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Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces - revised - clean.pdf (2.05 MB)
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Substituent Effects on the Thermal Decomposition of Phosphate Esters on Ferrous Surfaces

preprint
revised on 08.04.2020 and posted on 09.04.2020 by James Ewen, Carlos Ayestaran Latorre, Chiara Gattinoni, Arash Khajeh, Joshua Moore, Joseph Remias, Ashlie Martini, Daniele Dini

Phosphate esters have a wide range of industrial applications, for example in tribology where they are used as vapour phase lubricants and antiwear additives. To rationally design phosphate esters with improved tribological performance, an atomic-level understanding of their film formation mechanisms is required. One important aspect is the thermal decomposition of phosphate esters on steel surfaces, since this initiates film formation. In this study, ReaxFF molecular dynamics simulations are used to study the thermal decomposition of phosphate esters with different substituents on several ferrous surfaces. On Fe3O4(001) and α-Fe(110), chemisorption interactions between the phosphate esters and the surfaces occur even at room temperature, and the number of molecule-surface bonds increases as the temperature is increased from 300 to 1000 K. Conversely, on hydroxylated, amorphous Fe3O4, most of the molecules are physisorbed, even at high temperature. Thermal decomposition rates were much higher on Fe3O4(001) and particularly α-Fe(110) compared to hydroxylated, amorphous Fe3O4. This suggests that water passivates ferrous surfaces and inhibits phosphate ester chemisorption, decomposition, and ultimately film formation. On Fe3O4(001), thermal decomposition proceeds mainly through C-O cleavage (to form surface alkyl and aryl groups) and C-H cleavage (to form surface hydroxyls). The onset temperature for C-O cleavage on Fe3O4(001) increases in the order: tertiary alkyl < secondary alkyl < primary linear alkyl ≈ primary branched alkyl < aryl. This order is in agreement with experimental observations for the thermal stability of antiwear additives with similar substituents. The results highlight surface and substituent effects on the thermal decomposition of phosphate esters which should be helpful for the design of new molecules with improved performance.

Funding

EPSRC

U.S. Army Laboratory

History

Email Address of Submitting Author

j.ewen@imperial.ac.uk

Institution

Imperial College London

Country

UK

ORCID For Submitting Author

0000-0001-5110-6970

Declaration of Conflict of Interest

None

Version Notes

Revised version

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in The Journal of Physical Chemistry C

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