The propensity for aluminum nitride (AlN) to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in N2 to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption and dissociation, hydroxyl-mediated proton diffusion to form NH3, and desorption of NH3. We found activation barriers (Ea) for hydrolysis of 79 and 85 kcal/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al-N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic proton diffusion (Ea = 79 kcal/mol) and Grotthuss-like hydroxyl-mediated proton diffusion (Ea = 21 kcal/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 79 or 85 kcal/mol depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal-nitrogen bonds and thus more facile N liberation in the form of NH3.

Aluminum nitride hydrolysis enabled by hydroxyl-mediated surface proton hopping

253rd National Meeting (2017)

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