DFT-MD simulations of reaction mechanisms between tricresy phosphate and Fe₃O₄ (111) surface

Tricresyl Phosphate (TCP) used, e.g, as an anti-wear additive reacts to form surface layers on metal and metal oxide under high pressure and high temperature. So far, many experimental and theoretical studies have suggested principal factors that affect the reaction: configurational orientations of P=O group of TCP on the surface, water molecules on the surface, and oxygen atoms of oxidized metal. In a recent DFT study [1], three decomposition mechanisms of TCP on pure iron surfaces was investigated. We here perform DFT-MD simulations of gradually pressurized (~10 GPa) TCP on Fe₃O₄ (111) surfaces to understand the effects of exposed oxygen atoms of the Fe₃O₄ surfaces and of TCP orientations initially relaxed on the Fe₃O₄ surfaces. We thereby find that: (i) the P-O bond that connect to the toluene base of TCP breaks in all cases, (ii) the P-O bond breaking is attributed to the attacking by either oxygen or iron atom of the Fe₃O₄ surfaces, (iii) the adsorption behavior of TCP varies depending on the initial TCP orientations and the amount of exposed oxygen atoms of the Fe₃O₄ surfaces.
[1] E. Osei-Agyemang et al., Tribol. Lett. 66, 48 (2018).