Nonequilibrium Oxidation of Aluminum Nanoparticles in H2O–Air Mixtures: A Molecular Dynamics Study

Abstract Aluminum nanoparticles (ANPs) are promising high-energy-density fuels for amphibious propulsion systems. However, differences in mass transfer and chemical properties between H2O and O2 have led to uncertainty regarding the oxidation kinetics of ANPs, thereby limiting their performance regulation in water–air transmedia environments. In this work, the nonequilibrium oxidation and combustion of ANPs in mixed H2O–air atmospheres with various H2O/air ratios (i.e., 1:9, 1:4, 1:2, 1:1, 2:1, 4:1, and 9:1, namely, M1:9, M1:4, M1:2, M1:1, M2:1, M4:1, and M9:1) and temperatures (1500–3000 K) are investigated by molecular dynamics simulations. The results reveal that H2O acted as a critical promoter of ANP oxidation: it adsorbed and dissociated on Lewis acid sites of the Al surface, supplying OH– and O2– reactive oxygen species, which dramatically accelerated Al–O bond formation and enhanced energy release. The synergistic effect of high temperature and high-humidity environment accelerated H2O consumption and drove vigorous Al–water reactions, releasing substantial amounts of H2. The formation of hollow nanostructures was highly dependent on the H2O/air ratio: at M4:1, the pressure generated by internal H2 ruptured the oxide shell, destabilizing the hollow structure; at M1:1, a stable hollow shell emerged due to outward diffusion of Al3+ dominating over inward O2– diffusion; while at M1:4, a dense Al2O3 layer rapidly formed, which inhibited hollowing. Crucially, the oxidation of ANPs in humid environments proceeds via proton-coupled pathways involving hydroxyl intermediates, whereas it follows direct electron transfer in air. This investigation provides atomic-level insights into the reactive dynamics of ANPs in complex oxidizing environments, offering critical guidance for the design of high-performance and environment-adaptive metallic fuels.