The effect of oxidation of Al(1 1 1) surface on the catalytic efficiency of decomposition of ammonium perchlorate: A density functional theory study

The study of the decomposition behavior of ammonium perchlorate (AP) on Al surface is of crucial significance to the rational design of aluminum-based solid propellants containing AP. It is of interest to understand the underlying reaction pathways and primary reaction mechanism. Herein, the decomposition reactions of perchloric acid and ammonia molecules on oxidized Al(1 1 1) surface are investigated using density functional theory (DFT) to gain insight into the reaction mechanisms at the molecular-level. The co-adsorption configurations of perchloric acid and ammonia on Al(1 1 1) surface are obtained by energy minimization method, and the activation energy barriers of all elementary reactions are calculated through transition state searching method. In the co-adsorption configurations, perchloric acid decomposes and generates OH group and atomic oxygen and chlorine, whereas the ammonia molecule retains its integrity and forms a hydrogen bond with hydroxyl. When these molecules are adsorbed on oxidized Al(1 1 1) surface in isolation, the activity of HClOx(x = 1–4) is reduced by the formation of hydrogen bond between the 3-fold-coordinated oxygen atom and hydroxyl of HClOx. Furthermore, larger coverage by oxygen atoms adsorbed on Al surface leads to higher energy barriers of the decomposition reactions of HClO4. However, for NH3 the energy barriers of the dehydrogenation reactions are less affected by the degree of oxygen coverage of the Al surface, except for the completely oxidized Al substrate. A series of elementary decomposition reaction steps of AP is proposed to explain the decomposition mechanism of AP on Al(1 1 1) surface.