Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

Abstract

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.

The role of reducing agents in the nucleation and growth of Al metalloid clusters: Ab initio molecular dynamics study

“…It is also found that these reactions continue even after the end of the initial hydrogen transfer. [23,24,25]The same mechanism is observed in our simulations. A single H-transfer from the LAH reducing agent was able to initiate a continuous insertion of additional Al's that eventually lead to the growth of a larger aluminum complex.…”

High-Throughput Equilibrium Analysis of Active Materials for Solar Thermochemical Ammonia Synthesis

“…Two additional challenges to this reaction have the opposite dependence on temperature. The kinetics of hydrolysis are known to be slow and alleviated by increasing the temperature. , However, NH 3 is thermodynamically favored to decompose into N 2 + H 2 with decreasing Δ G r (increased driving force for decomposition) as temperature increases, requiring the fast capture and quenching of the liberated NH 3 . Although the thermodynamic conversion of nitride to oxide by steam exposure is thermodynamically favorable for ∼600 binary redox pairs, these additional kinetic and nonequilibrium challenges will further restrict the number of truly viable pairs for this step.…”

“…Along with thermodynamics, the kinetics of each reaction in STAS are critical to the viability of the NH 3 synthesis process, especially for reactions involving nitrogen. The corrosion of metal nitrides by H 2 O is a kinetically limited process for many nitrides below 1200 K, primarily associated with the need to break metal–nitrogen bonds that can have significant covalent character. ,, Similarly, the reduction of N 2 for metal nitride synthesis requires the activation and cleavage of the triply bonded N 2 molecule, which also poses a significant kinetic challenge. The difficulty in reducing N 2 can result in metals becoming only partially nitridated, limiting the NH 3 yield that can be achieved on a per cycle basis, as this quantity is directly linked with the amount N 2 that can be fixed to the metal.…”

Hydrolysis protection and sintering of aluminum nitride powders with yttria nanofilms

“…AlN degrades in water, forming Al 2 O 3 that dissolves into the AlN lattice to produce vacancies on aluminum sites (𝑉 ′′′ 𝐴𝑙 ) [7][8][9][10] :…”

“…AlN has many desirable properties, but it is expensive to produce due to the high sintering temperatures required to reach near‐theoretical density and reactivity with water. AlN degrades in water, forming Al 2 O 3 that dissolves into the AlN lattice to produce vacancies on aluminum sites ($$V_{Al}^{^{\prime\prime\prime}}$$) 7–10 : $$\begin{equation}2{\rm{AlN}} + 3{{\rm{H}}_2}{\rm{O}} \to {\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3} + 2N{H_3}\end{equation}$$ $$\begin{equation}{\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}\mathop \to \limits^{{\rm{AlN}}} 2{\rm{A}}{{\rm{l}}_{{\rm{Al}}}} + 3O_N^ \cdot + V_{Al}^{^{\prime\prime\prime}}\end{equation}$$…”