Mechanisms of the ammonia oxidation by hydrogen peroxide over the perfect and defective Ti species of TS-1 zeolite

Abstract

The catalytic performances of titanium species in TS-1 zeolite for the hydroxylamine formation have been investigated using the density functional theory with the ONIOM scheme. The reaction process for making hydroxylamine is divided into two steps: (i) the H₂O₂ decomposition over the Ti species to produce the peroxo titanium species and (ii) the NH₃ oxidation over the generated oxidizing species. Our results indicated that defective Ti species in the TS-1 zeolite are the dominant catalytic sites for H₂O₂ decomposition rather than perfect Ti species, leading to the formation of Ti–OOH species as oxygen-donating intermediates for NH₃ oxidation reaction. The energetic profiles for the ammonia oxidation over the TiOOH species and the catalytic effect from water were fully investigated, consisting of three proposed mechanisms. The most favored pathway was found to be: the adsorption of ammonia (NH₃/η¹TiOOH) → ammonia oxide complex (NH₃O/TiOH) → hydrated-titanium-oxyamine species (H₂O/TiONH₂) → hydroxylamine product (NH₂OH/TiOH), in which the highest energy barrier is 16.3 kcal mol⁻¹. Besides the hydrolysis of titanium-oxyamine species, the hydroxylamine was also generated through the second H₂O₂ decomposition over the titanium-oxyamine species whereas the activation energy for this step was slightly decreased to be 15.7 kcal mol⁻¹.