Research On Hydrothermal Deactivation Mechanism And Modeling Of Diesel-SCR Catalyst

Nowadays,Selective Catalytic Reduction?SCR?technology is the most widely used technology to reduce nitrogen oxides?NOx?emissions for diesel engines.To ensure the function of SCR system,the catalyst should have excellent performance.The vanadium-based SCR catalyst is the most widely used catalyst in diesel engines in China due to its good product maturity,low cost,good sulfur resistance and other advantages.However,it has poor hydrothermal stability.Improving its hydrothermal stability and durability is the key to meet the increasingly stringent emission regulations.In-depth research on hydrothermal deactivation mechanism of the vanadium-based SCR catalyst and the establishment of chemical reaction kinetic model that reflects its deactivation mechanism can provide theoretical basis for the improvement of catalyst composition,the formulation of rapid aging methods,the prediction of catalyst life,the evaluation of durability,etc.In this dissertation,hydrothermal aging experiments of vanadium-based SCR catalysts of different temperature and different time durations were carried out on a flow reaction test bench.The temperature-programmed desorption?NH₃-TPD?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,specific surface area test?BET?and many other characterization methods were used.Measurements of NOx conversion rate,NH₃ oxidation,changes in ammonia storage etc.were carried out to study and explore the hydrothermal aging mechanism of vanadium-based SCR catalysts.Based on the aging mechanism,a dual-active-site hydrothermal aging chemical reaction kinetic model was established.The equivalent and evaluation indexes for aging were discussed.The simulation results of the model were verified and compared with the experimental results to prove the effectiveness of the model.The main work and conclusions are as follows:?1?Research on hydrothermal aging mechanism of vanadium-based catalysts.The sintering of the catalyst carrier during hydrothermal aging process causes the reduction of the ammonia storage capacity and the reduce of specific surface area of the catalyst.The phase change of Ti O₂ only occurs when the catalyst is highly deactivated.The relationship between these aging factors and the reduction of NOx conversion rate of the catalyst was studied.The results show that the correlation coefficient between the ammonia storage capacity and the NOx conversion rate is the highest,indicating that the reduction of the catalyst active site number is the most important factor that causes the reduction of the NOx conversion rate.The experiment results also proved that hydrothermal aging caused a significant difference of the catalyst relative deactivation degree in the high temperature section and the low temperature section.Combined with characterization test results,it is shown that the sensitivity of Br?nsted and Lewis acid sites are different to the hydrothermal aging.This turned to be the most critical factor of the hydrothermal aging effects on the catalyst performance.?2?Research on the law of active sites and aging factors.In order to analyze the changes of the two active sites during hydrothermal aging quantitatively,the NH₃ desorption curves obtained from the NH₃-TPD experiments were divided,and the integrated areas of different peaks were used to represent the number of corresponding active sites.By conducting NH₃-TPD experiments on the catalysts with different hydrothermal aging degree,the variation law of the peak integral area representing different active sites was obtained.Based on the experimental data,the relationship among the number of the two active sites,the aging temperature and the aging time is obtained,which determines the aging factor of the catalyst and characterizes the change of different active sites during hydrothermal aging process.?3?Establish the dual-active-site hydrothermal aging chemical reaction kinetic model.Based on the aforementioned research,this paper builds a dual-active-site hydrothermal aging chemical reaction kinetic model of the catalyst.The model mainly includes NH₃ adsorption and desorption reaction,standard SCR reaction,fast SCR reaction,NH₃ conversion to N₂ reaction,NH₃ oxidation to NO reaction,NO oxidation to NO₂ reaction,and other reactions.The above-mentioned aging factor is added to the corresponding reaction rate equation.The parameters of the model were identified and verified through the reaction kinetic experiments.The results show that the dual-active-site hydrothermal aging reaction kinetic model reflects the hydrothermal aging process of the catalyst accurately.?4?Equivalent index of catalyst aging and application of hydrothermal aging model.On the basis of theory and data analysis,the priority ranking of the performance indexes reflecting different aging degrees of the catalyst after hydrothermal aging is obtained:aging factor?change in the number of active sites or the amount of ammonia storage?>NOx conversion rate,NH₃oxidation rate,NO Oxidation rate.Priority should be given to the aging factor when comparing the aging effects of different aging methods.Single index of the NOx conversion rate,NH₃oxidation rate and NO oxidation rate cannot accurately reflect the performance of the aged catalyst.These indicators need to be used together.Using the chemical reaction kinetic model in this paper and using the aging factor as the evaluation index,can provide help for the design of rapid catalyst aging methods,the prediction of catalyst life,the estimation of catalyst performance.