Adsorption and decomposition of HMX and CL-20 on Al(111) surface by DFT investigation

The adsorption and decomposition of HMX and CL-20 molecules on the Al(111) surface were investigated by the generalized gradient approximation of density functional theory. The calculations employed a supercell (6 × 6 × 3) slab model and three-dimensional periodic boundary conditions. The strong attractive forces between HMX (or CL-20) molecule and Al atoms induce the breaking of N-O and N-N bonds in nitro group. Subsequently, the dissociated oxygen atoms, NO2 groups, and radical fragments of HMX or CL-20 oxidize the Al surface. The largest adsorption energy is −1792.7 kJ/mol in B1, where CL-20 decomposes into four O atoms and a CL-20 fragment. With the number of the radical species in adsorption configurations increases, the corresponding adsorption energy increases greatly. We also investigated the decomposition mechanism of HMX and CL-20 molecules on the Al(111) surface. The activation energies (E_a) for the dissociations A2, A3, B1, and B6 are 31.2, 47.9, 75.5, and 75.9 kJ/mol, respectively. Although CL-20 is more sensitive than HMX in its gaseous state, the E_a of CL-20 is higher than that of HMX when they adsorb and decompose on the Al(111) surface, which indicates that the HMX is even easier to decompose on Al(111) surface as compared with CL-20.