Adsorption and Reaction of Trimethyl and Triethyl Phosphite on Fe 3 O 4 by Density Functional Theory
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ORIGINAL PAPER
Adsorption and Reaction of Trimethyl and Triethyl Phosphite on Fe3O4 by Density Functional Theory Resham Rana1 · Nicholas Hopper1 · Wilfred T. Tysoe1 Received: 11 August 2020 / Accepted: 16 September 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract First-principle density functional theory (DFT) calculations are used to calculate the most stable structures and heats of adsorption of trimethyl and triethyl phosphites on an Fe3O4 surface in order to understand their behavior as lubricant additives. Previous surface science studies of phosphate and phosphite esters on iron oxide surface have shown that they react by sequentially desorbing the corresponding alcohol and aldehyde in equimolar amounts. This implies that the reactions are limited by the rate of alkoxide removal, followed by a rapid reaction to form the alcohol and aldehyde. It is found that the trialkyl phosphites, dialkyl phosphite, and monoalkyl phosphite all adsorb with the phosphorus atoms bound to surface Fe3+ ions, with the alkoxy groups close to parallel to the surface. The heat of adsorption increases as the alkoxide groups are removed. The postulate that the rate-limiting step in the decomposition of the phosphate esters is the sequential removal of the alkoxide group is tested by plotting the experimental activation energy obtained from temperature-programed desorption experiments versus the corresponding heats of adsorption calculated by DFT. A linear dependence shows that the reaction obeys the so-called Evans–Polanyi relation, thereby confirming the above postulate. The slope of the plot is close to unity and thus implies that the structure of the transition state of the reaction resembles the product. Graphic Abstract
Keywords Phosphite ester · Density functional theory · Fe3O4 surfaces · Evans–polanyi relations
1 Introduction
* Wilfred T. Tysoe [email protected] 1
Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
Lubricant additives are mixed with the base oil of commercial lubricants to improve the friction and anti-wear properties of the sliding interface by producing a tribochemically formed layer [1]. Compounds containing phosphorus such as phosphate and phosphite esters are used to lubricate Fe
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(III) oxide surfaces where phosphate films grow during the tribological process [2–7]. It appears that the chemistry of phosphate and phosphite esters is controlled by whether molecular decomposition is initiated by P‒O or C‒O bond scission, where the reaction is dominated by C‒O cleavage on metallic iron [8–13] but is controlled by P‒O bond breakage on iron oxide [14, 15] and the surface chemistry of triethyl phosphite (TEPi) and trimethyl phosphite (TMPi) on oxidized iron in ultrahigh vacuum (UHV) has been correlated with the tribological behavior [16]. It is found that the phosphite esters adsorb onto a thin Fe3O4 film grown on an iron substrate by electron donation from the substrate to th
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