Theoretical study of surface dependence of NH3 adsorption and decomposition on spinel-type MgAl2O4

Abstract

Ammonia decomposition is a critical process in the production of renewable hydrogen energy. Although many studies have concentrated on ammonia decomposition, little is known about the effect of the spinel support on the activation of NH₃. The adsorption and dissociation of NH₃ on low-index (100), (110), and (111) MgAl₂O₄ surfaces were investigated using density functional theory. The interaction between NH₃ and MgAl₂O₄ was structure dependent, with the surface geometry and electronic structures determining the active sites and adsorption stability. The NH₃ was likely to point toward a surface protruding metal site with the formation of a hydrogen bond. For dissociation, the adsorption energy of the (111) surface was much more favorable than the energies of the (100) and (110) surfaces. The surface Al_3c-sp state at the Fermi level and the formation of the H–O_3c covalent bond of this surface were the main reasons for the higher adsorption energy in NH₃ dissociation. In particular, the hybridization between the Al_3c-sp state and N-p state on the (111) surface was larger than those of the other two surfaces. In view of the thermodynamics and dynamics, the (111) surface was more favorable for NH₃ adsorption and reaction. For the defective surfaces, the existence of an oxygen vacancy lowered the adsorption ability on metal sites. This was due to the destroyed surface symmetry structure and the reduced charge of the metal site. In addition, only on the (111) surface could the oxygen vacancy act as an active site for adsorption. The energy barrier of NH₃ dissociation on the Vo_3c (111) surface was the lowest, which indicated that the NH₃ reaction on this defective surface was dynamically the most favorable. These findings have an important implication for the decomposition of NH₃ on MgAl₂O₄ surfaces and could provide theoretical guidance for other catalytic reactions.