Theory On Catalytic Decompotion Mechanism Of NO Over Fe-ZSM-5

With the rapid development of modern industry and the increasing number of motor vehicles,the environmental problems caused by industrial exhaust gas and motor vehicle exhaust gas become more and more serious.As the main component of air pollution,the emission of resulting NOx is increasing,which has a serious impact on human normal life and physical and mental health and also prevents economic development and human living quality.At present,V₂O₅-MoO₃/TiO₂ and V₂O₅-WO₃/TiO₂ catalysts are employed to remove NOx with NH₃ as reductant in deNOx industry.However,the biological toxicity of V₂O₅ will threaten the ecosystem and human health under the condition that it is discharged easily with the exhaust into the atmosphere.In recent years,due to the continuous development and innovation of zeolite catalysts which employ alkali metal oxides or transition metal oxides as catalytic active center,the efficiency of direct decomposition reaction of NOx is improved and the cost is greatly reduced.Together with the advantages such as simple process,no side effects of secondary pollutants and low cost,the direct decomposition reaction of NOx is focused on once again.Based on the above-mentioned idea,in this paper,density functional theory methods are applied to investigate the sequential adsorption of two NO molecules and corresponding direct decomposition mechanism on the surface of 18 T Fe-ZSM-5 cluster model by means of the DMol3 module of Materials Studio6.0.The main conclusions are summarized as follows:?1?There are four stable configurations of the first NO molecule adsorption on Fe-ZSM-5,all of which are exothermic and stable chemisorption.The order of stability is as follows: the most stable configuration is NO adsorption as N-down on Fe atom.The next is NO adsorption when both N and O atoms adsorbed on Fe-ZSM-5.The least stable configuration is NO adsorption as O-down.Three of them are different adsorption configurations of NO molecule on Fe atom,while another one is in fact the reaction of NO molecule with lattice oxygen to produce NO₂ and then adsorption on Fe atom.NO molecules are activated after adsorption the adsorption reactivity is consistent with the results predicted by Fukui function index of Fe-ZSM-5.?2?All possible catalytic decomposition processes are studied by the exhaustion method in which four stable configurations of the first NO molecule adsorption on Fe-ZSM-5 act as initial reaction configuration respectively,adsorptions of the second NO molecule act as the initial reaction step,N₂ O molecule acts as the key reaction intermediate,N₂ and O₂ act as the target products.four direct decomposition mechanism of NO on Fe-ZSM-5 surface are investigated and confirmed by transition state search and frequency verification analysis.All the four obtained decomposition mechanisms can result in gaseous products N₂ and O₂.At the same time the Fe-ZSM-5 catalyst model will renew to the initial state to continue to catalyze reaction,thus the catalyst cycle is completed.In the four obtained reaction paths the energy barrier is the lowest and reaction steps are fewest while both N and O atoms of the first NO molecule adsorb on Fe-ZSM-5.The full reaction path is described as “NO?g?? NO?ads??NO?ads?+NO?ads??OONN?ads??O₂?ads?+N₂?ads??O₂?ads?+N₂?g??O₂?g?+N₂?g?” and the total reaction energy is-43.74 kcal/mol.The rate-determining step is the formation of OONN?ads?,i.e.NO?ads?+NO?ads?? OONN?ads?and its barrier energy is 25.73 kcal/mol.