Study On The Reaction Kinetics Of SCR At Low Temperature

Selective Catalytic Reduction (SCR) with ammonia/Urea is unanimously regarded as one of the most promising technologies for the abatement of NOx emissions from motor vehicles. As the diesel emission regulations increasing stringently, improving the conversion efficiency of NOx at low-temperature operating conditions has become the current research direction.In this paper, affect of operating conditions and reaction gas composition on the catalytic activity of V2O5-WO3/TiO2 catalyst at low temperature was tested in a fixed-bed catalytic reactor, with NH3 as the reductant, in order to analyze the factors that control the reaction rate.Then with combination of TPD and the TRM catalyst characterization techniques, the transient dynamics of SCR catalyst was studied to abtain the catalytic reaction mechanism. Finally the mathematical model of catalytic converter was established to estimate the reaction rate constants.At low temperature, the catalyst activity increased as the space velocity decreased; contact time analysis showed that under the conditions of the experiment, the surface reaction was the controll step; the catalyst activity was increased as O2 concentration increased; H2O would produce a certain inhibition on the conversion of NO; the activity of the catalyst was enhanced with the presence of SO2.Transient dynamic analysis for V2O5-WO3/TiO2 catalyst was carried on. The results showed that NH3 was strongly adsorbed to the catalyst surface, and NH3 adsorption capacity reduced as the temperature increased. NO adsorption had not occurred. Namely, the SCR reaction followed the Eley-Rideal mechanism. A "reservoir"of adsorbed ammonia species was present on the catalyst surface. When the NH3 coverage was below the "critical" NH3 coverage, the SCR reaction rate depended on the catalyst surface NH3 concentration; when the NH3 coverage was above the "critical" NH3 coverage, the SCR reaction rate was independed of the catalyst surface NH3 coverage. Based on the above analysis, the reaction path of NH3-SCR on the V205-W03/TiO2 catalyst was abtained and the equation of NO reaction rate was derived. One-dimensional single-channel transient dynamics model of converter was established to estimate the parameters of dynamic model. Simulation results were in good agreement with the activity tests. The results could be used to predict the NOX emission levels of the engine at different operating models and provide guidance to establish control strategy.