Passivation of Fe(110) via phosphorus deposition: the reactions of trimethylphosphite

Abstract

The reactions of trimethylphosphite, P(OCH 3) 3, with the clean and phosphorus pre-covered Fe(110) surface have been investigated by X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction spectroscopy (TPRS) and low-energy electron diffraction (LEED). P(OCH 3) 3 adsorbs molecularly on the clean surface and remains intact up to a temperature of 250 K at which point it begins to decompose to surface methoxy and atomic phosphorus. Subsequent reaction of methoxy yields gaseous CO and H 2 at 430 K, as well as adsorbed P, C, and O. CO is formed upon heating to ∼770 K from the recombination of atomically adsorbed C and O. The presence of phosphorus on the surface alters the decomposition pathway for molecular trimethylphosphite via a combination of electronic and geometric effects, leading to production of formaldehyde in addition to CO and H 2. The fraction of formaldehyde increases with higher phosphorus coverages. The dependence of the reactions on phosphorus coverage indicated that proximity to adsorbed phosphorus favors formaldehyde production while CO is formed on clean Fe sites. The effect of phosphorus on the reactions of trimethyl phosphite on Fe(110) is similar to the effect of sulfur and oxygen on the reactions of methanol on Fe(100), but different than the effects of sulfur and oxygen on the reactions of methanol on Fe(110), suggesting that adsorbed phosphorus may change the geometric structure of Fe(110). The deposition of phosphorus via irreversible decomposition of P(OCH 3) 3 is self limiting. A maximum of ∼0.35 ML of phosphorus is deposited on the surface at which point the trimethyl phosphite primarily reversibly 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.