Effect of Hydration on Promoting Oxidative Reactions with Aluminum Oxide and Oxyhydroxide Nanoparticles

To harness greater power from aluminum (Al) combustion, one approach is to chemically transform the aluminum oxide (Al₂O₃) passivation layer surrounding the Al core particle by exploiting surface reactions via a wet chemistry approach. The goal of this study is to identify key parameters affecting interfacial chemistry on Al particles. As models for the surface layer of Al particles, γ aluminum oxide (γ-Al₂O₃) and boehmite (γ-AlO(OH)) nanoparticles are examined. Density functional theory calculations of dehydroxylation and dehydration energies are conducted on γ-Al₂O₃ and γ-AlO(OH). The calculations predict that OH bridge bonds throughout γ-AlO(OH) require similar energy to break compared with terminal OH groups abundant on the surface of γ-Al₂O₃ and are the most reactive sites. Experimentally, the predictions are confirmed by isolating representative surface reactions using γ-Al₂O₃ and γ-AlO(OH) nanoparticles suspended in an iodic acid solution to synthesize an oxidizing salt, aluminum iodate hexahydrate (AIH). Powder X-ray diffraction and X-ray photoelectron spectroscopy identify species of whole materials and surfaces, respectively. The structural and compositional details are revealed by acquiring images, spectra, and determining component phase maps in the scanning mode via transmission electron microscopy. The AIH yield is higher for γ-Al₂O₃, AIH formation is linked to the removal of terminal OH bonds, and AIH appears isolated on the particle surface. AIH formation from γ-AlO(OH) is linked to the removal of OH bridge bonds, and AIH formation does not appear localized on particle surfaces. These results link dehydroxylation and dehydration energies to reactions that produce AIH and provide a fundamental understanding of how to use hydration to control Al₂O₃ surface reactions and transform the reactivity of Al particles. Also, AIH is newly discovered as a highly reactive oxidizer used in energetic material studies that show tremendous potential for increasing the energy release rate from Al combustion.