Adsorption of N₂ and H₂ at AlN(0001) Surface: Ab Initio Assessment of the Initial Stage of Ammonia Catalytic Synthesis

Adsorption of molecular nitrogen and molecular hydrogen at an Al-terminated AlN(0001) surface was investigated using ab initio simulation methods. It was shown that both species undergo dissociation during attachment to the surface, i.e., the adsorption is dissociative. Despite high bonding energies of both molecules, the dissociative adsorption is energetically highly favorable. In addition, both processes have negligible, close to zero, energy barriers. The adsorption sites were identified for both H and N adatoms. High adsorption energy is related to contribution from electron donation by the partially occupied Al broken bond state. The electron contribution terminates at the coverage equal to 1/4 and 3/4 monolayers for N and H, respectively. This is in accordance with the extended electron counting rule. Thus, the electron charge transfer role in ammonia catalysis at the surface is elucidated. Furthermore, as shown by ab initio calculations, the adsorbed species may react creating N-H radicals and the ammonia admolecules which desorbs from the surface. The ab initio modeling provides indication that AlN(0001) is a powerful catalyst for high-pressure-high-temperature synthesis of ammonia, indicating that AlN may be a candidate for industrial applications in ammonia synthesis.