Molecular dynamics simulation on the reaction of nano-aluminum with water: size and passivation effects

The reaction of aluminum and water is widely used in the field of propulsion and hydrogen production, but its reaction characteristics at the nanometer scale have not been fully studied. In this paper, the effect of particle size and surface passivation of aluminum particle on the reaction mechanism was studied by using reactive molecular dynamics (RMD) simulation. The reduction of aluminum particle size can accelerate the reaction rate in the medium term (20-80 ps) due to the increase of activity, but it also produces an agglomeration effect as the temperature increases. The presence of surface passivation reduces the proportion of active aluminum and the yield of hydrogen decreases by 30% and 33%, respectively, as the particle size decreases from 2.5 nm to 1.6 nm. The addition of AlH₃ can overcome these drawbacks when some aluminum powders are replaced by AlH₃. The hydrogen yield is increased by the reaction 2AlH₃ + 3H₂O → Al₂O₃ + 6H₂. In the reaction of surface passivated Al (1.6 nm in diameter) and H₂O, when the proportion of AlH₃ reaches 25%, the energy release and hydrogen yield increase from 59.47 kJ mol^-1 and 0.0042 mol g^-1 to 142.56 kJ mol^-1 and 0.0076 mol g^-1, respectively. This performance even approximates the reaction of pure aluminum with water: 180.67 kJ mol^-1 and 0.0087 mol g^-1. In addition, the surface passivation affects the reaction mechanism. Before the passivation layer melts, the reaction 4Al + Al₂O₃ → 3Al₂O occurs inside the nanoparticles.