Modeling adsorption reactions of ammonium perchlorate on rutile and anatase surfaces

In this work, the effects of two TiO₂ polymorphs on the decomposition of ammonium perchlorate (NH₄ClO₄) were studied experimentally and theoretically. The interactions between AP and various surfaces of TiO₂ 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 TiO₂ surfaces were very stable, indicating strong covalent and hydrogen bonding interactions, leading to highly energetic adsorption reactions. The calculated energy of adsorption (ΔE_ads) ranged from -120.23 to -301.98 kJ/mol, with highly exergonic calculated Gibbs free energy (ΔG_ads) of reaction, and highly exothermic enthalpy of reaction (ΔH_ads). The decomposition of adsorbed AP was also found to have very negative ΔE_dec values between -199.08 and -380.73 kJ/mol. The values of ΔG_dec and ΔH_dec reveal exergonic and exothermic reactions. The adsorption of AP on TiO₂ 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 TiO₂ polymorphs is entropy driven.