DFT Study On Catalytic Decomposition Mechenism Of NO_x Over V-ZSM-5

With the rapid development of global industrialization, the air pollution is increasingly serious. Nitrogen oxides as one of the main atmospheric pollutants, how effectively clear up nitrogen oxides attracting widespread interest. In this work, on the basis of the experimental and theoretical study experience, the NO direct catalytic decomposition and selective catalytic reduction methods over vanadium modified ZSM-5zeolite were studied using density functional theory (DFT) method.First, we emphatically studied the NO adsorption behaviors on the [VO] active site, which including adsorption of NO by N-end, O-end and parallel adsorption mode. The calculation datas show that the NO adsorption on V is mainly by N-end mode. Then we investigated in the NO direct catalytic decomposition reaction mechanism over [VO] active site and obtained the optimum decomposition path, respectively. The results showed that NO direct decomposition process is via N2O intermediate formation, and the activation energy (0.51eV) of N2O formation is low which indicated that the formation of N2O is relatively easy. As the N2O decomposition, NO decomposes to N2and O2. It implies that O2desorbs from the V-ZSM-5system with an active energy of2.65eV may be the rate-limiting step of NO direct decomposition. In order to investigate the activity of different sites in the role of NO direct catalytic decomposition, we further investigated in NO direct catalytic decomposition reaction mechanism over [V] active site, and obtained the optimum reaction path. The results show that the activation energy of N2O formation over the [V] active site (2.06eV) is larger than the activation energy of N2O formation over the [VO] active site. The activation energy of adsorbed O2formation on V (4.57eV) is also larger than the desorption energy of O2on the [VO] active site (2.65eV).Then, we systematically studied the V-ZSM-5zeolite catalytic performance in ammonia selective catalytic reduction (NH3-SCR) reaction. The NH3molecule reacts with the adsorbed NO to form NH2NO instable intermediate and new adsorbed OH. Then, the free NH3reacts with adsorbed NO2to form NH2NO and water. The unstable intermediate NH2NO can easily decompose into N2and H2O on the Br0nsted acid of ZSM-5zeolite. We also examine the possibility of NH2NO decomposition process on V. The results show that NH2NO decomposes on V with a not very high activation energy (1.75eV and1.63eV).