Effects of TiO2 Anatase Polymorph on Ammonium Perchlorate Decomposition

Ammonium perchlorate (AP) is a common oxidizer for solid propellants. It undergoes decomposition characterized by initial endothermic dissociation and gasification followed by an exothermic reaction of gaseous products. The acceleration of the initial gasification process contributes to an overall increase in the AP decomposition rate, thereby increasing the oxidizer's potential for power generation. In this study, we use density functional theory to examine the mechanism for the acceleration at two common surfaces of the titanium dioxide (TiO2) mineral anatase (001) and (101) on AP decomposition, as well as the impact of surface oxygen vacancies (O-vac) on anatase reactivity with AP. The adsorption of immediate AP gasification products, NH3 and HClO4, on the anatase surfaces was evaluated in terms of adsorption energies and calculated atomic Bader charges to elucidate the interactions between anatase surfaces and AP. The calculated enthalpy of adsorption (Delta H ads) revealed strongly exothermic adsorption reactions. At the experimentally observed highest decomposition temperature of (similar to 700 K), Delta H ads was -159 kJ/mol for the (001) surface and -172 kJ/mol for the (101) surface, respectively. The presence of an O-vac significantly altered these values, resulting in -243 kJ/mol for the (001) surface and -387 kJ/mol for the (101) surface at 700 K. These findings underscore the crucial role of oxygen vacancy defects in increasing adsorption, which in turn promotes the subsequent AP decomposition rate.