Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

Aluminum
nitride (AlN) is used extensively in the semiconductor industry as
a high-thermal-conductivity insulator, but its manufacture is encumbered
by a tendency to degrade in the presence of water. The propensity
for 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 nitrogen (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, hydroxyl-mediated proton diffusion to form
NH3, and NH3 desorption. We found activation
barriers (Ea) for hydrolysis of 330 and
359 kJ/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 (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion
is the predominant mechanism in AlN hydrolysis. The large activation
barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/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 NH3 liberation.