MP2 study on decomposition of nitrous oxide on the Ga‐ZSM‐5

The reaction mechanism for nitrous oxide decomposition has been studied on gallium site in Ga‐ZSM‐5 using the MP2/6‐31+G(d) method. The active centers were taken to be mononuclear [Ga] + , [GaO] + , and [GaO 2 ] + and the surrounding portion of the zeolite was represented by a 3T cluster, namely [AlSi 2 O 4 H 8 ] − . The first elementary step of N 2 O decomposition involves the formation of [GaO] + and the release of N 2 . The metal‐oxo species produced in this step then reacts with N 2 O again, to release N 2 and form [GaO 2 ] + . The calculated activation energies at MP2 level for N 2 O dissociation on Ga‐ZSM‐5 and GaO‐ZSM‐5 are 15.7 and 26.5 kcal/mol at 298 K, respectively. The third elementary step of N 2 O decomposition on GaO 2 ‐ZSM‐5 involves the formation of [GaO 3 ] + and the release of N 2 . The calculated activation energy at MP2 level for N 2 O dissociation on GaO 2 ‐ZSM‐5 is 43.7 kcal/mol. Four‐order perturbation theory (MP4//MP2) predicts that the activation barriers for nitrous oxide dissociation at 298 K on Ga‐ZSM‐5, GaO‐ZSM‐5, and GaO 2 ‐ZSM‐5 are 13.9, 13.0, and 34.4 kcal/mol, respectively. The calculated energy for desorption of singlet O 2 from the 3T − [Ga(O) 3 ] + cluster at MP2 level is 33.4 kcal/mol. When one takes into account the entropy gained on desorption of singlet O 2 , the contribution of entropy to the free energy of desorption is T Δ S = 12.3 kcal/mol at 298 K. The calculated energy of the singlet oxygen desorption from 3T − [OGaO 2 ] + cluster Δ H (298 K) = +43.57 kcal/mol and Δ G (298 K) = 30.13 kcal/mol is significantly higher than the barriers of the singlet molecular oxygen desorption from 3T − [Ga(O) 3 ] + cluster. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008