Experimental And Kinetic Study Of V₂O₅/TiO₂-based Catalysts In The Selective Catalytic Reduction (SCR) Of Nitrogen Oxides By Ammonia

Since the pollution of nitric oxides (NO_x) in china has been more and more serious in recent years, it is necessary to take measures to control emission of NO_x. In the dissertation,selective catalytic reduction (SCR) of NO_x by ammonia was adopted. DeNO_x performance of catalyst preparation was tested in a fixed-bed reactor. With combination of various surface characterization information and previous study results, how catalyst composition and operation parameters affect the behavior of conversion of NO_x was investigated. In addition, with the consideration of both internal and external diffusions of the reactants, kinetics modeling of catalyst has been established and can be applied for the operation optimization.In catalyst preparation, with V₂O₅-based catalyst grafted anatase TiO₂ and two V₂O₅-based catalysts grafted on different nano-grade anatase TiO₂ which two different corporations provided, V₂O₅, WO₃ or MoO₃ are introduced on the TiO₂ support through wet impregnation. In the activity experiments NO, SO₂, O₂ and N2 are used to simulate flue gas. NH₃, as reducing agent, is added in the stream to reduce NO.DeNO_x performance of catalyst grafted on nano-grade anatase TiO₂ provided by factory A is the worst. That of catalyst grafted on commercial anatase TiO₂ is better than it. Of all catalyst activity, the best one is catalyst grafted on nano-grade anatase TiO₂ provided by factory B. In addition, it is better than the commercial catalyst. The basic reason is that nano-grade TiO₂ A reacted with V₂O₅ during the preparation process, and that the surface area of commercial anatase TiO₂ became small. Therefore, activity and kinetics of catalyst grafted on nano-grade anatase TiO₂ B were researched in the study.Catalyst activity is increasing with V₂O₅ content, while DeNO_x performance has a limit and it also results in more N₂O emission when V₂O₅ content is greater than 1%. 95% ⁹9% NO conversion efficiency can be achieved in our experiment during temperature between 300-400℃, the peak value is at the point of 390℃.When NH₃/NO ratio in the feed is less than 1, NO_x reduction increases linearly with NH₃/NO_x. But when NH₃/NO_x is more than 1, the degree increases slowly. The experiment also shows that O₂ (O₂>2%) has little effects on NO_x reduction. NO_x in the flue gases from power plant (300⁴00℃, O₂ 5%) can be reduced efficiently in SCR process.For the catalysts of (1%w)V₂O₅-(10%w)WO₃-TiO₂(B, Nano-grade,Anatase), the SCR is a first order reaction and its intrinsic activation energy is 73.29 kJ/mol. The reaction is objected to Eley-Rideal mechanism——the NH₃ was first absorbed to the active site of the surface it reacted with the NO in the gas phase and then formed N2 and water. External mass-transfer phenomena are of no significance for the catalytic reduction of NO with NH₃ and O₂. But pore diffusion is very important to the reaction.The overall NO_x removal efficiency is expressed as an explicit function and a simple analyticsolution is obtained. There is great agreement between the experimental and modeling results. It has shown that the model equation can successfully reflect the conversion trend, and this provides the theoretical basis for designing a catalytic reactor.