Insights Into The Mechanism For The Selective Catalytic Reduction Of NO_x With NH₃ On Mn/ZSM-5 Catalysts:A DFT Study

The nitrous oxides?NO_x?from automobile exhaust emissions have caused devastating damage to environment and human.The vehicle numbers is dramatic increase with the rapid development of our country national economy and the treatment work of NO_x in vehicle exhaust emissions need to be done.The selective catalytic reduction with NH₃ is proved to be the best effective technology in the abatement of NO_x from vehicle exhaust,so that it is a crucial subject to develop new NH₃-SCR catalysts.Our research group prepared Mn/ZSM-5 catalysts,which possess many advantages incluing wide temperature range,excellent catalytic performance and selectivity,with the precipitation method.And on this basis,the density functional theory method is used to investage the mechanism of SCR reaction on Mn/ZSM-5 catalysts in this paper.We build the?MnO?⁺/ZSM-5 and?MnO?²⁺/ZSM-5 cluster models,which contain?MnO?⁺ and?MnO?²⁺ active species,respectively,to research the adsorption of NH₃ and NO and determine the initial step.Then we investigate the reaction pathways along E-R mechanism and L-H mechanism on these two models,the corresponding conclusions are shown as below:1.The adsorption energies,bond lengths,Mulliken populations,and partial densities of state were analyzed after NH₃ and NO adsorption on?MnO?⁺/ZSM-5 and?MnO?²⁺/ZSM-5 models.The results showed that the stability of the adsorbed NH₃ is much higher than the adsorbed NO,then the NH₃-SCR reaction of NO may begin with the adsorption of NH₃,which is consistent with other catalysts.2.On the?MnO?⁺/ZSM-5 and?MnO?²⁺/ZSM-5 models,there are three NH₃-SCR reaction pathways: reaction pathway 1 NH₃?ads??NH2?ads??NH2?ads?+ NO?g??NH2NO?ads??NHNOH?ads??N2 + H2 O along the E-R mechanism;reaction pathway 2 NH₃?ads??NH2?ads??NH2?ads?+ NO?ads??NH2NO?ads??NHNOH?ads??N2 + H2 O along L-H mechanism;reaction pathway 3 NH₃?ads??NH₃?ads?+ NO?ads??NH2NO?ads??NHNOH?ads??N2 +H2O along L-H mechanism.3.On the?MnO?⁺/ZSM-5 model,the elementary reaction in reaction pathway 1 along E-R mechanism: Z–[MnOH-NHNOH]+?Z–[MnOH-N2-H2O]+that the dissociation reaction of NHNOH species is the rate-control step with an activation barrier of 34.39 kcal/mol.There are two reaction pathways alongthe L-H mechanism on the?MnO?⁺/ZSM-5 models,and the both rate-determining steps are the same in reaction pathway 1 with higher activation barrier of 35.41 kcal/mol for the different configuration and dissociation of NHNOH species.The NH₃-SCR reaction along the E-R mechanism is more favorable on the?MnO?⁺/ZSM-5 catalysts.Meanwhile,the activation barriers of the rate-determining steps along different mechanisms are little difference,the reaction along two different mechanisms can occur at a certain temperature.4.On the?MnO?²⁺/ZSM-5 models,the elementary reaction in reaction pathway 1 along the E-R mechanism: Z^2-[MnO-NH₃]2+ ? Z^2-[MnOH-NH2]2+that the oxidation reaction of NH₃ species is the rate-control step with an activation barrier of 43.35 kcal/mol.The rate-determining step of reaction pathway 2 along the L-H mechanism is the same in reaction pathway 1,and the barrier energy of dissociation reaction of NH2 NO species is higher.The elementary reaction in reaction pathway 3 along the L-H mechanism:Z^2-[MnO-NH₃-NO]2+ ? Z^2-[MnOH-NH2NO]2+ that involves the oxidation reaction of NH₃ species is the rate-control step with a higher activation barrier of 44.73 kcal/mol.The NH₃-SCR reaction along the E-R mechanism is more favorable on the?MnO?²⁺/ZSM-5 catalysts.Meanwhile,the activation barriers of the rate-determining steps along different mechanisms are little difference,the reaction along two different mechanisms can occur at a certain temperature.5.The rate-determining step on?MnO?⁺/ZSM-5 catalysts,which contain?MnO?⁺ active species,has a smaller activation barrier.While,on the?MnO?²⁺/ZSM-5 catalysts with the?MnO?²⁺ active species,the rate-determining step has a smaller activation barrier.Then the?MnO?⁺ active species can improve the catalytic activity of Mn/ZSM-5 and promote the NH₃-SCR reaction.