The Research Of Action Mechanism On P-modified V-based NH₃-SCR Catalysts

The increasing emission of nitrogen oxides（NO_x）caused by the modern industry,not only brings damage to natural and ecological environment,but also seriously affects the human health.At present,NO_x is mainly derived from the exhaust emissions of stationary sources,mobile sources and industrial activities.Therefore,it is significant to manage and handle the NO_x emissions,except for the control from the source.So far,among various kinds of NO_x emission control technologies,ammonia selective catalytic reduction（NH₃-SCR）is thought to be one of the most effective techniques for removing NO_x,and the V-based catalyst,as a commercial catalyst,has been widely used in the NH₃-SCR field.However,those of commercial catalysts still have drawbacks of narrow active temperature window and alkali metal poisoning.To solve the problem,the modification with promoted elements doping has been widely applied to improve their catalytic performance.In current studies about NH₃-SCR catalyst modification,the phosphorus and some phosphorus-based compounds have been thought as a type of toxic substance,which leading to the deactivation of the SCR catalyst,but the effect mechanism has not been studied completely and accurately.In this thesis,different phosphorus species were introduced to modify the V-based catalyst and adjusted the coordination form between the additives and the active component on the catalyst surface at the molecular level.By virtue of the structural characteristics,physico-chemical properties of the P-doped NH₃-SCR catalyst were analyzed to reveal the influence of phosphorus species on the interaction between reactant molecules and catalysts,further elucidating the mechanism of phosphorus modification and providing a guidance for the modification and design of V-based commercial SCR catalysts.The main research contents and results were as follows:（1）Using the strong acidic phosphotungstic heteropolyacid as the object,V element was introduced into the molecular structure of heteropolyacid to obtain the V substituted phosphotungstic heteropolyacid,further loading into TiO₂support to obtain the PW₁₁V/TiO₂ catalyst.The activity result showed that the the PW₁₁V/TiO₂ catalyst show significantly better catalytic performance than that of the commercial phosphotungstic heteropoly acid catalyst with excellent N₂selectivity and the resistance of H₂O and SO₂ as well.The outcomes of characterization revealed that the enhancement of catalytic performance is owing to the successful insertion of V into the heteropoly acid structure,which greatly improved the redox property of phosphotungstic acid.（2）Taking phosphate as the research object,we investigated the effects of P doping on the structure and SCR performance of V₂O₅/TiO₂catalyst.Surprisingly,this work showed that the introduction of P element greatly promoted the NH₃-SCR catalytic activity of the V₂O₅/TiO₂ catalyst,which is not consistent well with previous studies.The related characterization results showed that the specific surface area and surface acidity of V₂O₅/TiO₂ catalyst distinctly increased after P doping.Furthermore,more highly active polymerized and reduced VO_xspecies were formed.In-situ DRIFTs revealed that although P doping occupied the original Lewis acid sites on the catalyst surface,it brought a large number of Br?nsted acid sites as a supplement.In addition,the introduction of P element greatly inhibited the adsorption of NO_x,and the NH₃-SCR reaction on the surface of the P-modified V₂O₅/TiO₂ catalyst mainly followed the E-R mechanism.（3）We further investigated the effects of P doping on the structure and SCR performance of V₂O₅-WO₃/TiO₂ catalysts.Similar with the result of last part,the addition of P also increased the specific surface area and surface acidity of the catalyst,and a twice increase in turnover frequency（TOF）per surface V atom was observed.Transient in-situ infrared showed that the Lewis acid site was the main active center,and kinetic studies showed that the P-modified V₂O₅-WO₃/TiO₂catalyst also followed the E-R mechanism during the NH₃-SCR reaction.Both in-situ Raman and ⁵¹V NMR indicated that the introduction of P promoted the generation of highly active polymerized VO_x species through a structural effect,which led to a higher NH₃-SCR reaction rate,and this structural effect probably induced by synergistic effect between P and W.