| NH3-SCR reaction mechanism over different DeNOx catalysts have been studied for a long time.The existing mainstream NH3-SCR reaction mechanism is mainly based on the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism.However,the following problems still remain in the current research process:(1)The metal ions in metal oxides catalysts had interaction with adsorbed NH3 species,and it could influence the NH3-SCR reaction.However,the specific mechanism is still unclear;(2)In the existing NH3-SCR reaction mechanism of Cu/SAPO-34 zeolite,the gaseous NO was first adsorbed by Cu2+ and then oxidized to form adsorbed NO2 prior to the SCR reaction.Therefore,the reaction order of SCR with respect to gaseous NO concentration should be 0.However,our preliminary experimental results showed that the reaction order of SCR with respect to gaseous NO concentration is obviously larger than 0.Therefore,the SCR reaction of Cu/SAPO-34 zeolite may have other ways.In this paper,we used steady-state kinetic models and in situ DRIFT as the main methods to study on NH3-SCR reaction mechanism over several catalysts.According to the model,we could obtain the kinetic rate constants of SCR reaction through the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism.By combining the kinetic rate constants,XRD,XPS,TPD and H2-TPR,the effects of Eley-Rideal mechanism and Langmuir-Hinshelwood,reaction conditions and properties of catalysts could be analyzed by semi quantitative analysis.The specific research results were as follows:(1)According to the the kinetic equation,the formation of NH2 could be expressed as the product of k3,the concentration of adsorbed NH3 and Mn4+.The H2-TPR results showed that the oxidation capacity of Cr/TiO2 is much higher than that of Mn/TiO2,so the k3 of Cr/TiO2 catalyst is much larger than that of Mn/TiO2-The XPS results showed that the concentration of Cr6+ on Cr/Ti02 is about 1.4 times than the concentration of Mn4+ on Mn/TiO2.The NH3-TPD results showed that the concentration of absorbed NH3 on Cr/TiO2 is about 0.75 times than that of Mn/TiO2.Therefore,the concentration of NH2 on Cr/TiO2 should be much higher than that of Mn/TiO2.However,the kinetic rate constants showed that the kE-R of Cr/TiO2 is much smaller than that of Mn/TiO2.kE-R could be expressed as the product of k1 and the concentration of NH2.Therefore,the k1 of Cr/TiO2 should be much smaller than that of Mn/TiO2.k1 was the kinetic rate constant of the reaction between NH2 and gaseous NO.The reaction needed to overcome the interaction between NH2 and the metal ions in the catalyst.It has been found that the interaction between Cr6+ and NH2 is stronger than that of Mn4+,which lead to the k1 of Cr/TiO2 much smaller than that of Mn/TiO2.Therefore,the kE-R of Cr/TiO2 is much smaller than that of Mn/TiO2.(2)The results of in situ DRIFT showed that both Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism can contribute to the SCR reaction of Cu/SAPO-34,which was similar to the mechanism of NH3-SCR reaction over metal oxide catalysts.The molecular NH3 adsorbed on the Lewis acid sites could directly participate in the NH3-SCR reaction,whereas ionic NH4+ adsorbed on the Br(?)nsted acid sites were used as a reserve species.The steady-state kinetic experiment results showed that the reaction order of SCR with respect to gaseous NO concentration is obviously larger than 0.The competitive adsorption between NH3 and NOx occured on the surface of Cu/SAPO-34 zeolite at high temperature. |
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