkyl intermediates that readily undergo β-hydride elimination and desorb into the gas phase as olefins, thus removing carbon from the surface. In contrast, the arylphosphates generate aryloxy intermediates by P–O bond scission and aryl intermediates by further C–O bond scission. Neither of these intermediates can undergo β-hydride elimination and thus they decompose to deposit carbon onto the Fe surface. The higher efficiency for carbon deposition may be the primary reason for the superior performance of the arylphosphates over alkylphosphates as vapor phase lubricants.