PASSIVATION OF FE(110) VIA PHOSPHORUS DEPOSITION - THE REACTIONS OF TRIMETHYLPHOSPHITE

The reactions of trimethylphosphite, P(OCH3)(3), with the clean and ph osphorus pre-covered Fe(110) surface have been investigated by X-ray p hotoelectron spectroscopy (XPS), temperature-programmed reaction spect roscopy (TPRS) and low-energy electron diffraction (LEED). P(OCH3)(3) adsorbs molecularly on the clean surface and remains intact up to a te mperature of 250 K at which point it begins to decompose to surface me thoxy and atomic phosphorus. Subsequent reaction of methoxy yields gas eous CO and H-2 at 430 K, as well as adsorbed P, C, and O. CO is forme d upon healing to similar to 770 K from the recombination of atomicall y adsorbed C and O. The presence of phosphorus on the surface alters t he decomposition pathway for molecular trimethylphosphite via a combin ation of electronic and geometric effects, leading to production of fo rmaldehyde in addition to CO and H-2,. The fraction of formaldehyde in creases with higher phosphorus coverages. The dependence of the reacti ons on phosphorus coverage indicated that proximity to adsorbed phosph orus favors formaldehyde production while CO is formed on clean Fe sit es. The effect of phosphorus on the reactions of trimethyl phosphite o n Fe(110) is similar to the effect of sulfur and oxygen on the reactio ns of methanol on Fe(100), but different than the effects of sulfur an d oxygen on the reactions of methanol on Fe(110), suggesting that adso rbed phosphorus may change the geometric structure of Fe(110). The dep osition of phosphorus via irreversible decomposition of P (OCH3)(3) is self limiting. A maximum of similar to 0.35 ML of phosphorus is depos ited on the surface at which point the trimethylphosphite primarily re versibly adsorbs below 500 K. These results indicate that P-containing moieties in lubricant additives form a passivating surface layer upon which the remaining additives only weakly bind. (C) 1998 Elsevier Sci ence B.V. All rights reserved.