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

Abstract

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.