Post‐Hartree‐Fock (MP2 and MP4) study on decomposition of nitrous oxide on the nonframework AlO(+) site in ZSM‐5 zeolite

The reaction mechanism for nitrous oxide (N 2 O) decomposition has been studied on AlO(+) site in AlO‐ZSM‐5 zeolite using the MP2/6‐31+G(d) method. The active centers were taken to be mononuclear [AlO] + and [AlO 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 [AlO 2 ] + and the release of N 2 . The metal‐peroxo species produced in this step then reacts with N 2 O again, to release N 2 and form [AlO 3 ] + . The calculated activation energies at MP2 level for N 2 O dissociation on AlO‐ZSM‐5 and AlO 2 ‐ZSM‐5 are 26.9 and 38.2 kcal/mol at 298 K, respectively. The calculated reaction enthalpies at MP2 level for N 2 O dissociation on AlO‐ZSM‐5 and AlO 2 ‐ZSM‐5 are −25.1 and −21.3 kcal/mol at 298 K, respectively. Four‐order perturbation theory (MP4//MP2) predicts that the activation barriers for N 2 O dissociation at 298 K on AlO‐ZSM‐5 and AlO 2 ‐ZSM‐5 are 12.9 and 29.5 kcal/mol, respectively. The calculated energy for desorption of singlet O 2 from the 3T − [Al(O) 3 ] + cluster at MP2 level is 46.4 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 = 11.1 kcal/mol at 298 K. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011