Dissociative Adsorption of 5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO) on Aluminum-Terminated (0001) Surface of α-Alumina As Predicted from Plane-Wave Density Functional Theory

Abstract

Plane wave density functional theory (DFT) was used to study the adsorption of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), an insensitive high performance explosive material, on Al-terminated (0001) surface of (4 × 4) α-Al2O3 using the PBE, PBEsol, BLYP, and recently developed van der Waals functional (vdW-DF2) within the Generalized Gradient Approximation. The interactions of valence electrons with atomic cores were approximated using the ultrasoft pseudopotentials. Various orientations of NTO with respect to the alumina surface were considered. It was revealed that the carbonyl site of NTO binds more strongly than the nitro group site with the surface aluminum ion. Important information revealed from the present investigation was that NTO undergoes dissociative adsorption on the Al-terminated α-alumina surface. During this process, both the carbonyl oxygen and the nitro oxygen are involved in binding with surface Al ions and a proton from the N4 site of the adsorbate (NTO) is dissociated and migrated to nearby surface oxygen site forming an OH bond. Consequently, the NTO adsorbed on the alumina surface is in the anionic form. Further, consequent to the adsorption, the interacting surface Al-atoms are pulled up toward the NTO with respect to the plane containing the rest of the Al atoms. Moreover, an analysis of charge density difference maps suggested a buildup of charge density in the NTO-alumina bonding region, indicating covalent nature of adsorption on the Al-terminated (0001) surface of α-alumina.