Density Functional Theory Study Of Two State Reactivity Of NO Activation On Iron Clusters Fe₂/Fe₄

As the main air pollutant, nitrogen oxides can cause the acid rain and photochenmistry, which exerts a significant influence on the human beings health and environment. At present, there will exists problems of secondary pollution and high cost when using NH3-SCR. As a consequence, it has been a sorely needed problem that developing an environment-friendly, lower cost way of nitrogen oxide removal.Based on the results of experiments of our group, nitric oxides can be reduced by iron directly. Nevertheless, the micromechanism of nitric oxide reduced by iron was lack. Whereas, the kinetic characteristic of reaction system can be revealed by density functional theory (DFT), which can analyse the micromechanisms between the iron clusters and nitric oxide.Based on the previous results of our group, the system of Fe atom and NO was considered as small, which make the intermediates and transition states different from the real reactions. In this paper, the mechanism of the reactions between iron clusters (Fe2 and Fe4) and NO were studied by the density functional theory (DFT) with the B3LYP methods combined with the 6-311+G (d, p) basis set. The geometry optimizations of reactants, transition states, intermediates and products of the reaction systems were completely confirmed, and all the transition states were verified by the vibration analysis and the intrinsic reaction coordinate calculations at Gaussian 09 suite of programs. The topological properties of bond critical points of stationery points were received by atom in molecule theory; in the light of activation energies, parameters of stationery points, the reaction rate constant (RRC) of reactions was calculated. Meanwhile, we can reached the following conclusions:1. In Fe2+NO system, there is a crossing point (CP) existing between the octet and dectet potential energy surface (PES), which belongs to the "Two State Reactivity" (TSR) and benefits the kinetic and thermodynamic aspects of this reaction. The cleavage of Fe-N bond is the rate-determining step. Before CP, bond orders of stationery points on octet PES are more stable than the dectet PESs’.After CP, bond orders of stationery points on dectet PES are more stable than the octet PESs’, which is consistent with the results of TSR. Utilizing the classical transition state theory, the RRCs of different PESs can be fitted, the existence of CPs can make the reaction rate constant larger.2. In Fe4+NO system, there is a crossing point(CP) existing between the 14-et and 16-et potential energy surface (PES), which belongs to the "Two State Reactivity" (TSR) and benefits the kinetic and thermodynamic aspects of this reaction. Before CP, bond orders of stationery points on 14-et PES are more stable than the 16-et PESs’; after CP, bond orders of stationery points on 16-et PES are more stable than the 14-et PESs’, which is consistent with the results of TSR. Utilizing the classical transition state theory, the reaction rate constants of different PESs can be fitted, the existence of CPs can make the rate constant of reaction larger. Comparing the activation energies, bond order and reaction rate constant of two reactions at same calculation level, the Fe4+NO system may be more favorable.In the light of the researches, the geometry and relative energy of intermediates, transition states of systems between Fe2/Fe4 and NO was received, which can offer theoretical basis the reactions of Fe2/Fe4 and NO.