Density Functional Theory Study of Mechanism of N2O Decomposition over Cu-ZSM-5 Zeolites

Abstract

The N2O decomposition mechanism is investigated over Cu-ZSM-5 using density functional theory (DFT). Though the mechanism is extended from Fe/Co-ZSM-5, the results show that a different step may be rate-determining over Cu-ZSM-5 compared to the Fe/Co-ZSM-5 system. In the beginning, Z[Cu] as active center decomposes the first N2O and generates Z[CuO] (process 1), and the energy barrier of N2O dissociation is 35.18 kcal/mol. Then Z[CuO] could decompose the second N2O and generate Z[CuOO] (process 2), and the energy barrier of N2O dissociation is 28.07 kcal/mol. In process 2, oxygen could desorb from Z[CuOO], and the desorption energy is 39.48 kcal/mol, which is only higher 4.30 kcal/mol than 35.18 kcal/mol in the process 1. However the corresponding rate constants show approximately that the rate-limiting step is O2 desorption in process 2 and not the N2O dissociation in process 1. Next, if Z[CuOO] could not desorb O2, it could decompose the third N2O and generate Z[CuO(O2)] (process 3). In this process, the energy barrier for N2O dissociation and the O2 desorption energy from Z[CuO(O2)] are 42.10 and 63.42 kcal/mol, respectively, which are much higher than the former processes. It indicates the presence of O2 could inhibit the N2O decomposition over Cu-ZSM-5, which is in line with the kinetic experiment. The results suggest the process 1 and 2 are the main catalytic cycle in N2O decomposition. Importantly, O2 desorption from Z[CuOO] shows that the mechanism over Cu-ZSM-5 is different from that over Fe/Co-ZSM-5 system.