A Computational Approach to the Analysis of the Initial Steps of Dehydration in Aluminum (Oxy)hydroxides

Modifying the surface chemistry of metal fuels is a strategy to promote greater power generation. Aluminum is a common fuel used in energy generation technologies and is a good candidate to study surface modification. Pyrophoric metals are passivated by their nascent oxide, and for aluminum core particles, the passivation shell is formed by aluminum oxide phases. A hydrated surface modification is a first step toward further transformation into tailored oxygen-rich and stable surface passivation chemistry. Various aluminum hydroxide species are involved in surface modification, of which Al(OH) 3 has been shown to be the most promising. Density functional theory (DFT) calculations reveal that removing surface hydroxyls is an endothermic process. In vacuum, boehmite requires ∼365–450 kJ/mol and bayerite ∼216–281 kJ/mol to release water (per site), indicating bayerite is intrinsically easier to dehydroxylate. Implicit solvation (water) reduces these energies by 5–20%, making boehmite and bayerite dehydration energetics more comparable (boehmite: ∼345–390 kJ/mol, bayerite: ∼195–250 kJ/mol for representative sites). Notably, under ambient-like conditions, boehmite’s dehydration energy approaches that of bayerite for some sites, although overall bayerite remains thermodynamically easier to dehydrate. These calculations explain why bayerite-coated aluminum particles show enhanced surface reactivity in experiments (faster ignition and oxygen release) compared to boehmite-coated particles. By highlighting the structural and chemical nuances between these shells, we aim to inform the design of improved passivation layers for metal particles that require both stability and chemical tunability.