Mechanistic Investigation Of Low Temperature SCR Over Hybrid Catalysts

The coal-based energy consumption structure of China will not change in the short term,and a large number of pollutants will be emitted during the use of coal such as Nitrogen oxides?NOx?.In order to reduce NOx emissions below national standards,various denitrification technologies have been developed and applied.Among them,the selective catalytic reduction with NH₃?NH₃-SCR?as a reducing agent has attracted widespread attention due to its high efficiency and stability.Catalysts are the focus and key of SCR technology,so a lot of research work revolves around developing efficient catalyst formulations.For efficient and rapid deployment of new catalysts,research on the reaction mechanism of SCR is essential.This article focuses on the low-temperature reaction mechanism of SCR,and applies this mechanism to develop and test oxidation-enhanced hybrid catalysts.At the same time,it conducts a mechanism-level study on the factors affecting the performance of hybrid catalysts.The full text of the research is as follows:Firstly,the intermediate species produced by NO oxidation activation during low-temperature SCR reaction and their effects on the low-temperature activity of the catalyst were studied.Through the NO fixed temperature adsorption+temperature desorption experiment,combined with chemical trapping method,the nitrite in the low temperature SCR process was captured and identified,and the in-situ infrared spectroscopic analysis was used to further identify the nitrite intermediate product.At the same time,by analyzing the relationship between the production of NO oxidation activation products on the surface of various supported metal oxides catalysts and the activity of the catalyst,it is proposed that NO oxidation activation is one of the important steps in the low temperature SCR reaction process.The NO oxidation ability of catalyst has an important effect to its activity.Then,combined with the theory of NO oxidation activation,using V₂O₅-WO₃/TiO₂,a classic SCR catalyst as a model catalyst,a conclusion that its poor low temperature activity ability is caused by its poor NO oxidize and activate ability during low temperature was proposed through intermediate product capture identification experiments.Based on this knowledge,by mixing an oxidation-type catalyst,the catalyst's ability to oxidize NO is enhanced,thereby improving its low temperature performance.The experimental results show that the low-temperature activity of V₂O₅-WO₃/TiO₂ can be greatly improved by the mixed oxidation catalyst.The results further validate the NO oxidation activation mechanism.At the same time,in order to find a better mixing method,this paper explores the factors that affect the performance of hybrid catalysts.Finally,based on the performance results of hybrid catalysts and the conclusions of previous studies on SCR mechanism,the effect of NO oxidation activation and surface NH₃ adsorption on hybrid catalysts were carried out through experiments of NH3 constant temperature adsorption+temperature desorption,NH₃ constant temperature adsorption+transient reactions.Based on the above research sesults and conclusions,the mechanism of enhanced low-temperature performance of hybrid catalysts was analyzed using chemical reaction kinetics.The analysis results show that the reason for the performance improvement of the hybrid catalyst lies in the synergy between the two catalyst components:the free nitrite produced by the oxidation and activation of NO on the surface of the oxidation-type catalyst can be transferred between the catalyst components,so as to interact with the surface adsorbed NH₃ on the SCR catalyst for further reactions to form N₂ and H₂O,completing the SCR reaction.