Preparation Of WO₃-DOPED TIO₂-ZrO₂-based NH₃-SCR Catalysts And Investigation On The Mechanism Of Nox Reduction From Flue Gas And Poisoning

NH₃-SCR（NH₃） has been considered as one of the most effecive methods for flue gas denitration since its introduction to China in 1980s. However, for the universality of SCR mechanism and the microscopic process of SO₂ and H₂O poisoning, there is still lack of systematic research on the above questions. In this paper, by taking TiO₂-ZrO₂ as carrier, WO₃ as additive, V₂O₅ as active component, employing such characterization means as BET、XRD、HRTEM and in-situ drift infrared, combined with the testing results of the denitration performance, and screening preparative catalysts, this paper conducts poisoning research of mechanism, SO₂、H₂O、SO₂+H₂O on the best catalyst in terms of denitration performance, and the major results are as follows:Taking TiO₂-ZrO₂ as carrier, using impregnation method to load WO₃ and V₂O₅ respectively, and synthesizing （x%）WO₃/TiO₂-ZrO₂ catalyst and （1%）V₂O₅-（x%）WO₃/TiO₂-ZrO₂ series catalysts, the characterization of catalyst and denitration performance testing results have demonstrated that:the Lewis acid strength increased with WO₃ addition, the BET area of catalyst is in little difference.When the capacity of WO₃ is 9 percent, the denitration of catalyst is most efficient, this is because under this capacity, the pore structure of catalyst is the most stable, with the most active species on the surface and the biggest absorbing capacity.With in situ FTIR experiment, through the comparative study of NH₃-SCR mechanism between （1%）V₂O₅-（9%）WO₃/TiO₂-ZrO₂ catalyst and commercial catalysts, the following conclusions can be arrived at:in standard process of NH₃-SCR, and when the temperature is 300℃, the surface of catalyst is mainly covered with Lewis acid,（the stability of Br（?）nsted in the center is weaker than that of Lewis）, the absorption capacity of NO and NO₂ on the surface is extremely weak, （NO is oxidized as NO₂）, and NO can only be absorbed on the surface of catalyst in the form of NO₂, the comparison of absorption intensity of NH₃（strong absorption） and NO （extremely weak absorption） has determined the fact that denitration process of catalyst can only abide by Eley-Ridel mechanism, in this process, O₂ plays an important part in oxidizing NO and participating in the formation of H₂O. The comparative results of NH₃-TPR and H₂-TPR show that:when the temperature is between 300 and 400℃, there only appears H₂ reduction peak of without NH₃, which means that in the process of denitration, NH₃ is not restored, it only absorbs on the surface of catalyst in the form of coordination compound, the activation of NH₃ lies in the difference of electronegativity between ligand and atom in the center, the above conclusions are applicable to （1%）V₂O₅-（9%）WO₃/TiO₂-ZrO₂ and commercial catalyst in the market. The difference between the two catalysts in the denitration process lies in:commercial catalysts have both Lewis acid site and Bransted acid site, leading to the difference of denitration paths for two catalysts:for （1%）V₂O₅-（9%）WO₃/TiO₂-ZrO₂, there only exists [NH₂-NO-L acid site]reaction path, while commercial catalyst has two reaction paths including [NH4+-NO-B acid site] and [NH₂-NO-L acid site]. The denitration process of two catalysts abide by different reaction paths under the same reaction mechanism.Designing a series of toxic experiment, combined with the corresponding in situ FTIR test, the single SO₂, H₂O and H₂O+SO₂ poisoning mechanism for V₂O₅-（9%） WO₃/TiO₂-ZrO₂ Catalyst is investigated, the conclusion are as follows:（1） The loading of catalyst is conducive to suppressing the absorption of SO₂ and SO₃.（2） When SO₂ is added to the mixed gas, NH₃ and SO₂ can both be absorbed on the surface of the catalyst, just like NH₃, SO₂ has also a pair of lone electron, which can be used as Lewis alkali, whether SO₂ or NH₃ is absorbed on the surface of catalyst first, the other gas will compete with the gas on the surface of the catalyst, and the adsorption capacity of them are equally matched, but there exists no competitive absorption between SO₂ and NO. （3）When H₂O is added to the mixed gas, NH₃ will prefer to combine with H₂O on the surface of the catalyst, the transition of NH₃�'NH4+�'NH₃ has delayed the activating time of NH₃, leading to the reduction of catalyst denitrification rate （4）When H₂O and SO₂ get into the exhaust gas, NH4HSO4 was observed on the surface of catalyst, (but when there is only SO₂ without H₂O, NH4HSO4 does not exist, therefore, H₂O plays an important role in the formation of NH4HSO4. （5）For catalyst, the poisoning of SO₂、H₂O was a slow process, the crystal phase and pore structure will remain unchanged in the short time.