Catalytic Oxidation Reaction Mechanism Of NO Over Cu, Fe Modified ZSM-5Zeolite By DFT Study

Density functional theory (DFT) as a vigorous theoretical research method has greatlyshortened development duration of catalysts and reduced development cost. At the same time,it also can be used to simulate the phenomenon and process that experimental method couldnot do, providing theoretical basis for experimental study. Nitrogen oxides(NOx) havebecome one of the most crucial gas to our environment. Therefore how to clear up NOxhasbeen an extremely concerned project. Many experiments about the elimination of NOx havebeen performed, but it is difficult to delve into the adsorption, oxidation, decompositionmechanism only rely on experimental methods. So in this work, quantum mechanics methodwas employed to study the reaction mechanisms of the adsorption-oxidation of NO ontransition metal Cu and Fe modified ZSM-5zeolite. In the process of calculation, DFT wasused at the levels of B3LYP/6-31G(d) and LANL2DZ with Gaussian03. The main content ofthis paper are listed below:1. We investigated the adsorption of NO on8T Cu and Fe modified ZSM-5zeolite. Thecalculated results showed that NO can be adsorbed on Cu+, Fe+species in two modes: η-Nadsorption mode andη-O adsorption mode. The adsorption eneries of NO in two adsorptionmodes is different, the adsorption eneries of NO on Cu-ZSM-5zeolite are-116kJ/mol and-44kJ/mol, the adsorption eneries of NO on Fe-ZSM-5zeolite are-256kJ/mol and-56kJ/mol. According to the contrast of the optimization results, adsorption eneries and infraredspectras, the configurations inη-N mode are more stable than inη-O modes.2. We also investigated the reaction mechanisms of the oxidation of NO on Cu and Femodified ZSM-5zeolite. The geometries for Cu-, Fe-ZSM-5zeolites represented as8Tcluster, The stationary points and the activation energies were obtained by the optimizecalculation. The results showed that the oxidation of NO on Cu-and Fe-ZSM-5zeolites wasin accordance with the adsorption-elementary reaction-desorption process. Firstly, the NOmolecule adsorbs on zeolite throughη-N mode.Then O2molecule adsorbs on NO to form the adsorbed ONOO*species. Through one step reaction, the ONOO*species forms chemicallyadsorbed NO3-intermediate through the transition state TS1, the activation energies were71kJ/mol and28kJ/mol. then NO3-reacted with the second adsorbed NO molecule to form theNO2product via the transition state TS2. The activation energies were140kJ/mol and158kJ/mol. Finally, the adsorbed NO2removed from ZSM-5zeolites. calculated and compared,According to the contrast of the activation energies, we found that the reaction of NO3-andNO was confirmed as the rate-limiting step.3. We also studied the reaction mechanism of NO and O2on Fe-ZSM-5zeolite by in situDRIFTS. According to the results of in situ DRIFT, NO3-、NO2and N2O4were formed in thereaction of NO and O2on Fe-ZSM-5zeolite and this is in good agreement with the DFTstudy.