Study On The Mechanism Of Cu-ZSM-5 Catalyzed N₂O Decomposition And No_X Reduction

Industrial production of adipic acid and caprolactam will emit a large amount of waste gas containing N₂O,NO and NO₂.Eliminating nitrogen oxides and reducing greenhouse gas emissions are the focus of green production of chemical products.At present,the combination of N₂O direct decomposition technology and NOx-NH₃-SCR technology is mainly used to treat this type of chemical flue gas.Although this combination achieves a respectable nitrogen oxide removal effect,it has disadvantages such as long process flow,high treatment cost and secondary pollution.If a new technology with short process and multi-pollutant removal is developed,which will reduce the operating costs and avoid the generation of secondary pollutants,the cost of waste gas purification treatment will be greatly reduced.Therefore,the development of a dual-functional catalyst for the direct catalytic decomposition of N₂O and the catalytic removal of NO_x is of great significance for the simultaneous removal of multiple nitrogen oxides in single industrial reactor.In addition,theoretical studying the catalytic process mechanism and the transition state structure and obtaining key structural parameters and thermodynamic data of important intermediate states will be of great value to the design and development of the catalyst.In present work,the mechanisms of the N₂O direct catalytic decomposition and the NO_x-NH₃-SCR over the Cu-ZSM-5 catalyst are systematically studied using the density functional theory method.The role of NO and NO₂ in promoting the conversion of active centers is discussed.At the same time,the feasibility of simultaneous removal of N₂O and NOx over the binuclear Cu-ZSM-5 is demonstrated,and the catalytic cycle path is proposed.The main work and conclusions are as follows.1.Based on the mono-nuclear Cu-ZSM-5,the O₂ formation mechanism and the NO formation mechanism of the N₂O direct catalytic decomposition are studied.The reaction paths of two N₂O molecules adsorbed on the active center with O or N terminal and decomposed to O₂ or NO are discussed,and the structural parameters,vibration frequency and thermodynamic data of each intermediate product,transition state,etc.are obtained.By comparing the energy barriers,it is found that both the O₂ formation mechanism and the NO formation mechanism can occur,and the NO formation mechanism has relatively higher activity.The rate-limiting step of the first N₂O molecule decomposition is the N₂O activation process.The rate-limiting step when the second N₂O molecule reacts according to the O₂ formation mechanism is the O₂desorption process,and when the NO formation mechanism is the second NO desorption.2.Based on the binuclear Cu-ZSM-5 active center,the reaction mechanism of the direct catalytic decomposition of N₂O is simulated.The reaction path of two N₂O molecules adsorbed on the active center in turn and decomposed to produce N₂ and O₂ is discussed,and the structural parameters,vibration frequency and thermodynamic data of each intermediate product,transition state,etc.are obtained.It is found that N₂O can be adsorbed and catalytically decomposed over the binuclear Cu-ZSM-5 active center in the sinusoidal channel.The activation of the first N₂O molecule has a low energy barrier,and the rate-limiting of the entire reaction is the second N₂O molecule activation.After the first N₂O molecule is decomposed,the active center has a stable（Cu₂O）²⁺structure,which can be used as the catalytic active center for the NOx-NH₃-SCR reaction.3.Based on the（Cu₂O）²⁺/ZSM-5 catalyst active center,the NO_X-NH₃-fast SCR reaction mechanism is simulated.Two reaction mechanisms of NO-NH₃-SCR are comparatively studied.It is found that the adsorption of NO helps to reduce the activation energy of NH₃,and the rate-limiting step is the NH₂ de-H process.Furthermore,two reaction mechanisms of NO₂-NH₃-SCR are comparatively studied.It is found that the different adsorption positions of NH₃have an effect on the structure of the intermediate product but have little effect on the reaction energy,and the rate-limiting step is the NH₃ de-H process.In addition,the structural parameters,vibration frequency and thermodynamic data of each intermediate product,transition state,etc.are obtained.4.Based on（Cu₂O）²⁺/ZSM-5 catalyst active center,the reaction mechanism of NO and NO₂ promoting the conversion of catalytic active center is simulated.The reaction path of the process of reducing the（Cu₂O）²⁺/ZSM-5active center to the binuclear Cu-ZSM-5 active center promoted by NO and NO₂is discussed,and the structural parameters,vibration frequency and thermodynamic data of each intermediate product,transition state,etc.are obtained.It is found that NO has extremely high oxygen carrying capacity,which can greatly increase the conversion rate of catalytic active centers,and NO₂ can also increase the conversion rate.5.The catalytic decomposition coupling behavior of N₂O and NO_Xco-feeding over the binuclear Cu-ZSM-5 is investigated.The results show that N₂O is adsorbed and decomposed on the active center first to generate N₂,and the active center is changed from binuclear Cu-ZSM-5 to（Cu₂O）²⁺/ZSM-5.Then the NO_X fast NH₃-SCR reaction occurs,and finally the active center is reduced to the initial state under the action of NO and NO₂.Since then,the catalytic cycle of combined removal of N₂O and NO_X has been completed.It can be seen that the combined removal of N₂O and NO_X can be achieved over the binuclear Cu-ZSM-5 catalyst.The catalytic cycle for the combined removal of N₂O and NO_X over the binuclear Cu-ZSM-5 is proposed.This conclusion will provide meaningful theoretical guidance for the design and development of a dual-functional catalyst for the integrated removal of N₂O and NOx.