Modeling adsorption reactions of ammonium perchlorate on rutile and anatase surfaces.

In this work, the effects of two TiO polymorphs on the decomposition of ammonium perchlorate (NHClO) were studied experimentally and theoretically. The interactions between AP and various surfaces of TiO were modeled using density functional theory (DFT) calculations. Specifically, the adsorption of AP on three rutile surfaces (1 1 0), (1 0 0), and (0 0 1), as well as two anatase surfaces (1 0 1), and (0 0 1) were modeled using cluster models, along with the decomposition of adsorbed AP into small molecules. The optimized complexes of the AP molecule on TiO surfaces were very stable, indicating strong covalent and hydrogen bonding interactions, leading to highly energetic adsorption reactions. The calculated energy of adsorption (ΔE) ranged from -120.23 to -301.98 kJ/mol, with highly exergonic calculated Gibbs free energy (ΔG) of reaction, and highly exothermic enthalpy of reaction (ΔH). The decomposition of adsorbed AP was also found to have very negative ΔE values between -199.08 and -380.73 kJ/mol. The values of ΔG and ΔH reveal exergonic and exothermic reactions. The adsorption of AP on TiO surfaces anticipates the heat release of decomposition, in agreement with experimental results. The most common anatase surface, (1 0 1), was predicted to be more reactive for AP decomposition than the most stable rutile surface, (1 1 0), which was confirmed by experiments. DFT calculations show the mechanism for activation of the two TiO polymorphs is entropy driven.