Investigation of ethanol oxidation over aluminum nanoparticle using ReaxFF molecular dynamics simulation

Adri C.T. van Duin,Yi Ran Zhang,Kai H. Luo

doi:10.1016/j.fuel.2018.06.119

Adri C.T. van Duin, Yi Ran Zhang + Show 1 more

Open Access

https://doi.org/10.1016/j.fuel.2018.06.119

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Abstract

Aluminum nanoparticles are an effective and economical additive for producing energetic fuels. In the present study, the state of the art ReaxFF molecular dynamics (MD) simulation has been used to uncover the detailed mechanisms of ethanol oxidation over aluminum nanoparticles with different oxidation states. The MD results reveal the dynamics process of ethanol oxidation reactions at nanoscales. The presence of aluminum nanoparticles is found to reduce the initial temperature of ethanol oxidation to 324 K. It is also found that compared to ethanol, oxygen molecules are more easily adsorbed on aluminum surfaces. Moreover, different oxidation states of aluminum nanoparticles influence the initial ethanol reactions on the nanoparticles’ surfaces. OH-abstraction is more commonly observed on pure aluminum nanoparticles while H-abstraction prevails on aluminum nanoparticles with oxide. The separated H atom from hydroxyl forms bonds with Al and O atom on aluminum nanoparticles surrounded by thin and thick oxide layers, respectively. Adsorptive dissociation of ethanol is hindered by the oxide layer surrounding the aluminum nanoparticle. Gas products like H2O and CO resulting from ethanol oxidation on aluminum nanoparticles with the thick oxide layer are observed while almost all the C, H and O atoms in ethanol diffuse into the nanoparticles without or with the thin oxide layer. For ethanol dissociation, a higher temperature is required than adsorption. In addition, the rate of ethanol dissociation increases with rising reaction temperatures. The activation energy for ethanol adsorptive dissociation is found to be 4.58 kcal/mol on the aluminum nanoparticle with the thin oxide layer, which is consistent with results from much more expensive DFT calculations.

Highlights

Metals have high energy density and are effective energetic additives in fuels and propellants [1,2,3]
AnAl nanoparticle of a diameter 2.8 nm containing 856 atoms is firstly prepared. The selection of this diameter is based on the objective of investigating aluminum melting temperature, oxidation process and carbon coating process by ReaxFF molecular dynamics without excessive computational cost
Two NVT molecular dynamics (MD) simulations with temperature of 298 K are performed to prepare aluminum nanoparticles with different oxide before investigating ethanol oxidation reactions. 300 and 600 oxygen molecules were distributed randomly around the aluminum nanoparticle in the simulation box

Summary

Introduction

Metals have high energy density and are effective energetic additives in fuels and propellants [1,2,3]. In particular, can be adopted as an energetic additive to enhance energy density and reduce the consumption of liquid fuels because of ready availability and low cost. Considerable efforts have been made to study the effects of added aluminum nanoparticles on liquid fuel ignition and combustion characteristics. Gan et al [11] found substantial enhancement in burning rates of ethanol due to the addition of 80 nm aluminum particles. A plausible explanation for this effect is the enhanced thermal conductivity due to nanoparticle additives [12] This explanation cannot explain a contrary observation that adding aluminum nanoparticles did not influence the burning rate of JP-8 [13]. Aluminum nanoparticles are not known as a good catalyst, but its catalytic effect may improve because of high specific surface areas

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